String Theory

Einstein's General Relativity

In 1905, Einstein proposed his theory of special relativity, which states that all uniform motion is relative and the laws of physics are the same in all reference frames, whether you are at rest or moving at a constant velocity. However, special relativity only applied to one particular situation—one where observers are moving at a constant velocity in relation to each other. In 1916, Einstein broadened his theory into a general theory of relativity that incorporated accelerated motion and thus gravity. This theory united special relativity and Newton's universal theory of gravitation. Einstein asserted that space and time form a single continuum and described gravity as a property of the geometry of space-time. Tests conducted during a 1919 solar eclipse proved Einstein right.

General relativity is extremely accurate on very large scales (the astronomical level), but falls apart when you try to apply it to very small scales (the ultramicroscopic level). To describe things on very small scales, scientists need quantum theory.

Quantum Theory

Quantum theory was first developed in the 1920s to explain the microscopic properties of the universe. In 1923, French physicist Louis de Broglie postulated that the wave–particle duality that Einstein put forth for light also applies to matter and that all particles also have a wave nature. Experimental physicists at Bell Telephone Company Laboratories confirmed de Broglie's theory with an experiment that showed that electrons form interference patterns, which is a property of waves.

In 1926, German physicist Max Born asserted that electron waves must be understood in terms of probability. He said that an electron is more likely to be found at points on the wave with high magnitude and is less likely to be found at points with low magnitude. This implies that at a fundamental level, matter must be described in a probabilistic manner, as opposed to the absolute manner of classical physics. Therefore, scientists can only predict the most probable outcome of a physics problem, not which outcome will actually occur.

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