time travel theory stephen hawking

September 2, 2014

Time Travel Simulation Resolves “Grandfather Paradox”

What would happen to you if you went back in time and killed your grandfather? A model using photons reveals that quantum mechanics can solve the quandary—and even foil quantum cryptography

By Lee Billings

On June 28, 2009, the world-famous physicist Stephen Hawking threw a party at the University of Cambridge, complete with balloons, hors d'oeuvres and iced champagne. Everyone was invited but no one showed up. Hawking had expected as much, because he only sent out invitations after his party had concluded. It was, he said, "a welcome reception for future time travelers," a tongue-in-cheek experiment to reinforce his 1992 conjecture that travel into the past is effectively impossible.

But Hawking may be on the wrong side of history. Recent experiments offer tentative support for time travel's feasibility—at least from a mathematical perspective. The study cuts to the core of our understanding of the universe, and the resolution of the possibility of time travel, far from being a topic worthy only of science fiction, would have profound implications for fundamental physics as well as for practical applications such as quantum cryptography and computing.

Closed timelike curves The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein's theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a "closed timelike curve," or CTC, a loop that could be traversed to travel back in time.

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Hawking and many other physicists find CTCs abhorrent, because any macroscopic object traveling through one would inevitably create paradoxes where cause and effect break down. In a model proposed by the theorist David Deutsch in 1991, however, the paradoxes created by CTCs could be avoided at the quantum scale because of the behavior of fundamental particles, which follow only the fuzzy rules of probability rather than strict determinism. "It's intriguing that you've got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away," says University of Queensland physicist Tim Ralph. "It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics."

Experimenting with a curve Recently Ralph and his PhD student Martin Ringbauer led a team that experimentally simulated Deutsch's model of CTCs for the very first time, testing and confirming many aspects of the two-decades-old theory. Their findings are published in Nature Communications. Much of their simulation revolved around investigating how Deutsch's model deals with the “grandfather paradox,” a hypothetical scenario in which someone uses a CTC to travel back through time to murder her own grandfather, thus preventing her own later birth. ( Scientific American is part of Nature Publishing Group.)

Deutsch's quantum solution to the grandfather paradox works something like this:

Instead of a human being traversing a CTC to kill her ancestor, imagine that a fundamental particle goes back in time to flip a switch on the particle-generating machine that created it. If the particle flips the switch, the machine emits a particle— the particle—back into the CTC; if the switch isn't flipped, the machine emits nothing. In this scenario there is no a priori deterministic certainty to the particle's emission, only a distribution of probabilities. Deutsch's insight was to postulate self-consistency in the quantum realm, to insist that any particle entering one end of a CTC must emerge at the other end with identical properties. Therefore, a particle emitted by the machine with a probability of one half would enter the CTC and come out the other end to flip the switch with a probability of one half, imbuing itself at birth with a probability of one half of going back to flip the switch. If the particle were a person, she would be born with a one-half probability of killing her grandfather, giving her grandfather a one-half probability of escaping death at her hands—good enough in probabilistic terms to close the causative loop and escape the paradox. Strange though it may be, this solution is in keeping with the known laws of quantum mechanics.

In their new simulation Ralph, Ringbauer and their colleagues studied Deutsch's model using interactions between pairs of polarized photons within a quantum system that they argue is mathematically equivalent to a single photon traversing a CTC. "We encode their polarization so that the second one acts as kind of a past incarnation of the first,” Ringbauer says. So instead of sending a person through a time loop, they created a stunt double of the person and ran him through a time-loop simulator to see if the doppelganger emerging from a CTC exactly resembled the original person as he was in that moment in the past.

By measuring the polarization states of the second photon after its interaction with the first, across multiple trials the team successfully demonstrated Deutsch's self-consistency in action. "The state we got at our output, the second photon at the simulated exit of the CTC, was the same as that of our input, the first encoded photon at the CTC entrance," Ralph says. "Of course, we're not really sending anything back in time but [the simulation] allows us to study weird evolutions normally not allowed in quantum mechanics."

