What is science? What makes a theory scientific?
There’s a picture we learn in high school. It’s not the whole story, certainly: philosophers of science have much more sophisticated notions. But for practicing scientists, it’s a picture that often sits in the back of our minds, informing what we do. Because of that, it’s worth examining in detail.
In the high school picture, scientific theories make predictions. Importantly, postdictions don’t count: if you “predict” something that already happened, it’s too easy to cheat and adjust your prediction. Also, your predictions must be different from those of other theories. If all you can do is explain the same results with different words you aren’t doing science, you’re doing “something else” (“metaphysics”, “religion”, “mathematics”…whatever the person you’re talking to wants to make fun of, but definitely not science).
Seems reasonable, right? Let’s try a thought experiment.
In the late 1950’s, the physics of protons and neutrons was still quite mysterious. They seemed to be part of a bewildering zoo of particles that no-one could properly explain. In the 60’s and 70’s the field started converging on the right explanation, from Gell-Mann’s eightfold way to the parton model to the full theory of quantum chromodynamics (QCD for short). Today we understand the theory well enough to package things into computer code: amplitudes programs like BlackHat for collisions of individual quarks, jet algorithms that describe how those quarks become signals in colliders, lattice QCD implemented on supercomputers for pretty much everything else.
Now imagine that you had a time machine, prodigious programming skills, and a grudge against 60’s era-physicists.
Suppose you wrote a computer program that combined the best of QCD in the modern world. BlackHat and more from the amplitudes side, the best jet algorithms and lattice QCD code, and more: a program that could reproduce any calculation in QCD that anyone can do today. Further, suppose you don’t care about silly things like making your code readable. Since I began the list above with BlackHat, we’ll call the combined box of different codes BlackBox.
Now suppose you went back in time, and told the bewildered scientists of the 50’s that nuclear physics was governed by a very complicated set of laws: the ones implemented in BlackBox.
Your “BlackBox theory” passes the high school test. Not only would it match all previous observations, it could make predictions for any experiment the scientists of the 50’s could devise. Up until the present day, your theory would match observations as well as…well as well as QCD does today.
(Let’s ignore for the moment that they didn’t have computers that could run this code in the 50’s. This is a thought experiment, we can fudge things a bit.)
Now suppose that one of those enterprising 60’s scientists, Gell-Mann or Feynman or the like, noticed a pattern. Maybe they got it from an experiment scattering electrons off of protons, maybe they saw it in BlackBox’s code. They notice that different parts of “BlackBox theory” run on related rules. Based on those rules, they suggest a deeper reality: protons are made of quarks!
But is this “quark theory” scientific?
“Quark theory” doesn’t make any new predictions. Anything you could predict with quarks, you could predict with BlackBox. According to the high school picture of science, for these 60’s scientists quarks wouldn’t be scientific: they would be “something else”, metaphysics or religion or mathematics.
And in practice? I doubt that many scientists would care.
“Quark theory” makes the same predictions as BlackBox theory, but I think most of us understand that it’s a better theory. It actually explains what’s going on. It takes different parts of BlackBox and unifies them into a simpler whole. And even without new predictions, that would be enough for the scientists in our thought experiment to accept it as science.
Why am I thinking about this? For two reasons:
First, I want to think about what happens when we get to a final theory, a “Theory of Everything”. It’s probably ridiculously arrogant to think we’re anywhere close to that yet, but nonetheless the question is on physicists’ minds more than it has been for most of history.
Right now, the Standard Model has many free parameters, numbers we can’t predict and must fix based on experiments. Suppose there are two options for a final theory: one that has a free parameter, and one that doesn’t. Once that one free parameter is fixed, both theories will match every test you could ever devise (they’re theories of everything, after all).
If we come up with both theories before testing that final parameter, then all is well. The theory with no free parameters will predict the result of that final experiment, the other theory won’t, so the theory without the extra parameter wins the high school test.
What if we do the experiment first, though?
If we do, then we’re in a strange situation. Our “prediction” of the one free parameter is now a “postdiction”. We’ve matched numbers, sure, but by the high school picture we aren’t doing science. Our theory, the same theory that was scientific if history went the other way, is now relegated to metaphysics/religion/mathematics.
