light can travel in

Speed of Light Calculator

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With this speed of light calculator, we aim to help you calculate the distance light can travel in a fixed time . As the speed of light is the fastest speed in the universe, it would be fascinating to know just how far it can travel in a short amount of time.

We have written this article to help you understand what the speed of light is , how fast the speed of light is , and how to calculate the speed of light . We will also demonstrate some examples to help you understand the computation of the speed of light.

What is the speed of light? How fast is the speed of light?

The speed of light is scientifically proven to be the universe's maximum speed. This means no matter how hard you try, you can never exceed this speed in this universe. Hence, there are also some theories on getting into another universe by breaking this limit. You can understand this more using our speed calculator and distance calculator .

So, how fast is the speed of light? The speed of light is 299,792,458 m/s in a vacuum. The speed of light in mph is 670,616,629 mph . With this speed, one can go around the globe more than 400,000 times in a minute!

One thing to note is that the speed of light slows down when it goes through different mediums. Light travels faster in air than in water, for instance. This phenomenon causes the refraction of light.

Now, let's look at how to calculate the speed of light.

How to calculate the speed of light?

As the speed of light is constant, calculating the speed of light usually falls on calculating the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

Source: Light
Speed of light: 299,792,458 m/s
Time traveled: 100 seconds

You can perform the calculation in three steps:

Determine the speed of light.

As mentioned, the speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s .

Determine the time that the light has traveled.

The next step is to know how much time the light has traveled. Unlike looking at the speed of a sports car or a train, the speed of light is extremely fast, so the time interval that we look at is usually measured in seconds instead of minutes and hours. You can use our time lapse calculator to help you with this calculation.

For this example, the time that the light has traveled is 100 seconds .

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m

What is the speed of light in mph when it is in a vacuum?

The speed of light in a vacuum is 670,616,629 mph . This is equivalent to 299,792,458 m/s or 1,079,252,849 km/h. This is the fastest speed in the universe.

Is the speed of light always constant?

Yes , the speed of light is always constant for a given medium. The speed of light changes when going through different mediums. For example, light travels slower in water than in air.

How can I calculate the speed of light?

You can calculate the speed of light in three steps:

Determine the distance the light has traveled.

Apply the speed of light formula :

speed of light = distance / time

How far can the speed of light travel in 1 minute?

Light can travel 17,987,547,480 m in 1 minute . This means that light can travel around the earth more than 448 times in a minute.

Speed of light

The speed of light in the medium. In a vacuum, the speed of light is 299,792,458 m/s.

How Does Light Travel? Does It Travel Forever?

Last Updated on Jan 27 2023

Light is almost always around us, but it’s not something that most people think twice about.

If you do start thinking about it, you might wonder how light travels and for how far. The answer is that light moves as a wave, and it can go on forever. Let’s go over what that means and dive a little deeper into the topic.

How Does Light Travel?

Since light travels like a wave, it can travel through a vacuum without interacting with anything. However, when light does go through something, that object can absorb some of it. Light travels through these objects, like glass and water, leaving heat behind.

Think of a flashlight . When you turn it on and face it toward a pool, the light can travel through the pool. However, if the flashlight isn’t bright enough, it won’t illuminate the bottom of the pool.

It’s also worth noting that light travels in all directions. If you have a lightbulb in the middle of the room with no lampshade, the light is going to travel in every direction. This is a basic principle of light, and it’s why when we create light to use, we must find ways to direct it.

Both flashlights and lamps do this, and it’s how we control where the light goes, by reflecting it off the surface of the object.

How Far Does Light Travel?

Light can travel for infinity. It doesn’t have a set range, and that’s why we can see light from billions of miles away.

It’s also why when we point the James Webb or the Hubble telescope out into deep space, we can see even farther, observing the light from galaxies from the very early universe.

However, when light hits objects like planets or other matter, this either reflects or absorbs the light. The most extreme example of this is a black hole. If light hits a black hole , the gravity is so strong that it can’t escape, thus ending the distance that the light travels.

Other Things That Affect the Movement of Light

Sometimes, gravity can pull on light without completely absorbing it or reflecting it.

This is common with black holes, neutron stars, and even large stars. As light passes by these objects, their respective gravities can pull on the light. If the light is far enough away, it will continue to move away from the object, but it will have a new trajectory.

If you look at a ray of light from above, what it will look like when this is happening is that the light will bend. So, while light travels in a straight line in all directions from an object, the gravitational pull of different objects can shift where that light ends up.

Final Thoughts

When you’re trying to figure out the science in the world, it can all seem a bit complicated. Hopefully, now that you know more about how light moves, you can move on to new questions and contemplate the ways that different things work.

That’s what science is all about, and if you’re asking questions like how light moves, an interest in science might just be in your future!

Featured Image Credit: nepool, Shutterstock

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About the Author Robert Sparks

Robert’s obsession with all things optical started early in life, when his optician father would bring home prototypes for Robert to play with. Nowadays, Robert is dedicated to helping others find the right optics for their needs. His hobbies include astronomy, astrophysics, and model building. Originally from Newark, NJ, he resides in Santa Fe, New Mexico, where the nighttime skies are filled with glittering stars.

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How Does Light Travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century’s BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern era, the 20th century led to breakthroughs that showed us that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein’s Theory of Relativity . However, there remains many unanswered questions about light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light to the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle’s theory of light, which viewed it as being a disturbance in the air (one of his four “elements” that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or “corpuscles”). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton .

$The first edition of Newton's Opticks: or, a treatise of the reflexions, refractions, inflexions and colours of light (1704). Credit: Public Domain.$

Newton’s corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise “ Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light “. According to Newton, the principles of light could be summed as follows:

Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to “wave theory”, which had been advocated by 17th century Dutch astronomer Christiaan Huygens . . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his “ Traité de la lumière “ (“ Treatise on Light “). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment , which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young’s version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation, would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter’s moon Io to show that light travels at a finite speed (rather than instantaneously).

Prof. Albert Einstein uses the blackboard as he delivers the 11th Josiah Willard Gibbs lecture at the meeting of the American Association for the Advancement of Science in the auditorium of the Carnegie Institue of Technology Little Theater at Pittsburgh, Pa., on Dec. 28, 1934. Using three symbols, for matter, energy and the speed of light respectively, Einstein offers additional proof of a theorem propounded by him in 1905 that matter and energy are the same thing in different forms. (AP Photo)

By the late 19th century, James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell’s equations ) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c ).

In 1905, Albert Einstein published “ On the Electrodynamics of Moving Bodies ”, in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity , which reconciled Maxwell’s equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect , Einstein based his idea on Planck’s earlier work with “black bodies” – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein’s photoelectric effect was attempt to explain the “black body problem”, in which a black body emits electromagnetic radiation due to the object’s heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes ).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein’s explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named “photons”. For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg’s “uncertainty principle” (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger’s paradox that claimed that all particles have a “wave function”.

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function , a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly “collapse”, or rather “decohere”, to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the “ Schrödinger Cat” paradox).

In this illustration, one photon (purple) carries a million times the energy of another (yellow). Some theorists predict travel delays for higher-energy photons, which interact more strongly with the proposed frothy nature of space-time. Yet Fermi data on two photons from a gamma-ray burst fail to show this effect. The animation below shows the delay scientists had expected to observe. Credit: NASA/Sonoma State University/Aurore Simonnet

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation ). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly divert it, or arrest it, is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

We have written many articles about light here at Universe Today. For example, here’s How Fast is the Speed of Light? , How Far is a Light Year? , What is Einstein’s Theory of Relativity?

If you’d like more info on light, check out these articles from The Physics Hypertextbook and NASA’s Mission Science page.

We’ve also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel .

56 Replies to “How Does Light Travel?”

“HOW DOES LIGHT TRAVEL?”

it travels lightly. 😀

Light doesn’t exist. This is an observation from light’s point of view and not ours. Traveling at the speed of (wait for it) light, absolutely no time passes between leaving it’s source and reaching it’s destination for the photon. This means, to the photon hitting your retina, it is also still on that star you are observing 10 light years away. How is this possible? Maybe John Wheeler was right when he told Richard Feynman that there is only one electron in the universe and it travels forward in time as an electron, then back in time as a positron and every electron we see is the same electron.

MY QUESTION IS: Whether light is a wave , particle or both.. where does it get the energy to move through space/time. In other words is the energy of light infinite? Does it continue on without lose of energy…..forever…….

I believe that Special Relativity says that the energy of light is infinite due to the very fact it has no mass. E=MC^2

In reverse, this is also why something with mass to begin with. If accelerated toward the speed of light, will see their mass and gravity increase to infinite points as they near relativistic speed (it actually starts around 95% with a steep upward curve from there), with a relative slowing to a stop of time.

Join the discussion

Light and the universe are only illusions that are formed in our minds via technology that sends information from the simulation program we’re living in. That information comes in the form of invisible wavelengths that includes wavelengths that we perceive as light. The visible retinas in our eyes are like tiny video screens where these particles are arranged into patterns that form into all the various objects we think are real objects. This information is also converted into thoughts within our minds which are like computer processors that process that information.

We are living in a computer simulation that is much more advanced than anything the characters in the program have built according to the information called the Beast.

Brad,…So You’re suggesting that “life” as we know and call it “is some kind of retro-virus” or “bio-intelligent format” heaped upon a perceived “set of accepted data sets” that are not in sync with each other in most cases with exception to Math 94% of the time….Even then it can vary which suggests Your idea would mean we all live in a fairy tale. That is what you suggest,…right?……

Brad has watched the Matrix too many times.

Correction: Even gravity doesn’t slow light down. Light (EM radiation of any wavelength) always travels at speed c, relative to any local inertial (Lorentz) frame. It could also be noted that the wavelength of an EM wave is not a characteristic of that wave alone; it also depends on the state of motion of the observer. You might even say, “One man’s radio wave is another man’s gamma ray.”

Light actually “slows down” every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a particular substance, like water. Light speed is not constant when traveling through any medium except pure vacuum. In fact that is why your pencil looks bent when you drop it in a glass of water. Light bends to find it’s fastest path through any medium, and it slows down in that medium.

if all you scientist could ever get it in your pie brain that there is no time, no light speed, no warping space, no black holes for the purpose of moving through space quickly, no smallest no biggest when it comes to space and that all of everything has always been in existence but not necessarily as it is now. you will never find the smallest because if it exist it has an inside, and you will never find the end of space because it is infinite.

What are you smoking?

The article started out nicely, but I lost interest as mistakes began to appear. First Einstein did not “propose” the photoelectric effect. The photoelectric effect was first observed by Heinrich Hertz in 1887. Einstein used the idea of photons to explain the photoelectric effect and derive the photoelectric equation. Also, Max Plank had already derived the blackbody distribution, by assuming that electromagnetic energy of frequency f could only be emitted in multiples of energy E=hf, by 1900. Einstein’s paper on the photoelectric effect was published in his “miracle” year of 1905. The photoelectric effect has nothing to do with black body radiation.

