The mystery of the missing antimatter
The Big Bang created equal parts matter and antimatter. So what happened to all the antimatter? The answer may finally be within reach, Cathal O’Connell reports.
The universe was a trillionth of a second old, and it was at war.
Out of the big bang's unimaginable light, two armies emerged and engaged in a frenzied attack. On one side were the particles of matter – electrons and protons. On the other side, antimatter – identical to the matter particles, except with opposite charge. Electrons clashed with their positron counterparts and destroyed one another. Protons duelled antiprotons, to the same violent end. It appeared the two sides were headed toward mutually assured destruction.
Then, a millionth of a second later, the fighting ceased.
In the aftermath lay a wasteland of photons and empty space. But when the dust settled, one side remained standing. For every billion pairs of matter and antimatter particles, a single particle of matter emerged unscathed from the melee. Today, billions of years later, everything we see – galaxies, stars, all of the atoms and molecules that make your body and mine – are descended from those surviving particles.
There’s just one problem: Everything we know about physics says we shouldn’t exist. Matter and antimatter are always created in equal parts, so the two armies should have wiped each other out, leaving the Universe empty, dark and lifeless. That it’s not means there’s something seriously wrong – or embarrassingly incomplete – about our best theory of how the Universe is screwed together.
The triumph of matter is one of the greatest mysteries in all of science. The big question that’s stumped physicists for decades is: what gave us the edge?
Now, using sophisticated new instruments that are as large as their targets are small, scientists are closing in on an unlikely solution. As it turns out, the secret may lie in a quiet corner of particle physics where a newly discovered quirk in the personality of neutrinos is challenging scientists to rethink the origin of the Universe.
Everything we know about physics says we shouldn’t exist.
Today antimatter is a well accepted ingredient in the recipe book of particle physics. We know that every matter particle has a twin antimatter particle that’s the same mass, but with opposite charge. We even use positrons in medical imaging (see box below ‘Everyday Antimatter’). But until 89 years ago, we had no inkling of antimatter.
It was the winter of 1927, and a young British physicist named Paul Dirac was in the com-mon room at the University of Cambridge, staring at the fire. As taciturn as Dirac was (his colleagues would later define a unit of a “dirac” as one word per hour) he was even more absorbed than usual. He was thinking about a thorny problem.
Physics was in the throes of a revolution. The new theory of quantum mechanics – which recast the subatomic world in terms of particles that could hop between energy levels – was beginning to explain previously mystifying effects. Just two years earlier, Erwin Schrödinger had placed the fledgling theory on a firm footing with his equation describing how electrons orbit an atomic nucleus. This equation allowed physicists to explain the structure of atoms with unprecedented detail. Yet some of those details, such as an electron’s magnetic readings, were off.
The problem was speed. Electrons whizz around the nucleus at about 2,000 kilometres per second and Schrödinger hadn’t accounted for how matter that is approaching the realms of light speed might warp the laws of physics. To address that question he needed Albert Einstein’s theory of special relativity. But nobody had been able to meld quantum mechanics and special relativity into one.
As Dirac stared at that fire, he had a sudden burst of insight – a piece of mathematical wizardry conjured from the flames that allowed him to skirt around the impasse that had stumped so many other physicists. He quickly wrote down a handful of mathematical symbols that combined the ideas of Schrödinger and Einstein in one equation. This new equation spat out a measure of the electron’s magnetism that was bang on. It also corrected some bugs in how Schrödinger’s equation dealt with the structure of the hydrogen atom. Most compelling of all, it provided a meaningful rationale for why electrons seemed to spin like a top – so-called “quantum spin”.
With this one elegant equation, Dirac laid the foundations for the electronics revolution of the 20th century. “If Dirac patented his equation,” Stephen Hawking later said, “he would have become one of the richest men in the world. Every television set or computer would have paid him royalties.”
But as successful as Dirac’s equation was, it posed a problem: it had two solutions. One was for a particle with negative charge, the electron. The other was for a particle that was identical in every way to the electron, except with positive charge. It was a ludicrous notion. As every scientist in 1927 knew, the Universe was made of only two kinds of particles: electrons and protons.
Dirac wrestled with the meaning of his equation’s second solution, first ignoring it out of “pure cowardice”. At last, in 1931, he proposed that a new particle, what he called the anti-electron, must exist. Within a year, the American physicist Carl Anderson announced he had found the tell-tale tracks of such a particle (“positively charged, and with the same mass as an electron”) while studying cosmic rays in a cloud chamber. He called it the positron (from “positive” and “electron”). The name stuck.
