Editing Zero-point energy (section)

{{Short description|Lowest possible energy of a quantum system or field}}
{{For|related articles|Quantum vacuum (disambiguation)}}
{{Distinguish|Zero Point (photometry)}}
{{Other uses|Zero point (disambiguation)}}
{{Use dmy dates|date=May 2020}}
[[File:2 Helium.png|thumb|[[Liquid helium]] retains [[kinetic energy]] and does not freeze regardless of temperature at standard atmospheric pressure due to zero-point energy. When cooled below its [[Lambda point]], it exhibits properties of [[superfluidity]].]]
{{Quantum mechanics}}

'''Zero-point energy''' ('''ZPE''') is the lowest possible [[energy]] that a [[quantum mechanical]] system may have. Unlike in [[classical mechanics]], quantum systems constantly [[Quantum fluctuation|fluctuate]] in their lowest energy state as described by the [[Heisenberg uncertainty principle]].{{sfnp|Sciama|1991|p=137}} Therefore, even at [[absolute zero]], atoms and molecules retain some vibrational motion. Apart from [[atom]]s and [[molecule]]s, the empty space of [[Vacuum state|the vacuum]] also has these properties. According to [[quantum field theory]], the universe can be thought of not as isolated particles but continuous fluctuating [[Field (physics)|field]]s: [[matter]] fields, whose [[Quantum|quanta]] are [[fermions]] (i.e., [[lepton]]s and [[quark]]s), and [[Force field (physics)|force field]]s, whose quanta are [[boson]]s (e.g., [[photon]]s and [[gluon]]s). All these fields have zero-point energy.{{sfnp|Milonni|1994|p=35}} These fluctuating zero-point fields lead to a kind of reintroduction of an [[Luminiferous aether|aether]] in physics{{sfnp|Sciama|1991|p=137}}{{sfnp|Davies|2011}} since some systems can detect the existence of this energy.{{citation needed|date=October 2024}} However, this aether cannot be thought of as a physical medium if it is to be [[Lorentz invariant]] such that there is no contradiction with [[Albert Einstein]]’s theory of [[special relativity]].{{sfnp|Sciama|1991|p=137}}

The notion of a zero-point energy is also important for [[cosmology]], and physics currently lacks a full theoretical model for understanding zero-point energy in this context; in particular, the discrepancy between theorized and observed vacuum energy in the universe is a source of major contention.<ref>See {{harvp|Weinberg|1989}} and {{harvp|Peebles|Ratra|2003}} for review articles and {{harvp|Shiga|2005}}, {{harvp|Siegel|2016}} for press comment</ref> Yet according to Einstein's theory of [[general relativity]], any such energy would gravitate, and the experimental evidence from the [[expansion of the universe]], [[dark energy]] and the [[Casimir effect]] shows any such energy to be exceptionally weak. One proposal that attempts to address this issue is to say that the [[fermion field]] has a negative zero-point energy, while the [[boson field]] has positive zero-point energy and thus these energies somehow cancel out each other.{{sfnp|Weinberg|2015|p=376}}{{sfnp|Sciama|1991|p=138}} This idea would be true if [[supersymmetry]] were an exact [[Symmetry (physics)|symmetry of nature]]; however, the [[LHC|Large Hadron Collider]] at [[CERN]] has so far found no evidence to support it. Moreover, it is known that if supersymmetry is valid at all, it is at most a [[Symmetry breaking|broken symmetry]], only true at very high energies, and no one has been able to show a theory where zero-point cancellations occur in the low-energy universe we observe today.{{sfnp|Sciama|1991|p=138}} This discrepancy is known as the [[cosmological constant problem]] and it is one of the greatest [[List of unsolved problems in physics|unsolved mysteries in physics]]. Many physicists believe that "the vacuum holds the key to a full understanding of nature".{{sfnp|Davies|1985|p=104}}