Posted on 01/09/2005 12:26:51 PM PST by snarks_when_bored




Nature 433, 10 (06 January 2005); doi:10.1038/433010a 

In search of hidden dimensions
So far, string theory has defied experiments, but Nima ArkaniHamed 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 ArkaniHamed hopes that particlecollision experiments will show that gravity leaks into other dimensions. 
But ask Nima ArkaniHamed, 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 particlephysics laboratory near Geneva, Switzerland. And if ArkaniHamed'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 32yearold physicist drinks no less than six cups of espresso during our hourandahalf 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.
ArkaniHamed'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, ArkaniHamed and his colleagues published calculations showing that some of these extra dimensions might be as large as a millimetre (N. ArkaniHamed, 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 ArkaniHamed'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 ArkaniHamed's collaborators.
The son of two Iranian physicists, ArkaniHamed 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, ArkaniHamed 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."

As a child, ArkaniHamed 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 highenergy 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 latenight phone calls and frequent emails, 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," ArkaniHamed says. But, he adds, as he reaches his caffeinefuelled 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.
That would depend on both the model and initial conditions. Most of the time it does. (And all randomly generatied objects can be reduced to a uniform sample by use of the inverse cumulative distribution function; like looking at percentiles.) That's why I was surprised by the guy's comment; it needed more explanation though perhaps it was filtered throught the Journalist Transformation.
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.
Why is "dimensions" so much more popular "properties of space"???
Stretch a sheet of waxed paper on an embroidery frame and place its four corners on gimbals and sprinkle a bit of water on it the surface; now play with it...
"our universe is not accidental"What does he mean, I wonder. Does he mean that the universe was created by some intelligence? Or that it was fated to exi[s]t?
I don't think Steinhardt is a theist, so his use of the words 'not accidental' is a bit odd. But consider his second paragraph:
Historically, most physicists have shared this pointofview. 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.
What he's worked up about is the Anthropic Principle and its apparent consequence that our universe is just one among a truly vast number of universes, all the others of which are unobservable. Steinhardt says, with good reason, that this is not science.
Forgive me if I sound incredibly stupid..
Isn't Gravity simply a function of Mass? ( function, for lack of a better word. )
Rather it seems that if space can bend than it is more like a material.
The basic equation in Einstein's General Relativity places (essentially) the geometric structure of spacetime on one side and stressenergy on the other, that is, there is an equivalence between the mathematical (geometry) and the physical (stressenergy). So, yes, spacetime (not just space) is more like a material, at least in General Relativity.
John Baez has a nice tutorial on General Relativity, but it's not for the faint of heart:
General Relativity is our current best theory of gravity, so I'll refer you to my post #47 on this thread.
Has there been any experimental verification of string theory? What if this is a colossal dead endwhat's Plan B?
No experimental verification of string theory yet (although certain properties of black holes have been deduced using string theory techniques). Perhaps string theory is a dead end, perhaps not. Either way, physics goes on. After all, what else is there to do?
I find it interesting that my original intent was to include "density" in my description..
( .. function of the density of mass.. )
While the short outline definition of StressEnergy Tensor mentions "density" of energy and momentum...
Also "Newton's gravitational constant"..
More reading/study.. although I think that will be a dead end for what I'm trying to understand..
Thanks..
Gravity itself is a field. So travel doesn’t apply. It’s like saying how fast is the ocean between SF and Tokyo. However, gravity waves, according to general relativity (and some pretty strong evidence), travel at exactly the speed of light.
Rather it seems that if space can bend than it is more like a material.
The basic equation in Einstein's General Relativity places (essentially) the geometric structure of spacetime on one side and stressenergy on the other, that is, there is an equivalence between the mathematical (geometry) and the physical (stressenergy). So, yes, spacetime (not just space) is more like a material, at least in General Relativity.
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if this is true then there should be some kind of equation with an = or an imbalanced >= which would do for space what E=MC2 has done for energy and matter...such that Space=Energy (something or other)
Space=Matter (something or other)
Quite correct. 'Gravitational disturbances' do the traveling.
...there should be some kind of equation with an = or an imbalanced >= which would do for space what E=MC2 has done for energy and matter...such that Space=Energy (something or other) Space=Matter (something or other)
Correct, there is—I just didn't put it in my post to you. On the left side of the equation is the socalled 'Einstein tensor' (this keeps quantitative track of the geometric curvature of spacetime); on the right side of the equation is the socalled 'stressenergy tensor' (this keeps quantitative track of the 'matter/energy' or 'stuff' in spacetime). There are also some multiplicative constants, usually written on the right side, to make the units work out.
Check out the reference in my post #47 for much more info.
And although it has a hokey background, here's an image of a coordinateindependent version of Einstein's general relativity field equation (that's the Einstein tensor on the left, the stressenergy tensor on the right):
What controls the number of dimensions?
That's a tough question that nobody really knows the answer to.
Standard general relativity works with a 4dimensional spacetime manifold (3 dimensions of space, 1 dimension of time). In order to get results that make sense mathematically, string theories require the postulation of an additional 6 spatial dimensions (these are the ones that are often characterized as being 'curled up' into tiny balls or other shapes). At the moment, it's not at all clear that it's going to be possible to describe the cosmos we inhabit using a string theory. So, as usual, we're not sure what the situation is as far as the dimensionality of the cosmos is concerned.
(I'm leaving out the stuff about 5dimensional branes and the like. For one thing, I don't know enough about it, and, for another thing, neither does anybody else, apparently.)
What controls the number of dimensions?
Does extra dimension theory interface with Hawking's latest statements on black holes and information ("Hawking changes his mind on black holes; Galactic traps may actually allow information to escape, author says")?
Not as far as I can tell. Hawking's work (which is still being checked, by the way) uses standard techniques of general relativity and quantum gravity. I'll refer you to another page by John Baez (who, incidentally, is performing a real service for webizens by trying to explain difficult ideas in physics and mathematics in a way that nonexperts have some hope of understanding). Baez attended the talk at which Hawking announced his result. Here's Baez' report on that talk (including a transcript of Hawking's entire presentation):
This Week's Finds in Mathematical Physics (Week 207)  July 25, 2004
You probably won't understand all of it (maybe not even much of it)—I only got glimpses and flashes of understanding when I read it, maybe a rough sense of the whole, but not many details. But just skip the parts you have trouble with and read the nontechnical bits if nothing else. Baez has some interesting remarks later in the piece about the role of authorities in science (there aren't any 'Popes of science', as it were—everything has to be checked).
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