Swiss physicists built an atomic cannon out of liquid helium
I read about this one this week and I keep thinking about it. Physicists at ETH Zurich and the Paul Scherrer Institute in Switzerland have built a thing that one of them actually calls an atomic cannon, and the cannon is made of liquid helium.
The setup goes like this. Muonium is a weird little atom: one electron going around an antimuon instead of a proton. So it is half normal matter and half antimatter, and it falls apart after about two millionths of a second. That short life is a big part of why nobody has ever properly measured how it responds to gravity.
The helium is the clever bit. They fire antimuons into a thin film of superfluid helium cooled to near absolute zero. Muonium forms inside the film, and the helium basically won't hold on to it. The atoms get pushed straight up and out, all at about the same speed. So instead of a messy spray going everywhere, you get a tidy vertical beam that you can actually watch. The method was published in Nature Physics.
What they want to know is simple to state. Einstein's theory says gravity pulls on everything the same way, whatever it is made of. Muons are second-generation particles, heavy cousins of the electron, and nobody has ever tested them against that rule. If muonium falls even a little differently than it should, that points to some force nobody has found yet.
I like that after all the accelerators and the cryogenics, the experiment itself is just dropping something and watching it fall. The hard part was making something worth dropping.