Photon dimensions

[anantchowdhary]

Main Question or Discussion Point

Does a photon have spatial dimensions.Also do two photons having different energy occupy the same volume?

Thnx

 

[country boy]

The motion (and location) of a photon is governed by its wavefunction, which contains all of the information about its "dimensions." To find the size of the region occupied by the photon, the uncertainty principle requires that you give up knowledge about its wavelength.

Two photons with different energies will have different wavefunctions, and these can certainly overlap.

 

[anantchowdhary]

umm
i didn quite understand.Wats the anser about the volume question:yes or No?

 

[country boy]

Yes, they can overlap in the same volume, but the fields add and you get just one combined wavefunction.

 

[anantchowdhary]

so photons hav different energy density?(yes i suppose)er..

 

[country boy]

You can't really define the energy density of a single photon. But you can calculate the number of photons per unit volume from the radiation intensity and the photon energy (E=h*freq).

 

[jtbell]

The "size of a photon" has been discussed regularly here. You might like to review some previous threads:

http://www.google.com/search?q=photon+size+site:physicsforums.com

Tip: concentrate on links that do not contain "archive" in the URL. The archive versions of threads don't have information about quoting, which can make reading them confusing, because it's hard to figure out who wrote what.

 

[country boy]

Very helpful. Thanks.

 

[anantchowdhary]

@jtbell

I went thru a few of the topics u had suggested,but i fail to understand how a photon is a point particle.This is as energy occupies space and time so shudnt a photon:zzz:

Also, duz a photon travel in a wave?

 

[Hootenanny]

I went thru a few of the topics u had suggested,but i fail to understand how a photon is a point particle.This is as energy occupies space and time so shudnt a photon:zzz:

I quote marlon from one of the threads in the Quantum Forums as he explains the crux of the matter so eloquently;

Just to add. Keep in mind that a photon is defined as a quantum of energy. So it is designated by a number in an energy-coordinate space, NOT in a spatial coordinate space. So a photon is not defined as an entity with fixed spatial boundaries, designated by numbers in a spatial coordinate base.

Just as an addendum

marlon

 

[coalquay404]

The "size of a photon" has been discussed regularly here. You might like to review some previous threads:

http://www.google.com/search?q=photon+size+site:physicsforums.com

Tip: concentrate on links that do not contain "archive" in the URL. The archive versions of threads don't have information about quoting, which can make reading them confusing, because it's hard to figure out who wrote what.

If you click on the "archive" links and then on the "View full version" link on the page that pops up, the threads are formatted properly.

 

[anantchowdhary]

But still there must be some way of comparing the energy density of a photon wudnt there be eh...?

 

[country boy]

You could measure the EM energy density before and after a photon is absorbed, and call the difference an equivalent energy density for the photon. But if the volume is different, you get a different answer. A photon really has no defined volume, so you are confusing classical and quantum concepts.

 

[MeJennifer]

Anantchowdhary, I think we should think how the properties of the macro world can be explained in terms of the sub-atomic world not how the sub-atomic world can be explained in terms of the macro world.

 

[anantchowdhary]

But still..Energy occupies space isnt it.I am sorry and thank all of you for being patient.But does energy occupy space or not(I think so as it also curves space-time)

 

[ZapperZ]

But still..Energy occupies space isnt it.I am sorry and thank all of you for being patient.But does energy occupy space or not(I think so as it also curves space-time)

Does "pain" occupy space? What about the color indigo or the saltiness flavor? Do they occupy space?

I'm not trying to be funny here, but it appears that this may be the only way to demonstrate to you why your assertion simply has no answer, unless you want to make one up.

I also don't understand why, after a link to a previous thread on the identical question, that we're repeating everything all over again.

Zz.

 

[anantchowdhary]

Please be more polite .I am being straight-forward .Please be the same with me and please explain to me why a photon cannot have volume even though it occupies spacetime

 

[ZapperZ]

Please be more polite .I am being straight-forward .Please be the same with me and please explain to me why a photon cannot have volume even though it occupies spacetime

Can you show me evidence that allows you to claim that "it occupies spacetime"? It appears that you used this several times, but you made no such support to how you would know such a thing.

Zz.

 

[anantchowdhary]

Sure.What i understand is tht if something curves spacetime then it occupies spacetime.Now please tell me whether that is correct or not

 

[ZapperZ]

Sure.What i understand is tht if something curves spacetime then it occupies spacetime.Now please tell me whether that is correct or not

Er.. there is a problem with logical consistency here. You simply cannot make a connection like this in physics and expect to get away with it. Just because A does something to B doesn't mean that A=B. A photon can also cause the emission of an electron. Does that mean it is also an electron? Or "occupies the space of an electron"?

Zz.

