Time Travel

The invisible dangers of travelling through time

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Split image of man shouting with arms raised (Credit: Taufenbach Pourtout)

The mishaps caused by time travellers exploring the past are a staple of science fiction. But what does physics think

One of the most common occurrences in Doctor Who is characters getting into trouble by changing, or trying to change, the past. As with so many time-travel stories, interfering with past events is represented as incredibly dangerous.

In the Season Six finale The Wedding of River Song, the Doctor’s friend and lover River Song changes the course of history when she refuses to shoot the Doctor – even though, this being a story about time travel, the audience has already seen her do it and knows it is inevitable.

Her actions completely break the Universe. Different time periods fuse together, so the characters find themselves in a version of London that has Victorian-era technology, but also flying cars and dinosaurs. Only when River goes through with her fated actions does the normal flow of time resume.

In reality, physicists and philosophers think that such paradoxical situations wouldn’t happen.

“A paradox isn’t really something you can create,” says theoretical physicist Barak Shoshany at Brock University in St Catharines, Canada. Instead, if our theory predicts a paradox, that means our theory is somehow inconsistent. “If your theory creates a paradox, that means your theory cannot be correct.”

It turns out there are many ways to resolve the potential paradoxes that arise from time travel. Some involve parallel universes and eldritch quantum phenomena; others rely on as-yet-undescribed mechanisms for creating logical consistency in the Universe. Which solution is correct depends in part on exactly what time is and how it works: something about which physicists are far from certain. But one way or another, the physics of time will see to it that we can’t create paradoxical situations.

The most obvious solution is that time travel to the past just isn’t possible. If we can’t travel into the past, we can’t mess it up.We are free to move up and down, forwards and backwards in space - but we can't do the same thing with time (Credit: Getty Images)

We are free to move up and down, forwards and backwards in space – but we can’t do the same thing with time

This seems natural when we consider our experience of time: it only goes in one direction. Monday leads to Tuesday leads to Wednesday, and so on throughout our lives. We never find ourselves doubling back from Tuesday to Monday. (Read more from BBC Future about why time travels forwards, not backwards.) 

However, it’s not obvious why time should operate like this. Space doesn’t: we are free to move up and down, forwards and backwards, left and right. Some directions of travel are difficult, because we have to overcome Earth’s gravity or dig through solid rock, but they are not impossible. Yet our movement through time is all one way.

There are two kinds of explanation for this “arrow of time”. The first is that it is a fundamental property of the Universe, something baked into the very nature of things. This has intuitive appeal, but it has been strongly challenged by studies of quantum mechanics: that is, the physics of the very tiny.

On quantum scales, processes can be driven in either direction, says quantum physicist Vlatko Vedral at the University of Oxford in the UK. “If you can excite an atom from one state to another, then quantum mechanics says you should be able to, with 100% efficiency, go backwards.” Experiments suggest this is correct. “At least in principle, everything ought to be reversible.”

If you drop a coffee cup onto a hard floor it will shatter, but you never see broken coffee cups spontaneously re-assembling themselves

Hence many physicists take a second approach to the arrow of time: they argue that it is not fundamental, but instead emerges from other phenomena.

“There are calculations that you can do, where you don’t include the ideas of space or time in the calculations, and it gives you the right answer anyway,” says cosmologist Katie Mack at the Perimeter Institute for Theoretical Physics in Waterloo, Ontario. “Maybe space and time are emergent properties. Maybe they’re even somehow illusory, and there’s some mathematical framework for the Universe that’s more fundamental than space and time.”

One such explanation can be traced back to the Austrian physicist Ludwig Boltzmann. He was interested in why certain processes, in the world of big objects that we live in, are irreversible. For example, if you drop a coffee cup onto a hard floor it will shatter, but you never see broken coffee cups spontaneously re-assembling themselves.

