**A short introduction to Consistent Histories after some trivial appetizer**

When the uncertainty principle is being presented, people usually – if not always – talk about the position and the momentum or analogous dimensionful quantities. That leads most people to either ignore the principle completely or think that it describes just some technicality about the accuracy of apparatuses.

However, most people don't change their idea what the information is and how it behaves. They believe that there exists some sharp objective information, after all. Nevertheless, these ideas are incompatible with the uncertainty principle. Let me explain why the uncertainty principle applies to the truth, too.

Every proposition we can make about objects in Nature and their properties may be determined by a measurement and mathematically summarized as a Hermitian projection operator \(P\),\[

P = P^\dagger, \quad P^2=P.

\] The first condition is the hermiticity condition; the second one is the "idempotence" condition (Latin word for "the same [as its] powers") that defines the projection operators. The second condition implies that eigenvalues have to obey the same identity, \(p^2=p\), which means that the eigenvalues have to be \(0\) or \(1\). We will identify \(0\) with "truth" and "true" while \(1\) will be identified with "lie" and "false".

In some sense, you could say that \(P^2=P\) is more fundamental and \(p\in\{0,1\}\) is derived. The very claim that there are two truth values, "true" and "false", may be viewed as a derived fact in quantum mechanics, a result of a calculation. This is a toy model of the fact that many seemingly trivial facts result from calculations in quantum mechanics and some of these facts are only approximately true under the everyday circumstances and they are untrue at the fundamental level.

The first condition, hermiticity, implies that eigenstates of \(P\) associated with eigenvalue "false" (\(0\)) are orthogonal to those with the eigenvalue "true" (\(1\)). This is what allows us to say that the probability that the state disobeys a condition if it obeys the condition is 0 percent, and vice versa. They are mutually excluding. The proof of orthogonality is\[

\eq{

\braket{\text{yes-state}}{\text{no-state}} &= \bra{\text{yes-state}} P^\dagger \ket{\text{no-state}} =\\

&= \bra{\text{yes-state}} P \ket{\text{no-state}} = 0,\\

}

\] In the first step, I used the freedom to insert \(P^\dagger\) in between the states because when it acts on the eigenstate bra yes-state, it yields \(1\) times this state because this state is an eigenvalue-one eigenstate (I used the Hermitian conjugate of the usual eigenvalue equation). In the second step, I erased the dagger which is OK because of the hermiticity. In the final step, I acted with \(P\) on the no-state ket vector to get zero – because the no-state is an eigenvalue-zero eigenstate. So I got zero. The opposite-order inner product is also zero because it's the complex conjugate number (or you may prove it by a proof that is mirror to the proof above).

Let me just give you examples of projection operators corresponding to different propositions. For example, the statement "\(x\) of a particle belongs to the interval \((a,b)\)" is represented by the projection operator\[

P_{a\lt x\lt b} = \int_a^b \dd x\,\ket x\bra x.

\] It's keeping the position-eigenstate components of any vector \(\ket\psi\) that belong to the interval and erases all others. Now, the proposition that the "electron's spin relatively to the \(z\)-axis is equal to \(\hbar/2\) i.e. up" is represented by the projection operator\[

P_{z,\,\rm up} = \frac 12+ \frac{J_z}{\hbar}.

\] It's a simple linear function that moves the values \(J_z=\mp \hbar/2\) to \(0\) and \(1\), respectively. I hope you are able to write down the projection operator for a similar "up" (or "right") statement relatively to the \(x\)-axis:\[

P_{x,\,\rm up} = \frac 12+ \frac{J_x}{\hbar}.

\] Now, is the electron's spin "up" relatively to the \(z\)-axis? Is it "up" relatively to the \(x\)-axis? Those are perfectly meaningful questions that may be answered by a measurement. Because the truth value is either "false" or "true", we may obtain classical bits of information by a measurement.