Those "weird evolutions" enabled by a CTC, Ringbauer notes, would have remarkable practical applications, such as breaking quantum-based cryptography through the cloning of the quantum states of fundamental particles. "If you can clone quantum states,” he says, “you can violate the Heisenberg uncertainty principle,” which comes in handy in quantum cryptography because the principle forbids simultaneously accurate measurements of certain kinds of paired variables, such as position and momentum. "But if you clone that system, you can measure one quantity in the first and the other quantity in the second, allowing you to decrypt an encoded message."

"In the presence of CTCs, quantum mechanics allows one to perform very powerful information-processing tasks, much more than we believe classical or even normal quantum computers could do," says Todd Brun, a physicist at the University of Southern California who was not involved with the team's experiment. "If the Deutsch model is correct, then this experiment faithfully simulates what could be done with an actual CTC. But this experiment cannot test the Deutsch model itself; that could only be done with access to an actual CTC."

Alternative reasoning Deutsch's model isn’t the only one around, however. In 2009 Seth Lloyd, a theorist at Massachusetts Institute of Technology, proposed an alternative , less radical model of CTCs that resolves the grandfather paradox using quantum teleportation and a technique called post-selection, rather than Deutsch's quantum self-consistency. With Canadian collaborators, Lloyd went on to perform successful laboratory simulations of his model in 2011. "Deutsch's theory has a weird effect of destroying correlations," Lloyd says. "That is, a time traveler who emerges from a Deutschian CTC enters a universe that has nothing to do with the one she exited in the future. By contrast, post-selected CTCs preserve correlations, so that the time traveler returns to the same universe that she remembers in the past."

This property of Lloyd's model would make CTCs much less powerful for information processing, although still far superior to what computers could achieve in typical regions of spacetime. "The classes of problems our CTCs could help solve are roughly equivalent to finding needles in haystacks," Lloyd says. "But a computer in a Deutschian CTC could solve why haystacks exist in the first place.”

Lloyd, though, readily admits the speculative nature of CTCs. “I have no idea which model is really right. Probably both of them are wrong,” he says. Of course, he adds, the other possibility is that Hawking is correct, “that CTCs simply don't and cannot exist." Time-travel party planners should save the champagne for themselves—their hoped-for future guests seem unlikely to arrive.

The Quantum Physics of Time Travel   (All-Access Subscribers Only) By David Deutsch and Michael Lockwood

Can Quantum Bayesianism Fix the Paradoxes of Quantum Mechanics?

Astrophysicist J. Richard Gott on Time Travel

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Stephen hawking on time travel, m-theory, and extra terrestrial life, invite time travelers to a party late "i sat there a long time, no one came.".

Michael Venables - Jul 1, 2012 7:00 pm UTC

Stephen Hawking is near-universally recognized as a brilliant scientist, and one small facet of his complex genius is the rockstar ability to democratize scientific knowledge. It's becoming increasingly important as purveyors of pop culture continue to eschew acknowledgment and respect for the most basic scientific principles. Hollywood in particular has continued its marginalization of scientific knowledge with blatant disregard in recent films .

But Hawking acts like a great counter force against anti-intellectual movements. He takes complex scientific principles and explains them so the general public can understand and, more importantly, appreciate the science behind them. He inspires people to want to know more about Calabi–Yau manifolds and multiverses. That is why Stephen Hawking rocks our scientific world.

So I (and everyone around me) felt a consummate air of curiosity in the Paramount Theater lobby on June 16, as Dr. Hawking arrived for an appearance at the Seattle Science Festival . But what was everyone curious about? Was it Dr. Hawking's motor neuron disease? Or how his speech synthesizer works, perhaps?

The following are the press questions from Seattle-area colleagues that I read to Dr. Hawking at a press event before his main program. The 70-year-old scientist keynoted a Saturday night symposium that also featured Leroy Hood and Jack Horner, with Hawking discussing his life and Nobel aspirations among other topics.

