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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"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."
Non-elitist bookmark bump.
It's not strong enough to provide a good signal. The EM spectrum is far superior.
Thanks for the ping!
Why does this guy think that a uniform distribution of matter and energy isn't the result of a random process? In many cases, this is exactly what would be predicted.
His comment about the universe being "accidental" just shows
how little our language and common understanding incorporates
all of the knowledge we do have.
One major question would be such:
Is there any such thing as an "accident?" If the universe
(no matter how many dimensions or what it is composed of)
is set on ONE solitary physical course, then the concept of
an accident(meaning something happened that wouldn't
"normally" happen) COULDN'T occur at all. EVERYTHING
would be an outworking of the physical plant we find ourselves
in...(e.g. every event which occurs is based on some
physical process which was initially set in motion when the
universe (or multiverse, if you will) began.
This of course raises questions about our belief, that
we can understand "NATURE" at it's core, since our very
thoughts could be considered as outworkings of the
physical processes initially set in motion when the universe
(or multiverse, if you will) began.
"I think, therefore I AM....er....do I really think?", would be
the edited version of that famous philosophical statement.......
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.
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however, observation has shown that the structure of space time is bent into "gravity wells" around massively large objects. Here are two sentences. 1.)Space tells matter how to move. 2.)Matter tells space how to bend. Sentence one describes "height, width, depth and time being the standard four dimensions of spacetime." Sentence one describes how we understand reality. However, all four dimensions seem to be subject to something else described in sentence two. The bend or gravity. But why is this bending of space described as a force rather than the character of space itself. It doesn't seem as if space is empty. Rather it seems that if space can bend than it is more like a material.
Either that or our notion of dimensions is a function of newtonian mathmatics. We're biased or our point of view is in favor of space time rather than matter time--if there is such a beast. I'm no mathmatician but I've heard recently some speculation at free republic that this bias had something to do with Newton's work and that it could be rectified or rebalanced by Newton's contemporary, Liebnitz, who did some work with infintessimals, a number system that is built around a one dimensional zero--or something like that. At this point I'm out of my depth.
atheist scientists don't believe in God since there is no "proof" yet fall all over themselves expousing the existence of cosmic strings and 23 different dimensions, for which there is
no experimental proof whatsoever
ah, hypocrisy. faith is OK, but only if politically correct
Random means uniform does it not? So I was taught in chemistry class.
Dr. Michio Kaku's latest book, Parallel Worlds...Bump for later
I'll never understand how it is that theoretical scientists fail to see the handwriting of God in the essence of their work. Just because something is of divine nature doesn't necessarily mean it defies measurement, it merely defies our current technology to make those measurements. And once measured, that doesn't mean that it has nothing to do with God's work.
God only gave Moses the rules for human behavior, not for the behavior of the universe. To assume that those 10 commandments are the only laws in existence which God created (or enforces) is arrogance in the extreme.
During my undergraduate degree studies in Physics, it became quite clear to me - nearly an atheist at the time - that the only plausible explanation to the universe rested with what we define as God. As I have aged over the last 25 years since, this belief has only become stronger.
Eventually, when humankind learns to overcome mortality, the human invented aspect of linear time will fall by the wayside, opening the doors which are closed today. Quantum theory begins to erase linear time, but time still keeps us "on the plantation" so to speak - locking us into Newtonian mechanics as the basis of our physical understanding.
This could be the answer to Fermi's paradox - in our local miniverse - advanced civilizations have come up with the physics to do this sooner rather than later. We're next.
Has there been any experimental verification of string theory? What if this is a colossal dead end--what's Plan B?
I imagine that we'll burn that bridge when we come to it.
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 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.
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 stress-energy on the other, that is, there is an equivalence between the mathematical (geometry) and the physical (stress-energy). 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 end--what'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 Stress-Energy 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..
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