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A conserved quantity is a property or value that remains constant over time in a system even when changes occur in the system. In mathematics, a conserved quantity of a dynamical system is formally defined as a function of the dependent variables, the value of which remains constant along each trajectory of the system.^[1]

Not all systems have conserved quantities, and conserved quantities are not unique, since one can always produce another such quantity by applying a suitable function, such as adding a constant, to a conserved quantity.

Since many laws of physics express some kind of conservation, conserved quantities commonly exist in mathematical models of physical systems. For example, any classical mechanics model will have mechanical energy as a conserved quantity as long as the forces involved are conservative.

Differential equations

[edit]

For a first order system of differential equations

{\frac {d\mathbf {r} }{dt}}=\mathbf {f} (\mathbf {r} ,t)

where bold indicates vector quantities, a scalar-valued function H(r) is a conserved quantity of the system if, for all time and initial conditions in some specific domain,

{\frac {dH}{dt}}=0

Note that by using the multivariate chain rule,

{\frac {dH}{dt}}=\nabla H\cdot {\frac {d\mathbf {r} }{dt}}=\nabla H\cdot \mathbf {f} (\mathbf {r} ,t)

so that the definition may be written as

\nabla H\cdot \mathbf {f} (\mathbf {r} ,t)=0

which contains information specific to the system and can be helpful in finding conserved quantities, or establishing whether or not a conserved quantity exists.

Hamiltonian mechanics

[edit]

For a system defined by the Hamiltonian ${\mathcal {H}}$ , a function f of the generalized coordinates q and generalized momenta p has time evolution

{\frac {\mathrm {d} f}{\mathrm {d} t}}=\{f,{\mathcal {H}}\}+{\frac {\partial f}{\partial t}}

and hence is conserved if and only if ${\textstyle \{f,{\mathcal {H}}\}+{\frac {\partial f}{\partial t}}=0}$ . Here $\{f,{\mathcal {H}}\}$ denotes the Poisson bracket.

Lagrangian mechanics

[edit]

Suppose a system is defined by the Lagrangian L with generalized coordinates q. If L has no explicit time dependence (so ${\textstyle {\frac {\partial L}{\partial t}}=0}$ ), then the energy E defined by

E=\sum _{i}\left[{\dot {q}}_{i}{\frac {\partial L}{\partial {\dot {q}}_{i}}}\right]-L

is conserved.

Furthermore, if ${\textstyle {\frac {\partial L}{\partial q}}=0}$ , then q is said to be a cyclic coordinate and the generalized momentum p defined by

p={\frac {\partial L}{\partial {\dot {q}}}}

is conserved. This may be derived by using the Euler–Lagrange equations.

References

[edit]

^ Blanchard, Devaney, Hall (2005). Differential Equations. Brooks/Cole Publishing Co. p. 486. ISBN 0-495-01265-3.{{cite book}}: CS1 maint: multiple names: authors list (link)

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In physics, a conserved quantity is a measurable property of an isolated physical system that remains constant over time, even as the system's internal state evolves through interactions or processes.^[1] These quantities, also known as constants of the motion, underpin fundamental conservation laws that govern the behavior of matter and energy in classical and quantum mechanics.^[1] The most prominent conserved quantities include total energy, which remains invariant in isolated systems due to the time-independence of physical laws; linear momentum, the vector sum of individual momenta that stays constant in the absence of external forces; and angular momentum, which is preserved under rotational symmetries.^[1] Other examples encompass electric charge, which is conserved in electromagnetic interactions,^[2] and baryon number, a quantity maintained in strong and weak nuclear processes.^[3] These conservation principles provide powerful constraints for predicting system evolution and are exact for closed systems, with no known violations in fundamental physics.^[1] The deep connection between conserved quantities and symmetries is formalized by Noether's theorem, which asserts that every continuous symmetry of the action (or Lagrangian) of a physical system implies a corresponding conserved quantity.^[4] For instance, translational invariance in space leads to momentum conservation, while rotational invariance yields angular momentum conservation, and time-translation invariance ensures energy conservation.^[4] This theorem, developed in 1918, extends across classical field theories, quantum mechanics, and relativity, highlighting symmetries as the origin of these enduring laws.^[4]

