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You might be confusing some issues. In special relativity, space and time do not stretch or compress. It really comes down to measurements with clocks and rulers made by people that are moving uniformly with respect to each other. One option that is consistent with observations for SR is that there is one family whose clocks and rulers are right and ...

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The speed of light in a medium depends on the frequency of the electromagnetic radiation $$v(\nu) = \frac{c}{n(\nu)}$$ where $n(\nu)$ is the refractive index. In a general case, $n(\nu)$ is a complex number, and its imaginary part accounts for the absorption of the medium (i.e. if a material is not transparent at frequency $\nu$, then $\textrm{Im } n(\nu) ... 2 Yes it is possible, but the difference would need to be miniscule. Effectively your question reduces to one of the following: "does light have a truly nonzero rest mass?" and / or "is there a highly diffuse optical medium all around us which modern repetitions of the Michelson-Morley experiment have not yet detected?". Look up "Experimental Checks on Photon ... 2 Why is the speed of light limited to (only) 299.792.458 m/s? Because space is the way it is. The speed of light is related to the permittivity and permeability of space via the expression: $$c_0={1\over\sqrt{\mu_0\varepsilon_0}}$$ See Wikipedia. There's a somewhat similar expression for shear wave velocity in mechanics: $$v_s = \sqrt{\frac {G} {\rho} ... 2 According to the principle of relativity, motion is relative. You can think about it this way: There could be no physical experiment which would tell you if your reference frame is moving or not. Who told you that we can attach a reference frame to the light ray? This is completely untrue. Yes, both times "slow down" in comparison to each other! And ... 1 The clocks are all in sync in the ground frame, but they are not in sync in the train's frame. An observer on the train would think that the clocks in the front of the train are ahead, while those to the rear are behind. Measuring the forward traveling beam against the nearby clocks will show a long time difference, while measuring the rearward traveling ... 1 Let me quote the relativistic velocity addition formula for easy reference:$$v_{AB} = \frac{v_A - v_B}{1 + \frac{v_Av_B}{c^2}}\tag{SR}$$I'm guessing you interpreted these quantities as follows:$v_A$is the speed of the light beam relative to George$v_B$is the speed of Gracie relative to George$v_{AB}$is the speed of the light beam relative to Gracie ... 1 The key to understanding this somewhat surprising result is that the relativistic velocity addition formula is not applicable to this calculation. As an example of when to apply the velocity addition formula, sssume there is an object with (1D) velocity$\mathbf u$in some inertial coordinate system. Now, what is the velocity$\mathbf u'$of that object in ... 1 Scale is not symmetrical in physics. Say, if you zoom in stuffs, you will eventually see atoms, which is totally different to the "continuous" things you see in your scale everyday. Therefore, there is nothing special to find light move "not impressively" in a cosmic scale. Not to mention that being "fast" or "slow" is a matter of comparison. Besides, ... 1 It's a good question, and illustrates an important principle in relativity. The proper time of an observer is equal to the length of their world line. For any observer the time shown by the clock they carry is called the proper time, and the proper time is an invariant i.e. all observers in all frames of reference will measure the proper time to have the ... 1 The expansion of the Universe has no effect on the local speed of light. Any local measurement of$c$will yield$c$, and$c$won't change. There is one thing that often causes confusion about the speed of light or faster-than-light travel. A photon moving in an expanding space-time appears to move at an average speed faster than$c\$. Consider a ...

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If I can expand a little bit on Sofia's answer the polarization of the medium opposes time variations in the electric field thus slowing down the phase velocity of the wave. This can be seen from Ampere's circuit law (the 4th Maxwell equation) which is central as you stated in arriving at the wave equation describing light. It can be written in vacuum as ...

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