What is the true nature of gravity? In 17th century, Sir Isaac Newton gave us the universal law of gravitation which stated that gravity is an inverse square force. In 1915, Albert Einstein recognised gravity as a curvature of space-time caused by the presence of mass and energy in it. Does that mean Newton was wrong? Is Newton's law not true? It did predict the motion of planets. But above all this, according to modern research on quantum gravity, gravity is transmitted by a particle called a graviton. Does this too violate Einstein's General Theory of Relativity? What is gravity then? What is its true nature?
 A: There are many questions here. I'll respond to as many as I can:
Newton's expression of the law of gravitation is true in the sense that it correctly accounts for observations of things like apples, planets, and galaxies- but it is silent on why. Einstein's formulation of general relativity does tell us why. This does not invalidate Newton's law, it extends it.
If we postulate the existence of a massless (infinite range), spin 2 (attractive force only) particle that couples to matter and energy, you get gravity as a consequence. This does not prove the existence of the graviton nor does it invalidate either Newton or Einstein; it simply lists the necessary characteristics of a (hypothetical) particle that mediates the force of gravity- a particle that is beyond our capability to detect, because its couplings are so weak.
A: If you observe something, measure it, test it and then formulate a description or formula, then you have described it.
Describing explains how, not why. Newton's laws explain how objects interact and how motions are interferred with. They don't explaint why. When new findings show up in physics, then we might get the why's. That doesn't alter the how, the way it actually works, it just adds to the description a look into the cause as well.
But keep in mind that when you have made your observation and formulated your description, like Newton did with his laws, then you only trust it to hold true within the scope you have tested it. Meaning, your law has a range of validity. In other words, your description can claim how it works within what you have tested - but you can only stipulate and assume that it will also work elsewhere, that it is actually a universal law that would also hold true under extreme conditions.
Take gravity, for instance. You can test, measure and observe how things fall, how strongly gravity pulls etc. here on Earth. You can formulate your exact description. You might figure out that everything falls with an acceleration of $g=9.8\,\mathrm{m/s^2}$. But then one day someone repeats your experiments on the top of a mountain in Mexico and again at the ground near the North Pole. He realises that gravity is weaker in the former but stronger in the latter location. It turns out that gravity varies with the distance from Earth's centre.
Later again, space travel is invented and we realise that there are points in space with no sense of gravitational pull when you are far from any large mass. All this doesn't mean that your assumption of a constant downwards-pulling gravitational force was wrong, it just only applied near the ground on Earth. To extend you description to become universal you must adjust it to new findings at the extremities. As a reallife example, Newton's laws have turned out to not hold true at the extremities e.g. when getting near the quantum scale. They, too, have a range of validity and must be adjusted if they were to apply universally.
A: Newton's law of gravity is an excellent approximation, but it does not produce correct predictions for some things (e.g. the orbit of Mercury or the bending of light near stars). Einstein's theory of gravity does predict those things correctly. In fact so far Einstein's theory has correctly predicted all gravitational effects we have observed. However, there are good theoretical reasons to think that it, too, may be incomplete and that if we ever are able to observe gravity on quantum scales that a different theory will give better predictions.
Does this mean that Newton was "wrong", or that Einstein may be "wrong"? Well, in some sense... but it's better to say that Einstein was "more right" than Newton, and some future quantum theory of gravity may be "more right" than Einstein. Newton's law of gravity works well enough to send spacecraft to planets, so as I said, it is an excellent approximation. The Einstein field equations describing curved spacetime reduce to Newton's laws in the case of relatively weak gravitational fields and "low" velocities. Similarly any future theory of gravity will have to produce answers very very close to Einstein's equations in all circumstances where those have been tested.
So, what is the "true" nature of gravity? The best answer we have right now is that it's due to the curvature of spacetime. Someday in the future we may have a better answer. That's the way science works... there's no absolute final truth, just better and better approximations to reality.
A: There is no such thing as "true nature of something" in physics. It is more of a phylosophical question (and phylosophers, please bounce the ball to someone else if I am wrong - I know it is hard).
Physics consists of models.
It is good when a model aligns with observations. The author gets citations.
It is even beter when model is used for successful predictions. The author sometimes gets Nobel.
Sometimes, a model is superseded by a better one (Newton gravity => Einstein relativity).
Sometimes, an older and not the best, but "good enough" and simple enough model is widely used. E.g. Newton gravity is pretty much used daily. Einstein relativity is only used when we need very good precision or when Newton gravity model is known to fail.
Sometimes, a model can explain another model (we say that the nature of chemical processes is electromagnetic or that electromagnetic and weak interactions can be fused in a common electroweak model).
But, with gravity, we don't have such a luxury.
It is not that we don't try. We have, e.g. "entropic gravity" - an attempt to explain gravity by thermodynamics. If this theory matures and the worst rough edges are cleaned, we could be able to say that the true nature of gravity is thermodynamic. But we don't say this right now.
