What is Gravity? The stuff that keeps our feet glued to the earth, the stuff that hit Isaac Newton’s head with an apple, the stuff that keeps our Earth revolving around the sun?
Isaac Newton’s Universal Law of Gravitation states that “Every particle in the universe attracts every other particle with a force along a line joining them. The force is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”. ‘
Gravity seems to be very simple in definition, in that it “is always attractive, and it depends only on the masses involved and the distances between them”.
The Newtonian definition of gravity seemed to fit most-Earthly situations. But when you account for massive, stellar objects, Newton could not properly explain their properties! For instance, the Newtonian definition could not explain Mercury’s distorted orbit around the sun.
Albert Einstein’s Theory of General Relativity accounts for these situations with ease, though! General Relativity proposes that all of space and time is composed in a fabric-like structure. Stellar objects are suspended in this fabric. As any Earthly being knows— a dense object (such as a tennis ball) laid on a cotton sheet will cause the sheet to sink and form a dent.
If you place say, an M&M or a small candy in that sheet, the candy piece will be drawn to the dent and will sink! Say that cotton fabric was actually really long and wide. If you place the M&M on the edge of the sheet, away from the tennis ball, it could probably sustain itself.
Einstein applied this concept to the Universal fabric of space-time. Space-time is 4 dimensional, so it is not only space that is warped.
General Relativity has been experimentally accurate. It is the most current and up-to-date knowledge on the nature of space and time. The hunt has not ended, though. With the recent progress scientists have made, quantum mechanics proposes theory on forces at the subatomic level.
The electromagnetic, strong, and weak forces can be explained on the quantum level. Gravity, though, remains excluded. Modern physics aims to unify the four forces of nature. General Relativity must fit in the picture of Quantum Mechanics! This is a search for the “theory of everything”.
The quest for Quantum Gravity is a relatively new, unique field of research. One hope lies in String Theory.
It is exciting to notice how even an ordinary question, “Why does the apple fall on my head?” has far more depth behind it. The question has lead scientists to venture into new fields of study for more than decades.