Young's slits with incandescent light source

[TheHutch]

TL;DR

All modern versions of the double slit experiment use lasers to produce a coherent light source, but the experiment also works with sunlight (as originally performed by Young). This light is made 'spatially coherent' by passing through a single slit, but the transmitted photons will still have random phase relationships. How does interference arise with random phase photons?

I have been looking at various sources, especially the (now closed) thread on PF classical optics "Coherence, Young's double slit experiments."
The trick to making the double slit experiment work with incandescent, as opposed to laser, light is to pass it through an initial single slit 'to make it coherent'. Confusingly, this does not seem to be at all the same coherence as coherent laser light.
@sophiecentaur explained the resulting interference as follows:
"real light [...] can be thought of as consisting of many short wave trains. Each wave train will interfere 'with itself' when it is split between the two slits and meets on the other side."
As far as I can see, real light (from an incandescent source) consists of a stream of uncorrelated photons, with entirely random phase relationships, so these short wave trains can only be the individual photons, and the only way interference patterns can emerge is if these photons interfere only with themselves, and not with each other. The purpose of the single slit is simply to constrain the spatial origin of photons making the interference pattern crisper. In other words, the interference is a quantum phenomenon of individual photons, and not some aggregate wave nature of light. The fact that interference is still seen when reducing the photon flux down to one at a time should be no surprise at all.
Is that understanding correct? I haven't seen it in any text books (maybe I just haven't read enough).
For laser light, perhaps the photons' coherence make cross-photon interference possible (by making photons indistinguishable?), a bit more like the traditional wave explanation given in school text books. That makes the experiments simpler to perform, but obscures what is really going on.

 

[anuttarasammyak]

Photons do not interfere. Coherence of light does not matter. Interference takes place in a single individual photon. In loose saying a photon interferes with itself.

FYI experiment video of single photon Young double slit experiment by Hamamatsu Photonics in 1982. https://photonterrace.net/en/photon/duality/

[EDIT]
>Coherence of light does not matter.

For many detection events to form a clear interference pattern, we need to maintain the temporal and spatial coherence of the incident light so that the relative phase between the probability amplitudes for each photon to travel through the left and right slits remains well defined.

 

[Cthugha]

Photons do not interfere. Coherence of light does not matter. Interference takes place in a single individual photon. In loose saying a photon interferes with itself.

FYI experiment video of single photon Young double slit experiment by Hamamatsu Photonics in 1982. https://photonterrace.net/en/photon/duality/

That is incorrect. Also, the experiment shown is not the double slit with single photons, but a double slit experiment done with a light field of an intensity of less than one photon on average. This is a huge difference.

Nobel prize winner Roy Glauber phrased this nicely (see this text on the ArXiv ):

" When you read the first chapter of Dirac’s famous textbook in quantum mechanics [8],
however, you are confronted with a very clear statement that rings in everyone’s memory.
Dirac is talking about the intensity fringes in the Michelson interferometer, and he says,

'Every photon then interferes only with itself. Interference between two different photons never occurs.'

Now that simple statement, which has been treated as scripture, is absolute nonsense.
First of all, the things that interfere are not the photons themselves, they are the prob-
ability amplitudes associated with different possible histories. You can obviously have
different histories that involve more than one photon at a time. "

The coherence you see in a double slit is classical first-order spatial coherence. This is a fully classical measure. It is similar to the classical first-order temporal coherence you see in a Michelson interferometer. Temporal coherence tells you how monochromatic a light source is. If you have different frequencies within your light field, these will necessarily go out of phase over time, which reduces the ability to observe interference and defines first-order temporal coherence. If you have a spatially extended light source, the phases of the light fields emitted from different points will run out of phase after some distance, which reduces the ability to observe interference and defines first-order spatial coherence.
Obviously, you can make any light source more coherent by filtering it - put a spectral filter in to increase temporal coherence. Put a narrow pinhole in to increase spatial coherence.
Loosely speaking, first order coherence is a measure of how well we can predict the amplitude and phase of a light field, if we know it at a single position. First-order coherence is completely classical and has absolutely nothing to do with photons. It is not sensitive at all to the existence of photons - as one can see in the expressions in Glauber's notes they only depend on fields and not on occupation numbers. This indeed also means that it is no surprise that first -order coherence persists even for feeble light levels.

