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The Great Filter: a possible solution to the Fermi Paradox

The Great Filter theory suggests that all life must overcome certain challenges, and at least one hurdle is nearly impossible to clear.

In 1950, the physicist and Nobel laureate Enrico Fermi famously asked his colleagues: “ Where are they? ” Fermi had been reflecting upon the vastness of the cosmos, and “they” in his question referred to extraterrestrials. With an almost unfathomable number of stars and planets in the universe, it seemed obvious that intelligent civilizations capable of developing radio astronomy and interstellar travel should speckle the distant stars. Yet, in Fermi’s day, no evidence of such civilizations existed — something that still holds true today.

The Fermi Paradox is the term used to describe the lack of evidence for extraterrestrial life in the face of a universe that should be, by the numbers, bursting with it. But we see no signs of alien technology, and our radio telescopes don’t pick up voices from other worlds.

Many hypotheses have been proposed to resolve the Fermi Paradox, but all of these remain unproven. And in the 1990s, another possible explanation for our apparent aloneness in the universe was formulated by Robin Hanson — a postulate that has become known as the Great Filter .

The hurdles to interplanetary life

Simply stated, the Great Filter says that intelligent interstellar lifeforms must first take many critical steps, and at least one of these steps must be highly improbable. Indeed, the premise of the Great Filter is that there’s at least one hurdle that is so high virtually no species can clear it and move on to the next. But while the term the Great Filter suggests the conscious action of some sort of exogenous entity, in reality, the hypothesis is more a way of thinking about the relative likelihood of certain events happening — or not happening — in their own natural course.

So, what basic hurdles must be cleared in order to become a truly advanced, spacefaring civilization? Hanson suggested a few, paraphrased below:

• A planet capable of harboring life must form in a star’s habitable zone.
• Life itself must develop on that planet.
• Those lifeforms must be able to reproduce, using such molecules as DNA and RNA.
• Simple cells (prokaryotes) must evolve into more complex cells (eukaryotes).
• Multicellular organisms must develop.
• Sexual reproduction, which greatly increases genetic diversity, must take hold.
• Complex organisms capable of using tools must evolve.
• Those organisms must create advanced technology needed for space colonization. (This is roughly where humans are today.)
• The spacefaring species must go on to colonize other worlds and star systems, while avoiding destroying itself.

While humans are not yet capable of interstellar travel in any meaningful sense (beyond a few small robotic probes like the Pioneer, Voyager, and New Horizons spacecraft ), we are capable of advanced radio astronomy, meaning we’re a relatively tech-savvy civilization. But even if it took the same inordinate amount of time for an alien civilization to make the technological leaps humanity has, given the age of the universe, there should be at least a few interplanetary species colonizing their entire galaxy by now.

But, again, astronomers see no evidence of such civilizations. When they look to the stars, the silence is deafening.

The biggest challenges to becoming a galactic civilization

The Great Filter is so difficult to identify because, among other reasons, other worlds experience wildly different environments than Earth. This artist’s concept depicts what the view may be like on a planet in the Trappist-1 system. 

So, what could the Great Filter be?

Well, perhaps abiogenesis (life arising from lifelessness) is wildly uncommon. Perhaps the extreme rarity of this event is in fact the Great Filter. Alternatively, perhaps it’s common for life to spontaneously arise, but the overwhelming majority of life never progresses beyond simple single-cell organisms. Maybe the universe is teeming with bacteria — but bacteria don’t build starships.

Alternatively, the Great Filter might be a consequence of technology itself. Perhaps advanced civilizations usually eradicate themselves via some sort of technology run amok, such as malevolent artificial intelligence, nanotechnology, or a doomsday machine. Humanity is already more than capable of destroying itself via global thermonuclear war. And sadly, it’s possible that such extinction events are virtually inevitable throughout the cosmos.

The Great Filter could also be a purely outside event that is not dependent on the species its testing, regardless of how advanced they may be. For instance, the impact of a giant asteroid or rogue planet, a nearby gamma-ray burst, or an intrusive supernova could potentially annihilate all life on Earth — or any other planet for that matter. No technology in our arsenal today could stop these events from occurring, even if we had forewarning.

Another possibility is that more than just one step of the Great Filter is extremely unlikely to occur. This would exponentially increase the difficulty of a civilization achieving the level of technology required to master interstellar travel.

Has humanity passed The Great Filter?

If the Great Filter is behind us, though, it bodes well for humanity as a species; the universe may be ours for the taking. If, however, the Great Filter still lies ahead, we may be doomed.

On the bright side, some have interpreted our apparent aloneness in the universe as a good sign — a blessing even — as it indicates we’ve safely made it through the bottleneck. Strange as it may seem, we may be the first species to have passed through the Great Filter (after all, someone has to be first).

On the other hand, if we were to detect a signal from a super-advanced technological species that makes us look primitive, it might imply that the Great Filter still lies ahead. Humanity could be destined to take a surprise cosmic test, one that we don’t know what to study for.