A: The job of physics is to construct models that are able to explain and predict empirical observations. You can never be completely sure that a given model is the "true" description, only that it, at the very least, captures facets of the truth by successfully accounting for certain observations.
Newton's law of gravity successfully models a wide range of gravitational phenomena but is not valid in extreme regimes. General relativity is a better description and reduces to Newtonian gravity in the limit of small masses and low speeds. But GR seems to have its own problems too and a future theory of gravity, if we find one, must similarly recover general relativity in an appropriate limit. Perhaps we'll eventually converge on the "true" description, but whether we will or whether such a thing is even meaningful is strongly debated.
All in all, it's a work in progress.
A: Its also not exactly fringe these days to alternatively postulate that space-time curvature emerges from quantum entanglement of fields. Energy necessarily follows. Gravity is then thrown in ‘for free’ as a statistical (emergent) property of the system, the entropy of this entanglement at each point.
A: What we know is that gravity is an effect that causes the motion of particles to deviate from straight (Euclidian) lines, that it is an attractive rather than a repulsive effect, that it weakens with distance, that the effect propagates at the speed of light, that the strength of the effect is related to the degree to which matter and energy are present; we know the form of the equations that accurately model the magnitude of the effect in terms of time, distance and the distribution of matter and energy. We have no idea why it exists.
Like many other fundamental phenomena in physics, you can analyse it so far, and answer questions about it to a certain point, but you eventually exhaust the scope for further explanation and have to just accept that things are how they are, until something new comes to light (experimental evidence, observations, theoretical breakthroughs, etc) that allows you to dig a little further. Most of the digging being done now is in the area of quantum gravity, which might eventually shed more light on 'why' gravity is the way it is, but even if it does, you will still be able to ask 'but why is quantum gravity like that?' and not have an answer.
A: I'll tell you that there are a number of very speculative concepts or theories on what actually causes gravity ranging all the way to leakage from alternative universes coming out of black holes.  Five years ago I decided to come up with my own concept, and here it is.  But remember that this is speculative so for gods sake don't quote it in any paper, but perhaps it can get you thinking.
The heart of the theory is that an excitation of the gluon field leads to a dilation of the particulate space of loop quantum gravity.  It is the size of these particles that defines the speed of light and thus the speed of time.  It is the warping of spacetime that causes gravity.  So what we have is a mechanism that causes the warping of spacetime, and thus gravity.
Now the details:
For the gravity of a planet or star, as you know, matter is made up of atoms, made up of protons and neutrons, in turn made up of quarks and gluons.  These gluons are spinning at c, the speed of light.  They also have a charge, called a colour charge, analogous to the electric charge of an electron.  So just as electron spin leads to an excitation of the electromagnetic field surrounding a bar magnet, gluon spin leads to an excitation of the gluon field surrounding a planet.
If you look at particulate space in the theory of loop quantum gravity, you will understand that energy causes these tiny particles to change shape.  I would say that energy causes these tiny particles to dilate.  When I read about LQG, the very first thing that crossed my mind was that in a different universe, if the particles were larger or smaller, then the speed of light would be different. This is because light must travel between the particles.  It is the speed of light that defines the speed of time so in essence, changing the size of the particles changes the speed of time.  Thus we arrive at a clear definition for the warping of spacetime that is at the heart of relativity.
So this is why the gravitational pull of a planet or star is directly related to the number of gluons inside. That is, a planet made of lead has more gravity than a similar sized planet made of tin.  It has more gluons, thus a greater excitation of the surrounding gluon field and a greater warping of spacetime surrounding the planet.
Now looking at black holes.  We know that under the Pauli exclusion principle, spin 1 particles like gluons can occupy the same space.  So it is certainly possible for a star's worth of gluons to collapse down to a singularity.
Edit to answer Eric's concerns in the comments.  This concept also explains kinetic time dilation and dark matter.
The gluon field is everywhere in the universe.  Anything moving through that gluon field, whether it be an electron or a rocket ship or a galaxy, is going to generate a "bow wave" in the field.  This bow wave is also an excitation of the gluon field, so will lead to kinetic time dilation and gravity in exactly the same way.
I came to understand this when I asked this question a couple of years ago.  I asked if a relativistic rocket passed very close by a clock, for the instant the rocket was passing, would the clock run slower.  The answer is yes.  So this led me to understand that is it not merely a relativistic object that experiences time dilation, but the surrounding space as well.
This actually explains dark matter, which should be called dark gravity, because there is no actual matter involved.  If you look at a map of dark matter in the universe you will see that it is always associated with a fast moving galaxy.  For example the Bullet Cluster of 40 galaxies is often cited as the proof of dark matter.  See this link  The Bullet Cluster is moving at about 1% of c, which is darned fast.  You can see in the image that the purple area around the galaxies looks exactly like a bow wave in space.  More recently, scientists have discovered that slow moving galaxies and more diffuse galaxies have little or no dark matter.  This is exactly what you would expect for a bow wave.  A group of row boats does not create the same bow wave as a ship.