The coherence that defines a laser is instead second-order coherence. It tells you how well you can predict the intensity or (or photon number) of a light field at some time and position if you know its intensity here and now by detecting one photon. This actually tells you something about single photons - if you have a single photon here and now and detect it, the photon number in the future will simply be zero because the photon is gone. This is the physical signature of a single photon: It is not about the mean occupation number being below one, but about the impossibility to have two simultaneous detection events.

Laser light has the remarkable quality that detecting a photon does not change the relative probability to detect another one in the future at all, which makes coherent light extremely robust to external perturbations and losses. If you have incandescent light, the detection of a photon instead increases the probability to detect more of them. This sounds odd, but it is not at odds with conservation of energy. Rather, think of photons being emitted in large bunches of many photons for a short duration followed by almost no emission for another short duration and so on. Detecting one photon then just means that you are within one of these bunches and are likely to find more photons within this bunch. Accordingly, this effect is also called photon bunching.

 

[anuttarasammyak]

Thanks @Cthugha from your comment would it be correct to say that, in a single-photon double-slit experiment, the probability amplitudes associated with the photon's two possible paths interfere?

 

[Cthugha]

Yes, absolutely!

 

[sophiecentaur]

so these short wave trains can only be the individual photons, and the only way interference patterns can emerge is if these photons interfere only with themselves, and not with each other

I think this approach is, at best, confusing. Getting involved with photons to describe the mechanism of interference (and diffraction) just takes you down a rabbit hole. Conventionally, the actual form of a diffraction pattern involves integrating over the whole bandwidth of the beam of light. You have an aperture (or any objet in fact) and you look at the contributions (vector-wise) of all wavelengths involved and all possible transmission paths. That will tell you the sum of the waves passing through in any direction. Hence you will get a set of 'ideal' Young's slit patterns which will lay on top of each other and display an imperfect Young's pattern as the result of all combinations of wavelength and real (not ideal) path.

Once you have dealt with the problem using the above approach it may be 'interesting' to try it with photons- though I'm not sure how you could even visualise trillions of photons with different energies passing through the real slits. Where and when they are going to affect each other will depend on probability functions. Afaiaa, the final maths will have the identical form that the conventional Fraunhofer calculation gives you. Comparing and contrasting the two approaches if more a matter of philosophy; which method is 'right'?

 

[PeterDonis]

@TheHutch you seem to be trying to model all possible states of light in terms of "photons". Unfortunately, that's not a workable approach.

The fundamental object, so to speak, where light is concerned is the quantum electromagnetic field. The term "photon" is much more restricted; it refers either to a certain set of possible states of the quantum electromagnetic field (which are called "Fock states" and are very hard to produce), or to the fact that, if we make light faint enough, we always detect it as individual particle impacts (for example, if you run a double slit experiment with a very low intensity source, you will see individual dots on the detector screen that gradually build up an interference pattern).

An incandescent light source doesn't fit into either of those cases. It doesn't produce Fock states or anything remotely close to them, and you can't turn its intensity down anywhere near enough to see individual dots on the detector screen, you just see the interference pattern. So there's not going to be any good way to model a double slit experiment with an incandescent light source using photons.

Even in the low intensity case of a double slit experiment, with a suitable light source, there are issues with trying to model things using photons, which have already been described in this thread.

 

[TheHutch]

Thanks everyone for the speedy replies. I can see that all these different perspectives are needed in practice depending on the scale of the scenario.

Just to clarify my original, loosely worded question: by 'incandescent' I was really thinking of a diffuse source where emitted photons don't have a way to conspire together to be in phase (I'm assuming that in all these scenarios, discrete photons are generated when electrons in atoms change energy levels and the aggregate emissions form the source for the double slit experiment).

In reply to:

I'm not sure how you could even visualise trillions of photons with different energies passing through the real slits

I'm definitely not trying to visualise trillions of photons! But I am wondering what happens as the intensity is dialled down to a (relatively) small number of photons passing simultaneously through the experiment, still giving rise to interference effects.

