No paradox for time travellers

[IChing]

NewScientist.com news service
Mark Buchanan
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Department of Physics, City College of New York
Karl Svozil, Vienna University of Technology
Greenberger, Svozil paper

THE laws of physics seem to permit time travel, and with it, paradoxical situations such as the possibility that people could go back in time to prevent their own birth. But it turns out that such paradoxes may be ruled out by the weirdness inherent in laws of quantum physics.

Some solutions to the equations of Einstein's general theory of relativity lead to situations in which space-time curves back on itself, theoretically allowing travellers to loop back in time and meet younger versions of themselves. Because such time travel sets up paradoxes, many researchers suspect that some physical constraints must make time travel impossible. Now, physicists Daniel Greenberger of the City University of New York and Karl Svozil of the Vienna University of Technology in Austria have shown that the most basic features of quantum theory may ensure that time travellers could never alter the past, even if they are able to go back in time.

The constraint arises from a quantum object's ability to behave like a wave. Quantum objects split their existence into multiple component waves, each following a distinct path through space-time. Ultimately, an object is usually most likely to end up in places where its component waves recombine, or "interfere", constructively, with the peaks and troughs of the waves lined up, say. The object is unlikely to be in places where the components interfere destructively, and cancel each other out.

Quantum theory allows time travel because nothing prevents the waves from going back in time. When Greenberger and Svozil analysed what happens when these component waves flow into the past, they found that the paradoxes implied by Einstein's equations never arise.
>>>>snip http://www.newscientist.com/article.ns?id=dn7535

[coloradodem2005]

1. So, I couldn't go back in time to 2000 to prevent Bush from being Pres.

That sucks.

[Mythsaje]

2. We're talking

multiple timelines here, aren't we? If you go back in time to change the past, you're simply creating another timeline with the altered reality?

That's the way I look at it, anyway.

[IChing]

4. Sorta like that SF show with the alternate earths (?)

In string theory, alternate dimension are possible within the matrix of the string
but from my understanding those dimensions are small.


The bosonic string :http://superstringtheory.com/


for a string propagating in flat 26-dimensional spacetime with coordinates Xm(s,t) can give rise to four different quantum mechanically consistent string theories, depending on the choice of boundary conditions used to solve the equations of motion. The choices are divided into two categories:
A. Are the strings open (with free ends) or closed (with ends joined together in a loop)?
B. Are the strings orientable (you can tell which direction you're traveling along the string) or unorientable (you can't tell which direction you're traveling along the string)?


"If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger.


so I think these must closed ends strings?

[Cerridwen]

3. Help with the rest of the article, please

It states

"Waves that travel back in time interfere destructively, thus preventing anything from happening differently from that which has already taken place (www.arxiv.org/quant-ph/0506027). "If you travel into the past quantum mechanically, you would only see those alternatives consistent with the world you left behind you," says Greenberger."

Does that mean I wouldn't be able to change anything or that I would be able to change those things consistent with my current time-line thereby creating some sort of a detour to my current time line?

OW! I think I strained a brain cell.

[Greyhound]

5. Kick for more.

:kick:

[Salviati]

6. It's essentially saying that the only type of time travel allowed

is the history fulfilling type of time travel. The kind where you can go back in time and do things, but only because that is what you've done already. This is the kind of time travel in "12 Monkeys", as opposed to the kind of time travel where you can change events that have already taken place, i.e. like "Back to the Future"

[Meldread]

7. The way I understand it...

...and it makes pretty good sense. I understand it to say that if you were to travel back in time, regardless of what you do, you'd never be able to get back to our "world". So, for example, if I stepped into a time machine and went back in time it would work, I would disappear, I'd just never be able to get back. I could be successful in my goals for traveling back in time, and then travel into the future, but my "future" would not alter what has already happened in this world. I'd be in a totally different parallel universe.

Of course, it might be possible to get back to this universe, but even if I could whatever I did in the past wouldn't have changed anything.

It explains why we haven't seen time travelers from the future as well. It seems to go along with what I understand about String Theory, and makes pretty good sense.

Of course, I thought this was old news.

[sutz12]

8. Are you sure....

that we've never seen time travelers from the future? ;)

[htuttle]

9. Sure we have -- they're UFOs

There ain't no difference between a flying saucer or a time machine.

People get so hung up on specifics, they miss out on seeing the whole thing. Take South America for example. Every year in South America thousands of people turn up missing. Nobody knows where they go. They just disappear. But if you think for a minute, realize something: there had to be a time when there was no people right? Well, where did all these people come from? I'll tell you where: the future. Where did all these people disappear to: the past. How did they get there? Flying saucers, which are really, yeah, you got it: time machines.