Time Travel

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"Once confined to fantasy and science fiction, time travel is now simply an engineering problem."
Michio Kaku

Time Travel

Time travel is no fantasy! It is a very real and intriguing phenomenon. The laws of physics do not forbid time travel. Not too long ago it was the 'stuff' of science fiction only. But now it is fair game or subject matter of many theoritical physicists. While we are all time travellers, in the sense that the future is continuously being converted into the past as we move through time, can we travel backwards and forwards in time at will. According to Einstein's general relativity equations, the laws of physics do not prevent time travel. They show how not only space is warped by gravity, but time too, and this is the key to time travel.

We can certainly go forwards in time. We can travel into the future. This prediction in the General Theory of Relativity can be witnessed as an everyday occurence now with the GPS satellites. If an object is moving fast enough through space then it can alter its passage through time. Einstein showed that the closer to the speed of light you travel, the more time would appear to slow down. This is what he called time dilation.

Experiments with atomic clocks in the early 1970s proved Einsteins predictions. These clocks were placed on jets which circumnavigated the world. When compared to previously synchronised ground based atomic clocks they showed a time discrepency in accordance to Einstein's equations.

GPS satellites in orbit are also affected by relativistic time dilation. Even thought the satellites speed is relatively slow in comparison to the speed of light, the on board clocks would quickly become unsynchronised with the 'stationary' ground based clocks due to time dilation, rendering the GPS system useless. The orbiting clocks are moving in a different frame of reference. Time runs ever so slightly slower for them, about 7 microseconds per day. To compensate, an offset is required on the satellite clocks which factors in time dilation. Without the offset, the accuracy of the GPS system would drift by over 2 Kilometers per day!

Cosmonaut Sergei Avdeyev holds the honorary role of being man's greatest time traveller. He spent a total of 748 days on the Russian space station Mir during three separate missions. Travelling at 17000 miles per hour for that length of time means he has aged one-fiftieth of a second less than us mere mortals on 'terra firma'. He has travelled one-fiftieth of a second into the future (Another cosmonaut has since taken over this mantle). The principle of course is what matters here. If a spacecraft travelled fast enough through space for a few years then returned to Earth, the astronauts would return to an Earth perhaps hundreds of years into the future.

Newton was one of the first Scientists to try and understand time. For him there was one universal time. It was the same for everyone and everywhere in the universe. Time was fixed. Its flow and pace never varied. But Einstein disagreed, he had other ideas about space and time. His theory gives the best description of space and time to-date. He showed that time was flexible, just like space. Time is relative. A clock at the top of a building runs a little bit faster than a clock at the bottom of a building (general relativity time dilation). And moving clocks tick slower (special relativity time dilation). And the faster they move the slower they tick. By controlling ones 'speed' through space it's possible to control and manipulate time itself.

But Einstein's time travel is limited. He said you could never travel back in time. To achieve this you would have to violate the speed-of-light barrier. And this is not possible according to relativity theory. This also makes much sense. Because travelling back in time has some weird consequences and would create many paradoxes. Suppose you travelled back in time and somehow prevented your parents from ever meeting. Then you would never be born. But if you were never born how could you travel back in time to prevent your parents from meeting? (More humane than the grandfather paradox I think). And doesn't this violate causality - the cause and effect principle in physics - where cause must always precede the effect (more on causality later).

Is there a way around Einstein's golden rule? The famous mathematician, Kurt Godel, showed in the 1940s that backwards time travel was indeed possible and was a conclusion of Einstein's theories and equations - provided the universe was rotating. Which it's not. But physicists now think that a rotating space will allow an object to break the light speed barrier, without violating any laws of physics. But where can we find rotating space? Black holes are one possibility. Non-rotating black holes won't suffice. At their heart is a singularity. Here space and time have been shown to cease to exist. Any matter drawn into the singularity would also cease to exist. But rotating black holes have a 'ring' singularity where, mathematically, matter drawn to the ring could pass through the ring and appear in a different region of space and time.

But time travel has bizzare consequences. It poses a few puzzles for the people who support it. If travelling back in time were possible then where are all the time travel 'tourists' from the future, as physicists Stephen Hawkings once asked. Surely the intrepid time travellers from the future, after the invention of the time travel machine, would reveal themselves to us. Surely there would be huge interest in significent events in history. Would time travellers not be queing up to witness first hand, and perhaps try and intervene, in events such as the crucifixation of Jesus, the assassination of John Kennedy (would they see anybody on the grassy knoll?) or Neil Armstrong's famous 'one small step' to name just a few.

Perhaps we have been visited from the future. Maybe they did intervene and history we look back on today is an accurate description of the outcome of their intervention. Maybe the future is where UFOs (the inexplicable ones) are from. Indeed some people believe there already are time travel tourists living amongst us! Or some people think that it may not be possible to travel back before the invention of the time travel machine, which is why there are no time travel 'tourists' around today.

And we are still stuck with the paradox of travelling back in time and preventing your own birth. How can this be possible? At the quantum level, all alternative outcomes are possible, according to quantum theory. This 'many-worlds interpretation' of the quantum world was first proposed by physicist Hugh Everett III where he said that all outcomes are not just possible, but do occur, each one generating a new universe. But does quantum theory apply to the macro level. Perhaps if it does, then travelling back in time and changing the course of history means you have created an alternative universe, and you can never return to the one you came from. Alternative universe theory solves or sidesteps the paradox problems posed by backwards time travel and doesn't seem to violate causality.

So when you travel back in time and alter the past, essentially you create a branch point or a fork in time for another universe, where in that universe, to revert back to the previously mentioned paradox, your parents do not meet and you are not born. But you are born in the universe you travelled back in time from. You create a new version of reality. It looks like we could end up with an infinite number of branch points and parallel universes. But if there are an infinite number of universes, where does all the energy and matter come from to make them up?


Wormholes offer another method of time travel. Einstein and Nathan Rosen realized that general relativity allows for the existence of 'bridges' called Einstein-Rosen bridges, even though there is no observational evidence of their existence to-date.

These are in effect space-time tubes, which give rise to not only a shortcut between two different regions of space, but also two different regions of time. By taking the path through the wormhole you could arrive at a different region of space-time faster than a beam of light travelling through normal space-time. In effect you are travelling faster than light which in 'reality' means you are travelling backwards through time.

So all outcomes are possible in quantum theory. Can this be applied to all of physics, all of nature? Does this mean that everything that can happen does happen, albeit in an alternate universe? Does this mean that in some universe, somewhere, a God exists? Can we stretch quantum theory this far? Is it saying to us that there is a possibility or probability of some degree that a God exists?