[jopie64 (OP)]

The answer is no. But consider this:

Suppose in a few days when earth is about to end, earth is approached by an anti-matter earth which starts orbitting our earth. (They actually orbit each other because same mass.) Anti-earth is getting closer and closer to earth. Freaked out by this news people start evacuating earth. Afraid of being radiated away by the energy from the anihilation, some brave people manage to build a giant mirror box around the earths.

Now the question:
The moon is now orbiting the giant mirrorbox with the earths in it. But what happens to the moon when the earths anihilate each other inside the box?

[Pmb]

The answer is no.

Whether its yes or now depends on what one means by "mass." Sometimes the term "mass" refers to inertial mass and at other times it refers to gravitational mass (of which there are two kinds, active g-mass and passive g-mass). Part of the sense you're using it, it means gravitational mass.

Now the question:
The moon is now orbiting the giant mirrorbox with the earths in it. But what happens to the moon when the earths anihilate each other inside the box?

The mass in the box remains constant. The only thing that would change is the distribution of mass. A box of radiation of total energy E contains mass an amount of mass m according to E = mc^2.

The moon remains in orbit of the box because the box still has mass (both inertial and gravitational mass)

[jopie64 (OP)]

The moon remains in orbit of the box because the box still has mass (both inertial and gravitational mass)

Ok I'd agree. But what is 'generating' the mass? Individual photons dont have mass, thats what I've been told. So alot of photons would still not have mass right?

[yor_on]

heh, welcome to the world of mathematics Photons are energy, matter are convertibles into energy too. Does 'energy' have mass?

[jopie64 (OP)]

Does 'energy' have mass?

Thats what I dont understand. Photons dont have mass. But they have energy. But energy IS mass (E=mc2). So they have mass.

??

[Pmb]

Ok I'd agree. But what is 'generating' the mass?

The photons and the box itself.

Individual photons dont have mass, thats what I've been told.

As I explained in post #2, whether a photon has mass or not depends on what one means by the term “mass.” In this case, i.e. the gravitational mass of one or more photons is nonzero. When it is said that a photon has zero it is referring to the proper mass of the photon. The proper mass of any particle of momentum p and proper mass m is given by

One of my pages with all the math worked out might help you understand these issues

So alot of photons would still not have mass right?

Wrong. See above.

If I were you I’d use the more precise term when you’re taking about an issue in which one or more of these definitions are being used at the same time.

[UltimateTheory]

The answer is no. But consider this:

Electron, according to Standard Model, has mass. Small, but still.

So the same Positron.

When Electron will hit Positron they will emit photons.

Conclusion must be simple- photons also must have mass.

Otherwise conservation of energy and conservation of mass we would have to consider as incorrect.

[Pmb]

Electron, according to Standard Model, has mass. Small, but still.

Let's focus in on the main problem with threads like this. People who make assertions like this always refrain from defining the quantity that they're defining. And when they do then the entire thread degenerates to a conversation where they assert that their definition is right and those who disagree with them and/or use another definition are just plain wrong/ignorant/don't understand "modern" definitions or something else derogatory.

This is really a thread whose subject matter has been repeated wy too many times in the past. The subject comes up on a monthly basis. That's why I wrote this article
http://arxiv.org/abs/0709.0687

It addresses all points of debate/discussion in nearly all these debates/discussions. And make no mistake - Both common definitions are widely used in the physics literature and in universities across the entire face of the earth.

All one really needs to do is read all the subject lines in this page that I created
http://home.comcast.net/~peter.m.brown/sr/sr.htm
and you'll be well versed on the topic. Do yourself a favor and give it a whirl

[lightarrow]

Photons are energy,

No. A physical object cannot be one of its properties.

Does 'energy' have mass?

If it's stationary, yes. If not, you need more informations to answer the question. Notice however that, even here, you shoud better have written "Does on object which has energy also have mass?" Because a property of a body cannot have another property!

[lightarrow]

Thats what I dont understand. Photons dont have mass. But they have energy. But energy IS mass (E=mc2).

Wrong. Energy is "energy" and mass is "mass".

So they have mass.

If you mean invariant mass, no, they don't have.

[lightarrow]

Electron, according to Standard Model, has mass. Small, but still.

So the same Positron.

When Electron will hit Positron they will emit photons.

Conclusion must be simple- photons also must have mass.

But unfortunately the conclusion is wrong...

Otherwise conservation of energy and conservation of mass we would have to consider as incorrect.

When a pair particle/antiparticle annihilates, it generates a couple of *two* photons. A system of such two photons *has* mass:

E² = (Mc²)² + (cP)²

E = energy of the two photons' system = E₁ + E₂ = 2E₁, with two equal photons, where E₁ is a single photon's energy (energy is additive).
M = mass of the two photons' system.
P = momentum of the two photons' system = P₁ + P₂ where P₁ and P₂ are the momenta of the  photon 1 and 2, respectively.

A single photon's momentum is, in modulus: |P₁| = |P₂| = E₁/c.

So, if the two photons are not travelling in the same direction:

|P| = |P₁ + P₂| < 2|P₁| = 2E₁/c

so

P² = |P|² < 4E₁²/c²   →   -P² > -4E₁²/c²

(Mc²)² = E² - (cP)² = (2E₁)² - c²P² > 4E₁² - c²4E₁²/c² = 0

so

(Mc²)² > 0

that is

M > 0.

[Pmb]

When Electron will hit Positron they will emit photons.

Electrons and positrons don’t emit photons. They annihilate each other and two or more photons are all that will remain after the annihilation.