Definition and Examples

Definition

In dynamical systems, a conserved quantity is defined as a function of the system's state variables that remains constant along the trajectories of the system, thereby invariant under its time evolution.^[5]^[6] This invariance implies that the value of the quantity does not change as the system progresses from one state to another, distinguishing it from other observables that may vary. For instance, in physical contexts, quantities such as total energy or linear momentum often serve as conserved quantities in isolated systems.^[7] Mathematically, if the state of the system is represented by

\mathbf{x}(t)

, where

t

denotes time, a conserved quantity

C(\mathbf{x})

satisfies the condition

\frac{dC}{dt} = 0

.^[5] This differential equation ensures exact conservation, meaning the quantity holds a strict constant value throughout the system's dynamics in ideal, closed scenarios. However, in real-world systems subject to external influences, dissipation, or modeling approximations, conservation may only be approximate, with the quantity varying slowly or within bounded fluctuations over relevant timescales.^[8]^[9] The presence of conserved quantities plays a crucial role in analyzing dynamical systems by effectively reducing the number of independent degrees of freedom, allowing for a lower-dimensional description of the system's behavior.^[10] This reduction simplifies the integration of equations of motion and aids in understanding long-term stability and qualitative features, without requiring exhaustive computation of all variables.^[11]

Physical Examples

In classical mechanics, total energy is conserved in isolated systems, where no energy enters or leaves, manifesting as the interconversion between kinetic and potential forms during motion.^[12] Linear momentum remains conserved in the absence of external forces, as seen in the uniform motion of objects or the balanced outcomes of interactions like billiard ball collisions.^[13] Similarly, angular momentum is conserved without external torques, evident in phenomena such as a spinning top maintaining its rotation rate until friction intervenes or a diver altering body position to control spin speed during flight.^[14] The recognition of these conserved quantities traces back to foundational developments in mechanics and thermodynamics. In mechanics, Isaac Newton's laws of motion, published in 1687, implied the conservation of linear momentum through the third law's action-reaction principle, providing a basis for analyzing interactions without net force changes.^[15] In thermodynamics, energy conservation emerged as the first law in the mid-19th century, with Julius Robert Mayer's 1842 observation that heat and work are interchangeable forms of energy in isolated processes, formalizing the principle for broader physical systems.^[16] Beyond classical mechanics, specific conserved quantities appear in other domains. Electric charge is conserved in all electromagnetic interactions, ensuring that the net charge in a closed system—such as during electron transfers in circuits or particle decays—remains unchanged, as verified through countless experiments in electrostatics and particle accelerators.^[17] In particle physics, baryon number is conserved, where protons and neutrons (baryons) carry a value of +1, maintaining balance in reactions like neutron decay or high-energy collisions, a principle upheld in observations from facilities like CERN.^[18] These principles are observable in simple experiments. A pendulum demonstrates energy conservation as it swings, converting gravitational potential energy at the highest points to kinetic energy at the bottom, with the total remaining constant in the absence of dissipative losses like air resistance.^[19] Collisions, such as those between gliders on an air track, illustrate momentum conservation, where the total momentum before impact equals the total after, whether the collision is elastic or inelastic, as measured by velocity changes.^[20] Such examples underscore how conserved quantities govern predictable behaviors in physical systems, often linked to underlying symmetries like time-invariance for energy.^[21]

