As far as i can tell light is affected by gravity. If light is affected by gravity, doesn’t that mean it has mass? If it does have mass, why doesn’t friction slow it down?
Photons have no rest mass, but a photon (travelling at the speed of light) does have mass . Friction is a phenomenon that only applies to objects made up of (many) atoms or molecules. Light does, in fact, slow down when traveling in a medium, but not because of “friction”. It slows down because it is absorbed and re-emitted by the atoms in the medium.
Light follows the short path in spacetime and mass makes that spacetime curve. So while the massless photon appears to bend towards a mass it’s only because that’s the shortest path.
If I recall correctly, there is something called relativistic mass.
The “mass=energy” nonsense follows from the hoary old equation:
E=M(c^2).
If photons have mass in any sense, it’s relativistic mass.
BTW, Relativity demands that photons have 0 rest mass*. And object with non-zero rest mass would have infinite mass if it traveled at the speed of light.
*The mass you measure when the photon an you are traveling at the same velocity.
Even travelling at the speed of light, it’s not really correct to say that photons have mass. They can carry momentum, nonetheless.
Photons are affected by gravity because gravity itself affects space, and photons follow the geodesic curves, going in what appears locally to be straight lines.
As for friction, it’s odd to try to apply the concept to a photon. “Friction” is an energy-dissipating mechanism. A big solid object can lose energy, but a photon that loses energy due to an inelastic process changes its identity. It doesn’t become a similar photon, travelling at a slower speed – it becomes a less energetic photon of a different wavelength (color) travelling at the same speed. But most people would view it as a different photon emitted by the process that absorbed the first one. When you get down to the quantum level, things get very strange, and concepts like simultaneity, identity, and precise position start to lose their meaning. The quantum world isn’t the world of our normal experience, writ small. It’s a wholly different environment.
Interesting choice of words. The Dancing Wu Li Masters has chapters on relativity entitled Special Nonsense and General Nonsense.
Does a photon at rest have any energy?
No such thing as a “photon at rest”. Photons move at the speed of light in their medium. If you lower the energy of a photon, it doesn’t slow down, it just goes to a longer wavelength (see my post above. equivalently, you can absorb a photon and re-emit one of lower energy = longer wavelength, the rest of it going into your system.)
Photons can’t be at rest.
In modern terminology, “mass” is generally used as synonymous with “rest mass”. What an object has when its moving, we just call “energy”.
By this standard, a photon does not have mass. However, a system of two or more photons in general does have mass (unless the photons are all moving in the same direction). Mass can be defined as the energy a system has in the reference frame where it has no momentum (also sometimes called the center of mass frame). With a single photon, there is no zero-momentum frame: If you take a frame that’s moving fast enough, you can get the momentum and energy arbitrarily small, but you can’t have a reference frame moving at the speed of light. So that’s why we say a photon is massless. But if you have multiple photons moving in different directions, then there is a zero-momentum frame, and in that frame, the energy still isn’t zero. So the energy that’s still left in that frame is what we call the mass.
How then, does a photon come into existance? Say you have a laser. You put some electricity through the noble gas and photons are emitted. Where were those photons before the electrons came barging through?
Photons get created when a system changes its ebnergy state. So a hydrogen atom going from an excited state top its ground state will lose the energy in the form of a photon. Energy is conserved. Later on, that photon can get absorbed elsewhere, and that system will gain energy.
Photons aren’t the only way this can happen. For instance, Energy can also be absorbed or emitted in packets of vibrational energy called phonons, which are quanta of lattice vibrations.
My head hurts.
Wouldn’t the mass of the universe be lost as it is converted to photons? If the universe is infinite, then all the photons that are created which don’t hit something and get reabsorbed will just keep going into the nothingness and eventually all the mass which is energetic enough to emit energy will “evaporate” away as energy.
Except that photons themselves can decay. For instance, they can decay into positron-electron pairs where the total energy of the system is equal to the energy of the original photon.
So would that return the mass back to the system? If it did, would the electron-positron pair still be hurtling through the nothingness leaving the rest of mass in the universe behind? I guess gravity might take hold and return it (eventually) to the stars, planets, quasars, etc… of the “known” universe.
I didn’t think so, but it was stated:
This is true only through mathematic extrapolation (as John Mace kinda indicates)
It made me think what a photon would be at rest: no mass and no energy? That’s like the definition of “nothing.”
(Maybe all that empty space in the universe is actually a bunch of photons at rest.)
But no mass* is created or lossed when a photon is emitted. The atom has gone from a higher energy state to a lower energy state, with the difference in energy being made up by the photon that was emitted.
Einsteins equations do tell us that mass can be converted into energy, but that isn’t what’s happening in this case.
*even in the representation where we say that photons have mass, they still have no rest mass, and so the net creation of (rest) mass = 0.
gaygnu: Yes, that’s correct. You can speak of rest mass vs mass as a way of keeping the math correct, but as others have stated, it’s more correct to just say that photons have no mass, period.
So let me see if I get this right: energy had to go into the atom to bump it into the higher state. So when it drops to a lower state, the energy given up as a photon balances the equation and no mass is lost.
I still wonder about those poor photons lost out in the unreachable void. ***Something *** is being lost, never to come back, surely. If it is just energy and not mass, then is this what they mean by the entropy death of the universe?
Yes!
Why is something being lost? As long as the photon exists in the universe, it doesn’t matter where it is. There is no such thing as an “unreachable void”.