This image, taken by the US Navy in 1999, captures an F/A-18 Hornet breaking the sound barrier in the skies over the Pacific Ocean. It’s a stunning image, but what exactly does ‘breaking the sound barrier’ mean?
Well according to Tobias Rossmann, a research engineer with Advanced Projects Research, the answer lies in our understanding of sound as a wave and its finite speed of propogation.
If you think about what an echo sounds like, this is a similar concept. What you are hearing are sound waves reflecting off a distant surface at a relatively slow rate of propogation.
However in the case of an F/A 18 Horner, the aircraft is able to move at a speed that is sometimes able to catch up with the sound waves it is emitting. As explained by Rossmann,
As its speed increases to the ‘sonic velocity’ (the local velocity of sound waves), the sound waves begin to pile up in front of the aircraft and, with sufficient acceleration, can burst through the barrier and move ahead of the radiated sound.
The result? A spectacular sight like the one shown above, and a sonic boom heard from the ground. So the next time you see a military aircraft or hear one emitting an explosive boom, you know exactly what, scientifically, is going down (hopefully not the aircraft as well).