Quantum modes - Ion Cotaescu

Quantum modes

Ion I. Cotaescu

1

The classical motion

The space of the classical physics is the Euclidean space E(3) where any
orthogonal basis {⃗1, ⃗2, ⃗3} can be associated to the system of Cartesian
e e e
coordinates (x1, x2, x3) having the origin in a fixed point O. In addition,
the time t ∈ R+ is considered to be universal. Thus, one defines the frame
(t, x1, x2, x3). In this notation the position vector reads ⃗ = xi⃗i (i, j, k, ... =
x e
1, 2, 3).

The classical mechanics studies the motion of massive point-wise particles.
A particular case is of a particle of mass m moving in an external field of
conservative forces
F (⃗ ) = −grad V (⃗ )
⃗ x x (1)
derived from the static potential V . Then the trajectory ⃗ (t) results from the
x
Newton dynamical principle which can be exploited in different formalisms
(Newton, Lagrange and Hamilton).

2

Newton Lagrange Hamilton
trajectory ⃗ (t) ∈ E(3)
x ⃗ (t) ∈ E(3)
x {⃗ (t), p(t)} ∈ E(3) × Ep(3)
x ⃗

function x x ˙ 1 x˙
L(⃗ , ⃗ ) = 2 m⃗ 2 − V (⃗ )
x 1 ⃗
H(⃗ , p) = 2m p 2 + V (⃗ )
x ⃗ x

momentum ˙
p = m⃗
⃗ x pi = ∂Li ˙
x ˙
⃗ → ⃗ (⃗)
xp
∂x
˙
∫ t2 ∫ t2
var. princ. δS = 0 , S = x x ˙
dt L(⃗ , ⃗ ) = ˙
dt [⃗ · p − H(⃗ , p)]
x ⃗ x ⃗
t1 t1

dynamics ¨
m⃗ = −gradV (⃗ )
x x ∂L − d ∂L = 0 xi = ∂H , pi = − ∂H
˙ ˙
∂xi dt ∂ xi
˙ ∂p i
∂xi

conserved E =T +V xi ∂Li − L = E
˙ H(⃗ , p) = E
x ⃗
∂x
˙

Remark: The observables (or physical quantities) are, in genral, functions
f : R+ × E(3) × Ep(3) → R , (2)
whose values are f (t, ⃗ , p).
x ⃗
3

In the canonical approach (Hamilton) the time evolution of the physical
observables is given by the general dynamical equation
df (t, ⃗ , p) ∂f (t, ⃗ , p)
x ⃗ x ⃗
= + {f (t, ⃗ , p), H(⃗ , p)}
x ⃗ x ⃗ (3)
dt ∂t
where the Poisson brackets are deﬁned as
( )
∑ ∂f ∂g ∂g ∂f
{f, g} = i ∂pi
− i i . (4)
∂x ∂x ∂p
i
Remark The conserved observables (or prime integrals) are functions
f (⃗ , p), independent on t, which satisfy {f, H} = 0.
x ⃗
Theorem Given any prime integrals f and g then {f, g} is a prime integral.

Deﬁnition A vector space equipped with an operation { , } with the
properties have the following properties:
{f, g} = − {g, f } , (5)
{f, {g, h}} + {g, {h, f }} + {h, {f, g}} = 0 , (6)
forms a Lie algebra.
4

Conclusion The set of the conserved observables (or prime integrals)
can be organized as a Lie algebra with respect to the Poisson bracket.
Moreover, there are other subsets of functions having this property.
Example 1. The coordinates and momentum accomplish
{ } { } { }
xi , p j = δij , xi , x j = 0, pi , pj = 0. (7)
Example 2. The components of the angular momentum L = ⃗ ∧ p satisfy
⃗ x ⃗
{L1, L2} = L3 , {L2, L3} = L1 ,
{L3, L1} = L2 . (8)
Example 3. The Laplace-Runge-Lenz vector R = p ∧ L − ϕ(r)⃗r , where
⃗ ⃗ ⃗ e
⃗r = ⃗ /r and r = |⃗ |, was recovered by Pauli and gives rise
e x x
to the nice algebra
{L1, R2} = R3 , {L2, R3} = R1 , {L3, R1} = R2 , (9)
{R1, R2} = χL3 , {R2, R3} = χL1 , {R3, R1} = χL2 . (10)
where
χ = −⃗
p 2 + 2ϕ(r) + dϕ(r) . (11)
r dr
5

Non-relativistic quantum systems

Experimental evidences

1. The states of the quantum system are determined by a macroscopic
apparatus which stops its causal evolution during the experiment. One says
that the apparatus prepares the quantum states.

