# Understanding the Massless Photon

• Lelan Thara
In summary: I've also come across references to the "virtual mass" of a photon. Can anyone shed more light on that?The larger question might be - is the massless photon massless by definition? In other words, "mass is a quality of matter, the photon isn't defined as matter despite being a particle, and therefore the photon can't... have mass?"I'm not sure what you're asking. Are you trying to ask if photons have a 'virtual mass'? If so, I'm not sure how to answer that.
Lelan Thara
(I hope I've chosen the right sub-forum for this question...)

Hi folks - I've recently joined here to see if people who are more knowledgeable than me can help me understand some physics issues I have struggled with for a long time.

My current question is a pretty basic one about how it is possible for a photon to have no mass.

We have the famous equation, "E = mc squared" My math knowledge is very limited, but from what I know - if I assign the value "0" to m, and multiply 0 by c squared, the answer for E should be zero.

Yet a photon possesses energy, and is said to have no mass.

I can see 3 possibilities:

- "E = mc squared" is not a standard algebra equation, and assigning the value "0" to m doesn't result in E being zero.

- "E = mc squared" does not apply to photons - something that seems very unlikely to me.

- photons do, in fact, have mass, or conversely, have no energy.

Can someone help me understand this? I would be very grateful. Thanks!

You need quantum mechanics too... E=h f

Thrice said:
It's usually a good idea to start out on wikipedia before asking here.

http://en.wikipedia.org/wiki/E=mc²

Actually I do usually start with Wiki, but it never occurred to me to look up "E = mc squared" there.

That was easy, wasn't it? Carry on, carry on...

Lelan Thara said:
Yet a photon possesses energy, and is said to have no mass.

Can someone help me understand this? I would be very grateful. Thanks!
If you’re just looking for a way to understand; there is a very simple way.
Don't thing of a photon as "possessing" energy --- thing of a photon AS energy.
Now when an atom absorbs a photon and moves one its electrons into a higher energy ‘obit’ it gains mass.
When it drops to a lower energy level the atom loses mass as a photon is emitted in some random direction.

Where was the mass? in the electron; or in the atom as whole – I’ll let you speculate.
But that mass cannot just appear and disappear; shouldn’t mass be “conserved”?
No conservation of mass and conservation of energy is “Old School”
In modern physics it is the net of Mass and Energy that is conserved.
So when a bit of mass disappears from an atom we can find it in the energy that escapes from it – we call it a photon.
When a lot of mass disappears very quickly we can describe it as a lot photons (aka energy) escaping – or more easily described as a nuclear explosion.

Lelan Thara said:
My current question is a pretty basic one about how it is possible for a photon to have no mass.

We have the famous equation, "E = mc squared" My math knowledge is very limited, but from what I know - if I assign the value "0" to m, and multiply 0 by c squared, the answer for E should be zero.

Yet a photon possesses energy, and is said to have no mass.
The complete relativistic expression for the energy of a particle is this:
$$E = \sqrt{p^2c^2 + m^2c^4}$$

Where p is the momentum. Note that for massive particles at rest (momentum = 0) that equation becomes the more familiar $E = mc^2$.

For a photon: the mass is zero, but the momentum and energy are non-zero. E = pc = hf.

Lelan Thara said:
Actually I do usually start with Wiki, but it never occurred to me to look up "E = mc squared" there.

That was easy, wasn't it? Carry on, carry on...
Glad I could help. I picked up this firefox extension to make sure I can't avoid searching on wikipedia.

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Thanks agains to all. The most fundamental answer to my question is that E=mc squared is not as universally applied as I assumed it was, at least not in its familiar simple form.

There is something that still confuses me a bit, and that's the concept of "rest mass". I was under the impression that the only time mass could be at rest was at a temperature of absolute zero, which doesn't really exist in nature (like a perfect vacuum). The photon is also never at rest - is the difference that the photon can't conceptually be at rest?

I've also come across references to the "virtual mass" of a photon. Can anyone shed more light on that?

The larger question might be - is the massless photon massless by definition? In other words, "mass is a quality of matter, the photon isn't defined as matter despite being a particle, and therefore the photon can't have a quality of matter - mass"?

Or is it something more concrete - that the photon doesn't exhibit inertia, acceleration and deceleration, and other properties of mass?

