Short Answer
p_mu p^mu = -(mc)^2 (sign convention dependent) is the best answer.
Relativity questions test whether space, time, energy, and simultaneity are being treated classically when they should not be. Start by deciding whether the question is special relativity, general relativity, or an observational consequence such as GPS timing.
The four-momentum norm is invariant and equals -(mc)^2 or +(mc)^2 depending on metric signature.
Why This Answer Is Correct
This is a Hard-level question in Relativity. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Most relativity mistakes come from mixing Newtonian intuition with relativistic invariants such as spacetime interval, proper time, or rest energy.
Choices At A Glance
- A. E^2 - p^2 = m^2
- B. p_mu p^mu = -(mc)^2 (sign convention dependent)
- C. p_mu p^mu = 0 for every particle
- D. Energy is not part of four-momentum
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Study Next
Review the related formula library
review the theoretical physics hub
Topic Snapshot
Topic: Relativity
Difficulty: Hard
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.