Short answers, pulled from the story.
Ludwig van Beethoven published metronomic indications for his eight symphonies in 1810. This publication marked a significant shift in how musicians understood the speed of their compositions.
Johann Nepomuk Maelzel invented the mechanical metronome during the early 19th century. This device allowed composers to specify exact beats per minute for musical performances.
The word tempo comes from Italian and means time. It describes the pace at which a piece unfolds alongside meter and articulation.
A tempo of 60 beats per minute equals one beat every second. Doubling that speed creates 120 beats per minute or two beats each second.
John Cage composed As Slow as Possible with one performance intended to last 639 years. His other work 4′33″ has a defined duration but contains no actual notes.