- Source: Mathematical Foundations of Quantum Mechanics
Mathematical Foundations of Quantum Mechanics (German: Mathematische Grundlagen der Quantenmechanik) is a quantum mechanics book written by John von Neumann in 1932. It is an important early work in the development of the mathematical formulation of quantum mechanics. The book mainly summarizes results that von Neumann had published in earlier papers.
Publication history
The book was originally published in German in 1932 by Springer. An English translation by Robert T. Beyer was published in 1955 by Princeton University Press. A Russian translation, edited by Nikolay Bogolyubov, was published by Nauka in 1964. A new English edition, edited by Nicholas A. Wheeler, was published in 2018 by Princeton University Press.
Table of contents
According to the 2018 version, the main chapters are:
Introductory considerations
Abstract Hilbert space
The quantum statistics
Deductive development of the theory
General considerations
The measuring process
No hidden variables proof
One significant passage is its mathematical argument against the idea of hidden variables. Von Neumann's claim rested on the assumption that any linear combination of Hermitian operators represents an observable and the expectation value of such combined operator follows the combination of the expectation values of the operators themselves.
Von Neumann's makes the following assumptions:
For an observable
R
{\displaystyle R}
, a function
f
{\displaystyle f}
of that observable is represented by
f
(
R
)
{\displaystyle f(R)}
.
For the sum of observables
R
{\displaystyle R}
and
S
{\displaystyle S}
is represented by the operation
R
+
S
{\displaystyle R+S}
, independently of the mutual commutation relations.
The correspondence between observables and Hermitian operators is one to one.
If the observable
R
{\displaystyle R}
is a non-negative operator, then its expected value
⟨
R
⟩
≥
0
{\displaystyle \langle R\rangle \geq 0}
.
Additivity postulate: For arbitrary observables
R
{\displaystyle R}
and
S
{\displaystyle S}
, and real numbers
a
{\displaystyle a}
and
b
{\displaystyle b}
, we have
⟨
a
R
+
b
S
⟩
=
a
⟨
R
⟩
+
b
⟨
S
⟩
{\displaystyle \langle aR+bS\rangle =a\langle R\rangle +b\langle S\rangle }
for all possible ensembles.
Von Neumann then shows that one can write
⟨
R
⟩
=
∑
m
,
n
ρ
n
m
R
m
n
=
T
r
(
ρ
R
)
{\displaystyle \langle R\rangle =\sum _{m,n}\rho _{nm}R_{mn}=\mathrm {Tr} (\rho R)}
for some
ρ
{\displaystyle \rho }
, where
R
m
n
{\displaystyle R_{mn}}
and
ρ
n
m
{\displaystyle \rho _{nm}}
are the matrix elements in some basis. The proof concludes by noting that
ρ
{\displaystyle \rho }
must be Hermitian and non-negative definite (
⟨
ρ
⟩
≥
0
{\displaystyle \langle \rho \rangle \geq 0}
) by construction. For von Neumann, this meant that the statistical operator representation of states could be deduced from the postulates. Consequently, there are no "dispersion-free" states: it is impossible to prepare a system in such a way that all measurements have predictable results. But if hidden variables existed, then knowing the values of the hidden variables would make the results of all measurements predictable, and hence there can be no hidden variables. Von Neumann's argues that if dispersion-free states were found, assumptions 1 to 3 should be modified.
Von Neumann's concludes:
if there existed other, as yet undiscovered, physical quantities, in addition to those represented by the operators in quantum mechanics, because the relations assumed by quantum mechanics would have to fail already for the by now known quantities, those that we discussed above. It is therefore not, as is often assumed, a question of a re-interpretation of quantum mechanics, the present system of quantum mechanics would have to be objectively false, in order that another description of the elementary processes than the statistical one be possible.
= Rejection
=This proof was rejected as early as 1935 by Grete Hermann who found a flaw in the proof. The additive postulate above holds for quantum states, but it does not need to apply for measurements of dispersion-free states, specifically when considering non-commuting observables. Dispersion-free states only require to recover additivity when averaging over the hidden parameters. For example, for a spin-1/2 system, measurements of
(
σ
x
+
σ
y
)
{\displaystyle (\sigma _{x}+\sigma _{y})}
can take values
±
2
{\displaystyle \pm {\sqrt {2}}}
for a dispersion-free state, but independent measurements of
σ
x
{\displaystyle \sigma _{x}}
and
σ
y
{\displaystyle \sigma _{y}}
can only take values of
±
1
{\displaystyle \pm 1}
(their sum can be
±
2
{\displaystyle \pm 2}
or
0
{\displaystyle 0}
). Thus there still the possibility that a hidden variable theory could reproduce quantum mechanics statistically.
However, Hermann's critique remained relatively unknown until 1974 when it was rediscovered by Max Jammer. In 1952, David Bohm constructed the Bohmian interpretation of quantum mechanics in terms of statistical argument, suggesting a limit to the validity of von Neumann's proof. The problem was brought back to wider attention by John Stewart Bell in 1966. Bell showed that the consequences of that assumption are at odds with results of incompatible measurements, which are not explicitly taken into von Neumann's considerations.
Reception
It was considered the most complete book written in quantum mechanics at the time of release. It was praised for its axiomatic approach.
Works adapted in the book
von Neumann, J. (1927). "Mathematische Begründung der Quantenmechanik [Mathematical Foundation of Quantum Mechanics]". Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse: 1–57.
von Neumann, J. (1927). "Wahrscheinlichkeitstheoretischer Aufbau der Quantenmechanik [Probabilistic Theory of Quantum Mechanics]". Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse: 245–272.
von Neumann, J. (1927). "Thermodynamik quantenmechanischer Gesamtheiten [Thermodynamics of Quantum Mechanical Quantities]". Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse. 102: 273–291.
von Neumann, J. (1929). "Allgemeine Eigenwerttheorie Hermitescher Funktionaloperatoren [General Eigenvalue Theory of Hermitian Functional Operators]". Mathematische Annalen: 49–131. doi:10.1007/BF01782338.
von Neumann, J. (1931). "Die Eindeutigkeit der Schrödingerschen Operatoren [The uniqueness of Schrödinger operators]". Mathematische Annalen. 104: 570–578. doi:10.1007/bf01457956. S2CID 120528257.
See also
Dirac–von Neumann axioms
The Principles of Quantum Mechanics by Paul Dirac
Notes
References
External links
Full online text of the 1932 German edition (facsimile) at the University of Göttingen.
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