The Great Filter is only a theory — yes. But from a logical perspective, it’s an appealing idea on many levels, offering a plausible explanation to the Fermi Paradox. So, although the question of “Where are they?” still remains unanswered, the Great Filter theory offers one of the best guesses we can dream up. Unfortunately, that doesn’t answer whether the Great Filter is already in our rearview mirror.

Doug Adler is the co-author of From The Earth to the Moon: The Miniseries Companion

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The Spacefaring Paradox

Deep-space human travel is a lose-lose proposition..

If there’s one collective lesson gleaned from the COVID pandemic so far, it may be the shared difficulty of being isolated in one’s own home—whether alone or with family members or roommates. The stresses of quarantine included crushing mundane routines, personal habits hypostatized, and all too familiar views (stove range, bathroom mirror, that solitary tree outside, changing while nothing changed). As Amanda Mull wrote in the Atlantic, after working from home for a year, her “ wallpaper has begun to yellow .” When space closes in, humans tend not to thrive. It can drive us to the brink of craziness.

I’ve been thinking about this problem in relation to SpaceX and its rapid advancements throughout the pandemic, including the most recent successful launch and landing of its largest rocket, Starship . This was a prototype of the ship that Elon Musk intends to travel to the Moon, to Mars, and eventually beyond. In “crew mode” it will be able to carry up to 100 passengers. As Marina Koren reported for the Atlantic, Musk suddenly seems a lot closer to his goal of making humans “a multiplanetary species.” If there was something vaguely cathartic or even inspiring in Musk’s tenacious drive to perfect the SpaceX Starship, especially during the pandemic , it may have been the fantasy of more space, out there, beyond the constraints of Earth, which were felt so heavily in 2020.

Yet there’s a paradox lurking at the core of SpaceX.

Before SpaceX will take passengers to space, the company plans to offer “ Earth to Earth transportation .” These would be ridiculously quick rides around the world—for instance, London to New York in a half-hour. The idea is to launch the rocket with paying travelers above Earth’s atmosphere, then speed around the globe and land promptly at the destination. As the SpaceX website boasts, “Imagine most journeys taking less than 30 minutes with access to anywhere in the world in an hour or less.” (Of course, this “anywhere in the world” really means major urban centers with an appropriate landing pad and equipment to service the rocket, but we’ll let the hyperbole slide.)

If achieved at commercial scale, this would turn the airline industry upside down—or at the very least, it would be a massive disruption for airlines that rely heavily on long-haul flights. No other airline or aircraft manufacturer is currently developing a similar mode of transit. A company called Boom recently made headlines for its attempt to bring back supersonic commercial flight, with a plane that is reminiscent of the Concorde, but for flyers on a budget. Yet SpaceX’s Starship flights, if realized, would make supersonic feel like the slow train.

The rationale for speeding up long flights, naturally, is that it is widely understood that people do not like to be in cramped airplane cabins for more than an hour. The less time, the better. The history of commercial aviation has been a race to shorten the time from origin to destination, and make more efficient all the steps in between. Still, there are some things that can’t be fixed. No one likes a tarmac delay or a long flight involving an annoying seatmate or constant turbulence. Time stretches out and plays tricks on the mind, when you’re sitting in an airplane.

Here is where the paradox enters. The same Starship that promises faster air travel around our planet—eliminating those pesky five-, 10-, or 15-hour flights—is also the aspirational repository for Musk’s would-be passengers to Mars. In other words, the Starship cabin is not ultimately intended for trips “under an hour,” but in fact for journeys of multiple months. If you think air rage is bad on a short hop from Las Vegas to San Diego, just wait until your seatmates are there beside you for weeks on end, in the black void of space. SpaceX describes the interior of these craft as including “private cabins, large common areas, centralized storage, solar storm shelters and a viewing gallery.” This makes it sound not so bad. Still, there’s no getting around the blunt truth of containment over a long period of time. Those “large” common areas are likely to shrink the longer the trip takes.

Then there’s sleep. Between 2007 and 2011 the European Space Agency worked with Russia to simulate the conditions of a trip to Mars, particularly as a psychological isolation experiment. Called Mars500 , the longest part of this study ran between 2010 and 2011, and revealed a significant degradation of the simulacral explorers’ sleep patterns. While on wide-body airliners, a business class cocoon seat can deliver comfort (and even luxury ) during an overnight flight, such ergonomic palliatives won’t be as easy for a yearlong journey. Space travel to Mars is supposed to be a bold and daring adventure. But what if it ends up feeling more like a superlong red-eye flight?

For years, Musk has compared his rockets to airliners, using the familiar sizes and thrust capacities of Boeing 737s and 747s as reference points for his future-bound ships. These comparisons circulate on social media , by way of making SpaceX craft both more graspable and more impressive. But the analogies are telling. As much as the goal is to reduce the time of feeling trapped inside a cramped cabin, the endgame is in fact more of this time. And let’s be honest: A hab on Mars is not going to be a whole lot more spacious than the interior of the ship.

