THE ONE LINE ANSWER AND MORE

If you simply want to know how gravity works (or what gravity is) according to our best-verified theory in terms of a one-line answer, here it is:

Gravity is the effect on the motion of any object (mass or massless) when it encounters a space-time warp caused by a mass object.

If you want to know how we figured that out, read on.

To know how we understand gravity we must first keep in mind that all our physical ideas depend mainly on mathematical formulations derived from observations and a simple idea of interaction. Once we use this point of view, we can get an accurate perspective on how scientists really understand how gravity works. This way, not only do we get to understand what gravity is, we also get to know why we believe in our understanding of gravity.

BEGINNINGS

As with all physical laws, the idea of gravity began with observations. Four hundred years ago, Kepler used astronomical observations to figure out the planets orbit the Sun in precise ellipses. This was the beginning: orbits follow elliptical paths.

Even though we could plot how the planets moved we did not know why they moved that way. This was when the idea of "action at a distance" interaction was used as a possible explanation. "Action at a distance" is simply a way of stating that one heavenly body is affecting the motion of another without touching it - i.e. acting upon another object at a distance. With this idea the concept of "force" was just a few leaps away. This is a very important step in our quest to understand because often what science really needs to move forward is an idea to latch upon, verify, and refine.

NEWTON'S IDEA OF GRAVITY

A hundred years after Kepler, Newton was working on this problem of "action at a distance". Starting from his ideas that later became his Laws of Motion (i.e. the ideas of force, acceleration, momentum, and mass) and a little bit of math (calculus and geometry) he figured out a mathematical formula that described this "action at a distance".

He found that this "action" was attractive - it pulled things close, rather than pushing away. This "action" increased in effect if the masses of the objects increased, and decreased in effect if the distance between them increased. This "action" came to be called the Gravitational Force, which can be defined as the action one heavenly body exerts on another at a distance. Later Newton realized the same "action" can be used to describe falling objects on Earth, and his formula extended to all objects, heavenly or mundane, and became the Universal Law of Gravitation.

Why did Newton believe his mathematical formula of Gravitation was the right one? Simple. That formula gave orbits for the planets that exactly matched Kepler's ellipses and his three Laws of Planetary Motion.

We now have an understanding of what gravity is according to Newton's ideas: it is an "action at a distance" - called a force by Newton - exerted by anything that has mass on anything else that has a mass.

It is an attractive force, where the result is each mass object imparts an acceleration on other mass objects. We call it attractive because this acceleration is directed towards the object exerting the force. Matehmatically, the acceleration that an object of mass M imparts on another mass a distance R away is found by the formula a=(G*M)/(R*R).

EINSTEIN'S REFORMS TO THE IDEA OF GRAVITY

Three hundred years later (or less than a hundred years ago), Einstein expanded this idea of gravity. Using two new assumptions - 1) the speed of light is the same whatever your own speed, and 2) the law of physics are the same no matter what your speed - he began to apply them to motion. These assumptions came about from previous observations and they forced Einstein to reformulate Newton's Laws of Motion. Suddenly there were new formulas for velocity, momentum, and mass. (Their values now depended on the object's speed in a frame of reference.)

More importantly, Newton's ideas of space and time as independent, i.e. a change it one has no affect on the other, was no longer valid. This is because the assumptions also changed the formulas for determining distances and times - they were now also dependent of speed which made them inter-dependent on each other. distance depended on time, and time depended on distance. Einstein had to discard the ideas of a separate space (a three dimensional x, y, z spatial axes) and separate time (a one dimensional time axis, t) in favor of space-time, a four-dimensional space. (All these ideas became Einstein's Special Theory of Relativity)

These changes to the Netwon's ideas must result in changes to his Gravitational Law. What are the changes to the formula of acceleration and force? Einstein knew he had to use a four-dimensional space-time to describe these quantities. As he applied 4D mathematics to acceleration and force he found something quite interesting. He found that the presence of a mass changed space-time - i.e. entering the mass into the equation generated a formula of a warped 4D space (similar to a dent a ball makes on a taut rubber sheet). When other masses come into the vicinity of the "dent", they simply move along the warping. We cannot see it move along the warp but the effect is observable: movement and orbits as predicted by Newton's and Kepler's formulas. (These ideas became Einstein's General Theory of Relativity)

This is a very important idea because now we have an alternative to the "action at a distance" idea. Instead of a mass affecting another mass at a distance, we now have a mass affecting something else: space-time.

This is a significant change in how gravity was previously formulated. No longer is gravity an "action at a distance" effect of one mass on another. Gravity is now an effect of the warping of space-time caused simply by the presence of mass objects. (It is important to remember that these changes in ideas are a consequence of making the two simple assumptions and then following the resulting changes in the mathematics!

MODERN QUESTS TO UNDERSTAND GRAVITY

The mathematical formalism of Einstein Relativity works well on large (interplanetary or intergalactic) distances and speeds near that of light. The formalisms of Newton's ideas work well on intermediate distances and speeds. But the math for both fail at atomic and subatomic distances (10^-9 meters and lower), sometimes called the Planck scale or Planck length.

The quest then is to find a mathematical formalism of gravity between two mass objects separated by very tiny distances. The additional requirement is that this formalism must agree with the already well-established laws of quantum mechanics.

The only theory today that claims a consistent theory of quantum gravity is String Theory. It has yet to have any experiments to verify its claims. Some scientists, however, believe it is a theory that can never be verified so it should be discarded in favor for another one. But let us look into how it views gravity.

String theory changes our idea of gravity once again. Instead of a Newtonian point-masses performing "action at a distance" or Einsteinian mass objects warping space-time, string theory starts off by defining one dimensional strings that exist in many dimensions. These strings vibrate and different vibrations represent different particles. The interactions between vibrations represent interactions between particles, and one of these interactions is supposedly of a type that would look "gravitational" on larger scales.

However, while string theory is a pretty theory, it is useless without some evidence that show strings are real objects of nature. Until then, string theory and, consequently, a complete theory of quantum gravity will remain a unvalidated theory waiting to become a Scientific Law.

SUMMARY

You now know that our knowledge of gravity deeply depends on a few observations, a concept of interaction, and a whole lot of mathematical derivations. Newton's theory treated gravity as an "action at a distance" force on mass particle exerts on another without touching. Einstein's theory treated gravity as any object following a warping of four-dimensional space-time caused by another mass object.

Einstein's theory is currently the best correct idea we have about gravity. We know this because it predicts something Newton's theory does not. If gravity is a warping of space-time, then even the paths of massless objects should be altered; if gravity were an action only between mass objects then the paths of massless object would not be altered. This was verified when, light particles, or photons, which are zero rest-mass objects from stars behind our sun was observed to have a deflection of their paths by our Sun's space-time warping.

So we finally have our best understand of gravity, The Einsteinian theory of Gravity: gravity is the effect on the motion of any object when it encounters a warping of space-time of a mass object.