As Andrew said, the action of gravity is thought to be (or to result from) the curvature of space-time.  I avoid "force" because once space-time is curved, force is only required to alter the natural trajectories due to gravity.

This begs the question of where does the force, action or other process come from to alter space-time, and what is space-time?  I just posted a question on the nature of space-time, see link below.

The more interesting answers to the force or action of gravity involve what today passes for fundamental explanations, i.e. something involving quantum mechanics.  There are several possibilities.  We actually do not have accurate enough measurements of strong gravity to distinguish among them, but should have in this century.

• The "standard" formulation is that gravity originates in an exchange of bosons, like other forces.  The hypothetical (never observed) gravity boson is the graviton.  This supports a momentum exchange model along lines of force, and upholds Gauss' Law on which the equation of general relativity is based.  It suggests the "strength" of gravity through any area enclosing the source mass is the same, called "conservation of source."  This is similar to the electromagnetic theory in that regard, and similar to strong and weak fields in the use of bosons.  However, those bosons are not massless and those fields do not obey Gauss' Law, but decrease too fast with distance.
• One (of many) alternative formulations of gravity is one that I developed several years ago which proposes that it comes from a quasi-measurement interaction.  I can provide links on this for interested readers.  It uses quantum states which have 1/R strengths in each of three orthogonal directions.  This matches a Newtonian potential at all distances, but at very short distances it is just a bit weaker than General Relativity, enough that instead of black holes one gets "black dots."  In this kind of gravity, the primary effect is Mach's Principle.  The "mass" and therefore spatial uncertainty of objects (mass or energy) is due to proximity to other masses.  It is entirely relativistic, however, the effect is through inertia, which dilates time, and only through recent arguments have I been able to relate this to spatial curvature (which is empirically verified and therefore must be addressed).

https://www.researchgate.net/post/Do_you_believe_in_the_block_universe_a_block_of_space-time_where_time_is_much_like_space?_tpcectx=qa_overview_asked&_trid=v2dSjY0cGEjKhTffJPuny46p_

Gravitational force arise when the force of will splits consciousness into particle and anti-particle. Force of will operate on individual scale as well as on universal scale. The split is apparent and the consciousness ever remains indivisible. The details of the theory are discussed in the following article and the references there in.

Article Unified field of consciousness

As Andrew said, the action of gravity is thought to be (or to result from) the curvature of space-time.  I avoid "force" because once space-time is curved, force is only required to alter the natural trajectories due to gravity.

This begs the question of where does the force, action or other process come from to alter space-time, and what is space-time?  I just posted a question on the nature of space-time, see link below.

The more interesting answers to the force or action of gravity involve what today passes for fundamental explanations, i.e. something involving quantum mechanics.  There are several possibilities.  We actually do not have accurate enough measurements of strong gravity to distinguish among them, but should have in this century.

• The "standard" formulation is that gravity originates in an exchange of bosons, like other forces.  The hypothetical (never observed) gravity boson is the graviton.  This supports a momentum exchange model along lines of force, and upholds Gauss' Law on which the equation of general relativity is based.  It suggests the "strength" of gravity through any area enclosing the source mass is the same, called "conservation of source."  This is similar to the electromagnetic theory in that regard, and similar to strong and weak fields in the use of bosons.  However, those bosons are not massless and those fields do not obey Gauss' Law, but decrease too fast with distance.
• One (of many) alternative formulations of gravity is one that I developed several years ago which proposes that it comes from a quasi-measurement interaction.  I can provide links on this for interested readers.  It uses quantum states which have 1/R strengths in each of three orthogonal directions.  This matches a Newtonian potential at all distances, but at very short distances it is just a bit weaker than General Relativity, enough that instead of black holes one gets "black dots."  In this kind of gravity, the primary effect is Mach's Principle.  The "mass" and therefore spatial uncertainty of objects (mass or energy) is due to proximity to other masses.  It is entirely relativistic, however, the effect is through inertia, which dilates time, and only through recent arguments have I been able to relate this to spatial curvature (which is empirically verified and therefore must be addressed).

https://www.researchgate.net/post/Do_you_believe_in_the_block_universe_a_block_of_space-time_where_time_is_much_like_space?_tpcectx=qa_overview_asked&_trid=v2dSjY0cGEjKhTffJPuny46p_

“…the action of gravity is thought to be (or to result from) the curvature of space-time. ”

- that  isn’t so, neither space, nor time and nor spacetime can be “curved”, the tribal Matter’s spacetime is 4(cannot be excluded 5) D absolute logical empty container – 4(5)D Euclidian manofold, which exists independently on – there is or not something in the container and if exists – on what happens with/ inside this something;