If the dream of space travel involves new horizons and feelings of unbound freedom—to explore, to discover, to spread humanity—a nightmare lurks just around the corner of consciousness. There will be no real “arrival” on this fantasy trip: It’s enclosures and pressurized chambers all the way down. When it comes to human space travel, the destination really is the journey. And the journey will be long, and claustrophobic. As far as “quarantine” goes, spacefaring may feel familiar to those who lived through the COVID pandemic—and certain survival tactics may crossover.

Musk wants to send humans to Mars (and beyond) because he believes that the species is doomed on Earth, sooner or later. This bleak assessment belies two haunting presuppositions: The miserable masses will wither on a climate-scorched and ecologically damaged planet back home; meanwhile, the spacefaring select will find themselves in a whole new purgatory of cramped isolation, en route and wherever they “land.”

The wish image of habitations on other planets is for simulated environments that feel as good as—if not better than—our home planet. The reality is bound to be precarious and highly contingent—no matter how awesome and intact space settlements might appear in artistic renderings. The motivation for spacefaring is, at least for Musk, premised on a desire to escape a planet in limbo; but the alternative is hardly a safe haven. This is the paradox of spacefaring: It’s a lose-lose proposition.

As anthropologist Lisa Messeri has found in her research on planetary scientists, ideas about inhabiting outer space can tend to revert back to making sense of our place on Earth. This isn’t necessarily a bad thing; in fact, one of the arguments for space exploration is to improve life back home. Yet as SpaceX moves closer to sending humans beyond the space station, beyond the moon, it’s worth pausing to consider the real implications of these endeavors. We’re already spacefaring, in a literal sense of the term. We know what it feels like to cram ourselves in tight vessels or rooms, and we don’t generally like it. And as the pandemic gradually (hopefully) subsides, our interconnectedness as a species and entanglements with other lifeforms has been made vivid. The adventures and challenges of spacefaring are right before our eyes, the spinning ground on which we’re already standing.

Future Tense is a partnership of Slate , New America , and Arizona State University that examines emerging technologies, public policy, and society.

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• Published: 28 December 2023

Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?

• Ian A. Crawford   ORCID: orcid.org/0000-0001-5661-7403 1 , 2 &
• Dirk Schulze-Makuch   ORCID: orcid.org/0000-0002-1923-9746 3 , 4 , 5 , 6  

Nature Astronomy volume  8 ,  pages 44–49 ( 2024 ) Cite this article

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The ‘Fermi paradox’ refers to the mismatch between a widely held expectation that advanced technological life should be common in the Universe—recently given impetus by the discovery that other planetary systems are common—and the absence of any evidence for it. Here we briefly review attempted solutions to the paradox and conclude that either (1) extraterrestrial technological civilizations are extremely rare (or absent) in the Galaxy or (2) they exist but are deliberately hiding from us, a scenario generally known as the ‘zoo hypothesis’. In this sense, we propose that the answer to the Fermi paradox is ‘the zoo hypothesis or nothing’. We argue that, given a strong commitment to the continued exploration of the Universe, humanity may be able to distinguish between these two alternatives within the next half-century.

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Crawford, I.A., Schulze-Makuch, D. Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?. Nat Astron 8 , 44–49 (2024). https://doi.org/10.1038/s41550-023-02134-2

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Jason Davis • Sep 09, 2021

The Fermi paradox and Drake equation: Where are all the aliens?

Step outside on a clear night and gaze up at the night sky. Depending on the sky conditions where you live, you may see as many as 2,000 stars .

That’s just a tiny fraction of the Milky Way galaxy, which may have between 100 billion to 1 trillion stars . Most of these stars host exoplanets, and we’ve already zeroed in on a few that may be Earth-like .

So where are all the aliens? This is a key question in the field of SETI, the Search for Extraterrestrial Intelligence . While SETI includes everything from listening for radio signals to examining odd fluctuations in starlight , theoretical work in the field has been dominated by two key concepts: the Fermi paradox and the Drake equation.

The Fermi paradox ponders why Earth has not been visited by aliens, while the Drake equation tries to estimate the number of intelligent civilizations in our galaxy. Both concepts involve a lot of uncertainty, because when it comes to extraterrestrial life, there’s a lot we don’t know.

“These two tools are foundational in the sense that they were made towards the beginning of the field and they are rather profound, but not foundational in the way, say, Newton's Laws are,” said Jason Wright, a SETI researcher at Penn State University.

What is the Fermi paradox?

The Fermi paradox is named after Enrico Fermi, a scientist best known for overseeing construction of the world’s first nuclear reactor and using it to conduct the first controlled nuclear reaction in 1942 .

During a 1950 visit to the Los Alamos National Laboratory in New Mexico, Fermi and some colleagues were discussing extraterrestrials and interstellar travel over lunch, in conjunction with a cartoon from “The New Yorker” showing aliens stealing New York City trash cans .