Einstein did not coin the name “photons” for light quanta, as stated in this article. This term was first used by Arthur Compton in 1928.

I have to say that I do not know what the author of the article means when he says ” calculating the wavelength at which light functioned” in reference to Louis-Victor de Broglie. Louis de Broglie used the dual nature of light to suggest that electrons, previously thought of as particles, also had wave characteristics and used this notion to explain the Bohr orbits in the hydrogen atom.

I gave up on the article after seeing these errors. I’m afraid I have a low tolerance for sloppy writing.

Oh, it’s BCE now, “Before the Common Era” BC has worked for 2000 years but now the PC police have stepped in so as not to offend who? Some Muslims?

mecheng1, you must be very young. BCE has been in used in academia for decades. It’s nothing “new”, just out of your circle of knowledge.

Decades??? Really?? How does that compare to 2000 years?

Only in Euro-centric texts have your assertions been true, McCowen. The rest of the world not influenced by Christianity have used their own calendars and a “0” year or a “year 1” from which to reckon the passage of time, largely based on their own religions or celestial observations.

Over the last century or so, through commerce, most of the world has generally accepted the use of a Western calendar (or use it along with their own for domestic purposes, like we here in the US still use Imperial units of measure that have to be converted to metric for international commerce). So, we are in a “common era” insofar as non-Christian societies are incorporating the Gregorian Calendar and the generally-accepted “year 1” established by that calendar (which is supposed to be the year of Jesus’s birth, but it probably isn’t according to current scholarship). Besides, the Gregorian calendar is an improved derivative of the Roman calendar – even the names of the months come from the Romans.

In short, it is more accurate, as well as respectful, to go with BCE in these global times.

Where is the information carried on a photon hitting my eye(s), or cluster/group/pack of photons hitting my eyes(s), that I see as other distant galaxies and planets going around stars?

That’s the mystery, isn’t it? Even in scattering, light remains coherent enough to convey an enormous amount of information.

Since the miniscule equal masses with opposite charges, that make up the photon structure, interact at 90 degrees, this induces a spin (a finding from the 80’s by the LANL plasma physics program) which creates a centrifugal force that counterbalances the charge attraction of the opposite charges. This establishes a stable structure for energies less than 1.0216 MeV, the pair-formation threshold, separating these “neutrino” sub-components by a specific distance providing wavelengths varying with photon energy. This composite photon propagates transversely at c/n, the speed of light divided by the index of refraction of the material traversed. In spite of the mass being defined as zero, for convenience in calculating atomic masses, there is actually an infinitesimal but non-zero mass for the photon that is required for calculations that describe its properties.

Tim, you poor guy! You have a discombobulated brain! Everything you wrote is just gibberish.

i would like to know the temperature in a black hole…maybe absolute zero? is absolute zero the moment that time stop?

I think the temp inside a black hole would be extremely high since temperature seems to increase with mass. Comparing absolute zero to time stopping is very interesting though. To the observer they would appear the same.

Theoretically there is no temperature in a black hole from any observer POV because time is stopped. Although JALNIN does bring up that point, and he also brings up the point of increasing mass corresponding to increasing energy. Everything in Hawking and Einstein’s equations though, suggest that any energy would be absorbed back by the singularity, so there wouldn’t be any heat. In fact it should be infinitely cold. But time is no more, so technically no heat or energy is emitted anyway from any observers POV. Yet recent images of black holes from Chandra show that they emit powerful Gamma Jets along their spin axis just like Neutron stars, and Pulsars. BTW edison. The accretion disk can reach temperatures of 20MN Kelvin on a feeding SM black hole (quasar). NASA just published an article on it through the Chandra feed a while back.

Light doesn’t travel, it just IS. It is we, the condensed matter, that travels, through time.

Oh really? Is this just your imagination/illusion or you have published a paper on it?

So you don’t believe you travel through time?

I wish I understood just a portion of I just read, love sicence so bad BUT, sighs

It would be easier to understand if it wasn’t pure gibberish written by someone with no science background.

I have two “mind-bending relativity side effects” to share. At least they are mind-bending to me.

1) Light travels the same speed relative to all particles of mass, regardless of how those particles move relative to each other:

I can conceptualize this if we are only talking about two mass-particles/observers and the examples I’ve seen always involve only two observers. But if you have many mass-particles/observers, how does the space-time seem to know to adjust differently for all of them. I am sure i am understanding this correctly as it is a basic concept of special relativity and nobody seems to bring this issue up. But it “bends my mind” when i try to include more than two observers. Maybe you can help.

2) General Relativity’s (“GR”) prediction that the big bang started with “Infinite” energy and now the universe appears to have finite mass energy and Regarding the first effect: How can something infinite turn into something finite? Is the answer that at that early in the universe, quantum takes over and GR’s prediction of infinite mass-energy at the start of the universe is just wrong?

I need to correct a typo in my previous comment. Where i say “i am sure am understanding this correctly” I meant to include the word NOT. so it should read “i am sure am NOT understanding this correctly” Mark L.

Mark,….I think you’re understanding it just fine from the standpoint of multiple observers, The point might be that in space, the density of “emptiness” or “lack of emptiness” might be impacted from one area of observation to another by an observer who’s perceptions are not equal but not being taken into consideration by each observer. ( an example if I may?) If you were to use a Clear medium which is oil based beginning with 5 gallons of mineral spirits in a large barrel and keep adding 5 gallons of thicker clear oil and then heavy grease and stop with using a clear heavy wax,…what happens is you end up with a barrel of clear fluid that begins with a floating substrate but the liquid begins to keep floating and the heaviest stuff goes to the bottom,…You end up with a sort of solid tube of clear fluids which if you could keep them in shape here on the earth, “you could observe them” from several positions, #1. the fluid end #2, the less fluid part, #3, the semi solid part #4. the seemingly solid part #5. the almost solid part & #6. the solid part……all of which would be transparent….You could then shine a laser through all of it and perhaps do that again from different places and see what happens at different angles…..I think what happens as a result would be, an observer would end up be influenced as per his or her ideas thusly because of the quasi-nature of what the density of space is at the point of space is where the observation is made. just a guess.

All Special Relativity really says about light is that it appears to move at the same rate from any observer POV. There are other more advanced rules relating to light speeds. One of them is the implication of infinite energy in a photon because of the fact it’s mass-less, therefore it can move at the maximum rate a mass-less particle or wave can (not necessarily that it does) Later when the electron was discovered (also mass-less particle or wave), it was also found to conform to the rules of special relativity.

As far as the big bang, there are a lot of cracks in that theory, and many different ones are beginning to dispute some of the common ideas behind the “Big Bang” as well as “Inflationary Cosmology”. Honestly though, both standard and quantum physics applied, and yet both went out the window at the same time at some point. That’s what all the theories really say. At some point, everything we know or think we know was bunk, because the math just breaks down, and doesn’t work right anymore.

i think until there is an understanding of the actual “fabric” of space itself, the wave vs particle confusion will continue. another interesting article recently was the half integer values of rotating light. planck’s constant was broken? gravity? a bump in the data? lol these are interesting times.

There’s no fabric.

Tesla insists there is an aether, Einstein says not. Tesla enjoyed far less trial and error than Einstein. The vast majority of Tesla’s projects worked the first time around and required no development or experimentation. I’ll go with Tesla; there is an aether as a fabric of space.

http://weinsteinsletter.weebly.com/aether.html

Maybe Special Relativity is not correct? 🙂

Feynman said unequivocally that QED is NOT a wave theory. In fact, the math only looks like Maxwell’s wave function when you are looking at a single particle at a time, but the analogy breaks down as soon as you start looking at the interactions of more than one, which is the real case. There’s no light acting alone, but always an interaction between a photon and some other particle, an electron, another photon, or whatever. He said “light is particles.” So the question re: how can light travel through a vacuum if it’s waves is a nonsensical question. There are no collapsing wave functions in light. There’s only probabilities of position that look like waves on a freaking piece of paper. Even calling light properties as “wavelengths” is nonsensical. Light comes in frequencies, i.e., the number of particles traveling tightly together. Higher frequency is more energy because it’s more particles (E=MC[squared]). “Wavicles” is pure bullshit.

I don’t agree with the John Wheeler theory that there is only one electron since the computer I am using was built by ion implantation and uses a very large number of them simultaneously to function.

Black holes don’t stop or slow light, if they even exist. A black hole could phase shift light, which is why we see things emitting xrays and call them black holes….but they could be something else too.

Photons have no mass but they do have energy. Energy and mass are transformable into each other. Gravity works on energy as well as mass. As massive particles approach the speed of light their measurable mass increases to infinity. But since energy is equivalent to mass, why doesn’t the photon, which has energy, not seem to have infinite mass?

NO other wave travels thru a vacuum? what about radio?

Radio waves are a specific frequency range of light.

Technically speaking, radio waves are emitted at various frequencies that share the same space time as light. They are not however light. They’re modulated electrons. Modulated photons certainly can be used to carry a vast amount of information a great distance. It cannot do it any faster or better than a radio wave though. Both electrons and photons are mass-less, therefore they both conform to the rules of Special Relativity in the same way. Both travel at the speed of light.

I just don’t understand is it a particle of a wave? It seems like it behaves like wave and sometimes like particle and in some situations is like a what ever you are going to call it.

So, the logical idea would to have formula Photon_influence * weight_for_particle + Wave_influence * weight_for_wave

Make it more compact.

This article is good but the title is bad as by the end we still weren’t told how light travels through space. Also, there are some historical mistakes as already pointed out. Now for my contribution: I think that light and Gravity have a lot in common; for one – an atom’s electrons transmit light and an atom contains the tiny heavy place that knows everything there is to know about gravity, that is, the nucleus. Light and Gravity are both related to the same entity, the atom. Unfortunately, we, still cannot grasp how what’s heavy brings about gravitation. For those of you with a creed for new ideas go to: https://www.academia.edu/10785615/Gravity_is_emergent It’s a hypothesis…

Gravity and light are infinite, like space and time… Mind the concept that there are waves within waves, motions within motion, vibrations within vibration, endless overtones and universal harmony…

From this article, I have “And in the end, the only thing that can truly slow down or arrest the speed of light is gravity”

Doesn’t light slow down in water and glass and other mediums. I was only a Physics minor, but I do remember coivering this though way back in the early 80’s. And in my quick checking online, I found the following.

“Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index = 1.3; see Figure 1) and 200,000 kilometers per second in glass (refractive index of 1.5).”

Were they saying something else here. I did like the article.

