Do Photons Have Mass? Exploring the Particle

[Phrak]

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.

It better be a medium. Relativity theory stips spacetime of Newtonian mechanistic properties but not the ability to support energy/momentum density, charge and current density, and so forth. Fields don't move in space. Charge doesn't move. Energy doesn't move from place to place. Mass doesn't move. The amplitudes of these quantities change from location to location. Like the water waves Low_Q was referring to where the water doesn't move with the wave, fields don't move; their amplitudes change over time.

Low_Q was much closer than you give him credit.

 

[dchris]

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.

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?

 

[harrylin]

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.

It just depends on the definition of mass that you prefer. As far as we know, light does not have rest mass.

 

[harrylin]

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

What do you mean with "light itself"? Compare "sound itself". Momentum is a property of the phenomenon that we call "light". Your question has been partly answered by Einstein: he modeled light as a wave that is bent due to the properties of space, using the Huygens construction - this is also called "gravitational lensing".

Einstein considered Space as a kind of medium, but apparently sophiecentaur uses a different, more modern model. It could be interesting to compare the two models.

 

[sophiecentaur]

It better be a medium. Relativity theory stips spacetime of Newtonian mechanistic properties but not the ability to support energy/momentum density, charge and current density, and so forth. Fields don't move in space. Charge doesn't move. Energy doesn't move from place to place. Mass doesn't move. The amplitudes of these quantities change from location to location. Like the water waves Low_Q was referring to where the water doesn't move with the wave, fields don't move; their amplitudes change over time.

Low_Q was much closer than you give him credit.

Bit of a problem with that one. This is contrary to experience, isn't it? Some of the statements are clearly true and other 'Zen' statements, I can go along with because you probably just need to look at things differently. But how do you square the idea of Energy not going anywhere? Are we re-defining what we mean by 'go anywhere'?

 

[Phrak]

Bit of a problem with that one. This is contrary to experience, isn't it? Some of the statements are clearly true and other 'Zen' statements, I can go along with because you probably just need to look at things differently. But how do you square the idea of Energy not going anywhere? Are we re-defining what we mean by 'go anywhere'?

We define electric and magnetic fields as fixed vectors, rather than free or sliding vectors. I'm not sure what sort of entities one would have with free vector fields. Taking Maxwell's equations at face value without metaphysical interpretation, charge and current are aspects--particular derivatives--of these fields, so comprise a fixed tensor in spacetime. Current density is a fixed tensor in space. Charge density is a fixed scalar (it's really a 3-form, but that's another matter).

Mass of a system can be defined as the norm of the energy momentum vector, or more properly the norm of theenergy momentum strength vector. This is a fixed tensor field over spacetime. I imagine one could find some consistent system using free vectors, or "free tensors", but these shouldn't have the same equations relating definitions of mass, momentum and energy, I would think. I'm not sure how it would all blend together without a great deal of mess, where vectors are free in space but where 4-vectors need to be fixed in spacetime.

 

[juanrga]

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 composed of photons and photons have momentum p and zero mass m, so you are right here.

Photons are the quanta of free electromagnetic fields, therefore the fields are not «The medium».

 

[sophiecentaur]

We define electric and magnetic fields as fixed vectors, rather than free or sliding vectors. I'm not sure what sort of entities one would have with free vector fields. Taking Maxwell's equations at face value without metaphysical interpretation, charge and current are aspects--particular derivatives--of these fields, so comprise a fixed tensor in spacetime. Current density is a fixed tensor in space. Charge density is a fixed scalar (it's really a 3-form, but that's another matter).

Mass of a system can be defined as the norm of the energy momentum vector, or more properly the norm of theenergy momentum strength vector. This is a fixed tensor field over spacetime. I imagine one could find some consistent system using free vectors, or "free tensors", but these shouldn't have the same equations relating definitions of mass, momentum and energy, I would think. I'm not sure how it would all blend together without a great deal of mess, where vectors are free in space but where 4-vectors need to be fixed in spacetime.

