Ultima Thule

In ancient times the northernmost region of the habitable world - hence, any distant, unknown or mysterious land.

Monday, June 12, 2006

Is String Theory Not Even Wrong? -- the not-so-elegant universe?

By Aussiegirl

Interesting article by Peter Woit based on his book entitled "Not Even Wrong". The phrase comes from a reputed quote by physicist Wolfgang Pauli, who upon being shown the work of a not very promising student remarked sadly -- "It is not even wrong."

As long as we are piling onto String Theory here, why not go a step further. The previous article dealt with some measurements of magnetic fields that placed the predictions of String Theory as being off by a factor of a billion. In this article it seems that the one measurable prediction of String Theory, that of the Cosmological Constant, appears to be off by a factor of an order of magnitude of 55.

Hmm, maybe that universe isn't so elegant after all?

American Scientist Online - Is String Theory Even Wrong?

For nearly 18 years now, most advanced mathematical work in theoretical particle physics has centered on something known as string theory. This theory is built on the idea that elementary particles are not pointlike objects but are the vibration modes of one-dimensional "stringlike" entities. This formulation hopes to do away with certain lingering problems in fundamental particle physics and to offer the possibility of soon explaining all physical phenomenaýeverything from neutrinos to black holesýwith a single theory. Fifteen years ago Edward Witten of the Institute for Advanced Study made the widely quoted claim that "string theory is a part of 21st-century physics that fell by chance into the 20th century," so perhaps it is now time to begin judging the success or failure of this new way of thinking about particle physics.click for full image and caption

The strongest scientific argument in favor of string theory is that it appears to contain a theory of gravity embedded within it and thus may provide a solution to the thorny problem of reconciling Einstein's general relativity with quantum mechanics and the rest of particle physics. There are, however, two fundamental problems, which are hard to get around.

First, string theory predicts that the world has 10 space-time dimensions, in serious disagreement with all the evidence of one's senses. Matching string theory with reality requires that one postulate six unobserved spatial dimensions of very small size wrapped up in one way or another. All the predictions of the theory depend on how you do this, but there are an infinite number of possible choices, and no one has any idea how to determine which is correct.

The second concern is that even the part of string theory that is understood is internally inconsistent. This aspect of the theory relies on a series expansion, an infinite number of terms that one is supposed to sum together to get a result. Whereas each of the terms in the series is probably finite, their sum is almost certainly infinite. String theorists actually consider this inconsistency to be a virtue, because otherwise they would have an infinite number of consistent theories of gravity on their hands (one for each way of wrapping up six dimensions), with no principle for choosing among them.

The "M" Word

These two problems have been around since the earliest work on string theoryýalong with the hope that they would somehow cancel each other out. Perhaps some larger theory exists to which string theory is just an approximate solution obtained by series expansion, and this larger theory will explain what's going on with the six dimensions we can't see. The latest version of this vision goes under the name of "M-theory," where the "M" is said variously to stand for "Membrane," "Matrix," "Mother," "Meta," "Magic" or "Mystery"ýalthough "Mythical" may be more appropriate, given that nearly eight years of work on this idea have yet to lead to even a good conjecture about what M-theory might be.

The reigning Standard Model of particle physics, which string theory attempts to encompass, involves at its core certain geometrical concepts, namely the Dirac operator and gauge fields, which are among the deepest and most powerful ideas in modern mathematics. In string theory, the Dirac operator and gauge fields are not fundamental: They are artifacts of taking a low-energy limit. String theorists ask mathematicians to believe in the existence of some wonderful new sort of geometry that will eventually provide an explanation for M-theory. But without a serious proposal for the underlying new geometry, this argument is unconvincing.

The experimental situation is similarly bleak. It is best described by Wolfgang Pauli's famous phrase, "It's not even wrong." String theory not only makes no predictions about physical phenomena at experimentally accessible energies, it makes no precise predictions whatsoever. Even if someone were to figure out tomorrow how to build an accelerator capable of reaching the astronomically high energies at which particles are no longer supposed to appear as points, string theorists would be able to do no better than give qualitative guesses about what such a machine might show. At the moment string theory cannot be falsified by any conceivable experimental result.click for full image and caption

There is, however, one physical prediction that string theory does make: the value of a quantity called the cosmological constant (a measure of the energy of the vacuum). Recent observations of distant supernovae indicate that this quantity is very small but not zero. A simple argument in string theory indicates that the cosmological constant should be at least around 55 orders of magnitude larger than the observed value. This is perhaps the most incorrect experimental prediction ever made by any physical theory that anyone has taken seriously.


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