Do Photons Have Mass and Momentum?

[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.

 

[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.