(b) The squared magnitude of a particle's four-momentum could be negative.

Question

Quizplus · Accepted Answer

The squared magnitude of a particle's four-momentum is equal to the square of its mass times the square of the speed of light, which cannot be negative. This quantity is invariant and always non-negative, reflecting the particle's rest mass energy. 
(b) The squared magnitude of a particle's four-momentum is its mass squared by definition. This squared mass must be positive.

(B) the Squared Magnitude of a Particle's Four-Momentum Could Be