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

Source: https://www.youtube.com/watch?v=DvpSnmRnHr0
Recap page: https://rapidrecap.app/video/DvpSnmRnHr0
Generated: 2025-07-21T20:33:53.137+00:00

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## Quick Overview

A new experiment has experimentally ruled out Bohmian mechanics, a long-standing interpretation of quantum mechanics that posits point-like particles guided by a wave, by demonstrating that tunneling photons exhibit non-zero velocity, directly contradicting Bohmian mechanics' prediction of zero velocity for such particles.

**Key Points:**
- Bohmian mechanics, a century-old interpretation of quantum mechanics, has been experimentally ruled out after 70 years of debate.
- Standard quantum physics describes everything by a wave function, which is unobservable and used to calculate probabilities, making it a non-deterministic theory.
- David Bohm's interpretation, Bohmian mechanics, posits point-like particles guided by a wave, where the wave function is split into a guiding field equation and a particle equation.
- In Bohmian mechanics, observations are probabilistic due to a lack of knowledge about the particle's initial position, but the particles themselves are considered physically real.
- A new experiment used photons traveling in a microcavity with a waveguide and a second adjacent guide to observe quantum tunneling.
- The experiment measured that tunneling photons spread from their point of appearance, indicating they possessed non-zero velocity, which directly contradicts Bohmian mechanics' prediction of zero velocity for such particles.
- This result is incompatible with Bohmian mechanics but aligns with the standard Copenhagen interpretation of quantum mechanics.

![Screenshot at 4:08: A graph showing photon spread over distance \(x\) for different energy levels, with a text overlay "They move!", indicating non-zero velocity.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-04-08.png)

**Context:** Quantum mechanics, over a century old, has long been debated regarding its interpretation, particularly concerning the nature of particles and their behavior. One prominent interpretation, Bohmian mechanics (also known as pilot-wave theory), proposed by David Bohm about 70 years ago, suggests that quantum mechanics describes point-like particles that are always at a definite position and follow a guiding wave. This deterministic view contrasts with the standard Copenhagen interpretation, which describes particles probabilistically via a wave function, leading to a non-deterministic theory.

## Detailed Analysis

Quantum mechanics, a foundational theory in physics, has been subject to various interpretations since its inception over a century ago. The standard formulation describes quantum systems using a wave function, which is not directly observable but serves as a tool to calculate the probabilities of measurement outcomes, leading to a non-deterministic understanding of reality. In contrast, Bohmian mechanics, or pilot-wave theory, proposed by David Bohm, offers a deterministic interpretation where particles are point-like entities that always have a definite position and are guided by a real, physical wave. This interpretation reformulates the Schrödinger equation into a guiding equation for the wave field and a particle equation for the particles moving within that field. While Bohmian mechanics still yields probabilistic observations, it attributes this to an initial lack of knowledge about the particle's exact starting position, rather than inherent randomness. A key distinction is that Bohmian mechanics primarily allows for the measurement of particle positions, whereas standard quantum mechanics permits various types of measurements like energy, spin, and momentum. The recent experiment, detailed in a Nature article, directly challenges Bohmian mechanics by investigating the energy-speed relationship of quantum particles during tunneling. Researchers used photons in a microcavity with a specially designed waveguide that included a sudden block and an adjacent second guide, allowing photons to quantum tunnel. Quantum tunneling is a phenomenon where particles can pass through energy barriers even if they classically lack sufficient energy. Bohmian mechanics predicts that particles undergoing tunneling would have zero velocity within the barrier region. However, the experiment measured that the photons spread out from their point of appearance in the second waveguide, demonstrating they possessed non-zero velocity. This experimental result directly contradicts the core prediction of Bohmian mechanics regarding particle velocity during tunneling, effectively ruling out its physical reality for such particles, while remaining consistent with the standard Copenhagen interpretation. The findings suggest that the "particles" posited by Bohmian mechanics do not correspond to the physically observable particles of the Standard Model, which have spatial extent and interact, unlike the theoretical point particles of Bohmian mechanics.

### Introduction to Quantum Mechanics

- Quantum mechanics is over a century old, with physicists debating its meaning for almost as long
- Standard formulation describes everything by a wave function, which is not observable but used to calculate probabilities of measurement outcomes
- Standard quantum mechanics is a non-deterministic theory with a fundamentally random element.

