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Relational Retrocausality as Necessity

Ahmed Albezawi

Published Apr 5, 2026• 0 citations
Publisher

Abstract

There is a particular kind of dissatisfaction that a physicist feels when they first encounter the measurement problem properly. Not the pedagogical version, wherethe textbook waves its hand and says 'and then the wavefunction collapses' andmoves swiftly on to the next topic. The real version. The version where you sit with the Schrodinger equation long enough to realize that nothing in it — not one term, not one symbol — implies or permits or even suggests a collapse. The equation evolves states unitarily. It has been doing so for a hundred years. And yet every experiment produces a definite outcome.Something happens at measurement. Something that the theory does not describe. That gap is the measurement problem, and it has been papered over with an astonishing variety of philosophical plasters: the Copenhagen interpretation's studied vagueness about the quantum-classical boundary; the many-worlds proliferation of realities too extravagant to take literally; the Bohmian ghost-wave that somehow knows where its particle is without looking; the GRW stochastic collapses that add new physics to plug a conceptual hole. Each of these is intelligent. None of them is satisfying. Each solves the problem by paying a price that a careful physicist should be unwilling to pay. This monograph proposes a different path. It begins with a question that almost nobody asks: what if the measurement problem is not about what happens to thefuture state, but about what happens to the past? What if the act of measurement does not merely update our predictions about what comes next, but retroactively assigns a consistent history to what came before?

DOI
10.6084/m9.figshare.31941099

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Relational Retrocausality as Necessity

Ahmed Albezawi

There is a particular kind of dissatisfaction that a physicist feels when they first encounter the measurement problem properly. Not the pedagogical version, wherethe textbook waves its hand and says 'and then the wavefunction collapses' andmoves swiftly on to the next topic. The real version. The version where you sit with the Schrodinger equation long enough to realize that nothing in it — not one term, not one symbol — implies or permits or even suggests a collapse. The equation evolves states unitarily. It has been doing so for a hundred years. And yet every experiment produces a definite outcome. Something happens at measurement. Something that the theory does not describe. That gap is the measurement problem, and it has been papered over with an astonishing variety of philosophical plasters: the Copenhagen interpretation's studied vagueness about the quantum-classical boundary; the many-worlds proliferation of realities too extravagant to take literally; the Bohmian ghost-wave that somehow knows where its particle is without looking; the GRW stochastic collapses that add new physics to plug a conceptual hole. Each of these is intelligent. None of them is satisfying. Each solves the problem by paying a price that a careful physicist should be unwilling to pay. This monograph proposes a different path. It begins with a question that almost nobody asks: what if the measurement problem is not about what happens to thefuture state, but about what happens to the past? What if the act of measurement does not merely update our predictions about what comes next, but retroactively assigns a consistent history to what came before?

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THE ROAD NOT TAKEN: A Different Kind of Physics

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Quantum mechanics is our most successful physical theory. It predicts the behavior of atoms, molecules, and light to ten decimal places. No experiment has ever contradicted it. It also contains a crack at its foundation. The theory’s central equation—the Schrodinger equation—describes a smooth, continuous, perfectly predictable evolution of the quantum state. But whenever anyone performs a measurement, they get a single, definite result. Which result they get appears to be random. The transition from smooth evolution to random outcome is called the “collapse of the wavefunction,” and after nearly a century of analysis, there is no agreed-upon explanation for what causes it, when it occurs, or whether it occurs at all. This is the measurement problem. The measurement problem is not a minor puzzle at the margins of the theory. It is a failure to explain the most basic operation in science: looking at the result of an experiment. Around it cluster other puzzles. Light behaves like a wave in some experiments and like a particle in others—two contradictory descriptions. Two particles that have interacted remain correlated across arbitrary distances, as though communicating faster than light. The probabilities that govern measurement outcomes appear as a postulate—the Born rule—inserted by hand with no deeper justification. And the theory offers no unified picture of reality: quantum mechanics, general relativity, and the standard model of particle physics speak incompatible mathematical languages. A century of effort has produced many interpretations of quantum mechanics. Copenhagen. Many-worlds. De Broglie-Bohm. Objective collapse. Quantum Bayesianism. None has achieved consensus. The measurement problem remains. This document proposes that the source of these puzzles is not quantum mechanics itself. It is a single, invisible assumption made at the very beginning of the theory in 1900—an assumption so deeply embedded that nobody noticed they were making it. Change that assumption, and the puzzles do not get solved. They dissolve. They stop being puzzles. They become predictable consequences of geometry.