As the story goes, Fermi famously asked, “where is everybody?” Three people who were part of the discussion later reported that Fermi was specifically talking about interstellar travel: If aliens exist and are capable of flying between the stars, then they should have visited us already.

Fermi never published any work on his off-the-cuff remark, and died just four years later. However, his question lived on and became known as the Fermi paradox.

What is the Drake equation?

The Drake equation is named after Frank Drake, an astronomer who in 1960 led the first official search for extraterrestrial radio signals at the National Radio Astronomy Observatory in Green Bank, West Virginia.

During a followup meeting with an eclectic group of thinkers at Green Bank in 1961 that included Planetary Society co-founder Carl Sagan , Drake introduced a formula that could conceivably calculate the number of civilizations currently transmitting signals out into the Milky Way: 

N = R * × f p × n e × f l × f i × f c × L

N , the number of civilizations currently transmitting signals, depends on seven factors:

R * is the yearly formation rate of stars hospitable to planets where life could develop

f p is the fraction of those stars with planets

n e is the number of planets per solar system with conditions suitable for life

f l is the fraction of planets suitable for life on which life actually appears

f i is the fraction of planets with life on which intelligent life emerges

f c is the fraction of planets with intelligent life that develops technologies such as radio transmissions that we could detect

L is the average length of time in years that civilizations produce such signs

“The Drake Equation goes in order from easiest to hardest,” said Kaitlin Rasmussen, an astrophysicist at the University of Michigan. While variables like L remain purely speculative, scientists can now answer with some certainty things like average star formation rates in the Milky Way, and the fraction of stars with planets.

Although some researchers have tried to estimate the number of habitable planets using statistics , Rasmussen is hopeful that upcoming generations of large telescopes will allow us to peer into the atmospheres of Earth-sized planets, giving us better estimates on the number of planets per solar system with conditions suitable for life.

Are the Fermi paradox and Drake equation still relevant?

More than half a century has passed since Drake proposed his famous equation, while it’s been over 70 years since Fermi asked where all the aliens are. Are these questions still the best way to think about intelligent life beyond Earth?

There are a myriad of possible solutions to Fermi’s paradox that have been proposed over the years. Perhaps aliens already visited Earth in the past. Interstellar travel may be either impossible or impractical. Or we may be all alone.

Another possibility is the so-called Great Filter : Intelligent species may run up against barriers like climate change or nuclear war that keeps their lifespans short and prevents them from spreading throughout the galaxy. Whether or not Earthlings have evolved beyond such a filter is anyone’s guess.

In a paper outlining the current state of SETI research , Wright says that while he believes theoretical work on the Fermi paradox is still important, “I fear we have approached a point of strongly diminishing returns that will persist until a detection is made.” He advises any would-be Fermi theorists to “stay close to the data.”

As for the Drake equation, any attempt to solve it requires guesswork for many of the variables, leading to wildly different results.

“I would say that its main drawback is that it cannot account for things which we have not even thought to consider,” said Rasmussen. For instance, a carbon dioxide-choked world like Venus could host life that’s completely different from Earth-based organisms, or what looks like a cluster of habitable exoplanets could be affecting each other’s orbits in unpredictable ways.

While some scientists have proposed tweaks to the Drake equation, the concept of using a mathematical formula to calculate the odds of life on other worlds still has merit. As the legendary SETI scientist Jill Tarter once put it , the equation is “a wonderful way to organize our ignorance.”

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Is NASA Actually Working On a Warp Drive?

Hyperspace, here we come!

• A former NASA propulsion physicist has, he says, resolved a certain paradox in a hypothetical warp drive.
• The Alcubierre drive uses a huge amount of energy to make a sort of fold-like pocket dimension.
• This technology is extremely far off , but thinking about it, experts say, could lead to breakthroughs .

Is NASA really working on . . . a warp drive ? An internal feasibility report suggests the agency might be, or at least that the idea of traveling through folded space is part of the NASA interstellar spaceflight menu.

The space agency isn’t building an engine that can approach the speed of light—yet. In the report, advanced propulsion physicist Harold "Sonny" White, PhD, now of Limitless Space , resolves a major paradox in the leading theoretical model for superluminal (faster than the speed of light) travel, what’s known as an Alcubierre warp drive.

The colloquial term “warp drive” comes from science fiction, most famously Star Trek. The faster-​than-light warp drive of the Federation works by colliding matter and antimatter and converting the explosive energy to propulsion. The show suggests that this extraordinary power alone pushes the ship at faster-than-light speeds.

The Alcubierre drive, first proposed by theoretical physicist Miguel Alcubierre, conforms to Einstein’s theory of general relativity to achieve superluminal travel. It works a bit like the classic “tablecloth and dishes” party trick: The spaceship sits atop the tablecloth of spacetime, the drive pulls the fabric around it, and the ship is situated in a new place relative to the fabric.

Alcubierre describes spacetime expanding on one side of the ship and contracting on the other, thanks to an enormous amount of energy and a requisite amount of exotic matter—in this case, negative energy. Alcubierre's theory creates a kind of pocket in spacetime where a spaceship can operate outside of physics. He insists the requirement for exotic matter is not implausible within quantum mechanics.

