Quantum Entanglement and Superluminal Particles: Preservation of Causality under Relativistic Constraints

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.