Do Photons Have Mass? Exploring the Particle

[Low-Q]

so can we say light has mass?

No.
Say that you have a flashlight that radiates X Watt of light. Say you put frictionless wheels on the flashlight. The flashlight weights Y Newton. Now after a given time, the flashlight should now roll a given distance due to the energy from the light. Since the flashlight actually don't move at all, it cannot be mass in the light that push the flashlight away.

Vidar

 

[Cantab Morgan]

No.
Say that you have a flashlight that radiates X Watt of light. Say you put frictionless wheels on the flashlight. The flashlight weights Y Newton. Now after a given time, the flashlight should now roll a given distance due to the energy from the light. Since the flashlight actually don't move at all, it cannot be mass in the light that push the flashlight away.

Vidar

Why do you contend that the flashlight will not roll? Of course, a real flashlight will not, but in your thought experiment, the flashlight with frictionless wheels will enjoy an impulse from the light.

 

[ZapperZ]

You seem very certain about the speed limit, and i understand it. But what do you think about the discovery about those neutrinos that traveled 60 nanoseconds faster than light? Do you think its some kind of mistake or error? Or maybe a revolution?

There should be NO discussion on the OPERA results outside of the existing thread in the Relativity forum. Please confine the discussion only to on-topic subject.

Zz.

 

[phinds]

In superconductors, photons are massive.

Huh? Could you expand on that?

 

[jetwaterluffy]

No.
Say that you have a flashlight that radiates X Watt of light. Say you put frictionless wheels on the flashlight. The flashlight weights Y Newton. Now after a given time, the flashlight should now roll a given distance due to the energy from the light. Since the flashlight actually don't move at all, it cannot be mass in the light that push the flashlight away.

Vidar

It would move. That's how solar sails work. Light has momentum, but no mass.

 

[Hootenanny]

It's true, but I don't think it's helpful to think of it as necessary. Rest mass is the norm of the energy-momentum four vector. This extends the concept seamlessly to massless particles, because even if you can't boost to a frame where the particle is at rest, every four vector has a norm.

You are of course right. Defining the mass as the norm of the energy-momentum vector is more convenient. Perhaps necessary is was too stronger word, I was merely trying to expand on sophie's point for the OP's benefit, who I fairly sure would benefit more from a physical explanation, than a more formal one.

 

[Phrak]

Do photons have mass?-

On shell photon fields have mass. Off shell fields have nonzero mass.

 

[sophiecentaur]

On shell? What shell?

 

[dchris]

First, I would caution you to develop a habit of not being sloppy in your phrasing of any statement purported to discuss science. "60 nanoseconds faster than light" is a meaningless phrase. What was observed was that neutrinos SEEM to have traveled over a certain distance in an amount of time that was 60 nanoseconds less than what it would have been had they been traveling at the universal speed limit (which is a speed limit that light obeys).

There has not as yet been any discovery that neutrinos travel faster than light. What there HAS been is a large number of experimental trials that give a result that the people who made the observations seem pretty convinced is an observational error, but they have tried six ways from Sunday to figure out where the error is and have not been able to do so, so they published their results with the specific request that other physicists (that is other than the 160 of them that had participated in the trials) see if they can figure out where the error is.

The popular press immediately pounced on this with such inanities as showing a picture of Einstein upside down and claiming a revolution in physics.

It certainly remains an open question until either the error is found or it is concluded that there is no error, in which case, THEN you can talk about faster than light, but it seems likely that an error will be found.

Thanks for pointing out my mistake. But also keep in mind I am just 15 years old.

 

[sophiecentaur]

But also keep in mind I am just 15 years old.

Perhaps you could also bear that in mind when you feel tempted to jump in with both feet. You can expect to get a good kicking, sometimes, if you appear to be making unfounded assertions. OK as long as you have a thick skin - if you're 'ard enough.

 

[phinds]

Thanks for pointing out my mistake. But also keep in mind I am just 15 years old.