The paradox holding back an Alcubierre, in addition to limitations like a lack of negative energy density, is that the direction of a spacecraft is arbitrary when the drive is used—there’s no steering it. Sci-fi has solved this paradox with “stable wormholes,” but NASA can’t fly a deus ex machina to Alpha Centauri.

So White suggests a different paradigm in his report. Instead of a stationary spacecraft engaging the Alcubierre drive from a stopped position, White explains, “In this modified concept, the spacecraft departs earth, establishes an initial subluminal velocity, then initiates the field. The field’s boost acts on the initial velocity as a scalar multiplier, resulting in a much higher apparent speed.” The ship would use a rolling start as a directional cue.

In 2011, White conducted a field sensitivity analysis on Alcubierre’s model to see if he could shake loose any new insights. He found that Alcubierre’s original drive creates a relatively weak field, with negative vacuum energy on the side of the craft being pushed through a fold in spacetime. By making a more robust field, White says, “you could reduce the strain on spacetime so the amount of energy the trick takes to work is significantly reduced. Think metric ton as opposed to Jupiter.”

White suggests the proving ground for warp speed could be closer to home than the nearest stars. If scientists can make the so-called “negative mass” required for an Alcubierre drive, even a tiny example could be deployed within Earth’s atmosphere. “[T]he idea of a warp drive may have some fruitful domestic applications ‘subluminally,’ allowing it to be matured before it is engaged as a true interstellar drive system,” he explains.

An early example could drastically increase speed and reliability of carrying payloads into space. Using a small “beginner” warp drive, White suggests, will give scientists something to iterate as they grow the technology.

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all. 

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Would you really age more slowly on a spaceship at close to light speed?

• Neel V. Patel archive page

Every week, the readers of our space newsletter, The Airlock , send in their questions for space reporter Neel V. Patel to answer. This week: time dilation during space travel. 

I heard that time dilation affects high-speed space travel and I am wondering the magnitude of that affect. If we were to launch a round-trip flight to a nearby exoplanet—let's say 10 or 50 light-years away––how would that affect time for humans on the spaceship versus humans on Earth? When the space travelers came back, will they be much younger or older relative to people who stayed on Earth? —Serge

Time dilation is a concept that pops up in lots of sci-fi, including Orson Scott Card’s Ender’s Game , where one character ages only eight years in space while 50 years pass on Earth. This is precisely the scenario outlined in the famous thought experiment the Twin Paradox : an astronaut with an identical twin at mission control makes a journey into space on a high-speed rocket and returns home to find that the twin has aged faster.

Time dilation goes back to Einstein’s theory of special relativity, which teaches us that motion through space actually creates alterations in the flow of time. The faster you move through the three dimensions that define physical space, the more slowly you’re moving through the fourth dimension, time––at least relative to another object. Time is measured differently for the twin who moved through space and the twin who stayed on Earth. The clock in motion will tick more slowly than the clocks we’re watching on Earth. If you’re able to travel near the speed of light, the effects are much more pronounced. 

Unlike the Twin Paradox, time dilation isn’t a thought experiment or a hypothetical concept––it’s real. The 1971 Hafele-Keating experiments proved as much, when two atomic clocks were flown on planes traveling in opposite directions. The relative motion actually had a measurable impact and created a time difference between the two clocks. This has also been confirmed in other physics experiments (e.g., fast-moving muon particles take longer to decay ). 

So in your question, an astronaut returning from a space journey at “relativistic speeds” (where the effects of relativity start to manifest—generally at least one-tenth the speed of light ) would, upon return, be younger than same-age friends and family who stayed on Earth. Exactly how much younger depends on exactly how fast the spacecraft had been moving and accelerating, so it’s not something we can readily answer. But if you’re trying to reach an exoplanet 10 to 50 light-years away and still make it home before you yourself die of old age, you’d have to be moving at close to light speed. 

There’s another wrinkle here worth mentioning: time dilation as a result of gravitational effects. You might have seen Christopher Nolan’s movie Interstellar , where the close proximity of a black hole causes time on another planet to slow down tremendously (one hour on that planet is seven Earth years).

This form of time dilation is also real, and it’s because in Einstein’s theory of general relativity, gravity can bend spacetime, and therefore time itself. The closer the clock is to the source of gravitation, the slower time passes; the farther away the clock is from gravity, the faster time will pass. (We can save the details of that explanation for a future Airlock.)

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5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

• Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
• Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

• Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
• Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

• Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
• Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

• Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
• Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

• Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

• The Solution: time travel is impossible because of the very paradox it creates.
• Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
• The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
• Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• time travel
• grandfather paradox

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Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

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Barak Shoshany does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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Paradox-Free Time Travel Is Theoretically Possible: Physicist “Squares the Numbers” on Time Travel

Paradox-free time travel is theoretically possible, according to the mathematical modeling of a prodigious University of Queensland (UQ) undergraduate student.

