Zero point energy is the lowest possible amount of energy that a quantum mechanical system can have. This is different than in classical physics. This is because, in quantum mechanics, even in lowest energy states, there are fluctuations. This is described by the Heisenberg uncertainty principle. Atoms, molecules and the vacuum of space all have these properties. In fact, the universe, in quantum mechanics, can be thought of as a series of these continually fluctuating fields. This would be opposed to a series of isolated particles. There are even matter fields. The quanta of these matter fields are the fermions, such as the quarks and leptons. The quanta of these force fields are the bosons, such as the gluon, photon, W and Z. All of these fields have zero point energy. This fluctuating zero energy field will give rise to kind of reintroduction of the notion of the aether in physics. Some systems can detect the existence of this energy. However, this aether could not be a physical medium, since, Lorentz invariance and special relativity can not be violated.
There is currently no full theoretical model at the present to understand zero point energy. This discrepancy between theorized and observed vacuum energy has been a serious source of conversation.
Paul Dirac's theory of emission and absorption was published in 1927. This was the first application of the quantum theory of radiation. This dealt with the very process by which particles are created. This process is knows as spontaneous emission. Dirac gave us a description of the electromagnetic field. It was a collection of harmonic oscillators. This also introduced the concept of creation and annihilation operators. In this theory, spontaneous emission depended on the zero point energy of fluctuations of the field. This is in order to get started. When a photon is annihilated or absorbed, the photon can be thought of as making a transition to a vacuum state. By the same token, when a photon is emitted or created, we can imagine that the photon has transitioned out of the vacuum state.
Hendrik Casimir, in 1948, showed a consequence of the zero-point energy field. This would result in a positively attractive force field between two uncharged, parallel, perfectly conducting metal plates. This phenomenon is known as the Casimir effect.