There are ghostly individual isolated hydrogen atoms drifting in interstellar space whose effective diameters are about 0.5 mm, while hydrogen atoms in our loca

There are ghostly individual isolated hydrogen atoms drifting in interstellar space whose effective diameters are about 0.5 mm, while hydrogen atoms in our local environment are more like 1.1 angstrom (1e-7 mm) in diameter. These are called Rydberg atoms and exist because the electon bound to the nucleus of the these atoms has climbed up to very high excitation levels, a state that is very unstable as any collision with another particle or interaction with electromagnetic fields will cause it to collapse down to lower energy levels. The extreme emptiness of interstellar space allows such atoms to persist for long periods. The density of interstellar hydrogen clouds is much lower than what can be achieved with the best vacuum systems on Earth, but laser cooling techniques have now made the production and maintenance of such atoms possible, to the point where they can be used as delicate sensors, and have possible applications in quantum computing.