#### What did Albert Einstein do? Albert Einstein works, discoveries, theories and contributions to science.

##### Capillarity and Brownian Movement:

Einstein s first scientific publication was a paper on capillary attraction that appeared in 1901. His papers on the statistical foundations of thermodynamics, published in 1902-1903, led him in 1905 to an application of the greatest significance. At that time the reality of molecules and the kinetic theory of matter, according to which the temperature of a body is due to the thermal agitation of its constituent molecules, was still under dispute. Einstein discovered by theoretical calculation that this thermal agitation can produce a detectable effect on particles suspended in a solution.

Such an effect had in fact been observed, without any realization of its cause, in 1827 by the botanist Robert Brown. Brown found that, even in the absence of currents and other external disturbances, pollen grains suspended in water can be seen under the microscope to be continually moving in an irregular zigzag fashion. Einstein showed that the Brownian movement can be used as direct evidence for the existence of molecules.

##### Photoelectric Effect:

Also in 1905, Albert Einstein made his first contribution to quantum physics. In 1900, Max Planck had assumed—in order to explain certain puzzling aspects of the radiation of light from hot bodies—that the emission (and absorption) of radiation by atoms can occur only in discrete packets of energy, or quanta, and not in purely arbitrary amounts. On this basis Planck succeeded in deriving a law of radiation, now known as Planck’s Law, that accorded with observation.

Planck was careful to restrict quantum effects to the interaction of matter and radiation, but Einstein came to the conclusion that radiation itself must have a corpuscular structure as well as a wavelike aspect and that it is actually composed of Planck’s quanta. Moreover, Einstein showed that a curious physical phenomenon known as the photoelectric effect, concerning the emission of electrons from a metal on which light is shone, that had previously defied explanation could be automatically accounted for if Planck’s hypothesis applied to radiation itself.

##### Special Relativity:

The third of Einstein’s great papers of 1905 arose from a problem that had puzzled him for many years—if one were to travel through space with the same velocity as a beam of light, how would one describe the beam? According to the usual idea of relative motion the beam would appear as a spatially oscillating electromagnetic field at rest, but such a concept was unknown to physics. Therefore, Einstein began to suspect that the laws of physics, including those concerning the propagation of light, must remain the same for all observers however fast they move relative to one another. Nevertheless, it was only after years of hard thought concerning this principle of relativity that he finally felt compelled to regard the velocity of light as independent of the motion of the observer.

This conclusion conflicted, however, with the traditional idea of relative motion on which Newtonian mechanics was based. Einstein realized that the measurement of motion depends on the idea of simultaneity. The crucial stage in his thinking occurred when he realized that this is a primitive idea only for events at the same place. When we observe a distant event we can only infer its time of occurrence by invoking assumptions about its distance and the velocity of light. Einstein therefore decided to abandon the traditional ideas of time and motion and instead to take the principle of relativity as fundamental.

In particular, he regarded the invariance of the velocity of light as a means of comparing time by observers in uniform relative motion. He found that a moving clock would appear to run slow compared with a clock at rest with respect to the observer. He also found that the inertial mass of a body increases with velocity, so that no particle can attain the velocity of light, which is therefore an upper limit to velocity. This dependence of mass on velocity led Einstein to conclude that mass and energy are different manifestations of the same thing, a conclusion that explains the enormous release of energy in nuclear transformations and, in particular, accounts for the sun’s radiation.

##### Specific Heats:

In 1907, Einstein made another important contribution to quantum theory by extending Planck’s hypothesis to the theory of solid bodies. In 1819, two French physicists, P. L. Dulong and A. T. Petit, had found empirically that the atomic heat (the product of specific heat and atomic weight) of monatomic solids is a constant, independent of temperature. However, it was later found that the atomic heat of all bodies diminishes at very low temperatures. Classical theory offered no explanation of this phenomenon, but Einstein showed that if the quantum hypothesis were applied to the energy of the vibrations of the atoms of a solid the difficulty could be resolved.

##### General Relativity:

At the end of 1907, Einstein published an important paper on gravitation in which he drew attention to the peculiar fact that, in any small region where the gravitational force can be regarded as uniform, all bodies fall with the same acceleration and so are unacceler-ated relative to each other. Motion in a uniform gravitational field is therefore equivalent to uniform motion with respect to a frame of reference that has the corresponding acceleration.

This “principle of equivalence,” as he later called it, became the foundation of his general theory of relativity. In this theory, which he first completely expounded in a paper published in 1916, he abandoned the restriction of the relativity principle to observers in uniform relative motion —which characterized his special theory of relativity of 1905—and argued instead that the laws of nature should be expressed in a form that is the same for all observers in any kind of relative motion. At the same time, since in Newtonian theory the distinction between accelerated and uniform motion is that the former is associated with the action of force, Einstein sought to eliminate this concept.

He eventually discovered that gravitational motions could be depicted as the analogues of straight lines in a four-dimensional combination of space and time (space-time) with a geometry of a non-Euclidean type known as Riemannian. He found that his theory could account for three small effects, not obtainable on Newton’s theory, concerning planetary motion and the influence of gravitational fields on the transmission of light, including the bending of its path from a straight line.

