# Making the wave observable in the double-slit experiment

• I
• Quantum Waver
In summary, Physicist Dr. Muthuna Yoganathan thought the wave function was just a calculation tool, which is the standard minimal interpretation of QM. But then she started doing her own versions of the double-slit experiment at home using a red laser, culminating in purchasing a smoke machine. With that, you can see the entire trajectory of the laser beam as it forms an interference pattern on a screen. This is in contrast to how the double-slit is usually presented, where the wave isn't visible, and the wave function is seen as a mathematical tool for making stochastic predictions.
Quantum Waver
TL;DR Summary
Physicist becomes convinced Schrodinger's equation describes real waves, because they can be made visible in the double-slit experiment using a laser and smoke machine.
Physicist Dr. Muthuna Yoganathan thought the wave function was just a calculation tool, which is the standard minimal interpretation of QM. But then she started doing her own versions of the double-slit experiment at home using a red laser, culminating in purchasing a smoke machine. With that, you can see the entire trajectory of the laser beam as it forms an interference pattern on a screen. This is in contrast to how the double-slit is usually presented, where the wave isn't visible, and the wave function is seen as a mathematical tool for making stochastic predictions.

At 10:02 in the video, she says:
You know, I've given this explanation to students for years, but I've never taken the whole wave picture very seriously. Yes, I know that to make the right prediction for the double-slit, you would have to use waves in your calculation. But i thought they were just that. Just calculation tools. Literally in quantum mechanics the waves are complex valued. So instead in my head, I thought of the light as particle when it exited the laser, and then a particle again when it got measured at the wall. But in between, it just seemed like there's nothing you could really say about the light besides a bunch of equations. But this next experiment changed my mind and made me finally feel that the waves are actually real.

Which is exactly the way I thought about the double-slit until this video. Because that's how it's been explained.

This does leave the measurement problem untouched, which is the basis for Bohr's complementarity and the probabilistic collapse of the wave function. Dr. Yoganathan has another video in which she discusses how Stern-Gerlach devices can be setup to determine whether a measurement has been performed using a single particle that has decohered with it's environment. Which is empirical evidence that you don't need collapse to perform a measurement.

The Copenhagen School of thought inspired treating the wave function as a calculation tool. Somehow, the classical/macroscopic world emerged from the quantum/microphysical, but the how was treated as a mystery. Perhaps it was just a random selection of possible values. But the wave function describes wave behavior. It would be extremely odd if something wasn't waving. Such as the excitations in quantum fields.

Or a statement?

Discussion starter based on those two videos. I've read a fair amount of the other threads in this forum debating interpretations. One point of contention that comes up a lot is whether the wave equation describes something real.

But one question I have is whether using two tubes at the barrier for the laser to travel down with only one filled with smoke would count as a measurement?

A second question would be whether a laser/smoke experiment would have changed Bohr or Heisenberg's opinions at all, being that lasers weren't invented until 1960.

Yes it is a measurement.
You don't need to use a laser.

hutchphd said:
Yes it is a measurement.
You don't need to use a laser.
Alright well I'm confused then as to why the wave equation gets treated as just a calculation tool in the minimal interpretation if you can observe the entire photon paths making interference? Other kinds of particles get treated the same in QM, so photons should be enough empirical evidence for there being a wave.

Quantum Waver said:
Alright well I'm confused then as to why the wave equation gets treated as just a calculation tool in the minimal interpretation
Reference please. I don't know what you mean.

hutchphd said:
Reference please. I don't know what you mean.
Where I quoted Dr. Yoganathan in the first video in my original post. It's been stated before in other threads in this forum. Perhaps I am not stating it precisely enough.

Let me rephrase with a question. Does Schrodinger's wave equation describe something ontic producing measure results (like waves in a field), or is it useful mathematical fiction for predicting the probability of making a measurement?

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Quantum Waver said:
if you can observe the entire photon paths making interference?
That's not what you are observing in the experiments you reference. You are observing the collective measurement results of huge numbers of photons. Each individual photon only gets observed as a single point. The "wave" or "interference" pattern only gets built up as the result of aggregating the single point measurements of huge numbers of photons.

In the standard double slit experiment, photons are only observed at the final detector screen, where an interference pattern is formed. In the experiments described in the OP, where a means of detecting photons is provided in the intervening space, you now can observe photons anywhere in the apparatus. But that doesn't change what I said above. If you could turn down the light source in the experiments described in the OP until it was so faint that only one photon was inside the apparatus at a time, you would not see an extremely faint photon path or wave pattern. You would see a single dot at some spatial point within the apparatus, wherever that particular photon got detected. And if you ran the experiment enough times to aggregate a huge number of dots, you would see the spatial pattern that is visible all at once when the light source is turned up so that a huge number of photons are inside the apparatus at one time.

mattt, PeroK and hutchphd
PeterDonis said:
That's not what you are observing in the experiments you reference. You are observing the collective measurement results of huge numbers of photons. Each individual photon only gets observed as a single point. The "wave" or "interference" pattern only gets built up as the result of aggregating the single point measurements of huge numbers of photons.

