Canonical Relational Quantum Mechanics from Information Theory

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by  Joakim Munkhammar

PreprinttypesetinJHEPstyle-HYPERVERSION
Canonical Relational Quantum Mechanics from
Information Theory
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0 Joakim Munkhammar
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Studentstaden23:230,75233,Uppsala,Sweden
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a E-Mail: [email protected]
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ABSTRACT:InthispaperweconstructatheoryofquantummechanicsbasedonShannon
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information theory. We define a few principles regarding information-based frames of
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- reference, including explicitly the concept of information covariance, and show how an
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ensembleofallpossiblephysicalstatescanbesetuponthebasisoftheaccessibleinforma-
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g tion in the local frame of reference. In the next step the Bayesian principle of maximum
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s entropyisutilizedinordertoconstrainthedynamics. Wethenshow,withtheaidofLisi’s
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i universal action reservoir approach, that the dynamics is equivalent to that of quantum
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y mechanics. Thereby we show that quantum mechanics emerges when classical physics
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issubjecttoincompleteinformation. Wealsoshowthattheproposedtheoryisrelational
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[ andthatitinfactisapathintegralversionofRovelli’srelationalquantummechanics. Fur-

1 thermorewegiveadiscussionontherelationbetweentheproposedtheoryandquantum
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mechanics,inparticulartheroleofobservationandcorrespondencetoclassicalphysicsis
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1 addressed. Inadditiontothiswederiveageneralformofentropyassociatedwiththein-
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formationcovarianceofthelocalreferenceframe. Finallywegiveadiscussionandsome
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. openproblems.
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1 KEYWORDS: InformationTheory,QuantumMechanics,Entropy.
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Contents
1. Introduction 2
1.0.1 RelationalQuantumMechanics 2
1.0.2 Universalactionreservoir 2
1.0.3 Relationalquantummechanicsandtheuniversalactionreservoir 3
2. Physicswithinformationcovariance 3
2.1 Principlesforinformation-basedframesofreference 3
2.2 Formalsetup 4
3. Connectionstoquantummechanics 7
3.1 Pathintegralformulation 7
3.2 Quantumproperties 9
3.3 Connectionstorelationalquantummechanics 11
3.4 Correspondenceprinciple 11
3.5 Observationsandwavefunctioncollapse 11
4. Notesonrelativisticinvariance 12
5. Entropy 12
6. Discussion 14
6.1 Openproblems 14
6.2 Finalcomments 14
7. Acknowledgments 15
–1–
”Informationistheresolutionofuncertainty.”
-ClaudeShannon
1. Introduction
Quantum mechanics constitutes a conceptual challenge as it defies many pivotal classi-
calconceptsofphysics. Despitethisthetheoreticalandexperimentalsuccessofquantum
mechanicsisunparallel[15]. Theintuitiveconstructionofclassicalmechanicsandtheper-
haps counter-intuitive quantum mechanical formulation of reality has thus amounted to
aproblemofinterpretationofquantummechanics. Thelistofinterpretationsofquantum
mechanics is long, but perhaps the most common interpretations are: Copenhagen [2],
consistent histories [7], many worlds [5] and relational [15]. Many of these approaches
share most central features of quantum mechanics, the difference is mainly the philo-
sophical context in which they are situated. The canonical problem in these approaches
is perhaps the counter-classic, and in many peoples views, counter-intuitive principle
that physics is fundamentally based on probability. The perhaps strongest opponent of
aprobability-basedtheoryofphysicswasEinsteinwhoconstructedseveralunsuccessful
thought experiments and theories in order to disprove the commonly accepted view of
quantum mechanics [4]. As a matter of fact all approaches to create a non-probabilistic
version of quantum mechanics have failed [3]. It has also been proven, via for example
Bell’s theorems, that the construction of such a deterministic or ”local hidden variable
theory”isimpossible[3]. Thusonehasnochoicebuttoconcludethatphysics,inevitably,
hasquantumproperties.
1.0.1 RelationalQuantumMechanics
It was Einstein’s revised concepts of simultaneity and frame of reference that inspired
Rovelli to formulate a theory of mechanics as a relational theory; relational quantum me-
chanics. In this seminal approach frames of reference were utilized and only relations
betweensystemshadmeaning. Thissetupgaveinterestingsolutionstoseveralquantum-
related conceptual problems such as the EPR-paradox [15]. It may be concluded that
Rovelli recast physics in the local frame of reference in such a way that the interactions
between systems amounted to the observed quantum phenomena. A particular facet of
Rovelli’sapproachwasthatanyphysicalsystemcouldbeobservedindifferentstatesby
different observers ”simultaneously”. This property can be interpreted as an extension
oftheconceptofsimultaneityin specialrelativity[15]. Thetheory ofrelationalquantum
mechanics was based on the hypothesis that quantum mechanics ultimately arose when
therewasalackofinformationofinvestigatedsystems.
