Why are we here? Or "An impossible nuclear reaction!"

March 22, 2018



A huge number of important events had to happen in the Universe for each of us to be here. From the Big Bang, the birth of our Sun, and the creation of the Earth, all the way to the fact that your parents met and the right sets of genes combined to make you who you are, everything leads to this moment, this Universe, these people, you and me. Together with all the events that HAD TO happen, there's also an equally large number of events that had to NOT happen. Some of them are truly random and some of them are what we simply call `` laws of nature''. Gravity comes to mind, without which no stars would have formed, no solar system, no us.


Among all the random events that have to happen or not happen for us to be here, there is a really important one that no one ever thinks about (unless you are a nuclear astrophysicist). The impossible nuclear reaction!


Think about it. If a star is really a giant ball of hydrogen, and the temperature is high enough for two hydrogen nuclei (protons) to fuse together, the whole fusion thing should be really easy. All hydrogen should fuse very quickly, most probably with a spectacular “boom” because of all the energy release in a short time. In this scenario there is no obvious reason why our Sun should be fusing hydrogen for billions of years. So why are we still here?


The answer is again: the impossible nuclear reaction!!!


You see, when two hydrogen nuclei fuse together, they make something that is not meant to last. The simple equation “proton + proton  = 2 protons = helium-2” doesn’t work here. The reason is because the moment a helium-2 nucleus is created, it breaks back into two free protons, and hydrogen fusion hits a dead-end. That’s why a star doesn’t fuse all of its hydrogen instantaneously. That’s why we are still here.


So then, if fusing two protons into helium-2 is impossible, what is actually happening at the center of our Sun?


Well, we know hydrogen fusion does happen! It just happens through a different and extremely rare type of fusion reaction, that helps stars burn their hydrogen fuel very very slowly. When two protons try to fuse together, every once in a while, very rarely, a proton is converted into a neutron. Magic? Not quite, but not as intuitive as the 1+1=2. To complete the picture, during this weird process a couple more particles are emitted, namely a positron (same as the electron but with opposite charge) and a neutrino.


At this point if you know your isotopes you should be complaining that this type of fusion I’m describing does not produce helium, but instead a heavier type of hydrogen. This is completely true. Indeed the final product of this rare fusion reaction is a hydrogen-2 made of one proton and one neutron. However, this was the hard part. Getting to helium from here is very easy because our newly formed hydrogen-2 is swimming in a sea of protons, and can easily capture one to become helium-3. Done! Hydrogen fusion into helium. The Sun can continue to burn happily for a few more billion years and we can continue to desperately try to figure out how the Universe works.


So, I hope next time you look at our Sun or at a distant star, or next time you are in philosophical mood and wonder about all the random events that brought us where we are, think about the impossible reaction too!!!





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