What Is Quantum Tunneling? Quantum Tunneling Explained Simply (Why the Sun Shines at All)

Quantum tunneling lets a particle cross an energy barrier that would stop it under classical physics. Its wave behaviour gives it a chance of crossing, which helps explain solar fusion, alpha decay, and several electronic devices.

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Why the Sun should not shine

The Sun's core reaches about 15 million degrees Celsius. The Sun formed about 4.6 billion years ago. Nuclear fusion, which joins light nuclei into heavier nuclei, supplies its long-lasting energy output. NASA describes these solar properties.

A nucleus forms the small central part of an atom. A hydrogen nucleus contains a proton, which carries positive electric charge. When two protons approach, their electric repulsion pushes them apart. They must approach extremely closely before nuclear reactions can occur.

Heat gives the protons motion, but their usual energies remain far below the electric barrier near nuclear distances. Classical physics predicts far too little fusion under these conditions. Harvard's stellar physics notes explain this energy mismatch.

The Sun therefore presents a puzzle for a purely classical model. Quantum tunneling supplies a missing part of the explanation. Another part involves the weak interaction, which can change a proton into a neutron.

Without tunneling, the present Sun could not sustain its normal fusion output. That does not mean a hot Sun would instantly become dark. Hot matter can still radiate stored energy. The question concerns how the Sun maintains its energy supply over billions of years.

The ball, the hill and the barrier

Imagine a ball rolling towards a hill without friction. Its kinetic energy, the energy of motion, decreases as it climbs. Its potential energy, the energy associated with its position, increases.

A slow ball stops before the top and rolls back. A faster ball can reach the top and continue down the other side. Energy conservation connects these changes: the ball cannot spend more energy than it has.

Physicists use an energy diagram to describe a similar problem for particles. Horizontal distance shows position. Vertical height shows potential energy. A raised part of the diagram marks a barrier, rather than a physical hill inside a chip. OpenStax explains the ball analogy and energy diagram.

Two magnets with their north poles facing provide another useful analogy. Pushing them closer requires work because they repel each other. Their magnetic interaction illustrates an invisible obstacle. The solar example instead involves electric repulsion.

A barrier can also come from the electronic properties of a real material. An insulating layer in a chip can present electrons with a high potential energy. Calling every barrier an invisible push simplifies the picture. The essential feature is the energy needed to cross the region classically.

For a small quantum particle, a finite barrier need not stop every attempt. An electron can have less energy than the barrier height and still reach the other side. It does not drill a hole or damage the material.

Tunneling also preserves energy. In the simple model with an unchanging barrier, a transmitted electron retains its total energy. Crossing does not require it to borrow extra energy briefly. MIT derives transmission through this kind of barrier.

Particles are also waves

In 1924, French physicist Louis de Broglie developed the idea that matter has wave properties. Electrons therefore need a description that goes beyond tiny moving balls. His Nobel biography records this work.

Experiments in 1927 supported that idea. Clinton Davisson and Lester Germer scattered electrons from a nickel crystal in the United States. George Paget Thomson studied transmission through thin films in Scotland. The results showed diffraction: patterns that depend on waves reinforcing or cancelling each other.

The crystal and thin films provided different ways to reveal electron waves. Neither experiment meant that electrons become ordinary water waves. De Broglie's Nobel lecture describes both experiments.

Quantum theory uses a wave function, a mathematical description that predicts possible measurement results. Its amplitude describes the wave's size. Squaring the magnitude of that amplitude gives probability density. This describes how likely a location measurement is within a small region.

A larger magnitude generally means a greater chance of finding the particle there. A zero value gives zero probability density there. Repeated measurements build a pattern, although each detection records a whole electron. OpenStax explains this probability rule.

Now consider a wave approaching an energy barrier. The solution can extend into the barrier instead of ending sharply at its edge. For a simple opaque barrier, its amplitude decreases rapidly with distance inside.

If the barrier extends far enough, very little amplitude reaches its far edge. Reflection then dominates. If the barrier is thin enough, appreciable amplitude can reach the far side and continue beyond it.

That transmitted part represents a chance of detecting the electron beyond the barrier. It does not represent a broken piece of an electron. The wave describes possible outcomes; the detector records one outcome. Feynman's lectures explain this distinction between amplitudes and detections.

No gap between atoms is necessary for tunneling. A smooth potential barrier can produce it. Particle mass, energy, and barrier shape matter more than the picture of a small ball squeezing through a crack.

Quantum tunneling experiment with light

A laboratory experiment with prisms makes the barrier idea easier to inspect. It uses frustrated total internal reflection, a classical wave analogy for quantum tunneling. It does not independently establish that electrons tunnel.

Light travelling through glass can reflect completely at its boundary with air. This requires an incidence angle above the critical angle. Physicists measure that angle from the normal, an imaginary line perpendicular to the surface.