Photons are not massless, but their mass is incredibly small even compared to a proton or neutron. So, by Einstein’s E=MC^2, the energy required for a photon to move is greatly reduced, but photons do have mass and are affected by gravity. If photons had no mass at all, then gravity would have no affect on them, but gravity does. Gravity bends light and can change it’s course through space. We see that in the actual test first performed to prove Einstein’s theory buy observing the distorted placement of stars as their light passes near the sun observed during an eclipse. We can also see it through gravitational lensing when viewing deeps space objects. And the fact that there are black holes that are black because light cannot escape it’s gravity. So photons do have mass, be it miniscule, and with that their propagation with light waves through space will eventually run out of energy and stop. but this would probably require distances greater to several widths of our universe to accomplish. Light from the furthest reaches of the universe are not as bright, or as energetic, as they are at anyplace between here and their origins. That reduction in their energy is also attributed to Einstein’s equation and the inverse square law, where the intensity of light is in relation to the inverse square of the distance. That proves that light looses energy the further it travels, but it still moves at the speed of light. As light looses energy, it doesn’t slow the light wave.

It has been proven that more energetic light does in fact travel slightly faster. You can find the experiments done with light that has traveled billions of light years, the more energetic is in fact faster over a number of seconds, around 10 -15 or so. As people encounter this information, they see that many accepted theories can now be debunked.

The point of the article is nothing new; light acts like a particle AND a beam. So when you sit behind a closed door and someone shines a light on the door, the light will engulf the door and wave through and around the edges, the particle does not just bounce straight back. You can focus a beam of light on an object, but it will sneak though the corners and underneath the door, through any opening,. And yes, light travels forever. It is a constant, that cannot be sped up. We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe: 13,8B light years away *the time that it takes for light to travel in one year, is one light year. So, it has taken 13,8B light years for the light of other galaxies to get here, so those galaxies could be gone by now, since it took so long to reach us, We are truly looking back in time as we see the light emitted from those galaxies and stars.

It propagates through the quantum mish-mash know as the aether . . .

If light is a particle and particles have mass why does not the mas increase with it speed?

Wow…there are errors in the article, yes…the enthusiasm demonstrated by all the comments is encouraging…but when I read these comments, I am a bit dismayed at the lack of understanding that is evident in most of them…confusing energy and intensity and wavelength…confusing rest mass and inertial mass…not to mention some off-the-wall hypotheses with no experimental evidence to support them. There are some great primers out there…books, documentaries, podcasts (like Astronomy Cast). Good luck!

Precisely correct. Sci-fi rules basic physics, which reflects on the poor education system. Pity.

First time I heard about A. A. and his theory about light I really didn’t like him. Why? Because light was the the fastest thing in the universe and there is no other thing faster than the light. Later, when I have red about angular speed I have asked my self if you have linear and angular speed and both of them are speeds how that will result in the maximum speed. Since then, I have not had a chance to get right answer.

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

Source: Universe Today

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NOTIFICATIONS

Light basics.

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Light is a form of energy produced by a light source. Light is made of photons that travel very fast. Photons of light behave like both waves and particles.

Light sources

Something that produces light is called a light source. There are two main kinds of light sources:

Blue and pink fireworks with black sky background.

Fireworks show how light travels faster than sound. We see the light almost instantly, but the sound arrives later. To work out how many kilometres away the fireworks are, count the seconds until you hear the bang and divide by 3.

Incandescent sources use heat to produce light. Nearly all solids, liquids and gases will start to glow with a dull red colour once they reach a temperature of about 525 °C. At about 2300 °C, the filament in a light bulb will start to produce all of the colours of the visible spectrum, so it will look white. The Sun, stars, a flame and molten metal are all incandescent.

Luminescent sources are normally cooler and can be produced by chemical reactions, such as in a glowstick or a glow-worm. Other luminescent sources include a computer screen, fluorescent lights and LEDs.

Light travels much faster than sound

Light travels at a speed of 299,792,458 m/s (that’s nearly 300,000 km/s!). The distance around the Earth is 40,000 km, so in 1 second, light could travel seven and a half times around the world.

Sound only travels at about 330 m/s through the air, so light is nearly a million times faster than sound.

If lightning flashes 1 kilometre away from you, the light reaches you in 3 millionths of a second, which is almost instantly. The sound of the thunder takes 3 seconds to travel 1 kilometre – to work out many kilometres away lightning is, count the seconds for the thunder to arrive and divide by 3.

Image showing jagged forks of lightning during a storm.

Lightning storms are important for converting nitrogen gas in the atmosphere through to forms that are biologically available.

Light takes about 8 minutes and 20 seconds to reach the Earth from the Sun. When we see the Sun, we are seeing what it looked like over 8 minutes ago.

Light can travel through empty space

Unlike sound, which needs a medium (like air or water) to travel through, light can travel in the vacuum of space.

Light travels in straight lines

Once light has been produced, it will keep travelling in a straight line until it hits something else.

Shadows are evidence of light travelling in straight lines. An object blocks light so that it can’t reach the surface where we see the shadow. Light fills up all of the space before it hits the object, but the whole region between the object and the surface is in shadow. Shadows don’t appear totally dark because there is still some light reaching the surface that has been reflected off other objects.

Once light has hit another surface or particles, it is then absorbed, reflected (bounces off), scattered (bounces off in all directions), refracted (direction and speed changes) or transmitted (passes straight through).

Models for light

Wave length, height and frequency

A wave can be described by its length, height (amplitude) and frequency.

Light as waves

Rainbows and prisms can split white light up into different colours. Experiments can be used to show that each of these colours has a different wavelength.

Prism showing 7 colours of the spectrum that make up white light

When white light shines through a prism, each colour refracts at a slightly different angle. Violet light refracts slightly more than red light. A prism can be used to show the seven colours of the spectrum that make up white light.

At the beach, the wavelength of water waves might be measured in metres, but the wavelength of light is measured in nanometres – 10 -9 (0.000,000,001) of a metre. Red light has a wavelength of nearly 700 nm (that’s 7 ten-thousandths of a millimetre) while violet light is only 400 nm (4 ten-thousandths of a millimetre).

Visible light is only a very small part of the electromagnetic spectrum – it’s just that this is the range of wavelengths our eyes can detect.

Light as particles

In 1905, Albert Einstein proposed that light is made of billions of small packets of energy that we now call photons. These photons have no mass, but each photon has a specific amount of energy that depends on its frequency (number of vibrations per second). Each photon still has a wavelength. Shorter wavelength photons have more energy.

The photoelectric effect

University of Waikato science researcher Dr Adrian Dorrington explains the photoelectric effect. He then describes how camera sensors can be designed on the basis of this effect to enable light energy to be converted into electric potential energy.

The photoelectric effect is when light can cause electrons to jump out of a metal. These experiments confirm that light is made of these massless particles called photons.

Simple explanations of some of these concepts can be found in the article Building Science Concepts: Shadows .

Nature of science

In order to understand the world we live in, scientists often use models. Sometimes, several models are needed to explain the properties and behaviours of a phenomenon. For example, to understand the behaviour of light, two models are needed. Light needs to be thought of as both waves and particles.

Useful links

Even though light doesn’t have mass, learn how it still has a tiny amount of momentum. Find out about NASA’s solar sails to power spacecraft.

Read about the LightSail project, a crowdfunded project from The Planetary Society, aiming to demonstrate that solar sailing is a viable means of propulsion for CubeSats (miniature satellites intended for low Earth orbit).

Explore solar sails more in your classroom, with this activity Solar Sails: The Future of Space Travel from the TeachEngineering website.

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The Nature of Light

Introduction.

Light is a transverse, electromagnetic wave that can be seen by the typical human. The wave nature of light was first illustrated through experiments on diffraction and interference . Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization .

In 1678, Christiaan Huygens (1629–1695) published Traité de la Lumiere , where he argued in favor of the wave nature of light. Huygens stated that an expanding sphere of light behaves as if each point on the wave front were a new source of radiation of the same frequency and phase.
Thomas Young (1773–1829) and Augustin-Jean Fresnel (1788–1827) disproved Newton's corpuscular theory.

Light is produced by one of two methods…

Incandescence is the emission of light from "hot" matter (T ≳ 800 K).
Luminescence is the emission of light when excited electrons fall to lower energy levels (in matter that may or may not be "hot").

Just notes so far. The speed of light in a vacuum is represented by the letter c from the Latin celeritas — swiftness. Measurements of the speed of light.

Veramente non l'ho sperimentata, salvo che in lontananza piccola, cioè manco d'un miglio, dal che non ho potuto assicurarmi se veramente la comparsa del lume opposto sia instantanea; ma ben, se non instantanea, velocissima…. In fact I have tried the experiment only at a short distance, less than a mile, from which I have not been able to ascertain with certainty whether the appearance of the opposite light was instantaneous or not; but if not instantaneous it is extraordinarily rapid …. Galileo Galilei, 1638 Galileo Galilei, 1638

Ole Rømer (1644–1710) Denmark. "Démonstration touchant le mouvement de la lumière trouvé par M. Roemer de l'Académie des Sciences." Journal des Scavans . 7 December 1676. Rømer's idea was to use the transits of Jupiter's moon Io to determine the time. Not local time, which was already possible, but a "universal" time that would be the same for all observers on the Earth, Knowing the standard time would allow one to determine one's longitude on the Earth — a handy thing to know when navigating the featureless oceans.

Unfortunately, Io did not turn out to be a good clock. Rømer observed that times between eclipses got shorter as Earth approached Jupiter, and longer as Earth moved farther away. He hypothesized that this variation was due to the time it took for light to travel the lesser or greater distance, and estimated that the time for light to travel the diameter of the Earth's orbit, a distance of two astronomical units, was 22 minutes.

The speed of light in a vacuum is a universal constant in all reference frames.
The speed of light in a vacuum is fixed at 299,792,458 m/s by the current definition of the meter.
The speed of light in a medium is always slower the speed of light in a vacuum.
The speed of light depends upon the medium through which it travels.The speed of anything with mass is always less than the speed of light in a vacuum.

other characteristics

The amplitude of a light wave is related to its intensity.

Intensity is the absolute measure of a light wave's power density.
Brightness is the relative intensity as perceived by the average human eye.

The frequency of a light wave is related to its color.

Color is such a complex topic that it has its own section in this book.
Laser light is effectively monochromatic.
There are six simple, named colors in English (and many other languages) each associated with a band of monochromatic light. In order of increasing frequency they are red, orange, yellow, green, blue, and violet .
Light is sometimes also known as visible light to contrast it from "ultraviolet light" and "infrared light"
Other forms of electromagnetic radiation that are not visible to humans are sometimes also known informally as "light"
Nearly every light source is polychromatic.
White light is polychromatic.

A graph of relative intensity vs. frequency is called a spectrum (plural: spectra ). Although frequently associated with light, the term can be applied to any wave phenomena.

Blackbody radiators emit a continuous spectrum.
The excited electrons in a gas emit a discrete spectrum.

The wavelength of a light wave is inversely proportional to its frequency.

Light is often described by it's wavelength in a vacuum .
Light ranges in wavelength from 400 nm on the violet end to 700 nm on the red end of the visible spectrum.

Phase differences between light waves can produce visible interference effects. (There are several sections in this book on interference phenomena and light.)

Leftovers about animals.