Thanks again.

Lelan Thara said:
Thanks agains to all. The most fundamental answer to my question is that E=mc squared is not as universally applied as I assumed it was, at least not in its familiar simple form.

There is something that still confuses me a bit, and that's the concept of "rest mass". I was under the impression that the only time mass could be at rest was at a temperature of absolute zero, which doesn't really exist in nature (like a perfect vacuum). The photon is also never at rest - is the difference that the photon can't conceptually be at rest?

That's in quantum mechanics. Special Relativity CAN be interfaced to quantum mechanics, but when we talk about SR by itself we mostly mean the classical model, where things can be at rest. You are right about the photon though, it has no rest frame, even in the classical theory, and no mass.

Sidebar.
I am one of the ones who do not use "rest mass" because that implies there is some kind of distinguished non-rest mass, which I deny. There are are other posters on this forum who vehemently disagree, and in order to get maximal use out of your experience here you have to be clear in your mind so the disagreement (which is a tempest in a teacup as far as physics is concerned) doesn't confuse you. In my view a particle has one mass, no matter how it is moving in relation to you, and this mass is a "scalar", that is to say the same in every frame.

I've also come across references to the "virtual mass" of a photon. Can anyone shed more light on that?

I don't know this one. Did you mean virtual photons?

The larger question might be - is the massless photon massless by definition? In other words, "mass is a quality of matter, the photon isn't defined as matter despite being a particle, and therefore the photon can't have a quality of matter - mass"?

ST postulates that the speed of light is invariant, the same for all inertial frames. It is a mathematical deduction from that postulate that if light is carried by a particle then that particle must be massless. In fact the two concepts "Massless" and "Moves at the Speed of Light" are synonymous in SR. Gluons in the Standard Model also have this property.

Or is it something more concrete - that the photon doesn't exhibit inertia, acceleration and deceleration, and other properties of mass?

As far as we can tell, it takes no acceleration to get a massless particle up to the speed c. But the experimental evidence for the masslessness of the photon is mostly in another area. From classical wave theory it follows that if light were carried by a massive particle, it would exhibit longitudinal (compression type) waves. Whereas we only find transverse light waves. Experimentalists do tests with fantastically refined versions of this distinction to examine the possibility that there are some very weak compression waves, that would allow a tiny mass to the photon. They've been doing this, with ever increasing precision, for generations. They've never found anything.

Of course no experiment can ever proves that the mass is exactly zero; every experiment has error bounds, but the error bounds on these tests have gotten really tiny, off the top of my head I want to say + or - 10-20 electron volts.

Thanks again.[/QUOTE]

Lelan Thara said:
Thanks agains to all. The most fundamental answer to my question is that E=mc squared is not as universally applied as I assumed it was, at least not in its familiar simple form.

There is something that still confuses me a bit, and that's the concept of "rest mass". I was under the impression that the only time mass could be at rest was at a temperature of absolute zero, which doesn't really exist in nature (like a perfect vacuum). The photon is also never at rest - is the difference that the photon can't conceptually be at rest?

I've also come across references to the "virtual mass" of a photon. Can anyone shed more light on that?

The larger question might be - is the massless photon massless by definition? In other words, "mass is a quality of matter, the photon isn't defined as matter despite being a particle, and therefore the photon can't have a quality of matter - mass"?

Or is it something more concrete - that the photon doesn't exhibit inertia, acceleration and deceleration, and other properties of mass?

Thanks again.
You made a good point here. The concept of a "massless particle" is so familiar in present day physics that people forget that it is in fact not such a good concept at all. As you pointed out, the photon is indeed never at rest, and can not be, so why should one talk of rest mass ? It is really nonsense if you think about it and it seems to cause a lot of confusion to many people (judging from the forum questions).So, the photon is massless by definition only with the convenient result that one can insert m=0 into the complete relativistic expression, such that one obtains the correct relation between energy and impulse. Maybe it would be better to use the term "c-particle" to indicate that it is a particle which moves at the speed of light.

notknowing said:
Maybe it would be better to use the term "c-particle" to indicate that it is a particle which moves at the speed of light.

There already is a term: "lightlike" particle.

