The String Theory Calamity

The status of string theory in physics has no historical analogue in modern science, at least so far as I can see.  The theory is at the same time a smashing “success” and a colossal, catastrophic failure.

String theory is a success in the sense that for all intents and purposes, for the subset of physicists concerned with theorizing about the unification of gravity and quantum theory, it appears to be the only game in town.  As documented in Peter Woit’s book “Not Even Wrong”, string theorists have swept the board, seizing control of faculty appointments and grant committees pertaining to high-energy theory (in the U.S., at least), dominating the publication of results, determining the research directions to which resources are allocated, and largely ensuring that graduate students are trained to regard string theory as the single worthwhile stream of research.  Alternative directions are marginalized, and students quickly understand that exploration cutting at cross-purposes to string theory will almost certainly result in unread work and unrequited careers.

String theory is a failure in the sense that it has reneged on every promise that has been made on its behalf since its emergence in the 1980s.  It has been unsuccessful in predicting the value of even one of the Particle Theory Standard Model’s physical constants, or in relating the value of any of them to any other, or even in demonstrating that the structure of the Standard Model emerges in a natural way as a low-energy limit from string theory.  Its one physical prediction — that the world is 10-dimensional — was hurriedly swept under the rug after a quick count.

String theory even had a once-in-a-century opportunity to prove its mettle, as its rise preceded by about a decade the greatest experimental (observational) surprise in about a century:  the discovery in 1998 that the universal expansion is accelerating, and that the universe behaves as if it were pervaded by a substance (termed “Dark Energy”) that behaves an awful lot like Einstein’s cosmological constant.  Had string theorists somehow predicted this by, say, 1995, it would be raining Nobel Prizes on the field’s leadership by now.  If they had even been able to give an ex-post-facto explanation of the dark energy phenomenon, that might at least have been the beginning of an experimental validation of the theory.

Of course, nothing of the sort occured.  In fact, the case is rather the reverse, in the sense that the discovery of cosmic acceleration triggered a wrenching crisis for string theory.  Prior to 1998, it was understood among particle theorists that to the extent that the magnitude of Einstein’s cosmological constant was estimable at all from back-of-the-envelope calculations of vacuum energy, it would have a magnitude exceeding the one eventually found by over 100 orders of magnitude.  It was also understood by string theorists that string theory itself would preclude a positive value of the cosmological constant, such as the one eventually measured (this is asserted by Lee Smolin in his book “The Trouble With Physics”).  So string theorists, to the extent that they were interested in predictions concerning the cosmological constant, were off by the most spectacular mis-estimate in the history of science, and had the wrong sign.

It gets worse.  As Smolin tells the story, in 2003, in a desperate effort to save the field, a group of physicists at Stanford constructed a very complex (even by the standards of the field) and rather ad-hoc string-theoretic structure (aptly described as a “Rube Goldberg contraption”) whose main purpose was to give rise to a positive cosmological constant.  They succeeded in this, but the resulting object had an awful lot of freedom in it — it wasn’t one theory, but rather many theories.  Many, many, many theories.  Probably on the order of 10500 completely distinct theories, each with totally different predictions about what the world is like.  And no way to select which one, if any, is correct.

Grasping at straws, string theorists decided to brazen it out, passing embarassment as triumph: they decided that all those theories were equally “correct”, and each represented a possible alternate world.  Appealing to the so-called “Anthropic Principle”, they announced that the reason that our own world has the structure and constants that it has is because we ourselves are alive to observe it, and if that structure were otherwise, we wouldn’t be here.

So, just to make sure you’re keeping score here: a theory that has failed to make any validated physical predictions whatsoever, and whose actual predictions (dimensionality of the world and size and sign of the cosmological constant) are in flat contradiction with observation and experience, is not to be discarded, as you might naively have expected.  No, the theory is to be saved.  What has to be given up instead are small things, that we’ll never miss:  the ideas that (a) scientific theories are only acceptable insofar as they can be validated against observations, and (b) the world runs according rules that are independent of our existence.  That is to say, the cost of saving string theory is a six-century regression in our scientific outlook, to one that would have been accepted and approved by any 14th-Century scholastic theologian: the view that observation has no bearing on truth, and that the world is the way it is because we live in it.

The catastrophe in physics is that this is in fact the prevailing view in high-energy theory.  Traditionally, theoretical physicists have been kept honest by the effect of pushback on their ideas from the real world.  Absent that pushback, it is easy for models to go badly off the rails.  The pathology here is that there is no prospect in our lifetimes of data at the the Planck scale, which is the energy scale required for data capable of providing the pushback that would get high-energy theorists back on the rails.  So the field has degenerated into an exploration of untethered ideas, and a sociological runaway instability has set in wherein an entire subfield of physics has declared independence from the guiding values of science, and is using its control of its social structures (money, jobs, and publications) to consolidate and perpetuate itself, an office usually performed by means of the values that it left behind.  This won’t end well.


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