[PBS Digital Studios chime] Winter may be coming, but be comforted; true, absolute zero is impossible.

We'll always have quantum fluctuations to warm our chilly bones.

[Space Time theme music] The mystical-seeming quality of heat is nothing more than the motion of a substance's component particles.

Temperature is just a measure of internal kinetic energy.

So then, the feeling of cold is the relative absence of internal kinetic energy.

But what if we reduced temperature so much that all particle motion ceases?

This state of absolute cold is the zero point in the Kelvin temperature scale, corresponding to -273.15 Celsius.

Many experimental physicists have spent their careers trying to cool things to absolute zero.

Using lasers and magnetic fields we've now managed to cool certain substances to less than a billionth of a Kelvin.

Doing so has revealed some bizarre quantum states of matter.

But quantum mechanics may also prevent us from ever reaching absolute zero.

Understanding the limit to cold will lead us to an understanding of the nature of the quantum vacuum itself.

We're all familiar with the states of matter; solid, liquid, gas.

Heat up any solid and eventually it'll melt into a liquid, pump in more energy and all liquids will vaporize into gas.

Now that's not the end of it.

Yet more heat causes electrons in any gas to escape the bonds of their atoms resulting in the less-known plasma state.

In these states of matter, particles have an enormous range of individual energies; some moving or vibrating fast, some slow.

Temperature just represents the average kinetic energy of the countless particles.

And while a substance can theoretically have any temperature above absolute zero its component particles cannot.

Those particles are quantum creatures; they can only occupy certain energy levels of vibration or motion.

Much like the discrete electron orbitals in an atom.

This quantum nature is revealed when we look at the spectrum of light produced as those particles hop between energy levels.

This is the black-body radiation described by Planck's law.

Its mathematical form was our first hint at the quantum nature of the subatomic world.

The influence of the quantum world becomes far more apparent in the strange states of matter that exist at the cold end of the heat spectrum.

An example is the Bose-Einstein condensate.

As we sap energy out of certain substances its particles drop into the lowest possible energy state.

Once nearly all particles occupy that one quantum state they share a single, coherent wave function.

This causes them to behave in a strange, collective way: they become immune to individual excitation.

Individual particles can no longer be bumped or jostled out of that lower state.

This means that they flow with no resistance whatsoever.

In certain solids, bonded pairs of electrons - Cooper pairs - condense into this state.

They flow unrestricted through the material, making it a superconductor.

However, if the entire substance can somehow remain a fluid when it reaches the critical temperature for Bose-Einstein condensation it becomes what we call a superfluid.

It has zero viscosity; it can pass through the smallest openings, sustain whirlpools that last forever, and even climb over the walls of its container.

Only one substance is known to produce a superfluid for conditions possible in a lab.

And that's Helium.

In particular, Helium-4.

Helium-4 has a total spin of zero, which makes it a boson.

So: a particle with integer spin.

Bosons are able to occupy the same quantum state as each other unlike the half-integer spin fermions which cannot.

The other unique property of Helium is that it can't be frozen - it remains a liquid down to the smallest possible temperature.

Every other substance freezes into a solid before it can become a superfluid.

The unfreezability of Helium reveals an even deeper quantum mystery.

See, there's an absolute limit to how cold a substance can become.

In theory, absolute zero temperature means no thermal energy so no internal motion of particles whatsoever.

But what does it mean for a particle to be completely still?

Well, its position relative to its neighbors would be fixed and its momentum would be zero.

However, the most fundamental law of quantum mechanics forbids this.

The Heisenberg Uncertainty Principle tells us that there's an absolute limit in the knowability of particular combinations of properties.

For example, the more precisely a quantum particle's position is defined, the less defined is its momentum.

And this isn't about measurement; a particle with a perfectly-defined position has a perfectly-undefined momentum.

So try to fix a particle's position perfectly - try to hold it still - and its momentum enters a state of quantum haziness.

That momentum can then fluctuate, potentially to very high values.

At the lowest temperatures particle motion acquires a sort of quantum buzz.

This translates to a very real minimum in average energy and to a minimum temperature.

That temperature is just a teensy bit higher than absolute zero.

We call the lowest-possible energy of a quantum system its zero-point energy.

For a group of particles that make up any form of matter that zero-point energy isn't actually zero.

There's always a little bit of kinetic energy remaining and so it's impossible to reach absolute zero in temperature.

Other quantum systems also have non-zero zero-points and that leads to even stranger phenomena.

For example: The quantum fields that fill our universe also fluctuate due to the uncertainty principle resulting in what we know as vacuum energy.

And some quantum fields have an intrinsic non-zero zero-point before even bringing Heisenberg into it.

This leads to the famous Higgs mechanism and possibly also the phenomena of inflation and dark energy.

To understand the universe we need to understand how it behaves absent heat, absent light, and absent matter.

But we're getting ahead of ourselves.

We'll need another episode to explore the quantum nature of nothing as we peer deeper into the coldest, darkest, and emptiest patches of spacetime.