Lets say today you had a test and you failed it. Next day, you buy a time machine and you go into the past. you tie up your past self and correct your test so that you pass the test. What would happen to the universe that you failed the test in?

This is a good question and show that we have lots of issues with time travel like this.

 

Some options are;

 

i) everything is consistent and whatever you do it was always like that. In this case you always did go back and interfere with yourself, but you failed anyway. (Novikov consistency)

 

ii) you can change the future. So the universe in which you failed never comes into existence. (the Marty McFly scenario)

 

iii) time travel places you into a different universe. Meaning that the original universe you were in now forks off into two branches, one in which you fail and one in which you don't. (Multiuniverse hypothesis)

 

Or maybe something else or the Universe will always arrange itself as not to allow time travel like this (Hawking chronological protection conjecture).

Well ajb has it covered from every angle as usual. I had only thought of iii that the first intelligence that can go back in time will cause "forks" to appear in their universe, where the universe branches into two versions. You, the time traveler, could encounter your version in this branching universe, and you would then be absent from your universe. As civilizations time travel they create a complicated branching series of universes, maybe never able to return to their original branch.

That question does not make any sense at all. You can't go back in time. So it doesn't matter what would happen to the first universe.

 

And why is this in the Astronomy/Cosmology forum?

You can't go back in time.

From a physics perspective it is not at all obvious that you cannot travel back in time. General relativity by itself does not rule it out, though when one adds the effects of quantum theory it becomes much more subtle.

From a physics perspective it is not at all obvious that you cannot travel back in time.

 

Breakage of energy conservation, momentum conservation, angular momentum, baryon number conservation, lepton number conservation, is not enough?

Breakage of energy conservation, momentum conservation, angular momentum, baryon number conservation, lepton number conservation, is not enough?

Energy and momentum conservation is much more subtle in general relativity and we have no reason why in general they would be globally conserved. The quantum numbers of the standard model maybe more interesting, but they apply to interactions. I don't see that one would necessarily require their global conservation in the presence of time machines (CTCs).