Thank you for the nice remarks on my essay.
Here are some thought about gravity that are a bit different.
Einstein, who, more than anyone else gave us our current view of the nature of gravity, said that gravity is not a force and yet in most of contemporary physics gravity is treated as if it were. It appears that the presently held view of gravity is that it does not pull you into the chair in which you are sitting but rather, because of the curvature of space-time, it pushes you into the chair. This is a bit absurd; Gravity is either a force or it isn't, it simply can't be both.
Einstein used the example of a man jumping from a building. The man would feel no force pushing or pulling him. The only way he would know he is moving is by the motion of the building that seems to be moving up and the friction of the wind. While nobody challenges this it seems to be almost universally ignored. The example of the man falling is a good one but gravity can be proved to not be a force by use of a very simple, basic physical law.
Suppose I hold a ball of a given weight stationary in the air. The understanding of vectors tells us that a force equal to the force I am supplying must be pushing down on the ball. Vector analysis also tells us that a resulting vector will appear in a direction opposite the acute angle formed by the two vectors. The acceleration of the resultant vector, if the forces are constant, is dependent upon the sine of the acute angle formed by the two vectors. In the case of my holding the ball the angle formed by my pushing up and the alleged force of gravity pushing down is 180°. The sine of 180° is zero so the resultant vector is zero. It is important to remember that the force and acceleration of both vectors is still very real.
Newton's second law of motion says that Force is equal to Mass times Acceleration - F=ma. If I hold a ball ten times heavier the force I supply must be ten times stronger as well. In order to the stationary position of the ball I must also increase the downward force ten times. Herein lies the problem.
Acceleration is dependent on force and mass. The only way acceleration can be changed is to alter either the force or the mass. We know that acceleration in a gravitational field is a constant. On the earth it is 32 feet per second squared. If the gravity of the earth is a force and created by the curvature of space-time then this force too must be constant. The only thing that is a variable is the mass however, if we change the mass we change either the force or the acceleration. Thus either heavier objects fall more slowly than lighter objects or the acceleration changes as a result of the change in mass. We know empirically that this cannot be true as both force and acceleration are constant. Therefore gravity cannot be a force.
The ball is now ten times heavier and thus the gravitational field (if indeed that is the correct term) is ten times as strong. The curvature of the space-time created by the ball is greater and so, if gravity is a force, the ball is pulling the earth with a stronger force. Actually the acceleration of the earth toward the ball has increased and so the earth is falling toward the ball at a greater velocity. We can see this in Newton's other formula: F = G(m1m2/r2) While this does not exactly hold in GR it is sufficient for this argument. The increase in the apparent attraction of the earth and the ten pound ball is so small as to be virtually immeasurable.
If gravity is not a force why do we feel our weight when sitting in a chair? Consider a situation where two opposing vectors are both forces, such as two cue sticks pushing on a billiard ball at two points in direct opposition.
The change in the position of the ball is zero and we can state that this is the resultant force of the two primary vectors. We have the mass of the cue ball and the force applied by the cue sticks. This means that there is in both cases an acceleration. An object can have any number of independent motions and in this case the ball is moving in two directly opposite directions but the ball is moving. The second law of motion states that force and mass will produce an acceleration. These two opposing accelerations do not 'cancel each other'. They create a vector with zero acceleration. Perhaps it may be more correct to say that they produce no vector.
Since gravity behaves much like a force, we feel our weight in a chair because we are still falling. Just because the chair stops a change in position does not mean we are not still falling. Our feeling of weight comes from momentum. A falling body has a certain momentum even if it does not actually change its position. It is this momentum we feel when sitting in a chair.
Since gravity is not in any way a force it has none of the properties of a force. It does not propagate. It would only propagate if it were a force. Contemporary physics not only thinks of gravity as a force but appears to think of it as an electromagnetic force. Many, many hours have been spent by really brilliant people trying to reconcile the 'force' of gravity with such forces as magnetism. The mass of an atom does not create the curvature of space-time any more than the nucleus creates the electron. The curvature is an integral part of the atom that was created when the atom was created. It cannot be modified nor removed.
Newton, when he worked out his gravitation theories, was concerned with action at a distance. Even though gravity is ubiquitous through the universe there is no action at a distance because there is no action. Gravity does not do anything, it simply is. It is not one of the elementary forces as it is not a force. There is no need for energy mediating bosons to mediate the force ergo, thus there is no graviton. I seriously doubt that the Large Hadron Collider will find any evidence of a massless, spin-2 boson.
It has been said that if the sun were to suddenly disappear we would not be aware of it for eight and a half minutes. That is true but has nothing to with the curvature of space-time and thus gravity. If the sun were to disappear instantly the curvature would disappear instantly as well. We would not sense this in any way, since the path of earth around the sun is a geodesic nothing would have changed; we would still be traveling in a straight line. Eight and a half minutes later everything would become instantly dark and start to quickly become very cold. That we would certainly sense and then we would know that the sun had disappeared.
The extent of a gravitational field appears to be limitless. It diminishes as described by the inverse square law but never completely disappears. Thus the entire universe is one large structure formed of a myriad of space-time curvatures.
Finally; since gravity is not a force why it is considered along with magnetism, the strong nuclear force and the weak nuclear force to be one of the primary force interactions of physical reality? Gravity is not a force, it is a condition.
If indeed gravity is not a force, are we correct is thinking that gravity functions at the quantum level? Does an elementary particle warp the space-time or is the concept of space even valid at the quantum level. It seems quite possible that gravity at quantum level may be a mathematical concept that would only be valid if gravity is a force.
