Once upon a time, I was at a book signing, and Sir Pterry asked me: "Does Now travel at the speed of light?" I asked around the Physics Department afterwards, and twice got the suggestion that Now travels at the speed of time; but then, both times. there was the rider "one second per second". Busted flush, thinks I: that's like checking the accuracy of a ruler with another ruler from the same production line.

Then I had two golden moments. The first was to realise that time is a machine-based measure, which we humans use to estimate the speed of our personal Nows, though it's a poor estimator when one is bored. Second: Now has a nonzero duration,* which varies from about a fraction of a second to something like a century. At the long end, Now starts tailing off into Then; at the short end, we see things like "too many things happening at once", or "What is it *now*?"

Eventual answer to Sir Pterry: Now doesn't travel at the speed of light, nor at the speed of time, but at the speed of surprise.

.... Many years later, I thought to ask an astrophysicist. He thought for a moment, then said: "It's one way of looking at spacetime in Special Relativity. If you're stationary, you're travelling along the axis of time at the speed of light; if you're moving, you trade off some of that to travel along one or more of the axes of space."

Second answer to Sir Pterry: To a relativist, Now does travel at the speed of light, but only if you sit still and *don't fidget*.

* "What?" says Herself. "When you say 'Do this now'," says I, "do you mean this minute or this month?" "... Ah," says she.