On Second Thought: How do we define time?

The story behind standard units of measurement is like TED Talks meets Jerry Springer. Very smart people have gotten very worked up about these seemingly obvious ideas for a long time. You don’t need to look any further than the United States’ stubborn use of imperial units to understand that once someone has an idea about measurement, they tend to stick with it until long after it stops making sense.

As much as I would love to expound on how every unit of measurement came to be set, I understand that you came here to read a blog and not a textbook. With that in mind, let’s set our gaze on the most comprehensible, familiar, and seemingly least arbitrary unit of measurement: the second.

Ask a third-grader what a second is and you might get a reasonably astute answer. Something along the lines of: “It is a piece of time.” Move up the academic echelon and things would get predictably more advanced. You would find out that it is one sixtieth of a minute, which is itself one sixtieth of an hour. An hour is one twenty-fourth of a day, and a day is how long it takes for the Earth to complete one full spin on its axis.

At this point we have gotten to the real heart of the matter. A second is a description of something real and meaningful that happens in nature – the rotation of the Earth. The problem with this definition of a second is that the Earth is less like a well-oiled machine than it is a mostly reliable but occasionally flaky roommate. The rent might get paid on time, but they have a bad habit of leaving windows open.

You see, the Earth doesn’t spin at a constant speed. It speeds up and slows down in ways that we don’t completely understand. A current day is roughly 24 hours long, but if you hopped in your Delorian and went back to the Triassic (220 million years ago) it would only be 23.5 hours. Go back a little (actually a lot) further to the Middle Cambrian 510 million years ago and you would only have 20.7 hours to get your errands run. But even that isn’t as bad as in the frigidly named Cryogenian Period of 900 million years ago when a day was a resoundingly speedy 18 hours.

The upshot of this inconsistency is that a second loses all meaning if we anchor it to the spin of the Earth. Scientists don’t like using units that don’t have meaning so since they realized that the Earth wasn’t cooperating, they have had to get creative.

For a long time (from AD 1000 until about 1960) the best they could come up with was saying that a second is 1/86,400 of the mean solar day. Unfortunately the “mean solar day” is just a fancy way of saying 24 hours and since we know that 24 hours is just a meaningless number, this definition doesn’t represent any actual improvement beyond sounding satisfyingly technical.

A better definition would incorporate something that is really and truly constant. Fortunately, there are a couple things that fit the bill. The best and most Star Treky is the speed of light. The cool thing about the speed of light is that it is the same everywhere in the universe (as long as you shoot your beam through a vacuum). Starting from there and following a few quick and painless calculations we could define a second as the time that it takes light to travel 299,792,458 meters in a vacuum.

That is a pretty good definition. It is precise, objective (if you ignore the definition of a meter which has something to do with a fraction of a line of longitude running through Paris), and easy to understand. The problem is, you can’t exactly measure it… The number of meters is just way too big.

We are almost there, though! I promise. We have all the ingredients we need for a solid definition of a second. We just need something constant that we can measure without building a lab that is bigger than the distance from the Earth to the moon. Enter atoms.

Atoms are great because they are as reliable as they are tiny. At a constant temperature and constant pressure atoms vibrate at measureable and consistent speeds. That is why the building blocks of matter are what scientists currently use to define a second. Next time someone is pestering you do to something and you want to tell them you will do it in a second, trip them up completely and say something like “Yeah, I’ll get to it in the time it takes an atom of Cesium 133 to oscillate 9,192,631,770 times!”

Assuming they don’t clue in that it took you longer than a second to say it (and if they can resist the urge to beat you up), you will have taught them something about the collision between the familiar and the strange. And THAT is what seconds are all about.