Should Unprovable Physics Be Considered Philosophy?

In some large part, science is powerful not because of ideas but because of how it treats ideas. Science asks, prove it. The distinction is what separates science from philosophy: falsifiable claims and experimentation. The Higgs boson was understood a half-century ago as a necessary component of physics, yet we spent $9 billion on a machine to observe it IRL. Until then the Higgs boson was only probably true.

Physics, cosmology in particular, is at an interesting and potentially dangerous crossroads, as argued in a ​recent, sharp piece in Nature by physicists Joseph Silk and George Ellis. In short, it would appear that theory, particularly neat-o ideas like string theory and the multiverse, has reached the outer limits of provability. We can't access the higher dimensions of string theory, nor can we observe (or not observe) our would-be sibling universes. Their fate is idea limbo, forever between notion and fact.

String theory and the multiverse are concepts that by definition defy experimentation, and yet a small movement within cosmology is attempting to make the case that they should be exempt. At stake, according to Ellis and Silk, is the integrity of science itself.

"This battle for the heart and soul of physics is opening up at a time when scientific results—in topics from climate change to the theory of evolution—are being questioned by some politicians and religious fundamentalists," the pair writes. "Potential damage to public confidence in science and to the nature of fundamental physics needs to be contained by deeper dialogue between scientists and philosophers."

The opposing view, popularly argued by cosmologist and writer Sean Carroll, is this: An idea might be exempt from experimentation if it's sufficiently elegant and explanatory. For example, string theory is (supposedly) the only framework that sufficiently unifies the four fundamental forces of nature (gravity, electromagnetism, the weak and strong forces), therefore it contains some grain of truth, even though that truth can never be experimentally demonstrated in its entirety. (​Supersymmetry doesn't prove strings.)

Another voice within this movement is that of philosopher and theorist Richard Dawid. Dawid argues that we can use probability as a stand-in for experiment. That is, using Bayesian analysis, it's possible to determine the probability that a set of facts fits a theory. If the probability is good enough, we can chuck testability. Dawid argues that, because, "no-one has found a good alternative" and "theories without alternatives tended to be viable in the past," string theory should be assumed legitimate.

In essence, he's arguing that theorized discoveries can be taken as evidence for fundamental theories. If we had the capability of conducting some experiment, it would probably have this outcome because the mathematics works out. Ellis and Silk argue simply that that's not good enough, for theoretical physics or any science.

The situation is similar for multiverse theories, which explain the fundamental constants of the universe (why everything is "just right" for human life) away as unspecial by claiming that in fact there are an infinite number of parallel universes composed of not just every alternative for those constants, but also any possibility for anything. Choices are never made in this reality, only new universes. There is an entire realm that exists in which I got two slices of pizza for lunch instead of three, and there is an entire realm that exists in which the strong force isn't strong enough to form atomic nuclei. Cool.

"Billions of universes—and of galaxies and copies of each of us—accumulate with no possibility of communication between them or of testing their reality," Ellis and Silk write. "But if a duplicate self exists in every multiverse domain and there are infinitely many, which is the real 'me' that I experience now? Is any version of oneself preferred over any other? How could 'I' ever know what the 'true' nature of reality is if one self favours the multiverse and another does not?" Stoners, beware.

"Post-empirical science is an oxymoron," the pair concludes. "Theories such as quantum mechanics and relativity turned out well because they made predictions that survived testing. Yet numerous historical examples point to how, in the absence of adequate data, elegant and compelling ideas led researchers in the wrong direction, from Ptolemy's geocentric theories of the cosmos to Lord Kelvin's 'vortex theory' of the atom and Fred Hoyle's perpetual steady-state Universe."

The scientific high-ground is at stake, with an ocean of pseudoscientists ready to flood the landscape, taking the public with them. The answer, according to the current paper, lies in a simple question. What observational or experimental evidence is there that would convince a theorist that their theory is wrong? If there is none, then the theory is not a scientific theory.