What would it take to make time travel a reality, and how would that affect the present reality?  (via Arik Korman, 95.7 KJR)

We are all travelling forward in time anyway. We can fast forward by going off in a rocket at high speed and return to find everyone on Earth much older or dead. Einstein's general theory of relativity seems to offer the possibility that we could warp space-time so much that we could travel back in time. However, it is likely that warping would trigger a bolt of radiation that would destroy the spaceship and maybe the space-time itself. I have experimental evidence that time travel is not possible. I gave a party for time-travelers, but I didn't send out the invitations until after the party. I sat there a long time, but no one came.

Horner and Hood at the symposium

So Horner's Montana lab is tweaking evolution's dialectic via complex procedures on embryos and making probes with extracted DNA. They then replace the probe to control the growth of "experimental atavisms"—long suppressed evolutionary dinosaur characteristics such as a long tail, three-fingered claws, and dinosaur teeth. Horner's goal is to grow the world's first "chickasaur." And he points out it's best to start out small because it's easier to catch if something goes wrong.

Leroy Hood's presentation touched on what he calls P4 medicine: predictive, preventative, personalized, and participatory. It is predictive because future medicine will draw from what will be a completed data set of the human genome in about ten years, and will make use of massive pools of digitized bio-information. "Preventative" because the approach to drugs will target discovery of illness via the use of such large repositories of digital genomic data. The P4 model should see the cost of medicine go down, allowing a focus on wellness from the continued mining of millions of data points for ongoing patient diagnoses. Each patient will have a unique genomic profile, so the approach can be tailored to each patient. Hood even imagines patient-driven social networks for individuals to share information on disease and wellness.

If M-theory is the only candidate for a complete theory of the universe, what's the best evidence that you think will be found to support the theory? Lacking that evidence, isn't M-theory just another kind of religion?  (via Alan Boyle, MSNBC )

M-theory is the only theory that seems to have all the properties we would expect of a complete and consistent "history of everything," but that may just reflect our lack of imagination. If M-theory is correct, it predicts that every particle should have a super-partner. So far, we have not observed any super-partners, but the hope is that they will be found at the LHC. If they are discovered, it will be strong evidence for M-theory. On the other hand, it they are shown not to exist, that will be exciting, because we will learn something new.

How would you describe your quality of life? What do you miss most from before the onset of ALS?  (via Arik Korman, 95.7 KJR)

Although I am severely disabled and on a ventilator, my quality of life is pretty good. I have been very successful in my scientific work, and have become one of the best known scientists in the world. I have three children and three grandchildren so far. I travel widely, have been to Antarctica, and have met the presidents of Korea, China, India, Ireland, Chile, and the United States. I have been down in a submarine and up on a zero-gravity flight, in preparation for the flight into space that I'm hoping to make on Virgin Galactic. Despite my disability, I have managed to do most things I want. My main regret is that it has prevented me from playing with my children and grandchildren as fully as I want.

John Gribben recently argued that we are almost certainly the only intelligent life in the Milky Way—do you think he's right or wrong, and why? Also, Seth Shostak argues that even if there are other intelligent civilizations out there, it's too late for us to keep quiet about our existence, because it's possible to pick up the signals we've sent out over the past seventy years. So, isn't it too late for us to keep quiet, and shouldn't we be thinking about upgrading our defenses against the alien hordes?  (via Alan Boyle, MSNBC)

We think that life develops spontaneously on Earth, so it must be possible for life to develop on suitable planets elsewhere in the universe. But we don't know the probability that a planet develops life. If it is very low, we may be the only intelligent life in the galaxy. Another frightening possibility is intelligent life is not only common, but that it destroys itself when it reaches a stage of advanced technology.

