A 2025 paper shows how certain equations for open quantum systems can retain two opposing time directions. Retrocausal models ask a different question: could later measurement settings help determine an earlier underlying physical state? The delayed-choice quantum eraser is often presented as experimental proof that the future changes the past, but its interference patterns appear in correlations obtained by sorting paired records. These are serious questions about time, causation, and quantum theory. They do not establish that we can rewrite recorded events or that consciousness controls the past.
The earlier post asked whether physics points beyond fundamental spacetime. This follow-up examines something more specific: whether the direction of time, and our usual account of causes preceding effects, belong to the deepest description of nature.
The difficulty begins with the phrase “time running backward.” It can refer to several different things, and the evidence for one does not automatically establish the others.
1. WHAT DOES REVERSING TIME MEAN?
| Idea | What it means |
|---|
| Time-reversal symmetry | The relevant laws admit an appropriately reversed physical history, with quantities such as momenta reversed too. |
| A thermodynamic arrow | A direction associated with macroscopic relaxation and entropy increase under particular physical conditions. |
| Retrodiction | Using information acquired later to infer an earlier event or state. |
| Retrocausality | In the models discussed here, later experimental settings enter the description of earlier underlying physical states. |
Retrodiction is familiar. Reading a fossil today changes what we know about an animal that lived long ago. It does not imply that examining the fossil caused the animal to exist. The harder question is whether a theory needs later settings to help specify what physically happened earlier, rather than simply to update our knowledge of it. Wharton and Argaman distinguish such dependence on future inputs from ordinary predictions and from sending messages into the past in their review of quantum reformulations (https://arxiv.org/abs/1906.04313).
Time-reversal symmetry is also distinct from the everyday arrow. A reversible microscopic law can coexist with overwhelmingly one-sided macroscopic behavior. There are vastly more microscopic arrangements corresponding to dispersed gas than to the same gas confined to a small corner. Starting from a suitable low-entropy condition, typical evolution leads toward the larger macrostate. Lebowitz explains how microscopic dynamics, typicality, and boundary conditions work together in the statistical account of irreversibility (https://arxiv.org/abs/2105.03470), including in his classic article “Boltzmann’s entropy and time’s arrow” (https://physicstoday.aip.org/features/boltzmanns-entropy-and-times-arrow).
That account does not remove the question of why the relevant earlier conditions were special. The universe’s special low-entropy starting condition is usually called the Past Hypothesis, and some philosophers, including Eddy Keming Chen, argue that it should count as a fundamental law in its own right (https://arxiv.org/abs/2008.00611). A dynamical law tells us how a state evolves; it does not, by itself, explain why that state was the one supplied. The boundary conditions carry part of the explanatory burden.
One qualification matters: exact time-reversal symmetry is not universal in nature. The BABAR collaboration directly observed time-reversal violation in neutral B-meson transitions (https://arxiv.org/abs/1207.5832). The discussion below concerns models with time-symmetric microscopic dynamics; it should not be inflated into the claim that every physical interaction is exactly time symmetric.
2. WHAT THE OPPOSING ARROWS PAPER FINDS
Thomas Guff, Chintalpati Umashankar Shastry, and Andrea Rocco published “Emergence of opposing arrows of time in open quantum systems” in Scientific Reports in 2025 (https://arxiv.org/abs/2311.08486; https://doi.org/10.1038/s41598-025-87323-x).
An open quantum system exchanges energy or correlations with an environment. Its effective evolution can sometimes be approximated as memoryless: the current state suffices to predict subsequent evolution without retaining the detailed history. This is the Markovian description, whose usefulness and limits are reviewed by Breuer and colleagues (https://arxiv.org/abs/1505.01385).
Guff and colleagues argue that a carefully applied Markov approximation preserves time-reversal symmetry in their equations for a system interacting with a heat bath, including Brownian, Lindblad, and Pauli dynamics. Relaxation can occur in opposing directions away from a special time origin at which system and environment are assumed uncorrelated; this does not make every irreversible process physically invertible.
The University of Surrey’s February 2025 press release presented the finding in its headline as “evidence” of two arrows of time. Its body describes opposing directions theoretically emerging from the equations and reports no observation of time running backward (https://www.surrey.ac.uk/news/physicists-uncover-evidence-two-arrows-time-emerging-quantum-realm).
