#497 – Biggest Mysteries in Physics: Antimatter, Dark Energy & ToE – Don Lincoln

Don Lincoln is a particle physicist at Fermilab who co-authored the 1995 paper announcing the discovery of the top quark, the heaviest known particle, based on roughly 38 candidate events with approximately half being background noise. He frames the history of physics as a history of unifications, from Newton merging terrestrial and celestial gravity in the 1650s, to Maxwell unifying electricity, magnetism, and light in the 1860s, to Einstein's special and general relativity, to the 1967 electroweak unification by Glashow, Salam, and Weinberg. Lincoln argues that taming electromagnetism is the foundational reason modern technological society, including the internet and computers, exists, and that nuclear power emerged directly from research into how protons and neutrons interact.

The Higgs field permeates all of space and has a nonzero value even in a vacuum. At approximately 10 to the minus 12 seconds after the Big Bang, the Higgs field turned on in an event called electroweak symmetry breaking, giving mass to the weak force carriers but not to the photon, which explains why the weak force operates only at distances smaller than a proton while electromagnetism has infinite range. The Higgs boson was announced at CERN on July 4, 2012, and after 14 years of measurement has been confirmed to match all decay modes predicted by the original 1964 theory. Lincoln describes the Higgs mechanism as a theoretical patch that makes electroweak theory work at low energies.

Lincoln considers a theory of everything almost certainly real in principle but predicts it will not be achieved within his lifetime or his grandchildren's grandchildren's lifetime. The unification scale sits around 10 to the 19 GeV compared to current accelerator capability of roughly 10 to the 4 GeV, a factor of 10 to the 15th. Accelerator energy increases by roughly a factor of seven every 20 years, implying around 500 years to close that gap. He considers string theory likely wrong not because its practitioners lack intelligence but because unknown physics will almost certainly intervene across a quadrillion-fold extrapolation, and because 50 years of work have produced no testable low-energy predictions. Loop quantum gravity originally predicted the speed of light would vary by frequency, a prediction falsified by gamma ray burster observations, though Lincoln notes the theory was subsequently revised to remove that prediction.

The antimatter mystery, called baryogenesis, remains unsolved. The Big Bang should have produced equal amounts of matter and antimatter, but the best current estimate is that for every one billion antimatter particles there were one billion and one matter particles, the pairs annihilated, and the leftover surplus is all observable matter. A matter-antimatter asymmetry was measured in the 1960s in accelerator experiments but is far too small in magnitude to explain the observed dominance of matter. Fermilab is currently racing a Japanese group to measure whether neutrinos and antineutrinos oscillate between their three types at the same rate, which could point toward leptogenesis as an alternative explanation. Lincoln personally expects the rates will be equal but says the measurement is necessary.

Producing antimatter at scale is prohibitively expensive. Fermilab required roughly 100,000 protons per antiproton and could collect approximately a nanogram of antimatter per year. NASA estimated antihydrogen production costs at roughly 62 to 63 trillion dollars per gram, making a one-megaton antimatter weapon cost approximately 1.5 quadrillion dollars compared to 10 to 50 million dollars for a conventional nuclear warhead of the same yield. Lincoln frames antimatter propulsion as an engineering problem rather than a physics problem, noting one gram could theoretically accelerate a spacecraft to Alpha Centauri at 0.2 times the speed of light in 20 years, but says he would be shocked if new physics made production meaningfully cheaper.

Dark energy was discovered in the late 1990s when astronomers found the universe's expansion was accelerating rather than slowing. The deepest problem is that quantum field theory predicts a vacuum energy density 10 to the 120 times larger than the measured value, which Lincoln calls the worst prediction in physics. A recent measurement suggests dark energy may be decreasing over time, but Lincoln says it is unconfirmed and should not yet be believed.

Dark matter is five times more prevalent than ordinary matter yet has not been detected through colliders, direct detection experiments, or indirect methods after roughly 30 years of searches. The bullet cluster observation, where gravitational lensing tracks galaxy positions rather than the gas cloud stopped by a collision, changed Lincoln's earlier personal preference for modified gravity explanations. The Dragonfly 2 and Dragonfly 4 galaxies rotate exactly according to Newton's laws, implying they lack dark matter entirely, and Lincoln argues a galaxy with no dark matter is strong evidence that dark matter is real because it demonstrates dark matter can be separated from ordinary matter. The viable mass range for dark matter candidates spans from roughly asteroid mass to far lighter than an electron, and an experiment targeting one part of that range is blind to all others, requiring many radically different experiments to run simultaneously.