Events, quantum field theory and the quantum substrate

The concepts of "event" and "measurement" in quantum physics have been discussed previously. In working physics, for practical purposes, the Born rule is used and is adequate for laboratory predictions. It is nonetheless mysterious what the measuring apparatus is doing physically. How does it sample a probability distribution? Outside the laboratory what triggers spontaneous events?  RTI [1] claims to answer these questions through an extension of transactional quantum mechanics to quantum field theory.

Quantum field theory (QFT) is usually considered a more fundamental development of non-relativistic quantum physics. And it is the case that it treats subjects such as relativistic invariance and particle creation and annihilation that standard quantum theory cannot cover. This has led some to tackle the formal ontological problems of quantum physics via field theory [2]. In addition, Kuhlmann argues for taking a formal representation of quantum field theory (Algebraic QFT) as fundamental for an ontological understanding. Whether or not this is the best approach is open to debate, but it is a worthwhile exploration with Kuhlmann taking analytical ontology seriously. One of the problems with Algebraic QFT, however, is that space-time regions are primary. That is, the physical information in quantum field theories is not contained in individual algebras but in the mapping \(\mathcal{O} \rightarrow \mathcal{A(O)}\) from spacetime regions \(\mathcal{O}\) to algebras \(\mathcal{A(O)}\) of local observables where the \(\mathcal{O}\)s are open and bounded regions in Minkowski spacetime [2]. Considering the previous posts covering relativistic transactional quantum theory (RTQT - which has been referred to as RTI, but we are dealing with more than just a reinterpretation) the role of the quantum substrate and the emergence of space-time was discussed but in what way is QFT ontologically more fundamental if it needs spacetime to get started. 

A field theory works with a function from a domain over which the field lies to whatever form the field values take. Classical electromagnetic theory provides a clear example. The domain is four-dimensional spacetime and the field values are three dimensional vectors.  This view of fields is usually carried over to the quantum domain. If space-time is needed to define the quantum fields, then the quantum substrate would be ontologically more fundamental than QFT.

I think Ruth Kastner avoids this. Particle position is also an important concept for the quantum substrate but here in the mathematical representation the position operators and the state of the system together constrain the set of possible spatial positions. This status of position as potentiality can be carried over to QFT. I think this agrees with Ruth Kastner's stance [1]. Interestingly, although he takes a very different approach, Kuhlmann also ends up proposing a dispositional ontology with observable take potential values that have a probabilistic propensity to occur [2].

So, how does RTQT use QFT to clarify the concept of quantum event? To understand this, I have found it useful to supplement the book length presentation of RTQT [1] with some of Ruth Kastner's papers. For the topics in this post, I found "On Real and Virtual Photons in the Davies Theory of Time-Symmetric Quantum Electrodynamics" [3] particularly enlightening. The discussion in the paper deals with photons but I believe the physics can be generalised to other particles.

Here are the main points extracted from [3] and wider reading on physical transaction in the relativistic quantum domain.

• A real photon is one that transfers empirically detectable energy 
• An absorber is a subsystem that can receive a transfer of energy while respecting conservation laws
• There may be many potential absorbers with the above characteristics and the probability of transaction will, in general, vary across them.
• The response of the absorber is what gives rise to the ‘free field’ that in the quantum domain is considered a ‘real photon’ 
• The virtual photon is just the potential for possible transaction between two currents—but one that was not realised
• A transaction is only attainable for virtual photons that satisfy the energy and momentum conservation constraints for the initial and final states of the system.
• The realisation of a photon happens due to an absorber response and random selection governed by the transaction probabilities
• In a realised transaction a virtual photon is elevated to a real photon
  
• In a situation that is not a prepared experiment it will not be possible to identify the subsystems that qualify as absorbers for a potential transaction.

In the sense understood here a transaction is an event. I believe this model can be generalised and the discussion of matter fields is important for this [4]. The creation and annihilation of photons is implicit in the points made above, but the role of creation and annihilation events needs to be made explicit in the generalisation to all particle types.

