How come photons and...

[Gawdzilla Sama]

Emitted - Yes, a better word, methinks.

I'm finding it difficult impossible to imagine anything (massive or not) moving that has not previously accelerated.
- I think I'm stuck in some Newtonian view of things or something. I'll just have to somehow resign myself to the fact that I'm never going to even get close to understanding any of this stuff, at least to a satisfactory extent.

[nellikin]

I think of photon emission as a transfer of energy, kind of like from a potential energy to a kinetic energy when you drop a stone. And if the photon has no weight, then there is nothing to slow it down, and since it is light, it has light speed..?

[Xamonas Chegwé]

Yeah, I thought about pointing that out Jim. The only problem with velocity or momentum implying mass is that it is light particles traveling at the speed of light. In a vacuum, the speed of light is invariant. As such they have no rest mass. This isn't to say that their rest mass is zero but simply they cannot have a rest mass. I must admit, though, on a personal level I want to think the photon must have some mass yet there is no evidential support for this. *shrug*

Well they definitely have momentum; when photons collide with an object, they transfer momentum to it, which is the whole principle behind light sails...

And in practical terms, rest mass = zero tends to mean the same thing as not being able to have a rest mass...

Photons have energy but not mass. Energy is related to mass by Einstein's famous equation E = mc2 - so, in a sense, they are the same thing. When a photon is absorbed by a system, the total mass of the system increases by E/c2 and when one is emitted, it decreases by the same amount. This doesn't mean that the photon weighs anything, merely that mass and energy are interchangeable.


(I think)

[JimC]

Yeah, I thought about pointing that out Jim. The only problem with velocity or momentum implying mass is that it is light particles traveling at the speed of light. In a vacuum, the speed of light is invariant. As such they have no rest mass. This isn't to say that their rest mass is zero but simply they cannot have a rest mass. I must admit, though, on a personal level I want to think the photon must have some mass yet there is no evidential support for this. *shrug*

Well they definitely have momentum; when photons collide with an object, they transfer momentum to it, which is the whole principle behind light sails...

And in practical terms, rest mass = zero tends to mean the same thing as not being able to have a rest mass...

Photons have energy but not mass. Energy is related to mass by Einstein's famous equation E = mc2 - so, in a sense, they are the same thing. When a photon is absorbed by a system, the total mass of the system increases by E/c2 and when one is emitted, it decreases by the same amount. This doesn't mean that the photon weighs anything, merely that mass and energy are interchangeable.


(I think)

They have no rest mass, but they do have a momentum, which is a function of their frequency and Plank's constant (h). The momentum is quantized, and is different in some ways to the classical momentum of an object like a cricket ball. However, when a photon interacts with a material object, momentum is transferred, and in ways which mimic the momentum transfer of classical collisions, including full conservation of momentum. For example, when a photon is absorbed by an object, its tiny momentum (call it x) is transferred to the object. However, a photon that is reflected from an object transfers twice that momentum. Light sails depend entirely on this effect, often called light pressure: http://en.wikipedia.org/wiki/Light_pressure

Given that a photon has momentum, then it has at least the equivalent of inertial mass (albeit very, very tiny), which I presume is connected to the observed gravitational effects on light photons. However, a deep mathematical knowledge of general relativity is needed to move further, which is well beyond me...

[Xamonas Chegwé]

Yeah, I thought about pointing that out Jim. The only problem with velocity or momentum implying mass is that it is light particles traveling at the speed of light. In a vacuum, the speed of light is invariant. As such they have no rest mass. This isn't to say that their rest mass is zero but simply they cannot have a rest mass. I must admit, though, on a personal level I want to think the photon must have some mass yet there is no evidential support for this. *shrug*

Well they definitely have momentum; when photons collide with an object, they transfer momentum to it, which is the whole principle behind light sails...

And in practical terms, rest mass = zero tends to mean the same thing as not being able to have a rest mass...

Photons have energy but not mass. Energy is related to mass by Einstein's famous equation E = mc2 - so, in a sense, they are the same thing. When a photon is absorbed by a system, the total mass of the system increases by E/c2 and when one is emitted, it decreases by the same amount. This doesn't mean that the photon weighs anything, merely that mass and energy are interchangeable.


