Do electrons actually spin? Here’s why the answer is really important

[Mar. 19, 2023: Whitney Clavin, CalTech]

Deep within all matter in the universe, electrons buzz and behave as if spinning on their axes like spinning tops. (CREDIT: Creative Commons)

Deep within all matter in the universe, electrons buzz and behave as if spinning on their axes like spinning tops. These “spinning” electrons are fundamental to quantum physics and play a central role in our understanding of atoms and molecules. Other subatomic particles also spin, and the study of spin has technical applications in chemistry, physics, medicine, and computer electronics.

But many physicists will tell you that electrons don’t really spin, they just act like them. For example, electrons have angular momentum, which is the tendency of something to keep spinning – like a moving bicycle wheel or a spinning skater – and because they have this property one might conclude that they turn. Further evidence comes from the fact that electrons act like small magnets and magnetic fields come from rotating charged bodies.

The problem with the notion that electrons spin is that due to their tiny size, electrons would have to spin faster than the speed of light to match the observed angular momentum values. (Think of an electron as a spinning skater with arms folded inward: the smaller the overall size, the faster it spins.)

Chip Sebens, assistant professor of philosophy at Caltech, wants to go back to the drawing board and rethink this notion. As a philosopher of physics, he wants to understand what is really going on at the deepest levels of nature.

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“Philosophers tend to be drawn to problems that haven’t been solved for a very long time,” says Sebens. “In quantum mechanics, we have ways of predicting the results of experiments that work very well for electrons and take spin into account, but important fundamental questions remain unanswered: why do these methods work and what happens? inside an atom?

To that end, Sebens explained why he thinks electrons and other subatomic particles actually spin. The answer has to do with fields.

In nature, there are both particles and fields. Physicists tend to think of fields as more fundamental than particles, but philosophers of physics still debate which entity is more fundamental. For example, light can be described as a beam of photons or as a wave in the electromagnetic field. This area of ​​science is called quantum field theory.

Chip Sebens. (Credit: Caltech)

The late Richard Feynman, a Caltech physicist and Nobel laureate, worked on aspects of this theory by creating his famous Feynman diagrams, which map interactions between particles like electrons and photons, indirectly describing fields. “Quantum field theory is the best physics we have,” says Sebens.

In several studies, including a recent article in the Synthesis review, Sebens explains why he thinks an electron is not a dot-sized particle that just acts as if it’s spinning, but rather a blob of spread out charge that actually spins. Going back to the ice skater analogy, the electron is more like a skater with arms outstretched.

In the field approach, a z-spin up electron is classically modeled as a concentration of energy and charge in the classical Dirac field (where the charge density is represented here by a gray cloud). (Credit: Caltech)

“In an atom, the electron is often represented as a cloud showing where the electron might be found, but I think the electron is actually physically distributed over this cloud,” says Sebens.

With the size of the electron spread out, the electron is now large enough to avoid the problem of having to move faster than the speed of light. In this case, explains Sebens, there are two important fields: the electromagnetic field as well as what is called the Dirac field, named after the physicist Paul Dirac. “Just as the electromagnetic field describes photons, the Dirac field describes electrons and positrons,” he says. The positron is the antiparticle of the electron.

In 1924, Wolfgang Pauli proposed that electrons possess “hidden rotation” with a symmetry of 720 degrees. (CREDIT: Wikimedia)

The research is part of an overall effort by Sebens to answer the question of whether nature is, at its core, constructed from fields or particles. In the same review article, Sebens argues that fields are more fundamental in nature.

Part of his argument is based on spin. As mentioned above, a field approach makes sense of the confusion that arises with spinning electrons. He also argues that the field approach helps answer another important question about electrons: how do electrons react to the electromagnetic fields they create?

If the electron is a point-sized ball of charge, the field it creates is infinitely strong at the location of the electron. This means that the field would have no defined direction and therefore no defined forces, which leads to problems in calculating the forces. But if the electron were instead an extended charge field, the forces on the different parts of the electron would be finite with well-defined directions.

“It makes the problem of self-interaction less severe,” writes Sebens in an Aeon essay on the fundamentals of nature. “But it’s not solved. If the charge of the electron is spread out, why don’t the different parts of the electron repel each other so that the electron explodes quickly?”

Sebens addresses this problem of self-repulsion in his ongoing research. Answers to this problem and others he studies could ultimately lead to new and better ways to calculate and measure quantities in quantum physics. The work could even lead to new ways to answer a recurring question in quantum physics called the quantum measurement problem.

When measuring a quantum system, such as an electron in a state of superposition (in two states at once), the system will collapse and the electron will take one state or another. Physicists are still debating why this happens. Research into the fundamentals of how particles and fields work could help solve the mystery.

Sebens writes in the Aeon essay, “Sometimes progress in physics first requires going back to re-examine, reinterpret, and revise the theories we already have. To do this kind of research, we need researchers who mix the roles of physicist and philosopher, as was the case thousands of years ago in ancient Greece.”

For more scientific news, see our New discoveries section on The bright side of the news.

Note: The documents provided above by CalTech. Content may be edited for style and length.

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