is it really impossible?
What might be the "right" circumstances? Nobody knows but one thing is all but certain ... they won't fail under ordinary mechanical circumstances, i.e. no one will ever design a mechanical apparatus which disobeys a conservation law while all of its parts do. If such a device could have been made, it would be known by now. It's all been done a million times over. Any such breakage must necessarily occur at the limits of our technical knowledge ... where unusual things occur ... like superconductivity at extremely low temperatures ... or at very high pressures ... or at very high velocity (think relativistic). Unusual things are found every year at the frontiers of physics but those frontiers have moved way past the mechanics of anyone's garage. Which brings me to my present subject ...
UFOs as violators of conservation laws
Since I'm not a scientist, I can hold the opinion that such things "might" exist. I say maybe because I have never personally seen one but my lifetime of experience tells me that some of the supposed "nutcases" are telling the truth. I find some of the witnesses compelling and responsible based on my experience as a human. Hence, I must entertain the possibility that such is the case even though the people with the sociologically correct papers say they are all liars or fools.
Based on their common descriptions, I have tried to find some way to duplicate their maneuvers that doesn't violate any conservation law. I am unable to envision such a mechanism. There are candidates but none seems satisfactory. For instance, an electromagnetic garbage sorter could conceivably make a craft hover but it would only be viable at close range (like a few feet off the ground) ... and would generate a lot of heat. In fact, one of the things missing from witness reports is that very dead giveaway of a mechanism operating within the boundaries of the conservation laws. Imagine the amount of gas required to make a helicopter hover in one place ... a veritable inferno of heat ... but none is detected by proximate witnesses. Neither is an air stream detected. In fact nothing is detected to be coming out of the craft whatsoever except a humming noise or complete silence.
By the law of linear momentum conservation, something must be thrown out to keep a craft hovering ... or ... it must push off from the earth in some way. No standard mechanisms are workable here. Many have tried to come up with something and all have failed. The preponderance of evidence indicates that these are impossibilities ... yet I see truth in some of their faces. The scientists who dispute the credibility of these witnesses also sense that they are telling the truth ... I see fear in their eyes ... not the happy face that normally comes when one is absolutely sure of one's position. What gives?
The only option left then is broken laws of physics ...
What makes a law of physics a "law". The answer is "We do". It is out conception of things that colors our judgment. We see that nature behaves in such and such a way ... all the time ... and conclude that there is a "law of nature" present. What exactly does this mean? It means that we then believe, falsely, that the law is logically primary to the observations, i.e. that the universe first had this law ... "Conservation of Linear Momentum" ... then applied movement to masses which then had to obey the "law". I've done this myself but never got any real satisfaction from such "laws" ... they always seem slightly contrived. A better description (the true one) is that we have noted that nature behaves in the manner perceived under the circumstances observed, and, in all similar circumstances we expect that it will continue to do so. In the second description, we do not impose our will upon the universe and presume to say what the "law" is. There are no laws ... just consistent observations. The better scientists think in these terms (the ones who appear worried by an apparently truthful witness) ... the lesser ones (the majority who hide behind the credibility of the former) lay down the Law and assert that everybody is a fool and a liar as if their will alone could dispose to the issue.
Therefore, if such violations are possible ... they will likely occur beyond the present limits of our technology. But certainly not beyond the limits of our understanding. If some alien (if such really exist) were to tell us what they did to achieve silent hovering and rapid acceleration ... there would be plenty of scientists who actually understood what they were saying and could begin a program of duplication.
Perhaps a better observation is this ...
The universe always behaves equitably wherever and whenever it must do something wherein multiple possibilities are conceivable.
... meaning that it always does something that least appears to favor one choice over another of apparent equality, i.e. it refuses to "choose". It is unconscious. It just reacts to everything pressing upon it at the moment in a particular place ... sums it all up ... and just goes thataway for not other reason than that it responds to "majority rule". This "equitable action" results in the observed conservation laws which are only the result of the unconsciousness of the universe. And that given the right sum of alternatives, it might just do something else since it is not bound by any law of nature beyond the which is "equitable". It might just violate energy conservation or linear momentum conservation or anything else. We just don't know yet for sure ... not until we can experiment completely at the limits of our technological abilities and those to come.
Some things we can know right now ...
If linear momentum conservation can be violated, angular momentum conservation will go with it because we could fix two linear violators on opposite sides of a wheel and it would rotate in violation of angular momentum conservation. Also, if linear momentum conservation goes, it's likely that energy conservation will go with it. Imagine a UFO hovering. It's accelerating with a force of it's mass times 9.8 meters per second per second. Presumably, if it is not pushing off on the earth or throwing something unseen in the other direction ... it could do the same in open space resulting in translational motion. And ... if it could hover for a couple months on its power source, it could also run in empty space for a couple month's worth of acceleration and that would get it to near light speed for an energy expenditure literally millions of time smaller than what the ship would then possess in kinetic energy.
You can see the dilemma here. If a violation occurs ... all our physics is wrong ... very wrong. If no violation occurs, the people who see these things must be loco or inveterate liars. If you choose to back the conservation laws, you must turn your back on your life's experience at judging human beings (it's all wrong) ... or ... you can turn your back on the Laws of Physics as presently understood and be laughed out of town (it's all wrong). In either case, it just doesn't sit too well with the most perceptive people, i.e. those who actually make genuine scientific discoveries.
The people who are most rabidly against the supposed existence UFOs and other such nonsense (like Bigfoot or the Loch Ness Monster) have a great deal in common with the people who send me hate mail. On the one hand my hate mailers are often vicious and lacking in education while those in the "debunking" business are much more polite, educated and often wear a tie. But they have this in common ... always ... none of these people has anything of his own to offer. No new ideas about anything at all. Everything they say is what would be considered philosophically safe, i.e. "indubitably true my good fellow". What they never do is ... stick their neck out.
For the world to move ahead ... somebody's got to stick their neck out. And if most get it chopped off, well, that's the price of real progress.
Here's how you stick your neck out ...
I'm still in a holding pattern here with my rotating sphere traveling through space. If special relativity is fundamentally correct then it makes no difference if a rotating object is moving in any reference frame. It will be no different than the frame in which it is traveling, i.e. it won't curve at all. However, I don't accept that there is no absolute reference frame and though special relativity will still work in such a frame for the most part, it will break down in this instance. The ball will curve because the fast side will have more "absolute" mass than the slow side, i.e. it's increased centrifugal force on one side will give it momentum in that direction in violation of linear momentum conservation. And you can have a hovering vehicle, silent ... after jumping over an enormous technological hurdle.
I'm gonna' hold my breath till they give me my Nobel Prize ...