Albert Einstein (pronounced /ˈælbərt ˈaɪnstaɪn/; German: [ˈalbɐt ˈaɪnʃtaɪn]; 14 March 1879 – 18 April 1955) was a theoretical physicist, philosopher and author who is widely regarded as one of the most influential and best known scientists and intellectuals of all time. He is often regarded as the father of modern physics.^{[2]} He received the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."^{[3]}
His many contributions to physics include the special and general theories of relativity, the founding of relativistic cosmology, the first postNewtonian expansion, explaining the perihelion advance of Mercury, prediction of the deflection of light by gravity and gravitational lensing, the first fluctuation dissipation theorem which explained the Brownian movement of molecules, the photon theory and waveparticle duality, the quantum theory of atomic motion in solids, the zeropoint energy concept, the semiclassical version of the Schrödinger equation, and the quantum theory of a monatomic gas which predicted Bose–Einstein condensation.
Einstein published more than 300 scientific and over 150 nonscientific works; he additionally wrote and commentated prolifically on various philosophical and political subjects.^{[4]} His great intelligence and originality has made the word "Einstein" synonymous with genius.
Physics in 1900
Einstein’s early papers all come from attempts to demonstrate that atoms exist and have a finite nonzero size. At the time of his first paper in 1902, it was not yet completely accepted by physicists that atoms were real, even though chemists had good evidence ever since Antoine Lavoisier’s work a century earlier. The reason physicists were skeptical was because no 19th century theory could fully explain the properties of matter from the properties of atoms.
Ludwig Boltzmann was a leading 19th century atomist physicist, who had struggled for years to gain acceptance for atoms. Boltzmann had given an interpretation of the laws of thermodynamics, suggesting that the law of entropy increase is statistical. In Boltzmann’s way of thinking, the entropy is the logarithm of the number of ways a system could be configured inside. The reason the entropy goes up is only because it is more likely for a system to go from a special state with only a few possible internal configurations to a more generic state with many. While Boltzmann’s statistical interpretation of entropy is universally accepted today, and Einstein believed it, at the turn of the 20th century it was a minority position.
The statistical idea was most successful in explaining the properties of gases. James Clerk Maxwell, another leading atomist, had found the distribution of velocities of atoms in a gas, and derived the surprising result that the viscosity of a gas should be independent of density. Intuitively, the friction in a gas would seem to go to zero as the density goes to zero, but this is not so, because the mean free path of atoms becomes large at low densities. A subsequent experiment by Maxwell and his wife confirmed this surprising prediction. Other experiments on gases and vacuum, using a rotating slitted drum, showed that atoms in a gas had velocities distributed according to Maxwell’s distribution law.
In addition to these successes, there were also inconsistencies. Maxwell noted that at cold temperatures, atomic theory predicted specific heats that are too large. In classical statistical mechanics, every springlike motion has thermal energy k_{B}T on average at temperature T, so that the specific heat of every spring is Boltzmann’s constant k_{B}. A monatomic solid with N atoms can be thought of as N little balls representing N atoms attached to each other in a box grid with 3N springs, so the specific heat of every solid is 3Nk_{B}, a result which became known as the Dulong–Petit law. This law is true at room temperature, but not for colder temperatures. At temperatures near zero, the specific heat goes to zero.
Similarly, a gas made up of a molecule with two atoms can be thought of as two balls on a spring. This spring has energy k_{B}T at high temperatures, and should contribute an extra k_{B} to the specific heat. It does at temperatures of about 1000 degrees, but at lower temperature, this contribution disappears. At zero temperature, all other contributions to the specific heat from rotations and vibrations also disappear. This behavior was inconsistent with classical physics.
The most glaring inconsistency was in the theory of light waves. Continuous waves in a box can be thought of as infinitely many springlike motions, one for each possible standing wave. Each standing wave has a specific heat of k_{B}, so the total specific heat of a continuous wave like light should be infinite in classical mechanics. This is obviously wrong, because it would mean that all energy in the universe would be instantly sucked up into light waves, and everything would slow down and stop.
These inconsistencies led some people to say that atoms were not physical, but mathematical. Notable among the skeptics was Ernst Mach, whose positivist philosophy led him to demand that if atoms are real, it should be possible to see them directly. Mach believed that atoms were a useful fiction, that in reality they could be assumed to be infinitesimally small, that Avogadro’s number was infinite, or so large that it might as well be infinite, and k_{B} was infinitesimally small. Certain experiments could then be explained by atomic theory, but other experiments could not, and this is the way it will always be.
Einstein opposed this position. Throughout his career, he was a realist. He believed that a single consistent theory should explain all observations, and that this theory would be a description of what was really going on, underneath it all. So he set out to show that the atomic point of view was correct. This led him first to thermodynamics, then to statistical physics, and to the theory of specific heats of solids.
In 1905, while he was working in the patent office, the leading German language physics journal Annalen der Physik published four of Einstein’s papers. The four papers eventually were recognized as revolutionary, and 1905 became known as Einstein’s "Miracle Year", and the papers as the Annus Mirabilis Papers.
