# Size of photon particle

1. ### billy_boy_999

131
how big is a photon? what are its dimensions?

if this is a meaningless question, please tell me why...

566
3. ### slyboy

206
Interestingly, I was asked this exact same question in my defense of my undergrad experimental project. At the time, I said it was a point particle.

It actually depends a bit on what you mean by "how big". An excitation of the electromagnetic field can extend arbitrarily far across space. However, we only ever detect them at points.

4. ### billy_boy_999

131
hmm...(thanks guys)...the article says the size of a photon is inversely proportional to it's wavelength...or is it a point?

is there any way that we can say all photons - no matter their wavelength - have the same dimensions? of course, if we take them as points we can...though is a point a dimension, per se?

i'm having trouble digesting the idea that the size of a photon depends on its wavelength...if photons really are the quanta, the bare constituency of light, then surely they should have a fixed size...isn't part of the wave-particle duality the idea that light always arrives in lumps - as feynman says - and that they always have the same amount of energy? since they are always traveling at the same speed and are massless, doesn't all their energy come from their speed/momentum/velocity/whatever? so shouldn't their size also be a constant?

5. ### Silverlancer

3
According to Quantum Mechanics, a photon, like any other point particle, has no size at all. It is a point, if treated as a particle. If it is treated as a wave, it has wavelength and amplitude instead, amplitude being the "size".

6. ### billy_boy_999

131
that is what the people at arizona.edu have to say...is this not a statement consistent with QM? is the point treatment reckoned to be a description of a physical particle? hardly, how can a point be a physical anything? is quantum mechanics the right theory to address the question of a photon's size?

7. ### arivero

3,026
Now, what about a massive boson? It has a momentum p, thus a characteristic length h/p, but it also has an energy $$E=\sqrt{m^2 c^4 + p^2 c^2}$$, thus a characteristic frequency E/h, thus another length hc/E. For the photon, m=0 and both sizes coincide.

71
Aren't monochromatic waves infinite? I dont understand the syntagm "characteristic size of a photon". Can you explain??

9. ### arivero

3,026
Tavi, he means, for sure, its wavelength.

### Staff: Mentor

It seems sloppily worded. Do you have a link so I can see the context?

It should be clear enough though, that since a photon can be a wave or particle depending on how you observe it, "how big is a photon?" is, if not meaningless, an incomplete question.

But, when viewing it strictly as a particle, it would have to be a point particle as stated.

11. ### techwonder

51
Point particles have a nasty habit of having infinite energy densities , but it is true that QM assumes particles to be point like. Super String Theory assumes all particles to be small strings with a length of just about the planck length and no width, i.e. one dimensional . In that case a photon is 10e-44 meters

12. ### billy_boy_999

131
and i was waiting for someone to mention planck length somewhere...the boson length is interesting, i'm going to have to read up on that...the sloppily worded quote comes from the first link that Imparcticle had posted...

to me it feels like there's something missing from QM's point particle treatment, it feels a deferment to the abstract...maybe that's just my bias...

13. ### reilly

1,079
Turns out that issues of photon size are very difficult -- in no small measure because photons, unlike massive particles, do not have a position operator - this was proved by Newton and Wigner in 1949. Nonetheless, it is possible to develop a probability density that tells where a photon is within a volume that is big compared to the photon's wavelength -- that is the probability of the photon triggering a photoelectric detector. Among the peculiarities of photons and QM, is the fact that the distribution of the photon's energy is spread out to regions where the probability of finding the photon is zero.

The bible on photon physics is Optical Coherence and Quantum Optics by Mandel and Wolf. It discusses, in great detail what I mentioned above. It assumes a sophisticated grasp of QM and statistics -- but it starts from ground zero, and does the basics -- state vectors, coherent fields, correlations,....-- albeit quickly. it is a great book, and it is worth the fight to read it.

Regards,
Reilly Atkinson

14. ### jtolliver

47
If the concept of photon size is meaningful and the size is the same for all photons then it is zero or infinite because (in the absence of charged particles at least) the field equations for the electromagnetic field are scale invariant(this is a consequence of the masslessness of the photon)

15. ### Redfox

19
It is essentialy an empty space. Current understanding is that in any case it is a complex multidimentional combination of EM field of various intensity spread within an unprecisely defined space, a worm that has very "blurry" head and tail and consists of EM field variations..... Again orientation of this worm will depend on the relative movement...HUH.

16. ### Revelationz

7
Question rephrased

OK, the QM world is admittedly unintuitive to macro-particles like me So can I try a re-phrase of the question?

I think, but am not sure, this statement is true: The more you try and 'pin down' the location of a QM particle, the more it tends to 'spread out'. So as the resolving power of your locating instrument increases, you find that you can detect the particle less often within a given bounding box.

That means, I think, that as resolving power of the instrument increases, a point will be reached at which the particle is detected exactly 99.00% of the time within the limits of resolution of the instrument.

So if I am looking at photons streaming in from a 'perfectly' collimated laser beam source, and using the Palomar telescope to do so, I will observe the photons a very high percentage of the time. As I shrink the aperture of the telescope, at some point I will only observe the photons 99.0% of the time, not because the beam isn't perfect, but because Heisenberg says I can't know the position of the photons that accurately.

If the above is true, then the question about size becomes something like:

A) For light of a specific frequency in the range of, say, visible red, what is the 2-D size of the aperture (3-D bounding box?) in meters, that will enclose the position of the photons 99.00% of the time?

If this question makes any more sense than the original one, then:

B)Is this aperture (bounding box?) different in size than that of photons of a different frequency, say visible blue light?

And for a somewhat related question about quantities in photons:

A photon is a wave of EM energy, oscillating at a given frequency. How many oscillations does a photon consist of?

17. ### Redfox

19
I think you are very close to finding your own answer. Optical devices cannot be used for observation of photons because they use photons as a "bounce off" partice for imaging purposes. It we had means of control over a free moving EM frield interacting particle much smaller than any estimated size of photon then such observations would become possible. It is practically impossible to determine the length of a photon in meters, because you have to set the minimum EM field magnitude where your measurements will start and stop. Because it is believed that EM magnitude gradually increases from zero to the photon's energy range and then decreases back to zero over the length of the EM "snail" that represents photon, it is impossible to set its true length. All calculations will give only approximate results. It happens becase human brain uses different principles in processing of any information by setting definite borders to share one from another, it works in a big world like our every day world, but fails to work in the world of particles. We cannot truly realise that on a particle level we live in a world that is illusive because it is essentially nothing, i.e. something which singular example we would not normally see, but a mega multuple expample of which comrises all that we see and touch every day.

18. ### CJames

355
It all depends on how you think of a photon. In quantum mechanics a particle can't be thought of in the sense of a little piece of something. It's one unit of energy and other quantum values, but other than that it's more a wave. Upon measurement you could define its position within the wavelength of whatever you used to measure it. It will condense to that space when it interacts with that particle. But afterward it will again expand as a wave of possible quantum states. The same goes for whatever you used to measure the photon.

19. ### Redfox

19
True and not. As much as anything. We are based on the knowledge of approximate nature that is few decades old. It doesn't mean it is wrong - it is developing. We won't ever know if we will never know. As it was just said: "within" -- is all it is. More alternative approaches give us a better chance.

20. ### jnorman

308
NO NO NO - where did you guys get the idea that a photon is a point particle? photons are NOT particles in the sense that they have some specific size, location, or physical attributes. they do not have a size. the bottom line here is:
you cannot know ANYTHING about a photon between the time it is emitted and the time it is absorbed.

Last edited: Aug 14, 2004