This is a theoretical result about effective equations, rather than an observation of clocks or detector records reversing. The authors’ proposed cosmological extension remains speculative.
My takeaway is that dissipation alone may not select a fundamental direction. That remains separate from later measurements influencing earlier states.
3. OTHER "BACKWARD TIME" HEADLINES
In 2022, teams in Austria and China reported photonic circuits implementing a “quantum time flip,” with an operation acting in a quantum superposition of forward and time-reversed input-output directions (https://arxiv.org/abs/2211.01283; https://arxiv.org/abs/2210.17046). These experiments built on Chiribella and Liu’s theoretical scheme for quantum operations with indefinite time direction (https://arxiv.org/abs/2012.03859). Laboratory clocks ran forward throughout, and nothing travelled back into the past. Researchers nevertheless disagree about whether the photons underwent a genuine reversal or a simulation of one, as their differing assessments in Quanta’s coverage make clear (https://www.quantamagazine.org/how-quantum-physicists-flipped-time-and-how-they-didnt-20230127/).
In a different experiment, Angulo and colleagues in Steinberg’s Toronto group measured the average time transmitted photons spent as atomic excitations in a cloud of atoms (https://arxiv.org/abs/2409.03680). For some pulses this average was negative, matching the light’s negative group delay; the quantity is a weak value conditioned on later detecting a transmitted photon, rather than an ordinary elapsed time read from a clock (https://journals.aps.org/prl/abstract/10.1103/gjfq-k9dv). The result supplies no channel for information to travel backward in time or faster than light, a distinction also emphasized in Scientific American’s reporting (https://www.scientificamerican.com/article/evidence-of-negative-time-found-in-quantum-physics-experiment). These headlines concern superpositions of process directions and unusual measured averages, not a recorded past being rewritten.
4. WHAT RETROCAUSAL MODELS PROPOSE
A familiar way to explain an experiment is to specify an initial state and evolve it forward until a measurement produces a result. A retrocausal account can instead allow the underlying state between preparation and measurement to depend on the later measurement setting.
The proposal concerns the relationship among physical variables across the experiment. It need not describe a past that first happened one way and was subsequently overwritten. A complete history can be constrained by conditions at more than one temporal boundary. Huw Price develops the motivation for allowing this possibility in quantum theory, while making clear that the argument depends on additional assumptions about the underlying description (https://arxiv.org/abs/1002.0906).
A well-known time-symmetric reformulation, the two-state vector formalism, describes a system between preparation and later measurement with one state evolving forward from the preparation and another evolving backward from the post-selection. Aharonov and Vaidman present it as a time-symmetrized approach to standard quantum theory, especially useful for pre- and post-selected experiments and weak measurements (https://arxiv.org/abs/quant-ph/0105101). Using a backward-evolving state in a formalism does not, by itself, commit us to a backward physical influence; that further claim depends on how we interpret the formalism.
Why consider it? Bell experiments put pressure on a familiar package of assumptions, including local causality and statistical independence between measurement settings and relevant hidden variables. A retrocausal model can relax that independence through dependence on later settings, while trying to keep influences locally mediated through spacetime. “Locally mediated” here is not the full Bell-locality condition that the observed correlations violate. Wharton and Argaman review concrete models and explain why dependence on future inputs can open this route (https://arxiv.org/abs/1906.04313). Their locally mediated toy models address specific entanglement scenarios, and they identify extending such models to all quantum phenomena as a major open challenge (https://arxiv.org/abs/1906.04313).
The attraction is understandable: if the underlying description treats temporal directions more symmetrically, perhaps some apparent action across distance can be represented through a different causal structure. The task is to turn that possibility into a coherent account of the actual correlations.
Superdeterminism offers another route to statistical dependence between settings and hidden variables. Hossenfelder and Palmer defend it as a research program (https://doi.org/10.3389/fphy.2020.00139). The shared statistical dependence does not make every such theory retrocausal: a model still has to specify whether the connection comes from earlier conditions, later boundary inputs, or some other structure.
5. DOES TIME SYMMETRY FORCE RETROCAUSALITY?
Leifer and Pusey sharpened this question in their 2017 Proceedings of the Royal Society A paper “Is a time symmetric interpretation of quantum theory possible without retrocausality?” They show that quantum predictions conflict with a package including no retrocausality, a particular symmetry of the underlying physical description, and other assumptions about physical states and their role in mediating correlations (https://arxiv.org/abs/1607.07871).
