A New Atom Beam Could Finally Put Einstein’s Gravity Rule To The Test

Einstein’s Gravity Rule Faces Its Toughest Test Yet, Thanks to a Super-Chilled Atom Beam

A hammer and a feather fall at the same rate in a vacuum, because gravity treats all matter equally. That rule sits at the core of Einstein’s theory of gravity, and scientists have checked it against atoms, neutrons and even antimatter. One strange particle pairing has always slipped through their fingers though, because it barely exists long enough to measure anything.

That particle is muonium, an atom built from an electron and a positively charged anti-muon instead of a normal proton. It survives for only about two millionths of a second before falling apart, and that impossibly short window has kept scientists from ever aiming a clean, steady beam of it at a detector to watch how it behaves under gravity.

A team of physicists reports in the journal Nature Physics that it has solved that problem. By stopping a stream of anti-muons in a two-millimeter layer of liquid helium chilled to nearly absolute zero, researchers created what they call a superthermal muonium beam, one that is unusually strong and unusually orderly, with atoms clustered tightly around a single speed of about 2,180 meters per second. That combination has never existed for muonium before, opening a real path toward finally putting Einstein’s rule to the test with this strange atom.

Muonium’s Fleeting Lifetime Blocked Every Gravity Test

Every previous gravity test, using everyday atoms, neutrons or antihydrogen, relied on particles built from the same basic ingredients as ordinary protons and neutrons. Muonium is different. Its mass comes almost entirely from the muon, a heavier cousin of the electron, so testing gravity with muonium tests Einstein’s rule on a kind of mass nobody has tried before.

Practical trouble stood in the way. A muon lasts only about 2.2 millionths of a second, and older methods of making muonium beams used porous materials like foamy glass or silica powder, which sent atoms flying off in all directions at all sorts of speeds, too scattered to focus for a gravity experiment. Cooling those sources down only made things worse: fewer atoms escaped at all, with almost none getting out below a certain very cold temperature.

A Super-Chilled Helium Trap Tames the Atom

Researchers at the Paul Scherrer Institute in Switzerland tried something different. Instead of a solid porous material, they used liquid helium in its superfluid state, a form the element takes only at extremely cold temperatures, where it flows without any friction. A beam of anti-muons was aimed into a thin layer of this superfluid helium, about two millimeters deep, chilled to roughly two-tenths of a degree above absolute zero.

As the anti-muons slowed to a stop, each one snagged a stray electron left behind in the trail of atoms it had ionized, forming a muonium atom. Superfluid helium strongly repels many impurities, and the researchers expected that if muonium atoms could reach the surface quickly enough, they would be ejected into the vacuum above it. Exactly how the atoms travel through the helium on their way up is still not fully understood.

To catch the atoms leaving the liquid, the team placed rows of particle detectors at different heights on either side of the chamber. Muonium atoms decay and shoot out telltale particles as they do, so watching where and when those decay signals showed up let researchers track the atoms rising through the vacuum, much like watching ripples move outward from a splash.

Detector Readings Confirm a Narrow Beam

Data matched what a narrow, superthermal beam would look like, rather than a spread-out thermal cloud. The atoms came out moving at an average speed of a little over 2 kilometers per second, with a narrow range of speeds around that average, tighter than what earlier, colder sources had achieved. Researchers tested the data against two computer models, one for a hot, scattered gas and one for a narrow, organized beam. The organized-beam model fit far better.

About 8 percent of the anti-muons that stopped in the helium ended up escaping as muonium atoms into the vacuum, a yield comparable to the best existing room-temperature sources but achieved with a beam far narrower in speed than any before it. That narrowness matters, since a beam that spreads out too much is nearly useless for delicate measurements like tracking gravity’s pull or aiming a precise laser at the atoms.

Based on these numbers, the team projects the beam could pin down muonium’s gravitational acceleration to about one percent precision using around 100 days of data at PSI. The same tightly bunched beam could also let scientists probe the atom’s structure with a laser far more precisely than before, sharpening measurements of the muon’s mass, a number used to test the broader rules governing how charged particles interact.

A Direct Gravity Test Is Finally Within Reach

Nothing here has actually measured how muonium falls yet. What this work shows is that one of the biggest missing pieces for that measurement, a bright, narrow muonium beam, now exists. Building it from an inert, super-chilled puddle of helium is a genuinely new approach that puts a direct gravity test on this unusual atom within reach. Whether muonium falls exactly the way ordinary matter does, or reveals a tiny crack in Einstein’s rule, is a question physicists may finally be positioned to answer.

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