Cold snap

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.

The Boomerang Nebula [Image is in the Public Domain courtesy of NASA/JPL]

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.

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Towards zero

Most scientifically-literate types know about the impossibility of reaching the temperature of -273.15 C, better known as "absolute zero."  The way most of us would explain why it's impossible goes back to one formulation of the Second Law of Thermodynamics.  Since the Second Law says that heat always flows from a hotter object to a colder one unless you put energy into the system (I always called this "the Refrigerator Principle" in my biology classes), once you get near absolute zero it takes exponentially more energy to remove that last bit of heat from the object you're cooling, and to go all the way to zero would require (1) an infinite amount of energy expended to accomplish it, and (2) an infinite heat sink in which to place the extracted energy.  (Despite having been conjectured for over a century, this way of looking at absolute zero was proven beyond question by the mathematics of known physical laws just three years ago.) 

There's another way to look at it, though, which is a little harder to wrap your brain around, because it hinges on one of the most misunderstood laws of physics: the Heisenberg Uncertainty Principle.  Developed in 1927 by German physicist Werner Heisenberg, the Uncertainty Principle does not mean what I saw someone claim on a woo-woo website a while back that "because of the Uncertainty Principle, we now know that science can't ever prove anything."

What the Uncertainty Principle does tell us is, despite its name, extremely specific.  What it says is for a particle, there are pairs of physical quantities called complementary variables, and for such a pair (call 'em A and B), the more we know about variable A, the less we can even theoretically know about variable B.  The most commonly cited pair of complementary variables is position and momentum/velocity.  If we know exactly where a particle is, we have no accessible information about its velocity, and vice versa.

Note that the Uncertainty Principle is not about the inaccuracy of our measuring techniques.  It's not that the particle has a specific position and velocity and we just don't know what they are, the same as watching a car speed by and thinking, "Okay, it was going so fast, at any given point along the road I don't know exactly how quickly it was moving."  This is a fundamental, built-in feature of the universe.  If I know a particle's position to a high degree of certainty, its velocity is equally uncertain, and in fact the particle exists in a superposition of all possible velocities simultaneously.  The reality is inherently blurry, and the more you home in on one piece of it, the blurrier the rest of it gets.

So what does this have to do with absolute zero?  Well, the temperature of an object is a measure of the average speeds of its constituent particles.  So if you had an object at absolute zero, you'd know the positions of the particles (because they're not moving) to 100% accuracy, and you'd also know their velocities (zero) to 100% accuracy.

About as huge a violation of the Uncertainty Principle as you can get.

The reason all this comes up is because of a study at the University of Vienna that was the subject of a paper last week in Science in which we read about a 150-nanometer bit of silica (made up of around a hundred million atoms) that was cooled down to twelve millionths of a degree above absolute zero.  This turns out to be the true temperature limit for a particle that size; every atom in the particle was in the ground state, the lowest allowable energy a particle can have without violating Heisenberg's law.  The particle was suspended in an "optical trap" -- the easiest way to describe it is they levitated it with lasers -- and then allowed to free-fall so they could observe its behavior.

What the researchers hope to do is to use such experiments to shed some light on the behavior of gravity in the quantum world, something that has been a dream of physicists for a very long time.  While the other three fundamental forces of nature (electromagnetism and the weak and strong nuclear forces) have all been shown to be manifestations of a single "electronuclear" force, gravitation has resisted all attempts to incorporate it into a "Grand Unified Theory" that could simultaneously explain the gravitational warping of space and the strange behavior of the quantum world.  None of the candidates for a Grand Unified Theory (the most famous contender is string theory) have as yet panned out, but the search continues -- and the ability to cool particles described in last week's paper give a bit of hope that physicists will be able to isolate and study systems under conditions that make the mathematics tractable.

So that's our quantum weirdness for today.  Thanks to the friend and long-term loyal reader of Skeptophilia who sent me the link to the study.  I would never claim to say I understand it at any kind of deep level -- after all, no less a luminary than Richard Feynman famously said, "If you think you understand quantum mechanics, you don't understand quantum mechanics."  The fundamental workings of the universe are counterintuitive and mind-blowingly odd, and that's kind of where we have to leave it.

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Author Mary Roach has a knack for picking intriguing topics.  She's written books on death (Stiff), the afterlife (Spook), sex (Bonk), and war (Grunt), each one brimming with well-researched facts, interviews with experts, and her signature sparkling humor.

In this week's Skeptophilia book-of-the-week, Packing for Mars: The Curious Science of Life in Space, Roach takes us away from the sleek, idealized world of Star Trek and Star Wars, and looks at what it would really be like to take a long voyage from our own planet.  Along the way she looks at the psychological effects of being in a small spacecraft with a few other people for months or years, not to mention such practical concerns as zero-g toilets, how to keep your muscles from atrophying, and whether it would actually be fun to engage in weightless sex.

Roach's books are all wonderful, and Packing for Mars is no exception.  If, like me, you've always had a secret desire to be an astronaut, this book will give you an idea of what you'd be in for on a long interplanetary voyage.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]