Are there any theories out there that explain this time travel paradox [duplicate]

Question

I was quite perplexed by a paradox that I thought of, for instance, say I am a person who completely understand einsteins field equations. In this case, Einstein formulated his theory. Now, say I have a time machine and I travel, back to the past and explained einstein the Idea of GR. So, Einstein wouldn't get to know about this Idea without me telling him.

So where is the information coming from? Is time travel to the past even possible?

Since, I was thinking about this, I could come up with no solutions and thought maybe time travel is not possible or there is a theory which already explains this situation.

So what does General relativity tell us about time travel to the past? Is it possible? and are the theories like may be the multiverse theory which can explain this phenomenon?

I know this is kind of related to the grandfather paradox, so there have to be theories since this is quite old right?

Answer 1

The same paradox has been stated in many different ways, and yes, it's basically a reformulation of the grandfather paradox.

In general relativity there is nothing wrong with solutions with closed time like curves, in fact general relativity can say very little about the topology of the space-time. We usually put by hand the constraint: "closed time like curves cannot exists", due to consistency-unitarity problems. I don't want to enter into much detail, but travel in time it's pretty easy with wormholes and spinning cosmic strings, even though they require unphysical (negative energy) matter that it's not available macroscopically in a classical theory. But it's possible to have this kind of matter in a quantum theory. Only a quantum gravity theory can really tell what happens at very small scale, and it's not even obvious that time will have a meaning there, a concrete possibility it's that time (and the whole space time) is an emergent concept.

By the way in a quantum theory, the grandfather paradox can be mitigated. Roughly speaking, you can be in a superposition of states in which 50% of the times you kill you grandparent and 50% of the times you are never born in the first place.

See:

http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.3197