References

1. Ruth E. Kastner, The Transactional Interpretation of Quantum Mechanics - A Relativistic Treatment, Cambridge University Press, second edition 2022
2.  Meinard Kuhlmann, The Ultimate Constituents of the Material World: In Search of an Ontology for Fundamental Physics, Ontos Verlag, 2010
3. Ruth E. Kastner, On Real and Virtual Photons in the Davies Theory of Time-Symmetric Quantum Electrodynamics, [v2] 2016 
4. Ruth E. Kastner, Antimatter in the Direct-Action Theory of Fields, 2015

There's lots of room left in Hilbert Space (*): events in the quantum substrate and empirical space

From the perspective of this blog, the most important advance provided by the Relativistic Transactional Interpretation (RTI, [1]) of quantum physics is the replacement of the ad hoc Born Rule with a physical process and freeing from its ties to measurement. This process involves an interplay between the quantum substrate (QS) and the space-time layer of empirical events, as discussed previously. Not only are there empirical events in space-time (constructed from past events) but also non-empirical QS events outside space-time. As shown in the previous post, the quantum state changes from that obtained through unitary propagation to a state associated with the projection on to the space-time event due to conditioning on that projection. So, there is an observable stochastic process in space-time and a hidden (not observable) QS stochastic process governed by the same transition probabilities. This interplay between the QS and space-time events needs further discussion.

Ruth Kastner explains in detail [1] how RTI quantum theory developed from the classical Wheeler-Feynman Absorber Theory of Radiation [2, 3]. In chapter 5 she reaches the technical core of the book that builds on the quantum adaptation of absorber theory by Davies [4,5]. The historical progression will not be our concern here but the formal structure of the argument.  In the book two further references are provided and they help in presenting the structure. They are a joint paper with John Cramer [6] and a paper explicitly on the measurement problem [7]. 

It is a theme of this blog that events in the micro-domain cannot be restricted to measurements so the argument will focus on absorption events with measurements as a special case. A helpful list of RTI terminology has been provided [7] and it will be adapted here to begin the structured argument. In a further adaptation I will seek to avoid mention of waves because pictures of wave propagation in empirical space have been a major source of confusion.

1. micro-emitter: an excited quantum object with the propensity to release of quantum of energy (\(\hbar \omega\)) 
2. micro-absorber: a quantum object with a propensity to be excited from a lower energy state to a higher one on receiving offered the quantum energy
3. macro-emitter: a collection of \(N_E : N_E \gg1 \) micro-emitters 
4. macro-absorber: a collection of \(N_A : N_A \gg 1\) micro-absorbers
5. emission: a micro-emitter generates an offer \(|\omega>\) of energy \(\hbar \omega\)
6. absorber response: a micro-absorber generates a confirmation (\(<\omega , k|\))
1. corresponding to the component of the offer received by it
2. instantiating a non-unitary transition 
7.  absorption: an actualised transaction in which conserved quantities (e.g. energy) are transferred from the emitter to a particular micro-absorber, resulting in excitation of the latter. This is irreversible (non-unitary) and random.

Setting aside the macro items for the moment, 1-2 indicate the elements in place to carry out a transaction. 5-7 is the transaction process resulting in absorption and a transfer of energy. The main part of the physics missing is the creation and annihilation of the boson that carries the quantum from the emitter to an absorber. In point 6 the presence of \(k\) was not explained. It has to do with the role of the photon. 

The photon

Although we are now working with a physical ontology consisting of a quantum substrate and a supervening space-time of events, there is a strong intuitional pull towards thinking of photons as quantised elements of electromagnetic fields in space-time. In standard classical electrodynamics it is possible to construct free waves solutions of the homogeneous (source free) Maxwell equations. A commonly held physical intuition would picture these free waves as having some distant source but this source only has a role in the physics narrative and does not affect the future propagation and eventual interaction of the wave. There are also solutions to the inhomogeneous Maxwell equations that show how the propagating fields are created and there is a theory of wave interaction with matter (charges). In contrast the Wheeler-Feynman classical theory of electromagnetism places emission and absorption in centre stage and not only diminishes but eliminates the free field [2, 3]. 

Both forms of classical electrodynamics are now replaced with some form of Quantum Electrodynamics (QED). Standard quantum electrodynamics [e.g., 8] carries over some intuitions from standard classical electrodynamics. Quantisation produces two kinds of photon commonly known as real and virtual. The real photon corresponds to quantising the freely propagating classical wave. The virtual photon mediates interaction between moving charges. The virtual photon is created and destroyed with a finite lifetime as it mediates the interaction. This virtual photon is sometimes referred to as internal because of how it is portrayed in Feynman diagrammes, as shown below for the electron-electron interaction [8, Fig. 7-3]

Davies recognises this virtual and real terminology is confusing [5] in direct action quantum theory. I can only agree. Transactional quantum mechanics has taken strong guidance from the mechanisms of classical Wheeler-Feynmann electrodynamics. In clarifying the physics of the transaction RTI builds on the direct-action QED developed by Davies [4, 5]. It is a theory of photon exchange but with ontological ambiguity about the status of the photon. As indicated above, in the RTI the physical stratum of the ontology includes two interplaying layers: the quantum substrate (QS) and empirical space-time with its entities. To decide on which layer entities belong requires some analysis. In all mathematical physical theory, there are constructs that correspond to entities in the ontology and some that do not - they are useful conceptual scaffolding. Just as in standard quantum mechanics, there may be versions of RTI that gives ontological preference to a wave picture or one that gives preference to a statistical transition picture. Ruth Kastner's presentation makes use of a wave picture, with offer and confirmation waves. I am interested in seeing how far a probabilistic transition picture can be developed. To help in this, rather than carrying over the "real" and "virtual" terminology from standard QED, it may be more productive to embrace fully the language of potentiality. 

