Physics: The Sound Barrier and Sonic Booms

After sobering up from his drunken walk home, Isaac Newton went to see his friend, Mr Science.  However, as Isaac went along, he noticed that the roadway to Mr Science's house was very busy; Mr Science lived in the middle of town, and it was market day, and Isaac found that there were large crowds of people milling around in front of him.  Still, walking was definitely the quickest way to see his friend, as although gravity had been invented, cars were still some way off in the future.


Isaac Newton was quite keen to get to Mr Science's house to discuss his adventures with apples, including his failed attempt to launch it into space, and started jogging and jostling through the crowd, shouting at people to move out of the way, instead of just meandering through it.  He bumped into people more frequently as he did so, but kept on jogging undeterred, and found that the faster he jogged, the more people he bumped into; in the end, he put his arms out in front of him like a wedge and started pushing his way through with more effort.  This continued until he found that crowds of people were gathering together in front of him, despite him shouting at them, they were barely unable to get out of his way before he started ploughing into them.  Finally, he realised he was going that fast, that the people, all bunched up in front him and desperate to get out of his way, were unable to stand aside and he sent the crowds tumbling left and right in front of him.


When he arrived at Mr Science's house, he recounted the strange behaviour of the crowd and the various stages he'd encountered. "That's interesting," commented Mr Science, "That reminds me of an experiment I've just been running."


Isaac's journey through the crowd is very similar to an aircraft (or a car) as it travels at speeds close to the speed of sound.  The atmosphere is made up of gas particles which travel around, silently bouncing off each other and generally behaving randomly (in the real sense of the word), at speeds which are close to - but less than - the speed of sound.  Particles in a gas are extremely small (as all particles are), and by comparison, the spaces between them are relatively large.  This means that there are large gaps between them, and if you move a large, solid object between them (or, for example, start walking through them) then you're able to push them aside and move through the gas.  


Walking at low speeds, you're not likely to notice this effect, but at larger speeds, for example running, you'll feel the air as it rushes past your face.  Cycling through the air, you'll feel this more strongly, and as you increase your speed, you'll begin to feel the effort of pushing through the air - it'll feel as if there's a wind blowing into you, pushing you back.  This is known as 'air resistance' and it increases as your speed increases.  You're pushing more and more air particles aside, as you cut through the air, and this takes more effort.  At these speeds, it becomes more and more important to get into an aerodynamic position - as low down as possible, elbows tucked in, and so on, to cut through the air as economically and as easily as possible.  In Isaac Newton's case, he put his arms out in front of him like a wedge, so that he could push through the crowds of people as easily as possible.

Now, the gas particles in the atmosphere are bouncing around, flying around at close to (but less than) the speed of sound, which is 330 metres per second (or thereabouts).  In an aircraft, it's possible to approach and exceed the speed of sound, but in order to do so, the aircraft has to push through the air particles as if they were a crowd.  At walking and cycling speeds, the air particles can easily move aside as you push through them, but at speeds close to the speed of sound, they particles are unable to get out of the way of an aircraft.  The aircraft has to shove the particles aside - this becomes very difficult at speeds close to the speed of sound - and break through the sound barrier.  

The air particles start to bunch up in front of the nose of the aircraft until eventually (if it continues to accelerate) they are pushed aside in a huge compression wave.  All these particles pushed together in one go produce a loud noise - a sonic boom - as the aircraft exceeds the speed of sound and goes supersonic.  

 This sonic boom continues to travel along the ground and will be heard along the line of the aircraft's flight path - it isn't produced just once and then stops.  Mr Science tried to explain all this to Isaac, but Isaac was extremely pleased with having discovered gravity, and wasn't in the mood to discuss ways of beating it in huge flying machines, let alone ones that could travel faster than sound.  "Maybe some other time," he explained to his friend, "When I've finished with the apples."