Jul 31, 2009
There’s not many absolutes in science, so absolute zero – the coldest temperature theoretically possible where entropy is reduced to zero – truly stands out. Indeed, things get really, really weird quantum mechanically as we approach absolute zero. Let’s take a look at what fun we can have going down the thermometer all the way to 0 Kelvin.
Let’s begin with the coldest place on Earth, Antarctica. The temperatures there reach a minimum of about -80 °C (-112 °F) in the winter, with the coldest ever recorded temperature of -89.2 °C (-128.6 °F).
In 2006, Anthony and Christine Powell of Frostbytes blog (fantastic photos there, by the way) recorded this video clip of what people in Antarctica consider a terrible weather (euphemistically called "Condition 1") at the McMurdo Station. So, next time you’re having some terrible winter weather where ever you are, just remember this video clip.
Purdue’s Senior System Engineer George Goble hated waiting for his BBQ to light. So, in 1995, he decided to find the fastest way to achieve barbecue ignition. He tried propane, acetylene torches, and even oxygen-fuel gas or racing fuel (the last one took 30 seconds). But that wasn’t fast enough – he wanted to set the world record of fastest ignition. (Source)
So Goble decided to get serious and reached for liquid oxygen (LOX, boiling point: 90.2 K or -183 °C). He doused 3 gallons of liquid oxygen (LOX) onto 60 pounds of charcoal and a smoldering cigarette*. Within 3 seconds about 40 pounds of the charcoal burned and the grill was vaporized.
For his creativity, George won the 1996 Ig Nobel Prize in Chemistry. He also attracted the attention of the West Lafayette, Indiana fire department who warned him never to repeat the stunt ever again.
*Actually it’s good that he had a lit cigarette in the pile. Pouring LOX onto unlit charcoal will cause it to explode at about the force of one stick of dynamite per charcoal. If you spill LOX on asphalt, it can detonate. Oh, did we mention that LOX is a rocket fuel? (The orange external tank of the Space Shuttle is filled with it.) Needless to say, don’t try this at home.
Nitrogen becomes liquid at 77 K (-196 °C), which is pretty darn cold. Liquid Nitrogen or LN2 is actually a very useful substance: it’s used in the laboratory to freeze things, in hospitals as a medical treatment to freeze and remove warts and skin lesions, and even in restaurants to make alcoholic ice cream.
Wait – make alcoholic ice cream? Yes, it turns out though you can’t freeze alcohol in the freezer (not cold enough), you can do so with liquid nitrogen. Here’s Ferran Adria, Head Chef of elBulli Restaurant using liquid nitrogen to make alcohol sorbets and frozen pistachio puree truffles. Yum!
Going down the temperature scale, we have liquid hydrogen at 20.28 K (-252.87°C). Liquid Hydrogen is good for one thing: fuel. It is a component of rocket fuel, and a perennial contender of zero-emission fuel (I’m looking at you, BMW H2R!)
Liquid hydrogen is used in one of the coolest (literally!) rocket engines ever created by NASA. Here’s the Common Extensible Cryogenic Engine ("CECE" for short), which generates a scalding 5,000 degree steam and a whopping 13,000 lb of thrust yet form icicles at the rim of its nozzle at the same time. It’s quite the fire and ice engine:
CECE is fueled by a mixture of -297 F liquid oxygen and -423 F liquid hydrogen. The engine components are super-cooled to similar low temperatures–and that’s where the icicles come from. As CECE burns its frigid fuels, hot steam and other gases are propelled out the nozzle. The steam is cooled by the cold nozzle, condensing and eventually freezing to form icicles around the rim. (Source)
Things get really, really strange with liquid helium. First of all, it’s the only element that remains liquid down to absolute zero (though you can solidify it with great pressure). It has two form of liquid phases – at 4.2K (-268.95 °C), helium-4 (an isotope of helium) becomes liquid. At 2.17 K, it turns into a superfluid.
And the fun begins: superfluid is weird – it has zero viscosity (a measure of friction for fluids), zero entropy, and infinite thermal conductivity. If a superfluid is placed in an open container, it will creep up the sides and flow over the top. If you rotate the container from stationary, the superfluid inside will never move.
And weirder still: if you place a capillary tube in a pool of superfluid, then shine light on it, you’ll get a frictionless fountain that will flow forever (no friction*, remember?)
*Actually, in bulk fluid, superfluid does have some viscosity whereas in capillary it has no viscosity. Scientists think the explanation of this paradox is that superfluid is composed of two components – the normal component, and the superfluid component. I told you it’s strange.
The Coldest Objects in Space
Quick: what’s the coldest object in space? A frozen comet or a chilly gas cloud? Nope, the coldest object in space is actually a manmade object – the Planck Telescope – launched by the European Space Agency.
As part of experiments to measure the cosmic microwave background (the afterglow of the Big Bang to you and me), the Planck Telescope is cooling its instruments to -273.05 °C or 0.1 °C above absolute zero.
But what about the coldest natural object in space? That title belongs to the Boomerang Nebula (aka the Bow Tie Nebula). The protoplanetary nebula located 5,000 light-years away from Earth has been spewing ultracold gas for 1,500 years. This cooled down the nebula to a mere 1 K above absolute zero (Source).
Boomerang Nebula, credit: European Space Agency/NASA
The Coldest Substance on Earth
In 2003, Nobel Laureate Wolfang Ketterle and colleagues at the Massachusetts Institute of Technology created the coolest man-made substance on Earth: they cooled a Bose-Einstein condensate of sodium atoms down to 450 picoKelvin (0.00000000045 K).
In 2009, Tauno Knuuttila and colleagues at the Helsinki University of Technology’s Low Temperature Lab used magnetic refrigeration (yes, using magnets to cool things down – ain’t physics interesting?) to cool rhodium to 100 pK (technically, it’s the temperature for nuclear spin, not its overall thermal energy).
Now that’s cold!