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Ok,this is a silly question. But for quite long I have been thinking about this. What is gravitation all about?? My book directly writes since force of gravitation is directly proportional to the masses and inversly proportional to the square of the distance ie. $$F = G\frac{M_1 . M_2}{r^2}$$ . So is this gravitation all about? What is the cause of it? The book then says

We are not in the scope in classical approach so as to discuss the cause of this. However,as a matter of fact that if there are more than one mass,then there is gravitation as it is their fundamental property.( Cause of gravitation can be attributed to exchange of non-classical particles between the masses) .

Then it ended the talkings. What are those non-classical particles? Why and how are they exchanged? So far I have studied physics , I never found such weird phenomenon like this. I want to know what is the cause of gravitation and what is the cause of exchange of those so-called non-classical particles? And how do they do it?

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    Those particles are supposed to be gravitons. – jinawee Oct 10 '14 at 21:18
  • There is no gravitational force (and there wasn't one in Newton's time, either). Gravity is an acceleration of a test mass by another mass. The best description we have makes gravity a distortion of the spacetime metric. Gravitons and more generally all approaches to microscopic explanations for gravity are pure speculation at the moment. – CuriousOne Oct 10 '14 at 21:23
  • Apparently gravitons are the particles that allow for the existence of gravity but they have never been experimentally confirmed to exist last I checked. – Sherlock Holmes Oct 11 '14 at 03:08
  • Possible duplicate: http://physics.stackexchange.com/q/4788/2451 – Qmechanic Oct 11 '14 at 17:19
  • Interestingly, you mention "square of the distance" along with "exchange of particles." If the latter were interaction, math. set theory would define the number of exchanges by square only if on both sides there were the same amount of particles, if the two bodies differ in number of interactive particels that would be the product, not the square. Interestingly, the formula has both. By intuition, masses involved differ, while, as it occurs both "sides" in spite of their being different in mass seem to be equal in "distance" (square of). It occurs: think of "center courts", concentrating. – Peter Bernhard Nov 09 '22 at 14:37

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Ultimately, the only thing that can explain a physical theory is a better theory. Newton's force law used to be the entire theory of gravitation up until the early 1900's. Newton himself had no explanation for what "caused" the force.

Einstein gave us General Relativity, which describes how gravity is the geometry of space and time. In GR, energy and momentum curve spacetime, and objects simply follow the straightest possible path through the curved spacetime! So Newton's law is "explained" by GR.

With the success of Quantum Field Theory at describing the other three fundamental forces (electromagnetism, weak nuclear, and strong nuclear), physicists have been trying very hard to create a quantum description of gravity, though none have been completely successful so far. In QFT each force is described by a field. For example, the electromagnetic field. When the field is "excited," or in other words when the field goes to a higher energy state, the excitations of this field are called photons. When you run through the math of QFT, you find that two like charges placed in the photon field will repel each other with a force that varies as $1/r^2$! This is because the two charges raise the energy state of the field. Similarly, what we do know is that if you put two objects in a simplified massless spin-2 field, you get an attractive force between the objects that varies as $1/r^2$! The force comes from each object coupling to the field and lowering its energy. Excitations of this field are called "gravitons." There is as of yet no direct evidence that gravitons exist, and it's highly unlikely that there will be any any time soon given how weakly they are predicted to interact.

Jold
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  • +1, a slightly more technical article on the possibilities of graviton detection here. It is basically impossible to directly detect a graviton and identify the signal. – Void Oct 11 '14 at 08:50
  • "So Newton's law is "explained" by GR." The equations of general relativity have been chosen from many possible relativistic equations so that they lead to the same result as Newton's theory in certain mathematical limit. For example, the Einstein gravitational constant $\kappa$ is chosen so that the the above limit leads to Newton's constant $G$. – Ján Lalinský Oct 11 '14 at 12:27
  • There is no more explanation for the value of $\kappa$ in Einstein's theory than for $G$ in Newton's theory, so it makes little sense to say Newton's gravitation has been explained by general relativity. It is more accurate to say that general relativity is one possible extension of Newtonian theory of gravitation to include the ideas of relativity. – Ján Lalinský Oct 11 '14 at 12:28
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    @JánLalinský - GR is anything but an extension of Newton's theory. It is a complete replacement. Look at the math, the descriptions of space and time. It is true that any theory of gravitation must yield results comparable to Newtonian mechanics in the Newtonian limit. Failure to do so would immediately falsify that new theory. This does not however mean that that theory is "an extension of Newtonian theory of gravitation." – David Hammen Oct 11 '14 at 13:46
  • In a similar vein, that quantum mechanics must also yield results comparable to Newtonian mechanics in the macroscopic limit does not mean that QM is an extension of Newtonian mechanics. Finally, that Newtonian mechanics needed to yield results comparable to Aristotelian physics in the Aristotelian limit does not mean that Newtonian mechanics is an extension of Aristotelian physics. Just as Newtonian mechanics was a complete replacement of Aristotelian physics, both QM and GR are complete replacements of Newtonian mechanics. – David Hammen Oct 11 '14 at 13:51
  • @David, I cannot agree with your view. This is not a good place to resolve the disagreement, so let's agree to disagree. – Ján Lalinský Oct 11 '14 at 15:43
  • This is nice, but I'd like to pick a nit. Where you say "when the field goes to a higher energy state, the excitations of this field are called photons." the naming of the excitation as a photon is specific to one particular field. Maybe better if you clarify that photons are excitations of the electromagnetic filed in particular. – dmckee --- ex-moderator kitten May 27 '16 at 17:09
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Currently, there is no generally accepted explanation of why objects attract each other. There are hypotheses such as Le Sage's, but none remained popular. The most basic (simplest) theory of gravitation is still Newton's law of universal gravitation.

Ján Lalinský
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