Reference: Evolution of Physics
This paper presents Chapter IV section 7 from the book THE EVOLUTION OF PHYSICS by A. EINSTEIN and L. INFELD. The contents are from the original publication of this book by Simon and Schuster, New York (1942).
The paragraphs of the original material (in black) are accompanied by brief comments (in color) based on the present understanding. Feedback on these comments is appreciated.
The heading below is linked to the original materials.
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Physics and Reality
What are the general conclusions which can be drawn from the development of physics indicated here in a broad outline representing only the most fundamental ideas?
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.
The theories of Physics try to describe the consistency between the physical reality and our mental structures.
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.
“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.
An object is a primitive concept. From objects to numbers is a movement from concrete toward abstraction.
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.
Our impressions are related to each other more as a matrix of “elements of knowledge”. They are not just a linear arrangement in terms of time.
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 nineteenth 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 behaviour of bodies, but the behaviour of something between them, that is, the field, may be essential for ordering and understanding events.
Particles are condensed inertia. The space between them is diffused inertia. The force between them probably represents inertial wave. This was Faraday’s view. This view can probably explain a lot of electromagnetic and quantum phenomena.
Later developments both destroyed old concepts and created new ones. Absolute time and the inertial co-ordinate 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 co-ordinate system was no longer needed. Every co-ordinate system is equally suited for the description of events in nature.
The relativity theory delves indirectly into the diffusion of inertia through diffusion of space and time.
The quantum theory again created new and essential features of our reality. Discontinuity replaced continuity. Instead of laws governing individuals, probability laws appeared.
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.
Quantum theory is dealing directly with the diffusion of inertia.
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.
There is inherent harmony in physical reality. We are trying to comprehend that harmony through physical theories.
WE SUMMARIZE:
Again the rich variety of facts in the realm of atomic phenomena forces us to invent new physical concepts. Matter has a granular structure; it is composed of elementary particles, the elementary quanta of matter. Thus, the electric charge has a granular structure and most important from the point of view of the quantum theory so has energy. Photons are the energy quanta of which light is composed.
Is light a wave or a shower of photons? Is a beam of electrons a shower of elementary particles or a wave? These fundamental questions are forced upon physics by experiment. In seeking to answer them we have to abandon the description of atomic events as happenings in space and time, we have to retreat still further from the old mechanical view. Quantum physics formulates laws governing crowds and not individuals. Not properties but probabilities are described, not laws disclosing the future of systems are formulated, but laws governing the changes in time of the probabilities and relating to great congregations of individuals.
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FINAL COMMENTS
The dimension being discovered through the theory of relativity and quantum theory is the dimension of inertia. We live on a plateau of inertia, where we have been oblivious of small changes in inertia. The theory of relativity considered the “diffusion” of space and time. The light quanta considered the diffusion of mass. Underlying this is the relationship between inertia, space and time. With diffusion of inertia there is a diffusion of space and time.
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