# The Meaning of Relativity

The Meaning of Relativity: Four Lectures Delivered at Princeton University, May 1921 is a book published in 1922 by Princeton University Press in the USA and by Methuen & Company in the UK. The 1922 book is a translation of the 1921 Stafford Little Lectures at Princeton University, given in German by Albert Einstein (1879–1955). Einstein's goal in the lectures was to give an overview of the physics, mathematics, and basic thinking for both special relativity theory and general relativity theory. The Princeton physics professor Edwin Plimpton Adams (1878–1950) translated the lectures into English. There are 4 subsequent editions: 2nd edition in 1945, 3rd edition in 1950, 4th edition in 1953, and 5th edition in 1955. Einstein added for the 2nd edition an appendix entitled On the "Cosmological Problem" and for the 3rd edition an Appendix II entitled Relativistic Theory of the Non-symmetric Field. For the 5th edition, he completely revised Appendix II, based upon simplifying the derivations and the form of the general relativistic field equations. The simplification was done in collaboration with his assistant Bruria Kaufman (1918–2010).

## Quotes from The Meaning of Relativity, 5th edition

### Chapter. Space and Time in Pre-relativity Physics

• The theory of relativity is intimately connected with the theory of space and time. I shall therefore begin with a brief investigation of the origin of our ideas of space and time, although in doing so I know that I introduce a controversial subject. The object of all science, whether natural science or psychology, is to co-ordinate our experiences and to bring them into a logical system. How are our customary ideas of space and time related to the character of our experience?
• p. 1

### Chapter. The Theory of Relativity

• Before the development of the theory of relativity it was known the principle of energy and momentum could be expressed in a differential form for the electromagnetic field. The four-dimensional formulation of these principles leads to an important conception, that of the energy tensor, which is important of the further development of the theory of relativity.
• p. 47

### Chapter. The General Theory of Relativity

• A little reflection will show that the law of the equality of the inertial and gravitational mass is equivalent to the assertion that the acceleration imparted to a body by a gravitational field is independent of the nature of the body. For Newton's equation of motion in a gravitational field, written out in full, it is:
(Inertial mass) ${\displaystyle \cdot }$ (Acceleration) ${\displaystyle =}$ (Intensity of the gravitational field) ${\displaystyle \cdot }$ (Gravitational mass).
It is only when there is numerical equality between the inertial and gravitational mass that the acceleration is independent of the nature of the body.
• p. 57

### Chapter. The General Theory of Relativity (continued)

• A material particle upon which no force acts moves, according to the principle of inertia, uniformly in a straight line. In the four-dimensional continuum of the special theory of relativity (with real time co-ordinate) this a real straight line. The natural, that is, the simplest, generalization of the straight line which is meaningful in the system of concepts of the general (Riemannian) theory of invariants is that of the straightest, or geodesic, line.
• p. 79

### Appendix for the Second Edition

• Some try to explain Hubble's shift of spectral lines by means other than the Doppler effect. There is, however, no support for such a conception in the known physical facts.
• p. 128

### Appendix II

• It is the essential achievement of the general theory of relativity that it has freed physics from the necessity of introducing the "inertial system" (or inertial systems). This concept is unsatisfactory for the following reason: without any deeper foundation it singles out certain co-ordinate systems among all conceivable ones. It is then assumed that the laws of physics hold only for such inertial systems (e.g. the law of inertia and the law of the constancy of the velocity of light). Thereby, space as such is assigned a role in the system of physics that distinguishes it from all other elements of physical description. It plays a determining role in all process, without in its turn being influenced by them. Though such a theory is logically possible, it is on the other hand rather unsatisfactory. Newton had been fully aware of this deficiency, but he had also clearly understood that no other path was open to physics in his time. Among the later physicians it was above all Ernst Mach who focussed attention on this point.
• pp. 139–140
• One can give good reasons why reality cannot at all be represented by a continuous field. From the quantum phenomena it appears to follow with certainty that a finite system of finite energy can be completely described by a finite set of numbers (quantum numbers). This does not seem to be in accordance with a continuum theory, and must lead to an attempt to find a purely algebraic theory for the description of reality. But nobody knows how to obtain the basis of such a theory.
• pp. 165–166

## Quotes about The Meaning of Relativity

• In May 1921 Albert Einstein delivered a series of lectures at Princeton University on the broad topic of relativity. The lectures form a unified survey of the basic concepts of relativity. Beginning with the pre-relativity physics of Newton (or perhaps, more correctly, the "three dimensional" relativity of Newton) Einstein lays the foundation for "four dimensional" relativity primarily from the postulational standpoint. The development of special relativity is followed by the formulation of the general theory leading up to the Schwarzschild line element and the cosmological problem.