What is the size and shape of single optical photon?

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Summary:
What is the shape and size of energy distribution of optical photon?
Optical photon is produced e.g. during deexcitation of atom, carrying energy, momentum and angular momentum difference.
So how is this energy distributed in space - what is the shape and size of single photon?

Looking for literature, I have found started by Geoffrey Hunter, here is one of articles: "Einstein’s Photon Concept Quantified by the Bohr Model of the Photon" https://arxiv.org/pdf/quant-ph/0506231.pdf
Most importantly, he claims that such single optical photon has shape similar to elongated ellipsoid of length being wavelength λ, and diameter λ/π (?), providing reasonably looking arguments:

1) Its length of λ is confirmed by:
– the generation of laser pulses that are just a few periods long;
– for the radiation from an atom to be monochromatic (as observed), the emission must take place within one period [10];
– the sub-picosecond response time of the photoelectric effect [11];

2) The diameter of λ/π is confirmed by:
– he attenuation of direct (undiffracted) transmission of circularly polarized light through slits narrower than λ/π: our own measurements of the effective diameter of microwaves [8,p.166] confirmed this within the experimental error of 0.5%;
– the resolving power of a microscope (with monochromatic light) being “a little less than a third of the wavelength”; λ/π is 5% less than λ/3, [12];

Is it the proper answer?
Are there other reasonable answers, experimental arguments?
 

Answers and Replies

  • #2
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A classic photon particle by definition will always be just a zero-dimensional point particle, meaning it has no length, width, or height. But looking at it in the QFT viewpoint, it's a disturbance in the photon field, then a photon will be a vibration along the width and height dimensions, but no vibrations along the length dimension. Basically just a flat pancake-like object with the vibrational flatness perpendicular to its direction of travel. The thickness of its vibrational edge might have maximum thickness of 1 Planck Length, if it isn't a perfectly zero length thickness.
 
  • #3
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I believe you are referring to Feynman diagrams: perturbative series approximation of QFT in momentum space - shouldn't we use nonperturbative in position space to get size of particles?
Perfect point particle would have infinite energy density - e.g. forming a black hole.

Photon is EM wave configuration, its hv energy is somehow distributed rho~|E|^2+|B|^2 over some volume - we should be able to describe at least its dimensions, e.g. averaged over quantum ensemble.

I have also found two more papers: https://arxiv.org/pdf/1604.03869
the length of a photon is half of the wave length, and the radius is proportional to square root of the wavelength
2021 "The size and shape of single photon" http://dx.doi.org/10.4236/oalib.1107179
 
  • #4
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A point particle isn't necessarily a black hole, it's just the average center of a packet of energy that we call a particle. Pack enough energy and mass into that point, then yes it becomes a black hole, but that energy level is the Planck Energy and the Planck Mass. Anyways, the whole concept of particles is now outdated, everything is now just a wave, and the point is just a mathematical abstraction useful for mapping the average locations of these "particles".

Particles moving at the speed of light will not have any wave motion in the direction of travel, all of the wave motion will occur in dimensions perpendicular to the direction of motion. Thus the wave motion will look like a pancake or a disk, very thin in the direction of travel, spread out in the perpendicular directions.
 
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The question of size of single photon seems similar as of the size of this packet of energy - distortion of rho~|E|^2+|B|^2 from center in position space (in momentum space we would ask for monochromaticity).

If this size as position distortion would be zero, then density would be infinite - forming black hole, but this boundary is really tiny like ~10^-57m for electron ( https://en.wikipedia.org/wiki/Black_hole_electron ).

Indeed everything is just a wave - of EM field in case of photon, its interference e.g. leads to reflection from mirror as explained in Feynman lectures.

The big question is understanding atomic processes from such EM wave perspective - e.g. there was observed ~22as delay in photoemission ( https://science.sciencemag.org/content/328/5986/1658 ) - what exactly happens during this time of e.g. photon creation?
 
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everything is now just a wave

Any source for that? I've read quantum mechanics and QFT textbooks and no one said such things. Of course defining particles in quantum context is a little bit tricky and not always possible, and we use a lot of wave-like equations, but thats far from "everything is a wave". That's good for pop-science talks, a poor ones if you ask me.


a photon will be a vibration along the width and height dimensions, but no vibrations along the length dimension. Basically just a flat pancake-like object with the vibrational flatness perpendicular to its direction of travel. The thickness of its vibrational edge might have maximum thickness of 1 Planck Length, if it isn't a perfectly zero length thickness.

Any source for that? What is a "vibrational edge"?

Photon is EM wave configuration

No, it's not. It's one-particle Fock state, EM waves are something different.
 
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Any source for that? I've read quantum mechanics and QFT textbooks and no one said such things. Of course defining particles in quantum context is a little bit tricky and not always possible, and we use a lot of wave-like equations, but thats far from "everything is a wave". That's good for pop-science talks, a poor ones if you ask me.
Yeah, text books are not really the greatest sources of understanding and interpretting, more just about "shut up and calculate". QFT pretty much gives up on the idea of particles. Particles are just more and more constricted waves on a field now.
 
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Yeah, text books are not really the greatest sources of understanding and interpretting

To the contrary, textbooks (or lectures) are the best (if not the only) sources you can get any understanding of QFT from.

