Antimatter idea offers scientists clue to cracking cosmic mystery Premium

Discover the fascinating world of antiparticles, their existence, scarcity, and implications for the universe's matter-antimatter asymmetry.

One of the most astonishing facts about the natural world is the existence of antiparticles. Theorised by the English physicist Paul A.M. Dirac in 1928 and observed in cosmic rays by American physicist Carl Anderson in 1932, an antiparticle is a ‘partner’ of a particle type that has the same mass but opposite charge. For example, the antielectron is the antiparticle of the electron; it has the same mass and is positively charged.

Antiparticles are an inevitable consequence of describing the world in terms of quantum mechanics and special relativity.

An antiparticle is a particle travelling backward in time. This is not an oversimplification. If it sounds eerie, that’s because it is.

But where is all the antimatter made of antiparticles? It is certainly scarce, or we would have discovered it a long time ago. Still, antiparticles are detectably numerous. Our own bodies make one antielectron every 20 seconds from the decay of potassium-40. Cosmic rays raining down on us supply antiprotons, antielectrons, and even antinuclei. Every proton and neutron — constituents of the nuclei that make up all the matter we can touch — is teeming with antiquarks.

But it is when we look out at the universe as a whole that antimatter’s scarcity becomes clear. All galaxies are made of matter, not antimatter. Even in the infant universe, there had to have been a small dissimilarity between the populations of protons and antiprotons for our predictions about the outcomes of the synthesis of nuclei in the early universe and the features of the cosmic microwave background (radiation leftover from the Big Bang) to hold.

That is, for every 1.7 billion proton-antiproton pairs, there should have been an extra unpaired proton.

Presumably the universe started out with equal amounts of matter and antimatter before something happened to distort this symmetry. That’s a good thing: otherwise matter and antimatter would have mutually annihilated to fill the universe with nothing but a fog of radiation — no raw material to make stars, planets or us.