Probably the most important question we need to answer is why a body traveling at constant speed feels no resistance while an accelerating one feels a strong resistance. Not until we can understand this will we ever have a true understanding of the mechanism of gravity. We badly need a theory of the source of inertia.
The definition of Antigravity
Before going much further we need to clarify what is generally the perceived meaning of the term 'antigravity'. In a sense a helicopter is an antigravity device but few of us would regard this a 'antigravity'. Also a helium balloon defies gravity but here again not really an antigravity device.
If, on the other hand you placed the helium balloon in a vacuum and it still went up instead of down then that would be antigravity.
Antigravity really refers to altering spacetime in a manner that creates an area of space isolated from the normal effects of gravity.
Is 'antigravity' possible?
A useful analogy to get an understanding of the magnitude of the problem is that it's like you are given the task of creating a machine made only of ice that can move against the flow of a waterfall without touching the rock face. In this analogy, ice equates to matter and the waterfall equates to gravity or the stuff that causes it.
There is one sure fact that if gravity can effect matter then matter can effect gravity.
It is the same logic as saying that if a bat can have an effect on a ball then the ball can have an effect on the bat.
If we ignore the engineering problem then antigravity is not so hard to achieve. This example is not to be taken seriously but it illustrates a point and may even offer a clue. All you have to do is get another planet with the mass of Earth and keep it suspended about 20 metres above your head by getting the two planets to orbit each other extremely fast.
The interesting thing is, apart from being practically weightless, you would not feel any centrifugal/centripetal effects. The only thing you would notice is the stars spinning round. In your vicinity, spacetime has been altered. To achieve antigravity, the clue here is to alter the vector of space without resorting to the use of mass.
I'm going to substitute the word 'gravity' for 'accelerating field'. Matter can influence the field or the vacuum of space but, unfortunately, very poorly, except at speeds approaching that of light. Matter is like a sieve, almost transparent to this mass-less substance.
A fast spinning disc will very slightly distort gravity. An exotic sounding term for this is 'frame-dragging'. It's really just making the energy field flow in a slightly different manner. This changes the local gravitational vector compared to when the disk is stationary.
A magnetised disc, provided it is magnetised in a sympathetic plane, will enhance this frame-dragging effect. This is due to the trajectory alignment of subatomic elements contributing to the frame-dragging mechanism - see Dr Eugene Podkletnov
We have very few tools to work with when it comes to manipulating local spacetime, which needs to be done if we want to counteract the effects of gravity.
so far so good ...
We know for sure we can influence the surrounding energy field. Pulse a current through a coil of wire and you produce an electromagnetic wave. Will it produce motion - no because it is pulsed in both directions. Bounce one side off a mirror and you have all the waves going in the same direction but you won't have much reactive force. Cranking the power up will help. Construct a loudspeaker type series of coils to try and accelerate the energy and you may get a bit more reactive force. But still not enough to get you anywhere.
Photon Effect and Spacetime
That said, you can't beat getting a chunk of matter and accelerating it in the opposite direction to where you want to go, and hence my opening statement, we need to know at the fundamental level why this should be. We need a theory of Inertia.
The Theory of Inertia
The following hypothesis requires that fundamental particles react in a manner that might be expected from a combination of mutually dependent high energy electromagnetic toroidal or ring vortices.
Please bear in mind that the following text is only my opinion.
This hypothesis proposes that when matter is accelerated, the fundamental particles that define matter as matter, in contrast to the raw material, notably dark energy, will attempt to precess in a plane at rightangles to the direction of acceleration. In addition the particles will change their kinetic rotational energy in the form of increased or decreased spin in order to maintain symbiosis with the local reference frame (in much the same way as the bottom of car tyre has a symbiotic relationship with the road surface). When the acceleration stops the spin velocity has adjusted to the objects new state and further resistance to the direction of motion ceases. In a sense, the object continues to freewheel at a constant velocity and direction.
Strange as it may seem, one example of an effect which may point to the source of inertia and show what I believe to be the dynamics of the fundamental particle, is shown to us by dolphins, see the YouTube video, www.youtube.com/watch?v=ks3aQhEohTE.
The first thing to notice is that the vortex rings don't rise to the surface as quickly as might be expected. The second observation is that vortex rings, once created, travel a surprisingly long way without loosing speed as a result of the viscosity of the surrounding medium. What appears to be happening is that the outer edge of the vortex is contributing to the forward motion of the vortex, a bit like the wheel on the car analogy above. The inner edge tends to redirect the oncoming external flow around the outside edge of the ring. Maybe difficult to see the link here but in a sense, the vortex ring is swimming through the water.
Assuming that this proposed theory of inertia is somewhere near reality then it doesn't take a lot of imagination to see how one could manipulate matter's inner toroidal fields to provide them with the polarised energy levels sufficient to counteract gravity - not permanently, I hasten to add. Forgive me for verging on science fiction here but one could foresee putting an object in something not unlike a microwave and watching it float away.
One observation that perhaps should be born in mind is that, if something is possible, then nature will have done it already. In other words, if something is possible to achieve then if you look in the right places, nature, with aeons of time at her disposal, usually shows how it can be done.
In the not too distant future, the human race will understand all things regarding the physical world in which we find ourselves. We will understand how matter is created, the cycles of the universe, all things both micro and macro.
However, there is one thing that is probably going to stump us and that is understanding the concept of Infinity. It is inconceivable that infinity can actually exist either spatially or in time. We know this to be true otherwise the universe would be crawling with mega advanced aliens spawned from aeons in the past. They are not here, thankfully.