Things only become tidy if energy is used to tidy them. “It takes more energy to untangle your headphones,” says astrophysicist Emma Osborne at the University of York in the UK. “Whereas there’s infinite ways they can be tangled.”Drop a coffee cup on the floor and it will likely beak; but it's almost impossible that nature will reassemble it unaided (Credit: Getty Images)

Drop a coffee cup on the floor and it will likely beak; but it’s almost impossible that nature will reassemble it unaided

Boltzmann explained this by saying that regenerating coffee cups or detangling headphone cables are not fundamentally impossible, but simply incredibly unlikely – so unlikely that you would never witness such a thing even if you waited for trillions of years. They’re unlikely because there are a huge number of ways the pieces of the broken coffee cup could be shuffled around, say by air currents, and hardly any of them would cause it to re-assemble. Most of them will just shuffle the mess around.

Decades later, this is still one of the best explanations for the arrow of time, says Vedral. It’s simply too difficult in practice to truly reverse most changes – and that means we consistently experience time going in one direction.

That’s probably true but there may also be a deeper explanation for the arrow of time, argues philosopher Emily Adlam at Chapman University in Orange, California.

Time: The Ultimate Guide

To mark the 60th anniversary of Doctor Who, we’re exploring the big questions about time, including the science of time travel, how clocks have shaped humanity, and even the mind-bending temporal consequences of flying into a black hole. Read and watch more from Time: The Ultimate Guide.

It’s necessary for the Universe to be internally consistent, Adlam says. “Consistency is the need to avoid logical contradictions.” At the most abstract level, we can think of the Universe as containing lots of different processes that can interact with each other. “What possible ways of connecting processes together will lead to logical contradictions?” she says.

“The answer is, basically, connecting processes around in loops will very reliably end up creating logical contradictions,” says Adlam. Going back in time and interfering with history would be an example of such a loop. The malign time traveller who tries to prevent Rosa Parks from carrying out her protest in the episode Rosa would, on this view, be violating some of the deepest laws of the Universe.

“If you want to avoid logical contradictions, what you have to require is that things don’t get connected in loops,” says Adlam. “They just get connected in linear ways and they never loop back around on themselves.”

This requirement to avoid causal loops sets limits on how time can work, says Adlam. “You get something like an arrow of time, purely as a result of this constraint that processes can’t loop back on themselves.”

In short, there may be two requirements that force time to flow in only one direction: the need to be logically consistent, as emphasised by Adlam, and the tendency for random processes to create more disorder rather than more order, as Boltzmann described.

With that in mind, let’s look at the two kinds of time travel paradoxes, both of which crop up in Doctor Who, and see how they might be resolved in practice.Going back in time to stop Rosa Park's landmark protest would contravene the most fundamental laws of the Universe (Credit: Getty Images)

Going back in time to stop Rosa Park’s landmark protest would contravene the most fundamental laws of the Universe

First, the bootstrap paradox. This occurs “when something is created out of nothing or something is causing itself”, says Shoshany. Suppose, he suggests, that a time machine appears in your room right now. An older version of you steps out, announces they are from 10 years in the future, and gives you the plans for the time machine. You spend the next decade building the time machine, then use it to go back to today to give yourself the plans. The question is, says Shoshany: “Who made the plans for the time machine?”

The Twelfth Doctor spells out a similar example involving the music of Beethoven in the opening of Season Nine’s Beyond the Flood.

The bootstrap paradox is a curious idea because it doesn’t create any inconsistency: the whole story ties up neatly. It’s just that somehow new information – a blueprint for a time machine – has appeared from nowhere.

Fortunately, there is a relatively simple escape from the bootstrap paradox. “Maybe this is just something I made up,” says Shoshany. “There’s no reason it would actually happen.

If you really could travel to the past, it would be virtually impossible to avoid changing anything

“The more worrisome paradox is the consistency paradox,” continues Shoshany. “Which is more colloquially known as the ‘grandfather paradox’.” This is the familiar problem where you go back in time and change a key event in your own personal history. For example, you kill your grandfather while he is still a child. This makes it impossible for you to be born, so there is nobody to go back in time and kill your grandfather – creating a paradox.