In the standard double slit experiment, photons are only observed at the final detector screen, where an interference pattern is formed. In the experiments described in the OP, where a means of detecting photons is provided in the intervening space, you now can observe photons anywhere in the apparatus. But that doesn't change what I said above. If you could turn down the light source in the experiments described in the OP until it was so faint that only one photon was inside the apparatus at a time, you would not see an extremely faint photon path or wave pattern. You would see a single dot at some spatial point within the apparatus, wherever that particular photon got detected. And if you ran the experiment enough times to aggregate a huge number of dots, you would see the spatial pattern that is visible all at once when the light source is turned up so that a huge number of photons are inside the apparatus at one time.
The first video goes over that with the particle beam animations. But particles don't explain cancellation, waves do. Since we can't observe the individual photon on it's way to the screen without a detector, which would cause decoherence, are we sure about that, or is that a classical interpretation of particles?

Let me rephrase with a question. Does Schrodinger's wave equation describe something ontic producing measure results (like waves in a field), or is it useful mathematical fiction for predicting the probability of making a measurement?

Instrumentalist interpretations would say something closer to the latter. And measurement processes have been studied intensively enough that (imo), measurement is no longer problematic or ambiguous to the instrumentalist.

This does not mean we won't see "wavy" characteristics in observations. These characteristics aren't a problem for instrumentalists.

Quantum Waver said:
particles don't explain cancellation, waves do.
No, quantum mechanics does. Quantum objects aren't either "particles" or "waves"; they are quantum objects. Some aspects of their behavior are particle-like and some aspects are wave-like.

Quantum Waver said:
Since we can't observe the individual photon on it's way to the screen without a detector, which would cause decoherence
Which, as I have already said, is precisely what happens in the experiments you referenced in the OP.

Quantum Waver said:
are we sure about that, or is that a classical interpretation of particles?
Are we sure about what? That QM explains the results of the experiments you referenced in the OP, while a classical particle model does not? Of course we are.

PeroK
PeterDonis said:
No, quantum mechanics does. Quantum objects aren't either "particles" or "waves"; they are quantum objects. Some aspects of their behavior are particle-like and some aspects are wave-like.
That would be Bohr's complementarity. Bohmian mechanics would say the wave guides the particles. MWI would say particle-behavior emerges from decoherence.

PeterDonis said:
Which, as I have already said, is precisely what happens in the experiments you referenced in the OP.
The decoherence from the smoke isn't destroying the interference pattern of the beam. The core issue here is that the double slit experiment is presented as classical-looking particles forming an interference pattern until their path to the screen is detected. But that doesn't explain cancellation unless the particles are waving on their way to the screen.

PeterDonis said:
Are we sure about what? That QM explains the results of the experiments you referenced in the OP, while a classical particle model does not? Of course we are.
Sure that the unobserved particle isn't a wave. The two videos are making a case for particles being waves, and only particle-like in a non-classical way.

Morbert said:
Instrumentalist interpretations would say something closer to the latter. And measurement processes have been studied intensively enough that (imo), measurement is no longer problematic or ambiguous to the instrumentalist.

This does not mean we won't see "wavy" characteristics in observations. These characteristics aren't a problem for instrumentalists.

What is the instrumentalist explanation for the "wavy" characteristics? Let's take the electron cloud. How does an instrumentalist account for chemistry if it requires the electron to be spread out in the atom? This is something Sean Carol brings up as being evidence for the reality of the wave (or field excitation).

Tim Maudlin claims that he doesn't know anyone in foundation of QM who thinks the wave is fictional in the double slit experiment.

You need to carefully define every new word you use.
What does "fictional" mean. What does "spread out" mean. If you are going to quote people then you need to footnote each quote. Science is fun and useful. Shadowboxing not so much.

hutchphd said:
You need to carefully define every new word you use.
What does "fictional" mean. What does "spread out" mean. If you are going to quote people then you need to footnote each quote. Science is fun and useful. Shadowboxing not so much.
I think this captures 'fictional' pretty well:
You know, I've given this explanation to students for years, but I've never taken the whole wave picture very seriously. Yes, I know that to make the right prediction for the double-slit, you would have to use waves in your calculation. But i thought they were just that. Just calculation tools. Literally in quantum mechanics the waves are complex valued. So instead in my head, I thought of the light as particle when it exited the laser, and then a particle again when it got measured at the wall. But in between, it just seemed like there's nothing you could really say about the light besides a bunch of equations. - Dr. Muthuna Yoganathan
Spread-out meaning the electron cloud shapes of their orbitals. The argument for them being more than a model of probability is theses shapes underly chemistry.

