Do Photons Have Mass and Momentum?

[sophiecentaur]

What worries me about this sort of discussion is the fact that people tend to ignore the caveats. There is 'implied mass', 'effective mass' and other descriptions, which apply in certain 'bound states' of photons. The innocent (young) reader will ignore the qualifying words and run away shouting "Photons have mass - yah boo" and suchlike, thinking their teachers (and all other elderly geezers) are totally wrong when they tell them that photons are massless.

The articles that deal with occasions where photons display the quality of having mass are all describing situations in which photons (even if they still can be called photons at the time) when interacting with massive entities are seen to produce mass-like effects. This must be taken into consideration and the whole thing put in proportion. Whether or not they 'really' have mass is quite irrelevant to whether photons 'really' have mass when they are buzzing around under normal conditions.

Fools rush in where angels fear to tread.
(I'm not, of course, referring to anyone who could possibly be reading this. haha)

 

[ZealScience]

I also have the same question. People rarely use this photon mass, instead they use photon momentum. But I think that it has effect of mass like gravitation.

And since energy in other particles give mass, then photons should also have similar effect to my perspective. I am not sure either.

 

[sophiecentaur]

I also have the same question. People rarely use this photon mass, instead they use photon momentum. But I think that it has effect of mass like gravitation.

And since energy in other particles give mass, then photons should also have similar effect to my perspective. I am not sure either.

They don't use it because it's not there. Momentum doesn't imply mass. You're doing exactly what I was warning about. Momentum is only mv where there is mass involved. For photons its h/λ.'

 

[ZealScience]

They don't use it because it's not there. Momentum doesn't imply mass. You're doing exactly what I was warning about. Momentum is only mv where there is mass involved. For photons its h/λ.'

I didn't say that momentum implies mass. I know that E/c=p where E=pc could be derived for massless particles.

But what is the exact definition of mass? Photons have gravitation. You can say that photon momentum is involved in the gravitation according to Einstein's equations. But isn't energy contributing to gravitation? And what is the difference between this energy and energy of a particle? It is a fact that energy in a fermion contributes to its mass (relativistic), right?

 

[DrDu]

I didn't say that momentum implies mass. I know that E/c=p where E=pc could be derived for massless particles.

But what is the exact definition of mass? Photons have gravitation. You can say that photon momentum is involved in the gravitation according to Einstein's equations. But isn't energy contributing to gravitation? And what is the difference between this energy and energy of a particle? It is a fact that energy in a fermion contributes to its mass (relativistic), right?

To describe correctly the coupling of particles which move at relativistic speeds (like free photons) to gravitational fields you have to use the equations of general relativity. There, it is the energy-momentum tensor which couples to the field, and no longer mass.

 

[ZealScience]

To describe correctly the coupling of particles which move at relativistic speeds (like free photons) to gravitational fields you have to use the equations of general relativity. There, it is the energy-momentum tensor which couples to the field, and no longer mass.

Yes, you need to apply Einstein's Equations. I think excluding gravitational waves, all other particles are included. Thus, fermions should be the same as bosons. Fermions have increase in mass when having more energy, so I think that bosons should also have more mass when having more energy.

 

[romsofia]

Even using the photon concept, photons are sometimes called "wave packets" that carry momentum.

What?? As far as I'm concerned, photons have a definite energy and wave packets are a range of possible energies and frequencies.

If I'm incorrect, please correct me!

 

[sophiecentaur]

Definite energy?
If you could tell me the exact frequency of one photon then I could tell you the energy, definitely.

"Wave packet" is just a rather tawdry attempt to give a photon some sort of a size.

 

[theman2000]

When is a photon not moving?

in its own rest frame...

 

[sophiecentaur]

is it valid to talk of a 'rest frame' for something traveling at c?

 

[Phrak]

Nevertheless below the article there is some interesting discussion as to whether these photons are to be considered free photons or merely some polariton. The authors seem to be of the first opinion. So it depends somehow on your definition of a photon. This definition does not necessarily coincide in high energy physics and in condensed matter physics.

