Time Travel

[Momotaro]

I've always been bugged by the concept of time travel. It is so preposterous to me that its really hard for me to enjoy stories that use it, even if they're otherwise interesting.

Off the top of my head I can think of the Terminator movies, Planet of the Apes, and many episodes of Star Trek. In both case it seems to me that the future justifies itself by its influence on the past. I can think of so many other logic problems that I don't understand how the concept has not been disposed of already.

I still hear from time to time about people saying time travel would be scientifically possible. I only need one word: causality. I don't know of a single story that uses the concept of time travel logically. Of course you can only hope so much out of work of fictions, but how come the concept is still going around like it has any validity.

Does anyone have a valid theory of how time travel would be possible? An explanation as to why it can't be ruled out would be fine too. Do you think time travel belongs in the same category with cold fusion and Ether?


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Only humans commit inhuman acts.

[Temujin]

Please step into my time machine:
http://www.straightdope.com/ubb/Forum3/HTML/001461.html

[Momotaro]

Oh.

In any case, that just confirms my belief. I honestly tried to find it with a search, but I guess I don't know how to use it 'cause I didn't even come close to something that was relevant.

[Temujin]

Don't worry! The problem is not with your ability to do a search; the problem is with the search engine:
http://www.straightdope.com/ubb/Forum2/HTML/000152.html

In any case, this is still a valid thread (even if a similar one already exists). I included the original link in case you weren't aware of it. I wasn't trying to nip this discussion in the bud.

In direct response to your OP, I think the answer to your final question is "yes."

[RealityChuck]

Yes. Causality is lost if you have time travel. That's one reason why scientists don't like the concept. It's also a reason why science fiction authors love the concept.

However, there are conditions where time travel seem to have a solid theoretical basis, though in order to take advantage of the conditions would be economically unfeasable.

There are many first-class time travel books and stories that handle the paradoxes well; your mistake is going to movies instead of the literature. Off the top of my head, I'd suggest the stories "Hobson's Choice" or "The Men Who Murdered Mohammed" by Alfred Bester, "The Weed of Time Bears Bitter Fruit" by George Alec Effinger, all of which give explanations ranging from logical to fanciful for why the paradoxes don't occur. David Gerrold's THE MAN WHO FOLDED HIMSELF probably covers the problems and paradoxes with the most rigor. Another good choice is MILLENNEUM by John Varley, with serves up a good adventure that takes into account the possible paradoxes (don't see the movie, though, which is really second rate).

You also should read "Sundials" in the next issue of ABORIGINAL SF.


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www.sff.net/people/rothman

[AWB]

I once had an excellent SF novel about time travel. Unfortunately, I can't recall the title or author. (My roommate loaned it to someone who never returned it. Grrr...)

In it, every time someone went back in time, they created a separate branch of reality. But, if they came prepared, they could create a gateway between the two realities, even across time.

It started in the 1970's, with the US still fighting WWII. President J.F. Kennedy, and his advisors determine that someone with time travel techology helped the Germans in the 1930's with rocketry and electronics. Finding some of the time travel equipment, a team goes back to help the Allies. But once back, they find that they can't return to the 1970's they knew.

Does this story sound familiar to anyone?

AWB, I don't recall reading the specific story you speak of, but something with a similar plot is Lightning by Dean Koontz, who comes up with a neat wrinkle. In his book, the Nazis develop time travel, but only to (and from) the future.

[moriah]

Saying "causality would be broken!" as a reason why time travel can not take place is circular argumentation. Causality and forward movement through time are two sides of the same coin. It would be like saying "you can't time travel because that would be breaking time!"

Relativity shows that anything with mass necessarily moves forward through time as a function of having mass -- in normal space-time. The 'normal space-time' is the important part. Theoretical speculation and calculations show that it may be possible, with the right amount of mass and the right type of mass in the right configuration to twist space-time so much that you travel through the 'wormhole' to a point in space-time that is actually pastward (same place but earlier) of where you once were in space-time.