(I think)

They have no rest mass, but they do have a momentum, which is a function of their frequency and Plank's constant (h). The momentum is quantized, and is different in some ways to the classical momentum of an object like a cricket ball. However, when a photon interacts with a material object, momentum is transferred, and in ways which mimic the momentum transfer of classical collisions, including full conservation of momentum. For example, when a photon is absorbed by an object, its tiny momentum (call it x) is transferred to the object. However, a photon that is reflected from an object transfers twice that momentum. Light sails depend entirely on this effect, often called light pressure: http://en.wikipedia.org/wiki/Light_pressure

Given that a photon has momentum, then it has at least the equivalent of inertial mass (albeit very, very tiny), which I presume is connected to the observed gravitational effects on light photons. However, a deep mathematical knowledge of general relativity is needed to move further, which is well beyond me...

I am not convinced. As I understand it, photons have no mass - ever. They have momentum but that momentum is NOT a result of their having mass. Photons only exist in motion - in fact, they only exist in motion at the speed of light - and anything that travels at the speed of light cannot have mass, since it would need an infinite amount of energy to overcome its inertia and accelerate it to that speed. Photons have no mass = no inertia = no acceleration (or, if your prefer, instantaneous acceleration.) Any mass at all would make this impossible.

[JimC]

XC wrote:

I am not convinced. As I understand it, photons have no mass - ever. They have momentum but that momentum is NOT a result of their having mass. Photons only exist in motion - in fact, they only exist in motion at the speed of light - and anything that travels at the speed of light cannot have mass, since it would need an infinite amount of energy to overcome its inertia and accelerate it to that speed. Photons have no mass = no inertia = no acceleration (or, if your prefer, instantaneous acceleration.) Any mass at all would make this impossible.

Well, certainly there is no acceleration involved - they are travelling at C (or the equivalent in whatever transparent material they are in) at the instant of emmision. However, they definitelky have momentum, which can be transferred, and the transfer obeys the law of conservation of momentum. You say no inertia, but momentum and inertia are closely linked in classical physics - the m in the mass times velocity function is inertial mass... And a light sail, bombarded with billions of photons per second, has its classical momentum altered just as much as if it were being bombarded with sub light speed particles...

However, quantised momentum is definitely a different beast to classical momentum, just like quantum spin is different to classical angular momentum. It may therefore not make a lot of sense to talk about the mass of even a travelling photon, and perhaps the affect of gravity on a photon is no more than it following the shortest geodesic in curved space-time...

From a wiki article on mass: http://en.wikipedia.org/wiki/Mass

Although mass must be distinguished from matter in physics, because matter is a poorly-defined concept, and although all types of agreed-upon matter exhibit mass, it is also the case that many types of energy which are not matter— such as potential energy, kinetic energy, and trapped electromagnetic radiation (photons)— also exhibit mass. Thus, all matter has the property of mass, but not all mass is associated with identifiable matter.

[JacksSmirkingRevenge]

Would I be correct in thinking, then, that a flashlight, for example, actually produces a tiny amount of thrust?

[Xamonas Chegwé]

XC wrote:

I am not convinced. As I understand it, photons have no mass - ever. They have momentum but that momentum is NOT a result of their having mass. Photons only exist in motion - in fact, they only exist in motion at the speed of light - and anything that travels at the speed of light cannot have mass, since it would need an infinite amount of energy to overcome its inertia and accelerate it to that speed. Photons have no mass = no inertia = no acceleration (or, if your prefer, instantaneous acceleration.) Any mass at all would make this impossible.

Well, certainly there is no acceleration involved - they are travelling at C (or the equivalent in whatever transparent material they are in) at the instant of emmision. However, they definitelky have momentum, which can be transferred, and the transfer obeys the law of conservation of momentum. You say no inertia, but momentum and inertia are closely linked in classical physics - the m in the mass times velocity function is inertial mass... And a light sail, bombarded with billions of photons per second, has its classical momentum altered just as much as if it were being bombarded with sub light speed particles...

However, quantised momentum is definitely a different beast to classical momentum, just like quantum spin is different to classical angular momentum. It may therefore not make a lot of sense to talk about the mass of even a travelling photon, and perhaps the affect of gravity on a photon is no more than it following the shortest geodesic in curved space-time...

From a wiki article on mass: http://en.wikipedia.org/wiki/Mass

Although mass must be distinguished from matter in physics, because matter is a poorly-defined concept, and although all types of agreed-upon matter exhibit mass, it is also the case that many types of energy which are not matter— such as potential energy, kinetic energy, and trapped electromagnetic radiation (photons)— also exhibit mass. Thus, all matter has the property of mass, but not all mass is associated with identifiable matter.

