After the warmest fall I can remember, we here in upstate New York finally are seeing cooler weather. I greet this with mixed feelings. As I've pointed out here many times, the abnormally warm temperatures we've had in the last few years are not good news. On the other hand, being a transplanted southerner, I can't say I'm fond of the cold, even after forty years of living in higher latitudes.
Our chilly winters, though, are nothing compared to a lot of other places. My Canadian friends, even the ones who live in the southern parts of that vast country, see cold temperatures the likes of which I've never had to deal with. The Rocky Mountain region, from Colorado up into Alberta, drops down to dangerous lows, often coupled with howling winds and snow. Scandinavia, Siberia, Greenland... there are a lot of places on Earth where the cold season is actively trying to kill you. The lowest temperature ever recorded on the surface of the Earth was -89.2 C, at Vostok Station, Antarctica, cold enough to freeze carbon dioxide into dry ice.
Makes our current 2 C seems like a gentle spring zephyr.
But I wonder if you've ever considered how much colder it can get?
Temperature is a measure of the average molecular motion of a substance. It is connected to, but not the same as, the heat energy; to prove that to yourself, put a pot of water on the stove and bring it to boil, and set your oven to 212 F/100C, and then decide which one would be less fun to stick your hand into. The water and the air in the stove are exactly the same temperature -- i.e., the molecules are moving at the same average speed -- but the water has a great deal more heat energy, because water molecules are so much harder to get moving than air molecules are.
So logically, there's a minimum temperature; absolute zero, where all molecular motion stops. This would occur at -273.15 C (0 on the Kelvin scale), but practically speaking, it's impossible to get there. Even if you could somehow extract all the heat energy from a substance, there's still the kinetic energy of the ground states of the atoms that can't be removed. Still, the scientists have gotten pretty damn close. The CUORE laboratory in Italy set a record in 2014, reaching a temperature of 0.006 K, but recently that's been broken on extremely small scales -- two years ago scientists working with an exotic form of matter called a Bose-Einstein condensate got it down to 38 picokelvin -- that's 0.000000000038 degrees above absolute zero.
But that, of course, is all done in a lab setting. What's the lowest naturally-occurring temperature ever measured?
You might think it's somewhere in deep space, but it's not. The temperature in deep space varies all over the place; recall that what matters is the average velocity of the atoms in an area, not how much heat energy the region contains. (The solar corona, for example, can reach temperatures of a million K, which is way higher than the Sun's surface -- there aren't many atoms out there, but the ones there are move like a bat out of hell.)
The coldest known place in the universe, outside of labs down here on Earth, is the Boomerang Nebula, a planetary nebula in the constellation of Centaurus, which has measured temperatures of around 1 K. The reason why is weird and fascinating.
A planetary nebula forms when a red giant star runs out of fuel, and the collapse of the core raises the temperatures to a ridiculously high one million degrees kelvin. This sudden flare-up blows away the outer atmosphere of the star, dissipating it out into space, and leaves the exposed core as a white-hot white dwarf star, which will then slowly cool over billions of years.
So how could a flare-up of something that hot trigger temperatures that cold? What's amazing is that it's the same process that heated up the core, but in reverse -- adiabatic heating and cooling.
Way back in 1780, French scientist Jacques Charles discovered that when you compress a gas (reduce its volume), it heats up, and when you allow a gas to expand (increase its volume), it cools. Volume and temperature turned out to be inversely proportional to each other, something we now call Charles's Law in his honor. If you've ever noticed that a bicycle pump heats up when you inflate your tire, you've seen Charles's Law in action.
This all happens because upon compression, the mechanical work of reducing the volume adds kinetic energy to the gas (increasing its temperature); when a gas expands, the opposite occurs, and the temperature falls. This is how compressors in air conditioners and refrigerators work -- the compression of the coolant gas increases its temperature, and the warmed gas is passed through coils where the heat dissipates. Then it's allowed to expand suddenly, reducing its temperature enough to cool the interior of a freezer compartment to below zero C.
This is what's happening in the Boomerang Nebula, but on a much larger scale. The outer atmosphere of the star is expanding so fast its temperature has dropped to just one degree above absolute zero -- making this peculiar nebula five thousand light years away the coldest spot in the known universe.
So that's our tour of places you wouldn't want to vacation. Top of the list: the Boomerang Nebula. Might be pretty to look at, but from a long way away, and preferably while warmly dressed.