Monday, June 18, 2012

Methodological Monday: A Few SI Units

Methodological Monday is a new (and hopefully regular) feature talking about some aspect of the physical world and the direct ways in which we come to understand it.

Measurements impact and describe everything we do. They allow us to figure out when it's time to put a fresh tank of gas in the car, and, moreover, to build that car and refine the gas that fuels us. We here in the 21st century can measure distances so small that no useful information would be gained by measuring any smaller.

This wasn't always the case. For quite a while, individual societies (often with areas no larger than individual cities) came up with their own definitions of their own units of measurement, often based off of physical constructs. A classic example of this is the Imperial System of units.

Growing up in Canada, I never learned much about imperial. I can tell you the inches to the foot, the ounces to the pound, and that's about as far as I go. When someone tells me it's ninety degrees out I look at them like they've lost their mind until I realize they mean ferenheit. In fact, it wasn't until I was studying the culinary arts in college that I learned anything about ounces at all.

Tradespeople love imperial measurements, and there's a good reason. In cooking, especially in baking, the precise proportions of ingredients can be crucial. Having a system of units, like pounds and ounces, that are easily manipulated in mental math makes converting recipes for yield a cinch.

What I've always been more comfortable working in, however, was SI, more commonly known as the metric system, though the two aren't quite the same thing. I'm sure everyone's heard about metric by now, so I won't get too boring on rehashing metres and celcius and so on. But there are three units I want to talk about in detail: the second, the kilogram, and the Kelvin.

The second might be my favourite unit. This is the SI unit for time. As we all know from school, it's possible to derive any larger or shorter stretch of time from the number of seconds therein. What's more, it's an integral component of almost every physics calculation I remember how to do, and that's because time is necessary to do, well, just about anything. The second also has a very generalized definition... it's not bound to any one measurement or observation, but merely the period of time it takes for an excited atom of Caesium-133 to cycle 9,912,631,770 times between two states coupled to its grounded state, at rest, at Absolute Zero (more on that later). This definition is astoundingly precise, and is the basis for function in atomic clocks worldwide, which most consider to be the gold standard of time measurement, and which have been essential for many of the great discoveries in physics of my generation.

This definition of a second is so precise, that from time to time, like this month, it becomes necessary to insert a Leap Second (like the extra day in Leap Years) into the civilian time stream... as will happen at the end of this month. To put a second into perspective for you, it's about the time it takes you get Never Gonna Give You Up stuck in your head when you get rickrolled on youtube.

The Kilogram, though, is pretty special, and for good reason. For one thing, it stands alone in being one of the only SI units for which the basic unit (gram) is not the standard unit. This is largely because a kilogram contains a thousand grams, and is the equivalent to about 2.2 pounds. A gram is a relatively fine unit of measurement... too fine to be of any grand use, and difficult to conceptualize on our scale. The other feature that makes it special is that the kilogram is the only SI unit still defined by a physical object... a single physical object.

The International Prototype Kilogram is a precisely-machined, one-kilogram cylinder of a 90-10 mix of Platinum and Iridium. This makes it reproducible... which is lucky, as even this relatively stable alloy, kept under hard vaccuum, suffers fluctuations in mass. One of the great, and probably uncelebrated, achievements of our time will be to find a suitable definition of a kilogram without this measurement. But let's put this into perspective for you - a kilogram is more than enough mass on a modern Hard Disk Drive to store all of Wikipedia's pages... and you'd be doing it in the heaviest possible way short of printing them.

Now we have the Kelvin. Think of Kelvin as a unit for measuring temperature (that's not quite what it is, mind you, but it's good enough). For our purposes, it's enough to say that a Kelvin is about one degree Celsius. The difference is that Kelvin puts its zero somewhere else... about -273C/-460F. To put that into perspective for you, it means water freezes at 273 Kelvin.

What's more, 0 Kelvin is Absolute Zero. The coldest temperature the universe could present us with... and one to which we frequently reach tantalizingly close to. Things we take for granted behave very weirdly at such low temperatures. Certain materials lose their resistance to electric current and become superconductors, allowing for neat tricks with magnetism and (potentially) computing. What's most interesting is the behaviour of Helium at around 2 Kelvin. It's a liquid that flows up surfaces and out of containers... defying gravity as other forces have a greater impact upon it.

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