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How different are matter and antimatter? This is a question that gets at the heart of modern particle physics and early-universe cosmology. The objects of everyday experience are made of ordinary matter. While some natural process produce limited amounts of antimatter, every known star and galaxy appears to be matter-based.

If matter and antimatter are physical mirrors of each other, then an atom of antihydrogen will behave in the same way as an atom of normal hydrogen. However, if there is an asymmetry between matter and antimatter, then the forces of nature may act differently on matter and antimatter.

A recent experiment by C. Amole et al. has used trapped antihydrogen atoms to measure a quantum transition in antihydrogen. The spin-flip transition involves placing antihydrogen in a microwave resonant cavity to manipulate the relative orientations of the antiproton and positron spins. While the experiment lacked the precision needed to distinguish any differences between matter and antimatter, it highlights how to proceed if we're going to make a successful measurement in the future.

If there are differences between matter and antimatter, they could make their presence felt through differences in the behavior of antihydrogen, which consists of a positron (the antimatter version of an electron) and an antiproton. However, to measure this, we need to trap antimatter atoms long enough to study them, which has proven difficult. But CERN is now keeping antihydrogen atoms around long enough to do some detailed measurements on them.

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