# What does a single photon look like

Since a photon travels at the speed of light it has no spatial existence in the way
we interpret all other physical particles to have. According to Einstein, who originated
the concept, the photon is not the wave but the particle of light which has a
definite quantified value of momentum.

Life and death of a photon:

http://www2.cnrs.fr/en/824.htm

The answer is fairly simple. A photon is a plain electromagnetic wave whose existence has a certain probability. This wave can give it's Energy in a single point to another quantum object. Mathematically this is it.

The confusion in the Feynman diagrams stems from the fact that they depict what happens in momentum space. The lines that are shown are not a tiny object anihilating with another tiny object producing a photon which flies distance and does something else. At least mathematically it's about infinite waves meeting in the universe and exchanging energy. On the other hand the tiny particles are, what you actually see in the detectors. This is caused by the fact, that you never have pure plain waves, but always spatial and temporal "envelopes" around them. So if you do experimental particle physics a photon is a tiny wavepacket. If you do quantum optics it's more often a standing wave in a box. And if you do theory it's an infinite plain wave filling the universe.

The answer is fairly simple. A photon is a plain electromagnetic wave ... Mathematically this is it.

Mathematically? That is not the question? The question is what is the physic(al)s object we term the photon?

The problem is that the present mathematics is a model of behavior not a model of the photon.

The present theory can not tell you if the EM mathematical description is a description of the photon (a property of the particle) or a description of the interaction of the photon (a property of the interaction - an inter-particle property) and thus lacks clearity.

That the EM description is likely not the photon is also evidenced by the fact that these can not be used when one tries to combine the photon and matter particles (QED), but requires the use of the vector potential. This unification can not be done using the EM description. Lastly the EM photon fields rise and fall together and thus do not continuously conserve total field energy.

To quote Einstein, Today every Tom, Dick and Hary thinks he understands the photon but they are wrong.''

reilly
Science Advisor
If you want to know what a photon looks like, then you must see it. And, virtually all methods of "seeing" a photon result in the destruction of the photon -- or substantial modification thereof. That's how our visual system works; photons create photo-dissociation currents in rods and cones, which lead to nerve pulses traveling up the optic nerve-- if the current is strong enough (this is a very simplified description).

Everything you see is generated by photons. It could be said that vision is seeing a bunch of photons as they impinge upon the retina; and that seeing is mediated by electric currents in the retina.

The photoelectric effect sees a photon by emitting an electron.
Regards,
Reilly Atkinson

Mathematically? That is not the question? The question is what is the physic(al)s object we term the photon?

The problem is that the present mathematics is a model of behavior not a model of the photon.

So you are replacing the well defined term photon with a fuzzy one that represents your idea of a photon. Sorry the name is taken, you may call your photon the enotstrebor-photon, and define it any way you like.

The present theory can not tell you if the EM mathematical description is a description of the photon (a property of the particle) or a description of the interaction of the photon (a property of the interaction - an inter-particle property) and thus lacks clearity.

It is a description of a state of the em field. Just like an electron is a state of the electron field. Your apparent disparity does not appear to me. I agree that introductory texts into particle physics, which don't use QFT are misleading, but once you read some dedicated Quantum optics books things should clear up.

That the EM description is likely not the photon is also evidenced by the fact that these can not be used when one tries to combine the photon and matter particles (QED), but requires the use of the vector potential.

Photons are excitations of the A-Field (aka vector potential). This is why we do quantum field theory. Electrons are also an excitation of the electron particle field. Still no contradictions.

This unification can not be done using the EM description.

Whatever that EM description might be. Maybe you are trying to tell us, that the fact that we use the Lorenz gauge in QFT which is something we are free to choose in classical EM. This still doesn't show that we don't understand photons.

Lastly the EM photon fields rise and fall together and thus do not continuously conserve total field energy.

I thought there were no photon fields in EM. If you are talking about QFT again, than I assure you that the em field conserves energy just like anything else in QFT.

To quote Einstein, Today every Tom, Dick and Hary thinks he understands the photon but they are wrong.''

And he liked dreaming, and had a lot of stupid ideas too:
I am enough of an artist to draw freely upon my imagination. Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world.
Albert Einstein

To summarize: We know the photon pretty well by now. Phenomenological text books and many QFT intros suck for reasons I don't understand, and this confuses many people, but once you have quantized the A field by hand and checked out things like quadrature squeezed states, I think you will have a better grip on the photons.

Many people falsely believe, that photons can only be measured by absorption, this is not the case. Theory gives no reason to assume this, and there are new experiments which manage to do these http://www.nature.com/nature/journal/v448/n7156/abs/nature06057.html". Half truths stated here are state of physics research many years ago. We have progressed people, photons are real and not just some clicks in a photomultiplier.

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I've always wondered what is considered to be 1 photon. We always see things like "When two of these particle collide, they produce this and that particle, and ONE photon." The hell does that look like?!

Is it like a monochromatic plane wave but just a line.. like a frozen segment of a sine wave propagating at c in some direction..?

I think that this is the best article about photon:

http://arxiv.org/PS_cache/quant-ph/pdf/0410/0410179v2.pdf" [Broken]

and 2 articles connected in triplet with this article. I hope you will find references for both of them.

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We would never be able to "see" a photon as the act of seeing things is our brain interpreting certain physical properties of photons in a certain way. These photons have bounced off of something which is what changes the properties. (wavelength in this case giving colour). We can only see objects as a difference in colours.

