Well Mike, when a rocket is launched it goes from sub to supersonic and apparently the sound waves move upwards and therefore not heard. That is what Professor Google told me.
They are out there and they are watching all the non believers Mike.
And that is to be expected with a rocket Dmaxer but are you suggesting all these aliens are so bored with us they are leaving? Surely one or two will want a look around.
Have you listened (because it was originally written as a BBC radio series) to the Hitch Hikers Guide to the Galaxy? Brilliant! Douglas Adams' imagination was something to behold. I was particularly taken by the concept that aliens hardly ever visited earth because it was so boring.
An exception being "Teasers": "Rich bored kids who land by the side of an old farmer's (who no one's going to believe) shack in the middle of nowhere and strut up and down making silly beep-beep noises". That's my kind of alien :)
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"I beseech you in the bowels of Christ think it possible you may be mistaken"
Oliver Cromwell, 3rd August 1650 - in a letter to the General Assembly of the Kirk of Scotland
I guess if they are advanced enough to travel here they know how to avoid sonic booms. Maybe they fly subsonic when nearby. Who knows, except them Mike