A quantum subsystem (QS entity) may have the potential to emit a quantum of energy. For example, it may be a QS atom in an excited state. It may also have potential to do other things such as bond with another atom to form a molecule. How that potential may or may not be realised depends not only on the atom but its environment. In the QS we must not think about this environment in four-dimensional space-time terms. Here the generalised probability formulation of quantum mechanics may help.  In this formulation the quantum sub-system of interest has potentialities that are conditioned on other entities in the QS. It is proposed that this conditioning is a QS physical effect and not just a mathematical manipulation in the theory. It is transactional. So, a quantum sub-system has a set of potentialities, and these potentialities form a complex that can separate into subsets of possibilities that have well defined probability spaces, in their mathematical description. Physically they are events that are conditioned to occur. Using a concept from Barbara Vetter's theory of Potentiality, we have a two-step iterated potentiality:

1. The potential possibilities that the sub-system could manifest are encapsulated within a complex (represented mathematically by the \(\sigma\)-complex)
2.  In the first step, the QS environment interacts with the sub-systems to realise sub-sets of statistical potentiality. This can be thought of as a step to propensity with statistical weightings realised through conditioning
3. In the second step the possibilities of the statistical potentiality are sampled by the other entities in the QS, and this can, but need not, lead to a space-time event. 

I have used the term "sampled" as it is familiar from statistics. It should be emphasised that this is not the observational sampling that statistical offices undertake or that a moderator may do by extracting a numbered ball from an urn in a lottery game. Here we have a physical active transactional sampling of one part of the QS by another. This can also be considered to be the physical interpretation of the role of projection operators associated with the sub-system's manifestation possibilities. This is an alternative to Ruth Kastner's symmetry breaking explanation as to why only one of a set of possibilities is realised. More generally [6] a representation of the physics is formulated in terms of creation and annihilation operators for photons acting on a Fock space [8]. This in turn has a natural representation in terms of projection operators, as used in the adapted Kochen re-formulation of standard quantum mechanics. 

The rich mathematical structure describing the QS cannot be accommodated in four-dimensional space-time. This was recognised previously (*) but then usually neglected to concentrate on empirical and technological success. Or work with an otology restricted to "be-ables" in four-dimensions. The four-dimensional space-time does, however, seem to represent the empirical space where events take place and experiments are carried out.  If empirical space supervenes on the quantum substrate, then to what extend does the QS determine the space-time structure? 

The structural laws of space-time may emerge in the empirical space and not be determined in anyway by the structure and laws of the QS but this should not be assumed prior to investigation. For both standard QED and RTI quantum theory the space-time structures developed in relativity theory strongly influenced the mathematical form of the theories. So, the relativistic mathematical structure of space-time has been inserted, or pre-supposed, in constructing the theories. As we only have direct access to events at the empirical level it is inevitable that the space-time structures describing physics at that level inform the mathematical representation of the QS. However, it has transpired that while locally observed quantum events obey relativistic locality the statistical corelations between space separated events do not always do so.  So, despite having used space-time relativistic structure to guide the construction of the mathematical representation of the QS, theory leads to predicted violation of relativistic principles at the empirical level. That is, robust statistical correlation is observed between space-like separated events. It is only through the construction of experiments based on the EPR "paradox", motivated by quantum theory, which has led to the confirmation of this effect. 

What has been developed in RTI quantum theory is a theory of emission and absorption of photons that provides a mechanism for no-unitary system dynamics.  However, more generally, other particles can be emitted and absorbed (or so it seems from second quantisation of standard quantum mechanics). For example, the electron dynamics also can be described using creation and annihilation operators acting on a Fock space. What we cannot do here is be guided by classical wave theory of emission and absorption of point particles - there is no such thing. Tackling these issues will need a deeper dive into the mathematical structure of the theory.  