In the light of that, I'm particularly interested in this:

The fundamental object, so to speak, where light is concerned is the quantum electromagnetic field. The term "photon" is much more restricted; it refers either to a certain set of possible states of the quantum electromagnetic field (which are called "Fock states" and are very hard to produce)

I'll read up on Fock states...

Thanks again.

 

[PeterDonis]

I'm definitely not trying to visualise trillions of photons!

The "trillions" part is not the issue; the "photons" part is:

I am wondering what happens as the intensity is dialled down to a (relatively) small number of photons passing simultaneously through the experiment, still giving rise to interference effects.

Unless the source is emitting Fock states (which it isn't in any double slit experiment that has been done to date), there are not photons passing through the experiment. There is light (the quantum electromagnetic field) passing through the experiment, but you can't usefully describe it as photons. It will get detected as individual impacts on the detector screen, but that does not mean it is photons before then. It's not.

 

[PeterDonis]

(I'm assuming that in all these scenarios, discrete photons are generated when electrons in atoms change energy levels and the aggregate emissions form the source for the double slit experiment).

Unfortunately, while this seems like it ought to work (textbooks do say, after all, that atoms "emit photons" when electrons change energy levels), it doesn't.

First consider a source like an incandescent lamp. How does it work? You have a filament that gets heated up, and it emits light because of its temperature. At the microscopic level, the light is not coming from distinct energy level transitions in atoms; it's coming from the kinetic energy of the atoms themselves as they jiggle around inside the filament. Heating them up makes them jiggle faster; emitting light makes them jiggle slower. The resulting light that gets emitted does not have a useful description in terms of discrete "photons", because the jiggling doesn't have a useful description in terms of a pair of energy levels, or even a reasonably small number of them. In quantum electromagnetic field terms, it's an incoherent state that doesn't have a well-defined "photon number" or even a well-defined frequency or phase. It's just a jumble. That's why, to even get a useful double slit experiment from such light at all, we have to first pass it through a filter, like a single slit, which at least gives it a somewhat useful frequency and phase (in classical terms--at the level of the quantum electromagnetic field it's still pretty much a jumble, but enough less of one that it can be used for a double slit experiment) so that it will form an interference pattern after the double slits.

Now consider a fancier source like a laser. In a laser, you do have what amounts to a two energy level system: you have a cavity with a bunch of particles in it (which can be atoms, usually of a gas like neon, or can be free electrons, or even something else) that normally sit there at the lower energy level, but if you pump energy into the laser, you pump them up to the higher energy level. Then they emit light when they drop back to the lower energy level, and because of the way Bose-Einstein statistics work, once one particle emits the light, it stimulates the others to emit it ("laser" stands for "Light Amplification by Stimulated Emission of Radiation"), so there is a very fast cascade where lots of particles emit light in a coherent state. (If you keep pumping energy into the laser, you can make the light emission continous, as particles are constantly jumping up to the higher energy level and then dropping back down to the lower one as they emit light. That's what a laser pointer is doing, for example--the energies involved are small, of course, but it's the same principle.)

However, even in the case of a laser, the light still cannot be said to be "made of photons". The light is in a coherent state--that's actually not just ordinary language, that's a technical term that describes a specific kind of state of the quantum electromagnetic field. In such a state, there is a well-defined frequency and phase (in practice there is always some spread, but it can be made very small), but there is not a well-defined photon number (it's not a Fock state).

 

[anuttarasammyak]

Just to clarify my original, loosely worded question: by 'incandescent' I was really thinking of a diffuse source where emitted photons don't have a way to conspire together to be in phase (I'm assuming that in all these scenarios, discrete photons are generated when electrons in atoms change energy levels and the aggregate emissions form the source for the double slit experiment).

Some further clarifications, and please correct me if my understanding is wrong.

Suppose that the incident light is monochromatic, i.e. it has essentially a single frequency. However, monochromaticity alone is not sufficient to guarantee a clear interference pattern. What is also important is that the relative phase of the light arriving at the two slits is sufficiently well defined.

For a point-like distant source, the incident wave is approximately a plane wave. Therefore, the phase difference between the two paths through the right and left slits is well defined. Many photon detection events can then accumulate to form a clear interference pattern on the screen.

