Probably an old theory, but perhaps there are infinitely many timelines, and time travel involves jumping between them? You go back in time and kill your grandfather, then return to your present time. You’re still alive since you’ve shifted timelines and in your timeline you didn’t kill your grandfather.
Then the question becomes, why can’t you stay on the same timeline going forward in time?
Au contraire! You most definitely can do so! We are doing it right now!
The trick is to do it faster than everyone else. One solution is to take a nap.
Seriously. Think back to the going-forward part of HG Wells’ “The Time Machine”. There’s no violation of causality, although we’d have to figure out why the traveller was not effected by events surrounding him. (Geordi would explain it as a “phase shift”, I’m sure.)
Would suspended animation count as a forward-only time machine?
Or maybe they have better sense than to interrupt us? Just because something isn’t happening doesn’t mean it can’t happen.
Humans having the sense to leave well enough alone? Either the time travelers are another species, or there’s some aspect of physics that lets the travelers observe the past, but prevents them from physically interacting with it.
That reminds me of one of Joe Haldeman’s short stories (I forget the name). Some crazy old guy is talking with a writer, claiming to be a tourist from the future who accidentally ‘materialized’ and now can’t return. He rambles on about events he’s seen (Apollo launch, WWII, etc) until the writer finally brushes him off. It ends with the writer saying to himself, “nobody’s going to listen to that nonsense these days. This is 1923, after all.”
–sublight.
Time travel is predicted by wormhole theory. A wormhole, if it exists, must also be a time hole.
Actually, I believe it was Einstien that stated that theoretically time travel is completly possible. This is how to do it:
A) Create a VERY large cylinder (by large I mean that it’s roughly the size of three of our suns laid end to end)
B) Free it of all gravity and pretty much everything else (including its own gravity)
C) Somehow get it to rotate (PLEASE don’t ask me how in the hell to rotate it (or for that matter how to accomplish part B (or for that matter how to build it in the firstplace))
D) Travel along the surface of said cylinder OPPOSITE it’s rotation direction to travel back in time, and IN THE SAME DIRECTION to travel forward in time.
In theory this will work. Potential problems:
If this doesn’t (and probably won’t) work, there’s the old standby, the Einstien-Rosen-Podawsky bridge. This is a wormhole that is mainly used to travel between alternate dimesnsions, but theoretically could be used to travel through time as well.
Any sufficiently advanced technology is indistinguishable from doubletalk, too.
(signed) “From Room 101”
What was wrong with Ether? Me and the other Sons love it.
My Time Displacemnt Device works fine. Paradox is only caused when I demonstrate it to Sleepers.
(This concludes this inside joke. To get the joke read Mage:The Ascenscion by White Wolf Games)
On a serious note, time and space are really time/space. Thus isn’t any movement movent through time? And the question that still blows my mind, we are all moving in the same time direction at the same speed (A basketball game is proof of this) Why? Why don’t some things move through time at a different rate? I don’t mean accelarating to relativistic speeds in order to move more slowly through time. I await incoming lectures.
You’ve answered your own question. What don’t you like about that answer? Time is a very powerful force. In this context, it might be akin to interia, in the sense that we are moving through time at a specific rate, and that rate will not change unless acted upon by a relevant (and sufficiently powerful) force. One such way, as you said, was by moving at close to to speed of light. Another way, IIRC, might involve use of ridiculously powerful gravitational fields.
And actually, if the question is “changing the rate at which time flows” rather than “significantly changing the rate at which time flows”, then this has actually been accomplished. On at least one of the Apollo flights, IIRC, they proved that about a half-second less time elapsed for the astronauts than for us here on earth, over the course of a couple of weeks. And other experiments as well. Significant? No. Real? Heck, yes.
Or, as I posted above, you can speed up time by taking a nap, making hours go by in minutes.
By accelerating to relativistic speed, you can temporarily alter the rate at which you move through time. As soon as the astronauts landed, they were back in sync with the rest of us. I want to know why there are no humans who are permanently at a different rate. Is this simply due to our travelling on the same planet? Why aren’t there people who live 1000 of our years on permanent slow mo?
The Six Fingers Of Time by R.A. Lafferty explores the idea of a man who can change the change the speed at which he moves forward in time. As for paradoxes, “Charlie’s machine was a very good tingler and flasher and noisemaker, but it was not a good time machine. It didn’t become that until later, after Charlie had gained enough insight from the future so that he could adjust the machine to work properly in the present.(There is a paradox involved in that, of course. Time travel is full of paradoxes. The universe runs on paradox-power)” Slaves Of Time by Robert Sheckley
While we’re on the subject, why aren’t there any people a hundred feet tall? I mean, sure, it’s possible to stretch a person, but what about people who’re just that tall when they’re not being stretched?
The book Timemaster by Robert L. Forward, though perhaps far from the best-written sci-fi book in the world, explains very well how time travel could work in a realistic way without violating casualty.