It was the first time anyone had predicted a new particle based on mathematical reasoning alone. “That to me is one of the great profound mysteries of science,” says Frank Close, a physicist at Oxford University. “It was a case of mathematics knowing about the Universe before we did.”
Dirac’s discovery won him the Nobel Prize for physics in 1933. In his Nobel lecture, he went on to predict that every particle of matter must have its own mirror-twin of antimatter. He was right: over the next two decades, repeated discoveries of the antiproton, the antineutron and more, bore out the theory.
But the discovery of antimatter raised a maddening question. Scientists soon realised that antimatter must be billions of times more scarce than matter. Why does our Universe seem to contain so little of the stuff?
It turns out there are three possible answers. Dirac himself put forth the first: the missing antimatter is out there, we just haven’t learnt to recognise it yet. Half the stars in the sky could be made of antimatter, he suggested, only they’d look exactly like regular stars, so we’d never be able to tell which was which. “The idea is we are just a little pocket of an even vaster universe in which there are huge galaxies of antimatter out there, but as yet not discovered,” says Close.
Though we may not be able to distinguish a given antimatter galaxy just by looking, there is one tell-tale sign that could help prove Dirac’s idea. Along the boundary between regions of matter and antimatter we’d see signs of their annihilation, a dividing line of gamma rays.
Yet attempts to find that dividing line have come up empty. The Compton Gamma Ray Observatory (CGRO), a satellite launched in 1991, orbited the Earth for 14 years and never detected any such boundary. Helen Quinn, an emeritus physicist at Stanford University, is not surprised. Our maps of the cosmic microwave background radiation – heat leftover from the Big Bang – paint a picture of the early Universe that is amazingly homogenous. The chance of regions of matter and antimatter sifting out from such a well-mixed Universe is “so improbable as to be, in everyday language, impossible”, she says.
The second possible answer is that the Big Bang simply created more matter than antimatter. Our best test of this idea is to recreate a miniature Big Bang by revving particles up to near light speed and ramming them into one another. The huge energy creates millions of particles of matter and antimatter. Since the 1960s, physicists have been recording the products of such collisions, and none have ever shown an imbalance. The result has left particle physicists in no doubt. “When we create fundamental particles … we make equal amounts of matter and antimatter,” says Jeffrey Hangst, a physicist at CERN (the European Organisation for Nuclear Research) in Geneva. “That’s an observed law of nature as far as we know.”
If universes cannot be born lopsided, the third possible solution is that just after the Big Bang, some mysterious process skewed the balance of matter and antimatter. This idea was proposed in 1966 by Russian physicist Andrei Sakharov. (Sakharov was somewhat of an enigma: while best known for developing humanity’s most destructive weapon, the hydrogen bomb, he also won the Nobel Peace Prize for his promotion of human rights in the Soviet Union.)
For Sakharov’s idea to pan out, it meant that antimatter could not be, as Dirac believed, matter’s mirror-twin. This assertion of an imbalance – a broken symmetry – struck at the heart of one of physicists’ basic creeds.
From Einstein onwards, physicists had achieved groundbreaking advances by building their theories on a foundation of symmetry. This culminated in the 1950s and 1960s when physicists built the Standard Model of particle physics – a sort of periodic table of the building blocks of the Universe. Much like the way composers use the rules of a key signature to plug in the notes of a melody, physicists used the rules of symmetry to plug in the missing notes of the Universe.
And now Sakharov was suggesting the Universe was built on a bum note? It was hard to fathom. But on the other side of the world, unbeknownst to Sakharov, other researchers were already finding hints of this broken symmetry.
In New York in 1963, physicists James Cronin and Val Fitch got the shock of their lives. They were measuring the properties of an unstable particle called the K-meson. Sixteen years before, they had been discovered in the showers of particles that rain down when cosmic rays strike the atmosphere. Cronin and Fitch found a way to create a beam of them by firing protons at a beryllium target. The experiment aimed simply to measure the decay rates of the neutral K-meson as it broke up into particles of matter and antimatter.
Mesons are weird particles because they contain both particle and antiparticle components. (The two components are not each other’s counterparts, so they don’t annihilate.) Like double agents playing both sides, they constantly switch identities between matter and antimatter. The laws of symmetry said they should spend exactly half their time in each camp. But what the Cronin-Fitch experiment showed was that the K-meson’s loyalty was skewed –they wore their antimatter uniforms slightly more often than their matter ones.
