Posted on 06/17/2005 12:06:22 PM PDT by LibWhacker
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 other words, you can pop back in time and have a look around, but you cannot do anything that will alter the present you left behind.
The new model, which uses the laws of quantum mechanics, gets rid of the famous paradox surrounding time travel.
Paradox explained
Although the laws of physics seem to permit temporal gymnastics, the concept is laden with uncomfortable contradictions.
The main headache stems from the idea that if you went back in time you could, theoretically, do something to change the present; and that possibility messes up the whole theory of time travel.
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.
So either time travel is not possible, or something is actually acting to prevent any backward movement from changing the present.
For most of us, the former option might seem most likely, but Einstein's general theory of relativity leads some physicists to suspect the latter.
According to Einstein, space-time can curve back on itself, theoretically allowing travellers to double back and meet younger versions of themselves.
And now a team of physicists from the US and Austria says this situation can only be the case if there are physical constraints acting to protect the present from changes in the past.
Weird laws
The researchers say these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time.
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.
So, if you know the present, you cannot change it. If, for example, you know your father is alive today, the laws of the quantum universe state that there is no possibility of him being killed in the past.
It is as if, in some strange way, the present takes account of all the possible routes back into the past and, because your father is certainly alive, none of the routes back can possibly lead to his death.
"Quantum mechanics distinguishes between something that might happen and something that did happen," Professor Dan Greenberger, of the City University of New York, US, told the BBC News website.
"If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him.
"But if you know he is alive, there is no chance you can kill him."
In other words, even if you take a trip back in time with the specific intention of killing your father, so long as you know he is happily sitting in his chair when you leave him in the present, you can be sure that something will prevent you from murdering him in the past. It is as if it has already happened.
"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind," said Professor Greenberger.
"You wouldn't be able to kill him because the very fact that he is alive today is going to conspire against you so that you'll never end up taking that path leads you to killing him."
Kewl. Does this mean I can travel back in time to 1980, to the night I met my ex-wife, and yell at myself "RUN, YOU IDIOT! RUN!"
"Any reliance on SciFi writting is laughable to the point of being ignored."
You're being a bit over-serious, to the point of dour, here. The author's reference was tongue-in-cheek, specifically due to the science fiction popularized in books and film.
The Earth and solar system are moving thru space at about 13 miles per second(IIRC) around the hub of the galaxy which is also moving thru space at an even faster speed. The positional calculations alone are a real bear, and when you look at the energy requirements, and that you have to take that energy supply and a computer, with you to get back, it gets unsurmountable real fast.
For those and other reasons we won't be time traveling anytime soon, if ever.
I just wish I could send a small piece of paper backwards to myself, about 30 minutes, with 6 numbers on it.
"You can't travel into the past. You can only travel into the future."
Then the big question is "can you get back?"
"You wouldn't be able to kill him because the very fact that he is alive today is going to conspire against you so that you'll never end up taking that path leads you to killing him."
Well! So much for my plans for father's day!
(ducking)
"Then there's that aforementioned predetermination argument. Am I really making choices, or do I only think I'm making choices?"
If you think you're making choices, then you are. That your choices were known beforehand changes nothing.
You could, but your wealth today has already been set. You would not be richer than before you went back to the past in order to alter the present (which then was in the future).
But, you could travel to the *future* and buy a copy of the Wall Street Journal, travel back to the present and invest wisely. Since you travelled to the future and now know with certainty what will happen, quantum mechanics says that the future, now, cannot be altered. Just make sure you do not meet yourself, else your future will also be already determined.
Now where's that Tylenol.
"you would have to move your mass backwards"
Bass mackwards? "Hey YOU!!! Move your mass!!!" LOL.
Wasn't this the theory used in the most recent movie "The Time Machine".
The guy creates a machine to go back in time to prevent his fiancee from dying, but she keeps getting killed, in different ways, EVERY TIME.
Turns out, the only reason he created the machine was because of her death. His existence in the past meant that SHE HAD TO DIE. Very clever.
So it is impossible to change anything. But then by simply going into the past you would be changing something, the air you breathe ?
You guys are kidding, right ?
Say what? What a stupid article.
"Since you travelled to the future and now know with certainty what will happen, quantum mechanics says that the future, now, cannot be altered."
Careful, the quantum cyber-harpies might swoop down and call you a Luddite idion, LOL. Or would that be a neo-Luddite?
This is apparently related to the laws of genetics, to wit: If your grandparents never had children, neither will you.
If you could find a way to generate 1.21 gigawatts of electricity. And have a DeLorean.
I travel into the future all the time. One second at a time.
"idion"
That would be idiot, idiot.
Just to be fair, they did kill Zarquon, Lifnerd, and Bullawayo -- and they were far worse.
"I travel into the future all the time. One second at a time."
Well, no. You're riding on the back of a now, which has always marched forward.
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