Scivillage.com Casual Discussion Science Forum

Problems with "The biggest lie about the double slit experiment"

I'm looking at a video from Looking Glass Universe - mostly about electrons.
In full here https://www.youtube.com/watch?v=fbzHNBT0nl0 .. full view not required to understand my comments below..

Back in 1801 Thomas Young used the 'double slit experiment' to prove (?) light was a wave - how? - by measuring it's wavelength.

The build up involves single slit interference.
The problem.. the two edges of the (single) slit are acting like sources and the single slit interference is itself a double source result which is what the double slit experiment is all about. In general you make the slit so narrow that both edges of the slit are effectively 'the same source' and you don't have to think about it any further.

At about 7:42 https://www.youtube.com/watch?v=fbzHNBT0nl0&t=460s
Unless, like the nice lady, you're talking about electrons and you can't see any actual 'double slit' interference so all you're left with is single slit interference.
At 8:06 she looks at the light version and mentions the "These weird little bits inside the bigger single slit pattern" - the weird bits are what Thomas Young used to measure the wavelength of light - worth a mention perhaps? In the electron version the wavelength of the electrons is so small that the weird little bits are too close together to be resolved in the demonstration. She just falls back on single slit interference without explanation (or understanding?).

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Young's Double Slit Experiment | Physics
To measure the wavelength of light using a double-slit experiment, you can use the equation λ = (d * y_m) / L, where λ is the wavelength, d is the slit separation, y_m is the distance from the central bright fringe to the m-th bright fringe, and L is the distance from the slits to the screen.

^^ Note the width of each slit doesn't come into it .. they just need to be 'narrow'. The bright fringes we're counting are the ones she calls the weird ones aka the double slit effect.

INTRO: In a new paper, researchers claim they’ve proven the experiment wrong, and that light is just a particle. Instead of light also being a wave that interferes with itself they say that there are both light photons and dark photons. Let’s take a look.

Did We Get the Double Slit Experiment All Wrong? ... https://youtu.be/DpMcC-E5l5c

^^^ If it were April I'd assume this was a joke - possibly even an attempt to fool a respected journal into publishing nonsense. In reality the authors seem unlikely to be deliberately 'wrong'.

A 'shallow' education can have 'interesting' consequences.

Not least Sabine claiming you need a laser to demonstate the effect that Thomas Young (13 June 1773 – 10 May 1829) famously(?) revealed to the Royal Society of London in 1803.

Why do I think the double slit effect ALWAYS a single photon effect..?

If the two paths are now made to interfere, we should require a photon in one path to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy.

I can't see the flaw in what seems (to me) to be obvious.

If there is only one photon to start with - present or absent .. what part do 'dark' photons play?

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Apologies for the tone/arrogance - I'm currently being driven mad by a rat.

(May 17, 2025 04:56 PM)confused2 Wrote: [ -> ]^^^ If it were April I'd assume this was a joke - possibly even an attempt to fool a respected journal into publishing nonsense. In reality the authors seem unlikely to be deliberately 'wrong'.

A 'shallow' education can have 'interesting' consequences.

Not least Sabine claiming you need a laser to demonstate the effect that Thomas Young (13 June 1773 – 10 May 1829) famously(?) revealed to the Royal Society of London in 1803.

Why do I think the double slit effect ALWAYS a single photon effect..?

If the two paths are now made to interfere, we should require a photon in one path to be able to interfere with one in the other. Sometimes these two photons would have to annihilate one another and other times they would have to produce four photons. This would contradict the conservation of energy.

I can't see the flaw in what seems (to me) to be obvious.

If there is only one photon to start with - present or absent .. what part do 'dark' photons play?

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Apologies for the tone/arrogance - I'm currently being driven mad by a rat.

With respect to lasers, it's as if this work is dependent upon or falls out of a whole genre of research using them in recent decades. But Sabine (and potentially the paper itself?) fails to distinguish or highlight that kind of historical demarcation, despite the video's title referencing the "double slit experiment" in a temporally general way.

"Dark photons" usually refer to some hypothesis associated with dark matter. So it's odd that there's no attempt at disambiguation from that meaning.

And while the paper itself is locked, neither the abstract of it nor the press release about it (below) use a "dark photon" term (it's "dark states" of light). The press release also introduces "entangled" to the mix (quantum entanglement?) which further befuddles. Apparently the abstract mentioned such as well.

New quantum optics theory proposes that classical interference arises from bright and dark states of light: "Their paper, published in Physical Review Letters, proposes that classical interference arises from specific two-mode binomial states, which are collective bright and dark entangled states of light."

