‘Spin’ is a fundamental quality of fundamental particles like the electron, invoking images of a tiny sphere revolving rapidly on its axis like a planet in a shrunken solar system.
Only it isn’t. It can’t. For one thing, electrons aren’t spheres of matter but points described by the mathematics of probability.
But California Institute of Technology philosopher of physics Charles T. Sebens argues such a particle-based approach to one of the most accurate theories in physics might be misleading us.
By framing the groundwork of matter primarily in terms of fields, he says, certain peculiarities and paradoxes that emerge from a particle-centric view melt away.
“Philosophers tend to be attracted to problems that have been unsolved for a really long time,” says Sebens.
“In quantum mechanics, we have ways of predicting the results of experiments that work very well for electrons and account for a spin, but important foundational questions remain unanswered: Why do these methods work, and what’s happening inside an atom?”
For the better part of a century, physicists have wrestled with the results of experiments that suggest the smallest pieces of reality don’t look or behave anything like objects in our everyday lives.
Spin is one of these qualities. Like a whirling cue ball colliding with the inner wall of a billiard table, it carries angular momentum and influences the direction of a moving particle. Yet, unlike the cue ball, a particle’s spin can never speed up or slow down – rather, it’s always confined to a set value.
To make the basic nature of matter even harder to picture, consider the fact an electron’s size is so small that it effectively lacks volume. If it were large enough to have volume, the negative charge spread throughout that space would push on itself, tearing the electron apart.
Significantly, even if we were to be charitable and grant the electron as a particle the largest radius experiments would allow for, its rotation would overtake the speed of light – something which might or might not be a deal-breaker on this scale, but for many physicists is enough to dismiss talk of rotating electrons.
One way to make the tapestry of fundamental physics a little easier to map is to describe points of matter as actions embedded into the weave of a field and then interpret these actions as particles.
Quantum field theory (QFT) does this successfully, weaving together aspects of Einstein’s special theory of relativity, classical field theory, and the particle propositions of quantum physics.
It’s not a controversial theory, yet there is still debate over whether those fields are fundamental – existing even if the blips that ripple through them were to fall silent – or if particles are the main actors that represent the vital information and fields are just a convenient script.
To us, it might seem like a trivial distinction. But to philosophers like Sebens, the consequences are worth exploring.
As he explained in a 2019 article featured in Aeon magazine: “Sometimes progress in physics requires first backing up to reexamine, reinterpret, and revise the theories that we already have.”
That reexamination of quantum field theory emphasizes several significant advantages to making fields a priority in physics over a particle-first approach, including a model that re-imagines electrons in ways that might give us better insights into their behavior.
“In an atom, the electron is often depicted as a cloud showing where the electron might be found, but I think that the electron is actually physically spread out over that cloud,” Sebens says.
By being physically spread out through a field rather than confined to a point, an electron might actually rotate in ways that are less mathematical constructs and more a physical description.
Although it would still not be anything like a tiny planet in a solar system, this rotating electron would at least move at a speed that doesn’t challenge any laws.
Just how this diffuse spread of negatively charged matter resists blowing itself apart is a question Sebens doesn’t have an answer for. But by focusing on the field aspects of a spread-out electron, he feels any solutions would make more sense than issues that arise from particles of infinite confinement.
There’s a quote that has become folklore in the halls of quantum theorists – “Shut up and calculate.” It’s become a saying synonymous with the aphantasic landscape of the quantum realm, where imagery and metaphor fail to compete with the uncanny precision of pure mathematics.
Every now and then, though, it’s important to pause our calculations and indulge in challenging a few old assumptions – and maybe even turn around for a new perspective on the fundamentals of physics.
This paper was published in Synthese.