What’s more, when the K-mesons decayed, they left behind more antimatter than matter. The finding didn’t entirely help Sakharov’s theory. K-mesons were decaying in the wrong direction to explain the abundance of matter in the Universe, and the effect was tiny. “But it did set a precedent,” says Close. If K-mesons could break the rules of symmetry, perhaps other particles could too.
In 1972, Makoto Kobayashi and Toshihide Maskawa at the University of Kyoto made a finding that shed suspicion on another double agent, the B-meson. Their calculations suggested that the decay of B-mesons should also break symmetry.
It took until 2001 for experiments to show that B-mesons did, indeed, violate symmetry. This time in the right direction – favouring matter over antimatter.
Mystery solved? “Actually not,” says Quinn. “Yes, you get a small imbalance of matter and antimatter, but too tiny to make even one star with the leftover matter.”
While other teams were exploring asymmetry in mesons, at CERN, physicists were hunting for clues to our unbalanced Universe in the properties of antimatter itself. In 1995 they produced the first atom of antimatter, an antihydrogen – though it immediately annihilated. By the early 2000s they had figured out how to trap antihydrogen long enough to study it directly (see graphic below). That has allowed CERN scientists to compare matter and antimatter in terms of its mass, how its subatomic particles are bound together, and in their electric and magnetic behaviour.
In 2014, for the first time, a CERN project called ALPHA (Antihydrogen Laser Physics Appa-ratus) measured the electric charge carried by an atom of antihydrogen – finding the charge is zero, just like its hydrogen counterpart. So no anomaly there. ALPHA’s next big experiment is to see how antimatter responds to gravity; some theories say that rather than falling down, antimatter could fall up! “Basically we’re looking everywhere we can,” says Hangst, a particle physicist at the ALPHA experiment.
So far, no new clues have been found; matter and antimatter have turned out identical in every property tested. But there is one more particle, so little understood, that it might harbour the secret behind our matter-dominated Universe – the mysterious neutrino.
Neutrinos are neutral particles produced during radioactive decay and in the centre of stars. They are famed for their ghostlike ability to pass through matter at the speed of light. In 1986 Masataka Fukugita and Tsutomu Yanagida of Tohoku University wondered if these mysterious particles might also hold the answer to the imbalance of the Universe. They came up with an extraordinary theory.
Their starting point was to propose that neutrinos actually travelled slightly slower than the speed of light and had a tiny mass. The reason for their extreme lightness, they suggested, is that neutrinos have a big brother – another, much heavier particle – that offsets their mass. The idea is called the seesaw theory because it reminded the physicists of the way a big child on a seesaw can suspend a smaller child high in the air. If you could not see the big brother on the other side, you’d wonder at the extreme lightness of the child perched high on the seesaw. So if the neutrino is extremely light, they reasoned, its brother must be superheavy.
The Japanese pair figured that these superheavy neutrinos would have been unstable. And that their decay may have been skewed toward matter. Given that they would have been created in huge numbers in the Big Bang, that might have tipped the balance towards matter over antimatter. Fukugita and Yanagida’s revolutionary idea was ignored. Their series of assumptions about neutrinos seemed tenuous, each stacked atop each other like a house of cards. Moreover there was no base to the stack because there was no evidence that the neutrino had mass.
Then, in 2001, the physics community was rocked by the discovery that neutrinos shapeshift between three possible forms as they zoom through the Universe. Think of neutrinos as a group of three close-packed riders in a bicycle peloton who take turns leading, with each leader showing a different face. This peloton behaviour could only be explained if neutrinos did carry mass (see Shape-shifting neutrinos that led to a Nobel Prize), though it must be miniscule even compared with the electron. Around the world, the physics community did a collective double-take.
Since the shapeshifting discovery, many other theories have been proposed to explain how the neutrino could be so incredibly light. But the idea of a secret neutrino big brother is the most popular.
Unfortunately, the mass of the proposed superheavy neutrinos is so large that researchers can’t produce them in a particle accelerator and watch them decay. But the seesaw theory does make two testable predictions about neutrinos. If they both prove true, they provide strong evidence that neutrinos are the hero behind matter’s victory in the war of creation.
