Ruth E. KastnerRuth 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 non-local. 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 Wheeler-Feynman 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 Sin-Itiro 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 Wheeler-Feynman attempt to describe the exchange of energy in the classical electromagnetic field as a time-symmetric process. Wheeler-Feynman proposed adding advanced field potentials (which look like never-seen-in-nature 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 InterpretationThe 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 "sum-over-paths" path-integral 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." 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 "sub-empirical" and pre-spatiotemporal (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 pseudo-Kantian "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 non-Euclidean 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 well-defined in terms of the conditions under which it occurs - in contrast to prevailing "orthodox" interpretations which suffer from an ill-defined micro/ macro "cut". The new understanding offered here is a rational account, in the sense of being well-defined and self-consistent, 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 notorious problem with the von Neumann formulation was that there seemed to be no way to determine when, why, or how the pure state should undergo such a transformation. If we take into account the physical process of absorption (i.e., state annihilation), "Process 1" becomes completely non-mysterious. It is just the process whereby the CW are returned to the emitter from all absorbers capable of responding, and a set of incipient transactions is established.
The If we adopt the approach that quantum theory tells us about many possibilities arising from interactions between offer waves and confirmation waves, then we gain a clearly defined set of possible outcomes missing in the standard account, which disregards the real physical process of absorption. Recall that one component of the measurement problem is the amplification of the quantum state through interactions with the measuring apparatus, the first observer, the second observer, etc., with no means of deciding when the measurement has been completed. The designation of a stage at which the measurement is "completed" is referred to as the "Heisenberg cut," which is notoriously arbitrary. The arbitrariness is removed once we notice that the original offer wave inevitably encounters one or more absorbers that generate confirmations in response to the offer. It is at that point that set of incipient transactions is established. 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" Normal | Teacher | Scholar