# What is mass

1. Jun 21, 2011

### bodhi

e=mc2
mass is same as energy along with a multiplier c2
when fusion takes place photon is lost n mass is decreased
can we then say mass is made of light,or mass is light conglomerated or saturated.

2. Jun 21, 2011

### Polyrhythmic

Mass is equivalent to a certain amount of Energy according to E=mc², and can can be transformed into other forms of energy through processes like fusion. To say that mass is made of light is not quite correct.

3. Jun 21, 2011

### bcrowell

Staff Emeritus
Please use standard punctuation and grammar when you post on PF, not text-speak.

Photons are typically emitted in nuclear fusion, but other things are emitted as well, typically neutrons.

It's true that a collection of photons has a nonzero rest mass. (Rest mass is not additive.) However, there is no way, for example, that you can model a proton or an electron as a collection of photons, since photons don't have charge.

Light does not play any fundamental role in relativity or in physics in general. Einstein's 1905 axiomatization of SR gives a fundamental role to light, but with 106 years worth of hindsight, that was a mistake. Light simply happened to be the only known fundamental field in 1905.

-Ben

4. Jun 21, 2011

### Polyrhythmic

Could you please elaborate? The speed of light is an upper bound on the spectrum of velocities we can access, how is this not fundamental? The speed of light is still a constant, even a century after its theoretical introduction.

5. Jun 21, 2011

### Staff: Mentor

The speed c is fundamental, but not because it is the speed of light. Light happens to travel at that speed because photons have zero invariant mass. Other particles with zero invariant mass would also travel at speed c. If photons had nonzero invariant mass, they would not travel at speed c, but c would still be a fundamental speed in relativity.

6. Jun 21, 2011

### WannabeNewton

When Einstein was around, the only other field known was EM so it was postulated that nothing could travel faster than light or a photon. Now we know that this is not unique to EM or to a photon because all massless particles travel at that speed. So while the value itself is equal to that of the speed of light, it is not unique to light and in that sense the speed of light is not fundamental. I don't know if that is the subtlety you were after.

7. Jun 21, 2011

### Passionflower

What other massless particles are you referring to?

8. Jun 21, 2011

### WannabeNewton

Even if they aren't all observed as free particles, all gauge bosons have no rest frame so them.

9. Jun 21, 2011

### Staff: Mentor

Among the gauge bosons, the W and Z have mass.

10. Jun 21, 2011

### WannabeNewton

Sorry I meant to say of the gauge bosons, so just the gluon and the photon.

11. Jun 21, 2011

### bcrowell

Staff Emeritus
I suppose the graviton could be included as well, although it can't be directly detected by any foreseeable technology.

-Ben

12. Jun 21, 2011

### Polyrhythmic

That is true, but when we talk about the speed of light, we mean c.

13. Jun 21, 2011

### Polyrhythmic

I know what you mean, but it's still correct to refer to that fundamental quantity as the speed of light.

14. Jun 21, 2011

### Polyrhythmic

If you're referring to the graviton as a quantized gravitational wave, this might be correct. Gravity hasn't successfully been quantized so far though.