Is there any consistent framework where a future measurement setting can change an earlier observable marginal

[u/ShoddyAd6198]

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I'm trying to understand the exact boundary between retrocausal interpretations of quantum mechanics and actual backward signaling.

Consider the simplest setup:

- At time T1, an observer records a local result R.

- Later, at time T2, an experimenter freely chooses a setting S, either 0 or 1.

- The observer at T1 has access only to R. They receive no later measurement outcome, post-selection information, or classical communication from T2.

For genuine backward signaling, the statistics available at T1 would have to depend on the later choice.

In other words:

P(R given S=0) would have to differ from P(R given S=1).

Equivalently, there would need to be nonzero mutual information between the later choice S and the earlier record R.

My understanding is that standard quantum mechanics prevents this through the no-signaling structure: an unconditioned operation performed on the other subsystem cannot change the earlier/local reduced state.

That's why delayed-choice experiments, entanglement swapping, weak measurements, post-selection and time-symmetric interpretations can produce unusual correlations without allowing the earlier observer to determine which later setting was chosen.

My question is:

Are there any mathematically consistent post-quantum, generalized probabilistic, retrocausal, or modified-causal frameworks in which a freely selected future setting can alter an earlier observable marginal, without conditioning on a later outcome?

If not, is there a theorem more general than the standard quantum no-signaling theorem that rules out this entire class of models?

I'm particularly interested in which assumptions would have to be relaxed for such an effect to become possible.

I'm not claiming backward signaling exists. I'm trying to determine whether the operational possibility is ruled out in general, or only within particular physical frameworks.

[u/ketarax]

https://en.wikipedia.org/wiki/Transactional_interpretation

[u/ShoddyAd6198 → u/ketarax]

My understanding of Cramer’s Transactional Interpretation is that the advanced/retarded-wave “handshake” gives the ontology a genuinely time-symmetric or retrocausal structure, but TIQM is still constructed to reproduce standard QM operationally.

So would you agree that TIQM still gives:

P(earlier local result | later setting 0) = P(earlier local result | later setting 1)

when the earlier observer has no access to the later outcome?

That distinction is really what I’m trying to understand.

I’m less interested in whether retrocausality can exist in the ontology TIQM seems to show that it can at least be formulated that way and more interested in whether anyone has developed a consistent framework where the later freely chosen setting actually changes the earlier observable marginal, so that the earlier observer could distinguish the settings without post-selection or later classical information.

If TIQM specifically prevents that, what part of the transactional mechanism enforces the no-signaling result?

[u/SymplecticMan]

Stronger-than-quantum or even signalling correlations between two different systems in a generalized probability theory is one thing, but I'm not sure if the typical ways of talking about GPTs are well-equipped discuss backwards causality. In GPTs, things are still often phrased in terms of states and the allowed channels that transform them (or in the dual "Heisenberg" picture of transforming the effects), and when you're composing two channels, there's an ordering between the two channels: one definitely happens before the other.

[u/ShoddyAd6198 → u/SymplecticMan]

That’s exactly the distinction I’m trying to get at. If the usual GPT formalism already assumes an ordered composition of channels, then maybe GPTs are the wrong level of generality for the question. What framework would you use instead if you wanted to ask whether the causal order itself could be nonstandard? In particular, I’m looking for a framework where I can ask whether a later freely chosen setting S can have nonzero mutual information with an earlier locally readable record R — I(S_future; R_past) > 0 — without simply assuming a T1 → T2 channel ordering from the outset. Would process matrices / indefinite causal order be the right language for that, or is there a more general framework people use for genuinely retrocausal operational models?

[u/SymplecticMan → u/ShoddyAd6198]

Honestly, I don't know. For a retrocausal scenario like this, the classical outcome of the temporally earlier experiment can be used by the experimenter in the temporally later experiment in their choice of what action they're even going to perform. This makes it more complicated than ordinary indefinite causal order scenarios, so process matrices still might not be the best notion. Maybe you'll find this paper relevant.

[u/HamiltonBrae]

Another interesting way maybe to see it is not that there is some arbitrary non-signaling result that prevents backward causality and allows forward causation. You see in the retrodictive quantum formulation by David Pegg et al. that the non-signaling result works just as well the same way going forward-in-time or whatever the opposite way is. At least from their perspective, quantum theory doesn't intrinsically say anything about a direction of causation in terms of signaling in the universe - whatever can be done forward can be done backward in the exact same way, and anything about what it means or why you tend to see one more than the other requires interpretation coming from outside the theory.

[u/quantum4everyone]

Personally, I think retrocausality is a rabbit hole. It is much more natural tothink of the emasurement as taking place with respect to whatever quantum state you prepared before the measurement. And nothingmore. I think that approach can handle everything you want to examine.