
#19
Feb2013, 07:38 AM

P: 5,634

From the OP:
Here are a few from others: Carlo Rovelli: Marcus quoting a prior post: Marcus : 



#20
Feb2013, 11:04 AM

P: 1

My understanding is that when we make a measurement we are really poking at the wavefunction, which holds all the measurable information about a particle within it. When the measurement is made it causes any probabilities to collapse and take on a definite value.




#21
Feb2013, 10:35 PM

P: 2

Let us assume that a photon propagate in z direction.
For the z direction size, we can make it as small as possible (at least theoretically) by superpositioning a various wavelength photon states, which becomes a delta function in position space while it's just a plane wave in momentum space. Moreover, we can also make it small in x and y direction by superpositioning standing waves, thereby make it small. If you express a photon as a wave packet, we can see that the wavefunction does not spread as time goes on.(for a mass zero particle while for electrons which as a finite mass it spreads out) I think, if the principle of superposition in quantum mechanics is valid in all circumstances, we can make it localized in position space. 



#22
Feb2013, 11:22 PM

Sci Advisor
P: 1,563

For details, read the famous quantum optics bible "optical coherence and quantum optics" by Mandel and Wolf. In my edition chapter 12.11 discusses the problems of a meaningful localization of photons. 



#23
Feb2113, 08:23 AM

P: 5,634

So while we are at it describing the 'size of a fundamental particle' and seeing there is no 'real' answer, at least no simple one, here is perhaps the craziest explanation of all. From Leonard Susskind whose work in black hole complementarity has won him widespread recognition, from THE BLACK HOLE WAR, Chapter 20:
[Susskind is relating here views of quantum field theory and string theory and while he uses 'atom' in the following description, he is could just as well have used 'particle' or 'photon' 



#24
Feb2113, 10:21 AM

P: 833

Size isn't something with a precise physical meaning in this context. The size of a photon depends on how you measure it.




#25
Feb2113, 05:51 PM

P: 824

There are basically two (compatible) definitions of a photon. The usual one is that it is an eigenmode of the electromagnetic spectrum. In the strict sense an eigenmode does not change in time. Therefore in a cavity the eigenmode fills the whole cavity, and in free space a photon is an infinite plain wave (with no amplitude... but well...) Inside these eigenmodes energy is stored, and that is the real idea of a photon. The intensity of the eigenmode drops by a quantized amount a multiple of [tex]\hbar \omega[/tex] when the light field interacts with something else.
When particle physics are discussed they are usually discussed in Fourier space. One infinite plain wave of say protons interacts with another infinite plain wave of protons and they exchange an infinite plain wave of photons or other stuff. The reason why one sees the particle traces in collider experiments is that protons in colliders are a short bit of such a plain wave: a wave packet. But the main physics is captured by the plain wave description. The interaction of a photon with the other elementary particles in the beams is point like, because its interaction does not depend on the momentum of the particles that it interacts width, leading to flat line in Fourier space and thus a delta peak in real space, for particles like neutrons which have an extend the interaction changes with the momentum of the interacting partners. So in a way photons are point like (in their interaction) but in another way they can be really large, in a mathematical description as large as the universe. Sorry if this reply is very technical, but we have gone a long way since the corpuscle theory of Newton, and this is all very much wave particle duality stuff. 



#26
Feb2213, 01:45 PM

Sci Advisor
PF Gold
P: 11,396

I think it's worth pointing out that 'the photon' cannot be pictured as some sort of 'burst of oscillations' passing through the aether or as a little bullet. These seem to be the most popular visualisations.
Old habits die hard and, before finally biting the bullet and realising that it's much harder than that, people tend to hang on to the idea that QM is, in fact, just like the old mechanical system but with a few inconsequential tweaks. No. It's 180 degrees different and you just have to get over it. "Physical Interpretation"??? Not possible. 



#27
Feb2313, 01:37 AM

P: 31

Size defined as the apparture of your measuring device? Being build of the same stuff your detector is made off, it becomes complex to measure size of your own building block. It then is going to depend on how well (a part of) the wave will interact with your detector, transfering just enough energy to make a difference.




#28
Mar413, 05:18 PM

Sci Advisor
PF Gold
P: 11,396

Sounds ever so much like a diffraction argument is creeping in, in disguise.




#29
Nov2213, 04:56 AM

P: 2

Perhaps what he was getting at is that when detected, the photon has size that interacts with the detector with pointlike properties, but while propagating before detection it has size that is described with wavelike properties distributed over the meter distance. IOW, I suppose he was trying to emphasize wave particle duality and that both are legitimate descriptions of the real world, with an equal claim to the concept of "size."
I know I tend to think of the particle as the "real" thing and the wave function as describing a probability for where that real thing is "really" located, but I know that's sort of a nonQM bias I have from growing up in a macro sized world. 



#30
Nov2213, 05:17 AM

Sci Advisor
PF Gold
P: 11,396

. . . . . Which leads to the conclusion that 'size' is just not a relevant property for a photon. That is, if it can be regarded as both a point and of infinite extent  haha.
The general picture that people seem to carry in their heads (and is how it's often drawn) is a a squiggle or short burst of oscillations, a few wavelengths long. I guess that fails in most respects. 



#31
Nov2213, 07:37 AM

Mentor
P: 16,485

What is the size operator? If there is no such operator then the question is poorly defined. If there is such an operator then it is just a matter of plugging it in to a singlephoton Fock state to see what the result is.




#33
Nov2213, 05:30 PM

P: 381

It has been shown that a photon can be arbitrarily "localized" in spacetime, i.e. there is not fundamental limit from the theory that forbids the complete localization of a onephoton state.
http://prl.aps.org/abstract/PRL/v79/i9/p1585_1 Edit: Localization is meant in the sense of arbitrarily small extent of its mode function as it travels with 'c', not that you localize it in a position eigenstate. 



#34
Nov2313, 07:55 AM

P: 5,634

Some comments on comments:
yes: /////////////////////// Other descriptions: Albert Messiah, Quantum Mechanics, pg 66: The wave packet... from Vanhees: From Wikipedia: http://en.wikipedia.org/wiki/Quantum..._and_particles Also, recall that VIRTUAL photons, complex numbered particles rather than the usual real numbers, are responsible for all electromagnetic interactions as described using quantum field theory. It's QFT that is used in the Standard Model of particle physics. So not only are photon characteristics 'fuzzy' in classical terms, so too they seem to be in mathematical terms. 



#35
Nov2313, 08:02 AM

P: 5,634

Anybody have any comments on the paper JK423 posted.....The description seems to conflict with what has been discussed in these forums...and it is not all that new.....1997....




#36
Nov2313, 08:19 AM

Mentor
P: 28,840

I always find it interesting that this question on the "size" of a photon (or the size of anything, really) has come up repeatedly. So let's try to narrow down what we DO know from straightforward quantum mechanics and see if people do agree on those:
1. There is a position operator in QM, and that gives you the value of the position of the particle. 2. The spread in position of the particle is not the same as the size of the particle. 3. There is no "size operator" yet welldefined in QM. Do we all not agree on those? Zz. 


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