2. The inﬂuence of this apparatus upon the measured quantum systems
is out of our hands and, therefore, this remains partially unknown. For this
reason there are quantum states with a natural statistical behavior.

3. In a quantum state some observables can not be measured with the
desired accuracy while other ones take discrete (quantized) values.

4. The quantum systems move (evolve) causally only out of the experiment.
Between two measured states the system makes a transition.
6

Principles

I. The ket vectors, | ⟩, of the space K, and the associated bra vectors ⟨ |,
of the space B, give rise to the hermitian forms ⟨ | ⟩. The quantum states
are represented by the ket vectors |ψ⟩ of ﬁnite norm (⟨ψ|ψ⟩ < ∞) forming
the Hilbert space H ⊂ K.

II. The quantum observables, denoted by A, B ,..., are time-independent
hermitian operators on the space K, i. e. A = A† or ⟨ψ|A|χ⟩ = ⟨χ|A|ψ⟩∗ .

III. When one measures the observable A one can obtain only values from
its spectrum S(A). The expectation value of A in the state |ψ⟩ is ⟨ψ|A|ψ⟩.

IV. The causal motion of any quantum system is governed by the
¨
Schrodinger equation
i ∂t|Ψ(t)⟩ = H(t)|Ψ(t)⟩ (12)
where the Hamiltonian operator H(t) can depend on time.
7

Correspondence

One takes over the classical dynamic associating to each physical quantity
a Hermitian operator,
xi → X i , pi → P i , f (⃗ , p) → f (X, P ) .
x ⃗ ⃗ ⃗ (13)
The next step is to postulate that each Lie algebra of classical quantities
is represented by a Lie algebra of operators whose skew-symmetric
operation is just the commutator, [A, B] = AB − BA. This enables one to
assume
{xi, xj } = 0 → [X i, X j ] = 0
{pi, pj } = 0 → [P i, P j ] = 0 (14)
{xi, pj } = δij → [X i, P j ] = i δij I .
where I is the identity operator on K.

Deﬁnition The algebra of quantum observables of the spinless systems
is freely generated by the operators X i and P i giving rise to orbital
operators.
8

Representations

Deﬁnition Each set of ket vectors which forms a bases in the space K
determines its own representation of the QM.
Remark The basis vectors can be deﬁned as common eigenvectors of some
complete sets of commuting operators (c.s.c.o.).

Example 1. The energy basis {|n⟩}n=0,1,2... of the one-dimensional
harmonic oscillator is formed by the eigenvectors obeying H|n⟩ = En|n⟩
and ⟨n|m⟩ = δn,m.

Example 2. The coordinate representation is given by the basis {|⃗ ⟩} x
of the common eigenvectors of the set {X 1, X 2, X 3}. These satisfy the
eigenvalues equations X i|⃗ ⟩ = xi|⃗ ⟩ and ⟨⃗ |⃗ ⟩ = δ 3(⃗ − ⃗ ). Any state
x x xy x y
|ψ⟩ ∈ H is represented now by the static wave-function ψ(⃗ ) = ⟨⃗ |ψ⟩
x x
while the scalar product has to be written as,
∫ ∫
⟨ψ|ϕ⟩ = d3x⟨ψ|⃗ ⟩⟨⃗ |ϕ⟩ =
x x d3xψ(⃗ )∗ϕ(⃗ ) .
x x (15)

9

Theorem In this representation the momentum operators act as the
ˆ
differential operators P i deﬁned as

⟨⃗ |P
x i|ψ⟩ = (P iψ)(⃗ ) = −i ∂ψ(⃗ ) .
ˆ x
x
(16)
∂xi
¨
Consequently, the Schrodinger equation can be written as a differential
ˆ
equation governed by the Hamiltonian operator H(t).

Assuming that there exists another discrete basis {|a, b, ...⟩} of common
eigenvectors of the c.s.c.o. {A, B, ...},
A|a, b, ...⟩ = a|a, b, ...⟩ , B|a, b, ...⟩ = b|a, b, ...⟩ , ...etc. , (17)
with the properties ⟨a, b, ...|a′, b′, ...⟩ = δ(a, a′)δ(b, b′)..., we obtain the
expansion
∑
ψ(⃗ ) = ⟨⃗ |ψ⟩ =
x x Ca,b,...⟨⃗ |a, b, ...⟩ ,
x Ca,b,... = ⟨a, b, ...|ψ⟩ . (18)
a,b,...

