Einstein is best known for relativity and his other 1905 breakthroughs – explaining the photoelectric effect and his work on Brownian motion – but his ideas also underpinned the development of the laser and the creation of a new state of matter called the Bose-Einstein condensate. These discoveries, which were made in 1916 and 1924, respectively, were based on Einstein's investigations into bosonic particles such as photons. Moreover, Bose – Einstein condensation was predicted to occur in one of the simplest physical systems: the ideal gas.
Einstein is best known for relativity and his other 1905 breakthroughs – explaining the photoelectric effect and his work on Brownian motion – but his ideas also underpinned the development of the laser and the creation of a new state of matter called the Bose-Einstein condensate. These discoveries, which were made in 1916 and 1924, respectively, were based on Einstein's investigations into bosonic particles such as photons. Moreover, Bose – Einstein condensation was predicted to occur in one of the simplest physical systems: the ideal gas.
In 2005 we celebrate the hundredth anniversary of the publication of five seminal papers by Albert Einstein establishing the basis of three fundamental fields of physics: the theory of relativity, quantum theory and the theory of Brownian motion. This happened at a time when the reality of atoms and molecules was still far from certain. The microscopic view Einstein took of molecular motion led to the calculation of Avogadro's number and the size of molecules by studying the motion of small particles in liquids. Combining kinetic theory and classical thermodynamics finally led Einstein to the conclusion that the displacement of a microparticle under Brownian motion varies as the square root of time. The experimental demonstration of this law three years later was considered as the striking proof that atoms and molecules are physically real. Today single atoms are probed directly in experiments, and we are able to observe the individual paths of colliding or dissociating particles. These new techniques are described in some of the papers of this issue in a very impressive way. An even more exciting development was initiated with the demonstration of Bose–Einstein condensation of dilute gases of ultracold atoms, a phenomenon first predicted by Einstein in papers published between 1916 and 1924. The first paper of Einstein's famous series of 1905, 'On an heuristic point of view about the creation and conversion of light', was an explanation of the photoelectric effect, demonstrated in several experiments over the previous few years. The explanation was based on the quantum hypothesis introduced by Max Planck five years earlier, and was considered as an additional and important proof of Planck's hypothesis. Today quantum theory and photons are the basis for much of modern science and technology. We have learned to experiment with single photons, and we have demonstrated the advantages of information transfer by single photons. Photons permit new and incredibly precise time and frequency standards. These are not only technically important, but may also lead to even more stringent tests of relativity and other fundamental laws and concepts of physics. Examples of these applications and many other uses of modern optics are discussed in this issue. Special relativity, the third of Einstein's 1905 topics, has always been closely connected with atomic, molecular and optical physics. Today atomic physics provides some of the most stringent tests of special relativity. Furthermore, fast electron beams are used in advanced light sources such as synchrotron radiation generators and free electron lasers. Those aspects and related applications are also discussed here. We hope that this special issue will be of great interest to the reader by highlighting recent advances in atomic, molecular and optical physics. This field continues to provide one of the most fertile areas for research one hundred years after it first emerged from the work of Einstein published in 1905, his 'miraculous year'.
This chapter comprises Einstein's explanation of the photoelectric effect, his analysis of the transition of electrons in atoms, his study of the behavior of bosons (named after their discoverer, Satyendra Bose), and his opposition to the standard interpretation of quantum mechanics. For the photoelectric effect, Einstein showed that light could behave as a particle, not only as a wave. Using this “photon” model, he explained how atoms absorbed and emitted light. He also predicted that a photon could stimulate the emission of many more, through (decades later) light amplification by stimulated emission of radiation. He studied bosons, particles that, unlike fermions (named after Enrico Fermi), such as electrons in atoms, had no “Exclusion Principle”: any number of them could “condense” into a single state. Finally, he opposed Bohr's “Copenhagen interpretation,” that a system such as an electron has a range of values for its properties such as spin, until a measurement is made, forcing it to choose one. Einstein argued for a “hidden variable theory” in which there was something deeper than quantum mechanics: that a system was in a specific state before a measurement. (He was wrong.)
S. N. Bose gave a new identity to the quanta of light, introduced into physics by Planck and Einstein, and derived Planck's formula for the radiation spectrum. Chance events led to Einstein's insightful use of Bose's result to build a quantum theory of atomic gases, with the remarkable prediction of a new type of condensation without interactions. Though the spectacular phenomena of superfluidity and superconductivity were identified as the consequences of Bose-Einstein Condensation, the phenomenon in its near pure form, as foresaw by Einstein, was observed 70 years later in atomic gases. The quest for atomic gas BEC was behind much of the developments of laser cooling and trapping of atoms. Since then, the atomic gas BEC has grown into a major research theme and versatile research tool all over the world. I outline the development that made BEC of laser cooled atoms the promised 'Swiss army knife' of laboratory physics, with mentions of some of the examples and applications. My involvement in this field of research at 'home and the world' is briefly reviewed.
Bose-Einstein condensation is a macroscopic quantum phenomenon that was first predicted by Albert Einstein in the 1920s, at a time when quantum theory was still developing and was being applied to microscopic systems, such as individual particles and atoms. Einstein applied the new concept of Bose statistics to an ideal gas of identical atoms that were at thermal equilibrium and trapped in a box. He predicted that at sufficiently low temperatures the particles would accumulate in the lowest quantum state in the box, giving rise to a new state of matter with many unusual properties.
In 1924 the Indian physicist Satyendra Nath Bose sent Einstein a paper in which he derived the Planck law for black-body radiation by treating the photons as a gas of identical particles. Einstein generalized Bose's theory to an ideal gas of identical atoms or molecules for which the number of particles is conserved and, in the same year, predicted that at sufficiently low temperatures the particles would become locked together in the lowest quantum state of the system. We now know that this phenomenon, called Bose-Einstein condensation (BEC), only happens for “bosons” – particles with a total spin that is an integer multiple of h, the Planck constant divided by 2π.
A century ago Albert Einstein transformed classical physics with his seminal papers on Brownian motion, the Photoelectric effect, and, of course, special and later general relativity. Lesser well-known are his contributions to BoseEinstein Condensation and the EinsteinPodolskyRosen paradox, the latter being a criticism of Quantum Mechanics. These later works were regarded even by physicists for decades as mere Gedanken or thought experiments. In recent years, not only have they been verified experimentally but revolutionary technological applications are emerging including quantum cryptography and possibly quantum computing. PACS Nos.: 03.65, 03.67, 03.75, 05.30.Jp