http://www.newscientist.com/article.ns?id=dn7535


No paradox for time travellers

    * 10:00 18 June 2005
    * NewScientist.com news service
    * Mark Buchanan


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. 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.

"This is a very nice idea," says physicist Avshalom Elitzur of the Weizmann
Institute in Rehovot, Israel, who also suggests that further work in the
area could help to clarify the nature of time itself. "Time is a very
mysterious thing."