Symmetries and Noether's Theorem

Noether's Theorem

Noether's first theorem establishes a profound connection between symmetries and conservation laws in physical theories. In 1918, Emmy Noether developed this result in her seminal paper "Invariante Variationsprobleme," addressing fundamental questions posed by David Hilbert and Felix Klein about the nature of conservation laws in Albert Einstein's general theory of relativity, where energy conservation appeared to manifest as an identity rather than a substantive law.^[22] Klein presented Noether's work to the Royal Society of Sciences in Göttingen on July 26, 1918, highlighting its resolution of these issues through the invariance of variational problems.^[23] The theorem's insight—that symmetries dictate conserved quantities—has since become a cornerstone of theoretical physics, applicable across classical and quantum regimes.^[22] The theorem states that for every differentiable continuous symmetry of the action functional, defined as the integral of the Lagrangian over time, there corresponds a conserved quantity.^[24] In the framework of Lagrangian mechanics for a system with generalized coordinates

q_i

and Lagrangian

L(q, \dot{q}, t)

, consider an infinitesimal transformation

\delta q_i = \epsilon \xi_i(q, t)

, where

\epsilon

is an infinitesimal parameter and

\xi_i

are the generators of the symmetry. The action

S = \int_{t_1}^{t_2} L \, dt

is invariant under this transformation if the variation of the Lagrangian satisfies

\delta L = \frac{d F}{dt}

for some function

F(q, t)

, known as the gauge function (often

F = 0

for strict symmetries).^[24] Along the solutions to the Euler-Lagrange equations, this invariance implies the existence of a conserved Noether charge. The general derivation proceeds from the principle of least action. The total variation of the action under the transformation is

\delta S = \int_{t_1}^{t_2} \left( \frac{\partial L}{\partial q_i} \delta q_i + \frac{\partial L}{\partial \dot{q}_i} \delta \dot{q}_i \right) dt + \left[ \frac{\partial L}{\partial \dot{q}_i} \delta q_i \right]_{t_1}^{t_2}

, assuming fixed endpoints where boundary terms vanish. Substituting

\delta \dot{q}_i = \frac{d}{dt} (\delta q_i)

and integrating by parts yields

\delta S = \int_{t_1}^{t_2} \left( \left( \frac{\partial L}{\partial q_i} - \frac{d}{dt} \frac{\partial L}{\partial \dot{q}_i} \right) \delta q_i + \frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_i} \delta q_i \right) \right) dt

. For

\delta S = 0

(or up to boundary terms), and using the Euler-Lagrange equations

\frac{d}{dt} \frac{\partial L}{\partial \dot{q}_i} = \frac{\partial L}{\partial q_i}

which hold on physical trajectories, the remaining term must satisfy

\frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_i} \delta q_i \right) = \delta L

. Given the symmetry condition

\delta L = \frac{d F}{dt}

, it follows that

\frac{d}{dt} \left( \sum_i \frac{\partial L}{\partial \dot{q}_i} \xi_i - F \right) = 0

, so the quantity

Q = \sum_i \frac{\partial L}{\partial \dot{q}_i} \xi_i - F

is conserved,

\frac{dQ}{dt} = 0

.^[24] This conserved charge

Q

generates the symmetry transformation in phase space. For instance, time-translation invariance yields conservation of energy as the associated charge.^[22] The theorem relies on several key assumptions to ensure its validity in classical mechanics. The symmetries must be continuous and differentiable, forming a Lie group whose infinitesimal generators

\xi_i

are smooth functions, allowing the expansion in

\epsilon

.^[4] The Lagrangian is assumed to be a first-order differential function, typically at most quadratic in velocities, leading to second-order Euler-Lagrange equations, and the action is varied with fixed endpoints.^[24] Invariance holds "on-shell," meaning conservation is guaranteed only for trajectories satisfying the equations of motion. The theorem extends naturally to quantum mechanics, where the conserved charge becomes a Hermitian operator commuting with the Hamiltonian, preserving the symmetry in the quantum Hilbert space.^[22] These conditions ensure the theorem's broad applicability while highlighting its foundational role in linking geometric symmetries to dynamical invariants.^[24]

