Question: Will we ever be able to travel in time?

Nathan Langford answered on 3 Jul 2012:

That’s a very difficult question to answer. Probably, if time travel does ever turn out to be possible, it probably means we will have discovered new theories of physics that go far beyond our current theories of quantum physics and general relativity, and which have opened up brand new possibilities.

However, I can say a couple of things. Firstly, one of the really strange things about quantum physics is that the basic laws don’t seem to depend on the direction of time flow. People say it is “symmetric to the arrow of time”. That means you can just as easily use quantum physics to calculate how systems evolve backwards in time as you can to calculate how they evolve forwards in time! This is really very strange, because *all* of our experimental observations and every day experience seem to demonstrate very strongly that time only flows in one direction and we don’t have any control over that.

So quantum physics really does have some quirky behaviour when it comes to time.

Also, you might be interested to know that there are very serious, well-respected researchers who are carrying out research into time travel of a certain specific sort – at least, theoretically. It turns out that general relativity does allow the concept of time travel in a particular, special sense. What we don’t know yet is whether it’s possible for these “time travel loops” to exist in our universe. Sometimes they lead to inconsistencies in the theory, like the classic “grandfather paradox”, where someone travels back in time and kills their own grandfather. Time travel would probably be much more believable if it could only happen in a way which avoided such paradoxes and inconsistencies.

Interestingly, although classical physics doesn’t seem to provide any general resolutions to such paradoxes, quantum physics might. It seems like we might be able to find answers to these questions by studying the flow of “quantum information”, which is the physical information that can be stored in quantum systems. For example, another way that quantum physics is a bit quirky with regard to time, is that it doesn’t seem to have any intrinsic problems to the idea of two particles at completely different times being entangled with one another. So perhaps you could end up building a time machine that was able to “teleport” particles back into the past or forwards into the future.

There are two last things we can probably say about limitations on how time travel might work within the realm of quantum physics. Firstly, it would probably not be possible to travel back in time to a point earlier than when the time-machine was first invented. Secondly, while it may be possible to send very carefully controlled, individual particles back or forward in time, it is much more unlikely that we would ever be able to send a person through time.

But the thing about ideas like this is that, at the end of the day, who knows what might be possible?