Have you ever been to an airshow and witnessed a fighter jet getting covered in what looks like a cloud of smoke, shortly before hearing a terribly loud bang? Well, that's a sonic boom. But, what exactly causes them?
The speed of sound is 1,225 km/h, which is most definitely fast, but, modern-day fighter planes are capable of exceeding this speed by quite a lot. The fastest plane in the world is the Lockheed SR-71 Blackbird, which is capable of travelling at over 3,500 km/h!
When a plane is flying overhead at a speed slower than the speed of sound, the sound reaches your ears before you see the plane. This is why you can tell if a plane is about to fly over. However, if the plane speeds up quite significantly, let's say to 1,500 km/h, you can see the plane fly over way before you can even hear it.
If you continuously speed the plane up, eventually all that sound energy that the plane produces gets compressed behind the aircraft. Eventually, all that sound energy gets released from behind it and we momentarily experience a sonic boom.
Take a look at the video below by the YouTube channel, Star Talk, on Niel deGrasse Tyson Explains Sonic Booms.