Why Is There Gravity?

When you pick up a stone and release it falls to the ground. This seemingly simple concept has been known throughout history as gravitation. Isaac Newton managed to explain gravity in terms of its effects, but few have come up with a working explanation for the driving force behind it. The mysterious nature of some of the more peculiar effects of gravity, as well as the simple ones, indicate that explaining why there is gravity will be a long, difficult, yet intriguing task.

Ohanian (1976) writes that without other forces interfering, mass attracts mass. This is the fundamental concept behind gravitation. Newton explained it as "there is a power of gravity pertaining to all bodies, proportional to the several quantities of matter which they contain… The force of gravity towards the several equal parts of any body is inversely as the square of the distance of places from the particles." This description of gravity creates the simple mathematical explanation of gravity: that the force of attraction, F, equals G * ((m1 * m2) / r^2).

This holds true for most gravitational interactions on earth, so any proper theory of gravity would have to include similar results for these interactions. Misner, Thorne, and Wheeler (1973) explain that there are, however, some more complex aspects of gravity that this law does not account for. One of these strange gravitational effects is observed in the "perihelion shift" movement of planets, the most dramatically affected of which is the planet Mercury. Feynman, Leighton, and Sands (1963) contest that another hole in Newton's predictions about gravitation is that repercussions of changes in gravitational state are felt instantaneously. In other words, gravitational effects travel faster than the speed of light, which is in direct contradiction with Albert Einstein's Special Theory of Relativity. This led Einstein to develop a more advanced theory of gravitation. This was known as the General Theory of Relativity and is the closest and only thing that mankind has as an answer to "Why is there gravity?"

Einstein's General Theory of Relativity introduces the idea that space itself can be curved. Just as the ground on earth appears to be flat though it is actually curved, Einstein suggested that space may behave much in the same way. Misner et al. (1973) compared the curvature of space to an ant walking on the surface of an apple whose direction was perfectly toward the stem of the apple. The shortest possible path on the surface of the apple to the stem was a curved line. Thus Einstein has theorized that the shortest possible path in a gravitational field is actually a curve, which is contrary to traditional Euclidean geometry. The other basic concept of general relativity is that gravity is essentially indistinguishable from acceleration. A person standing inside of a stationary elevator on earth could drop a ball and it would fall, accelerating at approximately 9.8m/s^2. A person standing in an elevator free from gravitational fields, but accelerating upwardly at 9.8m/s^2 would experience the same effect. Einstein used this similarity between gravity and acceleration in the creation of the General Theory of Relativity. One of the most stunning things about Einstein's theories is that they were created with virtually no experimental verification. Only later, after his death, have we been able to prove various aspects of the General Theory of Relativity experimentally. In this sense, Einstein was well ahead of his time. One such prediction that was ahead of its time was that time moves slower in the presence of a gravitational field. In Einstein's day, they simply did not have the resources to test this experimentally. In more recent times, however, through the use of atomic clocks we have shown this to be true. It is certainly amazing to make predictions about something so abstract on a purely theoretical basis and then later have those predictions verified through experimentation.

Unfortunately, Einstein's General Theory of Relativity is the only theory of gravity that has never failed experimentally. Other theories that work experimentally are essentially tweaked versions of the same ideas that Einstein had in the General Theory of Relativity. I feel this "theory monopoly" that Relativity has is detrimental to physics in general, being that it has closed too many doors too quickly. Nonetheless, I have no choice

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