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GianCarlo Ghirardi

Angelo Bassi, Tullio Weber, Detlef Dürr

On 1 June 2018, theoretical physicist GianCarlo Ghirardi, known for his work on the foundations of quantum mechanics, died unexpectedly in his summer resort home in Grado, Italy. GianCarlo Ghirardi MASSIMO SILVANO, ICTP PHOTO ARCHIVES, 2005PPT|High resolutionGhirardi was born in Milan on 28 October 1935. A few years after his 1959 graduation from the University of Milan with a PhD in physics, he moved to Trieste. In 1964 he met Alberto Rimini and one of us (Weber), and they worked on scattering theory. They parted after a brief period but remained in touch. In 1973 they wrote a paper that gave a new interpretation of the decay process and for the first time advanced a consistent explanation of the exponential decay law.Meanwhile, Ghirardi became more and more attracted to the foundations of quantum mechanics. The catalyst for him had been the 1970 International School of Physics conference, Foundations of Quantum Mechanics, in Varenna, Italy. Also fueling his interest was the 1971 book Conceptual Foundations of Quantum Mechanics by Bernard D’Espagnat. He was fascinated by the depth of the subject, and he suggested to Rimini and Weber that they join him in working on it. They accepted, with some hesitation. During the next roughly 15 years, Ghirardi, Rimini, and Weber succeeded in establishing a unique and enthusiastic collaboration.The main issue was the measurement problem: Quantum mechanics cannot explain the reduction of the wavepacket produced by a measurement; it must be appended ad hoc. Since collapse is not consistent with the dynamics of the linear Schrödinger equation, Ghirardi, Rimini, and Weber considered making the Schrödinger equation nonlinear. In 1984 they came up with an idea that works: Observing that all measurements involve measurements of the position of some sort of indicator, they imposed on the wavefunction a spatial localization that affects any particle, isolated or constituent, of any physical system.They wrote the desired wave equation effortlessly. It was stochastic and nonlinear, with the stochastic noise acting as a series of localizations occurring in time according to a Poisson distribution with some mean frequency. The nature and origin of the stochasticity were left undetermined. The remaining problem was to understand the equation’s physical import: What happens for a system of N particles? They found that the frequency of localizations becomes N times larger, meaning that the new dynamics mimics that of the Schrödinger equation for a small number of particles and that of the classical world for a large number. They called the modification quantum mechanics with spontaneous localization.Ghirardi, Rimini, and Weber submitted an abridged version of their paper for a 1984 workshop on quantum probability and applications held in Heidelberg, Germany. The proceedings were published in 1985; the final version appeared in Physical Review D in July 1986.The preprint, which came out in 1985, achieved wide circulation. In February 1986 Ghirardi, Rimini, and Weber, to their great surprise, received a letter from John Bell that began, “I read with very great interest and admiration your paper,” and informed them that he was preparing a review talk based on their work. Bell presented it at Imperial College London the following year, at a conference celebrating the centenary of the birth of Erwin Schrödinger. It was then that he named the theory GRW, an acronym that has been in use ever since.The GRW theory solves the quantum mechanics measurement problem by eliminating the need to appeal to an observer. But before GRW, two other quantum theories solved the same problem: Bohmian mechanics and many worlds, both from the 1950s. Nevertheless, GRW had a huge effect on physics, mainly for two reasons. First, Bohmian mechanics and many worlds were considered to be interpretations of quantum theory that offered no new predictions, but GRW was not. It was by all standards a new theory that made new predictions. After all, it destroyed one of the pillars of quantum theory: the linearity of wavefunction evolution. For the majority of physicists who acknowledged the measurement problem, GRW was a serious proposal with experimental implications, even if the technology was not yet in place to test them.Second, and more on the theoretical side, the theory seemed to ease some of the tension between relativity and quantum physics. Quoting Bell, from his Schrödinger centenary contribution, “I am particularly struck by the fact that the model is as Lorentz invariant as it could be in the nonrelativistic version. It takes away the ground of my fear that any exact formulation of quantum mechanics must conflict with fundamental invariance.” Indeed, a fully Lorentz invariant version of GRW had been constructed in 2006.Research on GRW is still vibrant, and the experimental community is now hunting its spontaneous collapses, which might explain why the microscopic quantum world seems not to reach the human scale.In addition to his journal editorial work, Ghirardi wrote a well-received popular book on quantum physics and had just finished a voluminous book on symmetries in nature, a subject that intrigued him throughout his scientific life. A theoretical physics professor at the University of Trieste for 42 years, Ghirardi will be remembered for his mentorship, sharp intellect, and inspiring passion for science.© 2018 American Institute of Physics.

