Quantum Tunnelling Is Happening Inside You Right Now

The Most Important Thing You’ve Never Heard Of

I’m going to make a bold claim: quantum tunnelling is the single most consequential quantum effect in your daily life. Not entanglement (fascinating, but it doesn’t make your toast). Not superposition (Schrödinger’s cat is a thought experiment, not a kitchen appliance). Tunnelling.

Without quantum tunnelling, the Sun wouldn’t fuse hydrogen and you wouldn’t be alive. Your flash drive wouldn’t store data. Radioactive decay would work differently — or not at all in some cases. The scanning tunnelling microscope, which images individual atoms, relies on it explicitly. Even some of the chemical reactions happening in your cells right now involve protons tunnelling through energy barriers they classically shouldn’t be able to cross.

And yet it gets way less press than entanglement or superposition. Probably because it’s harder to make spooky.

The Classical Picture (And Why It’s Wrong)

In classical mechanics, energy barriers are absolute. Imagine rolling a ball toward a hill. If the ball has enough kinetic energy to reach the top, it goes over. If it doesn’t, it bounces back. There’s a clear threshold, and it’s non-negotiable. You can push the ball a million times at just below the threshold energy, and it will bounce back every single time.

Quantum mechanics says: well, actually.

A quantum particle approaching an energy barrier doesn’t behave like a ball. It behaves like a wave. The wave function — which describes the probability of finding the particle at each location — doesn’t just stop when it hits a barrier. It decays exponentially inside the barrier, getting weaker the deeper it goes. But if the barrier is thin enough, the wave function is still non-zero on the other side. There’s a finite probability that the particle just… appears over there.

No extra energy. No secret passage. The particle simply has a probability of being found on the far side of something it shouldn’t be able to cross. The probability depends on the barrier height above the particle’s energy, the barrier width, and the particle’s mass. Higher barriers, wider barriers, and heavier particles all reduce the tunnelling probability exponentially.

That exponential dependence is why tunnelling matters for electrons and protons but not for baseballs. A tennis ball has a tunnelling probability through a net that is so fantastically small — something like 10^(−10^30) — that it will never happen in the lifetime of the universe. An electron through a nanometre-wide barrier? Happens all the time.

The Sun Runs on Tunnelling

This is the big one. The Sun fuses hydrogen into helium in its core, releasing the energy that sustains all life on Earth. But the core temperature — about 15 million kelvin — is not actually hot enough for protons to overcome their electrostatic repulsion classically.

Two protons approaching each other are both positively charged. They repel via the Coulomb force. To get close enough for the strong nuclear force to bind them (about 1 femtometre), they need to overcome an energy barrier of roughly 550 keV. But at 15 million kelvin, the average proton kinetic energy is only about 1.3 keV. Even the fastest protons in the thermal distribution — the ones in the far tail — rarely reach 10 keV. The vast majority of proton pairs don’t have nearly enough energy to overcome the barrier.

And yet fusion happens. Roughly 3.7 × 10³⁸ protons fuse per second in the Sun. Without tunnelling, the rate would be essentially zero — the Sun would be a cold ball of hydrogen.

What saves the day is quantum tunnelling. The protons’ wave functions extend through the Coulomb barrier. Even at energies far below the barrier height, there’s a small but non-zero probability of two protons finding themselves close enough for the strong force to grab hold. Multiply that tiny probability by the enormous number of proton-proton collisions per second in the core, and you get a fusion rate that matches what we observe.

Arthur Eddington predicted this in the 1920s, before quantum tunnelling was formally described. He knew the Sun wasn’t hot enough for classical fusion and suspected that some quantum effect must be helping. George Gamow worked out the tunnelling mathematics in 1928, and the explanation for stellar energy production fell into place.

Every star in the universe fuses hydrogen via tunnelling. Remove the effect, and the night sky goes dark.

Radioactive Decay: Escaping the Nucleus

Alpha decay — in which an atomic nucleus ejects a helium-4 nucleus (two protons and two neutrons) — was one of the first phenomena explained by quantum tunnelling, and it’s still one of the cleanest examples.

Inside a heavy nucleus like uranium-238, alpha particles are bound by the strong nuclear force but also feel the electrostatic repulsion of all those protons. There’s an energy barrier at the nuclear surface: the alpha particle is trapped in a potential well. Classically, if the alpha doesn’t have enough energy to clear the barrier, it stays put forever.

