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messytime
13th March 2003, 06:59 PM
gday.
i have seen it stated a few times that a time machine could not travel further into the past than the time it was built...could someone please explain this in lay terms?
thanks

rwald
13th March 2003, 07:17 PM
Think of a time machine not as a car, but as a doorway. You can't "drive" into the past, going places where the machine never existed; you can only visit parts of the machine's own past. You can only travel where the time machine already exists.

I can go into greater detail if this isn't clear.

messytime
13th March 2003, 07:24 PM
yes please Rwald,i still don't get it *L*

rwald
13th March 2003, 07:29 PM
OK, I'll use the example of a stable wormhole-based time machine to explain the concept. If you were unaware, a wormhole is basically a connection between two different points in space. You can travel throught the wormhole to get from point A to point B, sometimes transversing less space than actually exists between A and B (this isn't important for our present purposes). Anyway, we've got a wormhole connecting points A and B. Now, let us supposed that we take point B (along with the end of the wormhole), and accelerate it at nearly the speed of light. According to relativity, point B will travel into the future with respect to point A (which stayed stationary). So, if you want to travel into the past, all you need to do is go to point B and hop in the wormhole. Since A is in B's past, going from B to A will move you into the past. However, you clearly can't travel to a time before the wormhole existed. So, you can only travel into the wormhole's own past.

I hope that helped (though it might have just confused matters more).

spoonhandler
13th March 2003, 07:29 PM
I thought this was related to the paradox that if it was possible to develop a time machine, we'd know about it because someone would be bound to come back to our time and let us know. No time machine now = no time machine ever.

I could be wrong...

rwald - are you referring to the need to have the 'doorway' exist in both time points? That is, a mechanism is needed at both the 'entry' and the 'exit' points, rather than a machine that can open a doorway independently?

rwald
13th March 2003, 07:34 PM
Exactly, spoonhandler. All the reasonable proposals for time machines I've seen effectively create a "doorway" between the past and the future. The mechanism can only work if it exists both in the past and in the future.

And the proof that "Time travel can't exist, otherwise where are the future tourists?" is generally used along the same lines as Hawking's Chronology Protection Conjecture (which states that "The universe will protect itself from time paradox by not allowing time machines to be made, thus keeping the world safe for historians"). In other words, as a joke. But you might have already known that.

spoonhandler
13th March 2003, 07:54 PM
Actually, I wasn't aware the paradox thing was considered a joke (I've seen it bandied about by people who sounded serious), but I'm glad to hear it. It always sounded thin to me. :)

DrChinese
13th March 2003, 08:24 PM
There is a very practical reason why it would not be possible to go back to an earlier time on this planet using a time machine: Where - positionally in 3D space - would you go back to?

We live within a 4D spacetime continuum. Well, at least 4D, there may be a lot more. Our hypothetical time machine would really need to be a spacetime machine. The idea being, if you go back in time you plan to remain on this planet. So you need to return to where the planet was at that earlier point in time.

But it turns out that isn't such an easy task. The earth is moving quite rapidly relative to other celestial objects. And its path is anything but a straight line. Some have speculated that the earth could be traveling as fast a 5% of the speed of light relative to some other stars.

No problem, we'll simply go back to our original spot in absolute space. Oops! There is no such thing. Remember the Theory of Relativity? Says something about no preferred frame of reference etc. etc.

I cannot picture ever coming to any conclusion other that our movement through space is both unknown and unknowable. Which makes our time machine pretty much impossible. Or useless.

rwald
13th March 2003, 08:42 PM
You're absolutely right, DrChinese, and this is an important point which I forgot to mention. There is one caveat, however. If you're traveling into the time machine's own past, the question of where (spatially) you'll end up is easily answered: you'll end up right next to where the time machine was at that particular time. So, your argument in effect proves why you can only travel into the time machine's own past.

garys_2k
13th March 2003, 08:59 PM

Who can tell me the flaw in BackTo The Future II that, if exploited, would've made for a MUCH shorter movie?

BillyJoe
14th March 2003, 04:00 AM

Take a time machine into the future and return to the present with details of the Theory Of Everything. Publish the theory in a respectble journal - the one you consulted when you took that time machine into the future.

Someway or another, time travel into the past has to be impossible.

rwald
14th March 2003, 05:32 PM
Garys: Maybe if they just shot old Biff before he gave the book to young Biff? Yea, it'd affect history, but maybe not too much.

