Zero-point energy is the lowest-energy fluctuation that remains in a quantum system at absolute zero. It arises because quantum systems cannot generally have both perfectly fixed position and momentum.
For a quantum harmonic oscillator, the allowed energies are separated into levels, and the lowest level has energy one-half times the oscillator frequency multiplied by the reduced Planck constant. This ground-state energy is nonzero even when the oscillator has been cooled to 0 kelvin.
Zero-point energy is not ordinary thermal energy. Thermal energy decreases as temperature falls, whereas zero-point energy is a property of the quantum state and remains at the ground level. It appears in systems such as molecular vibrations, electromagnetic fields, and mechanical resonators.
The concept is also connected to the Casimir effect, in which closely spaced conducting plates experience a measurable force associated with altered electromagnetic modes. The interpretation of vacuum energy in cosmology remains a major unresolved problem because simple estimates differ enormously from the observed dark-energy density.