Time travel is a popular science fiction staple, from Back to the Future, to Dr. Who its use has captured the attention of many audiences. Although for now, time travel only exists in a fictional realm, there are many reasons why it could work. General relativity proposes that everything around us, space-time, is one set of dimensions. This suggests that as we can move through space by walking around or any other movement, there should be a way to move through time by moving along its dimension. This is obviously more complicated than it sounds, but theoretically there should be a way to do it.
Now, as is true in many cases, when general relativity seems to provide an answer, quantum mechanics is there to provide a problem. The first of these problems that I can see comes from quantum uncertainty. What I mean by this is, suppose you went back in time 65 million years ago, to the time of the dinosaurs. Currently we think we have a pretty good understanding of what this era would have looked like, based on fossils and carbon dating among other things. But right now this is just a probability of what things would have looked like at that time, and this probability eventually gave rise to our world today. However, since no one was there 65 million years ago, it is still just a probability. Probabilities are what quantum mechanics operate in. What this means is everything has a probability of being a certain way until it is observed as being one way or another, at which time it locks into a certain state of being. In the dinosaur case, the idea we have for that time is just a probability of what it may have looked like, but as soon as someone were to go back and observe how the world really was then, one probability will take form which may or may not be the one we imagined. In any case, there are going to be differences and these could have terrible consequences for today's world.
Another risk comes from the many worlds approach to quantum mechanics, in which every probability that exists does happen in some universe. For example if you flip a coin, quantum mechanics says that the 2 probabilities of heads or tails both happen, in their own universe one of which you happen to observe. So for time travel, when there is a probability of what a certain time looked like or will look like, if you were to travel to a certain time you may observe what you expected, but you may be in a completely different universe. This could make returning difficult because you would have to travel to one specific universe among and infinite possibility. So, even if you did travel to a different time you would be stuck there.
Obviously if anyone would like to reenact Marty McFly, we still have some work to do.
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