About Time Travel

[toss]

"If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger.

Can anyone clarify this sentence?

Article

[drudman]

Right now is time T. You can do anything you please for the next 24 hours. At time T+24h, you travel back to time T. Now, you find yourself at the same T you were at 24 hours ago. However, you can no longer do anything you please, because your actions the first time through created time T+24h. Whatever you do now, back at time T, for the next 24h cannot cause a paradox. I.e. the contention is that you cannot possibly do anything that will prevent time T+24h from happening exactly as it did. If you believe in chaos theory, you basically can't do ANYTHING other than exactly what you did, because even the slightest deviation could have an effect on time T+24h.

Did that make sense?

[Piper Tim]

Although off topic, a good time travel book, which deals with this question, in a way, is the Time Traveler's Wife. It is a good read. Highly recommended.

[toss]

Wait, does this mean we lose free will or are we even two separate entities when we travel back into time to meet ourselves?

And I think I'll check out that book.

[drudman]

Free will doesn't really enter into it, although in retrospect my analysis made it seem like it did. On the quantum level, there are a large number of possibilities, but as we, the observers, "observe" the actual outcomes of quantum events, we shrink the number of possibilities down to zero. The idea is that if you travelled back, the possibilities have already shrunk to zero, because you have already observed them so there would be no alternatives.

Now, say you were to go into absolute isolation, observing virtually nothing but the inside of a darkened room. If you then went back, there would, in theory, be a large number of possibilities once again, because you would have observed none of the outcomes.

The view that is presented in the first post of this thread is flawed, IMHO. It is avoidable completely if you subscribe to many-worlds hypothesis.

[toss]

I see, the probabilty of the past changing becomes zero. And the many-world hypothesis is another theory. An infinite amount of worlds.

[hbaromega]

Near the bottom of the article there is a link to a paper that I found useful in understanding the basic concept. I skipped most of the math in the middle but the conclusion was very interesting.

Basically we have a set of "choices", but once we make them and arrive in a state, we would only have the choices that allow us to get there if we went back in time.

[wheeler]

Didn't read the article but this is an old idea.

Easiest way to think of it is via the grandfather paradox: imagine you go back in time and are about to shoot your grandfather. Clearly, if you succeed in killing your grandfather he wasn't around to give birth to your mother etc., which would be contradictory. So something must *necessarily* go wrong, even though you'd think such things would have only a small probability of happening: either you trip over and drop the gun, the bullet jams, or the gun backfires etc. etc.

And if you did get the bullet off, it would diffract around or tunnel through your grandfather!!

[Quaalude]

[ QUOTE ]
Free will doesn't really enter into it, although in retrospect my analysis made it seem like it did. On the quantum level, there are a large number of possibilities, but as we, the observers, "observe" the actual outcomes of quantum events, we shrink the number of possibilities down to zero. The idea is that if you travelled back, the possibilities have already shrunk to zero, because you have already observed them so there would be no alternatives.

Now, say you were to go into absolute isolation, observing virtually nothing but the inside of a darkened room. If you then went back, there would, in theory, be a large number of possibilities once again, because you would have observed none of the outcomes.


[/ QUOTE ]

Incorrect. It doesn't matter what you have personally observed. According to the theory, once the past has occured, it is deterministic, it cannot be changed.

[ QUOTE ]

The view that is presented in the first post of this thread is flawed, IMHO. It is avoidable completely if you subscribe to many-worlds hypothesis.

[/ QUOTE ]

No, it isn't. From the paper. :

"The model also has consequences concerning the many-worlds interpretation of quantum theory. The world may appear to keep splitting so far as the future is concerned. However, once a measurement is made, only those histories consistent with that measurement are possible. In other words, with time travel, other alternative worlds do not exist, as once a measurement has been made confirming the world we live in, the other worlds would be impossible to reach from the original one."

The paper also states, "Of course, there is an equally likely explanation, namely that going backward in time is impossible. This also solves the paradox byavoiding it," which is my preffered explaination.

-Travas

[drudman]

You're wrong, I'm right.

It does matter, from a quantum point of view, that you observe. The past does not occur, that is to say all of the wave equations remain an large set of possible outcomes, until they are observed. Google Schrodinger's Cat, or Quantum Suicide Machine.

The second refutation you made is also wrong. Many-worlds hypothesis makes it so that you don't go back in time along the path you came, which you seem to imply. Branching of paths does not occur along a linear time.

"Going back" in time is a misnomer, you don't go backwards on the time line, you travel to a different world that is at an earlier time. This removes the problem of quantum determination, because nothing has been determined.

If time is linear, and many-worlds is either not true, or only good in the future direction, then time travel is impossible. So either it's bunk, and the whole discussion is moot, or it's not bunk, and you're wrong. Choose your fate.