Physicists Gotta Physics: Why New Experiments Are Inevitable

My copy of Sabine Hossenfelder's LOST IN MATH.

Chad Orzel

Sabine Hossenfelder has a new blog post in her ongoing series about how she thinks fundamental physics is on the wrong track, this one taking a dim view of dark-matter detection experiments. While I'm generally sympathetic to her call for new theoretical approaches, this is a nice demonstration of where I part company with her, and why.

In the post, Hossenfelder notes that past approaches to dark matter searches have pretty definitively failed to find anything. The response of the community to this has been a kind of doubling-down:

Instead, their strategy is now to fund any proposed experiment that could plausibly be said to maybe detect something that could potentially be a hypothetical dark matter particle. And since there are infinitely many such hypothetical particles, we are now well on the way to building infinitely many detectors. DNA, carbon nanotubes, diamonds, old rocks, atomic clocks, superfluid helium, qubits, Aharonov-Bohm, cold atom gases, you name it. Let us call it the equal opportunity approach to dark matter search.

As it should be, everyone benefits from the equal opportunity approach. Theorists invent new particles (papers will be written). Experimentalists use those invented particles as motivation to propose experiments (more papers will be written). With a little luck they get funding and do the experiment (even more papers). Eventually, experiments conclude they didn't find anything (papers, papers, papers!).

As you can tell by the tone, she's not really on board with this approach, though it's more (grudgingly) acceptable than building a bigger particle accelerator.

As an experimentalist by training and inclination, who knows a lot of people involved in different schemes of this general sort, I have a number of problems with this take. I think it overestimates the specificity of these approaches (many of which are not tied to specific theoretical predictions, but general classes of things that might happen), and underestimates their value as test beds for general experimental tools and techniques. I also think that a lot of these experiments have less to do with fundamental theory than a fundamental theorist might think-- that is, many of these searches are being undertaken now not because of any old failures or new developments on the theory side, but because a particular technology for manipulating some interesting system has advanced to a point where it can play a role. The exotic-physics searches in many cases are a fun sideline for a larger community developing high-precision sensors for more practical purposes.

At the most basic level, though, my problem is that I don't know what else Hossenfelder expects the physicists involved to do.

To some extent, I have the same question on the theory side. I'm sympathetic to the call for new theoretical approaches, but I have kind of a hard time picturing what this is supposed to look like, particularly when it comes to the "papers, papers, papers!" part of things. Should fundamental particle theorists be retreating into monastic seclusion, vowing to produce no new work in existing paradigms until they come up with something better?

I'm even less clear, though, on what experimental physicists are supposed to do while the theorists radically restructure their field. What does it mean to "think more carefully before commissioning experiments to search for hypothetical particles"? Just sit around and wait for the theorists to emerge from seclusion, like one of the outer layers of the concents in Anathem?

At the end of the day, to put it maybe too colloquially, physicists gotta physics. The nature of the business is to attract people who are curious, have ideas about how the universe, and want to test those ideas. By all means, we should have a greater diversity in the sorts of ideas we test. But just because you don't like the ideas we have and the paradigms by which they're generated doesn't make it invalid to want to test them using the tools available. That's what physicists do, particularly experimentalists.

And again, to some extent, I have a similar problem with denunciations of "bump chasing" on the theory side, where some tantalizing hint of a signal generates a flood of papers explaining it in terms of every individual pet theory, only to have the signal disappear when more data comes in. While I agree that it would be annoying to have to read all those papers (and I'm grateful that I don't need to), I just don't quite see what plausible activity would be a better use of those theorists' time.

As I said the last time I failed to escape this argument, I have some sympathy for the view that we could do a better job of allocating resources, and not a lot of patience for many of the counter-arguments. At the same time, though, I feel like a lot of the "you're Doing It Wrong" arguments end up denouncing physicists for simply doing what physicists do, and I don't think that's particularly helpful.

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