I was not aware of that and I'd say you're doing GREAT for a 15 year old, but as sophiecentaur pointed out, this is not a forum for the thinskinned and while I'm very serious in saying that you're doing well for a 15 year old, I would further caution you that if you want to play in the deep end of the pool you need to keep in mind that it was your own idea. Your statement "Thanks for pointing out my mistake. But also keep in mind I am just 15 years old." would really have been better as just "Thanks for pointing out my mistake". We ALL here make mistakes and part of playing in the deep end of the pool is to just fess up to them when we make them. You'll find my posts littered with them.

 

[sophiecentaur]

Yes. You may notice that people who start to start to 'wax lyrical' and 'alternative' very often get jumped on, here. There are always ways of saying things that avoid a bad response, at least initially. Asking questions rather than making statements and loads of IMHO's and things will help to oil the wheels. Remember, you may well be conversing with someone who knows a fair bit more about the particular topic than your teacher at School. And you would, of course, be verrrrry respectful about your teacher's knowledge.

There is a fair smattering of BS, too. You get to distinguish, soon enough, though.

 

[Low-Q]

It would move. That's how solar sails work. Light has momentum, but no mass.

OK. I did not take that into account, but you're right. However the force is very very weak. Vidar

 

[sophiecentaur]

Maxwell had all this sorted out before anyone discovered photons, you might be interested to know. My old classical electromag theory book derives the amount of 'light pressure' on a surface by just using the fields and how the surface modifies them. Surprise, surprise, old JCM got the same answer as the QM crowd. I bet that made the new boys pretty happy and relieved.

 

[Phrak]

On shell? What shell?

http://en.wikipedia.org/wiki/On_shell_and_off_shell"

 

[sophiecentaur]

http://en.wikipedia.org/wiki/On_shell_and_off_shell"

I don't get it. How does this imply that a photon has mass? That wiki article seems to be dealing with items that 'have mass'. Experiments seem to imply that a photon has none (or, at least they give it a very very low, higher limit - which is the best an experiment could ever do).

Does that equation, relating Energy, momentum and mass, really imply anything other than something about items that actually have mass? The article doesn't seem to mention photons so are you sure it applies here? Are you sure you aren't just using a sort of circular argument?

 

[Phrak]

I don't get it. How does this imply that a photon has mass? That wiki article seems to be dealing with items that 'have mass'. Experiments seem to imply that a photon has none (or, at least they give it a very very low, higher limit - which is the best an experiment could ever do).

Does that equation, relating Energy, momentum and mass, really imply anything other than something about items that actually have mass? The article doesn't seem to mention photons so are you sure it applies here? Are you sure you aren't just using a sort of circular argument?

There is no restriction in the Wikipedia article that m cannot equal 0 such as the case with photons.

In the pedagogy of quantum field theory, virtual particles, photons included, can be massive--including negative mass. These particles can give rise to Coulombic forces such as the repulsion of two like-charged pith balls, where the virtual photons have spacelike trajectories.

For any given particle the mass shell in energy-momentum space has a one to one correspondence with the 4-velocity vector of that particle in space-time space.

 

[sophiecentaur]

There is no restriction in the Wikipedia article that m cannot equal 0 such as the case with photons.

In the pedagogy of quantum field theory, virtual particles, photons included, can be massive--including negative mass. These particles can give rise to Coulombic forces such as the repulsion of two like-charged pith balls, where the virtual photons have spacelike trajectories.

For any given particle the mass shell in energy-momentum space has a one to one correspondence with the 4-velocity vector of that particle in space-time space.

OK then. The equation just loses the term with m in it if photons don't have mass. That agrees with experiment. Where did the on-shell / off-shell thing take us with regard to photons?

 

[jnorman]

inre: "there is no rest frame for a photon"

how about this - when a photon is absorbed by an atom (now at rest), the mass of the atom is increased. thus the photon has added mass to the system. i understand that this is because the photon has added energy to the system. and that e=mc2, implying that energy and mass are essentially the same, and interchangeable.

to me, the reason to believe that photons have no mass is that a photon cannot be accelerated, and that they ALWAYS travel at C (they do not accelerate or decelerate when emitted or absorbed).

i have no f'ing idea what the results of OPERA could mean, other than it is simply a mistake. given the nearly complete inability to detect neutrinos, i would guess they are making some inappropriate assumptions about the measurement setup - but i am an idiot and they are uber-physicists, so iam baffled.

