• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

time machine

messytime

Unregistered
M
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
 
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.
 
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).
 
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?
 
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.
 
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. :)
 
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.
 
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.
 
OK, enough about this crazy stuff!

Who can tell me the flaw in BackTo The Future II that, if exploited, would've made for a MUCH shorter movie?
 
How to discover the Theory Of Everything for free....

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.
 
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? ;)
 
rwald said:
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? ;)
Click to expand...
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.
 
Train? Wait, are you talking about Back to the Future III? That's the one with the train.
 
Who can tell me the flaw in BackTo The Future II that, if exploited, would've made for a MUCH shorter movie?
Click to expand...
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.
 
datacable said:

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.
Click to expand...
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?
 
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
 
You must log in or register to reply here.

Back
Top Bottom