Time stories have been around for years, and yet we have seen no time travellers. If there are time travellers, then they are hiding from us. That would be pretty hard, since society and language are changing all the time, and they would most likely look out of place.
If they came from the past, their mannerisms, clothing and use of language would likely give them away. Imagine someone from the time of Sir Francis Drake appearing in the current era. He would quickly be spotted.
For someone travelling from the past, the issue is that he or she would not know what to expect as the present is the future of times past, so they would not be able to prepare themselves for the future, as they would consider it.
If a citizen of the future where to travel to the current time, he or she could presumably prepare his/herself for what would be the past to him/her. The time traveller could learn about our era and equip him/herself with clothes, money and other things from our era and would be able to learn the idioms of the language of the current time, as well as the ethics and morals of the era.
There are web sites on the Internet which claim to have proof of time travel (I’m not going to link to one – a Google search will bring up many of them). In most cases the evidence is far from compelling, relying on blurry photographs and dubious eye witness accounts.
I’ve recently been scanning my old photographic slides and in one of them, from the early 1980s or late 1970s, the person in the picture appears to be holding an iPad! What in fact she is holding is a place mat, with a cream coloured border and relatively dark picture on it. This demonstrates how easily “evidence” of time travel can be found if you look hard, and if you strongly believe that time travel is possible.
Every person, every object, even every elementary particle has position which can be measured (leaving aside the issue of Heisenberg’s Uncertainty Principle and Quantum Physics for now) at any moment in time. The four dimensions, three of space and one of time uniquely identify an event in the life of the person, object or particle.
These four coordinates represent a single point in a four dimensional space. Since we find four dimensions hard to visualise, this space/time is usually represent by a depiction of a three dimensional space of two space and one time coordinate axes. The path of a person, object or particle through life consists of a single unbroken line in the four dimensional space.
Note that in time dimension, if time travel is not possible, for every value of the time coordinate there will only be one point of the person’s life line. In other words, while the person could visit and revisit the same three dimension spot in space many time, they will only pass through a particular time once and once only. A person’s now is unique.
Time travel means that a person could pass through the same moment in time multiple times, and the possibility arises of loops in time. It seems obvious that the same event could not appear on the time traveller’s life line. In other words the loops in a life line would not cross.
To get from one event in space/time to an earlier event in space/time, the person could either travel through the intervening times or just jump from the first event to the second. In other words time travel if it is possible would be represented by a line going backwards in time or it could be discontinuous, with a break the in the person’s time line.
From the point of view of an observer, a discontinuous time line for the time traveller would be seen as a sudden appearances from nowhere and a later sudden disappearance. If the time line is contiguous, the observer would again see a sudden appearance of the time traveller, and then two instances of the time traveller both apparently travelling into the future at one second per second.
One will be the time traveller doing just that, and the other will be the observer’s view of the time traveller as he travels backwards in time. Eventually the observer will see one of the instances (people!) merge with another instance of the time traveller and disappear. Since we don’t normally observe such sudden appearances and disappearances it’s very tempting to say that time travel does not happen.
To see what I mean, take a piece of paper and draw a line from top to bottom with a look in it. Now horizontal lines across the page represent time as seen by the observer. If you move a ruler down the page, at first there is a single line, the time line of time traveller. But at the point that the time travellers has travelled back to, suddenly there are (apparently) three lines travelling down the page. At the point that the time traveller travels from, two of the lines merge and disappear.
If time travel is discontinuous the observer would first see one instance of the traveller, and then another instance would pop into existence. The two would coexist for a time, and then one instance (the original one) would disappear, with the second continuing to exist.
As I said, it’s tempting to say that this proves that time travel is not possible. Certainly, a macro level we don’t see people appearing and disappearing so it is definitely very unlikely, though reasons for this not to be noticed can be constructed.
At an atomic level though, we do see spontaneous generation of particles, into a particle and its anti-particle. If the anti-particle the meet another particle and they annihilate one another, this could be construed to be a single particle with the anti-particle being the particle travelling back in time.
This is bad news for time travellers. To travel back in time by this method, the time traveller would have to be zapped into a burst of energy and an anti-traveller who would then travel back in time to the earlier time when a burst of anti-energy would be required to zap the anti-traveller into another instance of the time traveller. These occurrences would be likely to destroy the integrity of the time traveller’s body. That is it would destroy it.