There’s an article by Matthew R. Francis in Symmetry magazine garnering a lot of attention asking whether falsifiability is a useful criteria for scientific theories.
Popper wrote in his classic book The Logic of Scientific Discovery that a theory that cannot be proven false—that is, a theory flexible enough to encompass every possible experimental outcome—is scientifically useless. He wrote that a scientific idea must contain the key to its own downfall: It must make predictions that can be tested and, if those predictions are proven false, the theory must be jettisoned.
If you think about it, Popper’s criteria is simply that for a theory, a model, to be scientific, there must be something about observable reality that is different if it is true instead of false. There are a wide variety of conditions that could satisfy this criteria. It’s actually pretty broad. But apparently not broad enough for some.
But where does this falsifiability requirement leave certain areas of theoretical physics? String theory, for example, involves physics on extremely small length scales unreachable by any foreseeable experiment. Cosmic inflation, a theory that explains much about the properties of the observable universe, may itself be untestable through direct observations. Some critics believe these theories are unfalsifiable and, for that reason, are of dubious scientific value.
At the same time, many physicists align with philosophers of science who identified flaws in Popper’s model, saying falsification is most useful in identifying blatant pseudoscience (the flat-Earth hypothesis, again) but relatively unimportant for judging theories growing out of established paradigms in science.
Physicist Sabine Hossenfelder has a response up on her blog that is well worth reading. Both Francis and Hossenfelder discuss situations in which a rigid adherence to falsifiability is problematic, although Francis allows for a broader scope for it than Hossenfelder.
My own attitude, as a layperson and skeptic, is that it matters to me whether a theory has been tested, or can be tested in some plausibly foreseeable scenario. I’ll grant that scientists need space to work on speculative ideas, but as Hossenfelder notes, if there is never any pressure to eventually find testable predictions in those ideas, then they eventually become just metaphysical philosophy. Not that there’s anything wrong necessarily with metaphysics, but it doesn’t enjoy the credibility of science for a reason.
Anyway, there are a couple of points the Symmetry article makes that I want to comment on.
On that note, Caltech cosmologist Sean M. Carroll argues that many very useful theories have both falsifiable and unfalsifiable predictions. Some aspects may be testable in principle, but not by any experiment or observation we can perform with existing technology. Many particle physics models fall into that category, but that doesn’t stop physicists from finding them useful. SUSY as a concept may not be falsifiable, but many specific models within the broad framework certainly are. All the evidence we have for the existence of dark matter is indirect, which won’t go away even if laboratory experiments never find dark matter particles. Physicists accept the concept of dark matter because it works.
First is the observation that well established theories often make untested predictions. Sure they do. But those theories are well established because of the predictions that have been tested. And it’s well worth keeping in mind which predictions haven’t been tested yet, because it’s always possible that they reflect areas where even a well established theory might eventually have to be adjusted in the future.
But the other point is the use of the word “useful” here. In what way are the untestable theories useful? What about them makes them useful? How would they be different if they weren’t useful? Do they add value to other theories, value that makes the predictions of that other theory more accurate? If so, then congratulations, you’ve just made the useful theory falsifiable.
Or are they “useful” in some other manner involving their aesthetics or emotional appeal? Do they give us a feeling like we’ve explained something, plugged a hole in our knowledge of how something works, but without enhancing our ability to make predictions? If so, then this feels like what in psychology is often called a “just so” story.
Just-so stories are generally recognized as having little or no value. They’re just bias enforcing narratives we come up with that make us feel better about how something got to be the way it is, but not in giving us any real insights. The danger with such narratives is that if everyone is too satisfied with them, they might actually stifle investigation into areas that still need it.
(Of course, whether a particular theory has been or can be tested is inevitably a matter of judgment. I’ve had numerous people over the years point to a well accepted scientific theory they disliked and insisted that it either wasn’t falsifiable or that there was no evidence for it, and then insist that the evidence accepted by the vast majority of scientists wasn’t actually evidence. Even Popper had trouble with this, initially thinking that natural selection wasn’t falsifiable, a fact that delights the creationists who know about it.)
Falsifiability, in my understanding, is simply an insistence that a scientific theory must be epistemically useful, must enhance our ability to make more accurate predictions, directly or indirectly. If it doesn’t do that, or at least pave the way in some foreseeable manner for other theories that might do it, then the notion might have value as philosophy, but presenting it as settled science is misleading and, I think, puts the credibility of science in jeopardy.
Unless of course I’m missing something?