Do photons, phonons and electrons have mass?

[SSG-E]

TL;DR Summary

Do photons, phonons and electrons have mass?

These articles have energy but do they have mass?

 

[berkeman]

Summary:: Do photons, phonons and electrons have mass?

These articles have energy but do they have mass?

What does $m_e$ usually refer to?

 

[SSG-E]

What does $m_e$ usually refer to?

?

 

[berkeman]

?

Copy/paste your thread title into a Google search. Then do some reading in the search results. That should answer the $m_e$ part. You will likely still have a few questions about the mass of a photon and phonon, so reply with links to your reading and ask *specific* questions about that reading so that we can help you. Thank you.

 

[berkeman]

{crickets}...

 

[chemisttree]

Phonons? You mean lattice vibrations?

 

[SSG-E]

{crickets}...

It means electron rest mass. But an electron can't be at rest so how it has rest mass?

 

[berkeman]

It means electron rest mass. But an electron can't be at rest so how it has rest mass?

You are confusing electrons and photons. Can you post links to the reading you've been doing? That will help us to help you.

 

[SSG-E]

You are confusing electrons and photons. Can you post links to the reading you've been doing? That will help us to help you.

I know the differences between electrons and photons. I just replied about the symbol 'me'. So the correction is: A photon can't be at rest so how it has rest mass? Is 0 considered mass?

 

[berkeman]

A photon can't be at rest so how it has rest mass?

Where are you reading that?

 

[SSG-E]

Where are you reading that?

I have yet read at many websites and watched you tube videos.

 

[berkeman]

I have yet read at many websites and watched you tube videos.

Please post a link to such a source and highlight where they say a photon has rest mass...

 

[berkeman]

Maybe something like this?

https://www.physlink.com/education/askexperts/ae180.cfm

 

[SSG-E]

Maybe something like this?

https://www.physlink.com/education/askexperts/ae180.cfm

Electrons don't come at rest. Electron rest mass is the mass of an electron as measured when its speed is zero relative to an observer. A photon never comes at rest thus its rest mass is 0. But why can't be the rest mass of a photon be measured when its speed is zero relative to the observer?

 

[berkeman]

But why can't be the rest mass of a photon be measured when its speed is zero relative to the observer?

Because photons move at velocity c (the speed of light) relative to all observers. They never appear at rest to any observers. There are probably some Insights articles that we can point you to. Back in a minute...

 

[berkeman]

There are probably some Insights articles that we can point you to. Back in a minute...

https://www.physicsforums.com/insights/do-photons-have-mass/

https://www.physicsforums.com/insights/speed-light-frames-reference/

 

[binis]

Because photons move at velocity c (the speed of light) relative to all observers.

Not always i.e. inside glass they are moving slower,and inside heavy water of a nuclear reactor very slow.

 

[Borek]

and inside heavy water of a nuclear reactor very slow.

You've made that up, probably mixing and confusing several unrelated concepts. For most practical purposes the difference between the speed of light in water and heavy water is negligible, close to 0.75c.

 

[binis]

For most practical purposes the difference between the speed of light in water and heavy water is negligible, close to 0.75c.

You have right,might be inside the core,not in heavy water.

 

[weirdoguy]

Not always

Electromagnetic waves move slower in medium, but that says nothing about photons which are particular states of quantum electromagnetic field. Viewing EM waves as stream of bullet-like particles that slow down in medium is not a good idea.

 

[Borek]

You have right,might be inside the core,not in heavy water.

You are still making that up.

 

[binis]

Electromagnetic waves move slower in medium, but that says nothing about photons which are particular states of quantum electromagnetic field. Viewing EM waves as stream of bullet-like particles that slow down in medium is not a good idea.

A static EM field is practicaly not extending to infinite (math function has a definition field);how can EM waves do it,except viewing them as traveling particles? Moreover,you know that pair production is of one gamma-photon,not of a quantum EM field.

 

[weirdoguy]

A static EM field is practicaly not extending to infinite (math function has a definition field);how can EM waves do it,except viewing them as traveling particles?

How EM waves can do what? In your first sentence you wrote about static EM fields, EM waves are not static. Anyways, there are plenty of threads on PF about what photons are, and what they are not. For example this one: https://www.physicsforums.com/threads/what-is-a-photon.879128/ Take your time because it's not an easy topic and a lot of misconceptions circle around.

Moreover,you know that pair production is of one gamma-photon,not of a quantum EM field.

I'm sorry but I don't understand this sentence. Besides, pair production involves at least two photons.

 

[mfb]

A static EM field is practicaly not extending to infinite (math function has a definition field);how can EM waves do it,except viewing them as traveling particles? Moreover,you know that pair production is of one gamma-photon,not of a quantum EM field.