Falcon can see a 10 cm. object from a distance of 1.5 km.
Fly's Eye has a flicker fusion rate of 300/s. Humans have a flicker fusion rate of only 60/s in bright light and 24/s in dim light. The flicker fusion rate is the frequency with which the "flicker" of an image cannot be distinguished as an individual event. Like the frame of a movie… if you slowed it down, you would see individual frames. Speed it up and you see a constantly moving image. Octopus' eye has a flicker fusion frequency of 70/s in bright light.
Penguin has a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum.
Sparrow Retina has 400,000 photoreceptors per square. mm.
Reindeer can see ultraviolet wavelengths, which may help them view contrasts in their mostly white environment.

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The Magazine

What is the speed of light? Here’s the history, discovery of the cosmic speed limit

Time travel is one of the most intriguing topics in science.

On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it’s one of the most important constants that appears in nature and defines the relationship of causality itself.

As far as we can measure, it is a constant. It is the same speed for every observer in the entire universe. This constancy was first established in the late 1800’s with the experiments of Albert Michelson and Edward Morley at Case Western Reserve University . They attempted to measure changes in the speed of light as the Earth orbited around the Sun. They found no such variation, and no experiment ever since then has either.

Observations of the cosmic microwave background, the light released when the universe was 380,000 years old, show that the speed of light hasn’t measurably changed in over 13.8 billion years.

In fact, we now define the speed of light to be a constant, with a precise speed of 299,792,458 meters per second. While it remains a remote possibility in deeply theoretical physics that light may not be a constant, for all known purposes it is a constant, so it’s better to just define it and move on with life.

How was the speed of light first measured?

In 1676 the Danish astronomer Ole Christensen Romer made the first quantitative measurement of how fast light travels. He carefully observed the orbit of Io, the innermost moon of Jupiter. As the Earth circles the Sun in its own orbit, sometimes it approaches Jupiter and sometimes it recedes away from it. When the Earth is approaching Jupiter, the path that light has to travel from Io is shorter than when the Earth is receding away from Jupiter. By carefully measuring the changes to Io’s orbital period, Romer calculated a speed of light of around 220,000 kilometers per second.

Observations continued to improve until by the 19 th century astronomers and physicists had developed the sophistication to get very close to the modern value. In 1865, James Clerk Maxwell made a remarkable discovery. He was investigating the properties of electricity and magnetism, which for decades had remained mysterious in unconnected laboratory experiments around the world. Maxwell found that electricity and magnetism were really two sides of the same coin, both manifestations of a single electromagnetic force.

James Clerk Maxwell contributed greatly to the discover of the speed of light.

As Maxwell explored the consequences of his new theory, he found that changing magnetic fields can lead to changing electric fields, which then lead to a new round of changing magnetic fields. The fields leapfrog over each other and can even travel through empty space. When Maxwell went to calculate the speed of these electromagnetic waves, he was surprised to see the speed of light pop out – the first theoretical calculation of this important number.

What is the most precise measurement of the speed of light?

Because it is defined to be a constant, there’s no need to measure it further. The number we’ve defined is it, with no uncertainty, no error bars. It’s done. But the speed of light is just that – a speed. The number we choose to represent it depends on the units we use: kilometers versus miles, seconds versus hours, and so on. In fact, physicists commonly just set the speed of light to be 1 to make their calculations easier. So instead of trying to measure the speed light travels, physicists turn to more precisely measuring other units, like the length of the meter or the duration of the second. In other words, the defined value of the speed of light is used to establish the length of other units like the meter.

How does light slow down?

Yes, the speed of light is always a constant. But it slows down whenever it travels through a medium like air or water. How does this work? There are a few different ways to present an answer to this question, depending on whether you prefer a particle-like picture or a wave-like picture.

In a particle-like picture, light is made of tiny little bullets called photons. All those photons always travel at the speed of light, but as light passes through a medium those photons get all tangled up, bouncing around among all the molecules of the medium. This slows down the overall propagation of light, because it takes more time for the group of photons to make it through.

In a wave-like picture, light is made of electromagnetic waves. When these waves pass through a medium, they get all the charged particles in motion, which in turn generate new electromagnetic waves of their own. These interfere with the original light, forcing it to slow down as it passes through.

Either way, light always travels at the same speed, but matter can interfere with its travel, making it slow down.

Why is the speed of light important?

The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The old physics, dominated by the work of Isaac Newton, said that the universe had a fixed reference frame from which we could measure all motion. This is why Michelson and Morley went looking for changes in the speed, because it should change depending on our point of view. But their experiments showed that the speed was always constant, so what gives?

Einstein decided to take this experiment at face value. He assumed that the speed of light is a true, fundamental constant. No matter where you are, no matter how fast you’re moving, you’ll always see the same speed.

This is wild to think about. If you’re traveling at 99% the speed of light and turn on a flashlight, the beam will race ahead of you at…exactly the speed of light, no more, no less. If you’re coming from the opposite direction, you’ll still also measure the exact same speed.

This constancy forms the basis of Einstein’s special theory of relativity, which tells us that while all motion is relative – different observers won’t always agree on the length of measurements or the duration of events – some things are truly universal, like the speed of light.

Can you go faster than light speed?

Nope. Nothing can. Any particle with zero mass must travel at light speed. But anything with mass (which is most of the universe) cannot. The problem is relativity. The faster you go, the more energy you have. But we know from Einstein’s relativity that energy and mass are the same thing. So the more energy you have, the more mass you have, which makes it harder for you to go even faster. You can get as close as you want to the speed of light, but to actually crack that barrier takes an infinite amount of energy. So don’t even try.

How is the speed at which light travels related to causality?

If you think you can find a cheat to get around the limitations of light speed, then I need to tell you about its role in special relativity. You see, it’s not just about light. It just so happens that light travels at this special speed, and it was the first thing we discovered to travel at this speed. So it could have had another name. Indeed, a better name for this speed might be “the speed of time.”

Related: Is time travel possible? An astrophysicist explains

We live in a universe of causes and effects. All effects are preceded by a cause, and all causes lead to effects. The speed of light limits how quickly causes can lead to effects. Because it’s a maximum speed limit for any motion or interaction, in a given amount of time there’s a limit to what I can influence. If I want to tap you on the shoulder and you’re right next to me, I can do it right away. But if you’re on the other side of the planet, I have to travel there first. The motion of me traveling to you is limited by the speed of light, so that sets how quickly I can tap you on the shoulder – the speed light travels dictates how quickly a single cause can create an effect.

The ability to go faster than light would allow effects to happen before their causes. In essence, time travel into the past would be possible with faster-than-light travel. Since we view time as the unbroken chain of causes and effects going from the past to the future, breaking the speed of light would break causality, which would seriously undermine our sense of the forward motion of time.

Why does light travel at this speed?

No clue. It appears to us as a fundamental constant of nature. We have no theory of physics that explains its existence or why it has the value that it does. We hope that a future understanding of nature will provide this explanation, but right now all investigations are purely theoretical. For now, we just have to take it as a given.

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Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something. Then it can either be reflected or absorbed. Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.

Light has some interesting properties. It comes in lumps called photons. These photons carry energy and momentum in specific amounts related to the color of the light. There is much to learned about light. I suggest you log in to our website and type LIGHT into the search box. Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" , the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

Follow-up on this answer

Still Curious?

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Properties of Light
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Why is the speed of light the way it is?

It's just plain weird.

Einstein's theory of special relativity tells us the speed of light is 186,000 miles per second (300 million meters per second).

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights .

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics?

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply

Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more

rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.

rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply

Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?

Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?

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How Far is a Light Year?

How far is a light-year ? It might seem like a weird question because isn’t a ‘year’ a unit of time, and ‘far’ a unit of distance? While that is correct, a ‘light-year’ is actually a measure of distance. A light-year is the distance light can travel in one year.

Light is the fastest thing in our Universe traveling through interstellar space at 186,000 miles/second (300,000 km/sec). In one year, light can travel 5.88 trillion miles (9.46 trillion km).

A light year is a basic unit astronomers use to measure the vast distances in space.

To give you a great example of how far a light year actually is, it will take Voyager 1 (NASA’s longest-lived spacecraft) over 17,000 years to reach 1 light year in distance traveling at a speed of 61,000 kph.

Related Post: 13 Amazing Facts About Space

Why Do We Use Light-Years?

Because space is so vast, the measurements we use here on Earth are not very helpful and would result in enormous numbers.

When talking about locations in our own galaxy we would have numbers with over 18 zeros. Instead, astronomers use light-time measurements to measure vast distances in space. A light-time measurement is how far light can travel in a given increment of time.

Light-minute: 11,160,000 miles
Light-hour: 671 million miles
Light-year: 5.88 trillion miles

Understanding Light-Years

To help wrap our heads around how to use light-years, let’s look at how far things are away from the Earth starting with our closest neighbor, the Moon.

The Moon is 1.3 light-seconds from the Earth.

Earth is about 8 light-minutes (~92 million miles) away from the Sun. This means light from the Sun takes 8 minutes to reach us.

Jupiter is approximately 35 light minutes from the Earth. This means if you shone a light from Earth it would take about a half hour for it to hit Jupiter.

Pluto is not the edge of our solar system, in fact, past Pluto, there is the Kieper Belt , and past this is the Oort Cloud . The Oort cloud is a spherical layer of icy objects surrounding our entire solar system.

If you could travel at the speed of light, it would take you 1.87 years to reach the edge of the Oort cloud. This means that our solar system is about 4 light-years across from edge to edge of the Oort Cloud.

The distance between the Sun and Interstellar Space. NASA/JPL-Caltech .

The nearest known exoplanet orbits the star Proxima Centauri , which is four light years away (~24 trillion miles). If a modern-day jet were to fly to this exoplanet it would not arrive for 5 million years.

One of the most distant exoplanets is 3,000 light-years (17.6 quadrillion miles) away from us in the Milky Way. If you were to travel at 60 miles an hour, you would not reach this exoplanet for 28 billion years.

Our Milky Way galaxy is approximately 100,000 light-years across (~588 quadrillion miles). Moving further into our Universe, our nearest neighbor, the Andromeda galaxy is 2.537 million light-years (14.7 quintillion miles) away from us.

The Andromeda Galaxy is 2.537 million light-years away from us.

Light, a Window into the Past

While we cannot actually travel through time, we can see into the past. How? We see objects because they either emit light or light has bounced off their surface and is traveling back to us.

Even though light is the fastest thing in our Universe, it takes time to reach us. This means that for any object we are seeing it how it was in the past. How far in the past? However long it took the light to reach us.

For day-to-day objects like a book or your dog, it takes a mere fraction of a fraction of a second for the light bouncing off the object to reach your eye. The further away an object is, the further into its past you are looking.

For instance, light from the Sun takes about 8 minutes to reach Earth, this means we are always seeing the Sun how it looked 8 minutes ago if you were on its surface.

The differences between Lunar Distance, an Astronomical Unit, and a Light Year. Illustration by Star Walk .