 

[phinds]

to me, the reason to believe that photons have no mass is that a photon cannot be accelerated, and that they ALWAYS travel at C (they do not accelerate or decelerate when emitted or absorbed).

Yep, that's my understanding as well.

i have no f'ing idea what the results of OPERA could mean, other than it is simply a mistake. given the nearly complete inability to detect neutrinos, i would guess they are making some inappropriate assumptions about the measurement setup.

Well don't feel bad ... all 160 of them are JUST as puzzled, which is why they published their results in the first place ... they are asking the world of physicists to please help them find the error because no one, including them, really believes at this point that neutrinos travel faster than c.

 

[dchris]

inre: "there is no rest frame for a photon"

how about this - when a photon is absorbed by an atom (now at rest), the mass of the atom is increased. thus the photon has added mass to the system. i understand that this is because the photon has added energy to the system. and that e=mc2, implying that energy and mass are essentially the same, and interchangeable.

The photon doesn't add mass but increases the atoms gravity by adding energy. Or did i mistaken something?

 

[Drakkith]

inre: "there is no rest frame for a photon"

how about this - when a photon is absorbed by an atom (now at rest), the mass of the atom is increased. thus the photon has added mass to the system. i understand that this is because the photon has added energy to the system. and that e=mc2, implying that energy and mass are essentially the same, and interchangeable.

The photon doesn't add mass but increases the atoms gravity by adding energy. Or did i mistaken something?

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.

 

[sophiecentaur]

e=mc2, implying that energy and mass are essentially the same, and interchangeable.

"the same" is tantamount to saying that a photon must have mass, though. The 'mass' quality doesn't express itself in the photon so mass and energy are not so much "the same" as equivalent or interchangeable.

 

[Phrak]

OK then. The equation just loses the term with m in it if photons don't have mass. That agrees with experiment. Where did the on-shell / off-shell thing take us with regard to photons?

Some background may be helpful, maybe... You might recall Feynman's "sum over histories" paradigm. A photon takes all paths to get from point A to B. A good example is a photon bouncing off a reflecting surface. In Feynman's quantum electrodynamics, it bounces off the entire surface. There is only one tiny spot on the mirror where the angle if incidence is equal the angle of reflection.

For all this to work some of the paths, or various parts of some paths will be spacelike and some time like. For a real photon, the interference from the paths with trajectories not on the light cone, will cancel. But not all interactions of particles involve cancelation of the paths off the light cone. So there can be massive photons exchanged between two particles. They are not observed directly, or we would measure a spectum for the mass of photons, but are a necessary part of the theory and called virtual particles.

I'm not sure this answers you; I'm not sure what you were asking.

 

[Low-Q]

I don't get it. How does this imply that a photon has mass? That wiki article seems to be dealing with items that 'have mass'. Experiments seem to imply that a photon has none (or, at least they give it a very very low, higher limit - which is the best an experiment could ever do).

Does that equation, relating Energy, momentum and mass, really imply anything other than something about items that actually have mass? The article doesn't seem to mention photons so are you sure it applies here? Are you sure you aren't just using a sort of circular argument?

This reply made me start thinking on how sound waves are transferred through the air. The energy within the wave itself have no mass, but the air that is "energized" by the wave have mass. Therfor the soundwave can do practical work on an object near by. Could light-waves be just waves which travels through "something" that appears to have mass, and therefor it will appear that light have mass?

If light actually can propell a solar sail, that "mass" can be calculated. If the speed of that light is 299 792 458m/s, and we have 1kW of light pointed directly on a solar sail, this sail will accelerate at a given rate. The mass of the sail is known, so then it would be easy to find out the "mass" of that "somthing" which light travels trough and use to transfer energy into work(?).

