what is gravity????????????????????????????????????????????

Gravitation is a natural phenomenon by which physical bodies attract with a force proportional to their mass. In everyday life, gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, and coalesced matter to remain intact, thus accounting for the existence of the Earth, the Sun, and most of the macroscopic objects in the universe. Gravitation is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for natural convection, by which fluid flow occurs under the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena observed on Earth.

Gravitation is one of the four fundamental interactions of nature, along with electromagnetism, and the nuclear strong force and weak force. Modern physics describes gravitation using the general theory of relativity by Einstein, in which it is a consequence of the curvature of spacetime governing the motion of inertial objects. The simpler Newton's law of universal gravitation provides an accurate approximation for most physical situations.

The scientists don't really know what gravity is yet. They can describe its actions by the math formula G or Gravity equals Mass 1 plus Mass 2 divided by the distance between them squared. So the Earths mass plus the mass of your body divided by 4000 squared equals the number of pounds you weigh at the earth's surface. The distance is always from the average distance so you can treat the masses as point sources. This works for most newtonian situations. Once you start getting into stellar size masses or very high speeds you need to use Einstein's formulas to figure the effects more accurately. Einstein postulated that gravity was caused by the deformation of spacetime by the masses in it but that doesn't really tell what it is, it just describes its actions.

We have 2 equally good theories for what gravity is:

General Relativity: Gravity is mass warping the fabric of Space-Time

Quantum Mechanics: Gravity is the exchange of “Messenger Particles” called gravitons. It works like a game that combines Simon Says with Catch. The players (masses) throw balls (gravitons) to each other. Every time a player catches a graviton, if the player is free to move, the player moves one step closer AND adds a graviton to the number in play.

General Relativity: Another thought experiment: Einstein thought, "What exactly is gravity?" Newton described how gravity worked but could not figure out what it was. Example: The earth is attracted to the sun. But, how does the Sun do that? Where is the "rope" that allows the sun to pull on the earth? Newton ignored the question.

Einstein tackled it. And thought (My paraphrase): Suppose you were captured by aliens and drugged, you wake up in a sealed room. The room could be sitting motionless on the earth or you could be on the alien’s spaceship being taken back to their home planet. The space ship could have a soundless, vibration less rocket motor that we cannot tell if it is firing or not. If it is firing it is accelerating the room at 9.8 m/s^2 Is there any experiment that we can do that will allow you to figure out if the room is sitting motionless on the earth or accelerating through space at 9.8 m/s^2? Turns out there are no way to figure out if we are sitting on earth or accelerating through space. This means: If we cannot tell the difference, then there is no difference. ===> Gravity is not a force but acceleration that manifests itself as a warping (curving) of Space-Time. (A very profound conclusion. Newton caught a whiff of this insight, but missed its’ significants. F = MA To get a Force you multiply Mass times Acceleration).

Another way to say this: Masses are not attracted to each other, they just follow the curvature of space-time. The acceleration of gravity is the slope of the curvature of space-time.

Einstein knew from Special Relativity that acceleration would warp Time and Space ===> Gravity is mass warping Time and Space (Space-Time) ===> General Relativity

The warping of Space-Time is necessary for General Relativity. Einstein's reasoning requires it. If it was a force as Newton proposed, then if the mass of the sun disappeared the earth instantly leave its orbit. But, Special Relativity says that nothing, not even gravity can travel faster than light ====> It must take about 8 minutes for the earth to leave its orbit after the sun disappeared. With GR there would be a "ripple" in the fabric of Space-Time that when calculated actually moves at c. All this would be conjecture except we can test GR and its predictions are accurate

Very nice graphic of the Gravity Ripple at:

http://www.pbs.org/wgbh/nova/elegant/program.html

1st Hour – A New View of Gravity

The "proofs" are down in Section 4

http://en.wikipedia.org/wiki/Introductio%E2%80%A6

If Quantum Mechanics is correct and Gravitons exist, the answer to the original question remains the same: Gravitons travel at the speed of light ===> Still takes 8 minutes. But what is gravity? An acceleration due to the warp in Space-Time or a force "carried" by a Messenger Particle? The Graviton (if it exists) would be the "rope" linking the sun to the earth..====> The result of the Thought Experiment changes: We can now tell the difference because we can in theory detect Gravitons and tell if we are under gravity or acceleration. OPINION: Gravitons are NOT a foregone conclusion. Gravity is so different from the other 3 Fundamental Forces, its mechanism could easily be different. It will be interesting to see how it shakes out. For now: Opinion - Gravity is an acceleration that results from the warp of Space-Time.