The beam therefore approaches the surface at a sufficiently glancing angle. Optical fibres also use total internal reflection to guide light. OpenStax describes the angle rule and fibre application.

Total reflection does not mean that the electromagnetic field becomes zero immediately outside the glass. An evanescent field extends into the air and decreases rapidly with distance. Without another nearby material, it does not carry a travelling beam away across the gap.

Bring a second prism very close, and this field can couple into it. Light can then travel through the second prism. Increasing the gap strongly reduces the transmitted light. A laboratory clamp controls this small separation in the University of Colorado demonstration.

The relevant separation depends on wavelength, angle, and the materials. A visible-light wavelength is less than one thousandth of a millimetre. That gives the scale of the gap, rather than a universal cutoff where transmission suddenly vanishes.

Isaac Newton described an early version about three centuries ago. Modern researchers have repeated it with equipment that measures the field near the surface. Their study explains the experiment and its historical connection.

The useful comparison concerns waves that decay across a barrier region. Light supplies an electromagnetic field; an electron requires a quantum wave function. Similar equations connect the demonstrations, while the physical meanings of the waves differ.

The uranium puzzle: alpha decay

Some unstable nuclei release an alpha particle, a cluster of two protons and two neutrons. This process, alpha decay, changes the nucleus into a different element. It is one type of radioactivity, rather than an explanation for every radioactive process.

The attractive nuclear interaction helps hold the alpha cluster inside. Outside the nucleus, electric repulsion between positive charges creates the outer barrier. An escaping alpha can have less energy than the barrier's peak. Classical motion alone cannot explain its escape. OpenStax describes this nuclear barrier.

The decay times add another puzzle. Polonium-212 has a half-life of about 0.3 microseconds, or three tenths of a millionth of a second. Uranium-238 has a half-life of about 4.5 billion years. Evaluated polonium data and evaluated uranium data establish these scales.

Half-life means the time for about half the nuclei in a large sample to decay. It is not a scheduled waiting time for each atom. One uranium atom might decay soon; another might remain much longer. Radioactive decay follows a statistical law.

The outgoing alpha energies differ by only about a factor of two. The polonium alpha carries about 8.8 megaelectronvolts, while the main uranium alpha carries about 4.2 megaelectronvolts. A megaelectronvolt equals a million electronvolts. An electronvolt measures the energy an electron gains across a voltage of one volt.

The polonium value follows from the evaluated decay energy after allowing for the daughter nucleus's recoil. The uranium dataset directly lists the main alpha energy. These modestly different energies accompany vastly different half-lives.

In 1928, George Gamow explained alpha escape through tunneling. Ronald Gurney and Edward Condon independently developed the same central explanation. Their original paper connects escape probability with radioactive decay.

An alpha wave can extend through the outer barrier. A tiny escape probability can still produce decay because the nucleus supplies an extremely short internal timescale.

A simple polonium model estimates roughly a billion trillion barrier encounters each second. This is a model estimate, rather than a count of filmed collisions. Georgia State University shows the calculation.

Higher alpha energy can reduce the barrier width that needs tunneling. That can greatly increase the escape probability. Polonium and uranium also have different nuclear charges and structures, so energy alone does not set their exact lifetime ratio.

The thickness rule

For a simple, poorly transmitting barrier, tunneling probability falls exponentially as width increases. Exponential change means equal added widths produce roughly equal multiplying factors. It does not mean every small change has the same effect in every material.

Imagine an added thickness that cuts transmission to one tenth. Adding that thickness again can leave about one hundredth of the original transmission. This is an illustrative calculation: one tenth multiplied by one tenth gives one hundredth.

The size of the thickness step depends on the barrier and particle. Raising the barrier above the particle's energy also tends to reduce transmission. Increasing the particle's mass has a similar effect when energy and barrier shape stay fixed. MIT's barrier equations show these dependencies.

A proton has about 1,836 times an electron's mass, consistent with the video's rounded figure of about 1,800. NIST gives the measured mass ratio. The comparison predicts reduced proton transmission only when the other conditions match.

It does not establish that every electron tunnels more readily than every proton. Nor does tunneling concern only isolated light particles. A specially prepared collective system can also show it.

A tennis ball contains a vast collection of interacting particles. Its large mass and ordinary surroundings make tunneling through an everyday obstacle negligible. Shrinking a ball in a drawing therefore illustrates a change of physical regime, rather than a practical way to tunnel.

Quantum tunneling in the Sun

The solar core contains rapidly moving protons with a range of energies. Most lack enough energy to overcome their mutual electric barrier near nuclear distances. Tunneling allows some pairs to approach much closer than classical motion would permit.

That close approach does not automatically make a stable nucleus. Two protons alone do not form a stable bound pair. In the successful reaction, the weak interaction changes one proton into a neutron.