Vidar

 

[Drakkith]

This reply made me start thinking on how sound waves are transferred through the air. The energy within the wave itself have no mass, but the air that is "energized" by the wave have mass. Therfor the soundwave can do practical work on an object near by. Could light-waves be just waves which travels through "something" that appears to have mass, and therefor it will appear that light have mass?

If light actually can propell a solar sail, that "mass" can be calculated. If the speed of that light is 299 792 458m/s, and we have 1kW of light pointed directly on a solar sail, this sail will accelerate at a given rate. The mass of the sail is known, so then it would be easy to find out the "mass" of that "somthing" which light travels trough and use to transfer energy into work(?).

Vidar

The energy in a sound wave is carried as kinetic energy in the particles that make up the air. A collection of moving particles, such as found in a sound wave, do in fact have more mass than they would if they were stationary. As for finding the "mass" of light, it is irrelevant. We have defined mass to mean a specific thing and light does not meet that criteria, therefor it does not have mass. It has ENERGY and as such it does contribute and is effected by gravity, but it does not have mass.

 

[sophiecentaur]

This reply made me start thinking on how sound waves are transferred through the air. The energy within the wave itself have no mass, but the air that is "energized" by the wave have mass. Therfor the soundwave can do practical work on an object near by. Could light-waves be just waves which travels through "something" that appears to have mass, and therefor it will appear that light have mass?

If light actually can propell a solar sail, that "mass" can be calculated. If the speed of that light is 299 792 458m/s, and we have 1kW of light pointed directly on a solar sail, this sail will accelerate at a given rate. The mass of the sail is known, so then it would be easy to find out the "mass" of that "somthing" which light travels trough and use to transfer energy into work(?).

Vidar

It is the Momentum of the light and not any 'implied mass' that causes the force. It would be better to avoid making things up as you go along and read (from beginning to end and not just the odd sentence here and there) what Wiki has to say about this. Wiki is not wrong on this topic.

 

[Low-Q]

It is the Momentum of the light and not any 'implied mass' that causes the force. It would be better to avoid making things up as you go along and read (from beginning to end and not just the odd sentence here and there) what Wiki has to say about this. Wiki is not wrong on this topic.

I did put a question mark at the end. I had some thoughts. That's all. Wiki might be representing the todays facts until these facts is changed by new discoveries some time in the future. I just keep my mind open, and do not always trust well established facts - because they change all the time as we learn and discover new things.

Vidar

 

[sophiecentaur]

If you make sure of understanding, fully, the facts as accepted, at present, then you stand a chance of understanding any new facts, as they emerge. How will you be able to judge any new stuff if you have no basics?

 

[Low-Q]

If you make sure of understanding, fully, the facts as accepted, at present, then you stand a chance of understanding any new facts, as they emerge. How will you be able to judge any new stuff if you have no basics?

Good point! I do consider the basics, at least what we know about the behaviour / appearence of light under given conditions. I just play with some thoughts about WHY it behave/appear like it does. So I ask: Is it momentum in light itself, or is it the medium it travels trough, or the matter which absorbs or reflect light that has transformed light into momentum? Is light itself affected by gravity, or is it the path/eather the light travels through that is affected by gravity? I'll keep my mind open for any explanation, but basics will be important in any case to find answers.

Vidar

 

[sophiecentaur]

In our present understanding and in the model that works best, we don't consider Space as a 'medium'. If you want to go down a different road then you would have to start from a lot further back than here and build an entirely different model from scratch. Are you capable of that?. Idle speculation may be fun but, unless you accept quite a lot of the present state of knowledge, I can't see you getting very far.
I have a feeling that all this may be a lot harder than you imagine. No one, in history has built a whole model for themselves and that is what you seem to be proposing.

 

[jetwaterluffy]

Good point! I do consider the basics, at least what we know about the behaviour / appearence of light under given conditions. I just play with some thoughts about WHY it behave/appear like it does. So I ask: Is it momentum in light itself, or is it the medium it travels trough, or the matter which absorbs or reflect light that has transformed light into momentum? Is light itself affected by gravity, or is it the path/eather the light travels through that is affected by gravity? I'll keep my mind open for any explanation, but basics will be important in any case to find answers.

