Although spotting the Sun in Ireland can feel like quantum tunnelling itself – mysterious, unlikely and worthy of scientific explanation – it is worth pondering how the Sun is able to shine in the first place. The Sun is one of many massive balls of nuclear fire in the universe, pouring out the energy that lights up our world and powers life itself. There is also a weird twist, if the Sun only followed the regular rules of classical physics, it wouldn’t be shining at all.

So why does it? Well the answer surprisingly lies in one the most bizarre but magical ideals in physics: quantum tunnelling.

Nuclear Fusion

Deep in the sun’s core lies roughly 10^56 hydrogen atoms. Comprehending this number is practically impossible, if you were to imagine each hydrogen atom as a grain of rice then you could over the entire earth in rice up to a depth of millions of kilometres. All of these hydrogen atoms in the sun are constantly colliding and fusing into helium to release vast amounts of energy, and this energy is released from the suns surface in the form of sunlight.

But from the lens of classical physics this doesn’t make much sense. Hydrogen nuclei are made of protons which have positive charges and therefore repel each other. This repulsion is incredibly hard to overcome and is referred to as the Coulomb barrier.

Classically, these protons aren’t hot or fast enough to overcome this barrier and get close enough to collide and fuse. Even though the Sun’s core temperature is 15 million degrees Celsius, this is still not hot enough for fusion to occur. Well at least according to classical physics…

Quantum Mechanics

In the wonderfully strange world of quantum mechanics, we no longer imagine these protons as little balls but rather as waves whose exact position or momentum cant really be simultaneously sharply defined according to Heisenberg. This is what allows for the bizarre behaviour of quantum tunnelling.

Quantum tunnelling is the phenomenon where a particle can “tunnel through” a barrier that it classically shouldn’t be able to cross. A bit like throwing a ball at a brick wall and there being a small chance that the ball passes straight through the wall as if it wasn’t there.

Source: Isaac’s Science Blog

Quantum Tunnelling in the Sun

So in the sun’s core we have those extremely large number of protons zooming around due to high temperatures. Occasionally, two of these protons get very close. Classically, they’d bounce off the Coulomb barrier due to them both being positive charges. However, quantum mechanics say’s they have a small chance of quantum tunnelling through the Coulomb barrier causing nuclear fusion.

And that is exactly why the sun shines.

Even though the probability of quantum tunnelling is very low, there is an astronomically huge amount of hydrogen atoms in the Sun so even if one in a billion pairs of hydrogen atoms successfully quantum tunnel past the Coulomb barrier, it is enough to keep the sun shining for billions of years, as nuclear fusion creates a massive amount of energy.

Why Does it Matter?

Quantum tunnelling isn’t just this bizarre hypothetical idea. It is essential for the existence of stars. Without quantum tunnelling nuclear fusion would be practically impossible, and stars like our Sun couldn’t shine. Therefore we would have no sunlight and no life.

So next time you are enjoying this rare sunny weather remember that you are actually bathing in the glow of quantum mechanics.