Sound waves are like ripples in a pond that radiate around the object. An aeroplane travelling at sub-sonic speed will be travelling behind the sound that it has emitted. When the aeroplane reaches the exact speed of sound, the sound it is producing will accompany it on its journey. This can be very dangerous. Why? Because while it is travelling at the speed of sound, there will be a continuous build up of sound around the aircraft. As new sounds are produced, the aircraft will fly along with those new sounds - and so on. Eventually, the racket will break up the aircraft!

Because of this, aeroplanes break through the sound barrier. They will travel at subsonic speed for a while, and then accelerate rapidly until they have passed through and left the build up of sound behind them. Meanwhile anyone below will hear the sonic boom as sound waves radiate to the ground.

Light travels much faster than sound and no machine has ever been built that gets anywhere near the speed of light. It is theoretically impossible to reach this speed as it would require infinite energy (or limited energy over an infinite time period). If the laws of physics could be broken and the speed of light reached or surpassed, the forward momentum would become perpetual.

The difference in the speed of light and sound waves can be demonstrated by observing a large cannon being fired. If you used a powerful telescope to observe the cannon firing at a half mile distance (for example), you would actually hear the blast at a fraction of a second after seeing it being fired. A more common example is the gap between a lightening strike and thunder. We all know that the difference in time between the two represents the distance of the thunderstorm from the observer.

A final example is one of our fly killer machines. If you could film an unfortunate fly being zapped by the Insectocutor IND41 with the movie camera and microphone at one end of a large room, the slow motion replay would show a distinctive gap between the flash, as the insect is electrocuted and the fizzing sound that it makes.

Of course, nobody can hear an approaching aircraft travelling faster than sound. Only after it has passed will you hear anything. Luckily for us, wasps and flies are not this fast!