Timeline for How can the diameter of the universe be so big, if nothing can go faster than light?
Current License: CC BY-SA 3.0
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| Aug 9, 2016 at 9:21 | history | edited | Judge | CC BY-SA 3.0 |
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| Aug 9, 2016 at 9:06 | history | edited | Judge | CC BY-SA 3.0 |
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| Aug 7, 2016 at 14:33 | comment | added | Judge | Yes, or more simply: it's impossible for one of the photons to ever know about the other, because the messenger particle would have to catch up to something moving away at $c$, thus requiring a velocity greater than $c$. So in the rest frame of one photon, the other photon is not visible. Yes you're right, because if you try to Lorentz boost into the photon's rest frame ($v=c)$, the Lorentz factor $\gamma(v=\pm c)=\infty$ will make it very difficult to make sense of things (e.g. with infinite time dilation and infinite length contraction). | |
| Aug 7, 2016 at 12:18 | comment | added | Graipher |
Yeah, I think you are right, in the centre-of-momentum frame of the two photons they should both be receding with c. However from the rest frame of one of the photons, the other one would have to recede only with c. But I think that will fail because you can't boost into the restframe of a photon (?), because there is no frame where the photon is at rest, it is always moving with c.
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| Aug 7, 2016 at 10:51 | history | edited | Judge | CC BY-SA 3.0 |
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| Aug 7, 2016 at 10:33 | comment | added | Judge | I don't think so, but perhaps you know more than I do? If you fire one photon north, and another photon south. Both photons individually move at the speed of light, but because they're moving in opposite directions the distance between them is increasing at twice the speed of light. In other words, I'm not adding velocities I'm adding distances here. | |
| Aug 7, 2016 at 10:23 | comment | added | Graipher |
In the early universe, wouldn't they be only c*t apart, because velocity addition is different when moving with relativistic sppeds?
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| Aug 7, 2016 at 7:52 | history | answered | Judge | CC BY-SA 3.0 |