“…In this kind of gravity, the primary effect is Mach's Principle… ”

- the Mach’s principle is nothing more then some bare declaration that has no either  theoretical  grounds or experimental hints;

“…It is entirely relativistic, however, the effect is through inertia, which dilates time…”

- again - nothing can “dilate” time, including “inertia”; besides to obtain in the GR [non-existent in the reality] “spatial curvature” is necessary for the “time dilation” value be in accordance with the GR, when the GR predicts this value that is evidently twice larger then the real – “Newtonian” value of clocks’ tick rate (not, of course, “time dilation”) slowing down in the gravity field…

Cheers

From the perspective of black holes this is what gravity looks like

https://www.youtube.com/watch?v=8I9LQfWY6pA

Dear Colleagues,

What one can say reasonable about an old   classical problem  - the General relativity deals only with velocities and accelerations in the form of geodesic equations, mimicking the Newton law of motion, yet - they have nothing to do with the energy and momentum conservation laws, as the Hamiltonian  and momentum quantities are,  on the whole, within the GR not defined...

With best regards,

A.

In the article "Gravito-electromagnetism explained by the theory of informatons" it is shown that the gravito-electromagnetic (GEM) description of gravitational phenomena and laws perfectly can be deduced from the hypothesis that "information" is the substance of gravitational fields.

In that paper the idea is developed that any material object manifests itself in space by continuously emitting granular entities that run away with the speed of light, carrying information about the position ("g-information") and about the velocity ("beta-information") of their emitter.  Because they transport nothing else than "information", these entities are called "informatons".

From the hypothesis that informatons are the constituent elements it follows that the gravitational field is characterized by two vectorial quantities: the density of the flow of g-information that is defined by the field  Eg and the density of the cloud of beta-information that is defined by the gravitational induction Bg  (§1, 2, 3).  The laws of GEM are mathematically deduced from the kinematics of the informatons (§4);  the gravitational interactions are explained as effects of the trend of a material object to become blind for the flow of informatons emitted by other objects (§5);  it is shown that an oscillating mass is the source of a gravitational wave transporting packets of energy (gravitons - §6);  and the nature of gravitation is described (§7).

Article GRAVITO-ELECTROMAGNETISM EXPLAINED BY THE THEORY OF INFORMATONS

The force of gravity in General relativity  (GR) is thought to come form the curvature of space-time.

Since we have discovered that energy is inherent in the fabric of space-time then the true picture is an energy density gradient,.

The graviton is a guage particle which repels space-time in the vicinity of a gravitating mass

The force results form this energy density gradient

Additionally GR falls down in a number of areas

But given the new paradigm where space-time has an energy density gradient there are important answers to gravitation which you may find interesting.

Yes GR works in the weak fields of the the solar system, and Intermediate

fields of neutron stars. But

1). It produces infinite density singularities in the very strong fields

such as in black holes.

2).It does not necessarily explain dark matter in galaxy cores where we

know super-massive black holes exist and the galactic dark matter

therein,

3). It does not explain dark mater present in galactic halo's,and galactic

clusters such as the bullet cluster,

4).It does not explain the presence of cosmological dark matter as a whole.

5). It has not yet been fully corroborated by the studies of neutron stars

and in black holes.

6). It does not easily explain the presence of dark energy and cannot be

translated in quantum gravity.

Here I enclose a number of publications which explain all of these

phenomena, whilst agreeing with the results of GR where it has been

thoroughly tested.

1).First to obviate the infinite density singularities.

1a)

https://www.researchgate.net/publication/32897680_An_advanced_dynamic_adaptation_of_Newtonian_equations_of_gravity?ev=prf_pub

1b)

https://www.researchgate.net/publication/269689554_The_formulation_of_Dynamic_Newtonian_advanced_gravity_DNAg?ev=prf_pub

2).Secondly we explain the presence of dark matter at the centre of the

galaxy.

2a)

https://www.researchgate.net/publication/228858219_Advances_in_Black_Hole_Gravitational_Physics_and_Cold_Dark_Matter_Modelling._The_Gravity_of_Dark_Matter

3) Thirdly it is possible to explain the presence of dark matter in the

galactic halo.

3a)

https://www.researchgate.net/publication/269689929_Advances_in_Black_Hole_Physics_and_Dark_Matter_Modelling_of_the_Galactic_Halo?ev=prf_pub

4). It is able to explain the presence of cosmological dark matter

https://www.researchgate.net/publication/32897680_An_advanced_dynamic_adaptation_of_Newtonian_equations_of_gravity

5). It is corroborated in neutron stars (including Data from radiation

damping ) and is corroborated by black hole studies.

