Jump to content

Do photons carry/contain more than just energy, momentum and angular momentum?


Duda Jarek
 Share

Recommended Posts

Noether theorem says that with symmetries come conservation laws and so because of time, translation and rotation symmetry, EM field itself guards energy, momentum and angular momentum conservation.

While atom deexcitation there clearly appears energy and (orbital) angular momentum difference, so there should be created EM field configuration carrying this difference.

Photon's angular momentum is usually imagined as something only "quantum-transcendental", but in fact it is very real angular momentum.

For example Richard Beth in 1936 has measured the tiny reaction torque due to the change in polarization of light on passage through a quartz wave plate: http://prola.aps.org/abstract/PR/v50/i2/p115_1

Here is nice video of rotating macroscopic object using circularly polarized light - at about 20 second the polarization was switched to opposite one:

 

Is optical photon something more than just EM wave carrying energy, momentum and angular momentum?

If not - what more? Other than EM interactions? Some electric/magnetic moments?

Is it just a "twist-like wave"? - like behind marine propeller, but this time in viscosity-free environment and so does not dissipate - can travel undeformed for years (is soliton) from a concrete single atom to anther one ...

One would say that because of spin conservation, it has also "spin 1" - e.g. due to electron changing spin from -1/2 to +1/2 ... but isn't it just 180deg rotation - twist again? Especially that in opposite to other particles with spin, photon doesn't have magnetic dipole moment...

Another question: why it has momentum? Is it that it was just required for this kind of waves or maybe there is some momentum change required for atom deexcitation itself?

Edited by Duda Jarek
Link to comment
Share on other sites

Photon's angular momentum is usually imagined as something only "quantum-transcendental", but in fact it is very real angular momentum.

 

Usually? Really? How do you know this? That's not how I learned it.

 

 

Another question: why it has momentum? Is it that it was just required for this kind of waves or maybe there is some momentum change required for atom deexcitation itself?

 

Basic relativity. E = pc for particles with zero rest mass.

Link to comment
Share on other sites

So what do you think optical photon is?

 

Let's start with simpler question: what does it mean that photon has "spin 1"?

For particles with mass spin denotes magnetic dipole moment and that the beam is split in Stern-Gerlach experiment ... photon doesn't have such spin.

Its spin corresponds to electron changing spin e.g. from -1/2 to +1/2 - correspond to 180deg rotation - angular momentum again, which can be directly used to rotate macroscopic objects.

From the other side we rather (?) cannot use e.g. electron's spin to rotate macroscopic objects by shooting with polarized electron beam ...

 

My point is that spin of electron and of photon is something very different - the first corresponds to magnetic dipole moment, the second to angular momentum.

 

About momentum it's a different question - it is just seem unintuitive that while atom deexcitation, there had to be change of momentum ...

But ok, even while rotating something symmetric in water, twist like waves/vortices would be produced in both directions - carrying also momentum.

Link to comment
Share on other sites

So what do you think optical photon is?

 

I think the photon is the quantum of electromagnetic radiation

 

Let's start with simpler question: what does it mean that photon has "spin 1"?

 

It means it has 1 unit (h-bar) of intrinsic angular momentum

 

For particles with mass spin denotes magnetic dipole moment and that the beam is split in Stern-Gerlach experiment ... photon doesn't have such spin.

 

No, it means the photon does't have such a magnetic moment. It most certainly has spin.

 

Its spin corresponds to electron changing spin e.g. from -1/2 to +1/2 - correspond to 180deg rotation - angular momentum again, which can be directly used to rotate macroscopic objects.

From the other side we rather (?) cannot use e.g. electron's spin to rotate macroscopic objects by shooting with polarized electron beam ...

 

My point is that spin of electron and of photon is something very different - the first corresponds to magnetic dipole moment, the second to angular momentum.

 

No, it means that having a charge and angular momentum means having a magnetic moment. The magnetic moment for the electron, proton and neutron are not the same; they have different gyromagnetic ratios and different masses, both of which affect the magnetic moment. The problem here is in your interpretation or in how it was explained to you, not in the physics. It's not a general problem.

