# Knowledge of Quantum Hidden Variables Enables Backwards-In-Time Signaling

## TL;DR

This paper presents a protocol showing that knowledge of hidden variables in quantum mechanics could enable backward-in-time signaling, challenging causality regardless of relativity, unless hidden variables are dynamic.

## Contribution

It introduces a novel protocol demonstrating backward-in-time signaling with hidden variables, highlighting potential conflicts with causality in quantum theories.

## Key findings

- Knowledge of hidden variables can enable backward-in-time communication.
- Bohmian mechanics may prohibit such signaling if hidden variables change values.
- The protocol challenges the assumption that hidden variables are static.

## Abstract

Bell's theorem implies that any completion of quantum mechanics which uses hidden variables (that is, preexisting values of all observables) must be nonlocal in the Einstein sense. This customarily indicates that knowledge of the hidden variables would permit superluminal communication. Such superluminal signaling, akin to the existence of a preferred reference frame, is to be expected. However, here we provide a protocol that allows an observer with knowledge of the hidden variables to communicate with her own causal past, without superluminal signaling. That is, such knowledge would contradict causality, irrespectively of the validity of relativity theory. Among the ways we propose for bypassing the paradox there is the possibility of hidden variables that change their values even when the state does not, and that means that signaling backwards in time is prohibited in Bohmian mechanics.