For an extended incoherent source, such as a filament or the Sun, light from different points of the source arrives from slightly different directions. Each point on the source therefore produces a slightly different relative phase between the two slits and, consequently, a slightly shifted interference pattern. When the contributions from all these mutually incoherent source points are added together, the different fringe patterns can wash each other out, so the observed interference pattern becomes less visible or may disappear altogether.

This is what makes me interested in the Michelson stellar interferometer. A star is an extended and incoherent source, but interference can nevertheless be observed between light collected at two separated apertures. Since the star is very far away, the light from each sufficiently small point on its surface arrives approximately as a plane wave. However, light from different points on the stellar disk arrives from slightly different directions.

As the separation between the two apertures (the baseline) is increased, the relative phases associated with different points on the stellar disk become increasingly different. Their interference contributions therefore average out more strongly, and the fringe visibility decreases.

By measuring how the fringe visibility changes with baseline, we can determine the angular diameter of the star and, if its distance is known, estimate its physical radius.

Is this understanding basically correct?

[EDIT] Summary of my understanding oh the thread up to here

In Young’s double-slit experiment with a single photon, the probability amplitude for the photon to pass through the left slit interferes with the probability amplitude for it to pass through the right slit.

For two independent photons, the probability amplitudes associated with the left and right paths interfere for each photon separately. Normally, the probability amplitude of photon 1 does not directly interfere with that of photon 2.

However, in special cases of two-photon interference, such as Hong–Ou–Mandel interference, probability amplitudes corresponding to different indistinguishable processes involving the two-photon system as a whole can interfere with one another.

Ordinary coherence and incoherence can be explained without treating the incident light as photons, in terms of the superposition and phase relationships of classical electromagnetic waves. However, a quantum description involving photons is needed to explain the discrete detection of light one photon at a time.

 

[flippiefanus]

Nobel prize winner Roy Glauber phrased this nicely (see this text on the ArXiv ):

" When you read the first chapter of Dirac’s famous textbook in quantum mechanics [8],
however, you are confronted with a very clear statement that rings in everyone’s memory.
Dirac is talking about the intensity fringes in the Michelson interferometer, and he says,

'Every photon then interferes only with itself. Interference between two different photons never occurs.'

Now that simple statement, which has been treated as scripture, is absolute nonsense.
First of all, the things that interfere are not the photons themselves, they are the prob-
ability amplitudes associated with different possible histories. You can obviously have
different histories that involve more than one photon at a time. "

What Glauber is referring to (as far as I can tell) is multi-photon effects such as the Hong-Ou-Mandel effect. In such cases Dirac's statement is not valid. However, the slit-type experiments discussed here do not involve multi-photon effects. Therefore, Dirac's statement is applicable. One can understand (and calculate) the interference in such slit experiments based on the interference of photons with themselves.

 

[flippiefanus]

Loosely speaking, first order coherence is a measure of how well we can predict the amplitude and phase of a light field, if we know it at a single position. First-order coherence is completely classical and has absolutely nothing to do with photons. It is not sensitive at all to the existence of photons - as one can see in the expressions in Glauber's notes they only depend on fields and not on occupation numbers. This indeed also means that it is no surprise that first -order coherence persists even for feeble light levels.

The coherence that defines a laser is instead second-order coherence. It tells you how well you can predict the intensity or (or photon number) of a light field at some time and position if you know its intensity here and now by detecting one photon. This actually tells you something about single photons - if you have a single photon here and now and detect it, the photon number in the future will simply be zero because the photon is gone. This is the physical signature of a single photon: It is not about the mean occupation number being below one, but about the impossibility to have two simultaneous detection events.

First-order and second-order coherence/correlations are distinguished by the number of detections that are required. For first-order coherence, one needs only one detector. The coherence of all sources can be determined with the aid of an interferometer and one detector. Higher-order correlations require two (or more) detectors. Such cases include the Hong-Ou-Mandel effect, anti-bunching, etc.

 

[flippiefanus]

@TheHutch you seem to be trying to model all possible states of light in terms of "photons". Unfortunately, that's not a workable approach.