The resulting proton and neutron form deuterium, a heavy form of hydrogen. The reaction also releases a positron and an electron neutrino. A positron is the electron's positively charged counterpart; a neutrino is a very weakly interacting particle. Georgia State University explains this first fusion step.

Further reactions build helium and release energy. The complete sequence has the name proton-proton chain. Tunneling helps overcome electric repulsion, while the slow weak reaction limits the first step. Both processes belong in an explanation of the Sun's steady output.

The chance of fusion during any particular encounter is small. The core provides enormous numbers of protons and repeated encounters. Many rare successes therefore occur across the whole core. The process does not require every proton to fuse quickly.

Comparing typical thermal energies with a nuclear-scale barrier gives a temperature mismatch of hundreds of times. This is an order-of-magnitude comparison, not a precise alternative operating temperature for the Sun. A complete stellar calculation must also include energy distributions, reaction rates, density, and the star's structure.

Solar energy eventually reaches Earth as radiation. Plants capture some sunlight as chemical energy, which then passes through food chains. This connects nuclear physics with daylight and much of the food that supports us. NASA explains the role of sunlight in the carbon cycle.

Sunrise itself results from Earth's rotation. Tunneling helps explain the Sun's sustained energy source, rather than the Sun's apparent daily movement. Many physical and biological processes connect that source to life.

Quantum tunneling in real life: your phone, chips, seeing atoms

Flash memory

Phones, USB storage, and solid-state drives commonly use NAND flash memory. Each cell stores charge in a small region surrounded by insulation. Some designs use a conducting floating gate; others hold charge in an insulating storage layer.

An applied voltage changes the energy barrier and lets charge enter the storage region. In common NAND designs, tunneling supports programming and erasing. Removing the programming voltage makes charge escape much less likely. Kioxia explains charge storage and the voltage-controlled operations.

Stored charge changes the voltage needed to turn the cell's transistor on. Reading circuitry detects that change and interprets it as data. A simple cell distinguishes the states zero and one. Other designs distinguish several charge ranges and store more than one bit per cell. Kioxia describes these multiple storage levels.

The insulation allows storage without continuous power, but it does not hold charge perfectly forever. Useful retention can last years under suitable conditions. Temperature, wear, and the particular device affect that time. The manufacturer's retention guide explains these limits.

This explains how tunneling helps preserve a locally stored photograph. The tiny box is a useful drawing of charge storage, rather than a hollow container inside every memory chip.

Leakage in transistors

A transistor controls electrical current. In common chip transistors, an insulated gate controls a conducting channel through an electric field. The gate insulation should limit unwanted current into the gate.

As manufacturers made that insulation thinner, electrons increasingly tunneled through it. This gate leakage wasted power and produced unwanted heat. It was not simply current flowing through the intended channel while a switch was off.

In 2007, Intel announced a thicker hafnium-based gate insulator for its new transistors. Its stronger electrical response preserved gate control while the greater thickness reduced tunneling. Intel's technical account explains the material change.

The same physical effect can therefore help a memory cell and hinder another transistor. Engineers adjust the material, voltage, and thickness to suit the required operation.

Seeing atoms with a tunneling microscope

In 1981, Gerd Binnig and Heinrich Rohrer developed the scanning tunneling microscope at IBM in Zurich. They received a share of the 1986 Nobel Prize in Physics for this work. IBM records the development, and the Nobel site confirms the award.

A sharp conducting tip approaches a suitable conducting surface without touching it. A small applied voltage produces tunneling current across the gap. Increasing the gap strongly reduces that current.

For typical conditions, increasing the gap by 0.1 nm can reduce the current roughly tenfold. Here, nm means nanometre, one billionth of a metre. The exact response depends on the tip, sample, and applied voltage. University of Maryland research describes this sensitivity.

A feedback system raises or lowers the scanning tip to maintain a chosen current. Recording these adjustments builds a surface image. Purdue explains the scanning and feedback process.

The image reflects electronic structure as well as surface height. It is not a photograph of hard atomic balls. The method can resolve individual atoms on suitable surfaces, and researchers can also use the tip to move atoms.

The 2025 Nobel Prize: a whole circuit tunnels

During 1984 and 1985, John Clarke, Michel Devoret, and John Martinis studied superconducting circuits at the University of California, Berkeley. A superconductor carries current without electrical resistance under suitable conditions. Their experiment used very low temperatures.

A Josephson junction joins superconducting regions through a thin insulating barrier. Paired electrons, called Cooper pairs, share a coherent quantum state. Coherent means that the wave relationships remain organised across the relevant system. The Nobel explanation describes this collective behaviour.

The researchers arranged a circuit state with zero voltage across the junction. An energy barrier separated it from a state with a measurable voltage. At low temperatures, the circuit could escape the initial state through quantum tunneling.

