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# Consistent histories

In quantum mechanics, the consistent histories approach is intended to give a modern interpretation of quantum mechanics, generalising the conventional Copenhagen interpretation and providing a natural interpretation of quantum cosmology. The theory is based on a consistency criterion that then allows probabilities to be assigned to histories of a system so that the probabilities for each history obey the rules of classical probability while being consistent with the Schrödinger equation.

According to this interpretation of quantum mechanics, the purpose of a quantum-mechanical theory is to predict probabilities of various alternative histories.

## Histories

A homogeneous history $H_i$ (here $i$ labels different histories) is a sequence of propositions $P_\left\{i,j\right\}$ specified at different moments of time $t_\left\{i,j\right\}$ (here $j$ labels the times). We write this as:

$H_i = \left(P_\left\{i,1\right\}, P_\left\{i,2\right\},ldots,P_\left\{i,n_i\right\}\right)$

and read it as "the proposition $P_\left\{i,1\right\}$ is true at time $t_\left\{i,1\right\}$ and then the proposition $P_\left\{i,2\right\}$ is true at time $t_\left\{i,2\right\}$ and then $ldots$". The times $t_\left\{i,1\right\} < t_\left\{i,2\right\} < ldots < t_\left\{i,n_i\right\}$ are strictly ordered and called the temporal support of the history.

Inhomogeneous histories are multiple-time propositions which cannot be represented by a homogeneous history. An example is the logical OR of two homogeneous histories: $H_i vee H_j$.

These propositions can correspond to any set of questions that include all possibilities. Examples might be the three propositions meaning "the electron went through the left slit", "the electron went through the right slit" and "the electron didn't go through either slit". One of the aims of the theory is to show that classical questions such as, "where are my keys?" are consistent. In this case one might use a large number of propositions each one specifying the location of the keys in some small region of space.

Each single-time proposition $P_\left\{i,j\right\}$ can be represented by a projection operator $hat\left\{P\right\}_\left\{i,j\right\}$ acting on the system's Hilbert space (we use "hats" to denote operators). It is then useful to represent homogeneous histories by the time-ordered tensor product of their single-time projection operators. This is the history projection operator (HPO) formalism developed by Christopher Isham and naturally encodes the logical structure of the history propositions. The homogeneous history $H_i$ is represented by the projection operator

$hat\left\{H\right\}_i = hat\left\{P\right\}_\left\{i,1\right\} otimes hat\left\{P\right\}_\left\{i,2\right\} otimes cdots otimes hat\left\{P\right\}_\left\{i,n_i\right\}$

This definition can be extended to define projection operators that represent inhomogeneous histories too.

## Consistency

An important construction in the consistent histories approach is the class operator for a homogeneous history:

$hat\left\{C\right\}_\left\{H_i\right\} := T prod_\left\{j=1\right\}^\left\{n_i\right\} hat\left\{P\right\}_\left\{i,j\right\}\left(t_\left\{i,j\right\}\right) = hat\left\{P\right\}_\left\{i,1\right\}hat\left\{P\right\}_\left\{i,2\right\}cdots hat\left\{P\right\}_\left\{i,n_i\right\}$

The symbol $T$ indicates that the factors in the product are ordered chronologically according to their values of $t_\left\{i,j\right\}$: the "past" operators with smaller values of $t$ appear on the right side, and the "future" operators with greater values of $t$ appear on the left side. This definition can be extended to inhomogeneous histories as well.

Central to the consistent histories is the notion of consistency. A set of histories $\left\{ H_i\right\}$ is consistent (or strongly consistent) if

$operatorname\left\{Tr\right\}\left(hat\left\{C\right\}_\left\{H_i\right\} rho hat\left\{C\right\}^dagger_\left\{H_j\right\}\right) = 0$

for all $i neq j$. Here $rho$ represents the initial density matrix, and the operators are expressed in the Heisenberg picture.

The set of histories is weakly consistent if

$operatorname\left\{Tr\right\}\left(hat\left\{C\right\}_\left\{H_i\right\} rho hat\left\{C\right\}^dagger_\left\{H_j\right\}\right) approx 0$

for all $i neq j$.

## Probabilities

If a set of histories is consistent then probabilities can be assigned to them in a consistent way. We postulate that the probability of history $H_i$ is simply

$operatorname\left\{Pr\right\}\left(H_i\right) = operatorname\left\{Tr\right\}\left(hat\left\{C\right\}_\left\{H_i\right\} rho hat\left\{C\right\}^dagger_\left\{H_i\right\}\right)$

which obeys the axioms of probability if the histories $H_i$ come from the same (strongly) consistent set.

As an example, this means the probability of "$H_i$ OR $H_j$" equals the probability of "$H_i$" plus the probability of "$H_j$" minus the probability of "$H_i$ AND $H_j$", and so forth.

## Interpretation

The interpretation based on consistent histories is used in combination with the insights about quantum decoherence. Quantum decoherence implies that only special choices of histories are consistent, and it allows a quantitative calculation of the boundary between the classical domain and the quantum domain.

In some views the interpretation based on consistent histories does not change anything about the paradigm of the Copenhagen interpretation that only the probabilities calculated from quantum mechanics and the wave function have a physical meaning. In order to obtain a complete theory, the formal rules above must be supplemented with a particular Hilbert space and rules that govern dynamics, for example a Hamiltonian.

In the opinion of others this still does not make a complete theory as no predictions are possible about which set of consistent histories will actually occur. That is the rules of CH, the Hilbert space, and the Hamiltonian must be supplemented by a set selection rule.

The proponents of this modern interpretation, such as Murray Gell-Mann, James Hartle, Roland Omnès, Robert B. Griffiths, and Wojciech Zurek argue that their interpretation clarifies the fundamental disadvantages of the old Copenhagen interpretation, and can be used as a complete interpretational framework for quantum mechanics.

In Quantum Philosophy, Roland Omnès provides a less mathematical way of understanding this same formalism. The consistent histories approach can be interpreted as a way of understanding which sets of classical questions can be consistently asked of a single quantum system, and which sets of questions are fundamentally inconsistent, and thus meaningless when asked together. It thus becomes possible to demonstrate formally why it is that the questions which Einstein, Podolsky and Rosen assumed could be asked together, of a single quantum system, simply cannot be asked together. On the other hand, it also becomes possible to demonstrate that classical, logical reasoning often does apply, even to quantum experiments – but we can now be mathematically exact about the limits of classical logic.