The classical rule, and how quantum mechanics breaks it

Imagine rolling a ball toward a hill. If the ball doesn’t have enough kinetic energy to reach the top, classical physics says it simply rolls back down — there’s no way it ends up on the other side. This is true for everyday objects without exception.

But for particles governed by quantum mechanics — electrons, protons, and other subatomic particles — the rules are different. A particle’s behavior is described by a wavefunction (covered in the Learn topic on quantum superposition), and that wavefunction doesn’t drop to exactly zero the instant it hits a barrier it doesn’t have enough energy to climb over. Instead, it decays exponentially inside the barrier — meaning there’s a small but genuinely nonzero chance the particle appears on the other side, without ever having the energy to go “over” it. This is quantum tunneling.

Why “tunneling” is a slightly misleading name

The particle doesn’t dig a tunnel or pass through some hidden path — it’s more accurate to say its wavefunction extends into and through the barrier, and there’s a real probability of detecting the particle on the far side if you measure. The narrower and lower the barrier, the higher that probability.

Where tunneling actually matters, not just theoretically

  • The Sun’s fusion: protons in the Sun’s core don’t individually have enough energy to overcome their mutual electric repulsion and fuse, according to classical physics alone — quantum tunneling is what allows fusion to happen at the rate it does, and is part of why stars shine at all.
  • Scanning tunneling microscopes: these instruments image individual atoms on a surface by measuring a tunneling current between a sharp tip and the surface, sensitive to distances smaller than the width of an atom.
  • Radioactive alpha decay: some unstable atomic nuclei emit particles via tunneling, escaping a nuclear “wall” they wouldn’t classically have enough energy to overcome.
  • Flash memory and some transistors: modern computer chips rely on tunneling effects, both as something to exploit and something to carefully manage as components shrink to atomic scales.

Why this matters

Quantum tunneling is a genuinely strange effect with enormous real-world consequences — it’s part of why the Sun burns, why some electronics work the way they do, and a clear demonstration that at small enough scales, “possible” and “impossible” aren’t as sharply separated as classical physics would have you believe.