WATCH: Doctor Who explores the ‘grandfather paradox’

In Season One’s Father’s Day, the Doctor’s companion Rose Tyler creates an identical paradox when she goes back in time and saves her father from dying when she is little – transforming her life history and unleashing dangerous forces.

Unlike the bootstrap paradox, the grandfather paradox seems inescapable. If you really could travel to the past, it would be virtually impossible to avoid changing anything. Even if you have the common sense not to murder your own ancestors, it wouldn’t matter, because small changes can ripple out into the world and ultimately cause major disruptions.

How can we avoid it?

The simplest kind of explanation is that time travel to the past just isn’t possible. This “chronology protection conjecture” was set out by the late Stephen Hawking in a 1992 paper. Shoshany sums it up: “We don’t need to worry about paradoxes because time travel is impossible and that’s it.”

The problem here is that, while time travel to the past looks extremely unlikely and difficult, our incomplete understanding of physics means we can’t completely rule it out.

An alternative interpretation is that time travel to the past is possible, but you can’t change anything. This is called the Novikov self-consistency principle, after Russian physicist Igor Dmitriyevich Novikov who discussed the idea in a series of publications culminating in a study published in 1990.One theory states that by going back into the past and changing history, you create an alternate timeline - essentially an entirely different universe (Credit: Getty Images)

One theory states that by going back into the past and changing history, you create an alternate timeline – essentially an entirely different universe (Credit: Getty Images)

“Try to prevent your grandparents from meeting, you end up actually being the one who made them meet in the first place,” says Shoshany.

An example of this takes place in The Waters of Mars. The Tenth Doctor saves the life of astronaut Adelaide Brooke. This potentially rewrites human history, because her death is a pivotal event. However, Adelaide is appalled by the Doctor’s hubris and kills herself, restoring the timeline.

It’s not clear how this would work in practice. How would the Universe “know” that someone is interfering with history?

“This conjecture to me always seemed kind of weird, kind of unjustified,” says Shoshany. He has described scenarios in which objects move in and out of time machines, and which cannot be described in a mathematically consistent way – violating the conjecture. “This conjecture just cannot be correct.”

WATCH: Could your dreams predict the future?

Shoshany favours an alternative scenario. If time travel to the past is possible, he argues, it really should be possible to change things – but doing so will create an alternate timeline. Instead of creating a Universe-breaking paradox, you would create a second universe with a different history.

Parallel universes are a staple of Doctor Who, from the Third Doctor adventure Inferno to the alternative timeline explored by the Tenth Doctor and Rose Tyler in Rise of the Cybermen and Doomsday, and the sentient universe of It Takes You Away.

In reality, parallel universes are a much-debated concept. Although some theories of physics can be interpreted as implying the existence of them, there is as yet no physical evidence they exist.The theories about time travel have one main issue - there is no way to conduct experiments to see if any of them stack up (Credit: Getty Images)

The theories about time travel have one main issue – there is no way to conduct experiments to see if any of them stack up

Quantum mechanics offers one way that parallel universes could exist. A tiny subatomic particle can exist in a state called superposition, which means it is not in one specific place. Instead it exists in a range of spaces, each of which has some probability of being “correct”. Only when an experimenter observes the particle does its state become specified: it “collapses” from a range of possible positions into one.

Quantum physicists disagree over how to interpret the collapse. One interpretation, put forward by Hugh Everett in the late 1950s, is that the particle doesn’t really “choose” one single position. Instead, every possible option plays out in a different universe. Wherever we observe the particle is where it is in our Universe, but in other universes it is somewhere else. This became known as the “many worlds interpretation” of quantum mechanics.

There is no physical evidence that these other universes exist, and it is difficult to imagine what would constitute evidence. However, other interpretations of quantum mechanics have problems of their own, so the many worlds interpretation remains of interest.

If it is true, it offers a way to time travel into the past, and change things, without causing paradoxes. One of the first physicists to propose this was David Deutsch in a 1991 study. He argued that if you travel into the past and change something, you will create a second universe, parallel to the original one. In this second timeline, the altered version of history plays out – while in the original things go on as they always did.

