This story is from The Pulse, a weekly health and science podcast.
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Southeastern Louisiana University physicist Rhett Allain says warm is easy: Light a match, run current through a wire, rub your hands together.
“If you want to mechanically set up or engineer something to make things hot, I mean, it’s trivial,” he said.
Cool is much trickier than warm.
“Just have someone go in a room and say, ‘Try to find something to make something colder,’” Allain said. (Let’s imagine the AC is out of order.) “It’s pretty impossible.”
Go into a room now and look for something that makes heat. There’s the laptop this story was written on, every lightbulb in your house, a candle, your cat, even you.
“Why is it easier to go one way? Heat things up, increase the temperature, and it’s not so easy to decrease a temperature,” said Allain. “It’s a great question.”
At the most basic, to cool something, you can use something cool, obviously.
“Whenever you put two objects of different temperatures in contact, then they will transfer energy from the hotter object to the cooler object,” said Allain.
Put ice cubes in soda, the warm soda chills, of course. But think about it: You need an initial cold thing to cool the second warm thing. That speaks to why the cavemen had fire ages ago and we’ve only had refrigerators in our homes for a little more than 100 years.
If you want to understand why cool is complicated, you may first have to forget what you think you know about heat.
“We talk about heat in ways that makes it seem like it’s an actual substance that flows from one place to the other, and we even say heat flow, but it’s not,” Allain said. “It’s something that we made up to describe things.”
The average high temperature this time of year hovers around 90 degrees in Allain’s corner of Louisiana. Still, he doesn’t really believe in heat — at least not the way a lot of us think of it. He calls it a four-letter word.
“Heat is a leftover term,” he said. “Heat is a term from the early days of science, when we didn’t really have a good grasp of how everything worked and we didn’t really understand the quantum mechanical properties of matter that actually do really govern the way things work.”
Instead, he thinks of heat and cold as the transfer of energy. Here’s his “try this at home” experiment to dispel the whole idea of heat as a substance or a flowy thing.
“Take two water bottles and have them hot,” he said.
Fill them with hot water and cover one with a towel that’s soaked in hot water. Which bottle cools off faster: the one you added a hot towel to, or the other you left alone?
“It turns out the one that with a wet towel, even if it’s like a hot wet towel, is going to cool off faster than the other one,” he said.
Think energy transfer, not heat flow.
“So the water in that towel transitions from liquid state to gas state, which takes energy to evaporate,” Allain said. “And where does it get that energy from?”
It gets it from the water bottle.
This effect is how sweat cools us. The fan you have blasting in your face only really works if you’re sweaty — that is, if there’s something to evaporate into all that air.
Another type of cooling is the fundamental concept behind how our fridges work. Again, you can try this at home.
“So this one, you just take a rubber band and you stretch it and you hold it out stretched, and when you do that, you put it up against your lip cause your lips are sensitive,” he said.
The rubber band will feel warm because the polymers, the large molecules that make up the band, are being distorted, and that generates heat. If you keep holding it stretched out, it eventually goes back to room temperature — transferring energy to the air around it.
“And now once you reach the room temperature, you let it go back to its natural position,” he said. “And you know what happens? It gets cold, and it’s so awesome ’cause you can do this yourself.”
Back in its relaxed state, the rubber band gets even colder than normal because the universe demands that energy transfer. But how do you capture that cooling effect? This is why it took so long to invent refrigeration.
Your fridge uses gas to do it. The gas is compressed, which heats it up — all the atoms bunched up, bouncing off one another. But even compressed gas, like the stretched rubber band, eventually cools to normal. Then it’s decompressed and cools off to lower than normal, to refrigerator temperature, or freezer.
“It does seem like a magic trick,” Allain said. “And that’s what’s so awesome about it.”
Still, cooling and heating seem to be based on the same thing: energy transfer into and out of something. So why is one direction so much harder than the other?
“It probably has something to do with entropy, but I haven’t wrapped my head around it,” Allain said.
Roberto Ramos, an ultra-low-temperature physicist at the University of the Sciences in Philadelphia, said entropy is a thermodynamics term — it means disorder, randomness, and everything in nature gravitates toward it. More time passes, more entropy — messier and messier. And cool needs order.
