Actually there is a way. I know this because such craft regularly break the sound barrier without a sonic boom. In fact, I do not recall ever hearing about so much as a breeze caused by the movement of these things. And most are utterly silent. No engine noise, no, not so much as a rustled leaf as it passes by. They must have to dematerialize the air in front of them and re-materialize it again in the place the ship just vacated. Perhaps they merely exchange space/time for a different position in the future. The air surrounding the vacuum created when they leave their position has no time to implode before it is replaced by the air displaced by the new position of the spacecraft. This requires technical knowledge we do not have and my ridiculous “”theory” regarding exchanging space/time or dematerializing air is really silly. Or maybe it is not, I do not know. But there is a way…a realistic way…
All you have to do is create a vacuum on the leading edge of your craft and a pressure wave on the trailing edge. Do it just right and get the force and timing down pat and you can cancel any sonic boom. This means you have to have something to create a vacuum on the leading edge and something to create a pressure wave on the trailing edge. In a spherical or round craft there is no leading or trailing edge so you would have to have both vacuum and pressure ports evenly distributed. Of course distribution depends on the aerodynamics of the craft.
The occupants are also subject to G forces which would crush any living creature and the structure itself cannot withstand the G forces. So here is how they probably work:
They control gravity. They cancel extreme accelerations by using opposing gravitational forces. Gravitation is very much like an acceleration anyway so they just counter using an opposite force. This is like canceling inertia, but it really does not cancel inertia, it just creates an opposing force such that the net force on the occupants and structures is zero, or very close to it. Or maybe they just control time. You have the craft travel in its own time bubble, and the observers (that is, everyone else) in normal time. So our fighter pilots chasing the craft see the thing zoom off at an impossible speed. What is really happening is that the craft is taking its good old time, but time speeds up for them and them alone. One moment we see them here, the next they are somewhere else. Again, I do not really know so I am inventing impossible physical laws to prove that other physical laws are not being violated by the impossible maneuvers of alien craft.
Finally, how do they become invisible? With a cloaking device? Well, yes. We have the technology to do that now anyway, and in fact we probably are using it right now. Here are some techniques to make aircraft (or anything else) invisible at any wavelength:
1. (PASSIVE TECHNIQUE) Plaster your vehicle with LCD chips or any other paper thin technology used to generate pictures. Intersperse those LCD pixels with ambient light sensors and you can then measure the ambient (light) radiation. Having measured the “blueness” of the sky, you just display the same “blueness” on your pixel-painted craft. Want a desert sand painted battle tank to change color because it just rolled up to a watery oasis? Same technique.
2. (ACTIVE TECHNIQUE) Absorb incoming radar signals using materials which are transparent to radar wavelengths which they already do on the stealth fighters. Anything not absorbed due to materials limitations can be deflected, or there can be a honeycomb structure just underneath the skin which reflects the radar beam into a dissipation maze. By dissipation maze I mean a series of reflectors which cause the beam to bounce around until the power fades. The dissipation maze would intrinsically have to be very small so the number of reflections until the power dropped enough to not worry about it would be a large number. But who cares as long as it is trapped? (1/(r squared), right?). Analyze and retransmit a fake radar return which would look like a flock of birds, or nothing at all. Why not just measure the incoming radar power, and transmit a Doppler-shifted wave that is exactly 180 degrees of out phase with the incoming radar beam? Since the craft would be traveling it would have to be Doppler shifted on the fly. All this would take tremendous and fast computing power. Why not carry decoys in the form of small RPV’s? Send them off in another direction with a lot of noise (attention getting) while at the same time you switch on your steath mode? Remember the old SeaQuest show where a futuristic submarine deployed “whiskers”, which were small independent craft?
Am I just bullshitting you? Yes I am. I have no idea how this stuff works. Really.