Cesium the Moment

“Just a second,” said the young lady at the counter as she answered the phone.

She didn’t really mean my wait would be a second, of course. She meant it would be some indeterminate length of time that was short on a human scale. A second generally is too brief for humans to accomplish very much. However, it is long enough for a pygmy shrew’s heart to beat 14 times. A hummingbird can flap its wings 50 times per second while a bee averages 200. A photon will travel 299,792 km. Even for a human, it’s enough time to drive 88 feet in car going 60mph. I’m not hummingbird or a photon and I wasn’t in a car, so in the first second of waiting I didn’t flap my arms even once or travel anywhere that the entire earth wasn’t going anyway. Nor did I accomplish much in the next 50 or so seconds before she hung up the phone and turned her attention to my purchase. But it was enough time for my thoughts to wander to the matter of seconds, which I recalled were redefined sometime during my school years; a reference check once I was back home confirmed that they were.

The Sumerians came up with the traditional second (reflecting their peculiar sexagesimal predilection) even though they had no timepieces accurate enough to measure one. By definition an hour was 1/24 of a day; there were 3,600 seconds in an hour and 86,400 seconds in a day. This worked well enough for millennia, but by the 20^th century it posed a problem for scientists. The problem was that the earth’s rotation is variable, meaning the length of a second was variable too. This really wouldn’t do, so in 1967 a second was redefined as the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between two levels of the cesium 133 atom. An atomic clock that counts these cycles – using microwaves, resonance, and a crystal oscillator in a manner I don’t pretend to understand very well – is so accurate that it will drift no more than a second in 60,000,000 years, the length of time since the dinosaurs died. The first cesium clock was built in 1952, and so was well established technology by ’67.

The redefinition caused its own problems. That particular number of cycles was chosen because it matched the average second based on the earth’s rotation over the previous century. However, a slight slowing of the earth’s rotation caused by rising sea levels since the late 19^th century meant that by 1972 International Atomic Time already was off by a couple seconds from the common clock based on the earth’s rotation. In other words, by 1972 midnight came a couple seconds late. This wouldn’t do either, so in 1972 the leap second was introduced to re-synch the solar day while keeping the new definition of a second for all purposes. Whenever Universal Time (the common clock based on the earth’s rotation) drifts more than .9 seconds from International Atomic Time (which takes no account of earth’s rotation or its orbit) a “leap second” is added to the year, usually at midnight either on June 30 or on December 31. There have been 25 of these extra seconds since 1972. The last was in 2012, but we’ll have another in 2015 when June 30 will have 86,401 seconds.

The question “why cesium?” naturally came to mind. For once there was a fairly simple answer: the electron configuration of a cesium atom (a single electron in the outermost shell) makes it easy to manipulate for the purpose, and it has a higher cycle rate than other similarly suitable atoms such as rubidium. [Don’t get me started on “why cesium rather than caesium”; as a hangover from schoolboy Latin, my preference is the latter spelling but on these shores one gets corrected for using it; my Word 2013 has just underlined it in red.]

I also see that cesium has other interesting properties. For one thing it is yellow. This might seem a minor feature, but nearly all metals are grey or silver in color. The exceptions, notably gold and copper, tend to be valued for jewelry. So, why are there no cesium earrings, rings, and bracelets? For one thing it has a melting point of 28 degrees C (82 F), so the jewelry would turn to liquid at body temperature. This doesn’t really matter though, because it wouldn’t last long enough to melt. It would explode on contact with skin – more precisely on contact with the moisture of the skin – which would make short work of both the jewelry and the wearer. So, don’t give your sweetie a cesium necklace for Valentine’s Day this Saturday unless you are planning on ending the relationship.

Cesium and water