Neither an electron nor a photon can absorb or emit a photon. Although this is a common misinterpretation of what goes on in Feynman diagrams in QED it never actually happens in real life. Those are what can best be called virtual processes.

"photons also must have mass." - Inertial mass, yes. Proper mass, no. You're nearly always referring to inertial mass is the examples you give (inertial mass of photon is never zero) but when you speak of the mass of a photon being zero you've changed to the definition you’re speaking of to “mass” means “proper mass.” Don't you find that confusing?

Let me give you another example of having to be cautious with what a word means from other areas of physics: Non-Relativistic quantum mechanics. If we were talking about non-relativistic quantum mechanics and I asked you if the momentum of a charged particle in an EM field was a function of its charge, what would you say?

[Pmb]

I'm going to say this one more time and only one more time - Please stop confusing the fact of whether a photon has mass or not.The only thing I see you folks worry about and wasted an entire thread for is symantics. This whole thing has been only about terminology. Nothing about it is physics when all you're talking about is whether a photon has mass or not. No physicist worth his salt worries about this trivial nonsense. If you were to ask one who is worth his salt and has no alterior motives then all phycists would tell you that the proper mass of a photon is zero and that its inertial mass is nonzero (inertial mass is sometimes refered to as relativistic mass). Often the paticle physics people will steal the definition of "inertial mass" and define it for there own purposes. Mostly to win arguements of this trivial nonsense. I sure wish you'd all stop wasting space on this non-sense. All that you will wind up with is people in different fields wavig banners that they've won a debate and are now free to define mass as they want to. Then then the people from the other side of the debate will do the same thing and everbody will still end up with contradiction in terms.

So please stop dragging physics down with such trivial nonsense. It is unworthy of us all and a wase of breath. Can't you all find someting more constructive to do with your time then spend all day worring about semantics? Sheesh. Shame on you all.

[yor_on]

Maybe Lightarrow, but a photon definitely is a energy to me. In fact it is the closest thing I know to a classically, spatially localized, energy in its outcome (when measured). It has no size, no rest mass, it only consist of a momentum/energy. If I assumed a static universe the momentum and the energy (sh)would be undifferentiated as I think.
==

We have the definition of light quanta though, in where we have discrete 'energies' related to the frequency we find light to have. And that makes it hard on a static universe, as the speed of light is a constant one in a vacuum.  The momentum of a light quanta is not related to the intensity of light, or speed, but only to its frequency as I understands it. The intensity is, from a particle perspective, explained as a result of 'more photons over time' (of a same frequency) hitting the device measuring. And assuming a static reality one then has to find a reason to why this is possible. How would a light quanta be able to present different energies in such a universe?
=

I'm ignoring the speed of light for a reason, it's a constant 'c' in relativity, therefore the same no matter what energies we measure a light quanta to have. And using locality as a guiding principle then it is 'c' everywhere

[lightarrow]

Maybe Lightarrow, but a photon definitely is a energy to me. In fact it is the closest thing I know to a classically, spatially localized, energy in its outcome (when measured). It has no size, no rest mass, it only consist of a momentum/energy.

It's not true that it only consists of momentum/energy: it also has a spin, for example. Furthermore, photons with the same (average) energy can have different spectrum's broadness: more or less monocromaticity. I'm not talking of average energy of a set of different photons, but of average energy of photons *all equal*. Photons emitted by the source 1, all equal, will have a spectrum 1 of their energies; photons emitted by the source 2, all equal, will have a spectrum 2 of their energies (for example gaussian curves with different broadness).

[AndroidNeox]

Light has no rest mass, but it has momentum. Momentum is what generates gravity. If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.

[yor_on]

Maybe?

Are you defining a single photon/lightquanta as having a spectrum Lightarrow? That should be from a wave perspective if so, right? As you say it has what we call Spin/polarization but thinking of it, all of those definitions come from treating photons as waves, don't they?

To me it's a packet of energy, or excitation in a field maybe? And it definitely has a momentum.

And Android, If momentum is 'gravity' I don't know btw But as we assume that certain 'systems' of light quanta can present us with a proper mass, and as we know that proper mass and gravity goes together? But the universe we acknowledge is all made up from those light quanta, impressing themselves on our senses, so in one sense no. I think 'energy' is a better description for 'gravity' if so, but it's not perfect as gravity in itself is nothing at all except a preferred direction having different 'slopes', depending on the mass attracting.

You made me think, and wonder, some more there Lightarrow
How many photons does it take to measure a linear polarization? 

[Pmb]

Light has no rest mass, but it has momentum. Momentum is what generates gravity.

There are three things which generates gravity

1) mass-energy
2) momentum
3) stress

Light has all of these

If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.

Edit: I wrongly said "I disagree. The pressure inside the box would increase drastically and that pressure is a source of gravity."  The mass associated with the stress due to the box containing the photon gas cancels out the mass contribution due to the increase in pressure. Since there is no box containing a star then the pressure inside a star contributes to the mass.

[Pmb]

You're forgetting the relationship E = mc² which means that whatever has inertial energy E has inertial mass m and what has inertial mass m has inertial energy E.

[lightarrow]

Light has no rest mass, but it has momentum. Momentum is what generates gravity. If you put a kilogram of matter and one of antimatter into an impregnable box (like a Schrödinger cat box) and they annihilate each other, the gravitational mass of the box will not change, even if it's only full of light.

Yes, but what this has to do with the a photon's properties? For example, with that, do you believe to have proved that a photon warps spacetime?