Physics Today•Oct 1, 2018•0 citations

Hypothetical solution of the problem of measurement through the notion of quantum backward causality

Giuseppe Castagnoli

International Journal of Theoretical Physics•Aug 1, 1995•3 citations

Wheeler's Delayed-Choice Experiment as an

YOUNG HO GOH

Wheeler's delayed-choice experiment appears to show that a choice made now retroactively determines whether a photon behaved as a wave or particle in the past — even billions of years ago in the cosmic version. This has been interpreted as evidence for retrocausality, the role of consciousness in collapsing the wave function, or the many-worlds interpretation. We show that none of these interpretations is required. In the information processing load framework established in Papers 4 and 7, the photon propagates with n ≈ 0 environmental interactions throughout its journey. Since n = 0 < n* for all physical photons, the condition C_wave < C_particle holds: the wave state is maintained for the entire propagation duration, regardless of whether that duration is nanoseconds or 10 Gyr. The experimenter's detector setting determines only n_detector at the moment of detection — not the photon's state during propagation. The interference visibility function is V = max(0, 1 − n_detector/n*): full interference when n_detector = 0, no interference when n_detector ≥ n*. We verify numerically that a photon from a quasar 10 Gly distant accumulates n ≈ 2.6 × 10⁶ CMB scatterings during propagation, while n* ≈ 82 for a 500 nm photon — confirming n << n* and wave state maintenance. The apparent retrocausality is a consequence of a misidentification: the question 'which path did the photon take during propagation?' presupposes a definite path during propagation, but in the load framework the photon was not in a path-definite state. There is no fact of the matter about the path during propagation. The detector choice at detection determines what question can meaningfully be asked — not what happened in the past.Wheeler's delayed-choice experiment appears to show that a choice made now retroactively determines whether a photon behaved as a wave or particle in the past — even billions of years ago in the cosmic version. This has been interpreted as evidence for retrocausality, the role of consciousness in collapsing the wave function, or the many-worlds interpretation. We show that none of these interpretations is required. In the information processing load framework established in Papers 4 and 7, the photon propagates with n ≈ 0 environmental interactions throughout its journey. Since n = 0 < n* for all physical photons, the condition C_wave < C_particle holds: the wave state is maintained for the entire propagation duration, regardless of whether that duration is nanoseconds or 10 Gyr. The experimenter's detector setting determines only n_detector at the moment of detection — not the photon's state during propagation. The interference visibility function is V = max(0, 1 − n_detector/n*): full interference when n_detector = 0, no interference when n_detector ≥ n*. We verify numerically that a photon from a quasar 10 Gly distant accumulates n ≈ 2.6 × 10⁶ CMB scatterings during propagation, while n* ≈ 82 for a 500 nm photon — confirming n << n* and wave state maintenance. The apparent retrocausality is a consequence of a misidentification: the question 'which path did the photon take during propagation?' presupposes a definite path during propagation, but in the load framework the photon was not in a path-definite state. There is no fact of the matter about the path during propagation. The detector choice at detection determines what question can meaningfully be asked — not what happened in the past. Keywords: delayed-choice experiment; Wheeler; retrocausality; information processing load; wave function collapse; quantum eraser; cosmic photon; weak measurement; n < n*; no consciousness required

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Bohr is said to have remarked that anybody who is not shocked by quantum theory has not understood it. No physicist, however, challenges its practical usefulness in predicting experimental results and providing an explanatory framework for an enormous range of study. Nevertheless, more thoughtful physicists have been astonished and disturbed by what is implied when attempts are made to interpret its meaning. As a result a variety of ingenious, and sometimes bizarre, schemes have been devised since its inception in order to make these implications more acceptable to the scientific mind. Some of these schemes have resulted in proposals which appear stranger than the problems they have been designed to resolve. After some sixty years there is no consensus of opinion upon the solution of the epistemological problems posed. Indeed the debate keeps on resurfacing in the scientific literature and continues to generate library shelves of books. Some progress, however, has been made and the late John Bell himself and the experimental work of Alain Aspect have demonstrated that some epistemological positions cannot be sustained.

