When causality breaks

Louis Osgood Kattsoff

Lev Vaidman A

We report results of an investigation of relativistic causality constraints on the measurability of nonlocal variables. We show that measurability of certain nondegenerate variables with entangled eigenstates contradicts the principle of causality, but that there are other, certainly nonlocal, variables which can be measured without breaking causality. We show that any causal measurement of nonlocal variables must erase certain local information. For example, for a system of two spin-1/2 particles, even if we take the weakest possible definition of verification measurement, verification of an entangled state must erase all local information.

Miklós Rédei, Iñaki San Pedro

Andreas Wichert

We redefine free will as the concept of identity in a deterministic universe. Causality is related to a meaningful explanation. For impossible explanations, causality does not exist, and the identity of the self-breaks. Only in meaningful causal worlds may personal identities exist. We are one person described by the meaningful path in the multiverse that is correlated with our free choices.

Xiao Yuan, Syed M. Assad, Jayne Thompson +6 more

Abstract In general relativity, closed timelike curves can break causality with remarkable and unsettling consequences. At the classical level, they induce causal paradoxes disturbing enough to motivate conjectures that explicitly prevent their existence. At the quantum level such problems can be resolved through the Deutschian formalism, however this induces radical benefits—from cloning unknown quantum states to solving problems intractable to quantum computers. Instinctively, one expects these benefits to vanish if causality is respected. Here we show that in harnessing entanglement, we can efficiently solve NP-complete problems and clone arbitrary quantum states—even when all time-travelling systems are completely isolated from the past. Thus, the many defining benefits of Deutschian closed timelike curves can still be harnessed, even when causality is preserved. Our results unveil a subtle interplay between entanglement and general relativity, and significantly improve the potential of probing the radical effects that may exist at the interface between relativity and quantum theory.

Joe Henson C