Applications to Symmetries

Noether's theorem links continuous symmetries of a physical system's action to corresponding conserved quantities, providing a foundational explanation for many conservation laws in classical and field theories. A prime example is time-translation symmetry, which asserts that the laws of physics are invariant under shifts in time. This symmetry yields the conservation of energy, where the total energy—typically the Hamiltonian for closed systems—remains constant throughout the system's evolution.^[25] Spatial translation symmetry, meaning the laws of physics are the same everywhere in space, leads to the conservation of linear momentum. In systems exhibiting this isotropy, the total linear momentum vector is preserved, as demonstrated in the motion of isolated particles or collections of particles without external forces.^[25] Rotational symmetry, or the invariance of physical laws under arbitrary rotations in space, corresponds to the conservation of angular momentum. For instance, in central force problems like planetary orbits, the total angular momentum about the center remains fixed, explaining phenomena such as the stable elliptical paths predicted by Kepler's laws.^[25] Internal symmetries extend these principles beyond spacetime. Gauge symmetries, such as those in electromagnetism under U(1) transformations, give rise to the conservation of electric charge through Noether's second theorem, where the associated current is covariantly conserved on-shell. In non-Abelian gauge theories like quantum chromodynamics, similar symmetries conserve color charge, though it is confined within hadrons. Scale invariance, present in certain massless field theories, leads to the conservation of the dilation current, relating to the trace of the energy-momentum tensor vanishing.^[26] When symmetries are broken, conservation laws become approximate. In the weak nuclear interaction, parity symmetry—invariance under spatial inversion—is violated, as established experimentally in beta decay processes, leading to slight deviations from perfect conservation in parity-related quantities, while stronger conservations hold in electromagnetic and strong interactions.^[27]^[28]

Mathematical Formulation in Dynamical Systems

Ordinary Differential Equations

In dynamical systems described by ordinary differential equations (ODEs) of the form

\dot{\mathbf{r}} = \mathbf{f}(\mathbf{r}, t)

, where

\mathbf{r}

is the state vector and

\mathbf{f}

is the vector field, a scalar function

H(\mathbf{r}, t)

serves as a conserved quantity if its total time derivative vanishes along trajectories. This condition is given by

\nabla H \cdot \mathbf{f} + \frac{\partial H}{\partial t} = 0

, ensuring that

H

remains constant for solutions of the system.^[29] Geometrically, this implies that the level sets of

H

are invariant under the flow generated by

\mathbf{f}

, providing a foliation of the phase space that constrains the motion. Algebraically, it equates the directional derivative of

H

along

\mathbf{f}

to the negative of any explicit time dependence, allowing identification of conserved quantities through solvability of this partial differential equation, known as the transport equation for

H

. Conserved quantities, often termed first integrals, act as constants of motion that reduce the dimensionality of the ODE system. For an

n

-th order ODE or an equivalent first-order system in

n

dimensions, each independent first integral restricts the dynamics to an

(n-1)

-dimensional manifold, effectively lowering the order of integration required.^[30] This reduction facilitates solving the system by successive quadratures, as the problem decouples into lower-dimensional subsystems on the invariant surfaces defined by the constants. In non-autonomous cases, time-dependent first integrals similarly preserve this utility, though explicit

t

-dependence complicates the reduction. For non-Hamiltonian ODE systems, where standard symplectic structures do not apply, Darboux theory provides a framework for identifying conserved quantities using integrating factors. This approach, originating from classical methods for polynomial vector fields, involves constructing cofactors and Darboux polynomials—functions whose directional derivatives along

\mathbf{f}

yield multiples of themselves—to generate first integrals via products or ratios.^[31] Integrating factors in this context multiply the vector field to yield an exact form, whose primitives are conserved quantities, extending applicability to dissipative or irregular systems without relying on variational principles. A representative example is the simple harmonic oscillator, governed by the second-order ODE

\ddot{x} + \omega^2 x = 0

, or equivalently the first-order system

\dot{x} = v

\dot{v} = -\omega^2 x

. Here, the total energy

H(x, v) = \frac{1}{2} v^2 + \frac{1}{2} \omega^2 x^2

satisfies the conservation condition, as

\frac{dH}{dt} = v \dot{v} + \omega^2 x \dot{x} = v(-\omega^2 x) + \omega^2 x v = 0

, reducing the system to motion on elliptical level curves in the

(x, v)

-plane.^[32] This first integral halves the effective order, yielding the solution

x(t) = A \cos(\omega t + \phi)

directly from quadrature on the energy manifold.