Lelan Thara said:
Thanks agains to all. The most fundamental answer to my question is that E=mc squared is not as universally applied as I assumed it was, at least not in its familiar simple form.

it could be more universally applied if the $m$ in $E = m c^2$ is always considered the relativistic mass and $E$ is the total energy, kinetic energy plus rest energy. or the energy of the particle in the frame of the observer watching it whiz by. or the momentum of the particle divided by speed. assuming photons travel at the speed of the wavespeed of electromagnetic radiation, $c$, the (relativistic) mass of the photons is

$$m = \frac{E}{c^2} = \frac{h \nu}{c^2}$$

but here $m$ is not the "rest mass" or "invariant mass". for particles that move more slowly that $c$, special relativity says their momentum is

$$p = m v = \frac{m_0 v}{\sqrt{1 - \frac{v^2}{c^2}}}$$

where $m_0$ is the rest mass (and $E_0 = m_0 c^2$ is the rest energy and the total energy $E_0 = m_0 c^2$ is the rest energy plus kinetic energy). so the relationship between rest mass and relativistic mass is

$$m = \frac{m_0}{\sqrt{1 - \frac{v^2}{c^2}}}$$

or turned around is

$$m_0 = m \sqrt{1 - \frac{v^2}{c^2}}$$

or

$$m_0 = \frac{E}{c^2} \sqrt{1 - \frac{v^2}{c^2}}$$ .

now, no matter what finite energy that particle has, if it moves at the speed of light, that equation says that the rest mass of the particle has to be zero.

the equation

$$E = m c^2$$

is just as general as

$$E^2 = m_0^2 c^4 + p^2 c^2$$

if the rest mass $m_0$ is related to relativistic mass $m$ as per the equation above and momentum is stiil the same $p = m v$.

There is something that still confuses me a bit, and that's the concept of "rest mass".

particles moving past an observer at very high speeds "appear" to that observer to have a larger mass than they do if the observer is moving alongside the particle. the mass that the particle has in the same reference frame of the particle is the rest mass.

I've also come across references to the "virtual mass" of a photon. Can anyone shed more light on that?

dunno if it means

$$m = \frac{h \nu}{c^2}$$

of if it means that the jury might be out as to whether photons, the particle manifestation of light, travels as fast as the wavespeed of light. some have posted is upper bound for the rest masses of photons (if i recall this upper bound was somewhere around 10-52 kg which is virtually nothing. so the difference in speeds are not measureable if there is such a difference at all. if it turns out that photons are known to travel at precisely the speed of light (waves), then the rest mass of the photons would have to be zero.

The larger question might be - is the massless photon massless by definition? In other words, "mass is a quality of matter, the photon isn't defined as matter despite being a particle, and therefore the photon can't have a quality of matter - mass"?

it's because they (are believed to) move at the speed of light that their rest mass has to be zero.

Or is it something more concrete - that the photon doesn't exhibit inertia, acceleration and deceleration, and other properties of mass?

it exhibits inertia, but no acceleration or deceleration if it always flies by at a speed of $c$, no matter who the observer is (this is a postulate of special relativity). they have momentum of

$$p = m v = \frac{h \nu}{c^2} v$$

which, if they move at speed $v = c$, then the momentum is

$$p = \frac{h \nu}{c}$$

they have non-zero momentum if that is what you mean by "inertia".

i know that i am presenting this from a POV that is discouraged in modern physics pedagogy (i don't think that Doc Al will like it), but it's correct given the definition of relativistic mass.

Thank you, folks.

I wish I could tell you where I saw the "virtual mass" for a photon mentioned. Most of my physics reading is books for laymen, but once I did try to wade through articles by physicsts themselves in the library - I think it might have be in an article by Bohr where I saw it.

Here's a related question - if a photon has no mass, what pushes a solar sail? Is it other forms of radiation, as opposed to photons?

Lelan Thara said:
Here's a related question - if a photon has no mass, what pushes a solar sail? Is it other forms of radiation, as opposed to photons?

Photons have momentum, as well as energy.

Lelan Thara said:
(I hope I've chosen the right sub-forum for this question...)

Hi folks - I've recently joined here to see if people who are more knowledgeable than me can help me understand some physics issues I have struggled with for a long time.

My current question is a pretty basic one about how it is possible for a photon to have no mass.

