Scientists revel in exploring mysteries, and the bigger the mystery, the greater the enthusiasm. There are many huge unanswered questions in science, but when you’re going big, it’s hard to beat “Why is there something, instead of nothing?”
That might seem like a philosophical question, but it’s one that is very amenable to scientific inquiry. Stated a little more concretely, “Why is the universe made of the kinds of matter that makes human life possible so that we can even ask this question?” Scientists conducting research in Japan have announced a measurement last month that directly addresses that most fascinating of inquiries. It appears that their measurement disagrees with the simplest expectations of current theory and could well point toward an answer of this timeless question.
Their measurement seems to say that for a particular set of subatomic particles, matter and antimatter act differently.
Matter v. Antimatter
Using the J-PARC accelerator, located in Tokai, Japan, scientists fired a beam of ghostly subatomic particles called neutrinos and their antimatter counterparts (antineutrinos) through the Earth to the Super Kamiokande experiment, located in Kamioka, also in Japan. This experiment, called T2K (Tokai to Kamiokande), is designed to determine why our universe is made of matter. A peculiar behavior exhibited by neutrinos, called neutrino oscillation, might shed some light on this very vexing problem.
Asking why the universe is made of matter might sound like a peculiar question, but there is a very good reason that scientists are surprised by this. It’s because, in addition to knowing of the existence of matter, scientists also know of antimatter.
In 1928, British physicist Paul Dirac proposed the existence of antimatter — an antagonistic sibling of matter. Combine equal amounts of matter and antimatter and the two annihilate each other, resulting in the release of an enormous amount of energy. And, because physics principles usually work equally well in reverse, if you have a prodigious quantity of energy, it can convert into exactly equal amounts of matter and antimatter. Antimatter was discovered in 1932 by American Carl Anderson and researchers have had nearly a century to study its properties.
However, that “into exactly equal amounts” phrase is the crux of the conundrum. In the brief moments immediately after the Big Bang, the universe was full of energy. As it expanded and cooled, that energy should have converted into equal parts matter and antimatter subatomic particles, which should be observable today. And yet our universe consists essentially entirely of matter. How can that be?
By counting the number of atoms in the universe and comparing that with the amount of energy we see, scientists determined that “exactly equal” isn’t quite right. Somehow, when the universe was about a tenth of a trillionth of a second old, the laws of nature skewed ever-so-slightly in the direction of matter. For every 3,000,000,000 antimatter particles, there were 3,000,000,001 matter particles. The 3 billion matter particles and 3 billion antimatter particles combined — and annihilated back into energy, leaving the slight matter excess to make up the universe we see today.
Since this puzzle was understood nearly a century ago, researchers have been studying matter and antimatter to see if they could find behavior in subatomic particles that would explain the excess of matter. They are confident that matter and antimatter are made in equal quantities, but they have also observed that a class of subatomic particles called quarks exhibit behaviors that slightly favor matter over antimatter. That particular measurement was subtle, involving a class of particles called K mesons which can convert from matter to antimatter and back again. But there is a slight difference in matter converting to antimatter as compared to the reverse. This phenomenon was unexpected and its discovery led to the 1980 Nobel prize, but the magnitude of the effect was not enough to explain why matter dominates in our universe.