How Big is a Photon and What are its Dimensions?

[peter0302]

The "amplitudes" of the EM field of a photon at any given point refer not to physical space in which there is a field, but rather to the strength of the EM field at that point, right?

 

[Mentz114]

agkyriak :

Nice post. It's always good to see one's prejudices (only mass can be localised) supported by good maths. I've long argued against ascribing 'particle' properties to photons.

 

[Alfi]

You do know that this thread is more than a year old, don't you?lol - It's brand new to me. I've only just read just now.
In a year from my 'now' this thread will be a year old.
time must be some kind of illusion. The 'now' is very much like a point particle.

but interesting none the less :)

 

[lightarrow]

The 'now' is very much like a point particle.

...or as a non-localized field.
Yes, it's a completely different point of view...

 

[heterotic_]

I'm bringing this conversation back from the dead, because I feel like it has some sort of meaning.

If we talk about size in a non-conventional sense of the word, but as the average minimum amount of space it occupies, doesn't everything have to have a size? If two variables "try to" occupy the same amount of space, at points defined by x, y, z, in our universe, don't they interact (either by repulsion [in which they do not occupy the same space], attraction [bind to form something "new"] or annihilation), instead of occupying the same point in space?

 

[Eynstone]

I wonder what we'd get if we tried to calculate the 'radius' of photon on the lines of the classical radius of electron.
On the other hand, don't we treat particles as points right at the outset of quantum mechanics?

 

[heterotic_]

The actual radius of an electron is calculable, it is just dependent on its environment, so shouldn't this be also true of a photon?

Everything has a center, even if it is so small, its entire volume is its center. It is how far it is spread out that is changeable (or rather, how concentrated the energy is in relation to its center, or a "point") I believe that point must be the minimum space it can occupy, and therefore must have a size, even if we have no way to fathom it.

 

[PhilDSP]

The wave function of photon is here introduced as follows. The vectors of the EM field \vec {{\rm E}} and \vec {{\rm H}}, as the solutions of the wave equation of the second order, which follow from the Maxwell equations, are considered as the classical wave functions \vec {\varepsilon }\left({\vec {r},t} \right) and \vec {H}\left( {\vec {r},t} \right).

That's a wonderful analysis! But isn't it dependent on the simple assumption that the photon IS the wave rather than the alternative that the photon is the structure that generates the wave?

P.S. Sorry, I didn't realize at first that that post was several years old.

 

[Jivesh]

I was also perplexed as to what possible answer could be there when we talk about the size of the photon. Even the concept of measuring the radius of the electron is not justified in quantum theory, because if we consider the electron to be a sphere with the charge smeared on its surface or throughout the volume of the sphere, then we will have to explain the spin of the electron in terms of the an actual spin, as we do for the case of, say, the Earth's spin. But that is completely wrong description of the electron, as it results in absurd values of speed of the surface of the electron.
I think that quantum mechanics starts with the assumption that the elementary particles are point like objects, which do not have any dimensions. Furthermore, if we talk about quantum mechanics, then visualising the electrons, photons, etc. as classical particles would be a mistake. The only thing that we have to guide us in QM is the wavefunction of these supposed "particles", and their evolution according to the Schrodinger's equation.

 

[heterotic_]

That's a wonderful analysis! But isn't it dependent on the simple assumption that the photon IS the wave rather than the alternative that the photon is the structure that generates the wave?

P.S. Sorry, I didn't realize at first that that post was several years old.

Redundancy would be silly, so I brought it back.

I would say that we cannot calculate the "classical" radius of a photon like an electron, because a photon always moves at the speed of light.

But what about calculating the mass of a photon depending on its energy? I know people say the photon has no mass, based on the definition of mass, but it does. It doesn't have a "rest mass", because it is never at rest, but the best definition of mass is that it is "the measure of inertia". Inertia is "the amount of resistance to change in velocity". Change in velocity is "an increase or decrease in Kinetic Energy (and Potential Energy, since Total energy is conserved)".

So, inertia is also "resistance to change in Energy"; the inertia of a body is dependent on its energy content. So mass then, is a measure of resistance to change in energy.

So instead of asking, can we calculate the mass of a photon, we are asking, can we calculate the measure of a photon's resistance to change in energy? YES.

Next, if we talk about the fact that a photon exhibits the characteristics of both a particle and wave, we cannot assume that a photon is only a wave. How it behaves is what it is. So how does it behave?

Now, bear with me, I'm only an undergrad, so if I'm way off base, feel free to correct me. If the photon has a center, and a wavelength, if we look at an electron, which has similar properties of a photon in that its radius is variable and it exhibits the wave-particle duality, we can get an idea of what a photon probably is. In a Hydrogen atom, the wavelength of its electron is equal to the Bohr circumference of the electron's orbit. Can we use this information to describe the duality of a photon?

