Physics

# Confounded by Quantum? Try Bohmian Mechanics on for Size

To accept quantum mechanics, you have to accept some pretty strange things: the idea that matter can be both a particle and a wave at once, for instance. Despite this, quantum mechanics has successfully predicted the result of every experiment we've thrown at it so far. But there's an alternative view, one that's a century old but still championed by its share of physicists: Bohmian mechanics, or pilot-wave theory. In this theory, particles are always particles and waves are always waves. Simple, right? Don't make up your mind yet — it definitely has its drawbacks.

## Surf's Up!

To understand the differences between quantum mechanics and Bohmian mechanics, you can think about the infamous double-slit experiment, which you can read more about here. As a refresher, the experiment sends a beam of electrons at two parallel slits in a sheet of metal that sit in front of a detector. If you measure the electrons to see which ones went through which slits, you get two bright spots, one in front of each slit, as if the electrons were behaving like particles. But if you don't observe the electrons at all, the detector reveals a pattern of light and dark bands, as if the electrons were behaving like waves.

Someone who sticks with the so-called Copenhagen interpretation of quantum mechanics would explain this by saying that electrons don't have definite positions — they behave as both particles and waves until they're observed. That's why they spread out like waves on the detector when they aren't being measured, but once they're observed, each one "chooses" a position and behaves more like an individual particle. Someone who adheres to the pilot-wave interpretation of Bohmian mechanics, however, would say that electrons always have a definite position, but are pushed around on a "pilot wave." It's that pilot wave that changes its behavior: When unobserved, it takes electrons along all possible paths to create a wave-like interference pattern, but when observed, it collapses and shows the true location of each electron. The big difference between the two theories is that Bohmian mechanics is deterministic: it makes predictions about where particles will be at any given point. Quantum mechanics only deals in probabilities.

## Not So Fast, Bohm

It's easy to see the appeal in Bohmian mechanics. The theory "gives you a pretty straightforward account of how the world is," as Griffith University physicist Howard Wiseman told Quanta Magazine. "You don't have to tie yourself into any sort of philosophical knots to say how things really are." Except ... you kind of do. For one thing, it relies even more heavily than quantum mechanics does on a principle called nonlocality, or the idea that the behavior of one particle depends on the behavior of another. A much larger issue, however, is that it doesn't account for Einstein's theory of relativity. "That means it's at best incomplete," says Matt O'Dowd of PBS Space Time. Even Einstein himself, an early proponent of the theory, eventually became jaded. "Have you noticed that Bohm believes (as de Broglie did, by the way, 25 years ago) that he is able to interpret the quantum theory in deterministic terms?" he wrote to physicist Max Born in 1952. "That way seems too cheap to me."

Still, the debate over quantum and Bohmian rages on. In 1992, the so-called ESSW paper was hailed as the final nail in the theory's coffin because it explained how a Bohmian explanation for the double-slit experiment was impossible. But in 2016, researchers published a study in Science Advances in which they actually performed the experiment and showed how it was possible. Even so, Bohmian mechanics lacks some important elements. It still has some explaining to do.