# Do Photons have Width?

Can a photon be said to have width? I believe their length can be determined in any given frame of reference.
Cheers, Keith

Photons have no physical dimensions. If you want to discuss “wavelength,” there are a lot of physicsits on this board.

I am really sorry. That was an unforgivably snarky post.

A “photon” is just a mathematical construction that we use to describe the interaction between light and objects, other light, etc. on a quantum level.

Or you can look at it like it’s a wave…

This is too complicated for a quick apology post.

So the photon has zero length as well. What appart from an electric (and corresponding magnetic) field does it have as attributes? I can think of frequency (or by calculation wavelength) and direction or position to within the accuracy bounds of Heisenbergs uncertainty principal.
So what gives it the macroscopic property of polarisation?
My appologies for bad spelling and bad physics (I’ve been computing to many years for either to be propperly up to scratch

Cheers, Keith

I have already stated that I am not a physicist (and mispelled it in the process). But I’ll take a crack at your question.

Your follow-up question doesn’t make a great deal of sense. I speak American English, and a smattering of German. If your native language is domething other

Are you screwing with me?

> "That was an unforgivably snarky post. "
Don’t even think about it, I didn’t take it that way.
I had enougth background to know what I was asking wasn’t simple, I guess I really hope for links to a site discussing this and similar features of photons or else directions to an old thread that covered this (my search for photon width brought up nothing of use).
I allways tended to think of the light wave as the mathematical construct of light on a macroscopic level Maybe I should have asked about light having width, but that would cause confusion with the idea of the width of a light beam. I asked when I realised that my view of light being made up of perpendicular electric and magnetic waves had lead me to thinking about light (and photons) having a width which I kind of doubt.
Can a photon be said to posses an electric and magnetic wave perpendicular to its direction of travel?
Here I am thinking of the photon as the smallest amount of light producable at a particular frequency by a particular source. Quantum Mechanics would render such an object ‘smeared out’ over all its possible paths making width a concept only if it is forced to colapse into a particular configuration by observation.
Help please, from anyone whom has kept up with their quantum physics.
Cheers, Keith

>Your follow-up question doesn’t make a great deal >of sense. I speak American English, and a >smattering of German. If your native language is >domething other

I would email this to you, but your account dissallows direct emails, so I’m putting it here on the board
:Moderator please turn this into an email to Exgineer if you feel that is appropriate.

I speak only English English. My second question is what property of a photon or a group of photons gives it polarization? Does it have something which is at the polarization angle, is the electric field at an angle compared to the direction the photon is travelling?

Imagining O being the point at which a photon emmerges from this screen can the electric field of the photon be thought of like …

| /
O O -O- O
| / \

0 deg 45 deg 90 deg 135 deg

having an angle?

Is this what light polarization is on the quantum scale?

> Are you screwing with me?
Certainly not, especially not in the English English sence, as I hardly know you, and this isn’t even like the first date

Cheers, Keith

dang, html ate my ascii art.

|
O
|

0 deg

0
/nbsp \

45 deg

-O-

90 deg

etc …

The electric and magnetic fields of light are always perpendicular to the direction of propagation (and to each other). Polarization tells you just what the direction of the fields is. Usually, you give the direction of propagation and the direction of the E field, but you could also use the B field instead.

Photons don’t have length or width, but in many cases where you would classically be interested in length or width, you can use the wavelength and get a good “feel” for the answer. For instance, if you have a slit in a screen that’s much wider than a wavelength, then light will pass straight through with little effect from the edges, but if you have a slit that’s about the size of the wavelength, then light will diffract going through the slit.

Photons actually have remarkably few properties. Well, they have properties, but most of them are zero. They have no charge, no mass, no lepton or baryon number, no shape or size. You can measure the polarization or frequency of light, but those depend on your frame of reference. About the only nonzero property I can think of for photons is that they have an intrinsic spin of 1.

And for the record, Exgineer, it’s spelled “Fizisist”.

Photons, like all fundamental particles, were once said to be
point-like constructs (zero dimensions). However, Grand Unification theories such as Supergravity/supersymmetry/superstring theories don’t work unless a particle is defined as a one dimensional construct… a tiny
line segment. This line segment could be straight, wiggly, or tied
end-to end, like a string (hence, the name). The way the string
is presented to the universe defines what kind of particle it is.

The size of this string? The Planck Length.
The width? Zero.

[nitpick]
String theories require one-dimensional objects instead of particles (by definition). SUSY and GUTs work just fine with point particles, thankyouverymuch.

Another way of looking at it:

A photon (or any particle) with a momentum defined with perfect precision can be characterized as a plane wave. Plane waves are infinite in extent in space-time (all dimensions). Of course, no photon has a perfectly precise momentum; the less precise, the smaller the region the plane wave characterization is good. For a real photon, one could express the extent of the plane wave region as a function of the photon’s frequency and bandwidth (uncertainty in frequency).

Well, there’s spatial coherence and temporal coherence of light, and these correspond to width and length. However, since individual photons get swallowed by individual charged particles within atoms, they must be fairly “dot-like.”

“Temporal coherence” is how long an EM wave train can go without having a jump in phase (or wiggling phase.) Laser light can act as if the photons are a few inches or feet long. If samples of a beam are taken at two different spots, if the spots are too far apart then the sampled light won’t make interference patterns if later combined. It’s as if the light came from different photons. The more monochromatic the light, the better (longer) the temporal coherence.

“Spatial coherence” is the width of the light wavefront that lacks jumps or wiggles in phase. When people say that laser light is “monochromatic and coherent”, what they really mean is that it has large temporal coherence and large spatial coherence. If the light source was a perfect point-source, then the spatial coherence of the light would be very large. But if the source was wider than a point, then the wavefront could contain many lobes of an interference pattern, and only within each lobe would there be spatial coherence.

With stellar interferometers sometimes people talk as if photons from stars were several feet wide. They’re talking about the spatial coherence of starlight. Even after travelling all the way from Betelguese(sp?), the width of the interference patterns is only a few meters. But that’s impressive considering that the light is so coherent, yet it’s not monochromatic.