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View Full Version : Where have all the photons gone; Long time passing . . .



mkline55
2013-Jan-28, 04:20 PM
While you are humming along, maybe someone could answer this. I think I could get a consensus that every photon has a source. Can a photon exist without an ultimate destination? If a photon is a thing which carries energy from one place to another, then can a photon exist if that "another" place does not exist? Is this question addressed anywhere in QM, GR, SR, etc.?

From our own reference frame, all the photons we see have a known speed, and exist for a length of time. As I understand it, the current math does not provide/permit a reference frame for photons. So there is no way to say that a photon is created at a source and destroyed at a destination simultaneously in its own reference frame.

If photons can exist without a destination, then though a photon carries energy, is that energy simply carried away from all matter never to be "seen" again?

Hornblower
2013-Jan-28, 07:09 PM
A photon that doesn't encounter another particle of some sort is like the Eveready bunny. It just keeps on going, and is a component of the total energy that is present in the cosmos. That energy will be "seen" only when it interacts with something, at which time it is transformed into another form of energy, such as heat or chemical energy.

mkline55
2013-Jan-28, 09:34 PM
Thanks, Hornblower. That's the natural conclusion. But the question was more one of QM, and in my limited experience, QM has so far defied my naturally limited somewhat-logical conclusions. Can a photon even be created if it has no destination? Has that question been addressed in mainstream theory?

Tensor
2013-Jan-28, 10:06 PM
QM has discrete operators (creation and annihilation operators). The creation operator increases the number of particles, the annihilation operator decreases the number of particles. There are separate rules concerning bosons and fermions(since fermions cannot occupy the same quantum state).

Take a look here (http://www.phys.ksu.edu/personal/wysin/notes/quantumEM.pdf) for an introduction.

mkline55
2013-Jan-29, 01:58 PM
Thanks, Tensor.
The paper addresses the rates of emission and absorption but does not appear to make a statement about a direct connection between the two. Would it be fair to conclude that QM makes no statement that requires that the emission of a photon requires a subsequent absorption?

profloater
2013-Jan-29, 03:10 PM
Looking around, there are lots of stars in every direction. If we can see one star at millions of miles distance then assuming spherical emission there are really a lot of photons criss crossing in every direction. If space is continuous and effectively infinite, then I suppose it all balances out, every photon eventually hits a star or interstellar dust. If they can escape the physical bound universe, then that would be a net loss. If the "energy" universe is expanding at the speed of light, then there is an advancing frontier of photons. If the photons curve in space time it becomes more complicated. If a photon is a light speed particle, then it's receiver has no knowledge of the source until it gets hit. That's what Einstein said. So it would seem each photon sets out on on a voyage with no known destination. And that must always be true if Einstein was right about that. But then that weird action at a distance thing.............

neilzero
2013-Feb-23, 08:34 PM
When a photon is created it's destination is often unknow, but I suppose half of the photons that have escaped the atmosphere, be it planet, star or other, will find a destination in a million times a million times a million = 10^18 years if not sooner. Possibly there are a google photons enroute in our galaxy; 10^120 in the knowable Universe including those traveling between the galaxies. Neil