 

(*) Gentleman: there's lots of room left in Hilbert Space. Sanders Mac Lane

References

1. Ruth E. Kastner, The Transactional Interpretation of Quantum Mechanics - A Relativistic Treatment, Cambridge University Press, second edition 2022
2. John A. Wheeler, Richard P. Feynman, (1945). Interaction with the Absorber as the Mechanism of Radiation. Reviews of Modern Physics. 17 (2–3): 157–181.
3. John A. Wheeler, Richard P. Feynman, (1949). "Classical Electrodynamics in Terms of Direct Interparticle Action". Reviews of Modern Physics. 21 (3): 425–433. 
4. Paul C. W. Davies, (1971) Extension of Wheeler-Feynman Quantum Theory to the Relativistic Domain I. Scattering Processes, J. Phys. A: Gen. Phys. 4, 836.
5. Paul C. W. Davies, (1972) Extension of Wheeler-Feynman Quantum Theory to the Relativistic Domain II. Emission Processes, J. Phys. A: Gen. Phys. 5, 1025-1036.
6. Ruth E. Kastner, and John G. Cramer, (2018). Quantifying absorption in the Transactional Interpretation. International Journal of Quantum Foundations 4:3, 210–22.
7. Ruth E. Kastner, (2018) On the status of the measurement problem: recalling the relativistic transactional interpretation. International Journal of Quantum Foundations 4:1, 128–41.
8. James D. Bjorken and Sidney D. Drell, (1964) Relativistic Quantum Mechanics, McGraw-Hill Book Company

Conditional states, potentiality and experiment

In this post some of my earlier posts will be reviewed using insights from Ruth Kastner's Relativistic Transactional Interpretation (RTI) [1] of quantum physics, which was discussed in the previous post. In fact, the term interpretation under sells this reformulation and its physical insights. Relativistic Transactional Formulation or even Relativistic Transactional Theory could be more appropriate.  However, it is referred to, I will be using it to re-examine the concept of quantum conditional probability and Bohm's version of the Einstein, Podolsky, Rosen (EPR) thought experiment, that use space-like separated Stern-Gerlach detectors. Of course, the experiment has now been carried out using various physical implementations and the results are generally considered to be robust.

In an earlier post, on quantum chance, I used the Kochen formulation of quantum mechanics [2] to discuss a generalisation of conditional probability,

$$\begin{eqnarray}\label{eq2:reduction}
p(X\mid Y) =  \text{tr}(Y \rho YX)/ \text{tr}(Y \rho Y).  \kern 4pc 
\end{eqnarray}$$

Where $p$ is a state of the system of interest on the \(\sigma\)-complex $Q(\mathcal{H})$ and $Y$ a projection operator in $ Q(\mathcal{H}) $ such that $p(Y) \ne  0$. For the mathematical background see the post A Mathematical Foundation of Quantum Mechanics or Kochen's paper [2]. The equation is shown to have the form of classical conditional probability when \(X\) and \(Y\) commute. Otherwise there is an additional interference term. Kochen uses this quantum conditional probability to analyse the Measurement Problem. Conditioning the outcome on the set of possible detections is a mathematically elegant way of seeming to derive the Born Rule. Kochen recognises that unitary evolution of the system state does not give results in agreement with what is observed but then he goes on to say

The present interpretation stands the orthodox interpretation on its head. We do not begin with the unitary development of an isolated system, but rather with the results of a measurement, or, more generally, of a decoherent interaction.

p(Y)≠0

That is, the observable \(X\) of the system of interest is conditioned by the projection on to the set of detection projection operators \({Y_i}_i\). This is interpreted by Kochen as state reduction. However, just as in classical probability the quantum generalisation of conditional probability describes an association rather than causation. Kochen provides no physical mechanism for measurement to cause the reduction or why one of the set of possible outcomes occurs. The mathematical formalism shows that if an event is observed then the description of the system is reduced to the corresponding state. He does then say

... symmetry-breaking processes do take place in isolated compound systems with internal decoherent interactions during reduction of state.

 However, decoherence is at best a research topic rather than an established mechanism. Kochen's is firstly a mathematical description but can be interpreted as the detector taking an active role, just as in the Transactional Interpretation, but he does not provide the physics for this.

The Relativistic Transactional Interpretation (RTI) [1] does provide an explanation for the physical mechanism for identifying what set of physical objects play the active detector role for an object of interest. It does this by going outside non-relativistic quantum theory. Relativistic quantum theory provides a description of particle creation and annihilation, and boson mediated interaction (or transaction). However, it is still possible to do useful calculation in the non-relativistic formulation, but it is an approximate theory in a new sense now. It has long been recognised that it is a low energy theory but in addition it must now be recognised that the Born rule (von Neumann-Luders Projection Rule) is explained outside of formulation although it can be used mathematically within the non-relativistic formulation through conditioning the quantum state on an event. Or more precisely conditioning on the projection on to the event.

