Short answers, pulled from the story.
Albert Einstein published the paper titled Does the Inertia of an object Depend Upon Its Energy Content on the 21st of November 1905. The document appeared in the journal Annalen der Physik as one of his four groundbreaking annus mirabilis papers.
Einstein stated that if a body gives off energy L by emitting light its mass diminishes by L divided by c squared. This formulation related only a change in mass to a change in energy without requiring an absolute relationship between the two quantities.
Eighteenth century theories included work by Isaac Newton published in 1717 while Swedish scientist Emanuel Swedenborg proposed ideas in 1734. Nineteenth century contributors included Nikolay Umov in 1873 Samuel Tolver Preston in 1875 Olinto De Pretto in 1903 Henri Poincaré in 1900 J.J. Thomson in 1881 Oliver Heaviside in 1889 Wilhelm Wien in 1900 Max Abraham in 1902 and Hendrik Antoon Lorentz in 1904.
The atomic bombings of Hiroshima and Nagasaki occurred in 1945 linking Einstein's equation directly to nuclear weapons in public consciousness. About one kilogram of the approximately 6.15 kilograms of plutonium fissioned into lighter elements totaling almost exactly one gram less after cooling as electromagnetic radiation and thermal blast energy released carried the missing gram of mass.
The solar eclipse of the 29th of May 1919 provided the first major test confirming gravitational effects of energy while the Pound-Rebka experiment performed in 1960 emitted a beam of light from the top of a tower detecting it at the bottom. NASA announced in 2018 the Parker Solar Probe reached speeds of 170 kilometers per second making it fastest ever spacecraft with differences between approximations accounting for four parts per hundred million energy correction.