I'm not sure what you mean here, but I'm sure it won't help OP understand the concept of mass, so I don't think it's a good idea to continue that.

 

[PeterDonis]

Moderator's note: Thread moved to quantum physics forum.

 

[PeterDonis]

These articles have energy but do they have mass?

If by "mass" you mean rest mass, then electrons do have it, photons do not, and for phonons, AFAIK, the question isn't well-defined since phonons are not fundamental particles.

 

[PeterDonis]

why can't be the rest mass of a photon be measured when its speed is zero relative to the observer?

Because a photon can never be at rest relative to any observer. It moves at $c$ relative to all observers.

 

[AndreasC]

A static EM field is practicaly not extending to infinite (math function has a definition field);how can EM waves do it,except viewing them as traveling particles? Moreover,you know that pair production is of one gamma-photon,not of a quantum EM field.

You can think of light moving through matter as photons slamming into atoms, being absorbed and then being emitted again, many times over. This process has the effect of slowing down the propagation of the wave as a whole but each individual photon still moves at c while being scattered from atom to atom, it's just that this whole process makes it take longer to get to the other side of the material.

 

[binis]

being absorbed and then being emitted again

Isn't this the fluorescent effect?

 

[PeterDonis]

Isn't this the fluorescent effect?

No, that's not what @AndreasC was describing. He was simply describing one heuristic way of explaining why light appears to slow down when passing through a material medium as compared to vacuum.

 

[AndreasC]

Isn't this the fluorescent effect?

I guess it is a kind of fluorescence. I am not sure though if the term is 100% accurate when applied here. Basically what happens when light is scattered is that it is absorbed by electrons, launching them temporarily to an excited state. But that state is unstable, so soon enough the electrons fall back to their stable state by emitting photons. Usually that changes the energy of the photons to something lower than the original. The fluorescence you probably have in mind is when that frequency is much lower. Then materials re-emit UV light as regular light that we can see. There is also phosphorescence where the electrons are stable enough in their excited states to emit the photons much later.

 

[AndreasC]

No, that's not what @AndreasC was describing. He was simply describing one heuristic way of explaining why light appears to slow down when passing through a material medium as compared to vacuum.

I might have a gap in my understanding, is it not strictly true that photons are absorbed and emitted again while passing through a material or is it just a heuristic that is onyl partially true?

 

[binis]

this whole process makes it take longer to get to the other side of the material.

Divide mentally material in layers. So there is a slight delay from layer to layer.Therefore this delay must appears as a slowing of the speed of light INSIDE the medium: But,light speed remains constant inside a medium.

 

[PeterDonis]

I am not sure though if the term is 100% accurate when applied here.

It isn't.

is it just a heuristic that is onyl partially true?

Yes.

Divide mentally material in layers. So there is a slight delay from layer to layer.

As noted, this is a heuristic picture. It's not 100% accurate.

light speed remains constant inside a medium.

In the sense that photons still have zero rest mass inside a medium, to the extent the "photon" concept makes sense inside a medium, yes.

 

[binis]

But that state is unstable, so soon enough the electrons fall back to their stable state by emitting photons. Usually that changes the energy of the photons to something lower than the original.

Has the emitted photon a longer wavelength? There must be a little loss of energy because the ray is totally absorbed by the material at last if width is quite long (Lambert's law).

 

[PeterDonis]

Has the emitted photon a longer wavelength?

You are pushing the heuristic picture to the point where it breaks down.

 

[AndreasC]

It isn't.
Yes.
As noted, this is a heuristic picture. It's not 100% accurate.
In the sense that photons still have zero rest mass inside a medium, to the extent the "photon" concept makes sense inside a medium, yes.

Alright, but what is actually happening? It is a question I've had, it's just that every class I've had so far has left me with that impression. Is it beyond the scope of this thread?

 

[PeterDonis]

what is actually happening?

The only ultimate answer to any question like that is, we don't know. We can't directly observe the micro-level details of light propagation in a medium. The best we can do is construct models and compare their predictions with observed data.

The most fundamental model we currently have would be quantum field theory; the QFT answer to your question is that the quantum EM field is interacting with the quantum electron field in each atom in a way that makes the expected "speed" of propagation from a source on one side of the medium to a detector on the other side (as shown by the probability amplitude for detection as a function of the time lapse from source to detection) slower than the speed of light in vacuum. The "photons getting absorbed and re-emitted" heuristic is more or less based on perturbative QFT, but all of the caveats about virtual particles (about which we have several good Insights articles) apply to that heuristic.

Is it beyond the scope of this thread?

