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Terry s Archive Online

Light Cones as Quantum Phenomena

Terry B. Bollinger

Published Nov 9, 2021• 0 citations
Publisher

Abstract

Quantum erasure experiments push the boundary between the quantum and classical worlds by letting delayed events influence the state of previously recorded and potentially widely distributed classical information. The only significant restriction to such unsettling violations of forward-only causality is that the distribution of forward-dependent information cannot cross out of the light cone boundaries of the event in the past, a feature that ensures no violations of causality — no rewriting of anyone else's recorded histories — can occur. The erasure interpretation of this conundrum requires rewriting of information recorded and distributed in the past, which would itself be a violation of causality. The quantum predestination interpretation removes the causal rewriting issue. However, quantum predestination requires detailed coordination of inputs from outside of the forward-dependent event's light cone, thus massively violating the same limit that prevents causality violations in such events. Yet another approach is to invoke the Schrödinger's cat variant of quantum erasure in which arbitrarily complex classical events within the light cone become quantum dependent upon the future event. As with all Schrödinger's cat interpretations of quantum mechanics, this variant of quantum erasure violates causality by discarding local classical histories such as the information-rich state of the cat's body. The most straightforward interpretation of erasure experiments is to follow the lead of the equations themselves, which transform on paper as if their components are independent of ordinary space and time limits, up to the limits imposed on them by the speed of light. Interpreting the light cone of each quantum system as an atemporal, aspatial unit in which classical time and space have no meaning results in a multi-scale, matter-dependent definition of spacetime in which every light cone is a singular quantum entity. In such a universe, both time and space are defined not as pre-existing, mass-independent continuums but as the consensus of vast numbers of constantly interacting and mutually limiting quantum-entity light cones.