Do you have other semantic hoops you wish me to jump through?

Quantum Waver said:
That would be Bohr's complementarity.
What I said is actually just basic QM since I was describing the mathematical model, not anything about "reality". Complementarity is one interpretation of QM. Bohmian and MWI are other interpretations.

Quantum Waver said:
The decoherence from the smoke isn't destroying the interference pattern of the beam.
I didn't say it should. Nor does decoherence theory say it should. But each individual photon is decohered when it is measured somewhere in the apparatus. What you are calling "the interference pattern of the beam" is the pattern formed by measurements of a huge number of photons. (Btw, your quotes from the physicist who made the videos in the OP indicates that they do not fully grasp this.)

Quantum Waver said:
Sure that the unobserved particle isn't a wave.
It's a quantum object. That is all that the mathematical model says. Claims about what the particle "really" is are QM interpretations, and different interpretations make different and incompatible such claims, as you have already noted.

hutchphd said:
You're familiar with this debate, so I don't know why you're wanting definitions. Are you irritated with the MWI implication of the videos?

Quantum Waver said:
I think this captures 'fictional' pretty well:
Yes, but until you gave that quote, nobody knew what you meant by "fictional". Now we do.

Quantum Waver said:
Do you have other semantic hoops you wish me to jump through?
This is uncalled for. You can't use words that have no well-defined technical meaning in the subject area under discussion and expect others to understand you. If you can't be bothered to use the correct technical terminology, the very least you can do is to explain what you mean by the words you choose to use.

PeterDonis said:
It's a quantum object. That is all that the mathematical model says. Claims about what the particle "really" is are QM interpretations, and different interpretations make different and incompatible such claims, as you have already noted.
This is a forum for debating interpretations, right? Some physicists (along with a few philosophers and mathematicians) disagree that's all the mathematical model says. I'm inclined to agree with them, but I understand the matter is undecided.

PeterDonis said:
Yes, but until you gave that quote, nobody knew what you meant by "fictional". Now we do.
I quoted the video in my first post where she discusses thinking it was just a mathematical tool, and mathematical 'fictions' are used in this debate. I thought people were familiar with the terminology from haivng read other threads. But alright, I'll try to be more precise.

Quantum Waver said:
This is a forum for debating interpretations, right?
Yes, based on published descriptions in the literature of what those interpretations say. It's not clear to me that the physicist in the videos you referenced in the OP is using any particular interpretation, however.

Quantum Waver said:
Some physicists (along with a few philosophers and mathematicians) disagree that's all the mathematical model says.
No, physicists disagree about the "reality" that underlies the mathematical model. No reputable physicists disagree about what the mathematical model itself says: the mathematical model is what they all use to make predictions, and they all make the same predictions, so they're all using the same mathematical model.

Quantum Waver said:
mathematical 'fictions' are used in this debate
Really? If you think this is a recognized technical term in this debate, please give a reference that defines it.

PeterDonis said:
Really? If you think this is a recognized technical term in this debate, please give a reference that defines it.
Would instrumentalist have worked better? Or minimal standard interpretation?

I'm not sure why Dr. Muthuna Yoganathan is so impressed with her experiment or why she always thought of the wave nature of light as purely mathematical. We can 'see' EM waves as actual oscillations in our measurement tools at longer wavelengths and down in the microwave and radio wave regions the particle aspect disappears almost entirely for most practical purposes since the energy of each photon is so tiny. Interference effects are second nature when dealing with frequencies in this range.

mattt, PeroK, PeterDonis and 1 other person
PeterDonis said:
No, physicists disagree about the "reality" that underlies the mathematical model. No reputable physicists disagree about what the mathematical model itself says: the mathematical model is what they all use to make predictions, and they all make the same predictions, so they're all using the same mathematical model.
Understood. I should have just started by asking questions about the first video.

Drakkith said:
I'm not sure why Dr. Muthuna Yoganathan is so impressed with her experiment or why she always thought of the wave nature of light as purely mathematical.
Based on the first video, I think it's because when the double slit is performed by emitting one particle at a time, you can't observe the particle's path without destroying the interference that is built up. Leading to debates over the measurement problem and the shut and calculate response to said debates. The math provides accurate predictions, and nothing more can be said about what the particle is between the emitter and screen, so trying to visualize what's going on with a mental picture isn't helpful.