 

One of the issues with causing rotation is that electrons are not Bosons, while photons are. Electron spin does't get transferred because spin is intrinsic angular momentum and electrons don't simply disappear, but the photons do, and something has to happen to their angular momentum.

 

 

About momentum it's a different question - it is just seem unintuitive that while atom deexcitation, there had to be change of momentum ...

But ok, even while rotating something symmetric in water, twist like waves/vortices would be produced in both directions - carrying also momentum.

 

Few people learning QM claim that QM is intuitive.

Link to comment
Share on other sites

I think the photon is the quantum of electromagnetic radiation

Quantum in the sense that optical photon is individable? - e.g. produced by concrete single atom and even much later absorbed by another one - undeformed configuration of the field (soliton) - ok, we agree on that ... but what exactly is this configuration????

It means it has 1 unit (h-bar) of intrinsic angular momentum

I also agree with that answer to 'what does it mean that photon has "spin 1" ' question - its spin doesn't denote having anything more than angular momentum - so basically photon is just a twist-like wave of EM field.

No, it means the photon does't have such a magnetic moment. It most certainly has spin.

Whow, once again we agree :)

The only thing what is misleading here is what does "spin" mean - it depends on the context - means something different for electron than for photon.

No, it means that having a charge and angular momentum means having a magnetic moment. The magnetic moment for the electron, proton and neutron are not the same; they have different gyromagnetic ratios and different masses, both of which affect the magnetic moment. The problem here is in your interpretation or in how it was explained to you, not in the physics. It's not a general problem.

 

One of the issues with causing rotation is that electrons are not Bosons, while photons are. Electron spin does't get transferred because spin is intrinsic angular momentum and electrons don't simply disappear, but the photons do, and something has to happen to their angular momentum.

I see you interpret electron's spin as literally spinning and so creating magnetic moment - here are some problems with such picture: http://www7b.biglobe.ne.jp/~kcy05t/spin.html

People see charge as something more fundamental than magnetic moment, while there are much lighter fermions with spin and so probably magnetic moment: neutrinos.

So maybe it's magnetic moment what is more fundamental ... ? Why charge?

Few people learning QM claim that QM is intuitive.

Ok, here is some quote from Feynman about interference: "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way and which is at the heart of quantum mechanics. In reality it contains the only mystery."

Surprise! In 2006 there was made classical analogue of objects having wave-particle duality and observed interference for them and later also tunneling and even orbit quantization: http://www.scienceforums.net/topic/65504-how-quantum-is-wave-particle-duality-of-couders-walking-droplets/

There are clear pictures, intuitions and I haven't seen any reasonable counter-arguments for such equivalent pilot-wave intuitive view ... but it seems there is some strange need for quantum transcendentalism in the society ... it can be intuitive if only you want it to.

Link to comment
Share on other sites

The only thing what is misleading here is what does "spin" mean - it depends on the context - means something different for electron than for photon.

 

No, it means angular momentum for both.

 

I see you interpret electron's spin as literally spinning and so creating magnetic moment

 

No, I never said anything about literally spinning.

Link to comment
Share on other sites

I'm a bit confused - so in the first statement here you say that electron has angular momentum: is spinning, while in the second that it is not? :)

It's a part of quantum transcendentalism? Like it is both wave and particle, but isn't? :)

Link to comment
Share on other sites

I'm a bit confused - so in the first statement here you say that electron has angular momentum: is spinning, while in the second that it is not? :)

It's a part of quantum transcendentalism? Like it is both wave and particle, but isn't? :)

 

I said it has angular momentum. I did not say it was physically spinning.

Link to comment
Share on other sites

Angular momentum conservation says that if we would take a lot of "tiny angular momentums", we should be able to rotate a macroscopic object - and it works for photons. So shouldn't it also work for electrons?

If electron's angular momentum doesn't correspond to some kind of rotational motion, so ... what does it even mean? It's not a real angular momentum, but only some kind of quantum-transcendental one? :)

 

Quantum rotation operator says precisely what does e.g. electron's spin means - that while rotating the system by alpha angle, quantum phase changes by spin*alpha - it is also definition of Conley topological charge.

