Wednesday, December 3, 2014

Is String Theory Too Timid?

Lately I've been watching the series of YouTube videos put out by Stanford University in which Leonard Susskind lectures about string theory and M-theory.  He really is an excellent lecturer, so even though I have had little cause to use advanced quantum mechanics over the past two decades, let alone particle physics or string theory, it's easy to follow his lectures.  (They are not lectures for the general public, though, because they do require having had at some time a pretty good background in physics, but they are OK for an advanced undergrad or someone who is very rusty.)

I have tended to be a skeptic regarding superstrings, mostly because it is too often presented to the general public in exactly the same way ancient Greek philosophers presented their physics theories:  long on an appeal to what a beautiful idea it is, but almost completely lacking any evidence from experiment or observation.  That is very bad form for a scientist!  It hasn't helped that some of the theory's proponents have even suggested that perhaps it is the ONLY possibility, so that "God had no choice" but to build a universe out of superstrings -- a statement that is at least borderline blasphemous and which displays a mind-boggling absence of imagination.  Happily, though, the actual science behind string theory is not quite so muddle-headed as the popularizations. 

One thing that is still surprising, though, is how reliant string theory seems to be on physics that is backed up mostly by our experience with molecules, atoms, and nuclei.  In particular, Susskind starts off with a string of masses connected by springs, considers that string in a reference frame in which it is moving close to the speed of light, then applies the standard techniques to quantize it.  This is all pretty basic stuff, very similar to what is used in the Debye model of a generic crystal.  The only thing is, he wants this to apply all the way down to the Planck scale.

To me that seems to be unjustified optimism.  Quantum mechanics has of course been phenomenally successful in explaining or predicting all kinds of phenomena, but there are, after all, a number of foundational problems about it, and it requires a huge jump to go from they physics of hadrons and mesons to the Planck scale.  Somehow, it seems likely to me that along the way, we will have to replace Quantum Mechanics itself with something new that will be even weirder.  Quantum Mechanics would remain as a "low energy" approximation for whatever comes next, just as Newtonian Mechanics remains as an approximation for Quantum Mechanics.

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