If hydrogen did not exist there could be no water, H2O, no sunshine and probably no life. Yet the existence of hydrogen in our universe was touch-and-go. It depended on a minuscule mass difference between two subatomic particles called quarks.
Hydrogen is the simplest atom, with a tiny electron orbiting a bigger, but still tiny, proton. Our most powerful “microscopes,” particle accelerators, show no size at all for electrons, while protons are spheres about 10-15 m across, ten trillionths the diameter of a human hair. They are bound together by the electrical attraction between opposite electric charges. Protons have particles inside that seem point-like like electrons, called quarks, with two types that were named up-quark (u) and down-quark (d). Four more types are known, two of which were discovered at nearby Fermilab.
The proton is made of two ups and one down, so uud, and has a slightly heavier neutral sibling called the neutron made of one up and two downs, so udd. (Up has electric charge +2/3 and down -1/3 so protons have charge +1.)
Neutrons and protons have almost, but not quite, the same mass. Physicists use a mass unit called MeV (million electron volts) for particles. The neutron mass is 939.565 MeV, only 1.293 MeV more than the proton mass, 938.272 MeV. That little difference allows a down-quark to decay to an up-quark, changing a neutron into a proton while emitting an electron (0.5 MeV) and a very much lighter antineutrino to take away the charge and energy. Free neutrons decay with a half-life of about 15 minutes. So, hours after the Big Bang there were very few left, only protons, electrons and (anti)neutrinos. Some neutrons managed to survive by sticking to protons to make helium and lithium; the others all decayed. Most neutrons stuck inside atomic nuclei do not have the energy to decay; others make elements radioactive. As the expanding universe cooled, protons and electrons found each other and made hydrogen atoms.
Nobody knows why the down-quark is that tiny bit heavier than the up-quark, just enough to allow neutrons to decay. Since the up-quark has more electric charge one might expect the opposite. But then all the free protons from the Big Bang would decay to neutrons and positrons, with neutrinos. No hydrogen, no water, no stars as we know them – a very different, probably sterile, universe. Perhaps most universes are like that.
Aren’t we the lucky ones?
