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There has been zero observational data demonstrating gravity nullification as an effect anywhere in the observable universe.<<
While that is true, many things exist only as man made. There are no spontaneously made AC electric motors in the observable universe.
Some modification of gravity does exist in a form of gravitational lenses.
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Lets stay on the firm ground.
If we keep away from unproven mathematical poetry, like string theory, parallel universes, traveling back in time etc. and stick with mundane commercial grade engineering physics, than by the tried-and-tested and 100 years old Special Relativity, gravity is simply curvature of space-time. Lets not forget that Special Relativity is used in everyday gadgets like GPS and satellite communications and it is quite a plain vanilla for those who know how to use it.
Whatever Gravity is, it is very unlikely that UFO electromagnetic effects have nothing to do with producing UFO's modified gravity. One can read various papers published by Bruce Maccabee and Ray Stanford that illustrate strong presence of EM fields.
Design of any flying machine, like aeroplane or UFO, will always be constrained by imperative to save weight and increase payload. UFOs are designed to cross wast voids between stars and they can not afford to carry fancy stuff of no use. Gravitational effects that UFOs produce are restricted to a small area in a vicinity of the craft. UFO's modified gravity is not spreading out further than 1 or 2 UFO diameters. Contrary to that, UFO's EM effects can be felt many miles away.
In other words, it takes UFO a lots of EM to create a very little gravity modification. Hypothetically, but within a reason, we might assume that ratio of the EM's energy to Modified Gravity's energy is similar to the ratio of UFOs mass devoted to EM vs UFOs mass devoted to modified gravity. For example if we have 30 tons UFO, than we can say that 29 tons are used for EM energy and maybe 1 ton is used for modifying gravity. Now, this is only a very crude, probably wrong working assumption, but importantly very down to earth, without any fancy stuff.
A chief aerodynamicist of McDonald Douglas, Paul Hill, estimated a weight of 10m (30ft) UFO close to 30 tons. That UFO would be beaming EM field strong enough to stop car's petrol engines from 100-200m (300-400ft) away. We are simply talking about levels of EM power that is the same order of magnitude as the output of a whole hydroelectric dam, packed in a single 10m craft. There was even a case, in Portugal, of 3m (9ft) UFO actually knocking a hydroelectric dam's generator for few hours.
What I am trying to illustrate is that EM output is so huge, it can't be just a side-show. A very large percentage of UFO's mass is devoted to creation of EM energy. So it must be of some use in modifying gravity. Otherwise UFOs wouldn't waste so much of its mass to lumber heavy EM components accross distances measured in light years.
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There are few primary considerations here. Foremost, we are dealing with a statistical analysis. Witnesses reports might not be exact, but undeniably contain statistically significant trends that are firmly grounded in known lows of nature.
Second, we have to assume that all the UFOs are not made in the same factory. As it is unlikely that we are the only developed species in universe, it is equally unlikely that there is only one more developed species. Most likely there are dozens of civilizations that are flying their crafts around in variety of UFO types. Some of them might use plasma, some will not. Some of them will use landing gear, while others will not etc. etc. But 99% of them manifest gigantic EM fields.
So we have few hundred of physical observations by witnesses and maybe a dozen of different UFO types. We have no choice but to use statistical approach here and count how many times each observed physical effect featured in witness testimonies.
Unfortunately for the flames that
@marduk mentioned, I only know of two cases where flames were seen, in comparison with hundreds, if not thousands of observed EM effects.
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Regarding the repulsive field, it actually appears in few cases as the most prominent observation. But it appears that they are always either on the ground with engines off or flying close to the ground ready to land, when repulsive field is in action. Repulsive field is new to me, because I was only aware of cases with attractive modified gravity field.
UFOs seemingly don't care about the air drag at all. They have at leas two methods to deal with it: plasma and possibly repulsive field.
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UFOs have plasma around themselves, there is no second guessing about that. They simply manage that plasma carefully enough to avoid short circuits and lightning bolts to the ground. That would be easy even with our own engineering.
Now the fact that electric field and plasma are off when UFO is about to land gives you one very important clue. Because UFO is still hovering above the ground when plasma is off (to avoid lightnings into the ground), that means that electric field doesn't have big role in creating modified gravity. That would mean that modified gravity is mostly related to magnetic field.
Now, just this single detail is consistent with General Relativity. Because one of 24 differential equations in GR describes direct connection between gravity and magnetism.
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Now, this is why it is worth carefully studying UFO physical effects. If one follows clue after clue, he or she can finally discover how to "take ET back home" as Ben Rich, Lockheed Martin's CEO once said.