Vidar

I think light itself has momentum, but that is different to it having mass. The medium of light is electromagnetic fields, and to me, it seems even more unlikely that they have mass than light has mass. Light is affected by gravity, that is the basis of the general theory of relativity and the definition of a black hole. The light follows the curves in space.
EDIT: just looked on wikianswers for some reason. They seem to have got it into their head that light does have mass.

 

[sophiecentaur]

I think light itself has momentum, but that is different to it having mass. The medium of light is electromagnetic fields, and to me, it seems even more unlikely that they have mass than light has mass. Light is affected by gravity, that is the basis of the general theory of relativity and the definition of a black hole. The light follows the curves in space.

Light just does what it always does (follows a straight line in space, in very crude terms). It's just space that is messed about by the presence of masses.

 

[DrDu]

Huh? Could you expand on that?

In a superconductor, local gauge symmetry is broken. The original gauge degree of freedom becomes the longitudinal component of the magnetic vector potential. I.e. the kinetic energy
operator is $\frac{1}{2m_e}(p-eA)^2$. In a homogeneous superconductor, the ground state wavefunction of the condensate can be written as $\psi=\rho^{1/2} \exp(i\phi)$ where rho is approximately constant. So the expectation value of the kinetic energy is
$\rho\frac{e^2}{2m_e}(\frac{\hbar}{e}\frac{\partial \phi}{\partial x_i}+A)^2$. Now we re-define $\tilde{A} =\frac{\hbar}{e}\frac{\partial \phi}{\partial x_i}+A$ and $\rho\frac{e^2}{2m_e}\tilde{A}^2$ looks now like a mass term for the electromagnetic field. Note that the gradient of phi makes a longitudinal contribution to $\tilde{A}$.
That's the famous Anderson-Higgs mechanism which was found by Anderson in 1958 in superconductors before it was transferred to elementary particle physics by Higgs in the 1960's.
The non-zero effective mass of the electromagnetic field is responsible for the rapid exponential decay of the magnetic field on the surface of a superconductor, i.e. the Meissner effect.
You can find an elementary introduction e.g. at the end of vol. 3 of the Feynman lectures.

That's not the only way to generate effectively massive photons. E.g. about half a year there was a report in Science (I think) where a group obtained a superfluid condensate of (massive) photons who gained there mass by being restricted to a wave-guide.

 

[phinds]

In a superconductor, local gauge symmetry is broken. The original gauge degree of freedom becomes the longitudinal component of the magnetic vector potential. I.e. the kinetic energy
operator is $\frac{1}{2m_e}(p-eA)^2$. In a homogeneous superconductor, the ground state wavefunction of the condensate can be written as $\psi=\rho^{1/2} \exp(i\phi)$ where rho is approximately constant. So the expectation value of the kinetic energy is
$\rho\frac{e^2}{2m_e}(\frac{\hbar}{e}\frac{\partial \phi}{\partial x_i}+A)^2$. Now we re-define $\tilde{A} =\frac{\hbar}{e}\frac{\partial \phi}{\partial x_i}+A$ and $\rho\frac{e^2}{2m_e}\tilde{A}^2$ looks now like a mass term for the electromagnetic field. Note that the gradient of phi makes a longitudinal contribution to $\tilde{A}$.
That's the famous Anderson-Higgs mechanism which was found by Anderson in 1958 in superconductors before it was transferred to elementary particle physics by Higgs in the 1960's.
The non-zero effective mass of the electromagnetic field is responsible for the rapid exponential decay of the magnetic field on the surface of a superconductor, i.e. the Meissner effect.
You can find an elementary introduction e.g. at the end of vol. 3 of the Feynman lectures.

That's not the only way to generate effectively massive photons. E.g. about half a year there was a report in Science (I think) where a group obtained a superfluid condensate of (massive) photons who gained there mass by being restricted to a wave-guide.

OK, that's over my head, but I'll check it out further. Thanks for the reference to the Feynman lectures.