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HISTORY OF PHYSICS: EINSTEIN, LORENTZ, AND THE ETHER

The following points are made by John Stachel (Nature 2005 433:215):

1) During the 19th century, the mechanistic world-view -- based on Isaac Newton's formulation in the Principia (1687) of the kinematics and dynamics of corpuscles of matter, and crowned by his stunningly successful theory of gravitation -- was challenged first by the optics, then by the electrodynamics of moving bodies. By the mid 1800s Newton's corpuscular theory of light was no longer tenable. To explain Snell's law of refraction, this theory assumed that light corpuscles speed up on encountering a medium of higher refractive index. But in 1849, Leon Foucault (1819-1868) and Hippolyte Fizeau (1819-1896) showed that, in fact, light slowed down, as predicted by the rival wave theory espoused by Newton's contemporary Christiaan Huygens (1629-1695). The problem now was to fit the wave theory of light into the newtonian picture of the world.

2) Indeed, the ether -- the medium through which light waves were assumed to propagate in the absence of ordinary, ponderable matter -- seemed to provide a physical embodiment of Newton's absolute space. But elucidating the relation between ether and ponderable matter presented grave problems: did moving matter drag the ether with it -- either totally or partially -- or did the ether remain immobile? It proved impossible to reconcile the consequences of any of these hypotheses with all the experimental results on the optics of moving bodies. By the last third of the nineteenth century, many physicists were acutely aware of this problem.

3) By 1865, James Clerk Maxwell (1831-1879) had demonstrated that light could be interpreted as wave-like oscillations of the electric and magnetic fields, obeying what we now call the Maxwell equations for these fields. It was realized that the optical problems were only a special case of similar problems in reconciling the electrodynamics of moving bodies with newtonian kinematics and dynamics. Towards the end of the century, however, Hendrik Antoon Lorentz (1853-1928) seemed to overcome all these problems through his interpretation of Maxwell's equations. Lorentz assumed that the electromagnetic ether is entirely immobile, in which case there would be no dragging of the ether.

4) Although in newtonian mechanics it is impossible to distinguish any preferred inertial frame (this result is often referred to as the galileian principle of relativity), at first the situation seemed different for electrodynamics and optics. The rest frame of the ether provided a preferred inertial frame, and motion through it should have been detectable. Yet all attempts to detect the translational motion of the Earth through the ether by means of optical, electrical or magnetic effects consistently failed. Lorentz succeeded in explaining why: according to his theory, no such effect should be detectable by any experiment sensitive to first order in (v/c), where v is the speed of the moving object through the ether and c is the speed of light in that medium. Until the 1880s, no experiment with greater sensitivity had been performed, and Lorentz's explanation of the failure of all previous experiments was a crowning achievement of his theory.

5) Newton's mechanics now seemed to have successfully met the challenge of optics and electrodynamics. But the seeds of its downfall had already been planted. Lorentz's explanation led him to introduce a transformation from newtonian absolute time to a new time variable in each inertial frame moving through the ether. As the relation between absolute time and this time varied from place to place in each inertial frame, Lorentz called this new variable the "local time" of that frame, regarding the local time as a purely formal expression. But Henri Poincare (1854-1912), the great mathematician who concerned himself extensively with problems of physics, was able to give a physical interpretation of this time variable within the context of newtonian kinematics: it is the time that clocks at rest in a frame moving through the ether would read if they were synchronized using light signals, without taking into account the motion of that frame. This was an important hint that the problems of the electrodynamics and optics of moving bodies were connected with the concept of time. But it was Einstein who made the final break with the concept of absolute time by asserting that the local time of any inertial frame is as physically meaningful as that of any other, because there is no absolute time with which they can be compared.[1-5]

References (abridged):

1. Einstein, A. Ann. Phys. (Leipz.) 17, 132-148 (1905)

2. Einstein, A. Ann. Phys. (Leipz.) 17, 549-560 (1905)

3. Einstein, A. Ann. Phys. (Leipz.) 17, 891-921 (1905)

4. Einstein, A. Ann. Phys. (Leipz.) 18, 639-641 (1905)

5. Einstein, A. Ann. Phys. (Leipz.) 19, 289-306 (1906)

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

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EINSTEIN ON PHYSICS AND REALITY

The following points are made by A. Einstein and L. Infeld (citation below):

1) What are the general conclusions which can be drawn from the development of physics? Science is not just a collection of laws, a catalogue of unrelated facts. It is a creation of the human mind, with its freely invented ideas and concepts. Physical theories try to form a picture of reality and to establish its connection with the wide world of sense impressions. Thus the only justification for our mental structures is whether and in what way our theories form such a link.

