So, why should we worry about the speed of light adding then? If there is some mysterious field effect and drain of energy, then surely that would stop ET from going past c and developing FTL craft. If ET does have FTL craft, then clearly there would be something in physics we don't understand yet. But, then again... we need to show that there are ETs and they have craft.
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The laser beam starts at the left, passes through the glass, then exits the right of it. We know that light slows down as it passes through a transparent medium. The amount of slow-down is determined by the reciprocal of the refractive index of the medium. In the case of glass it’s around 1.52. But let’s simplify things by making it 1.43. This makes the slow-down amount 0.7, i.e. 70% of light speed.
So as the beam moves through the glass it is going at 0.7c. But what is this speed relative to? To the glass of course. Once the beam enters the glass it starts moving from atom-to-atom within the glass. Each atom becomes a new launch point for the light and that is what the beam moves relative to. To make a weak analogy, it is like someone running first on dry land then through waist-deep water: the water is the medium that determines current speed, not the dry land.
Thus if you had a laser (white light) hitting a pane of glass out in space somewhere between Earth and Alpha Centauri (a near vacuum), the laser would hit the surface of the glass and slow to about 66% of c (in a vacuum). Some energy would be imparted to the glass at this point. Then the laser would move through the glass as white light (wavelength is the same as original emitted laser), hitting the other side of the pane of glass. It then would revert back to c (in a vacuum) and go on its way. But the wavelength of the laser would now be greater, as the energy absorbed by the glass, when the light was slowed to 66%, must be accounted for. Here is it possible to see that c is a hard limit, as light always goes back to c.
For light to go faster then c, then you would need a medium with a cooefficent of absorbtion/transparency greater then 1. Or, you'd have to have something, some region of space, that is more clear then vacuum, which is impossible according to modern science. What can be more clear then nothing?
200000000 / 299792458 = 0.667 = 66.7%
We’ll now complicate the situation a little as shown:
Answer: they both slow to 0.7c. The beam is now inside the glass and is moving relative to it. The initial speed of the laser can no longer have any effect on the current beam speed because, as before, the beam is now moving from atom to atom within the glass. Those atoms are what control the speed.
The beams then reach the other side of the glass and exit. The beams now go back to full speed: c. But relative to what – the lasers? No, the glass of course! Like the earlier example, the original beam speed is no longer important. The beams exiting the glass now move with identical speed.
This is not to say the beams will be identical in all aspects. The beam from the moving laser strikes the glass at a higher velocity and its light waves will appear to have a frequency 10% higher. This frequency will be preserved throughout the process. And the observer will see the moving laser beam as having a higher frequency – a Doppler shift! But the final velocity of both beams will be the same: c.
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If you want to assume that space/time is different in intergalactic space... fine, but that has no baring on physics inside the galaxy, inside out solar system and here on Earth. Any ETs out there will have to live with the same Rules we do.