How big is a photon

DaTario

It seems quite to provoke us that light quanta has no position but travels with c and moreover obeys kinematics rules dictated by correlation experiments with twin photon's (spontaneous down conversion for instance).

Of course we may learn this lesson and start building the ontology of photon with such subtleties. Photon is not a typical kind of being and etc...

Weisskopf Wigner theory for spontaneous emission produces the notion of a particle like object, which is corroborated by ecrans. If we are to abandomn this particle view, we should provide explanation for ecrans and the punctual appearance of light on it.

best wishes

DaTario

Anonym

That is not what I have in mind. Zz is the experimentalist.

You need the standard references and standard terminology if you want that the other people will understand what you want to say.

What is the problem? Just apply to Zz at ANL to be his student/assistant. They need youngers with one leg in the experiment and the other in the theory.

Regards, Dany.

lightarrow

What does "The particle you are describing is spread out all over that range" mean? Does it mean that a particle with very short-range coherence has a quite well defined position in space? And the same for a photon? It's just a question.

ZapperZ

Er.. I did ask you to find <x>, didn't I? Try it. When you find <x>, that is exactly what I meant when I said it is spread out.

We are trying to put "words" to something that is essentially and foremost, has a mathematical description. So unless you understand what it looks like mathematically, word description will never be able to convey the exact scenario.

Zz.

lightarrow

Since it seems difficult for you to give direct answers, I will give them for you (so, you can criticize them, it's your preferite task).

1. A photon's wavefunction with coherence lenght L represents an object travelling in space with a position indefiniteness = L, that is, the photon can be detected in a any instant of time from when the initial end of the train wave reach the detector to when the last end reach it.

1.B The photon is pointlike. But in this case, why differently prepared photons (of the same energy) can have different L?

2. A photon's wavefunction with coherence lenght L represents an object travelling in space which is spread out in the lenght L, that is, the entire photon is actually located in all that lenght.

3. ...


Which do you prefer?

ZapperZ

It is difficult because the term "coherence length" has different physical meaning in different scenario, very much like "phase space". In superconductivity, it is of the scale of the extent of the Cooper Pairs wavefunction.

Since I wasn't the one who introduced the coherence length concept for a photon, I shouldn't be the one who has to answer this for you as far as photons are concerned. All I have given you is some generic concept on how something with long-range coherence can be considered to be spread out all over that range. That's all. If you wish to look beyond this but not care about the mathematical formalism, then I am the wrong person to ask.

Zz.

rewebster

that is he that--and probably more


hopefully you know that was not what I was saying


and from post 51:

we could BOTH apply and be ROOM MATES


--but the problem would be, if we did apply and be the junior peon preons in the accelerator world, from what I'm reading that we STILL wouldn't find out 'how big a photon is'

rewebster

I don't have problem with the part about you toeing the party line--I think I got that part. --and there is, also to me, a fine line between being subjective in an opinion (from knowledge) and speculative; in that you (and the other mentors) have to define from time to time.


anyway, post 202 was answering to your question, not me just spewing (and I wasn't arguing either)

Anonym

Honestly, I have no any idea what you have in mind and what does it mean.

Go ahead. I promise it will be great fun not only for Zz but for whole ANL!

However, you missed something. I know 'how big a photon is' in QED but don’t know that in CED yet. I do not need help from them. This is my job.

Regards, Dany.

rewebster

(shhhhhh-----don't say that aloud around here-----shhhhh)

Schrodinger's Dog

I think it was Zz who lead me from the dark side. He made a rather good post that answered this question in terms I felt made the most sense. If people really want to find out good answers to this they've all been given numerous times before and will suit most people.

To be frank though I was never really that convinced it had mass, I just had questions like a alot of people do.

In this particular case and with our current level of technology, the well trodden path is best . Words of wisdom do not cover everything, at some point you have to accept that to proceed further along the unbeaten trail you need better machinery

rewebster

-----LHC ?

Schrodinger's Dog

I don't think it's going to suffice, it's not so much smashing them apart but how you detect and measure such tiny portions of energy amongst the mass of energetic particles created in a collision; besides establishing the mass or ever more precise upper limits of a photon is not as important as establishing the existence or non existence of say the Higgs Boson, so since detection methods aren't precise enough scientists probably have better things to do with it. I'm not an expert not even close, but I don't think we are anywhere near being refined enough with the machinery atm. I'd ask Zz...

rewebster

I wasn't too serious when I suggested it---just the idea of 'bigger and better' came up.

I wonder how many betting 'pools' have come up as to how many new particles will be 'discovered'/'created' with the first year there.


well, that's good for you though that's one less worry/thing to think about if it is 'at rest' now.

LorentzR

No! I don’t agree with this statement. The idea of the photon was at least partly introduced as a solution to the old problem of what causes “action” at distance. But the question presupposes the notion of locality. This in the context of modern physics would be the assumption that the “particles” composing matter can be assigned, for any given time, a unique position in space. However, correlation experiment results showing the violation of Bell’s inequality suggest that our world has a non local flavour; casting doubts on the idea that particles of matter can be assigned exact locality relative to our inertial reference frames. This in turn puts into doubt the need for carrier particles to mediate action at a distance. If the locations of “particles” are not fixed exactly relative to our inertial reference frames then neither are the gulfs that separate them and direct interaction between pairs of particles becomes possible when the gulf between them has zero magnitude.

The “particles”/ “fields” of matter must remain in our philosophy it is only the photon (the suggested mediator of force) that is surplus to the explanation of electromagnetism.

LorentzR

No one has ever observed the position of a photon or any other quantum particle. What you observe is a macroscopic response in a particular experimental set up. The timing and position of the response may be measured with exquisite accuracy but the observable response exists at the end of a sequential causal chain that is necessary to amplify a quantum event to an observable event.

You are observing the location of a macroscopic event that is initiated by a quantum event. There is no clear logical path that links what is observable to where and when the quantum event took place. To give a photon or other quantum particle a definite position on this evidence is unsustainable. All we can say is that somewhere in the four dimensional manifold an event has occurred that has affected our detection device in a particular way. There is no justification based on experimental evidence for assigning a definite position to a quantum object.

ZapperZ

Then you need to write a rebuttal to all the experimental high energy physics papers, and even to condensed matter papers using anything that measures electron and neutron spectroscopy such as what I've shown in my avatar. Look under ANY papers in any physics journal that actually detects these particles and tell them that they're not really detecting that 'quantum event'. Remember, in angle-resolved photoemission experiment, the location where the electron hits the detector just doesn't signify that this electron was there at that time, but also carries information about its energy and "in-plane" momentum while it was in the solid!

Did I see electrons? Of course not! But did I detect that at the instant the detector made its detection, that quantum particle was there? Sure I did! Is this macroscopic? Of course it is! All our measurements are not only macroscopic, they are also classical! That is why I can talk about "position" and "momentum" and "energy", because these are all classical concepts. It is also the CAUSE of the weirdness at the quantum level, because we are asking for properties that are not that well-defined at the quantum world.

Note that in the HUP, there's nothing here that says that you cannot measure, as accurately as technologically possible, the position and momentum of just ONE measurement. The accuracy of these measurements do not depend on the HUP, but depends on the accuracy of the instrumentation and technique. The HUP says nothing about a single measurement. The HUP, however, tells you (i) the accuracy that you can predict the NEXT measurement, given the accuracy that you have imposed on one of the non-commutating observable, and (ii) the spread in the value of that observable after repeated identical measurement.

Zz.