We have the famous equation, "E = mc squared" My math knowledge is very limited, but from what I know - if I assign the value "0" to m, and multiply 0 by c squared, the answer for E should be zero.

Yet a photon possesses energy, and is said to have no mass.

I can see 3 possibilities:

- "E = mc squared" is not a standard algebra equation, and assigning the value "0" to m doesn't result in E being zero.

- "E = mc squared" does not apply to photons - something that seems very unlikely to me.

- photons do, in fact, have mass, or conversely, have no energy.

Can someone help me understand this? I would be very grateful. Thanks!
The answer to your question regards your confusion as to the definition of the term "mass." Sometimes the term is used to refer to a particles proper mass while sometimes its used to refer to inertial mass (aka relativistic mass). The photon has zero proper mass and an inertial mass m = E/c2. For details please see

http://www.geocities.com/physics_world/mass_paper.pdf

Best wishes

Pete

Note that Pete's paper, with the URL given above, while mostly correct, has not been peer reviewed, and that some of us (like me) have disagreements with him on certain technical points and usages. Most of these are rather "fine" points, though.

Pervect - given what you said above, my question would be - do you agree with Pete that photons have inertial mass?

Photons have momentum and carry energy.

Garth

Lelan Thara said:
Pervect - given what you said above, my question would be - do you agreew ith Pete that photons have inertial mass?

I would say that photons have a zero invariant mass. If pressed, I would admit that they have a non-zero "relativistic mass", though I would be quick to point out that I personally did not like relativistic mass.

But you asked about "inertial mass". I'm not quite sure what you mean by "inertial mass". At a guess, you are trying to divide the momentum of a photon by its speed (which is always 'c') and come up with a number. This number will depend on the frame of reference - it will not be a property of the photon alone.

My general remark would be this: photons carry momentum and energy. This should be clearly understood. The idea of "mass" is actually somewhat of an "umbrella concept" - the name "mass" is an "umbrella" which covers a large number of closely related, but different, concepts. You'll really need to learn about mass in Newtonian mechanics, mass in special relativity (invariant mass and perhaps relativistic mass) and mass in general relativity (ADM mass, Bondi mass, Komar mass) separately. To quote Max Jammer, "Mass is a mess".

For some online reading, samples of Max Jammer's two books on mass are available on Google. They appear to be some of the better non-technical references out there.

You can find the remark about "mass is a mess" at http://books.google.com/books?vid=I...x+jammer+mass&sig=rvmUlL1YxTTmzI_MxSEwoJB7fzo

The umbrella analogy is AFAIK mine, however.

Lelan Thara said:
Pervect - given what you said above, my question would be - do you agree with Pete that photons have inertial mass?
That question cannot be answered until you have a definition of inertial mass. A.P. French in his SR text defines "inertial mass" as the ratio of the particle's momentum to its speed. Many others use this definition as well. So according to Frenchl anything that has momentum has speed. Therefore mass, just like momentum, will be dependant on the observer.

Best wishes

Pete

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pmb_phy said:
That question cannot be answered until you have a definition of inertial mass. A.P. French in his SR text defines "inertial mass" as the ratio of the particle's momentum to its speed. Many others use this definition as well. So according to Frenchl anything that has momentum has speed. Therefore mass, just like momentum, will be dependant on the observer.

Best wishes

Pete

OK, using that definition, please tell me what is the "speed" of a crystal momentum.

Zz.

ZapperZ said:
OK, using that definition, please tell me what is the "speed" of a crystal momentum.

Zz.
That is very different usage of the term "momentum." It isn't even used in classical physics and this is a forum on cassical mechanics, not quantum mechanics. You will also note that I used the term "partcle" which a crystal is not.

Best wishes

Pete

pmb_phy said:
That is very different usage of the term "momentum." It isn't even used in classical physics and this is a forum on cassical mechanics, not quantum mechanics. You will also note that I used the term "partcle" which a crystal is not.

Best wishes

Pete

pmb_phy said:
So according to Frenchl anything that has momentum has speed.

Then something is amiss here because in QM, the momentum operator is one of the central observable of our world.

And note that the word "crystal" here isn't what you you think it means. If you don't care for it, then let me ask you to replace "crystal" with, for example, "momentum of conduction electron in metals". Would that qualify as "particles" in your book?

