Reference: Einstein’s 1920 Book
Section XVI (Part 1)
Experience and the Special Theory of Relativity
Please see Section XVI at the link above.
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Summary
The theory of relativity explains the phenomenon known as “aberration”—the yearly movement of the apparent position of the fixed stars resulting from the motion of the earth round the sun. It also explains the slight displacement of the spectral lines of the light transmitted to us from a fixed star due to radial motion (Doppler principle). The various experimental arguments in favor of the Maxwell-Lorentz theory, also favor the theory of relativity.
The electrons are a substance of very small inertia and large velocity. The electrodynamic theory is unable to give an account of their nature, but it hypothesizes that the electron experience a contraction in the direction of motion in consequence of that motion. This hypothesis is not justifiable by any electrodynamical facts, but it is easily justified by the theory of relativity.
The reference body K refers to aether in electrodynamics, which is equivalent to infinite inertia and zero motion. The reference body K’ refers to “æther-drift” and electrodynamics assigns complicated laws to it. Electrodynamics then looked for “æther-drift” with respect to earth’s surface (K’), which was not to be found. It requires overcoming the inherited prejudices about time and space to resolve this dilemma.
The theory of relativity overcomes the inherited prejudices about time and space, and discards the ideas of “æther” and “æther-drift.” Here the contraction of moving bodies follows from the motion with respect to the body of reference chosen in the particular case in point. Thus for a co-ordinate system moving with the earth the mirror system of Michelson and Morley is not shortened, but it is shortened for a co-ordinate system which is at rest relatively to the sun.
Comments
Maxwell-Lorentz theory of electromagnetic phenomena deals with lines of force, or force fields. An analysis of these force fields leads to the velocity of light of 3 × 108 m/s. This may be interpreted to mean that the force field is the substance which fills the space in the absence of matter. This substance is of the same nature as light.
The velocity of light is a measure of the flexibility of this force field, just like the velocity of material bodies is a measure of their flexibility (the opposite of inertia). Einstein extrapolates the flexibility, as related to motion, from matter to light by assuming the velocity of light to be a universal constant. This, makes it possible to relate the velocity of matter to imperceptible changes in its flexibility, or inertia.
Electrons are not the same as charged particles (ions), so, in cathode ray, they cannot be distinguished from each other as “separate particles;” and, therefore, there is no repulsion to scatter the cathode ray. A cathode ray simply consists of “electronic substance” of certain inertia as dealt with in the theory of relativity.
The idea of “contraction” simply reflects the change in the consistency (inertia) of the substance filling the space, instead of change in some abstract idea of coordinate systems. Space denotes the extents of the substance; and time denotes the duration of the substance. Therefore, any notion of spacetime coordinates is just a reflection of the consistency of the substance filling the space.
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