Atom to the positively charged nucleus by the attraction created from opposite electric charges. If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged as a whole; a charged atom is called an ion. Electrons have been known since the late 19th century, mostly thanks to J.J. Thomson; see history of subatomic physics for details. Protons have a positive charge and a mass 1,836 times that of the electron, at . The number of protons in an atom is called its atomic number. Ernest Rutherford (1919) observed that nitrogen under alpha-particle
Atom laser an optical and an atom laser are that atoms interact with themselves, cannot be created as photons can, and possess mass whereas photons do not (atoms therefore propagate at a speed below that of light). The van der Waals interaction of atoms with surfaces makes it difficult to make the atomic mirrors, typical for conventional lasers. A pseudo-continuously operating atom laser was demonstrated for the first time by Theodor Hänsch, Immanuel Bloch and Tilman Esslinger at the Max Planck Institute for Quantum Optics in Munich. They produce a well controlled continuous beam spanning up to 100 ms, whereas their predecessor