In quantum mechanics, the angular momentum operator is one of several related operators analogous to classical angular momentum. The angular momentum operator plays a central role in the theory of atomic and molecular physics and other quantum problems involving rotational symmetry. Being an observable, its eigenfunctions represent the distinguishable physical states of a system's angular momentum, and the corresponding eigenvalues the observable experimental values. When applied to a mathematical representation of the state of a system, yields the same state multiplied by its angular momentum value if the state is an eigenstate (as per the eigenstates/eigenvalues equation). In both classical and quantum mechanical systems, angular momentum (together with linear momentum and energy) is one of the three fundamental properties of motion.Introductory Quantum Mechanics, Richard L. Liboff, 2nd Edition,
There are several angular momentum operators: total angular momentum (usually denoted J), orbital angular momentum (usually denoted L), and spin angular momentum (spin for short, usually denoted S). The term angular momentum operator can (confusingly) refer to either the total or the orbital angular momentum. Total angular momentum is always conserved, see Noether's theorem.
Overview
In quantum mechanics, angular momentum can refer to one of three different, but related things.
Orbital angular momentum The classical definition of angular momentum is . The quantum-mechanical counterparts of these objects share the same relationship:
where r is the quantum position operator, p is the quantum momentum operator, × is cross product, and L is the orbital angular momentum operator. L (just like p and r) is a vector operator (a vector whose components are operators), i.e. where Lx, Ly, Lz are three different quantum-mechanical operators.
In the special case of a single particle with no electric charge and no spin, the orbital angular momentum operator can be written in the position basis as: where is the vector differential operator, del.
Spin angular momentum
There is another type of angular momentum, called spin angular momentum (more often shortened to spin), represented by the spin operator . Spin is often depicted as a particle literally spinning around an axis, but this is only a metaphor: the closest classical analog is based on wave circulation. All elementary particles have a characteristic spin (scalar bosons have zero spin). For example, electrons always have "spin 1/2" while photons always have "spin 1" (details below).
Total angular momentum Finally, there is total angular momentum , which combines both the spin and orbital angular momentum of a particle or system:
Conservation of angular momentum states that J for a closed system, or J for the whole universe, is conserved. However, L and S are not generally conserved. For example, the spin–orbit interaction allows angular momentum to transfer back and forth between L and S, with the total J remaining constant.
Commutation relations
Commutation relations between components The orbital angular momentum operator is a vector operator, meaning it can be written in terms of its vector components . The components have the following commutation relations with each other:
where denotes the commutator
This can be written as
where l, m, n are the component indices (1 for x, 2 for y, 3 for z), and denotes the Levi-Civita symbol. Alternatively Einstein's summation convention can be used to write this as:
A compact expression as one vector equation is also possible:
The commutation relations can be proved as a direct consequence of the canonical commutation relations , where is the Kronecker delta.
There is an analogous relationship in classical physics:H. Goldstein, C. P. Poole and J. Safko, Classical Mechanics, 3rd Edition, Addison-Wesley 2002, pp. 388 ff.
where Ln is a component of the classical angular momentum operator, and is the Poisson bracket.
The same commutation relations apply for the other angular momentum operators (spin and total angular momentum):
These can be assumed to hold in analogy with L. Alternatively, they can be derived as discussed below.
These commutation relations mean that L has the mathematical structure of a Lie algebra, and the are its structure constants. In this case, the Lie algebra is SU(2) or SO(3) in physics notation ( or respectively in mathematics notation), i.e. Lie algebra associated with rotations in three dimensions. The same is true of J and S. The reason is discussed below. These commutation relations are relevant for measurement and uncertainty, as discussed further below.
In molecules the total angular momentum F is the sum of the rovibronic (orbital) angular momentum N, the electron spin angular momentum S, and the nuclear spin angular momentum I. For electronic singlet states the rovibronic angular momentum is denoted J rather than N. As explained by Van Vleck, the components of the molecular rovibronic angular momentum referred to molecule-fixed axes have different commutation relations from those given above which are for the components about space-fixed axes.
Commutation relations involving vector magnitude Like any vector, the square of a magnitude can be defined for the orbital angular momentum operator,
is another quantum operator. It commutes with the components of ,
One way to prove that these operators commute is to start from the [Lℓ, Lm] commutation relations in the previous section:
Mathematically, is a Casimir invariant of the Lie algebra SO(3) spanned by .
