Tom Befuddled by Speed of Light

My editor, Tom, whose last contribution to the blog brought him only misery and derision, has hit me with an imponderable speed-of-light question, to wit:

"According to Einstein (I think), the velocity of light is the one constant in the universe, and everything else, including time, is relative. But isn't velocity a function of time? You can't say what the velocity of anything is without using units of time. So if time is relative, how can velocity be absolute? OK science wonks, and I know you are out there, let the savaging begin!"

I have to confess that the question makes my head hurt. Just like Tom's editing. It would take me several hours to put together an answer for Tom, and then it would be wrong, and I'd have to call up Brian Greene, who probably has better things to do than be pestered by the likes of me. But it seems to me that Tom is fussing over a measurement issue. How can you measure the speed of light if "lightspeed" sets your standard for time? He senses an illogical loop-de-doop. I believe he overthinks the matter. Light moves at a constant speed through a vacuum, regardless of the speed of the observer -- this is Einstein's great insight (1905). Einstein didn't have to measure the speed of light, though by then people knew it moved about 186,000 miles per second. He just accepted that light has this very strange quality -- contrary to common sense -- of moving at a constant speed even if, say, the lantern is mounted on the front of a speeding train. From that emerge the weird implications of special relativity.

Because light moves at this constant speed, it has become the basis for all of our official measurements, not only in terms of time but also distance. We can theoretically call these units of measurement anything we want (foot, meter, second, whatever), but the ultimate standard of length (temporally, spatially) is not some gold bar in the King's chamber, but the distance light travels in that particular unit of time, or the amount of time it takes light to travel for that particular unit of distance. Obviously we have to measure these things and any measurement is going to be inexact and a little bit of an approximation.

Oh, never mind.