# Wave function collapse

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In quantum mechanics, wave function collapse occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an "observation". It is the essence of a measurement in quantum mechanics which connects the wave function with classical observables like position and momentum. Collapse is one of two processes by which quantum systems evolve in time; the other is the continuous evolution via the Schrödinger equation. [1] Collapse is a black box for a thermodynamically irreversible interaction with a classical environment. [2] [3] Calculations of quantum decoherence show that when a quantum system interacts with the environment, the superpositions apparently reduce to mixtures of classical alternatives. Significantly, the combined wave function of the system and environment continue to obey the Schrödinger equation. [4] More importantly, this is not enough to explain wave function collapse, as decoherence does not reduce it to a single eigenstate. [2] [5]

## Contents

Historically Werner Heisenberg was the first to use the idea of wave function reduction to explain quantum measurement. [6]

## Mathematical description

Before collapsing, the wave function may be any square-integrable function, and is therefore associated with the probability density of a quantum mechanical-system. This function is expressible as a linear combination of the eigenstates of any observable. Observables represent classical dynamical variables, and when one is measured by a classical observer, the wave function is projected onto a random eigenstate of that observable. The observer simultaneously measures the classical value of that observable to be the eigenvalue of the final state. [7]

### Mathematical background

The quantum state of a physical system is described by a wave function (in turn—an element of a projective Hilbert space). This can be expressed as a vector using Dirac or bra–ket notation  :

${\displaystyle |\psi \rangle =\sum _{i}c_{i}|\phi _{i}\rangle .}$

The kets ${\displaystyle |\phi _{1}\rangle ,|\phi _{2}\rangle ,|\phi _{3}\rangle ,\dots }$ specify the different quantum "alternatives" available—a particular quantum state. They form an orthonormal eigenvector basis, formally

${\displaystyle \langle \phi _{i}|\phi _{j}\rangle =\delta _{ij}.}$

Where ${\displaystyle \delta _{ij}}$ represents the Kronecker delta.

An observable (i.e. measurable parameter of the system) is associated with each eigenbasis, with each quantum alternative having a specific value or eigenvalue, ei, of the observable. A "measurable parameter of the system" could be the usual position r and the momentum p of (say) a particle, but also its energy E, z components of spin (sz), orbital (Lz) and total angular (Jz) momenta etc. In the basis representation these are respectively ${\displaystyle |\mathbf {r} ,t\rangle =|x,t\rangle +|y,t\rangle +|z,t\rangle ,|\mathbf {p} ,t\rangle =|p_{x},t\rangle +|p_{y},t\rangle +|p_{z},t\rangle ,|E\rangle ,|s_{z}\rangle ,|L_{z}\rangle ,|J_{z}\rangle ,\dots }$.

The coefficients c1, c2, c3, … are the probability amplitudes corresponding to each basis ${\displaystyle |\phi _{1}\rangle ,|\phi _{2}\rangle ,|\phi _{3}\rangle ,\dots }$. These are complex numbers. The moduli square of ci, that is |ci|2 = ci*ci (where * denotes complex conjugate), is the probability of measuring the system to be in the state ${\displaystyle |\phi _{i}\rangle }$.

For simplicity in the following, all wave functions are assumed to be normalized; the total probability of measuring all possible states is one:

${\displaystyle \langle \psi |\psi \rangle =\sum _{i}|c_{i}|^{2}=1.}$

### The process of collapse

With these definitions it is easy to describe the process of collapse. For any observable, the wave function is initially some linear combination of the eigenbasis ${\displaystyle \{|\phi _{i}\rangle \}}$ of that observable. When an external agency (an observer, experimenter) measures the observable associated with the eigenbasis ${\displaystyle \{|\phi _{i}\rangle \}}$, the wave function collapses from the full ${\displaystyle |\psi \rangle }$ to just one of the basis eigenstates, ${\displaystyle |\phi _{i}\rangle }$, that is:

${\displaystyle |\psi \rangle \rightarrow |\phi _{i}\rangle .}$

The probability of collapsing to a given eigenstate ${\displaystyle |\phi _{k}\rangle }$ is the Born probability, ${\displaystyle P_{k}=|c_{k}|^{2}}$. Immediately post-measurement, other elements of the wave function vector, ${\displaystyle c_{i\neq k}}$, have "collapsed" to zero, and ${\displaystyle |c_{i}|^{2}=1}$. [note 1]

More generally, collapse is defined for an operator ${\displaystyle {\hat {Q}}}$ with eigenbasis ${\displaystyle \{|\phi _{i}\rangle \}}$. If the system is in state ${\displaystyle |\psi \rangle }$, and ${\displaystyle {\hat {Q}}}$ is measured, the probability of collapsing the system to eigenstate ${\displaystyle |\phi _{i}\rangle }$ (and measuring the eigenvalue ${\displaystyle q_{i}}$ of ${\displaystyle |\phi _{i}\rangle }$ with respect to ${\displaystyle {\hat {Q}}}$ would be ${\displaystyle |\langle \psi |\phi _{i}\rangle |^{2}}$. Note that this is not the probability that the particle is in state ${\displaystyle |\phi _{i}\rangle }$; it is in state ${\displaystyle |\psi \rangle }$ until cast to an eigenstate of ${\displaystyle {\hat {Q}}}$.