Zz.

Oh, puleeze, not another thread-war on relativistic mass versus rest mass

To the OP:

There are different ways to define "mass". All of them are motivated in one way or another and they are not always equivalent. So the best way to get confused is to take values or formulas for one, and use them as another. There are religious wars going on over what is the "right" way to define mass.

As Doc Al pointed out, there is a universal "dispersion relation" which links energy and 3-momentum of a particle, and it is given by:

E^2 = k^2 c^4 + c^2 p^2

with E the energy, c the light speed and p the norm of the 3-momentum of the particle. k turns out to be a constant which depends only on the type of particle. Consider that this form of the relationship between momentum and energy is experimentally established. We call this constant k the "rest mass", and usually write it like m_0 or the like.
Turns out that certain particles have this constant equal to 0. In that case, the relationship simplifies to E = |p| c
It turns out that these things can only travel at light speed.
For other particles, where the constant m_0 turns out to be a non-zero number, it turns out that they can travel at any speed below c. In the specific case where they are at rest (which is possible), the momentum is 0 too, and the relationship becomes E = m_0 c^2
But mind you that this is only 1) in the rest frame of the particle and that 2) only particles with non-zero m_0 can have a rest frame.

Now, in how much it is a good or a bad idea to call this m_0 the "rest mass" in the case of m_0 = 0 is a matter of semantics.

There are people who are fond of the relation E = m c^2 even in cases where we are not in the rest frame, and there are some good reasons to do so. The number m, which is nothing else but E / c^2, is called "relativistic mass". Clearly, it is another name for "energy" (in other units).
It has some good uses in extending Newton's second law into the relativistic domain.

There are people who are fond of the relation p = m v. The number m (which is different from the number m representing "relativistic mass") is called "inertial mass".

There are still other ways to define "mass".
They have different values, they have different meanings, and they are used in different formulas.

vanesch said:
There are people who are fond of the relation p = m v. The number m (which is different from the number m representing "relativistic mass") is called "inertial mass".
Why? It should be the same.

lightarrow said:
Why? It should be the same.

Uh, yes. You're right.

I wanted to point out that there was a difference in definition (starting point), and then wrote something silly.

ZapperZ said:
Then something is amiss here because in QM, the momentum operator is one of the central observable of our world.

And note that the word "crystal" here isn't what you you think it means.
I think it means what it is defined to mean as defined in Wikipedia.
If you don't care for it, then let me ask you to replace "crystal" with, for example, "momentum of conduction electron in metals". Would that qualify as "particles" in your book?
It does.

I don't understand your confusion. Are you telling me that you don't know what momentum or speed is?

By the way, when I wrote "So according to Frenchl anything that has momentum has speed." It as a gross error on my part. That should have read "So according to French anything that has momentum and speed has inertial mass." Perhaps this is the source of your confusion??

This is not a unique place to see the term "inertial mass" defined in this way. Schutz also uses the term in the same way as French.

Oh, puleeze, not another thread-war on relativistic mass versus rest mass
No such war has taken place here in quite some time. People come here and always ask ther same, or similar, questions and the answer lies in definitions. The most reasonable thing to do is to provide the definitions. The "war" that you speak of has to do with how the term "mass" should be defined or someone whining about the way someone else uses the term.

Re - The statement Clearly, it is another name for "energy" (in other units). may be wrong. Please see

http://www.geocities.com/physics_world/sr/mass_momentum_density.htm
http://www.geocities.com/physics_world/sr/inertial_energy_vs_mass.htm

Best wishes

Pete

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pervect said:
But you asked about "inertial mass". I'm not quite sure what you mean by "inertial mass". At a guess, you are trying to divide the momentum of a photon by its speed (which is always 'c') and come up with a number. This number will depend on the frame of reference - it will not be a property of the photon alone.

that isn't true, is it? the speed of the "massless" photon is always c no matter the frame of reference but, if you define the mass of a particle as momentum divided by speed, then that mass does not depend on the frame of reference and is a property of the photon, namely

$$m = \frac{p}{c} = \frac{h \nu}{c^2}$$

hmmm, i guess because the frequency $\nu$ of the wave is a function of frame of reference (doppler effect), but then also the momentum $p = \frac{h \nu}{c}$ of the photon is dependent on the frame of reference in the same manner and no one is saying that photons have no momentum.
pmb_phy said:
Oh, puleeze, not another thread-war on relativistic mass versus rest mass
No such war has taken place here in quite some time.

i dunno. i try to get a few licks in.

