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).