As an aside, the term that Kochen uses to refer to the properties in the \(\sigma\) -complex is extrinsic and for properties of the type familiar from classical physics intrinsic. They look like they correspond to the quantum substrate (QS) of potential properties and the actual space-time events, respectively, RTI theory. Kochen provides a mathematical description of consequences of an actual event but not the physics.

Now that we have a theory (RTI) that provides a physics of actual events it will be instructive to revisit a specific example.

In Locality and Quantum Mechanics, the experimental configuration of two space-like separated Stern-Gerlach measurement systems was discussed, as illustrated below.

The mathematical treatment will not be repeated here but commented on using the language of quantum substrates, potentiality, possibility, actuality, and events. However, the first use that I will make of the language is to augment the ontology presented in Strata of Real Being. The inorganic stratum on which all other levels supervene now gets split into two levels. They are the more fundamental quantum substrate and supervening space-time. It is possible that eventually there may be the need to introduce a yet more fundamental level but ontology strata have considerable autonomy and so we can put that consideration to one side for now. For example, understanding and explanation of most of social life, through social entities such institutions, individuals, and groups, need not take account of quantum physics. 

The discussion of a specific experiment should help to develop a better understanding of the relationship between these two levels in the inorganic layer.

The experiment

A system of two particles is prepared in a state of total spin zero. This state exists in the quantum substrate. Its preparation would, however, involve apparatus and presumably scientists existing and acting at higher levels of the ontology. The experiment as a whole involves the preparation of two Stern-Gerlach setups so that they are space-like separated when the particles arrive. This arrangement is situated in the space-time domain of actual events.  In the analysis, the magnetic field is treated classically, this is an approximation to the field description in quantum electrodynamics and can be traced to its origin in the QS although that plays no role in the analysis where the magnetic field is is an actual field. This allows the Schrödinger equation to be solved using a semi-classical coupling of the magnetic field to the spin operators of the particles.  The \(x\) coordinate (see the figure) plays no explicit role in the mathematical analysis. The fact that the two Stern-Gerlach arrangements are space-like separated is merely part of the thought experiment narrative. The spin operators and the associated spinor states have no \(x\) component. The position coordinate \(z\) comes into play only through the coupling to the classical magnetic field. The coupling parameter for the magnetic field in the Hamiltonian \(\mu = \frac{e \hbar}{2 m c}\) is made up of what are usually termed fundamental constants. RTI puts \(e\) and \(m\) (the electron charge and rest mass) in the QS. \(c\) is the speed of light and, as RTI puts actualised photons in the space-time level, it is an actual space-time constant. \(\hbar\) is associated with the dynamics in the QS.  

At least in the non-relativistic approximation, the QS may not be a domain of pure potentiality because \(e\) and \(m\) always have actual values (there is no uncertainty). However, Barbara Vetter's theory of potentiality [3] allows for a potential property that has only one possible actualisation. This could be interpreted just as a form tidiness in the associated modal logic, but it should be recognised that such degenerate potentialities are permitted and can exist in the QS.

In RTI what seemed like non-causal non-local effects in the EPR scenario, using standard quantum theory, are robust correlations in the quantum substrate. In the QS what is going on is not much more mysterious than blue and green card example at a less fundamental level of reality. In the card example, as discussed before, the two cards are put in enveloped each. An associate takes one of the envelopes at random and leaves with it. I open my envelope and know immediately the colour of the card my associate has.  At higher levels of the ontological hierarchy properties are actual (cards have a specific colour) in the QS potential property values can be robustly correlated prior to taking actual values. The RTI theory predicts that this correlation makes an appearance in the statistical correlation of the space-time events that are the result of an EPR experiment. Other formulations give this result, but they do not provide an explanation in physics for the appearance of the events.

References

1. Ruth E. Kastner, The Transactional Interpretation of Quantum Mechanics - A Relativistic Treatment, Cambridge University Press, second edition 2022
2. Simon Kochen. A Reconstruction of Quantum Mechanics. In: ArXiv e-prints (June 2015).
3. Barbara Vetter, Potentiality: From Dispositions to Modality, OUP, Oxford 2015

Potentiality, possibility, and probability

In the previous post I returned to Heisenberg and his discussion of potentia in quantum theory and examined Barbara Vetter's modal metaphysics of potentiality in that context. The intention being to relate this metaphysics to the formulation of quantum mechanics as a \(\sigma\)-complex of potentiae, that captures the possible manifestations of quantum objects. The occurrence of actual events is then governed by the probability of actualisation, which depends on the physical context that the object finds itself.