People come here and always ask ther same, or similar, questions and the answer lies in definitions. The most reasonable thing to do is to provide the definitions. The "war" that you speak of has to do with how the term "mass" should be defined or someone whining about the way someone else uses the term.

rbj said:
that isn't true, is it? the speed of the "massless" photon is always c no matter the frame of reference but, if you define the mass of a particle as momentum divided by speed, then that mass does not depend on the frame of reference and is a property of the photon, namely

$$m = \frac{p}{c} = \frac{h \nu}{c^2}$$

hmmm, i guess because the frequency $\nu$ of the wave is a function of frame of reference (doppler effect), but then also the momentum $p = \frac{h \nu}{c}$ of the photon is dependent on the frame of reference in the same manner and no one is saying that photons have no momentum.
Frankly I don't see why being observer dependant is a problem or concern to anyone in relativity. Many things are observer dependant such as energy and the lifetime of a particle, value of electric/magnetic field, relative velocity of on object relative to an observer etc. etc. etc.

Best wishes

Pete

rbj said:
i dunno. i try to get a few licks in.
There is plenty of discussion of it yes. But nobody getting angry at another poster. There are posters who get angry when the subject comes up, e.g. Oh, puleeze, not another thread-war on relativistic mass versus rest mass. vanesch makes it sound as if when a subject is discussed a few times it shouldn't be discussed again. However he doesn't seem to realize that we get newcomers all the time and referring them to an old thread is rather like blowing the OP off by pasting a reference to such post. I respond as simply as I can as a matter of curtisy the precise point/solution to their problem regarless if somene thinks I should or shouldn't speak about "that" subject. Otherwise what could be a short simple answer gets trampled on by someone who doesn't like the topic to come up. I recall one time that a poster was upset with me because I didn't say how many people use what in their work etc. Frankly that's a matter of statistics. However there is a paper which does give some statistics if anyone is curious.

Pete

pmb_phy said:
I think it means what it is defined to mean as defined in Wikipedia.

And Wikipedia is always right and has a better definition than Ashcroft and Mermin's Solid State text? I am the last person you want to shove a Wikipedia reference to.

I don't understand your confusion. Are you telling me that you don't know what momentum or speed is?

No, I asked YOU to use the definition that you have adopted as being universal and make sense of it when applied to the crystal momentum.

By the way, when I wrote "So according to Frenchl anything that has momentum has speed." It as a gross error on my part. That should have read "So according to French anything that has momentum and speed has inertial mass." Perhaps this is the source of your confusion??

This is not a unique place to see the term "inertial mass" defined in this way. Schutz also uses the term in the same way as French.

Then give me what I want with the crystal momentum!

You cannot argue something is wrong by citing your unpublished work. You are giving the misleading impression that someone ELSE also agrees with this and that this is part of a larger, accepted work. This is also in violation of our rules that you have to cite valid sources. And we define valid sources as being either peer-reviewed published works, or work that are part of standard accepted physics. Please do not do this anymore or those references will be edited out.

Zz.

ZapperZ said:
And Wikipedia is always right and has a better definition than Ashcroft and Mermin's Solid State text? I am the last person you want to shove a Wikipedia reference to.
I was merely telling you what definition I was going by. This is a relativity forum so what makes you think that I know what crystal momentum is? Do you actually think I claim to know every single term and concept that exists in physics today? If so then you need a reality check. Actually I've never even heard of this crystal momentum and I don't have a text on solid state physics and I never learned of it in either as an undergraduate or as a graduate student.
No, I asked YOU to use the definition that you have adopted as being universal and make sense of it when applied to the crystal momentum.
Tell you what. Why don't you provide the definition of momentum as it is used in relativity and apply it to your little example.