10

Time evolution
¨
The causal evolution is given by the Schrodinger equation (12). This has
a well-determined solution if one know the initial state |Ψ(t0)⟩ = |ψ⟩
prepared at time t0.
Definition The evolution operator U (t, t′) is unitary, has the algebraic
properties U (t, t) = I and U (t, t′)U (t′, t′′) = U (t, t′′) and satisfies the
equation
i ∂tU (t, t′) = H(t)U (t, t′) . (19)
With this operator we can separate the time evolution from the preparation
of the initial state as |Ψ(t)⟩ = U (t, t0)|ψ⟩.
Turning back to the coordinate representation we find that the wave-function
which describes the causal time evolution can be expanded in the basis
{|a, b, ...⟩} as
∑
Ψ(t, ⃗ ) = ⟨⃗ |Ψ(t)⟩ =
x x Ca,b,...⟨⃗ |U (t, t0)|a, b, ...⟩ ,
x (20)
a,b,...

11

Quantum modes

Equation (20) can be put in the form
∑
Ψ(t, ⃗ ) =
x Ca,b,...ua,b,...(t, ⃗ )
x (21)
a,b,...
where the wave-functions ua,b,...(t, ⃗ ) = ⟨⃗ |U (t, t0)|a, b, ...⟩ are called often
x x
the quantum modes determined by the c.s.c.o. {A, B, ...}.

Remark Whether all the operators of the c.s.c.o. {A, B, ...} commute
with H(t) and implicitly with U (t, t′) then the quantum modes are common
eigenfunctions of the differential operators {A, B, ...} which act as
ˆ ˆ
[Aua,b,...(t)](⃗ ) = ⟨⃗ |AU (t, t0)|a, b, ...⟩
ˆ x x (22)
= ⟨⃗ |U (t, t0)A|a, b, ...⟩ = aua,b,...(t, ⃗ ) .
x x (23)
The conclusion is that in this case the quantum modes are solutions of the
¨
Schrodinger equation and simultaneously common eigenfunctions of the
c.s.c.o. of differential operators {A, B, ...}.
ˆ ˆ
12

Relativistic quantum modes
Looking for relativistic quantum modes

1. We focus on the relativistic free ﬁelds since these satisfy
homogeneous differential equations which transform covariantly under
isometry transformations.

2. We must identify complete sets of commuting differential operators,
which commute with the operator of the ﬁeld equation, being thus able to
determine the quantum modes as systems of common eigenfunctions.

3. We need to introduce a relativistic scalar product exploiting an internal
symmetry. This will help us to normalize the quantum modes.

Remark The only operators which play the role of conserved quantities
are the generators either of the isometries of the background or of
several internal symmetries. These operator can be derived in Lagrangian
theories using the Noether theorem.
13

Covariant fields in special and general relativity

Let (M, g) be a physical 4-dimensional Riemannian manifold of metric
tensor g whose flat model is just the Minkowski space-time (M0, η) with
the metric η = diag(1, −1, −1, −1). In special relativity we have M = M0
and g = η .

Definition The group G(η) = SO(1, 3) which preserves the metric η is
called the gauge group of (M, g).

The manifold (M, g) may have isometries which transform the local
coordinates without to affect the form of the metric tensor. The isometries,
xµ → x′ = ϕξ (x) = xµ + Ka (x)ξ a + ... ,
µ µ µ
(24)
form the Lie group I(M ) depending on the real parameters ξ a, a = 1, 2...N ,
where N ≤ 10. The Killing vectors Ka associated to these isometries satisfy
the Killing equation Ka µ;ν + Ka ν;µ = 0. (The notation f;µ = ∇µf stands
for the covariant derivatives).
14

Definition The field ψ : M → V is called covariant if it transforms under
isometries as
ψ(x) → ψ ′(x′) = Q(x, ξ)ψ(x) = ψ(x′) − (iξ aXaψ)(x′) + .... (25)
where Q are the matrices of a finite-dimensional representation of the
gauge group G(η) = SO(1, 3) carried by the vector space V .

Theorem The representations Q : I(M ) → Aut(V ) are induced by the
gauge group G(η) such that the generators of the transformations (25) are
given by Carter and McLenaghan formula
µ 1
Xa = −iKa ∇µ + Ka µ;ν S µν , (26)
2
where the S µν are point-wise matrices generating the finite-dimensional
representations D of the gauge group.

