# How do we know that there is more than one photon in the universe?

Excuse this possibly dumb question and correct me - i am not a physicist, but i wondered: A photon is massless and travels with lightspeed, which means for the photon itself, that time stands still and space shrinks to zero. If i would be a photon, then i could travel to everywhere in no time. So, for us as humans to observe multiple photons would it not be sufficient that only one single photon really exists to give us an observation of multiple photons, since it can zap around in no-time-no-space making it look like there are millions of it? How do we know there is more than one photon?

• energy and conservation of energy – Nikos M. Sep 29 '14 at 18:24
• See Wheeler's "one-electron universe" for a similar idea en.wikipedia.org/wiki/One-electron_universe (but it does not really make sense because there can be many isolated loops in the combined worldline and interactions that change electrons into other particles, similarly for photons). – Philip Gibbs - inactive Sep 29 '14 at 18:42
• A photon is massless and travels with lightspeed, which means for the photon itself, that time stands still and space shrinks to zero. Not true. See physics.stackexchange.com/q/29082 – Ben Crowell Sep 29 '14 at 20:37
• " it can zap around in no-time-no-space ", well not exactly. Its speed is still limited and not sufficient to be everywhere anytime – mattecapu Sep 30 '14 at 13:16

You are using the concept of time in a mixture between Newtonian ideas and Relativistic.

It is true, that for the photon time is slowed to a standstill, however, for an observer who has mass, time still flows, one can measure simultaneously two separate photons, that are not causally connected and know these are two separate entities since having measured one, it can't possibly be measured in the other locale at the same time.

It would need to travel faster than light to get there...

for instance a detector at Canada and a second detector on the moon detect photons at times, say half a second apart.

I have personally measured many photons, certainly more than one. Not once did a photon tell me that it was the same one that I measured an hour earlier. :-)

That time stands still for a photon is not true, by the way. Photons simply make a full rotation from the time-like to the space-like coordinate axis. To a photon "when" becomes "where". You can see that in every interference experiment, because photons "track" time with their wave count, which is an invariant under relativistic boosts. Now, there is ONE photon for which this is not true: that's the photon with frequency zero. But since the visible universe is not infinite, that photon can't live in this part of the universe, to begin with.

• Photons simply make a full rotation from the time-like to the space-like coordinate axis. No, this is wrong. – Ben Crowell Sep 29 '14 at 20:36
• @BenCrowell: It's wrong to say that a classical photon knows "what time" it is by knowing "where" it is? I don't think so. It just doesn't care about the differences on the clocks of massive folks like us. "Where" and "when" are the same thing in the "photon's view" of the universe. – CuriousOne Sep 29 '14 at 20:46

It is an interesting question but somewhat moot. Photons really only exist at the interactions of EM and material. In between EM is a wave. Hence the wave particle duality of light. However, from a point of view of Feynman diagrams you could be right. It might be possible that all photons in the universe in Feynman diagrams are traced by a single photon. This explains why the two slit experiment can produce an interference pattern either with multiple or single photons. A single photon can retrace its path as an anti-photon and then reproduce a second path which interferes with itself. But really this is just a more complicated model of how a wave works.