Thus, it would be, in fact, time travel (through spatial travel in twisted-up time-space). And thus, causality can be broken. Causality would be twisted up as much as the wormhole has twisted up space-time. Just as a line in twisted space-time can become a loop, so too can causality become loop (the classic time-loop paradox).

Peace.

[Atrael]

Another good S/F book about this is The TimeGod by L. E. Modesitt Jr.

Deals with time travel from the point that the characters can't travel back and forth on their timeline, but can visit other planets at different points in time...to collect different technology....the book is so-so, but I liked the concept behind it.

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<i>I haven't lost my mind, I have a tape backup around somewhere.</i>

[tanstaafl]

AWB - The Proteus Operation by James P Hogan.

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"You can't run away forever; but there's nothing wrong with getting a good head start." --- Jim Steinman

Dennis Matheson --- Dennis@mountaindiver.com
Hike, Dive, Ski, Climb --- www.mountaindiver.com

[rjk]

RealityChuck gives a good start at a bibliography but missed a couple: one that might be the best paradox story ever, All You Zombies by Robert Heinlein, and one of the best-written, The Garden of Forking Paths by Jorge Luis Borges.


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Bob the Random Expert
"If we don't have the answer, we'll make one up."

[Undead Dude]

Saying "causality would be broken!" as a reason why time travel can not take place is circular argumentation. Causality and forward movement through time are two sides of the same coin. It would be like saying "you can't time travel because that would be breaking time!" -- moriah

Well, not totally. But to avoid that, you need to accept one of two principles, neither of which are very acceptable scientifically. You could accept that all events are predestined, thus time-travel will only result in previously established causes. You can also accept that there is some sort of magical causality protecting nature of the universe that protects us from our murderous posterity.

Of course these ideas don't generally fly to well.

It seems that every vaguely reasonable time travel theory that I have heard revolves around wormholes, and they are all major long shots.

Carl Sagan threw a time travel idea out in the '80s, that is conceptually fairly simple. Of course it sure as heck isn't simple in the practical sense.

Here's Sagan's idea

First you need a stable wormhole that somehow you can travel trough without getting ripped to pieces. That's a pretty severe condition as it is, that probably can't happen.

Second, you need to have the ability to move one of the ends of a wormhole in circles at close to the speed of light. Your pilot enters the wormhole at the non-moving end and returns at the moving end. If the distance between the two ends of a wormhole are close enough, the pilot could emerge at an earlier time and get back to his starting point before he started.

Of course Sagan didn't even begin to suggest a way of moving a wormhole at close to the speed of light. This would be comparable to moving a black hole around at nearly the speed of light.

[RealityChuck]

I also forgot one of the most memorable of all (with the most ummemorable title, since everyone asks about it): Ray Bradbury's "A Sound of Thunder."

Another great Time Travel (sort of) is Ken Grimwood's REPLAY, which takes the question "What if you could live your life over?" and comes up with some surprising answers.

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www.sff.net/people/rothman

Anyone here remember "The Lincoln Hunters"?

[Keeves]

AWB - are you thinking of the second PHILADELPHIA EXPERIMENT movie?

[Maureen Birnbaum]

Sure, I remember.

"The Weed of Crime Bears Bitter Fruit" forms the first part of THE NICK OF TIME or THE BIRD OF TIME, I forget which one. And I wrote both of them. A long time ago.

(signed) "Time is the simplest thing/unless your short-term memory has been misplaced"

[Milton De La Warre]

I believe Stephen Hawking suggested time travel must be impossible (now and forever) because if it weren't, we'd be seeing a lot of tourists, explorers, etc. from the future now, in our present.

[Maureen Birnbaum]

You can't logic your way through time travel. Just because we don't appear to see time travelers, that doesn't mean time travel is impossible.

It probably is, but that doesn't mean we can't write and read stories about it. There's no reason not to write a "first landing on the moon" story, either, just because we've been there and done that. Make it an alternate universe.

Speaking of which, one solution of a bit of Einstein says that parallel universes MUST exist. That's the good news. The bad news is that we can't get to them.

Much like the airport at rush hour.


(signed) "I have enough trouble in three dimensions, especially if there is no escalator"

[Chronos]

The best time travel story I've ever read is "By his Own Bootstraps", by Heinlein. His The Door into Summer is another good one.

