Exotic atom to weak, electromagnetic and gravitational forces, muonic atoms are governed to very high precision by the electromagnetic interaction. Since a muon is more massive than an electron, the Bohr orbits are closer to the nucleus in a muonic atom than in an ordinary atom, and corrections due to quantum electrodynamics are more important. Study of muonic atoms' energy levels as well as transition rates from excited states to the ground state therefore provide experimental tests of quantum electrodynamics. Muon-catalyzed fusion is a technical application of muonic atoms. A "hadronic atom" is an atom in which one or more of the
Captain Atom of alternate Captains Atom, including Earth-13's Brigadier, the Captain Atom as depicted in the graphic novel "Kingdom Come" (Earth-22), and a Captain in a red/silver variant of the Monarch's costume, one similar to the Charlton Comics Atom (Earth-4), and a Hulked out variant named Attum from an unknown Earth. Several more variants are shown in "Countdown: Arena" #4, including a President Atom, a robot called Quantum Mechanix, Kid Quantum of the Legion of Super-Heroes, Captain Adamma, Quantum Boy, an anthropomorphic wolf version, a from Earth-30, a Doctor Manhattan-lookalike, an energy based-Atom who makes calculations during his attacks, and a giant-sized