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A thought experiment I was pondering - Say there's a photon source and two observers, one at rest and one in motion relative to the photon source.

It would seem that at the point the photon is emitted, it 'knows' where it will be observed, because it will travel at c for the particular observer.

I was thinking that this might be because the wavefunction for the photon hasn't collapsed until it is actually observed. Does that mean that the wavefunction of a photon contains the possibilities of all paths including all observers at the endpoints of each path, and is that any different to saying the wavefunction includes all paths?

joshua.thomas.bird
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  1. you are right, the photon will be observed to have traveled with speed c by any observer

  2. yes the wavefunction is not collapsed until observed

  3. the wavefunction is the probability distribution of the photon

  4. there is no new information created by observing the photon, because like you write, the wavefunction already has the probability of the photon being at all locations

  5. when you observe the photon, you are just mapping the probability distrbution

  6. yes as you write, in this case you could say that the wavefunction maps the paths probability distribution

  7. you are right, it does not matter if an observer is in motion or at rest, they will all see the photon travel at speed c

  8. you are thinking this the wrong way, because you think the photon moves faster then the observers. But it is the photon that moves at speed c, that is constant, and all the observers are just relative to that.

Árpád Szendrei
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  • All of these points make sense, but can you go into a bit more detail on 8? I'm not sure I understand the premise that the observers move relative to the photon – joshua.thomas.bird Apr 30 '18 at 14:44
  • Sure. It took me a long time until I understood, but when you do, your view of the Universe will change. So at the Big bank, there was a sea of photons, that was created first as energy. Just photons, flying around. Those were all traveling at the speed c. There was no other speed, since there were no other particles, just particles with no rest mass. Now these particles were not moving in the time dimension, their speed was 0. Their spatial speed was c. Now this will sound very hard to imagine and most physicists would say this is not commonly accepted, but imagine the photon box. – Árpád Szendrei Apr 30 '18 at 16:00
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    A photon box is a photon in confinement. If you want to see time flow, you need to slow down in space. Your spatial speed needs to be slower then c to start moving in the time dimension. You just have to say OK, this is how the universe works, the magnitude of the four speed vector has to be c. Now if you want to see time flow, so like we do, you need to slow down in the spatial dimensions, and since your four speed vector's magnitude stays c, your speed vector in the time dimension will compensate, and you will start moving in the time dimension. – Árpád Szendrei Apr 30 '18 at 16:03
  • So after the sea of photons, there was pair creation, and baryon asymmetry, and pointlike particles were created, with rest mass. Now everything that you see as matter is built up of these, quarks, and electrons. These particles have rest mass, they move in the time dimension, so everything they build up will move in the time dimension too, so matter as you see it. Now of course the higgs field gives these rest mass, I understand, but what makes up quarks and electrons? Again, here a physicist would say there is no commonly accepted theory. – Árpád Szendrei Apr 30 '18 at 16:06
  • But look at the photon box. If you imagine the photon box, why does it have rest mass? Because the photon inside is in confinement. Like photons absorbed by the electron. Or maybe quarks too are built up like that, energy in confinement. So now if the only speed at which energy can move (because permettivity and permeability makes it) is c, then imagine everything that has rest mass moving relative to the photon. – Árpád Szendrei Apr 30 '18 at 16:11
  • In this case, it does not matter what particle with rest mass or what more complex object (that is built up by quarks and electrons) with rest mass moves at a certain speed. we could call it speed 0.1c or 0.2c. So all objects in the universe move at a speed relative to c. In this case it does not matter who is looking at the photon from what angle and how fast the observer moves. The observer will always move at a certain speed relative to c like 0.1c or 0.2c. – Árpád Szendrei Apr 30 '18 at 16:11
  • And so all observers regardless of their speed see the photon moving at speed c. – Árpád Szendrei Apr 30 '18 at 16:11
  • Now the more tricky question is, what speed are two observers moving towards each other, each with 0.9c. Intuitive thinking says they would have to move relative to each other 1.8c, but the math shows they are not, they are still moving slower then c relative to the other observer. – Árpád Szendrei Apr 30 '18 at 16:15