Integrable Systems

In dynamical systems, particularly Hamiltonian systems, integrability refers to the existence of a complete set of conserved quantities that allow for exact solutions. For a Hamiltonian system with

n

degrees of freedom, it is defined as integrable if there are

n

independent conserved quantities

I_1, \dots, I_n

(including the Hamiltonian

H = I_1

) that are in involution, meaning their Poisson brackets vanish:

\{I_i, I_j\} = 0

for all

i, j

.^[33]^[34] This condition ensures the system can be reduced to quadratures, enabling analytical integration of the equations of motion.^[35] The Liouville-Arnold theorem provides a geometric characterization of such systems. It states that for a compact, connected component of the level set defined by the conserved quantities, the phase space foliates into invariant

n

-dimensional tori, on which the motion is quasi-periodic with frequencies determined by the gradients of the integrals.^[36]^[37] This structure implies that trajectories are confined to these tori, avoiding ergodic filling of the energy surface and enabling action-angle coordinates where the Hamiltonian depends only on the actions.^[38] A classic example is the Kepler problem, describing the motion of a particle in a

1/r

potential, which has three degrees of freedom. Its conserved quantities include the total energy

E

, the angular momentum vector

\mathbf{L}

(with three components, though one is redundant due to rotational invariance), and the Laplace-Runge-Lenz vector

\mathbf{A} = \mathbf{p} \times \mathbf{L} - m k \hat{\mathbf{r}}

, where

\mathbf{p}

is momentum,

m

mass, and

k

the coupling constant.^[39]^[40] These quantities are in involution, confirming integrability and explaining the closed elliptical orbits.^[41] Another example is the finite Toda lattice, a one-dimensional chain of

n

particles with nearest-neighbor exponential interactions

V(q_{i+1} - q_i) = e^{-(q_{i+1} - q_i)}

. This system possesses

n

independent conserved quantities in involution, derivable from its Lax pair formulation, allowing exact solution via spectral invariants of the associated tridiagonal matrix.^[42]^[35] The Toda lattice illustrates how integrability persists in nonlinear chains, with applications to soliton dynamics.^[43] While integrable systems are solvable, small perturbations often lead to non-integrability. The Kolmogorov-Arnold-Moser (KAM) theorem addresses this by showing that, for sufficiently small non-integrable perturbations of an integrable Hamiltonian, a set of positive measure of the invariant tori persists, with slightly deformed frequencies, provided the unperturbed frequencies are non-degenerate.^[44]^[45] However, the complementary set of tori is destroyed, giving rise to chaotic regions and highlighting the fragility of full integrability.^[46] This partial persistence underscores the distinction between integrable and generic dynamical systems.^[47]

In Classical Mechanics

Lagrangian Mechanics

In Lagrangian mechanics, conserved quantities emerge naturally from the structure of the Lagrangian function

L(q, \dot{q}, t)

, where

q

represents generalized coordinates and

\dot{q}

their time derivatives. When the Lagrangian does not depend explicitly on time,

L = L(q, \dot{q})

, the system exhibits time-translation invariance, leading to the conservation of the energy function. This energy

E

is defined as

E = \sum_i \dot{q}_i \frac{\partial L}{\partial \dot{q}_i} - L,

which represents the total energy in many systems, such as the sum of kinetic and potential energies for scleronomic constraints.^[48]^[49] To demonstrate conservation, consider the Euler-Lagrange equations,

\frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_i} \right) = \frac{\partial L}{\partial q_i},

for each coordinate

i

. The time derivative of

E

\frac{dE}{dt} = \sum_i \left[ \ddot{q}_i \frac{\partial L}{\partial \dot{q}_i} + \dot{q}_i \frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_i} \right) - \frac{\partial L}{\partial \dot{q}_i} \ddot{q}_i - \frac{\partial L}{\partial q_i} \dot{q}_i \right] - \frac{\partial L}{\partial t}.

