Einstein, Albert

German-born US physicist whose theories of relativity revolutionized our understanding of matter, space, and time. Einstein established that light may have a particle nature. He was awarded the Nobel Prize for Physics in 1921 for his work on theoretical physics, especially the photoelectric law. He also investigated Brownian motion, confirming the existence of atoms. His last conception of the basic laws governing the universe was outlined in his unified field theory, made public in 1953.

Brownian motion
Einstein's first major achievement concerned Brownian movement, the random movement of fine particles that can be seen through a microscope, which was first observed in 1827 by Robert Brown when studying a suspension of pollen grains in water. The motion of the pollen grains increased when the temperature increased but decreased if larger particles were used. Einstein explained this phenomenon as being the effect of large numbers of molecules (in this case, water molecules) bombarding the particles. He was able to make predictions of the movement and sizes of the particles, which were later verified experimentally by the French physicist Jean Perrin. Einstein's explanation of Brownian motion and its subsequent experimental confirmation was one of the most important pieces of evidence for the hypothesis that matter is composed of atoms. Experiments based on this work were used to obtain an accurate value of Avogadro's number (the number of atoms in one mole of a substance) and the first accurate values of atomic size.

The photoelectric effect and the Nobel Prize
Einstein's work on photoelectricity began with an explanation of the radiation law proposed in 1901 by Max Planck: E = hν, where E is the energy of radiation, h is Planck's constant, and ν is the frequency of radiation. Einstein suggested that packets of light energy are capable of behaving as particles called ‘light quanta’ (later called photons). Einstein used this hypothesis to explain the photoelectric effect, proposing that light particles striking the surface of certain metals cause electrons to be emitted. It had been found experimentally that electrons are not emitted by light of less than a certain frequency ν⁰; that when electrons are emitted, their energy increases with an increase in the frequency of the light; and that an increase in light intensity produces more electrons but does not increase their energy. Einstein suggested that the kinetic energy of each electron, 1/2mv², is equal to the difference in the incident light energy, hν, and the light energy needed to overcome the threshold of emission, hν⁰. This can be written mathematically as: 1/2mv² = hν - hν⁰

The speed of light and the special theory of relativity
The special theory of relativity started with the premises that (1) the laws of nature are the same for all observers in unaccelerated motion, and (2) the speed of light is independent of the motion of its source. Until then, there had been a steady accumulation of knowledge that suggested that light and other electromagnetic radiation does not behave as predicted by classical physics. For example, various experiments, including the Michelson–Morley experiment, failed to measure the expected changes in the speed of light relative to the motion of the Earth. Such experiments are now interpreted as showing that no ‘ether’ exists in the universe as a medium to carry light waves, as was required by classical physics. Einstein recognized that light has a measured speed that is independent of the speed of the observer. Thus, contrary to everyday experience with phenomena such as sound waves, the velocity of light is the same for an observer travelling at high speed towards a light source as it is for an observer travelling rapidly away from the light source. To Einstein it followed that, if the speed of light is the same for both these observers, the time and distance framework they use to measure the speed of light cannot be the same. Time and distance vary, depending on the velocity of each observer. From the notions of relative motion and the constant velocity of light, Einstein derived the result that, in a system in motion relative to an observer, length would be observed to decrease, time would slow down, and mass would increase. The magnitude of these effects is negligible at ordinary velocities and Newton's laws still hold good. But at velocities approaching that of light, they become substantial. As a system approaches the velocity of light, relative to an observer at rest, its length decreases towards zero, time slows almost to a stop, and its mass increases without limit. Einstein therefore concluded that no system can be accelerated to a velocity equal to or greater than the velocity of light. Einstein's conclusions regarding time dilation and mass increase were verified with observations of fast-moving atomic clocks and cosmic rays. Einstein showed in 1907 that mass is related to energy by the famous equation E=mc², which indicates the enormous amount of energy that is stored as mass, some of which is released in radioactivity and nuclear reactions, for example in the Sun.

Gravity and the general theory of relativity
In the general theory of relativity (1916), the properties of space–time were to be conceived as modified locally by the presence of a body with mass; and light rays should bend when they pass by a massive object. A planet's orbit around the Sun arises from its natural trajectory in modified space–time. General relativity theory was inspired by the simple idea that it is impossible in a small region to distinguish between acceleration and gravitation effects (as in a lift one feels heavier when it accelerates upwards). Einstein used the general theory to account for an anomaly in the orbit of the planet Mercury that could not be explained by Newtonian mechanics. Furthermore, the general theory made two predictions concerning light and gravitation. The first was that a red shift is produced if light passes through an intense gravitational field, and this was subsequently detected in astronomical observations in 1925. The second was a prediction that the apparent positions of stars would shift when they are seen near the Sun because the Sun's intense gravity would bend the light rays from the stars as they pass the Sun. Einstein was triumphantly vindicated when observations of a solar eclipse in 1919 showed apparent shifts of exactly the amount he had predicted.