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The quantum handshake welcoming you to spacetime

In today’s blog we talk about the Transactional Interpretation of quantum theory with the author of its relativistic extension, philosopher and physicist Dr. Ruth Kastner, PhD from the University of Maryland.

David: Where does the Transactional Interpretation, or version of quantum theory, (TI) lie between operational and realist?

Ruth: TI is 100% realist. All the formal elements of the theory refer to entities and processes occurring in the real world. This includes the Born Rule.

David: Does TI take time reversal symmetry more seriously than other approaches?

Ruth: TI has a form of time symmetry, but I wouldn’t characterize it as ‘time reversal symmetry,’ because no process is being ‘reversed in time.’ Rather, the elementary field generated by charges is isotropic in a temporal sense, i.e., has both future-directed and past-directed components. However, TI includes temporal symmetry breaking, in that the actualized transaction always involves the delivery of positive energy in the future direction.

David: Why have most people ignored this approach?

Ruth: Here, I’ll quote from a paper I presented at Chapman University in 2019 (below is a lengthy excerpt):

“It has been established in the literature that TI and its relativistic extension (RTI) provide a physical, quantitative account of measurement... That is, TI provides a specific non-unitary physical interaction corresponding to 'measurement,' which distinguishes measurement-type interactions from unitary interactions (such as elastic scattering not involving transfer of real energy). … Moreover, this account also explains why we don't see macroscopic superpositions (Kastner 2018). So we seem to have a clear solution to the measurement problem here. Why is this solution still being generally overlooked?”

Firstly, there has long been a stigma attached to the "absorber theory" or Direct-Action Theory (DAT) for historical reasons, which could be summarized as follows:

“Wheeler & Feynman were two smart guys who abandoned their direct-action theory.”

The first response to this sort of concern is to note that in fact Wheeler had returned to it with enthusiasm in 2003, when (with Wesley) he promoted the direct-action theory as a possible approach to quantum gravity:

“[The Wheeler-Feynman theory] swept the electromagnetic field from between the charged particles and replaced it with “half-retarded, half advanced direct interaction” between particle and particle. It was the high point of this work to show that the standard and well-tested force of reaction of radiation on an accelerated charge is accounted for as the sum of the direct actions on that charge by all the charges of any distant complete absorber. Such a formulation enforces global physical laws, and results in a quantitatively correct description of radiative phenomena, without assigning stress-energy to the electromagnetic field.” (Wesley and Wheeler 2003, p. 427).

It's also important to recall in this regard that the reason that Wheeler and Feynman originally lost interest in the DAT was because their main concern in developing it was to solve the problem of self-energy. When they realized that some form of self-action was necessary at the quantum level, this purpose no longer seemed to be served. However, as this author has noted elsewhere (Kastner 2016a), the self-action applying to the quantum form of the DAT is not self-energy; it is only self-force, and this feature of the DAT becomes very useful for solving consistency problems of standard quantum field theory (Kastner 2015).

Having established that the reluctance of Wheeler and Feynman to further develop their direct-action theory is no reason to consider it inadequate, we turn now to another reason for the general disregard of the transactional picture: its purported refutation by Maudlin (1996). In fact, the Maudlin challenge has now been multiply refuted (Marchildon 2006, Kastner 2014); and it completely evaporates when the fully relativistic level (RTI) is considered (Kastner 2019). In particular, at the relativistic level, the 'slow-moving offer wave' required for the objection does not exist.

Finally, besides the lingering, but unjustifed, worries about Wheeler and Feynman's attitudes and the Maudlin scenario, more deeply seated cultural views need to be identified, and their pervasive influence recognized. First, recall a foundational concept of Asian thought and culture: the duality of Yin and Yang.

These elements are complementary aspects of reality that are seen as balancing and even interpenetrating one another. In Asian metaphysics, there is no process in nature that is without both elements. Yang (symbolized by the white area) is the element corresponding to initiating, giving, and creating, while Yin (symbolized by the black area) is the element corresponding to responding, receiving, and dissolving.

With the help of this concept of Yin and Yang, it can be seen that the unilateral propagation picture is an approach consisting of 'all Yang and no Yin'. This preponderance of the initiating, creating, Yang element and neglect of the responding, receiving Yin element characterizes much of Western thought and metaphysics. It is taken for granted that objects or substances can be created autonomously (the Yang element) and have an independent existence and career without any element of Yin. Yet this contradicts much of what we actually see in the world. For example, if we want to have tulips in our garden, we can't just toss the bulbs out. They must be received in the earth, and interact with the soil in specific and crucial ways in order for anything to grow. Similarly, there are always two elements in a financial transaction: the seller and the buyer. Nobody sells a house (Yang) without someone buying and receiving it (Yin). However, the standard approach to field propagation takes all physical processes as Yang-only. Exemplifying this attitude is the idea that a photon can be emitted unilaterally and continue on autonomously, without ever being absorbed. In contrast, in the direct-action theory, both Yin (response; reception) and Yang (initiation; emission) are required for the photon to exist. A photon, which is a transfer of energy from one system to another, can no more exist based on a unilateral impulse than someone can sell a house without a buyer.

David: Can you describe your contribution to this framework?

