Gravity is asserted to be the natural force of attraction that exists between any two objects that have mass - one of the four fundamental forces of nature. It causes objects to fall to the ground, the tides, and keeps the planets in orbit around the sun. But these things always happen; objects always fall towards the centre of the earth, tides always occur, and planets always orbit the sun. Another thing that always happens is energy is always conserved; could this be what causes gravity? The first point to note is that what is moving is a matter of perspective as the following diagrams show:

To an observer on the ground (in contact with the earth), the ball will appear to be falling down.

To the ball observer (not in contact with the earth) it is the ground that is moving up towards him.

To an observer at the barycentre of the ball and earth, both bodies are in motion, with velocities that are proportional to their masses.

The first diagram shows a ball drop as observed by a person on the ground. They see the ball falling. The second diagram shows the perspective of the ball observer (or an observer falling with the ball). They see the ground moving up towards them. The different perspective are because an observer never sees themselves as being in motion, always the other body. Diagram 3 is the interesting one; an observer is placed at the barycentre of the ball and ground. Such an observer will see both bodies moving. This is of course correct; it not possible to have only one body moving, even though this is what any real observer will observe. To the observer at the barycentre, the momentum (mass x velocity) of the two bodies will be equal.

Rotating through 90 degrees.

Doing this results in straight line motion being observed, rather than a body falling, or rising. Of course it is possible to rotate through any angle, not just 90°. So falling or rising or direction of any kind in a co-ordinate system is purely an arbitrary choice. The exception to this is the barycentre of the system, which is a single point and always in the same position.

To the rotated earth observer, the ball is now moving in a straight* line towards them. The earth observer does not see themselves moving, so this is their frame of reference.

To the rotated ball observer, the earth is now moving in a straight* line towards them. This is in the ball observers frame of reference.

To the observer at the barycentre, just as before, the two bodies are moving with velocities relative to their masses.

Conservation of energy.

When two are bodies are moving relative to each other, the momentum is of the system is conserved, i.e, it is always zero at their barycentre. Of course, conservation of momentum is just a special case of conservation of energy. Diagram 7 shows the change of energy from potential to kinetic as a body falls (from the ground observers point of view). The fact that nett energy levels of system must remain zero at all times, implies that the rate of change of energy over time must be a constant. This is the gravatational/inertial constant G. So G is the change of a bodies' energy profile over time - i.e, G=Δe/t. When the ball and ground come into contact, kinetic energy is converted back to potential energy, with the potential energy of the ball now part of the potential energy of the earth.

Potential energy is converted into kinetic energy as a body falls. When contact is made with the ground the kinetic energy of the ball is converted into the potential energy of the earth.

The black object has a mass of 10kg and the green one a mass of 5kg. As their distance from the barycentre increases, their potential energy increases.

When two objects are dropped from the same height, their potential energy at any point on the way down must be the same as it was on the way up. This means that to ground observer, both objects fall at the same rate.

The importance of contact.

When two bodies are in physical contact with each other, they will eventually come to rest relative to each other - the kinetic energy between the two bodies will become zero. But the point at which the nett energy of the two bodies is zero is not their point of contact, but their common centre of mass - where their barycentre used to be. In the case of the earth, to all intents and purposes, this is the centre of the earth. So objects on the surface of the earth will continue to try to move towards the centre - pushing against the ground. The ground will push back, and this is what gives objects weight. When contact with the ground is lost, motion is resumed and the objects become weightless (practically).

Conclusion

If the law of conservation of energy is the fundamental law of nature, then it follows that all the laws of nature, including gravity are the local effects of this one universal law. Gravity was invoked to explain why objects fall towards the ground. But the idea of falling depends solely on an arbitrary choice of direction, which will depend on an observers' perspective. To the ground observer, the ball is falling, to the ball observer it is the earth which is moving up towards them. Although no real observer can be at the barycentre of themselves and a body they observe moving, neither can they be stationary relative to a body they are observing moving. The only fixed point in the system is the barycentre, which is the point where the nett energy of the system is zero. Thus the path of the two bodies is determined by the law of conservation of energy rather than an entity called gravity. Gravity is the observed effect rather than the cause.

* The path is a spiral rather than a straight line.