< Photon

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A Photon is a particle of very little mass that likes to travel at the speed of light. After all that it is what light is, photons. These particles appear to be everywhere. They carry energy with them when they are let loose. Photons can come from such places as combusting fuel (gas). In order to see fire, photons are given off. How exactly this mechanism takes place, we do not know. There are many theories on how such occurrences take place, but until you can prove something it is nothing more than a bedtime story.

Science is all bedtime stories, eh? In a main article like photon?

As a result of their size photons can pass through several different types of matter (with certain structures, one being glass) without even seeing it.

This isn't true. Photons, even photons of visible light, interact with glass - you can tell because they move slower through it, are refracted by it, and so forth. And size has nothing to do with it, as all subatomic particles are more than small enough to slip through glass.

Changed 'diffraction' back to 'refraction'. It really is refraction of light that causes rainbows. I'll explain the mechanism when I get around to doing rainbow.

Photons do not have mass. Photons have energy. Some claim that E=mc2 means that they are the same thing, but this is not a widely accepted theory. Gravity simply couples energy, of which mass is a type. Photons do not have that type of energy. It is interesting to note that two photons can have mass, even though one does not.--BlackGriffen

Does it matter if one or both of them is Catholic?

--- BlackGriffen: you write "It is interesting to note that two photons can have mass, even though one does not." I'd be very interested to know how this works. -- SJK

In relativity mass and energy are the same thing. It is customary to separate out kinetic energy, which stems from the motion of the system in question as a whole, and invariant mass, which stems from its simple existance. In composite systems, though, there can be binding energy which raises the mass relative to the sum of the components. Actually, though, I'm not sure how photons can get stuck together like that... --- My bad, guys, I think. But it's nice to see that Wikipedia is working in removing the (accidental!) piece of bullshit that I put in. Any chance of expanding upon why photons don't have mass, and why they're affected by gravity, which only affects mass-carrying particles if they don't?

And is invariant mass just a better, non-flawed description for rest mass, or are they something subtly different? -- Same thing, but invariant mass is the preferred term, as rest mass carries the strong connotation of the mass at rest and that doesn't make any sense for a photon.

With thanks, Dave McKee.

I'm not sure how two photons having mass arises. I recall two things a theoretical physicist said at a colloquium: 1, two photons can have mass; 2, it has something to do with the physical arrangement. My guess would be that when two photons are moving in opposite directions through the same space, they can create a standing wave. At least, two hertzian waves can. In order to be standing and still have energy, it must have mass. Don't quote me yet, though.

"Any chance of expanding upon why photons don't have mass, and why they're affected by gravity, which only affects mass-carrying particles if they don't?"

Certainly. I don't recall the experimental evidence, but here is the theoretical background:

p = γmov
E = γmoc2
γ = 1/√(1-(v/c)2)

Photons travel at c, right? Well, sticking that in the above equations, which all particles (including photons) must obey, yields an infinite γ. If the photons have any finite non-zero mass, they would have an infinte energy and momentum! Last time I was knocked over by a photon... ;) In all seriousness, the reason they are affected by gravity is because gravity doesn't couple with mass, it couples with energy (of which mass is a type).

The idea that energy and mass are the same comes from one of these two fallacies:

mv = γmov
mc2 = γmoc2

leading to:

m = γmo

Essentially, an attempt to redefine mass so that the old equation for momentum works, the idea that gravity couples to mass still holds, and that it was consistent with the new rest mass energy formula (sticking in v = 0). The only thing Einstein said was that the total energy of the system is:

E = γmc2

So you might say that in order to measure mass you need to bring the system being measured to rest. Already the idea of a photon having mass runs in to trouble since a photon can't be brought to rest (the easiest way to imagine trying to stop a photon is by trying to move your reference frame at c in the same direction [never mind the practical problems] as the photon. The dopler effect and other relativistic spatial transformations will reduce the frequency [and thus the energy] to zero).--BlackGriffen

Is it really true that photons are slowed down when moving through a medium? I thought that they are constantly absorbed and reemitted, so it's not really one and the same photon moving through and being slowed down. --AxelBoldt

Not sure about in QM, but IIRC in classical oscillator theory, photons move at c (i.e. vacuum speed of light) between atoms. They're not really absorbed and re-emitted by the atoms, though. What happens is that the electric field of the photon drives charges into oscillation, and those oscillating charges radiate a field which is slightly out-of-phase with the photon. The superposition of the photon and the radiated field is slightly retarded w.r.t. the original field, and so the photon is 'delayed' a bit at each atom. On a large enough scale, this looks like the photon is slower. -- DrBob