© Image of Newton: Wikimedia Commons, Public Domain; Image of Einstein: Marcelo Gleiser.

Sir Isaac Newton (1643–1727, left) developed the universal law of gravitation, published in his manuscript *Philosophiae Naturalis Principia Mathematica*. Albert Einstein (1879–1955, right) created the theory of general relativity. Einstein's general relativity is the best theory of gravity we have, describing phenomena ranging from black holes to the gravitational time delay affecting clocks on GPS satellites. Einstein's theory is equivalent to Newton's universal law of gravitation applied to the everyday situations Newton used as examples in developing his theory. Both Newton and Einstein made numerous contributions to other fields of physics in addition to establishing the foundation of our understanding of gravity. The two physicists are considered to be among the most influential scientists of all time. (Unit: 3)