Quantum Tunneling: The Physics Phenomenon That Lets Matter Pass Through Impossible Barriers

🚧 The Impossible Barrier

Imagine a ball rolling toward a hill. If it doesn't have enough energy, it stops and rolls back — that's what classical physics says. In our world, a particle with energy E can never overcome an energy barrier (potential barrier) with height V > E. It is simply impossible.

And yet, in quantum mechanics, the impossible happens. Particles "pass through" barriers that are classically impenetrable. This phenomenon is called quantum tunneling and is one of the most impressive and counterintuitive phenomena in modern physics.

Quantum tunneling is not a theoretical curiosity — it is the reason the Sun shines, radioactive elements decay, and modern technology works. Without this phenomenon, the world as we know it would not exist.

🌊 The Wave Explanation

Classical physics treats particles like small balls. Quantum mechanics, however, describes them through wavefunctions — mathematical expressions that give the probability of finding a particle at each point in space.

When a particle approaches an energy barrier, the Schrödinger equation reveals something astonishing: the wavefunction does not abruptly drop to zero at the boundary. Instead, it exhibits exponential decay inside the barrier — decreasing exponentially but never becoming exactly zero.

If the barrier is sufficiently thin, the wavefunction emerges on the other side with non-zero amplitude. This means there is a non-zero probability of finding the particle beyond the barrier — without ever having “climbed” over it. The probability depends on three factors: the barrier height, its width, and the particle's mass.

For macroscopic objects — a tennis ball, a person — the tunneling probability is so astronomically small (e.g., 10⁻³⁸ for an electron through a 1 nm barrier, even less for larger objects) that it practically never happens. But at the atomic scale, tunneling is an everyday occurrence.

☀️ Tunneling in the Sun

The most spectacular application of quantum tunneling occurs in the core of the Sun. There, the temperature reaches 15 million °C — sounds enormous, but in reality it is far too low for classical nuclear fusion.

For two protons to fuse, they must overcome the Coulomb barrier — the electrostatic repulsion between two positively charged particles. Classically, the thermal energy in the Sun's core is about 1,000 times lower than what would be required.

The solution was provided by George Gamow in 1928, just two years after the formulation of the Schrödinger equation. Gamow showed that protons don't need to “climb” over the Coulomb barrier — they can tunnel through it. Even with low energy, there is a small but non-zero probability that two protons will find themselves close enough for the strong nuclear force to take over.

With billions upon billions of protons attempting fusion every second, even the small probability is enough: the Sun converts 620 million tons of hydrogen into helium every second, releasing energy that illuminates the entire solar system.

💡 Technological Applications

Quantum tunneling is not just an astrophysical phenomenon — it lies at the heart of many technologies we use daily.

The tunnel diode, invented by Leo Esaki in 1957, directly exploits electron tunneling through a thin semiconductor barrier. It operates at exceptionally high frequencies and Esaki was awarded the Nobel Prize in Physics in 1973.

The Scanning Tunneling Microscope (STM), developed by Gerd Binnig and Heinrich Rohrer, uses tunneling current between an extremely thin metallic tip and a surface to “see” individual atoms. Their discovery was honored with the Nobel Prize in Physics in 1986. The STM allowed scientists for the first time to image and even move individual atoms.

Flash memory found in every smartphone and USB stick relies on tunneling: electrons tunnel through a thin insulating layer (silicon oxide) to store or erase data. And alpha radioactive decay — the emission of helium-4 particles from unstable nuclei — is fully explained by quantum tunneling, something first understood by Gamow.

⏱️ How Fast Is It?

One of the most interesting and controversial questions about tunneling is: how long does it take a particle to pass through a barrier?

This question, known as "tunneling time", has remained open for decades. Some theoretical models predict that the process is nearly instantaneous. The so-called Hartman effect shows that tunneling time does not increase as the barrier becomes thicker — seemingly implying superluminal speeds.

Experiments by physicist Aephraim Steinberg at the University of Toronto (2019–2020) showed that photons “pass through” a barrier in what appears to be zero time. However, this does not violate relativity — no information is transmitted faster than light. The apparent “superluminality” is due to how the wave packet reshapes during passage.

🔬 Current Research

Quantum tunneling is today at the cutting edge of multiple research fields.

In quantum biology, researchers have discovered that proton and electron tunneling plays a role in enzyme catalysis. Enzymes such as alcohol dehydrogenase use quantum hydrogen tunneling to accelerate chemical reactions — biology has been exploiting quantum mechanics for billions of years before we “discovered” it.

In quantum computing, tunneling plays a dual role. On one hand, it poses a problem — electrons can “escape” via tunneling from transistors as these shrink below 5 nm. On the other hand, quantum annealing deliberately exploits tunneling to find solutions to complex optimization problems, as D-Wave computers do.

In low-temperature physics, macroscopic quantum tunneling in SQUIDs (Superconducting Quantum Interference Devices) represents one of the few cases where quantum phenomena manifest at macroscopic scales. SQUIDs can detect magnetic fields billions of times weaker than Earth's, with applications in medical brain imaging.