It’s a warm July day when I meet Fabian Kislat in his lab. Kislat, an experimental astrophysicist and a professor at the University of New Hampshire, is dressed for the summer weather: T-shirt, shorts.
But I’m here to see something almost impossibly cold. Something called a dilution refrigerator.
It takes up one side of the lab: a silver rack with a maze of golden plates and coils, not much larger than a water cooler, suspended inside.
It looks both futuristic and old-fashioned at the same time, like an elaborate steampunk dollhouse with three floors.
The refrigerator is off today, so we can see inside. But when it’s on, it gets really cold. Colder than the open freezer section at Market Basket. Colder than a winter dip at Rye beach. Even colder than the top of Mount Washington in February.
“Less than 100th of a degree above absolute zero — the lowest temperature that can theoretically be reached,” Kislat explains.
That’s about -460 degrees Fahrenheit.
And in addition to being very cold, this refrigerator allows Kislat to do something very cool: study the elements formed when huge stars die.
“What I'm really interested in is studying how the elements that the whole world, the universe is made of, how they are formed in supernova explosions,” he says. “To do that, we need to observe the supernova explosions. And we’re using gamma rays to do that.”
He’s working with special detectors that can measure the energy of gamma rays very precisely. They have to be super cold to work, because they’re made out of a material that becomes a superconductor of electricity at very low temperatures.
“When a gamma ray hits that detector, the energy of the gamma ray gets converted into heat, heating up that device just a little bit, moving along this transition from superconducting to normal conducting,” he says.
If that sounds complicated to you, don’t worry, it is. But what it takes to get to temperatures this cold — absolute zero cold — is a little more familiar.
Kislat says the dilution refrigerator uses pretty much the same process our kitchen refrigerators use. It’s based on an idea called phase transition. He says you can think of it like lemonade.
“If you put ice cubes into lemonade, they're not immediately going to melt, nor are they immediately going to freeze the lemonade. But we know they're trying to get at the same temperature,” he says.
The ice cubes cool the lemonade by melting — transitioning phases — using heat from the lemonade itself. Normal refrigerators use the same process, evaporating a fluid using the heat from the inside of the refrigerator. This is sort of the same idea for Kislat’s golden steampunk machine, but using two types of helium.
“Helium four, which is the normal helium that you'll have in your party balloons, and helium three, which does not occur in nature,” he says.
Mixed together just right, the two kinds of helium will separate into two phases. The process of transitioning phases uses the heat from inside of the refrigerator. That’s how it gets so cold.
In order to run, the refrigerator has to be covered with a vacuum vessel, which looks like a white can. At these temperatures, everything — every known element on earth except for helium — freezes solid. And it’s important to keep hands and feet outside of the refrigerator.
“If you somehow manage to touch it, anything would just freeze into a solid. Air would freeze into a solid,” Kislat says.
But when the machine is on, it’s still room temperature, on the outside.
“You never get to feel how cold it is,” he says. “And that is probably a good thing.”
Eventually, Kislat’s goal is to send his gamma ray detectors into space. The dilution refrigerator needs gravity to work, so that would stay on earth. But Kislat says there’s other machines that can make sure the detectors stay cool.
