Tuesday, 25 November 2014

Absolute Zero: The never-ending quest to get atoms to sit still

Last month at a laboratory in Italy a group of scientists cooled a cubic meter of copper to a temperature of 6 milliKelvins (-273.144 C, -459.66 F). According to the researchers involved, for 15 days that 400 kg (880 lbs) of copper was the coolest object in the universe. Of course, they had to say something that sounded impressive because they had invested millions of dollars in grant money to create arguably the most useless thing on the planet. The feat was significant because it was the first time an object so large had been brought close to the temperature of absolute zero (0 Kelvin, -273.15 C).

Temperature itself is a surprisingly tough concept to pin down. Thanks to an influential nation with a reputation for being stubborn when it comes to measurement, we are forced to work with three different temperature scales: Celcius, Kelvin, and Fahrenheit. Two of these scales are useful and one is arbitrary to the point of being infuriating.

Celcius is a useful scale that is grounded in practicality and common sense, but it is not without its arbitrary aspects. It’s inventor, a Swede named Anders Celcius, based the scale on water and set 0 as the point at which water freezes and 100 as the point at which is boils. That means at any point on the scale 1 degree equals 1% of the change needed to bring water from freezing to boiling. The Fahrenheit scale, invented by German Daniel Gilbert Fahrenheit, by contrast and for complicated reasons, sets the freezing point of water at 32 degrees, the boiling point at 212 degrees, and historically also tried to incorporate human average body temperature for no apparent reason. The result is a mess of a scale that is really only used in the US, but for some reason we all acknowledge it and record F temperatures in parentheses next to their Celcius values.

The Kelvin temperature scale is the scale of science. While everyday scales based on the behaviour of water make good sense for most of us, scientists like to have more inarguable reasons for setting values. The Kelvin scale is based on the core principle of temperature: the movement of molecules. At its root, that is all temperature is. The faster the molecules in a substance are moving, the hotter it feels and the higher we say its temperature is. For that reason, 0 on the Kelvin scale is the point at which molecules stop moving completely, the infamous “absolute zero.” Beyond that, 1 degree K is equal to 1 degree C. Nice and simple and sciencey.

So what is with all the hubbub about scientists trying to cool things to absolute zero? Well, as it turns out, reaching absolute is a tough thing to do... actually it’s impossible. The problem is that for each degree you move down on any temperature scale, the work you need to do to move down another degree increases. Logically and mathematically it plays out that by the time you get to 1 degree K, the amount of work you need to do to go down one more degree and reach zero is infinite. That is why the Italian scientists were so excited to reach 6 milliKelvins. Unfortunately for them this isn’t the coldest temperature ever achieved in a lab. In 2003 scientists as MIT used heat shields and a process called laser cooling to chill a cloud of sodium atoms to 450 picoKelvins, that is 450 trillionths of a degree.

That is all very cool (puns!), but what is the point of cooling something down to such a degree (okay, stop)? Well it turns out that very very very cold things behave differently than we would expect them to. Atoms that are cooled to within a billionth of a degree of absolute zero can exchange electrons and from chemical bonds at distances 100 times greater than they can at room temperature. Also, at such low temperatures, atoms don’t exchange energy the way they do when things are warmer. Instead of zipping around and bouncing off one another, waves of energy called quantum mechanical waves overlap with each other, allowing groups of atoms to behave identically in a spooky choreographed dance as a kind of super-atom. Substances where this happens are called Bose-Einstein condensates. The first Bose-Einstein condensate was created in 1995 in Colorado when researchers cooled a rubidium cloud to 170 nanoKelvins.

So I guess there actually was a point to the Italian experiment. If there is one thing research into temperatures as taught us it is to expect the unexpected. So even though the cubic meter of copper didn’t form a united zombie-esque super-atom, maybe it was worth doing. At the very least, we can claim to have created the coldest piece of copper in the universe. Take that, aliens.


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