Going into more detail about how QFT models light propagation in a medium would be best done in a new thread, yes.

 

[AndreasC]

The "photons getting absorbed and re-emitted" heuristic is more or less based on perturbative QFT, but all of the caveats about virtual particles (about which we have several good Insights articles) apply to that heuristic.

Would you mind linking me to some of said articles?

 

[PeterDonis]

Would you mind linking me to some of said articles?

https://www.physicsforums.com/insights/?s=virtual+particles

 

[Vanadium 50]

You can think of light moving through matter as photons slamming into atoms, being absorbed and then being emitted again, many times over.

Sure, you can think of it, but you'd be wrong.

If this were true, the refractive indices in gasses would depend on pressure, but not temperature. They depend on both. If this were true, the refractive index of graphite would be much lower than diamond (because of densities). In fact, it's higher.

I note that we have drifted quite some way from the original topic, perhaps because multiple people have attempted to answer the original question but have instead injected their own misunderstandings into the thread.

 

[AndreasC]

Sure, you can think of it, but you'd be wrong.

If this were true, the refractive indices in gasses would depend on pressure, but not temperature. They depend on both. If this were true, the refractive index of graphite would be much lower than diamond (because of densities). In fact, it's higher.

I note that we have drifted quite some way from the original topic, perhaps because multiple people have attempted to answer the original question but have instead injected their own misunderstandings into the thread.

We drifted away from the original topic because the op asked how light appears to slow down when it passes through matter even though its speed is supposed to be always the same. I tried to answer to the best of my knowledge but someone explained that it is not entirely correct. Now after that I started looking around a bit and I found out that there is actually a section of the FAQ of this website which discusses this and explains why my version is incorrect. Imo the FAQ is a little bit harder to find than it should for a FAQ but beyond that, OP can look there!

 

[berkeman]

I found out that there is actually a section of the FAQ of this website which discusses this and explains why my version is incorrect. Imo the FAQ is a little bit harder to find than it should for a FAQ but beyond that, OP can look there!

Link?

 

[Lord Jestocost]

You can think of light moving through matter as photons slamming into atoms, being absorbed and then being emitted again, many times over. This process has the effect of slowing down the propagation of the wave as a whole but each individual photon still moves at c while being scattered from atom to atom, it's just that this whole process makes it take longer to get to the other side of the material.

When light travels through a medium like a glass plate, it appears to slow down. The apparent "slower speed" is the result of the superposition of two radiative electric fields:
The incoming light, traveling at speed c, and the light re-radiated by the atoms in the medium (oscillating charges driven by the incoming light) in the forward direction, traveling at speed c, too.

The superposition shifts the phase of the radiation in the air downstream of the glass plate in the same way that would occur if the light were to go slower than c in the glass plate. In case one wants to understand the essential aspects of the phenomena, I recommend to read chapter 31 “The Origin of the Refractive Index” in “The Feynman Lectures on Physics, Volume I". (http://www.feynmanlectures.caltech.edu/I_31.html). On Bruce Sherwood’s homepage (https://brucesherwood.net/) you find an article “Refraction and the speed of light” dealing with this question, too.

 

[AndreasC]

Link?

Sure! https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

 

[vanhees71]

From a QFT point of view an in-medium photon is a photon-like quasiparticle which is described by the in-medium photon Green's function. If you want to visualize a photon (no matter whether in vacuum or in medium) it's rather better to think in terms of waves, and the wave properties are described by the corresponding propagator. If the propagator is sufficiently peaked in energy-momentum space you have a "quasiparticle" which can be described by a dispersion relation $E=E(\vec{p})$, which you can find by looking for poles of the Green's function. Whether or not the photon has an effective mass is decided by this dispersion relation.

In a homogeneous medium you usually have two such "plasmon modes", a longitudinal and a transverse one with different dispersion relations.

Interestingly in a superconductor the photons really get a mass due to the Higgs mechanism, caused by the formation of a charged condensate of Cooper pairs. This mass is responsible for the Meissner effect and defines the penetration length of the London theory (which is a possible effective electrodynamics in superconductors). That's why the Higgs mechanism has been first formulated by Andersen in thinking about spontaneous symmetry breaking in condensed-matter physics.

 

[Vanadium 50]

We drifted away from the original topic because the op asked how light appears to slow down when it passes through matter even though its speed is supposed to be always the same.

Which message?

 

[AndreasC]

Which message?

#17 and #22.

 

[mfb]

binis didn't start the thread, and I already asked to stay on topic as response to that.

 

[AndreasC]

binis didn't start the thread, and I already asked to stay on topic as response to that.

Oh sorry then, I got confused.