Accordingly to it, situation near +1/2 (left) and -1/2 (right) spin looks something like here:

fqm8.jpg

This picture also solves another transcendental "quantum mystery" - that some objects rotated by 2pi becomes something different ... it can be repaired using some covering groups ... or just using field with some symmetry: saying that phase and minus phase is the same as on this picture.

And something very similar we have while explaining that magnetic flux going through superconducting ring is quantized: because the phase around has to make integer multiplicity of 2pi:

flux_quant_rs.jpg

... or understanding quants of magnetic flux in superconductor: fluxons/Abrikosov vortices - configurations stable because of topology - topological solitons.

Their magnetic field is because of topological singularity of quantum phase - like for electron.

Edited by Duda Jarek
Link to comment
Share on other sites

Angular momentum conservation says that if we would take a lot of "tiny angular momentums", we should be able to rotate a macroscopic object - and it works for photons. So shouldn't it also work for electrons?

 

You have to flip the spin and transfer that angular momentum. Photons can do that easily; in the experiment they passed through a wave plate. Doing the equivalent for an electron is more problematic.

 

If electron's angular momentum doesn't correspond to some kind of rotational motion, so ... what does it even mean? It's not a real angular momentum, but only some kind of quantum-transcendental one? :)

 

The problem is that putting a smiley on this just reinforces that it's a sarcastic question rather than a serious one. Spin is intrinsic angular momentum. There is a wealth of scientific evidence to support this. Mockery doesn't change that. What mockery does is make me much less inclined to do any legwork for you to go find it.

Link to comment
Share on other sites

You have to flip the spin and transfer that angular momentum. Photons can do that easily; in the experiment they passed through a wave plate. Doing the equivalent for an electron is more problematic.

You could also just absorb these photons/electrons with their angular momentum - I think it's doable experiment:

 

Shoot with polarized electrons at object floating on surface of conductive liquid - it would absorb polarized electrons, then release unpolarized ones - so the difference of average angular momentum should make it rotate.

 

I think you couldn't rotate it this way, but maybe I'm wrong ?

 

My sarcasm was only about "universal answer" when physicists don't understand something: "it's quantum.".

While quantum mechanism is kind of extension of classical one with the wave nature of particles and in first approximation (h=0) of QM you still get classical mechanics - quantum concepts are not unreachable for our minds, if only we stop basing on such imaginary limit of our understanding.

We shouldn't just "shut up and calculate", but still try to understand e.g. dynamics behind wavefunction collapse, field configurations behind particles ... and many other important fields ignored because of "it's quantum" universal answer.

Have a good weekend.

Link to comment
Share on other sites

  • 4 weeks later...

I think the photon is the quantum of electromagnetic radiation

I'm too lazy to do the work but I do recall that the stress-energy-momentum tensor for a EM wave traveling in one direction has a non-zero stress component. If my memory is correct then wouldn't that be the answer to Do photons carry/contain more than just energy, momentum and angular momentum? or would the stress component just be part of the angular momentum, if an EM wave has angular momentum? Let's consider a sinusoidal wave movingin the x-direction, i.e. E_z = E_0 * sin( x - wt) where E_0 = a number whose value is the magnitude of the maximum value of the electric field.

Link to comment
Share on other sites

I don't know how this breaks down for the stress-energy-momentum tensor. That's beyond my area of study.

 

You could also just absorb these photons/electrons with their angular momentum - I think it's doable experiment:

 

Shoot with polarized electrons at object floating on surface of conductive liquid - it would absorb polarized electrons, then release unpolarized ones - so the difference of average angular momentum should make it rotate.

 

I think you couldn't rotate it this way, but maybe I'm wrong ?

 

 

You would have to be sure that you weren't doing anything because of the electron motion once it's in the conductor. The photon experiment ensures that this is not an issue, since there photon merely passes through, or if its absorbed, it no longer exists.

Link to comment
Share on other sites

I don't know how this breaks down for the stress-energy-momentum tensor. That's beyond my area of study.

I guess I'll have to stop being lazy and do the work. :(

 

I think it'd be good to do the work here. It'd be a good way to refresh my memory of how to do Latex. That will come later this week (weekend) or next week ... or .. today. :unsure: Yikes!

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.