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Faster than light ... again?

The Slashdot set is buzzing over a new experiment that seems to indicate light can move faster than … um, the speed of light. The fact that the last statement sounds so strange hints at the bizarre caveats that surround such experiments, and mainstream scientists have argued for years that the phenomena really don't break Einstein's rules of the relativistic road. At the same time, they admit that the results are pretty darn weird. And weirder experiments are on their way.

The latest controversy focuses on research conducted by physicists Günter Nimtz and Alfons Stahlhofen of the University of Koblenz in Germany. They set up an apparatus using microwaves and two prisms to look for a phenomenon called quantum tunneling - basically, a phenomenon in which a particle can sneak through places where it's not supposed to go and pop up on the other side.

The outcome is described in their research paper and summarized in a New Scientist report. First the two prisms were set against each other with a small gap between them. When the microwaves were sent through the apparatus, most of the signal was reflected internally by the first prism - but some of the signal tunneled through the gap and went through the second prism.

Nimtz and Stahlhofen found that the reflected signal and its quantum-tunneling doppelgänger arrived at their respective photodetectors at the same time. That led them to the conclusion that, in effect, the tunneling photons bridged the gap between the prisms instantly, violating the 186,000-mile-per-second speed limit laid out in the special theory of relativity.

"This is the only violation that I know of," Nimtz is quoted as saying.

The research hasn't appeared in a publication yet. In fact, the online version was just submitted for review a couple of weeks ago. But it's already drawn plenty of comments in the geek world, from A (for Ars Technica) to Z (for ZDNet).

"Unfortunately, the claim is worse than weak; it is silly," Chris Lee writes on Ars Technica.

Others are more cautious in their criticism, but the bottom line is that physics can play tricks when you use quantum phenomena to look for loopholes in relativity. Over the past several years, Nimtz has taken aim at this subject multiple times, and he hasn't convincingly hit the bull's-eye yet.

A seemingly faster-than-light effect can arise from the way a signal is shifted as it travels through different media. The crest of a wave in one medium may become a valley in another medium, and the valley may become a crest. It may look as if that crest has zipped ahead faster than the speed of light - but in actuality, it's just the same old wave with its shape shifted.

We delved into this explanation more fully four years ago and revisited the subject specifically with regard to Nimtz's research in a follow-up. The phenomenon of quantum tunneling makes the latest results even fuzzier, figuratively and literally. To learn more about how physicists play fast and loose with faster-than-light experiments, check out this entry from the Physics FAQ and this discussion of group velocity on the MathPages.

When it comes to assessing the latest faster-than-light research, I particularly like the down-to-earth explanation that New Scientist was given by Aephraim Steinberg, a quantum optics expert at the University of Toronto:

"Steinberg explains Nimtz and Stahlhofen's observations by way of analogy with a 20-car bullet train departing Chicago for New York. The stopwatch starts when the centre of the train leaves the station, but the train leaves cars behind at each stop. So when the train arrives in New York, now comprising only two cars, the centre has moved ahead, although the train itself hasn't exceeded its reported speed.

"'If you're standing at the two stations, looking at your watch, it seems to you these people have broken the speed limit,' Steinberg says. 'They've got there faster than they should have, but it just happens that the only ones you see arrive are in the front car. So they had that head start, but they were never travelling especially fast.'"

For further reflections on faster-than-light experiments and quantum tunnelling, check out this this Alternate View column (and this one) from University of Washington physicist John Cramer.

Speaking of Cramer, I figured this was a good day to check in on the progress of his own weird quantum experiment. Cramer has been gearing up to test whether causality can go backward in time, thanks to quantum entanglement.

The last time I checked, Cramer was hoping to wrap up his laser-and-mirrors experiment by Sept. 15, because the apparatus he was working with was supposed to be dismantled by that time to make room for the next occupant. "Probably that was optimistic," he told me today. He isn't even finished assembling the rig for doing the entanglement test. 

Fortunately, the deadline pressure has gone away because Cramer has found alternate lab space at the university. "Last week we successfully moved the laser and the rest of the equipment two doors down," Cramer said. He no longer worries about having to move just when the experiment is reaching its retrocausational climax.

"I feel a lot more relieved," the physicist told me.

Will Cramer's experiment turn up fantastic new twists at the intersection of quantum mechanics and relativity? Will it spark a fuzzy controversy, like the debate over seemingly faster-than-light communication? Or will it simply fizzle? Cramer doesn't know if his experiment will result in new physics - but he's anxious to find out.

"There are things that can move faster than light, but signals don't seem to be among them," Cramer said. "Unless our experiment works."

Update for 8:40 p.m. ET Aug. 17: The debate over causality would seem quite peculiar to some of the characters in Kurt Vonnegut's classic, "Slaughterhouse-Five." On one level, the book is a semiautobiographical novel about the 1945 firebombing of Dresden and the outrages of war. On another level, it's a science-fiction story about the nature of time and Tralfamadorian timelessness. And on an even deeper level, it's a philosophical meditation on the human condition, with all its wonders and horrors.

That makes it a perfect selection for the Cosmic Log Used-Book Club, our mostly monthly offering of books with cosmic themes that have been around long enough to become available at public libraries and used-book shops. In fact, "Slaughterhouse-Five" has been around long enough to become available as an audio book and a movie on DVD. But it's still a particularly timely selection, due to the current debate over the war on Iraq as well as Vonnegut's recent passing.

Judith Moore suggested "Slaughterhouse-Five" in response to last month's CLUB Club offerings, and as a reward I'm sending her a copy of "Rocketeers," Michael Belfiore's just-published book about the private-sector space race. Do you have a nomination for future CLUB Club selections? Leave your suggestion as a comment below, and you might just earn a book as well.

Update for 8:50 p.m. ET Jan. 9, 2008: For an update on Cramer's experiment, check out this progress report.