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Time Travel - Is it possible?                                                        The Week

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Time travel: four ways in which it could be possible

Some physicists are convinced that time travel is possible. Here's how we might move through the fourth dimension

Time travel has long been a science fiction staple – and an affore t' common sense. But physicists have not yet been able t' prove or disprove that humans may one day be able t' manipulate t' fourth dimension.

"Time travel was once considered scientific heresy," writes Professor Stephen Hawking in the Daily Mail. "I used to avoid talking about it for fear of being labelled a crank." However, these days he's far less cautious, admitting he believes human will one day figure out how to travel into the future.

How could time travel be possible?

"The question of time travel features at the interface between two of our most successful yet incompatible physical theories," explains physicist Martin Ringbauer. "Einstein's general relativity and quantum mechanics." Using both of these theories, scientists have suggested several ways they believe time travel could be possible – at least theoretically. These include:


Einstein suggested t' theoretical existence o' "bridges" through time and space – often referred t' as wormholes. His theory has been further developed by numerous physicists includin' Stephen Hawkin' and Kip Thorne.

"The basic idea if you're very, very optimistic is that if you fiddle with the wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time," Professor Brian Greene, a prominent string-theory physicist, told Live Science.  

: No wormhole has ever been discovered, and even if it was, it would be far too small for scientists to manipulate for the purposes of time travel – measuring just a billion-trillion-trillionth of a centimetre across. They also pose a significant risk, bringing with them the threat of sudden collapse, high levels of radiation and contact with dangerous exotic matter, warns.

Wormholes are unstable because of the feedback created by this radiation, explains Hawking. In the same way that excessive feedback between a microphone and a speaker will fry the equipment, a wormhole is damaged by the radiation feedback it generates. "As soon as the wormhole expands, natural radiation will enter it, and end up in a loop," he explains. "So although tiny wormholes do exist, and it may be possible to inflate one someday, it won't last long enough to be of any use as a time machine."

Cosmic strings

Described as one-dimensional "cracks in the universe" and some of the strangest structures observed by cosmologists, cosmic strings could help us navigate through time. "Cosmic strings are either infinite or they're in loops, with no ends", explains J Richard Gott, an astrophysicist at Princeton University. "So they are either like spaghetti or Spaghetti Os."

They are thought to have formed billions of years ago, moments after the Big Bang, and because they contain such large amounts of mass, some scientists believe they could potentially "warp" space-time around them. "The approach of two such strings parallel to each other, will bend space-time so vigorously and in such a particular configuration that [it] might make time travel possible – in theory," according to Live Science.  

Problem: Again, cosmic strings only exist only in theory. "This is a project a super civilisation might attempt," says Gott. "It's far beyond what we can do. We're a civilisation that's not even controlling the energy resources of our planet."

Supermassive black hole

Described by Professor Hawking as natural time machines, black holes are so dense that they have a dramatic impact on time, slowing it down more than anything else in the universe. If a spaceship were to orbit a black hole, those on board would only experience eight minutes of time for every 16-minute orbit.

"Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be travelling through time," he explains. "Imagine they circled the black hole for five years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more that they had."

Black holes are more practical than wormholes because they don't present the same paradoxes and won't be destroyed by feedback. "But it's pretty dangerous," concedes Hawking. "It's a long way away and it doesn't even take us very far into the future.

Travelling at the speed of light

Another possibility would be travelling at the speed of light, a constant, finite speed of 186,000 miles per second. "If you go fast, your clock runs slow relative to people who are still," explains physicist Professor Brian Cox in the Daily Mail. "As you approach the speed of light, your clock runs so slow you could come back 10,000 years in the future."

Problem: According to the laws of physics, nothing can travel as fast as the speed of light – let alone a spaceship. Even the Large Hadron Collider, the strongest particle accelerator in the word, can't make protons move that fast. "If a proton did achieve that speed, it would need infinite energy to go any faster, and we don't have an infinite supply of energy," explains the BBC's Jennifer Ouellette. Also, the human body would not be able to withstand time travel at all, as travelling at nearly the speed of light would kill you.


Why time travel may be impossible by any means

Apart from physical problems, several paradoxes stand in the way of time travel. These include the "grandparent paradox", which has long flumoxed physicists and philosophers

As Science Alert explains, a time traveller could in theory prevent his or her grandparents from meeting, "thus preventing the time traveller's birth". This would make it impossible for the time traveller to have set out in the first place and kept the grandparents apart.

However, cosmologists believe they have figured a way around this by suggesting that there is more than one universe in existence – the 'multiverse' model. This allows for every possible version of an event to take place. The Independent's science editor Steve Connor gives this example: "a woman who goes back in time to murder her own granny can get away with it, because in the universe next door the granny lives to have the daughter who becomes the murderer's mother." 

This, and other paradoxes, are situations that "give cosmologists nightmares," writes Hawking. But, "even if it turns out that time travel is impossible, it is important that we understand why it is impossible," he says.

