For the lay reader, this is an excellent summary of Einstein's (Einstein wrote the forward) key theories (See below for an an addition to this review, August 27, 2022):
1. The dual nature of light - particles and waves; the beginnings of quantum theory.
2. Everything is in motion; there's no absolute, fixed point in space from which to measure motion; motion is relative, of one body to another ("Motion can be detected only as a change of position with respect to another body.").
3. The velocity of light is constant, unaffected the source of the motion or the receiver.
4. Time and space are not absolute; they contract with increased velocity; time stops at the speed of light (and there's no space?); space is an "order of material objects"; time is an "order of events"; "an hour is actually a measurement in space."; time (to cover space/distance) and space (a difference in space inherently involves time) are integral to each other (e.g., as a way to locate coordinates -- latitude, longitude, altitude, time - of moving objects in space).
5. Mass is concentrated energy a la E=MC squared; mass is resistance to a change in motion; "heaviness" is resistance to changes in motion; mass is relative to velocity - the faster the motion, the more energy that is involved and more energy means more mass.
6. Bodies move uniformly in a straight line and resist opposing forces that would deviate us from such movement.
7. Gravity is not a force (attraction) but involves paths that follow the geometric structure of space (created by "the mass of a gravitating body and the structure of the field around it...."
8. Light has mass a la E=MC squared (energy and mass are equivalent, with mass being degrees of energetic content) but light has kinetic (not rest) mass, the energy of motion.
OR, something like the above....
The book closes with a description of Einstein's long quest to develop a Unified Field Theory and thereby identify "a profound simplicity" to "supplant the surface complexity of nature." He distilled and united universal essences down to energy and matter, space and time, and gravity. The relationship of these to quantum theory at the micro level remained his challenge up to his death. Barnett recasts that quest into Platonic terms - the search for "the ultimate immutable essence that undergirds the mutable illusory world." Then Barnett notes the risks involved, as Einstein acknowledged, with tossing aside the senses so that one is left only with the highest theory devoid of content, as illustrated by a Hegal quote, "'Pure Being and Nothing are the same.'" But isn't it the Big Bang, the ultimate contraction into a singularity, that gives birth to All?
August 27, 2022 Addition to this review:
Barnett’s 1948 book (with a foreword by Einstein) filled a niche. As the book’s editors write of the author, he “read everything he could find on Einstein and relativity, and he read with a growing sense of discouragement. The popular works skirted the periphery of Einstein’s theories without ever coming to grips with them. The books and articles that really tried to explain the substance of Einstein’s work were too technical for him to understand.”
A few points to highlight:
Einstein’s science was about measurement of things at the macro level (and attempts to measure position and direction of particles at the micro scale). It was about position in space and time and movement across them. As Barnett writes, “The world is a space-time continuum; all reality exists both in space and in time, and the two are indivisible. All measurements of time are really measurements in space, and conversely measurement in space depend on measurements of time.”
The fundamental units of the universe are radiation (light) and matter, and the fundamental causes of their motion are electromagnetism and gravitation. Einstein’s life-long quest was to unite gravitation and electromagnetic forces. (1)
Where Newton had mass as a fixed quantity, Einstein sees “the relativity of mass” as it increases with velocity: matter’s concentrated mass-energy state becomes converted to a liberated mass-energy state, void of matter, i.e. light’s mass (energy content) is expressed as movement only.
As a moving body has more mass, and as the faster it is the more mass it has, at c that body has infinite mass and the principle of inertia kicks in full time: Infinite mass means there’s infinite resistance to increases in speed. In effect, the respective infinities cancel each other out, and it is this that sets a finite limit to the speed of light (“no material body can travel with the speed of light”).
Regarding a scientific-philosophical picture of big physics, the most intriguing part of this book is something that does not get discussed much these days: “the great closed cosmic curve.” (2) In elaborating, Barnett writes that in the Einstein universe there are no straight lines. There are only great circles” so that “a sunbeam setting out through space at the rate of 186,000 miles a second would, in this universe, describe a great cosmic circle and return to its source after a little more than 200 billion terrestrial years.”