All classical states are also quantum states. Therefore, even classical light sources produce photons. All scenarios can be analysed using quantum theory. It may not always be efficient to do a calculation with quantum theory when a classical calculation would suffice. However, often it is equally efficient. Sometimes a quantum point of view can actually be helpful in a classical scenario. The scenario of the current discussion may be such a case.

Incandescent light can be modelled as a thermal state. One can model the slit as a kind of beamsplitter where one output port represents the photons that survived the slit and the other output port represents those photons that are lost and are traced out. Then we propagate the surviving photons with a unitary operation representing free space propagation to the screen where they are detected with an observable in a final trace operation. Such a calculation is not too difficult and it should reveal the interference. (However, it may need a previous slit following the same process to introduce the spatial coherence via the Van Cittert-Zernike theorem.)

 

[flippiefanus]

The term "photon" is much more restricted; it refers either to a certain set of possible states of the quantum electromagnetic field (which are called "Fock states" and are very hard to produce), or ...

Since the Fock basis spans a subspace of the Hilbert space that is dense in the Hilbert space, all states can be expanded in terms of the Fock basis. That is not to say that classical light is a Fock state. It just means that one can model classical light as a superposition of Fock states.

 

[PeterDonis]

All classical states are also quantum states.

Yes.

Therefore, even classical light sources produce photons.

No. Not all states of the quantum electromagnetic field have a useful description as photons.

one can model classical light as a superposition of Fock states.

One can of course model any state of the quantum electromagnetic field (not just "classical light") as a superposition of Fock states. That does not justify the claim that all states of the electromagnetic field are made of photons. One could equally well express the same states of the field in any of an infinite number of other bases.

 

[flippiefanus]

Heating them up makes them jiggle faster; emitting light makes them jiggle slower. The resulting light that gets emitted does not have a useful description in terms of discrete "photons", because the jiggling doesn't have a useful description in terms of a pair of energy levels, or even a reasonably small number of them. In quantum electromagnetic field terms, it's an incoherent state that doesn't have a well-defined "photon number" or even a well-defined frequency or phase.

This scenario is precisely the situation that led Max Planck to realise that radiation from such a thermal source is quantised. These quanta came to be called photons. An incandescent light source is a source of thermal light producing a black-body radiation spectrum. Planck showed that the shape of this spectrum can only be explained if the radiation is quantised. That does not mean that a thermal state of light has a fixed number of photons. It consists of a superposition of Fock states with different numbers of photons. The same applies for coherent states. So, a fixed photon-number is not a prerequisite for the existence of photons in a state.

 

[flippiefanus]

Some further clarifications, and please correct me if my understanding is wrong.

Suppose that the incident light is monochromatic, i.e. it has essentially a single frequency. However, monochromaticity alone is not sufficient to guarantee a clear interference pattern. What is also important is that the relative phase of the light arriving at the two slits is sufficiently well defined.

For a point-like distant source, the incident wave is approximately a plane wave. Therefore, the phase difference between the two paths through the right and left slits is well defined. Many photon detection events can then accumulate to form a clear interference pattern on the screen.

For an extended incoherent source, such as a filament or the Sun, light from different points of the source arrives from slightly different directions. Each point on the source therefore produces a slightly different relative phase between the two slits and, consequently, a slightly shifted interference pattern. When the contributions from all these mutually incoherent source points are added together, the different fringe patterns can wash each other out, so the observed interference pattern becomes less visible or may disappear altogether.

This is what makes me interested in the Michelson stellar interferometer. A star is an extended and incoherent source, but interference can nevertheless be observed between light collected at two separated apertures. Since the star is very far away, the light from each sufficiently small point on its surface arrives approximately as a plane wave. However, light from different points on the stellar disk arrives from slightly different directions.

As the separation between the two apertures (the baseline) is increased, the relative phases associated with different points on the stellar disk become increasingly different. Their interference contributions therefore average out more strongly, and the fringe visibility decreases.

By measuring how the fringe visibility changes with baseline, we can determine the angular diameter of the star and, if its distance is known, estimate its physical radius.

Is this understanding basically correct?