5a)

https://www.researchgate.net/publication/32888463_An_advanced_modification_of_dynamicgravitation

5b).

https://www.researchgate.net/publication/269667939_Corroboration_of_Dynamic_Black_Hole_Gravitational_Physics_from_Observations_of_Cygnus_X-1

6) it explains dark energy and translates into quantum gravity

https://www.researchgate.net/publication/32899306_String_quintessence_and_the_formulation_of_advanced_quantum_gravity

1 2

Is Gravity a Push or a Pull? - ResearchGate. Available from: https://www.researchgate.net/post/Is_Gravity_a_Push_or_a_Pull [accessed Oct 19, 2015].

Article An advanced dynamic adaptation of Newtonian equations of gravity

Article The formulation of Dynamic Newtonian advanced gravity, DNAg

Article Advances in Black Hole Gravitational Physics and Cold Dark M...

Article Advances in Black Hole Physics and Dark Matter Modelling of ...

Article An advanced modification of dynamic gravitation

Article Corroboration of Dynamic Black Hole Gravitational Physics fr...

Article String quintessence and the formulation of advanced quantum gravity

As Sergey Shevchenko wrote, it is absurd to claim that the gravity force comes from the "curvature of space and time".

A force must have a purely mechanical reason of existence. For instance, F=ma can be obtained by the fact that two atoms can not exist at the same place. So, when one pushes an object, one tends to place an atom where another one is staying. Whithout knowing exactly how the inner structure is and how it works, this explanation should be acceptable in a first time.

To really explain the gravity force, one needs to use a process that is mechanical too. I mean purely mechanical. There are but two mechanical processes that can occur: one is what I have explained above, the second one is the Coriolis force. The advantage of the Coriolis force is moreover that it is a vector product between an angular vector and a velocity vector from another origin.

In practice, that means that there must be an action between a spinning particle and an incoming tiny particle, mostly called gravitons, that comes from another particle.

Only such a process, with the rest of the picture still to be defined, can truly be a candidate to explain gravity.

Einstein's theory of gravity is the only one explaining that gravitational forces are proportional to the mass of inertia, a remark made already by Newton but not explained by him. Einstein's theory (General relativity) is based on a geometrical picture of the four-dimensional space and it gives a this space  curvature due to masses. This theory is very successful and describes many experimental facts, like the observed gravitational lensing of light by the field of Galaxies. Incidentally this theory was published by Einstein exactly hundred years ago, in November 1915.  

Yves Pomeau

@ Yves: I am afraid I have to totally disagree with you. The General relativity was fabricated to comply with the double bending of light and with the Newtonian limit.So, it doesn't predict the garvitational lensing.

Moreover, it appears that the alleged compliance of the Schwarzschild metric with Mercury's perihelion advance is wrong, since that metric is a very incomplete solution of the original equation, after successive convenient approximations in order to get the 43"/100years. But Anatoli Vankov has proven that when the approximations are not applied, the resulting advance becomes exactly zero.

So, in reality, the General relativity doesn't prove anything and is not successful at all. The alleged compliances are just reasonings based upon the theory without any observed facts. Several examples exist. The distant binaries for instance have several unknown parameters, so it is easy to make fit the results with the General relativity. Other reasonings like the alleged expansion of the universe and the Big Bang are also not directly observed, but they are made compatible with existing theories.

No, there is no complying fact at all that is directly observed. No close-by event in the cosmos is explained by it.

The Gravity Probe B experiment proves that the Thirring-Lense effect is absolutely ineffective, since the error is 20%. Instead, when using gravitomagnetism, it complies within an error range of 1%.

GR relativity explains proportionality between gravitational and inertial mass. This is something and it is also closely observed very accurately. 

Actually, GR takes the "equivalence" of inertial and gravitational mass as a postulate.  So it is accurate to say that equivalence (partly) explains GR, but not the other way around.

In GR, the equivalence is a consequence, not an assumption. It follows from general assumptions about the  space-time geometrical structure + the assumption that masses follow geodesics. This makes the theory fully coherent with local Lorentz invariance and consistent with equivalence principle. In Newtonian mechanics equivalence has to be postulated without being linked to a more general principle. In this sense GR explains equivalence: because this is about physical theories one has to assume something to get something else. The same is true of Newtonian physics, all derived from postulates clearly stated by Newton. Einstein did the same for GR. 

@ Yves. The "universal" gravitational constant is the only link between inertial mass and the gravitational mass. It has been defined as such by Newton. Einstein's gedanken experiment is just a trivial confirmation of this definition. Hence, it is not a consequence. The Eötvös experiment is in fact a trivial check of this as well, because it only compares the masses in the Newtonian limit.

