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Ruth E. Kastner
Ruth Kastner is a physicist and philosopher known for her defense and possibilist extension of John G. Cramer's "Transactional" Interpretation of quantum mechanics.
The transactional interpretation makes no experimental predictions different from standard quantum mechanics. But it does remove some of the puzzling and perhaps unnecessary assumptions that are part of other Interpretations of quantum mechanics. In particular, it denies that conscious observers are needed to cause the "collapse of the wave function" (without which there is no actual "outcome" in the measurement process). The transactional interpretation adds nothing ad hoc to the standard theory, such as "hidden variables or additional terms to the Schrōdinger equation to force a collapse. It is explicitly indeterministic and nonlocal. Cramer has explored the radical possibility of sending information between entangled particles faster than the speed of light, as well as causal relations that go backwards in time (retrocausality). And, like Schrōdinger and the decoherence advocates, Cramer denies the existence of particles! The core physics in the transactional interpretation is a way of looking at photon emissions and absorptions as an exchange of advanced and retarded waves that is based on the 1945 WheelerFeynman Absorber Theory of radiation, which was abandoned by Feynman, who went on to develop the Path Integral formulation of quantum mechanics and later, with Julian Schwinger and SinItiro Tomonaga, the theory of Quantum Electrodynamics (QED). While QED is a powerful theory that allows precise calculations of physical observables such as the motions of photons and electrons and the emission and absorption of a photon by an electron, the transactional interpretation is simply a way of looking at the emission and absorption of photons based on the WheelerFeynman attempt to describe the exchange of energy in the classical electromagnetic field as a timesymmetric process. WheelerFeynman proposed adding advanced field potentials (which look like neverseeninnature incoming spherical waves converging on light sources) to the normal outgoing spherical waves (with retarded potentials) of classical electrodynamics. Their goal was to symmetrize electrodynamics with respect to time. There is nothing inherent in electromagnetic theory that explains the time asymmetry of radiation propagation (we see outgoing waves only). Cramer's transactional interpretation describes an electron as sending out probabilistic "offer waves" (OW) to potential absorbers. He adds what he calls "confirmation waves" (CW) incoming to an emitter from the many possible absorbers of an emitted photon. An offer wave is not an actual photon emission, and a confirmation wave is not an actual absorption or "detection" of a photon. But Cramer did see the two waves as connecting events in spacetime. Eventually, one advanced potential confirmation wave "handshakes" with the retarded potential offer wave and produces an actual absorption.
Kastner's Possibilist Transactional Interpretation
The offer wave going out in all directions and the many confirmation waves returning are a sort of subset of the infinite number of virtual photons traveling all possible paths between emitters and absorbers in Feynman's "sumoverpaths" pathintegral formulation of quantum mechanics. Kastner proposes to regard the outgoing offer wave and many incoming confirmation waves as "possible" transactions, only one of which indeterministically becomes "actual."
In the information interpretation of the wave function as a "possibilities" function, the possibilities are real in the sense that they can directly interfere with one another. Some thoughts are also real in the sense that they may lead to empirically observable actions.
Kastner is a possibilist who argues that OWs and CWs are possibilities that are "real." She says that they are less real than actual empirically measurable events, but more real than an idea or concept in a person's mind. She suggests the alternate term "potentia," Aristotle's that she found Heisenberg had cited. For Kastner, the possibilities are physically real as compared to merely conceptually possible ideas that are consistent with physical law (for example, David Lewis' "possible worlds." But she says the "possibilities" described by offer and confirmation waves are "subempirical" and prespatiotemporal (i.e., they have not shown up as actual in spacetime). She calls these "incipient transactions." Kastner describes actual spacetime events as emergent from the transaction process. She correctly thinks that Niels Bohr and Werner Heisenberg were mistaken to renounce any attempts to visualize a quantum reality underlying quantum phenomena. She describes her "new realism:" To assume, like Bohr, that a realist understanding must be in terms of the usual "classical," causal account is to limit ourselves to a pseudoKantian "category of experience" which is shown to be obsolete by scientific advance, much as Kant's own prescribed "categories" became obsolete when (for example) it was discovered that theories of spacetime had to allow for nonEuclidean forms. The new realist understanding may not be in terms of causal, mechanistic processes. It may instead encompass a fundamental indeterminism at the heart of nature, but one which is welldefined in terms of the conditions under which it occurs  in contrast to prevailing "orthodox" interpretations which suffer from an illdefined micro/ macro "cut". The new understanding offered here is a rational account, in the sense of being welldefined and selfconsistent, even while it lacks certain features, such as determinism and mechanism, that have been traditionally assumed to be requirements for an acceptable scientific account of phenomena. The subtitle of Kastner's book is ""The Reality of Possibility." She says that her main thesis is that "it is perfectly reasonable to be realist about the subject matter of quantum theory" (p.28). And she calls for a new metaphysical category to describe "not quite actual...possibilities" and the Heisenberg/Aristotle idea of "potentia." ￼ Heisenberg took a further step in "listening" to quantum theory when he made the following statement: "Atoms and the elementary particles themselves are not real; they form a world of potentialities or possibilities rather than things of the facts." Kastner claims that the transactional interpretation removes the "mystery" in John von Neumann's Process 1.
She says that her "possibilities" view provides outcomes that are "clearly defined" where standard quantum theory does not.
The information interpretation of quantum mechanics identifies the Heisenberg/von Neumann "cut" unambiguously as the irreversible creation of stable information in the world (e.g., a detector click or spot on a photographic plate) that may later be observed and constitute a "measurement." For the location of the "cut," see John Bell's possible locations for the shifty split"
Kastner's Explanation of Irreversible Processes
In 2017, Kastner published an article in the journal Entropy, in which she argued correctly that the "possibilities" of her transactional interpretation would result in indeterministic "collapses" of the deterministically evolving wave function.
Many interpretations of quantum mechanics, especially those popular today, say that deterministic evolution of the Schrödinger wave equation makes the theory "unitary." "Unitarity" conserves many expectation values, including the total information at all times. These interpretations deny the "collapse" of the wave function and the appearance of a particle somewhere. Many say that there are no particles, only fields. She writes...
It has been argued that if the nonunitary measurement transition of Von Neumann is a physically real component of quantum theory, then the representation of the system(s) under study by proper mixed states, subject to a probabilistic master equation description relative to a distinguished basis, becomes physically justified. This rectifies a weakness in the usual approach, which helps itself to the convenient basis and accompanying probabilistic description (effectively Pauli’s “random phase assumption”) as a “for all practical purposes” approximation. However, the utility of a probabilistic expression for calculational purposes does not constitute theoretical justification for the probabilistic description, which is needed in order for the “coarsegraining” and resulting entropy increase to describe what is physically occurring in a system. Once we have that justification, through real nonunitary collapse, we have the microscopic irreversibility needed to place the Htheorem on sound physical footing.
References
On Quantum Collapse as a Basis for the Second Law of Thermodynamics, Entropy, 2017, 19, 106
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