1.0.2 Universalactionreservoir
In a recent paper Lisi gave an interesting approach to quantum mechanics based on in-
formation theory and entropy [12]. He showed that given a universal action reservoir and
–2–
a principle for maximizing entropy quantum mechanics could be obtained. In his paper
the origin of the universal action reservoir was postulated as a principle and was given
no deeper explanation. This was addressed in recent papers by Lee [10, 11] where he
suggestedthatitwasrelatedtoinformationtheorycoupledtocausalhorizons.
1.0.3 Relationalquantummechanicsandtheuniversalactionreservoir
In this paper we shall recast Rovelli’s relational approach to quantum mechanics on the
conceptofinformationcovarianceandconnectittoLisi’scanonicalinformationtheoretic
approach. Intheprocessofthisweshowthattheuniversalactionreservoirisaninevitable
consequence of incomplete information in physics. This theory is, in spirit of Rovelli’s
approach,ageneralizationofthespecialrelativisticconceptregardingframesofreference.
2. Physicswithinformationcovariance
2.1 Principlesforinformation-basedframesofreference
Letusassumethatwehaveasetofobservationsofcertainphysicalquantitiesofaphysical
system. This is the obtained information regarding the system. For all other events that
hasnotbeenobservedweonlyknowthatsomethingpossiblehappened. Ifweknewwith
certainty what happened then we as observers would be in a frame of reference based
on complete information regarding the explored system. However such a theory would
requiresomeformofaconservationlawwhichwouldrequirethataphysicalsystemhas
apredeterminedstatewhennotobserved. Letusinsteadconsidertheopposite: Assume
that no observation is made of the system, then information regarding thatsystem is not
accessible and thus not inferable without more observations. It is then reasonable that
anythingphysicallypossiblecouldhaveoccurredwhenitwasnotobserved. Ifonecanbase
the laws of physics on the premise of only what is known in the frame of reference, in
termsofinformation,regardinganyphysicalsysteminconnectionwiththeobserverand
her frame then one has attained a high from of generality in the formulation of the laws
ofphysics. Thisamountstoaseeminglytautologicalyetpowerfulprinciple:
Ifasysteminphysicsisnotobserveditisinanyphysicallypossiblestate.
Weshallcallthisprincipletheprincipleofphysicalignorance. Inonesensethisprinciple
is intuitive and trivial: the unknown is not known. In another more intricate sense it
violates a number of basic principles of physics such as many of the laws of classical
physics. Thelawofinertiaisonesuchprincipleforexample;theinertiaofabodydoesnot
necessarilyholdwhenwedonotobserveit. HoweveronecannotassumethatNewton’s
lawsoranyotherclassicallawsholdinasystemforwhichlimitedinformationisknown.
Forallweknowregardingclassicalphysicsisthatitholdsinacertainclassicallimit. The
definition of ”known” here is what information has become accessible in ones frame of
reference obtained through interaction with another system. The laws of physics should
–3–
thusbe formulatedinsucha waythatthey holdregardlessof information contentinthe
local frame of reference; the formulation of the laws of physics should be invariant with
respecttotheinformationcontent. Thisamountstoaprinciplefortheformulationofthe
lawsofphysics:
Thelawsofphysicsaredefinedonthebasisoftheinformationintheframeofreference.
Thisprincipleshallbecalltheprincipleofinformationcovariance. It’sscopeofgeneral-
ityissimilartotheprincipleofgeneralcovariance,whichisthenaturalgeneralizationofthe
framesofreferenceusedinspecialrelativity. Theprincipleofinformationcovariancesuggests
that the frame of reference is entirely based on information; thus any new observations
will alter the information content and thus re-constrain the dynamics of the studied sys-
tem”projected”withintheframeofreference. Oneshouldalsokeepinmindthatphysics
cannot, in a frame of incomplete information, ”project” any information through physi-
cal interactions that is not inferable from the accessible information. This creates a form
of locality of physics: Physics only exists in every frame of reference. Thus there exists
no such thing as an objective observer. It should be noted here that there exists a sim-
ilar notion of frames of reference in traditional relational quantum mechanics [15]. The
dynamics is derived from a different set of principles and formal setup but arrives at a
similarconclusions[15].
