[paul.fr]

does it?

[ukmicky]

NO

[syhprum]

According to my calculations a cubic meter of sunlight in the vicinity of the earth has a mass of 0.5*10^-22 Kg or at least that is the mass equivalent of the energy it contains

[lightarrow]

does it?

If it moves in one direction only no.

[Bored chemist]

Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

[ukmicky]

As far as rest mass goes the answer is no as nothing with rest mass can ever travel at C

[ukmicky]

Just found this which explains it much better than i could.

http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

[Bored chemist]

Since photons are never at rest, the rest mass of a photon probably isn't what was being asked about.

[ukmicky]

Since photons are never at rest, the rest mass of a photon probably isn't what was being asked about.

But the only other type of mass is relativistic Mass but that isn't really Mass in the correct sense of the word so therefore a photon has 0 Mass.

[syhprum]

I have read the article quoted by Ukmiky and can now to be consided a convert, I realise that I have to capture my cubic meter of sunlight in a perfectly mirrored box before its mass becomes apparent.

[lightarrow]

This page also explain very well what is mass and its strangeness:
http://www.mathpages.com/home/kmath232/kmath232.htm

A photon has no rest mass, which implies that the Minkowskian norm of its energy-momentum vector is zero. However, it does not follow that the components of its energy-momentum vector are all zero, because the Minkowskian norm is not positive-definite. For a photon we have E2 - px2 - py2 - pz2 = 0 (where E = hn), so the energy-momentum vectors of two photons, one moving in the positive x direction and the other moving in the negative x direction, are of the form [E, E, 0, 0] and [E, -E, 0, 0] respectively. The Minkowski norms of each of these vectors individually are zero, but the sum of these two vectors is [2E, 0, 0, 0], which has a Minkowski norm of 2E. This shows that the rest mass of two identical photons moving in opposite directions is m0 = 2E = 2hn, even though the individual photons have no rest mass.

[syhprum]

I have read the article quoted by Lightarrow and my head is in a whorl, I am not going in future even to read any questions concerning mass, energy etc.

[lightarrow]

I have read the article quoted by Lightarrow and my head is in a whorl, I am not going in future even to read any questions concerning mass, energy etc.

The concept of mass is indeed less obvious than what we thougth; we should "digest" it a little at a time.
(My compliments to have read all the paper!)

[ghh]

The principle of equivalence comes from the equation
      E^2 = m^2c^4    +    p^2c^2                
In which m is mass, p is defined as momentum, and c is an absolute velocity:   
It is generally accepted that electromagnetic phenomena have “momentum, but no mass”, the proof being
E2 = 02c4    +    p2c2
therefore    √E2 =  √( p2c2)
and so       E = pc                        (1.0.1)

and similarly “rest” mass has no momentum, therefore
      E2 = m2c4    +   02c2
      E2 = m2c4   
      E  = mc2                         (1.0.2)               
However, the definition of p, momentum, is mass x velocity, albeit in this case the velocity has the absolute value “c”.

So in SI units    pc = Kg.(m.sec-1)(m.sec-1), and mc2  =  (Kg.m2.sec-2)      (1.0.3)      
                           
These are dimensionally identical, so the solutions (1.0.1) and (1.0.2) do not seem to be legitimate, although they are claimed by experiment to be “proved”. If this is the case, and E = pc and E = mc2 are valid then either (pc)2 + (mc2) = E2 and-a-bit, or if equation (1.0) is to be upheld, the possibility has to be considered that  m has to be non-zero, and c has to be variable.

[lightarrow]

The principle of equivalence comes from the equation
      E^2 = m^2c^4    +    p^2c^2                
In which m is mass, p is defined as momentum, and c is an absolute velocity:   
It is generally accepted that electromagnetic phenomena have “momentum, but no mass”, the proof being
E2 = 02c4    +    p2c2
therefore    √E2 =  √( p2c2)
and so       E = pc                        (1.0.1)

and similarly “rest” mass has no momentum, therefore
      E2 = m2c4    +   02c2
      E2 = m2c4   
      E  = mc2                         (1.0.2)

It was good up to here.

