*Reference: Einstein’s 1920 Book*

*This paper presents Part II, Chapter 6 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.*

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## Behaviour of Clocks and Measuring Rods on a Rotating Body of Reference

Hitherto I have purposely refrained from speaking about the physical interpretation of space- and time-data in the case of the general theory of relativity. As a consequence, I am guilty of a certain slovenliness of treatment, which, as we know from the special theory of relativity, is far from being unimportant and pardonable. It is now high time that we remedy this defect; but I would mention at the outset, that this matter lays no small claims on the patience and on the power of abstraction of the reader.

*Einstein admits that he has
omitted the physical interpretation of space-time, which I have interpreted as “force
in motion” in my comments on the last chapter.*

We start off again from quite special
cases, which we have frequently used before. Let us consider a space-time
domain in which no gravitational fields exists relative to a reference-body ** K**
whose state of motion has been suitably chosen.

**is then a Galileian reference-body as regards the domain considered, and the results of the special theory of relativity hold relative to**

*K***. Let us suppose the same domain referred to a second body of reference**

*K***, which is rotating uniformly with respect to**

*K’***. In order to fix our ideas, we shall imagine**

*K***to be in the form of a plane circular disc, which rotates uniformly in its own plane about its centre. An observer who is sitting eccentrically on the disc**

*K’***is sensible of a force which acts outwards in a radial direction, and which would be interpreted as an effect of inertia (centrifugal force) by an observer who was at rest with respect to the original reference-body**

*K’***. But the observer on the disc may regard his disc as a reference-body which is “at rest”; on the basis of the general principle of relativity he is justified in doing this. The force acting on himself, and in fact on all other bodies which are at rest relative to the disc, he regards as the effect of a gravitational field. Nevertheless, the space-distribution of this gravitational field is of a kind that would not be possible on Newton’s theory of gravitation.**

*K**[The field disappears at the centre of the disc and increases proportionally to the distance from the centre as we proceed outwards.]*But since the observer believes in the general theory of relativity, this does not disturb him; he is quite in the right when he believes that a general law of gravitation can be formulated—a law which not only explains the motion of the stars correctly, but also the field of force experienced by himself.

*Instead of a rotating
disk of Einstein, I see the field as a uniformly rotating whirlpool in which
the centrifugal force is replaced by a centripetal force toward the center. *

The observer performs experiments on
his circular disc with clocks and measuring-rods. In doing so, it is his
intention to arrive at exact definitions for the signification of time- and
space-data with reference to the circular disc ** K’**, these definitions being
based on his observations. What will be his experience in this enterprise?

To start with, he places one of two
identically constructed clocks at the centre of the circular disc, and the
other on the edge of the disc, so that they are at rest relative to it. We now
ask ourselves whether both clocks go at the same rate from the standpoint of
the non-rotating Galileian reference-body ** K**. As judged from this body, the
clock at the centre of the disc has no velocity, whereas the clock at the edge
of the disc is in motion relative to

**in consequence of the rotation. According to a result obtained in Section XII, it follows that the latter clock goes at a rate permanently slower than that of the clock at the centre of the circular disc, i.e. as observed from**

*K***. It is obvious that the same effect would be noted by an observer whom we will imagine sitting alongside his clock at the centre of the circular disc. Thus on our circular disc, or, to make the case more general, in every gravitational field, a clock will go more quickly or less quickly, according to the position in which the clock is situated (at rest). For this reason it is not possible to obtain a reasonable definition of time with the aid of clocks which are arranged at rest with respect to the body of reference. A similar difficulty presents itself when we attempt to apply our earlier definition of simultaneously in such a case, but I do not wish to go any farther into this question.**

*K**At the center of the whirlpool
the velocity shall be much slower, and, accordingly, the time duration shall be
much greater. In other words, space as force is condensing into thicker
substance toward the center.*

Moreover, at this stage the definition
of the space co-ordinates also presents unsurmountable difficulties. If the
observer applies his standard measuring-rod (a rod which is short as compared
with the radius of the disc) tangentially to the edge of the disc, then, as
judged from the Galileian system, the length of this rod will be less than 1,
since, according to Section XII, moving bodies suffer a shortening in the
direction of the motion. On the other hand, the measuring-rod will not
experience a shortening in length, as judged from K, if it is applied to the
disc in the direction of the radius. If, then, the observer first measures the
circumference of the disc with his measuring-rod and then the diameter of the
disc, on dividing the one by the other, he will not obtain as quotient the
familiar number π = 3.14 …, but a larger
number, *[Throughout this consideration we
have to use the Galileian (non-rotating) system K as reference-body, since we may only assume the validity of the
results of the special theory of relativity relative to K (relative to K’ a
gravitational field prevails).] *whereas of course, for a disc which is at
rest with respect to

**, this operation would yield exactly. This proves that the propositions of Euclidean geometry cannot hold exactly on the rotating disc, nor in general in a gravitational field, at least if we attribute the length 1 to the rod in all positions and in every orientation. Hence the idea of a straight line also loses its meaning. We are therefore not in a position to define exactly the co-ordinates x, y, z relative to the disc by means of the method used in discussing the special theory, and as long as the co-ordinates and times of events have not been defined we cannot assign an exact meaning to the natural laws in which these occur.**

*K**In truth, faster a body
moves the less condensed it is (lesser inertial density). Therefore, space
shall be more condensed at the center of the “whirlpool” field. When there is
no whirlpool type rotation there is no inertial condensation, and the space
appears as a uniform void made up of extremely diluted force.*

Thus all our previous conclusions based on general relativity would appear to be called in question. In reality we must make a subtle detour in order to be able to apply the postulate of general relativity exactly. I shall prepare the reader for this in the following paragraphs.

.

*Einstein has avoided
interpreting space-time physically. He rejects Descartes view that space-time
represent characteristic of substance. Faraday, on the other hand, agreed with
Descartes. He saw space as filled with a field of force, where force was “inertially”
diluted form of substance.*

*Einstein has the vectors reversed. The velocity increases in the field domain compared to the material domain because matter has “diluted” as field. Therefore, the contraction of length occurs with decrease in velocity and not the other way around.*

*The same reversal of
vectors occurs in general relativity when Einstein sees field as a rotating
disk and not as a rotating whirlpool. This is because Einstein is using matter
as his frame of reference, and not space (diluted force) as Faraday did.*

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