But when she performed experiments herself with the laser, it changed her thinking from a classical particle being emitted and detected to thinking of the particle as a wave, because only waves can cancel. The second video discusses how decoherence can produce a measurement, so you don't need the classical particle picture at all. She has other videos discussing probability in the MWI, so it's pretty clear what sort of interpretation Dr. Yoganathan has adopted.

The discussion I was trying to start is the idea that the measurement problem could be the result of treating observation classically instead of quantum mechanically, which is how it's usually presented in the double slit experiment. If we think of particles as decohered waves, the problem goes away, although it does leave other implications to work through, depending on the interpretation.

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Quantum Waver said:
What is the instrumentalist explanation for the "wavy" characteristics? Let's take the electron cloud. How does an instrumentalist account for chemistry if it requires the electron to be spread out in the atom? This is something Sean Carol brings up as being evidence for the reality of the wave (or field excitation).
"Spread out in the atom" needs to be unpacked. A common assumption in quantum chemistry is the electronic charge density of the ground state determines all properties of the system. But an electronic charge density isn't a wavefunction (see density functional theory). This is made especially clear in multi-electron systems, where the wavefunction becomes a function of many positions, while the charge density remains a function of one position.
Tim Maudlin claims that he doesn't know anyone in foundation of QM who thinks the wave is fictional in the double slit experiment.
The statistical undulation present in the ensemble of experimental outcomes in the double slit experiment is real. But this wavelike characteristic is distinct from the wavefunction.

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vanhees71 and Quantum Waver
Quantum Waver said:
The discussion I was trying to start is the idea that the measurement problem could be the result of treating observation classically instead of quantum mechanically, which is how it's usually presented in the double slit experiment. If we think of particles as decohered waves, the problem goes away, although it does leave other implications to work through, depending on the interpretation.
Instrumentalists can have their cake and eat it. Quantum theory is mature enough that we can extend the theory to the relevant collective degrees of freedom of the measurement apparatus and establish the correlations between macroscopic properties of the apparatus and microscopic properties of the measured system (having the cake). We can also acknowledge that the measurement process produces irreversible, classical records registered by the experimenter (eating the cake).

Quantum Waver said:
The discussion I was trying to start is the idea that the measurement problem could be the result of treating observation classically instead of quantum mechanically, which is how it's usually presented in the double slit experiment. If we think of particles as decohered waves, the problem goes away, although it does leave other implications to work through, depending on the interpretation.
Although there was some early optimism along these lines, it is not at all clear that thinking in terms of decohered waves helps with the measurement problem. Yes, that line of thought explains why macroscopic measuring devices will only produce classical results (the cat is dead or alive and we don't know which until we look, but not in a coherent superposition of dead and alive before we look). How do we get from the quantum mechanical prediction of various probabilities of various outcomes to the post-measurement fact that we got this result rather than that? What physical process justifies discarding some components of the wave function in a collapse interpretation, or confining our attention to one branch of the wave function in MWI interpretations?

Quantum Waver said:
Alright well I'm confused then as to why the wave equation gets treated as just a calculation tool in the minimal interpretation if you can observe the entire photon paths making interference? Other kinds of particles get treated the same in QM, so photons should be enough empirical evidence for there being a wave.
Let's replace light with electrons and look at the double-slit experiment from a purely QM viewpoint.

The electron initially is constrained by a narrow uncertainty. In the rest frame of the electron it is perhaps a narrow Gaussian.

When it reaches a single slit it is effectively in an infinite potential well. The Gaussian must then be decomposed into eigenstates of the potential well, each of which has a charteristic lateral momentum.

When it exits the slit it is in a superposition of these eigenstates and already has a superposition of quantized lateral momenta.

This is how the wave pattern immediately after the slits is explained.

Note that the usual heuristic explanation in terms of the uncertainty principle does not fully explain the single or double slit in terms of the quantized lateral momenta.

PeroK said:
The electron initially is constrained by a narrow uncertainty. In the rest frame of the electron it is perhaps a narrow Gaussian.
This is momentum uncertainty, or more precisely lateral momentum uncertainty, correct? In the usual idealized case the electron's state is a plane wave with zero lateral momentum.

vanhees71
PeterDonis said:
This is momentum uncertainty, or more precisely lateral momentum uncertainty, correct? In the usual idealized case the electron's state is a plane wave with zero lateral momentum.
Yes, effectively zero lateral momentum. The Gaussian would spread out only slowly laterally relative to the time to the slit.

PS and the breadth of the Gaussian must encompass both slits.

vanhees71

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