A wave guide can be modeled as the interference generated by an array of stub antennae between parallel conducting plates or a defraction gratting at optical frequencies. In any case, we can say that all that is required to produce 'massive' photons is a little interference. This is the situation common in nature, outside the laboratory, where incoherent states are the rule. They're not really massive.

Mass can be inferred from the increased wave velocity (or reduced group velocity). But these velocities are directionally depended. The inferred mass, greater than 0, is a function of a direction vector.

There is nothing really interesting going on with this. Simply take the energy momentum 4-vector of a photon with a norm of zero--meaning zero mass--and project it onto any preferred spacetime ray--like down the axis of a wave guide. The projection will, in general, be nonzero.

 

[Phrak]

There have been some misconceptions floating about, on this thread. Energy and mass should not be confused unless talking about the center of mass frame, where we can use E=mc^2.

In other cases, the nontensoral equation for regions of spacetime applies (for a sufficiently flat spacetime), m^2 = E^2/c^4 - p^2/c^2.

m is the mass within a region, E is the energy, and p is the momentum. For some region containing a particle, the equation applies to the particle.

E = h \nu and p = k \lambda, where \nu is the angular frequency, and k is the reduced wave number. To find the mass in terms of \nu and k, (or lambda and omega) do the substitutions.

 

[dchris]

But previously you said that photons don't have mass, just energy that contributes to gravity. So how can something that has no mass add mass to a system?

Hey, will some of you smart guys answer my question?

 

[jaketodd]

When is a photon not moving?

In the reference frame of another photon traveling along side it. However, I've heard that photons are considered not to have a reference frame, so ya.

Oh, and how about at the center of a black hole?

 

[sophiecentaur]

Hey, will some of you smart guys answer my question?

I don't think there is an answer which involves just familiar concepts. It's outside the set of things that we are used to.

 

[dchris]

I don't think there is an answer which involves just familiar concepts. It's outside the set of things that we are used to.

Does my previous question outgrow you all? Cause its funny how you go on talking about stuff and can't answer such an easy question.

 

[Phrak]

Does my previous question outgrow you all? Cause its funny how you go on talking about stuff and can't answer such an easy question.

Which question?

 

[sophiecentaur]

Does my previous question outgrow you all? Cause its funny how you go on talking about stuff and can't answer such an easy question.

If it's easy as you say then why can't you answer it yourself? Or perhaps you don't understand all its ramifications.
Does a mocking tone give a post gravitas?

 

[dchris]

If it's easy as you say then why can't you answer it yourself? Or perhaps you don't understand all its ramifications.
Does a mocking tone give a post gravitas?

My opinion is that photons don't add mass to any system, as they don't posses any mass. They just add their energy to the system, which increases the systems gravity (curvature of spacetime).

Drakkith wrote:

"No, the energy does add to mass. And as such the photon does add to the mass of the system by adding that energy.
I think a key here is that when you talk about a system of particles you can talk about mass increasing. A single particle cannot have energy or mass added without being in a larger system."

and then i replied:
"But previously you said that photons don't have mass, just energy that contributes to gravity. So how can something that has no mass add mass to a system?"

 

[dchris]

So we are emerging with two different opinions, Drakkith's and mine, and I am simply asking which is correct.

 

[Astronuc]

See - https://www.physicsforums.com/showthread.php?t=511175

 

[phinds]

So we are emerging with two different opinions, Drakkith's and mine, and I am simply asking which is correct.

Yours is based on a personal theory and is incorrect, his is based on currently accepted theory and is correct.

 

[sophiecentaur]

"Correct" doesn't necessarily come into it. Why do you expect it to be all cut and dried, like the Victorians did?

