Like many things in physics a simple mental experiment Official Kurt Cobain was right we’re stupid and contagious shirt tells you all, you need to know. Suppose the source of sound moves faster than the sound itself can travel. Think of sound at the tip of the airplane nose. It tries to “jump off” the airplane. Normally it would travel so as to spread: Subsonically at any time it would lie in a ring (more precisely a spherical shell) like a wave of water when you throw a rock from a pier. Now, what happens if the jet moves faster than sound? The sound at the tip of the jet “wants” to move forward. Can it? No! Why? Because sound is the pushing (pressure) of air molecules. Think of a line of people (the “molecules”) and one of them pushes the person in front of them. The “push” travels from person to person. That is sound. If the plane is moving faster than sound there is no “room to push” that can be had!
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Essentially all the sound piles up at the tip and can’t Official Kurt Cobain was right we’re stupid and contagious shirt move forward. It can though go elsewhere and indeed do as we now see. Put another way there is plenty of room for air molecules to push back and forth, aft of the direction of jet travel, more in fact than if the jet didn’t move at all! Below is a picture from NASA that shows the region where sound can travel when going supersonic. observing that along the Mach line sound exactly “catches up” with (but can’t surpass) forward displacement. This stuff is exciting to me! That makes me a nerd and therefore unfit for proper society. But in Quora land I am liberated.