We don't measure time. We define it.
It sounds a bit egotistical, but in fact, it is quite correct. I was recently reading an interesting article on the Discover Magazine website. If you have the time on your hands, I would recommend reading the original article (linked at the bottom). Here are a few excerpts:
“I recently went to the National Institute of Standards and Technology in Boulder,” says Lloyd. (NIST is the government lab that houses the atomic clock that standardizes time for the nation.) “I said something like, ‘Your clocks measure time very accurately.’ They told me, ‘Our clocks do not measure time.’ I thought, Wow, that’s very humble of these guys. But they said, ‘No, time is defined to be what our clocks measure.’ Which is true. They define the time standards for the globe: Time is defined by the number of clicks of their clocks.”
Rovelli, the advocate of a timeless universe, says the NIST timekeepers have it right. Moreover, their point of view is consistent with the Wheeler-DeWitt equation. “We never really see time,” he says. “We see only clocks. If you say this object moves, what you really mean is that this object is here when the hand of your clock is here, and so on. We say we measure time with clocks, but we see only the hands of the clocks, not time itself. And the hands of a clock are a physical variable like any other. So in a sense we cheat because what we really observe are physical variables as a function of other physical variables, but we represent that as if everything is evolving in time.
Very interesting indeed.
PS: This blog isn't dead. I've been extremely busy, and the upcoming articles are quite detailed, so they are taking a considerable amount of time to produce. More to come, I promise.