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As a SciShow viewer, you can  keep building your STEM skills for 20% off an annual premium  subscription at Brilliant.org/SciShow. [ intro ] 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 understand reality. Atoms are bizarre quantum objects  that make the world go round but also break the rules of  classical mechanics and probability.

But that doesn’t mean they have to be a mystery, because there’s stuff like the  Brilliant course on Quantum Objects that explores those atomic contradictions  through 18 interactive lessons. Brilliant is an online learning platform  with puzzles and lessons in science, computer science, and math. And Brilliant  is also supporting this SciShow video!

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By clicking that link or going  to Brilliant.org/SciShow, you’ll get 20% off an annual  premium Brilliant subscription. The world is full of unavoidable  complexity at the atomic level and beyond, and we’re so glad you choose  to explore it all with us! [ outro ]