Wow, mocking a theory you don't understand. Since about the third grade I've been asking teachers, "what causes gravity" and they always told me "gravity is the attractive force between two objects" never siting how or why. I have yet to get a good answer from a teacher on this.
The reason your teacher kept saying that is because She/He thought you did not have enough understanding of science to comprehend the actual explanation, or they themselves didn't know.
There is a proper explanation along with experiments done that prove the explanation is not false.
I ahve tried to explain it several times here, but as I can see form your continued lack of understanding of it, that you don't actually understand it (or you are just trolling).
Yes, these experiments are not easily reproducible by just any one and do require a lot of high precision and high powered machines. But the concepts behind the equipment is understandable, at least if the engineering simplified.
So here I go again (yes there will be a large wall of text, so take your time):
A theory is only as good as its ability to allow us to use it. This means that a theory has to make testable predictions. Now a test does not ahve to be a specific experiment, but it can be use of that principal in an engineering capacity. For instance the theory of electricity tell us about how various materials will behave in electric fields. Using this we can design devices that behave in specific ways. Say one that resists the flow of an electric current (flow of electrons through potential difference of an electric field), or that we can store a charge (capacitors), or even that some devices will one become conductors in certain circumstances (transistors).
So a theory is only as good as its ability to allow us to use it. If a theory gives an explanation, but does not allow us to derive anything useful form it, then it is a poor theory. All this means is just explaining the Facts is not enough, a theory has to be able to not only explain the facts, but make predictions about future "facts" that are covered by it.
So now we enter into the realm of scientific theory.
Isaac Newton saw objects fall (not necessarily apples). And came up with an idea: There is some force that exists between objects that cause them to be attracted to each other.
Now, as an Idea, this is pretty "out there". A "Spooky action as a distance" as someone put it. But there were precedents. Magnets were known and as could be seen, a magnet could affect another at a distance without them touching. So such an idea was not completely unheard of.
Newton proposed an Effect, not a cause of falling. Namely Gravity, and like Magnetism, it acted at a distance without any direct contact.
But, they were wrong about magnetism. There was direct contact in the form of small packets of energy that we call Photons. This is what Einstein won the Nobel prize for, not gravity, or relativity (or why tea leaves in a stirred cup move towards the centre when centrifugal force should send them outwards - yes, this was another of Einstein's contributions to science
), but for proof of Photons through the Photoelectric Effect.
But how does the Photoelectric effect prove that the earth can not be flat and is indeed round?
Now, for the Photoelectric effect to work, light must behave in a certain way, it must obey Maxwell equations on the prorogation of electromagnetic waves. One of the properties of these waves is that they must always travel in a straight line in free space. The path a Photon takes can not bend unless Space Time itself bends (and then the light is not bending, it is still travelling in a straight line, it is just that Euclidean Geometry is wrong).
What I need to explain here now is Geodesics. A Geodesic is a straight line in Non-Euclidean Space.
Euclidean space is the space we normally associate with geometry. You know at school drawing circles and triangle and so forth. Euclid came up with some postulate about geometry, but it the Fifth one, known as the Parallel postulate (
http://en.wikipedia.org/wiki/Parallel_postulate) that is of importance. Essentially, this postulate is
wrong. It is actually a special case of what is now known as Non-Euclidean Geometry, that is one where the surface is flat.
A Straight line in Euclidean Space is defined as the Shortest line between two points. However, as Euclid assumed a flat surface, he didn't include the idea that the surface itself could be curved.
If we correct for this we come up with: The shortest path between two points
that follows the surface.
This is known as a Geodesic.
So if the Surface is flat, a Geodesic is identical to a Euclidean Straight line. But if the surface is curved, then Euclid's Fifth Postulate is not correct.
This is all probably not new to you, but it is important as these are the things that are needed to explain what causes gravity. If you don't understand these and how they are "useful" (see above) then this previous section is needed.
Now, on to the proof of gravity:
As Maxwell's equations state, light must travel in a straight line, but that was assuming a Euclidean space. In non-Euclidean Space, the fabric of Space can be bend and this would cause any light travelling through it to follow the Geodesic. It would be incorrect to say that light was bent, but as an analogy that is a good one.
The light is not bent by gravity, or anything, in fact according to Maxwell's equations which if they were incorrect, then we would not get the results that we do from the Photoelectric effect. We would get different amounts of energy, and particles kicked off would not travel in the directions that they do.