By the way, I don't think that Sagan came up with the idea for the wormhole time machine... I believer that was Kip Thorne.

[Saint Zero]

Originally posted by JCHeckler
I believe Stephen Hawking suggested time travel must be impossible (now and forever) because if it weren't, we'd be seeing a lot of tourists, explorers, etc. from the future now, in our present.

Or maybe they have better sense than to interrupt us? Just because something isn't happening doesn't mean it can't happen.

[Caldazar]

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?

[Keeve]

Originally posted by Caldazar
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?

[Sublight]

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.

[Lazarus7]

Time travel is predicted by wormhole theory. A wormhole, if it exists, must also be a time hole.

[bouv]

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:
1) How to build it
2) How to put it in a constant, variable free, enviroment
3) How to rotate it
4) How to travel along the surface without succombing to its immense gravity (which shouldn't be there in the first place, but oh well...)

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.

[Maureen Birnbaum]

Any sufficiently advanced technology is indistinguishable from doubletalk, too.

(signed) "From Room 101"

[DocCathode]

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.

[Keeve]

Originally posted by DocCathode
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.

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.

[DocCathode]

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

[Chronos]

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?

[JasonFin]

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.

[DrMatrix]

Originally posted by JasonFin
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. [/b]

"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.

[JasonFin]

Originally posted by DrMatrix

"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.

[Keeve]

"Negative time delay" sounds an awful lot like Heinlein's "thiotimoline".

Loved that story! Beautiful satire of technical papers as an art form.

[Maureen Birnbaum]

"Thiotimoline" was Asimov's, not Heinlein. How soon we forget.

[Kilraven]

Originally posted by JCHeckler
I believe Stephen Hawking suggested time travel must be impossible (now and forever) because if it weren't, we'd be seeing a lot of tourists, explorers, etc. from the future now, in our present.

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!)

[Keeve]

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?)

[Chronos]

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...

[bpaulsen]

I believe Stephen Hawking suggested time travel must be impossible (now and forever) because if it weren't, we'd be seeing a lot of tourists, explorers, etc. from the future now, in our present.

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.

[ZenBeam]

JCHeckler says

I believe Stephen Hawking suggested time travel must be impossible (now and forever) because if it weren't, we'd be seeing a lot of tourists, explorers, etc. from the future now, in our present.

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.

[AWB]

Originally posted by tanstaafl
AWB - The Proteus Operation by James P Hogan.

Yes, that was it! I'll have to see if I can get a reprint. That was a great story.

[davesink]

OK... I know this thread has been done to death, but one point I seldom heard mentioned re. feasavbility of time travel is the issue of creating something from nothing. When traveling back in time, wouldn't that amount to creating matter, when faced with your old self. I thought matter could be neither created nor destroyed, only change form.

BTW since someone earlier mentioned Robert Heinlein, his excellent short story on the subject, "All you Zombies" can be found at the following link:

http://www.geocities.com/CapeCanaver...5/all_you.html

[ZenBeam]

When traveling back in time, wouldn't that amount to creating matter, when faced with your old self.

Good point. perhaps you'd have to send as much matter an equal amount forward in time, or twice as much matter half as far forward. Or maybe spend some time in the future to balance how much you spend in the past.

Of course, all our conservation laws are only known to hold when time travel is not occurring. Since we don't know the physics of time travel, we can only guess that some form of the conservation laws will hold.

[Phobos]

You can fake-time-travel to the future by travelling at relativistic speeds (a la Twins Paradox). But I assume that this is not what the OP is about.

Backward time travel seems to need a wormhole to work. But wormholes may not exist. IIRC, Kip Thorne and/or Stephen Hawking believe that backward time travel would still be impossible this way because there would be some kind of energy feedback through the wormhole that would destroy you & the wormhole. Unfortunately, I forget the details at the moment. (sorry! IIRC, I think I read about it in the book "Carl Sagan's Universe")

[hijack]
If forward-time-travel is possible, then do you think that would mean that the universe is predetermined?
[/hijack]