Evidence that intelligent life is very short-lived is that we don't seem to have been visited by extra terrestrials. I'm discounting claims that UFOs contain aliens. Why would they appear only to cranks and weirdos? Do I believe that there is some government conspiracy to conceal the evidence and keep for themselves the advanced technology the aliens have? If that were the case, they aren't making much use of it. Further evidence that there isn't any intelligent life within a few hundred light years comes from the fact that SETI, the Search for Extra Terrestrial Life, hasn't picked up their television quiz shows. It is true that we advertise our presence by our broadcast. But given that we haven't been visited for four billion years, it isn't likely that aliens will come any time soon.

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November 9, 2018

Stephen Hawking's final book suggests time travel may one day be possible – here's what to make of it

by Peter Millington, The Conversation

"If one made a research grant application to work on time travel it would be dismissed immediately," writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions . He was right. But he was also right that asking whether time travel is possible is a "very serious question" that can still be approached scientifically.

Arguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic. So where does this leave us? We cannot build a time machine today, but could we in the future?

Let's start with our everyday experience. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now . But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us.

Our inability to access the "now" of someone far away is at the heart of Albert Einstein's theories of space and time.

Light speed

Einstein told us that space and time are parts of one thing – spacetime – and that we should be as willing to think about distances in time as we are distances in space. As odd as this might sound, we happily answer "about two and half hours", when someone asks how far Birmingham is from London. What we mean is that the journey takes that long at an average speed of 50 miles per hour.

Mathematically, our statement is equivalent to saying that Birmingham is about 125 miles from London. As physicists Brian Cox and Jeff Forshaw write in their book Why does E=mc²? , time and distance "can be interchanged using something that has the currency of a speed". Einstein's intellectual leap was to suppose that the exchange rate from a time to a distance in spacetime is universal – and it is the speed of light.

The speed of light is the fastest any signal can travel, putting a fundamental limit on how soon we can know what is going on elsewhere in the universe. This gives us "causality" – the law that effects must always come after their causes. It is a serious theoretical thorn in the side of time-travelling protagonists. For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth).

Now, if the speed of light is universal, we must measure it to be the same – 299,792,458 metres per second in vacuum – however fast we ourselves are moving. Einstein realised that the consequence of the speed of light being absolute is that space and time itself cannot be. And it turns out that moving clocks must tick slower than stationary ones.

The faster you move, the slower your clock ticks relative to ones you are moving past. The word "relative" is key: time will seem to pass normally to you. To everyone standing still, however, you will be in slow motion. If you were to move at the speed of light, you would appear frozen in time – as far as you were concerned, everyone else would be in fast forward.

So what if we were to travel faster than light, would time run backwards as science fiction has taught us?

Unfortunately, it takes infinite energy to accelerate a human being to the speed of light, let alone beyond it. But even if we could, time wouldn't simply run backwards. Instead, it would no longer make sense to talk about forward and backward at all. The law of causality would be violated and the concept of cause and effect would lose its meaning.

Einstein also told us that the force of gravity is a consequence of the way mass warps space and time. The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick. If we squeeze in enough mass, spacetime becomes so warped that even light cannot escape its gravitational pull and a black hole is formed. And if you were to approach the edge of the black hole – its event horizon – your clock would tick infinitely slowly relative to those far away from it.

So could we warp spacetime in just the right way to close it back on itself and travel back in time?

The answer is maybe, and the warping we need is a traversable wormhole . But we also need to produce regions of negative energy density to stabilise it, and the classical physics of the 19th century prevents this. The modern theory of quantum mechanics , however, might not.

According to quantum mechanics, empty space is not empty. Instead, it is filled with pairs of particles that pop in and out of existence. If we can make a region where fewer pairs are allowed to pop in and out than everywhere else, then this region will have negative energy density.

However, finding a consistent theory that combines quantum mechanics with Einstein's theory of gravity remains one of the biggest challenges in theoretical physics. One candidate, string theory (more precisely M-theory) may offer up another possibility.

