Einstein 1920: General Results

Reference: Einstein’s 1920 Book

This paper presents Part 1, Chapter 15 from the book RELATIVITY: THE SPECIAL AND GENERAL THEORY by A. EINSTEIN. The contents are from the original publication of this book by Henry Holt and Company, New York (1920).

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.


General Results of the Theory

NOTE: These comments are temporary as more work is required. Better results are expected if velocity is replaced by acceleration, kinetic energy is replaced by force, and potential energy and mass are replaced by inertia.

It is clear from our previous considerations that the (special) theory of relativity has grown out of electrodynamics and optics. In these fields it has not appreciably altered the predictions of theory, but it has considerably simplified the theoretical structure, i.e. the derivation of laws, and —what is incomparably more important —it has considerably reduced the number of independent hypotheses forming the basis of theory. The special theory of relativity has rendered the Maxwell-Lorentz theory so plausible, that the latter would have been generally accepted by physicists even if experiment had decided less unequivocally in its favour.

The theory of relativity has grown out of the effort to align the laws of electrodynamics with the laws of mechanics. This goes back to Einstein’s paper on light quanta, from which it may be inferred that radiation is a substance that quantizes into matter. The laws of electrodynamics apply to radiation, whereas, the laws of mechanics apply to matter.

The special theory of relativity has succeeded in explaining the observed motions better than classical mechanic, by somehow accounting for inertia. Classical mechanics assumes inertia to be uniform because velocity is uniform. This is not so because velocity can be different while uniform, and so can inertia. Not accounting of inertia in classical mechanics is causing the error that is being corrected by special theory of relativity.

Classical mechanics required to be modified before it could come into line with the demands of the special theory of relativity. For the main part, however, this modification affects only the laws for rapid motions, in which the velocities of matter vare not very small as compared with the velocity of light. We have experience of such rapid motions only in the case of electrons and ions; for other motions the variations from the laws of classical mechanics are too small to make themselves evident in practice. We shall not consider the motion of stars until we come to speak of the general theory of relativity. In accordance with the theory of relativity the kinetic energy of a material point of mass m is no longer given by the well-known expression


but by the expression

mc2 / √(1 – v2/c2).

Velocity is different for different reference-bodies, and it can be chosen arbitrarily. This makes kinetic energy dependent on the reference-body too. This arbitrary dependency goes away when acceleration is chosen in place of velocity;  force is chosen in place of kinetic energy, and inertia is chosen in place of potential energy and mass.

This expression approaches infinity as the velocity vapproaches the velocity of light c. The velocity must therefore always remain less than c, however great may be the energies used to produce the acceleration. If we develop the expression for the kinetic energy in the form of a series, we obtain

Each velocity seems to represent a certain “potential force”, or inertia that is in balance. This is the case with both v and c. The value of c is very high because the inertia in balance is very small.

When v2/c2 is small compared with unity, the third of these terms is always small in comparison with the second, which last is alone considered in classical mechanics. The first term mc2 does not contain the velocity, and requires no consideration if we are only dealing with the question as to how the energy of a point-mass depends on the velocity. We shall speak of its essential significance later.

The first term (mc2) is more representative of the inertia of the body. The second term seems to be more representative of force in some manner.

The most important result of a general character to which the special theory of relativity has led is concerned with the conception of mass. Before the advent of relativity, physics recognised two conservation laws of fundamental importance, namely, the law of the conservation of energy and the law of the conservation of mass; these two fundamental laws appeared to be quite independent of each other. By means of the theory of relativity they have been united into one law. We shall now briefly consider how this unification came about, and what meaning is to be attached to it.

Faraday had combined the two conservation laws of mass and energy into one single law of the conservation of force. In Faraday’s framework mass corresponds to inertia. Active energy corresponds to force.

The principle of relativity requires that the law of the conservation of energy should hold not only with reference to a co-ordinate system K, but also with respect to every co-ordinate system K’ which is in a state of uniform motion of translation relative to K, or, briefly, relative to every “Galileian” system of co-ordinates. In contrast to classical mechanics, the Lorentz transformation is the deciding factor in the transition from one such system to another.

Lorenz transformations somehow account for the difference in inertia, when velocities are different.

By means of comparatively simple considerations we are led to draw the following conclusion from these premises, in conjunction with the fundamental equations of the electrodynamics of Maxwell: A body moving with the velocity v, which absorbs 1 an amount of energy E0 in the form of radiation without suffering an alteration in velocity in the process, has, as a consequence, its energy increased by an amount

E0 / √(1 – v2/c2).

1 E0 is the energy taken up, as judged from a co-ordinate system moving with the body.

This is more like an increase in inertia with the absorption of radiation force.

In consideration of the expression given above for the kinetic energy of the body, the required energy of the body comes out to be

Thus the body has the same energy as a body of mass (m + E0/c2) moving with the velocity v. Hence we can say: If a body takes up an amount of energy E0, then its inertial mass increases by an amount E0/c2; the inertial mass of a body is not a constant, but varies according to the change in the energy of the body. The inertial mass of a system of bodies can even be regarded as a measure of its energy. The law of the conservation of the mass of a system becomes identical with the law of the conservation of energy, and is only valid provided that the system neither takes up nor sends out energy. Writing the expression for the energy in the form

we see that the term mc2, which has hitherto attracted our attention, is nothing else than the energy possessed by the body 2 before it absorbed the energy E0.

2 As judged from a co-ordinate system moving with the body.

The special theory of relativity interprets mass as the configuration of substance formed from an equivalent amount of energy. In the relationship E0 = m0c2 , however, c cannot be taken to its limiting value of infinity. A finite solution must exists when c is taken to infinity.

A direct comparison of this relation with experiment is not possible at the present time, owing to the fact that the changes in energy E0to which we can subject a system are not large enough to make themselves perceptible as a change in the inertial mass of the system. E0/c2 is too small in comparison with the mass m, which was present before the alteration of the energy. It is owing to this circumstance that classical mechanics was able to establish successfully the conservation of mass as a law of independent validity.

Let me add a final remark of a fundamental nature. The success of the Faraday-Maxwell interpretation of electromagnetic action at a distance resulted in physicists becoming convinced that there are no such things as instantaneous actions at a distance (not involving an intermediary medium) of the type of Newton’s law of gravitation. According to the theory of relativity, action at a distance with the velocity of light always takes the place of instantaneous action at a distance or of action at a distance with an infinite velocity of transmission. This is connected with the fact that the velocity c plays a fundamental rôle in this theory. In Part II we shall see in what way this result becomes modified in the general theory of relativity.


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