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Einstein's General Theory of relativity states that space is not always what it appears to us. Suppose for example you did not know the worlds was round. Would you think that it was? Probably not. After all, the earth looks pretty flat all around you. Thus, you would probably think the world was flat. We know that this is not the case though. Despite what it looks like from our vantage point, we know that the earth is round. In the same way, Einstein postulated that although it does not appear to change at all, space actually bends in the presence of an object with mass.

In the first part of your experiment, the seat cushion represents space. With nothing on the seat cushion, it stayed relatively flat. However, when a massive object (the bowling ball) was placed there, the entire seat cushion bent.The bend was greatest in the middle and and was least around the edges, but never the less, the entire seat cushion bent. In response, the marble rolled towards the bowling ball. This is Einstein's picture of gravity. Space (the seat cushion) bends in the presence of mass (the bowling ball). As a result, all objects (the marble) accelerate towards the mass. This makes it look like a force is being applied to an object.

In the second part of your experiment, you watched the marble roll straight across the flat seat cushion. When the bowling ball was once again placed on the seat cushion, the marble did not roll straight. Instead, it rolled in a curved path. According to Einstein, this is why planets orbit the sun. In his theory, all planets are actually moving in a straight line. Because space is so bent by the mass of the sun, however, that straight line is deformed until it becomes a circle. Thus as far as the planets are concerned, they are traveling in a straight line. Space itself, however, causes that straight line to become a circle.

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Now, if you and your helper were forced to do this continually, what would you do? You would move closer to one another, to make it easier, right? Well, if massive objects do exchange gravitons, and if they must be exchanged in within a certain small time interval, massive objects would probably move closer to one another as well. That, could be what causes the attractive gravitational force. Massive objects are attracted to one another because the exchange gravitons.

There is evidence for Einstein's General Theory of Relativity. Is there any evidence for the gravitation theory? Well, not exactly. Gravitons have never been observed, and we have never witnessed massive objects exchanged particles. Thus, there is no evidence as of yet for this theory. Why would anyone believe it? Well, the other forces in nature (the electromagnetic force, the weak force, and the strong force) all can be explained by the idea of particles exchange. Thus, it is natural to think that that other force in creation should be explained that way as well.

Wait a minute. Did I just say that this strange theory if particle exchange explains the other forces in creation? I sure did! (Oh Wile, you goon.) Believe it or not, negatively-charged particles are attracted to positively charged particles because they exchange small particles (called "photons") with one another. I will tell you all about that at length in an upcoming module. In addition, the other two forces are explained the same way. Thus, even though there is no evidence at this point that gravity is caused by the exchange of gravitons, it seems like a likely explanation, since the other forces in creation are explained in a similar way.

Indeed, all physicists agree that gravitons would be very, very hard to detect. Thus, even if they do exist, we should not be surprised that we haven't seen them yet. Of course, most physicists admit that Einstein's explanation is better right now, since it at least has some evidence supporting it. In fact, there are some that postulate both theories are true. They assume that curved space induces the exchange of gravitons. Thus it is even possible that both theories are true. At the same time, of course, it is possible neither theory is true. After all, even the data that backs up Einstein's theory is is scant at best.

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Gravity, in the sense of forces makes sense in any case if you think about the other forces and Newtonian physics.

Gravity is the weakest of all forces. This is because gravity relates on the macroscopic universe, that being what we can see. We experience gravity, but in reality we are truly far away from the main source of gravitational force.

Electrostatic force is the next weakest force (if I remember my forces correctly). Electrostatic deals with the attraction and repulsion between electrons, which are SIGNIFICANTLY more close together than our bodies are to the center of the Earth. Also this deals with photons and charge and crap.

Weak Force is the next. The weak force describes why nuclei don't fall apart because of their immensely polar charge (positive). It's the force that keeps protons and neutrons in the nuclei of atoms

Finally we have Strong Force, which holds the protons and neutrons together. Gluons hold quarks together inside neutrons and protons. Blah, blah, blah. Anyway, this force relates close than any of the other forces, and therefore makes sense to be the strongest force!