I said, suppose you take a light - I was thinking of just light bulbs because, in those days, lasers were not yet really there - and sent a light pulse between two masses. Then you do the same when there's a gravitational wave. Lo and behold, you see that the time it takes light to go from one mass to the other changes because of the wave.

Every time you accelerate - say by jumping up and down - you're generating gravitational waves.

If the wave is getting bigger, it causes the time to grow a little bit. If the wave is trying to contract, it reduces it a little bit. So, you can see this oscillation in time on the clock.

Rainer Weiss Quotes