"Stop being lazy. PS your parents have severe mental issues, stop listening to them as if they know what they're talking about. Get a job regardless of what they say. Your grades won't suffer and you know it."
One of my favorite thought experiments from physics is described by Igor Novikov, in The Future of Spacetime. He describe a game of billiards in which a ball entering a certain pocket will enter a time machine, and then emerge in such a way that it interferes with its own path so as to prevent the ball from entering the pocket first place. Were the ball's path changed so that it misses the pocket, we would have a paradox. But it is found that while the ball may be deflected by its future self, it cannot be deflected enough to miss the pocket. THAT is an amazing result, even for a thought experiment.
Why? Just saying that is one way out of a paradox, but it would be more interesting if he'd at least suggest some reason the paradox is avoided. Conservation of momentum across temporal dimensions would be kind of an interesting avenue to explore as a thought experiment - especially since the entire Earth, the Solar System, and the Galaxy are screaming through space at incredibly high speeds, so real time travel would require a substantially large change in position as well as time.
Wrong book. Yes, Novikov refers to Kip Thorne's ideas, but those are discussed in Kip Thorne's "Black Holes and Time Warps" along with Hawking's views on the subject (even though they are briefly mentioned in the book you mentioned) .......Novikov refers to rigorous calculations first discovered by Kip Thorne. [p.78]
I checked just to be sure.
..... and, yes, Dave's question is not just a spurious question. It makes a difference.Relative to what?
It's not just a spurious question. One can deduce that, regardless of how the time travel might or might not work, we cannot escape the conservation of momentum issue and have to deal with it somehow.
And, in light of the previous post, my message to myself from the future would be to buy a good helmet to protect myself from all those damn billiard balls! (I'm suddenly filled with doubt about my tin foil helmet being quite sturdy enough.)OK, I've got a corollary question now:
If you were to receive a message right now from your future, what would it probably be?
Just curious.BobG said:Instead of the wormhole being destroyed by a runaway feedback loop, you have a stable wormhole creating an imbalance in the entire universe.
It's the same for meIt would be either "Do it" or "Don't do it." And I would know exactly what I'm talking about.
No, don't do it!Maybe I should have tried some things with some girls when I had the opportunity.
Number 2 is a good one. You can rarely change a person's destination, but you can at least enjoy the time you have with them.2) Dance with your mother at your wedding; you know, I know, and everyone that has ever met you knows you can't dance. Just do it this once.
This is a good one, but be careful. I likely got hitched because of the awkward hug a friend (now husband) and I shared... it was awkward for him because he thought it was rather long, and me because I thought it was rather short... and odd enough so that we both knew something was different....A neat tradition of three hugs...