Alex McColgan

So much so, that even the kinetic energy provided by the extreme conditions at the heart of the sun

where protons are travelling around 500 km per second, is wildly insufficient.

In fact, protons have only around 1,000th of the kinetic energy they require to overcome this barrier.

It turns out, the only reason the Sun is able to sustain fusion at all is because quantum effects come into play.

Now, you may remember that according to quantum physics, protons don't just act as particles, they also act as waves.

This wave behavior means that in 10 to the power 28 proton-proton interactions, the protons can overcome this energy barrier, getting close enough that the strong nuclear force pulls them together.

This is called quantum tunneling.

But even when protons are drawn together this way,

There is yet another effect to contend with.

The force that mediates the conversion of one of the protons into a neutron is the weak force, which, because it is controlled by the massive W boson, is very inefficient.

This leads to the more likely product of the proton-proton reaction being a proton pair, which immediately decays back into single protons.

Together, these effects mean that the rate of proton conversion in our star is extremely slow.

On average, a proton will wait 10 billion years before undergoing fusion.

Indeed, the only reason the proton-proton reaction proceeds at all is because there are a heck of a lot of protons in the Sun, allowing it to convert 600 million tonnes of hydrogen

to 596 tonnes of helium every second.

This sounds like a lot, but it's actually only a tiny fraction of the hydrogen available for fusion.

Now, stopping for a moment to look at the big picture, this is very good news for us earthlings.

It means, instead of annihilating itself in a giant thermonuclear explosion, the sun has been gently burning through its hydrogen stocks for around 4.6 billion years, and will continue to do so for at least a few billion more.

It's only because protons are so slow at converting into deuterium that we are here at all.

For fusion on Earth, on the other hand, it is bad news.