Slashdot Mirror
New Model Solves Grandfather Paradox
goldfishy writes "If you went back in time and met your teenage parents, you could not split them up and prevent your birth - even if you wanted to, a new quantum model has stated. Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is 'complementary' to the present. In theory, you could go back in time and meet your infant father but you could not kill him." From the article: "Quantum behaviour is governed by probabilities. Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated."
Sounds like the Novikov self-consistency principle to me.
Please, for the good of Humanity, vote Obama.
"Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious."
Oh yes quite obvious!
Or not.
If you went back in time and changed the past, those in the future would instantly be changed and would have no reccolection of past events being different!
1's and 0's should be free.
Look up "closed timelike loop."
Basically all this article is saying is that all time travel must consist of closed timelike loops. That is, you "fulfill" the present, rather than altering it. This isn't news - it's the only kind of timelike loop that can exist in GR anyway. The difference here is that quantum mechanics also forces them to be the only ones that exist.
Point of note, however: as far as I know, we don't actually have the math to deal with the formation of a topological change in a surface (i.e., the "alteration" of a timeline). This is very much akin to a wave crashing - fluid dynamics works up until the exact point when the top of the wave touches the rest of the ocean. After that point, the math breaks down. So it's a little difficult to say "X isn't possible, because the math won't allow it" when theorists are in fact only using math that won't allow it. So it's moderately circular. That's GR. In QM, we don't actually have the math that deals with the collapse of the wavefunction (the 'measurement'), and so again, it's moderately circular. If you instead suppose that the wavefunction doesn't actually collapse, then of course you can change the past - you just end up following a different course in probability the second time around.
Examples of closed timelike loops actually are more common than you think in modern scifi/fantasy. Harry Potter and the Prisoner of Azkaban had an excellent example of a CTL, and the timetravel used in Anne McCaffrey's Pern series are entirely CTLs. This leads to statements like "I know I can do this, because I've done it already."
The problem with CTLs is that they muck with certain people's belief in free will.
If, for example, you knew a picture would be taken, you could reflect light from your body and appear in that picture, thereby altering the future.
Assuming you didn't exist in the picture before you went back in time.
And it just moves up from there for all other physical effects. Nothing touched, no air breathed, no light disturbed, nothing.
Unless it was already disturbed to begin with.
Again, there's no real logical problem here. Just the fact that you would have to disassociate yourself from the fact that all of your future actions are possible.
I don't necessarily agree with time travel. A closed timelike loop is essentially the equivalent of a monkey popping out of thin air, and then disappearing a few seconds later. It seems idiotic, and completely counter to all natural laws. But that wouldn't be the first time nature did that to us.
You are quite correct, in fact, this is the only way people make money in the stock market.
-- I was raised on the command line, bitch
This is hardly a "new" theory. At best it is a restatement of the decades old theories that are expressed here:http://plato.stanford.edu/entries/time-travel -phys/
Non sequitur: Your facts are uncoordinated.
In the many-worlds theory, if you alter the past a new timeline is created, with your changes in place. If you kill your mother, then the other you doesn't exist, but since he's not the one who went back in time, no problem.
In the self-consistent theory (there might be a better name, but I don't know it), any alterations you make in the past have already been made. They are part of the history that led up to your time travel in the first place. Paradoxes are impossible - the probability of such an event is zero, as it assumes multiple, inconsistent events occur. One way to think of this is as similar to many-worlds, except with no branching - every world which is self-consistent exists, and every one with a paradox does not. While it appears to you you're going back in time to meet/kill/observe your mother, you're in fact just following a closed timelike curve through spacetime. The eventualities in which there is a paradox do not exist - even if you get to the past with killing intent, you will not be able to carry it out. Something will happen to prevent you carrying out your mission, from a simple attack of conscience to a sudden meteor strike.
At least give the authors of the song credit: Dwight Latham and Moe Jaffe.
Anonymous Cowards suck.
You're thinking of "--All You Zombies--". "By His Bootstraps" is a similarly structured story about a man who is visited by future versions of himself, who give him advice. It is also a closed time loop, but I think the one in "All You Zombies" is considerably more convoluted.
The circumstances of the protagonist's conception and birth are an elaborate setup which can exist only because of the interference of the protagonist as an older man in his own past - he is his own mother and father, and in various other ways responsible for his own existence. He feels as if he is the only real person in the world, hence the title: "I know where I came from, but where did all you zombies come from?"