The chip did not move through a physical wall. A collective electrical variable changed across an energy barrier. This distinction explains how a macroscopic system can tunnel while its solid hardware remains in place.

The experiment also separated tunneling from ordinary escape caused by heat. At sufficiently low temperatures, the escape rate stopped depending on temperature as thermal activation would predict. The measured rate agreed with the quantum calculation. The original paper reports this test.

Related measurements showed discrete energy levels. The circuit absorbed energy at particular microwave frequencies, like a system moving between ladder steps. It did not accept every possible energy change within the measured transition pattern.

The Nobel committee announced the prize on 7 October 2025. It recognised both macroscopic tunneling and energy quantisation in an electric circuit. The official announcement states the award's scope.

This work helped establish superconducting quantum technology. Google and IBM use superconducting circuits in quantum processors. Martinis later led Google's quantum hardware work. UC Santa Barbara describes his team's processor. Other quantum computing approaches use different physical systems.

Can you walk through a wall?

A human body does not behave like the collective electrical variable in that carefully prepared circuit. Moving an intact person across a wall requires a vastly different physical transition. The circuit experiment gives no evidence for that possibility.

A rough human-body estimate is about seven octillion atoms. This means seven followed by twenty-seven zeros. NASA gives this approximate scale. The number varies with body size and composition. Its purpose here is to show how many parts must remain organised.

The body and wall interact through many particles, forces, and possible states. Treating every atom as an independent tunneling attempt misses those interactions. Simply multiplying identical single-particle probabilities does not provide a reliable prediction for a person.

The barrier also extends across ordinary physical distances, far beyond a microscope's tiny gap. Even simple models predict extreme suppression for a heavy object crossing such a barrier.

Warm objects constantly exchange energy and information with their surroundings. This interaction causes decoherence, the loss of the wave relationships needed for certain collective quantum effects. It helps explain why everyday objects do not behave like carefully isolated quantum systems. Zurek's review explains environmental decoherence.

Repeatedly walking into a wall does not offer a practical route through it. A comparison with attempts over the universe's lifetime illustrates the scale, rather than a measured human tunneling rate. No supported calculation here assigns an exact probability or waiting time.

Electrons can tunnel across suitable microscopic barriers within materials and devices. That fact does not mean they freely cross every wall, or that a person's intact body can follow them.

The whole picture

The examples connect through a common sequence.

1. A particle or collective quantum state has wave properties.
2. Its available energy lies below a barrier that would stop classical passage.
3. A finite barrier can allow a nonzero transmitted wave amplitude.
4. The transmitted amplitude determines a probability, rather than guaranteeing an outcome.
5. Barrier width, barrier height, particle energy, and mass affect that probability.

The physical role changes between examples. Alpha decay involves escape from a nucleus. Solar fusion requires close nuclear approach. Flash memory transfers stored charge. A tunneling microscope measures a tiny current, while a superconducting circuit changes its collective electrical state.

Classical physics remains useful for ordinary motion. Quantum theory explains why its barrier rule has limits. Chance enters through definite mathematical rules, rather than permission for anything to happen.

What this means

Tunneling connects basic quantum theory with measurable effects and working technology. It helps explain both why certain processes occur and why their rates vary so strongly.

The conditions matter. A wider barrier can protect stored data, while a controlled thin gap can reveal a surface. A slow nuclear reaction can sustain a star even when individual encounters rarely succeed.

Simple pictures help when their limits stay clear. The ball illustrates energy. The prism illustrates a decaying field. The circuit illustrates collective quantum behaviour. None makes an intact person passing through a wall a practical consequence.

FAQ

What is quantum tunneling, explained simply?

It is a chance for a quantum particle to cross a barrier that classical physics would forbid at its energy. Its wave function can extend through a finite barrier. Detection beyond the barrier does not require a hole, extra borrowed energy, or a broken particle.

What makes the Sun shine?

Nuclear fusion supplies the Sun's sustained energy. Tunneling helps positively charged nuclei approach closely despite electric repulsion. The weak interaction enables the first successful proton-proton reaction. Later reactions build helium and release energy.

What is a quantum tunneling experiment?

A tunneling microscope measures electron current across a tiny gap. Superconducting circuit experiments measure escape between electrical states. A double-prism light experiment illustrates related wave mathematics, but classical optics also explains its result.

Where does quantum tunneling occur in real life?

Examples include alpha decay, solar fusion, NAND flash memory, transistor gate leakage, and scanning tunneling microscopy. Engineers sometimes encourage tunneling and sometimes reduce it. The barrier and operating conditions determine whether it serves a useful purpose.

Can you walk through a wall?

No practical physical process lets an intact person walk through an ordinary wall by tunneling. A human body differs greatly from an isolated particle or a controlled superconducting state. Microscopic tunneling does not supply a usable exception to everyday contact with solid objects.