# massless particles ?

## Recommended Posts

I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship..

because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass..

and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy..

##### Share on other sites

Important things are what is mass and what is photon. We could describe well photoelectronic effect without photon mass.

##### Share on other sites

I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship..

All physical particles must obey the following:

$E^{2}-p^{2}c^{2} = m^{2}c^{4}$,

where p is the 3-momentum and m the mass. So for a massless particle we have

$E^{2}-p^{2}c^{2} =0$.

No violation of relativity here. Note we do not have a photon rest frame so we cannot set p=0.

For massive particles we can chose the rest frame and the mass-shell condition becomes

$E = mc^{2}$,

in that frame.

##### Share on other sites

I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship..

because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass..

and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy..

Generally speaking, massless particle is DM(dark matter). And other concept is unparticle .

##### Share on other sites

Generally speaking, massless particle is DM(dark matter).

Dark matter must be massive otherwise it would not "clump together". It would not form halos if massless.

And other concept is unparticle.

Unparticles are a hypothetical so far hidden sector the the standard model that cannot be described in terms of particles. The idea is that "stuff" maybe scale invariant, but not massless. In standard QFT the particles must be massless to have this scale invariance. Loosing the notion of a particle can "cure" the insistence of zero mass.

##### Share on other sites

I was wondering how a particle could be massless, such as the photon. Because its energy isn't it and wouldn't that violate special relativity by means of the energy-mass relationship..

because since energy and mass are interconnected, doesn't that mean that any particle would have to have mass regardless wether or not its physically energy or mass..

and by the way how i a photons physical particle topology held together (if thats the right wording), since its only energy..

As far as I know, light only has mass in the sense that is has relative mass, i.e. at any moment, if it suddenly turned into matter, it would have a specific amount of mass it would turn into under the relationship E=mc^2

Edited by steevey

##### Share on other sites

I asked pretty much the same exact question about a week ago in this thread right here,

http://www.scienceforums.net/topic/54756-size-of-a-black-hole/

I personally feel light does have mass but its to small for us to really care most of the time.

##### Share on other sites

I personally feel light does have mass but its to small for us to really care most of the time.

For sure the best one can really do is put experimental limits on the mass of a free photon. Directly from the validity of Coulombs law the photon mass must be less than $10^{-14}$eV.

Theoretically the photon mass is exactly zero as required by the gauge symmetries of Maxwell's equations.

##### Share on other sites

If a photon has mass, you can slow it down or speed it up. This you can't do. So, the mass =0.

However, a photon has momentum: p = E/c.

A photon's spin =1. But it has only two orientations: +1, -1. If it were to have s=0, it would not be relativistically invariant.

A photon is its own antiphoton.

Pretty special boson! Isn't it?

I don't vision any other 'particle' like it. Do you?

By the way, neutrinos have mass.

##### Share on other sites

If a photon has mass, you can slow it down or speed it up. This you can't do. So, the mass =0.

However, a photon has momentum: p = E/c.

A photon's spin =1. But it has only two orientations: +1, -1. If it were to have s=0, it would not be relativistically invariant.

A photon is its own antiphoton.

Pretty special boson! Isn't it?

I don't vision any other 'particle' like it. Do you?

By the way, neutrinos have mass.

Spin 1 can have a zero angular momentum projection. You can have an electric dipole transition with [imath]\Delta J = 0 [/imath] as long as you don't have a 0—>0 transition.

##### Share on other sites

Of course, spin 1 could have an s=0 orientation. But I had a photon on my mind. Photon is a very special boson! It does not have s=0.

##### Share on other sites

i don't think that it is massless....

##### Share on other sites

i don't think that it is massless....

I refer you to post #8.

##### Share on other sites

All physical particles must obey the following:

$E^{2}-p^{2}c^{2} = m^{2}c^{4}$,

where p is the 3-momentum and m the mass. So for a massless particle we have

$E^{2}-p^{2}c^{2} =0$.

No violation of relativity here. Note we do not have a photon rest frame so we cannot set p=0.

For massive particles we can chose the rest frame and the mass-shell condition becomes

$E = mc^{2}$,

in that frame.

Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass?

##### Share on other sites

Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass?

Only by redefining what you mean by mass. Which renders all of the equations of relativity moot.

Edited by swansont
typo

##### Share on other sites

Can't energy also be a measurement of mass though? Or is it only ever referring to a single part of a particle such as the amount of matter OR the amount of energy and never both when describing mass?

If the particle is massive, the mass is equal to the energy in the rest frame of the particle. This is $E = mc^{2}$. So, in this sense energy is a measure of the mass. However, energy is not a relativistic invariant, it depends on the frame employed. So, you cannot equate mass with energy absolutely,

##### Share on other sites

A photon has no (gravitational) mass. That's why no force can act on it and (de)accelerate it. BUT, ... there is usually a but, ...

A photon has inertial mass = (Planck constant * frequency)/(speed **2).

See my previous responses.

Is that okay?

##### Share on other sites

A photon has no (gravitational) mass. That's why no force can act on it and (de)accelerate it. BUT, ... there is usually a but, ...

Gravity accelerates photons. Null geodesics are not straight lines, gravity does influence the motion of light.

A photon has inertial mass = (Planck constant * frequency)/(speed **2).

This is really just the energy or momentum of the photon.

I am not sure that trying to interpret this as a mass is going to be very useful. Unless you have some idea in mind?

Is that okay?

Maybe you are looking for an interpretation that really is not going to be fruitful. The idea that photons have no rest mass should not unduly worry you.

Having the inertial and gravitational mass of a photon being different means that the equivalence principle is violated. Which maybe the truth of nature, or it more likely means you are using an nonstandard definition of mass.

Edited by ajb

##### Share on other sites
!

Moderator Note

The "reflections have no mass" tangent has been split off http://www.scienceforums.net/topic/55783-reflections-have-no-mass/

## Create an account

Register a new account