You cannot argue something is wrong by citing your unpublished work.
But I can argue something with logic and a mathematical derivation using the principles of relativity. Everybody does it here so there's no reason why I can't. I am not citing something by posting a link to a derivation which can't fit into one of these posts. It turned out to be much better for me since people are often asking the same questions so instead of re-writing the same answers over and over and over again I create a web page for it. Many people here have very much appreciated those pages and have bookmarked them. So you're saying you want to deprive work because you have a personal problem with me? No other mentor has had a problem with this for the time I arrived here except you today.

But the worst part of your attitude is that you're speaking from a point of ignorance. You should have looked at the pages before you complained about them. I never said that those particular pages were my work. In fact I post the relevant published work on which it is based at the bottom of the page. Otherwise I state that it is my derivation. There is nothing wrongwith posting derivations.
You are giving the misleading impression that someone ELSE also agrees with this and that this is part of a larger, accepted work. This is also in violation of our rules that you have to cite valid sources. And we define valid sources as being either peer-reviewed published works, or work that are part of standard accepted physics. Please do not do this anymore or those references will be edited out.

First off you really need to look at the pages you're whining about before posting this kind of irritating post.

In the second place there is is nothing wrong with posting a derivation/proof of something in a post. We all do it. However some derivations are too long and difficult to post in Latex in a post. It was for that reason I created those web pages - ease of explanation.

And if you delete these links in the future then that is my invitation from you to leave and I will happily do so. I come here only to help others in a field I love so much. I did not come here to listen to comments such as the ones you're providing today. I am not in a place in my life where I want to tolerate such comments such as these.

Since its rather easy to predict what the response to this post is then I say thee fairwell and I will end my posting career here at this point.

Bye bye.

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pmb_phy said:
I was merely telling you what definition I was going by. This is a relativity forum so what makes you think that I know what crystal momentum is? Do you actually think I claim to know every single term and concept that exists in physics today? If so then you need a reality check. Actually I've never even heard of this crystal momentum and I don't have a text on solid state physics and I never learned of it in either as an undergraduate or as a graduate student.

But you seem to have ignored my original complaint, that you have somehow adopted A UNIVERSAL DEFINITION of momentum and speed. I know what forum this is. However, to say that your definition only works for so-and-so and not when QM kicks in makes it a highly dubious claim. THAT was my argument.

Please note that in many papers regarding the definition of "mass" and "momentum", the QM issue was never neglected! How can they when it is also such a central issue of what we actually measure! How do you think a measurement of a photon's momentum is made? Would you care to speculate on the assumption made to enable the extraction of the photon interaction that produces such momentum? Or what about the "momentum" and "speed" of charge particles, especially elementary particles? What do you think is the most accurate means to determine such a thing?

ALL of those experimental observations on which anyone tries to based his/her ideas on ARE quantum measurements! So I find it highly incredulous that you think it is justifiable that somehow THAT part of physics can be safely ignored!

Tell you what. Why don't you provide the definition of momentum as it is used in relativity and apply it to your little example.

I'm NOT the one who goes around saying that I have a clear-cut definition of anything - YOU DO! I'm also not hung-up on any form of definition when the mathematical formulation is so clear that physicists don't go around using the WRONG formalism. Have you seen such issues popping up in physics? For example, when papers kept being published on the measurement of the upper limit of the photon "mass", do you see them using the WRONG formulation, regardless of what being measured is called? No? Thus, *I* personally have no use in those word games, so I am not the one here who is making a claim of anything regarding how people define "mass", "momentum", and "speed". Again, in case you forgot, you did! That was why I asked you to apply it in a particular situation.

But I can argue something with logic and a mathematical derivation using the principles of relativity. Everybody does it here so there's no reason why I can't. I am not citing something by posting a link to a derivation which can't fit into one of these posts. It turned out to be much better for me since people are often asking the same questions so instead of re-writing the same answers over and over and over again I create a web page for it. Many people here have very much appreciated those pages and have bookmarked them. So you're saying you want to deprive work because you have a personal problem with me? No other mentor has had a problem with this for the time I arrived here except you today.

But the worst part of your attitude is that you're speaking from a point of ignorance. You should have looked at the pages before you complained about them. I never said that those particular pages were my work. In fact I post the relevant published work on which it is based at the bottom of the page. Otherwise I state that it is my derivation. There is nothing wrongwith posting derivations.

First off you really need to look at the pages you're whining about before posting this kind of irritating post.

