1920

• In February, physicist William D. Harkins posits the existence of the neutron, a subatomic particle with a neutral charge and a mass equal to that of a proton. In 1932, British physicist James Chadwick will discover the neutron.
• In March, physicist Otto Stern announces that electrons have a spin expressible in either whole numbers (bosons) or half numbers (fermions).
• In July, American inventor Earl Charles Hanson invents the first hearing aid using vacuum tubes. It is marketed in 1921 as the Vactuphone.
• In November, John Thompson, a retired U.S. Army officer, receives a patent for his machine gun, later nicknamed the "tommy gun."
• On November 2, Station KDKA in Pittsburgh transmits the first regular licensed radio broadcast.
• On December 13, at the Mount Wilson Observatory in California, physicist Albert Michelson uses a stellar interferometer to calculate the diameter of the...