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Although he is regarded as one of the most brilliant mathematical
physicists of the century, Einstein thought of himself as much as a philosopher as a scientist. Certainly his theories relating matter, energy,
space, time and gravity have guided much of the work in theoretical physics since 1905. His famous "thought experiments," based
on intuition and imagination rather than laboratory work, propelled us beyond
the mechanistic, unchanging "clockwork universe" of Newton and the other
classical physicists into a **relativistic universe**. Here clocks run slower or faster depending on the speed of travel or location in the universe, and "true" distances are stretched or shrunk by gravity.

Einstein's legacy is a universe in which space and time are woven into a single fabric -- spacetime. It is matter that causes spacetime to curve and whose motion and properties are, in turn, altered by that curvature.

If all this seems a bit baffling, relax. With a bit of persistence you'll get the gist.

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One of the foremost pioneers of modern science, Isaac Newton developed his three laws of motion and a theory of gravity, not to mention the calculus needed to develop and express these theories in math! He set his concepts in a framework of space and time which he (like everyone else at that time) assumed to be absolute.

For two centuries that omission was overshadowed by his triumphs in celestial mechanics and optics.

What, in essence, were the key concepts in Newton's theory of gravitation?
Masses experience an attractive force between them, a force which acts
at a distance, resulting in their acceleration toward each other.
The strength of that force depends on the size of the masses and is
inversely proportional to square of the distance between them.

In Newton's universe, space existed independent of the matter in it. Both space and time were absolute, regardless of the motion of the observer and the matter contained within space. No substance controlled the motions of the moon, Earth and planets; only the force of gravity. But Newton's theory of gravitation was a "descriptive" theory; it didn't explain how the force of gravity was exerted, a fact that has an interesting parallel in Einstein's work and which remains a hot topic for future research.

Newton's laws satisfactorily explained most phenomena studied for the next two hundred years. Toward the end of the 19th century, however, as measuring devices grew more and more precise, the list of puzzling inconsistencies was growing.

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