In the second place there is is nothing wrong with posting a derivation/proof of something in a post. We all do it. However some derivations are too long and difficult to post in Latex in a post. It was for that reason I created those web pages - ease of explanation.

And if you delete these links in the future then that is my invitation from you to leave and I will happily do so. I come here only to help others in a field I love so much. I did not come here to listen to comments such as the ones you're providing today. I am not in a place in my life where I want to tolerate such comments such as these.

Since its rather easy to predict what the response to this post is then I say thee fairwell and I will end my posting career here at this point.

Bye bye.

If you have noticed, pervect even had to point out your earlier reference to your unpublished papers. I would strongly suggest that you do not cite this UNTIL it is published.

I HAVE read your links, and quite a few times since you used it quite often. There is a difference between stating standard facts and stating OPINIONS! If you think YOUR definition of what a "mass" is is simply "logic", then it is you who are showing your arrogance, because there are point of views that disagree with you. Just look at the discussion on PF on this each time you brought it up! This is ample proof that it isn't just a matter of mathematical derivation that is contained in your links - you assert YOUR point of view of the physical meaning of such-and-such which may or may NOT be in line of what you'd find in a standard text.

If all you care about is truly trying to bring out the logical derivation of something that is constantly asked, then there are plenty of other legitimate sites that stick to addressing such issues. This includes the often-asked questions about photon mass! In fact, when such a question is being asked, it is often done by someone who is NOT familiar with basic SR. I put it to you that by pointing out what you have on your webpage will cause even MORE confusion than necessary. Why you can't just use the answer given in practically all standard SR textbooks (and already repeated many times on here) is beyond me.

Zz.

Jammer's virus

pervect said:
I would say that photons have a zero invariant mass. If pressed, I would admit that they have a non-zero "relativistic mass", though I would be quick to point out that I personally did not like relativistic mass.

But you asked about "inertial mass". I'm not quite sure what you mean by "inertial mass". At a guess, you are trying to divide the momentum of a photon by its speed (which is always 'c') and come up with a number. This number will depend on the frame of reference - it will not be a property of the photon alone.

My general remark would be this: photons carry momentum and energy. This should be clearly understood. The idea of "mass" is actually somewhat of an "umbrella concept" - the name "mass" is an "umbrella" which covers a large number of closely related, but different, concepts. You'll really need to learn about mass in Newtonian mechanics, mass in special relativity (invariant mass and perhaps relativistic mass) and mass in general relativity (ADM mass, Bondi mass, Komar mass) separately. To quote Max Jammer, "Mass is a mess".

For some online reading, samples of Max Jammer's two books on mass are available on Google. They appear to be some of the better non-technical references out there.

You can find the remark about "mass is a mess" at http://books.google.com/books?vid=I...x+jammer+mass&sig=rvmUlL1YxTTmzI_MxSEwoJB7fzo

The umbrella analogy is AFAIK mine, however.
I fully aggree with your first four lines.
Proposing Jammer somebody will tell you the following
High Energy Physics - Phenomenology, abstract
hep-ph/0602037
From: Lev Okun [view email]
Date: Fri, 3 Feb 2006 15:58:53 GMT (19kb)
The Concept of Mass in the Einstein Year
Authors: L.B. Okun
Comments: 19 pages, Presented at the 12th Lomonosov conference on Elementary Particle Physics, Moscow State University, August 25-31

Various facets of the concept of mass are discussed. The masses of elementary particles and the search for higgs. The masses of hadrons. The pedagogical virus of relativistic mass.
Full-text: PostScript, PDF, or Other formats
As I see, discussions on physics help take sometimes a very unpolite character. That is why I finish with
sine ira et studio

pmb_phy said:
And if you delete these links in the future then that is my invitation from you to leave and I will happily do so. I come here only to help others in a field I love so much. I did not come here to listen to comments such as the ones you're providing today. I am not in a place in my life where I want to tolerate such comments such as these.

Since its rather easy to predict what the response to this post is then I say thee fairwell and I will end my posting career here at this point.

Bye bye.
To me that is very unfortunate, I very much appreciated your explanations on GR issues. Let me know which forum you will post in the future by private email.

ZapperZ:
Zapper, I fail to understand your frequent harshness to people here. You are supposed to be a mentor but instead you come over as a very arrogant person.

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