Physics World•May 1, 1991•0 citations
There is a particular kind of dissatisfaction that a physicist feels when they first encounter the measurement problem properly. Not the pedagogical version, wherethe textbook waves its hand and says 'and then the wavefunction collapses' andmoves swiftly on to the next topic. The real version. The version where you sit with the Schrodinger equation long enough to realize that nothing in it — not one term, not one symbol — implies or permits or even suggests a collapse. The equation evolves states unitarily. It has been doing so for a hundred years. And yet every experiment produces a definite outcome.Something happens at measurement. Something that the theory does not describe. That gap is the measurement problem, and it has been papered over with an astonishing variety of philosophical plasters: the Copenhagen interpretation's studied vagueness about the quantum-classical boundary; the many-worlds proliferation of realities too extravagant to take literally; the Bohmian ghost-wave that somehow knows where its particle is without looking; the GRW stochastic collapses that add new physics to plug a conceptual hole. Each of these is intelligent. None of them is satisfying. Each solves the problem by paying a price that a careful physicist should be unwilling to pay. This monograph proposes a different path. It begins with a question that almost nobody asks: what if the measurement problem is not about what happens to thefuture state, but about what happens to the past? What if the act of measurement does not merely update our predictions about what comes next, but retroactively assigns a consistent history to what came before?
There is a particular kind of dissatisfaction that a physicist feels when they first encounter the measurement problem properly. Not the pedagogical version, wherethe textbook waves its hand and says 'and then the wavefunction collapses' andmoves swiftly on to the next topic. The real version. The version where you sit with the Schrodinger equation long enough to realize that nothing in it — not one term, not one symbol — implies or permits or even suggests a collapse. The equation evolves states unitarily. It has been doing so for a hundred years. And yet every experiment produces a definite outcome. Something happens at measurement. Something that the theory does not describe. That gap is the measurement problem, and it has been papered over with an astonishing variety of philosophical plasters: the Copenhagen interpretation's studied vagueness about the quantum-classical boundary; the many-worlds proliferation of realities too extravagant to take literally; the Bohmian ghost-wave that somehow knows where its particle is without looking; the GRW stochastic collapses that add new physics to plug a conceptual hole. Each of these is intelligent. None of them is satisfying. Each solves the problem by paying a price that a careful physicist should be unwilling to pay. This monograph proposes a different path. It begins with a question that almost nobody asks: what if the measurement problem is not about what happens to thefuture state, but about what happens to the past? What if the act of measurement does not merely update our predictions about what comes next, but retroactively assigns a consistent history to what came before?
Quantum mechanics is our most successful physical theory. It predicts the behavior of atoms, molecules, and light to ten decimal places. No experiment has ever contradicted it. It also contains a crack at its foundation. The theory’s central equation—the Schrodinger equation—describes a smooth, continuous, perfectly predictable evolution of the quantum state. But whenever anyone performs a measurement, they get a single, definite result. Which result they get appears to be random. The transition from smooth evolution to random outcome is called the “collapse of the wavefunction,” and after nearly a century of analysis, there is no agreed-upon explanation for what causes it, when it occurs, or whether it occurs at all. This is the measurement problem. The measurement problem is not a minor puzzle at the margins of the theory. It is a failure to explain the most basic operation in science: looking at the result of an experiment. Around it cluster other puzzles. Light behaves like a wave in some experiments and like a particle in others—two contradictory descriptions. Two particles that have interacted remain correlated across arbitrary distances, as though communicating faster than light. The probabilities that govern measurement outcomes appear as a postulate—the Born rule—inserted by hand with no deeper justification. And the theory offers no unified picture of reality: quantum mechanics, general relativity, and the standard model of particle physics speak incompatible mathematical languages. A century of effort has produced many interpretations of quantum mechanics. Copenhagen. Many-worlds. De Broglie-Bohm. Objective collapse. Quantum Bayesianism. None has achieved consensus. The measurement problem remains. This document proposes that the source of these puzzles is not quantum mechanics itself. It is a single, invisible assumption made at the very beginning of the theory in 1900—an assumption so deeply embedded that nobody noticed they were making it. Change that assumption, and the puzzles do not get solved. They dissolve. They stop being puzzles. They become predictable consequences of geometry.
