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Einstein's Theory of Relativity
Einstein's most famous contribution to physics is his theory of relativity. He proposed two main theories of relativity. The special theory was formulated in 1905 and the general theory in 1916.
In the system of mechanics formulated by Sir Isaac Newton, the laws of mechanics are the same for all uniformly moving systems. Einstein extended this concept to include the laws describing the motion of light. In addition he suggested that the velocity of light is constant regardless of the velocity of the source of the light.
On the basis of these two ideas Einstein deduced that the apparent physical properties of an object in uniform motion were different from those of the same object at rest. When the moving body is observed from a stationary point the body's length decreases in the direction in which it is moving, it gets heavier, and time passes more slowly.
It also follows from the two basic ideas that no body can travel at a speed greater than that of light in a vacuum. Another important deduction that he was able to make on this basis was the relationship between mass and energy, which he stated in the equation E = mc2. In this equation, E represents energy, m mass, and c the speed of light in a vacuum. Many deductions made from his basic ideas have since been verified by precise experiments.
The general theory is concerned with acceleration and gravitation. Einstein showed that any effect due to one of these might equally well be attributed to the other. He explained that the inertial mass of an object, which accounts for its resistance to any change in its state of uniform motion, is the same as its gravitational mass, which produces the gravitational field of the object.
This theory proposed that all the laws of physics could be formulated in terms of a new system of geometry involving four dimensions, the fourth dimension being time. According to the general theory, the effects of gravitation are not evidence of physical forces, but are explained by the curved shape of the four-dimensional universe.
Unified Field Theory
After publishing his theories of relativity, Einstein continued his work in theoretical physics in an attempt to construct a unified field theory. In such a theory he hoped to show a relation between gravitation and electromagnetism, in much the same way that his special theory of relativity had showed the relation between mass and energy on the one hand and space and time on the other. He published the mathematical formulas of his unified field theory in 1953. However, the full significance of this complex theory has not been evaluated, and the theory has not been confirmed by experiment.