Quantum vacuum state

In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. However, the quantum vacuum is not a simple empty space, but instead contains fleeting electromagnetic waves and particles that pop into and out of the quantum field.

The QED vacuum of quantum electrodynamics (or QED) was the first vacuum of quantum field theory to be developed. QED originated in the 1930s, and in the late 1940s and early 1950s, it was reformulated by Feynman, Tomonaga, and Schwinger, who jointly received the Nobel prize for this work in 1965. Today, the electromagnetic interactions and the weak interactions are unified (at very high energies only) in the theory of the electroweak interaction.

The Standard Model is a generalization of the QED work to include all the known elementary particles and their interactions (except gravity). Quantum chromodynamics (or QCD) is the portion of the Standard Model that deals with strong interactions, and the QCD vacuum is the vacuum of quantum chromodynamics. It is the object of study in the Large Hadron Collider and the Relativistic Heavy Ion Collider, and is related to the so-called vacuum structure of strong interactions.

Non-zero expectation value

Main article: Vacuum expectation value

In simple terms, the quantum vacuum is like the lowest energy state in a special kind of science called quantum field theory. Usually, this state has no particles in it. But sometimes, even in this empty space, certain fields can have small, invisible waves or particles that appear and disappear quickly.

When the rules used to study these fields become tricky at very low energies, some fields can have values that are not zero even in the vacuum. This happens because of something called spontaneous symmetry breaking. In a big theory called the Standard Model, this is how some particles get their mass.

Energy

Main article: Vacuum energy

The quantum vacuum, the emptiest space possible, still has energy called zero-point energy. Scientists can see this energy in experiments, like the Casimir effect. In the study of the universe, this vacuum energy is linked to something called the cosmological constant. Even a tiny bit of empty space has a very small amount of energy.

Symmetry

In a theory that describes how fast things move and where they are, the empty space has a special balance called Poincaré invariance. This means that only certain simple parts of the theory can show special behavior.

Sometimes, the empty space does not follow all the rules of the theory. When this happens, we say that spontaneous symmetry breaking has occurred. This means the empty space has less balance than the theory itself allows.

Non-linear permittivity

Main article: Schwinger limit

When very strong electric fields are applied to what we normally think of as empty space, scientists believe that space itself might behave a little differently. This idea comes from theories that combine electricity and magnetism with the rules of quantum physics. These theories suggest that under extreme conditions, empty space could show very slight changes in how it allows electric fields to pass through.

Researchers are still trying to test if this is true, as the electric fields needed are incredibly powerful. If proven, it would mean that even "empty" space has hidden complexities that only show up under very special conditions.

Virtual particles

Main article: Virtual particle

In the quantum vacuum, there are tiny waves and particles that appear and disappear quickly. These are called virtual particles. They happen because the space isn’t completely empty. Even though, on average, there seems to be nothing there, there are small changes in the energy of the space. These changes let particles appear for very short times before disappearing again. This idea helps scientists understand some of the strange behaviors in the tiny world of particles.

The idea comes from a rule in physics that says energy and time are linked in a special way, but not everyone agrees on exactly what this means. Some think it means particles can “borrow” energy for a short time and then give it back. Others are not sure this is the right way to think about it. Scientists are still working to understand these ideas better.

Physical nature of the quantum vacuum

When we imagine a space with no matter and cool it to the coldest possible temperature, we create what scientists call the quantum vacuum state. Even though it seems empty, this space is full of tiny, quick changes. These changes happen because of energy that always exists in empty space.

Scientists have studied how very small parts of atoms interact with this special empty space. One example is the Casimir effect, where two special plates move toward each other because of forces from this empty space. These ideas help us understand how tiny parts of atoms and energy work together in the world of very small things.