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Why does the light of objects such as the Sun, stars etc. take time to reach us?

E.g. Why does sunlight take 8 minutes to reach us, instead of reaching our eyes as soon as it rises.

K. M.
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For the very simple reason that light travels very fast, but not infinitely fast.

It takes light about 1 nanosecond to travel 30cm. That is so fast that you don't normally notice it. However it does take some time for light to reach your eyes, even from just across the room.

Objects in space are a lot further away. It takes light about 1.2 seconds to reach the moon, and as you note it takes light 8 minutes to come from the Sun. The reason is that the sun is 150 million km away, and it takes about 8 minutes for light to go that far.

If light did have infinite speed, it would be a weird universe. Effects would happen at the same time as causes, so the basic nature of cause preceding effect would be broken. The "speed=frequency*wavelength" formula for light would be broken. Refraction and diffraction wouldn't exist. A photon would be emitted and absorbed at the same time. Could something that is absorbed at the same time as it is emitted actually exist?

James K
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  • In photon's field inertial frame it is emitted and absorbed at the same time, yet it plainly exists. There is no reason I am aware of to suggest that the photon's inertial frame is less 'valid' than our own! – adrianmcmenamin Aug 01 '16 at 10:48
  • The photon's frame of reference is moving at the speed of light. It is a highly singular frame. You can't use the photon's frame of reference to model the behaviour of matter. – James K Aug 01 '16 at 18:52
  • I accept that nothing with a rest mass can move at c, but my point is that the argument something that can be absorbed and emitted at the same time cannot exist is flawed. – adrianmcmenamin Aug 01 '16 at 20:25
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I sense in your question that you wonder why speed of light is not infinite as many persons think it is. If the speed of light was infinite (no delay) then there would not have any refraction phenomenon and the immediate consequence is that your eye wouldn't be able to focus so you would be blind somehow.

Carthusianorum
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