Fourth-year Bachelor of Advanced Science (Honours) student Germain Tobar has been investigating the possibility of time travel, under the supervision of UQ physicist Dr. Fabio Costa.

“Classical dynamics says if you know the state of a system at a particular time, this can tell us the entire history of the system,” Mr. Tobar said. “This has a wide range of applications, from allowing us to send rockets to other planets and modeling how fluids flow.

“For example, if I know the current position and velocity of an object falling under the force of gravity, I can calculate where it will be at any time.

“However, Einstein’s theory of general relativity predicts the existence of time loops or time travel – where an event can be both in the past and future of itself – theoretically turning the study of dynamics on its head.”

Mr. Tobar said a unified theory that could reconcile both traditional dynamics and Einstein’s Theory of Relativity was the holy grail of physics.

“But the current science says both theories cannot both be true,” he said. “As physicists, we want to understand the Universe’s most basic, underlying laws and for years I’ve been puzzled on how the science of dynamics can square with Einstein’s predictions.”

“I wondered is time travel mathematically possible?”

Mr. Tobar and Dr. Costa say they have found a way to “square the numbers” and Dr. Costa said the calculations could have fascinating consequences for science.

“The maths checks out – and the results are the stuff of science fiction,” Dr. Costa said.

“Say you traveled in time, in an attempt to stop COVID-19 ’s patient zero from being exposed to the virus . However if you stopped that individual from becoming infected – that would eliminate the motivation for you to go back and stop the pandemic in the first place.

“This is a paradox – an inconsistency that often leads people to think that time travel cannot occur in our universe.

“Some physicists say it is possible, but logically it’s hard to accept because that would affect our freedom to make any arbitrary action. It would mean you can time travel, but you cannot do anything that would cause a paradox to occur.”

However, the researchers say their work shows that neither of these conditions has to be the case, and it is possible for events to adjust themselves to be logically consistent with any action that the time traveler makes.

“In the coronavirus patient zero example, you might try and stop patient zero from becoming infected, but in doing so you would catch the virus and become patient zero, or someone else would,” Mr. Tobar said.

“No matter what you did, the salient events would just recalibrate around you. This would mean that – no matter your actions – the pandemic would occur, giving your younger self the motivation to go back and stop it.

“Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency.

“The range of mathematical processes we discovered show that time travel with free will is logically possible in our universe without any paradox.”

Reference: “Reversible dynamics with closed time-like curves and freedom of choice” by Germain Tobar and Fabio Costa, 21 September 2020, Classical and Quantum Gravity . DOI: 10.1088/1361-6382/aba4bc

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“Try as you might to create a paradox, the events will ALWAYS adjust themselves, to avoid any inconsistency.”

What if someone were to got back in time and kill their mother before she gave birth to him? How does that get ‘adjusted?’

You decide to go back and end Covid-19; it is Sept 24 2020: If you stopped “person-zero” from getting COVID-19, then “person-one” will get it in the “adjustment”. Back in Sept 24, 2020, COVID-19 is still around, and you still decide to go back to stop it. So then you are stopping “person-one”, and “person-two” gets the adjustment. Back in Sept 24, 2020, COVID-19 is still around, and you still decide to go back to stop it. So then you are stopping “person-two” and “person-three” gets the adjustment. So ….. you get trapped in an infinite time-loop from which you can never escape. OR this is a poorly written article in which (a) the author did not understand the mathematical model, or (b) the mathematicians did a poor job explaining what they are modeling, or (c) both, or (d) the MODEL is an incorrect model of the “real” universe, even though the “math” that describes the model is error free. Whatever the answer, this article left me with more questions than answers regarding the title of the article. WHAT?

I don’t think it’s neccessary to get trapped in endless loop. If I would like to try to stop the Patient-Zero from being infected by jumping back in time I would see the futility of the failed attempts and give up after a few loops. More over, jumping back-and-back-to-the-future-again should not cause memory erasure so all the trials would appear to me as a normal cause-effect chain from my perspective, so why should I try again and again? But now we know that timespace could be this self-correcting data stream and I should take this theory into consideration before I try to lose a chunk of >my< lifespan in the history. But that's just me trying to sound logical to myself 🙂

It seems that the poison made by Einstein continues toxicating physicists who blindly believe the current orthodox. They use general relativity as a tool to mathematically speculate the possibility of the existence of closed time curves in the so-called four-dimensional spacetime, but have never seriously defined what time travel really means. When they say somebody time travels to the past, which is the boundary of the person and his surroundings, and how do his surroundings continuously connect from the present to the past? Obviously, there is no way to have a logically consistent definition for so-called time travel.

