Although quantum theory and relativity are two of the monumental achievements in modern physics, their exact relationship remains somewhat uncertain. Indeed, while the nature of quantum theory is subjective at the fundamental level, relativity involves a rule that is invariant under observers’ different reference frames. This paper argues that quantum theory and relativity are connected in such a way that Alice’s and Bob’s choices of coordinate bases, which correspond to the apparatus outcome in quantum measurement, are related to Einstein’s field equation. That is, the phenomenon of gravity corresponds to a commonality among observers, thereby involving an intersubjective aspect, rather than being objective. This shared feature of intersubjectivity is also discussed in terms of entanglement and locality.
Although quantum theory and relativity are two of the monumental achievements in modern physics, their exact relationship remains somewhat uncertain. Indeed, while the nature of quantum theory is subjective at the fundamental level, relativity involves a rule that is invariant under observers’ different reference frames. This paper argues that quantum theory and relativity are connected in such a way that Alice’s and Bob’s choices of coordinate bases, which correspond to the apparatus outcome in quantum measurement, are related to Einstein’s field equation. That is, the phenomenon of gravity corresponds to a commonality among observers, thereby involving an intersubjective aspect, rather than being objective. This shared feature of intersubjectivity is also discussed in terms of entanglement and locality.
Abstract Quantum mechanics and relativity are two of the most successful theoretical frameworks in modern physics, yet their conceptual foundations remain difficult to reconcile at the deepest level. Quantum theory describes physical systems through states, observables, probabilities, measurement contexts, superposition, entanglement, and nonclassical correlations. Relativity describes physical processes through space-time structure, causal order, invariant physical law, observer-dependent decomposition of time and space, and constraints on signal propagation. This article examines the conceptual relationship between quantum measurement, relativistic causality, and space-time description. The purpose is not to present a mathematical unification but to analyze how the two frameworks organize physical explanation differently and where their interpretive tensions arise. Special attention is given to the measurement problem, the status of quantum states, the role of observers and reference frames, the meaning of locality, the distinction between correlation and causal influence, and the challenge of describing quantum processes without privileging a single global time. The article argues that many apparent conflicts between quantum theory and relativity result from importing classical intuitions into domains where they no longer apply. Quantum entanglement does not permit controllable faster-than-light communication, yet it challenges classical separability. Relativity forbids superluminal signaling, yet it does not require that all physically meaningful correlations be reducible to local classical properties. Measurement introduces another difficulty because standard quantum theory often describes state update in a way that appears instantaneous, while relativistic space-time does not provide a universal present. The discussion develops a conceptual framework in which quantum measurement is treated as a physical interaction, relativistic causality as a constraint on operational influence, and space-time description as an organizing structure for events rather than a simple background stage. This analysis is relevant for quantum field theory, quantum information, relativistic quantum foundations, and future approaches to quantum gravity.
Abstract The subject of quantum reference frame transformations gets popular lately with some interesting new theoretical development partly for the reason that the physics involved is becoming experimentally accessible. The position of a position eigenstate when observed from an object with ‘uncertainty’ in position would be seen with ‘uncertainty’. In fact, even the existence of entanglement is reference frame-dependent. We present an improved formulation of such a transformation and give a novel way to describe exactly by ‘how much’ the ‘value of the position’ has changed which fully encodes all information about the changes, including the ‘uncertainty’ and entanglement. That is an application of the notion of noncommutative values of physical quantities we introduced to understand the reality of quantum physics and beyond. Some implications on fundamental physics will also be discussed. In particular, we suggest thinking about quantum gravity as a theory of general quantum relativity, alleviating Penrose’s notion of incompatibility of qauntum mechanics with the relativity principle.
The quantization of gravity remains one of the most important---yet extremely elusive---challenges at the heart of modern physics. Any attempt to resolve this long-standing problem seems to be doomed, as the route to any direct empirical evidence (i.e., detecting gravitons) that sheds light on the quantum aspect of gravity is far beyond the current capabilities. Recently, it was discovered that gravitationally induced entanglement, tailored in the interferometric frameworks, can be used to witness the quantum nature of gravity. Even though these schemes offer promising tools for investigating quantum gravity, many fundamental and empirical aspects of the schemes are yet to be discovered. Considering the fact that, besides quantum entanglement, the quantum uncertainty and complementarity principles are the two other foundational aspects of quantum physics, the quantum nature of gravity needs to manifest all of these features. Here, we lay out an interferometric platform for testing these nonclassical aspects of quantum mechanics in the quantum gravity setting, which connects gravity and quantum physics in a broader and deeper context. As we show in this work, all of these fundamental features of quantum gravity can be framed and thoroughly analyzed in an interferometric scheme.
One of the most basic notions in physics is the partitioning of a system into subsystems and the study of correlations among its parts. In this Letter, we explore this notion in the context of quantum reference frame (QRF) covariance, in which this partitioning is subject to a symmetry constraint. We demonstrate that different reference frame perspectives induce different sets of subsystem observable algebras, which leads to a gauge-invariant, frame-dependent notion of subsystems and entanglement. We further demonstrate that subalgebras which commute before imposing the symmetry constraint can translate into noncommuting algebras in a given QRF perspective after symmetry imposition. Such a QRF perspective does not inherit the distinction between subsystems in terms of the corresponding tensor factorizability of the kinematical Hilbert space and observable algebra. Since the condition for this to occur is contingent on the choice of QRF, the notion of subsystem locality is frame dependent.
Whether the principles of general relativity extend to a quantum regime remains one of the open questions in modern physics. In classical general relativity, Einstein's equivalence principle underpins the interpretation of gravity as spacetime geometry. However, it is not known whether the principle remains valid when the source of gravity could be quantum. Here we show that gravity-induced entanglement (GIE) of quantum clocks provides a framework to characterize local position invariance (LPI) in such a regime, which constitutes one of the subprinciples of the equivalence principle. By adapting the existing formulation of quantum LPI to the context of GIE, we analyze two schemes that address complementary aspects of LPI and differ only in the choice of the initial clock state: one in which entanglement will be generated if and only if there is a genuinely quantum violation of LPI, and the other where classical-like LPI violation manifests in the frequency of entanglement oscillation. Our results suggest that GIE of quantum clocks offers an approach to investigating fundamental principles of general relativity in the quantum regime, shedding new light on the interplay between quantum mechanics and the theory of gravity.
Einstein's general theory of relativity dominates the science of the very large, including gravity and cosmology. Quantum theory reigns supreme on the scale of atoms and smaller. What is more natural than to try to unify these two theories into a single quantum theory of gravity? Modern theories of gravity, including superstring theory, give quantum properties to space and time. These theories have been inspired by Dirac's principles of beauty and elegance. But Dirac had the advantage of experimental evidence, and modern theories of quantum gravity do not. Moreover, beauty and elegance are subjective – they are a guide to the correct theory, not a guarantee. For any scientific theory, experiment is the final test.