However, we never observe collapse to a single eigenstate of a continuous-spectrum operator (e.g. position, momentum, or a scattering Hamiltonian), because such eigenfunctions are non-normalizable. In these cases, the wave function will partially collapse to a linear combination of "close" eigenstates (necessarily involving a spread in eigenvalues) that embodies the imprecision of the measurement apparatus. The more precise the measurement, the tighter the range. Calculation of probability proceeds identically, except with an integral over the expansion coefficient ${\displaystyle c(q,t)dq}$. [8] This phenomenon is unrelated to the uncertainty principle, although increasingly precise measurements of one operator (e.g. position) will naturally homogenize the expansion coefficient of wave function with respect to another, incompatible operator (e.g. momentum), lowering the probability of measuring any particular value of the latter.

### Quantum decoherence

Quantum decoherence explains why a system interacting with an environment transitions from being a pure state, exhibiting superpositions, to a mixed state, an incoherent combination of classical alternatives. [5] This transition is fundamentally reversible, as the combined state of system and environment is still pure, but for all practical purposes irreversible, as the environment is a very large and complex quantum system, and it is not feasible to reverse their interaction. Decoherence is thus very important for explaining the classical limit of quantum mechanics, but cannot explain wave function collapse, as all classical alternatives are still present in the mixed state, and wave function collapse selects only one of them. [2] [9] [5]

## History and context

The concept of wavefunction collapse was introduced by Werner Heisenberg in his 1927 paper on the uncertainty principle, "Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik", and incorporated into the mathematical formulation of quantum mechanics by John von Neumann, in his 1932 treatise Mathematische Grundlagen der Quantenmechanik. [10] Heisenberg did not try to specify exactly what the collapse of the wavefunction meant. He, however, emphasized that it should not be understood as a physical process. [11] Niels Bohr also repeatedly cautioned that we must give up a "pictorial representation", and perhaps also interpreted collapse as a formal, not physical, process. [12]

Consistent with Heisenberg, von Neumann postulated that there were two processes of wave function change:

1. The probabilistic, non-unitary, non-local, discontinuous change brought about by observation and measurement, as outlined above.
2. The deterministic, unitary, continuous time evolution of an isolated system that obeys the Schrödinger equation (or a relativistic equivalent, i.e. the Dirac equation).

In general, quantum systems exist in superpositions of those basis states that most closely correspond to classical descriptions, and, in the absence of measurement, evolve according to the Schrödinger equation. However, when a measurement is made, the wave function collapses—from an observer's perspective—to just one of the basis states, and the property being measured uniquely acquires the eigenvalue of that particular state, ${\displaystyle \lambda _{i}}$. After the collapse, the system again evolves according to the Schrödinger equation.

By explicitly dealing with the interaction of object and measuring instrument, von Neumann [1] has attempted to create consistency of the two processes of wave function change.

He was able to prove the possibility of a quantum mechanical measurement scheme consistent with wave function collapse. However, he did not prove the necessity of such a collapse. Although von Neumann's projection postulate is often presented as a normative description of quantum measurement, it was conceived by taking into account experimental evidence available during the 1930s (in particular the Compton-Simon experiment was paradigmatic), but many important present-day measurement procedures do not satisfy it (so-called measurements of the second kind). [13] [14] [15]

The existence of the wave function collapse is required in

On the other hand, the collapse is considered a redundant or optional approximation in

The cluster of phenomena described by the expression wave function collapse is a fundamental problem in the interpretation of quantum mechanics, and is known as the measurement problem.

In the Copenhagen Interpretation collapse is postulated to be a special characteristic of interaction with classical systems (of which measurements are a special case). Mathematically it can be shown that collapse is equivalent to interaction with a classical system modeled within quantum theory as systems with Boolean algebras of observables [16] and equivalent to a conditional expectation value. [17]

Everett's many-worlds interpretation deals with it by discarding the collapse-process, thus reformulating the relation between measurement apparatus and system in such a way that the linear laws of quantum mechanics are universally valid; that is, the only process according to which a quantum system evolves is governed by the Schrödinger equation or some relativistic equivalent.