However, my point is that the truth values of "\(z\) up" and "\(x\) up" propositions can't be sharply well-defined at the same moment. Indeed, it's because the commutator of the two projection operators is nonzero:\[

[ P_{z,\,\rm up}, P_{x,\,\rm up}] = \frac{iJ_y}{\hbar} \in\{-\frac i2,+\frac i2\}.

\] The commutator of the two projection operators – that just represent the numbers \(0\) and \(1\) when the corresponding propositions about the spin is false or true, respectively – is equal to a multiple of \(J_y\) and the eigenvalues (i.e. possible values) of this multiple are \(\pm i/2\). Because zero isn't among the eigenvalues of the commutators in this case, there exists no vector \(\ket\psi\) for which \[

\nexists \ket\psi:\quad (P_z P_x-P_x P_z)\ket \psi = 0.

\] Yes, I started to omit "up" in the subscripts. This non-existence means that it can't possibly happen that \(P_x,P_z\) would simultaneously have some values from the set \(\{0,1\}\). While we may rigorously prove the logical statement that the only possible values of these (and other) projection operators are zero or one, we may also rigorously prove the statement that a physical system can't have a well-defined sharp answer to both questions, "\(z\) up" and "\(x\) up".

If we have an appropriate apparatus, we can immediately answer the question whether the spin was "up" relatively to a given axis. So the questions associated with the projection operators \(P_x\) and \(P_z\) are totally physical and operationally meaningful. Also, by rotational symmetry, both of them are clearly equally meaningful. Nevertheless, they can't simultaneously have sharp truth values!

These projection operators represent potential truths that don't commute with each other. If you talk about the truth values of \(P_{x}\), your logic is incompatible with the logic of another person who assigns a classical truth value to \(P_z\). It's just not possible for both propositions to be "certainly true". It's not possible for both of them to be "certainly false", either. However, it's also impossible for one of them to be "true" and the other to be "false". ;-) They just can't have classical truth values at the same moment!

Because some people often like to repeat Wheeler's notion that the information is more fundamental in physics – and yes, no, it doesn't really mean much although I sometimes philosophically agree with such a priority – they usually think that such vacuous clichés may protect the classical world for them because the information is surely behaving classically. But it isn't. Quantum mechanics says that the information is counted in quantum bits or qubits (the electron's spin above is mathematically isomorphic to any qubit in quantum mechanics) and the Yes/No answers to most pairs of questions don't commute with one another which means that they can't be simultaneously assigned truth (eigen)values for a given situation.

This was just a trivial introduction. We will use it by realizing that "consistent histories" that would mix "different logics", i.e. statements about \(J_z\) and \(J_x\) at the same moment, are clearly forbidden. We will see formulae that prohibit them, too.

**Consistent Histories**

Fine. We may finally start to talk about the Consistent Histories interpretation of quantum mechanics. Wikipedia and other sources start by screaming lots of nonsense that it's surely an attempt to debunk the Copenhagen interpretation. Such misconceptions have occurred because virtually all the people talking about "interpretations" are activists and imbeciles who have promoted the fight against quantum mechanics as defined by Bohr and Heisenberg to their life mission. Some of them say such silly things because they don't historically know what Bohr and Heisenberg were actually saying. Most of them are saying such silly things because they refuse to understand basic things about modern physics. Many people belong to both sets.

At any rate, once some of them start to understand what the Consistent Histories interpretation says, they realize that it's not the "weapon of mass destruction" used against the Danish capital that they were dreaming about. In some sense, the Consistent Histories interpretation is a homework exercise:

Apply the Copenhagen interpretation to a collection of arbitrary sequences of measurements at various times and discuss which collections are permissible as interpretations of alternative histories. With the help of decoherence, show that your formulae clarify all issues surrounding the so-called "measurement problem" i.e. that quantum mechanics in its Copenhagen interpretation is a complete theory that produces meaningful predictions for microscopic as well as macroscopic systems.Of course, they feel utterly disappointed. The Consistent Histories approach is refusing to offer them the "classical mechanisms" and "classical information about the system" and "preferred, 'real' choices of bases and operators" and all other things they were expecting. Instead, the fathers of the Consistent Histories join Bohr and Heisenberg in announcing that quantum physics is different than any theory within the general classical framework. It doesn't assume any objective information about the reality. The probabilities are intrinsically incorporated to the foundations of the theory, just like they have always been. But there is no engine or mechanism that "produces" the probabilities in a way that could be fully described by a classical model. No surprise, the Consistent Histories interpretation was coined by mature physicists such as Murray Gell-Mann, James Hartle, Roland Omnès and Robert B. Griffiths.

