Heisenberg’s starting point was the philosophical judgment, that a physical theory should not concern itself with things like electron orbits in atom… - Steven Weinberg

Acceleration is the rate of change of . ...The units are ...velocity per unit time, or distance-per-time per time. ...[F]alling bodies ...near ...earth fall with an acceleration or 9.8 meters-per-second per second ...after the first second ...falling at speed ...9.8 meters per second, after two seconds... 19.6 ...and so on. [T]he units of velocity are length/time... and units of acceleration... (distance/time)/time, or equivalently distance/time² ...[T]he acceleration near... Earth would be written 9.8 m/sec² for short.

Electrons are believed to be absolutely stable, and protons and neutrons (when bound...) live at least 10³⁰ years. With few exceptions, all other particles have very short lifetimes...

By the end of the nineteenth century the idea of the atom had become familiar... but not yet universally accepted. Partly because of the heritage of Newton and Dalton, there was a disposition to use atomic theories in England. ...Resistance to atomism persisted in Germany ...under the influence of an empiricist school... centered on Ernst Mach... many [German physicists and chemists] held back from incorporating into... theories anything that—like atoms—could not be observed directly. ...It is said that the opposition to Boltzmann's work by the followers of Mach contributed to Boltzmann's suicide...