Yes, interesting! So, what you are revealing here is the difference between temporal and spatial coherence. The monochromatic assumption addresses the temporal coherence requirement. To address the spatial coherence one needs to limit the size of the source, or strictly speaking the allowed variation in the angle of the propagation vector. That is why one needs to use another slit at some distance before the actual slits. However, such a slit only addresses the spatial coherence. It cannot make the light monochromatic. For the interference in a double slit experiment, the spatial coherence is more important than the temporal coherence. If the light is not quite monochromatic but the spectrum is not too broad one can still see interference fringes, but with some dispersion effect.

 

[Cthugha]

What Glauber is referring to (as far as I can tell) is multi-photon effects such as the Hong-Ou-Mandel effect. In such cases Dirac's statement is not valid. However, the slit-type experiments discussed here do not involve multi-photon effects. Therefore, Dirac's statement is applicable. One can understand (and calculate) the interference in such slit experiments based on the interference of photons with themselves.

No, this is what Glauber is of course famous for, but this is not what he is referring to. The key point is that all completely indistinguishable events that begin from the same initial configuration and end up with the same physical situation (which in this case means a detector click at some detector position behind the double slit) will give you an interference pattern. This is also the case if you illuminate the two slits of the double slit with different light sources, but (and this is a huge but) stabilize them to the point that their spectrum and relative phase are well defined. This was first shown by Magyar and Mandel in 1963 using masers (See this famous article ). One would need to do quite a stretch to explain this in terms of individual photons - then the photon would have to be emitted by both light sources.

This again shows why it is much more sensible and sufficient to discuss these effects in terms of fields. And if one really wants to talk about photons, it is sufficient to talk about probability amplitudes which behave fully like the electromagnetic fields from a mathematical point of view, but avoid the problem of discrete detection events by describing the probabilities for photon detection (when considering the modulus squared as the equivalent of the classical intensity) instead of intensities. Of course everyone is free to choose a more complicated treatment, but in a thread marked as undergrad level, I would be hesitant to do so.

 

[gentzen]

However, a quantum description involving photons is needed to explain the discrete detection of light one photon at a time.

The goal of a quantum description should first of all be to compute and predict measurable results. To first order, stimulated „discrete“ emission (rates) (of an excited electron dropping to a lower state) and „discrete“ absorption (rates) (causing excitation of an electron) can be computed and predicted with a classical wave picture of light.
To compute and predict spontaneous „discrete“ emission (rates) (of an excited electron dropping to a lower state), you need a quantum description of light.
Where you also need a quantum description is for understanding energy and momentum conservation in single events, for scenarios where it is applicable.

Of course, there are many more scenarios where a quantum description is needed. Maybe you want to compute more accurate than to first order, or you have a scenario with important intermediary quantum states (like in a quantum computer), or …

 

[PeterDonis]

This scenario is precisely the situation that led Max Planck to realise that radiation from such a thermal source is quantised. These quanta came to be called photons.

As a description of a key event in the history of quantum theory in ordinary language, this is fine. But physics is not the same as the history of physics, and we don't do physics in ordinary language.

a fixed photon-number is not a prerequisite for the existence of photons in a state.

I disagree, for reasons I've already explained. As you can see from, for example, @Cthugha's post #19, I am not the only one who sees issues with your claim.

 

[sophiecentaur]

I'm definitely not trying to visualise trillions of photons!

But, if you want to describe the mechanism of light diffraction in this way then that is precisely what you are implying; the statistics of a huge number of photons which, of course, you haven't described adequately (they are not little bullets. Treating the wave travelling through the slits can be done perfectly well using fields and waves. Trying to introduce (your) photons in an effort to get 'nearer the truth' is pointless.

 

[flippiefanus]

This is also the case if you illuminate the two slits of the double slit with different light sources, but (and this is a huge but) stabilize them to the point that their spectrum and relative phase are well defined. This was first shown by Magyar and Mandel in 1963 using masers (See this famous article ). One would need to do quite a stretch to explain this in terms of individual photons - then the photon would have to be emitted by both light sources.

The stabilisation of the two source makes this scenario much more complicated and thus cannot be compared with the single source scenario. This strictly speaking becomes a multiple-photon effect.