BillyJoe: But what if you need a Theory of Everything to build your time machine? ;)

garys_2k
14th March 2003, 06:41 PM
Originally posted by rwald
Garys: Maybe if they just shot old Biff before he gave the book to young Biff? Yea, it'd affect history, but maybe not too much.

BillyJoe: But what if you need a Theory of Everything to build your time machine? ;)
No, just a way that would've killed the plot line to the movie and have the main problem solved in ten minutes. No need to use that whole train gimmick.

rwald
14th March 2003, 06:47 PM
Train? Wait, are you talking about Back to the Future III? That's the one with the train.

garys_2k
14th March 2003, 06:51 PM
Originally posted by rwald
Train? Wait, are you talking about Back to the Future III? That's the one with the train.
DOH! Yes, that's the one! I got 'em mixed up.

rwald
14th March 2003, 06:54 PM
I only saw bits and pieces of that one. Sorry.

datacable
15th March 2003, 05:18 AM
Who can tell me the flaw in BackTo The Future II that, if exploited, would've made for a MUCH shorter movie?
Is this, by any chance, a reference to the gaping temporal logic hole that Marty (and Jennifer) should have arrived in an alternate 2015 in which neither of them had existed for the past 30 years? That one always drove me nuts.

garys_2k
15th March 2003, 05:56 AM
Originally posted by datacable

Is this, by any chance, a reference to the gaping temporal logic hole that Marty (and Jennifer) should have arrived in an alternate 2015 in which neither of them had existed for the past 30 years? That one always drove me nuts.
Well, my original point was for Back To The Future III, but yes, you may be right about that. The future they arrived at never existed because, at the end of the movie, they avoided the accident that determined that very future.

But do you see the flaw in the third movie that would've made the whole train business completely unnecessary?

BillyJoe
15th March 2003, 07:57 AM
Originally posted by rwald
BillyJoe: But what if you need a Theory of Everything to build your time machine? ;) Then you are **********. :cool:

Stimpson J. Cat
15th March 2003, 08:39 AM
Well, they could've pulled a Bill & Ted. You know, the Doc could've decided that after he gets back, he will build another time machine, and send it back to save them. Then presto, a brand-spankin' new time machine arrives...

That's what I loved about Bill & Ted. While being an incredibly silly and pointless movie, it simultaneously managed to point out the huge gaping logic flaw in nearly any time-travel story, which turns the entire thing into a big game of Calvin-Ball.

Dr. Stupid

Latimer
15th March 2003, 10:28 AM
Originally posted by garys_2k

Well, my original point was for Back To The Future III, but yes, you may be right about that. The future they arrived at never existed because, at the end of the movie, they avoided the accident that determined that very future.

But do you see the flaw in the third movie that would've made the whole train business completely unnecessary?

The fact they had a whole buried Delorean to grab parts and gas from, but didn't dig it up, grab the parts, split for 1985, get more parts, and put it back?

Jonathan
-Temporal Klutz-

garys_2k
15th March 2003, 01:38 PM
Latimer got it! Yes, once whatsisname (Michael J. Fox's character) appeard back in the 1800's there was no longer a need to keep the DeLorean buried and hidden away. Just uncover it and either use it or fix the broken one.

Kilted_Canuck
15th March 2003, 02:12 PM
I also heard somewhere that if you could theoretically exceed the speed of light, you would actually be travelling backwards in time. I have to find the source though...

rwald
15th March 2003, 02:41 PM
Well, if you were traveling at the speed of light, you'd be moving into the future infinitely fast. So, I'm not sure if going faster than the speed of light would make you move backwards in time.

Of course, it could be like the function y = 1/x, where x is how close to the speed of light you are (x=0 is when you're traveling at c), and negative values of x represent speeds faster than c. The y value would be how fast (and in what direction) you move in time. If you could somehow get into the negative region, you'd be traveling backwards in time. However, that large asymptote in the middle would make crossing the threshold rather diffucult...

Actually, some theories suggest the existence of "tachyons," particles which travel faster than the speed of light and move backwards in time. For tachyons, the speed of light is still the universal speed limit -- they can never go slower than it, or even reach it. So, even if moving faster than light would let you move backwards in time, it's still impossible for baryonic matter (what you're made of) to move faster than light.