Now as I said the only way we can see things is by bouncing photons off of the object. To see a photon would require bouncing a photon off of another photon. But because of this the most we could get is a colour? (Which would be incorrect. A photon does not have a colour. Essentially it IS colour.)

Congratulations to anyone who manages to follow that. Its very difficult for me to explain. :D

So you are replacing the well defined term photon with a fuzzy one that represents your idea of a photon. Sorry the name is taken, you may call your photon the enotstrebor-photon, and define it any way you like.

I am not replacing anything! Your just being supersilious.

So lets start again.

It is a description of a state of the em field.

Answer one question: Can you say conclusively that the Maxwell equation photon solution(EM wave description) or QFT version; 1) represents the photon or 2) represents the interaction (inter-particle) effect of the photon.

Study the history of physics. These are (historically and experimentally) purely models of the inter-particle behavioral effects (and not from an set of underlying elements of a particle theory and the resultant mathematical model designed to produce the resultant behavioral effects), i.e the observables.

Thus you can not say that Maxwell's equations and QFT represent the particle when all that is known is that they represent (reproduce) the inter-particle behavior!

It is a description of a state of the em field.

As to the question of conservation of energy at any instant in time, just look at (EM wave description) http://en.wikipedia.org/wiki/Image:Onde_electromagnetique.svg (from http://en.wikipedia.org/wiki/Electromagnetic_radiation)

As both the E field and B field can both be given by an in-phase sine wave (or both by in-phase cosine) then at some instant of time say t=0 the normalized E,B field energy is zero (i.e. sin(0)) and at a later instant in time the normalized E,B field energy is one (i.e. sin(90).

This is why the original version of the photon had the E and B fields 90 degrees out of phase (field energy = cos^2 +sin^2=constant) until it was shown experimentally that the E and B fields were in phase (field energy=sin^2+sin^2).

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I am not replacing anything! Your just being supersilious.

Yeah. Physics is more fun that way. To paraphrase Asimov: Those who think they know everything, are a great annoyance to those of us who do.

[...]
Answer one question: Can you say conclusively that the Maxwell equation photon solution(EM wave description) or QFT version; 1) represents the photon or 2) represents the interaction (inter-particle) effect of the photon.
I don't understand your question, and by virtue of my haughtiness I blame you for posing a bad question. Does the kinetic gas theory describe a kinetic gas or the interaction of the kinetic gas with the world? What's the difference? Ontology? Then this thread needs to move to philosophy now.
Study the history of physics. These are (historically and experimentally)
... but not theoretically and modernly...
purely models of the inter-particle behavioral effects (and not from an set of underlying elements of a particle theory and the resultant mathematical model designed to produce the resultant behavioral effects), i.e the observables.
Again I don't understand. Are you saying Maxwell doesn't follow from a subset of QFT? First course in QFT we derived Maxwell from the Lagrangian. What's your point?

Thus you can not say that Maxwell's equations and QFT represent the particle when all that is known is that they represent (reproduce) the inter-particle behavior!

Inter charge behavior, inter electron behavior but no inter photon behavior.

[something about no energy with zero poynting vector]

So if your point is that we don't know if there are further details to photons stemming from some underlying theory, I'd agree. And then you might say we don't know what a photon really is, but than we don't need an argument about how e/m theory was discussed 100 years ago, but one about QFT.

But in the end it doesn't matter. We can make beautiful theories that describe things well without any underlying theory. We don't need to know about atoms for the description of rigid bodies.

So if your point is that we don't know if there are further details to photons stemming from some underlying theory, I'd agree.

This is the essence of what I am saying.

Remember the original question I was addressing was, "What does a single photon look like?"

If a behavior involves only the particle itself then one can say the behavior is fundamental to the nature of the particle. If the behavior is the result of one particle (massed or massless) with another particle then the behavior is an inter-particle behavior.

The mathematical modelling of the behavior may attribute an aspect to the particle (in this case the photon) which is actually the resultant effect on the second particle (e.g. the electron). For example (hypothetical example), as an inter-particle effect, the photon could have a single field which produces two effects on the other particle (e.g. an electron). Noting the similarity between the signs of the Fuv matrix elements and the four dimensional torsion matrix of a gyroscope is suggestive that given particle spin, the E is potentially an in-plane (spin) gyroscopic action and the B effect a plane-normal gyroscopic reaction. Thus hypothetically the E,B effects are not two fields but two resultants of a single field.

Which brings us back to the question, "Can you say conclusively that the Maxwell equation photon solution (EM wave description) or QFT version; 1) represents the photon or 2) represents the interaction (inter-particle) effect of the photon."

But if the mathematical model (QFT or Maxwell's) is not a model of the photon but of its effects on the particle this goes to the heart of the question "What does a single photon look like?"

The answer is we don't and can't know from the present models.

but than we don't need an argument about how e/m theory was discussed 100 years ago, but one about QFT..

Does QFT tell us anything more about the photon than we knew 100 years ago? QFT can't tell use anything fundamentally true about the nature of the photon as it can not answer the question, does the photon in QFT ; 1) represents the photon or 2) represents the interaction (inter-particle) effect of the photon.

That is to say that in 100 years of effort we still cann't answer the question "What does a single photon look like?"

What's the difference? ......But in the end it doesn't matter..

If it doesn't matter, then why does the question still arise?

Why when the question is asked, do not those who are knowledgeable just simply explain/admit that the present mathematical models can not tell us what the photon looks like.

said:
"What does a single photon look like?",

The answer is, after 100 years of theoretical developement, we haven't got a clue.

But we have a great model of its inter-particle behavioral effects.