2.2 Formalsetup
Let us assume that we have a set of observations A regarding physical quantities of a
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systemC fromaframeofreferenceK. TheavailableinformationregardingthesystemC
inK is givenby theinformation inthe observedphysical quantitiesA and whatcan be
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inferred from them, the rest of the properties of the system are by the principle of physical
ignorance unknown. Formally according to the principle of physical ignorance we may con-
clude that the possible configurations of the explored physical system C in the frame of
reference K has to form a set of all possible physical configurations under the constraint
of the set of observations A for the system. Consider a set of configuration parameters
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for each possible physical configuration or path in configuration space path = q(t). The
setofconfigurationsparametersisparameterizedbyoneormoreparameterst. Thenfor
each physical system we may associate an action S[path] = S[q(t)] defined on the basis
of the configuration space path q(t). The structure of the action is here assumed to be
something of a universal quantity of information inherent to every frame of reference, it
is the paths in the configuration space of objects that is unknown to every observer until
(cid:82)
observed. The action of a system is defined classically as S = L(q,q˙)dt, where L(q,q˙)
is the Lagrangian for the system [17]. Traditionally with the aid of a variation principle
the expected action of a classical system is used to derive the dynamics of the system
[17]. Inquantummechanicsaprobabilisticsetupisperformed,ideallyviaapathintegral
formulation. In our situation we shall instead look for all possible configurations under
–4–
condition of the observations A in accordance with the previously defined principle of
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physicalignorance. Thepossibleactions,basedonthepossibleconfigurations,areeachas-
sociated with a probability of occurring, p[path] = p[q(t)]. The sum of these probabilities
needtosatisfytheubiquitousnormalizationcriterion:
(cid:90)
(cid:88)
1 = p[path] = Dqp[q], (2.1)
paths
Along with this we may conclude that any functional, or observable quantity Q, has
anexpectedvalueaccordingtotheexpression[12]:
(cid:90)
(cid:88)
(cid:104)Q(cid:105) = p[path]Q[path] = Dqp[q]Q[q]. (2.2)
paths
Wehavetheexpectedaction(cid:104)S(cid:105)accordingto[12]:
(cid:90)
(cid:88)
(cid:104)S(cid:105) = p[path]S[path] = Dqp[q]S[q]. (2.3)
paths
Asameasureoftheinformationcontent, orratherlackthereof, wecanconstructthe
entropyofthesystemaccordingto:
(cid:90)
(cid:88)
H = − p[path]logp[path] = − Dqp[q]logp[q]. (2.4)
paths
So far we have merely utilized information theoretic concepts, we shall deal with its
physicalconsequencesfurtheroninthispaper. Althoughtheprobabilitiesfortheevents
insystemC observedfromreferencesystemK areyetundefinedwehaveasetofpossible
configurations that could occur for a system and we have associated a probability of oc-
currencewitheachbasedontheensemblesetupabove. Inordertodeducetheprobability
associatedwitheachpossibleeventweneedsomeformofrestrictionontheensembleof
possibilities. InaBayesiantheoryofinterferencethereisamaximizationprincipleregard-
ing the entropy of a system called the Principle of maximum entropy [8] which postulates
thefollowing:
Subjecttoknownconstraints,theprobabilitydistributionwhichbestrepresentsthecurrentstate
ofknowledgeistheonewithlargestentropy.
This principle is utilized in several fields of study, in particular thermodynamics
where it serves as the second fundamental law [8]. We shall assume that this principle
holdsandweshallutilizeitasarestrictiononourframework. In[12]Lisiperformedthe
followingderivationwhichisworthrepeatinghere. ByemployingLagrangemultipliers,
λ ∈ Candα ∈ C,theentropy(2.4)ismaximizedby:
(cid:90) (cid:16) (cid:90) (cid:17) (cid:16) (cid:90) (cid:17)
H(cid:48) = − Dqp[q]logp[q]+λ 1− Dqp[q] +α (cid:104)S(cid:105)− Dqp[q]S[q] , (2.5)
–5–
whichsimplifiedbecomes:
(cid:90)
H(cid:48) = λ+α(cid:104)S(cid:105)− Dq(p[q]logp[q]+λp[q]+αp[q]S[q]). (2.6)
Ifweperformvariationontheprobabilitydistributionweget:
(cid:90)
δH(cid:48) = − Dq(δp[q])(logp[q]+1+λ+αS[q]) (2.7)
whichisextremizedwhenδH(cid:48) = 0whichcorrespondstotheprobabilitydistribution:
1
p[q] = e−1−λe−αS[q] = e−αS[q] (2.8)
Z
which is compatible with the knowledge constraints [12]. By varying the Lagrange
multipliersweenforcethetwoconstraints,givingλandα. Especiallyonegets: e−1−λ = 1
Z
whereZ isthepartitionfunctionontheform:
(cid:90)
(cid:88)
Z = e−αS[path] = Dqe−αS[q] (2.9)
paths
andtheparameterαisdeterminedbysolving:
(cid:90) (cid:90)
1 ∂
(cid:104)S(cid:105) = DqS[q]p[q] = DqS[q]e−αS[q] = − logZ. (2.10)
Z ∂α
In order to fit the purpose Lisi concluded that the Lagrange multiplier value; α ≡
1. Lisi concluded that this multiplier value was an intrinsic quantum variable directly
i(cid:126)
related to the average path action (cid:104)S(cid:105) of what he called the universal action reservoir.
