In 1905, while working as a clerk in a Swiss patent office, Albert Einstein (German-born theoretical physicist, 1879-1955) published "On the Electrodynamics of Moving Bodies." This paper contained his special theory of relativity that would forever change how we think about time, space, and our place in the universe.
Europeans had long used the Ptolemaic model to explain the movement of the planets, which held that the Earth was at the center of the universe inside a series of concentric spheres. Accordingly, the Earth remained stationary as the planets rotated around it. This model had been challenged by scientists of the Renaissance, including Nicolaus Copernicus (Polish astronomer, 1473-1543), Johannes Kepler (German mathematician and astronomer, 1571-1630), and Galileo Galilei (Italian physicist and astronomer, 1564-1642) who proposed that the Earth and other planets revolved around the sun. The fact that the church persecuted Galileo demonstrates just how closely related our notions of space-time are to our world view: by challenging the idea that the sun rotated around the Earth, Galileo was doing no less than shaking the very foundations of the Catholic Church.
By the mid-seventeenth century, in the midst of the wonder of the scientific revolution, Sir Isaac Newton (English physicist and astronomer, 1643-1727) and RenŽ Descartes (French mathematician and astronomer, 1596-1650), among others, formulated laws that described motion, inertia, and other properties of space and time. The development of the visual models of perspective—paintings created from a single point of space and time—was directly related to the notion of a single, detached observer, the person who is the measure of all things. So, too, were the works of Newton and others: they created a universe that was easily comprehensible and, perhaps more importantly, easier to depict in the rules of painting that had evolved during the Renaissance.
Newton postulated the law of gravity, from which it follows that there is no absolute standard of motion. That is, if you were in a moving carriage you would have no way to tell you were in motion without looking out the window. The laws of physics were the same for you as they were for those outside of the carriage. Newton's law of gravity replaced the notion of absolute position in space, but not with the notion of absolute time.
Scientists as far back as the late seventeenth century had speculated that light travels at a constant speed. In the nineteenth century, James Clerk Maxwell (Scottish mathematician and physicist, 1831-79) developed a set of equations describing the electromagnetic spectrum, which included visible light. Light travels at a fixed speed for any observer and can be measured at 186,000 miles per second. But if there were no absolute rest, what was this speed relative to? Through a series of thought experiments involving no apparatuses, only the question "What would happen if ..." Einstein demonstrated that space was relative and so, too, was time. If two observers are traveling in relation to one another—that is, if their frames of reference are moving—they will observe time and space differently. Neither observation is correct or incorrect, only different. At the speeds we experience in everyday life, these differences are hardly noticeable, but as an observer approaches the speed of light, they become more pronounced. Einstein also showed that gravity was in fact a curvature of space-time caused by the mass of objects. Objects travel in a straight line in four dimensions (space-time), though they appear to us to travel in a curved line in three dimensions.
