The man was a genius and probably a bit insane, but this was an interesting read! ( )

The two brief lectures in this volume are each interesting in their own right, although there is little to unify them beyond concern for the most foundational aspects of the science of physics.

The first, on "Ether and the Theory of Relativity," treats the now-quaint-sounding topic of the luminiferous ether, which seems to have gone the way of phlogiston and other obsolete scientific topics. Surprisingly, Einstein insists that there is a role for the ether concept in relativistic physics! It is, however, a "gravitational ether" from which every mechanical characteristic has been eliminated, and which is therefore just as easily denominated as space. Although this paper avoids mathematical and experimental details, a certain prior familiarity with the history of modern physics is very useful in appreciating it, since Einstein races through an extensive series of theorists in his summary of the evolution of the ether concept.

The second paper begins by offering the distinction between geometry as an axiomatic philosophical undertaking, and the empirical physical science of "practical geometry." Einstein explains that the effort to adjust physical laws to accommodate known events and behaviors to the axiomatic system of Euclidean geometry was in fact a driving force behind the theory of relativity, even though it eventualy became necessary to posit non-Euclidean space as a result of that theory. The later part of the paper is concerned to permit novices to acquire an imaginative appreciation of finite but unbounded spaces--in particular spaces curved through the fourth dimension in a hyperspherical fashion.

This little book, Sidelights on Relativity, presents two lectures given by Einstein. The first, concerns the relation between the ether concept and general relativity. Einstein concludes that the concept of empty space in general relativity is an existing entity, which may have properties (in analogy to the ether), but these properties are not mechanical or material in any sense, neither solid nor fluid.

The second lecture, Geometry and Experience, gives a perfect example of the kind of property that empty space (modern version of the ether) may have. This property is that of 'curvature' which is the central concept of general relativity theory. He relates this concept to the geometrical measurements which are actually made in practice, in the sense of geometry as a physical science. He also gives some hints which may help the reader to visualize higher dimensional spaces. In addition, this lecture includes Einstein's often quoted remark "As far as the laws of mathematics refer to reality, they are not certain; and as far as they are certain, they do not refer to reality".

For Newton, the space between particles was absolutely empty, consisting of exactly nothing. In the 19th century some physicists considered the possibility that space could be filled with a medium (the ether) with material properties (solid or liquid) which could support vibrations (oscillations of motion). Einstein rejected both of these views and introduced the idea of a space which could have non-material properties but not material properties. This key concept has had an influence in both relativity theories and quantum theories, but its full implication has not yet been assimilated by the scientific culture. Thus this book may be of greater significance than the title suggests.

This book is reminiscent of the book Essays in Science (Philosophical Library, 1930's) which is a collection of writings by Einstein on various scientific subjects. That book is abstracted from a still earlier work Mein Weltbild which gives Einstein's views on many topics including social issues. ( )