In similarity with Lisi’s approach we shall also assume that the arbitrary scaling-part of
the constant α is in fact 1/(cid:126). Lisi also noted that Planck’s constant in α is analogous
to the thermodynamic temperature of a canonical ensemble, i(cid:126) ↔ k T; being constant
B
reflects its universal nature - analogous to an isothermal canonical ensemble [12]. This
assumptionalongwith(2.9)bringsustothefollowingpartitionfunction:
(cid:90)
Z = (cid:88) eiS[p(cid:126)ath] = DqeiS(cid:126)[q]. (2.11)
paths
Byinserting(2.11)into(2.2)wearriveatthefollowingexpectationvalueforanyphys-
icalquantityQ:
(cid:90) (cid:90)
(cid:104)Q(cid:105) = (cid:88) p[path]Q[path] = DqQ[q]p[q] = 1 DqQ[q]eiS(cid:126)[q], (2.12)
Z
paths
Thissuggeststhataconsequenceoftheincompleteinformationregardingthestudied
system is that physics is inevitably based on a probabilistic framework. Conversely, had
physicsnotbeenprobabilisticinthesituationofincompleteinformationtheninformation
of the system could be inferred. But a process of inferring results from existing limited
informationdoesnotprovidemoreinformationregardingthatsystemthanthelimitedin-
formationhadalreadyprovided. Thatwouldhaverequired,aswepreviouslyargued,an
–6–
K’
K
Figure 1: This illustration shows the path of a particle from one point to another in a complete
information frame of reference K (essentially a particle that is observed along its path). It also
showssomeofthepossiblepathsaparticletakesintheincompleteinformationframeofreference
K(cid:48).
additionalprincipleofperfectinformation. Insteaditisonlyinteractionthatcanprovide
new information. We may conclude that by the principle of information covariance physics
islocalandbasedonlyontheavailableinformationinthelocalinformation-basedframe
of reference. In turn this this creates an ensemble of possible states with a definite and
assignedexpectationvalueforeachphysicalquantityinthestudiedsystemaccordingto
(2.12). This formalism, which might be called information covariant, is then directly com-
patiblewiththegeneralprincipleofrelativitywhereinAllsystemsofreferenceareequivalent
withrespecttotheformulationofthefundamentallawsofphysics.
3. Connectionstoquantummechanics
3.1 Pathintegralformulation
Thepathintegralformulation,originallyproposedbyDiracbutrigorouslydevelopedby
Feynman[6],isperhapsthebestfoundationalapproachtoquantummechanicsavailable
[12]. It shows that quantum mechanics can be obtained from the following three postu-
latesassumingaquantumevolutionbetweentwofixedendpoints[6]:
1. The probability for an event is given by the squared length of a complex number
calledtheprobabilityamplitude.
2. The probability amplitude is given by adding together the contributions of all the
historiesinconfigurationspace.
3. ThecontributionofahistorytotheamplitudeisproportionaltoeiS/(cid:126),andcanbeset
equal to 1 by choice of units, while S is the action of that history, given by the time
integraloftheLagrangianLalongthecorrespondingpath.