However, the definition of p, momentum, is mass x velocity,

That's wrong.

albeit in this case the velocity has the absolute value “c”.

So in SI units    pc = Kg.(m.sec-1)(m.sec-1), and mc2  =  (Kg.m2.sec-2)      (1.0.3)      
                           
These are dimensionally identical, so the solutions (1.0.1) and (1.0.2) do not seem to be legitimate, although they are claimed by experiment to be “proved”. If this is the case, and E = pc and E = mc2 are valid then either (pc)2 + (mc2) = E2 and-a-bit, or if equation (1.0) is to be upheld, the possibility has to be considered that  m has to be non-zero, and c has to be variable.

I didn't understand anything of this.

[felixtheferret]

Slightly tangential, but interestingly mass still remains the one SI unit that cannot yet be defined in terms of something else:  'mass' is DEFINED by the kilogram sample of platinum alloy kept in a box in Paris !   No-one really knows, at a fundamental level, what mass 'is.' You may as well ask what a quark is made of , or what a magnetic field is made of, these questions have no answers in current physics.  Think of it like this:  as a particle (say an electron) speeds up, it slowly gains mass according to GR. just exactly how is this 'mass' added?  - little tiny bits of electron magicking themselves out of nothing and sticking to the surface of the moving electron?!!  Very unlikely.  'mass' at the particle level becomes a purely mathematical abstraction - don't let anyone kid you otherwise :-)  Therefore for all intents and purposes a photon DOES have mass, purely by virtue of it's motion, i.e. its energy.   

[lightarrow]

Slightly tangential, but interestingly mass still remains the one SI unit that cannot yet be defined in terms of something else:  'mass' is DEFINED by the kilogram sample of platinum alloy kept in a box in Paris !

Time is not also defined in such a way? Can you define time in terms of something else? And current intensity?

   No-one really knows, at a fundamental level, what mass 'is.' You may as well ask what a quark is made of , or what a magnetic field is made of, these questions have no answers in current physics.  Think of it like this:  as a particle (say an electron) speeds up, it slowly gains mass according to GR.

Sorry, that's wrong. Most physicists (Einstein too) agree on calling "mass" just the rest mass and not the relativistic mass (I have barred that name just because it's now considered wrong).

just exactly how is this 'mass' added?  - little tiny bits of electron magicking themselves out of nothing and sticking to the surface of the moving electron?!!  Very unlikely.  'mass' at the particle level becomes a purely mathematical abstraction - don't let anyone kid you otherwise :-)  Therefore for all intents and purposes a photon DOES have mass, purely by virtue of it's motion, i.e. its energy.

A photon is mass-less.

[JP]

I don't think physicists yet understand what generates inertial (rest) mass.  They've proposed a "Higgs field" in high energy physics, which is the most promising theory (the field couples to fundamental particles and the interaction gives them mass), and I'm sure there are other models out there. 

On another note, I'm pretty sure there are cases where a photon can interact with things as if it has mass: via wierd quantum mechanical fluctuations in which the photon generates fleeting "virtual particles" which have mass. 

But on the original topic, a photon itself has no mass.

[lightarrow]

Last time I checked mass and energy were equivalent; doesn't that mean that light, which clearly has energy, must have mass?

I have discovered not long ago how the equation E = mc² is very often misunderstood (and it was also by me); it must be considered in this way:

If you have a stationary body or physical system with a total rest mass M, and you give it some energy E, so that the system remains stationary, then it gains a rest mass m equal to E/c².
So the new mass is M + m = M + E/c².

Examples:
1. You give heat to a stat. body.
2. You spin it.
3. You make a cavity with internal reflecting walls, then, through a tiny hole, you shoot a light beam inside of it.
4. You compress a spring and you let it stay compressed with a clamp. The mass of the system "compressed spring + clamp" is greater than that of the system "non-compressed spring + clamp".

[edward2007]

If light has no mass, how would you explain the working of a solar sail?