However at Medum, there is a summation of a paper for which no link is even provided, published the same month as the other one, that sounds suspiciously similar. But it differs in that it clarifies that entanglement is indeed key to its proposal (not that the paper that the video is about fails to likewise do that, but the #$#$ thing can't be accessed for details). I seriously doubt they are the same, but at least the possible "paper mill" candidate below is up-front about the significant role of "entangled" in it...

A Unified Entanglement-Based Interpretation of the Double-Slit Experiment
https://medium.com/@abhijit.mahale/a-uni...2a42eb8568

ABSTRACT (more elaboration following it): This paper proposes a new interpretation of the double-slit experiment by introducing a fundamental role of entanglement at the point of particle generation. The central idea is that particles, such as electrons or photons, emerge from the source in an entangled state — not with external systems, but internally within their own probability distribution.

This internal entanglement forms a unified quantum wave that exhibits interference, even when particles are emitted individually. The act of measurement is interpreted as an interaction that collapses or disentangles this internal structure, thereby localizing the particle and removing the wave-like behaviour. This model provides an intuitive and consistent explanation for the wave-particle duality and the collapse of the interference pattern upon observation.

The abstract of the paper that Hossenfelder addressed:

Bright and Dark States of Light: The Quantum Origin of Classical Interference
https://journals.aps.org/prl/abstract/10...134.133603

ABSTRACT: Classical theory asserts that several electromagnetic waves cannot interact with matter if they interfere destructively to zero, whereas quantum mechanics predicts a nontrivial light-matter dynamics even when the average electric field vanishes. Here, we show that in quantum optics, classical interference emerges from collective bright and dark states of light, i.e., particular cases of two-mode binomial states, which are entangled superpositions of multimode photon-number states. This makes it possible to explain wave interference using the particle description of light and the superposition principle for linear systems only. It also sheds new light on an old debate concerning the origin of complementarity.

Being published "in a top science journal" means little today, since the whole industry is riddled with retractions. If "entangled states" really are central to what these authors propose, then the lack of most of these "science intepreters" focusing on that seems to hint that they are likewise suffering cognitive problems with respect to what the actual setup of these authors is, or what they're really talking about.

Science Reader: Despite its publication in a top physics journal, the interpretation has limitations. As physicist Sabine Hossenfelder points out, both “light” and “dark” photon states still pass through both slits, and the energy in dark regions equals zero – contradicting the particle-only claim. This debate highlights how even century-old experiments continue to spark new interpretations in quantum physics, though this new perspective likely won’t make quantum mechanics any less perplexing for the rest of us.

(May 17, 2025 08:47 PM)C C Wrote: [ -> ]The abstract of the paper that Hossenfelder addressed:
[indent=3]Bright and Dark States of Light: The Quantum Origin of Classical Interference
https://journals.aps.org/prl/abstract/10...134.133603

Here’s the full text.

https://arxiv.org/pdf/2112.05512

(May 18, 2025 04:00 PM)Secular Sanity Wrote: [ -> ]

(May 17, 2025 08:47 PM)C C Wrote: [ -> ]The abstract of the paper that Hossenfelder addressed:
Bright and Dark States of Light: The Quantum Origin of Classical Interference
https://journals.aps.org/prl/abstract/10...134.133603

Here’s the full text.

https://arxiv.org/pdf/2112.05512

Thanks, SS.

Like the abstract ("entangled"), only a single occurrence of "entanglement" in the entire body of the paper.

Also, a lone mention of "lasers" in the paper.

Hossenfelder (video): "Why does one need laser light? Is it just that really, really like lasers? They do, but one actually does need the laser light, because you need the phases of the light, that is the places where there are crests and troughs, to all be the same, otherwise they add up and cancel in random places, and you don’t see the pattern."
- - - - - - -

PAPER EXCERPT: In light of our findings, we emphasize that the single-mode case has a unique PDS, namely the vacuum state |0⟩. It may be trivial since there is no photon to excite the detector, yet its interest lies in its uniqueness: any other state excites the sensor atom. Thus, our interpretation in terms of dark and bright states provides a new way to explain why single-mode Fock states |N ⟩ with N > 1 do excite the sensor atom, even for zero mean electric fields.

The multi-mode case, however, is fundamentally different since it possesses an infinite family of dark states with an arbitrarily large number of photons, which do not couple to the sensor atom in the ground state. In addition, the two-mode case also predicts bright and intermediate states, the latter having no correspondence in classical physics.