The first prediction is that neutrinos are their own antiparticle. Right now we know that as neutrinos zip through space, they always spin anticlockwise while antineutrinos spin clockwise. But a thing that spins clockwise coming toward you spins anticlockwise going away from you. This means neutrinos and antineutrinos could be two sides of the same coin. The smoking gun to prove this theory would be detecting a special radioactive process called neutrinoless double beta decay.
Regular double beta decay happens when two neutrons in the same nucleus decay simultaneously, spitting out two electrons and two neutrinos. Normally the path of the emitted electrons is unbalanced because the neutrinos carry away some of the energy – just as two struck billiard balls have unequal paths because the cue ball carries some of the energy. But if a neutrino can act as its own antiparticle, then occasionally those two neutrinos should annihilate. If this happens within the nucleus, it’s as if no neutrino was emitted at all: that’s neutrinoless double beta decay. The absence of neutrinos would leave a unique signature on the electrons that were released. Instead of being imbalanced, the paths of the electrons will be perfectly balanced. By measuring the paths of emitted electrons, physicists hope to nail the dual nature of neutrinos. “This is what the next generation of experiments are going to try to crack,” says Simon Peeters, a physicist at the University of Sussex. He is part of an international collaboration based at Canada’s Sudbury Neutrino Observatory (SNO) where some of the shapeshifting Nobel prize work was performed. When a new detector, SNO+, is operational next year, the team will watch for two tell-tale flashes of light that indicate a tellurium nucleus has decayed by emitting two electrons simultaneously.
The decay is incredibly rare and difficult to detect. “What you’re looking for is basically a handful of these decays in a tonne of material,” says Peeters.
Far from the ridicule that first greeted Fukugita and Yanagida, there is fierce competition to be the first to nail neutrinos as double agents. The Enriched Xenon Observatory in New Mexico is already probing 200 kilograms of liquid xenon for similar tell-tale flashes. And in Italy, the GERDA (Germanium Detector Array) uses huge crystals of the semiconductor germanium to detect the elusive decay.
pursuing neutrinos sounds like a huge gamble to try to crack antimatter, but they’re the best chance we’ve got.
Prove that neutrinos are their own antiparticle would be the biggest coup for particle physics since the discovery of the Higgs Boson. Yet this would only be one step toward proving the seesaw theory.
The theory’s second prediction is that neutrinos, like K-mesons and B-mesons, break the rules of symmetry between matter and antimatter. We can’t directly test whether neutrinos decay in a lopsided manner. But we can check whether they break the symmetry rules. A major project called DUNE (Deep Underground Neutrino Experiment) at Fermilab, in Illinois, could soon put this idea to the test by firing the world’s most intense beam of neutrinos and antineutrinos at a detector buried 1,300kilometres away in a South Dakota mine. The long distance gives the neutrinos time to shapeshift. DUNE will probe the beam twice along the way to measure such shifts.
The laws of symmetry dictate that neutrinos and antineutrinos should shapeshift at the same rate. If they don’t, then they are part of a select club of symmetry-breakers (see graphic Profile: Matter vs antimatter below) that might break all sorts of other rules. And if they behave badly, their big brother probably does too – possibly decaying in a lopsided manner, leading to our matter-dominated universe.
The US has already devoted $1 billion to the DUNE project, which should be operational by 2022, and a huge list of international collaborators, including CERN, have signed up too.
Admittedly pursuing the weirdness of neutrinos sounds like a huge gamble to try to crack the mystery of antimatter. But they’re still the best chance we’ve got. “Most particle physicists are betting on them,” says Quinn. What’s at stake is nothing less than our reason for being. But for now, we must wait, as two of the most sophisticated physics experiments on the planet run their course. What’s another few years to a mystery 13.8 billion years in the making?
Antimatter sounds like something right out of science fiction – probably because it is. In Star Trek, it was the fuel for the USS Enterprise. In Dan Brown’s Angels and Demons it was the stuff threatening to blow up Vatican City. Yet antimatter is not as exotic as it sounds. In fact, it’s all around you. A banana, for instance, produces a positron about every 40 minutes (through the decay of a particular potassium isotope). Your body also contains potassium, so even you are generating antimatter. And ever heard of a PET scan? That stands for positron emission tomography. It’s a medical imaging technique where doctors inject you with a small radioactive dye that gives off positrons. The positrons immediately annihilate in your bloodstream, and by detecting the gamma rays they emit, doctors can map your circulatory system – even the inner structure of your brain.