### Bohmian Mechanics Explained

- David Bohm proposed Bohmian mechanics (pilot-wave theory) about 70 years ago, suggesting quantum mechanics is about point-like particles following a guiding wave
- The wave function is split into a guiding field equation and a particle equation for particles moving in that field
- Observations in Bohmian mechanics are still probabilistic, but this is attributed to a lack of knowledge about the particle's true initial position, making it a deterministic theory at its core.

### Distinction from Standard QM

- Bohmian mechanics primarily allows measurement of particle positions, unlike standard quantum mechanics which measures various properties like energy, spin, and momentum
- Explaining other measurements in Bohmian mechanics requires complex interpretations, effectively reducing them to position measurements
- If the formalism is taken seriously, Bohmian particles are considered physically real.

### The Experiment Setup

- The experiment investigated the energy-speed relationship of quantum particles, specifically photons, challenging Bohmian mechanics
- Photons traveled in a microcavity, a thin gap between two mirrors, one of which had carved valleys creating a one-dimensional waveguide
- The mirrors were not perfectly reflective, allowing a small part of the photons to leak, enabling scientists to measure what was happening inside the waveguide
- The waveguide design included a section that first deepened, then made a sudden jump to become thinner, acting as a block for photons
- A second guide was added next to the block, close enough for photons to quantum tunnel from the blocked path to the other.

### Quantum Tunneling and Predictions

- Quantum tunneling allows a particle to pass through a barrier even if its energy is classically insufficient
- Bohmian mechanics predicts that particles undergoing tunneling would have zero velocity within the barrier region, essentially sitting still.

### Experimental Results and Implications

- The experiment measured how the second waveguide filled up, observing that photons spread from the point where they appeared
- This spreading indicates that the tunneling photons had non-zero velocity, directly contradicting the prediction of Bohmian mechanics
- The results are incompatible with Bohmian mechanics but are neatly compatible with the standard Copenhagen interpretation of quantum mechanics
- This implies that the point particles of Bohmian mechanics cannot be identified with the actual particles we observe, which have spatial extent, interact, and are not infinitesimally small.

![Screenshot at 0:00: A woman speaking in front of a blue and purple background with a world map, with a thought bubble saying "No one understands quantum mechanics!" and a man yelling "Yes we do!" in a smaller inset.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-00-00.png)
![Screenshot at 0:11: The woman speaking, with "Bohmian Mechanics" text overlay and an abstract image of purple and blue light particles and lines.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-00-11.png)
![Screenshot at 0:33: A wave function equation "H\|Ψ\> = iħ∂t\|Ψ\>" with a green arrow pointing to \|Ψ\>, overlaid on an abstract wave pattern.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-00-33.png)
![Screenshot at 0:48: A blue banner with the symbol "\|Ψ\>" and an orange arrow pointing to "Probabilities", overlaid on a colorful, textured background.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-00-48.png)
![Screenshot at 1:16: Two equations, "Guiding equation" and "Particle equation", overlaid on an abstract image of swirling golden lines and a clear sphere.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-01-16.png)
![Screenshot at 2:07: A man in a party hat dancing in a room with string lights, with text overlays "energy?", "momentum?", "spin?", "multiplici-what?", illustrating the complexity of measuring these in Bohmian mechanics.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-02-07.png)
![Screenshot at 2:27: A scientific paper title "Energy–speed relationship of quantum particles challenges Bohmian mechanics" with author names and publication details.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-02-27.png)
![Screenshot at 2:30: A diagram showing a microcavity with a green laser beam entering from the top, and red dotted lines indicating photon paths between two blue mirror layers.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-02-30.png)
![Screenshot at 3:10: A 3D graph showing two potential wells with red dots labeled "ψm" and "ψa", and a double-headed arrow labeled "J₀" between them, illustrating tunneling.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-03-10.png)
![Screenshot at 4:08: A graph showing photon spread over distance \(x\) for different energy levels, with a text overlay "They move!", indicating non-zero velocity.](https://ss.rapidrecap.app/screens/DvpSnmRnHr0/00-04-08.png)