Most physics textbooks will tell you that electrons do not really spin, but behave as if they do. What is actually going on? Charles T. (Chip) Sebens, a new assistant professor of philosophy at Caltech, ponders questions like this—he does in fact think electrons do spin—in addition to other mysteries lying at the roots of physics. He hopes his philosophical tools will help make theories of quantum physics more precise and help science progress.
Sebens started out in physics as an undergraduate at MIT, but then ended up double majoring in both physics and philosophy, earning dual bachelor's degrees in 2009. He went on to earn a master's degree in philosophy of physics from the University of Oxford (2010), a master's degree in physics from the University of Michigan (2013), and a PhD in philosophy, also from University of Michigan (2015). Sebens served as an Ahmanson Postdoctoral Instructor in the History and Philosophy of Physics at Caltech from 2015 to 2016, during which he began a collaboration with Sean Carroll, Research Professor of Physics at Caltech. He then served as an assistant professor of philosophy at UC San Diego from 2016 to 2018 before returning to Caltech as a faculty member.
We sat down with him to talk about how philosophy differs from theoretical physics, his science fiction and philosophy class, and more.
How did you become interested in the philosophy of physics?
I had wanted to be a theoretical physicist and started out in physics as an undergrad. At MIT, we were required to take humanities classes and so I took philosophy, which I thought would just be tolerable. But I ended up really liking it and ultimately switched over. However, I try to keep a foot in both disciplines as much as possible.
Can you tell us about your science fiction and philosophy class?
Yes, I recently taught a class called Philosophy Through Science Fiction. Science fiction often poses important philosophical questions. For example, in the television series Black Mirror, in an episode called "San Junipero," a future is depicted where some people live happy lives in a virtual world. Watching this episode may lead one to wonder how we ought to judge the quality of a person's life. What makes a life good? Science fiction does a great job raising questions like this and inspiring you to think about them. But the fiction isn't trying to actually answer these deep questions. That's where philosophy comes in. Philosophers are doing their best to find answers. In my class, we use science fiction as motivation and then dive into the philosophy.
What are some of the areas of physics you focus on?
A lot of my earlier work was on the interpretation of quantum mechanics, and I've written some papers on that subject with Sean Carroll. The big question is to figure out what's really going on in quantum mechanics. Take the idea of quantum wave functions collapsing when you observe them. This is commonly explained by saying something like, "A particle can be in two places at once until you look. Then, the particle's wave function collapses and the particle jumps to a particular location." If this collapse is supposed to be a real physical process that only happens when someone makes an observation, quantum mechanics would be completely unlike any other kind of physics. I don't think the rules that nature follows should depend on whether you are looking or not.
Taking collapse less seriously, you might think of quantum mechanics more like a recipe or a set of tools. The equations are useful for making predictions and calculating probabilities. But they don't really tell you why things turn out that way or what's actually happening. This might be fine for some practical purposes, but it's an incomplete picture. Why must we give up on the project of understanding what's actually happening in nature?
Philosophers are interested in versions of quantum mechanics that don't refer to observers in the laws. I've studied a handful of options, one of which is the many-worlds interpretation. This interpretation includes multiple parallel universes. The wave function never really collapses, but there is an appearance of collapse when the wave function splits into different parts (as will happen during quantum measurements). This splitting is a natural physical phenomenon, governed by the same laws as any other process.
What are you working on now?
I've just finished a few papers. One is on the question of whether electrons really spin. In quantum mechanics, electrons and other fundamental particles have a property called "spin." But the textbooks will tell you that electrons are not literally spinning. They use the word "spin" because electrons act like little bar magnets and have angular momentum, just as you'd expect for rotating charged bodies. However, it's thought that electrons can't really be spinning because they are too small—they're often treated as point-size. That leaves us in an awkward position: electrons act like they spin but don't actually spin.
I've written a paper arguing that the electron really does spin. Basically, using something called the Dirac field, I show that the mass and charge of an electron flow in circles around its center—sort of like the clouds in a hurricane. In a full quantum theory, you can think of the electron as being in a superposition of different states of spinning.
What do you see as the purpose of the philosophy of physics?
I think it's helpful to think of philosophy as being on the border of physics. Some people do philosophy of physics with the idea that they want to solve philosophical problems using physics. I am actually more interested in going the other way. I'm hoping that this kind of philosophical questioning can actually help physics progress. My goal is to produce outputs that are of interest to physicists. As an example, the question of quantum gravity is a big one right now in physics—how to make the laws of the macroscopic world (i.e., general relativity) compatible with the laws of the microscopic world (i.e., quantum physics). But to do that, we need clear foundations. With quantum physics, we still have a lot to figure out.