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 if your father goes missing at some point and you can't remember where you were or what you were doing at some time when he was missing it is possible to go back in time and kill him, otherwise there is no possiblity that some dope can go back and save JFK only to see a nuclear war start between the US and USSR.
Science has now proved predestination…
Yes, but when you return, everyone will be a vegetarian.
No you science-hating luddite idiot. This is a hypothesis, not a theory tested by experiments. Furthurmore it concerns the past and the present saying nothing about what will happen in the future.
But wheeeee! this traveling forward is a blast!
"I never was comfortable with the idea of an infinite number of possible universes resulting from time travel to the past."
The number of universes ceases to be infinite once one has been "fixed" into place. So, if time travel is at all possible, know that you're destined to do it, if an older version of you ever pops in to say "hi." And nobody will be erased from existence due to your having done it. I still wonder why this would be the case, if there is no deity, though.
Only if you are a female democrat member of the Arkansas Mafia.
Ok, now where can I get my hands on a Tardis?
Awesome!
Sign me up!
"No you science-hating luddite idiot."
Well hi there, buddy. I happen to love science. I only hate the demagogues and ideologues who've been passing themselves off as scientists of late. Theory is taken as virtual "proof" of evolution, so why would this not be taken as proof of predestination?
Does this mean that if Darth Vader had done a lot more faster-than-light travel than his son Luke, that Luke would have been younger than his dad and .... ah if he.... ah I mean the dad had killed the ah no... um the younger guy would have eh... becau...
Does this mean that if Darth Vader had done a lot more faster-than-light travel than his son Luke, that Luke would have been younger than his dad and .... ah if he.... ah I mean the dad had killed the ah no... um the younger guy would have eh... becau...
Fantasy is OK. I've been telling my kids that I'm from the future ever since they were old enough to understand what that meant.
No it isn't because if the past was changed we'd never know. The new past would seemlessly become our past and anyone "erased" would never have existed.
Does this mean that if Darth Vader had done a lot more faster-than-light travel than his son Luke, that Luke would have been younger than his dad and .... ah if he.... ah I mean the dad had killed the ah no... um the younger guy would have eh... becau...
The point is that people wouldn't suddenly diappear in front of you if their grandparents were killed before their parents were born. They would simple not even exist in your memory to begin with, since they never would have existed. Any reliance on SciFi writting is laughable to the point of being ignored.
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.
Hmmm... I still don't understand; If someone went back and killed Marx's grandparents, Marx's contemporaries wouldn't suddenly see him "fade into the ether." He just simply would never have existed for them. And we here in the 21st century wouldn't be aware that anything amiss had happened either. In fact, couldn't time travelers be traveling backward in time constantly killing people whose offspring they don't like -- thereby instantly wiping out our memories of those people and anything they might have accomplished during their lifetimes? I just wish they'd get around to Stalin, Mao and Hitler!
I LOVE that Bradbury story - I can't wait to see it on the Big Screen. I'm also anxiously awaiting Gibson's rendition of F451.
The aliens said that you can travel forward in time, but not backwards...
An absurd statement. Our inability to perceive something does not disprove its existence.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.