While on an airplane earlier today, a friend of mine and I got talking about air travel vs. train travel, the new super fast trains like the the Maglev, advantages and disadvantages of supersonic flight, and the possibility of someday making a supersonic train to possibly compete with airliners. During the course of this conversation my friend presented me with a question, a kind of a puzzle, that left me somewhat confused. It’s a bit complicated, but try to keep up, and never mind any practicalities involved in actually building such a train. Here’s the deal:
Consider the following: you are a budding young tenor on a hypothetical futuristic supersonic Japanese train, on your way to take part in a performance of, say, Mozart’s “The Marriage of Figaro” somewhere in Japan. To be precise, you are on an open platform at the rearmost carriage of the train, exposed to the open air. The train speeds up and breaches the sound barrier. While moving along supersonically, you decide to spend your time polishing your singing skills, and break into a rendition of the aria “Non piu andrai”. The sound thus produced travels away from the train in all directions at, obviously, the speed of sound, about 340 m/s. At the same time, the train moves along at a supersonic speed, let’s say at twice the speed of sound, outrunning the sound waves. Now, suddenly the train slows back down to subsonic speed again, to pull into a station. The sound waves from your performance will then catch up with you, and you will get to hear yourself sing. The thing is, and here is the conundrum, will you then hear the aria backwards?
You might think “why would you?”, but here’s the tricky part. Picture the point where you sing the word “non”. The sound starts moving along at the speed of sound, but at the same time you are moving twice as fast. At a point a little bit further down the line you sing the word “piu”, and a bit further again you sing the word “andrai”. When the train slows down, the sound waves that are closest to you, and that will catch up with you first, are from the last bit of the aria that you sang. The next waves will be from an earlier part, etc. Now, in this simplified model, the sound of the aria should come back to you as first “andrai”, then “piu”, then “non” - turning “non piu andrai” into “andrai piu non”.
Am I missing something here, or is this really a way to reverse Mozart’s aria, getting to hear yourself sing it backwards, and possibly get to find out if the great man put some hidden satanist messages in there?
my only contribution to the topic at hand is to wonder whether or not the sound would dissipate so thoroughly because of dispersion before the train slowed down that you would actually hear nothing at all?