It’s true that time scale is different for different physical processes. For example, a rabbit thinks that one year is a long time, but a turtle feels one year is just a blink. Some physical processes can even be reversed. Thus we define our physical time to be irreversible with a fixed scale as shown on a physical clock and use a rate to customize the description of the change of each physical process. For example, when a car has driven 100 km at a speed of 100 km/h, we know it takes one hour time, but then after it has driven for another 100 km backward at the same speed, we still think that the time it takes is another hour rather than a negative one hour because we consider the backward speed is negative, rather than time is negative. If somebody got healthier and looked younger after eating some special food, we would not think that time was reversed but just his aging rate was negative during the period. Reversible physical processes exist everywhere, but our physical time is always irreversible as we have defined.

Einstein made a fatal mistake in his special relativity. He assumed that the speed of light should be the same relative to all inertial reference frames, which requires the change of the definition of space and time. But he never verified that the newly defined time was still the time measured with physical clocks. Then many mathematicians and theoretical physicists think that time is like a playdough which can be made to fit all kinds of theories, while our physical time measured with physical clocks is stiff and absolute, which won’t change with the change of the definition of the space and time. Actually, the newly defined relativistic time is no longer the time measured with physical clocks, but just a mathematical variable without physical meanings, which can be easily verified as follows:

We know physical time T has a relationship with the relativistic time t in Einstein’s special relativity: T = tf/k where f is the relativistic frequency of the clock and k is a calibration constant. Now We would like to use the property of our physical time in Lorentz Transformation to verify that the relativistic time defined by Lorentz Transformation is no longer our physical time.

If you have a clock (clock 1) with you and watch my clock (clock 2) in motion and both clocks are set to be synchronized to show the same physical time T relative to your inertial reference frame, you will see your clock time: T1 = tf1/k1 = T and my clock time: T2 = tf2/k2 = T, where t is the relativistic time of your reference frame, f1 and f2 are the relativistic frequencies of clock 1 and clock 2 respectively, k1 and k2 are calibration constants of the clocks. The two events (Clock1, T1=T, x1=0, y1=0, z1=0, t1=t) and (Clock2, T2=T, x2=vt, y2=0, z2=0, t2=t) are simultaneous measured with both relativistic time t and clock time T in your reference frame. When these two clocks are observed by me in the moving inertial reference frame, according to special relativity, we can use Lorentz Transformation to get the events in my frame (x’, y’, z’, t’): (clock1, T1′, x1’=-vt1′, y1’=0, z1’=0, t1′) and (clock2, T2′, x2’=0, y2’=0, z2’=0, t2′), where T1′ = t1’f1’/k1 = (t/γ)(γf1)/k1 = tf1/k1 = T1 = T and T2′ = t2’f2’/k2 = (γt)(f2/γ)/k2 = tf2/k2 = T2 = T, where γ = 1/sqrt(1-v^2/c^2). That is, no matter observed from which inertial reference frame, the events are still simultaneous measured with physical time T i.e. the two clocks are always synchronized measured with physical time T, but not synchronized measured with relativistic time t’. Therefore, our physical time and the relativistic time behave differently in Lorentz Transformation and thus they are not the same thing. The change of the reference frame only makes changes of the relativistic time from t to t’ and the relativistic frequency from f to f’, which cancel each other in the formula: T = tf/k to make the physical time T unchanged i.e. our physical time is still absolute in special relativity. Therefore, based on the artificial relativistic time, special relativity is wrong, so is general relativity. For more details, please check: https://www.researchgate.net/publication/297527784_Challenge_to_the_Special_Theory_of_Relativity

Once we know our physical time is absolute, there is no room for those mathematicians and theoretical physicists to speculate.

I find it disconcerting that space is never discussed while musing over time travel. The arrow of time moves with space, forward. To travel back in time, one would also need to move in space. So, you may go back in time (if you had a sci-fi apparatus), but, the rest of the universe would proceed on its merry way. Thus, you and your apparatus, going back in time one hour would find that you are out drifting in space because the earth and the rest of the universe continues on it way. Time and space do not readily part ways to accommodate a fantasy. There are probably more profitable areas of conjecture for intelligent students, professors and scientists to spend their time.

And because time travel implicitly requires space travel, the amount of energy required becomes mind boggling. Also, because there is a finite limit on the speed of any material object, it means that one can’t go back in time instantaneously. It would require a very long time to move to the position in space occupied by Earth 100 million years ago, even at the speed of light. And, the calculations would have to be unobtainably accurate or one might end up outside the atmosphere, or in the interior of the Earth.

Perhaps we should be changing the name of the website from “Sci-Tech Daily” to “Sci-Assume Daily”?

You fail no matter what you do. It could turn out that it wasn’t really your mother but a lookalike. Or that you hallucinated it, which wouldn’t be far fetched for someone seriously trying to kill their mother.

So this is still wrong. The events wouldn’t “shift” to prevent the paradox. There IS no paradox. You did it because in your timeline the events happened. But in doing so, youve created a timeline where they didn’t. There is no need for your younger self to go back, but you did. This isn’t a paradox, it’s a different timeline. You crated a branch in the timeline. An alternate universe.

What evidence do you have that alternate universes exist?

