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Is time travel possible? Well, we might find the answer in this clip.
Professor Brian Cox, a Doctor Who admirer, makes a stunning experiment to prove time travel.
His conclusion: Time travel is possible, but not in the past but only into the future!
Time travel possible or not?
In 1895 the Englishman H. G. Wells published his first novel “The Time Machine”. A few years later, not only the six-decade reign of British Queen Victoria ended but also Newtonian physics, two centuries old. When Albert Einstein presented his special theory of relativity in 1905, he threw the traditional notion of space and time out of control. Something that previously only existed in the imagination of science fiction writers like Wells was now possible: journeys into the future. According to Newton’s imagination, time was uniform everywhere. It never went faster or slower. For Einstein, on the other hand, time was relative.
Time travel is not only possible, it has even taken place – albeit not as Wells had imagined. The previous record is held by the astronaut Sergei Konstantinowitsch Krikaljow. In the course of his career, the Russian cosmonaut spent 803 days in space. As Einstein proved, time passes faster for objects at rest than for those in motion relative to them. On board the Mir space station, Krikalyov raced around the world at 27,000 kilometers per hour. All in all, the cosmonaut has aged less than his fellow human beings who remained on the ground – the 48th part of a second. In other words, Krikalyov traveled a 48th of a second into the future.
Greater speeds and distances make the effect more obvious. If an astronaut left Earth for a flight at 99.995 percent of the speed of light to the Beteigeuze star 520 light years away and returned just as quickly, he would need a total of just ten years for this journey in his view – the distance would seem much shorter to the almost light-fast astronaut due to the relativistic length contraction occurring here. But more than 1000 years would have passed on Earth by now.
Jumping into the future in fractions of a second or even centuries is theoretically possible, despite practical technical hurdles. Travelling to the past, on the other hand, is much more difficult. In Einstein’s special theory of relativity, it was even impossible until the physicist presented his general theory of relativity after another decade. She lifted that ban, at least on paper. However, how someone could actually travel backwards through time is an unsolved problem, since the general theory of relativity has many different solutions. They correspond to universes with different properties, and only a few allow such journeys.
Whether any of these solutions actually describe our own cosmos is an open question that also raises further fundamental puzzles. How much would we have to work on the basics of physics in order to travel back in time? And will space prevent such excursions in any other way, even if Einstein’s equations allow them? Theoretical physicists do not deal with such highly speculative questions because they actually believe in journeys into the past as we know them from science fiction. Rather, this reflection leads to astonishing insights into the nature of the universe in which we live – and perhaps even into how it came into being.
Universal speed limit
Einstein’s special theory of relativity made time deformable. He came to his revolutionary conclusions by analyzing two very basic ideas: Every law of physics must always be the same for all observers in the universe, regardless of their state of motion. The second consideration was that the speed of light must also be an unchangeable quantity, wherever and however one determines it. Because if the same physical laws apply to everyone, everyone must come to the same result when measuring the speed of light.
For this universal speed limit, however, Einstein had to throw two assumptions deeply rooted in everyday experience overboard. Now different observers were allowed to come to different results both when measuring lengths and time intervals. A watch that passes you ticks slower than one that is at rest. And very similarly, the length of a ruler flying past is smaller than that of an identical one holding still. But for an observer who moves with one of the objects, the clock ticks normally, and the ruler also has its usual length.
Under normal conditions, the space and time-distorting effects of the special theory of relativity are negligibly small. But when something moves at a considerable part of the speed of light, they are no longer overlooked. This has been shown in many experiments. For example, the lifetime of an unstable particle, the muon, is considerably longer when it moves at almost the speed of light.
Back in the past
So astronauts and muons can travel into the future. Can the direction be reversed? The famous Austrian mathematician and logician Kurt Gödel was the first to use general relativity theory to design a universe in which journeys into the past were possible. He presented the theoretical model to his close friend Einstein as a present for his 70th birthday. It has two special properties: On the one hand, it rotates, which prevents the collapse by the gravity of the contained matter. Einstein wanted such stability from every cosmological model. On the other hand, it allows journeys into the past, which in turn caused the recipient a deep uneasiness. In Gödel’s cosmos, an astronaut can always fly straight ahead and thus return to his original place – but at a time in the past. Such paths physicists call “closed temporal curves”.
