Drop a PC onto Newton's or Heavyside's or Lord Rayleigh's desk.... Now that I'd pay to see...
~
Go to 1939-1940
Really just drop a case of Helium Neon laser tubes with a text on how to coat the dielectric mirrors onto a desk at the Tizzard Mission, Radlab, Bell Labs, NCR, Berkely, Delco, MIT, NRL, or Tuxedo park.
Its something they could almost make, can be powered by technology prevalent at the time, and gives you an unbeatable measurement standard and diagnostic tool in surveying, construction, physics, chemistry, bio, ME and EE... Truly a device that changed the world nearly as much as the transistor. Or just teach the chemists to make a simple, easy to build atmospheric pressure Nitrogen laser in the near uV.. Impact not as big, but lasers revolutionized chemistry.
Steve