The Nature of Nothing | Space Time

In our last episode, we talked about the nature of absolute cold.

We saw that it's actually impossible to reduce any substance to absolute zero in temperature.

Zero Kelvin means no motion whatsoever in a substance's constituent particles.

But that perfect stillness implies that a particle's position and momentum are simultaneously perfectly defined, and this is impossible according to the Heisenberg uncertainty principle.

Fix a particle's position and its momentum, and so its motion becomes a quantum blur of many possible momenta.

This results in a real minimum average kinetic energy called a zero-point energy.

So the walls of our empty jar will always radiate a faint heat glow.

But hypothetically, what would perfectly empty space look like, far from the nearest particle of matter or radiation? The answer will bring us closer to understanding the nature of space itself.

Our modern understanding of the quantum nature of space is described by quantum field theory.

We've talked about QFT a lot recently, but for a refresher, this episode is especially useful.

In short, space itself is comprised of fundamental quantum fields, one for each elementary particle.

Those fields oscillate, vibrate with different energies, and those oscillations are the electrons, quarks, neutrinos, photons, gluons, et cetera, that comprise the stuff of our universe.

Now, these fields are quantum fields, which means their oscillations can't just have any old energy.

They can only be excited in quantized chunks, integer multiples of some baseline energy.

In each quantum state, so each combination of particle properties, there's a ladder of energy levels, a bit like electron orbitals in an atom.

Each new rung of the ladder represents the existence of one additional particle in that quantum state.

In fact, the math of quantum field theory is all about going up and down this particle ladder using so-called creation and annihilation operators.

We'll come back to those when we talk about Hawking radiation in the future.

The bottom of this energy ladder corresponds to these quantum oscillators having no energy, which means there are no particles in the given quantum state.

We call this the vacuum state of the field.

Inside a perfect vacuum, all of the fields at all locations should be in the vacuum state, exactly zero energy at all times.

But here we run up against that pesky Heisenberg uncertainty principle once again.

We saw that it's impossible to simultaneously fix position and momentum.

Well, it's also impossible to simultaneously perfectly define time and energy.

The more tightly we try to define the time window for the behavior of a quantum oscillator, the less certain we can be of its energy state in that time window.