There are many ideas about the interaction between em radiation and matter. Sometimes it is convenient to think in terms of particle-like photons, sometimes it is convenient to think in terms of waves. I think there is a lot to be said for totally ignoring the photon particle model and just think of the photon as a quantum of energy which only exists during the interaction process. I think it has to be true that the photon completely loses its identity once the energy has been absorbed into an Aton / charge system.
So what happens inside some resonating / waveguide system when a photon 'enters it' could be looked at in the same terms as what happens in other absorption situations. I.e. the photon no longer has an identity and its energy is just part of the system. So you don't need a frame of reference for a photon, ever, because it is onle in existence at the point and time of energy transfer into or out of a system.

 

[dchris]

Yours is based on a personal theory and is incorrect, his is based on currently accepted theory and is correct.

care to explain why?

 

[dchris]

His opinion based on current theories says that photons add mass to a system (thats what he wrote) but in his previous posts he wrote that photons don't have mass. So how can something without mass add mass to any system?

 

[Redbelly98]

Anything with energy, like a photon, can add mass to a system, in accordance with Einstein's equation E=mc².

When people say that a photon itself has no mass, they mean it has zero rest mass, as discussed earlier in this thread.

 

[DrGreg]

His opinion based on current theories says that photons add mass to a system (thats what he wrote) but in his previous posts he wrote that photons don't have mass. So how can something without mass add mass to any system?

Because mass isn't conserved in relativity, only energy and momentum.

In relativity, the mass of an object is given by<br /> m = \sqrt{\frac{E^2}{c^4} - \frac{|\textbf{p}|^2}{c^2}}<br />where E is the total energy of the object and p is the total momentum of the object.

Now consider a photon with momentum p, and therefore energy |p|c (and zero mass), that is absorbed by an object with momentum 0 and energy m₁c². By conservation of momentum the object's momentum becomes p and by conservation of energy its energy becomes |p|c + m₁c². Then its mass becomes<br /> m_2 = \sqrt{\frac{\left(|\textbf{p}|c + m_1c^2 \right)^2}{c^4} - \frac{|\textbf{p}|^2}{c^2}} = \sqrt{\frac{2|\textbf{p}|m_1}{c} + m_1^2} &gt; m_1<br />

 

[phinds]

I think there is a lot to be said for totally ignoring the photon particle model and just think of the photon as a quantum of energy which only exists during the interaction process.

I don't know enough about this stuff to have a "professional" opinion on your point of view but I sure do like it and I think it would avoid lots of apparently pointless discussions (it's the "pointless" that I'm not 100% sure about)

 

[Astronuc]

His opinion based on current theories says that photons add mass to a system (thats what he wrote) but in his previous posts he wrote that photons don't have mass. So how can something without mass add mass to any system?

It's more than a theory. A deuteron can absorb a photon of a certain minimum energy and dissociate into its constitutive nucleons, a proton and a neutron.

The sum of the proton mass and neutron mass exceeds that of the deuteron.

m(p) = 1.007276466812(90) u ref: http://physics.nist.gov/cgi-bin/cuu/Value?mpu
m(n) = 1.00866491600(43) u ref: http://physics.nist.gov/cgi-bin/cuu/Value?mnu

m(d) = 2.013553212724(78) u ref: http://physics.nist.gov/cgi-bin/cuu/Value?mdu

The difference in energy is the equal to the energy given off as a gamma ray when a neutron combines with a proton to form a deuteron, or the minimum energy of a gamma ray required to dissociate a deuteron into the two nucleons. Actually the energy is slightly higher than the mass difference because one must account for the kinetic energies of the masses involved.

 

[sophiecentaur]

This is a typical example of people trying to 'explain' or come to terms with an advanced concept whilst still holding on to 'Victorian' ideas of Science. Why do you think the changes in the last hundred years or so have been call 'Earth Shattering'? It's because you just can't make do with the old notions if you want to get nearer to Scientific 'truth'.
To get a grasp of modern Science one has, constantly, to hurt the brain considerably and go with the flow. There's one paradox after another to deal with on this road.
If you think you've understood it then you haven't.