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Time-travel physics: Stranger than fiction
Physicists say future tests could tell whether we can change the past
By Alan Boyle
Science editor
Updated: 3:18 a.m. ET Nov 21, 2006

That time-honored standby of science fiction — traveling back in
time — has come back into fashion in Hollywood, on the big screen
("Deja Vu," premiering Wednesday) as well as the small screen ("Day
Break," which made its debut last Wednesday on ABC).

The plot twists have their parallels in classics as old as H.G.
Wells' "The Time Machine" (1895) and as recent as Bill Murray
in "Groundhog Day" (1993). But is there really anything new under the
sun in time travel lore, or are we caught in an infinite time loop?

Despite years of debate, scientists still haven't completely ruled
out the possibility of going back in time. "Many physicists have a
gut feeling that time travel to the past is not possible," said
Columbia University theoretical physicist Brian Greene. "But many of
us, including me, are impressed that nobody's been able to prove
that."

Over the next few years, some experiments hold out a chance of
finally being able to show whether or not time can move backward as
well as forward. Theoretically, at least, it might be possible for
the future to influence the past, said John Cramer, a physicist at
the University of Washington. He and his colleagues plan to try just
such an experiment next year.

Cramer acknowledged that the concept of retro-causality doesn't seem
to make sense, "but I don't understand why not."

Both Greene and Cramer know the science as well as the fiction side
of the time-travel issue: Greene is the author of "The Elegant
Universe," a best-selling book on string theory — but he also played
a cameo role in "Frequency," a time-travel movie released in 2000,
and served as a scientific consultant for "Deja Vu."

"It was a kick to be in the room with [producer] Jerry Bruckheimer
and [director] Tony Scott and the writers, talking about special
relativity and general relativity and wormholes," he told MSNBC.com.

Cramer, meanwhile, has done research into ultra-relavistic heavy-ion
physics at CERN and Brookhaven National Laboratory — but he's also
written two science-fiction novels and pens a regular column for
Analog magazine called "The Alternate View." If his experiments show
that retro-causality is a reality — that one event can determine the
outcome of another event taking place 50 microseconds earlier — it
could lend support to the ultimate alternate view of quantum physics.

"It opens the door to doing all kinds of really bizarre things," he
said.

It's all relative?
Over the past 100 years or so, physicists have come to understand
that time travel is all relative: In a sense, we're all traveling
through time, and depending on your reference frame, some would seem
to be doing it more quickly than others. For example, astronauts
returning from a space station mission might find that their watches
were a few nanoseconds behind earthly timepieces, thanks to
relativity.

But most time-travel plots involve more than just slowing down or
speeding up the forward pace of time. What we're talking about here
is reversing time's flow, and perhaps influencing the stream of
causality to follow another course: one in which, say, Hitler died in
childhood, or 9/11 never happened, or Britney Spears stayed happily
married.

If you assume that such reversals are possible, Greene said physics
would allow for two possibilities:
# Nature would conspire against changing causality, something
Cambridge physicist Stephen Hawking has called the "chronology
protection conjecture": For example, if you tried to shoot your
father before you were born, somehow the gun would fail to go off.
# Causality can be changed, sending the universe down different forks
in the road. You could go back and shoot your father, creating a
universe where you were never born. But it wouldn't be the same
universe you came from. You'd just be an alien visitor from a
different reality, living out a scenario that's called the "many-
worlds interpretation."

Without giving away the plot, Greene said that the writers of
the "Deja Vu" movie "took a very creative approach. ... They
said, 'This is an open issue. Let's allow both of these possibilities
to have a little bit of play.'"

Reality checks are in the works
Sometime next year, Cramer is hoping to nail down one of those
possibilities — or at least find out why those possibilities are
actually impossible.

The experiment he's devised to test for backward causality plays off
the idea that the states of two photons can become "entangled": Even
if the photons are separated by great distances, what happens to one
photon is reflected by the other one.

Einstein called this behavior "spooky action at a distance," and held
it up as evidence that quantum physics was wrong. However, in recent
years physicists have shown that quantum entanglement is indeed an
actual phenomenon.

Cramer and his colleagues propose using a special lithium iodate
crystal to create two streams of photons. First they would conduct
what Cramer called "a rather simple tabletop experiment" to
demonstrate quantum entanglement with a two-slit screen and two
detectors, much as other researchers have done already.

If they're successful with the first experiment, they would try a
more elaborate demonstration: One stream would be sent directly
through the screen to a detector — while the other would be split by
the slits, then take a detour through two 6-mile (10-kilometer)
lengths of optical cable before reaching a second detector.

The researchers would adjust the position of the second detector to
find the spot where the stream changes from a fuzzy diffraction
pattern to a more defined, wavy interference pattern. Theoretically,
because the streams contain entangled photons, the first detector
should register the same changes 50 microseconds earlier.

Cramer said some unanticipated factor could spoil the experiment he's
planned out. "It's fairly likely that we've missed something
someplace that is going to prevent this kind of measurement from
being possible," he told MSNBC.com.

But if the entangled photons act as theory dictates, that opens the
door to the kind of paradoxes usually found in time-travel
fiction. "What happens if you receive the signal that's supposed to
be sent 50 microseconds later, and you decide not to send it?" he
asked.

In the end, Cramer might find new twists in quantum theory that make
retro-causality impossible. "You can't have inconsistent time loops,"
he said. "It's called a bilking paradox. ... The general consensus is
that nature refuses to be bilked."

But sometimes nature turns out to be stranger than fiction.

More fuel for fiction
Over the past few years, advances in quantum theory and string theory
have provided plenty of additional fuel for science-fiction
speculations. One emerging view holds that time as well as the three
spatial dimensions we perceive are all embedded in a higher-
dimensional space. If this is so, the universe next door could be on
another "brane" just a fraction of an inch away — but in the
direction of another dimension we can't sense directly.

Greene says physicists are just beginning to work out the larger
implications of brane theory.

"None of it has been parlayed into any stunning revelations about
time quite yet," he said. "I think that's the next revolution."

Greene is looking forward to results from Europe's Large Hadron
Collider, due to begin operation in 2007, as well as data from the
Wilkinson Microwave Anisotropy Probe and the Planck satellite, which
are studying the afterglow from the universe's first moments of
existence.

Such observations were unlikely to provide direct evidence whether or
not time can flow backward as well as forward. However, if they point
to the existence of new classes of supersymmetric subatomic
particles, or extra dimensions, that would tell physicists that the
workings of time — and the universe itself — are more mysterious than
scientists thought.

"What they would do more directly is allow us to understand these
questions about the origin of the universe," Greene said. "Did time
begin with what we consider to be the universe? Or is our universe
just one bubble in this big bubble bath?"

An earlier version of this report listed the incorrect network
for "Day Break."
© 2006 MSNBC Interactive

URL: http://www.msnbc.msn.com/id/15817394/
© 2006 MSNBC.com