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    A New Clue in the Hunt for the Universe's Missing Antimatter

    Written by

    Michael Byrne


    The ALPHA antihydrogen trap. Image: CERN

    Among the more esoteric mysteries of the universe you'll find what is also one of its biggest: missing antimatter. The physics of subatomic particles teaches us that the creation of matter from energy, in which some high energy massless particle collides with another particle to create some massive particle like an electron or proton, results in not just the expected particle but a particle pair. One of those particles is of the "normal" sort that comprises most of our cosmic reality, and the other is its antimatter partner.

    Think of the collision and resulting matter as knocking out a divot of grass with a golf club. There's the airborne chunk of sod, but also the hole left in the ground. The sod is a typical elementary particle, like a proton or electron, but the hole is its own very real particle as well. The difference is that the hole has entirely opposite properties. It's antimatter, the result of every single reaction that creates matter, from the birth of the universe until eternity.

    It's simply impossible to kick out a divot of grass without leaving a hole, and the swing of the golf club must have enough energy behind it to create the mass of both, the regular matter chunk and the antimatter hole. They both have the same mass, and so require the energy to create two equal masses, but some minority of physicists suspect the antimatter particle really has negative mass (which would theoretically give rise to things like antigravity). We see this antimatter/matter pair creation in nature regularly in the upper atmosphere, where high-energy cosmic rays bomb atomic nuclei, resulting in showers of particles and antiparticles.

    The not-quite-obvious mystery implied by antimatter creation is that there should be a whole lot of antimatter out there somewhere in the universe, just as much as there is regular matter in fact. However, that appears not to be the case. While it's true that antimatter has the same potential to form into all of the amazing structures of the universe, from black holes to human life, we can reasonably say that what we're looking at out there is regular matter with little to no antimatter interspersed.

    Even if we could make out some distant antimatter galaxy, it wouldn't solve the fundamental problem of why some matter continues to exist at all: Equal parts anti- and plain old matter should have left nothing but energy, yet here we are. The imbalance problem is known as baryon asymmetry and it's among the most immediate why are we here?-type physics mysteries. One hope is that by looking very closely at the properties of matter and its antimatter counterparts, we might find some difference beyond simple opposition. Perhaps antimatter charges are slightly different in magnitude, or the antiparticle particles have slightly different masses.

    Charge is one such property being probed at CERN in its ALPHA experiment, specifically the charge of an antihydrogen atom. As reported in this week's Nature Communications, the project's latest results confirm that antihydrogen carries the same (if opposite sign) charge as its hydrogen counterpart up to eight decimal places of precision. It's what should be expected from a perfect antimatter opposite, essentially confirming existing antimatter theories. Unfortunately, those theories, which say the charge magnitudes should be identical, are the same theories that suggest there should be no material universe at all and we should not even exist to ponder antimatter in the first place.

    As it is now, the universe we observe appears to be entirely material, with zero significant concentrations of antimatter. Even a slight asymmetry between the two varieties of matter could be enough to account for this universe, as all it would take for the vast material universe to exist is just the slightest, tiniest bit of undestroyed matter. The preference for matter over antimatter so far observed in other experiments is one particle in a billionan imbalance seen in certain types of particles that morph into their anti- or "normal" counterparts​but the actual source of that preference is unknown.

    "Though the result is not surprising it is a fundamental test that matter and antimatter have equal and opposite electric charges," said John Womersley, the Chief Executive of the UK's Science and Technology Facilities Council, in an emailed statement. "It is reassuring that nature behaves as expected, but as scientists we should never take anything for granted and measurements like this are therefore very important indeed."

    Are we so sure those galaxies and stars and "godzillas of Earths" we see with our modern cosmos-spanning telescopes are actually our kind of matter and not antimatter imposters? Yes, mostly. A starscape of mixed matters would be extremely obvious to observers because of the simple fact that when antimatter meets regular matter, both annihilate, giving up their entire massive existences in exchange for pure energy, which cares not a bit about antimatter/matter oppositions (it's not matter after all).

    Basically, you can't hide the stuff, and even trapping antimatter for long enough to examine is a challenge.

    If, say, a person made of antimatter were to teleport onto Earth, the result would be the release of enough energy to blow a sizable hole in the solar system; a matter-antimatter collision liberates nearly 100 percent of the energy equivalent of a given mass, which is some three orders of magnitude greater than the energy release seen from a nuclear bomb. If antimatter galaxies were out there, so too would there be a matter/antimatter boundary region of the most intense cosmic violence ever witnessed. There isn't.

    Instead, we look for the missing antimatter in laboratories manifested as asymmetry. The ALPHA experiment doesn't damn the idea in the least, and will soon enough seem primitive as antimatter-trapping mechanisms get better and better. ALPHA, which measures particle charges by charting their trajectories through a repulsive electric field, will soon be upgraded to ALPHA-2.

    Moreover, ALPHA will also soon be joined by the AEgIS project, which will examine one of the most tantalizing questions about antihydrogen: how it behaves in a gravitational field, e.g. does antimatter fall up? If so, that might help provide an alternative explanation as to why the sky isn't full of antimatter/matter collisions beyond there just not being any antimatter up there. Also: the possibility of antigravity.

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