Weinberg-Witten theorem

The Weinberg-Witten theorem states that certain massless particles with specific properties cannot carry a conserved Lorentz-covariant energy-momentum or have a conserved Lorentz-covariant higher-spin current. In practical terms, it implies that massless particles with spin greater than one (like hypothetical particles beyond photons and gravitons) cannot have a well-defined, gauge-invariant energy or momentum if they are also to respect the principles of relativity. This theorem constrains theories of fundamental particles, indicating, for example, that composite theories cannot produce massless particles with high spins that carry energy in a straightforward, gauge-invariant way.