A general description of the evolution of quantum mechanical systems is possible by using density operators and quantum operations. In this formalism (which is closely related to the C*-algebraic formalism) the collapse of the wave function corresponds to a non-unitary quantum operation. Within the C* formalism this non-unitary process is equivalent to the algebra gaining a non-trivial centre [18] or centre of its centralizer corresponding to classical observables. [19]

The significance ascribed to the wave function varies from interpretation to interpretation, and varies even within an interpretation (such as the Copenhagen Interpretation). If the wave function merely encodes an observer's knowledge of the universe then the wave function collapse corresponds to the receipt of new information. This is somewhat analogous to the situation in classical physics, except that the classical "wave function" does not necessarily obey a wave equation. If the wave function is physically real, in some sense and to some extent, then the collapse of the wave function is also seen as a real process, to the same extent.

## Notes

1. Unless the observable being measured commutes with the Hamiltonian, the post-measurement state will in general evolve as time progresses into a superposition of different energy eigenstates as governed by the Schrödinger equation. Unless the state projected onto upon measurement has a definite energy value, the probability of having the same measurement outcome a non-zero time later will in general be less than one.

## Related Research Articles

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Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.

In particle physics, the Dirac equation is a relativistic wave equation derived by British physicist Paul Dirac in 1928. In its free form, or including electromagnetic interactions, it describes all spin-½ massive particles such as electrons and quarks for which parity is a symmetry. It is consistent with both the principles of quantum mechanics and the theory of special relativity, and was the first theory to account fully for special relativity in the context of quantum mechanics. It was validated by accounting for the fine details of the hydrogen spectrum in a completely rigorous way.

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Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two quantum states can be added together ("superposed") and the result will be another valid quantum state; and conversely, that every quantum state can be represented as a sum of two or more other distinct states. Mathematically, it refers to a property of solutions to the Schrödinger equation; since the Schrödinger equation is linear, any linear combination of solutions will also be a solution.

A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements made on the system can be derived from it. The most common symbols for a wave function are the Greek letters ψ and Ψ.

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In physics, an operator is a function over a space of physical states onto another space of physical states. The simplest example of the utility of operators is the study of symmetry. Because of this, they are very useful tools in classical mechanics. Operators are even more important in quantum mechanics, where they form an intrinsic part of the formulation of the theory.

In quantum mechanics, a probability amplitude is a complex number used in describing the behaviour of systems. The modulus squared of this quantity represents a probability density.

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In quantum mechanics, einselections, short for "environment-induced superselection", is a name coined by Wojciech H. Zurek for a process which is claimed to explain the appearance of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions. In this approach, classicality is described as an emergent property induced in open quantum systems by their environments. Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable due to entangling interaction with the environment, which in effect monitors selected observables of the system. After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale, a generic quantum state decays into an uncertain state which can be decomposed into a mixture of simple pointer states. In this way the environment induces effective superselection rules. Thus, einselection precludes stable existence of pure superpositions of pointer states. These 'pointer states' are stable despite environmental interaction. The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglement and superposition.

The Born rule is a key postulate of quantum mechanics which gives the probability that a measurement of a quantum system will yield a given result. In its simplest form, it states that the probability density of finding a particle at a given point, when measured, is proportional to the square of the magnitude of the particle's wavefunction at that point. It was formulated by German physicist Max Born in 1926.

The theoretical and experimental justification for the Schrödinger equation motivates the discovery of the Schrödinger equation, the equation that describes the dynamics of nonrelativistic particles. The motivation uses photons, which are relativistic particles with dynamics described by Maxwell's equations, as an analogue for all types of particles.

The Ghirardi–Rimini–Weber theory (GRW) is a spontaneous collapse theory in quantum mechanics, proposed in 1986 by Giancarlo Ghirardi, Alberto Rimini, and Tullio Weber.

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 in time exhausts all that can be predicted about the system's behavior. A mixture of quantum states is again a quantum state. Quantum states that cannot be written as a mixture of other states are called pure quantum states, while all other states are called mixed quantum states. A pure quantum state can be represented by a ray in a Hilbert space over the complex numbers, while mixed states are represented by density matrices, which are positive semidefinite operators that act on Hilbert spaces.

This is a glossary for the terminology often encountered in undergraduate quantum mechanics courses.

The Koopman–von Neumann mechanics is a description of classical mechanics in terms of Hilbert space, introduced by Bernard Koopman and John von Neumann in 1931 and 1932, respectively.

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