Such a curve returns to its four-dimensional beginning with a time delay. In Gödel’s rotating universe, this orbit circles the whole universe, similar to the equator of the globe. Since then, physicists have come up with a whole series of closed temporal curves that allow journeys into the past, at least in theory. A real flight along such a route would be completely unspectacular. The space traveler would see the universe with stars and planets passing by normally, and for him, time would pass as usual. In contrast to some films, for example, the hands-on board would not turn backward. And yet he would arrive in his own past.
The possible existence of such curves leads to paradoxical scenarios. How can we change the past when it has already happened? The followers of causality should reassure us that astronomers have found no evidence of a rotating universe. Gödel himself has apparently unsuccessfully searched through galaxy catalogs for signs that would support his theory. He did not develop a realistic model of the universe, but at least he showed that closed temporal curves are in perfect harmony with the equations of general relativity. The formulas do not prohibit travel to the past. Recently, time travel enthusiasts have also devised possibilities that make do with local curvatures of space-time. According to the general theory of relativity, planets, stars, galaxies and other massive bodies distort space-time. This, in turn, influences the movement of these objects. In extreme cases, space-time could bend so much that it creates a path from the present to the past.
Theorists have proposed some exotic mechanisms to put such paths through time. In a work published in 1991, the US astrophysicist J. Richard Gott of Princeton University showed how cosmic strings – infinitely long structures thinner than an atom that could have been formed in the young universe – allow closed temporal curves at their intersections. And Kip Thorne of the California Institute of Technology began studying so-called wormholes in 1983, which could be a kind of tunnel between different regions of space-time and thus allow journeys into the past. “If you combine two different regions of space in general relativity, you simultaneously combine two different regions of time,” explains Sean Caroll, a colleague of Thorne.
The entrance of a wormhole would be spherical – a three-dimensional entrance into a four-dimensional tunnel. As with all closed temporal curves, the journey through a wormhole would be unspectacular, says Caroll. “You don’t dissolve or reassemble at any other time. “Such visions from science fiction are not possible in any recognized theory.” Although physicists can formulate equations that describe wormholes and other closed temporal curves, all models offer serious pitfalls. Negative energy is needed to create a wormhole at all. Without them, the spherical entrance and the four-dimensional tunnel would immediately implode. Such a structure would be virtually unstable or perhaps even fundamentally impossible, says Caroll: “Negative energy causes a lot of problems in physics.”
But even if a wormhole could be kept open, the next difficulties would come. Particles, which move through, would pass this way loop-like again and again. In the process, their energy grows beyond all limits. And since energy and mass deform space-time, the wormhole collapses into a black hole in this scenario. “We are not 100% sure that this will happen,” admits Caroll, “but it seems a reasonable assumption that the universe is preventing the construction of a time machine by turning it into a black hole.
Unlike black holes, which are a natural consequence of general relativity theory, wormholes and closed temporal curves are artificial constructs intended to test the limits of theory. Even if wormholes are not physically plausible, it is important that they do not contradict the general theory of relativity. “It’s strange that we’re so close to excluding the possibility of time travel, and yet we can’t really make it. I find it annoying,” says Caroll, unnerved by the fact that such an elegant theory as Einstein’s allows something so obviously unbelievable. But thinking about it also leads physicists to a better understanding of our laws of nature. It is possible that our universe could not have been created in the first place if a way backward in time would have been forbidden in principle.
