Time travel and "The Terminator"

[Beerina]

Supposedly, in the Terminator movies, the war against the Machines is led by John Connor, who's father came back through time to procreate him in the first place.

So basically the future is based on the future already happening. But this is not how time travel works..right?

If time travel did exist, and we went back in time and changed things, it would create an alternate time line, with the original time line moving forward as if nothing happened...right?

And you can't have a reality based on time travel having already happened..in the future..since the future has not happened yet..right?

Where is Doc Brown when you need him???

[qimg]http://thm-a01.yimg.com/image/a286060fd8f01924[/qimg]

An "alternate timeline" is a hypothesis to explain a difficult problem. Since you can't go back and kill your grandfather because then you wouldn't be alive to go back and to it so he'd be alive so you would be born after all so therefore you can go back and kill him and...


But this is only a paradox in a universe with one time axis. If you can go back in time, you're really leaving that time axis and moving out into another one. Then all problems disappear. Destroy the whole original universe if you want. It won't affect you. You've moved on to a different coordinate in a 3-D, 2-T, 5 dimensional universe.


I don't know why people have to make things so difficult!

 

[Aitch]

Similar concent in Asimov's The End of Eternity, IIRC.

 

[Vorpal]

Supposedly, in the Terminator movies, the war against the Machines is led by John Connor, who's father came back through time to procreate him in the first place.

So basically the future is based on the future already happening. But this is not how time travel works..right?

Well, maybe it does. In the Terminator storyline, John Connor et al fail to stop the apocalyptic Judgment Day, and JD itself is apparently necessary for John's very existence.
That sort of fatalistic situation is a bit analogous to the "closed timelike curve" solutions of general relativity, in which something can interact with its past but not change it, because the interaction has already happened. So in this sense Terminator version of time travel is slightly more realistic than that of some other sci-fi stories. (Although in the storyline they apparently do manage to change some details, so the analogy is very loose.)

P.S. Sol doesn't agree that CTCs constitute genuine time travel, but although I understand his reasons and that it isn't as exciting as his preferred version, I don't see why it deviates from the general idea of time travel. This is, however, little more than an issue of semantics.

 

[HeyLeroy]

What I want to know is: Where does their clothing go?!

Dunno about the rest, but their socks end up in my dryer.

 

[Singularitarian]

Supposedly, in the Terminator movies, the war against the Machines is led by John Connor, who's father came back through time to procreate him in the first place.

So basically the future is based on the future already happening. But this is not how time travel works..right?

If time travel did exist, and we went back in time and changed things, it would create an alternate time line, with the original time line moving forward as if nothing happened...right?

And you can't have a reality based on time travel having already happened..in the future..since the future has not happened yet..right?

Where is Doc Brown when you need him???

[qimg]http://thm-a01.yimg.com/image/a286060fd8f01924[/qimg]

There is a chronological principle by Hawking which suggests that time travel to a point before the time machines creation is impossible, because the machine has not yet been created yet. Moving on... yes, if time travel was actual and if the human being can even endure such a trip in time, we could certainly alter the possibilities in the timeline, so that you may even change events located in the future or pasts (present times). Suffice to say, i don't believe time travel is possible on the scales we normally desire, but we actually time travel everyday when we jump in our cars to go to work; even at the small speeds we move at, we dilate time along the way, distorting the fabric every passing moment. That last point is never really considered, or missed out.

 

[Ziggurat]

Time travel within one universe would cause equally severe difficulties, both with the laws of thermodynamics and every other law of physics (particularly conservation of energy).

There are possible ways around the energy and momentum conservation problem: when you project something to another time, grab something from that time and pull it back to now to balance it. Entropy is solvable too, if the time machine generates enough entropy in the process.

 

[sol invictus]

There are possible ways around the energy and momentum conservation problem: when you project something to another time, grab something from that time and pull it back to now to balance it. Entropy is solvable too, if the time machine generates enough entropy in the process.

Maybe you could balance energy that way. But I think there's a more basic problem, one which you're not going to be able to get around.

Energy conservation follows from the time-translation invariance of the laws of physics. Energy is the charge associated with the Hamiltonian, and it's the Hamiltonian which generates motion through time. A good Hamiltonian is hermitian, which implies that time evolution is unitary, which in turn means time evolution is unique - time lines never bifurcate or merge. But that means you can't have time travel in the Terminator sense, where you go back and change the past.

Those various properties are all so closely entwined that I strongly doubt you'll be able to change one without changing all the rest. But I admit it's not a completely solid argument.