“Nature abhors a vacuum” — you’ve probably heard this phrase in one version or another. It’s true to some extent, especially within the boundaries of our planet, but in the grand scheme of things, it has more to do with how physicists tackle things than with nature itself.

The vacuum you have in mind is emptiness — the complete absence of matter, that’s the classical definition of a vacuum. But what we typically call a ‘vacuum’ is not exactly empty.

Fields

The problem with talking about nothing is that we need to discuss everything. Let’s take it step by step. The first thing we have to go through is the concept of a field. The most familiar ones are probably the gravitational field, the one which makes us stay on the Earth, or the electric field that helps transmit energy for our houses. 

In quantum mechanics, fields get very tricky. Instead of thinking of the world as made of particles, quantum physicists see everything as fields whose interactions combine to produce what we call a particle. Things get even trickier when we look at a vacuum in this light, because even if it is ‘nothing’, a field oscillates on the quantum scale.

According to quantum mechanics, a vacuum is not simply empty space, but rather a place with the lowest possible energy, where fleeting electromagnetic waves still pop in and out. These fluctuations can become energetic enough to make a particle appear.

Zero-point energy

A key concept in quantum mechanics is Heisenberg’s uncertainty principle, which states that you can’t be fully accurate when predicting all the physical parameters of a particle. Heisenberg himself famously described it thusly:

“One can never know with perfect accuracy both of those two important factors which determine the movement of one of the smallest particles—its position and its velocity. It is impossible to determine accurately both the position and the direction and speed of a particle at the same instant.”

According to the uncertainty principle, a system cannot be absolutely motionless, because then you’d know its speed, and since it doesn’t move, you’d also know its position. So particles always need to fluctuate — even at absolute zero. There is always some energy, it is never actually zero.  

For microscopic phenomena (keep in mind, we’re only talking microscopic here), there are specific energy levels that can happen — the particle is either in one energy level or the next, there is no between. Think of energy levels as a staircase: you can either be on a stair or not, not in between. There’s also a lowest level of energy for the quantum harmonic oscillator — which is never zero, but rather something called the zero-point energy.

Casimir effect

The way we can measure the zero-point energy in the macroscopic world is through Casimir forces. Hendrik Casimir theorized an attraction between two metallic neutral plates in the vacuum. This is counter-intuitive, we know that opposite charges attract and similar charges repel, how could this result in attraction?

Before this idea, physicists were aware of the interaction between two neutral structures (atoms or molecules), called the van der Waals force. This is possible as long as the charges in the structure could be arranged in order to form a polarity, like with our batteries, which have a positive and a negative pole.

To explain that, we need to imagine that the atoms are connected like a knotted rope, each atom in the knots. This connection is not rigid like ropes, but very elastic, when one atom is pushed downwards, the other is pulled upwards. This constitutes oscillations that create a field possible to make the neutral atoms attract each other.

In 1958, Marcus Sparnaay published results from an experiment measuring the Casimir force. He used two mirrors perfectly parallel to each other to do so, they were small at about 1 cm² ad separated by 0.000001 meters. That is because the Casimir force is too small to be detected at larger distances. To make the mirrors neutral, they had to touch each other. 

A more sophisticated experiment was designed in 1997 by Steve Lamoreaux who used a sphere and a plate, which is much easier to align instead of trying to make two mirrors perfectly aligned. The sphere was a 4cm diameter lens made of quartz painted with metallic material and the plate with the same materials. If the Casimir force made the sphere and the plate attract, a small bar would be twisted. His measurement was accurate, demonstrating that the theory is correct.

Cosmology

The weirdness of vacuums also has implications on the largest of scales. The vacuum energy is thought to be, in cosmological terms, the cosmological constant introduced by Einstein. He believed the universe was static, so there should be some constant added to Einstein’s Field equations to keep the universe fixed. Except the universe isn’t static — it’s expanding, and at an accelerated pace.

This could be the explanation for what dark energy is — something that has “negative pressure,” a completely counterintuitive process here on Earth, but which could explain why the universe keeps expanding faster and faster, despite the gravitational pull of objects. We have evidence of dark energy through many probes like the cosmic microwave background radiation or cosmic distance ladder. In other words, the negative pressure is accelerating the universe’s expansion. 

But things are rarely clear when we’re trying to explain the universe. In this case, the idea seems valid, and the principle seems sound, but the numbers don’t really add up.

The concept of vacuum energy from particle physics, when applied to the physics that describes the cosmological scales, which is general relativity, has a value originally obtained as 10 with 117 zeros greater than observed by the cosmic microwave background radiation — so there’s still a lot of finessing to do with this theory. 

Ultimately, the big question is how to connect the quantum scale to the macroscopic scale. It seems weird, but in order to understand the biggest things in the universe, we have to understand the smallest things in the universe. Or, to put it differently, in order to understand nothing, we must understand everything.