On this view, you might be able to travel into the past, but “you can’t go back on your own worldline,” says Osborne.

Shoshany and his colleagues have developed this idea further. In a 2020 study, they showed that this version of time travel need not lead to infinite numbers of timelines. Infinities are generally a sign that a theory is wrong, says Shoshany, so this means it is more plausible. In February this year, they published a more realistic version of this model, which again avoided paradoxes and infinities. The following month, they explored the scenario in more detail, showing that a branch in the timeline would start out localised and gradually expand to affect more of the Universe.

It turns out that there are many possible scenarios that could take place if we were somehow to travel into the past. Right now, there is no way of conducting an experiment to determine which, if any, of these scenarios of time travel is correct. Finding the correct answer will require understanding how and why the arrow of time really exists. Until we have greater clarity on the laws underpinning the Universe, we can’t choose between our ideas about time travel.

Researchers successfully sent a simulated elementary particle back in time.

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6 Most Peculiar Time Travel Paradoxes of All Time.

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https://interstellaruniverse.in/6-most-peculiar-time-travel-paradoxes-of-all-time/

What Did Stephen Hawking Do? The Physicist’s 5 Biggest Achievements

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On Wednesday, world-renowned astrophysicist Stephen Hawking died at age 76 in his home in Cambridge, England. He lived for 55 years with the neurological disease amyotrophic lateral sclerosis (ALS), and as a result, he spent most of his life using a wheelchair, which, for the last decade, also included hands-free communication capability that gave him the computerized voice with which so many people now associate him.

As a working physicist and prolific public figure, Hawking helped revolutionize the field of astrophysics. His scholarship helped elucidate our modern understanding of the universe and its origins, and he was quick to share his views on humanity and society. While his achievements are many, there are five in particular worth noting.

Space

5. Stephen Hawking Theorized How Black Holes Emit Information

Black holes are notoriously hungry phenomena, distorting spacetime and sucking in any matter that passes within their event horizon. But Hawking theorized that black holes actually radiate energy as a result of quantum effects near the event horizon. We could only observe this theoretical energy, which is referred to as “Hawking radiation,” in smaller black holes that are about the same mass as our sun. In larger black holes, it would be overwhelmed by the gas falling into the black hole. Hawkin’s hypothesized phenomenon hasn’t been directly observed, but as Inverse previously reported, physicists are working on it.

'Big Bang Theory' loved its nerdy celeb cameos, and Stephen Hawking's was an absolute treasure.

4. Stephen Hawking Proposed That the Singularity Was an Essential Element of the Big Bang Theory

The Big Bang Theory — the physics one, not the television one — proposes the universe began with a powerful expansion that started with one point, the singularity. Before Hawking’s time, physicists tried to reconcile the apparent paradox of the singularity. The idea of a single point of infinite density simply didn’t mesh with the conventional views of physics in the middle of the 20th century. In 1970, though, Hawking co-authored a paper with Roger Penrose that began to reconcile this notion.

This paper, titled “The singularities of gravitational collapse and cosmology,” countered the widely discussed notion that the Big Bang was preceded by the universe contracting. Physicists generally accept this version of the Big Bang Theory, in which there was nothing before the beginning of the universe.

Ripples in spacetime.

3. Stephen Hawking Proposed There Was No Meaningful Distinction Between Space and Time in the Early Universe

In his 1988 best-selling book, A Brief History of Time, Hawking proposed that at the very beginning of the universe, space existed, but time as we know it did not yet exist. Astrophysicists continue to describe space and time as being intrinsically tied to one another, but Hawking hypothesized that at the very beginning of everything there was no meaningful distinction. The curious public digested this hypothesis in Hawking’s book, but physicists continue to debate his idea.

