# Photon dimensions

anantchowdhary
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).

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

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.

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country boy

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?

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Staff Emeritus
Gold Member
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.

marlon

coalquay404
The "size of a photon" has been discussed regularly here. You might like to review some previous threads:

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.

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anantchowdhary
But still..Energy occupies space isn't 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)

Staff Emeritus
But still..Energy occupies space isn't 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

Staff Emeritus
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

Staff Emeritus
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 I am 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...

Staff Emeritus
Might be.And I am 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.

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

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 I am 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!

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Staff Emeritus
Gold Member
is mass a form of energy or not?

https://www.physicsforums.com/showpost.php?p=1203679&postcount=7"
https://www.physicsforums.com/showpost.php?p=1204278&postcount=36"
https://www.physicsforums.com/showpost.php?p=1204345&postcount=40"

All indecently by Zz. If there is anything that you don't understand, please feel free to ask specific questions. But just repeating or restating your original question, over and over, either in the same thread or different ones such as this isn't going to get you anywhere. You won't gain any understanding and the people replying to your threads are just going to become more frustrated. I urge you (as cristo did in the above thread) to read the thread carefully and pay particular attention to the posts I outlined above.
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.
The important point to note from Chris' excellent post above; is that you are attempting to reconcile two theories, QFT and GR, which as yet have been found to be incompatible. The photon has no place (as yet) in GR and curved space time.

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Staff Emeritus
All indecently by Zz.

All indecently by me? Hoot, how could you? I assure you that while I was typing that without wearing any pants, I was wearing a bright smile!

:rofl:

Zz.

anantchowdhary
I don't get how mass is Converted to energy!Thats it.And thanks for being patient

anantchowdhary
T
is mass a form of energy or not?

I just asked this again as i read this on page 941 in the physics text by Resnick,Halliday and Walker(Fundamentals of Physics:Extended 6th edition)

In the last paragraph it is clearly mentioned:
mass can be considered to be another form of energy

country boy
To add to Chris' very nice explanation, I might suggest that going back to the photoelectric effect could help in understanding the photon. This is the experiment that led to the photon concept. If the incident light has a frequency below the threshold for atiomic emission, then no amount of increased intensity will eject an electron. But when the frequency of the light is raised above the threshold, electrons flow. The electrons are absorbing the radiant energy in discrete amounts equal to the the threshold energy (plus kinetic energy). Each amount is equal to h*freq, leading to the idea that the light consists of a bunch of photons and each photon has this energy. The quantized nature of light comes from how it interacts with matter. There is no requirement that the photon in the radiant beam has a size or a location. The only location is where the absorption occurs.

Staff Emeritus
Gold Member
All indecently by me? Hoot, how could you? I assure you that while I was typing that without wearing any pants, I was wearing a bright smile!

:rofl:

Zz.
One of my more amusing typos! :rofl: Damn this Google toolbar

Staff Emeritus
Your question is still unclear. Imagine that you had an unlimited budget, a very large laboratory, and a trained staff. What sort of experiments would you do to determine whether or not a photon "occupied space"?

The question is so vague and general in its current form that it's not really clear what you are interested in. I wouldn't know what sort of experiments you'd perform, nor what answers would convince you that the answer was that the photon did occupy space, or what answers would convince you otherwise.

Without this understanding of how you would reduce the question to experiment, I can't answer the question. Any answer would be highly misleading, esp. if you are using it to attempt to derive other results.

Science is supposed to be about questions that are ultimately testable. If you can't come up with a scenario that explicitly answers your questions through experiment, you are not doing science, but philosophy.

I suspect very much that the questions you have may be more related to quantum mechanics than relativity. But it's possible (because of the vagueness of the question) that I'm wrong. So I'll try and outline the relevant aspects relating to GR

1) GR is a classical theory, while photons are a quantum concept. We can certainly talk about whether or not light interacts via gravity though. The answer is basically yes. Light has energy and momentum. The "stress-energy" tensor that is the source of gravity in General relativity arises from energy and momentum, and how it is transported. Thus light contributes to gravity via its contributions to the stress-energy tensor.

That is about as much of an answer as you are going to get out of GR.

For a taste of the quantum questions (which should go in another forum, either philosophy or quantum mechanics) - suppose we have a light source that shines through two slits. We can experimentally see that it generates an interference pattern. We say that the light is due to waves, which interfere with each other. Suppose we make the light beam so weak that only one photon is ever in the apparatus at any given time. Does the light beam still generate an interference pattern.

The answer to this question is yes, and one philosophical interpretation of this result is that photons, regarded as particles, take multiple paths, and interfere with themselves.

Some people don't like this philosophical interpretation, which is in general fine, there are many. This particular philosophical interpretation is due to Feynman. People with different philosophies of QM often have a hard time communicating with each other, though if they both know what they are doing, they can both come up with the same answers to a well-posed experimental problem.

anantchowdhary
I was just wondering...

Is it possible to a see a photon in an absolutely dark room with a very high speed camera?

Staff Emeritus