Experimental Theoretical Physics

I was talking with some other physicists about my “Black Box Theory” thought experiment, where theorists have to compete with an impenetrable block of computer code. Even if the theorists come up with a “better” theory, that theory won’t predict anything that the code couldn’t already. If “predicting something new” is an essential part of science, then the theorists can no longer do science at all.

One of my colleagues made an interesting point: in the thought experiment, the theorists can’t predict new behaviors of reality. But they can predict new behaviors of the code.

Even when we have the right theory to describe the world, we can’t always calculate its consequences. Often we’re stuck in the same position as the theorists in the thought experiment, trying to understand the output of a theory that might as well be a black box. Increasingly, we are employing a kind of “experimental theoretical physics”. We try to predict the result of new calculations, just as experimentalists try to predict the result of new experiments.

This experimental approach seems to be a genuine cultural difference between physics and mathematics. There is such a thing as experimental mathematics, to be clear. And while mathematicians prefer proof, they’re not averse to working from a good conjecture. But when mathematicians calculate and conjecture, they still try to set a firm foundation. They’re precise about what they mean, and careful about what they imply.

“Experimental theoretical physics”, on the other hand, is much more like experimental physics itself. Physicists look for plausible patterns in the “data”, seeing if they make sense in some “physical” way. The conjectures aren’t always sharply posed, and the leaps of reasoning are often more reckless than the leaps of experimental mathematicians. We try to use intuition gleaned from a history of experiments on, and calculations about, the physical world.

There’s a real danger here, because mathematical formulas don’t behave like nature does. When we look at nature, we expect it to behave statistically. If we look at a large number of examples, we get more and more confident that they represent the behavior of the whole. This is sometimes dangerous in nature, but it’s even more dangerous in mathematics, because it’s often not clear what a good “sample” even is. Proving something is true “most of the time” is vastly different from proving it is true all of the time, especially when you’re looking at an infinity of possible examples. We can’t meet our favorite “five sigma” level of statistical confidence, or even know if we’re close.

At the same time, experimental theoretical physics has real power. Experience may be a bad guide to mathematics, but it’s a better guide to the mathematics that specifically shows up in physics. And in practice, our recklessness can accomplish great things, uncovering behaviors mathematicians would never have found by themselves.

The key is to always keep in mind that the two fields are different. “Experimental theoretical physics” isn’t mathematics, and it isn’t pretending to be, any more than experimental physics is pretending to be theoretical physics. We’re gathering data and advancing tentative explanations, but we’re fully aware that they may not hold up when examined with full rigor. We want to inspire, to raise questions and get people to think about the principles that govern the messy physical theories we use to describe our world. Experimental physics, theoretical physics, and mathematics are all part of a shared ecosystem, and each has its role to play.

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