The terms involving

\ddot{q}_i

cancel, and substituting the Euler-Lagrange equation yields

\frac{dE}{dt} = -\frac{\partial L}{\partial t}

. For time-independent

L

\frac{\partial L}{\partial t} = 0

, so

\frac{dE}{dt} = 0

, proving

E

is conserved along the system's trajectory.^[48]^[50] Similarly, spatial translation invariance manifests through cyclic coordinates, where the Lagrangian is independent of a particular

q_j

, so

\frac{\partial L}{\partial q_j} = 0

. The Euler-Lagrange equation then simplifies to

\frac{d}{dt} \left( \frac{\partial L}{\partial \dot{q}_j} \right) = 0

, implying the conjugate momentum

p_j = \frac{\partial L}{\partial \dot{q}_j}

is constant. This conserved momentum corresponds to linear momentum in Cartesian coordinates for translationally invariant systems.^[51]^[48] A simple example is the free particle in one dimension, with Lagrangian

L = \frac{1}{2} m \dot{x}^2

, where

x

is the position. Here,

L

is independent of

x

, making

x

cyclic; the conjugate momentum is

p = \frac{\partial L}{\partial \dot{x}} = m \dot{x}

, which is conserved, reflecting constant velocity in the absence of forces.^[51] More generally, for infinitesimal symmetry transformations

\delta q = \epsilon K(q, \dot{q}, t)

and

\delta t = \epsilon \Lambda(q, \dot{q}, t)

, where

\epsilon

is small, the Lagrangian transforms such that the action integral remains invariant up to a total derivative. This leads to a conserved Noether current in the Lagrangian framework: the quantity

Q = \frac{\partial L}{\partial \dot{q}} \delta q - L \delta t

satisfies

\frac{dQ}{dt} = 0

on solutions to the Euler-Lagrange equations, yielding a conserved charge associated with the symmetry. These currents generalize the earlier cases, with time independence (

\delta t = \epsilon

\delta q = 0

) recovering energy conservation and coordinate independence (

\delta q_j = \epsilon

, others zero) recovering momentum.^[52]^[53] This approach connects directly to Noether's theorem by linking symmetries to these conserved quantities without requiring Hamiltonian reformulation.^[54]

Hamiltonian Mechanics

In Hamiltonian mechanics, conserved quantities are identified within the framework of phase space, where the dynamics are governed by the Hamiltonian function

H(q, p)

, with

q

representing generalized coordinates and

p

generalized momenta. This formulation leverages the symplectic structure of phase space, endowed with a non-degenerate, closed 2-form

\omega = \sum_i dq_i \wedge dp_i

, which ensures the preservation of phase space volume under time evolution. The Poisson bracket, a bilinear operation derived from this symplectic geometry, provides the algebraic tool for detecting conservation: for two smooth functions

f

and

g

on phase space, the Poisson bracket is defined as

\{f, g\} = \sum_i \left( \frac{\partial f}{\partial q_i} \frac{\partial g}{\partial p_i} - \frac{\partial f}{\partial p_i} \frac{\partial g}{\partial q_i} \right).

