Gravity accompanies us in our everyday lives—from early morning, when we get out of bed, to late evening, when we drop tiredly onto the mattress. Although no other force of nature shapes our lives as much as gravity, we still know little about it. Many scientists around the world are working to uncover the secrets of gravity. One of them is researching in Canton Aargau: the 32-year-old particle physicist Anna Soter.
In Dan Brown’s thriller ‘Angels & Demons’ a quarter of a gram of antimatter is stolen at CERN and finally exploded high above Rome by annihilation. The American author has richly decorated his story about the intriguing secret society of the 'Enlightened Ones,' but his fiction is somewhat based on reality: antimatter really exists; its existence was proven in 1932 when Carl David Anderson discovered the antielectron as a component of cosmic rays. Antielectrons are today used in medicine—in PET scanners for cancer diagnosis for example. Antiprotons were first produced and detected in 1955 in accelerator experiments at the Lawrence Berkeley National Laboratory in the USA. But antiprotons also occur naturally in cosmic radiation. If antielectrons are brought together with antiprotons, antihydrogen atoms can be produced; CERN is the only laboratory in the world that can do this. The lifetime of antimatter is just as long as that of conventional matter. Only when matter and antimatter are brought together do they annihilate each other.
However, antimatter has by no means revealed all its secrets. Still open, for example, is the question of whether antimatter is subject to gravitational force to the same extent as matter. Would an antimatter apple, if it existed, fall to the ground in the same way as the famous apple, thanks to which Isaac Newton, according to legend, discovered the law of gravity? Many physicists assume that matter and antimatter actually behave in the same way. This fact - physicists speak of the 'equivalence principle' - has so far been confirmed in many respects. However, this principle as a general rule has not been proven by experimental evidence. Antielectrons and antiprotons are electrically charged, which makes it impossible to measure their gravitational interaction in isolation. The gravitational force is much too weak in relation to the electromagnetic force to allow reliable measurement. Antihydrogen atoms, on the other hand, are electrically neutral and therefore gravitational effects could be detected in carefully performed experiments. Similarly, electrons and antimuons can be brought together to produce neutral muonium, in which gravitational effects can be detected. Muons are elementary particles that are very similar to electrons. However, they have a mass about 200 times heavier and a lifetime of 2.2 microseconds. Antimuons are the antiparticles to muons. Both are contained in cosmic rays and can also be generated artificially in particle accelerators. Muonium - discovered in 1960 by the US physicist Vernon Hughes - consists mainly of antimatter.
A Muonium Ray
Such a sophisticated experiment may soon be possible. And while Isaac Netwon is said to have observed his apple in his English hometown in 1660, this experiment could now take place in the canton of Aargau, more precisely in Villigen north of Brugg, where the Paul Scherrer Institute (PSI) is based. In principle, this experiment is quite simple: researchers produce a horizontal beam of muonium and then observe whether the electron-antimuon particles are pulled downwards by the Earth’s gravitational pull, according to Newton's law, like the water droplets spraying from a garden hose.
“In my current research project, I am investigating whether such a gravitational experiment is fundamentally feasible,” says Anna Soter. The 32-year-old scientist grew up in Hungary and earned her doctorate in 2016 at the Ludwig Maximilian University in Munich. Her thesis concerned the overlapping area between particle physics and quantum optics. She joined PSI in autumn 2017, where she now works in the research group of ETH Professor Klaus Kirch.
Search for a Rich Muonium Source
If you want to turn the gravitational experiment into reality, however, it quickly becomes tricky. Where to get Muonuim from is perhaps the most difficult question: “It is precisely this feature that is interesting for us when we look at the gravitational experiment,” says Anna Soter. “For this experiment to succeed, we need a uniform muonium beam--that is a beam in which individual particles have the same direction and the same velocity. Whether the PSI researcher will achieve this goal remains to be seen, but the first steps have already been taken. “For the production of muonium we use superfluid helium at 2.1 Kelvin. In this very cold environment, fast antimuons originating from the accelerator can be strongly cooled so that they can form muonium with an electron originating from a helium atom. In experiments last autumn with one cubic centimeter of superfluid helium, 70% of the muons were converted into muonium. It is still uncertain whether this will lead to a sufficiently abundant source of muonium in a vacuum.”
An Existence of Two Microseconds
PSI provides an ideal environment for Anna Soter’s research. No other laboratory in the world produces so many muons and antimuons artificially. But muons are very volatile elementary particles. Their lifetime is just 2.2 millionths of a second. So the scientists have very, very little time for the gravitational experiments with muonium. And since a muonium covers just 14 mm during its lifetime, an experiment must be set up in a very small space. The researcher is not deterred by such obstacles; over the next few months, she plans to work towards creating an efficient source of muonium. In addition, she will develop an atomic mirror that directs the muonium beam in a horizontal direction. “Hopefully in two years’ time we will know whether we can produce a muonium beam suitable for a gravitational experiment. If this succeeds, it would be a huge success. If not, the effort would not have been in vain: We would then have a novel source of muonium that could be used for other experiments.”
Author: Benedikt Vogel