… Yeah, “Paradox-Free Time Travel Is Theoretically Possible” … it doesn’t mean that we actually have a time travel, or that some advanced civilization has developed a time travel, after all… So! The next idea it might be that aliens are the humans from the future! Yeah, that is a great idea, a really great one…

The act of time traveling is a paradox within itself. It has to happen in the greater time line for everything afterwards to occur. Also, the Russians already achieved a six minute time travel a decade ago…

… “I wondered: “is time travel mathematically possible?”

Well, we all have asked our self the very same question. However, we have no evidence of such a thing happening, or ever happened. Though, it would be great. Second thing is that time dilatation, which has been proven correct, but was it perceived in a proper way, or it is just the numbers that mean something else. …

According to current physical theory, is it possible for a human being to travel through time?

… ask Goedel, he would know about paradoxes that might arise from theory of relativity…

It would correct itself if you tried to kill your mother by someone stopping you from doing it or you leave thinking your mother is dead but isn’t or even that you’re giving birth to buy a different woman. And this time travel thing kind of plays along in the theory that our universe is actually the interior of a black hole just inside the horizon. A wormhole would be like getting right to the edge of the horizon and then it bending you back around to a point in the past as you try to cross it. Which kind of goes along with the fact that if you tried to travel in a straight line away from the Earth fast enough for long enough you would get back to the Earth except if the speed is fast enough you would get back to the Earth in the future though you would not have aged. But anyway going back in the past and killing your mother would be made impossible by whatever happened to stop you from killing her. Moreover you would never even be allowed to meet her at least not where she’s aware that it’s you because that would change the future. So impossible to kill your mother in the past no matter how hard you tried. And people like to say well if sometime people invent time travel to the past why haven’t they come back and talk to us. Maybe it’s impossible because of time correcting itself. Or if creating a paradox is possible people of the future realize that they could wipe themselves out and interference with the past is not something they do. They only observe. Maybe even have rules for exactly when they’re allowed to go to. Maybe they’re only allowed to travel to prehistory. Not travel to somewhere where it could cause a paradox just by knowing someone’s from the future.

All of these comments are interesting and informative.

But they all assume a past Paradigm that only Time Moves. We are holding onto a rock in the river of Time as the current flows past us from the Past through the Present and into the Future.

We are not holding onto the rock of Earth as the Sun, planets, stars and galaxies flow around us. We move through moving Space.

ALL OF THESE VIEWS also assume we can change Time with our actions. WE NEVER CHANGE TIME WITH OUR ACTIONS. WE CHANGE OUR JOURNEY THROUGH TIME WITH OUR ACTIONS. (Drop your egocentric notions.)

We move through moving spacetime.

A Paradigm Shift from geocentric to heliocentric helped explain our neighboring space.

A Paradigm Shift from Newton to Einstein helped explain our neighboring spacetime.

Publishing my spacetime / mattergy Paradigm Shift Work will solve all your Paradoxes.

Time travel could very well be possible in this day and age. But it may be a whole lot slower than people would imagine.

https://www.youtube.com/watch?v=qCEwPT80cSA

I’ve been travelling forward in time for sixty years.

Time travel? The paradox that these two words conjure in one’s analytical mind are endless. No two people imagine what these words mean in the same way. Some believe only a single body jumps through time with all the cognitive capabilities and memories of another time left in tact. Where others imagine the time travelers mind and memories also change to synchronize with the time visited. Does this insinuation limit time travel for someone to the number of years they have been alive? And does the visitation of a time where the traveler was an infant subject that person to a life of Deja-Vu episodes? Does the number of times one has time traveled have a direct correlation with an elevated intelligence to reasoning capability? If so, would that be a reason to assume an increased intellectual capability of anyone who leaps through time to visit our planet. And No, the mere virtue of them having the technology to travel in that method does not imply they must understand how it works. Look how many use modern conveniences regularly with no clue as to their engineering. Everything a person does in this modern society is by the grace of someone else’s knowledge and occupation. My guess is that if time travel is possible, the ancillary effects of doing so would be unavoidable. Nothing in this universe has only positive result. For every action does have an equal and opposite reaction. My luck I would leap to a time in the future, and find its one day after my burial. Oh Chit!

How could you travel back to a period of time from which you have no memory of to a place you’ve never seen or have no accurate visual representation of in your memory I can travel anywhere I’ve never been in the universe but will only become aware of its spatial and physical and visual properties once I have arrived at that location in space and time or have at least observed it at a distance before arriving there so how could you ever go to a place or time in the universe that never entered your light cone

You can mathematically prove anything just like you could make anything bad sound good if you shuffle the words around enough numbers and ideas are all creations of our minds to help us understand our existence the truth is our ideas and our perception of reality is the universe we are nothing more than a way for the universe to know itself for it to feel and see what it is and in that we are stuck in an infinite unbreakable loop of never being able to break out of the constraints the universe (us/we) have put on ourselves or fully understand the nature of our own existence because we all came from nothing and therefore are nothing but what the universe imagines itself to be

I’ll admit it is fun to try to imagine anything is possible and maybe that’s enough to make anything possible

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