New Experiment Rules out Bohmian Mechanics. It’s Serious.

New Experiment Rules out Bohmian Mechanics. It’s Serious.

Sabine Hossenfelder • 7:13 minutes • Published 2025-07-20 • YouTube

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Quantum Mechanics, Bohmian Mechanics, and a New Experiment That Challenges Old Ideas

Quantum mechanics, the fundamental theory describing the behavior of particles at the smallest scales, has fascinated and puzzled physicists for over a century. One of the longstanding debates revolves around how to interpret the strange mathematics and probabilistic outcomes that quantum mechanics predicts. Among the many interpretations, one particularly beloved by some physicists is Bohmian mechanics, also known as the pilot-wave theory, proposed by David Bohm about 70 years ago.

What is Bohmian Mechanics?

In standard quantum mechanics, the state of a system is described by a wave function—a mathematical object that encodes the probabilities of different measurement outcomes. Crucially, the wave function itself is not directly observable. Instead, it serves as a tool to calculate the likelihood of finding a particle with a certain property upon measurement. The theory is inherently probabilistic and non-deterministic; it cannot predict exact outcomes but only probabilities.

Bohmian mechanics offers a different perspective. It breaks down the wave function into two parts: a guiding wave and pointlike particles that move according to this wave. The randomness in outcomes, in this view, arises not from fundamental indeterminism but from our ignorance about the precise initial positions of these particles. In other words, if we knew exactly where the particles started, we could in principle predict their trajectories and outcomes deterministically. This interpretation is appealing for those who prefer a particle-centric view of quantum phenomena.

However, Bohmian mechanics differs from standard quantum mechanics in important ways. Notably, it asserts that particle positions are the only true observables, whereas standard quantum mechanics allows measurements of various properties such as energy, spin, and momentum. This mismatch forces proponents of Bohmian mechanics to reinterpret measurements like spin in terms of particle positions, which can be conceptually challenging.

The New Experiment and Its Findings

Recently, an experimental breakthrough has put Bohmian mechanics to the test—and the results are striking. The experiment involved photons (particles of light) traveling through a narrow gap between two mirrors, forming a one-dimensional waveguide. One mirror had tiny carved valleys, shaping the waveguide's depth. The waveguide first widened, then suddenly narrowed, creating a barrier that photons could tunnel through quantum mechanically—a phenomenon where particles pass through barriers they classically shouldn't be able to cross.

Crucially, just before the barrier, a second waveguide was placed close enough for photons to tunnel into it. The experimenters measured how photons spread into this second waveguide after tunneling. According to Bohmian mechanics, the particles should have essentially zero velocity immediately after tunneling, meaning they would remain localized. Instead, the photons were observed to spread out, indicating they did have velocity and moved away from their initial position.

This observation directly contradicts the predictions of Bohmian mechanics but aligns perfectly with the standard quantum mechanical description, which treats the wave function's evolution as fundamental and probabilistic.

What Does This Mean?

Does this experiment mean Bohmian mechanics is completely wrong? The answer is nuanced. While the experiment rules out the idea that Bohmian particles correspond to the particles we observe in nature (like electrons or photons in the standard model), it doesn't necessarily discard all possible variants of hidden-variable theories. It highlights a fundamental mismatch: Bohmian particles are pointlike and deterministic, but the particles we measure have spatial extent, interactions, and complex quantum properties.

From a philosophical standpoint, this challenges the appeal of theories relying on point particles with deterministic trajectories. After all, the infinite precision required for such point particles is physically questionable.

Final Thoughts

This new experimental result marks a significant step forward in our understanding of quantum foundations. It narrows down the viable interpretations and pushes physicists to refine or rethink their models of reality at the quantum level. While Bohmian mechanics offered an intuitive particle picture, nature seems to resist such a simplistic view.

Protecting Your Privacy in a Digital World

On a related note, just as physics experiments seek to uncover hidden truths, many of us face challenges protecting our own personal information online. Data leaks and privacy invasions are common, such as unwanted scam calls resulting from phone numbers being exposed on the internet.

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Thanks for reading! Stay curious, and see you in the next post.