“Basically to cool things, it means to put them in ordered form. And that takes work,” Ramos said. “It’s not natural for things to become cold. You have to do work, or you have to do work to make things cold. That’s why it’s harder to cool things than to warm it.”
In a universe ruled by entropy, it’s harder to cool something in sort of the same way it’s harder to put an egg back together — rather than crack one.
And when Ramos talks about cool, he means really, really cool. Most of his research takes place below 4 kelvins.
Kelvin is a temperature scale with no negative, stopping at zero, at the bottom of temperature. Zero kelvin is about minus 460 degrees Fahrenheit.
“From the visual point of view, if one were to try to imagine atoms, right, jiggling back and forth, vibrating back and forth, when it stops, that has to be the bottom of temperature,” Ramos said.
The bottom, absolutely no jiggling at all, is called absolute zero, and it’s theoretically impossible thanks to something called the Heisenberg Uncertainty Principle. But there are ways to get close, Ramos said. A very common route is liquid helium, similar to what’s in birthday balloons as a gas.
It’s also in MRI machines, which need something called superconductivity to work. When certain materials get very, very cold, their electrical resistance drops away — and resistance is what causes stuff to heat up when you run electricity through it.
“For example, we know aluminum, when you lower its temperature to about 1 degree kelvin, it’s not just the resistance becoming less and less,” Ramos said, “At 1 degree kelvin, all the resistance vanishes.”
That’s important because MRIs use incredibly powerful magnetic fields that take a lot of electricity. If there were normal, non-superconducting resistance at play, they’d cook patients rather than help treat them.
But perhaps the most exciting thing in cold these days is what it can do for quantum computers, which dwarf the power of current supercomputers by using atomic particles to do computation instead of digital ones and zeroes.
Those quantum bits are very sensitive, though, any little thing can throw them off.
“You have to isolate the physical quantum system from the universe,” Ramos said. “And one way to do that is using cryogenics, basically ultra-low-temperature physics techniques.”
How cold is he going these days?
“So right now, in my lab, we have a small cryocooler that when we plug it in the electrical outlet, it can go to about 2 degrees kelvin in about 1 ½ hours,” he said. “So at that, we’re actually colder than outer space, and we can do that in about an hour and a half.”
The only warmth in deep, empty space, Ramos said, comes from background cosmic radiation left over from the big bang. His lab can get down to 0.3 kelvin, but it isn’t even the coldest.
Other labs go way lower.
“Sometimes, it’s a bit mind-boggling,” said Pertti Hakonen of the Low Temperature Lab at Aalto University in Helsinki. “You start thinking that maybe there are some other intelligent beings and they have produced even lower temperatures perhaps.”
In 1999, his lab set the cold world record — possibly actually the cold known-universe record.
“The smallest temperature that we have reached is around 100 picokelvin, and that’s one-tenth of a billion of degree above absolute zero,” he said. “That’s like point one and then nine zeros.”
The process starts with liquid helium to get down to a few kelvin, and then it gets pretty complicated. Basically, the researchers apply and then remove powerful magnetic fields, which causes a temperature drop in certain materials. In this case, some bits of the metal rhodium.
“It’s not very small,” Hakonen said. “I mean, it’s two grams of metal.”
It took decades of work to get those two grams of metal that cold, and years to even measure it after it was achieved. And there are other labs always striving to go colder, to add another zero after the decimal point.
But it’s not as if scientists are out there freezing things just for the sake of an announcing an impressively small number.
“You always want to have some kind of physical question that you want to test in the experiments, and the record temperatures, they are then byproducts of the research,” Hakonen said.
When things get very cold at the subatomic level, he said, a kind of mask falls away.
“If you do research at room temperature, then many things are masked by the thermal motion of atoms,” he said. “But when you go to lower temperatures, then you get rid of this, and you can really study the basic properties of material.”
The more scientists learn about cold, the lower they can go, and the more they can uncover about the fundamental nature of everything. It’s pretty cool.
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May 29, 2020 at 05:12PM
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Heat's easy, but cool is complicated, scientifically speaking - WHYY
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