In the heavy version, the helium atoms captured the muons, which are much heavier than electrons. The negatively charged particles orbited very close to the helium atoms' nuclei, where they effectively canceled out one of the protons. The other electron is thereby tricked into seeing a nucleus with just one positively charged particle, just like a hydrogen atom. But the nucleus is really 4.1 times heavier than normal. The light and heavy versions of hydrogen differ in size by a factor of 36, according to Fleming's study, published Friday in the journal Science.