An unidentified flying object, or UFO, is a moving or stationary anomaly observed over the earth's surface that is not immediately identifiable to the observer.
Although UFO can refer to any object, including natural phenomena, popular culture has made the term synonymous with spacecraft that have an extraterrestrial origin.
Supporters of this hypothesis argue that UFOs cannot be natural phenomena because their flying behavior suggests propulsion and intelligent control. Furthermore, UFOs are unlikely to be human made craft such as planes, missiles or satellites because they perform maneuvers that are impossible using today's flight technology. This line of thinking gives rise to various theories aiming to explain UFO propulsion.
Those who support these theories believe that UFOs alter the ambient gravitational field and are able to produce a matter-attracting gravity potential just beyond the vehicle's front end. This amounts to an attractive force that always tugs the craft forward as if there was a massive planetary mass just beyond its "windward" end. Think of the carrot-stick effect.
Not only would a craft be able to float in the Earth's atmosphere through such gravity control but also accelerate close to the speed of Light with a minimum expenditure of energy. This type of propulsion would make interstellar travel feasible and economic.
Mainstream science scoffs at the idea of UFOs as extraterrestrial vehicles and of unconventional forms of propulsion.
The creation of an artificial gravitational field of the magnitude required by a UFO is not possible according to the main theories of physics including Newton's Laws, Einstein's Theory of Relativity and Quantum Mechanics.
According to Relativity Theory gravitational fields are only created by mass. Abrupt gravitational fields, like those required by UFOs, could only be created around huge chunks of mass, like Black Holes. Furthermore, no theory of Physics predicts a reversal of gravity, or anti-gravity, as it is commonly referred to. Unlike the magnetic and electrical fields, gravity is a monopolar force that does not have an opposite pole and cannot be cancelled.
This does not stop various theorists who are not bound by the speed limits set by official science to speculate that UFOs employ an advanced form of propulsion that is best understood in the context of a Unified Field Theory. A Unified Field Theory is said to be the future state of Physics, once the fragmentation separating Classical Physics, Relativity Theory and Quantum Mechanics is resolved.
Some of these theorists have written books on UFO propulsion and on unified field theories but the claims made in these books are generally not taken seriously by mainstream academics.
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