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HISTORY OF SCIENCE: EINSTEIN ON PHYSICS AND PROGRESS

The following points are made by Albert Einstein (Physics Today 2005 June):

1) If philosophy is interpreted as a quest for the most general and comprehensive knowledge, it obviously becomes the mother of all scientific inquiry. But it is just as true that the various branches of science have, in their turn, exercised a strong influence on the scientists concerned and, beyond that, have affected the philosophical thinking of each generation. Let us glance, from this point of view, at the development of physics and its influence on the conceptual framework of the other natural sciences during the last hundred years.

2) Since the Renaissance, physics has endeavored to find the general laws governing the behavior of material objects in space and time. To consider the existence of these objects as a problem was left to philosophy. To the scientist, the celestial bodies, the objects on Earth, and their chemical peculiarities, simply existed as real objects in space and time, and his task consisted solely in abstracting these laws from experience by way of hypothetical generalizations.

3) The laws were supposed to hold without exceptions. A law was considered invalidated if, in a single case, any one of its properly deduced conclusions was disproved by experience. In addition, the laws of the external world were also considered to be complete, in the following sense: If the state of the objects is completely given at a certain time, then their state at any other time is completely determined by the laws of nature. This is just what we mean when we speak of "causality." Such was approximately the framework of the physical thinking a hundred years ago.

4) As a matter of fact, the framework was even more restrictive than it has been sketched. The objects of the external world were considered to consist of immutable mass points, acting upon each other with well-defined forces eternally attached to them and, under the influence of these forces, carrying out incessant motions to which, in the last analysis, all observable processes could be reduced.

5) From a philosophical point of view, the conception of the world, as it appears to those physicists, is closely related to naive realism, since they looked upon the objects in space as directly given by our sense perceptions. The introduction of immutable mass points, however, represented a step in the direction of a more sophisticated realism. For it was obvious from the beginning that the introduction of these atomistic elements was not induced by direct observation.

6) With the Faraday-Maxwell theory of the electromagnetic field, a further refinement of the realistic conception was unavoidable. It became necessary to ascribe the same irreducible reality to the electromagnetic field, continually distributed in space, as formerly to ponderable matter. But sense experiences certainly do not lead inevitably to the field concept. There was even a trend to represent physical reality entirely by the continuous field, without introducing mass points as independent entities into the theory.

7) Summing up, we may characterize the framework of physical thinking up to a quarter of a century ago as follows: There exists a physical reality independent of substantiation and perception. It can be completely comprehended by a theoretical construction which describes phenomena in space and time -- a construction whose justification, however, lies in its empirical confirmation. The laws of nature are mathematical laws connecting the mathematically describable elements of this construction. They imply complete reality in the sense mentioned before.

8) Under the pressure of overwhelming experimental evidence concerning atomistic phenomena, almost all of today's physicists are now convinced that this conceptual framework --notwithstanding its apparently wide scope -- cannot be retained. What appears untenable to physicists of our times is not only the requirement of complete causality but also the postulate of a reality which is independent of any measurement or observation.

Physics Today http://www.physicstoday.org

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HISTORY OF PHYSICS: EINSTEIN AND BROWNIAN MOTION

The following points are made by Giorgio Parisi (Nature 2005 433:221):

1) On 30 April 1905, Einstein completed his doctoral thesis on osmotic pressure, in which he developed a statistical theory of liquid behavior based on the existence of molecules. This work, together with his subsequent paper on "brownian motion", constitutes one of the most important, but often overlooked, contributions that Einstein made to physics.

2) In the closing decades of the 19th century, theoretical physics was in a state of turmoil. The big outstanding questions of that time have been much discussed. Such questions culminated in relativity and quantum mechanics -- theoretical developments in which Einstein's key role is being justly celebrated this year. But it should not be forgotten that the seemingly innocuous observations of Robert Brown (1773-1858) of the irregular motions of a suspension of pollen grains in water -- now known as brownian motion -- also heralded a revolution in physical thought.

3) Although the concepts of atoms and molecules are now universally accepted, this was not the case at the turn of the 20th century. The statistical interpretation by Ludwig Boltzmann ((1844-1906) of the laws of thermodynamics -- a body of work deeply rooted in the ensemble dynamical motion of material atoms -- had many adherents. But there were also many heavyweight dissenters (for a time including Max Planck (1858-1947)), who did not accept that thermodynamics had its origins in the reversible motion of invisible hypothetical particles. And many distinguished physicists of the time (among them Wilhelm Roentgen (1845-1923)) suspected that brownian motion indicated a clear failure of Boltzmann's formulation of the second law of thermodynamics.

