Quantum Tunneling
Definition
Particles leaking through barriers they classically couldn't climb — the reason the Sun shines and flash drives remember.
Technical definition
The wavefunction decays exponentially inside the classically forbidden region but stays nonzero, giving transmission T ≈ e^(−2κL).
Example
The Sun's core is too cold for protons to classically overcome their repulsion — tunneling lets them fuse anyway, at a slow, steady, life-giving rate.
Common confusion
Tunneling particles don't 'dig through' the barrier or borrow energy inside it — the wavefunction simply extends through. No energy is gained or spent in transit.