Stephen Hawking, Big Bang

2. Stephen Hawking Provided Evidence That Time Travel Is Impossible

Back in 2009, Hawking hosted a time traveler party, inviting time travelers to join him for a reception to celebrate their achievements. Here’s the catch, though: He didn’t send out the invitations until the next day. The idea was that anyone who actually showed up would clearly be legit since nobody knew about the party before it happened. On Hawking’s 75th birthday in 2017, he announced that nobody had shown up to his party. While this isn’t definitive proof that time travel doesn’t exist, it’s pretty strong evidence. After all, if you discovered how to travel through time, wouldn’t Hawking’s time travel party be one of your first destinations?

Stephen Hawking on 'The Simpsons'

1. Stephen Hawking Played Himself Four Times on The Simpsons

Sure, revolutionizing astrophysics is great, but what about having your cartoon avatar immortalized for posterity? In addition to playing himself on Star Trek, Hawking appeared on The Simpsons four times between 1999 and 2010. Sure, this achievement wasn’t scientific, strictly speaking, but it does embody the character and public image of one of the best-known scientists in modern history. As a physicist, Hawking didn’t create much original work in his later years. But as a science popularizer, he continued to inspire people to learn about the world around them. And as far as monumental achievements go, that one’s hard to overstate.

Scientists find Time Travel is mathematically POSSIBLE

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Mathematically, time travel is possible. Scientists have created a new mathematical model that dictates how time travel is theoretically possible. Experts used Einstein’s Theory of General relativity as basis for a hypothetical device which they named a Traversable Acausal Retrograde Domain in Space-time (TARDIS).

In other words, they’ve come up with a mathematical model of a theoretical time machine box that has the ability to move back and forth through space and time.

For centuries have humans imagined traveling in time. This idea resulted in countless movies, series and books produced and science fiction seems to have figured out everything there is about Time Travel.  But now scientists decided to see whether they could learn something more about time travel and whether this is just an idea possible in science fiction.

 “People think of time travel as something as fiction. And we tend to think it’s not possible because we don’t actually do it,” said Ben Tippett, a physicist and mathematician from the University of British Columbia, said in a UBC news release, adding “But, mathematically, it is possible.”

What Tippett and his colleague from University of Maryland astrophysicist David Tsang created was a mathematical formula based on Einstein’s General Relativity theory to show how Time Travel is in fact possible, at least in theory.

 According to the abstract of the scientific paper, which was published in the journal Classical and Quantum Gravity, “we present geometry which has been designed to fit a layperson’s description of a ‘time machine.’ It is a theoretical box which allows those within it to travel back and forth through time and space, as seen by an external observer.”

Graciously, they’ve named it TARDIS—which stands for Traversable Acausal Retrograde Domain in Space-time.

Tippet further explained how: “My model of a time machine uses the curved space-time to bend time into a circle for the passengers, not in a straight line. That circle takes us back in time.”

In other words, their newly formulated model ‘assumes’ how time could curve around high-mass objects just as physical space does in the universe.

Tippet and Tsang refer to their TARDIS as a space-time geometry “bubble” that has the ability to move fast than the speed of light. They explain in their paper how: “It is a box which travels ‘forwards’ and then ‘backwards’ in time along a circular path through space-time.”

“Delighted external observers would be able to watch the time travelers within the box evolving backward in time: un-breaking eggs and separating cream from their coffee,” explain scientists in their paper.

But don’t get all excited, it’s still not possible to build—at least not yet.

“While is it mathematically possible, it is not yet possible to construct a space-time machine because we need materials—which we call exotic matter—to bend space-time in these impossible ways, but they have yet to be discovered,” Tippet explained.

Science world: AUSTRALIAN PHYSICISTS PROVED TAHT TIME TRAVEL IS POSSIBLE

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Scientists from the University of Queensland have used photons (single particles of light) to simulate quantum particles travelling through time. The research is cutting edge and the results could be dramatic!

Their research, entitled “Experimental simulation of closed timelike curves “, is published in the latest issue of Nature Communications. The grandfather paradox states that if a time traveler were to go back in time, he could accidentally prevent his grandparents from meeting, and thus prevent his own birth.

