An energy–time uncertainty relation like Δ Δ E Δ Δ t ≳ ≳ ℏ ℏ / 2 , has a long, controversial history; the meaning of Δ Δ t and Δ Δ E varies and different formulations have different arenas of validity. However, one well-known application is both well established and experimentally verified: the connection between the life-time of a resonance state, τ τ 1 / 2 and its energy width Δ Δ E : τ τ 1 / 2 Δ Δ E = π π ℏ ℏ / 4. In particle-physics, widths from experimental fits to the Breit–Wigner energy distribution are used to characterize the lifetime of quasi-stable or decaying states.