In
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
, an observable is a
physical quantity that can be measured. Examples include
position and
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...
. In systems governed by
classical mechanics
Classical mechanics is a physical theory describing the motion of macroscopic objects, from projectiles to parts of machinery, and astronomical objects, such as spacecraft, planets, stars, and galaxies. For objects governed by classi ...
, it is a
real-valued "function" on the set of all possible system states. In
quantum physics, it is an
operator, or
gauge
Gauge ( or ) may refer to:
Measurement
* Gauge (instrument), any of a variety of measuring instruments
* Gauge (firearms)
* Wire gauge, a measure of the size of a wire
** American wire gauge, a common measure of nonferrous wire diameter, es ...
, where the property of the
quantum state
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...
can be determined by some sequence of
operations. For example, these operations might involve submitting the system to various
electromagnetic field
An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical ...
s and eventually reading a value.
Physically meaningful observables must also satisfy
transformation laws that relate observations performed by different
observers in different
frames of reference. These transformation laws are
automorphisms of the state space, that is
bijective transformations that preserve certain mathematical properties of the space in question.
Quantum mechanics
In
quantum physics, observables manifest as
linear operators on a
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
representing the
state space of quantum states. The eigenvalues of observables are
real numbers that correspond to possible values the dynamical variable represented by the observable can be measured as having. That is, observables in quantum mechanics assign real numbers to outcomes of ''particular measurements'', corresponding to the
eigenvalue
In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denote ...
of the operator with respect to the system's measured
quantum state
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution i ...
. As a consequence, only certain measurements can determine the value of an observable for some state of a quantum system. In classical mechanics, ''any'' measurement can be made to determine the value of an observable.
The relation between the state of a quantum system and the value of an observable requires some
linear algebra
Linear algebra is the branch of mathematics concerning linear equations such as:
:a_1x_1+\cdots +a_nx_n=b,
linear maps such as:
:(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n,
and their representations in vector spaces and through matrice ...
for its description. In the
mathematical formulation of quantum mechanics, up to a phase constant, pure states are given by non-zero
vectors in a
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
''V''. Two vectors v and w are considered to specify the same state if and only if
for some non-zero
. Observables are given by
self-adjoint operators on ''V''. Not every self-adjoint operator corresponds to a physically meaningful observable. Also, not all physical observables are associated with non-trivial self-adjoint operators. For example, in quantum theory, mass appears as a parameter in the Hamiltonian, not as a non-trivial operator. For the case of a system of
particle
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from ...
s, the space ''V'' consists of functions called
wave functions or
state vectors.
In the case of transformation laws in quantum mechanics, the requisite automorphisms are
unitary
Unitary may refer to:
Mathematics
* Unitary divisor
* Unitary element
* Unitary group
* Unitary matrix
* Unitary morphism
* Unitary operator
* Unitary transformation
* Unitary representation In mathematics, a unitary representation of a grou ...
(or
antiunitary)
linear transformations of the Hilbert space ''V''. Under
Galilean relativity or
special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The law ...
, the mathematics of frames of reference is particularly simple, considerably restricting the set of physically meaningful observables.
In quantum mechanics, measurement of observables exhibits some seemingly unintuitive properties. Specifically, if a system is in a state described by a vector in a
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
, the measurement process affects the state in a non-deterministic but statistically predictable way. In particular, after a measurement is applied, the state description by a single vector may be destroyed, being replaced by a
statistical ensemble. The
irreversible nature of measurement operations in quantum physics is sometimes referred to as the
measurement problem
In quantum mechanics, the measurement problem is the problem of how, or whether, wave function collapse occurs. The inability to observe such a collapse directly has given rise to different interpretations of quantum mechanics and poses a key s ...
and is described mathematically by
quantum operations. By the structure of quantum operations, this description is mathematically equivalent to that offered by the
relative state interpretation where the original system is regarded as a subsystem of a larger system and the state of the original system is given by the
partial trace of the state of the larger system.
In quantum mechanics, dynamical variables
such as position, translational (linear)
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass ...
,
orbital angular momentum,
spin, and
total angular momentum are each associated with a
Hermitian operator
In mathematics, a self-adjoint operator on an infinite-dimensional complex vector space ''V'' with inner product \langle\cdot,\cdot\rangle (equivalently, a Hermitian operator in the finite-dimensional case) is a linear map ''A'' (from ''V'' to i ...
that acts on the
state of the quantum system. The
eigenvalues of operator
correspond to the possible values that the dynamical variable can be observed as having. For example, suppose
is an eigenket (
eigenvector) of the observable
, with eigenvalue
, and exists in a
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
. Then
This eigenket equation says that if a
measurement
Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determining how large or small a physical quantity is as compared ...
of the observable
is made while the system of interest is in the state
, then the observed value of that particular measurement must return the eigenvalue
with certainty. However, if the system of interest is in the general state
, then the eigenvalue
is returned with probability
, by the
Born rule.
The above definition is somewhat dependent upon our convention of choosing real numbers to represent real
physical quantities
A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For exam ...
. Indeed, just because dynamical variables are "real" and not "unreal" in the metaphysical sense does not mean that they must correspond to real numbers in the mathematical sense.
To be more precise, the dynamical variable/observable is a
self-adjoint operator in a Hilbert space.
Operators on finite and infinite dimensional Hilbert spaces
Observables can be represented by a Hermitian matrix if the Hilbert space is finite-dimensional. In an infinite-dimensional Hilbert space, the observable is represented by a
symmetric operator, which
may not be defined everywhere. The reason for such a change is that in an infinite-dimensional Hilbert space, the observable operator can become
unbounded, which means that it no longer has a largest eigenvalue. This is not the case in a finite-dimensional Hilbert space: an operator can have no more eigenvalues than the
dimension
In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coord ...
of the state it acts upon, and by the
well-ordering property, any finite set of real numbers has a largest element. For example, the position of a point particle moving along a line can take any real number as its value, and the set of
real numbers is
uncountably infinite. Since the eigenvalue of an observable represents a possible physical quantity that its corresponding dynamical variable can take, we must conclude that there is no largest eigenvalue for the position observable in this uncountably infinite-dimensional Hilbert space.
Incompatibility of observables in quantum mechanics
A crucial difference between classical quantities and quantum mechanical observables is that the latter may not be simultaneously measurable, a property referred to as
complementarity. This is mathematically expressed by non-
commutativity
In mathematics, a binary operation is commutative if changing the order of the operands does not change the result. It is a fundamental property of many binary operations, and many mathematical proofs depend on it. Most familiar as the name of ...
of the corresponding operators, to the effect that the
commutator
In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.
Group theory
The commutator of two elements, ...
This inequality expresses a dependence of measurement results on the order in which measurements of observables
and
are performed. Observables corresponding to non-commuting operators are called ''incompatible observables''. Incompatible observables cannot have a complete set of common
eigenfunctions. Note that there can be some simultaneous eigenvectors of
and
, but not enough in number to constitute a complete
basis.
See also
*
Measure (physics) The measure in quantum physics is the integration measure used for performing a path integral.
In quantum field theory, one must sum over all possible histories of a system.
When summing over possible histories, which may be very similar to each ...
*
Observable universe
The observable universe is a ball-shaped region of the universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because the electromagnetic radiation from these ob ...
*
Observer (quantum physics)
*
Table of QM operators
*
Unobservable
References
Further reading
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{{Quantum mechanics topics
Quantum mechanics