–7–
In order to find the overall probability amplitude for a given process then one adds
up(orintegrates)theamplitudesoverpostulate3[6]. Inanattempttolinktheconceptof
information-basedframesofreference-developedinthispaper-toquantummechanics
we shall utilize Lisi’s approach wherein the probability for the system to be on a specific
path is evaluated according to the following setup (see [12] for more information). The
probabilityforthesystemtobeonaspecificpathinasetofpossiblepathsis:
(cid:90)
(cid:88)
p(set) = δset p[path] = Dqδ(set−q)p[q]. (3.1)
path
paths
Here Lisi assumed that the action typically reverses sign under inversion of the pa-
rametersofintegrationlimits:
(cid:90) t(cid:48) (cid:90)
St(cid:48) = dtL(q,q˙) = − dtL(q,q˙) = −St(cid:48). (3.2)
t(cid:48)
This implies that the probability for the system to pass through configuration q(cid:48) at
parametervaluet(cid:48) is:
(cid:32) (cid:33)(cid:32) (cid:33)
(cid:90) (cid:90) q(t(cid:48))=q(cid:48) (cid:90)
p(q(cid:48),t(cid:48)) = Dqδ(q(t(cid:48))−q)p[q] = Dqpt(cid:48)[q] Dqp [q] = ψ(q(cid:48),t(cid:48))ψ†(q(cid:48),t(cid:48)),
t(cid:48)
q(t(cid:48))=q(cid:48)
(3.3)
inwhichwecanidentifythequantumwavefunction:
ψ = (cid:90) q(t(cid:48))=q(cid:48)Dqpt(cid:48)[q] = √1 (cid:90) q(t(cid:48))=q(cid:48)Dqe−αSt(cid:48) = √1 (cid:90) q(t(cid:48))=q(cid:48)DqeiS(cid:126)t(cid:48). (3.4)
Z Z
The quantum wave function ψ(q(cid:48),t) defined here is valid for paths t < t(cid:48) meeting at
q(cid:48) whileitscomplexconjugateψ†(q(cid:48),t(cid:48))istheamplitudeofpathswitht > t(cid:48) leavingfrom
q(cid:48). Multiplied together they bring the probability amplitudes that gives the probability
of the system passing through q(cid:48)(t(cid:48)), as is seen in (3.3). However, just as Lisi points out
[12], this quantum wave function in quantum mechanics is subordinate to the partition
function formulation since it only works when t(cid:48) is a physical parameter and the system
is t(cid:48) symmetric, providing a real partition function Z. Indeed, the postulate of an infor-
mationcovariantsetuponthelawsofphysicsaccordingtotheprevioussectionsuggests
thatphysicsisruledbythegeneralcomplexpartitionfunction(2.9):
(cid:90)
Z = (cid:88) eiS[p(cid:126)ath] = DqeiS(cid:126)[q]. (3.5)
paths
Howdoesthisrelatetothepathintegralformulation? Thesuminthepartitionfunc-
tion(3.5)isasumoverpaths. Letustakethecommonsituationwhenthepathisthatofa
particlebetweentwopoints. WecanthenconcludethateachtermisontheformeiS[path]/(cid:126)
which is equivalent to postulate 3. Furthermore all paths are added, thus postulate 2 is
alsochecked. Also,atleastforthesituationwherep(q(cid:48),t(cid:48)) = ψ(q(cid:48),t(cid:48))ψ†(q(cid:48),t(cid:48))thesumadds
–8–
Figure 2: This illustration shows on the left hand side the uncertainty of path of a particle from
onepointtoanotherandontherighthandsidethattheparticletakesallpossiblepathsfromone
point to another. These two interpretations are equivalent under the general interpretation that
information is incomplete. Uncertainty in path means in practice that it takes any possible path
untilweobserveit,asuperpositionofstatesisinevitablewheninformationisincomplete.
up to the probability density, checking postulate 1 as well. Thus we may conclude that
theinformationcovariantapproachisequivalenttothecanonicalpathintegralformulation
ofquantummechanicsunderthecircumstancesprovidedforit.
3.2 Quantumproperties
The path integral formulation is canonical for quantum mechanics and covers the wide
variety of special features inherent to quantum mechanics [6, 12]. Since the approach in
thispaperisequivalenttothepathintegralformulationinmostaspects,someproperties
are be worth discussing. A pivotal component of quantum mechanics is the canonical
commutationrelationwhichgivesrisetotheHeisenberguncertaintyprinciple[3,6]. For
examplethefamouscommutationrelationbetweenpositionxandmomentumpofapar-
ticleisdefinedas:
[x,p] = i(cid:126). (3.6)
This can be obtained through the path integral formulation by assuming a random
walk of the particle from starting point to end point [6]. This works with this theory as
wellunderthesameconsiderationssincearandomwalkisequivalenttoawalkwithno
information about direction. In the path integral formulation it is also possible to show
that for a particle with classical non-relativistic action (where where m is mass and x is
position):
(cid:90) mx˙2
S = dt, (3.7)
2
that the partition function Z in the path integral formulation turns out to satisfy the
followingequation[6]:
∂Z (cid:104) 1 (cid:105)
i(cid:126) = − ∇2+V(x) Z. (3.8)
∂t 2
–9–