The above discussion, originally for two radiation modes, extends directly to M modes/slits, where any interference is described via collective bright, dark, and intermediate states. A pulsed light from mode-locked lasers exemplifies this, with photons forming bright during pulses and dark states between them.

In the context of C2’s quantum eraser video:
     

Before erasing information: The photon will behave as if it has a definite path, and the interference pattern may disappear.

After erasing the information: When you erase the path information, you can restore the interference pattern, suggesting that the photon didn't "choose" a definite path after all. It’s as if the photon was still in a superposition of states (even after traveling through the setup) and interference can still occur because the "which-path" information isn't available.

This doesn’t mean that the photon ceases to exist during destructive interference; rather, it’s in a superposition of states where its position and momentum are uncertain until you measure it. The act of measuring (or "erasing" information) changes the outcome of the experiment.

In classical waves, troughs don’t show up because the fields cancel out perfectly at those points, resulting in zero intensity (and no light).

In quantum interference, troughs don't show up because the probability amplitude for detecting a photon at those points is zero — the photon has a zero chance of being detected there due to destructive interference.

From the paper:

"We then discuss the rather counter-intuitive result that a vanishing photon-detection probability at locations of destructive interference does not prove the absence of photons. We argue, instead, that these photons are in a state that is perfectly dark for the employed sensor atom. In other words, we replace Glauber’s explanation of constructive or destructive interference in terms of superposed transition amplitudes by a description where the light is in a state that can or cannot excite the atom, respectively. This leads to a new view on which-path detection in double-slit experiments as, contrary to the standard notion, photons always reach the dark regions, independently of the presence of the detector."

Me: 

In quantum mechanics, a single photon can interfere with itself due to the wave-like nature of its wavefunction.

This is a manifestation of quantum superposition, where the photon is in a superposition of paths until it is measured, and the interference happens because of the probability amplitudes of these paths.

It looks like the paper uses Glauber’s framework to demonstrate that quantum interference (based on probability amplitudes) doesn’t mean the photons are absent at destructive interference points; rather, they are in states that are undetectable by the atom or sensor at those points, (dark states).

Glauber's theory clarifies that the interference pattern isn't the result of particles interfering with themselves. Instead, it arises from the interference of probability amplitudes, which are mathematical quantities that describe the likelihood of a particle being at a particular location at a given time.

From Phys.org:

"A notable feature of dark states is that they contain photons. The new theoretical framework outlined by the researchers suggests that these photons are present at the nodes of an interference pattern. As the state they are associated with is dark, however, these photons were hypothesized to be unobservable using conventional experimental methods.

"This is a highly counterintuitive picture which initially made us doubt that our description can be correct," explained Rempe. "Support came from an experiment that I conducted in my group in the late 1990s, which concerned the role of a which-path observer in double-slit experiments.

"As had been controversially discussed at that time, which-path observation (of a particle through the double slit) can be so gentle as to not exert a momentum kick on the interfering particle. This raises the so far open question of how the observation can steer the particle from a bright into a dark region in order to wash out the interference pattern."

The new theoretical approach outlined by the researchers provides a quantum optics-based alternative explanation for classical interference. Specifically, it suggests that which-path detection changes the state in dark regions to a bright state. Namely, without necessarily altering the trajectory of a particle, a which-path observer can alter the state in such a way that the particle becomes detectable." Read more

Interesting to think about. Thanks, CC!

Quote:A notable feature of dark states is that they contain photons. The new theoretical framework outlined by the researchers suggests that these photons are present at the nodes of an interference pattern. As the state they are associated with is dark, however, these photons were hypothesized to be unobservable using conventional experimental methods.

If all the photons are detected elsewhere and the number detected matches the number available for detection then the number of photons in the dark regions must be zero. If you kill all your rats then there's none in the kitchen* - there's no 'unrats' lurking there.
*hoping.

Actual experiment..


Figure 4: Photon Counting Data for Single and Double Slit Configurations

From: https://www.teachspin.com/two-slit

Note that (channeling Syne) .. each slit gives a maximum count of about 600, with both slits open the maximum count is 2400 .. the photons really have .. done it.

(May 19, 2025 07:02 PM)confused2 Wrote: [ -> ]

Quote:A notable feature of dark states is that they contain photons.

Or the photons were potentially present and prepared to try, but had no effect.

Dark state: "A dark state refers to a state of an atom or molecule that cannot absorb (or emit) photons. [...] A state that cannot absorb an incident photon is called a dark state."