As above, there is an analogous relationship in classical physics:
where is a component of the classical angular momentum operator, and is the Poisson bracket.Goldstein et al, p. 410
Returning to the quantum case, the same commutation relations apply to the other angular momentum operators (spin and total angular momentum), as well,
Uncertainty principle
In general, in quantum mechanics, when two observable operators do not commute, they are called complementary observables. Two complementary observables cannot be measured simultaneously; instead they satisfy an uncertainty principle. The more accurately one observable is known, the less accurately the other one can be known. Just as there is an uncertainty principle relating position and momentum, there are uncertainty principles for angular momentum.
The Robertson–Schrödinger relation gives the following uncertainty principle:
where is the standard deviation in the measured values of X and denotes the expectation value of X. This inequality is also true if x, y, z are rearranged, or if L is replaced by J or S.
Therefore, two orthogonal components of angular momentum (for example Lx and Ly) are complementary and cannot be simultaneously known or measured, except in special cases such as .
It is, however, possible to simultaneously measure or specify L2 and any one component of L; for example, L2 and Lz. This is often useful, and the values are characterized by the azimuthal quantum number (l) and the magnetic quantum number (m). In this case the quantum state of the system is a simultaneous eigenstate of the operators L2 and Lz, but not of Lx or Ly. The eigenvalues are related to l and m, as shown in the table below.
Quantization
In quantum mechanics, angular momentum is quantized – that is, it cannot vary continuously, but only in "quantum leaps" between certain allowed values. For any system, the following restrictions on measurement results apply, where is reduced Planck constant: If you measure... ...the result can be... Notes , is sometimes called azimuthal quantum number or orbital quantum number. , is sometimes called magnetic quantum number.
This same quantization rule holds for any component of ; e.g., .
This rule is sometimes called spatial quantization.Introduction to quantum mechanics: with applications to chemistry, by Linus Pauling, Edgar Bright Wilson, page 45, google books link , s is called spin quantum number or just spin. For example, a spin- particle is a particle where s = . , is sometimes called spin projection quantum number.
This same quantization rule holds for any component of ; e.g., . , j is sometimes called total angular momentum quantum number. , is sometimes called total angular momentum projection quantum number.
This same quantization rule holds for any component of ; e.g., .
Derivation using ladder operators
A common way to derive the quantization rules above is the method of ladder operators. The ladder operators for the total angular momentum are defined as:
Suppose is a simultaneous eigenstate of and (i.e., a state with a definite value for and a definite value for ). Then using the commutation relations for the components of , one can prove that each of the states and is either zero or a simultaneous eigenstate of and , with the same value as for but with values for that are increased or decreased by respectively. The result is zero when the use of a ladder operator would otherwise result in a state with a value for that is outside the allowable range. Using the ladder operators in this way, the possible values and quantum numbers for and can be found.
Since and have the same commutation relations as , the same ladder analysis can be applied to them, except that for there is a further restriction on the quantum numbers that they must be integers.
Visual interpretation
Since the angular momenta are quantum operators, they cannot be drawn as vectors like in classical mechanics. Nevertheless, it is common to depict them heuristically in this way. Depicted on the right is a set of states with quantum numbers , and for the five cones from bottom to top. Since , the vectors are all shown with length . The rings represent the fact that is known with certainty, but and are unknown; therefore every classical vector with the appropriate length and z-component is drawn, forming a cone. The expected value of the angular momentum for a given ensemble of systems in the quantum state characterized by and could be somewhere on this cone while it cannot be defined for a single system (since the components of do not commute with each other).
Quantization in macroscopic systems The quantization rules are widely thought to be true even for macroscopic systems, like the angular momentum L of a spinning tire. However they have no observable effect so this has not been tested. For example, if is roughly 100000000, it makes essentially no difference whether the precise value is an integer like 100000000 or 100000001, or a non-integer like 100000000.2—the discrete steps are currently too small to measure. For most intents and purposes, the assortment of all the possible values of angular momentum is effectively continuous at macroscopic scales.
Angular momentum as the generator of rotations
The most general and fundamental definition of angular momentum is as the generator of rotations. More specifically, let be a rotation operator, which rotates any quantum state about axis by angle . As , the operator approaches the identity operator, because a rotation of 0° maps all states to themselves. Then the angular momentum operator about axis is defined as:
where 1 is the identity operator. Also notice that R is an additive morphism : ; as a consequence
where exp is matrix exponential. The existence of the generator is guaranteed by the Stone's theorem on one-parameter unitary groups.
In simpler terms, the total angular momentum operator characterizes how a quantum system is changed when it is rotated. The relationship between angular momentum operators and rotation operators is the same as the relationship between Lie algebras and Lie groups in mathematics, as discussed further below.
Just as J is the generator for rotation operators, L and S are generators for modified partial rotation operators. The operator