M-theory requires spacetime to have 11 dimensions: the one of time and three of space that we move in and seven more, curled up invisibly small. Could we use these extra spatial dimensions to shortcut space and time? Hawking, at least, was hopeful.

Saving history

So is time travel really a possibility? Our current understanding can't rule it out, but the answer is probably no.

Einstein's theories fail to describe the structure of spacetime at incredibly small scales. And while the laws of nature can often be completely at odds with our everyday experience, they are always self-consistent – leaving little room for the paradoxes that abound when we mess with cause and effect in science fiction's take on time travel.

Despite his playful optimism, Hawking recognised that the undiscovered laws of physics that will one day supersede Einstein's may conspire to prevent large objects like you and I from hopping casually (not causally) back and forth through time. We call this legacy his " chronology protection conjecture ".

Whether or not the future has time machines in store, we can comfort ourselves with the knowledge that when we climb a mountain or speed along in our cars, we change how time ticks.

So, this " pretend to be a time traveller day " (December 8), remember that you already are, just not in the way you might hope.

Provided by The Conversation

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JUICE mission to explore Jupiter's icy, ocean-bearing moons lifted off aboard Arianespace's Ariane 5 rocket at the Guiana Space Center in Kourou, French Guiana today (Jody Amiet/AFP via Getty Images)

JUICE spacecraft will visit Jupiter’s moons

Today, the Jupiter Icy Moons Explorer (JUICE) successfully launched on its eight-year journey to Jupiter, where it will study three of the planet’s four Galilean moons: Ganymede, Callisto and Europa. The launch required a precise lift-off time to insert the spacecraft into the correct orbit around the Sun: exactly 12:14 UT. “There is no launch window, only one launch instant,” said programme director and launch operator Véronique Loisel.

The European Space Agency’s spacecraft will be the first to orbit a moon of another planet when it circles Ganymede in search of a hidden ocean beneath the icy surface. A complex manoeuvre around the Sun and Earth will slingshot JUICE towards the outer Solar System. The mission will eventually end with a crash landing on Ganymede’s surface.

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Calls to reinstate gender-equality researcher

Around 4,000 academics have signed an open letter demanding that the University of Groningen in the Netherlands reinstate social-psychology professor Susanne Täuber . She was sacked after publishing an article on how her experiences in the university’s prestigious Rosalind Franklin Fellowship programme caused her to believe that initiatives “set up to promote gender equality might inadvertently work against women” at the university. Groningen would not comment on Täuber’s case, but court documents show that a manager described the article as “inappropriate and damaging”. Supporters of Täuber say her firing is a blow for academic freedom and has made many fear similar treatment if they criticize their own institutions.

Reference: Journal of Management Studies article written by Täuber

How to sleep like a bear and not get clots

Brown bears ( Ursus arctos ) don’t develop dangerous blood clots during hibernation — despite months-long periods of inactivity that would put humans at great risk of thrombosis. The bears’ blood platelets, the component that causes clotting, produce significantly less of the clotting protein HSP47 during the animals’ winter snooze than when they are active. In people, a similar mechanism seems to lower the risk of thrombosis during long periods of immobility: lowered levels of HSP47 were found in people with spinal-cord injuries and in participants of a space-flight-simulation study who spent a month in bed.

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Reference: Science paper

Astronomers spot runaway black hole

Some 40 million years ago, a supermassive black hole escaped the clutches of its host galaxy, leaving a sparkling trail of young stars as it sped away into space. The rogue black hole was first picked up by the Hubble Space Telescope as a faint linear trail . Further observations made with the W. M. Keck Observatory in Hawaii revealed that the streak was a stream of blue stars stretching across a staggering 200,000 light years. The object weighs 20 million Suns and is travelling at more than 1,500 kilometres a second. “If this is a runaway black hole..., it is travelling very fast,” says astronomer Christopher Reynolds.

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Reference: Astrophysical Journal Letters paper

Features & opinion

Six steps to cleaner lithium extraction.