Heisenberg's book on the philosophy of quantum mechanics is well known and I may have missed some recent work by others that is inspired by or develops his thinking.  A search found a number of papers but one, "Taking Heisenberg’s Potentia Seriously" by R. E. Kastner, Stuart Kauffman and Michael Epperson [1], provided an analysis that is close to the position that I have arrived at 5 years later and goes further in working out some consequences for our understanding of quantum theory. 

To better compare my position with that of Kastner et al, here is a brief list of my main points:

1. Physics is about the physical and not about, information, knowledge, or psychology (of course physics is knowledge and provides information.)
2. Ontology needs to encompass potentiality so that the dispositional nature of quantum processes can be captured in a physical theory
3. The quantum state is represented by a complex of probability models (modified Kochen formulation) and is not merely statistical
4. The manifestation (or actuality) of quantum events depends on the physical context (the mechanism for this is the outstanding puzzle)
5. The process of going from the potential to the actual is a probabilistic transition
6. Measurment is a physical process but there are physical events in the absence of measurement that the theory must be able to describe
7. The mathematical structure of quantum theory can provide important clues to the underpinning ontology.

The main points extracted from Kastner et al are:

1. A realist understanding of quantum mechanics calls for the metaphysical category of res potentia
2. Res potentia and res extensa are interdependent modes of existence
3. Quantum states instantiate in quantifiable form res potentia; ‘Quantum Potentiae’
4. Quantum Potentiae are not spacetime objects, and they do not obey the Law of the Excluded Middle or the Principle of Non-Contradiction.
5. Measurement is a real physical process that transforms Quantum Potentiae into elements of res extensa, in a non-unitary, acausal process
6. Spacetime (the structured set of actual events) emerges from a quantum substratum
7. Spacetime is not all that exists
8. There is a mathematical theory covering the above.

Although the above only provides the briefest summary of both points of view, the similarities should be obvious. 

The modifications I made to Kochen's formulation, although modest, were aimed at eliminating any temptation to think that we are dealing with some non-standard logic. So, point 4 by Kastner et al poses a problem and I don't think that it is well argued in the paper that potentiae do not obey the Law of the Excluded Middle or the Principle of Non-Contradiction (I note that in her book [2] Ruth Kastner makes no mention of this point). I think the appropriate logic for potentiality is far better captured in the proposal of Barbara Vetter. However, within point 4 the proposal that potentiae are not space-time objects is interesting and potentially fruitful. However, it does indicate an ontology that may be as extravagant as the multiverse interpretation. In mitigation the ontology captures a rich substratum of all possibilities rather than infinity of actual universes. 

Where they clearly go beyond my points is with their point 9. Whereas I have been comparing and contrasting standard quantum theory, the propensity interpretation, Kochen's reformulation, various Bohmian proposals, GRW collapse theories, the multiverse interpretation, and Fröhlich's ETH research project; Ruth Kastner has developed a specific theory of quantum potentiality [2]. She starts with an interpretation that I have neglected so far; the Transactional [3]. It is an interpretation that gives physical importance to an aspect of the mathematical formulation that is usually considered a mere calculation device. In the Dirac notation, for standard quantum mechanics, a state \(\Psi\) is denoted by the ket \(|\Psi>\). The observables, represented by Hermitian operators, act on the ket as follows \(\Omega |\Psi>\) and in standard theory it is simply a calculation device to gain the expected value of the observable in the state to use the complex conjugate of the state, the bra, \(<\Psi|\Omega |\Psi>\).

However, in the transactional interpretation \(<\Psi|\) gains a physical significance. It is the confirming echo from the absorber (or detector) to the emitter's potential for observable properties to become actual. Despite this attractive proposal Cramer's original formulation has some issues. There is backward causation. This is what many would consider anti-causation because the effect precedes the effect. There are some other issues that Ruth Kastner proposes a solution for in her book and we will examine later. She takes Cramer's formulation and, building on other work, develops a relativistic formulation. She shows that this is needed to avoid some of Cramer's difficulties. 

I now believe that Kastner's formulation and ontology provides a very promising approach to gaining a deeper understanding of the quantum domain and it will therefore provide the focus of the coming posts. Even if it turns out to have some flaw the analysis should rewarding.

Some of the points I address will be presentation and terminology. For example, Vetter's metaphysics indicates a process going from potential to possibility and (through weighting) probability. So, for me quantum states will represent potentiality with the set of possible events represented by the spectrum of the associated observables. The probabilities are gained by decomposing the potentiality in the basis associated with a particular observable. I will als try to eliminate reference to the wave concept. I will stick by my preference for a Heisenberg picture because the wave concept to too closely tied to the space-time continuum.

