Phenomenon/Gravity

Gravity

Gravity (Force, Physics, Theory):

The Newtonian theory of gravity is based on an assumed force acting between all pairs of bodies—i.e., an action at a distance. When a mass moves, the force acting on other masses had been considered to adjust instantaneously to the new location of the displaced mass. That, however, is inconsistent with special relativity, which is based on the axiom that all knowledge of distant events comes from electromagnetic signals.

In Einstein’s theory of special relativity, inertial mass is a manifestation of all the forms of energy in a body, according to his fundamental relationship $E=mc^{2}$, $E$ being the total energy content of a body, $m$ the inertial mass of the body, and $c$ the speed of light. Dealing with gravitation, then, as a field phenomenon, the weak principle of equivalence indicates that all forms of nongravitational energy must identically couple to or interact with the gravitational field, because the various materials in nature possess different fractional amounts of nuclear, electrical, magnetic, and kinetic energies, yet they accelerate at identical rates.

Newton's law of universal gravitation

The equation for universal gravitation thus takes the form: $F=G{\frac {m1,m2}{r^{2}}}$

Where $F$ is the gravitational force acting between two objects, $m1$ and $m2$ are the masses of the objects, $r$ is the distance between the enters of their masses], and $G$ is the gravitational constant ($G=6.67430(15)\times 10^{-11}{\rm {\ m^{3}{\cdot }kg^{-1}{\cdot }s^{-2}}}$).

Henry Cavendish

The most famous of "experiments" for "proving" gravity is known as the Cavendish experiment. In 1798, Henry was to determine the density of the Earth using a modification of the torsion balance built by geologist John Michell, who died before he could begin the experiment. The apparatus was sent in crates to Cavendish, who completed the experiment in 1797–1798 and published the results.

The problem with the Cavendish experiment was it doesn't converge to any specific value. The idea behind the experiment is that it is measuring the universal gravitational constant G. Despite our advances in technology over the last 200 years we still can't seem to all agree on a value.

Gravimeter

Using a device called a gravimeter you can measure downward acceleration directly by dropping a metal ball in a vacuum tube and keeping time with a cesium clock. People have found that gravity varies from location to location and even in the same location from day to day.

Here is one webpage detailing this but there are several on this topic: nist.gov Big G Redux: Solving the Mystery of a Perplexing Result

“Big G has been a frustrating problem, The more work we do to nail it down, the bigger the divergences seem to be. This is an issue that no metrologist can be pleased with.”
- Carl Williams, Deputy Director of NIST’s Physical Measurement Laboratory (PML).

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