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| This image represents the evolution of the Universe, starting with the Big Bang. The red arrow marks the flow of time. |
One of the deepest and most enduring mysteries in the study of the universe is why anything exists at all.
It really shouldn’t, not according to the best current physics. What really should exist is nothing but a flash of light. And yet, just look around. There is stuff everywhere. Physicists do not understand why, and after a major new discovery out of Europe’s nuclear research program, they are still in the dark.
The key to this mystery is antimatter. Antimatter is just as it sounds, the opposite of matter. Matter is made of different kinds of particles, like the proton, antimatter of corresponding kinds of antiparticles, like the antiproton. They have the same mass, but opposite charge, and if they meet, they annihilate each other, converting their mass into pure energy according to Einstein’s famous equation E=MC2.
Matter and antimatter are kind of like identical twins of the opposite sex, similar but opposite, says Makoto Fujiwara, a leading researcher at TRIUMF, Canada’s national laboratory for particle physics in Vancouver, who runs experiments at CERN in Geneva to create, detect, control, and trap antimatter.
This work is tricky, not only because of the problem of what to store antimatter in (one answer: cool it down so it gets very sluggish, then trap it with with magnets so that it does not touch anything). Antimatter is also very rare. It is used in medicine for PET scanning (positron emission tomography) but naturally it is produced only in thunderstorms, radioactive decay, and cosmic ray collisions. This rarity compared to regular matter is the core of the mystery.
In the latest research, published in the journal Nature, a team led by Christian Smorra used CERN’s oldest accelerator, a proton synchrotron, to slam a beam of protons into metal to create a shrapnel of particles, including antiprotons, which were then “tamed” into a coherent beam by a ring of magnets that guide them. This beam was then slowed to a speed that is still very fast, about one tenth of the speed of light, before particles could be trapped and measured.
By doing this, the team managed to measure a physical property of antimatter, known as the magnetic moment, to an unprecedented precision of 1.5 parts per billion, hundreds of times better than any previous measurement.
It is both a breakthrough and a frustrating setback, a cutting edge experiment that reveals the vast depths of human ignorance about the cosmos.
What they discovered is that both the proton and the antiproton have a magnetic moment of 2.7928473. The only difference is that the proton is positive, the antiproton negative. No matter how closely scientists measure, they still look identical, only opposite.
It is a glorious frustration, an experimental success but a theoretical red herring.
Current physics predicts that when the universe expanded from an infinitely hot and dense point nearly 14 billion years ago in the Big Bang, particles of matter and antimatter were created in equal measure. They should have annihilated each other completely, perfectly, leaving nothing but a flash of light.
Clearly, this is not what happened. Some matter was left over. What seems to have happened is that for every billion or so annihilations in the moment after the Big Bang, a single regular particle was left behind, with no antimatter counterpart to annihilate it. This left over matter was like the lingering smoke after a flash, and it is the key to all material existence today, everything from stardust to supernovas to spaghetti bolognese and the people who eat it.
The rest of the story is on solid observational footing. The flash of those early annihilations can still be seen today in the Cosmic Microwave Background, a faint glow in the universe, the literal first light. But the cosmic surplus of regular matter remains a total mystery.
“There’s got to be some difference between matter and antimatter in order to explain what happened to antimatter,” Fujiwara says.
This is why he and other researchers at CERN, home of the Large Hadron Collider, have been looking for some physical difference between matter and antimatter, other than being perfect mirror images. The hypothesis is that if you measure precisely enough, there will be some difference that explains the cosmic bias toward regular matter. Finding it is one of the greatest experimental challenges of modern physics.
Fujiwara, for example, is at CERN this week to work on an experiment measuring gravity’s effect on anti-hydrogen, to see whether maybe it falls up instead of down, or behaves differently in some other way. But every difference remains conjecture. So far, the physical properties of anti-hydrogen line up exactly with regular hydrogen.
Even though it is vastly more common, regular matter still seems almost like a cosmic afterthought, a rounding error. It is not even the predominant stuff in the universe, far from it. As it is today, the newborn universe was mostly made of dark matter, a strange substance that does not interact with other matter except to exert gravity. So the diffuse dusting of leftover regular matter, the “smoke” that lingered after the Big Bang, was shaped by the gravity of dark matter first into stringy filaments, like cheese off a hot pizza just before it breaks.
Those strings coalesced into clumps, all in motion under gravity’s influence. These clumps became stars, and as they burned, the intense heat and pressure in their cores fused the lighter elements like hydrogen and helium into heavier elements like carbon and iron. When the stars burned out and exploded, those new elements were freed in clouds of stardust. From that stardust came more stars, arranged in billions of swirling galaxies, and eventually planets in their orbit, with satellite moons of their own. The rest is history.
Ever since, antimatter has been exceedingly rare.
To physics, this imbalance is weird. For example, there is no theoretical reason to think the universe should have, on average, an electrical charge, says A.W. Peet, professor of physics at the University of Toronto. So there should be equal numbers of electrons and anti-electrons (known as positrons). But there does not seem to be.
Or maybe there is no imbalance. Maybe it just looks that way. Many scientific problems have turned out to be tricks of perspective.
“Sometimes people have wondered if maybe the antimatter sort of went somewhere else, and it’s in some region of the universe that is just antimatter making up anti-protons and anti-neutrons and anti-electrons, even anti-atoms and anti-molecules, and all that sort of stuff,” Peet says. “Anti-planets, anti-humans, you know, perhaps.”
Unfortunately for this intriguing scenario, if that region did exist, then the boundary between it and the rest of the universe “would produce lots of bursts of energy from annihilation, where the edges of the conglomeration of antimatter would find some matter, and we don’t see that. So that’s one of the reasons why astrophysicists and particle physicists are fairly convinced that there are not large quantities of antimatter around the universe,” Peet says.
So physicists are left with this puzzle — a theory that predicts nothing should exist but pure energy, but a universe that obviously has a lot of stuff in it.
The hypothesis remains that, somewhere down beyond one part in a billion, there is some discrepancy waiting to be discovered. But that is just a hope, unfulfilled by this latest research.
“We have not been able to find an asymmetry yet,” Fujiwara said.
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