What do you consider as the “greatest” invention or discovery in the last 500 years? Something that really changed the world at its time.
I nominate two:
The marine chronometer. This was/is a clock-like device which enabled sailors to determine what longitude they were at in an endless expanse of water. (you could tell your latitude from the height of the sun in the sky) In 1714, the British government offered a prize of up to 20,000 pounds to a successful inventor - which John Harrison collected 50 years later.
The telegraph. This made the Pony Express obsolete instantly (it shut down two days after transcontinental telegraphy was introduced). You could now send a long-distance message instantly, rather than waiting days, weeks - even months - for it to be received.