A Good Light

I wish all my students well in their final physics exam this year. Classes ended today with a session on Special Relativity. One bright student really surprised me with the remark, "Yes, but the speed of light is slowing down, isn't it?" I thought it best to ask her exactly what she meant by that, and she replied enthusiastically that her Dad (a school science teacher) had told her that we now have evidence that the speed of light has been slowing down. I carefully pointed out that, yes, indeed, there were physicists willing to contemplate this evidence with a critical and fair eye, but that the vast majority would entirely disagree with the idea. She said, "Yes, but Relativity is only correct until it is shown to be wrong, right?" I pointed out that, yes, a physical theory only had a certain domain of applicability, beyond which it would not apply, and she said, "Well, we've seen that already with Newtonian mechanics!" And it was true that I did have to work hard this week explaining that Newtonian clocks simply didn't work when things started moving at high speeds, so I confessed that I was actually personally fond of the idea of a slowing speed of light, until another student impatiently drew me back to the problems in hand.