Extending that observation, and given the arguments these days about the Big Bang formation of this universe, is there not an explosion outward from a center point that thereby provides an overall curved cosmic structure. Matter and energy move outward in their straight-line (inertial) motion that is curved by space-time, dissipating more (progressively becoming less dense) as they move further from the cosmic center. While the gravitational centers of attraction is lessened (expansion increases the distance, per the inverse square law) and space becomes flatter (less curved) because there is an escape from gravitational effects. This is another way that mass-energy becomes liberated from matter, but light and its mass-energy continues its straight-line motion that follows geodesic, curved paths back to the beginning point – the geometric center. If the cosmic structure is a “globe,” all light paths lead back to the center – in this case, the big bang’s original point, becoming denser per the inverse square law as the paths of motion converge. In other words, regarding the fate of this particular cosmos, there is no contraction back to the beginning point but, rather, there’s a continuation of mass-energy around the curved cosmic structure back to its beginning point. (3)
Does this way of looking at the cosmic structure have implications for the notion of entropy as well? The big bang that begins as a low entropy state (concentrated energy), dissipation across space and time into high entropy states. Yet, because the cosmic structure is curved, mass-energy continues as radiation around that curved structure, to become, per the increase square law, more concentrated, a low entropy state again?
(1) Barnett writes that in the “vast cosmic picture, when fully revealed, the abyss between macrocosmos and microcosmos – the very big and the very little – will surely be bridged, and the whole complex of the universe will resolve into a homogenous fabric in which matter and energy are indistinguishable….” Toward the end of the book, Barnett adds: “in the same way that Relativity reduced gravitational force to a geometrical property of the space-time continuum, a Unified Field Theory will reduce electromagnetic force – the other great universal force – to equivalent status. ‘The idea that there are two structures of space independent of each other, the metric-gravitational and the electromagnetic,’ Einstein once observed, ‘is intolerable to the theoretical spirit.’ And, then Barnett comments that “What his Unified Field Theory undertakes is to show that gravitational and electromagnetic force are not independent of each other – that they are in a very real physical sense inseparable. More specifically, it attempts to describe gravitational and electromagnetic force in terms of a deeper reality that undergirds both – a basic universal field within which gravitational and electromagnetic fields are merely particular ephemeral forms or conditions of state.” Later, Barnett adds: “The distinctions between gravitational force and electromagnetic force, matter and energy, electric charge and field, space and time, all fade in the light of their revealed relationships and resolve into configurations of the four-dimensional continuum which Einstein revealed the universe to be. Thus all man’s perceptions of the world and all his abstract intuitions of reality would merge finally into one, and the deep underlying unity of the universe would be laid bare.” Whatever else this means, Barnett seems to be saying that collected masses at the macro level were formed by electromagnetic forces at the micro level.
(2) “Einstein concluded that the universe is neither infinite nor Euclidean, as most scientists supposed, but something hitherto unimagined. It has already been shown that Euclidean geometry does not hold true in a gravitational field. Light rays do not travel in straight lines when passing through a gravitational field, for the geometry of the field is such that within it there are no straight lines; the shortest course that the light can describe is a curve or a great circle which is rigorously determined by the geometrical structure of the field. Since the structure of a gravitational field is shaped by the mass and velocity of the gravitating body – star, moon, or planet – it follows that the geometrical structure of the universe as a whole must be shaped by the sum of its material content. For each concentration of matter in the universe there is a corresponding distortion of the space-time continuum. Each celestial body, each galaxy creates local irregularities in space-time, like eddies around islands in the sea. The greater the concentration of matter, the greater the resulting curvature of space-time. And the total effect is an overall-curvature of the whole space-time continuum: the combined distortions produced by all the incomputable masses of matter in the universe cause the continuum to bend back on itself in a great closed cosmic curve.”
(3) Of those who operate within the classical paradigm, Einstein and Infeld write that “A journey along a straight line never leads them back to the starting point” but in Einstein’s world “They will…find that starting on a journey straight ahead, they ultimately return to their point of departure. ‘Straight ahead’ means along the great circle of the sphere.” From the big bang and inflation, three typical scenarios are proposed - contraction, heat death and infinite expansion – regarding descriptions of the fate of the cosmos. These are cast more or less implicitly in terms of Newton’s flat space diagrams, i.e., not curved as a globe is curved (it is hard to draw such). It’s interesting that magnetic lines flow in such a circular fashion as well.