Possibility of time travel

The Theory of Relativity of Albert Einstein offers several options for time travel:


Traveling into the future

If you travel with nearly the speed of light in a spaceship (under circumstances  speeds by 10% the speed of light should be enough) from earth and return after a certain period of time, on earth, a longer period of time will have elapsed as on board the spacecraft. The reason for this is the time dilation that occurs after the theory of relativity by Albert Einstein at such high speeds. More on that under twin paradox.


At sufficiently high cruising speed and acceleration any distant future on earth would thereby be reachable in any (short) time. However considering a reasonable acceleration acceptable for the human body, a time shift of years requires a travel time from the perspective of the spaceship crew of over a year (about 347 days each for acceleration and deceleration of 9.81 m / s²).


According to the general theory of relativity, the passage of time is also dependent on the gravitational acceleration and conditions to which a system is subjected. Meaning the time on a high mountain passes slightly faster than at sea level. This phenomenon could be described as time travel into the future, but not only faster, a travel at slower speed is also possible.


On a neutron star, the gravitational time dilation can be substantial. A hypothetical resident of a neutron star could do a time-consuming task in an orbit around the star in order to keep an appointment on the surface.


Travel into the past

In the current state of science time travel into the past is principally not possible. Existing theories, according to which such travel is possible, are speculative and controversial. Undeniably though, in any event, is that the practical implementation of such theories is impossible in the foreseeable future.


1949 Kurt Gödel discovered that a solution of general relativity in which the universe is rotating, that its possible for an object to return to its own past. Such a universe is known as a Gödel universe (R-universe).


Even though it can be proved that our universe is not rotating,  the R-universe shows that Einstein's field equations admit a universe with closed time like curves. Consequently, this means that it’s not necessary to have a uniform passage of time.


According to the general theory of relativity, it is conceivable that two different regions of space-time could be interconnected via so-called wormholes. If the two outputs of such wormhole would connect two regions of different time, time travel into the past would be possible. However, calculations show that wormholes are not normally stable and collapse so quickly that a passage is not possible. However, with negative energy density, the so-called exotic matter, one could stabilize a wormhole. Nevertheless the required amount of exotic matter is, according to current knowledge, not available in the entire known universe.


Possibly a journey into his past would be possible on a specific trajectory in the environment of a sufficiently rapidly rotating black hole. However, it is believed that there are no such a rapidly rotating black holes.


A time travel into the past would be possible with the vicinity of two cosmic strings that pass sufficiently fast to each other. However the existence of such strings is controversial.


After a very controversial interpretation of a time reversal was achieved by Superluminare tunnels for particles or photons, that is, from the experimental arrangement – in the "tunnel" the particles seemed to come out before they were blasted into it. However, in these experiments, a very controversial definition of signal timing was used.


Should it be possible to travel into the past, questions would arise as how the paradoxes are to be avoided which may arise in this context from the violation of causality, such as the Grandfather paradox. A possible answer would be Everettsche Much worlds theory. In that theory, the past, in which one travels, is located in a parallel world. The original course of events and by intervening in the past modified sequence would both happen. In particular, it would be impossible for the traveler to return to his original version of the present, but into a parallel world that would be almost identical to his.


Occasionally, if traveling back in time is mentioned, hypothetical “faster than light” particles called tachyons, are brought into play. When a particle with superluminal velocity moves from A to B, you will always find an observer who sees the movements from B to A. Since the observers evaluate the temporal order of the events of A and B different, the tachyon moves from the past to the future for all participating.

Time travel 'may be possible', says new research

Physicists studying the behaviour of single particles of light say they can now discount one of the main theoretical objections to time travel.

During research published in Nature Communications, scientists at the University of Queensland designed an experiment that simulated the effect of a photon – a particle of light – travelling back in time and interacting with its older self.

"Time travel was simulated by using a second photon to play the part of the past incarnation of the time-travelling photon," said University of Queensland physics professor Tim Ralph.

The lead author of the paper, PhD student Martin Ringbauer, said the experiment aimed to examine the intersection of quantum mechanics and Einstein's theory of relativity.

"The question of time travel features at the interface between two of our most successful yet incompatible physical theories," Ringbauer said. "Einstein's general relativity and quantum mechanics."

Einstein's theory of relativity suggests that it may be feasible to travel backwards in time by following a "space-time path" that doubles back and returns to its own starting point in space, but at an earlier time, NDTV news reports. Physicists refer to such paths as closed time-like curves (CTCs).

Researchers ran two versions of the experiment. In the first version, they looked at what might happen when a photon "travels through a wormhole into the past, then interacts with its older version". In the second case, they wanted to investigate what might happen when a photon "travels through normal space-time, but interacts with another photon that is trapped inside a CTC forever".

"The results revealed that time travel on a quantum level seems to be possible," the Daily Mail reports.

Physicists and philosophers have long struggled with the grandparent paradox. As Science Alert explains, a time traveller could in theory prevent his or her grandparents from meeting, "thus preventing the time traveller’s birth". This would make it impossible for the time traveller to have set out in the first place and kept the grandparents apart.

The new research suggests that such paradoxes may not render time travel impossible, the Mail notes, "albeit only on a quantum level for now".