This bracket satisfies antisymmetry, bilinearity, and the Jacobi identity, endowing the space of functions with a Poisson structure that underlies the Lie algebra of Hamiltonian vector fields.^[55]^[56] A function

f(q, p)

is a conserved quantity along the trajectories of the system if its Poisson bracket with the Hamiltonian vanishes,

\{f, H\} = 0

, assuming

H

is time-independent. This condition implies that the total time derivative

\dot{f} = \{f, H\}

is zero, so

f

remains constant under the flow generated by Hamilton's equations

\dot{q}_i = \frac{\partial H}{\partial p_i}

and

\dot{p}_i = -\frac{\partial H}{\partial q_i}

. In the broader Poisson geometric context, this extends to Poisson manifolds, where the symplectic leaves foliate the phase space, but in standard Hamiltonian mechanics, the invertible Poisson tensor corresponds directly to the symplectic form. Conserved quantities thus label invariant tori or level sets in phase space, facilitating the analysis of integrable systems./15%3A_Advanced_Hamiltonian_Mechanics/15.02%3A_Poisson_bracket_Representation_of_Hamiltonian_Mechanics)^[57]^[58] Symplectic invariance arises because conserved quantities generate canonical transformations that preserve the Hamiltonian and the symplectic form. Specifically, if

f

is conserved, the time-

\tau

flow

\phi_\tau

generated by the Hamiltonian vector field

X_f

(defined via

\omega(X_f, \cdot) = -df

) leaves

H

invariant, as

\mathcal{L}_{X_f} H = \{f, H\} = 0

. Such transformations are symplectic maps, maintaining the Poisson structure and thus the form of Hamilton's equations. This generation property links conservation directly to the symmetries of the phase space geometry.^[59]^[55]^[60] A representative example is the conservation of angular momentum in central force problems, where the potential depends only on the radial distance

r = |\mathbf{q}|

. The Hamiltonian is

H = \frac{|\mathbf{p}|^2}{2m} + V(r)

, and the angular momentum

\mathbf{L} = \mathbf{q} \times \mathbf{p}

satisfies

\{\mathbf{L}, H\} = 0

due to the rotational invariance of

H

in phase space. Each component

L_i

generates infinitesimal rotations that preserve

H

, confirming conservation and enabling reduction to effective one-dimensional radial motion.^[61]^[55]

Extensions to Other Theories

Quantum Mechanics

In quantum mechanics, conserved quantities are represented by Hermitian operators

\hat{C}

that correspond to observable physical properties. Such an operator is conserved if it commutes with the system's Hamiltonian

\hat{H}

, satisfying

[\hat{C}, \hat{H}] = 0

. This commutation relation ensures that the operator remains time-independent in the Heisenberg picture, where the time evolution of operators is governed by the equation

\frac{d\hat{C}}{dt} = \frac{i}{\hbar} [\hat{H}, \hat{C}]

(assuming no explicit time dependence in

\hat{C}

). As a result, the expectation value

\langle \hat{C} \rangle

in any state does not change over time, preserving the associated physical quantity during the system's dynamics.^[62] Noether's theorem extends to quantum mechanics by linking continuous symmetries of the Hamiltonian to conserved operators via unitary transformations. A symmetry generated by an infinitesimal unitary operator

\hat{U} = e^{-i \epsilon \hat{G}/\hbar}

(where

\hat{G}

is the generator and

\epsilon

is a small parameter) implies that

[\hat{H}, \hat{U}] = 0

, making

\hat{G}

a conserved charge that also commutes with

\hat{H}

. For instance, translational invariance of the Hamiltonian under spatial shifts generates the total momentum operator

\hat{p}

as the conserved quantity, while rotational invariance yields the angular momentum operator

\hat{\mathbf{L}}

. This quantum adaptation of Noether's theorem underscores how symmetries dictate the existence of conserved operators, facilitating the classification of quantum states and the prediction of measurement outcomes.^[63] Certain conserved quantities impose superselection rules, which restrict the allowable quantum superpositions by decomposing the Hilbert space into orthogonal sectors. These rules arise when the conserved operator, such as the electric charge

\hat{Q}

, commutes not only with

\hat{H}

but also with all local observables, forbidding coherent superpositions between states in different eigenspaces of

