How Big is a Photon and What are its Dimensions?

[billy_boy_999]

how big is a photon? what are its dimensions?

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

 

[Imparcticle]

http://ethel.as.arizona.edu/~collins/astro/subjects/electromag6.html

I'm not sure if this is any good...http://www.fervor.demon.co.uk/photons.htm

 

[slyboy]

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.

 

[billy_boy_999]

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?

 

[Silverlancer]

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".

 

[billy_boy_999]

The characteristic size of a photon is inversely related to its frequency: (c/f)

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?

 

[arivero]

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.

 

[tavi_boada]

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

 

[arivero]

Tavi, he means, for sure, its wavelength.

 

[russ_watters]

that is what the people at arizona.edu have to say...is this not a statement consistent with QM?

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.

 

[techwonder]

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

 

[billy_boy_999]

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...

 

[reilly]

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

 

[jtolliver]

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)

 

[Redfox]

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.

 

[Revelationz]

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?

 

[Redfox]

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 realize 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.

 

[CJames]

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.

 

[Redfox]

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.

 

[jnorman]

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.

 

[Redfox]

No-one said it's a point particle. It cannot be a point particle because inbetween its emission and absorption points it is in motion at C. It can have a location, because any ray of light can be located. Ultimate conclusions that something "cannot be done" are limits of the accepted theory. Humans have been only progressing for about 300 years as a culture.

 

[danitaber]

I don't know if Billy boy 999 is even reading this anymore, but just in case, I will say that what Billy Boy 999 just witnessed is an example of a physicist's aversion to the phrase "I don't know" (ha!)

Personally, I think it's a meaningless question for the following reason:
We can't measure its "size". No one knows what "size" even is in regard to fundamental or quasi-fundamental particles. We can talk about wavelength, we can talk about points, we can talk about distributions and wave-packets, we can talk about strings, and we can talk about the loudness of a "blip" on a computer. We can't talk about "size". We have a hard time talking about "reality", even! (see many worlds thread, LOL)

We are less concerned with what it is than what it does. I'm sure I'm going to get jumped on (I'm slowly getting used to it) but there it is. OK, who's first for the danitaber-kabob? Refutations are welcome.

 

[ZapperZ]

I don't know if Billy boy 999 is even reading this anymore, but just in case, I will say that what Billy Boy 999 just witnessed is an example of a physicist's aversion to the phrase "I don't know" (ha!)

Personally, I think it's a meaningless question for the following reason:
We can't measure its "size". No one knows what "size" even is in regard to fundamental or quasi-fundamental particles. We can talk about wavelength, we can talk about points, we can talk about distributions and wave-packets, we can talk about strings, and we can talk about the loudness of a "blip" on a computer. We can't talk about "size". We have a hard time talking about "reality", even! (see many worlds thread, LOL)

We are less concerned with what it is than what it does. I'm sure I'm going to get jumped on (I'm slowly getting used to it) but there it is. OK, who's first for the danitaber-kabob? Refutations are welcome.

Danitaber kabob... YUM! I'll have seconds if it comes with couscous.

Actually, I don't find anything wrong with what you just said (or maybe I'm not in such a bad mood this morning). There is one thing I will pick on, though. "What it is" is DEFINED by "what it does". If you consider carefully, everything that we define is based on its physical properties. An electron has a set of properties that defines what it is. The same with a photon. You recognize a friend or family member based on physical characteristics that you observe, the sound of his/her voice, his/her behavior, etc. These are all physical properties and characteristics that DEFINE what it is.

A photon was NEVER defined as a "particle" with definite spatial boundaries, like a ping-pong ball. It was defined as a particle in the sense that each one of them carry a definite quantity of energy. The implication from this is very subtle but important, especially when we delve into things like "single-photon" sources.

Zz.

 

[jcsd]

kebab! kebab! kebab! or kebap to absoultely correct, or even kabab, but never kabob!

And while I'm on the subject how can you have vegtable kebabs? kebap literally means 'broiled meat'.

 

[danitaber]

There is one thing I will pick on, though. "What it is" is DEFINED by "what it does". If you consider carefully, everything that we define is based on its physical properties. An electron has a set of properties that defines what it is. The same with a photon. You recognize a friend or family member based on physical characteristics that you observe, the sound of his/her voice, his/her behavior, etc. These are all physical properties and characteristics that DEFINE what it is.

Thanks for not slaughtering and broiling me, Zapper.
OK, I'll agree with you there on a technical level; I'll stand by my original phrasing for metaphysical purposes only. (i.e., I am not the sum of red hair and stunning good looks , etc. I am essentially something that I don't believe can be quantized.)

And I'll amend "kabob" to "kebab", but that's all the ground I'm giving.

 

[Tau_Muon_PlanetEater]

Reillly said'

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.


- Question, How can the probability of finding the photon be zero at the extremes of the distribution? Wouldn't they approach zero but never hit it?

 

[Fredrik]

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...

A photon has a perfectly well-defined wavelength only when it's in a momentum eigenstate, i.e. when it has a perfectly well-defined momentum (and energy). This never happens. A photon is always in a superposition of momentum eigenstates:

$$\lvert\alpha\rangle=\int{d^3p}\ f(\mathbf{p}) \lvert \mathbf{p}\rangle$$

The only quantity that we might want to call the "size" of the photon is the width of the Fourier transform of the momentum-space wave function, f, i.e. the uncertainty in the photon's position. This uncertainty could be anything between zero and infinity. (I'm ignoring Planck-scale effects here). Since it can be arbitrarily close to zero, it makes sense to call the photon a "point particle".

However, if we assume that the uncertainty in momentum is proportional to the magnitude of the momentum (which is the only thing we can assume if we know nothing about the state), the uncertainty in position is proportional to Planck's constant divided by p (the magnitude of the momentum). Since p is inversely proportional to the wavelength, the uncertainty in position is proportional to the wavelength.

So it makes sense to think of the wavelength as the "size" of the photon (or at least as something proportional to it). This may seem strange, but it is at least consistent with e.g. the fact that microwaves (with wavelengths of order 1 cm) won't go through a metal net with millimeter-sized holes (like the net that covers the window of your microwave oven), but they will go through a net with much larger holes.

You might wonder what the effect of the Planck scale is, since I said I ignored it. A Planck-energy photon will have a wavelength (i.e. "size") near the Planck length. This implies that the energy density of the region where the photon is located will be just large enough to form a black hole.

This is a pretty good reason to expect QED (and all other quantum field theories) to break down at the Planck scale.

 

[Fredrik]

Reillly said'

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.

- Question, How can the probability of finding the photon be zero at the extremes of the distribution? Wouldn't they approach zero but never hit it

You are absolutely right, but the numbers we are talking about are so ridicilously small that for all practical purposes they are zero. One meter away from the expected position, the probability is less than

$$\frac{1}{10^{10^{30}}}$$

 

[teal4two]

I have been wondering about the size of photons as well, or even what sense that question makes.

1) My microwave oven has metal mesh in the door. Can visible light photons from a bulb in the interior pass through the holes in the mesh? Can microwave photons pass through the holes in the mesh?

2) X-rays are scattered by crystals, but they are only scattered by part of the crystal. How big is this part?

 

[inha]

2) X-rays are scattered by crystals, but they are only scattered by part of the crystal. How big is this part?

uh.. where'd you get this idea from?