There’s an old particle physics joke, “Don’t trust atoms, they make up everything.” While that’s actually not entirely true, most of the stuff we interact with on a daily basis is made of atoms. So it makes sense that we all have to learn about them. And in school, we are taught that atoms are made of some combination of protons, neutrons, and electrons. The simplest atom has just one proton and one electron buzzing around it. And That is hydrogen. But it turns out that there’s an ‘atom’ out there that’s even simpler than hydrogen.
It’s called muonium, and it may help researchers understand the deepest mysteries in physics. Muonium gets its name from a particle called a muon. So let’s start by explaining what that even is. Basically, it’s a particle that’s almost exactly like an electron, with the same negative electric charge. But there are two key differences between them. First, a muon is about 200 times more massive than an electron, though that still makes it about nine times lighter than a proton.
And second, it is unstable. After about two-millionths of a second, it will spontaneously decay, leaving behind an electron and other weird subatomic particles. Those two millionths of a second are imperceptibly short to humans like us, but for muons, it is long enough for them to interact with other particles, and not just that, but also to have relationships with them. And that includes making something that looks a lot like a hydrogen atom.
But this doesn’t make any sense. If a muon is negatively charged, and hydrogen is supposed to have a positively charged proton in its center, how is muonium another kind of hydrogen? Well, that’s because a traditional muonium atom doesn’t actually contain a muon. I know, it’s wild. Instead, it uses an antimuon. Yep. We’re dealing with an atom that’s partly made of antimatter. While visions of matter-antimatter explosions may have just popped into your head, antimatter is really just a different kind of matter. Every type of subatomic particle has an ‘anti-’ counterpart with the same mass but opposite electric charge. An antimuon has the same mass as a muon, and it decays in the same amount of time, but it has a positive charge instead of a negative one. And here’s the key point: the strength of that positive charge is exactly the same as the positive charge of a proton. So if a regular ol’ electron is in orbit around an antimuon, that’s a muonium atom.
Those things don’t annihilate each other because they’re not each other’s matter-antimatter counterpart. And when it comes to chemistry, the difference in mass between a proton and an antimuon doesn’t matter nearly as much as their identical electric charges do. So an atom of muonium, ignoring its super short lifespan, acts as a chemical in almost the exact same way that a proton-based hydrogen atom does. This is why some scientists consider muonium to be the lightest form of hydrogen! And just like other forms of hydrogen, like deuterium and tritium, chemists gave muonium an honorary chemical symbol: Mu. If you prefer to think about muonium as its own element, and not like a quirky kind of hydrogen, that could make muonium the simplest element in the universe.
Just like muons, antimuons are fundamental. They’re as simple as a single subatomic particle can get. Meanwhile, an individual proton is…complicated. It’s not fundamental Each one is made of smaller particles called quarks. So even if the simplest hydrogen atom has only one proton and one electron, muonium is even simpler. These days, antimuons are fairly easy to create in particle accelerators, although they come out traveling pretty quickly. So making muonium requires taking a concentrated beam chock full of antimuons, and then slowing them down. This can be done by literally just putting aluminum or gold foil in the beam’s path, which slows the antimuons down by making them bump into electrons and the like. When they get nice and slow, the old adage of “opposites attract” kicks in. The positively charged antimuons can peel off some of the negatively charged electrons orbiting other atoms to make a bunch of muonium. Now, physicists aren’t making all these exotic atoms just for fun.
They want to make muonium because it lets them use experimental techniques from the well-developed field of atomic physics to study our murky subatomic reality. For instance, each kind of atom has a unique sequence of colors that it emits and absorbs. It’s called a spectrum, and it’s basically a barcode that lets scientists learn more about the subatomic structure and properties of a given atom. Muonium has its own spectrum, too, which is easier to calculate because the atom is simpler. So scientists can study the spectrum in exquisitely precise detail, and use that to test what physics says muons should look and act like. That lets them look for places where their theories about muons break down. And since those theories tend to describe how other particles are supposed to act, it turns into a testbed for all of particle physics.
But muonium might even help answer questions about a phenomenon that’s a little more tangible to human minds: Gravity. Specifically, does gravity pull on antimatter the way it pulls on regular matter? No one knows, because no one’s made enough antimatter to properly ‘weigh’ it. But muonium may be the perfect solution. Not only is muonium relatively easy to make, the atom as a whole is electrically neutral. That means it will be easier for scientists to screen out any effects caused by the electromagnetic force, and focus solely on how gravity is acting. And because we’re in the antimatter game, antimuonium is also a thing. Instead of an antimuon forming a bond with a regular electron, this involves a negatively charged, regular muon combining with a positively charged antielectron.
So if scientists ever spot atoms of muonium and antimuonium ‘falling’ in different ways under the same conditions… either faster, slower, or even in different directions… that’ll be a clear sign that there’s some new, unexplained physics going on! Meaning that yes, the atom that breaks all the rules might break one of the biggest rules in the universe, and literally fall upwards. We may never see muonium get its own box on the periodic table, or taught to the next generation of elementary school students, but it could turn out to be one of the most important atoms in our quest to under
stand reality.
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