Thursday, December 25, 2014

Thoughts on the scientific method (IV): string theory

This post (sorry for the delay) is the concluding part of a four-part series, the first three parts can be found here, here and here.

To summarise the main points that I argued previously: (i) logical positivism and the criterion of falsifiability are not necessarily appropriate guides to how science should be practised, (ii) the word "theory" is inconsistently used and best avoided in such a discussion. What science does, in practice, is to pursue goals, develop frameworks and formulate models. In applying criteria like testability, verifiability and falsifiability, one needs to keep this classification in mind, (iii) Quantum Field Theory (QFT) is the framework used to describe fundamental particles and forces. Quantum Electrodynamics (QED) and Quantum Chromodynamics (QCD) are two distinct (but similar in some ways) models formulated to describe the electromagnetic and strong interactions in nature, and both have been extremely successful when confronted with experiment (QED is the unique model in all of science that produces predictions from first principles in agreement with experiment to an accuracy of nine decimal places).

I also pointed out that Classical Mechanics is already "falsified" since it cannot explain a large class of phenomena -- precisely those phenomena which we refer to today as "quantum" or "relativistic". To explain such phenomena we need to invoke Quantum Mechanics and/or Special Relativity. However, I argued that frameworks should not be thought of as falsified even when they have been contradicted by experiments. Classical Mechanics is still taught as a subject in school and college for a very good reason: it is a very successful framework. We understand that it is successful in the domain where it should be applied, namely the domain where it works! Such reasoning may appear circular but is, quite rightly, accepted and followed in science. It is questionable only if you insist on a literal application of logical positivism and falsifiability.

QFT is a difficult framework and a large fraction of physicists remain unfamiliar with its depths. Perhaps for this reason, it is not widely realised that QFT has also beem falsified, by the experimental fact that gravity exists. QFT with gravity is not "UV complete" and this means it will inevitably break down at very high energies. A number of physicists believe we should not worry about this until we are able to perform measurements at those energies. And we may have remained in this mode but for a fortuitous accident. A model (not framework) had been proposed by Nambu, Susskind and Nielsen to qualitatively describe the binding of quarks in a proton, which has a strange property: the force between quarks is stronger at large distances and weaker at short distances. One (perhaps the only) place where we encounter this in classical physics is the behaviour of a rubber band. So the above scientists (one of whom now has a Nobel prize, though not for this idea) proposed that quarks behave as if they are connected by rubber bands. The formalism to describe this is string theory. Thus, much as in the case of QED, a model led to the development of a framework.

As has happened many times before in physics, string theory began to emerge as a framework that could encompass far more than was originally imagined. From its initial role as a model for quark interactions it grew to provide a consistent framework of quantum gravity, satisfying the very criterion ("UV completeness") that the framework of QFT failed to satisfy. The framework is able to naturally accommodate other "gauge forces" and a bold proposal was made in 1984 that one may be able to find a "theory of everything", a unified model of all fundamental forces including gravity, within the string framework. This proposal may be right or wrong, we don't have much guidance today - neither from experiment nor from theory. There is no compelling model (though models with specific compelling features exist) and no positive experimental result (such as proton decay). And indeed, work on this proposal has been scaled down considerably since the early days.

Through the above developments, a framework emerged which reduces to ordinary QFT in the limit of small strings (exactly as quantum mechanics reduces to classical mechanics in the limit of small quantum effects). It is a very powerful framework that addresses a wide variety of topics in theoretical physics. Indeed, a string theorist today is not someone who necessarily does string theory, but someone who has expertise with the framework and is able to use it wherever needed. In the last ten days alone, I have attended talks on topics ranging from quantum phase transitions in superconductors, to black hole physics, to fluid dynamics, to quantum entanglement, to cosmological inflation. But during all this time I was only at "string theory" conferences, and the speakers were all "string theorists". Many of the talks made no mention of string theory but the theoretical techniques were, sometimes very dimly, related to string theory. The goals in these conferences were not connected to the unification of fundamental forces - a goal that will be revived if and when there is a strong new motivation either from theory or experiment - but rather, to a wide variety of physics goals. The topics listed above are firmly rooted in empirical reality, but theoretical work (without string inspiration) has often been too limited in its ability to provide an understanding of them.

Skepticism is integral to science. But skepticism does not mean that one points to, say, the special theory of relativity and says "I don't like it" (as many people did in the time of Einstein). You may not like it, but you need to present something as good or better and defend that scientifically. In the medium to long term, frameworks or models that are not compelling and serve no useful purpose simply fade out. They do not get stamped out by someone writing an article in Nature whose title is a call to arms and which casually tries to push an unpopular competing theory due to one of the authors.

I'll conclude by providing three possible developments which could easily be around the corner and which would force even skeptics to sit up: (i) the discovery of supersymmetry - this would be an entirely new corner of physics that arises most naturally from string theory, (ii) the discovery of a controlled model of quark confinement within string theory, (iii) the discovery of a tensor-scalar ratio r greater than, say, 0.01 in the cosmic microwave background. If you are interested in physics but don't know about one or more of these topics I suggest you read about them, because they are important.

And if you are a physicist, the question you should be asking is: can I learn something from string theory about the scientific goals important to me?


Anonymous said...


I am going to foolishly blunder in where angels fear to tread. My excuse is that I don't pretend to be a physicist. Just an interested lay bystander.

Would it be fair to make an analogy with the view that the earth is flat? We all drive our cars as if it is. Below a particular scale of distance, the earth is indeed, for practical purposes, flat. There is no need to frighten learner drivers with the possibility that they might shoot off into space if they drive fast enough.
Yet we know that the earth isn't really flat.

Is it a matter of scale when one contrasts classical mechanics with quantum whatsit or relativity whatsit? Classical mechanics is good enough at most everyday scales.


Sunil Mukhi said...

Joel: You are completely correct and this is a great analogy. Studying classical physics is like studying the flat earth: one can calculate things and function very well indeed, as long as one doesn't stray far enough that the curvature of earth is an issue. In this analogy, "straying far enough" is like going to very high energy scales or short distance scales, where classical ideas break down and one has to replace them with quantum physics. Also in your analogy, as in physics, there is no definite point where the familiar laws break down: they gradually become less and less applicable.

Anonymous said...

Phew. Lucky hit. I am off to join the Flat Earth Society.

More seriously, what was once obviously and completely true in the eyes of hard-nosed physicists turns out to be true only if qualified by various clauses.

No flat-earth physicist could justifiably have precluded the possibility that they were seeing only part of the picture.

Perhaps a "theory of everything" physicist will always suspect that it may turn out to be a "theory of less than everything".

I await the day when the theory of everything will successfully predict how my wife will feel about any given event.

sukratu said...

It was nice to read a post concerning science or scientific enquiry on this blog. It was especially nice since it is a philosophically oriented post (or rather a set of such posts)

While I have my own (very hyper-strong!) prejudices regarding the scientific theories (I hope the use of term 'theory' is clear from the context here!) mentioned like QFT or string theory or quantum gravity (as in its usual connotation) it is still a good reprieve to see an attempt to introduce ideas like 'framework' or 'model' which are overarching and seek to put in place some kind of a philosophical semantic valuation system for all the various terms like
'string theory' or 'renormalization' which end up rather like being bandied about more like buzzwords on pop media than philosophically properly evaluated concepts in scientific discourse.