Cooling is hard

Why did air conditioning appear so long after fire?

It is now the season of the air conditioner (in the northern hemisphere).  Window units and centralized machines hum into action as their thermostats require, the muted roar of fans sounding on all sides as the summer proceeds.  It’s quite familiar to nearly everyone.  And yet, in historical terms it’s very recent.

In one of Alistair Cooke’s Letter From America radio talks, he describes summer life in New York City before the advent of air conditioning.  The wife and children were sent off to the cool air of the mountains.  The remaining workers lived a bachelor, primitive existence, sweating in undershirts much of the time as the normal social conventions were discarded.  Across the river from us, in Washington DC, it was the time of the summer recess.  The southern states were not the chosen destination for retired people.

That only changed in the last half of the twentieth century, as machines to produce cool air reliably became widespread.  But we’d been able to produce heat at will for quite some time.  About as long as our species has been around, we’ve been able to warm ourselves by the fire.  Cooling is recent.

If you’d asked, say, Plato, he probably would have said that an object can partake of the form of Cold just as easily as that of Hot, and any difficulties were a matter for mere artisans to work out.  Our tutoring colleague has spent much time convincing students that energy transfers effortlessly from potential to kinetic and back again, as you throw a ball up in the air or swing a pendulum back and forth.  So it certainly seems logical that one should be able to cool something off as easily as heat it up.  There is some technology involved, but the essential part, pumping liquids and gases, was possible centuries ago.  Maybe even the ancient Greeks could have done it, were they so inclined.

Well, in the nineteenth century it was worked out that Heat and Cold are not opposed principles, or fluids running in and out of bodies.  Heat is the very small-scale random motion of atoms or molecules.  There is a zero of temperature, when everything stops moving, but no upper limit.  Cold is less heat, not something of its own.  You can’t light an anti-fire that will radiate coolness over the campsite.

That’s part of the problem, but not the important part.  The real unfairness is due to entropy.  You’ve probably heard the word, and maybe know that it’s a measure of disorder, or even seen a formula.  For now, the easiest way to think of it may be to realize that things like to spread out and mix, not separate themselves into different piles.  When you light a fire, the chemical energy in the wood spreads throughout the vicinity.  When you cool your drink with an ice cube, the warmth and coolness are initially separate, but mix.  To take air all at one temperature and cool part of it while warming another (which is what an air conditioner does), you have to unmix, to separate, and so work against entropy.  It’s hard to do, which is why it took so long to make a machine to do it.

Conceiving the world in equal and opposite principles, Yin and Yang, is intellectually tidy and satisfies our desire for fairness.  But sometimes the world is unfair.