Remark In the Minkowski case the gauge group SO(1, 3) is a subgroup of
the isometry one which is the Poincare group I(M0) = T (4) SO(1, 3). For
this reason the fields on (M0, η) transform manifestly covariant.
15

The Lagrangian formalism
The free fields satisfy field equations which can be derived from actions
∫
¯ 4x √g L(ψ, ψ , ψ, ψ ) ,
S[ψ, ψ] = d ;µ ¯ ¯;µ g = |det gµν | , (27)
∆
¯
depending on the field ψ and its Dirac adjoint ψ whose components play the
role of canonical variables in the Lagrangian density L.
¯
Theorem The action S is extremal if the fields ψ and ψ satisfy the Euler-
Lagrange equations
√ √
∂L 1 ∂( g L) ∂L 1 ∂( g L)
¯ − √g ∂µ ∂ ψ,µ = 0 ,
∂ψ ¯ ∂ψ
− √ ∂µ
g ∂ψ,µ
= 0. (28)

Example If ψ is the scalar field of mass m then ψ = ψ ∗ and the Lagrangian
¯
density takes the form L = g µν ∂µψ ∗∂ν ψ − m2ψ ∗ψ giving the Klein-Gordon
equation
1 √ µν
√ ∂µ( g g ∂ν ψ) + m2ψ = 0 . (29)
g
16

Definition Any transformation ψ → ψ ′ = ψ+δψ leaving the action invariant,
S[ψ ′, ψ ′] = S[ψ, ψ], is a symmetry transformation.
¯ ¯

Theorem Noether Each symmetry transformation ψ → ψ ′ = ψ + δψ gives
rise to the current
Θ µ ∝ δ ψ ∂L + ∂L δψ
¯ (30)
¯
∂ ψ,µ ∂ψ,µ
µ
which is conserved in the sense that Θ ;µ = 0.

Remark As mentioned before, there are two types of symmetries:

1. External symmetries (isometries) transforming simultaneously the
coordinates and the field components according to equations (24) and (25).
In this case δψ = −iξ aXaψ where Xa are defined by equation (26).

2. Internal symmetries when only the field components transform as
ψ → ψ ′ = D(ζ)ψ = ψ − iζAZ Aψ + ... according to the representation
D (generated by the matrices Z A) of a unitary group of internal symmetry
having the parameters ζ A (A = 1, 2...Nint). Now δψ = −iζAZ Aψ .
17

Conserved quantities and the relativistic scalar product

According to the Noether theorem each isometry of parameter ξ a and
each internal symmetry parametrized by ζ A produce the corresponding
conserved currents
( )
µ
Θa = i ¯ ψ ∂L − ∂L Xaψ
Xa , a = 1, 2...N (31)
¯
∂ ψ,µ ∂ψ,µ
( )
µ
ΘA = i ¯ ψ ∂L − ∂L ZAψ
ZA , A = 1, 2...Nint . (32)
¯
∂ ψ,µ ∂ψ,µ

Theorem For each conserved current Θµ there exist a corresponding
conserved quantity ∫
√
C= dσµ g Θµ , (33)
∂∆
called often conserved charge.

18

Deﬁnition The relativistic scalar product ⟨ , ⟩ is deﬁned as
∫ ( )
√ ∂L′ ∂L ′
⟨ψ, ψ ′⟩ = i dσµ g ¯
ψ ¯′ − ∂ψ,µ ψ . (34)
∂∆ ∂ ψ ,µ

Corollary The conserved charged can be represented as expectation
values of the symmetry generators using of the relativistic scalar product,
∫
√ µ
Ca = dσµ g Θa = ⟨ψ, Xaψ⟩ , (35)
∫∂∆
√ µ
CA = dσµ g ΘA = ⟨ψ, ZAψ⟩ , (36)
∂∆
Remark 1. The operators Xa and ZA are self-adjoint with respect to this
scalar product, i. e. ⟨Xψ, ψ ′⟩ = ⟨ψ, Xψ ′⟩.
Remark 2. The ’squared norm’ ⟨ψ, ψ⟩ represents the conserved electric
charge since the generator of the gauge group U (1)em is the identity
operator I .

19

Concluding remarks
Remark 1. The relativistic covariant fields are classical fields but which can
be interpreted in the quantum manner as forming spaces of wave-functions
equipped with invariant scalar products. The quantum observables are the
isometry generators Xa which are differential operators.

Remark 2. For deriving quantum modes one must start with a c.s.c.o.
{E, X1, X2, ...} including the operator of the field equation, E . The quantum
modes are the common eigenfunctions of these operators. Finally, these
modes must be correctly normalized with respect to the relativistic scalar
product. Otherwise, the physical meaning could be dramatically affected.

Remark 3. These modes are globally defined so that the vacuum state is
well-defined and stable eliminating thus the kinetic welling effects (e. g. the
Unruh and Hawking-Gibbons ones).

Query: What happens when there are no isometries ? I don’t know !
20

Quantum modes - Ion Cotaescu

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