Quote:

Quote:The new theoretical framework outlined by the researchers suggests that these photons are present at the nodes of an interference pattern. As the state they are associated with is dark, however, these photons were hypothesized to be unobservable using conventional experimental methods.

If all the photons are detected elsewhere and the number detected matches the number available for detection then the number of photons in the dark regions must be zero. If you kill all your rats then there's none in the kitchen* - there's no 'unrats' lurking there.
*hoping.

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(May 20, 2025 03:07 PM)confused2 Wrote: [ -> ]Actual experiment..


Figure 4: Photon Counting Data for Single and Double Slit Configurations

From: https://www.teachspin.com/two-slit

Inspiration from Robert H Dicke runs amok in their paper.

They are apparently using "two light fields each in a superposition of having zero or one photon", which the physics dot org article also indicated. Whereas, that traditional setup:

Creating a Single Photon Source. As anyone who has ever used a light dimmer can attest, decreasing the light output of a given incandescent bulb shifts the spectrum toward the longer, red wavelengths. By placing a narrow band green filter in front of a standard light bulb, students use this "obvious" phenomenon to create a source of single photons.

Quote:Note that (channeling Syne) .. each slit gives a maximum count of about 600, with both slits open the maximum count is 2400 .. the photons really have .. done it. EDITED INSERT: I tried to add the last part of C2's previous post (above the horizontal rule) to this for the sake of continuity, but it was too much of a CC&P and BBcode mess to keep straight.

Given that their paper lacks precise details for that area of interest -- I suppose it can't be wholly excluded that they might count the total number of photons -- including those not having an effect in the dark states context. But where there's no smoke, why assume there's fire gone unmentioned...

https://www.sciencenewstoday.org/researc...terference

EXCERPT: Even when the average electric field of a light wave drops to zero, its constituent particles—photons—continue to exist. They are not banished into oblivion by destructive interference. Instead, they hover in silent, invisible readiness, capable of interacting with matter in ways classical theories could not foresee.

This deep contradiction between classical and quantum views has fascinated physicists for decades. Now, a remarkable study carried out by researchers from the Federal University of São Carlos, ETH Zurich, and the Max Planck Institute of Quantum Optics sheds new light on this mystery. Their work, recently published in Physical Review Letters, rewrites our understanding of interference, proposing that the classical picture of wave interference masks a far richer quantum story—one filled with bright and dark entangled states of light.

The seeds of this theoretical revolution were sown years ago, through a long-standing collaboration between Celso J. Villas-Boas and Gerhard Rempe, two physicists whose shared passion for quantum optics opened a new path through a very old forest of ideas. Rempe, a pioneer in cavity quantum electrodynamics (QED), had spent years exploring how single atoms interact with single photons inside optical cavities—microscopic theaters where light and matter perform their delicate dances.

It was against this backdrop that Villas-Boas posed a deceptively simple question: What would happen if an atom were exposed not to classical fields of light, but to quantum fields—specifically, two light fields each in a superposition of having zero or one photon?

The answer lay in revisiting the concept of bright and dark states—a concept first formulated by Robert Dicke in the 1950s to describe collective states of atoms. In Dicke’s picture, groups of atoms could enter a “bright” state, radiating light collectively, or a “dark” state, hidden from the world due to destructive interference among their emissions. Villas-Boas and Rempe realized they could transplant this idea, but with a profound twist: instead of atoms, they considered two modes of light itself—each containing at most a single photon.

In this new scenario, a bright state is a quantum superposition where an atom exposed to the light can absorb energy and become excited. A dark state, in contrast, is a superposition where the quantum amplitudes for excitation cancel perfectly, rendering the atom inert to the light. Astonishingly, even though photons are present in a dark state, the atom cannot detect them—they are hidden, as if wearing a quantum cloak.

This shift in thinking led Rempe and Villas-Boas down an exhilarating, if sometimes contentious, intellectual journey. In traditional physics, interference patterns—like those seen in the famous double-slit experiment—are described as waves reinforcing or cancelling each other. Where light waves amplify, bright fringes appear; where they cancel, darkness falls.

But the quantum world speaks a different language. In their new model, Rempe and Villas-Boas propose that interference patterns arise not from waves but from entangled states of photons. In their vision, the alternating bright and dark bands of an interference pattern correspond to regions where photons form bright or dark states, respectively.

This is a staggering idea: in the dark regions where classical theory says “no light,” there are, in fact, photons. These photons are real and present—but they are in dark states, meaning they cannot interact with the detector. It’s not that the photons cease to exist; it’s that their quantum superposition renders them invisible to conventional means of detection.