📝 Transcript (171 entries):

Quantum mechanics is more than a century old and physicists have argued about its meaning for almost as long. One of the most beloved interpretations comes from David Bow. It has it that quantum mechanics is really about pointlike particles that follow a guiding wave.

This interpretation now called bombian mechanics has now been ruled out by experiment. That's a pretty big deal after 70 years of argument. You could almost call it progress. In the standard formulation of quantum physics, everything is described by a wave function. The wave function itself is not observable. We just use it as a tool to calculate the probabilities with which we observe measurement results. In quantum physics, we can't predict exactly what the measurement outcome is, just this probability. It's therefore a non-deterministic theory with a fundamentally random element. Bowmanian mechanics. sometimes called pilot wave theory was proposed by David Bow about 70 years ago. At first glance, it seems to be just a reformulation of the mathematics of quantum physics. One takes the wave function apart into an equation for a guiding field plus the equation for particles moving in that field. One then says, well, but we didn't know exactly where the particles started. we just have a probability distribution of starting positions and correspondingly we get a probability for the possible end positions. This means that the observations in bowan mechanics are still probabilistic but one chalks up the probability to our lack of knowledge about where the particle really was. This rewriting of quantum mechanics is appealing if you prefer working with particles. But it's not just a reinterpretation. For one thing, in bombian mechanics, you can really only measure particle positions. Whereas in standard quantum mechanics, we measure all kinds of different things, energy, spin, momentum. And this is why in bowian mechanics, you have to dance around quite a bit to explain why a spin measurement is really just a measurement of the position of something. The other issue is that if you take the formalism seriously, then bombian particles are physically real. It's this letter point that the new experiment has just bluntly ruled out. What did they do for this experiment? They let photons quant of light travel in a thin gap between two mirrors. In the one mirror, they've tiny valleys carved in. So that creates a basically one-dimensional wave guide.

And the mirrors are not perfectly reflective. a small part of the photons leak and that allows the scientists to measure what's going on inside the wave guide. The waveguide first gets deeper, so the photons have more space and then

it suddenly makes a jump and gets thinner again. This is like a sudden block. And for the photons, it's like hitting a wall. But just before the block, they add a second guide next to the first. This is close enough so that the photons can tunnel from the blocked path to the other. What will happen then is that the photons will tunnel into this other wave guide and then distribute into this. A tunneling process in quantum mechanics means that it's possible for a particle to go through a barrier even though its energy isn't high enough. If the particle didn't have quantum properties, tunneling would be impossible. But with quantum properties, it has a chance. The question they want to answer in this experiment is what do the photons do after they tunnel? This is an interesting question because bombian mechanics makes a prediction. It says the particles just sit there. They have basically zero velocity. So what they measure in this experiment is how the second wave guide fills up the one into

which the photons have tunnneled. and they find that the photons spread from the point where they appeared. This means they didn't have zero velocity. That is their result directly disagrees with the bombian mechanics interpretation.

But it is of course neatly compatible with standard quantum mechanics. But what does it mean? Does it really mean that bombian mechanics is wrong? Well, yes and no. It means that you can't identify the particles of bomian mechanics with the particles that we actually observe like the particles in the standard model. But in some sense we knew this already because the particles in bomian mechanics are point particles and the particles that we measure are not. They have spatial extent. They interact. They smear. Some of them have careers in politics. Personally, I never understood why people like a theory with point particles. I don't think that something infinite decimally small can be real. That said, I was thinking we'd never make progress on that. And yet, here we are. My meter. I give it a 1 out of 10. It's a really neat experiment, though I think they're slightly overstating the relevance for bombian mechanics. So bombian particles don't match the particles we measure which I suppose makes them like most of my life goals well definfined internally consistent and entirely unobservable. I used to get a lot of scam calls. I found out that this happened because my phone number had leaked from some websites I must have once signed up to. I now have a new phone number and I'm signed up to incognite to prevent that from happening again. You see, each time you open my website, it'll try to collect data about who you are and where you are and what other websites you've visited. If you then sign up for a website and fill in your personal details, they can and often do make money by selling your private information to data brokers.

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See you tomorrow.

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