The challenge of sustainably mining and processing lithium for batteries and other green technologies “represents a rare opportunity in which the needs of fundamental research and global policy are aligned”, write six researchers. The time is ripe for the industry to upgrade its inefficient, wasteful and damaging methods , which have changed little over the past century. “If nothing changes, simply ramping up lithium production at existing sites could negate the benefits of the clean technologies they power,” they write.

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Source: IEA

Stephen Hawking’s final theory

The Universe is a four-dimensional membrane in a five-dimensional space , and a small part of a much vaster hidden reality. This is Stephen Hawking’s final theory, described in On the Origin of Time by Hawking’s last collaborator, Thomas Hertog. The book is a fascinating tour of cosmology, writes reviewer and science philosopher Robert Crease. In accessible language and with colourful anecdotes, Hartog describes how Hawking flip-flopped on whether the Universe had a beginning. Still, Hertog’s “Hawking worship” and his scorn for philosophy could be a source of irritation for readers, Crease says.

Futures: The forever family

The temptation to create the perfect home clashes with all-too-real love and grief in the latest short story for Nature ’s Futures series.

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Podcast: Octopuses taste by touch

Octopuses’ suckers are covered with receptors that allow them to taste by touching things. “We describe this as being a chemotactile sense, which basically combines chemosensation, sensation of chemicals, with tactile sensation, or touch,” explains cell biologist Corey Allard on the Nature Podcast . Squid have similar receptors, but there are differences that mirror differences in the animals’ hunting behaviours: octopuses feel around for food in areas they can't see into, whereas squid ambush prey and pull it towards themselves before deciding whether to eat it.

Nature Podcast | 27 min listen

Subscribe to the Nature Podcast on Apple Podcasts , Google Podcasts or Spotify .

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“when you go back to the question of what makes us different to a fly or a worm, we've increasingly realised that the answers lie in the dark genome.”.

On the 20-year anniversary of its completion, scientists look back at how the Human Genome Project changed our understanding of the non-protein-coding-genes that were once written off as ‘junk DNA’. ( BBC | 10 min read )

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Fast forward: How we learned to love time travel

A classic H. G. Wells story and key mathematics made time travel such a popular idea that it fundamentally changed our thinking, argues James Gleick in Time Travel: A history

By Jonathon Keats

7 September 2016

The time travel trope can be traced to H. G, Wells’s The Time Machine

Ronald Grant Archive

ON 28 June 2009, at 1200 UTC, Stephen Hawking hosted a party for time travellers at the University of Cambridge. Among the careful preparations, the most important was to invite guests only after the event had taken place. Much to Hawking’s disappointment, nobody showed up.

Travelling through time may never be feasible, but it remains a perennially popular topic in science fiction, philosophy and theoretical physics. In Time Travel , James Gleick provides an absorbing history of the idea, eloquently elucidating the reasons for its enduring appeal.

The concept of time travel is surprisingly recent. “Though the ancients imagined immortality and rebirth and lands of the dead,” Gleick observes, “time machines were beyond their ken.” In fact, he traces the trope back to a single work of fiction: The Time Machine , written by H. G. Wells between 1888 and 1895.

It tells of an unnamed time traveller who rides into the future on an apparatus resembling a bicycle. His voyage is made possible by the fact “there is no difference between Time and any of the three dimensions of Space except that our consciousness moves along it” (as the Traveller helpfully explains to his temporally challenged friends).

His reasoning (and Wells’s story) were inspired by the investigations of 19th-century mathematicians such as Bernhard Riemann. But as Gleick observes, Wells’s vision was equally driven by technology: “Time became vivid, concrete, and spatial to anyone who saw the railroad smashing across distances on a coordinated schedule.”

If the literary roots of time travel are The Time Machine , then scientific interest originated with Einstein’s special theory of relativity, published in 1905. In Gleick’s account, these two foundations were mutually reinforcing. Science gave credibility to the fiction, which made the science more accessible. The combination was so potent, and expanded so quickly, that time travel began to seem like a truly timeless principle.

