Editing Transactional interpretation (section)

== Debate ==
In 1996, [[Tim Maudlin]] proposed a [[thought experiment]] involving [[Wheeler's delayed choice experiment]] that is generally taken as a refutation of TIQM.<ref name="Maudlin 1996">
{{cite book
 |last1=Maudlin |first1=Tim
 |date=1996
 |title=Quantum Nonlocality and Relativity: Metaphysical Intimations of Modern Physics
 |publisher=Wiley-Blackwell
 |edition=1st
 |isbn=978-1444331271
}}</ref> However Kastner showed Maudlin's argument is not fatal for TIQM.<ref name="Kastner 2006">
{{cite journal 
 |last1=Kastner |first1=Ruth E
 |date=May 2006
 |title=Cramer's Transactional Interpretation and Causal Loop Problems 
 |journal=Synthese 
 |volume=150 
 |issue=1
 |pages=1–14 
 |arxiv=quant-ph/0408109
 |doi=10.1007/s11229-004-6264-9
 |s2cid=5388235
}}</ref><ref name="Kastner 2012">
{{cite journal 
 |last1=Kastner |first1=Ruth E
 |date=2012
 |title=On Delayed Choice and Contingent Absorber Experiments
 |journal=ISRN Mathematical Physics 
 |volume=2012
 |issue=1
 |pages=1–9
 |arxiv=1205.3258
 |doi-access=free |doi=10.5402/2012/617291
 |bibcode=2012arXiv1205.3258K
 |s2cid=72712087
}}</ref>

In his book, ''The Quantum Handshake'', Cramer has added a hierarchy to the description of pseudo-time to deal with Maudlin's objection and has pointed out that some of Maudlin's arguments are based on the inappropriate application of Heisenberg's knowledge interpretation to the transactional description.<ref name="Cramer book">
{{cite book
 |last1=Cramer |first1=John G.
 |date=2016
 |title=The Quantum Handshake: Entanglement, Nonlocality and Transactions
 |publisher=Springer Science+Business Media
 |isbn=978-3319246406
}}</ref>

Transactional Interpretation faces criticisms. The following is partial list and some replies:
{{ordered list
| "TI does not generate new predictions / is not testable / has not been tested." {{pb}}

TI is an exact interpretation of QM and so its predictions must be the same as QM. Like the [[many-worlds interpretation]] (MWI), TI is a "pure" interpretation in that it does not add anything ad hoc but provides a physical referent for a part of the formalism that has lacked one (the advanced states implicitly appearing in the [[Born rule]]). Thus the demand often placed on TI for new predictions or testability is a mistaken one that misconstrues the project of interpretation as one of theory modification.<ref>''The Quantum Handshake'' by John G. Cramer, p. 183: "No consistent interpretation of quantum mechanics can be tested experimentally, because each is an interpretation of the same quantum mechanical formalism, and the formalism makes the predictions. The Transactional Interpretation is an exact interpretation of the QM formalism. Like the Many-Worlds and the Copenhagen interpretations, the TI is a "pure" interpretation that does not add anything ''ad hoc'', but does provide a physical referent for a part of the formalism that has lacked on (e.g. the advanced wave functions appearing in the Born probability rule and amplitude calculations). Thus the demand for new predictions or testability from an interpretation is based on a conceptual error by the questioner that misconstrues an interpretation as a modification of quantum theory. According to Occam's Razor, the hypothesis that introduces the fewest independent assumptions is to be preferred. The TI offers this advantage over its rivals, in that the Born probability rule is a result rather than an independent assumption."</ref>

| "It is not made clear where in spacetime a transaction occurs." {{pb}}

One clear account is given in Cramer (1986), which pictures a transaction as a four-vector standing wave whose endpoints are the emission and absorption events.<ref>''The Quantum Handshake'' by John G. Cramer, p. 183: The TIQM "pictures a transaction as emerging from an offer-confirmation handshake as a four-vector standing wave normal in three-dimensional space with endpoints at the emission and absorption verticies. Kastner has predicted an alternative account of transaction formation in which the formation of a transaction is not a spatiotemporal process but one taking place on a level of possibility in a higher Hilbert space rather than in 3+1-dimensional spacetime."</ref>