If the Photoelectric effect was incorrect, then we would not have the devices that we do. Digital cameras, smoke alarms, automatic doors, etc, etc all use the photoelectric effect and would not work as they do without it. Digital Cameras are an important one as they require precise energies of photons, and if these were different, then they would not work as they do. If Maxwell's equations were incorrect, then the Photoelectric effect would produce different energies and so Digital Cameras woudl not work as they do.
So if Digital Cameras work as the do, then Maxwell's equations must also work as stated.
However, if Maxwell's equations work as stated, then this can be allowed as proof of Gravity.
As I said, Light must travel in a straight line, or if space is non-Euclidean, then along the geodesic.
If space is Euclidean, then light will always travel in a straight line, never deviating from it. But if space is non-Euclidean, then it will deviate form the Euclidean straight line because space itself is bent.
To put it crudely (and incorrectly) light will
appear to bend.
This means that if we see light deviating from a Euclidean Straight line, then Space itself must be bent by something.
If space is bent, then there is probably a cause. If we can find something common to all incidents of where light deviates from the Euclidean path, then what is common would be a reasonable cause for that distortion of space.
The thing that we have seen that causes this distortion is what Newton called "Gravity". And what Newton said was the cause of gravity was Mass. Everywhere we see mass we see gravity, where we see gravity we see light deviating form the Euclidean path.
In this respect "Mass" is just the "Charge" of matter that produces gravity. Just like Electricity has a charge, Gravity has the Charge of "Mass".
But the results of careful experiments did not quite match up with the result of this theory. The theory was wrong.
One of the results was that the stronger the field of gravity the slower time seemed to move. Reactions that took known times were slowed down if they were in a stronger field, or would appear to operate faster if they were in a weaker field.
This is what Einstein proposed, that it is not just space that is distorted by gravity, but Time as well.
And this is what gravity is:
The field of Gravity is space/time (just like the electric field is the field for electricity). Distortions of this field produce distortions in the Geodesic path through it. The same can be shown for the Electric and Magnetic fields. As we can easily manipulate the Electric and Magnetic field, we can introduce such distortions and even get results that confirm that this is the case.
IF we treat the Electromagnetic force as a fabric Space/Time, we can reproduce all the effects that we associate with Distance, Time, Geodesic Paths, a form of Inertia, a form of "Gravity" (which matches with the inertia effect produced) and much more. In other words, we know the cause of Gravity as much as we know the causes of the Electromagnetic force.
They have even gone as far as manipulating the Electromagnetic force to simulate a black hole's event horizon. Of course, as it is not truly gravity that they are generating, and that they can switch it off, there is absolutely no danger, all this is a simulating (or better yet, emulation) of the event horizon of a black hole by treating the electromagnetic force as if it is a space/time.
What it shows is that Space and Tim are the results of a Field, just like the electromagnetic field, only that it uses a different form of Charge, that of Mass.
Now, one might speculate that we don't exactly know what an electric charge is. We know that electrons have it, and so do Quarks, but we don't exactly know what it is. And the same is for the Mass "charge" of matter. We know it is possessed by matter, we can directly measure the effect and we can even indirectly measure the effects, but we don't exactly know what causes the charge, only that this charge does indeed exist.
So although we don't know what causes this "charge" called Mass, we do know that it really does exist, and if it does exist, then we have Gravity and all that comes with it. We don't know what causes the charge we call "Electric Potential" but we do know that it does exist, and if it does exist then we have electricity and all that comes with it.
And as a final part, although we have detected the Photon, we have not detected the Graviton, but we might just be about to detect the actual cause of Mass, that of the Higgs Particle.
The Higgs field (of which the Higgs particle is the carrier) is actually more analogous to the experiments where the Electromagnetic field was manipulated to cause the effects of Space and Time. The Higgs, field, although not actually the cause of gravity would allow the Higgs field to create the effects of a space time, just as was done with the electromagnetic field. Space and Time might not literally exist, but be just an artefact of the metrics of the Higgs field. This will be settled by the LHC, if they do detect the Higgs particle, then this will mean that space and time are not what they seem, but are an artefact of this Higgs field.
The next few years might actually allow us to find out what
exactly Gravity is made from.