# Time’s arrow

I have always enjoyed science fiction on TV, movies and books. One facet of the genre that I find particularly intriguing is time travel. I am thinking of shows like Dr Who, Star Trek, Time Tunnel and Quantum Leap. I also recently read [and very much enjoyed] The Time Traveler’s Wife. In all these cases, we are asked to accept the possibility that travel, both forwards and backwards in time, is a possibility. Interestingly, in all these cases, time travel does seem a bit haphazard, with some challenges about hitting a specific date. But they all manage to make the journey.

For the most part, when reading any kind of fiction, I expect a lot from the author. I want to be entertained, but I also want to learn something. So, I anticipate a high level of integrity and knowledge to be displayed. If I find numerous technical flaws or “continuity” errors, I get frustrated [and do not get me started on “The Da Vinci Code”!]. With older science fiction, I have to suspend belief, as science may have overturned the knowledge and assumptions of the time; this is hard, but worth it for a good story. Somehow, I am always happy to accept the idea of time travel, even though it is counter-intuitive and at variance with a lot of scientific research …

As far as I can tell, modern scientific thinking refutes almost any form of time travel – the mathematics indicates that it is not possible to freely move backwards and forwards in time.

Traveling to the future is easy. Just sit and wait and the future will come to you. If you are impatient, you can move things along. Einstein’s theory of General Relativity indicates that the faster an object or person moves in space, the faster time moves forward from their viewpoint. To get a useful effect, you need to achieve a velocity close to the speed of light, but then you can fast-forward as much as you like. Measurements have shown that the equations hold true.

The problem is that you cannot travel back in time. Einstein’s equations indicate that this would require a velocity beyond the speed of light, which is strictly impossible. Although there is a suggestion that a class of particles, tachyons, might exist which intrinsically travel at faster-than-light speeds and, hence, appear before they have been created. There are hints that they might actually exist, but no ideas on how they might be harnessed. [Although, in Star Trek, they seem to be an everyday commodity.]

If such time travel were possible, I am sure that “time tourism” would be popular. The fact that we are not infested with visitors from the future suggests that the problem will never be solved [or mankind will be annihilated before finding a solution]. This does solve the problem of time paradoxes, like traveling back and killing your own grandfather.

This leaves open an interesting question, to which physicists, cosmologists and mathematicians have no answer: why can we not travel freely in time? We know the equations saw we cannot, but it does seem odd that we can move freely in the 3 spatial dimensions, but not in the fourth dimension, time. I believe that the answer lies in the Big Bang.

The Universe is expanding as a result of the Big Bang. Whichever way you look in the sky, distant objects appear to be moving away from us. The further away they are, the faster they seem to be moving. This makes it sound as if we are at the center of the Universe, but this view would appear to be the same from anywhere. Imagine being on the surface of an inflating balloon – this is the 2-dimensional analog of the 3-dimensional expanding Universe. It is space itself that is expanding. It seems to me that, if three dimensions are expanding, it is safe to assume that the fourth one is doing so as well and this expansion is what we perceive to be the forwards movement of time.

I am not a physicist, cosmologist or mathematician. I am just interested in science and read a lot about what these smart people are doing. So, it is entirely possible that I have got it all wrong. If you can correct or clarify, please comment or email.

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