QFT pretty much gives up on the idea of particles.

Most certainly it does not, as most of the textbooks show.

So, what are the sources for your claims?
 
  • #9
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First of all, QFT is usually used through Feynman diagrams - approximation with perturbative series in momentum space.
We can ask about monochromacity there, but this is still an approximation - to really understand shapes of particles with QFT, you would need to use nonperturbative in position space.
 
  • #10
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First of all, QFT is usually used through Feynman diagrams - approximation with perturbative series in momentum space.

You can do your calculations in position space, it's just more convenient to that in momentum space.
 
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Like the claims of electron being a perfect point, which make no sense from so many reasons

Well, it makes sense to particle physicists and others involved. At least in some contexts.

and experimental evidence don't confirm it:

Experimental evidence is in agreement with it.
 
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berkeman
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Thread closed temporarily for Moderation...
 
  • #15
PeterDonis
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how is this energy distributed in space - what is the shape and size of single photon?
There isn't any. A photon is not a classical object; it doesn't have a "distribution in space". It doesn't even have a well-defined spatial wave function, which is the closest thing a quantum particle with nonzero rest mass, like an electron, has to a "distribution in space".

Is it the proper answer?
Not really. The model being used in the paper you reference is classical, not quantum. The paper claims to be able to account for one particular phenomenon--interference in the double slit experiment when done with light--using its model. This is hardly a breakthrough, since the fact that classical Maxwell electrodynamics predicts interference in this experiment has been known since the 19th century. However, there are still many, many experimental results involving photons that classical Maxwell electrodynamics cannot account for; one key phenomenon is the very fact that we can build "photon detectors" that emit single clicks (or show single flashes of light) when single photons hit them--such detectors are of course common in quantum optics experiments.

This means that the classical Maxwell model of light is only an approximation, valid in certain cases, but not a valid general theory of all phenomena involving photons. (Even the concept of "photon" used in the paper, i.e., a classical "wave packet", is not the same as the quantum concept of a photon.)

Are there other reasonable answers, experimental arguments?
The "reasonable answer" is the one I gave at the start of this post. (The more technical version of that answer is that Newton-Wigner localization fails for photons, but discussing that would take this thread into "A" level territory.)

I have also found two more papers
They both have the same problem as the other paper, which I explained above. Basically, none of the papers you are reading are talking about photons at all. They are talking about the classical model of light, and they are concentrating on experiments where that model makes reasonably accurate predictions. The reason that is the case is that those experiments don't even involve "photons" at all--the light sources being used are nothing at all like the "single photon" sources that quantum physicists use. They are just ordinary classical light sources that are useless for exploring any of the quantum properties of light.
 
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  • #16
PeterDonis
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A classic photon particle
There is no such thing.

looking at it in the QFT viewpoint, it's a disturbance in the photon field, then a photon will be a vibration along the width and height dimensions, but no vibrations along the length dimension.
This is nonsense. I don't know where you are getting your "QFT viewpoint" from, but wherever it is, it does not appear to be a reliable source.

Pack enough energy and mass into that point, then yes it becomes a black hole, but that energy level is the Planck Energy and the Planck Mass.
This is all just speculation. Plausible-sounding speculation that a number of physicists have made, but still speculation.

Particles moving at the speed of light will not have any wave motion in the direction of travel, all of the wave motion will occur in dimensions perpendicular to the direction of motion. Thus the wave motion will look like a pancake or a disk, very thin in the direction of travel, spread out in the perpendicular directions.
Again, I don't know where you are getting your information from, but it does not appear to be a reliable source.

QFT pretty much gives up on the idea of particles.
This will come as a great surprise to all of the people who call themselves "particle physicists".

The concept of "particle" in QFT is certainly more complicated than just "little billiard ball" or "point particle". But that doesn't mean the concept has been "given up on".

Particles are just more and more constricted waves on a field now.
The QFT concept of "field" is, if anything, less like its classical counterpart than the QFT concept of "particle". I think you need to check your understanding.
 
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  • #17
PeterDonis
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Thread closed temporarily for Moderation...
I am reopening the thread in case any members have useful references on quantum optics, which should be much better sources for the OP to learn from than the papers so far linked to in this thread.

Everyone, please bear in mind the PF rules on personal theories and personal speculations.
 
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  • #18
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Saying that particles are perfect points in Feynman diagrams: Dirac deltas in perturbative QFT in momentum space ... wouldn't it mean that they are plane waves in position space?
Plane waves cover entire universe - should we imagine that e.g. photons now going from screen to our eyes are plane waves of infinite size?

If not, please provide better arguments, preferably based on experiments which are the best arguments - like lower bound for duration of ultrashort laser pulse, or weakening transmission through hole in wavelength scale?
 
  • #19
PeterDonis
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Dirac deltas in perturbative QFT in momentum space
Delta functions in perturbative QFT in momentum space do not "say that particles are perfect points". They are there for momentum conservation, not to model point particles.

please provide better arguments
Unfortunately, your whole approach to this question is flawed, and given your responses so far to having that pointed out, you are unable to recognize "better arguments" when they are presented to you. Given that, I am closing this thread again, this time permanently.
 

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