 

[flippiefanus]

This again shows why it is much more sensible and sufficient to discuss these effects in terms of fields. And if one really wants to talk about photons, it is sufficient to talk about probability amplitudes which behave fully like the electromagnetic fields from a mathematical point of view, but avoid the problem of discrete detection events by describing the probabilities for photon detection (when considering the modulus squared as the equivalent of the classical intensity) instead of intensities. Of course everyone is free to choose a more complicated treatment, but in a thread marked as undergrad level, I would be hesitant to do so.

Yes, fields indeed, but unless one introduces the notion of quantised fields (photons are not classical point particles), one would just be doing classical field theory that cannot explain all phenomena. It is not sufficient to just use probability amplitudes without considering quanta. There are experiments (violation of Bell's inequality) where the quantised nature of the EM field plays an important role.

Replacing the intensity with a probability amplitude that is normalised under $L^2$ (modulus square integrates to 1), one implies a quantum of the field, i.e., a photon. So one can just as well call it by that name. Reading through the description of the calculation procedure, I get the feeling that it is still based on photons, just without calling it that.

 

[flippiefanus]

But physics is not the same as the history of physics, and we don't do physics in ordinary language.

The historical significance of Planck's discovery is precisely in providing a deeper understanding of the physics, which led to the development of the formalisms of quantum theory, all of which incorporates the notion of quantisation.

 

[sophiecentaur]

one would just be doing classical field theory that cannot explain all phenomena.

You seem to be implying that classical field theory cannot be used for any phenomena. That is throwing the baby out with the bath water. It seems to me that this question about diffraction and how to explain it in terms of photons can only be be done with a thought experiment.

The practicalities of 'proving' the idea are actually far too difficult. There are plenty of experiments that justify the results of diffraction in terms of streams of individual photons so the basic idea is justified but (using the information from classical theory) it would be necessary to create a source of photons (i.e. a light beam) with the characteristics that are being proposed here would the equivalent of standing up in a hammock. What would be the point? Sure we can be confident that the result would be exactly the same (as far as the resulting diffraction pattern is concerned) as with a conventional experiment.

Can anyone think of an example where the two approaches conclusively disagree? Not in the context of 'normal conditions, I think. There seems to be a misconception that photons 'do everything better' The big-endians and little-endians are showing themselves here. Spending some time on practical use of diffraction equations (antenna and optics design) could be a better use of time and improve the soul.

 

[TheHutch]

Throwing another pebble in the pond...

Unless the source is emitting Fock states (which it isn't in any double slit experiment that has been done to date), there are not photons passing through the experiment. There is light (the quantum electromagnetic field) passing through the experiment, but you can't usefully describe it as photons. It will get detected as individual impacts on the detector screen, but that does not mean it is photons before then. It's not.

Would everyone's answers change if I said 'electron' instead of 'photon'?
As I understand it, we get just the same interference effects in 'double slit' experiments using electrons. Is that just a coincidence? Maybe the effects aren't the same - has anyone done other optics-like experiments with electrons? There must be lots of diffraction patterns out there, for example.

 

[anuttarasammyak]

FYI experiment video of Double-slit experiment for electrons https://www.hitachi.com/rd/research/materials/quantum/doubleslit/index.html

​

 

[sophiecentaur]

we get just the same interference effects in 'double slit' experiments using electrons. Is that just a coincidence?

I imagine that, as electrons are fermions and photons are bosons, the effects of any real aperture could be different. A slit behaves much as a n ideal slit for photons but electrons would be affected differently in detail when passing an edge of matter with a distribution of charge.

So yes and no. If you can specify the model for each case then the wave solution would be the same as the low density particle model. But is that a surprise or an annoyance?

 

[PeterDonis]

Would everyone's answers change if I said 'electron' instead of 'photon'?

Electrons are fermions, not bosons, so the structure of the Hilbert space is different, but more importantly, the kinds of states produced by common sources are different. Most importantly for this discussion, fermion Fock states are much easier to produce than boson Fock states. For example, the electron states produced by a cathode ray tube, AFAIK, are electron Fock states. So viewing what comes out of a CRT as a beam made of electrons is a much more viable approach than viewing what comes out of a flashlight as a beam made of photons.