Your quote mentions "trapped" electromagnetic radiation. This would equate to a photon bouncing between two mirrors. However, this is not actually a single photon bouncing back and forth but a series of photons, continually being created on one reflective surface, travelling to the other at c and being absorbed by that surface. The mass increase comes from the absorbed photon energy changing the state of each surface in turn. (Again, I think - I have been reading a lot about such things lately but my understanding is patchy.)

Incidentally, it is the two different concepts of momentum (mass x velocity and quantised momentum) that, when equated, lead to E = mc2 - showing that mass and energy are interchangeable.

[mistermack]

Well, you either accept the curvature of space, or you don't.
If you do, it explains the effect of gravity on photons, and everything else.
You dont need to postulate some mass-like property. The light merely follows the path of space, as it always does. But in the absence of significant gravity, this is a straight line. In strong gravitational fields, it's curved.

Photons obviously don't have mass, but they are perfectly convertible into mass, through pair production.
It should hardly be surprising that gravity acts on a packet of energy, in a similar way that it acts on a particle with mass. As mass is convertible to energy, and energy to mass, why the surprise? Is there a fundamental change, or is the energy just constrained, in a particle with mass, ready to be let loose?

[JimC]

Well, you either accept the curvature of space, or you don't.
If you do, it explains the effect of gravity on photons, and everything else.
You dont need to postulate some mass-like property. The light merely follows the path of space, as it always does. But in the absence of significant gravity, this is a straight line. In strong gravitational fields, it's curved.

Photons obviously don't have mass, but they are perfectly convertible into mass, through pair production.
It should hardly be surprising that gravity acts on a packet of energy, in a similar way that it acts on a particle with mass. As mass is convertible to energy, and energy to mass, why the surprise? Is there a fundamental change, or is the energy just constrained, in a particle with mass, ready to be let loose?

Whether they can be said to have mass is a moot point, really. They definitely have zero rest mass; rest mass is important, because (relative to an observer), all particles increase their mass over and above their rest mass via relativistic effects.

The issue that is of interest to me is photon momentum. Photons have a clear and well defined momentum, which they can transfer to material objects via collisions. For particles, momentum is intimately involved with mass; but whether the quantised momentum of photons implies anything sensible about their mass as they travel, I am unsure...

[colubridae]

Well, you either accept the curvature of space, or you don't.
If you do, it explains the effect of gravity on photons, and everything else.
You dont need to postulate some mass-like property. The light merely follows the path of space, as it always does. But in the absence of significant gravity, this is a straight line. In strong gravitational fields, it's curved.

Photons obviously don't have mass, but they are perfectly convertible into mass, through pair production.
It should hardly be surprising that gravity acts on a packet of energy, in a similar way that it acts on a particle with mass. As mass is convertible to energy, and energy to mass, why the surprise? Is there a fundamental change, or is the energy just constrained, in a particle with mass, ready to be let loose?

Whether they can be said to have mass is a moot point, really. They definitely have zero rest mass; rest mass is important, because (relative to an observer), all particles increase their mass over and above their rest mass via relativistic effects.

The issue that is of interest to me is photon momentum. Photons have a clear and well defined momentum, which they can transfer to material objects via collisions. For particles, momentum is intimately involved with mass; but whether the quantised momentum of photons implies anything sensible about their mass as they travel, I am unsure...

photon momentum isn't quantised, it can take any value.

[JimC]

Well, you either accept the curvature of space, or you don't.
If you do, it explains the effect of gravity on photons, and everything else.
You dont need to postulate some mass-like property. The light merely follows the path of space, as it always does. But in the absence of significant gravity, this is a straight line. In strong gravitational fields, it's curved.

Photons obviously don't have mass, but they are perfectly convertible into mass, through pair production.
It should hardly be surprising that gravity acts on a packet of energy, in a similar way that it acts on a particle with mass. As mass is convertible to energy, and energy to mass, why the surprise? Is there a fundamental change, or is the energy just constrained, in a particle with mass, ready to be let loose?

Whether they can be said to have mass is a moot point, really. They definitely have zero rest mass; rest mass is important, because (relative to an observer), all particles increase their mass over and above their rest mass via relativistic effects.

