  Quantum entanglement and relativistic causality are key concepts in
theoretical works seeking to unify quantum mechanics and gravity. In this
article, we show that the interplay between relativity theory and quantum
entanglement has intriguing consequences for the spacetime surrounding
elementary particles with spin. Classical and quantum gravity theories predict
that a spin-generated magnetic dipole field causes a (slight) bending to the
spacetime around particles, breaking its spherical symmetry. Motivated by the
apparent break of spherical symmetry, we propose a very general gedanken
experiment that does not rely on any specific theory of classical or quantum
gravity, and analyze this gedanken experiment in the context of quantum
information. We show that any spin-related deviation from spherical symmetry
would violate relativistic causality. To avoid the violation of causality, the
measurable spacetime around the particle's rest frame must remain spherically
symmetric, potentially as a back-action by the act of measurement. This way,
our gedanken experiment proves that there must be a censorship mechanism
preventing the possibility of spacetime-based spin detection, which sheds new
light on the interface between quantum mechanics and gravity. We emphasize that
our proposed gedanken experiment is independent of any theory and by allowing
spacetime to be quantized its purpose is to be used for testing present and
future candidate theories of quantum gravity.