Traveling back through our solar system, Jupiter is approximately 30 light-minutes from Earth, so we see Jupiter how it looked 30 minutes ago if you were on its surface. Extending out into the Universe to our neighbor the Andromeda galaxy, we see it how it was 2.537 million years ago.

If there is another civilization out in the Universe watching Earth, they would not see us here today, they would see Earth in the past. A civilization that lives 65 million light-years away would see dinosaurs roaming the Earth.

Helpful Resources:

How big is the Solar System? (Universe Today)
What is an Astronomical Unit? (EarthSky)
How close is Proxima Centauri? (NASA Imagine The Universe)

Pusat TOEFL

Soal tes toefl dan pembahasan jawaban written expression (complete test 1 by longman), kunci dan pembahasan jawaban, menu halaman statis.

December 5, 2023

Light Can Travel Backward in Time (Sort Of)

Light can be reflected not only in space but also in time—and researchers exploring such “time reflections” are finding a wealth of delightfully odd and useful effects

By Anna Demming

creative artist's concept showing a traditional alarm clock encircled by a laser light effect

Ekrem EDALI/Alamy Stock Photo

Can we turn back time? Ask a savvy physicist, and the answer will be “it depends.”

Schemes for retrograde time travel abound but usually involve irreconcilable paradoxes and rely on outlandish theoretical constructs such as wormholes (which may not actually exist). Yet when it comes to simply turning back the clock—akin to stirring a scrambled raw egg and seeing the yolk and white reseparate—a rich and growing subfield of wave physics shows that such “time reversal” is possible.

Reversing time would seem to fundamentally clash with one of the most sacred tenets of physics, the second law of thermodynamics, which essentially states that disorder—more specifically “entropy”—is always increasing, as humbly demonstrated in the incessant work needed to keep things tidy. This inexorable slide toward mess and decay is what tends to make unscrambling eggs impossibly difficult—and what propels time’s arrow on a one-way trip through our day-to-day experiences. And although so far there’s no way to unscramble an egg, in certain carefully controlled scenarios within relatively simple systems, researchers have managed to turn back time.

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The trick is to create a certain kind of reflection. First, imagine a regular spatial reflection, like one you see in a silver-backed glass mirror. Here reflection occurs because for a ray of light, silver is a very different transmission medium than air; the sudden change in optical properties causes the light to bounce back, like a Ping-Pong ball hitting a wall. Now imagine that instead of changing at particular points in space, the optical properties all along the ray’s path change sharply at a specific moment in time. Rather than recoiling in space, the light would recoil in time, precisely retracing its tracks, like the Ping-Pong ball returning to the player who last hit it. This is a “time reflection.”

Time reflections have fascinated theorists for decades but have proved devilishly tricky to pull off in practice because rapidly and sufficiently changing a material’s optical properties is no small task. Now, however, researchers at the City University of New York have demonstrated a breakthrough: the creation of light-based time reflections .

To do so, physicist Andrea Alù and his colleagues devised a “metamaterial” with adjustable optical properties that they could tweak within fractions of a nanosecond to halve or double how quickly light passes through. Metamaterials have properties determined by their structures; many are composed of arrays of microscopic rods or rings that can be tuned to interact with and manipulate light in ways that no natural material can. Bringing their power to bear on time reflections, Alù says, revealed some surprises. “Now we are realizing that [time reflections] can be much richer than we thought because of the way that we implement them,” he adds.

Such structural properties are also found in nature—for example, in the radiant iridescence of a butterfly’s wing. Picking up where nature left off, however, researchers studying metamaterials have engineered structures that can render objects invisible , and applications range from better antennas and earthquake protection to building light-based computers . Now scientists are trading in spatial dimensions of these structural features for temporal ones. “We design metamaterials to do unusual things, and this is one of those unusual things,” says Nader Engheta, a professor at the University of Pennsylvania and a pioneer in metamaterial-modulated wave physics.

Waves Gone Weird

The device Alù and his collaborators developed is essentially a waveguide that channels microwave-frequency light. A densely spaced array of switches along the waveguide connects it to capacitor circuits, which can dynamically add or remove material for the light to encounter. This can radically shift the waveguide’s effective properties, such as how easily it allows light to pass through. “We are not changing the material; we are adding or subtracting material,” Alù says. “That is why the process can be so fast.”

Time reflections come with a range of counterintuitive effects that have been theoretically predicted but never demonstrated with light. For instance, what is at the beginning of the original signal will be at the end of the reflected signal—a situation akin to looking at yourself in a mirror and seeing the back of your head. In addition, whereas a standard reflection alters how light traverses space, a time reflection alters light’s temporal components—that is, its frequencies. As a result, in a time-reflected view, the back of your head is also a different color. Alù and his colleagues observed both of these effects in the team’s device. Together they hold promise for fueling further advances in signal processing and communications—two domains that are vital for the function of, say, your smartphone, which relies on effects such as shifting frequencies.

Just a few months after developing the device, Alù and his colleagues observed more surprising behavior when they tried creating a time reflection in that waveguide while shooting two beams of light at each other inside it . Normally colliding beams of light behave as waves, producing interference patterns where their overlapping peaks and troughs add up or cancel out like ripples on water (in “constructive” or “destructive” interference, respectively). But light can, in fact, act as a pointlike projectile, a photon, as well as a wavelike oscillating field—that is, it has “ wave-particle duality .” Generally a particular scenario will distinctly elicit just one behavior or the other, however. For instance, colliding beams of light don’t bounce off each other like billiard balls! But according to Alù and his team’s experiments, when a time reflection occurs, it seems that they do.

The researchers achieved this curious effect by controlling whether the colliding waves were interfering constructively or destructively—whether they were adding or subtracting from each other—when the time reflection occurred. By controlling the specific instant when the time reflection took place, the scientists demonstrated that the two waves bounce off each other with the same wave amplitudes that they started with, like colliding billiard balls. Alternatively they could end up with less energy, like recoiling spongy balls, or even gain energy, as would be the case for balls at either end of a stretched spring. “We can make these interactions energy-conserving, energy-supplying or energy-suppressing,” Alù says, highlighting how time reflections could provide a new control knob for applications that involve energy conversion and pulse shaping, in which the shape of a wave is changed to optimize a pulse’s signal.

Unscrambling the Physics

Readers who are well versed in the laws of physics can be reassured that Alù’s device does not violate the tenets of thermodynamics. The waveguide does not, for instance, create or destroy energy but simply transforms it efficiently from one form to another—the energy gained or lost by the waves comes from that which is added or subtracted to change the metamaterial’s properties. But what about the inescapable increase of disorder—entropy—over time, as prescribed by thermodynamics? How is a light beam’s time reflection not the equivalent of unscrambling an egg?

As John Pendry, a metamaterial-focused physicist at Imperial College London, explains, however odd reversing a light beam may look, it’s wholly consistent with ironclad thermodynamic principles. The rise of entropy is really a matter of losing information, he says. For instance, line schoolchildren up in alphabetical order, and someone will know exactly where to find each child. But let them loose in the playground, and there’s a vast number of different ways the children could be arranged, which equates to an increase in entropy, and what information you had for locating each child is lost. “If [something is] time-reversible, it means you’re not generating entropy,” Pendry says, even if it looks like you are. Going back to the playground analogy, although the children still run off to play, they know what lines to form to return to class at the bell—so no entropy is generated. “You don’t lose the information,” he says.

Reflection is far from the only optical phenomenon to receive the time-domain treatment. In April Pendry and a team of researchers, including Riccardo Sapienza of Imperial College London, demonstrated a time-domain analogue of a classic experiment from centuries ago that ultimately played a key role in establishing light’s wave-particle duality. First performed by physicist Thomas Young in 1801, the “ double-slit experiment ” provided such irrefutable evidence of light’s wavelike nature that in the face of subsequent evidence for light acting as a particle, scientists could only conclude that both descriptions applied. Send a wave at a barrier with two slits, and waves fanning out from one slit will interfere with those emanating from the other. With light, this constructive and destructive interference shows up on a screen beyond the double slit as multiple bright stripes, or “fringes.” Sapienza, Pendry and their colleagues used indium tin oxide (ITO), a photoreactive substance that can rapidly change from transparent to opaque, to produce “time slits.” They showed that a beam of light interacting with double time slits would produce a corresponding interference pattern in frequency, which was used as a time analogue—that is, there were bright light fringes at different frequencies.

According to Engheta, what motivates experiments that swap time and space in optical effects are the “exciting and novel features we can find in the physics of light-matter interaction.” And there are plenty. Pendry describes with a chuckle how he and his colleagues’ temporal explorations with metamaterials have revealed “some very strange things,” including what he calls a “photonic compressor.” Pendry’s photonic compressor is a metamaterial that is striped with regions of different optical properties that affect the speed at which light propagates. The stripes are adjustable, forming a sort of “metagrating,” and when this metagrating moves through the metamaterial alongside light, it can act to trap and herd the photons together, effectively compressing them. Further investigation has also revealed that this kind of photonic compressor shares characteristics with black holes , potentially providing a more manageable lab-scale analogue for studying those extreme astronomical objects. Having unfurled a whole new time dimension to metamaterials, photon-compressing black hole analogues are just one avenue of curious phenomena to delve into, and the possibilities are legion.

“It’s really assembling a toolbox,” Pendry says, “and then showing this to the world and saying, ‘What can you do with it?’”

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Flooding causes no-travel advisory in southeast sd; portions of i-29 blocked, by: searchlight staff - june 21, 2024 3:31 pm.

A flooded southeast South Dakota road on June 21, 2024. (Courtesy of South Dakota Department of Transportation)

The Department of Transportation issued a no-travel advisory this afternoon for all state highway sections in southeastern South Dakota experiencing flooding, including a blocked portion of Interstate 29.

Some locations in the area received more than 5 inches of rain in the past two days, with 2 to 4 additional inches of rain in the forecast for some locations today into Saturday.

The Department of Transportation said heavy rainfall and saturated ground conditions are causing widespread flooding, making travel difficult to impossible in the southeastern region. A majority of state and local routes are impassable due to standing or flowing water across the roadway, and many of the major streams and rivers will continue to rise and are not anticipated to peak until this weekend or later.

A rainfall totals map published on the morning of June 21, 2024, showing totals from the prior 48 hours. (Courtesy of National Weather Service)

Interstate 29 is blocked at exit 50 (Centerville) and exit 62 (Canton). An alternate route has been created in conjunction with the Iowa Department of Transportation and the Minnesota Department of Transportation to allow motorists to safely travel from Sioux Falls to Sioux City, and Sioux Falls to the South Dakota Highway 48 junction north of Junction City.

The alternate route diverts motorists in Sioux Falls to eastbound I-90 to Worthington, Minnesota, turning south on U.S. Highway 59, turning southwest on State Route 60 through Sheldon, Iowa, and continuing south on U.S. Highway 75 from Le Mars, Iowa, to access Sioux City, Iowa. Motorists can also use Iowa State Highway 3 heading west out of Le Mars to access South Dakota Highway 48 and I-29.

