This guy thinks it's possible, in this century. He takes all the fun out of it, though:

...Mallett – an advocate of the Parallel Universes theory – assures us that time machines will not present any danger.

“The Grandfather Paradox [where you go back in time and kill your grandfather] is not an issue,” said Mallett. “In a sense, time travel means that you’re traveling both in time and into other universes. If you go back into the past, you’ll go into another universe. As soon as you arrive at the past, you’re making a choice and there’ll be a split. Our universe will not be affected by what you do in your visit to the past.”

So what's the point? Does that mean that I can go to another universe and affect it, and then come back to this one? It would certainly be great for historical research (though you'd kind of screw over the denizens of that other universe). Anyway, I won't be investing in this guy's theories any time soon.

I'm not sure I would even call this time travel as most people use the term. Isn't the whole point of time travel to go back into the past to see what really happened (in my past) or go to the future to see what is going to happen (in my future)? Looking at the past or future of a different universe may be interesting and educational, but that's not really what I had in mind.

Wasn't it Steven Hawking who asked if time travel is possible why aren't we inundated by tourists from the future? Can we call this Hawkins's paradox (reference: Fermi's paradox).

Wasn't it Steven Hawking who asked if time travel is possible why aren't we inundated by tourists from the future?

That paradox assumes two things:

1. This era would actually be interesting to humans from the future. Maybe life gets more interesting than we can possibly imagine. Or maybe future humans live in complete virtual-reality addiction, caring very little for reality.

2. The human race won't destroy itself as soon as time travel becomes accessible to a large enough group of people. I imagine people would drive themselves insane trying to constantly rewrite their own histories. Forget intellectual enrichment in studying ancient history.

Even a non-invasive approach to time travel has problems. Isaac Asimov wrote a short story in which the creators of a time travel "viewer" (where you can only see the past) realize in horror that not only is it possible to view events large distances into the past, but it is possible to travel back just a few seconds, and view events happening anywhere, thus privacy is completely lost.

From what I understand of Relativity and time travel, several possible methods of twisting space time to produce a time machine (a sort of ripple in space time that you can go through to pass forwards or backwards in time). These methods even allow one to go back to when they came from. However, the major problem they have from most people's perspective is that none of them can travel past the point they were created or destroyed, meaning you are limited in how far back or forward you could go. It is possible, but extremely unlikely, for one of these types of ripples in space to occur naturally.

Also, the way I've read paradoxes not occuring is that they can't. I could go back in time (if a time machine existed) and try and stop JFK from being assassinated, for instance. I would fail. I might very well go back in time, I might slip into the crowd, and later photos would show that I was indeed there at JFK's assissination, but something happened and I just wasn't able to stop the event and instead was in the crowd, and just a bit too late or something.

None of it suprises me. I was always convinced that alternate universes was the only way out of causal paradoxes, if time travel is possible at all. And Everett's many-worlds interpretation of quantum physics seems to lend itself to this.

And you *would* be visiting the past of your own universe, like traveling from a branch, down to the trunk of a tree. However, anything you do there (presumably even your eyes absorbing those photons that were not otherwise absorbed) esentially puts you on a slightly-to-very different path forward again. Changing events doesn't destroy history as you know it. If those changes would seem to negate your own existence (the 'grandfather paradox') it simply makes you an unknown 'orphan' in a different timeline...

Not that you can't have interesting possibilites, even so. See the Larry Niven short story; 'All The Myriad Ways.'

But let's see if it actually works...

I can't remember the title, but Issac Asimov had a short, intentionally tongue-in-cheek story about the discovery of a substance that would dissolve 1.3 seconds before water hit it, and what happened when experimenters tried to prevent the water from hitting it after the "pour" had started. That story will get thoughts about causation, paradox and alternate universes running around in your head. (Make them stop, please!)

No one goes to the 20th Century except for the wars.

ray_g: the Asimov story you're thinking of is "The Endochronic Properties of Resublimated Thiotimoline," if I recall correctly. That's the kind of memory I have. I forgot to set my clock ahead this weekend, but I remember thiotimoline.

If you could go back and buy GM stock in 1932, then trade it in for Texas Instruments at its IPO, do you really care which universe you're in?

I think the whole time travel mental image that the public has betrays an extreme lack of imagination. Parallel universes are a cheap escape, not a serious attempt to resolve time paradoxes. The very idea that when our mental image of a process glitches up, it reflects the nature of reality is silly. Reality doesn't glitch. Furthermore, the idea that there's some cosmic significance to going in the other direction in time, as opposed to the usual one, or to human decision-making, also strikes me as antrhopomorphic and sloppy.

The norikov self-consistency principle most nearly mirrors my views on the subject:http://en.wikipedia.org/wiki/Time_Travel#The_possibility_of_paradoxes. You can have a tangled up knot of looping cables in 3-D space, and yet never encounter a space paradox. I think time paradoxes resolve in a similar manner: not possible - mere artifacts of people thinking sequentially about something where they have to envision a set of events in space-time.

Novikov self consistency principle, sorry. In any case, whether or not time travel is possible is no excuse to be logically sloppy or anthropocentric in thinking about the implications. The idea that entire universes must be spawned specially, just to accomodate your reversal of direction in space-time seems absurd to me.

Of course from our perspective, our timeline could have been altered innumerable times and still appear normal from our perspective.

I give the case of "Chuck" Cunningham and his inexplicable dissappearance from apparent existance on Happy Days as an example of this phenomenon.

So what's the point?

The point is that too many people think too big when it comes to time travel. "Meet Jesus!" they say. Or "Kill Hitler", or "Save JFK", or any number of variations.

The intelligent thing to do is go "back" two hours and buy the winning lottery ticket. Then when your analog in the new universe goes "back" to buy his own ticket you step into his shoes and go on with what is now your life:-).

History can take care of itself.

....and of course you must use your lottery winnings to fund the time machine so you can go back and buy the ticket so you can fund the time machine to go back and buy the ticket....................

The deep question is why is math predictive of reality?

The idea that you can change your environment by changing the past is passable, but you would not be effected, you might kill your grandfather in the past but that won't kill you, I'm saying this because if you are dead who would have killed your grandfather. Thus your alive and like it was said you would be an unknown orphan.

The idea of time travail is passable and we do see all the time but are never aware of it when it happens.

I will talk about this more another time