You know, if you’ve got kids, the last couple of years you’ve probably been forced to watch
the movie Frozen like a gazillion times. But what with me being a scientist and all,
this got me to thinking. How cold is cold? Is there a coldest temperature possible? And,
if so, how close have scientists gotten to having achieved it? These are all interesting questions
with fascinating answers, so let’s look into it. And, I promise, I won’t break out into song...
well, okay- maybe at the end.
(intro music)
If we’re going to talk about cold, we need to talk about temperature scales.
Americans are familiar
with the Fahrenheit scale, which sets the freezing and boiling points of water to be thirty two and
two hundred and twelve degrees, respectively. Most of the rest of the world uses the Celsius
or centigrade scale and sets those points at zero and one hundred degrees. Because there is nothing
unique about water as a chemical, beyond being familiar, there is really nothing special about
either way to report temperatures.
However, there is a scale, called the Kelvin scale, after Lord Kelvin, that is much more
natural. To start with, Kelvin postulated that there was a coldest possible temperature.
Thinking on a molecular or atomic scale, the coldest possible temperature would be
the one in which the motion of the atoms was at an absolute minimum. Classically, we’d say
that this temperature is when molecules inside the object weren’t moving at all, although the quantum
picture is a little different.
Physicists use the Kelvin scale for most things. In the Kelvin scale, this lowest temperature,
which is called absolute zero by the way, is defined to be zero.
Zero degrees Kelvin is equal to minus 273.15 degrees Celsius and minus 459.67
degrees Fahrenheit. And, to be complete, in the Kelvin scale, water freezes at 273.15 and boils
at 373.15 Kelvin.
Okay, with those scales defined, how close can we get to absolute zero? Is it merely an ideal,
or is it achievable?
Well, let’s think of some ridiculously cold things, like the temperature Fermilab gets
in the dead of a particularly harsh winter. The recorded coldest temperature in the laboratory’s
vicinity is -36 degrees Fahrenheit, which is -37.78 degrees Celsius and 235.37 Kelvin.
235.37 is way higher than zero, so we’re going to have to get much colder than that.
What about the coldest recorded
temperature in Earth out in nature? That was back in 1983 in Antarctica, when the temperature hit a
bone-chilling negative 128.5 degrees Fahrenheit, which is negative 89.2 degrees Celsius and 184
Kelvin. Still a long way to go to absolute zero.
What about liquid nitrogen, which is what you get when the dominant substance in ordinary air
is chilled until it's liquid? That’s colder, coming in at minus 320.4 degrees Fahrenheit,
minus 195.8 degrees Celsius and 77 Kelvin. That’s better, but we’re not there yet.
Alright- let’s get serious. Helium is
the gas that fills children’s balloons; however, it can be cooled until it's a liquid. That
happens at -452.2 degrees Fahrenheit, -269 degrees Centigrade, and 4.2 Kelvin. And that is about
the temperature at which the Fermilab Tevatron operated until it was decommissioned in 2011.
So, this is great – we’ve gotten into single digit temperatures. But, of course, four-ish Kelvin
isn’t the same as zero, so can we do better?
Well, sure. 4.2 Kelvin is the temperature at which helium condenses from gas to liquid.
The temperature at which water condenses from gas to liquid is 100 Celsius or 212 Fahrenheit, and we
know we can cool water below that temperature. We should be able to get helium colder than the
temperature at which it boils.
Using the same conceptual tricks that we use to run commercial refrigerators and freezers,
we can cool helium more. At CERN, our sister laboratory over in Europe, they’ve built a
huge ring of magnets that is 27 kilometers or sixteen-plus miles around, down to 1.9 Kelvin.
It’s hard work, but we can make big things very cold. Using this advanced refrigeration
technology, we can get things down to something like 1 Kelvin. One-ish Kelvin is the lowest
temperature you can do on an industrial scale. To go lower, we need to do some new tricks.
At such low temperatures, helium
does some funny things. For instance, normal helium like you might find in a party balloon
is called helium 4 because it has four particles in the nucleus – two protons and two neutrons. On
the other hand, there’s a version of helium called helium 3, which has two protons and one neutron.
Helium 4 can be extracted from natural gas, but helium 3 is rare on Earth. Luckily,
it's a byproduct of nuclear power generation.
Helium 3 and helium 4 have very different properties, especially at temperatures just
under one Kelvin. If you put them in a container, the two liquids separate into two layers. Then,
and slowly, a bit of the helium 3 diffuses into the helium 4, which carries away some energy and
that can cool the remaining helium 4 even colder. This technology is called dilution refrigeration
and it was proposed back in the 1950s and first demonstrated in 1964 at Leiden University in the
Netherlands.
