Thursday, December 18, 2014

Thoughts about the scientific method (I)

After a long time away from this blog, I felt compelled to return to it. The proximate reason is a recent article in Nature about what does - and does not - constitute science. But I'll first introduce the topic and make some comments on it, then get to the article in a subsequent posting.

The issue at hand is "what is the scientific method?" and "what constitutes science?" (as opposed to pseudo-science or non-science). This keeps popping up for a variety of reasons. On one side, there are schools of thought that would like astrology to be included with science. This worries many (not all) scientists. On the other, there is a concern that some branches of science as currently practiced are "not scientific". This also worries some (not all) scientists. In both cases, the empirical method and the concepts of testability and falsifiability are produced to justify the arguments made. My concern is that there is not enough reflection on what this method and these words really mean, and I would like to put forward some thoughts on this.

Let me start by quoting from a popular article I wrote for the Times of India  about seven years ago:

The notion that scientific theories must be tested experimentally is fundamental to the doctrine of positivism, which also requires that theories must always deal with quantities that are observable. [...] But Steven Weinberg, a Nobel Laureate and one of the greatest living physicists, asserts that "positivism has done as much harm as good". To make this point, which he develops at length in his excellent book "Dreams of a Final Theory", he argues that it was positivism that kept a number of scientists from believing in atoms, in electrons and much later in quarks.

Weinberg supports his claim with a comparison of two scientists. The British physicist J.J. Thomson is credited with the discovery of the electron, but Walter Kaufman in Germany performed the same experiment independently at the same time, and even managed a more precise measurement of the electron's properties. While Thomson reported the discovery of a new particle, which he named the electron, Kaufman merely reported the phenomenon he had observed (the bending of cathode rays). Exercising a positivist's restraint, he did not assume it corresponded to a new particle. [...] The harm caused by a positivist approach, in Weinberg's view, is this: unless one is willing to make - and believe - a hypothesis based on the limited information available, there tends to be a lack of direction in one's subsequent research. Only if one makes a conjecture about what is happening, defying positivism at least temporarily, is one motivated to perform experiments that can confirm or deny the conjecture.
The physicists I know routinely claim to be positivists. However, when confronted with the above argument, which contradicts their point of view, they instantly agree with it. I don't mean to only criticise others: I'm an early example of this phenomenon. In 1998 I attended a workshop in Santa Barbara where Stephen Hawking gave an informal talk to a round table of around a dozen people. He started by saying he was a logical positivist and then added "this is also how most physicists would describe themselves, if only they knew what it means". I was in awe of Hawking and was seeing him for the first time. So it never crossed my mind that he could be talking through his hat, but now I'm pretty sure this was the case.

This is not to say that I disrespect the spirit of positivism. Experiment and experimental verification are fundamental parts of science. The problem arises, just as in the example above, when one forgets some key factors: (i) the complexity of the interplay between theory and experiment, (ii) the presence of (sometimes long) periods where one or the other of these has to surge forward without any support from the other, (iii) the role of intuition, guesswork and leaps of faith in temporarily furthering the cause of science.

To explain point (i): there is no simple mechanical routine where someone does an experiment, then someone writes a theory to explain it and predicts a new experiment, then another experiment is done etc. The interplay is complicated and often incomprehensible to both theorists and experimentalists at the time they are working. This is why every great scientist confesses in her retirement speech that she didn't really understand what was going on while she was making great discoveries.

On point (ii), there are periods in science when theory is stuck for lack of ideas or computational tools, or experiment is stuck for lack of ideas or experimental equipment. At present there is little theoretical understanding of "dark matter" but experiment must continue until the theorists catch up. Conversely there is little experimental understanding of "quantum gravity", so theorists must do their work and wait until experimental understanding is achieved.

Finally about point (iii), the word "temporarily" is crucial. A fundamentalist who blindly follows positivist ideology might not be able to do what everyone in science routinely does: mull over a hypothesis at night and resolve to test it the next morning. Just 12 hours of speculation might, in his overly pedantic view, be considered non-positivist! Of course, no scientist would consider such speculation wrong or unscientific. But if it's OK to speculate overnight, is it OK to do so over a month, a year or a decade? And this is a very crucial point. The scientists who designed and built the Large Hadron Collider relied on theoretical speculations from 1964 about the existence of a Higgs boson (this is why the experiment was optimised to find this boson, and found it nearly half a century later). Do we consider fifty years of speculation about the Higgs to be unscientific? I think not.

I know the standard response to this line of argument: "The Higgs speculation was focused on creating an experimental test. If the Higgs were eventually found it would validate the speculation, while if it were not then the speculation would have been falsified. Thus the entire process followed Karl Popper's criterion of "falsifiability" and is therefore admissible as science."

I disagree on two counts. The Higgs hypothesis was not initially focused on an experimental test. Englert-Brout and Higgs did not suggest any experimentally verifiable picture of their idea in 1964. It took many years of theoretical research to arrive at the possibilities and predictions that were lined up by the time the LHC started to operate. So the fifty years until verification were not merely due to experimental limitations. There were theoretical issues that needed to be understood, in a speculative (i.e no Higgs particle was known to exist) but totally scientific context.

My second disagreement though is more significant. I simply don't agree with Popper's falsifiability criterion. It sounds nice the first time you hear it. However - like all the lapsed positivists described above - you may change your mind when you consider its implications.

Continued.... Click here for the next part.


Rahul Siddharthan said...

I think this is a strawman. Popper did not say that you should not make speculative statements. Thomson reported findings (bending of cathode rays) and his interpretation (that they were composed of particles), and even calculated the charge-to-mass ratio. This had consequences down the line, ie it was a falsifiable claim. More to the point, within perhaps 10 years a whole bunch of other experiments pointed to the same conclusion, and Millikan's experiment calculating the charge (therefore the mass, from their known ratio) was less than 15 years away. If anything, Thomson was the positivist here -- he made interpretations of his observations that were subject to falsification. Your story about Thomson and Kaufman doesn't contradict anyone's point of view: it is exactly how science is supposed to work!

Meanwhile, the theory-that-you-do-not-name has been around for about 35 years now, and I think claims that it does not need to be subject to falsification (likewise multiverse, inflation, etc) are seriously problematic.

Unknown said...

Thank you! An excellent start to what I am sure will be an excellent post (and welcome back to your blog; let me assure you that you've been missed). Now waiting eagerly for the continuation! Talk about cliff-hangers... leaving us in suspense...

Sunil Mukhi said...

Rahul, I'll explain all the confusions in the next couple of articles. Also I will name string theory and discuss its role, but only after some context has been established.

Gaurav Goswami said...

I had been waiting for a post on this blog for a very long time. Moreover, the issues you raise are increasingly becoming important not only for fundamental physics but also to cosmology (e.g. inflation and cosmological constant problem), so thanks a lot for beginning to share your views here.