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.

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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."

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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."

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