On 1 June 2018, theoretical physicist GianCarlo Ghirardi, known for his work on the foundations of quantum mechanics, died unexpectedly in his summer resort home in Grado, Italy. GianCarlo Ghirardi MASSIMO SILVANO, ICTP PHOTO ARCHIVES, 2005PPT|High resolutionGhirardi was born in Milan on 28 October 1935. A few years after his 1959 graduation from the University of Milan with a PhD in physics, he moved to Trieste. In 1964 he met Alberto Rimini and one of us (Weber), and they worked on scattering theory. They parted after a brief period but remained in touch. In 1973 they wrote a paper that gave a new interpretation of the decay process and for the first time advanced a consistent explanation of the exponential decay law.Meanwhile, Ghirardi became more and more attracted to the foundations of quantum mechanics. The catalyst for him had been the 1970 International School of Physics conference, Foundations of Quantum Mechanics, in Varenna, Italy. Also fueling his interest was the 1971 book Conceptual Foundations of Quantum Mechanics by Bernard D’Espagnat. He was fascinated by the depth of the subject, and he suggested to Rimini and Weber that they join him in working on it. They accepted, with some hesitation. During the next roughly 15 years, Ghirardi, Rimini, and Weber succeeded in establishing a unique and enthusiastic collaboration.The main issue was the measurement problem: Quantum mechanics cannot explain the reduction of the wavepacket produced by a measurement; it must be appended ad hoc. Since collapse is not consistent with the dynamics of the linear Schrödinger equation, Ghirardi, Rimini, and Weber considered making the Schrödinger equation nonlinear. In 1984 they came up with an idea that works: Observing that all measurements involve measurements of the position of some sort of indicator, they imposed on the wavefunction a spatial localization that affects any particle, isolated or constituent, of any physical system.They wrote the desired wave equation effortlessly. It was stochastic and nonlinear, with the stochastic noise acting as a series of localizations occurring in time according to a Poisson distribution with some mean frequency. The nature and origin of the stochasticity were left undetermined. The remaining problem was to understand the equation’s physical import: What happens for a system of N particles? They found that the frequency of localizations becomes N times larger, meaning that the new dynamics mimics that of the Schrödinger equation for a small number of particles and that of the classical world for a large number. They called the modification quantum mechanics with spontaneous localization.Ghirardi, Rimini, and Weber submitted an abridged version of their paper for a 1984 workshop on quantum probability and applications held in Heidelberg, Germany. The proceedings were published in 1985; the final version appeared in Physical Review D in July 1986.The preprint, which came out in 1985, achieved wide circulation. In February 1986 Ghirardi, Rimini, and Weber, to their great surprise, received a letter from John Bell that began, “I read with very great interest and admiration your paper,” and informed them that he was preparing a review talk based on their work. Bell presented it at Imperial College London the following year, at a conference celebrating the centenary of the birth of Erwin Schrödinger. It was then that he named the theory GRW, an acronym that has been in use ever since.The GRW theory solves the quantum mechanics measurement problem by eliminating the need to appeal to an observer. But before GRW, two other quantum theories solved the same problem: Bohmian mechanics and many worlds, both from the 1950s. Nevertheless, GRW had a huge effect on physics, mainly for two reasons. First, Bohmian mechanics and many worlds were considered to be interpretations of quantum theory that offered no new predictions, but GRW was not. It was by all standards a new theory that made new predictions. After all, it destroyed one of the pillars of quantum theory: the linearity of wavefunction evolution. For the majority of physicists who acknowledged the measurement problem, GRW was a serious proposal with experimental implications, even if the technology was not yet in place to test them.Second, and more on the theoretical side, the theory seemed to ease some of the tension between relativity and quantum physics. Quoting Bell, from his Schrödinger centenary contribution, “I am particularly struck by the fact that the model is as Lorentz invariant as it could be in the nonrelativistic version. It takes away the ground of my fear that any exact formulation of quantum mechanics must conflict with fundamental invariance.” Indeed, a fully Lorentz invariant version of GRW had been constructed in 2006.Research on GRW is still vibrant, and the experimental community is now hunting its spontaneous collapses, which might explain why the microscopic quantum world seems not to reach the human scale.In addition to his journal editorial work, Ghirardi wrote a well-received popular book on quantum physics and had just finished a voluminous book on symmetries in nature, a subject that intrigued him throughout his scientific life. A theoretical physics professor at the University of Trieste for 42 years, Ghirardi will be remembered for his mentorship, sharp intellect, and inspiring passion for science.© 2018 American Institute of Physics.