It’s highly recommended that motorists use the alternate route, said the South Dakota Department of Transportation, as other secondary highways in the area are impassable due to high water.

A map of the alternate route as well as additional flooding traveler information can be found at https://dot.sd.gov/travelers/travelers/flooding-information . Travelers can also find road condition information for the state of Iowa at https://www.511ia.org/ and the state of Minnesota at https://511mn.org/ .

Interstate 90 is currently open but is anticipated to close this evening as the weather system becomes stronger and rainfall increases. Areas on I-90 near Salem and Mount Vernon are significantly impacted by flooding.

Travel impacts are expected to increase throughout the evening hours, with a high likelihood of rain continuing through Saturday. An additional storm system is anticipated to move into the area around 5 p.m. Central today, which will bring heavy rainfall, damaging winds, hail and possibly a tornado. Motorists can expect additional road closures if conditions continue to worsen.

Motorists are reminded to respect all road closures and not drive around barricades. They should not use secondary highways to avoid road closures or highway obstructions. Driving into floodwater areas can lead to potentially dangerous or life-threatening situations. Due to the expected length of this weather event, all motorists are asked to plan their travel accordingly and not travel in southeast South Dakota if possible.

Visit https://sd511.org or download the SD511 mobile app to view all current road closures, no-travel advisories and highway obstructions.

EDITOR’S NOTE: This story has been updated since its initial publication with additional information about the blocked segment of I-29 and its detour.

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Searchlight staff

How To Pack Light & Travel With Only Your Carry-On

While traveling is fun, flying can be a headache. One of the hardest things is figuring out what to pack, especially when airlines put so many restrictions on traveling with luggage. The worst for me is to put the luggage on the scale and find out that there will be an exorbitant charge because the bag is overweight. Because then I get to find myself in the middle of the airport trying to rearrange items between different bags so that all of them stay under the weight limit.

Luckily, I happily avoid all this nowadays by just bringing a carry-on. I’ve traveled quite extensively over these past couple of years and have only needed to check a bag once (only because I bought a bottle of wine as a souvenir). It’s taken me many trips to perfect the art of fitting everything I need in just one carry-on, but I now feel freer not having so much stuff with me. If you want to avoid high airline fees, take a look at these tips to pack light and only travel with a carry-on.

Start with the Essentials

Don’t just dump anything and everything into your bag and then try to scale back right from the get-go. That’s the wrong approach. Instead, start with the essentials first. Pack your undergarments, a change of clothes for each day you’re away (or less if you will have access to a washer/dryer), a couple pairs of shoes, and necessary toiletries. Then gauge how much room you have left and what ‘nice to haves’ you can bring. When you start with the essentials, you realize that you might not need as much as you thought.

Choose Versatile & Light Pieces

How much you bring has a lot to do with where you are going. While warmer climates allow for minimal clothing, it’s much more difficult to pack for colder weather. Instead of bringing large bulky coats, opt for light pieces you can layer for warmth. Also take with you versatile pieces in neutral colors that you easily take from day to night and wear multiple times.

Use Packing Cubes

Packing a suitcase correctly is an art. You waste a lot of space if you’re just throwing items in. Be sure to fold every piece of clothing. You should also invest in a good set of packing cubes. These bags will help you organize and pack everything neatly . It will help you save a lot of space and make packing much easier.

Take Advantage of Your ‘Personal Item’

Aside from your carry-on luggage, airlines will also allow you to bring one additional personal item. This could be a purse, laptop bag, or backpack. Most personal items have to be around 9 inches x 10 inches x 17 inches. That’s actually a lot of space. Take advantage of your personal item by bringing a large purse or backpack.

Be Realistic

Lastly, be realistic. While it may be nice to have five pairs of shoes with you, do you really need it? Only bring what you will actually use. Plan out your outfits ahead of time so that you know what you need to bring.

Do you like to travel light? What are you best packing tips?

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5 Best Pepper Mills, Tested and Reviewed

Our spicy take on the best pepper grinders for your kitchen.

We've been independently researching and testing products for over 120 years. If you buy through our links, we may earn a commission. Learn more about our review process.

Best Overall

Oxo good grips contoured mess-free pepper grinder.

Best Manual

Peugeot paris u'select 9-inch pepper mill.

Best Electric Salt and Pepper Mill

Cuisinart stainless steel rechargeable salt, pepper and spice mill.

Grinding your own pepper is an easy way to improve the flavor of your dishes. Whole peppercorns contain volatile oils that are released when the peppercorn is cracked. Because those oils dissipate quickly, the flavor diminishes over time (meaning the pre-ground stuff in the can isn't very flavorful). A pepper mill, also referred to as a pepper grinder, passes peppercorns through burrs to grind them into fragments, releasing oils and aromas that deliver a fresher and more potent pepper flavor. In short: Pepper from a mill is better.

In the Good Housekeeping Institute Kitchen Appliances and Innovation Lab, we tested 15 pepper mills from top brands to find the best of the best. Our picks include manual and electric models so you can find the right style for your kitchen.

Thanks to its compact and intuitive design, this grinder performed well in our Lab tests. Though the fine setting was not the finest in our testing, we liked that there is a differentiation between the fine and coarse grinds.

The five settings for choosing the grind size are clearly marked and easy to select. It holds 62 grams (about 1/2 cup) of peppercorns, which is an efficient use of space for a grinder of this size.

This pick was one of the easiest to refill in our test: The bottom unscrews, allowing access to a wide mouth. It then grinds from the top, which means no mess when you set the grinder down after use.

The clear body lets you see when it’s time to reload, but take note that it does expose the peppercorns to light, which can degrade their potency over time.

Given that Peugeot arguably invented the pepper mill back in 1874, it's no surprise that this pick has a classic silhouette, making it perfect for kitchen-to-table use. It's carved from beechwood that's sourced in France and has a smooth, matte finish, so it looks and feels good.

It's engineered with a two-stage grinding process: First, sharp teeth crack the whole peppercorns; then, a smaller set of teeth mill the cracked peppercorns. According to the brand, this allows a more uniform output, and we did find in our tests that the finest setting was very fine, and the coarse setting was nicely coarse.

It has six fixed grinding settings that are easy to select and clearly labeled on the base of the mill. The grinding action was smooth. But it was somewhat difficult to refill, since it's designed with a center post to drive the grinder, and you must add the peppercorns around it.

If you're looking for a more rugged model to season foods while grilling outdoors , Peugeot's BBQ All-Terrain mill has a handle so it can hang from your grill. It also includes a built-in, automatic LED light for low-light conditions and rubber bumpers to make it more impact-resistant.

If you prefer to push a button rather than use a twisting motion, this electric grinder is for you. It's d esigned with two chambers so you can keep both salt and pepper ready for use .

The unit is rechargeable: Simply store it in the charging base — which includes a removable crumb tray that catches loose grounds for easy cleanup — and an indicator light will let you know the grinder is ready. The grind size is adjustable from fine to coarse, but the dial does not include preset grind sizes.

The clear spice holders let you see when it’s time to refill. It includes a cap with a one-teaspoon capacity to make grinding to measure easier — but in our tests, we found that the slow grinding meant it took a fair amount of time to hit a teaspoon.

Männkitchen Pepper Cannon

The cannon name is accurate! The output dwarfed the others in our test , with the weight of the coarse grounds more doubling the next highest weight. This means it's easy to season large dishes and to generate heftier amounts needed for recipes.

The Pepper Cannon also has noteworthy nuance in its grind sizes: The coarse is very coarse, and the fine is very fine — the finest in our test — so you can crust a steak with large cracked pepper or dust on a powdery grind.

Despite the center post inside the mill, we found it easy to fill with a push-bottom release on the lid and a wide mouth. It includes a cap on the bottom for mess-free storage as well as for collecting grounds for sprinkling by pinch or from the lid itself, which holds up to 1/3 cup of ground pepper.

It's somewhat slippery since it's all metal, so it's not the most ergonomic, but the tradeoff is that you won't need to produce as many turns to get the pepper you need.

FinaMill Pro Plus Pepper Mill & Spice Grinder

FinaMill's grinder stood out for its versatility. It's designed as a universal motor that you click onto different pods that hold peppercorns, salt or some other types of spices, such as cumin seeds and dried rosemary.

It's very easy to fill, since each pod unscrews and you simply add the spices to an open cup. The interchangeable pods in this set allow the grinder to work for two seasonings without affecting the flavor of either. You can also purchase pods separately if you want more options.

We like how easy it is to change pods: Press down to release the current pod, then press the empty handle onto the next pod. And the single-button, battery-operated motor makes it easy for cooks of all kinds.

The downside is that there's not a lot of differentiation in coarseness levels, and it's not always easy to dial back to fine from coarse. Also take note that it requires two AA batteries but does not include them.

How we test pepper mills

We tested 15 pepper mills for this story. We filled each grinder to determine its maximum capacity and noted the weight of the full grinder.

We compared the performance by grinding pepper from each mill for a controlled amount of time on the fine, medium and coarse settings. We weighed the output of each setting to help gauge how long — and how much effort — it would take to season foods and measure for recipes.

We then evaluated the ease of returning to the fine setting from the coarse setting, which can be a challenge for many grinders: The coarse grounds can get stuck and make the adjustment difficult.

a handwritten chart showing the grinds from several pepper mills

We assessed whether there was a differentiation between the size of the fine, medium and coarse grinds for each as well as how the sizes compared across models.

We also considered the ease of operation — whether the model was difficult to turn, if applicable, or slippery — and whether the grind sizes were preset or manually selected.

What to look for when buying a pepper grinder

✔️ Manual vs. electric: To use a manual pepper mill, you'll fill a chamber and then turn a portion of the mill to grind the pepper. For electric grinders, you’ll fill a hopper and then push a button to grind. An electric grinder can be a good choice for those with dexterity or hand-strength issues, as the repetitive motion of grinding can get tiring with manual models.

✔️ Grind size settings: Being able to adjust your grind size is important; it allows you to control the seasoning and texture in your dishes. Some pepper mills are designed with a dial that sets the grind size; you adjust it by rotating the base or by tightening or loosening a pin or screw. This gives you control, but you'll sacrifice consistency. Other pepper mills have preset settings: This ensures you’ll get the same grind results each time, but it lacks nuance. Electric grinders may also have preset settings or a dial that lets you swing between coarse and fine.

✔️ Capacity: Mills are often sold with a height measurement listed. Generally speaking, a 12-inch mill will hold more peppercorns than a 6-inch mill. This affects how often you need to refill, the storage space required and how comfortable it will feel in your hand. Larger mills and grinders are often heavier as well, so if you have limitations, you might want to opt for a smaller model.

✔️ Ease of refill: Some mills and grinders have a narrow opening that makes it harder to smoothly funnel peppercorns into the chamber. Look for models with a large and easy-to-open access point.