I don’t know about you, but I’m uncomfortable with the idea that what is or is not science depends on historical chance. I don’t like the idea that we could be stuck with a theory that doesn’t explain everything, simply because our experimentalists were able to work a bit faster.
My second reason focuses on the here and now. You might think we have nothing like BlackBox on offer, no time travelers taunting us with poorly commented code. But we’ve always had the option of our own Black Box theory: experiment itself.
The Standard Model fixes some of its parameters from experimental results. You do a few experiments, and you can predict the results of all the others. But why stop there? Why not fix all of our parameters with experiments? Why not fix everything with experiments?
That’s the Black Box Theory of Everything. Each individual experiment you could possibly do gets its own parameter, describing the result of that experiment. You do the experiment, fix that parameter, then move on to the next experiment. Your theory will never be falsified, you will never be proven wrong. Sure, you never predict anything either, but that’s just an extreme case of what we have now, where the Standard Model can’t predict the mass of the Higgs.
What’s wrong with the Black Box Theory? (I trust we can all agree that it’s wrong.)
It’s not just that it can’t make predictions. You could make it a Black Box All But One Theory instead, that predicts one experiment and takes every other experiment as input. You could even make a Black Box Except the Standard Model Theory, that predicts everything we can predict now and just leaves out everything we’re still confused by.
The Black Box Theory is wrong because the high school picture of what counts as science is wrong. The high school picture is a useful guide, it’s a good rule of thumb, but it’s not the ultimate definition of science. And especially now, when we’re starting to ask questions about final theories and ultimate parameters, we can’t cling to the high school picture. We have to be willing to actually think, to listen to the philosophers and consider our own motivations, to figure out what, in the end, we actually mean by science.
It’s for these kinds of reasons that I think students should be required to take a “philosophy of science” course. It’s unfortunately all too common for physicists to adopt a naively scientistic and falsificationist stance that breaks down under scrutiny. I’ve always found it ironic that the dissonance you are pointing out is particularly striking in the case of physics pedagogy, where unfalsifiable realist mental models that coherently and parsimoniously unify, explain, and postdict previously encountered facts, are taken for granted by virtually every physicist as synonymous with “conceptual understanding” and “physical insight.”
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“First, I want to think about what happens when we get to a final theory, a “Theory of Everything”. It’s probably ridiculously arrogant to think we’re anywhere close to that yet…”
This is a common sentiment among physicists and I want to push back against it. We might be close to a final theory of everything or we might not be. It’s not arrogant to suspect either possibility is true. It is arrogant to assert with high confidence that there’s something fundamental we’re missing, just as it’s arrogant to assert that the final theory is right around the corner.
The argument people usually make is that there have been so many times that when physicists were confident they were on the cusp of a theory of everything only to discover they were completely missing something fundamental. First of all, it’s actually not that many times: I dare you to name more than four. The only example I usually see is the period right before quantum mechanics. This history doesn’t prove much: if modern physics has been going for 500 years without finding a fundamental theory of everything then the naive estimate is that the chance it will be found in the next 50 year is no more than 10%, which can still be a significant chance.
Second, the state of our knowledge today is genuinely better than it was 100 years ago or earlier. As Sean Carrol points out, we completely understand the laws of physics underlying everyday life, which is not something any previous paradigm before our own claimed. The reason why physics research was so slow in the last decades is that we understand physics so well that if we want to find anything at all we can’t account for it takes thousands of collaborators running giant machines and even that usually doesn’t work. The number of remaining mysteries is so small that a single physicist can know about all of them, and many do. Except perhaps for the cosmological constant problem, every mystery can in principle be explained using existing paradigms, although we have no way of knowing which if any of the explanations is correct. (I’m counting the quantum measurement problem and the problem of consciousness as philosophy rather than physics.) Although fundamental physics is currently at a standstill and it’s possible we’re missing some fundamental stuff I also think it’s possible a theory of everything is near.
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Science has always seemed as a bit of magic compared to maths.
In maths everything is a given
But in science you have to decide what assumptions to make, which are the critical assumptions and parameters etc
So if a model predicted everything without explaining anything then it would be maths rather than science
In science, I expect, first you have to explain and then predict new things which you didn’t explain
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