Wheeler's delayed-choice experiment appears to show that a choice made now retroactively determines whether a photon behaved as a wave or particle in the past — even billions of years ago in the cosmic version. This has been interpreted as evidence for retrocausality, the role of consciousness in collapsing the wave function, or the many-worlds interpretation. We show that none of these interpretations is required. In the information processing load framework established in Papers 4 and 7, the photon propagates with n ≈ 0 environmental interactions throughout its journey. Since n = 0 < n* for all physical photons, the condition C_wave < C_particle holds: the wave state is maintained for the entire propagation duration, regardless of whether that duration is nanoseconds or 10 Gyr. The experimenter's detector setting determines only n_detector at the moment of detection — not the photon's state during propagation. The interference visibility function is V = max(0, 1 − n_detector/n*): full interference when n_detector = 0, no interference when n_detector ≥ n*. We verify numerically that a photon from a quasar 10 Gly distant accumulates n ≈ 2.6 × 10⁶ CMB scatterings during propagation, while n* ≈ 82 for a 500 nm photon — confirming n << n* and wave state maintenance. The apparent retrocausality is a consequence of a misidentification: the question 'which path did the photon take during propagation?' presupposes a definite path during propagation, but in the load framework the photon was not in a path-definite state. There is no fact of the matter about the path during propagation. The detector choice at detection determines what question can meaningfully be asked — not what happened in the past.Wheeler's delayed-choice experiment appears to show that a choice made now retroactively determines whether a photon behaved as a wave or particle in the past — even billions of years ago in the cosmic version. This has been interpreted as evidence for retrocausality, the role of consciousness in collapsing the wave function, or the many-worlds interpretation. We show that none of these interpretations is required. In the information processing load framework established in Papers 4 and 7, the photon propagates with n ≈ 0 environmental interactions throughout its journey. Since n = 0 < n* for all physical photons, the condition C_wave < C_particle holds: the wave state is maintained for the entire propagation duration, regardless of whether that duration is nanoseconds or 10 Gyr. The experimenter's detector setting determines only n_detector at the moment of detection — not the photon's state during propagation. The interference visibility function is V = max(0, 1 − n_detector/n*): full interference when n_detector = 0, no interference when n_detector ≥ n*. We verify numerically that a photon from a quasar 10 Gly distant accumulates n ≈ 2.6 × 10⁶ CMB scatterings during propagation, while n* ≈ 82 for a 500 nm photon — confirming n << n* and wave state maintenance. The apparent retrocausality is a consequence of a misidentification: the question 'which path did the photon take during propagation?' presupposes a definite path during propagation, but in the load framework the photon was not in a path-definite state. There is no fact of the matter about the path during propagation. The detector choice at detection determines what question can meaningfully be asked — not what happened in the past. Keywords: delayed-choice experiment; Wheeler; retrocausality; information processing load; wave function collapse; quantum eraser; cosmic photon; weak measurement; n < n*; no consciousness required
Bohr is said to have remarked that anybody who is not shocked by quantum theory has not understood it. No physicist, however, challenges its practical usefulness in predicting experimental results and providing an explanatory framework for an enormous range of study. Nevertheless, more thoughtful physicists have been astonished and disturbed by what is implied when attempts are made to interpret its meaning. As a result a variety of ingenious, and sometimes bizarre, schemes have been devised since its inception in order to make these implications more acceptable to the scientific mind. Some of these schemes have resulted in proposals which appear stranger than the problems they have been designed to resolve. After some sixty years there is no consensus of opinion upon the solution of the epistemological problems posed. Indeed the debate keeps on resurfacing in the scientific literature and continues to generate library shelves of books. Some progress, however, has been made and the late John Bell himself and the experimental work of Alain Aspect have demonstrated that some epistemological positions cannot be sustained.