2) We have seen new realities created by the advance of physics. But this chain of creation can be traced back far beyond the starting point of physics. One of the most primitive concepts is that of an object. The concepts of a tree, a horse, any material body, are creations gained on the basis of experience, though the impressions from which they arise are primitive in comparison with the world of physical phenomena. A cat teasing a mouse also creates, by thought, its own primitive reality. The fact that the cat reacts in a similar way toward any mouse it meets shows that it forms concepts and theories which are its guide through its own world of sense impressions.

3) "Three trees" is something different from "two trees." Again "two trees" is different from "two stones." The concepts of the pure numbers 2, 3, 4..., freed from the objects from which they arose, are creations of the thinking mind which describe the reality of our world.

4) The psychological subjective feeling of time enables us to order our impressions, to state that one event precedes another. But to connect every instant of time with a number, by the use of a clock, to regard time as a one-dimensional continuum, is already an invention. So also are the concepts of Euclidean and non-Euclidean geometry, and our space understood as a three-dimensional continuum.

5) Physics really began with the invention of mass, force, and an inertial system. These concepts are all free inventions. They led to the formulation of the mechanical point of view. For the physicist of the early 19th century, the reality of our outer world consisted of particles with simple forces acting between them and depending only on the distance. He tried to retain as long as possible his belief that he would succeed in explaining all events in nature by these fundamental concepts of reality. The difficulties connected with the deflection of the magnetic needle, the difficulties connected with the structure of the ether, induced us to create a more subtle reality. The important invention of the electromagnetic field appears. A courageous scientific imagination was needed to realize fully that not the behavior of bodies, but the behavior of something between them. that is, the field, may be essential for ordering and understanding events.

6) Later developments both destroyed old concepts and created new ones. Absolute time and the inertial coordinate system were abandoned by the relativity theory. The background for all events was no longer the one-dimensional time and the three-dimensional space continuum, but the four-dimensional time-space continuum, another free invention, with new transformation properties. The inertial coordinate system was no longer needed. Every coordinate system is equally suited for the description of events in nature.

7) The quantum theory again created new and essential features of our reality. Discontinuity replaced continuity. Instead of laws governing individuals, probability laws appeared.

8) The reality created by modern physics is, indeed, far removed from the reality of the early days. But the aim of every physical theory still remains the same. With the help of physical theories we try to find our way through the maze of observed facts, to order and understand the world of our sense impressions. We want the observed facts to follow logically from our concept of reality. Without the belief that it is possible to grasp the reality with our theoretical constructions, without the belief in the inner harmony of our world, there could be no science. This belief is and always will remain the fundamental motive for all scientific creation. Throughout all our efforts, in every dramatic struggle between old and new views, we recognize the eternal longing for understanding, the ever-firm belief in the harmony of our world, continually strengthened by the increasing obstacles to comprehension.

Adapted from: The Evolution of Physics: From Early Concepts to Relativity and Quanta. A. Einstein and L. Infeld. Simon and Schuster 1938, p.254.

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THE YEAR 1905: EINSTEIN'S ANNUS MIRABILIS

The following points are made by Arthur I. Miller (citation below):

1) By the spring of 1905, the 26-year-old Einstein had decided that physicists were "out of their depth". From calculations based on Planck's radiation law, Einstein drew the astounding "general conclusion" that light can be a particle and a wave, and in fact both at once, a wave/particle duality. Therefore the electromagnetic world-picture could not succeed, because Lorentz's theory could represent radiation, or light, only as a wave, and so could never provide a way to explain how the electron's mass is generated by its own radiation.

2) Whereas Planck had discovered certain peculiarities about the energy of radiation, Einstein set out to explore the structure of radiation itself. Einstein's particles of light differed fundamentally from Newton's in ways that even he did not yet fully realize. Around the third week of May 1905, Einstein sent his friend Habicht what are surely some of the greatest understatements in the history of science. He wrote that he had only some "inconsequential babble" for his friend, whom he lambasted for neither writing nor visiting him during Easter:

"So what are you up to, you frozen whale, you smoked, dried, canned piece of soul... I promise you four papers."

3) The first paper is the light quantum paper that Einstein referred to as "very revolutionary". The second suggested a means to measure the size of atoms using diffusion and viscosity of liquids. The third paper explored Brownian motion using methods of the molecular theory of heat. Einstein wrote: "The fourth paper is only a draft at this point, and is an electrodynamics of moving bodies which employs a modification of the theory of space and time; the purely kinematic part of this paper will surely interest you."

4) What is so incredible about this outburst of creativity is that by late May two papers were completed and the third was in draft form." [Editor's note: The fourth paper, the so-called relativity paper, was completed a few weeks later in June 1905.]

Adapted from: Arthur I. Miller: Einstein, Picasso: Space, Time, and the Beauty That Causes Havoc. Basic Books, New York 2001, p.189.

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