Modern day fighter jets can travel faster than the speed of sound without breaking a sweat, but until 70 years ago on October 14 of 1947; no human had ever traveled that fast.
Those who came close saw their aircraft become uncontrollable, or shake themselves to pieces, leading some to believe that manned flight faster than the speed of sound was impossible. There seemed to be a "sound barrier" and we wanted to break it.
You may be wondering why the speed of sound is important in aviation at all, and to understand that, you have to remember what sound is in the first place.
In a gas, sound travels as a longitudinal wave, meaning that it propagates when air molecules bunch together and then space out again. Watching a wave travel down a slinky is a good visualization.
As planes fly through the air, they create sound waves, but if the planes go fast enough, they start to catch up with their own sound waves. Air molecules get rammed into each other faster than they can get out of the way, and the waves pile up to form a powerful shock wave -- that famous sonic boom observers hear when a supersonic jet goes by.
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