Quantum particles tunnel through classically forbidden barriers. T = 1/(1 + V²sinh²(κa)/(4E(V−E))). WKB: T ≈ exp(−2κa). Applications: alpha decay (Gamow 1928), proton fusion in the Sun (without tunneling the Sun wouldn't shine), scanning tunneling microscope (STM), tunnel diodes. The Sun's Coulomb barrier is ~500 keV but mean thermal energy ~1 keV — tunneling via the Gamow peak at ~20 keV makes stellar fusion possible.
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