However, if he had never been born, he could never have traveled back in time, in the first place. The paradoxes are largely caused by Einstein’s theory of relativity, and the solution to it, the Gödel metric.

How relativity works

Einstein’s theory of relativity is made up of two parts – general relativity and special relativity. Special relativity posits that space and time are aspects of the same thing, known as the space-time continuum, and that time can slow down or speed up, depending on how fast you are moving, relative to something else.

Gravity can also bend time, and Einstein’s theory of general relativity suggests that it would be possible to travel backwards in time by following a space-time path, i.e. a closed timeline curve that returns to the starting point in space, but arrives at an earlier time.

It was predicted in 1991 that quantum mechanics could avoid some of the paradoxes caused by Einstein’s theory of relativity, as quantum particles behave almost outside the realm of physics.

“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics,” said Martin Ringbauer, a PhD student at UQ’s School of Mathematics and Physics and a lead author of the paper.

“Einstein’s theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules.”

Simulating time travel

The scientists simulated the behavior of two photons interacting with each other in two different cases. In the first case, one photon passed through a wormhole and then interacted with its older self. In the second case, when a photon travels through normal space-time and interacts with another photon trapped inside a closed timeline curve forever.

“The properties of quantum particles are ‘fuzzy’ or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” said co-author Professor Timothy Ralph. “Our study provides insights into where and how nature might behave differently from what our theories predict.”

Although it has been possible to simulate time travel with tiny quantum particles, the same might not be possible for larger particles or atoms, which are groups of particles.

New Research Shows That Time Travel Is Mathematically Possible

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IN BRIEF

Physicists have developed a new mathematical model that shows how time travel is theoretically possible. They used Einstein’s Theory of General Relativity as a springboard for their hypothetical device, which they call a Traversable Acausal Retrograde Domain in Space-time (TARDIS).

BENDING TIME

Even before Einstein theorized that time is relative and flexible, humanity had already been imagining the possibility of time travel. In fact, science fiction is filled with time travelers. Some use metahuman abilities to do so, but most rely on a device generally known as a time machine. Now, two physicists think that it’s time to bring the time machine into the real world — sort of.

The Future According to H. G. Wells [INFOGRAPHIC]

“People think of time travel as something as fiction. And we tend to think it’s not possible because we don’t actually do it,” Ben Tippett, a theoretical physicist and mathematician from the University of British Columbia, said in a UBC news release. “But, mathematically, it is possible.”

Essentially, what Tippet and University of Maryland astrophysicist David Tsang developed is a mathematical formula that uses Einstein’s General Relativity theory to prove that time travel is possible, in theory. That is, time travel fitting a layperson’s understanding of the concept as moving “backwards and forwards through time and space, as interpreted by an external observer,” according to the abstract of their paper, which is published in the journal Classical and Quantum Gravity.

Oh, and they’re calling it a TARDIS — yes, “Doctor Who” fans, hurray! — which stands for a Traversable Acausal Retrograde Domain in Space-time.

FEASIBLE BUT NOT POSSIBLE. YET.

“My model of a time machine uses the curved space-time to bend time into a circle for the passengers, not in a straight line,” Tippet explained. “That circle takes us back in time.” Simply put, their model assumes that time could curve around high-mass objects in the same way that physical space does in the universe.

For Tippet and Tsang, a TARDIS is a space-time geometry “bubble” that travels faster than the speed of light. “It is a box which travels ‘forwards’ and then ‘backwards’ in time along a circular path through spacetime,” they wrote in their paper.

Unfortunately, it’s still not possible to construct such a time machine. “While is it mathematically feasible, it is not yet possible to build a space-time machine because we need materials — which we call exotic matter — to bend space-time in these impossible ways, but they have yet to be discovered,” Tippet explained.

Image credit: Tippet and Yang

Indeed, their work isn’t the first to suggest that time traveling can be done. Various other experiments, including those that rely on photon stimulation, suggest that time travel is feasible. Another theory explores the potential particles of time.