Ruth: I suppose my main contribution is to extend Cramer’s original TI to the relativistic domain (RTI). This has two key aspects: (1) recognizing the coupling amplitude (charge) as the amplitude to generate an offer or confirmation wave, which yields a concrete physical basis for the onset of measurement (non-unitarity); and (2) elaborate the role that the Feynman propagator plays in the quantum analog of the idea of “absorber response.” Further specifics are given in my books and papers (see References below). I’m currently finalizing a 2nd edition of my 2012 Cambridge University Press work, The Transactional Interpretation of Quantum Theory: The Reality of Possibility, which lays out in more detail the latest developments of RTI.

David: Can you better describe this idea that the space, time, matter, picture of the world emerges out of a complex, multi-dimensional, world of transactions?

Ruth: This is a huge question, and I can’t hope to do it justice in a brief reply. It is addressed in considerable detail in my forthcoming CUP 2nd edition. But for now, I’ll note that actually, matter (in the sense of rest mass) exists in what I call the ‘quantum substratum.’ This realm has no spacetime features, although rest-mass systems have their own internal periodicities that act as internal clocks, defining rest (inertial) frames. This means that inertial frames, as references for the application of spacetime indices to actualized events, are not themselves elements of spacetime. The rest-mass quantum systems (as emitters and absorbers) give rise to photon transactions that establish actualized emission and absorption events; these events are elements of spacetime, and together they define invariant spacetime intervals (corresponding to the photon transfer connecting the emission and absorption events). Quantum systems themselves comprise the multidimensional realm (quantum substratum), while transactions herald the domain of 3+1 spacetime. It is photon transfer (via actualized transactions) that create the spacetime manifold. This gives a clear physical interpretation of the standard physical concept that conserved currents generate spacetime displacements (with corresponding symmetries). In RTI, the transfer of those conserved currents (via photons) literally create those spacetime displacements. For a less verbose and more visual picture of this process, see:

David: From an outsider’s perspective, one gets the impression that understanding in quantum physics appears to get diluted over time. Do you see improvement in understanding over time?

Ruth: I would have to say that in my view, the “mainstream” understanding regarding quantum theory is still stuck in something of a dead end, because it does not recognize that quantum theory has genuine non-unitarity, which is needed to account for measurement. One only gets real physical non-unitarity in the direct-action theory of fields. This issue was discussed in my Chapman talk excerpt above.

David: Thank you Dr. Kastner!


Kastner, R. E. (2014). “Maudlin’s Challenge Refuted: A Reply to Lewis,” Studies in History and Philosophy of Modern Physics 47: 15-20. Preprint version:

Kastner, R. E. (2015). “Haag’s Theorem as a Reason to Reconsider Direct-Action Theories,” International Journal of Quantum Foundations 1(2), pp. 56-64. arXiv:1502.03814

Kastner, R. E. (2016a). "Antimatter in the direct-action theory of fields," Quanta 5(1), pp. 12-18. arXiv:1509.06040

Kastner, R. E. (2016b). "The Born Rule and Free Will," in C. DeRonde et al (Eds). Probing the Meaning of Quantum Mechanics: Superposition, Dynamics, Semantics and Identity. Singapore: World Scientific (2016), pp. 231-243.. Preprint version:

Kastner, R. E. (2018). “On the Status of the Measurement Problem: Recalling the Relativistic Transactional Interpretation,” Int'l Jour. Quan. Foundations, Volume 4, Issue 1, 128-141.

Kastner, R. E. (2019). “The Relativistic Transactional Interpretation: Immune to the Maudlin Challenge,” In C. de Ronde et al., eds, Probing the Meaning of Quantum Mechanics: Information, Contextuality, Relationalism and Entanglement. Singapore: World Scientific. arXiv:1610.04609

Kastner, R. E. (2020b). "Why the Quantum Absorber Condition is Not a Light-Tight Box." Preprint,

Kastner, R. E. and Cramer, J. G. (2018). “Quantifying Absorption in the Transactional Interpretation,” IJQF.

Marchildon, L. (2006). “Causal Loops and Collapse in the Transactional Interpretation of Quantum Mechanics,” Physics Essays 19, 422.

Martin. E. 1991. "The egg and the sperm: How science has constructed a romance based on stereotypical male/female roles." Signs 16: 485 - 501.

Maudlin, T. (2002). Quantum Nonlocality and Relativity: Metaphysical Intimations of Modern Physics. (Second Edition); (1996, 1st edition.) Wiley-Blackwell.

Pearle, P., 1997, ‘True Collapse and False Collapse’, in Da Hsuan Feng and Bei Lok Hu (eds.), Quantum Classical Correspondence: Proceedings of the 4th Drexel Symposium on Quantum Nonintegrability, Philadelphia, PA, USA, September 8–11, 1994, Cambridge, Mass.: International Press, pp. 51–68. [Preprint available online]

Schatten, G. and Schatten, H. (1984). "The Energetic Egg," Medical World News 23 (January 23, 1984).

Wesley, D. and Wheeler, J. A., “Towards an action-at-a-distance concept of spacetime,” In A. Ashtekar et al, eds. (2003). Revisiting the Foundations of Relativistic Physics: Festschrift in Honor of John Stachel, Boston Studies in the Philosophy and History of Science (Book 234), pp. 421-436. Kluwer Academic Publishers.

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