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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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    $\begingroup$ 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. $\endgroup$
    – Qmechanic ♦
    Commented May 29, 2013 at 17:53
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    $\begingroup$ Partial duplicate of physics.stackexchange.com/q/2230 $\endgroup$
    – Brandon Enright
    Commented May 29, 2013 at 17:54
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    $\begingroup$ 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? $\endgroup$
    – Dilaton
    Commented May 29, 2013 at 19:44
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    $\begingroup$ @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 :-). $\endgroup$
    – Dilaton
    Commented May 29, 2013 at 21:52
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    $\begingroup$ @Dilaton: Okay, done. $\endgroup$
    – Qmechanic ♦
    Commented May 29, 2013 at 22:15

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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 $c$.

A spacecraft couldn't travel at the speed of light in the first place, because as it goes faster and faster (in the observer's frame), its mass effectively gets bigger and bigger, meaning it requires more energy just to increase its speed a tiny bit. It effectively would take infinite energy for a spaceship to reach the speed of light.

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: $c$.

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  • $\begingroup$ To be precise, Einstein sorta plagiarized from Maxwell ;-) $\endgroup$
    – Waffle's Crazy Peanut
    Commented May 29, 2013 at 17:57
  • $\begingroup$ @ϚѓăʑɏβµԂԃϔ it's more complicated than that. They knew light was a wave but they didn't know "what was waving". The proposal was the"aether" but the Michelson-Morely experiment contradicted an aether explanation. $\endgroup$
    – Brandon Enright
    Commented Jun 2, 2013 at 17:56
  • $\begingroup$ No, Einstein did not "plagarize" Maxwell. His equations, inasfar as they could be laid down consistently (e.g. using Thomson's correction to Maxwell's constitutive laws) were non-relativistic and did not assert the existence of any finite absolute speed. Quite the opposite. A distinction was made between the "stationary" and "moving" forms of Maxwell's equations, the former being in the frame for light propagation. See my reply here: physics.stackexchange.com/questions/281485/… $\endgroup$
    – NinjaDarth
    Commented Jul 29, 2023 at 19:39

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