As hinted at in the introduction, our ideas about the universe have not always been based on evidence. During the historical time when religious belief overruled common sense, some outright fantasies stood in for testable hypotheses. We've come a long way from the time when celestial objects were thought to ride on crystal spheres, and when a stone was said to fall to earth for the reason that it was eager to rejoin its mother. Since that time, cosmology has become a marriage of observation and physical theory, in particular those theories having to do with how gravity works.

On the basis of observations and a substantial amount of sheer intellectual power, Isaac Newton came up with a description of gravity (not by any means an explanation of gravity). In Newton's system, a concise statement of the force of gravity is that it is equal to the product of two masses and a gravitational constant G, divided by the square of the distance between the masses:

(1)    f = G	m1 m2
	r2

Where:

f	Force, Newtons.
G	The Universal Gravitational Constant, presently estimated to be equal to 6.6742x10-11 N m2 / kg2.
m1,m2	The masses of the two bodies in question.
r	The distance between m1 and m2.

All in consistent units.

This scheme produced satisfactory results when used to model and predict the motion of celestial bodies, in particular when the static force equation above was joined with the methods of Calculus, a combination that allowed dynamic predictions to be made.

After working out the inverse square law of gravitation attraction, Newton produced the essentials of Calculus and used it to model the behavior of planets moving in orbits, and by so doing realized those orbits would be elliptical. Newton, being Newton, concealed his methods and only hinted at his results. When he was asked how he knew the planets' orbits were elliptical, he replied "Why, I have calculated it."

Edmund Halley overcame Newton's secretiveness and persuaded him to publish both his methods and his results in a tome named Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), possibly the single most important work in the history of science.

In Newton's system, time and space were absolute and immutable, and light waves as well as gravitation were thought to convey force by way of a mysterious fluid — the "ether" — that filled all of space. This system prevailed for about 300 years, until a certain experiment cast doubt on its assumptions.

In 1887, Albert Michelson and Edward Morley attempted to measure the Earth's motion through the ether. The ether was believed to be moving with respect to the Earth due to the Earth's proper motion in its orbit, and it was expected that this velocity would be detectable in some kinds of optical experiments. But this well-designed, careful experiment failed to detect the ether, which caused many physicists to begin to doubt its existence.

This was a time of great anxiety. Light waves moved through space in a very predictable way, but waves of all kinds were assumed to require a medium, like water for ocean waves and air for sound waves — what was the medium that carried light waves? And gravitation reliably held planets in their orbits, but with no apparent medium for that force, now that doubt had been cast on the ether theory.

In hindsight, we can see the approach of a revolution in how people thought about the world, an overthrow of much of contemporary physics. It is at such times that one of the more cynical sayings about scientific progress comes to mind — "Science proceeds one funeral at a time."

The year the Michelson-Morley experiment was conducted, Albert Einstein was eight years old. About this time Einstein's father gave Albert a compass, thus igniting his imagination — the compass needle responded to an invisible force, reliably pointing the way north, with no physical connection involved, no air, no water, no ether.