Where is antimatter hiding? - Physics explains the mystery of missing antimatter | Don Lincoln

The conversation begins with the historical prediction and discovery of antimatter. Paul Dirac's 1928 attempt to merge quantum mechanics and relativity produced equations with two solutions — one representing the electron and another representing an unknown positively charged counterpart, later called the positron. Carl Anderson confirmed its existence in 1932, and subsequent decades saw the discovery of the antiproton (1956) and antineutron (1957). Scientists have since created increasingly complex antimatter structures, including antihelium nuclei and even antimatter hydrogen atoms at CERN, which have been shown to emit light with the same spectral characteristics as ordinary hydrogen.

A 2023 CERN experiment tested whether antimatter falls up or down under gravity. The results showed antimatter falls downward, consistent with regular matter, though the measurement precision was not yet tight enough to confirm the exact equivalence of gravitational strength (measured at 75% with large uncertainties).

The discussion then turns to the extreme difficulty of producing antimatter. Fermilab, formerly the world's leading antiproton production facility, required smashing 100,000 protons to yield a single antiproton. Running continuously, it could produce roughly one nanogram per year. Since a gram of antimatter contains 10^23 antiprotons, producing a single gram would take approximately one billion years at that rate. NASA estimates the cost of producing enough antimatter for a one-megaton equivalent explosion (~25 grams) at around $1.5 quadrillion, compared to $10–50 million for a conventional nuclear warhead of equal yield.

Despite the impracticality, antimatter has theoretical applications in spacecraft propulsion — potentially enabling travel to Alpha Centauri in 20 years at 0.2 times the speed of light. However, containment remains a near-insurmountable engineering challenge, as any contact with regular matter would result in immediate annihilation. Lincoln emphasizes this is an engineering problem, not a physics one, and that no new theoretical breakthroughs are expected to make production cheaper — the key bottleneck is concentrating energy at subatomic densities.

The final and deepest topic is the matter-antimatter asymmetry mystery. The Big Bang should have produced equal amounts of matter and antimatter, yet the observable universe is composed almost entirely of matter. By comparing proton counts in galaxies with photon counts in the cosmic microwave background, physicists calculate that for every one billion antimatter particles created, there were one billion and one matter particles — the surviving 'one' is everything we see. The origin of this asymmetry is unknown. Theories include baryogenesis (matter-antimatter oscillation with a slight bias) and leptogenesis, which Fermilab is actively investigating by comparing neutrino and antineutrino oscillation rates. If neutrinos and antineutrinos oscillate at different rates, it could be a major clue to explaining why matter dominates the universe, though Lincoln candidly admits the answer remains unknown.