4) It was in this context that Einstein's explanation for brownian motion made an initial impression. In particular, Einstein showed that the irregular motion of the suspended particles could be understood as arising from the random thermal agitation of the molecules in the surrounding liquid: these smaller entities act both as the driving force for the brownian fluctuations (through the impact of the liquid molecules on the larger particles), and as a means of damping these motions (through the viscosity experienced by the larger particles). This connection between displacement and the viscosity can be quantitatively expressed in one dimension as a relationship between displacement, viscosity, the universal gas constant, Avogadro's number, the Boltzmann constant, the temperature, and the radius of the suspended particles. This finding went beyond simply confirming the existence of atoms and molecules, and provided a new way of determining Avogadro's number. As Einstein himself remarked, the consequence of this relation is that one can see, directly through a microscope, a fraction of the thermal energy manifest as mechanical energy. By proving that a statistical mechanics description could explain quantitatively brownian motion, all doubts concerning Boltzmann's statistical interpretation of the thermodynamic laws suddenly faded.(1-3)

References (abridged):

1. Pais, A. Subtle is the Lord... (Oxford Univ. Press, 1982)

2. Kuhn, T. S. Black Body Theory and the Quantum Discontinuity 1894-1911 (Oxford Univ. Press, 1978)

3. Mezard, M., Parisi, G. & Virasoro, M. A. Spin Glass Theory and Beyond (World Scientific, Singapore, 1987)

Nature http://www.nature.com/nature

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HISTORY OF PHYSICS: EINSTEIN AND RADIATION

The following points are made by Daniel Kleppner (Physics Today 2005 February):

1) Albert Einstein had a genius for extracting revolutionary theory from simple considerations: From the postulate of a universal velocity he created special relativity; from the equivalence principle he created general relativity; from elementary arguments based on statistics he discovered energy quanta. His 1905 paper on quantization of the radiation field (often referred to, inaccurately, as the photoelectric-effect paper) was built on simple statistical arguments, and in subsequent years he returned repeatedly to questions centered on statistics and thermal fluctuations.

2) In 1909, Einstein showed that statistical fluctuations in thermal radiation fields display both particle-like and wave-like behavior. His was the first demonstration of what would later become the principle of complementarity. In 1916, when he turned to the interplay of matter and radiation to create a quantum theory of radiation, he once again based his arguments on statistics and fluctuations.

3) Einstein's theory of radiation is a treasure trove of physics, for in it one can discern the seeds of quantum electrodynamics and quantum optics, the invention of masers and lasers, and later developments such as atom-cooling, Bose-Einstein condensation, and cavity quantum electrodynamics. Our understanding of the Cosmos comes almost entirely from images brought to us by radiation across the electromagnetic spectrum. Einstein's theory of radiation describes the fundamental processes by which those images are created.

4) Einstein's 1905 paper on quantization endowed Max Planck's quantum hypothesis with physical reality. The oscillators for which Planck proposed energy quantization were fictitious, and his theory for blackbody radiation lacked obvious physical consequences. But the radiation field for which Einstein proposed energy quantization was real, and his theory had immediate physical consequences. His paper, published in March 1905, was the first of his wonder year. In rapid succession he published papers on Brownian motion, special relativity, and his quantum theory of the specific heat of solids.

5) In 1907, his interest shifted to gravity, and he took the first tentative steps toward the theory of general relativity. His struggle with gravitational theory became all-consuming until November 1915, when he finally obtained satisfactory gravitational field equations. During those years of struggle, however, Einstein apparently had a simmering discontent with his understanding of thermal radiation, for in July 1916, he turned to the problem of how matter and radiation can achieve thermal equilibrium. One could argue that 1916 was too soon to deal with that problem because there were serious conceptual obstacles to the creation of a consistent theory. Einstein, in his Olympian fashion, simply ignored them. In the next eight months, he wrote three papers on the subject, publishing the third, and best known, in 1917.[1,2]

References (abridged):

1. A. Einstein, Phys. Z. 18, 121 (1917); English translation On the Quantum Theory of Radiation, by D. ter Haar, The Old Quantum Theory, Pergamon Press, New York (1967), p. 167

2. A. Pais, Rev. Mod. Phys. 49, 925 (1977)

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