The general theory of relativity describes the universe on its largest scales. Quantum mechanics, on the other hand, is a kind of manual for atomic scales and offers a playground for particularly surprising phenomena with closed temporal curves. The phenomena can even affect the origin of the cosmos. At very short lengths of about 10-30 centimeters, researchers like John Friedman from the University of Wisconsin-Milwaukee expect the topology of space-time to bubble. And such random fluctuations could also lead to closed temporal curves if nothing fundamental prevents it. Can such fluctuations be increased to build a time machine? “There is no formal proof that macroscopic, closed temporal curves are completely impossible,” Friedman answers. “But most researchers who have dealt with these general questions would probably bet against it.”
A cosmos that creates itself
Creating a loop in space-time, whether on the quantum level or on the cosmic scale, undoubtedly requires extreme physical conditions. And the most likely place and time such extreme circumstances might have prevailed is the birth of the universe.
In a paper published in 1998, God and Li-Xin Li, an astrophysicist now working at Beijing University, argued that closed temporal curves are not only possible but necessary to explain the birth of the cosmos. “We examined the question of whether a time loop at the beginning of the universe could allow outer space to create itself,” God explains.
The universe of God and Li, like the standard model of cosmology, contains a period of inflation: an all-pervading energy field drives the initial rapid expansion of the cosmos. Many cosmologists think that the process produces countless different universes next to our own. “It is difficult to stop inflation once it has started,” God explains. “Rather, we obtain an eternally ramifying structure. One of these branches is our universe. But we have to ask ourselves how the trunk of the tree is formed. We suspected one of the branches could form a loop and become the trunk.” A simple two-dimensional representation of this self-starting cosmos of God and Li resembles the number 6, with the space-time loop at the bottom and our universe at the top. Inflation allowed the universe, the two researchers argue, to break out and develop into our cosmos.
While it is difficult to imagine this complex model, God sees a decisive advantage in eliminating the need to create the universe from nothing. On the other hand, Alexander Vilenkin from Tufts University, Stephen Hawking from the University of Cambridge and James Hartle from the University of California at Santa Barbara, for example, have certainly designed models in which the universe emerges from nothing. According to the laws of quantum mechanics, the seemingly empty space is filled with virtual particles that arise spontaneously and destroy themselves again. Hawking and his colleagues think the universe could have emerged from a quantum fluctuation in the same way.
Currently, we have no way to verify that any of these theories are correct. The famous physicist Richard Feynman compared the universe to a game of chess played by gods. The scientists observe it and try to understand the rules. For example, you can see how farmers only ever move one field forward. If the researchers never watch the opening of a game, they don’t know that the pieces can then move two fields far. And if they witness a farmer being turned into a queen, they’d think that’s a violation of the rules, even though such a move is possible. Except no one’s ever watched him before.
The exploration of time travel is in a similar situation. We investigate the limits of physical laws by looking at extraordinary circumstances. Journeys to the past are not forbidden, they only do not occur in the universe familiar to us. The transformation of a farmer into a queen could be one of the rules of relativity and thus be important for the question of how the universe came into being.
Such speculative ideas seem to be more philosophical than physical. But right now quantum mechanics and general relativity – two powerful theories that contradict everyday experience – are all we have to describe our universe. Scientists have no clear idea what happens when they try to combine quantum mechanics and relativity theory. It does not always provide conclusive results to mix mathematics from both worlds bit by bit. But the theorists can hardly do anything else, for they do not know how else they could move forward in a meaningful way.
Will a future theory eliminate the possibility of journeys to the past once and for all? Or will it once again become apparent that the universe is even stranger than we can imagine? Physics has made enormous progress since Einstein’s redefinition of time and space. Time travel, for Wells pure fiction, has become a proven reality, at least in one direction. Is it really impossible to imagine a cosmic symmetry that allows us to steer in all directions both in space and time? God answered this question with an anecdote. Einstein once talked to a colleague. Suddenly he took out a notebook. “What is that?” Einstein asked. “One notebook,” replied the other, “into which I immediately write every important thought.” Einstein replied: “I never needed one. I’ve only had three good ideas in my whole life.” What about the moral of the story? “I think,” God concluded, “we’re waiting for another good idea.”