📜 Module 3 · Lesson 3/3

👻Quantum Tunnelling

Walking through walls — the reason the Sun shines.

⏱️ 15 minStart

🎯 By the end you will…

✓Describe what tunnelling is
✓Know how barrier width and height change the odds
✓Name real technologies that rely on it

Roll a ball at a hill without enough speed and it rolls back. Always. But a quantum particle is a wave, and a wave doesn't stop dead at a wall — it fades away inside it. If the wall is thin enough, a little of the wave is still alive on the other side. That means there's a real chance the particle simply appears beyond the wall. This is quantum tunnelling.

🎛️Interactive experiment

Launch a wave packet at the barrier. Make the barrier thinner or lower and watch more of the wave leak through.

🧮Equation, decoded

Tunnelling probability (rule of thumb for thick walls)

T \approx e^{- 2 κ L}

What each symbol means

$T$ The chance of getting through
$e^{\dots}$ Exponential: the chance shrinks *very* fast
$L$ Thickness of the wall
$κ$ “kappa”: how much taller the wall is than the particle's energy

📖 Say it like a story

Each extra slice of wall multiplies the chance by the same small factor. Double the thickness and you don't halve the odds — you square them. That's why tunnelling happens across a billionth of a metre but never through your bedroom wall.

🌍Where you meet it

☀️

The Sun

Protons tunnel close enough to fuse. Without tunnelling, the Sun would be far too cool to shine.

🔍

Tunnelling microscope

The STM (1981, Nobel 1986) images single atoms by measuring tunnelling current.

💾

Flash memory

USB sticks and SSDs store bits by tunnelling electrons through thin insulators.

✅ Check your understanding

Answer all questions to complete the lesson and earn XP.

1. What makes tunnelling more likely?
2. Which natural process depends on tunnelling?
3. What did the 2025 Nobel Prize show?

Tunnelling probability (rule of thumb for thick walls)

✅ Check your understanding

📚 Sources & further reading