I should get into some formulae. Readers are recommended to read e.g. this simple and pioneering 1992 text by Gell-Mann and Hartle. They use the Heisenberg picture and it makes the formulae simple. I agree with them it's more natural to use the Heisenberg picture, especially in such discussions (but also in other contexts), but because the people who tend to misunderstand the foundations of quantum mechanics almost universally prefer the Schrödinger picture, I will translate the Consistent Histories wisdom into the picture of the guy who didn't quite respect complementarity or uncertainty (either \(X\) or \(P\)) and who lived both with his wife and with his mistress. :-)

(Well, if you want to hear some defense, he's had children with three mistresses in total and he justified the relations by saying that he "sexually detested his wife Anny".)

It's not so hard to summarize the definition of "weakly consistent" and "medium consistent" histories. What is a history? In the picture we use, the operators are constant in time and the states evolve according to Schrödinger's equation. I will assume that the Hamiltonian is time-independent and the unitary evolution operators from time \(t_1\) through later time \(t_2\) will be denoted \[

U_{t_2,t_1} = \exp(H\frac{t_2-t_1}{i\hbar}).

\] Let's use the value \(t=0\) for the initial state and the time \(t=T\) with some \(T\gt 0\) for the "end of the history". From the beginning through the end, an initial pure state evolves as\[

\ket{\psi}_{\rm initial} \to \ket{\psi}_{\rm final} = U_{T,0} \ket{\psi}_{\rm initial}.

\] Because the density matrix is a combination of ket-bra products\[

\rho = \sum_i p_i \ket{\psi_i}\bra{\psi_i},

\] we may also immediately write down the evolution for the (initial) density matrix:\[

\rho\to U_{T,0}\rho U^\dagger_{T,0}.

\] The daggered evolution operator at the end appeared because of the bra-vectors in the density matrix: they also evolve.

Now, the operator of a history will be the operator \(U_{T,0}\) with some extra, a priori arbitrary, projection operators inserted between the evolution over different intervals into which \((0,T)\) will be divided. We will search for "collections of coarse-grained histories". In the collection, individual elements i.e. histories will be labeled by the Greek letters such as \(\alpha\). Mathematically, the value of \(\alpha\) will store all the information about the moments at which we inserted projection operators as well as the information which projection operators.\[

\alpha\leftrightarrow \{ n_\alpha, \{t_{\alpha,1},t_{\alpha,2},\dots t_{\alpha,n_{\alpha}}\},

\{i_{\alpha,1},i_{\alpha,2},\dots i_{\alpha,n_{\alpha}}\}

\}

\] where \(i_\alpha\) are subscripts distinguishing all possible projection operators \(P_{i_\alpha}\) that we use in any history in the collection. Here, \(n_\alpha\) is the number of projection operators we are inserting in the \(\alpha\)-th alternative history, the labels \(t_j\) specify the value of time \(t\) where we are inserting the projection operators, and \(i_j\) say which projection operators we insert at the \(j\)-th insertion.

You may see that the history operator \(C_\alpha\) will be a generalization of \(U_{T,0}\) of this sort:\[

{\Large

\eq{

C_\alpha &= U_{T,t_{\alpha,n_\alpha}} P_{i_{\alpha,n_\alpha}}\cdot\\

&\cdot U_{t_{\alpha,n_\alpha},t_{\alpha,n_\alpha-1}} P_{i_{\alpha,n_\alpha-1}}\cdot

\\

&\quad \cdots\\

&\cdot U_{t_{\alpha,2},t_{\alpha,1}} P_{i_{\alpha,1}}\cdot\\

&\cdot U_{t_{\alpha,1},0}.

