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. It usually has no physical particles. But the quantum vacuum is not just empty space. It has quick electromagnetic waves and particles that appear and disappear in the quantum field.

The QED vacuum of quantum electrodynamics (or QED) was the first vacuum of quantum field theory to be developed. QED started in the 1930s. In the late 1940s and early 1950s, it was changed by Feynman, Tomonaga, and Schwinger. They shared a Nobel prize for this work in 1965. Today, the electromagnetic interactions and the weak interactions are combined (only at very high energies) in the theory of the electroweak interaction.

The Standard Model is a broader idea that includes all known elementary particles and their interactions (except gravity). Quantum chromodynamics (or QCD) is part of the Standard Model that deals with strong interactions. The QCD vacuum is the vacuum of quantum chromodynamics. Scientists study it at the Large Hadron Collider and the Relativistic Heavy Ion Collider. It is linked to the vacuum structure of strong interactions.

Non-zero expectation value

Main article: Vacuum expectation value

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

When studying these fields 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 the Standard Model, this is how some particles get their mass.

Energy

Main article: Vacuum energy

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

Symmetry

In theories that explain how fast things move and where they are, the empty space has a special balance called Poincaré invariance. This means only certain simple parts of the theory can behave in a special way.

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 allows.

Non-linear permittivity

Main article: Schwinger limit

When very strong electric fields touch what we think of as empty space, scientists think space might act a little differently. This idea comes from theories that mix electricity and magnetism with the rules of tiny particles. These theories say that under very strong conditions, empty space might change a tiny bit in how it lets electric fields pass through.

Researchers are still working to see if this is true, because the electric fields needed are extremely strong. If it is true, it would mean that even "empty" space has hidden secrets that only appear under very special conditions.

Virtual particles

Main article: Virtual particle

In the quantum vacuum, tiny waves and particles appear and disappear quickly. These are called virtual particles. They happen because space isn’t completely empty. Even though, on average, it seems like there is 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. 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.