What do the Iridium satellites have to do with the element iridium? As far as I can tell, absolutely nothing -- the name just sounded impressive. (Which leads to an interesting question: can you trademark the name of an element?)

So what is the story with iridium? It's one of three sister elements with similar chemical properties; osmium, iridium, and platinum. Platinum is the Cinderella princess of the three -- a "precious metal" used in jewelry -- while osmium and iridium are the ugly step-sisters. But iridium plays a central role in science....

Sitting in a French vault is a cylinder composed of platinum and iridium -- the "standard kilogram." It's the basis of the fundamental unit of mass that we use in all of science. But it may soon be on the way out.

An American copy of the standard kilogram. Note the double layer of protection! |

There are three fundamental units in science: mass, time, and distance. But the one that we can measure most precisely is time. Modern units of time aren't based on astronomical units like the day or the year -- instead, we rely on the fact that atoms give off radiation with very precise frequencies. So the "second" is now defined as a certain number of oscillations from a particular form of atomic radiation. All you need to do is count up those oscillations, and you have your second. (Even I know how to count!)

The second key unit is distance. Until the middle of the last century, scientists used a "standard meter", which was also made from platinum and iridium -- it was a bar with two scratches on it to mark the distance. But that isn't the most precise way to measure distance. So later, distance was defined as a multiple of a certain wavelength of light. But even this could be improved on -- remember that we can measure time best of all. So scientists

*defined*the speed of light to have a particular fixed value, and then simply took the meter to be a fraction of the distance that light travels in one second. It sounds like a cheat, but it really isn't.

And the standard kilogram might soon be on the way out, too. There's a move to use Planck's constant, which plays a fundamental role in quantum mechanics. If we simply define Planck's constant to have a fixed value, then we can calculate a value for the kilogram directly in terms of the second. If that happens, the

*all*of our units of measurement will be defined in terms of the second. It's about time.

My apologies for the awful pun. Metrology -- the study of the fundamental units of measurement -- is a subject that I find both endlessly fascinating and deeply tedious at the same time. I wrote a short story on the subject many years ago -- I will post it next time.

## 7 comments:

I know this one!

The original plan for the satellite constellation called for 77 satellites, equal to iridium's atomic number. Ergo the name. By the time it was deployed, fewer satellites were used, but the name stuck.

BTW Asimov's robots' positronic brains were made of a platinum-iridium alloy.

Thanks! I didn't know that. It looks like there are 66 satellites in orbit, which implies the name "dysprosium satellites". I see why they stuck with iridium!

Agreed. Iridium is a much more colorful name (all good puns should be bilingual )

"If we simply define Planck's constant to have a fixed value, then we can calculate a value for the kilogram directly in terms of the second."

Can you elaborate? I know Planck's constant defines the minimal size of space / unit of time. How do you get to mass?

Start with the speed of light. If you define it to have a certain known value in meters per second, then a unit of time (seconds) can be converted exactly into a unit of distance (meters). Now remember that Planck's constant has units of energy times time. So if you define it to have a fixed value, then you can convert you unit of time into a unit of energy. But E = mc^2, and we've already defined c to have a known value, so now we convert our unit of energy into a unit of mass.

Oh I see! Thanks so much for the follow up. Also, mind blown :)

Post a Comment