DOI
10.48034/20211109

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Quantum erasure experiments push the boundary between the quantum and classical worlds by letting delayed events influence the state of previously recorded and potentially widely distributed classical information. The only significant restriction to such unsettling violations of forward-only causality is that the distribution of forward-dependent information cannot cross out of the light cone boundaries of the event in the past, a feature that ensures no violations of causality — no rewriting of anyone else's recorded histories — can occur. The erasure interpretation of this conundrum requires rewriting of information recorded and distributed in the past, which would itself be a violation of causality. The quantum predestination interpretation removes the causal rewriting issue. However, quantum predestination requires detailed coordination of inputs from outside of the forward-dependent event's light cone, thus massively violating the same limit that prevents causality violations in such events. Yet another approach is to invoke the Schrödinger's cat variant of quantum erasure in which arbitrarily complex classical events within the light cone become quantum dependent upon the future event. As with all Schrödinger's cat interpretations of quantum mechanics, this variant of quantum erasure violates causality by discarding local classical histories such as the information-rich state of the cat's body. The most straightforward interpretation of erasure experiments is to follow the lead of the equations themselves, which transform on paper as if their components are independent of ordinary space and time limits, up to the limits imposed on them by the speed of light. Interpreting the light cone of each quantum system as an atemporal, aspatial unit in which classical time and space have no meaning results in a multi-scale, matter-dependent definition of spacetime in which every light cone is a singular quantum entity. In such a universe, both time and space are defined not as pre-existing, mass-independent continuums but as the consensus of vast numbers of constantly interacting and mutually limiting quantum-entity light cones.
As indicated by the name "quantum erasure," the most common interpretation of certain classes of delayed choice quantum experiments is that they, in some fashion, erase or undo past decisions. Unfortunately, this interpretation cannot be correct since the past decisions were already classically and irreversibly captured as recorded information or datums. A datum is information that, through temporal entanglement, constrains future events. The correct interpretation of such experiments is stranger than erasure: Recordings made early in such quantum experiments predestine choices made later through arbitrarily complex and often human-scale classical choices. Since this process of quantum predestination occurs only within the future light cone of datum creation, another (possibly) less radical way to interpret such experiments is that time is multiscale, granular, and impossible to define outside of the quantum state of the entities involved. The continuum time abstraction is not compatible with this view.
Causality has never gained the status of a 'law' or 'principle' in physics. Some recent literature has even popularized the false idea that causality is a notion that should be banned from theory. Such misconception relies on an alleged universality of the reversibility of the laws of physics, based either on the determinism of classical theory, or on the multiverse interpretation of quantum theory, in both cases motivated by mere interpretational requirements for realism of the theory. Here, I will show that a properly defined unambiguous notion of causality is a theorem of quantum theory, which is also a falsifiable proposition of the theory. Such a notion of causality appeared in the literature within the framework of operational probabilistic theories. It is a genuinely theoretical notion, corresponding to establishing a definite partial order among events, in the same way as we do by using the future causal cone on Minkowski space. The notion of causality is logically completely independent of the misidentified concept of 'determinism', and, being a consequence of quantum theory, is ubiquitous in physics. In addition, as classical theory can be regarded as a restriction of quantum theory, causality holds also in the classical case, although the determinism of the theory trivializes it. I then conclude by arguing that causality naturally establishes an arrow of time. This implies that the scenario of the 'block Universe' and the connected 'past hypothesis' are incompatible with causality, and thus with quantum theory: they are both doomed to remain mere interpretations and, as such, are not falsifiable, similar to the hypothesis of 'super-determinism'.This article is part of a discussion meeting issue 'Foundations of quantum mechanics and their impact on contemporary society'.
In general relativity, the causal structure between events is dynamical, but it is definite and observer-independent; events are point-like and the membership of an event A in the future or past light-cone of an event B is an observer-independent statement. When events are defined with respect to quantum systems however, nothing guarantees that the causal relationship between A and B is definite. We propose to associate a causal reference frame corresponding to each event, which can be interpreted as an observer-dependent time according to which an observer describes the evolution of quantum systems. In the causal reference frame of one event, this particular event is always localised, but other events can be ‘smeared out’ in the future and in the past. We do not impose a predefined causal order between the events, but only require that descriptions from different reference frames obey a global consistency condition. We show that our new formalism is equivalent to the pure process matrix formalism (Araújo et al 2017 Quantum 1 10). The latter is known to predict certain multipartite correlations, which are incompatible with the assumption of a causal ordering of the events—these correlations violate causal inequalities. We show how the causal reference frame description can be used to gain insight into the question of realisability of such strongly non-causal processes in laboratory experiments. As another application, we use causal reference frames to revisit a thought experiment Zych et al (arXiv: 1708.00248 ) where the gravitational time dilation due to a massive object in a quantum superposition of positions leads to a superposition of the causal ordering of two events.
In recent years decoherence has been widely discussed and accepted as a mechanism responsible for the emergence of classicality in quantum measurements and the absence in the real world of Schrödinger-cat-like states [ 2 , 6 , 7 , 11 , 12 ]. Let me start with recalling the Schrödinger cat paradox. It was a hypothetical experiment proposed by Schrödinger in 1935 which can be briefly described as follows. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
This paper examines the possibility that past and future events are fundamentally alike in their ontological openness, while differing in their causal accessibility. The conventional distinction between a fixed past and an open future becomes problematic in light of the relativity of simultaneity, which provides no universal present at which openness could turn into ontological definiteness. As an alternative to the fully determined history of a block universe, the paper considers a symmetric conception in which neither past nor future is necessarily ontologically invariant. The proposed distinction separates ontological openness from causal accessibility. While past events cannot be influenced from our present position, parts of the causal future remain accessible to intervention. This asymmetry is compatible with the invariant causal structure of relativistic spacetime and does not require a universal transition from openness to definiteness. Schrödinger's cat provides a further illustration. If the cat is found dead and already in rigor mortis, the observed traces indicate an earlier death. Nevertheless, this does not by itself establish when the corresponding history became ontologically definite. The paper explores the possibility that a later measurement may be associated with the ontological articulation of an earlier, causally ordered history, without requiring backward causation. The argument is situated in relation to relational quantum mechanics, Wheeler's delayed-choice experiments, and philosophical discussions of retrocausality. The paper does not claim to prove an ontologically open past. Rather, it develops this possibility as a serious philosophical alternative to an invariant block universe, distinguishing physical evidence from ontological interpretation and clarifying the conceptual limits of what can be established.