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What if space isnt the final frontier? What if beyond our galaxy, far, far away, there is a whole other universe parallel to ours, perhaps in another dimension? Fans of comic books and science fiction have long fantasized about such possibilities. Remember Supermans Earth-One and Earth-Two? Or how that transporter malfunction brought us an evil Kirk and Spock?
Turns out such awesomeness could transcend mere fanboy musings. Its also the stuff of serious and popular science. So much so that one Monday night last month, two lecture halls at the American Museum of Natural History were packed with more than 900 curious New Yorkers, who came to watch five physics professorsand one flat-screen TVdebate the merits of string theory, the scientific theory of everything. For nearly 30 years, many in physics circles have believed that string theory might help unlock the secrets of the universe and possibly reveal hidden realities beyond our own. But skeptics have charged that instead of aiding scientific discovery, and helping young physics post-docs get on the path to professorships, string theory has actually become a dead end.
That conflict was reflected in the title of the museum event, the Hayden Planetariums 10th annual Isaac Asimov Memorial Debate: The Theory of Everything . . . Still Searching? The five professors were introduced and seated, and the TV was turned on. There appeared the head of a sixth professor: Brian Greene of Columbia University, author of several popular science books including The Hidden Reality (which just hit stores and discusses extra dimensions and parallel universes), who joined the panel from out of town via Skype. Planetarium director Neil deGrasse Tyson reminded the crowd that Greene had appeared on the very first Asimov panel back in 2001, which also tangled with string theory. That was shortly after he helped found Columbias Institute of Strings, Cosmology and Astroparticle Physics (ISCAP).
Tyson asked Greene to boil string theory down to one sentence. It will be a long sentence, Greene replied.
A unified theory is something of a Holy Grail in physics, and a long line of brilliant minds from Einstein on down have failed to craft one that can be proven to work. The trick has been to find a single theory that explains all of physics, from the quantum forces that control subatomic particles to the relativity that holds sway over the greater cosmos.
Consider, for example, black holes. These objects, formed from collapsed stars, are believed to be compressed to a size that reaches the atomic level. But no physical law has ever been devised to explain how gravity works at that scale. Einsteins laws and those of quantum dynamics each work well in their own realm, explained Greene, but become fierce antagonists when you try to meld them.
String theory, first proposed in the mid-1980s, proposed a possible solution. It began, he explained, as a new way to envision the inner structure of the tiniest particles. According to conventional theories of physics, matter can be parsed down to molecules, then atoms, then protons and neutrons, then electrons and quarks, then . . . what? String theory suggests a deeper level of tiny filaments of energy that vibrate in certain patterns. A string vibrates one way, you get a quark. Another way, an electron. Yet another, gravity.
With that, and through fancy mathematical footwork, Greene told the Planetarium panel that we have united gravity and quantum mechanicsat least on paper.
String theory produced some unusual side effects. To make the math behind the strings work, theorists needed extra dimensions beyond our four (three for space, one for time). Those dimensions became dubbed branes, short for membranes. Imagine that we live on a giant slice of bread, that all reality is on our slice of bread, Greene tells the Voice. Now imagine that our slice is in a giant cosmic loaf. We can see only whats on our slice, but there could be other slices out there.
If so, Evil Spock and Earth-Two Superman could live elsewhere in the so-called multiverse on one of those cosmic brane slices. But other academics beg to differ. One prominent string-theory skeptic, in fact, resides on Greenes own campus: Peter Woit, an instructor in Columbias mathematics department, who in 2006 published his own popular book called Not Even Wrong: The Failure of String Theory and the Search for Unity in Physical Law. String theory, Woit believes, is so nebulous that it cant even be used to make predictions that could be proven or disproven, making it effectively useless.
String theorists would often talk a good game, says Woit, but their ideas have consistently failed to produce experimental proof (something that many string theorists themselves have long acknowledged). But, he adds, science comes down to whether you can actually predict something with your theory, and they just havent been able to meet that challenge. His message: Lets drop strings and move on to more fruitful areas of research.
Woit says that theres been a backlash to string theory in academe over the past decade, and that both universities and up-and-coming physics post-docs are starting to lose interest. I feel bad for young people trying to get jobs, he says. Its hard to get a job doing string theory at this point.