Time loops and real quantum possibles: Ruth Kastner

In the transactional interpretation (TI) of quantum mechanics proposed by John Cramer, physical reality emerges from feedback loops across time.

First proposed in 1986, Cramer’s transactional interpretation was enthusiastically reviewed by John Gribbin in “Schrödinger’s Kittens” (1995). According to Gribbin, the transactional interpretation “wins hands down” when it comes to providing a more intuitive understanding of what is really going on.

“[The transactional interpretation] provides the best all-round picture of how the world works at the quantum level… and with any luck at all it will supersede the Copenhagen interpretation as the standard way of thinking about quantum physics for the next generation of scientists.”

In his science fiction novel “Einstein’s Bridge” (1997), Cramer summarizes the transactional interpretation as:

“This universe… moves forward in time at the quantum level by a chain of handshakes between past and future… the future reaching back to make an accommodation with the past that allows a quantum event to happen, to become reality. Each quantum event emerges into reality as the result of a feedback loop between past and future. These are allowed timelike loops that bring the universe into being.”

In his book “The Quantum Handshake” (2016), Cramer illustrates the transactional interpretation with feedback loops between light emission (in the past) and absorption (in the future), taken as elementary events that always come in pairs (“an atom never emits light except to another atom”).

Offer waves from an emitter propagate to possible absorbers in the future. The possible absorbers reply with advanced confirmation waves that propagate backward in time toward the emitter, reaching it at the moment of emission. One confirmation wave may be selected and accepted, in which case a photon of light is exchanged between the emitter and the selected absorber. Cramer describes the transactional process using these sequential steps in “pseudotime,” but notes that the “process itself is atemporal,” and everything happens all at once in time.

“When you go out at night and look at the stars, consider that… the advanced light waves from your eyes have reached a thousand years into the past to encourage that star to shine in your direction.”

In Cramer’s words, “the freezing of possibility into reality as the future becomes the present, is… a fractal-like surface that stitches back and forth between past and present, between present and future… like the progressive formation of frost crystals on a cold windowpane.”

Cramer shows that the transactional interpretation permits making intuitive sense of “weird” quantum behavior, including the double slit experiment, entanglement, and other usual suspects. But the transactional interpretation hasn’t received the wide acceptance that Cramer and Gribbin hoped for.

Enter Ruth Kastner, who has developed a subtly different formulation of the transactional interpretation, called Relativistic Transactional Interpretation (RTI) or Possibilist Transactional Interpretation (PTI), and written three books to promote it (1, 2, 3).

Kastner de-emphasizes Cramer’s attempt to visualize the transactional process against a spacetime background, emphasizing instead the more general concept, suggested e.g. by Werner Heisenberg, that quantum possibilities (“potentiae”) are really real and exist alongside the actual reality of things in space and time.

But quantum possibles are not constrained by space, time, and causation. In Stuart Kauffman’s analogy (source), the possibility for a group of friends to meet in store vanishes “instantaneously and also acausally” if the store closes. With Kauffman and Michael Epperson, Kastner has written a paper titled “Taking Heisenberg’s Potentia Seriously” (republished in 3).

Actual reality is formed when possible realities, which follow quantum rules and live in the Hilbert spaces of quantum mechanics and quantum field theory, are selected and frozen in place by measurement-like interactions.

According to Kastner, Cramer’s handshakes and feedback loops take place in the nonlocal Hilbert space, beyond actual space and time (which are derived concepts).

Cramer, who describes himself as “an experimental nuclear physicist, not a philosopher of science or an abstract theoretician,” focuses on the transactional interpretation’s ability to provide a more intuitive visualization of quantum processes. He says that Kastner’s approach is “perhaps not incorrect, but… an unnecessarily abstract roadblock to visualization.”

Kastner counters (arXiv) that her approach IS necessary “if TI is to be a realist interpretation.” She notes that Cramer himself seems to agree, but is held back by his insistence on visualization and “his attempt to preserve a simple spacetime picture.”

In another paper (arXiv), Kastner emphasizes that reality is not contained in spacetime, and that Cramer’s “pseudotime” account is, even according to him, not a description of something that really occurs.

