Nature 433, 10 (06 January 2005); doi:10.1038/433010a
Posted on 01/09/2005 12:26:51 PM PST by snarks_when_bored
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Nature 433, 10 (06 January 2005); doi:10.1038/433010a |
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In search of hidden dimensions
So far, string theory has defied experiments, but Nima Arkani-Hamed thinks he has found a way to put the idea to the test. Geoff Brumfiel finds out how.
J. IDE/HARVARD UNIV. NEWS OFFICE |
String fellow: Nima Arkani-Hamed hopes that particle-collision experiments will show that gravity leaks into other dimensions. |
But ask Nima Arkani-Hamed, a physicist at Harvard University, and he will give you a far closer date: 2008. That is when the first results from the Large Hadron Collider, the world's most powerful particle accelerator, are expected to be released by CERN, the European particle-physics laboratory near Geneva, Switzerland. And if Arkani-Hamed's predictions are correct, then that is when an experiment will detect the first evidence to support string theory — a vision of the cosmos that has never been verified experimentally. "The field is going to turn on what happens at the collider," he says.
Pacing his sparse Harvard office, the 32-year-old physicist drinks no less than six cups of espresso during our hour-and-a-half interview, as he tries to explain why he thinks string theory can now be tested.
String theory emerged in the 1980s as a way to answer questions that still baffle modern physics, such as why is gravity so much weaker than other fundamental forces? By imagining that everything is composed entirely of strings ten billion billion times smaller than atomic nuclei, theoretical physicists were able to create a model of the Universe that unified all fundamental forces into one, and described most of the particles we see today. Unfortunately, these strings are far too small to be detected by even the most powerful particle accelerators. And so, critics say, they are more philosophy than physics.
Arkani-Hamed's ideas have very little to do with strings themselves. Instead, he is hoping to detect the extra dimensions predicted by the theory, which, like the strings, are thought to be vanishingly small. But in 1998, Arkani-Hamed and his colleagues published calculations showing that some of these extra dimensions might be as large as a millimetre (N. Arkani-Hamed, S. Dimopoulos and G. Dvali Phys. Lett. B 429, 263–272; 1998). Such large dimensions, they argued, have escaped detection because everything we know — except for gravity — is confined to the three dimensions of space and one of time. But gravity, they think, might be able to seep into these extra dimensions. This would explain why it seems so weak to us. And, as a result, unexpected variations in gravity could allow researchers to detect the hidden dimensions.
Leaking away
"It was a watershed event in the field," recalls Joe Lykken, a theoretical physicist at Fermilab near Chicago in Illinois. Suddenly, a theory that most thought could never be tested was within experimental reach. Some groups rushed to look for deviations in gravity at small scales. So far, they have nothing to report, but the hope created by Arkani-Hamed's work is enough to win him wide praise. "The word 'genius' is overused, but I think it is easily applicable in the case of Nima," says Savas Dimopoulos, a Stanford theorist and one of Arkani-Hamed's collaborators.
The son of two Iranian physicists, Arkani-Hamed was born in Houston, Texas, and grew up in Boston. After the Iranian revolution of 1979, his family returned to their homeland, but as religious fundamentalists took over the government, his father was forced to go underground and the family eventually had to flee across the border to Turkey. By 1982, Nima was living in Toronto, Canada.
Recalling his early life, Arkani-Hamed says that his time in Iran was largely a positive experience. "The strange thing is that I have mostly wonderful memories," he says. If anything, he adds, it taught him to worry less about what others thought of him. "Given that so many aspects of my life have been unusual, I've never had a problem with feeling different or being different or doing different things."
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As a child, Arkani-Hamed loved physics, but he initially disliked almost everything about string theory. "String theory just seemed like abstruse junk to me," he says. "What I really liked was physics that explained things about the world around me."
That changed when he began studying quantum field theory at the University of Toronto. At first, this complex theory — which underlies high-energy physics and much of string theory — seemed too arcane, but as he studied it more carefully, he found a level of order and explanation far beyond anything he had learned before. "Clearly, there was something very deep going on," he says.
It captivated him, and by the time he finished graduate school in 1997, he knew he wanted to try to make string theory experimentally verifiable. He found an ally and mentor in Dimopoulos, who has devoted his career to seeking testable versions of string theory. "We believe that the only way to make progress is to take an idea, and push its consequences to find observations," Dimopoulos says.
These days, in late-night phone calls and frequent e-mails, the two are thinking about what might emerge at the Large Hadron Collider. Their current calculations show that some of the energy created by particle collisions in the machine could escape into extra dimensions, carried off by leaking gravity, if those dimensions are large enough. The result would be an apparent violation of the conservation of energy — a dramatic sign that string theorists are on the right track.
