Retrocausal models of quantum mechanics add further weight to the conflict between causality and the possible existence of free will. We analyze a simple closed causal loop ensuing from the interaction between two systems with opposing thermodynamic time arrows, such that each system can forecast future events for the other. The loop is avoided by the fact that the choice to abort an event thus forecasted leads to the destruction of the forecaster's past. Physical law therefore enables prophecy of future events only as long as this prophecy is not revealed to a free agent who can otherwise render it false. This resolution is demonstrated on an earlier finding derived from the two-state vector formalism, where a weak measurement's outcome anticipates a future choice, yet this anticipation becomes apparent only after the choice has been actually made. To quantify this assertion, weak information is described in terms of Fisher information. We conclude that an already existing future does not exclude free will nor invoke causal paradoxes. On the quantum level, particles can be thought of as weakly interacting according to their past and future states, but causality remains intact as long as the future is masked by quantum indeterminism. Quanta 2016; 5: 53–60.
Retrocausal models of quantum mechanics add further weight to the conflict between causality and the possible existence of free will. We analyze a simple closed causal loop ensuing from the interaction between two systems with opposing thermodynamic time arrows, such that each system can forecast future events for the other. The loop is avoided by the fact that the choice to abort an event thus forecasted leads to the destruction of the forecaster's past. Physical law therefore enables prophecy of future events only as long as this prophecy is not revealed to a free agent who can otherwise render it false. This resolution is demonstrated on an earlier finding derived from the two-state vector formalism, where a weak measurement's outcome anticipates a future choice, yet this anticipation becomes apparent only after the choice has been actually made. To quantify this assertion, weak information is described in terms of Fisher information. We conclude that an already existing future does not exclude free will nor invoke causal paradoxes. On the quantum level, particles can be thought of as weakly interacting according to their past and future states, but causality remains intact as long as the future is masked by quantum indeterminism. Quanta 2016; 5: 53–60.
Quantum weak measurement is presented as shedding new light on the retrocausality question. It is shown to leave a system almost unaffected while gathering information about it. Next, an EPR experiment is studied where each particle undergoes a few weak measurements along some pre-set spin orientations. These weak outcomes are individually recorded. Then the particle undergoes a strong measurement along a spin orientation freely chosen at the last moment. Bell-inequality violation is expected between the two final strong measurements within each EPR pair. At the same time, agreement is expected between these strong measurements and the earlier weak ones performed on that pair. A contradiction thereby ensues: i) Bell’s theorem forbids spin values to exist prior to the choice of the spin-orientation to be measured; ii) A weak measurement cannot determine the outcome of a successive strong one; and iii) Indeed no disentanglement is inflicted by the weak measurements; yet iv) The weak measurements’ outcomes agree with those of the strong ones, suggesting the existence of pre-determined values. The most reasonable resolution seems to be that of the Two-State-Vector Formalism, namely, that the experimenter’s choice has been encrypted within the weak measurement’s outcomes, even before the experimenter themselves knows what their choice will be. Causal loops are avoided by this anticipation remaining encrypted until the final outcomes enable to decipher it.
This paper presents a conceptual and analytical investigation into the limits of temporal access through the combined perspectives of causality, relativity, quantum mechanics, thermodynamics, and information theory. The work introduces the concept of the “Temporal Causality Barrier,” a selective and logically grounded framework that distinguishes between passive temporal observation and active temporal intervention. The paper argues that passive observation of the past is both physically permissible and continuously occurring through ordinary causal processes such as light propagation, memory, historical records, and astronomical observation. In contrast, active intervention into the past including backward information transfer, causal alteration of previous events, and closed causal loops is shown to generate logical contradictions and violations of causal consistency. The analysis critically examines major theoretical approaches related to time travel and retrocausality, including closed timelike curves, Novikov’s self-consistency principle, Deutsch’s quantum closed timelike curve model, and the two-state vector formalism in quantum mechanics. The paper argues that while some interpretations may mathematically describe retrocausal correlations, none currently provide an operational mechanism for backward information transmission without encountering paradoxes or information-theoretic inconsistencies. The framework further connects the causal structure of spacetime with the thermodynamic arrow of time, emphasizing the role of entropy, light cone geometry, and information propagation in preserving temporal coherence. The work also explores broader philosophical implications concerning the nature of time, consciousness, causality, and agency within a causally structured universe. Rather than proposing a new physical force or replacing existing theories, this paper provides a logically structured analysis of why temporal observation is compatible with known physics while active temporal intervention remains fundamentally constrained by both physical law and logical consistency.
