# What causes gravity?

What causes gravity?

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dextercioby
Homework Helper
There are 2 accepted theories which describe,unfortunately at classical level,the gravitational interaction:
*Newton's theory does not answer the question "why?",but the questions "how?" and "how much?".
*Albert Einstein's theory answers every possible question (except the ones regarding behavior at quantum level) including "why?".
He states that gravity is determined by (present whenever there is) a nontrivial (nonzero) spacetime curvature (for empty spacetime (no matter) the field equations for the gravitational field are:Riccie tensor=0).He asserts that in a flat spacetime (as the ones encountered in special relativity and quantum field theory) gravity is absent.
The logics is:curved spacetime->gravity and the other way around gravity->curved spacetime.

Daniel.

Garth
Gold Member
Welcome to these Forums afton, and keep asking questions!

quasar987 your question, "But what curves spacetime?" is easy - mass, energy and momentum curve spacetime, but I think you already know that. Perhaps your question should be, "Why does mass, energy and momentum curve spacetime?"?

We have now taken the explanation to a deeper level and perhaps a quantum gravity theory will answer that question eventually, but it will probably throw up an even deeper question in doing so!
"Turtles all the way down!"
Garth

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Chronos
Gold Member
Empirically, mass causes gravity. Under GR, energy does too. Empty space is allergic to any nearby field of matter or energy. It curves in response.

Einsteins theory is probably the best yet explanation - but there are competing notions - like gravitrons, string theorys, spatial inflow, quantum loop gravity, quantum field theory and cosmic expansion. Einsteins theory is a field theory as opposed to Newtons idea that material bodies act directly upon one another instantaneously. In Einstein's theory (General Relativity) matter conditions local space which Einstein regarded as a continuum. It has met every test so far, but it is incomplete in one major aspect - it does not predict what the Gravitational constant is - this factor must be put into the equations by hand - it is determined from experimental measurment, not from theory.

turbo
Gold Member
yogi said:
In Einstein's theory (General Relativity) matter conditions local space which Einstein regarded as a continuum. It has met every test so far, but it is incomplete in one major aspect - it does not predict what the Gravitational constant is - this factor must be put into the equations by hand - it is determined from experimental measurment, not from theory.
GR's gravitational model is incomplete in another major aspect. There is no mechanism by which gravitation arises. The curved space-time model works for simple systems (like the solar system), but it falls apart on galactic (and larger) scales. Also, curved space-time is incompatible with the flat fields of quantum physics, which prevents the extension of gravitation to small scales.

DB
afton said:
What causes gravity?
Let's put the space-time curvature aside for now. I think what you are asking is what force keeps our feet planted on the ground (you know that it's gravity) and why is that so.

As we walk on earth an accelerating force of ~9.8 m/s^2 is acting upon us, keeping us grounded. This is because we are forced to be following a geodesic (a straight line). As our planet is curved, our geodesic path is a curve aswell. The bigger the spherical planet (as are all) the stronger the force must be to keep its atmosphere at a geodesic pace. It's the curved path in space time that creates the acceleration.

If you are asking about stars and there orbits then (although there are different theories in development) General Relative solves the problem.
A star, our sun for example, has its own force in space.
Take a look at this: http://nrumiano.free.fr/Images/bh_warp1_E.gif
Every object, depending on it's mass and density, has a "deformation of space and time, "space-time fabric". So earth is orbiting the sun because it is caught in the deformation of space and time, circling around the star, as would all the vegetables in a soup would if you stuck a big spoon in and started swirling it around. :tongue2: Planets have the same effect in space, which is why they have moons.

The curvature of space and time is due to matter curving space-time itself. For example, light will always travel at "c" 299792 km/s. But when light enters and escapes a massive object, it loses it energy simply to enter or escape the object's force of gravity and remain at "c". Let's say an orange light beam was speeding through space and encounters a huge star. In order for the orange light beam to enter the star gravitational force it must use up some of its energy. When light loses energy, it is redshifted, that is it moves to the left of the electromagnetic spectrum (not that it just turns red). So if you were standing on the surface of the star the light beam would hit your eyes and seem redder than it actually is. The same happens to time. It is warped in strong gravitational fields.

Hope that helps.

turbo 1 - good point(s). Don't know however if the galactic rotational velocities may still be explained w/i the famework of GR - the answer to the question of just what space is, and what happens to it on the galactic scale of many interacting moving masses may require another theory or a modification to GR.