Quantum physics

The Sun Only Shines Because Of Quantum Physics

Ethan SiegelSenior Contributor

Starts With A BangContributor Group

Science

The Universe is out there, waiting for you to discover it.

The Sun is the source of the overwhelming majority.

Earth, as we know it, is only teeming with life because of the influence of our Sun. Its light and heat provides every square meter of Earth — when it’s in direct sunlight — with a constant ~1500 W of power, enough to keep our planet at a comfortable temperature for liquid water to continuously exist on its surface. Just like the hundreds of billions of stars in our galaxy amidst the trillions of galaxies in the Universe, our Sun shines continuously, varying only slightly over time.

But without quantum physics, the Sun wouldn’t shine at all. Even in the extreme conditions found in the core of a massive star like our Sun, the nuclear reactions that power it could not occur without the bizarre properties that our quantum Universe demands. Thankfully, our Universe is quantum in nature, enabling the Sun and all the other stars to shine as they do. Here’s the science of how it works.      Starlight is the single greatest source of energy in the Universe throughout its entire 13.8 billion year history, subsequent to the hot Big Bang. These large, massive concentrations of hydrogen and helium contract under their own gravity when they first form, causing their cores to become denser and denser all while heating up. Eventually, a critical threshold is reached — at temperatures of ~4 million kelvin and densities exceeding that of solid lead — where nuclear fusion begins in the star’s core.     But here’s the puzzle: you can determine exactly how much energy the particles in the Sun must have, and calculate how those energies are distributed. You can calculate what types of collisions occur between protons in the Sun’s core, and compare that with how much energy is required to actually bring two protons into physical contact with one another: overcoming the electric repulsion between them.

And when you do your calculations, you find a shocking conclusion: there are zero collisions happening there with enough energy to lead to nuclear fusion. Zero. None at all.

A                                                     At first glance, this would appear to make nuclear fusion — and hence, the ability of the Sun to shine — completely impossible. And yet, based on the energy we observe coming from the Sun, we know that it does, in fact, shine.

Deep inside the Sun, in the innermost regions where the temperature ranges between 4 million all the way up to 15 million kelvin, the nucleus of four initial hydrogen atoms (i.e., individual protons) will fuse together in a chain reaction, with the end result producing a helium nucleus (made of two protons and two neutrons), along with the release of a significant amount of energy.

That energy is carried away in the form of both neutrinos and photons, and while the photons might spend over 100,000 years before they make it to the Sun’s photosphere and radiate into space, the neutrinos exit the Sun in mere seconds, where we’ve been detecting them on Earth since the 1960s.

You might think about this scenario and be a bit puzzled, since it isn’t obvious how energy is released from these reactions. Neutrons, you see, are ever so slightly more massive than protons are: by about 0.1%. When you fuse four protons into a nucleus containing two protons and two neutrons, you might think that reaction would require energy instead of emitting it.

If all of those particles were free and unbound, that would be true. But when neutrons and protons are bound together into a nucleus such as helium, they wound up being bound together so tightly that they’re actually significantly less massive than their individual, unbound constituents. While two neutrons have about 2 MeV (where an MeV is one million electron-volts, a measure of energy) more energy than two protons are — via Einstein’s E = mc² — a helium nucleus is the equivalent of 28 MeV lighter than four unbound protons.

In other words, the process of nuclear fusion releases energy: about 0.7% of whatever protons fuse together gets converted into energy, carried by both neutrinos and photons.

The most straightforward and lowest-energy

When the wavefunction of two protons in the Sun’s core overlap, there’s only a minuscule chance that they’ll do anything other than return to being two protons. The odds of them fusing together to make a deuterium nucleus are about the same as winning the Powerball lottery three times in a row: astronomically small. And yet, there are so many protons inside the Sun that this successfully occurs so often that it powers not only our Sun, but practically all of the stars in the Universe.

however, nuclear fusion wouldn’t occur in the Sun at all, and Earth would simply be a cold, lifeless rock floating in the abyss of space. It’s only because of the uncertainty inherent to position, momentum, energy, and time, that our existence is possible at all. Without quantum physics, the Sun wouldn’t be able to shine. In a very real sense, we really did win the cosmic lottery.

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