I’ve read, here and elsewhere, quite a lot about the paradoxes of time travel. It’s enough to give one a headache if you let it.
One point, though, that I haven’t seen (or heard) discussed is the matter of, well, matter. It’s impossible (so not a paradox) to travel in time because matter cannot exist in two places at once. To go back in time you would have to re-create yourself out of thin air, and physics won’t let you do that. Your atoms have existed since the Big Bang, right?
Peace,
mangeorge
Well, sort of. When you get down to the quantum level, you get particles popping into existence from nothing, existing in two places at once, and having zero mass, which doesn’t make much sense. But I can’t see anything like that working for something as big as a human.
I don’t know if this makes sense, but what if the conservation of mass and energy is unaffected by time. Like, when you disappear from Time A and go to Time B, no mass is lost, it’s just moved to a different time.
If you could go back in time and prevent this thread from happening …
Well I say that Yogi Bear is smarter than the…
Hey wait!
When I started this post the thread was about cartoons, now it’s about time travel…
What happened?!?!?!?!?!?!?!?!?!?!?!
Or, there could be formulations of time travel where there’s a path to the past, so wherever you are, your matter exists continuously in time. That is, you don’t just disappear and reappear, but go for a long walk and end up on tuesday. Whereever you are, time is normal locally, but in the big picture, you’ve looped.
<GR hat on> stop imagining space as flat 
Imagine a teapot:
______
_/ \=\
|_ | \\
\ /==/
\____/
Sorry for the crude drawing, but you get the idea. Imagine 1d people living on the surface, where space is round, and time is down on the main pot, but space is round and time is up on the handle. Wherever you are, there’s always a direction of space you can move in, and a direction of time you must, but if you position yourself under the handle, your progression through time will carry you round the handle and back up to the top - ie. a loop in time!
Whereever you are, space/time looks like it would if you were on a sphere or an infinitite cylinder or plane (cf. big bang/crunch, always existing universe finite or infinite universe.) So your trip involves no spontaneous creation (except at the top - the big bang 
) but you do move in time.
Does that make any sense? I don’t know if the universe does work like this, but I think it makes sense like it could.
Physics dopers - anyone have a more formal answer?
ah yes, but what do you mean by ‘two places at once’, because of relativistic ‘failure of simultaneity at distance’ the phrase doesn’t mean much.
At the moment ther is no solid disproof of backwards in t ime travel, indeed there are a couple of solutions in general relativity that allow for closed temporal loops. These solution refer to situations that are not physically possible, but it shows that there is nothing in relativty that disallows time travel in principle.
What I mean is, right now there are 220lbs of me sitting here typing this. Ten minutes ago that same mass was in the kitchen brewing a cup of excellent coffee. If I were to attempt to go back ten minutes, but to my backyard, I couldn’t do it because those 220lbs of atoms would already be in the kitchen brewing coffee. If I tried to go back to anywhere, eons ago, I’d encounter the same problem. You can’t replace yourself either, because of problems with accelerating mass, but that concept is (also :)) way over my head.
I’m addressing possibilities in reality, as do paradoxes. Theoretical crutches for math geeks to play with don’t count. 
We can observe the past, I think, but we cannot take part in it.
Yes but for different obsevers different events are simultaneous, which also means they may disagree on the ordering of events too (though importantly this doesn’t lead to casuality violations).
Bakwards time travel is probably impossible, but there’s no conclusive proof of this yet.
Well, how about if you took part in something that was, for all intents and purposes, the past?
Warning: No Real Science Beyond This Point
Suppose the big bang is followed eternally by the big crunch. In that big crunch, under the high gravitational pull and heat, matter is in its simplest form and in its smallest possible space. Given that this crunch contains all matter in the universe in the tighest possible arrangement, there’s probably an ideal construction for this, at which time, BANG, everything starts over again, and everything happens in precisely the same way in some bizarre Newtonian reversion of chaos theory. The same simple matter in the same shape under the same pressure and heat as before yields the same result; history duplicates itself.
Now suppose you could somehow travel forward in time by a few trillion years or so – one whole universe tick, that is, minus the time you wanted to go backward. You go amazingly far forward and find yourself in a world identical to the one you knew except this time, you’re not you, you’re an observer of another lump of atoms that looks and behaves exactly as you did at that time.
Ah, but in order for this to work you have to step out of the universe during the Big Crunch and subsequent Bang. So you’d need to find a way to step out of the universe while that happened – and since the absence of your 220 pounds of atoms would change the ideal construction of the Crunch, you have to precisely replace your atoms with 220 pounds of Stuff From Somewhere Else that will act as a placeholder in the Big Crunch. (And you’d need to find a way to move yourself forward a few trillion years, a mere piffling detail for someone who can step out of the universe.)
Ah, but this just occurred to me. If you invented a machine that could do this, you couldn’t possibly have invented it for the first time. If the ideal construction of the Big Crunch always yields the same history, you’d already have invented this machine an eon of universe ticks ago, right? So you’re merely replicating the invention a different You already invented; if you intend to visit yourself it means you probably have been visited, right? Unless you stop your own mad genius, there will be an infinite number of Yous, each creating his own mad time-forward-leap machine and trying to stop his future selves from creating further machines…
Ow. My head hurts.
FISH
…you wouldn’t feel obligated to reply to it.
Here, FISH, take a hit of this. It’ll make you feel much better.
There’s no conclusive evidence that the Easter Bunny doesn’t exist either.
Ah, but what if you’re not actually introducing new matter? What if for every instant each bit of matter is distinctly different from the matter that was there the previous instant? So that each ‘location’ of time would contain different matter. Then imagine that matter conservation really worked over all space and time. Then it seems like the issue you mention would be resolved. Your matter is always existing simultaneously with your matter of all other times. So, just existing in the same time as another you wouldn’t upsed the balance.
I actually considered that, Eonwe. Trouble is, it’s the same matter, which has existed since the Beginning Of Time, aka The Big Bang. It wasn’t me all that time, but it (they) was (were) the same atoms. Pretty much, anyway.
I guess the same would apply to travel to the future. We can’t do it because we didn’t. The idea of time travel simply doesn’t stand up to common sense scrutiny, does it.
Dang! 
well what about quantum electrodynamics and the possibilty that antiparuicles are simply particles travelling back in time, in this case it may even be possible that there is only one electron in existance travelling back and forth in time.
Two hundred twenty pounds of antiparuicles, in perfect harmony, traveling back and forth in time, at my command? Beautiful, my friend.
Now I’m gonna go see what the heck an antiparuicle is.
Hmmm.
Google asks;
Well???
I did indeed.
Consider that as opposed to “one particle cannot exist in two places at one time” we might have “one particle cannot exist in two places at the same time in its history.”
That means that without time travel, no atom can exist in two places at the same time. If we could take the same atom back and observe it next to itself, they would be the “same” atom in that they are made up of the exact same particles, but exist at different points along that atom’s “lifetime.” This would allow the atom to be sent back in time - we just can’t get two copies of an atom to have the same “age” (however it is that we gauge the age of subatomic particles… picoseconds since the Bang?)
Since we don’t have time travel, we can’t find out. Nyah.
I travel through time all the err…time
I travel at one second per second forward.
therefore time travel is possible. you might want to change the rate, but the traveling part is possible.