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We know that the propagation of light does not involve the oscillations of particles instead Electric and Magnetic fields oscillate. The variation of E and B fields results in the propagation of light, it does not require any medium for its propagation.

Question:

  • Since speed of sound is measured w.r.t some medium which is 330 m/s, what about speed of light? W.r.t what it is measured and calculated as 2.98 x 10^8 m/s?
Dale
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Cosmos22
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    Acording to special relativity, all observers, whether stationary or moving, will measure same value of speed of light. Moreover this standard value is for vaccum medium. – Kshitij Kumar Oct 31 '21 at 11:03

4 Answers4

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The speed of light in vacuum is measured with respect to any inertial frame.

Dale
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As already said in the comments, it doesn't really matter because the speed of light is constant1. In other words, every observer will measure the same value for $c$.

There is however a slight nuance to this which is that this only applies to inertial observers, i.e. moving at a constant velocity without acceleration. Non-inertial observers might measure velocities greater than $c$, for example.

TL;DR: every inertial observer

1 This is the second postulate of special relativity: "The speed of light in vacuum is the same for all observers, regardless of the motion of the light source or observer."

jng224
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  • Is it correct to say that for a non-inertial observer, the apparent inconstancy of $c$ is a consequence of mis-defining distance? That is, I measure my current distance, acceleration, and relative velocity towards a mirror and I say, "light emitted towards the mirror reaches me in less of my own proper time than $s'(t_1)+s'(t_2)/c$ where $t_1$ and $t_2$ are in my proper time but $s'$ is measured in an inertial frame initially comoving with me... so the speed of light is greater than $c$" - when I should have been measuring my own proper length if I was going to be using my proper time? – g s Oct 31 '21 at 18:34
  • @gs I'm honestly not sure. Maybe post it as an actual question? – jng224 Oct 31 '21 at 19:07
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    I found an in-depth answer to that question here. If anybody else is interested: TLDR, not as simple as what I said, but yes, it's a consequence of adopting mathematically convenient coordinate systems that define distance differently. https://physics.stackexchange.com/questions/33816/does-the-speed-of-light-vary-in-non-inertial-frames – g s Oct 31 '21 at 19:23
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Another way to think of this is that the speed of light is measured relative to nothing. This is because there is no way possible to distinguish between an inertial frame of reference which is at rest and one which is coasting at a constant velocity. That means that everyone, irrespective of their velocity, will measure the speed of light as the same, no matter what.

niels nielsen
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The speed of light in vacuum is measured with respect to the measuring device, not only in an inertial frame.

Earlier there was a hypothesis of a medium for the electromagnetic waves, just as sound waves are transported by air. This medium was called aether. But nowadays this hypothesis has been discarded and the waves are thought to not be waves of a medium. https://en.wikipedia.org/wiki/Luminiferous_aether

Also, nowadays the length unit meter has been defined from the time unit second in such way that the speed of light in vacuum has been given the exact value $299\ 792\ 458\ \mathrm{m}/\mathrm{s}$. This could be done because the speed of light in vacuum seems to be independent of the observers speed through universe.

md2perpe
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