How do you measure time?

Well, these days, It's a simple matter. You use a clock. In Galileo's day, clocks were more primitive. Sometimes he used his pulse. He described using his pulse to measure a slow pendulum and was surprised to to learn that the period was independent of mass. We also know that sometimes he sang a song. That's how he timed his experiments with an incline plane. Galileo was a trained musician, and he was pretty good a judging time from a musical beat.

Actually, the clock problem today is similar to that in Galileo's day. Our clocks are fabulously improved, as discussed by my friend, Bill Harter, but in the end we need something that goes through some repeated motion. Your heart beat, a pendulum, a balance wheel, a vibrating quartz crystal, or ultra-precise laser spectroscopy of molecules such as HO₂, the radical cousin of the more common H₂O.

Since our problem in relativity is related to the finite velocity of light, it makes sense to use light bouncing up and down in an empty tube between mirrors be our clock. Now, this is what Einstein would call a Gedanken clock, a thought clock rather than a practical timepiece. On the other hand, it's sort of how a laser works. Of course, in a laser the tube is not empty. It contains excited atoms to keep the light going.

But we can think in terms of an empty tube do a Gedanken experiment, a thought experiment, in the spirit of Einstein. We will let our clock move sideways at about 2/3 the velocity of light.

Click below to see what you might think you would see. But then you must think about it to answer a question. Do you think it will work?

Moving clock

Galileo used his own body as a clock