✔️ Versatility: Consider the material of the grinding mechanism. If it doesn’t absorb odors (ceramic, for example), you may be able to switch between pepper and, say, cloves without mixing or tainting flavors. (But our pros recommend dedicating the grinder to pepper to ensure clear flavor.) Be careful when using a pepper grinder for salt — salt can be corrosive, and some salt-specific grinders are a better choice for the task.

✔️ Opacity: Like olive oil, peppercorns are sensitive to light. Exposure to light can degrade the oils, and therefore the flavor, over time. Opaque grinders protect the peppercorns, but it's harder to tell when to refill, and they can be heavier. Grinders with a clear body expose the peppercorns but show you how much pepper you have left. If you choose a grinder with a clear body, opt for something smaller so you are more likely to use the peppercorns quickly.

What is the difference between a pepper mill and a pepper grinder?

This frequently asked question has an easy answer: There is no difference! Our kitchen experts confirm that these are "interchangeable terms for the same device, and you can use whichever name you prefer.”

How do you clean a pepper mill?

If needed, you can wipe the outside of the mill with a damp cloth, but you want to avoid getting water inside the grinding mechanism. And never put a pepper grinder in the dishwasher. You don't want to trap water or debris inside the mill.

If there’s a jam, you can open the mill and tap to dislodge any partially ground peppercorns. Depending on the model, you can also loosen the nut that controls the grind size to help any trapped debris escape.

To maintain the longevity of your pepper mill, be sure to clean it according to the manufacturer's instructions only.

Why trust Good Housekeeping?

Sarah Wharton is a deputy editor for the Good Housekeeping Institute . She conducted the testing for these pepper grinders and has led the testing of kitchen gear such as fish spatulas and grill thermometers . She has worked as a recipe developer and holds a certificate in professional culinary arts from the International Culinary Center (now the Institute of Culinary Education). She also grinds a lot of pepper in her personal life thanks to a love of cacio e pepe.

Alexandra Kahn is an editorial intern for Good Housekeeping . She just finished her sophomore year at Tulane University where she is pursuing a Communications major and a double minor in Strategy, Leadership & Analytics and Political Science.

Sarah (she/her) is a deputy editor in the Good Housekeeping Institute , where she tests products and covers the best picks across kitchen, tech, health and food. She has been cooking professionally since 2017 and has tested kitchen appliances and gear for Family Circle as well as developed recipes and food content for Simply Recipes, Martha Stewart Omnimedia, Oxo and Food52. She holds a certificate in professional culinary arts from the International Culinary Center (now the Institute of Culinary Education).

Alexandra Kahn is an editorial intern for Good Housekeeping . She just finished her Sophomore year at Tulane University where she is pursuing a Communications major and a double minor in Strategy, Leadership & Analytics and Political Science.

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La Jolla Light | Let Inga Tell You: The sisterhood of the…

La Jolla Light

La jolla light | let inga tell you: the sisterhood of the traveling underpants.

My many friends who travel a lot have been lamenting for some time that they just can’t seem to resolve the underwear problem, especially if they’re going to be staying at a different place every night. You wash out your dainties, but depending on the climate, they don’t quite dry before you have to pack them up and move on.

My friend Linda says she toured Scotland and Ireland for 17 days with a plastic bag of clean but soggy unmentionables that were never truly dry until she got home and put them in her dryer.

The nightly washing ritual has a number of other downsides, not the least of which is having one’s undies draped all over one’s hotel bath, particularly if you’re staying in the $1,000-a-night Scottish castle/golf resort. It just looks so, well, low class. And might explain why those Scots don’t wear anything under their kilts. They couldn’t get it to dry in that damp climate either.

What can a traveler do when away from the comforts of one's home dryer? (Getty Images)

The main issue, of course, is that underwear takes up so much room in your suitcase — room you’d rather have for souvenirs. So several of my friends, including Linda, have been test-driving other solutions, including disposable underwear specifically meant for traveling. Wear it once and toss it.

Apparently, it is much more comfortable than one might imagine for cheap underwear, and thus leads to the question of why one would ever buy expensive underwear that needs to be handwashed if the cheap disposable stuff is just as comfy. But ours is not to reason why.

Another friend says she has tried saving all her old ratty underwear to bring with her to just throw away each night. Yet another says she hits up the dollar store and buys a three-pack for $1.

But here’s the problem: While the plan is excellent, the execution has turned out to be less so. At the moment of truth, they can’t quite bear to throw away perfectly good underwear. Or even serviceable if elastically challenged lingerie. It just seems so wasteful.

The ratty underwear solution is even more problematic. You’ve left a nice tip for the maid at the pricey French chateau, so do you really want her to find your shabby dainties in the trash? One can almost hear her mumbling under her breath: “Merci, mais il vaut mieux peut-etre que vous gardiez votre argent pour vous offrir du linge moins fatigués.” (“Thanks, but maybe you should keep the money and buy yourself some new underwear.”) The French can be so sarcastic.

On a more fundamental basis, wearing ratty underwear goes against everything that is holey, er holy. Didn’t your mother always exhort you to wear good underwear in case you were in an accident? Do you really want to end up in the Cap Ferrat Urgent Care in tattered u-trou?

Yet another friend says she is planning to solve the problem by buying the super-lightweight travel underwear that is guaranteed to dry within hours, even in Indian monsoons. The problem is, it is seriously expensive. Of course, if it truly dries that fast, you wouldn’t need very many pairs. But if that monsoon thing is a bit of advertising hyperbole, you could be spending your trip feeling like a human terrarium.

Stories of depending on a hotel laundry service are legion and usually involve sagas of a three-week trip with one’s clean underwear doggedly following two days behind.

My husband, who traveled a lot on business, knew too well the perils of depending on a hotel laundry, especially in out-of-the-way places. Olof tells of traveling to Indonesia and, after a certain period of time, needing to get his laundry done. His underwear had obviously enjoyed the pampering of a U.S. washing machine, but when he got it back from his Yogyakarta hotel, it was clear it had undergone a much more vigorous manner of washing. Best case, it had been beaten with rocks. More likely, it had been subjected to a local cleansing method involving stampeding water buffalo. Suffice to say, it was full of holes.

On the rest of his travels in Asia, he didn’t dare send his underwear out again, not only out of the sheer embarrassment that a “rich American” would have such shredded skivvies but also his conviction that it would never survive a second experience.

Weighing all the options, there’s really only one obvious conclusion. If you really want to travel light, you’re going to have to go commando.

Inga’s lighthearted looks at life appear regularly in the La Jolla Light. Reach her at [email protected]. ◆

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Travel Advisory in effect

Please visit our guest updates page for more information. Please only call if you’re travelling within the next 72 hours. 

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The WestJet Group begins flight cancellations in anticipation of WestJet Aircraft Maintenance Engineers and other Tech Ops employees' strike

As the WestJet Group prepares for labour action by WestJet Aircraft Maintenance Engineers and other Tech Ops employees, the airline has started cancelling and consolidating flights, in order to park aircraft in a safe and organized manner. This action enables proactive communication with guests and crew to minimize the potential for being stranded and ensures the airline can avoid abandoning aircraft in remote locations.

The decision to cancel flights comes as the WestJet Group awaits a response on behalf of the Canadian Industrial Relations Board (CIRB) to intervene under the Canada Labour Code. If accepted, this action would refer both WestJet and AMFA to arbitration for a first collective agreement and prevent labour action by either party.

“We are immensely disheartened that we are in a position where we must activate our contingency plan and begin parking aircraft, as a result of the strike notice served by AMFA. We deeply regret the disruption this will have on the travel plans of our guests, communities and businesses that rely on our critical air service,” said Diederik Pen, President of WestJet Airlines and Group Chief Operating Officer. “Following the memberships’ nearly unanimous decision to reject a generous tentative agreement that would have made our Aircraft Maintenance Engineers the highest paid in the country, with a take-home pay increase of 30 to 40 per cent in the first year of the proposed agreement, it is clear that the bargaining process has broken down.”  

In the coming 48-hours the WestJet Group will work to park aircraft, in a measured, phased and safe approach, resulting in the following cancellations.

Total cancellation summary

Tuesday, June 18 – Wednesday, June 19:

~40 cancellations

Guest impact*

~6,500 guests impacted

*WestJet is making every effort to reaccommodate all impacted guests

“We will continue to manage our operations to the highest degree of safety and will never compromise in this area,” concluded Pen.

Guests travelling are advised to check the status of their flight prior to leaving for the airport. Please visit WestJet’s Guest Updates page for more information regarding flight status, travel changes and more. 

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Westjet receives second strike notice from amfa within hours of returning to the bargaining table targeting busiest travel weekend of summer.

Union continues to reject Canadian industry-leading agreement, which would make WestJet AMEs best paid in country

Guten tag, Germany! WestJet directly connects five Canadian cities to Frankfurt through Condor Airlines codeshare agreement

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Practical Guide: How to Make Full Use of 144-Hour Visa-Free Transit in Beijing

Hey, dear friends, do you want to immerse yourself in Beijing's ancient charm and modern flair within 144 hours? This super practical guide will take you through the streets and alleys of Beijing, exploring all the must-see attractions!

Starting from November 2023, visitors from 54 countries can enjoy the 144-hour visa-free transit policy! As long as you have a valid passport and booked connecting flights within 144 hours, you can roam around Beijing and feel its pulse.

Palace Museum: Back to the Ming and Qing Dynasties

Route: Visitors can enter through the Meridian Gate (Wumen) and leave from the Gate of Divine Prowess (Shenwumen), and tour major palaces such as the Hall of Supreme Harmony and the Palace of Heavenly Purity along the central axis.

Recommended time: At least half a day, preferably in the morning, to avoid crowds at peak time.

Ticket: (combined ticket) CNY 60 in peak season and CNY 40 in off-season. Tickets must be booked in advance. Reservations start at 20:00, seven days before the visit. The Palace Museum (Forbidden City) is closed every Monday throughout the year except for statutory holidays.

Travel guide: Tian'anmendong, Subway Line 1.

Tian'anmen Square: Feel the Heartbeats of China

Route: Visitors can start from the southern end of the square and head north for a view of the Forbidden City in the distance.

Recommended time: 1-2 hours, best enjoyed in the soft morning or evening light, perfect for photography.

Ticket: Free admission. Reservations are required 1-9 days in advance.

Travel guide: Tian'anmenxi, Subway Line 1.

Great Wall: Wonder of the World

Route: Badaling or Mutianyu section is recommended. You can travel by cable car or hiking.

Recommended time: 1 day. Depart early in the morning for optimal experience and return in the afternoon.

Ticket: CNY 40 for Badaling or Mutianyu in peak season. Reservations can be made up to 15 days in advance.

Travel guide: S2 train, bus No. 877 or high-speed rail to Badaling; tour bus or self-drive to Mutianyu.

Summer Palace: Strolling through the Imperial Garden

Route: Visitors can enter through the East Palace Gate and tour the Long Corridor, Tower of Buddhist Incense and Kunming Lake.

Recommended time: Half a day to a full day.

Ticket: CNY 30 in peak season, and CNY 20 in off-season. Tickets can be purchased 1-7 days in advance.

