Quantum mechanics implies the existence of correlations which cannot be described by any local theory. These nonlocal correlations possibly suggest the existence of particles traveling faster than light (tachyons). But the potential paradoxes associated with their existence imply that, even i f these tachyons do exist, it should not be possible to control them in a manner necessary for human superluminal (faster than light) communication. Indeed, the unitary nature of the time development operator precludes the possibility of using non-local correlations to communicate superluminally within the quantum formalism. A simplified model which preserves unitary time development i s shown to be useful in studying 'thought experiments' which appear init ially to allow for superluminal communication. The consistency of quantum mechanics is questioned when measurements of retarded fields are considered. I f certain idealized measurements are possible in principle, then the quantum formalism appears inconsistent. I f the measurements are not possible in principle, then 'state reduct ion' (non-unitary time evolution of the state operator) in a pragmatic sense can be said to have occurred. The suggestion that tachyons be used to explain the non-local correlations i s abandoned in favor of another model whose philosophy is supported by the tenets of relativity theory. Such a model may imply the existence of a more general quantum theory where a quantum system itself can be taken as a (generalized) frame of reference.
Quantum mechanics implies the existence of correlations which cannot be described by any local theory. These nonlocal correlations possibly suggest the existence of particles traveling faster than light (tachyons). But the potential paradoxes associated with their existence imply that, even i f these tachyons do exist, it should not be possible to control them in a manner necessary for human superluminal (faster than light) communication. Indeed, the unitary nature of the time development operator precludes the possibility of using non-local correlations to communicate superluminally within the quantum formalism. A simplified model which preserves unitary time development i s shown to be useful in studying 'thought experiments' which appear init ially to allow for superluminal communication. The consistency of quantum mechanics is questioned when measurements of retarded fields are considered. I f certain idealized measurements are possible in principle, then the quantum formalism appears inconsistent. I f the measurements are not possible in principle, then 'state reduct ion' (non-unitary time evolution of the state operator) in a pragmatic sense can be said to have occurred. The suggestion that tachyons be used to explain the non-local correlations i s abandoned in favor of another model whose philosophy is supported by the tenets of relativity theory. Such a model may imply the existence of a more general quantum theory where a quantum system itself can be taken as a (generalized) frame of reference.
As shown in the famous EPR paper (Einstein, Podolsky e Rosen, 1935), Quantum Mechanics is non-local. The Bell theorem and the experiments by Aspect and many others, ruled out the possibility of explaining quantum correlations between entangled particles using local hidden variables models (except for implausible combinations of loopholes). Some authors (Bell, Eberhard, Bohm and Hiley) suggested that quantum correlations could be due to superluminal communications (tachyons) that propagate isotropically with velocity v t > c in a preferred reference frame. For finite values of v t , Quantum Mechanics and superluminal models lead to different predictions. Some years ago a Geneva group and our group did experiments on entangled photons to evidence possible discrepancies between experimental results and quantum predictions. Since no discrepancy was found, these experiments established only lower bounds for the possible tachyon velocities v t . Here we propose an improved experiment that should lead us to explore a much larger range of possible tachyon velocities V t for any possible direction of velocity of the tachyons preferred frame.
Two statements in physics are frequently conflated: that nothing can travel faster than light, and that two entangled particles behave as if "instantaneously" connected regardless of their separation. This paper addresses both questions together: whether a faster-than-light particle could exist and, if so, whether it would count as matter; and whether quantum entanglement actually violates the light-speed limit. Starting from the energy–velocity relation of special relativity, it is shown that massive particles cannot reach the speed of light, and the properties of hypothetical tachyons (imaginary mass, absence of a rest frame and violation of causality) are examined. Using the singlet state, the non-separability, reduced state and correlation properties of entanglement are derived, and the quantum violation of the CHSH inequality up to 2√2 is computed. Finally, via the no-signaling theorem, it is shown that entanglement produces non-local correlations without permitting superluminal communication. In conclusion, nature accommodates non-local correlations without allowing any object to travel faster than light.