Am I dumb and is that an answer to my question?

 

[Phrak]

Am I dumb and is that an answer to my question?

Yes, it's an answer. Fields don't move in field theory and energy is a component of a field.

 

[sophiecentaur]

I can disagree with that ok. Whilst a field can be regarded as storing energy (energy is required to produce a field), the essence of a wave is fields continually changing - not moving. So we can accept Energy moving from place to place as the fields change but don't need to move.
How's that.?

 

[Phrak]

It's rare in physics that the word "move" is not associated with velocity.

 

[sophiecentaur]

Incontravertable.
But apropos of what?

 

[Low-Q]

What do you mean with "light itself"? Compare "sound itself". Momentum is a property of the phenomenon that we call "light". Your question has been partly answered by Einstein: he modeled light as a wave that is bent due to the properties of space, using the Huygens construction - this is also called "gravitational lensing".

Einstein considered Space as a kind of medium, but apparently sophiecentaur uses a different, more modern model. It could be interesting to compare the two models.

What I ment is that sound energy appears to move from one place to another, but the air itself does not go anywhere. So by saying "light itself" I meant the waves that appears to travel throug space, without the actual space going anywhere. I see space as a "carrier" of the energy of light. This carrier might as well have the momentum, and not the energy of light itself. I am therefor questioning the theory that light have momentum, when it MIGHT not be the case at all. The recent experiments with neutrinos, which preliminary concludes that they travel faster than light, they could likely use another "carrier" that has different properties than the "carrier" of the energy of light.

Just assumtions from my side, because I do not fully understand space, time, and the transmission of energy/waves over distance. I just try to make sense of all this.

Vidar

 

[sophiecentaur]

Just assumtions from my side, because I do not fully understand space, time, and the transmission of energy/waves over distance. I just try to make sense of all this.

Vidar

Despite what you may read on these pages, you are not alone!
It's a brave person who claims to understand 'all' or even 'any' of this stuff. If you can feel 'comfortable' or, maybe, 'familiar' with some of it then you are doing well. Remember - many people in this world don't even think about these matters at all.

Many people rant away, using their own personal model, and come to amazing conclusions (me. too, at times). Hopefully they take on board the sharp replies that people can dish out in response, without being too offended.

It often comes down to semantics. You can choose to call light the thing that moves or the thing that carries energy. I think it's reasonable to say that the waves move but the fields don't. - they just change.

 

[harrylin]

[..]
It often comes down to semantics. You can choose to call light the thing that moves or the thing that carries energy. I think it's reasonable to say that the waves move but the fields don't. - they just change.

I agree that light is like a wave that moves ("propagates") through space. Even using the photon concept, photons are sometimes called "wave packets" that carry momentum.

 

[Low-Q]

I agree that light is like a wave that moves ("propagates") through space. Even using the photon concept, photons are sometimes called "wave packets" that carry momentum.

Interesting. I must investigate further

 

[DrDu]

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.

Here is the link to the paper I mentioned (not Science but Nature):
http://www.nature.com/nature/journal/v468/n7323/full/nature09567.html

 

[sophiecentaur]

I agree that light is like a wave that moves ("propagates") through space. Even using the photon concept, photons are sometimes called "wave packets" that carry momentum.

The issue that I was addressing was whether Fields Move. Waves / Energy move - in anybody's book, I think but fields don't need to move in order for the wave to move.

 

[juanrga]

Here is the link to the paper I mentioned (not Science but Nature):
http://www.nature.com/nature/journal/v468/n7323/full/nature09567.html

From the article (bold style from mine):

The cavity mirrors provide both a confining potential and a non-vanishing effective photon mass, making the system formally equivalent to a two-dimensional gas of trapped, massive bosons.

Effective mass is the mass that a particle would carry in a semiclassical model of transport. The article does not say/suggest that photons have any mass.

 

[DrDu]

Effective mass is the mass that a particle would carry in a semiclassical model of transport. The article does not say/suggest that photons have any mass.

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.

 

[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.