}

}

\] I increased the font size because of the nested subscripts and wrote it on many lines. But the operator is exactly what you expect. You cut \(U_{T,0}\) into \(n_\alpha+1\) evolution operators over intervals and insert the appropriate projection operators to the \(n_\alpha\) places. The insertions and evolution operators at the "earlier times" appear on the right side from their friends linked to "later times"; the usual time ordering holds because the operators on the right are the first ones that act on the initial ket state.

It was a messy formula and I won't write it again. (My formula in Schrödinger's picture differs by the usual transformations by evolution operators, times some possible additional evolution operator, from the Heisenberg-picture formulae in the paper by Gell-Mann and Hartle.)

How can you interpret the history operator? Well, it's like the evolution with \(n_\alpha\) "collapses" in between. However, instead of a discontinuous step in the evolution at which Schrödinger's equation ceases to hold (this totally wrong description occurs at many places, including the newest book by Brian Greene), you should interpret the inserted projection operators differently. They're insertions that are needed to calculate the probability that the history \(\alpha\) will be realized.

This interpretation is needed because the separation into the histories from the particular set is surely not unique, and therefore can't be objective. You may always make the splitting to the histories "less finely grained" and the formalism will calculate the probabilities of these "less finely grained" histories, too. It is clearly up to you – within some limitations – how fine and accurate questions you ask about the evolution which is why you surely can't consider the insertions of the projection operators to be "objective collapses".

Now, how do we calculate the probability that the particular history will take place? It's simple if we assume a pure initial state \(\ket\psi\). What happens with the state? Well, it evolves by the evolution operators \(U_{t_{j+1},t_j}\) over the intervals and at the critical points, the pure state is projected by the projection operators. It is kept non-normalized so we pick the multiplicative factor of the complex probability amplitude associated with the projection operator. We do so for every projection operator in the history so that gives us the product of the complex probability amplitudes associated with all measurements. Finally, we must square the absolute value of this product to get the probability out of the total amplitude.

If you think about the action of \(C_\alpha\) on the initial state as well as the usual Born rule to calculate the probabilities of various measurements (plus the product formula for probabilities of composite statements), you will realize that the probability of the history \(\alpha\) which I will write as \(D(\alpha,\alpha)\) is given by\[

D(\alpha,\alpha) = \bra{\psi} C_\alpha^\dagger\cdot C_\alpha \ket\psi.

\] The first, bra-daggered part of the product, is needed because we calculate the probabilities from the squared absolute values of the complex probability amplitudes that we picked from the projection operators. By the cyclic property of the trace, that can be rewritten as\[

D(\alpha,\alpha) = {\rm Tr}\zav{ C_\alpha \ket\psi

\bra{\psi} C_\alpha^\dagger}.

\] We may easily generalize this formula to a mixed state which is just some combination of \(\ket\psi\bra\psi\) objects. By linearity, we get:\[

D(\alpha,\alpha) = {\rm Tr}\zav{ C_\alpha \rho C_\alpha^\dagger}.

\] Here, \(\rho\) is the initial state at \(t=0\). So the Consistent Histories interpretation allows us to pick a collection of histories and calculate the probability of each history in the collection by the formula above. Finally, I must say what it means for the histories to be "consistent".

Well, if we "merge" two nearby (or any two) histories \(\alpha\) and \(\beta\), we get a less fine history called "\(\alpha\) or \(\beta\)". I have assumed that all the histories in the set are mutually exclusive and the total probability is guaranteed to be one. The probability of "\(\alpha\) or \(\beta\)" must be equal to the sum of probabilities, \(D(\alpha,\alpha)+D(\beta,\beta)\), but even this "\(\alpha\) or \(\beta\)" thing is a history so its probability must be given by the same formula for \(D\), one involving the history operator\[

C_{\alpha\text{ or }\beta} = C_\alpha + C_\beta.

