FTL Characteristics Seen by Observer

That is an interesting question, Gary. Before I start, I should mention that to my knowledge, no one has actually seen anything go faster that light. Therefore, there may be effects I have no knowledge of. Although the results are a bit different, for a “bubble/spaceship” (for simplicity we will call this item a “ship”) accelerating away from an observer and one decelerating to an observer, the answer here is only for the first case since the other is basically the reverse of the one given. Therefore, we are assuming the ship is initially stationary with respect to the observer and then accelerates away. Initially the observer will see nothing unusual (assuming of course that seeing a spaceship is not unusual). We then allow the ship to accelerate away, to and then past the speed of light. At the start, the light from the ship (both reflected and generated within) will become more and more red-shifted. By that it is meant the wavelength becomes longer and longer. All visible light from the ship becomes redder to the observer. Eventually all “normal” light from the ship will be shifted into the infrared and the ship will effectively disappear. As it disappears (and a little into the disappearance) it will probably sparkle some as the high-energy radiation in the sky gets reflected and shifted down into the visible range. There also may be a visible trail left by the action of the bubble if there is a lot of water vapor in the atmosphere. As the ship further speeds up all normally visible light will be shifted down into the microwave and then radio and TV ranges and it’s possible it could be “heard” by those receivers. Once the ship reaches and then exceeds the speed of light, we can forget anything being reflected off of it since it will basically outrun that light. Internally generated light is, however, a different story. That radiation will continue to have its wavelength expanded and it may be possible to locate an FTL spaceship through it. I should also mention that in air, the compression of space in the front of the bubble may produce an audible sound. The air molecules would remain in their own little volume of air but the compression would rapidly push everything together. It seems reasonable to expect that to cause some sound but I don’t know what it would be like.