Posted on 02/26/2003 8:40:57 PM PST by vannrox
Just ahead of a bandwagon of theoreticians suggesting the discovery of extra dimensions might be just around the corner, a streetwise inquiry into the potential effects of these additional "spaces" has come up as empty as a gas tank during an oil embargo. Theorists are unlikely to be sobered by the new study of possible effects on gravity in tiny spaces, however. The research is useful in that it puts an upper limit on the distance at which strange new physical behaviors might yet be detected. Further, it explored only one possible manifestation of extra dimensions.
There are three known and obvious spatial dimensions -- the x, y, and z coordinates of basic geometry. Time is considered a fourth dimension. Various exotic theories suggest one or several other, hidden dimensions that would explain mysterious aspects of cosmology, like the relatively recent and inexplicable observation that the universe is accelerating at an ever-faster pace. At the annual meeting of the American Association for the Advancement of Science, held in Denver earlier this month, top physicists and cosmologists expressed optimism that extra dimensions would be soon be uncovered. "We have a number of hints from experiments and theoretical ideas that make us think they?re probably out there," said Fermi National Accelerator Laboratory physicist Joseph Lykken. "That?s why we?re so excited about looking for them." String theory Firm signals of invisible spaces might tie one of the most seemingly outlandish ideas, string theory, into reality. String theory is the leading contender seeking to unify the age-old theory of gravity with physical explanations for how very small particles behave. In string theory, elementary particles are not point-like, but instead are more akin to strings. The various forms of string theory each require the existence of at least six extra spatial dimensions. Each is invisible, theory holds, because it is rolled up, stringlike, into a space too small to see. Physicists call this "compactification." String theorists have suggested that some of these extra dimensions might be detectable by the effect they have on gravity at bantam distances. Perhaps, they say, gravity is diluted as it propagates through several wee spatial dimensions. Another idea is that the extra dimensions could be revealed through "moduli fields," which describe the size and shape of the compact dimensions at each point in ordinary 3-D space, explains John Price of the University of Colorado at Boulder. Moduli fields can act in a way similar to the gravitational field, but are expected to generate forces somewhat stronger than gravity. Advances in technology now make it possible to search for these tiny forces under such micro-conditions. Price and his colleagues, led by Joshua Long, report the results of the latest experiment, involving a newly built lab device called a high-frequency resonator, in the Feb. 27 issue of the journal Nature. The researchers observed no new forces at distances of about one-tenth of a millimeter. This puts an upper limit on the moduli effects that theorists call the dilaton and radion forces, which remain hypothetical (not to mention highly complex). With funding from the National Science Foundation, the University of Colorado team looked for effects so tiny they're about equal to the weight of one-billionth of a grain of sand. Price was careful to point out that the tests were not designed to prove string theory. "The ideas we test are just 'string inspired scenarios,' not precise predictions of string theory," he told SPACE.com. "It is not yet possible for string theory to make precise predictions of this kind, and I would say that no one knows if string theory will ever be able to that." He said further experiments at even smaller distances may yet "add more patches to the quilt of physics," and therefore it is worthwhile to continue the line of research because "something new and very fundamental could be discovered." Extra-dimension revolution? Whatever the fate of string theory, the idea of extra dimensions is gaining steam. Introduced nearly a century ago, the notion was initially laughed at by many physicists. Most aren't laughing any more. Why? For one reason, something is going on that standard physics models can't explain. Among the more compelling bits of inexplicable evidence is the fact that the universe is expanding at an accelerating rate. Some mysterious force, dubbed dark energy for now, is driving the cosmos apart, acting like antigravity. Nobody knows what it is. What they do know is that while gravity holds galaxies together at the local level, some mysterious force is driving galaxies apart on broad scales. Dark energy may involve interactions between dimensions we see and those we don't, some theorists say. At the AAAS meeting, several top cosmologists and physicists expressed optimism that the field would be revolutionized before long. "There is the tantalizing possibility that a complete change of perspective makes all of the problems collapse at once," said Sean Carroll, an assistant professor in physics at the University of Chicago. These problems inevitably surround gravity, whose force was calculated by Newton more than three centuries ago. "Gravity may have been the first of the fundamental forces to be described mathematically, but it is still the most poorly characterized," says C. D. Hoyle of the University of Trento in Italy. That's because though pervasive, gravity is relatively weak compared to the other three fundamental forces (electromagnetism, and the strong and weak nuclear forces that govern atoms). In particular, scientists have long sought to determine whether Newton's inverse square law -- that the gravitational force between two bodies depends on the inverse-square of the distance between them -- holds up at very small distances, in the so-called quantum world. Newton developed it for astronomical distances, as between the Sun and a planet. Hoyle, who analyzed the University of Colorado study for Nature, said the "elegant experiment" set the best constraints yet on any possible deviations in the behavior of gravity at small scales. "So far, Newton is holding his ground," Hoyle added. An understanding of gravity at the quantum level is expected to tie it into quantum mechanics and the full descriptions of the other forces. Developed in the 1920s, quantum mechanics describes the behavior of objects at the atomic level. "What?s going on right now in particle physics, gravitational physics and cosmology is like when quantum mechanics started coming together," said Maria Spiropulu, a University of Chicago researcher who organized the AAAS session on the physics of extra dimensions. |
Testing the inverse square law for gravity at distances down to a tenth of a millimeter is tricky indeed. However, they will have to get down to much smaller scales to exclude all *large-extra-dimension models capable of solving the "Hierarchy Problem" (i.e., the problem of why gravity is so much weaker than the other forces).
(* "Large" here means "large compared to the Planck length", which is of order 10^-33 cm.)
The idea posits that there are extra dimensions, but that gravity alone is permitted to travel through "the bulk" (the extra dimensions). Our universe is but a thin slice through the bulk, so it "sees" only a small part of the true gravitational force.
We're bound by our perceptual abilities. It's hardwired into the brain. Who can know what else exists? Or, more scientifically, how could they know it?
This is an off-hand reference to Kaluza-Klein theory. Kaluza (as I recall a math teacher?) was fooling around with 5-D space time, and discovered to this amazement that Maxwell's equations 'fell out' with no a priori assumptions from pure geometry. He and Klein (of Klein bottle fame) published a paper, which was promptly forgotten, until it was rediscovered in the 1970s and somebody went "Holy S**t!"
--Boris
Do fish know they're wet?
--Boris
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