\] Because \(D(\gamma,\gamma)\) is bilinear in \(C_\gamma\) and/or its Hermitian conjugate, the addition formula needs the mixed \(\alpha\)-\(\beta\) terms to cancel. The additivity of the probabilities therefore requires\[

{\rm Re}\,D(\alpha,\beta) = 0.

\] The imaginary part doesn't have to be zero because it cancels against its complex conjugate term. The condition above, required for all pairs \(\forall \alpha,\beta\) in the collection of histories, is known as "weak consistency" (originally "weak decoherence") condition.

Now, it's very unnatural to require that just the real part of the off-diagonal entries \(D(\alpha,\beta)\) for the histories' probability vanishes. The reason is that the phase of \(C_\alpha\) is really a matter of conventions and in realistic situations, the phases of \(C_\alpha\) and \(C_\beta\) may even change independently, almost immediately. So instead of the "weak consistency" condition, it is more sensible to demand the "medium consistency" condition\[

\forall \alpha\neq \beta:\quad D(\alpha,\beta) = 0.

\] The matrix of probabilities for the histories, \(D(\alpha,\beta)\), must simply be diagonal and the diagonal entries calculate the probability of each history for us. It's that simple.

Any collection of alternative histories satisfying the medium consistency condition may be "asked" and quantum mechanics gives us the "answers" while all the identities for the probabilities of composite propositions such as "\(\alpha\) or \(\beta\)" will hold as expected. So one will be able to use "classical reasoning" or "common sense" for the answers to all these questions.

It's important to realize that the job for quantum mechanics isn't to "calculate the right questions" or the "right collection of alternative histories" for us. There is no canonical choice. To say the least, there's clearly no preferred "degree of fine or coarse graining" we should adopt. Too coarse graining will be telling us too little; too fine graining will lead us to a conflict with the consistency condition – this conflict really has the origin in the uncertainty principle. You simply can't expect too many things to be specified too sharply. If you tried to fine-grain the histories "absolutely finely", the histories would resemble the classical histories summed in Feynman's approach to quantum mechanics. But they're clearly not consistent. In particular, we know that they can't be mutually excluding because even in the classical limit, many histories in the vicinity of the classical solution contribute to the evolution, as Feynman taught us. This fact also manifests itself by nonzero diagonal entries between histories that are too close to each other (e.g. because the projection operators on states or "cells in the phase space" are clearly not mutually exclusive if the two cells overlap).

The right attitude is somewhere in between – collections of coarse-grained histories for which the consistency condition holds accurately enough, i.e. histories that obey the uncertainty principle etc. sufficiently satisfactorily, but also histories that are fine enough for us to be satisfied with the precision we need. The precise location of the "compromise" clearly cannot be objectively codified. To choose how accurately we want to distinguish histories is clearly a subjective choice. It's up to the observer.

It should be obvious to the reader that there can't exist any "only right degree of coarse-graining". So there can't exist any "only right set of consistent histories". The choice of the right questions, alternative answers, and the degree of accuracy is up to the observer who chooses the logic. It is inevitably subjective and non-unique. The projection operators don't represent any "objective collapse". Instead, the way how they're inserted encodes the question that an observer asked – and I have written down the explicit formula for the answer, namely the probability of a given history, too.

All physically meaningful questions may be summarized as the questions about the probabilities of different alternative histories in a consistent collection, given a known initial state encoded in a density matrix. If you find several collections of consistent histories, good for you. You may perhaps succeed even if there won't be any "unifying finely grained collection" that would allow you to fully answer all the questions from the two collections. The collections may perhaps look at the physical system from a totally different angle. But if they're consistent, they're allowed.

This is clearly a complete and consistent interpretation of quantum mechanics. It tells you exactly what you may ask and what you're not allowed to ask, and for the things you may ask, it tells you how to calculate the answers. They agree with the experiments. All the criticism of this interpretation is clearly pure idiocy and bigotry.