An EPR experiment is studied where each particle undergoes a few weak measurements along some pre-set spin orientations, whose outcomes are individually recorded. Then the particle undergoes a strong measurement along a spin orientation freely chosen at the last moment. Bell-inequality violation is expected between the two final strong measurements within each EPR pair. At the same time, agreement is expected between these measurements and the earlier weak ones within the pair. A contradiction thereby ensues: i) Bell's theorem forbids spin values to exist prior to the choice of the spin-orientation to be measured; ii) A weak measurement cannot determine the outcome of a successive strong one; and iii) Indeed no disentanglement is inflicted by the weak measurements; yet iv) The weak measurements’ outcomes agree with those of the strong ones. The most reasonable resolution seems to be that of the Two-State-Vector Formalism, namely, that the experimenter’s choice has been encrypted within the weak measurement's outcomes, even before the experimenter themselves knows what their choice will be. Causal loops are avoided by this anticipation remaining encrypted until the final outcomes enable to decipher it.
This paper discusses experimental findings from the Princeton Engineering Anomalies Research project supporting the existence of retrocausal information flow. Possible alternative explanations to retrocausation are examined for each experiment. In each case it can be shown that retrocausal information flow is either a parsimonious explanation, or the only currently available explanation, for the observational data. Gedankenexperiments are then examined to demonstrate that regardless of observational effect size, empirical phenomena of the type seen in these experiments cannot produce time paradoxes.The theoretical discussion pursuant to the experimental data covers some of the basic problems in the physical representation of time. Human perception of time incorporates a dichotomy: the past is known but cannot be changed, the future is presumed to be indeterminate and hence controllable but unknowable in advance. Physical formalisms have difficulty in representing this dichotomy, arriving in the most extreme case at the completely deterministic Einsteinian block universe view of spacetime. Strict physical determinism, however, currently seems implausible due to apparently essential indeterminism at the quantum scale.Retrocausal phenomena attack the dichotomy on empirical rather than theoretical grounds, since if past events can be causally influenced by future events either the past is malleable or the future is knowable. This approach allows a resolution of the past/future dichotomy by additional approaches other than strict determinism: the past may be indeterminate, or both past and future can contain a mixture of deterministic and indeterminate events.
The Delayed-Choice Quantum Eraser experiment is commonly interpreted as implying that in quantum mechanics a choice made at one time can influence an earlier event. We here suggest an extension of the experiment that results in a paradox when analysed in a local realist interpretation combined with backward causation (``dynamical retrocausality''). We argue that resolving the paradox requires giving up the idea that, in quantum mechanics, a choice can influence the past in this way, and that it instead requires a violation of Statistical Independence without (what most people think of as) retrocausality. Finally, we propose an implementation of the experiment that we believe to be possible with existing technology. This new experiment can distinguish between different types of hidden-variables theories in a way that Bell-type tests cannot: unlike in a Bell test, the measurement setting here is set by an earlier outcome, which makes the consistency of the backwards influence itself testable. We classify the fixed-point (consistency-enforcing) hidden-variables models of the experiment, which can reproduce quantum mechanics only if the backwards influence has no observable effect.
We analyze an idealized experiment in which weak which-path measurements do not prevent consecutive weak interference measurements, and then again the which-path information is recovered by strong measurements. We also show how the same effect can be obtained even by all strong and weak measurements being carried out on a single particle. The simplified setting enables critically assessing competing interpretations of the results. The most natural one turns out to be that of the Two-State-Vector Formalism, according to which the quantum two-state between an earlier and a later measurement is equally determined by both of them.