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Imagine you made yourself a cup of tea, stirred it, left it for five minutes…
and came back to see that it was still spinning.
Now, imagine you picked up that cup, and the tea fell straight through the bottom…
while also starting to climb up the sides of the cup and flow out the top.
It may sound impossible, but if your cup was full of helium cooled to about -270°C,
that’s exactly what you’d see.
Because at that temperature, helium becomes a weird substance known as a superfluid.
It’s part of a bizarre world of ultracold phenomena that have won theoretical and experimental
physicists alike several Nobel Prizes.
And it might even hold the key to understanding the nature of spacetime itself.
To understand how superfluids work, it helps to know a little about how particles behave.
Thanks to quantum mechanics, things like atoms and molecules can’t have just any old amount
of energy
. Instead, their energy comes in discrete levels.
In other words, an atom or molecule can jump between different energy levels,
but it can’t ever be in between those levels.
This is true for any molecule, but in everyday life, you don’t actually notice.
There are just so many randomly-moving particles out there that other effects outweigh the
quantum ones.
But when you cool things down, the situation starts to get a bit strange --
especially when it comes to helium.
At low temperatures, most things just freeze and become boring blocks of ice.
But helium is unique among the elements in that it basically never freezes.
At atmospheric pressure, it remains a liquid pretty much all the way to absolute zero,
which is -273.15°C -- also called zero Kelvin.
But things get weird even before you hit that point.
When you get to the -270° range, or just above 2 Kelvin, it’s like liquid helium
totally forgets how matter is supposed to work.
This happens because the atoms are falling into lower and lower energy states as they
cool down.
And as liquid helium gets cold, more of the atoms fall into the same low-energy state.
This is when the weirdness really starts.
Because the rules of quantum mechanics mean that when the atoms are on the same energy
level, they start to behave in unison.
They literally become mathematically indistinguishable, and all behave the same way.
So they don’t bump into each other or even move in different directions like a regular
liquid.
Instead, all that is replaced with the perfectly coordinated unison of a superfluid.
Because they’re all moving together, there are no atoms bumping into and sliding off
one another, which means there is zero friction between them.
Start stirring them, and they’ll basically never stop spinning.
The liquid can also slip past anything: itself, the walls of its container --
even microscopic cracks in the bottom of the beaker it’s in.
And if you don’t have a perfectly tight lid on your container, the helium will straight-up
go rogue.
All liquids tend to climb up the walls of the container they’re in, but usually,
the friction between the walls and the liquid are enough to counter the effect.
That is not true with superfluids:
They will get pushed right up the walls and over the top of the container.
This can happen even with the septillion atoms in a beaker on a lab bench --
which is inconvenient, but also amazing, because it’s a purely quantum phenomenon you can
see with your eyes.
Now, to be totally clear, these effects don’t work with just any helium atom.
It specifically needs to be helium-4.
Helium-4 has two protons, two electrons, and two neutrons.
And that configuration means the atom behaves like a type of particle called a boson,
which is known for its ability to occupy the same energy level as its neighbors.
Only these kinds of atoms can slip into that low-energy state together and become a superfluid.
Helium-3, which has one fewer neutron, can’t do that, because on a quantum level, that
missing neutron means it behaves differently.
Well, for the most part.
It turns out that helium-3 atoms can sort of “team up” in pairs called Cooper pairs.
And those can behave like a boson, meaning they can condense just like atoms of helium-4.
This means you can make a superfluid out of them.
But the helium-3 and helium-4 superfluids behave differently.
To get helium-3 to work, you have to cool it down to less than 3 millikelvin.
Yes, less than three thousandths of a degree above absolute zero.
By contrast, helium-4 only needs to go down to 2.1 Kelvin.
Which sure is warmer, although, y’know, still colder than most of deep space.
and to make things more complicated, helium-3 is only 0.0001% of all the helium found in
nature.
Most of the stuff around today comes from nuclear reactors.
So making any superfluid is hard, but making one from helium-3 that is worthy of a Nobel
Prize or two!
Even though they’re weird and amazing, superfluids are more than just an interesting quantum
quirk.
Research suggests that superfluids might make up the cores of neutron stars, which are tiny,
dense objects in space made almost entirely of neutrons.
Superfluidity is also closely related to other weird quantum effects, like superconductivity,
where electricity can flow through wires with zero resistance.
If we could control superconductors better, we could make batteries that never degrade,
better quantum computers, lots of other useful things.
There’s even an idea out there that spacetime itself might be a superfluid,
which may help in the long quest for a theory of everything.
This is a theory that would explain both very large and very small systems, and scientists
have been searching for it for years.
This just goes to show that sometimes in physics, messing around with some equations on a chalkboard,
or messing around with some chemicals on a lab bench, can lead to discoveries that can
change the world.
All of these discoveries started with one big idea and one basic step, whether it was
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