In principle, physics doesn't forbid traveling back in time. We don't have the technology to do it, but maybe someday we will. But what about the paradoxes it causes? Well, if you go back in time and, for example, land in a distant galaxy where you don't do anything that would contradict what you already know about that place, then there is no problem. The paradox arises when you create a time loop, which means a closed chain of cause and effect.

Now, if the time loop is consistent, then there is no paradox. Good examples of consistent time loops in fiction are the ones that happen in "Harry Potter and the Prisoner of Azkaban", and in the MLP episode "It's About Time". If every link in the chain fits the previous one, then it's all good. Although there is another apparent paradox, because this kind of loop means that some things can seemingly appear out of nothing. In "It's About Time" it's the information where the time spells are kept. Future Twilight tells it to Past Twilight, but how does Future Twilight know it? Well, when he was Past Twilight, Future Twilight told her. So there is no clear source of this information. But it's not actually a paradox, it doesn't break any laws of logic or physics, so we just have to accept that such things can happen.

Actually, this kind of time loops happen all the time. There is a phenomenon called "pair creation". It's not only a theoretical concept, it causes an effect that has been measured. Even in vacuum, due to quantum fluctuations, a pair of "virtual particles" can appear. It's a particle and an anti-particle, they appear close to each other, and annihilate after some time. That's the standard description of the process, but, according to Feynman, an anti-particle is a particle moving back through time. If we take that into account, we can describe it as one particle moving on a closed loop through space and time. So, in the micro scale, the consistent time loop happens all the time. And these are the kind of loops where things (namely particles) appear "out of nothing".

But what will happen if we have the ability to create a macroscopic time loop, and we deliberately try to make it inconsistent? Examples in literature usually include drastic measures, such as killing one of our ancestors. There are many theories what would happen in such case, and I have one as well. Note that I'm not a professional physicist, and it's based on intuition rather than any true knowledge, so it can be complete rubbish. But I think it's interesting enough to share with you here.

First, let's consider the "orbitals" of electrons in atoms. Only some sizes and shapes of them are allowed, because the quantum mechanics requires that the wave function after "going around" the atom matches the one from the previous "round". Otherwise the wave cancels itself, which sets the probability of such process to zero. I think it's the same in the pair creation process, the virtual particle's wave function after going around the loop in space-time must match itself. And my theory is that it also happens in potential macroscopic time loops. If we try to set up an experiment that allows an inconsistent time loop, then we will observe some large scale quantum effects that will work like a force that prevents us from doing it. It's like the Pauli exclusion principle, where if we try to do something that is improbable in quantum mechanics, like putting two electrons too close to each other, we will get something that works like a repulsive force that prevents it. Actually I suppose that large scale time loops may be very hard to do in practice, because we would have to make sure that every particle involved in the process behaves in a consistent way. So I'm afraid these fictional examples I mentioned will remain fiction for a long time...

Whoa, that was a long entry! But I hope you liked it, next week we'll talk about extra dimensions!