Then again, they might not be. "You can spend ten years of your life and every idea you come up with can be wrong, and that's gratifying in its own way," Arkani-Hamed says. But, he adds, as he reaches his caffeine-fuelled conclusion: "If this thing turned out to be true, it could be the biggest discovery in science in, say, 300 years."
GEOFF BRUMFIEL
Geoff Brumfiel is Nature's Washington physical sciences correspondent.
Ping
That's not a hidden dimension!
Wired while studying strings.
This is interesting. I've always thought that besides height, width and depth, time and gravity are also dimensional
The free world benefits from oppression once again, though Canada barely qualifies.
E = MC5 ... no.
E = MC4 ... nope, not that.
E = MC3 ... nah, doesn't work.
Ah, screw it.
You're right about height, width, depth and time being the standard four dimensions of spacetime. But gravity is not a dimension; it's one of the four fundamental forces, the other three being electromagnetism, the strong nuclear force and the weak nuclear force.
I should have added that current versions of string theory require that there be additional spatial dimensions (up to 6 or even 7 more). Since these dimensions haven't been observed, it has been conjectured that they're quite tiny (curled up into up into any of a myriad of possible shapes). The present article discusses the possibility of getting experimental confirmation of these extra dimensions.
Right on both counts.
PAUL STEINHARDT
Albert Einstein Professor of Physics, Princeton University.I believe that our universe is not accidental, but I cannot prove it.
Historically, most physicists have shared this point-of-view. For centuries, most of us have believed that the universe is governed by a simple set of physical laws that are the same everywhere and that these laws derive from a simple unified theory.
However, in the last few years, an increasing number of my most respected colleagues have become enamored with the anthropic principle—the idea that there is an enormous multiplicity of universes with widely different physical properties and the properties of our particular observable universe arise from pure accident. The only special feature of our universe is that its properties are compatible with the evolution of intelligent life. The change in attitude is motivated, in part, by the failure to date to find a unified theory that predicts our universe as the unique possibility. According to some recent calculations, the current best hope for a unified theory—superstring theory—allows an exponentially large number of different universes, most of which look nothing like our own. String theorists have turned to the anthropic principle for salvation.
Frankly, I view this as an act of desperation. I don't have much patience for the anthropic principle. I think the concept is, at heart, non-scientific. A proper scientific theory is based on testable assumptions and is judged by its predictive power. The anthropic principle makes an enormous number of assumptions—regarding the existence of multiple universes, a random creation process, probability distributions that determine the likelihood of different features, etc.—none of which are testable because they entail hypothetical regions of spacetime that are forever beyond the reach of observation. As for predictions, there are very few, if any. In the case of string theory, the principle is invoked only to explain known observations, not to predict new ones. (In other versions of the anthropic principle where predictions are made, the predictions have proven to be wrong. Some physicists cite the recent evidence for a cosmological constant as having anticipated by anthropic argument; however, the observed value does not agree with the anthropically predicted value.)
I find the desperation especially unwarranted since I see no evidence that our universe arose by a random process. Quite the contrary, recent observations and experiments suggest that our universe is extremely simple. The distribution of matter and energy is remarkably uniform. The hierarchy of complex structures ranging from galaxy clusters to subnuclear particles can all be described in terms of a few dozen elementary constituents and less than a handful of forces, all related by simple symmetries. A simple universe demands a simple explanation. Why do we need to postulate an infinite number of universes with all sorts of different properties just to explain our one?
Of course, my colleagues and I are anxious for further reductionism. But I view the current failure of string theory to find a unique universe simply as a sign that our understanding of string theory is still immature (or perhaps that string theory is wrong). Decades from now, I hope that physicists will be pursuing once again their dreams of a truly scientific "final theory" and will look back at the current anthropic craze as millennial madness.
Maybe that's a floating, er, cake crumb??
Let's say a star is formed a thousand light years from earth. How long before the gravity from the newly formed star reaches earth? IOW, how fast does gravity travel?
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PAUL STEINHARDT |
Go figure. Now the original posting looks okay. Sorry...
Gravity is currently thought to travel at the speed of light.
Is it possible, in theory at least, to use gravity to transmit and receive information as is done with electricity and light?
Is it possible, in theory at least, to use gravity to transmit and receive information as is done with electricity and light?
In theory, it's possible, but in practice it would be devilishly difficult (and would require an engineering expertise which is almost impossible to imagine). Almost the only events that produce gravitational disturbances that we could hope to detect are collapses of stars into neutron stars or black holes, or the orbiting of a neutron star around a black hole. So we'd have to figure out a way of using such phenomena to send gravitational signals.
I'm not holding my breath over here.
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