Particles can pass through walls they shouldn't

In classical physics, a ball that can’t clear a wall simply bounces back. In the quantum world, particles behave as spread-out waves, and that wave doesn’t stop dead at a barrier — it leaks through. If the barrier is thin enough, there’s a real chance the particle appears on the other side. This is quantum tunnelling.

It isn’t a curiosity; it keeps the Sun shining. At the temperatures of the solar core, two protons can’t classically overcome their mutual electrostatic repulsion — they simply lack the energy to touch. Fusion happens anyway because the protons occasionally tunnel through that repulsive barrier. Tunnelling also cracked one of physics’ early puzzles: in 1928, George Gamow explained radioactive alpha decay as alpha particles tunnelling out of the nucleus, one of quantum mechanics’ first real triumphs.

The key to all of it is barrier thinness. Tunnelling probability falls off exponentially with width, so a barrier twice as thick is dramatically harder to cross. That ferocious sensitivity is exactly what makes the scanning tunnelling microscope (STM) work. Built by Gerd Binnig and Heinrich Rohrer at IBM Zürich in 1981, it sends electrons tunnelling across a tiny vacuum gap between a sharp tip and a surface.

Because the current changes so steeply with distance, the STM resolves features below 0.1 nanometre — fine enough to image and even reposition individual atoms. The invention won the 1986 Nobel Prize in Physics.

The same exponential dependence underlies the flash memory in your phone and the tunnel diode, devices engineered to coax electrons through barriers that classical physics would have sealed shut.