The symmetry premise is stronger than observing that a familiar evolution equation can be reversed. It asks for a relationship between symmetry in experimental probabilities and symmetry in the underlying account of reality. Their result requires at least one assumption in the package to be relinquished; it does not identify retrocausality as an experimentally established mechanism.
The distinction generated a substantive exchange. Tim Maudlin argued that the proof contained an error and proposed a model he considered a counterexample (https://arxiv.org/abs/1707.08641). Leifer replied on both points: Maudlin’s model does not satisfy the particular time-symmetry premise used in the theorem, and the alleged error in the proof rests on a mistaken objection (https://arxiv.org/abs/1708.04364).
That is why the slogan “quantum mechanics is time symmetric, therefore the future causes the past” goes too far. The interesting argument concerns which additional principles we should require of an underlying theory.
6. WHAT DELAYED CHOICE CHANGES
Wheeler’s delayed-choice idea concerns when an experimental arrangement is selected. In an interferometer, recombining two paths can reveal interference, while a different arrangement can distinguish them. Delaying the selection challenges a simple picture in which the photon arrives with an advance commitment to behave as either a classical wave or a classical particle. Ma, Kofler, and Zeilinger review the proposals and their experimental realizations (https://arxiv.org/abs/1407.2930).
In this setting, the late choice changes which measurement is performed. It does not require an already recorded detector result to be rewritten. The quantum eraser adds entanglement and correlations between two measurement records, making the interpretation more tempting—and the distinction between a complete dataset and a selected subset more important.
In Kim and colleagues’ experiment, reported in a 1999 preprint and published in 2000, photon pairs were generated in two possible source regions. One member, the signal photon, was detected at a position labeled D0; its partner, the idler, followed optical paths leading to other detectors. The signal was detected about eight nanoseconds before the idler. The paper specifies what appeared in the coincidence records (https://arxiv.org/abs/quant-ph/9903047).
| Partner detector used to select matching D0 records | Pattern in that selected set |
|---|
| D1 | Interference fringes |
| D2 | A complementary fringe pattern with a shifted phase |
| D3 or D4 | No interference fringes |
The original apparatus used beam splitters to route idlers among these outcomes. Calling it “delayed choice” should not imply that a person could freely select the individual detector click after seeing D0.
7. WHY THE FRINGES APPEAR AFTER SORTING
Qureshi’s analysis of a closely related Mach–Zehnder interferometer version of the eraser shows why the two erasing outcomes reveal complementary patterns: bright regions in one align with dark regions in the other. Combining the appropriately weighted subsets removes the interference structure (https://arxiv.org/abs/1908.03920).
The probability bookkeeping can be written simply:
P(x) = Σⱼ P(j) · P(x | j)
Here x is the signal position, and j labels the partner outcome. The complete distribution includes every outcome with its actual frequency; a selected fringe pattern describes a joint correlation.
Imagine retaining the original list of D0 positions and timestamps. Later, the partner results let you label entries and plot subsets. The positions in the original list remain the same. What changes is which question you ask of the paired data.
This does not reduce entanglement to ordinary classical ignorance. It identifies how the experiment displays its quantum correlations. The distinction matters because a striking conditional pattern need not be a controllable change in the earlier local statistics.
A later laboratory implementation by Ashby, Schwarz, and Schlosshauer compared delayed erasure with an arrangement without the delay and found agreement, as quantum theory predicts. Their experiment used physical optical delays and photon detections; the timing of erasure did not supply a new effect beyond the predicted correlations (https://arxiv.org/abs/1601.06089).
8. WHY THIS DOES NOT SEND A MESSAGE INTO THE PAST
Suppose the idler operator wants to transmit a bit to someone who already holds the signal records. For a usable message, the earlier signal data alone would have to distinguish the later choice. Standard quantum theory supplies no such channel. Changing the partner’s measurement basis does not change the signal’s unconditioned local distribution when all partner outcomes are included. This is the no-signaling distinction discussed in the Bell-nonlocality review by Brunner and colleagues (https://arxiv.org/abs/1303.2849).