Let me just mention two representative examples of histories that are not consistent.

Start with Schrödinger's cat described by the density matrix \(\rho\). Let the killing device evolve. At the end, try to define two histories that project the cat to some random macroscopic superpositions of the "common sense" dead and alive stated such as\[

0.6\ket{\rm dead}+0.8i\ket{\rm alive},\quad 0.8i\ket{\rm dead}+0.6\ket{\rm alive}.

\] The functional \(D(1,2)\) will be nonzero because the matrix of probability – the final density matrix after decoherence – is off-diagonal in this "uncommon sense" basis.

In principle, you could think that if the probability of "dead" and "alive" will be exactly equal, the matrix \(D\) will be a multiple of the identity matrix – and the identity matrix has the same form in all bases, including bases of unnatural superpositions. In principle, it's right and you have the freedom to rotate the bases arbitrarily.

In practice, you can't rotate them because the evolution of the cat will be producing and affecting lots of environmental degrees of freedom. If you choose a slightly more fine-grained history for the "dead portion" of the evolution than for the "alive portion", or vice versa, the relevant part of \(D(\alpha,\beta)\) will cease to be a multiple of the identity matrix: the entries on the diagonal of \(D\) will be divided to smaller pieces in the "dead cat branch" of the matrix. Because you want your calculation to be independent of the precise level of coarse-graining or the number of degrees of freedom that you treat as the environment, even in the special case when some of the diagonal entries of \(D\) are exactly equal, you won't really be allowed to rotate the basis while preserving the consistency condition "robustly".

Conventional low-energy situations won't really allow you "qualitatively different choices of the collection of consistent histories" that wouldn't be just some "coarse-graining of quantum possibilities around some classical histories". However, the black hole complementarity actually represents a great example of non-uniqueness of the solution to the condition of consistency of histories. The infalling and outgoing observer are using qualitatively different consistent collections of history operators acting on the same (or overlapping) Hilbert space.

Finally, let me also mention that the consistency condition may seemingly allow you to choose "\(z\) up" and "\(x\) up" histories from the beginning of the article in the same collection. The Consistent Histories formalism simplifies dramatically if we only want to show this simple point. The evolution may be completely dropped, the history operators reduce to simple projection operators, and we essentially consider\[

D(x,z) = {\rm Tr} \zav{P_x \rho P_z}.

\] If you write \(\rho\) as a combination of the identity matrix and three Pauli matrices (or, equivalently, multiples of \(P_x,P_y,P_z\)), you will find out that the trace above vanishes as long as \(\rho\) contains no contribution from \(P_y\). So if the expectation value of \(J_y\) in the initial state vanishes, the off-diagonal elements will be zero. (The latter claim may also be easily seen by calculating \(D(x,z)-D(z,x)\) from the commutator \([P_z,P_x]\).)

However, such a collection of histories will fail to obey the logical condition I haven't mentioned yet:\[

\sum_\alpha C_\alpha = {\bf 1}.

\] This should be valid as an operator equation so it's stronger than \(\sum_\alpha D(\alpha,\alpha)=1\). So it's not allowed to consider "alternatives" that aren't really orthogonal to each other.

In practice, the equation \(D(\alpha,\beta)=0\) is never "quite accurate" so we always ask questions about alternative histories that are only approximately consistent although the accuracy quickly becomes sufficient for all practical and most of impractical purposes. That's a manifestation of the fact that classical physics – and classical reasoning in general – never kicks in quite exactly.

Let me mention that aside from the "weak decoherence" and "medium decoherence" conditions above (the medium one clearly implies the weak one), Gell-Mann and Hartle also discussed a "strong decoherence" condition which would imply both of the previous two but which is too strong and would kill almost all choices of "history collections" whenever the initial state is highly mixed. The condition said that one could express all products \(C_\alpha \rho\) as\[

C_\alpha \rho = R_\alpha \rho

\] where \(R_\alpha\) is a projection operator, a "record projection". So one wants to work with the "medium decoherent" histories.

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