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During the early decades of the 20th century, the field of theoretical physics was revolutionized by some startling new discoveries. These discoveries-quantum theory and relativity-had profound implications for our understanding of the universe and for the development of new technology. Quantum theory is concerned with the absorption and emission of energy by matter, and with the wavelike motion of matter.
This area of physics was developed by several scientists over a period of about thirty years, beginning at the start of the 20th century. Before the development of quantum theory, physicists had believed that energy could be absorbed or emitted in any amount, and that matter occupied a definite location in space. However, quantum theory showed that these conceptions were inaccurate. According to quantum theory, energy can only be absorbed or emitted in certain, discrete amounts, which are called quanta.
It is as if energy is transported in small “packets” that only exist in particular sizes. This finding can be seen in the frequencies of light and other radiation that are emitted by different kinds of atoms. Each atom only emits radiation having certain frequencies. These frequencies correspond to the amounts of energy that are released when the electrons that orbit around the nucleus of an atom move from a higher orbital path to a lower orbital path.
Another astonishing feature of quantum theory is that matter can exist both as a particle and as a wave. Experiments have shown that the electrons of an atom can behave as waves; for example, electrons can diffract, or bend, in the same way that light waves bend. One surprising implication of this wavelike property is that the precise location of a particle cannot be known with certainty. The other major discovery of the early 20th century physics was the theory of relativity.
Unlike quantum theory, relativity was largely the work of one man, a physicist named Albert Einstein. Einstein demonstrated that the speed of light is constant, regardless of the motion of the observer. Einstein showed that, contrary to the assumptions of classical physics, time and motion are not constant, but relative to the observer. If a spaceship could move at an extremely high speed, time would pass considerably more slowly on that spaceship than for people who stayed on the earth. Moreover, the spaceship will appear to become shorter as its speed increases, and the mass of the spaceship would increase as its speed increased.
One of the bewildering ideas from Einstein’s theory of relativity is the notion that time can be added to the three dimensions of space-length, width, and height-as a fourth dimension. According to the theory of relativity, massive objects cause a distortion, or warping, of this four-dimensional space-time continuum.
But because the speed of light is constant, light will follow a straight line through space-time, and its motion will appear to be warped as it moves through space that is distorted by massive objects such as stars or planets. Another astonishing implication of the theory of relativity is that matter and energy are interchangeable.
This is the basis for Einstein’s famous formula, E = mc2, which states that energy equals mass times the speed of light squared. This idea is the basis for atomic energy, which allows for the release of energy by destroying a small amount of matter when the nucleus of an atom is divided, or split.
Today, thanks to the efforts of Einstein and the other physicists of the early 20th century, the study of theoretical physics is based largely on the ideas of quantum theory and relativity. Also, much of our modern technology-from electronics to nuclear power-is based on the ideas that were developed during this exciting period in the history of science.