Abstract Particles cannot travel faster than the speed of light, nor can information. Nonetheless, this assumption has been frequently questioned over the years. This includes radical proposals like tachyons that can be observed in particle accelerators to more subtle ideas like studying superluminal causation to find classical explanations for quantum correlations. Most recently, it has been argued (Dragan and Ekert 2020 New J. Phys. 22 033038) that in a world with superluminal observers local determinism is impossible, linking the two pillars of physics-quantum theory and relativity-suggesting that the latter serves as the foundation for the former. Motivated by this approach, in this work, we extend the framework of quantum reference frames to incorporate superluminal Lorentz transformations. We apply this conceptual result to examine an apparent paradox where particles acquire negative energies after undergoing a superluminal Lorentz boost and propose a resolution within our framework. A consequence of this resolution is that different observers no longer agree whether a particle is incoming or outgoing or in a superposition of the two. We further find a similar apparent paradox for the entropy and/or temperature under superluminal Lorentz boosts and resolve it in an analogous fashion. Finally, we discuss Bell experiments under superluminal quantum reference frame transformations, showing that the involved probabilities still remain conserved. By applying the toolbox of quantum reference frames, we start charting an unexplored territory of superpositions between sub-and-superluminal quantum observers which gives rise to new conceptual questions.
It may well be that a relativistic version of the theory, while Lorentz invariant and local at the observational level, may be necessarily non-local and with a preferred frame (or aether) at the fundamental level. – John Bell [1] Introduction Bell's theorem establishes a contradiction between locality (and a few other assumptions) and certain predictions of quantum mechanics [2]. There have been two main issues surrounding the deep implications of this celebrated theorem. The first issue is basic, and it concerns whether the theorem really establishes that nonlocality is a necessary feature of any empirically viable theory and hence a feature of nature itself. One aspect of the issue concerns exactly what the underlying assumptions of Bell's theorem are. For example, some argue that these assumptions include the assumption of counterfactual definiteness [3–5], while others disagree [6–8]. The other aspect of the issue concerns which assumption should be dropped due to the resulting contradiction. Some believe that standard quantum mechanics may avoid nonlocality by denying counterfactual definiteness [3–5]. The second issue is much deeper, and it concerns how to make sense of the strange nonlocality when assuming that Bell's theorem establishes that our world is nonlocal. In particular, it has yet to be determined whether such quantum nonlocality is compatible with the theory of relativity, e.g., whether the nonlocality requires the existence of a preferred Lorentz frame [7, 9, 10]. It seems that “there is a preferred frame of reference, and in this preferred frame of reference things do go faster than light … Behind the apparent Lorentz invariance of the phenomena, there is a deeper level which is not Lorentz invariant” [11]. But it also seems possible that the existence of a preferred Lorentz frame is an illusion, since it cannot be detected according to standard quantum mechanics. A more subtle issue is whether quantum nonlocality permits superluminal signaling. It is widely thought that the answers to these questions will lead to a deeper understanding of Bell's theorem and quantum nonlocality. In this paper, we will present a new analysis of quantum nonlocality and its apparent incompatibility with relativity.
This work presents a conceptual and semi-formal analysis of quantum entanglement, locality, and determinism, motivated by a thought experiment involving double-slit measurements performed on entangled particles at spacelike separation. While quantum correlations appear instantaneous, the paper demonstrates that no superluminal information transfer or causal influence is required to explain observed outcomes.Building upon the Einstein–Podolsky–Rosen framework and Bell’s theorem, the study distinguishes between local dynamical processes and nonlocal informational correlations encoded in the global quantum state. It argues that measurement outcomes are locally realized yet globally constrained, leading to an effectively deterministic universal description while preserving epistemic uncertainty for observers.The paper reframes quantum nonlocality as a manifestation of global consistency rather than physical influence, offering insights into the relationship between realism, free will, and relativistic causality. The discussion is interpretational in nature and does not introduce new experimental claims, aiming instead to clarify foundational assumptions underlying quantum theory.This work is intended for researchers and students interested in quantum foundations, the philosophy of physics, and the conceptual structure of modern physical theories.
Causal quantum theory is an umbrella term for ordinary quantum theory modified by two hypotheses: state vector reduction is a well-defined process, and strict local causality applies. The first of these holds in some versions of Copenhagen quantum theory and need not necessarily imply practically testable deviations from ordinary quantum theory. The second implies that measurement events which are spacelike separated have no nonlocal correlations. To test this prediction, which sharply differs from standard quantum theory, requires a precise definition of state vector reduction. Formally speaking, any precise version of causal quantum theory defines a local hidden variable theory. However, causal quantum theory is most naturally seen as a variant of standard quantum theory. For that reason it seems a more serious rival to standard quantum theory than local hidden variable models relying on the locality or detector efficiency loopholes. Some plausible versions of causal quantum theory are not refuted by any Bell experiments to date, nor is it evident that they are inconsistent with other experiments. They evade refutation via a neglected loophole in Bell experiments---the collapse locality loophole---which exists because of the possible time lag between a particle entering a measurement device and a collapse taking place. Fairly definitive tests of causal versus standard quantum theory could be made by observing entangled particles separated by $\ensuremath{\approx}0.1$ light seconds.