Like many philosophers before him, Einstein was intrigued by the contrast between the imperfect arena of sensory experiences and the ideal realm of abstract concepts. He wondered which aspects of the world required hands-on experimentation and which could be deduced through pure thought. His life’s journey stepped carefully between these two positions. Ultimately, the latter would win out, and his mathematical side would overtake his more practical side. He would become obsessed by the idea of finding inviolable mathematical principles, elegant and beautiful in their simplicity of expression, that could explain all of nature.

One of Einstein’s first “thought experiments” involved a seeming contradiction between Newtonian physics and the known properties of light. At the age of 16 he imagined chasing a light wave and trying to catch up with it. He pictured himself running faster and faster until he precisely matched the speed of the flash. Then, he wondered, would the signal seem still to him, like two trains keeping pace?

Newtonian physics would suggest the affirmative. Any two objects moving at the exact same velocity should observe each other to be at rest—that is, their relative velocity should be zero. However, by Einstein’s time, physicists knew that light was an electromagnetic wave. James Clerk Maxwell’s well-known equations of electro-magnetism made no reference to the velocities of observers. Anyone recording the speed of light (in a vacuum) must measure the same value. Hence, two of the giants of physics, Newton and Maxwell, appeared locked in a conceptual battle.

Others tried to find a way out of this dilemma by proposing effects due to the invisible aether (which by that time had experimentally been discredited), but it was Einstein who developed the definitive solution. In a breakthrough known as the special theory of relativity, he demonstrated that Newtonian mechanics and Maxwellian electrodynamics could be reconciled by abandoning the notions of absolute space and time. By asserting that measured distances and durations depend on the relative velocities of the observer and the observed, Einstein developed dynamical equations that preserve the constancy of the speed of light.

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