It is well established that the light speed in a perfect vacuum is roughly $3\times 10^8 \:\rm m/s$. But it is also known that outer space is not a perfect vacuum, but a hard vacuum. So, is the speed limit theoretically faster than what we can measure empirically, because the hard vacuum slows the light down? Is this considered when measuring distances with light?
Special Relativity – Is Light Faster Than Current Measurements Suggest? Understanding Speed of Light in Cosmology
cosmologyexperimental-physicsrefractionspecial-relativityspeed-of-light
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Jon Custer hinted at something, which I think is best explained via an analogy.
Imagine you can walk along a pavement at 4mph. When the pavement is empty, it takes you an hour to travel four miles. But when the pavement is crowded, you're dodging around people and bumping into them. You're still walking at 4mph, but it takes you an hour and a half to travel the four miles. And if you're a little old lady with short little steps walking at 4mph, you're held up more than if you're a big guy with long strides walking at 4mph. Now let's look at your questions again:
But I wonder why their speeds differ in any other medium?
Because the light interacts with the material, and those interactions are wavelength dependent.
And why Red light travels faster while it has less energy than blue light?
Because the light interacts with the material, and those interactions are wavelength dependent!
I always thought, that it is actually the other way around. There is a maximum speed limit in our universe. This maximum speed limit of causality is necessary in order to have the world as we have it. So in principle it is possible to have a world with infinite speed, it's just not ours and it would look completely different than ours. And in our world with an upper limit of the speed of causality everything which doesn't have a mass can travel as fast as this limit. It is true therefore that the speed of light is the maximum speed in our universe. However, this is because there is an upper speed limit and the photons have no mass which allows them to travel that fast.
Best Answer
If we take air, then the refractive index at one atmosphere is around $1.0003$. So if we measure the speed of light in air we get a speed a factor of about $1.0003$ too slow i.e. a fractional error $\Delta c/c$ of $3 \times 10^{-4}$.
The difference of the refractive index from one, $n-1$, is proportional to the pressure. Let's write the pressure as a fraction of one atmosphere, i.e. the pressure divided by one atmosphere, then the fractional error in our measurement of $c$ is going to be about:
$$ \frac{\Delta c}{c} = 3 \times 10^{-4} \, P $$
In high vacuum labs we can, without too much effort, get to $10^{-10}$ torr and this is around $10^{-13}$ atmospheres or 10 nPa. So measuring the speed of light in this vacuum would give us an error:
$$ \frac{\Delta c}{c} \approx 3 \times 10^{-17} $$
And this is already smaller than the experimental errors in the measurement.
So while it is technically correct that we've never measured the speed of light in a perfect vacuum, the vacuum we can generate is sufficiently good that its effect on the measurement is entirely negligible.