Zero-point energy may not exist

In a previous post, I talked about how there might be more ordinary explanations for the Casimir effect that do not rely on vacuum energy. I hinted in that post that other phenomena supposedly attributed to vacuum energy might also have alternative explanations. In this post, I go into more detail about why, after 100 years, it may be time to put vacuum energy to rest.

Vacuum energy has for decades held the promise of limitless energy hiding below the surface of reality. Numerous zero-point energy patents have been filed making wild technological claims that this energy could be made available to humanity. Some start-up companies even claim to be able to do exactly that successfully.

But why believe that this energy exists at all?

It all has to do with quantum ground state. In classical physics, the ground state of an object is simply the energy it has when it has no energy. A spring that is neither stretched nor compressed, a weight that is lying on the ground, all have zero energy.

A quantum spring, however, also called a quantum harmonic oscillator does not have a zero ground state. Instead, if you calculate the lowest energy it can have, it is a positive number.

This seems to imply that any quantum system, no matter what state it is in, has energy. In fact, this lack of a zero ground state causes problems in quantum physics because it means that nothing has non-zero energy and because there is an infinite amount of nothing there must be an infinite amount of energy. Physicists, therefore, to give sensible results to quantum fields, had to invent a way to lower the ground state to zero. This is called normal ordering.

This leads us to question whether that non-zero ground state is a real thing or simply a byproduct of how we turn classical theories into quantum theories, a process called quantization.

Quantum field theory, which is the best theory of how matter works in the universe that we have, suggests that all matter particles are excitations of fields. The fields permeate the universe and matter interacts with those fields in various ways. That is all well and good of course. We are not questioning that these fields exist. The question is whether a field in a ground state has any measureable effect on matter.

The first effect that people readily point out is the Casimir effect. When two plates of metal are brought close enough together in a vacuum, there is a measureable force trying to close that gap. The explanation that has been given out for decades is that the plates cut off vacuum fluctuations with wavelengths larger than the gap. Because the density of fluctuations is lower inside the gap than outside, there is a measurable pressure from vacuum energy pushing the plates together.

This explanation is a cartoon at best and misleading at worst. The Casimir effect behaves differently depending on what you bring close together. A plate and a tiny sphere, for example, will experience either attractive or repulsive forces depending on how close they are together. This means that a nanosphere can levitate above a plate a few hundred nanometers. This is really good if you want to make a frictionless surface, but it has nothing to do with cutting off vacuum fluctuation wavelengths.

Instead, it has to do with the forces between atoms when quantum effects are taken into account. Every atom is surrounded by an electron cloud, which is a like quantum probability field for the position of the electrons. These clouds of electrons are repulsive against one another. Thus, two atoms brought close together should repel one another unless they are forced so close together the strong force takes over and their nuclei fuse. This only happens, however, at very high temperatures where electrons are stripped off the atoms in a state of matter called a plasma as in the Sun.

The electron clouds, however, are not perfectly spherical. They have a shape. If the electrons are biased in a particular way, the atoms can become attracted or repel one another. This is how many molecular bonds form.

With large collections of molecules that are relatively far apart (more than a nanometer), this force, called the van der Waals force, becomes extremely weak because of relativistic effects. Basically, the forces have to cross the gap between the two materials (metallic plates for example). The force interaction loses strength because of the finite speed of light which puts the action and reaction between the materials out of phase. This means that they do not have the strength of molecular bonds. Instead, there is a weak force between them which can be attractive or repulsive depending on distance and shape of the objects. Nevertheless, vacuum energy doesn’t come into the picture at all. It is simply the bulk interaction of the molecules.

Two other effects have been attributed to vacuum energy. Although neither has been observed, both are predicted by well-known principles of physics. These are Hawking radiation and the Unruh effect.