Demonstrations of quantum entanglement which confirm the violation of Bell's inequality indicate that under certain conditions action at a distance is possible. This consequence seems to contradict the relativistic principle of causality, which asserts that an effect never precedes its cause, in any reference frame. By analyzing a numerical example of Bell's experiment with entangled pairs of photons, we show how observers in two inertial reference frames can disagree about the causality relation between two events. One observer claims that event 1 is the cause of event 2, while the other claims that event 1 is the result of event 2. The solution we suggest to the paradox is that in entangled systems, one can find pairs of "entangled events" which have symmetrical causality relations. Each of the events can serve as a cause or as an effect, depending on the frame of reference in which they are observed.
Demonstrations of quantum entanglement which confirm the violation of Bell's inequality indicate that under certain conditions action at a distance is possible. This consequence seems to contradict the relativistic principle of causality, which asserts that an effect never precedes its cause, in any reference frame. By analyzing a numerical example of Bell's experiment with entangled pairs of photons, we show how observers in two inertial reference frames can disagree about the causality relation between two events. One observer claims that event 1 is the cause of event 2, while the other claims that event 1 is the result of event 2. The solution we suggest to the paradox is that in entangled systems, one can find pairs of "entangled events" which have symmetrical causality relations. Each of the events can serve as a cause or as an effect, depending on the frame of reference in which they are observed.
Demonstrations of quantum entanglement (QE), which confirm the violation of Bell’s inequality, indicate that under certain conditions action at a distance is possible. This consequence seems to contradict the relativistic principle of causality, which asserts that an effect never precedes its cause, in any reference frame. By analyzing a numerical example of Bell’s experiment with entangled pairs of photons, we show how observers in two inertial reference frames can disagree about the causality relation between two events. One observer claims that event 1 is the cause of event 2, while the other claims that event 1 is the result of event 2. The solution we suggest to the paradox is that in entangled systems, one can find pairs of “entangled events” which have symmetrical causality relations. Each of the events can serve as a cause or as an effect, depending on the frame of reference in which they are observed.
One of the most fundamental phenomena of quantum physics is entanglement. It describes an inseparable connection between quantum systems, and properties thereof. In a quantum mechanical description even systems far apart from each other can share a common state. This entanglement of the subsystems, although arising from mathematical principles, is no mere abstract concept, but can be tested in experiment, and be utilized in modern quantum information theory procedures, such as quantum teleportation. In particular, entangled states play a crucial role in testing our understanding of reality, by violating Bell inequalities. While the role of entanglement is well studied in the realm of nonrelativistic quantum mechanics, its significance in a relativistic quantum theory is a relatively new field of interest. In this work the consequences of a relativistic description of quantum entanglement are discussed. We analyze the representations of the symmetry groups of special relativity, i.e. of the Lorentz group, and the Poincaré group, on the Hilbert space of states. We describe how unitary, irreducible representations of the Poincaré group for massive spin 1/2 particles are constructed from representations of Wigner's little group. We then proceed to investigate the role of the Wigner rotations in the transformation of quantum states under a change of inertial reference frame. Considering different partitions of the Hilbert space of 2 particles, we find that the entanglement of the quantum states appears different in different inertial frames, depending on the form of the states, the chosen inertial frames, and the particular choice of partition. It is explained, how, despite of this, the maximally possible violation of Bell inequalities is frame independent, when using appropriate spin observables, which are related to the Pauli-Ljubanski vector, a Casimir operator of the Poincaré group.
*Special Relativity places severe constraints on any possibility for faster-than-light (FTL) phenomenon. There are four hopes for FTL; warp drive via General Relativity Theory (GRT), wormholes via GRT, tachyons, and quantum mechanics. This talk focuses on why quantum mechanics makes the list. The nonlocal nature of matter demonstrates correlations between distant events that cannot be explained by current concepts of causality. For two classical events to be correlated, either one causes the other, or both possess a common cause in their past light cones. Neither kind of causality can explain the correlations observed in entangled quantum systems. Reality seems to support a third mysterious type of causality, with an essential spacelike nature, but direct spacelike causality should be impossible as it would lead to temporal paradox. The two concepts of classical causality, timelike causes and common cause, are presented along with a review of the EPR experiment showing that something like spacelike causality exists between the distant parts of an entangled quantum system. Bell’s Inequality shows that the EPR correlations cannot be attributed to common causes. Gisin’s EPR-Relativity experiment shows that timelike causes cannot explain the EPR correlations either. The metaphor of Quantum Tic-Tac-Toe is used to show a third type of causality applicable only to quantum situations. This third type of causality is an indirect spacelike cause acting through multiple classical realities, permitting FTL but prohibiting temporal paradox. I. Introduction HIS paper considers the issue of causality in entangled quantum systems. The Background section reviews the two classical notions of causality, EPR type entanglements, and Special Relativity. The Common Cause section reviews Bell’s Inequality theorem and shows how this type of causality cannot explain the correlations seen in entangled systems. The Einsteinian Causality section reviews Gisin’s experiment demonstrating that time-like causality cannot explain these correlations either. Gisin’s conclusion is that quantum systems support a new mysterious 3 rd type of causality. The Quantum Causality section introduces a situation in Quantum Tic-Tac-Toe that exhibits this same kind of causality
A still widely debated question in the field of relativistic quantum information is whether entanglement and the degree of violation of Bell's inequalities for massive relativistic particles are frame independent or not. At the core of this question is the effect that spin gets entangled with the momentum degree of freedom at relativistic velocities. Here, we show that Bell's inequalities for a pair of particles can be maximally violated in a special-relativistic regime, even without any postselection of the momentum of the particles. To this end, we use the methodology of quantum reference frames, which allows us to transform the problem to the rest frame of a particle, whose state can be in a superposition of relativistic momenta from the viewpoint of the laboratory frame. We show that, when the relative motion of two particles is noncollinear, the optimal measurements for violation of Bell's inequalities in the laboratory frame involve "coherent Wigner rotations." Moreover, the degree of violation of Bell's inequalities is independent of the choice of the quantum reference frame. Our results open up the possibility of extending entanglement-based quantum communication protocols to relativistic regimes.
The relation between quantum theory and special relativity is peculiar. On the one hand it is close and essential. Steven Weinberg [1], for example, takes the position that the standard model is an inevitable consequence of the marriage of the two theories. On the other, the action at a distance implied by the famous paper by Einstein, Podolsky, and Rosen [2] is certainly contrary to the implications of special relativity in spite of the fact that it does not allow transmission of information. The setting of quantum theory is a Hilbert spaceH and a path inH whose development in time is governed by a one parameter group of unitary transformations while the setting for special relativity can be thought of as Minkowski space with the time development not absolute, but dependent on the inertial system of the observer. The problem is to show how these two very different viewpoints can be reconciled in a way that includes an explanation of the "action at a distance" conundrum.
Relativistic bipartite entangled quantum states is studied to show that Nature doesn't favor nonlocality for massive particles in the ultra-relativistic limit. We found that to an observer (Bob) in a moving frame S', the entangled Bell state shared by Alice and Bob appears as the superposition of the Bell bases in the frame S' due to the requirement of the special relativity. It is shown that the entangled pair satisfies the Bell's inequality when the boost speed approaches the speed of light, thus providing a counter example for nonlocality of Einstein-Podolsky-Rosen(EPR) paradox.