The Gravity Probe B experiment has found that there indeed is a velocity-dependent gravity effect, which is perfectly described by gravitomagnetism within an error range of 1%. However, this theory doesn't change the value of the "universal" gravitational constant, nor its original definition.

The idea that masses would increase with velocity has been falsified by Fermilab. In the following page http://www.fnal.gov/pub/today/archive/archive_2014/today14-04-04_NutshellReadMore.html , it is written:

"For instance, some people hear the words "mass increases" and think that particles are getting heavier in the gravitational sense. In fact, it is more accurate to say that inertia increases as velocity increases. At low velocities, or nonrelativistic velocities, it is perfectly reasonable to equate inertia and mass. This is the reason even scientists sometimes say that mass changes with velocity. They use the phrase "relativistic mass" to label this fallacious idea of increasing mass.

In fact, there is only one mass, and that is what is often called "rest mass," which is the mass of an object when it is not moving. Even though the idea of relativistic mass is, strictly speaking, not correct, it is a valuable mental picture and helps us get used to the fact that objects cannot exceed the speed of light. So if you prefer to think about relativistic mass, go ahead and do so without feeling guilty. Just realize that if you push the idea too hard, it will lead you astray."

Hence, Fermilab admits there is no real increase of inertial mass, since there is no gravitational increase, provided the equivalence principle should remain.

The real problem here is that the GR theory itself is the cause of the mismatch and of the "paradoxes", not the masses themselves, since the link between them is given by the definition of the "universal" gravitational constant.

The only solution I know to correctly interpret gravity is the application of gravitomagnetism, which has absolutely nothing to do with GR, but which is fully coherent with observation, and which explains a multitude of gravitational events.

Again, to write something as “…assumptions about the  space-time geometrical structure + the assumption that masses follow geodesics…” is necessary to define reasonably – what are “space”, “time”, and “spacetime” and further to explain/ground– why/how /by what way these notions / phenomena can change their “geometrical structure” and can force something that evidently differ from these notions, i.e. – material bodies “to follow geodesics”.

The GR – and the SR – don’t contain any definitions/explanations of notions above, i.e. – all postulates, at least relating to the spacetime are only some bare declarations.

Such situation isn’t quite unusual in physics and is permissible on initial stages of development of a theory, but all such suggestions must be analysed logically and experimentally. The SR at once doesn’t  pass the logical testing – the Dingle problem. The GR seems as more logically resistant since implicitly admits an existence of an absolute spacetime (though 4D Riemannian with imaginary time or space, what is nonsense),  but seems doesn’t pass experimental checking.

Corresponding experiments must be well controlled, when this rule at some testing the GR seems is accepted as inessential. An example – “Gravity Probe B experiment”, where a pair of gyroscopes moved in Space around  Earth constantly being impacted by uncontrolled and possibly unknown forces besides the gravity; thus for experimenters turned out be necessary more 3 years (!) to “work out” the evidently questionable “experimental data” and “to confirm the GR” (as well as “to found some confirmations of the gravitomagnetism”, etc., though)

When there is rather simple, reproducible and well controlled experiment that directly checks the GR, moreover, the full experiment consists of two ones, when the first one was made more 50 years ago,

when Pound and Rebka have measured that if photons move between points with different gravity potentials then measured difference of proper [relative] frequences of radiating and recieving atoms in these points differ on the value gh/c^2 – “quite in accordance with the GR”;.

including – in accordance with the GR claim that photons doesn’t change their energy when moving asross their “geodesics” , so  alll differnce occurs only because of prdicted by GR “gravitational time dilation”.

Nonetheless this result seems as quite non-physical, because of from this follows the value of gravitational mass defect  twice larger then possible (“Newtonian”) value. As well as it seems rather strange that photons don’t change their energy when moving in the gravity.

To clear the situation seems enough to measure directly the lapsed proper thick rates of a pair of clocks in different points of the gravity potential and compare the clocks’ showings (see https://www.researchgate.net/publication/277710038_The_informational_model__gravity_a_next_experiment) .

It is very possible that the differnce will be twice lesser then Pound and Rebka result, i.e. gh/2c^2 – as that is in accordance with “Newtonian”  gravity impact on the clocks’ tick rate; besides the real photons’ energy losses will be measured and will be equal  to gh/2c^2 also, as these two effects were summed in the Pound and Rebka results.

And this experiment would cost 1-2 $millions, thausand times lesser then the the Gravity Probe B…

Cheers

Research The informational model – gravity; a next experiment