New model 'permits time travel'

BBC ^ | 6/17/05 | Julianna Kettlewell

[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."

[RightWhale]

The idea that gravity is leaking out of our universe into a nearby universe is still awaiting laboratory confirmation. The idea might not work out, most don't, but multiverses are entering the domain of public discourse. It's practically mainstream cosmology already.

[LibWhacker]

I think some bumblebum time traveler may have tried to interfere with that conception by getting the old man drunk, hoping bubba senior would flip his car over into a ditch full of water before that particular union was consummated. Unfortunately for humanity, our bumblebum picked the wrong night, and the rest is history. I hope the Dept. of Time Travel fired his incompetent butt!!! ;-)

[rmmcdaniell]

I happen to love science.

I doubt that.

Theory is taken as virtual "proof" of evolution, so why would this not be taken as proof of predestination?

This is a dead giveaway that you are a religious nut. Your types purposefully blur the distinction between hypothesis and theory, attack all fields of science by bringing evolution into debates where it is not relevant, and claim the scientists are saying something they are not.

[RegulatorCountry]

"This is a dead giveaway that you are a religious nut."

LOL. And your statement is a dead giveaway that you are a "nut" of what sort? Highly scientific, this "nut" talk.

[rmmcdaniell]

The entire concept is flawed, unless you are not visible and cannot interact with the environment. Your mere presence could set of an unknown sequence of events, period.

Unless your affect on the past sets off a series of different events that eventualy converges to a "present" universe that is exactly the same as if you weren't there. Multiple paths to the same goal, is that possible?

[perfect stranger]

Hilarious.

[plain talk]

That's fine. Defend your sloppy sentence like Durbin defends his gaffe. That's the trend these days. I don't really care and usually don't get anal about such petty things. Sorry for bugging you about it. Have a nice evening.

[edsheppa]

There's nothing sloppy about that sentence. It's your reading that is sloppy.

[RegulatorCountry]

You are correct that there is nothing "sloppy" about your sentence, but clarity might have been enhanced by adding a comma.

"Usually, if you can't understand something, it's not that thing, that's stupid."

I think the two "thats" confused him.

[plain talk]

Usually, if you can't understand something, it's not that thing that's stupid.

OK. ha ha ha

[spetznaz]

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.

Actually no.

Think of it this way. Let's say we COULD affect the past (and thus the buffer that the physicists are theorizing does not exist), and the following scenario takes place. We have a guy called Joe, and for some reason he wants to take out his great-great-grandfather (who we shall call Ancestor J).

Ok, so Joe hops into his thingimajig TimeMachine and zips to 1855 and shoots Ancestor J with a blunderbuss, and thus no more Ancestor J.

Well, the moment he does that the time-continuum (I CANNOT believe I just said that) will shunt, and thus there will be no great grandfather, no grandfather, no father, and thus no Joe. Hence Joe will immediately cease to exist the moment Ancestor J breathes his last.

Now, the moment Joe disappears into the ether the time continuum will shift again! Since Joe ceases to exist that means he never got into his time machine thingimajig, and thus it means he never went back to 1855, and consequently it means he never killed Ancestor J. And because Ancestor J was not killed he does not die, and because Ancestor J is alive then that means that greatgranddaddy is, as is g.dad, as is dad ....as is Joe.

Hence Joe now exists, and because he exists it means that he is angry with his great great grand dad Ancestor J, and thus he hops into a time machine with a vintage blunderbuss and heads off to kill him.

Now, the moment he terminates Ancestor J then Joe ceases to also exist, and thus he never gets into his time machine, and thus ........

Well, you get the picture.

Just do not ask me why he decides to use a blunderbuss!

[Junior]

Two things. First, this is known as the "grandfather paradox" and there is no reason that the timeline won't shift back and forth between the two and still appear seamless to folks in those timelines. Second, the blunderbuss was about 200 years out of date in 1855.

[goldstategop]

Its a paradox. If you kill your father, you remove yourself from the timeline. Of course no one in their right mind would be crazy enough to kill themselves. All things being equal, quantum mechanics holds there is a parallel universe in which you don't exist. There's a parallel universe where you're a monster. And so forth. Time travel gives one a temporal headache!

(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)

[goldstategop]

Reminds me of that classic scene in Star Trek: The Voyage Home where Admiral Kirk tells a beautiful marine biologist: "I am an admiral. I just happen to come from Iowa!"

(Denny Crane: "Sometimes you can only look for answers from God and failing that... and Fox News".)

[spetznaz]

Two things. First, this is known as the "grandfather paradox" and there is no reason that the timeline won't shift back and forth between the two and still appear seamless to folks in those timelines. Second, the blunderbuss was about 200 years out of date in 1855.

I said no questions about the blunderbuss (LOL). I knew it was way out of date by 1855 anyways. As for the Grandfather's paradox what i was trying to say was that if the changes would be as seamless as you say then the continuous loop inherent in it would basically lead to all affected parties simply ceasing to exist. They would constantly be popping in and out of 'seamless existence' in a perpetual loop, and if people did not notice any changes they would simply cease to exist by the start of the second cycle. A crude analogy would be like making a photocopy of a photocopy, with each copy being a less clear iteration of its master ....with the only difference in this case being that the next full iteration basically fades completely.

Which is why (before this article at least) the consensus among most physicists was that if time travel would basically embody travelling into what could best be described as paralled realities. That is the only was the present reality could stay in situ after one cycle. And while the whole 'multiple dimension' thing had its own set of flaws it at least allowed fluidity of motion when it came to time travel.

Going back to the blunderbuss ....hey, not many people in the last couple of centuries can claim to having had their lights switched off with a blunderbuss.

[patton]

"Knowledge is impotence"

that bears thinking about.

[RightWhale]

Goedel's time loops are physically possible, although there doesn't appear to be the requisite net rotation in the universe.

The ability to travel in time, in that solution, also makes time impossible--an illusion. You want to believe Goedel or your lying eyes?

[thoughtomator]

In order for this to be true, there can be no free will. What if you went back in time, told your father that nothing could kill him (because he was alive in the future), and your father therefore decided to take wanton risks with life and limb in the knowledge that he could not be killed?

[spetznaz]

LOL.

[bannie]

Now, there's a question:

Could we (theoretically) have contact with anyone in this supposed future-travel? What about traveling forward? If we went forward and came back, we could take the same measures (ie: investing, etc.) as we could have done in traveling backwards.

I've never bought time travel: We'd see effects of it now.