| Sound waves are like ripples in a pond that radiate | | | | light reached or surpassed, the forward momentum |
| around the object. An aeroplane travelling at | | | | would become perpetual. |
| sub-sonic speed will be travelling behind the sound | | | | The difference in the speed of light and sound |
| that it has emitted. When the aeroplane reaches the | | | | waves can be demonstrated by observing a large |
| exact speed of sound, the sound it is producing will | | | | cannon being fired. If you used a powerful telescope |
| accompany it on its journey. This can be very | | | | to observe the cannon firing at a half mile distance |
| dangerous. Why? Because while it is travelling at the | | | | (for example), you would actually hear the blast at a |
| speed of sound, there will be a continuous build up of | | | | fraction of a second after seeing it being fired. A |
| sound around the aircraft. As new sounds are | | | | more common example is the gap between a |
| produced, the aircraft will fly along with those new | | | | lightening strike and thunder. We all know that the |
| sounds - and so on. Eventually, the racket will break | | | | difference in time between the two represents the |
| up the aircraft! | | | | distance of the thunderstorm from the observer. |
| Because of this, aeroplanes break through the sound | | | | A final example is one of our fly killer machines. If |
| barrier. They will travel at subsonic speed for a while, | | | | you could film an unfortunate fly being zapped by the |
| and then accelerate rapidly until they have passed | | | | Insectocutor IND41 with the movie camera and |
| through and left the build up of sound behind them. | | | | microphone at one end of a large room, the slow |
| Meanwhile anyone below will hear the sonic boom as | | | | motion replay would show a distinctive gap between |
| sound waves radiate to the ground. | | | | the flash, as the insect is electrocuted and the fizzing |
| Light travels much faster than sound and no machine | | | | sound that it makes. |
| has ever been built that gets anywhere near the | | | | Of course, nobody can hear an approaching aircraft |
| speed of light. It is theoretically impossible to reach | | | | travelling faster than sound. Only after it has passed |
| this speed as it would require infinite energy (or | | | | will you hear anything. Luckily for us, wasps and flies |
| limited energy over an infinite time period). If the | | | | are not this fast! |
| laws of physics could be broken and the speed of | | | | |