“Today the time machine is no longer obligatory to the game of altering the past – it has been internalised“

Gleick traces its literary pedigree, sometimes to the point of tedium, from Wells to writers such as Robert Heinlein and Jorge Luis Borges. He also delves into pop culture, ranging from Star Trek to Woody Allen.

Then there’s the science. While Einstein remained sceptical of voyaging through the space-time continuum, his close friend Kurt Gödel mathematically described an alternate universe in which time warped to loop back on itself. Gödel gave the calculation to Einstein for his 70th birthday, often checking later whether his theory had been proven.

It wasn’t, but Gödel’s “closed time-like curves” continued to bedevil physics long after his death, ultimately inspiring a rebuttal by Hawking, who claimed that time loops violated established laws of physics. Hawking organised his Cambridge party as experimental evidence.

Whether or not Hawking has the final say, the concept of time travel has proven phenomenally productive. Within physics, Gleick captures some of the intellectual ferment in his account of the debate about whether time is an illusion. Within literature, he’s particularly incisive in his account of alternative histories, which originated as an accident of time travel. “Travel to the past begins as tourism in the extreme,” he writes. But the sightseers “start tinkering”. Eventually they aim at history’s greatest villains, and murder Hitler, or slay his mother.

Beyond the adrenaline, what makes this compelling is the chance to imagine what might have been. Counterfactual narratives let us examine the past more speculatively, and explore how things can go awry in the present. Is despotism a function of personal charisma or socio-economic conditions? How do we prevent a holocaust? “Nodal points must exist,” says Gleick, “just not… where we think.”

Today the time machine is no longer obligatory. The game of altering the past has been so internalised that Gleick suggests the notion of time travel has fundamentally changed the way we think.

To illustrate, he cites our habit of burying time capsules. Only since the 20th century have we sought ways to communicate with the future. Now we tend to interpret any box of coins found under a cornerstone as an effort by ancestors to send us a message. In fact, those old cornerstone caches were votive offerings, not meant to be discovered.

In contrast to his enthusiasm for SF, Gleick finds a time capsule “a tragicomic time machine”, moving through time at a rate of one second per second. Few capsules survive, he notes, and why should the future care about us in the first place?

But here Gleick neglects the wisdom of his book, forgetting that time travel is experienced in the traveller’s present. Time machines are instruments for exploring the past and future, to augment our current knowledge or enrich our lived experience. Placing items in a time capsule is an opportunity for self-appraisal. Considering how we would like to be perceived by the future is a way of examining what we most cherish.

Time Travel: A history

James Gleick

Random House

This article appeared in print under the headline “Fast forward”

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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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Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical: Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “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.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

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On the Origin of Time: Stephen Hawking's Final Theory

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NEW YORK TIMES BESTSELLER • Stephen Hawking’s closest collaborator offers the intellectual superstar’s final thoughts on the cosmos—a dramatic revision of the theory he put forward in A Brief History of Time . “This superbly written book offers insight into an extraordinary individual, the creative process, and the scope and limits of our current understanding of the cosmos.”—Lord Martin Rees Perhaps the biggest question Stephen Hawking tried to answer in his extraordinary life was how the universe could have created conditions so perfectly hospitable to life. In order to solve this mystery, Hawking studied the big bang origin of the universe, but his early work ran into a crisis when the math predicted many big bangs producing a multiverse—countless different universes, most of which would be far too bizarre to ​harbor life. Holed up in the theoretical physics department at Cambridge, Stephen Hawking and his friend and collaborator Thomas Hertog worked on this problem for twenty years, developing a new theory of the cosmos that could account for the emergence of life. Peering into the extreme quantum physics of cosmic holograms and venturing far back in time to our deepest roots, they were startled to find a deeper level of evolution in which the physical laws themselves transform and simplify until particles, forces, and even time itself fades away. This discovery led them to a revolutionary idea: The laws of physics are not set in stone but are born and co-evolve as the universe they govern takes shape. As Hawking’s final days drew near, the two collaborators published their theory, which proposed a radical new Darwinian perspective on the origins of our universe. On the Origin of Time offers a striking new vision of the universe’s birth that will profoundly transform the way we think about our place in the order of the cosmos and may ultimately prove to be Hawking’s greatest legacy.

