Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have and it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of the Heisenberg uncertainty principle.
Because of the uncertainty principle all physical systems, even at absolute zero temperature, have a zero-point energy that is greater than zero. Liquid helium-4 (4He) remains liquid—it does not freeze—under atmospheric pressure no matter how low its temperature is, because of its zero-point energy.
The concept of zero-point energy was developed in Germany by a group of physicists, among them Max Planck (1911), Albert Einstein and Otto Stern (1913). In 1916 Walther Nernst postulated that the vacuum of space is filled with zero-point electromagnetic radiation. The term zero-point energy originates from the German Nullpunktenergie.
Vacuum energy is the zero-point energy of all the fields in space, which in the Standard Model includes the electromagnetic field, other gauge fields, fermionic fields, and the Higgs field. It is the energy of the vacuum, which in quantum field theory is defined not as empty space but as the ground state of the fields. In cosmology, the vacuum energy is one possible explanation for the cosmological constant. The variation in zero-point energy as the boundaries of a region of vacuum move leads to the Casimir effect, which is observable in nanoscale devices. A related term is zero-point field, which is the lowest energy state of a particular field.