The Apple Didn't Fall
"Professor Marshal, what causes gravity?"
"Let's see, you're Troy aren't you?" The professor was always amused by the new freshman and their naive ideas of science. "The fact is, we don't know. No one has ever proposed a reasonable hypothesis that is consistent with all the things we can observe."
"Yeah, but" Troy blurted in the time honored way that made the professor wince, "the textbook says, scientific laws must be true, universal, and absolute. They're the cornerstone of scientific discovery, because if a law ever did not apply, then all science based upon that law would collapse.
"If we don't know what causes it, doesn't that mean that all of science is based on an assumption?"
"Okay, normally we wouldn't have time to talk about such frivolous things as the validity of scientific laws, but since this is the first day, maybe a discussion will help me to get to know you better. If we're going to have a discussion though, let's do it right, by defining our terms. Newton's law of universal gravitation states that every massive particle in the universe attracts every other massive particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. We can measure the acceleration of a falling object like Newton's apple and it's 32 feet/second squared."
At this point, Troy put up his hand, but the Professor ignored it. "When we observe the planets and moons in the solar system, it's obvious that there's a force holding them in their orbits, otherwise, by another of Newton's laws-of-motion, they would fly off into space."
Becoming impatient, Troy began waving. With a sigh, the Professor finally acknowledged him, "Yes, Troy?"
"I think we understand the basic grade school stuff." Troy looked around to see if any of the other students objected. "What we don't know is how they determined what the mass is of the sun or the moon. There are three unknowns in the equation, the mass of both objects and the force required to maintain them in orbit. You can't solve the equation for all three at the same time."
Slightly more impressed, the Professor elevated the age at which his lecture was aimed. "Of course, we can't prove what the mass is, but each planet or moon is affected by more than one object. The earth, for example, revolves around the sun and the moon revolves around the earth. Each of the three affects the others. So we don't have just one equation with three unknowns, we have three equations with four unknowns."
With a smile, Troy sprang his trap. "But how do you know the unknowns are the mass? Has there ever been an experiment where the mass of both the objects drawn together by gravity was measured directly? What if the unknowns in the equations represent something else other than the mass? How could we tell?"
"Actually, we couldn't tell, but no one has ever suggested any other property of the planets or moons that could be responsible," the Professor answered blithely.
"What about the electrical charge of a body? If the charges are opposed, they'd exert the same kind of force," Troy offered, refusing to let go.
Without so much as a by-your-leave, another student jumped in, "That's a really stupid idea.