About the Author

Thomas Hertog is an internationally renowned cosmologist who was for many years a close collaborator of the late Stephen Hawking. He received his doctorate from the University of Cambridge and is currently professor of theoretical physics at the University of Leuven, where he studies the quantum nature of the big bang. He lives with his wife and their four children in Bousval, Belgium.

Praise for On the Origin of Time: Stephen Hawking's Final Theory

“[A] wonderful book about Stephen Hawking's Hawking’s ‘biggest legacy’.” — Spectator “Truly mind-stretching . . . Immensely immensely rewarding.” —The Times “Why is our universe the way it is? How did everything begin? How might it end? Thomas Hertog probed these overwhelming questions in collaboration with Stephen Hawking, achieving a privileged perspective into how, struggling against daunting physical odds, Hawking’s imprisoned mind yielded astonishing insights even in his later years. This superbly written book offers insight into an extraordinary individual, the creative process generally, and the scope and limits of our current understanding of the cosmos.” —Lord Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of Cambridge, and author of Just Six Numbers “Like his mentor and colleague Stephen Hawking, Thomas Hertog has never shied away from being ambitious in theorizing about the universe. This sweeping book provides an accessible overview of both what we know about cosmology and some audacious ideas for moving into the unknown. It is an introduction to Hawking’s final theory, but also a glimpse into even grander theories yet to come.” —Sean Carroll, author of The Biggest Ideas in the Universe: Space, Time, and Motion “Stephen Hawking’s final theory is lucidly explained in this splendidly accessible book. Author Thomas Hertog, one of Hawking’s closest collaborators, gives us a vivid insight into Hawking as both a brilliant physicist and an astonishingly determined human being.” —Graham Farmelo, Churchill College, University of Cambridge, and author of The Strangest Man “A beautifully written, thought-provoking account of both the physics and the personalities involved in Hawking’s visionary struggle to comprehend the cosmos. Thomas Hertog has provided a fascinating insider’s view.” —Neil Turok, co-author of Endless Universe

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How scientists have reshaped the Big Bang theory

Regina Barber

Scientists have long been interested in the early history of the universe. Famed physicist Stephen Hawking helped popularize that the Big Bang was a singular point in time — but that's not how many cosmologists think of the Big Bang today.

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NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

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Time Travel

• Episode aired Apr 25, 2010

The promise of time travel has long been one of the world's favorite scientific "what-ifs?" Hawking explores all the possibilities, warping the very fabric of time and space as he goes. From... Read all The promise of time travel has long been one of the world's favorite scientific "what-ifs?" Hawking explores all the possibilities, warping the very fabric of time and space as he goes. From killing your grandfather to riding a black hole, we learn the pitfalls and the prospects ... Read all The promise of time travel has long been one of the world's favorite scientific "what-ifs?" Hawking explores all the possibilities, warping the very fabric of time and space as he goes. From killing your grandfather to riding a black hole, we learn the pitfalls and the prospects for a technology that could quite literally, change everything.

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• Benedict Cumberbatch
• Christopher Goh
• Stephen Hawking
• 1 User review

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Did you know

• Goofs Near the end of the episode, when they are traveling on their hypothetical time travel ship, the narrator says they travel farther than possible. After two years they are traveling at 50% the speed of light. After another two years, they pass the nearest neighboring star, Alpha Centauri. Alpha Centauri lies about 4.25 light years distant. It would be impossible to pass it after only 4 years even if they started off at the speed of light.

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• Dec 12, 2023
• April 25, 2010 (United States)
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• Runtime 43 minutes

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