# The Speed of Light [duplicate]

I know that the speed of light is constant for some reason.

1. But why is it that when shining light from a spacecraft that is moving at (almost) the speed of this light, the speed of this light wouldn't seem to travel at (almost) twice the speed of light to an outside observer, or does it?

2. I also would like to know why the speed of light is constant?

3. Finally, I also would like to know why the speed of light considered the fastest speed possible? I read that there is no explanation, it is just a physical law of our universe but is there a possible explanation?

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## marked as duplicate by Qmechanic♦May 29 '13 at 22:13

Hi user2357. Welcome to Phys.SE. This post (v1) asks hypothetical questions outside accepted standard physics theories, such as, e.g., asking about a massive object traveling at the speed of light. Fictional questions are off-topic, cf. faq. If you (or somebody else?) can modify your question, so it stays within the realm of standard physics, I would be happy to reopen it. – Qmechanic May 29 '13 at 17:53
Partial duplicate of physics.stackexchange.com/q/2230 – Brandon Enright May 29 '13 at 17:54
Huh @Qmechanic, I think he is asking about nothing else than special relativity and wants in particular know why velocities can not naively added for objects traveling with the speed of light. Many textbooks make use of this or very similar thougth experiments to discuss relativistic addition of velocities. So I see really nothing off topic in this question. Why do you think it is off topic exactly? I think if it is not a duplicate, it should be legitimate? – Dilaton May 29 '13 at 19:44
@Qmechanic ok, can the close reason be changed to remove the darn unneeded automatic downvote, that goes along with "off topic"? This really picks me everytime I see it happen, this bad feature should really be turned off on our site :-/. They should not have implemented that network wide but let the moderators of each site decide if they want it instead! I always contrevote it if I see it happen and it is very unjust :-). – Dilaton May 29 '13 at 21:52
@Dilaton: Okay, done. – Qmechanic May 29 '13 at 22:15

The idea that speed of light is measured to be the same constant, $c$, in all reference frames, is a postulate that Einstein put forth in his seminal paper, On the Electrodynamics of Moving Bodies. So according to the theory of special relativity, it doesn't really have a "reason" for being true; it's a first principle. It's true to the best of our knowledge because all the experiments we've done to measure the speed of light in various reference frames have returned the answer $c$.
If, however, the spaceship were going half the speed of light, and someone in the spaceship turned on a flashlight, the person on earth and the person in the spaceship would both measure the same speed of the light from that flashlight: $c$.