We all know gravity. It's what makes things fall if you don't hold onto them.
Gravity also keeps our feet on the ground – and provides the ground itself, since gravity holds planets together. Stars form by the gravitational collapse of huge clouds called nebulae, so we can thank gravity for our life-giving Sun. Beyond the Solar System, galaxies stay together through gravity and form clusters. It seems as though gravity is the glue that holds the Universe together, but what is it?
Aristotle: Perfect heavens and imperfect Earth
If we were living at the time of Aristotle (384-322 BC) – or even for about eighteen centuries afterward – no one would have asked what gravity was, let alone been able to answer. Gravity is a relatively modern idea and so is the idea that physical laws apply to the heavens as well as the Earth.
According to Aristotle and his centuries of successors, the Earth was imperfect, but was still the center of the cosmos. It was surrounded by the perfect heavens which were based on spheres and circles, the perfect geometrical shapes.
Heaviness related to falling, an earthly phenomenon. Aristotle had said that heavy things fall faster than light ones and no one saw any reason to doubt it. You could easily demonstrate it by dropping a feather and a hammer, which makes sense if you don't think about air resistance.
Copernicus, Kepler and Galileo: a new cosmos
Although the old worldview persisted for well over one and a half thousand years, it became ever harder to reconcile it with what people were actually observing.
Johannes Kepler (1571-1642) used the careful observations of Tycho Brahe (1546-1601) to test Solar System models. They fit if he started with the idea of Copernicus (1473-1543) that planets orbited the Sun as long as the orbits weren't circular. They had to be ellipses (squashed circles).
Galileo (1564-1642) experimented with falling bodies and discovered that they all fell at the same rate. By the way, he didn't do it by dropping cannonballs off the Tower of Pisa, though that makes a good story! He would have loved seeing David Scott drop a feather and hammer together on the Moon - what do you think happened with no air? You can see it here.
Isaac Newton describes gravity
It needed Isaac Newton (1642-1727) to bring the new findings together and describe gravity mathematically as the force of attraction between all the masses in the Universe. His theory was published in 1687 as Philosophiae Naturalis Principia Mathematica (known as "the Principia"). Keep in mind that a scientific theory isn't a good guess. It's a well thought out explanation that accounts for existing evidence and makes testable predictions.
Newton's theory applied to heaven and Earth. He was prompted to connect apples falling to the ground (not on his head) with the movements of heavenly bodies. His equation says that the gravitational force increases with the mass, so the Sun's force is stronger than the Earth's. However it decreases rapidly with distance. If Earth were three times farther from the Sun than it is, the gravitational force wouldn't be one-third of its current strength, but one-ninth.
Newton's equations work – NASA sent men to the Moon using them. But it isn't the whole story. Even Newton was unhappy that his theory was based on a force that acted everywhere in the Universe instantaneously. How could that work? It took over two hundred years for someone to do a complete rethink of gravity and space.
Einstein takes up the challenge
In the General Theory of Relativity, Albert Einstein (1879-1955) shows that what looks like a force is actually an effect of the geometry of space.
To Newton, space was the flat unchanging nothingness between things of interest like stars and planets. This fits pretty well with the way most of us experience the world.
However Einstein said that space and matter affect each other. Matter curves the space around it and the greater the mass, the greater the curvature. But then matter moves along the easiest path through the curved space. It's expressed as "Matter tells space how to curve. Space tells matter how to move."
It's not easy to imagine curved space, so it's often suggested that we imagine a rubber sheet held flat in a frame. If we put a bowling ball on it, the sheet sags. The surface is no longer flat. If you rolled a marble onto it, you could get the marble to circle the bowling ball for a while, something like this. However eventually, it would fall into the depression. If you couldn't see the rubber sheet, it might look as though the bowling ball were pulling on the marble.
At last, a way of looking at gravity that removes the problem of universal instantaneous action! But it's rather strange. Does it work? Yes, the theory has been tested and has passed all of the tests, starting with a famous one.
One consequence of mass curving space would be that everything would move along the curved paths – including light. That means that light should bend a certain amount as it passes by the Sun.
English astrophysicist Arthur Eddington successfully tested this prediction during a total solar eclipse when, uniquely, the stars on the far side of the Sun are visible. He compared his photographs of the star positions with ones taken six months before when the Sun wasn't in the way. When the results fit the predictions, world media reported the story and Einstein became an international celebrity.
We know that Newton's theory of gravity works, but is incomplete. It's contained in Einstein's more comprehensive theory. We know that Einstein's theory works – your GPS system has to take it into account to work correctly. However scientists realize that it too is incomplete. I wonder if it will take another century for someone to include Einstein's theory in an even more amazing one.