\hat{Q}

. For example, a system cannot be placed in a superposition of states with differing total charge, as such states would be unobservable due to the inability to couple them via physical operations. Superselection rules thus enforce classical-like behavior for these quantities, limiting quantum coherence while preserving conservation.^[64] A prominent example occurs in the quantum hydrogen atom, where the central Coulomb potential preserves rotational symmetry, rendering the angular momentum operators

\hat{L}^2

and

\hat{L}_z

conserved since they commute with the Hamiltonian

\hat{H} = -\frac{\hbar^2}{2m} \nabla^2 - \frac{e^2}{4\pi \epsilon_0 r}

. This conservation leads to degeneracy in the energy eigenstates: for a given principal quantum number

n

, states with the same

n

but different azimuthal quantum numbers

m_l

(from

-l

+l

) share identical energies, as

\hat{H}

is independent of the orientation specified by

m_l

. The

2l + 1

-fold degeneracy in

m_l

for fixed orbital angular momentum quantum number

l

directly stems from this symmetry, enabling the atom's spectral lines to exhibit characteristic multiplet structures upon further perturbations.^[65]

Field Theory and Relativity

In field theories, Noether's theorem extends the concept of conserved quantities to continuous systems by associating symmetries of the action with conserved currents in spacetime. For a Lagrangian density

\mathcal{L}(\phi, \partial_\mu \phi)

invariant under infinitesimal transformations

\delta \phi = \epsilon K(\phi)

and

\delta x^\mu = \epsilon \Xi^\mu

, the theorem yields a conserved Noether current

J^\mu = \frac{\partial \mathcal{L}}{\partial (\partial_\mu \phi)} \delta \phi - \mathcal{L} \delta x^\mu

, which satisfies the continuity equation

\partial_\mu J^\mu = 0

on solutions to the equations of motion.^[66] This framework applies to relativistic field theories, where the integral of the time component over a spatial slice gives a conserved charge, reflecting the flow of the quantity through spacetime. Quantum field theory builds on this by promoting currents to operator-valued distributions, ensuring conservation in the sense of equal-time commutators.^[66] Relativistic examples prominently feature the energy-momentum tensor arising from spacetime translation symmetries. Under translations

\delta x^\mu = \epsilon^\mu

, the Noether current is the canonical energy-momentum tensor

T^\mu{}_\nu = \frac{\partial \mathcal{L}}{\partial (\partial_\mu \phi)} \partial_\nu \phi - \delta^\mu_\nu \mathcal{L}

, conserved via

\partial_\mu T^\mu{}_\nu = 0

.^[67] In general relativity, the symmetric stress-energy tensor

T_{\mu\nu}

sources the curvature through the Einstein field equations, and its conservation

\nabla_\mu T^{\mu\nu} = 0

follows from the contracted second Bianchi identity

\nabla_\mu G^{\mu\nu} = 0

, where

G^{\mu\nu}

is the Einstein tensor, independent of Noether's theorem due to the diffeomorphism invariance of the theory.^[68] In gauge theories, local symmetries lead to conserved charges via the global subgroup, as captured by Noether's first theorem. For quantum electrodynamics (QED), the U(1) gauge symmetry under phase transformations

\psi \to e^{i\alpha} \psi

(with

\alpha

constant globally) yields the conserved electromagnetic current

j^\mu = \bar{\psi} \gamma^\mu \psi

, satisfying

\partial_\mu j^\mu = 0

and integrating to the electric charge.^[69] Local gauge invariance enforces this through the covariant derivative, ensuring anomaly-free conservation at the classical level. The Standard Model of particle physics incorporates numerous conserved quantities from accidental global symmetries, such as baryon number

B

and lepton number

L

, preserved perturbatively but violated non-perturbatively via sphaleron processes that change both

B

and

L

by three units while conserving

B - L

.^[70] Beyond the Standard Model, extensions like seesaw mechanisms for neutrino masses introduce

L

-violating terms, such as Majorana mass operators, potentially observable in processes like neutrinoless double beta decay, while maintaining approximate conservation for low-energy phenomenology.^[71]

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