1. Ruth  E. Kastner, Stuart Kauffman and Michael Epperson, Taking Heisenberg’s Potentia Seriously, International Journal of Quantum Foundations, March 27, 2018, Volume 4, Issue 2, pages 158-172
2. Ruth E. Kastner, The Transactional Interpretation of Quantum Mechanics - A Relativistic Treatment, Cambridge University Press, 2nd edition 2022
3. J. G. Cramer, (1986). The transactional interpretation of quantum mechanics, Reviews of Modern Physics 58, 647–88.

Metaphysical potentiality and physics

Heisenberg in one of his excursions into philosophy (Physics and Philosophy) discusses epistemology, logic, and ontology. In developing his ideas on how the innovations of quantum mechanics impact on philosophy he introduces (or rather reintroduces with reference to Aristotle) the concepts of potentiality.

He comes to potentiality through considering the logic required to refer to things that quantum mechanics describes. That is, it ...

... concerns the ontology that underlies the modified logical patterns. If the pair of complex numbers represents a "statement" in the sense just described, there should exist a "state" or a "situation" in nature in which the statement is correct. We will use the word "state" in this connection. The "states" corresponding to complementary statements are then called "coexistent states" by Weizsäcker. This term "coexistent" describes the situation correctly; it would in fact be difficult to call them "different states," since every state contains to some extent also the other "coexistent states." This concept of "state" would then form a first definition concerning the ontology of quantum theory. One sees at once that this use of the word "state," especially the term "coexistent state," is so different from the usual materialistic ontology that one may doubt whether one is using a convenient terminology. On the other hand, if one considers the word "state" as describing some potentiality rather than a reality —one may even simply replace the term "state" by the term "potentiality" —then the concept of "coexistent potentialities" is quite plausible, since one potentiality `may involve or overlap other potentialities.

In the version of quantum mechanics that I have been developing these "coexistent potentialities" should be understood in relation to the \(\sigma\)-complex. Despite his hints, Heisenberg does not succeed in including potentiality in the required logic. What is needed is a modal logic that includes a POTENTIALITY operator. Barabara Vetter constructs such a modal logic and defends it with modal metaphysical arguments in her book Potentiality: From Dispositions to Modality. There is a need for a theory going beyond the naive position that there are objects that simply have the potential to have certain properties. In quantum mechanics in particular it is not sufficient to say than an electron has the potential to being in some place. If this is elaborated to saying that it has some position with a certain probability, we get a deceptive oversimplification. The theory of potentiality developed by Vetter give potentialities the status of properties of objects. Contrary to Heisenberg 's view that "one considers the word "state" as describing some potentiality rather than a reality". Vetter's theory allows us to enlarge the domain of what is real to potentialities. 

In standard quantum mechanics observables can take possible values. The possibility is to be described by the theory and the values are represented by the spectrum of the self-adjoint operator that in turn represents the observable.  Vetter proposes, in general, to define possibility as follows: 

POSSIBILITY: It is possible that p \(=_{\mbox{df}} \) Something has an iterated potentiality for it to be the case that p.

We have now moved on to potentiality and more generally its iteration.  I have discussed dispositions in previous posts and now explore Vetter's proposal that we call those properties which form the metaphysical background for disposition ascriptions potentialities. In the definition the something also need explanation. Something can be an object or any collection of objects. 

Potentialities are individuated, not by a pair of stimulus and manifestation and a corresponding conditional, but by their manifestation alone. The manifestation of a potentiality is the property that the potentiality’s possessor would have if the potentiality were to be manifested. A manifestation can be a property of and object (or collection of objects) or a relation that the object (collection) has with something else. 

Joint potentiality

The relation between a potentiality and its manifestation, the relation ‘...is a potentiality to ...’. Manifestations individuate potentialities and therefore the classification of manifestations as relations or properties provides a classification of the joint potentialities:

Type 1 joint potentialities are joint potentialities whose manifestation is a relation between, or a plural property of, all its possessors. 

Type 2 joint potentialities are joint potentialities whose manifestation consists in a property or relation of only some, but not all, of its possessors.

In type 1 joint potentialities, the manifestation is a relation (or plural property) holding non-trivially between all its possessors, such as for an example of the door key's joint potentiality with the door for the former to unlock the later. In type 2 joint potentialities, however, the manifestation of a joint potentiality concerns only some of its possessors; such are the cases of 

• a uranium pile (with a potential to go unstable) plus the rods and fail-safe mechanism
• a glass (with a potential to brake) but protected by styrofoam,
• a city (with a potential to be destroyed) and its defence system. 