Gamow showed in 1928 that the alpha particle tunnels through this barrier. The probability of tunnelling depends exponentially on the barrier height and width, which depend on the nuclear charge and the alpha’s energy. Small differences in energy produce enormous differences in tunnelling probability — which is why radioactive half-lives vary from microseconds (high-energy alphas, thin barriers) to billions of years (low-energy alphas, thick barriers). Uranium-238 has a half-life of 4.5 billion years. Polonium-212 has a half-life of 0.3 microseconds. Same physics, different barrier geometry.

The Geiger-Nuttall law — an empirical relation between alpha energy and half-life discovered in 1911 — was a puzzle until Gamow explained it as a natural consequence of exponential tunnelling probabilities. The physics predicted the observations perfectly.

Flash Memory: Tunnelling by Design

Your USB stick, your phone’s storage, your SSD — they all use flash memory, and flash memory stores data using quantum tunnelling.

A flash memory cell consists of a transistor with a floating gate — a conducting layer surrounded by insulating oxide. To write data, a high voltage is applied that causes electrons to tunnel through the thin oxide barrier (~8 nm) onto the floating gate. Once the voltage is removed, the electrons are trapped — the oxide barrier is too thick for them to tunnel back out spontaneously. The trapped charge changes the transistor’s threshold voltage, which encodes a bit (or multiple bits, in modern multi-level cells).

To erase the data, a voltage is applied in the opposite direction, causing the electrons to tunnel back out. After many write-erase cycles (typically 3,000–100,000), the oxide layer degrades, which is why SSDs have a limited write lifespan. But the fundamental read-write mechanism is quantum tunnelling of electrons through an insulating barrier — pure quantum mechanics, engineered into a $20 consumer product.

There’s a certain irony in the fact that every photo on your phone, every saved document, every text message is preserved by a quantum effect that Einstein spent years arguing about. Quantum mechanics doesn’t care whether you believe in it. It stores your data regardless.

The Scanning Tunnelling Microscope

In 1981, Gerd Binnig and Heinrich Rohrer at IBM Zurich built the scanning tunnelling microscope (STM), which uses quantum tunnelling as its primary measurement mechanism. They won the Nobel Prize for it in 1986.

The STM brings an atomically sharp metal tip to within about 1 nanometre of a conducting surface. At this distance, electrons from the surface tunnel across the vacuum gap into the tip (or vice versa), producing a measurable current. The tunnelling current depends exponentially on the gap distance — a change of just 0.1 nm (roughly one atomic diameter) changes the current by about a factor of 10.

By scanning the tip across the surface and mapping the tunnelling current, the STM produces images with sub-atomic resolution. It was the first instrument capable of imaging individual atoms on a surface, and it opened the entire field of nanotechnology.

The exponential sensitivity is both the STM’s greatest strength and the reason it was so hard to build. The tip position must be controlled to a precision of about 0.01 nm — a hundredth of an atomic diameter. This is accomplished with piezoelectric actuators, in an environment isolated from vibrations, in vacuum. The engineering is extraordinary, but the physics principle is simple: tunnelling current depends exponentially on distance, so measure the current and you know the distance to atomic precision.

Tunnelling in Biology

This one is still being worked out, and some of it is controversial, but there’s growing evidence that quantum tunnelling plays a role in biological systems.

Enzymes — the protein catalysts that accelerate chemical reactions in living cells — sometimes work faster than classical transition state theory predicts. One proposed explanation is that hydrogen atoms (protons) involved in enzyme-catalysed reactions can tunnel through reaction barriers rather than going over them. The evidence comes from kinetic isotope effects: when you replace hydrogen with deuterium (which is heavier and tunnels less effectively), the reaction rate decreases more than classical theory predicts. This extra sensitivity to mass is a signature of tunnelling.

Proton tunnelling may also play a role in DNA mutations. The hydrogen bonds holding DNA base pairs together involve protons sitting in double-well potential energy landscapes. If a proton tunnels from one well to the other — switching from the normal position to a “tautomeric” position — it can cause a mismatch during DNA replication. This is speculative and the contribution to actual mutation rates is debated, but the physics is sound in principle.

Not Spooky, Just Quantum

Tunnelling doesn’t get the pop-science headlines that entanglement and superposition do. Nobody makes movies about it. There’s no cat in a box. But in terms of real, tangible, happening-right-now consequences — the Sun shining, your data persisting, radioactive isotopes decaying at the rates they do, enzymes catalysing the reactions that keep you alive — tunnelling might be the most consequential quantum effect there is.

It’s not magic and it doesn’t violate energy conservation. It’s just what happens when you take the wave nature of matter seriously. Particles aren’t hard little balls that either clear a hurdle or don’t. They’re probability amplitudes, and probability amplitudes leak through barriers. That leakage built the universe as we know it.