However, some think that a time machine wouldn’t be feasible because time traveling itself isn’t possible. One points to the intimate connection between time and energy as the reason time traveling is improbable. Another suggests that time travel isn’t going to work because there’s no future to travel to yet.

Whatever the case may be, there’s one thing that these researchers all agree on. As Tippet put it, “Studying space-time is both fascinating and problematic.”

Australian Scientists Prove Time Travel Is Possible.

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There are some physicists who believe that time travel can be done.

At the University of Queensland, Australia a team of scientists have examined how time-traveling photons react; proving that, at the quantum level, the grandfather paradox-which makes time travel impossible-could be fixed. In the study, photons (single particles of light) to play out quantum particles traveling back from time. Through behavioral study, scientists have unveiled much stranger aspects of modern physics. 

“The properties of quantum particles are ‘uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations. Our study provides insights into where and how nature might behave differently from what our theories predict,” says co-author Professor Timothy Ralph.

The Daily Mail further states: In the simulation, the researchers examined the behavior of a photon traveling through time and interacting with its older self. In their experiment they made use of the closely related, fictitious, case where the photon travels through normal space-time and interacts with another photon that is stuck in a time-travelling loop through a wormhole, known as a closed timelike curve (CTC).

Simulating the behavior of this second photon, they were able to study the behavior of the first – and the results show that consistent evolutions can be achieved when preparing the second photon in just the right way.

Physicists believe that due to Albert Einstein’s theories of general and special relativity, time travel is indeed a possibility. Special Relativity means that time and space are the same aspect, known as the space-time continuum, and time can either speed up or slow down, depending on your speed, relative to something other.

General Relativity states that it would be entirely possible to go back in time through a space-time path. In 2012, physicists Serge Haroche and David Wineland shared the Nobel Prize in Physics for the demonstration of “quantum weirdness” and how it can’t exist at the subatomic micro-world level, and how it can appear itself in the macro-world.

“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics. Einstein’s theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules,” says Martin Ringbauer, a PhD student at University of Queensland’s School of Mathematics and Physics, and a lead author of the paper.

In a documentary from the BBC, astrophysicist Stephen Hawking finds that it isn’t possible to go back into time. There isn’t a lot to look forward to at all.

Now, there are some developments in quantum theories that could deliver understanding of how to take on time travel paradoxes.

 

Let’s Meet Yesterday

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Let’s Meet Yesterday 

An engaging but limited examination of one of the most mysterious and elusive concepts in physics—time travel.

“WHAT IS TIME?” asks James Gleick, in this history of time travel. “We know that it is imperceptible. It is immaterial. We cannot see it, hear it or touch it. Time is what clocks measure. But what is a clock? An instrument for the measurement of time. The snake swallows its tail again!” This is the kind of circular logic that the author tries to break, in this engaging foray into one of the most mysterious concept that has intrigued man since the 19th century.

Scholarly journal papers that announce new breakthroughs invariably begin with a review of past literature. Once in while, the literature review becomes bigger than any new concept that is being announced and in extreme cases, we end up with what is known as a review paper that merely surveys the subject without offering anything new. So is the case with this book. Rather than offering any new insight or even a clear exposition of any specific point of view, the author leads us through a grand tour of the various perspectives that scientists, philosophers and literary personas have explored in their efforts to put a structure around this most mysterious and elusive idea of time.

Given the breadth of subjects addressed, the depth is limited, but at least it creates a map of the terrain that the reader can explore on his own. This is the true value of the book under review.

Time as a matter of discussion entered the public domain with the HG Wells classic The Time Machine, that set the tone for a whole genre. A science fiction story set in a different era—complete with gadgets and behaviours that are dramatically different from what the author and his readers are accustomed to—is one way of travelling into the future or the past. But the real flavour of time travel is revealed when the protagonists move forward and backwards in time, into other eras or epochs. Such travel creates contradictions, like a man meeting his self in the past or the future or murdering his father and negating his existence, which forms the backbone of many interesting novels that are discussed in this book.

