There seems to be a lot of confusion about the speed of light. For instance some die hard Quantum Mechanics adherents, still believe that the speed of light follows Galilean transformations:

Depending on what you mean you could be wrong. If you mean that if, in the inertial frame S, a beam of light was moving from along the x-axis in the positive direction with speed c and another beam was moving along the same x-axis in the negative direction with speed c then their relative speed of the two beams is 2c. What SR states is that that will be the same in any inertial frame of reference.

This statement indicates such an extremely poor understanding of special and general relativity, that it is quite depressing to realize that the most eager supporters of Quantum Mechanics, the ones who are willing to stand up for it no matter what, are the ones who do not realize that Quantum Mechanics does not claim to yield answers that satisfy ‘reality’. This is true regardless of whether one is thinking in terms of multiple dimensions, or of the invariance of the speed of light in a vacuum. Supporters of the type I refer to will insist that it is not dimensions that are in question but “degrees of freeedom’. In the same way these supporters will also argue about the speed of light being constant, in the terms quoted above, Apart from this they are extremely abusing and lofty in their self regard and disparaging of those who try to state the facts as they exist. He following are examples:

Quote from: McQueen

This means that if they were 300,000 km apart, they would meet after 0.5 seconds and not after 0.25 seconds as would have been the case had Galilean transformations held good. This was extremely puzzling,

PmbPhyThat's because you have a misunderstanding of the invariance of c postulates of relativity.

Having pointed out this seeming unwillingness on the part of certain QM supporters to accept the fact that the very definition of Quantum Mechanics is that nothing can be defined in the classical sense of the term, it is time to return to the question of what does the term speed of light as constant mean ?

I had demonstrated earlier by mathematical examples , how the speed of sound is invariant in a given medium provided the medium does not undergo a change:

For instance take the example of two cars; car (a) at 60 kmh and car (b) at 30 kmh approaching each other from opposite directions at a distance of 10 Km. Their combined speed would be 60kmh + 30 kmh = 90 kmh. This can easily be verified by calculating at what point the two cars would meet if they maintain the same constant speeds. Since the distance between them is 10 Km, it will take 10/90 = 0.111 hours for the two cars(a) and (b) to meet. During this time Car (s) would have travelled 3.33 Km and car (a) would have traveled = 6.6666 Km Thus the time taken for the two cars meet is the time calculated according to their combined velocities 60 kmh and 30 kmh = 90 kmh. The same kind of relationship holds true in general for any two bodies moving with respect to each other. If two people (a) and (b) are walking towards each other from an initial starting distance of 1 km and their speeds respectively are 5 kmh and 3 kmh, then their speed with respect to each other is 8 kmh and to verify this it is found that if they continue to maintain these same speeds that they will meet after 7.5 minutes, during this time (a) would have travelled 0.625 Km, and (b) would have travelled 0.375 Km. making a total of 1 Km.

Yet coming back to the first example take car (a) travelling at 60 kmh is 1 km distant from car (b) travelling at 30kmh. Suppose that at this time car(a) activates a siren and also suppose that the speed of sound is 340m/s . At what time will the sound reach car(b) supposing that car(a) and car(b) continue to approach each other at the same speeds. Would Galilean transformations apply and the combined speed with which car(a) and car(b) together with siren sound be 16.6m/s + 340m/s + 8.3m/s = 365m/s approx and they would meet at 1000/365 = 2.75 seconds. The answer is NO. Because the speed of sound is invariant in a medium it means that it does not obey Galilean transformations, thus the time taken for the sound of the siren to reach car (b) would be 1000/340 = 2.95 secs. During this time car (b) would have travelled 24.5 m approx. This is a completely unexpected result, until one realizes that light also exhibits such invariance:

That is, if two beams of light were approaching each other they would not follow Galilean transformations and have a combined speed of 600,000 km per second but each beam of light would have a constant speed of 300,000 km per second. This means that if they were 300,000 km apart, they would meet after 0.5 seconds and not after 0.25 seconds as would have been the case had Galilean transformations held good.

The natural conclusion that one comes to on witnessing this circumstance, is, that the invariance that light demonstrates as it travels through a vacuum could very well be because it is travelling through a medium whose presence has thus far managed to elude us. The point being that this ( to some) is a more acceptable explanation than to believe in time dilation or the fact that objects shrink as they approach close to the speeds of light. The irony being that these physical properties may very well manifest themselves, even if a medium (aether) for light to travel through does exist. But they would manifest themselves not because the speed of light is a constant but because an aether exists.