##### Quantum Theory:

Shortly after this success, Einstein turned his attention again to quantum theory. In 1913, Niels Bohr had applied quantum principles to explain the spectral lines emitted by radiating atoms. He assumed that an atom can exist permanently only in a discontinuous series of stationary states and that the radiation absorbed or emitted during a transition between two stationary states possesses a frequency that accords with the quantum principles of Planck and Einstein. Bohr’s theory, despite its success in accounting for the observed spectral lines of hydrogen and other atoms, left many questions unanswered.

In particular, it threw no light on the laws governing the probability of transitions between stationary states. In one of his most brilliant papers, published in 1917, Einstein investigated this question, obtaining a new and much more satisfactory derivation of Planck’s radiation law. But perhaps the most remarkable feature of this paper was that in it Einstein postulated the process known as stimulated emission and inferred its properties. This is the process that is now employed in the maser and the laser.

In 1924, the Indian physicist S. N. Bose derived Planck’s radiation law by regarding radiation as a kind of gas made up of photons. Einstein realized that this treatment could be extended to ordinary gases consisting of atoms if one assumed that atoms, like photons, had simultaneously wave and particle properties. This idea had already been formulated independently by Louis de Broglie, and two years later it stimulated Erwin Schrodinger to develop his wave mechanics, which is now widely used for solving problems of atomic physics.

##### Unified Field Theory:

During the latter part of his life Einstein was concerned mainly with problems arising out of his general theory of relativity and with unsuccessful attempts to produce what he called a “unified field theory” embracing electromagnetic as well as gravitational forces.

##### Cosmology:

In 1917, Einstein made the important application of general relativity to cosmology. In a pioneering paper he laid the foundations of modern theoretical work on the structure of the universe as a whole. In place of the infinite Newtonian universe he constructed a world model, since known as the Einstein universe, that was finite but unbounded. This model was static, but following the discovery by Edwin Hubble in 1929 of the expansion of the universe Einstein, in collaboration with the Dutch astronomer Willem de Sitter, constructed the simplest form of expanding world model that accords with the laws of general relativity. This is now known as the “Einstein-de Sitter universe.”

##### Equation of Motions:

The most important work that Einstein did after settling in the United States was in collaboration with Leopold Infeld and Banesh Hoffmann. In 1938 they succeeded in proving that in general relativity, unlike in Newton’s theory, the fundamental equations prescribing the strength of the gravitational field also determine the equations of motion of the particles present.

### PHILOSOPHY OF NATURE

Although Einstein was primarily concerned with problems of physics, he was influenced by philosophical as well as purely technical considerations, and his work in turn has had a considerable philosophical influence.

##### Early Interest in Philosophy:

The critical reasoning that led him when a young man to abandon the previously unquestioned concept of worldwide simultaneity was stimulated by his interest in philosophy. According to his own account he was particularly influenced by Hume and Mach, the former through his critical attitude toward traditional commonsense assumptions and beliefs, the latter through his criticism of Newton’s ideas concerning space and time and also by his critical examination of Newtonian mechanics.

Broadly speaking, Einstein’s philosophical standpoint when developing the special theory of relativity can be described as “positivistic.” He refused to accord physical significance to concepts that in principle cannot be measured or determined directly, such as the simultaneity of distant events with events in the observer’s immediate neighborhood.

##### Philosophy of Quantum Mechanics:

Although Einstein attached great practical importance to the use of statistical techniques for dealing with problems concerning large numbers of particles, he always retained the view that the ultimate laws are causal and deterministic. This led to his rejection of the new quantum theory developed between 1925 and 1927 by Werner Heisen-berg, Max Born, P. A. M. Dirac, and others. He believed, contrary to the opinion of most physicists, that the statistical interpretation of quantum mechanics was a consequence of the incompleteness of the description of physical systems in that theory. He had many arguments with Niels Bohr over this.

Einstein’s most subtle and penetrating paper on this question was written jointly with Boris Podolsky and Nathan Rosen in 1935. In this paper, the correctness of a theory, as judged by the degree of agreement between the conclusions of the theory and human experience, was carefully distinguished from its completeness, a necessary requirement for the latter being that every element of the physical reality concerned must have a counterpart in the theory.

The main point of the paper was to produce an example of a physical situation in which it was possible in principle to make two precise measurements of complementary physical quantities, such as position and momentum, in defiance of Heisen-berg’s Uncertainty Principle. This says that in the case of two such complementary quantities, the more accurately we measure either quantity the greater becomes the uncertainty in our measurement of the other. Einstein concluded that the quantum-mechanical description of physical reality is incomplete. Although many physicists have rejected this conclusion, others believe that the difficulty exposed by Einstein, Podolsky, and Rosen has never been satisfactorily resolved.

##### Retreat from Positivism:

With the development of general relativity, which would not have been possible without recourse to sophisticated mathematical techniques, Einstein turned away from positivism. He came to realize that the growth of physics depends on theories that are often far removed from our observational experience. In his view, although the consequences of a theory must be tested empirically, its axioms are not automatic inferences from experience but free creations of the human mind, guided by considerations of mathematical simplicity and beauty. To the end of his life he retained a profound belief in the harmony and ultimate “knowability” of nature.