Dilution refrigerators can cool objects quite a bit colder. The physical limit using dilution
refrigeration is about 2 milliKelvin, which is 0.002 Kelvin. That’s close
enough to absolute zero that I’m not going to keep giving the Fahrenheit and Celsius
numbers.
And these dilution refrigerators can be super pretty, as you can see here. This one is located
at Fermilab and it operates at 5.5 milliKelvin. By the way, I say that the 2 milliKelvin limit is
a physical one because at about 2 milliKelvin, helium 3 changes phase and that diffusion trick
no longer works. Since that trick is the one we rely on to make continuous-use refrigerators,
that’s as cold as we can go for what we might call physical objects- that is to say things big enough
for humans to touch and manipulate.
Because of the need for super cold temperatures to do quantum computing, bigger dilution
refrigerators are being built. IBM and others are building big ones that should operate at
temperatures under 20 millikelvin, and my colleagues at Fermilab is building a big dilution
refrigerator – over two meters in diameter – big enough that they call it Colossus – that will
operate in the 10 to 20 millikelvin range.
Now, the millikelvin range is about the lowest we can go if you want to cool an actual object
and do some sort of laboratory-sized thing, but it is possible to cool small numbers of
atoms to even lower temperatures. That is done to study the nature of matter very,
very close to absolute zero.
Using magnets to align the spin of atomic nuclei and lasers to reject fast moving atoms,
it's possible to cool relatively small numbers of atoms to a microkelvin, that is to say a millionth
of a degree above absolute zero. Scientists at the National Institute of Standards and Technology
and other facilities routinely achieve that, and often into the nanokelvin range, which is
a billionth of a degree above absolute zero. And the current record is held by a group of
European scientists who in 2021 used every trick in the book to cool 100,000 rubidium atoms to 38
picoKelvin – that’s 38 trillionths of a Kelvin. They held the temperature for about ten seconds.
That’s just ridiculously cold.
So that’s in the lab. What's the coldest thing in the natural universe?
Well, if we look at the temperature of space itself, we see that it’s a relatively balmy
2.7 Kelvin. And even the coldest spot every observed in the universe is the Boomerang Nebula,
which has a temperature of about 1 Kelvin.
Okay, is 38 picoKelvin the limit? Well, no, probably not. Scientists are clever,
after all. However, what about literally, exactly, absolute zero. Is that achievable
and what happens then?
Well, if we could reach absolute zero, that would mean that the molecules and atoms would
not be moving and they would settle down into a completely orderly configuration. However,
quantum mechanics doesn’t allow for objects to be simultaneously stationary and precisely located.
This means that there is always some small quantum motion, and thus absolute zero is an
unachievable goal.
Now, you may have heard about negative temperatures on the Kelvin scale and they exist,
but the meaning of temperature is very different in that realm. For instance, one mind-bending
fact is that all negative temperatures are hotter than positive temperatures. I say that because if
you have an object with negative temperatures and touch it to an object with positive temperatures,
the energy from that negative temperature object will move into the positive temperature one.
Negative temperatures are just messed up and to explain them would take its own video. They’re
just like, weird, man.
So, there we go. That’s how cold we can go. The coldest natural place in the universe is
just a single degree Kelvin above absolute zero. In the lab, the coldest we can make
industrial scale things is a Kelvin or so. The lowest range of laboratory-scale things
is a couple of millikelvin and if we want to study a small number of atoms,
we can achieve temperatures as low as nanokelvin or even as low as a few tens
of picokelvins. Zero is impossible, and negative temperatures will bend your brain.
Okay, so that’s a quick look into how cold
things can get. I think what this all boils down to is don’t complain next winter how cold it is.
It could be way, way, way, worse. Alright, you know the drill- please subscribe to the channel.
You do that by poking that button down there. If you do, I’ll keep making more physics videos,
which is what all of us want, right? And physics videos is where it’s at, because, after all,
physics is everything.
Alright, what do you think Ian? Can I sing that song now?
(Frozen-style music starts playing)
(offscreen) Uh, yeah, we just ran out of time, sorry...
Oh, no, nah, nah, nah, I tell you what- I'm going to do it.
(offscreen) Yeah, I gotta-
They want it, they want to hear it-
(offscreen) I gotta cut though, Don, sorry, yeah-
(static)
(Frozen-style outro music plays)