Authors writing about time travel usually drift into philosophical discourses on the nature of time. Is it like a river? And if it is, is the observer standing on the bank or a boat floating along with the river? Is it just the flexibility of the English language that allows us to save time, to spend time or even to waste time or do these verbs connect with certain real properties of time? These are questions that appear again and again, but answers remain elusive to the original authors, the current author and certainly to the reader. In fact, the author admits: “I doubt any phenomenon…has inspired more perplexing, convoluted and ultimately futile philosophical analysis that time travel has.”

The book becomes more interesting when it eventually moves into science. The publication of The Time Machine by Wells was nearly simultaneous with some far-reaching scientific study of time as a physical dimension that eventually culminated in Einstein’s relativity. This, paradoxically, demolished the concept of simultaneity, which forms the basis of all mechanisms to measure time. All laws of physics, except the second law of thermodynamics, are indifferent to the direction of time and in principle, should allow people to move back and forth in time as they do as in left and right, or up and down. But of course, the ability to do so comes with the paradox of going back to the past and changing the course of history and hence it is ruled out, not by science but by logic. However, Godel, the man who had upended the apple cart of mathematics with his Theorems of Incompleteness, has shown that such situations are not logically impossible and there could be physical worlds where there is no logical bar on time travel. This, along with the loss of simultaneity, leads to the concept of retro-causation where effect “precedes” cause and makes us wonder whether our language can support a discussion of such constructs.

Unfortunately, most of these deeper concepts are glossed over as the author James Gleick regales us with descriptions of time travel that appear in various literary works. These I am sure are worth reading, not to understand the concept of time, but for the sheer pleasure of reading well-written novels.

Source:swarajyamag.com

Simulation Shows Time Travel Is Possible

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Australian scientists created a computer simulation in which quantum particles can move back in time. This might confirm the possibility of time travel on a quantum level, suggested in 1991. At the same time, the study revealed a number of effects which are considered impossible according to the standard quantum mechanics.

Using photons, physicists from the University of Queensland in Australia simulated time-traveling quantum particles. In particular, they studied the behavior of a single photon traveling back in time through a wormhole in space-time and interacting with itself. This time-traveling loop is called a closed timelike curve, i.e. a path followed by a particle which returns to its initial space-time point.

simulation-shows-time-travel-is-possible

The physicists studied two possible scenarios for a time-traveling photon. In the first, the particle passes through a wormhole, moving back in time, and interacts with its older self. In the second scenario, the photon passes through normal space-time and interacts with another photon which is stuck in a closed timelike curve.

According to the researchers, their study will help to find a link between two great theories in physics:the Einstein’s general theory of relativity and quantum mechanics.

“The question of time travel features at the interface between two of our most successful yet incompatible physical theories – Einstein’s general relativity and quantum mechanics,” said Martin Ringbauer of the University of Queensland who led the study. “Einstein’s theory describes the world at the very large scale of stars and galaxies, while quantum mechanics is an excellent description of the world at the very small scale of atoms and molecules.”

Einstein’s General Relativity suggests the possibility of moving back in time if the time-traveling object is stuck in a closed timelike curve. Yet, this possibility is known to cause a number of paradoxes, such as the famous “grandfather paradox”, in which a time traveler prevents his own existence by preventing his grandparents from meeting each other.

In 1991, the concept of time travel in the quantum world was suggested. It was said that traveling through time on a quantum level can prevent such paradoxes, since the properties of quantum particles are not precisely defined, in accordance with Heisenberg’s uncertainty principle.

“The properties of quantum particles are “fuzzy” or uncertain to start with, so this gives them enough wiggle room to avoid inconsistent time travel situations,” said professor Timothy Ralph who participated in the study.

Thus, the experiment conducted by the Australian scientists shows that such kind of time travel might be possible.

At the same time, some new bizarre effects were discovered, which are forbidden by standard quantum mechanics. For instance, it appears that it is possible to accurately distinguish various states of a quantum system, despite the fact that it violates Heisenberg’s uncertainty principle.