gravity

[g12]

how does gravity work?

[Tormod]

Who are you? Have you tried googling for "how does gravity work"?

Tormod

[g12]

who are you?wait I know a retard tormod

[deamonstar]

uh oh... watch the name calling, please.

as to how gravity works....
gravity is a psuedo-force much in the same way that centrifugal force is a psuedo-force. it is caused by the temporal inertia of mass (its tendancy to remain at rest in a given moment in time) being accelerated through space-time.
the acceleration is provided by the accelerated expansion of the universe. as a body of mass exists in space, the 'outward' expansion of space tries to push the body of mass through time as well. as space expands, it encounters resistance from the temporal inertia of the body of mass resulting in a curvature of space-time around it.
other bodies of mass are not actually 'pulled' towards another body of mass, so much as space-time is expanding around them. 'realatively' speaking, the distance between the bodies of mass will become less. (of course, other factors such as total mass and velocity and trajectory influence the behaviours of the various bodies of mass which interact with each other.

[Roberto]

Our present understanding of gravity suggests that it is only a manifestation of spacetime curvature. Mass curves the space around it. The mechanism is unclear, but it happens.

According to General Relativity, a body subject only to gravitation, simply describes a path in the space that corresponds to the shortest distance between two points of spacetime. When there is no body in the neighborhoods, it describes a straight line at constant velocity. When a body happens to be next, the curvature of spacetime makes the shortest path between to points to be distorted and the moving body will simply follow this distortion.

Pretty simple, isn't it? This is equivalent to say that all bodies subject only to gravity travel in geodesics in the spacetime. Matter curves spacetime and distort these geodesics from a straight line to other curves, depending on the distribution of matter.

[Anthony]

but can you prove it and how?

[Freethinker]

Jump

[TeleMad]

Anthony: but can you prove it and how?

What the above people are describing is gravity according to Einstein's general theory of relativity. General relativity has been confirmed by several observations.

Because the Sun is massive, it should warp the spacetime surrounding it. Thus, light that passes by the Sun should be deflected from a straight line course; it should follow the straightest possible path in the curved spacetime. In short, a star that appears to be next to the Sun should have its light bent during its travel past the Sun. To test this, some such stars were observed during a eclipse and their positions were found to be slightly off of where they should have been - their light had been bent by the Sun's mass. Since then other such observations have confirmed the bending of light by mass: one has even come to be known as Einstein's Cross - it is a single star (or is it a galaxy?) that appears to be multiple positions at once, forming a cross-like shape.

Another has to do with orbital precession. Mercury's orbit had been known to have a very slight deviation from that predicted by Newton's theory of universal gravitation. That is, the ellipse of Mercury's orbit slowly moved around the Sun. Einstein's general relativity described Mercury's orbital precession quite nicely whereas Newton's didn't.

Einstein also stated that strong gravitational fields - large curvatures of spacetime due to mass - would cause time dilation, and this too has been experimentally confirmed.

Another prediction of Einstein's general relativity is that gravitational waves exist and that they travel at the speed of light. I know that experiments were planned to detect gravitational waves, but I stopped keeping track of that kind of thing a couple of years ago. If the experiments have been carried out, then they would add to the pile of evidence that confirms Einstein's view of gravity as being warped spacetime.

[TeleMad]

Someone above mentioned that gravity is a pseudo-force in general relativity...how can that be?

Einstein introduced the equivalence principle which states that gravity and acceleration are indistinguishable - they are equivalent. Mass has two interpretations. Gravitational mass determines how strongly an object attracts others gravitationally and inertial mass determines how much an object resists attempts to change its state of motion: i.e., to accelerate it. Yet these two seemingly unrelated masses turn out to be not only proportional to each other, but to be exactly equal (that's why heavy and light objects fall at the same rate: the gravitational attraction for the more massive object is greater, but it also takes a greater force to accelerate the more massive object - since the two opposing factors are exactly equal, they cancel out). Many people had pondered on why this was but Einstein explained it.

Here's the idea in a nutshell. Picture yourself in a regular elevator. Naturally, you are standing on the floor because gravity pulls you down and holds you to it. Now, imagine that the elevator is not here on Earth but is instead out in deep intergalactic space, where there is no gravity. You are floating around inside the elevator. But if the elevator were to be accelerated at 9.80 m/s^2 in a direction towards the top of the elevator, you would find yourself standing on the floor and being held there by the accelerating elevator. You'd feel a force on your feet equal to the force of gravity if you were on Earth and the elevator motionless. The two situations are indistinguishable. Even if you perform an experiment, you can't tell the difference. For example, if you had a ball and dropped it, in either situation it woud fall towards to the floor with an acceleration of 9.80 m/s^2. Einstein concluded that since there is no experiment you could perform that would tell which is the case - on Earth and motionless or out in space and accelerating - that gravity and acceleration can be considered equivalent and gravity can be transformed away by replacing it with a suitable acceleration (or vice versa).


PS: Technically, tidal forces can be used to determine which is the case if the reference frames are large. For example, if you had 2 balls and dropped them on opposite sides of a very wide and tall elevator, you would see the two balls approach each other horizontally as they fell because of their non parallel paths directed towards Earth's center of mass. However, in space, this would not occur: the balls would fall perfectly parallel to each other. So the equivalence principle holds truly for only very small reference frames.

[Freethinker]

Originally posted by: TeleMad

Here's the idea in a nutshell. Picture yourself in a regular elevator. Naturally, you are standing on the floor because gravity pulls you down and holds you to it. Now, imagine that the elevator is not here on Earth but is instead out in deep intergalactic space, where there is no gravity. You are floating around inside the elevator. But if the elevator were to be accelerated at 9.80 m/s^2 in a direction towards the top of the elevator, you would find yourself standing on the floor and being held there by the accelerating elevator. You'd feel a force on your feet equal to the force of gravity if you were on Earth and the elevator motionless. The two situations are indistinguishable. Even if you perform an experiment, you can't tell the difference. For example, if you had a ball and dropped it, in either situation it woud fall towards to the floor with an acceleration of 9.80 m/s^2. Einstein concluded that since there is no experiment you could perform that would tell which is the case - on Earth and motionless or out in space and accelerating - that gravity and acceleration can be considered equivalent and gravity can be transformed away by replacing it with a suitable acceleration (or vice versa).

This and a good number of other thought experiments are covered in his own book on the subject "Relativity (The Special and General Theory)" Albert Einstein.

Not only do you benefit from sharing his understanding and specific apporach, but just to actually read HIS words as he felt they best explained his ideas. Great book.