To quote Newton, in the original Latin of the Principia:

"... hypotheses non fingo ..."

'about the basic nature of these things, I frame no hypotheses ...'

Newton was asserting that the task of science (which in those times was called, "natural philosophy") was to figure out the mathematical laws that govern natural phenomena, not to try to invent some mechanism to explain them.

In Newton's theories, gravity was a universal force, exerted by every mass in the universe on every other mass, according to his Law of Gravity -- the force on mass #1 due to mass #2 is proportional to the product of the masses, inversely proportional to the square of the distance between them, and directed toward the other mass.

In Einstein's theory of General Relativity, after recognizing that Newtonian gravity was instantaneous, and that this was impossible according to Special Relativity, he hit upon the idea that all the effects we know as gravity could be described in a mathematically consistent way by specifying that mass causes spacetime to curve, and that the curvature of spacetime dictates how other masses (as well as light, which is massless!) will move.

EDIT:

Old Pilot brings up some excellent aspects of this question. In General Relativity (GR), gravitation isn't really a force; it's a pseudoforce, like the so-called centrifugal force in circular motion. Pseudoforces arise from trying to work in a non-inertial frame, like one that's rotating. In the point of view of GR, the surface of the Earth is not inertial; it is accelerating away from the Earth's center at 1 g.

And he (O.P.) alludes to this point, but let me say explicitly, that there is still no comprehensive, consistent theory that combines quantum mechanics and general relativity. A few things are known about any such theory, including the necessity of the graviton as the 'carrier' of the force, along with some of its properties (it must be massless, and have intrinsic spin=2). Also, spacetime itself in any such theory will have to cease being smooth on a very tiny scale, and become more foam-like, due to the Heisenberg Uncertainty Principle (when I say tiny, think 20 orders of magnitude smaller than a proton!). The most likely candidate(s) at present for a quantum theory of gravity, seems to be the various versions of string theory, the mathematics for which makes General Relativity look like grade-school arithmetic.

I would also point out that in GR, acceleration alone does not curve or warp spacetime. GR, borrowing from earlier work by mathematicians on curved manifolds, provides a clear recipe for computing the intrinsic curvature of a manifold, which is independent of the choice of coordinates laid onto that manifold. Accelerating coordinates (e.g., Rindler coordinates), do not introduce intrinsic curvature to a flat spacetime manifold; it is still intrinsically flat.

In fact, there is a way to distinguish accelerating at 1g in a spaceship from standing in a closed room on Earth, but it requires *very* careful measurement and observation. The easier case to describe, is how to distinguish non-acceleration in a spaceship from non-rotating free-fall (say, falling toward Earth) near Earth's surface. If you 'float' three pairs of small bodies, initially perfectly at rest inside your 'room,' one pair separated along each of three coordinate axes, and sit back and watch, then if they remain exactly where they were placed, then you're in free-fall in empty space.

But if one pair start ever so slowly accelerating away from each other, and the other two pairs ever so slowly accelerate toward each other, then that is a direct observation of curvature of the spacetime you're inhabiting, and you are near a gravitating body. (This happens because, in the Newtonian view, in the pair that's aligned with Earth's radius, the 'downward' one accelerates relative to the surface a little faster than the 'upward' one, because of the 1/r^2 dependence of the acceleration of gravity. For the other two pairs, each body is falling toward Earth's center, and they will therefore be seen accelerating toward each other.)

The force that pulls you down to the ground. That's why you can't float in earth, sucks doesn't it.

Space/time warped by mass.

Keeps you on your feet

your mother !