 

[anantchowdhary]

Might be.And im not saying if a photon curves spacetime it is spacetime(A=B)

.So please explain that y cannot it be that a photon occupies space-time

 

[anantchowdhary]

and what is meant by e=mc^2.Just for my knowledge,does it signify that masss and energy are related and mass is a form of energy.I am confused....

 

[ZapperZ]

Might be.And im not saying if a photon curves spacetime it is spacetime(A=B)

.So please explain that y cannot it be that a photon occupies space-time

But why should it?

We are trying to tackle the SOURCE of your misunderstanding here, so don't change the subject. You are using a wrong "connection" between "A" causes spacetime curvature" to mean "A" must occupy spacetime also. As I've pointed out that if I apply your logic, it can lead to absurd conclusion. Do you STILL stand by such claim? Because if you do, then I want the exact physics that allows you to stand by that claim. If you don't, then this point should be declared DEAD and you cannot use it again in trying to insist that a photon must have a dimension.

Zz.

 

[Chris Hillman]

Point particles, null geodesics, and plane waves in classical physics

Hi, anant, you asked:

Does a photon have spatial dimensions.Also do two photons having different energy occupy the same volume?

i fail to understand how a photon is a point particle.This is as energy occupies space and time so shudnt a photon?

Also, duz a photon travel in a wave?

What i understand is tht if something curves spacetime then it occupies spacetime.Now please tell me whether that is correct or not

This might be a good time/place to point out that mention of "volume", "dimension", and "curved spacetime", plus the fact that you posted on the "special and general relativity" board at PF, could be taken to suggest that you are asking about the relativistic but classical (as in "nonquantum") viewpoint taken in special and general relativity.

If so, strictly speaking the notion of a photon belongs to the domain of quantum field theory, so if you want to discuss the classical physics perspective it is probably better to refer to a "laser pulse" or "radar pip" rather than a "photon".

Essential caveat: as you probably know, quantum theory has proven hard to reconcile with the curved spacetime models used in general relativity, so it is currently difficult to say very much with certainty about how photons and other particles might be treated in the yet unknown quantum theory of gravity. This is one reason why I suggest that you restrict yourself, at least initially, to trying to understand the classical perspective.

In classical (as in "nonquantum") physics, the notion of a point particle arises as an approximation to an object whose mass or other "charge" (say, electrical charge) is so small that it does not appreciately disturb the ambient field (say, an electromagnetic field) under study. Assuming also that this object is very small in comparison to the region of interest, under these conditions, it can usually be treated as a point particle.

The notion of a "point particle" in classical physics is not free of objections, but it has proven so useful that physicists can hardly reject simply because it ultimately leads to self-contradictions involving issues like "self-forces" or "back-reaction", particularly in non-linear classical field theories like gtr.

Also, in classical physics, waves (as in electromagnetic waves or gravitational waves) are often first encountered as idealizations called "plane waves" (an additional twist: in most gtr textbooks, these are treated using a "linear approximation", although this is not neccessary). However, with more effort, one can try to model classical "wave packets" which can be used to idealize a laser pulse or radar pip as a wave rather than as a particle, which is probably more realistic from the classical perspective. However, point particles whose world lines are null geodesics (often called "photons" for short in the literature, but this is potentially misleading as I just explained) are much easier to work with. One thing they are useful for is thinking about upper bounds on how quickly "information" can be transmitted, or about how changes in initial conditions can influence particular events in a particular spacetime model.

Might be.And im not saying if a photon curves spacetime it is spacetime(A=B)

So please explain that y cannot it be that a photon occupies space-time.

Well, if you follow my advice and rephrase this as the assertion that according to gtr, the electromagnetic field energy in a "light beam" (think of a projector in a movie theater) should gravitate, and therefore should curve spacetime, you are correct, but to verify that you need to postulate a "legal" model according to gtr. One such model is the Bonnor beam, an exact "null dust" solution in gtr. See http://en.wikipedia.org/w/index.php?title=Bonnor_beam&oldid=47130395

what is meant by e=mc^2.Just for my knowledge,does it signify that masss and energy are related and mass is a form of energy.

Yes. In another and very recent thread, I mentioned how Einstein arrived at this conclusion from expanding the expression for the relativistic kinetic energy of a particle, namely
$$E = m \, \cosh \arctanh v = m + m \, v^2/2 + O(v^4)$$
as a power series in $v$ (pedantic quibble: actually, Minkowski's geometric interpretation wasn't adopted by Einstein until a bit later, so he didn't use this geometric language in his original paper, but this is the best way to understand where E = m, in the rest frame where v=0, comes from).

I hope this helps to clear the air.

 

[anantchowdhary]

Thanks Chris.I now can understand a bit of what ur saying.Ill surely do some research on the topic.

Also,I didnt understand the last part about explaining e=mc^2.

is mass a form of energy or not?
Thanx and also you were the first person to actaully recognize my name Anant

Cheers!