The issue that is of interest to me is photon momentum. Photons have a clear and well defined momentum, which they can transfer to material objects via collisions. For particles, momentum is intimately involved with mass; but whether the quantised momentum of photons implies anything sensible about their mass as they travel, I am unsure...

photon momentum isn't quantised, it can take any value.

From Wikipedia: http://en.wikipedia.org/wiki/Quantizati ... n_momentum

To me, that shows that calculations of photon momentum involve quantum mechanics (eigenvalues, Planck's constant), which I assumed always meant that the quantity in question was restricted to quantised amounts. I may be mistaken in that, but quantum momentum is different in some respects to the classical momentum of particles, even though it can be converted to it via collisions...

[mistermack]

Whether they can be said to have mass is a moot point, really. They definitely have zero rest mass; rest mass is important, because (relative to an observer), all particles increase their mass over and above their rest mass via relativistic effects.

Yes, but of course this can never happen for a photon, because it has the same velocity relative to any possible observer, no matter what is the velocity of the observer.

The issue that is of interest to me is photon momentum. Photons have a clear and well defined momentum, which they can transfer to material objects via collisions. For particles, momentum is intimately involved with mass; but whether the quantised momentum of photons implies anything sensible about their mass as they travel, I am unsure...

I'm not sure that momentum is a good word for what a photon posesses. Because we normally think of momentum as a combination of mass and velocity, it seems to say that if the photon has momentum, it must have mass of some sort.
If we were to say that photons posess momentum POTENTIAL, it might be more accurate I suppose. It posesses energy that can BECOME momentum, if transferred to a particle.

[JimC]

Whether they can be said to have mass is a moot point, really. They definitely have zero rest mass; rest mass is important, because (relative to an observer), all particles increase their mass over and above their rest mass via relativistic effects.

Yes, but of course this can never happen for a photon, because it has the same velocity relative to any possible observer, no matter what is the velocity of the observer.

The issue that is of interest to me is photon momentum. Photons have a clear and well defined momentum, which they can transfer to material objects via collisions. For particles, momentum is intimately involved with mass; but whether the quantised momentum of photons implies anything sensible about their mass as they travel, I am unsure...

I'm not sure that momentum is a good word for what a photon posesses. Because we normally think of momentum as a combination of mass and velocity, it seems to say that if the photon has momentum, it must have mass of some sort.
If we were to say that photons posess momentum POTENTIAL, it might be more accurate I suppose. It posesses energy that can BECOME momentum, if transferred to a particle.

Both in classical and quantum physics, energy and momentum are completely seperate properties of a given system, so energy can never "become" momentum.

There is absolutely no doubt that photons posses momentum, it has a clearly defined value in quantum mechanics. (http://en.wikipedia.org/wiki/Theoretica ... of_photons)Also, as I have explained several times in this thread, the momenta of a stream of photons is readily converted to the momentum of an object as they strike it, hence light sails...

However, I agree that one cannot jump from the existence of momentum to a clearly defined mass for a photon; the relationship "momentum = mass x velocity" may only make sense for a particle of matter, rather than a packet of electromagnetic waves... (this is an answer to your sentence "Because we normally think of momentum as a combination of mass and velocity, it seems to say that if the photon has momentum, it must have mass of some sort.")

And certainly, the behaviour of photons under the influence of the gravity of a massive object is perfectly explained by the concept of tracing a geodesic in curved space, as described by general relativity, so mass in the classical sense is not required to explain this property of light.

[mistermack]

No, what I was saying that photons don't have momentum, if momentum is mass x velocity. People may use the word momentum, but what they have isn't that. It may convert to ordinary momentum, when the light hits a particle.

That's why I offered the term "momentum potential", because although the momentum of a photon may be a perfectly convertible equivalent, it's clearly not the same thing.

I suppose I'm just surprised that such an exact science can use the word momentum for two such different phenomena. Momentum of a particle is not the same as momentum of a photon. That's why in my opinion, momentum potenial would have been a better expression for the "momentum" of photons.

Both in classical and quantum physics, energy and momentum are completely seperate properties of a given system, so energy can never "become" momentum.

Yeh, I didn't express myself very well there. I think that it posesses energy that has a PROPERTY that can become momentum, if transferred to a particle. I'm suggesting that it would be better to call that property momentum potential, instead of momentum, in view of the obvious difference.