I’ve also seen a good quantum-mechanical justification of time-travel: “solutions” of the Schrödinger equation in which contradictions occur have a magnitude of zero, indicating that they have a zero probability of occurring. If time travel were possible, therefore, there would be no possibility of causing a paradox.
Also, Hans Moravec, in his book Robot: Mere Machine to Transcendent Mind (which may be where I saw the above argument; I can’t recall) includes a detailed analysis of the uses time travel would have in computers. By including components with a “negative time-delay,” quantum computers could be built to carry out such previously intractable computations as solving the game of chess using a very small number of iterations.
“Negative time-delay” 
Delay as a function of time is a strictly increasing function.
With negative time delay, the more iterations you do, the sooner you would get the result. If you do enough, you should get the answer before you input the problem. And if you got the answer before you input the problem, why bother inputting the problem?
Quantum Mechanics does not allow information to travel faster than light or back in time.
A “negative time delay” component would be one that outputs the answer before it receives input. It is indeed a form of time travel. The use of such components in a computer wouldn’t be to receive the answer before asking a question, but to easily solve many types of “intractable” problems.
Quantum computers, as I understand it, can be looked at as working by setting up the components of a computation such that the output of the correct solution to a problem is the only possibility that does not result in a contradiction. As I mentioned in my previous post, a contradiction or paradox in physics has 0 chance of occurring.
A time-travel computer would be an extension of this. If there is a difficult problem for which, given a proposed solution, it is simple to see if the solution is correct, a computer designed to solve this problem could be set up as follows:
Connect the output of an algorithm designed to check a solution to that same algorithm’s input. Give the algorithm a negative time delay that exactly cancels out the time it takes to perform this check, so that at any time the input of this algorithm must be identical to the input.
Now (and here’s the kicker) program the algorithm so that if the answer it receives is correct, it is to display that answer to the user and send that answer back to its own input. If the answer is incorrect, it is to generate a different proposed answer to send back to the input, resulting in a paradox.
If the answer is incorrect, then a situation in violation of the laws of physics would result—two different signals would be simultaneously sent to the algorithm. The probability of this occurrence is 0. Therefore the answer must be correct.
If one had a machine like this, however, it would need to be built with an extreme degree of reliability. If the machine were more likely to break down than to pull the correct answer from “thin air,” then it would frequently break down, even if in the normal course of events a malfunction would be very unlikely. Time travel, if possible, would distort probability in much the same way as do quantum computers. The primary difference would be that such distortion would occur at a macroscopic instead of a microscopic scale.
Note that I’m explaining this from memory, so may not be entirely accurate. This discussion of the possibilities of time travel was a very interesting section in a particulary fascinating book.
“Negative time delay” sounds an awful lot like Heinlein’s “thiotimoline”.
Loved that story! Beautiful satire of technical papers as an art form.
“Thiotimoline” was Asimov’s, not Heinlein. How soon we forget.
I remember once reading an essay that suggests time travel is impossible because of quantum mechanics. If one travels from the future to the present, that would mean the future is already determined… but the Uncertainty Principle in quantum mechanics would preclude determinism, therefore no time travel. That’s what I remember, anyway.
Still, Time Travel is just about my favourite sub-genre of Science Fiction.
(Someone mentioned Ken Grimwood’s novel REPLAY – I highly recommend it to all time travel fans!)
REPLAY — Oh, yes, the absolute best time travel story I have ever read, bar none. I finished it in under 3 days, and it’s still the only novel that my 17-year-old son has ever read cover-to-cover.
I still daydream about waking up as a child in my parents’ home, and they don’t believe my story; but then I tell them what Walter Cronkite is going to say tomorrow when he interrupts As The World Turns…!
I often wonder if Grimwood is a replayer himself, and this is his latest attempt to find others…
(Maureen, thanks for the correction. My apologies to Dr. Asimov. How’d I mess up that one?)
A quantum computer is not based on a “negative time delay”. That’s just ludicrous. What a quantum computer does, is it essentially handles all possible cases simultaneously, in a finite, positive amount of time. They would, if sufficently complex, be able to do things like solving chess, but they would not use anything remotely resembling time travel to do so.
Of course, the only “quantum computer” yet built can only handle two bits, but they’re working on that. We’ll probably have them in Only Twenty More Years, just in time to be powered by fusion power plants… :rolleyes:
Who knows? Living in NYC, I see a lot of tourists who don’t look like they are living in the present. I just haven’t had the nerve to ask them if they are from the future. Maybe I’ll do that some time when I’m bored in Times Square. 
JCHeckler says
They wouldn’t be everywhere/everywhen. But surely, if they exist, now and in Florida are the time and the place to look for them. Cameras are everywhere down there, unlike previous historic events. It’d be hard for them to avoid getting photographed. Plus, it’s an on-going thing. Dallas, November 1963, who knew? Tallahasse, Nov./Dec. 2000 we know in advance. Tommorrow at the US Supreme Court would be a good oppurtunity, also.
Review the video, find the same person two places at once, you’ve got 'em nailed! Let’s not squander this oppurtunity.