We know that a photon is polarized and has a spin. So what if, it is a sphere (energy concentrated at a center, and pointing outward in all directions to create a force field) that swells and contracts? Wouldn't that explain the particle-wave duality? If this were true, the radius of a photon would then be variable, but at its swell would be its greatest radius, which we could calculate using its wavelength.

My thoughts:

r(photon-E) \leq \frac{ch}{2E}

and

m(photon-E) = \frac{h^2}{Ec}

I was also perplexed as to what possible answer could be there when we talk about the size of the photon. Even the concept of measuring the radius of the electron is not justified in quantum theory, because if we consider the electron to be a sphere with the charge smeared on its surface or throughout the volume of the sphere, then we will have to explain the spin of the electron in terms of the an actual spin, as we do for the case of, say, the Earth's spin. But that is completely wrong description of the electron, as it results in absurd values of speed of the surface of the electron.
I think that quantum mechanics starts with the assumption that the elementary particles are point like objects, which do not have any dimensions. Furthermore, if we talk about quantum mechanics, then visualising the electrons, photons, etc. as classical particles would be a mistake. The only thing that we have to guide us in QM is the wavefunction of these supposed "particles", and their evolution according to the Schrodinger's equation.

My problem with QM and GR for that matter, is that they both work, but not really together. That just is not a good answer for me.

We talk about the spin of an electron and a photon, but what if it's not a spin, like you said in terms of the Earth? What if it is more like... the electricity shifting in a uniform motion throughout its volume in one direction or the other, as a result of the polarization?

 

[Salman2]

From this book: Lecture Notes in Physics: The Nature of the Elementary Particle, 1978. Malcolm H. MacGregor, it is stated (in quotes):

"The particle properties of the photon emerge most clearly from Compton scattering, in which a high-energy photon makes a billiard-ball type of collision with an electron, and hence delivers its energy and momentum into a volume that has dimensions on the order of 10^-11 cm (the Compton wavelength of the electron)."

The "particle" aspect of the photon from Compton scattering experiments represents a "MeV photon" compared to an "optical-frequency wave photon" that is about 10^6 times larger in dimension.

"In dealing with the photon, we must take into account both its wave aspects and its particle aspects. But since these two aspects differ dimensionally by many orders of magnitude, they are in a practical sense separate entities, so that we can discuss the wave properties of the photon without having to consider its particle properties, and vice versa."

 

[heterotic_]

From this book: Lecture Notes in Physics: The Nature of the Elementary Particle, 1978. Malcolm H. MacGregor, it is stated (in quotes):

"The particle properties of the photon emerge most clearly from Compton scattering, in which a high-energy photon makes a billiard-ball type of collision with an electron, and hence delivers its energy and momentum into a volume that has dimensions on the order of 10^-11 cm (the Compton wavelength of the electron)."

The "particle" aspect of the photon from Compton scattering experiments represents a "MeV photon" compared to an "optical-frequency wave photon" that is about 10^6 times larger in dimension.

"In dealing with the photon, we must take into account both its wave aspects and its particle aspects. But since these two aspects differ dimensionally by many orders of magnitude, they are in a practical sense separate entities, so that we can discuss the wave properties of the photon without having to consider its particle properties, and vice versa."

Realistically, that is only a good conclusion when you discuss the outcomes the two produce independent of each other.

However, we are discussing bridging the gap, so saying not to, is just illogical. If we think about what a wave really is, we envision that oscillating line we produce with technology, or a ripple from a linear perspective so we see the amplitude of a physical wave. The problem with that is that it is a perspective, it isn't the entirety of the actual wave itself. It is only a description from a reference point, and requires ignoring the rest of the "picture", as it were. It is also representing a point · moving across an oscillating line from the "side", but a straight line from the "top". You really cannot picture the wave that way, it is merely a representation to represent wavelength and amplitude, based on what we observe as physical waves from a sideview, like the water oscillating in a ripple.I think a "real view" of the wave would produce a different picture more like:

O>·<O>·<O>·<O>·<O>·<O

With that (granted, think of the swells with more of the same wave length of a particular frequency), we could even find the radii of all the "sizes" that the photon takes on spherical calculations of each moment of the swell.

 

[Pinewater]

Just an fyi, in scientific literature the 'size' of a photon generally refers to its coherence length. (which is definitely NOT to say that this is the only correct way to think about a photon's size!)

You can look up coherence length on the web, but I understand it best in the context of an interferometer. In this setting, the coherence length is the maximum distance that a single photon can travel and still interfere with itself (which means that it produces an interference pattern at the screen).

 

[Pianoasis]

I think the two-slit expirements should be done in a vacuum. The molecules in the air obviously cause some interference with the photons natural behavior. In fact it could be the only reason the photons act like a particle. The photon waves hit the molecules in the air, send them along the photon wave's path, and hit the screen in the appearance of a particle. If these tests were to be done in a vacuum we would have much clearer results.

 

[Vanadium 50]

This thread is two years old.