Gravity

**Gravity** is a property of objects with mass that causes these objects to move towards each other. It the principle force besides propulsion in orbital mechanics and is also the force described in Einstein's theory of General relativity.

## Pre-Newtonian Gravity

Before Isaac Newton, it was common sense that things fell, and everybody knew gravity existed. There were a lot of theories of why things fell before the inverse-square law, but the inverse-square law is cooler than all of them, so we should skip to that.

## Newtonian Gravity

Isaac Newton, the famous alchemist-physicist, proposed the inverse square law, which stated that the force of attraction experienced between two objects was proportional to the product of their masses divided by the square of the distance between them. This means if two objects (let's say the Earth with a mass of 5.9722 x 10^24 kilograms and a book weighing 0.5 kilograms) were a distance of 6,371,000 meters apart (roughly the radius of Earth), we should expect a force of ((5.9722 x 10^24)x(0.5))/(6371000^2) = 73568031804.

73568031804 of *what* though?

To make sense of this result we introduce the proportionality constant between this law (using kilograms and meters for mass and distance) known as the gravitational constant, which is equal to 6.674 x 10^-11 kgm3s2 and is commonly denoted G. This G is a measure of the strength of gravity, and due to its value in just a little over 66 trillionths, it happens to make gravity an extremely weak force compared to the other fundamental forces. Multiplying 73568031804 by 6.674 x 10^-11 gives a force of about 4.91 newtons. A force is just mass times acceleration however, so the acceleration of the 0.5 kilogram book when let in free fall should be this force divided by it's mass, coming out to about 9.8 m/s2. This is the same acceleration that all objects at sea level experience on Earth and means that objects experience a constant force as they fall, speeding them up over time.

Using the inverse square laws and other physics laws, it is possible to predict the orbits of various planets and moons with great precision. There are limits to this however. . .

## Einsteinian Gravity

While Newtonian gravity under the auspices of the inverse-square law sufficed for many purposes, it proved incapable of resolving puzzling phenomenon like the procession of Mercury's perihelion. In 1915, a decade after the *Annus Mirabilis* of 1905 wherein he debuted the Special Relativity theory, Albert Einstein would describe how energy and momentum warps the shortest paths between two points in an initially flat spacetime, leading to motion that is straight within the frame of the moving object but curved and acceleration from an outsider's standpoint. Since energy is a property all mass possesses, all mass warps spacetime in this way. So does all energy, even energy without mass such as the photon of the electromagnetic field, commonly referred to as "light." Light has no mass but due to its momentum still has energy and therefore one can create a black hole by getting enough light together in one place.

Black holes are an interesting part of Einstein's theory, as since nothing can travel faster than light it implies that an object with enough density could have a region of gravity so strong that light couldn't escape, and it would therefore appear black. Black holes were not first thought of by Einstein and had been thought of before, but Einstein shared the math which showed effects that weren't known before then if one got close to a black hole. One of these involves time dilation, wherein the clock of the world away from the black hole appears to speed up from the reference point of someone falling into a black hole. All gravity affects time in this way, where clocks will run slightly slower (or a lot slower depending on the density of the object) the closer they are to a source of gravity.

## Post-Einsteinian Gravity

Einstein's theory of general relativity has shed a lot of light on how gravity operates in the post-Newtonian regime, but there are still some corners of the universe it fails to light up, like Penrose's unilluminable room. These include questions such as "what is the singularity of a black hole *really* like?" and "does gravity have a fundamental particle like light and the photon?" To answer these questions it is likely scientists will have to formulate theories of gravity that account for the quantum world such as loop quantum gravity or string theory. So far there is no foremost clear contender for a "grand unified theory."