The term virtual state is commonly used to refer to two different types of states in physical systems. It may refer to a very short-lived, unobservable quantum state or a real, but unstable, state. Early definitions of the term (for example, see ) appear to distinguish the virtual state from the "virtual quantum."

Quantum State

In many quantum processes a virtual state is an intermediate state, sometimes described as "imaginary" in a multi-step process that mediates otherwise forbidden transitions. Since virtual states are not eigenfunctions of anything , normal parameters such as occupation, energy and lifetime need to be qualified. No measurement of a system will show one to be occupied, but they still have lifetimes derived from uncertainty relations. While each virtual state has an associated energy, no direct measurement of its energy is possible but various aproaches have been used to make some measurements( for example see and related work on virtual state spectroscopy) or extract other parameters using measurement techniques that depend upon the virtual state's lifetime. The concept is quite general and can be used to predict and describe experimental results in many areas including Raman spectroscopy, non-linear optics generally, various types of photochemistry , and/or nuclear processes . The term is often invoked in speculative treatments of such controversial topics as cold fusion, zero-point energy extraction[ citations to follow] , time-travel, anti-gravity, and consciousness.

Unstable State

The term virtual state can also be used to refer to bound or transient states which can decay into free states or relax at some finite rate. This state may be the metastable state of a certain class of Feshbach resonance

Feshbach resonance

In physics, Feshbach resonance, named after Herman Feshbach, is a resonance of a many-body system in which a bound state is achieved if the coupling between an internal degree of freedom and the reaction coordinates which lead to dissociation vanish...

 , "A special case of a Feshbach-type resonance occurs
when the energy level lies near the very top of the
potential well. Such a state is called 'virtual'" and may be further contrasted to a shape resonance

Shape resonance

A shape resonance is a metastable state in which an electron is trapped due the shape of a potential barrier.Serhan describes a state as being a shape resonance if, "the internal state of the system remains unchanged upon disintegration of the quasi-bound level."A more general discussion of...

 depending on the angular momentum.. Because of their transient existence, they can require special techniques for analysis and measurement, for example.

See also

• Two photon absorption
• Virtual particle
  Virtual particle
  In physics, a virtual particle is a particle that exists for a limited time and space. The energy and momentum of a virtual particle are uncertain according to the uncertainty principle...
  
  
• Feshbach resonance
  Feshbach resonance
  In physics, Feshbach resonance, named after Herman Feshbach, is a resonance of a many-body system in which a bound state is achieved if the coupling between an internal degree of freedom and the reaction coordinates which lead to dissociation vanish...
  
  
• Shape resonance
  Shape resonance
  A shape resonance is a metastable state in which an electron is trapped due the shape of a potential barrier.Serhan describes a state as being a shape resonance if, "the internal state of the system remains unchanged upon disintegration of the quasi-bound level."A more general discussion of...