| "Maudlin (1996, 2002) has demonstrated that TI is inconsistent." {{pb}}

Maudlin's probability criticism confused the transactional interpretation with Heisenberg's knowledge interpretation.  However, he raised a valid point concerning causally connected possible outcomes, which led Cramer to add hierarchy to the pseudo-time description of transaction formation.<ref>Berkovitz, J. (2002). ``On Causal Loops in the Quantum Realm", in T. Placek and J. Butterfield (Ed.), Proceedings of the NATO Advanced Research Workshop on Modality, Probability and Bell's Theorems, Kluwer, 233–255.</ref><ref name="Kastner 2006" /><ref><!--While Physics Essay often publishes hot garbage, Louis Marchildon is far from a nutcase -->{{cite journal | last1 = Marchildon | first1 = L | year = 2006 | title = Causal Loops and Collapse in the Transactional Interpretation of Quantum Mechanics | journal = Physics Essays | volume = 19 | issue = 3| pages = 422–9 | doi=10.4006/1.3025811| arxiv = quant-ph/0603018 | bibcode = 2006PhyEs..19..422M | s2cid = 14249516 }}</ref><ref>''The Quantum Handshake'' by John G. Cramer, p. 184: "Maudlin raised an interesting challenge for the Transactional Interpretation by pointing out a paradox that can be constructed when the non-detection of a slow particle moving in one direction that modifies the detection configuration in another direction. This problem is dealt with by the TI ... by introducing a hierarchy in the order of the transactional formation ... Other solutions to the problem raised by Maudlin can be found in the references."</ref><ref>''The Quantum Handshake'' by John G. Cramer, p. 184:  Maudlin also made the claim, based on his assumption that the wave function is a representation of observer knowledge, that it must change when new information is made available. "That Heisenberg-inspired view is not a part of the Transactional Interpretation, and introducing it leads to bogus probability argument. In the Transactional Interpretation, the offer wave does not magically change in mid-flight at the instant when new information becomes available, and its correct application leads to the correct calculation of probabilities that are consistent with observation."</ref>
Kastner has extended TI to the relativistic domain, and in light of this expansion of the interpretation, it can be shown that the Maudlin Challenge cannot even be mounted, and is therefore nullified; there is no need for the 'hierarchy' proposal of Cramer.<ref>{{Cite arXiv |eprint = 1610.04609|last1 = Kastner|first1 = R. E.|title = The Relativistic Transactional Interpretation: Immune to the Maudlin Challenge|class = quant-ph|year = 2016}}</ref>
Maudlin has also claimed that all the dynamics of TI is deterministic and therefore there can be no 'collapse'. But this appears to disregard the response of absorbers, which is the whole innovation of the model. Specifically, the linearity of the Schrödinger evolution is broken by the response of absorbers; this directly sets up the non-unitary measurement transition, without any need for ad hoc modifications to the theory. The non-unitarity is discussed, for example in Chapter 3 of Kastner's book ''The Transactional Interpretation of Quantum Mechanics: The Reality of Possibility'' (CUP, 2012).<ref name="Kastner, R. E 2012"/>

| "It is not clear how the transactional interpretation handles the quantum mechanics of more than one particle." {{pb}}

This issue is addressed in Cramer's 1986 paper, in which he gives many examples of the application of TIQM to multi-particle quantum systems.  However, if the question is about the existence of multi-particle wave functions in normal 3D space, Cramer's 2015 book goes into some detail in justifying multi-particle wave functions in 3D space.<ref name="quantum_handshake">''The Quantum Handshake'' by John G. Cramer, p. 184. Cramer's earlier publications "provided many examples of the application of the TI to systems involving more than one particle. These include the Freedman-Clauser experiment, which describes a 2-photon transaction with three vertices, and the Hanbury-Brown-Twiss effect, which describes a 2-photon transaction with four vertices. [Other publications contain] many examples of more complicated multi-particle systems, including systems with both atoms and photons.

But perhaps the question posed above is based on the belief that quantum mechanical wave functions for systems of more than one particle cannot exist in normal three-dimensional space and must be characterized instead as existing only in an abstract Hilbert space of many dimensions. Indeed, Kastner’s "Possibilist Transactional Interpretation" takes this point of view and describes transaction formation as ultimately appearing in 3D space but forming from the Hilbert-space wave functions.

... The "standard" Transactional Interpretation presented here, with its insights into the mechanism behind wave function collapse through transaction formation, provides a new view of the situation that makes the retreat to Hilbert space unnecessary. The offer wave for each particle can be considered as the wave function of a free (i.e., uncorrelated) particle and can be viewed as existing in normal three-dimensional space. The application of conservation laws and the influence of the variables of the other particles of the system on the particle of interest come not in the offer wave stage of the process but in the formation of the transactions. The transactions "knit together" the various otherwise independent particle wave functions that span a wide range of possible parameter values into a consistent ensemble, and only those wave function sub-components that are correlated to satisfy the conservation law boundary conditions at the transaction vertices are permitted to participate in this transaction formation. The "allowed zones" of Hilbert space arise from the action of transaction formation, not from constraints on the initial offer waves, i.e., particle wave functions. Thus, the assertion that the quantum wave functions of individual particles in a multi-particle quantum system cannot exist in ordinary three-dimensional space is a misinterpretation of the role of Hilbert space, the application of conservation laws, and the origins of entanglement. It confuses the "map" with the "territory". Offer waves are somewhat ephemeral three-dimensional space objects, but only those components of the offer wave that satisfy conservation laws and entanglement criteria are permitted to be projected into the final transaction, which also exists in three-dimensional space."</ref> A criticism of Cramer's 2015 account of dealing with multi-particle quantum systems is found in Kastner 2016, "An Overview of the Transactional Interpretation and its Evolution into the 21st Century, Philosophy Compass (2016).<ref>{{Cite arXiv |eprint = 1608.00660|last1 = Kastner|first1 = R. E.|title = The Transactional Interpretation and its Evolution into the 21st Century: An Overview|class = quant-ph|year = 2016}}</ref> It observes in particular that the account in Cramer 2015 is necessarily anti-realist about the multi-particle states: if they are only part of a 'map', then they are not real, and in this form TI becomes an instrumentalist interpretation, contrary to its original spirit. Thus the so-called "retreat" to Hilbert space (criticized also below in the lengthy discussion of note<ref name="quantum_handshake"/>) can instead be seen as a needed expansion of the ontology, rather than a retreat to anti-realism/instrumentalism about the multi-particle states. The vague statement (under<ref name="quantum_handshake"/>) that "Offer waves are somewhat ephemeral three-dimensional space objects" indicates the lack of clear definition of the ontology when one attempts to keep everything in 3+1 spacetime.}}