Travel guide: Xi Yuan or Beigongmen, Subway Line 4.

Sanlitun: Fashion Clusters

Route: Visitors can explore the surrounding fashion shops, bars, restaurants, and art spaces from Taikoo Li.

Recommended time: Evening, especially on weekends, when Sanlitun comes alive with its vibrant nightlife.

Ticket: The business district is open to the public. No tickets required.

Travel guide: Tuanjiehu, Subway Line 10, within walking distance to Taikoo Li, Sanlitun.

798 Art District: Collision of Creativity and Art

Route: Free tour to galleries and art studios.

Recommended time: Half a day. It is recommended to go on weekdays to avoid crowds.

Ticket: Admission is free, but some exhibitions may have entry fees.

Travel guide: Wangjing Nan (S) or Jiangtai, Subway Line 14.

Food experience: From Peking Duck to Zhajiangmian (noodles with fried bean sauce), and from Douzhi (fermented bean drink) to Luzhu (wheaten cake boiled in meat broth), Beijing offers countless surprising flavor combinations to delight your taste buds.

Hutong tour: Explore old alleys like Nanluoguxiang by riding a human-powered tricycle, immersing yourself in the charm of ancient Beijing.

Cultural experience: Participate in cultural activities such as Peking Opera and tea ceremonies to deeply understand Beijing's rich cultural heritage.

Shopping hubs: Visit shopping districts like Wangfujing and Xidan to purchase unique souvenirs.

From ancient palaces to modern art districts, from bustling business hubs to tranquil gardens, every facet of Beijing is worth exploring. Plan your Beijing trip today!

Limited Time Offer! Celebrate Children's Day with Special Discounts! Today is International Children's Day, and many attractions in Beijing are offering limited-time free admission, discounted ticket prices, and a variety of exciting activities for you to participate in.
Self-portraits of 50 Art Masters Including Raphael Debut at National Museum of China The National Museum of China (NAMOC) collaborates with the Embassy of the Italian Republic in China and esteemed cultural institutions and museums in Italy to present an exhibition titled "Self-Portrait Masterpieces from the Uffizi Galleries Collections". The exhibition started at 10:00 on April 27 and will last until September 10. This significant museum partnership project launched by China and Italy marks the end of the China-Italy Year of Culture and Tourism and serves as a testimony to the continuity and development of Sino-Italian friendship.

IMAGES

How Far Can Light Travel in One Second? Exploring the Incredible Speed
How Light Energy Travels
Light travels in straight lines outwards from its source
Overview
Light travels in a straight line with explanation
Learn about light 🤔💡| How does light travel?

VIDEO

Faster than light can travel
Does Traveling at Light Speed Have a Limit? A Journey Beyond Time
Ever wondered how long a light year is? #spaceexploration #space
Light can Travel in Vacuum but not Sound, Why?🙄 w/ Neil deGrasse Tyson #physics
The Incredible Distance Light Travels in a Second!
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COMMENTS

How Does Light Travel?
A Ray of Light. When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray.
Speed of Light Calculator
The final step is to calculate the total distance that the light has traveled within the time. You can calculate this answer using the speed of light formula: distance = speed of light × time. Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m. FAQs.
How Does Light Travel? Does It Travel Forever?
Since light travels like a wave, it can travel through a vacuum without interacting with anything. However, when light does go through something, that object can absorb some of it. Light travels through these objects, like glass and water, leaving heat behind. Think of a flashlight. When you turn it on and face it toward a pool, the light can ...
How Does Light Travel?
We can slow it down by focusing it through prisims or crystals. But it still is traveling at 186,000/MPS.and that speed does not change. So, that is why we can see the outer edge of the universe ...
How fast does light travel?
The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light.
How does light travel?
Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums ...
Light basics
Light travels much faster than sound. Light travels at a speed of 299,792,458 m/s (that's nearly 300,000 km/s!). The distance around the Earth is 40,000 km, so in 1 second, light could travel seven and a half times around the world. Sound only travels at about 330 m/s through the air, so light is nearly a million times faster than sound.
Light: Electromagnetic waves, the electromagnetic spectrum and photons
Electromagnetic radiation is one of the many ways that energy travels through space. The heat from a burning fire, the light from the sun, the X-rays used by your doctor, as well as the energy used to cook food in a microwave are all forms of electromagnetic radiation. While these forms of energy might seem quite different from one another ...
The Nature of Light
introduction. Light is a transverse, electromagnetic wave that can be seen by the typical human. The wave nature of light was first illustrated through experiments on diffraction and interference. Like all electromagnetic waves, light can travel through a vacuum. The transverse nature of light can be demonstrated through polarization.
Speed of light: How fast light travels, explained simply and clearly
On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it's one of the most important constants that appears in nature and defines the relationship of ...
Understanding light and other forms of energy on the move
All light shares three properties. It can travel through a vacuum. It always moves at a constant speed, known as the speed of light, which is 300,000,000 meters (186,000 miles) per second in a vacuum. And the wavelength defines the type or color of light. Just to make things interesting, light also can behave as photons, or particles. When ...
Does Light Travel Forever?
This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light isabsorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range ...
Why is the speed of light the way it is?
That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of ...
Speed of light
The speed of light can be used in time of flight measurements to measure large distances to extremely high precision. Ole Rømer first demonstrated in 1676 that light does not travel instantaneously by studying the apparent motion of Jupiter's moon Io. Progressively more accurate measurements of its speed came over the following centuries.
energy
A photon will travel "at the speed of light" until obstructed. From the speed, and elapsed time, you can calculate how far the light will travel. Laser light consists of more than one photon "in phase", which has exactly the same property in this respect, as a solitary photon.
How Far is a Light Year?
A light-year is the distance light can travel in one year. Light is the fastest thing in our Universe traveling through interstellar space at 186,000 miles/second (300,000 km/sec). In one year, light can travel 5.88 trillion miles (9.46 trillion km). A light year is a basic unit astronomers use to measure the vast distances in space.
How far can light travel?
Light dissipating. The fact that we can see the Sun and stars shows that light can travel over enormous distances (150 million kilometres from the Sun). In fact there is no known limit to how far light can travel. However, as you will be aware from observing torch beams or car headlights, there is a limit to the distance over which these are ...
Soal Tes TOEFL dan Pembahasan Jawaban Written Expression ...
16. Light can travels from the Sun to the Earth in eight minutes and twenty seconds. Pembahasan Jawaban: travels --> travel Modal + Verb 1 murni tanpa embel-embel: Modal (can) + verb 1 (travel) 17. Every human typically have twenty-three pairs of chromosomes in most cells. Pembahasan Jawaban: have --> has Subject Verb Agreement: every termasuk dalam kategori singular.
A 'quantum time flip'? Scientist explains how light can travel back and
Scientist explains how light can travel back and forth in time. It's more complex than a photon simply "traveling into the past". Published: Jan 02, 2023 07:44 AM EST. Chris Young.
How do we know that light can travel through a vacuum?
The first is by observation of the Sun and other stars. Astronauts have measured the pressure in outer space and found that there is a very good vacuum, much better in fact than that which we can easily make on earth. The second is through observations on earth. Scientists have measured the speed of light in a vacuum very carefully, and they ...
How far can light travel?
Thinking about the learning. On the one hand pupils are quite prepared to accept that light can travel 150 million kilometres from the Sun to the Earth and yet at the same time believe that light from their torch beam gets used up in a matter of a few metres. The light from the torch becomes progressively more spread out and may also be ...
Light Can Travel Backward in Time (Sort Of)
Light can be reflected not only in space but also in time—and researchers exploring such "time reflections" are finding a wealth of delightfully odd and useful effects
Maximum distance light can travel?
On Earth, we receive about 10^17 photons per square CM. ( source ). Using the inverse square rule, to receive 10 photons per second on a pair of human eyes, that distance would be 100 million AU - give or take, or about 1,580 light years. We probably can't see just 10 photons, but in pitch black, with eyes adjusted to the dark, maybe we could.
Flooding causes no-travel advisory in southeast SD; portions of I-29
Travel impacts are expected to increase throughout the evening hours, with a high likelihood of rain continuing through Saturday. An additional storm system is anticipated to move into the area around 5 p.m. Central today, which will bring heavy rainfall, damaging winds, hail and possibly a tornado.
How To Pack Light & Travel With Only Your Carry-On
While traveling is fun, flying can be a headache. One of the hardest things is figuring out what to pack, especially when airlines put so many restrictions on traveling with luggage. The worst for ...
5 Best Pepper Mills, Tested and Reviewed
️ Opacity: Like olive oil, peppercorns are sensitive to light, and exposure to light can degrade the oils, and therefore flavor, over time. Opaque grinders protect the peppercorns, but it's ...
Video: Why Knoxville, Tennessee celebrates Dolly Parton and the
After celebrating all things Dolly Parton with the locals, CNN's Derek Van Dam catches a firefly-powered light show in the woods. For more, check out America's Best Town's to Visit .
Let Inga Tell You: The sisterhood of the traveling underpants
If you really want to travel light, you're going to have to go commando. Inga's lighthearted looks at life appear regularly in the La Jolla Light. Reach her at [email protected].
The WestJet Group begins flight cancellations in anticipation of
We deeply regret the disruption this will have on the travel plans of our guests, communities and businesses that rely on our critical air service," said Diederik Pen, President of WestJet Airlines and Group Chief Operating Officer. "Following the memberships' nearly unanimous decision to reject a generous tentative agreement that would ...
Practical Guide: How to Make Full Use of 144-Hour Visa-Free Transit in
Route: Visitors can start from the southern end of the square and head north for a view of the Forbidden City in the distance. Recommended time: 1-2 hours, best enjoyed in the soft morning or evening light, perfect for photography. Ticket: Free admission. Reservations are required 1-9 days in advance. Travel guide: Tian'anmenxi, Subway Line 1.

Speed of Light Calculator

What is the speed of light? How fast is the speed of light?

How to calculate the speed of light?

What is the speed of light in mph when it is in a vacuum?

Is the speed of light always constant?

How can I calculate the speed of light?

How far can the speed of light travel in 1 minute?

How Does Light Travel? Does It Travel Forever?

About the Author Robert Sparks

Related Articles:

How Does Light Travel?

Theory of Light to the 19th Century:

Double-Slit Experiment:

Electromagnetism and Special Relativity:

Einstein and the Photon:

Wave-Particle Duality:

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How does light travel?

Theory of Light in the 19th Century:

Double-Slit Experiment:

Wave-Particle Duality:

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Light sources

Light travels much faster than sound

Light can travel through empty space

Light travels in straight lines

Models for light

Light as waves

Light as particles

Nature of science

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The Nature of Light

other characteristics

What is the speed of light? Here’s the history, discovery of the cosmic speed limit

How was the speed of light first measured?

What is the most precise measurement of the speed of light?

How does light slow down?

Why is the speed of light important?

Can you go faster than light speed?

How is the speed at which light travels related to causality?

Why does light travel at this speed?

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