The operator also cannot choose the particular random partner outcome needed to select one fringe pattern rather than its complement. To identify the subsets, the two sides must compare their records through an ordinary communication channel. By that stage, the receiver has acquired additional information; the earlier standalone dataset never contained a readable message selected from the future.
There is an even stronger reason to avoid a literal story in which a later event reaches backward and changes an earlier image. Ma and colleagues implemented quantum erasure with actively selected settings that were spacelike separated from the relevant interference events, using both a 55-metre fibre arrangement and a 144-kilometre free-space link between islands. A light-speed signal could not connect them within the experiment, and different reference frames need not agree on their temporal ordering (https://arxiv.org/abs/1206.6578).
These arrangements preserve the correlations while undermining an intuitive account that depends on one detector universally happening first. They do not supply an operational channel across that separation.
9. CAN A RETROCAUSAL INTERPRETATION STILL EXPLAIN THE ERASER?
Yes. The absence of a backward message does not, by itself, eliminate retrocausal accounts of hidden physical variables. An interpretation can describe the entire experiment using a different underlying temporal structure while preserving the same observed probabilities.
For example, Fearn analyzes Kim’s experiment through Cramer’s transactional interpretation, using retarded and advanced waves to describe the process. That account reproduces the experimental predictions while explicitly denying backward communication (https://arxiv.org/abs/1501.00970).
The distinction is between accommodating a result and being uniquely supported by it. Standard quantum mechanics predicts the eraser correlations without requiring a retrocausal ontology, while a transactional interpretation can describe them in its own terms. The experiment alone does not choose between those accounts.
Kastner likewise argues that delayed-choice eraser experiments do not demonstrate retrocausation beyond the entangled correlations already involved (https://arxiv.org/abs/1905.03137). We can take the experimental result seriously while leaving the underlying interpretation open.
The same discipline applies to the opposing-arrows paper. Its time-symmetric dissipative equations and a retrocausal interpretation of measurement answer different questions. Neither result can serve as a substitute for the other’s argument.
10. WHAT WOULD STRENGTHEN THE CASE?
A serious retrocausal model has to reproduce the quantum probabilities and explain why its underlying dependencies never become a usable signaling channel. This is a substantive constraint, not something resolved merely by drawing an influence backward along a timeline.
Wood and Spekkens show that, within the causal-modeling framework they analyze, explanations of Bell correlations face a fine-tuning problem: underlying causal connections have to coexist with the absence of observable signaling, including in explanations using superluminal influences, superdeterminism, or retrocausal influences without causal loops (https://arxiv.org/abs/1208.4119). A model needs an account of that coexistence, whether through its symmetries, dynamics, or other structural principles.
My standard would be to ask what the proposed account explains more clearly, which assumptions it replaces, and whether it yields a distinctive test. A confirmed prediction separating it from competing accounts would be especially informative. Where two accounts give exactly the same observable predictions, the experiments they share cannot by themselves establish which underlying picture is correct.
Consciousness introduces a further question. In the experiments discussed here, the relevant operations involve sources, beam splitters, measurement settings, detectors, and the comparison of records. My inference is that calling those operations “observation” supplies no additional evidence that awareness selects the temporal direction or changes an earlier event.
Claims that minds can sense future events can also be tested: in 2011, Daryl Bem reported experiments suggesting that later events influenced earlier responses. Three independent, pre-registered replications of one key experiment, concerning whether later practice improved earlier recall, found no significant effect (https://pmc.ncbi.nlm.nih.gov/articles/PMC3303812/). My takeaway is that awareness reaching backward in time is a testable claim, and this prominent result did not hold up in those replications.
Someone could propose a theory assigning consciousness that role. The proposal would then need to show what awareness contributes to the physical model and how that contribution could be distinguished from the apparatus and dynamics already described.
The possibilities remain interesting without collapsing them into a single claim. An emergent arrow of time concerns how temporal asymmetry arises. Retrocausality concerns the dependence of underlying states on later inputs. The quantum eraser concerns which interference correlations appear when measurement records are compared.
My conclusion is that quantum physics gives us good reasons to examine our assumptions about time and causation. It has not established that the future rewrites recorded history, and the results discussed here do not select consciousness as the explanation. The deeper question is how a physical description of the whole experiment produces the relationships we observe.
