Photon 'size'

To a class lecturer at Princeton U. I once asked, "how large is a photon?" There was not much answer. Cool to read a neat-oh article from this past March 24, in Science News, March 24, on photon sizes. (I am speaking to my brother.)
Most interesting esp. after our discussions. "Physicists have been uncertain as to how long these quanta are, but by some it was believed that they were as much as a yard in length. <They create 3-inch long pulses>... [now] researchers see that until a definite question is asked, and the experimental setup specified, there is no definite answer to how big a photon is...'What is the extent of the photon wave
packet as it is emitted from the atom? ' Multiplying the lifetime of the excitied atom by the SOL gives an answer of around a meter".
FUN, HUH????? ¿¿¿¿ I have sensed that yes it certainly depends upon your context. If you have microwaves in a metal box, the box walls are part of what would be a wider field, for instance.

This post has been edited by Albers on Jun 29 2012, 04:51 PM

My brother is an astronomer, and we go back and forth over light and photons. In this example we are looking, I think, at a somewhat near field of an atom emitting a photon. If the wave function is describable as roughtly a meter in length, this means that somewhere within this meter, a photon will be detected, measured, or snagged, whatever. I have read in an advanced Q.E.D. text, that photon absorptions at some detector cannot influence the EM field at spacelike distances, relativistically speaking. Can we then see the absorption event as being radiated outward at the SOL? My brother speaks of the collapse of the wavefunction... (thwp, thwp) . I understand that non-locality is outside of time, regarding the sharing of INFORMATION. Thus my question.

This post has been edited by Albers on Jul 1 2012, 07:03 PM

The photon (particle) size would be separate from the EM wave field. I guess there is some debate as to whether there is a position operator for a photon:

http://arxiv.org/abs/0711.0112

For $120 you can get the answer here:

http://www.amazon.com/The-Nature-Light-Opt...g/dp/1420044249

Otherwise, maybe the position uncertainty is defined by its momentum, a la Heisenberg?

The photon wave function size spreads out to a much larger dimension and would collapse with infinite speed I believe.

This post has been edited by scalbers on Jul 1 2012, 08:04 PM

Just a thought .. if a photon is caught in one place then (maybe) it's simply 'not anywhere else' - this is slightly different from a trick involving infinite speed. Not my field (shopkeeper).-C2.

It seems that somehow the wave function knows not to create a second photon somewhere else along the wavefront once it has collapsed in a certain location. Perhaps that is an example of the infinite speed. It's the radial spread of the wavefront that would be at the speed of light.

And by the way, this paper argues that quantum wavefunctions are physically real if mysterious, even though we are discussing that they can do certain things faster than light:

http://xxx.lanl.gov/abs/1111.3328

This post has been edited by scalbers on Jul 1 2012, 09:03 PM

A far out, nice answer, thank you SCALBERS, I know I am with most minds, feeling kinda strange here. I would like to see further here, beyond the light-cone. I'd seek to study further here but 'solidspin' never returned my advanced QED text, the Dreaded Little Green Book. Richard Feynman, at some point in Vol.III of his Lectures, says that the 4-vector potential expression is the q. wave function. This is what I used in 2005, to construct both an inhomogeneous electron and a photon. The photon study is very good, and just depicts an assumed wave packet. The electron study has much to offer but is a mess, because a friend composed it when I first got a computer in late 2005, and I cannot edit the MS source codes. It would be a large effort to compose this on my Apple. With a simple inhomogeneous field of charge assumed, I came out within 4-5%, on field totals like the fine structure constant.


This post has been edited by Albers on Jul 1 2012, 10:24 PM

Just a question: Has the length of a photon ever been measured behind the slits of the double slit experiment? Both with and without detection in front of the slits.

Fast gates ??? ¿¿¿

The reason for the question is that you started of this thread with the possibility - as I understand it - that a photon could have a length of a yard. Well that must have been measured then. It can't be that difficult to place a double slit (an experiment I can even do at home BTW) in the path of the photon. Shoot one of with and shoot one of with out the slit in place. And in both instances measure the length.

What's the problem in combining that in the length measuring experiment that clearly must have taken place?



This post has been edited by kristalris on Jul 5 2012, 09:17 AM

I am not in bad company here, as I could quote an article by a theorist who admits, "I really don't know much about photons, but I know one when I see it !" We actually are speaking about the length of the quantum wave packet. If the excited emission time gives a meter-long wave, still, at some point within that meter one will absorb the photon. Further: Feynman says the vector potential is the wave function. Therefore I do have a valid analysis, from 2005, of a single wave packet. It may not be Nature's trick, tho, I don't know.

This post has been edited by Albers on Jul 5 2012, 07:48 PM

Thanks

Maybe someone can remind me whether photons actually do any traveling at all. Perhaps it is the E&M fields (in quantized form) that do the traveling and photons are merely the sites of interaction of these fields with matter?

These E&M fields from a photon emission would be localized in the radial direction but still spreading out in a spherical wave front with an associated large transverse dimension.

This post has been edited by scalbers on Jul 7 2012, 02:55 PM

I have long obsessed over just this. We are the quantizers, as are the emitters, yah?

It seems the wavefunction is always localized radially, the more significant quantization / localization would collapse from the transverse direction. Here's an experimental report that we can check to see if it is consistent with my simple description of a photon / wave function...

http://www.nature.com/nature/journal/v474/...re10120_F1.html

This Wikipedia entry gives me some pause though about the simple interpretation:

http://en.wikipedia.org/wiki/Photon_dynami...n_of_the_photon

Is the vector potential a better description of the wave function partly because it is always positive, unlike the E&M fields?

This reference from the Wikipedia article provides more grist for the mill:

http://arxiv.org/abs/quant-ph/0508202

This post has been edited by scalbers on Jul 7 2012, 03:41 PM

Photons cannot be sharply localized?

http://pra.aps.org/abstract/PRA/v79/i3/e032112