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."
So this is saying that I couldn't go back in time to the night of Bill Clinton's conception and scream "please no, don't do it" at his father?
If TERMINATOR can do it, so I can I!!!
Einstein objected to that based on physics grounds, but Goedel's solution is valid.
"So it may be technically possible, extremely expensive and utterly useless. Sounds like a gov't program."
It would be invaluable in determining cause. Terrorist attacks, crime...
"We could all occupy the same space and never bump into each other, yet still remain well within the bounds of the speed of light."
We're a self-absorbed lot. Out of sight, out of mind.
Ooooh. That'll be good.
There can't be parallel universes or as he said "Thats why they call it a UNI-Verse" :)
Good point, but it would also obliterate privacy. I read a story once, might have been Bradbury. There was a machine that supposedly let you view the past. You could dial up, say Congress on July 4, 1776 and see what happened.
But it turns out that you could also dial up your neighbor's bedroom three picoseconds ago.
SD
"So this is saying that I couldn't go back in time to the night of Bill Clinton's conception and scream "please no, don't do it" at his father?"
It's saying that you could do it, but you'd find that Slick still got pleasured by Monica, while he talked on the phone about the urgent military situation in Bosnia.
I think you know what I was trying to say better than I do...
ping
Time travel is possible, but not probable since you would have to move your mass backwards or forward to the point that the Earth was in during the time period you wish to visit and then back to the point and time of origin.
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.
Now that's funny!
This is a textbook example of what happens when you give immature physicists with no social lives (i) plenty of time on their hands and (ii) access to alcohol.
"This is a textbook example of what happens when you give immature physicists with no social lives (i) plenty of time on their hands and (ii) access to alcohol."
LOL. Sounds like a blast. Sign me up.
No. Although I occasionally make mistakes, there is no grammatical error in that sentence. Was it just too long for you?
There was nothing of substance in that article.
While nothing in it is fundamentally new thinking, what it says is quite substantial. I'd say it is the prevailing view of scientists who think about the meaning of closed, time-like loops which GR does not prohibit.
Such as Randolph Hearst becoming President of the US?
I already read this yesterday. Re-post it for the day before.
I'm going to put in a sell order for Enron at 90 1/8.
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