Einstein's Books (New Translations)

Einstein starts the book with a brief account on the relationship between physics and geometry. Relativity is essentially a theory of space (and time), and hence it is no surprise that Einstein dedicates the first chapter of his popular book on explaining the physical meanings of geometrical concepts and theorems in geometry, which is a branch of mathematics that also studies space (and, geometrical shapes and their placement in space, etc.).

In this second chapter, Einstein introduces the coordinate system as an analytical tool for doing geometry, which we owe to Rene Descartes, among others. Many readers of this book should be already familiar with the basic ideas of coordinate systems. In physics, the coordinate system is closely related to the _reference frame,_ which is one of the most fundamental concepts in mechanics, as well as in other branches of physics.

In the first two chapters of the book, Einstein further emphasizes the importance of using reference frames, and coordinate systems, for instance, when we specify the location/position of an event or object. He continues his argument along this line in this chapter, now with respect to motion, and time, in addition to position in space, but still within the context of classical mechanics.

Most of the readers who have taken any kind of physics class in school should have learned what inertia is. But, it was again none other than Einstein who discovered its deeper meanings, which all other people had overlooked. His brilliant insight is the topic of this chapter. As we briefly mentioned in the previous chapter, if you go even deeper, then it is still an open question, e.g., as to where inertia really comes from in the first place, which even Einstein couldn't get a definite answer to in his lifetime.

In this chapter, Einstein introduces the special principle of relativity, through a series of generalizations, starting from the fact that motion is relative. The special theory of relativity is a simple consequence of the special principle of relativity. Everything in special relativity, literally everything, follows from this seemingly "obvious" principle.

Motions are relative: The motion of a body A can be described relative to another body B, or an inertial frame of reference.<!-- --> The motion of the body B can also be described with respect to yet another body, say, C. Now, the question is, how do you describe the motion of the body A relative to the body C, based on the descriptions of the two known motions, that of A relative to B and that of B relative to C?

In the previous chapter, Einstein explained how the velocities of two motions are added when we view the combined motion from a single frame of reference. At the end of the chapter, Einstein also mentioned that this simple, and rather intuitive, law of classical mechanics was not completely accurate in reality. In fact, this simple law is in conflict with other fundamental laws of nature, which is the topic of this chapter

In this chapter, Einstein defines time more precisely by taking into account of the fact that light propagates at a finite and constant speed, among other things. In order to do this, he first deconstructs the common idea of simultaneity, and then redefines it so that it is consistent with the special principle relativity. This story continues in the next chapter.

In Chapter 8, Einstein described how to determine whether two events have occurred simultaneously in a given inertial reference frame. Now, in this chapter, Einstein goes on to show that the concept of "the same time" is relative, and hence time itself, according to its definition in special relativity, is also _relative._ This is what ultimately differentiates Einstein's time from Newton's time.

In the previous two chapters, Einstein explains how to define simultaneity, and time. In order to define time Einstein relies on the very concept of distance. In particular, to define the same time between two distant locations, at least conceptually, we need to measure the distance between them and find their middle points first. In this sense, space is more fundamental than time. On the other hand, as Einstein describes in this chapter, relativity of distance essentially comes from relativity of time. So, which is it? Is it a chicken and egg problem again?

A new English translation 2024. This article was published as a short booklet titled "Einstein: On the Principle of Relativity".

The new translation of this article was first published in 2024, with the title, "Einstein: On Theoretical Physics".

A short ebook, "Einstein: What is the Theory of Relativity?" (2024), includes the new English translation of this newspaper article.

This article was originally published in Nature, in 1921, and its new translation was published, in 2024, in a booklet, "Einstein: A Brief History of the Theory of Relativity".

A completely new English translation 2025, which is also available as a short ebook, "Einstein: On Scientific Methods in Physics". The quality of the copy of this short newspaper article is rather poor, and this provides the best translation of this Einstein's article.

This lecture note, also published as a short ebooklet, "Einstein: On Aether and Relativity" (2025), is one of the rarest writings/views of Einstein with respect to then-pervasive idea of the Aether by Maxwell and Lorentz.

Non-Euclidean geometry, which was invented some 200 years ago, still remains to be an esoteric subject to this day. Einstein was one of the very first people who used non-Euclidean geometry, especially that of Gauss and Riemann, for the "real world" problems. This lecture note by Einstein includes his general views on non-Euclidean geometry and its application to modern cosmology, the subject that he practically invented himself. The new English translation of this article was first published in an ebook, "Einstein: Geometry in the Real World" (2025).