In all three only one of the objects manifests the disposition or potentiality but they have jointly the potentiality, to a certain degree.

Extrinsic potentiality

An object has an extrinsic potentiality if the object has a joint potentiality whose co-possessors are the dependees of the extrinsic potentiality. To return to the key and door example, in the case of the key’s extrinsic potentiality to open the door, the dependee and co-possessor (the door) was part of the potentiality’s manifestation (unlocking the door). In general, where a potentiality’s manifestation consists in a relation to a particular other object—such as opening this particular door, as opposed to opening some door with a lock of a specific shape—the potentiality will be extrinsic, and the object involved in the manifestation will function as its dependee and as the co-possessor of the relevant joint potentiality.

Iterated potentiality

As well as joint or extrinsic the potentialities can be iterated. That is, objects can have the potential to have potentials.

Objects and collections of objects have potentialities to possess properties. Potentialities themselves are properties. So, things should have potentialities to have potentialities and the latter potentialities might themselves be potentialities to have potentialities. So, there is nothing to prevent things from having potentialities to have potentialities to have potentialities, or potentialities to have potentialities to have potentialities to have potentialities ... and so forth. These are iterated potentialities.

Formalising potentiality 

In Vetter's theory potentialities include dispositions and abilities as properties of individual objects or collections of objects. However, in this theory POTENTIALITY cannot be defined because it is the metaphysical underpinning of dispositions and consequently possibilities. Vetter uses examples such as those I have discussed in explaining dispositions in earlier posts. Ordinary language semantics is also used as when the potential to do or be something is discussed.  It is on this bases that potentiality is then posited to be metaphysically fundamental. 

Although POTENTIALITY cannot be formally defined it can be formalised to show how it operates. Just as there are modal operators for necessity and possibility that act on propositions, the modal potentiality operator is called POT. 

POT must [therefore] be a predicate operator which takes a predicate—specifying the potentiality’s manifestation—to form another predicate, which can then be used to ascribe the specified potentiality to an object.

Formally, where upper case Greek letters are predicates,

$$ \mbox{POT} [\Phi](t)$$

ascribes to t the potentiality to \(\Phi\), where \(\Phi\) is an \(n\)-place singular predicate and \(t\) is a singular term, or \(\Phi\) is an \(n\)-place plural predicate and \(t\) is a plural term. Potentialities can be described by more general sesntences rather than just predicated. For the POT operator to work with sentences rather than predicates, let the sentence \(\phi\) be for something to be such that \(\phi\) is true is equivalent to it have the potentiality to \(\Phi\).  To express logically complex predicates and to turn closed sentences into ‘such that’ predicates, we need to introduce a standard predicate-forming operator, \(\lambda\).

$$ \mbox{POT} [\Phi](t) \rightarrow  \mbox{POT} [\lambda x.\phi](t)$$

 Where \(\phi\) is a sentence, open or closed, and \(\phi[t/x]\) is the result of substituting a term \(t\) for any free occurrence of \(x\) in \(\phi\), the sentence \(\lambda x.\phi (t)\) is true just in case \(\phi[t/x]\) is true. Intuitively, \(\lambda x.\phi\) turns the sentence \(\phi\) into a predicate meaning ‘is such that \(\phi\)’. 

Although there are further subtleties it is now possible to formulate a simple form of iterated potentiality.

$$ \mbox{POT}[\lambda x. \mbox{POT}[F](x)](a) $$

which ascribes to \(a\) an iterated potentiality to be \(F\). This formalisation can be iterated further.

The electron again

Returning to an electron described by quantum mechanics. An electron has the potential to be somewhere, to have a momentum, to have charge and a mass. But the electron description in quantum mechanics has even more structure. It appears somewhere with a certain probability in certain circumstances. Quantum mechanics does not just provide bare probability distributions for each potential property but has those probability models combined in a \(\sigma\)-complex; corresponding, I propose, to the "coexistent states" of Weizsäcker. The electron has a joint potentiality with other objects to manifest one of the probability distributions from the complex, in certain circumstances. Then that probability distribution weights the appearance of a specific set of values of the property with a numerical probability. Therefore, the probability that the electron will appear in a specific region of space in certain circumstance is an iterated potentiality. The electron has the potential to manifest one of a complex of probability distributions and a probability distribution captures in mathematical form the potential for the electron to appear in a region of space.

This example indicates how the concept of potentiality, as formalised, can be used to describe quantum processes. It remains to be seen how useful this will be when we introduce considerations of spin, entanglement, interference etc. This will be explored in future posts.