NIST F1 atomic clock as measurement standard

One part in 10¹⁵ accuracy — one second in 30 million years. Returns at end of lecture as the time-measurement upper bound.

Any questions? If not, I'll go back to what we were talking about, which was observables. We were going from the sublime to what I call the ridiculous. The sublime is the NIST F1 atomic clock — one part in 10 to the 15th accuracy, which is one second in 30 million years. So we have a pretty good handle on time today.