# Massless particles

1. Jun 28, 2009

### monty37

in what way are gauge bosons ,photons and gluons massless?
please explain the significance of the massless nature,i mean every particle is supposed
to have mass?

2. Jun 28, 2009

### Civilized

In the standard model, particles are described as excited states of a (quantum) field. A simplified picture of a field would be wiggly sheet of rubber. We can distinguish between two kinds of fields:

(Massless) This is a sheet of rubber that is so free to wiggle that no matter how lightly you touch it you will set off a vibrational motion.

(Massive) This is a sheet of rubber that is not so free to wiggle, and in fact if you poke it with a strength below some threshold (i.e. the mass gap) it will not wiggle at all.

Therefore gluons and photons are massless because they can be formed with arbitrarily low amounts of energy (i.e. you get a wiggle no matter how softly you push).

It's not true that every particle must have mass, all a particle needs in order to interact with matter is to carry energy/momentum.

3. Jun 28, 2009

### Fredrik

Staff Emeritus
Every particle satisfies $E^2=\vec p^2c^2+m^2c^4$, where m is the mass, p the momentum and E the energy. So if a particle satisfies $E^2=\vec p^2 c^2$, it's massless.

The fact that every particle must satisfy that equation is a part of the definition of "particle" in relativistic quantum mechanics, which is rather technical.

4. Jun 28, 2009

### diazona

Interesting analogy, I like it

5. Jun 28, 2009

### samalkhaiat

6. Jun 28, 2009

### Civilized

7. Jun 28, 2009

### daisey

Monty,

This revelation surprised me, too. Please - Anyone please correct me if I say this wrong.

One thing to remember about mass is that in the quantum world it is a gauge of an objects inertia. A particle that is mass-less instantaneously accelerates to the speed of light, and travels only at this speed. Surprisingly, though, a mass-less particle CAN be affected by the force of gravity, as photons are.

In the macro world most non-physicists (including myself) tend to relate mass to weight. I don't believe this correlation is as closely related in the quantum world.

Daisey

8. Jun 28, 2009

### Civilized

It's true that massless particle travel only at the speed of light, but there is nothing in physics that corresponds to an 'instantaneous acceleration' for massless particles - they never accelerate at all.

This does not have to be so mysterious -- if you cast a ripple in a shallow pool of water then the ripple does not accelerate, it always propagates at a characteristic speed (that depends only on the density of the material and the depth of the water, for shallow water waves).

In the classical theory of light as an EM wave, there is no notion of the waves accelerating to speed c, they just always travel at that speed. This is the way that waves work in general, they travel at a characteristic speed that depends on the medium (vacuum in the case of light waves).

9. Jun 28, 2009

### monty37

well,how were these particles detected experimentally,as they have no mass?
iam sorry but i just can't digest that"massless" factor,please elaborate

10. Jun 29, 2009

### Civilized

The thing is that they carry energy and momentum. Those massless particles can travel through space and give us sunburn at the beach! A laser can cut through steel using massless particles, because they can carry lots of energy. More generally, massless particles are detected when they smash into targets and transfer energy and momentum to them.

11. Jun 29, 2009

### daisey

Even with no mass, they have other attributes that make them "noticeable" to experiments.

12. Jun 29, 2009

### Bob_for_short

A massless particle (a photon, for example) can travel with v<c in water, glass, and in other "transparent" materials with n>1. There is a reference frame where such a photon is at rest: there the photon is just a standing electromagnetic wave.

In relativistic mechanics the systems exchange with energies while interactions, not with masses, so the notion of mass is somewhat secondary. The main feature of a particle is posessing the energy-momentum to exchange (to receive and to give away).

Bob.