I don’t disagree, but I think naive pictures can be useful starting points. Cramer’s more naive and Kastner’s more elaborate formulations of the transactional interpretation don’t exclude each other, as shown e.g. in a joint paper (arXiv, republished in 3) by Cramer and Kastner.

I think quantum mechanics and its interpretation will likely evolve, but the core insights of Cramer, Kastner, Kauffman, and others are valid. The past is frozen, but the future is open. Quantum reality transcends space and time, and nonlocal processes in the background quantum reality are needed to make sense of observed physical reality.

I reached out to Ruth to hear her thoughts.

Q/A with Ruth Kastner

GP: You don’t think that reality is a block universe where the future “exists.” So how can something that exists here and now interact with something that doesn’t exist in the future?

RK: I view quantum entities as potentiae that live in a pre-spacetime fundamental domain. But these quantum possibilities ARE REAL. I.e., they are JUST AS REAL as spacetime actualities, just different, not as concrete and not as tangible. They are less tangible than spacetime actualities (events).

The “HERE AND NOW” is the domain in which possibilities are actualized — like the needle knitting the yarn into a garment. The yarn represents the possibilities, and the extruded garment represents the actualized spacetime events. So, interactions take place among these real quantum entities (possibilities), and some of them are actualized as events in the spacetime realm, which is like an extruded fabric from the knitting process. See also this animation.

GP: So the real pre-spacetime of possibilities (Hilbert space) is the bedrock of fundamental reality, from which spacetime actualities are derived. But isn’t this an abstract restatement of quantum mechanics that loses the intuitive visual power of John Cramer’s original formulation of the TI?

RK: No. First, it’s not an abstraction — it’s a specific physical ontology. In any case, Cramer’s picture arguably didn’t have all that much ‘intuitive visual power,’ since he appealed to a ‘pseudotime’ process that didn’t correspond to anything physical. That is arguably more of an abstraction than are the real physical possibilities or ‘res potentia’ of my approach (PTI or RTI).
Again, these are not abstractions. They are given a specific physical characterization. They are simply not part of the spacetime level. So we need to expand our ontology instead of viewing the res potentiae as ‘abstractions’. They are not just ideas. They are part of physical reality.

GP: How would describe the core differences between your and Cramer’s formulations of the TI?

RK: Cramer’s picture really requires a block world ontology since he restricts everything to spacetime. There’s no room in that ontology for any real dynamics, so the dynamical ‘pseudotime’ story doesn’t have any real physical content to it.
In contrast, my approach (PTI or RTI) provides clear physical content to the transactional process, with an ontology that allows for genuine dynamics, so that the transactional process corresponds to real dynamical processes.
RTI also extends the original TI into the relativistic domain such that it explains how and why ‘offers’ and ‘confirmations’ are produced. In Cramer’s original version, these are taken for granted. This expansion allows RTI to decisively resolve some of the earlier challenges to TI, such as Maudlin’s challenge.

GP: It seems to me that in Cramer’s picture, if we don’t assume a block universe where the future is actual, we should keep track of confirmation waves from all possible positions (etc.) of the absorber, not just the actual future position (etc.) of the absorber, which isn’t defined yet. What do you make of this?

RK: Cramer’s picture and RTI in general both involve confirmations from multiple absorbers. If you’re referring a particular one of these, I don’t think this applies, since the CW comes from an absorber in a way that is not dependent on all its possible future positions. To fully understand the circumstances of CW generation, you must go to the relativistic level and use the applicable Hamiltonian interaction between absorbers and emitters, which involves a decay rate based on the relationship of the systems. This relationship doesn’t involve the entire future career of the absorber. It’s defined by the applicable state transition and momentum of a photon going from the emitter to that particular absorber.

GP: What happens to discarded possibilities (e.g. the possibility of spin-down when spin-up becomes actual)? Are they removed from pre-spacetime?

RK: Yes, they no longer exist if they are not actualized and some contrary possibility is actualized. That is, if spin-up is actualized for a system, it is no longer possible for it to be spin-down, so the latter no longer exists as a possibility.

Cover picture from Pexels, picture from Needpix, picture from Pexels.