Antigravity
Antigravity is a very complicated subject, because we don’t even really know what gravity is, let alone anti-gravity. In fact from Newton to this day we only know basic concepts about gravity, it is considered to be one of the most mysterious phenomenon, and therefore discovering something to prevent it or shield the object in question seems to be one of the hardest tasks, even in theory.
What is Antigravity?
Having mentioned the complexity of finding antigravity, we can still discuss the concept surrounding it, which is the idea of creating a place or object that is free from the force of gravity. Note that there is a difference between antigravity and countering the gravitational force by an equal and opposite force as an airplane, helicopter, balloons … Anti-gravity requires that the gravitational force be partially or entirely neutralized.
Also note that magnetic levitation (maglev) suspends an object against gravity by use of electromagnetic forces. While visually impressive, gravitation itself functions normally in such devices. Critics of various alleged anti-gravity devices often suggest that unusual effects observed around them are due to electromagnetism.
Antigravity Machine?
An antigravity machine is something that we apparently failed to produce so far. At least that’s how the world understands it. Science has, as yet, failed in its attempts to shield or nullify the effects of gravity, though some continue to study possible methods of success in this endeavor.
Newton's Law of Gravitation considered gravity to be a force between two objects, causing attraction in proportion to the objects' mass and distance between them. Under this interpretation, an object with negative mass would repel ordinary matter, and could be used to produce an anti-gravity effect. Alternatively, depending on the mechanism assumed to underlie the gravitational force, it may seem reasonable to postulate a material that shields against gravity or otherwise interferes with the force. A fictional example of such a material is cavorite, which is a major element in H. G. Wells' famous book The First Men in the Moon, although cavorite isn't consistent with even a Newtonian view of the universe by causing violations of conservation laws.
Most physicists believe that at extremely high energies, gravity and the other fundamental forces unify, which would allow gravity to be manipulated in ways that are not readily apparent now.
Some models of anti-gravity claim to be based on quantum gravity models, though the connection of these to mainstream quantum gravity models is often tenuous.