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Physics, Astronomy & Cosmology / How long would it take two photons, from Earth and the Sun, to collide?
« on: 05/11/2011 11:47:17 »Essentially, if you shot one photon of light towards the Sun, and simultaneously a photon was shot from the Sun towards Earth how long would they take to collide, considering that they're travelling towards eachother?
From my understanding of relativity, it would take 8 minutes 19 seconds regardless of the fact that they're travelling towards eachother, not 4 minutes 9.5 seconds as you would think intuitively.
Anyway. Help/explanations appreciated.
We have to make a distinction between the speed of objects (like photons) and the *relative* speed of objects. For example, galaxies cannot move within spacetime faster than the speed of light (despite space expanding greater than the speed of light) however if you have two galaxies hurtling towards one another and each one is travelling, let's say, at 3/4 of the speed of light then their *relative* speed is more than the speed of light. This does not violate Einstein because neither galaxy is actually travelling more than the speed of light. So two photons travelling towards one another would take half the time to reach each other than would be the case if one of the photons was at rest.