The first book I encountered after deciding I wanted to learn Einstein’s theory of relativity was Bertrand Russell’s ABC of Relativity, a supposedly layman introduction to the concept. I must be stupider than the layman because I had severe trouble grasping the concepts Russell was trying to explain. It didn’t help that he was layering explanations on top of each other, so that if you didn’t understand just a paragraph of what he was describing, you could be lost for the rest of the book. A third of the way through I had no choice but to give up and find another resource.

I’m still not sure if the above book is intended for the true layman, but it was much easier to understand. It begins by explaining how motion is relative:

There is no apparent way to chart the absolute motion of anything; that is to say, there is no fixed, final frame of reference by which all motions can be measured. Motion and rest, like large and small, slow and fast, up and down, left and right, seem to be completely relative. There is no way to measure the motion of one object except by comparing it with the motion of some other object.

[…]

…the greater the distance between two events, the greater the difficulty of deciding about simultaneity. It is important to understand that this is not just a question of being unable to learn the truth of the matter. There is no actual truth of the matter. There is no absolute time throughout the universe by which absolute simultaneity can be measured. Absolute simultaneity of distant events is a meaningless concept.

[…]

The instant “now” has meaning only for the spot you occupy. You cannot assume that a “now” exists simultaneously fir all spots of the universe.

I learned that the reason we can’t travel at the speed of light is because inertial mass increases with speed. At the speed of light you would need an infinite amount of energy to push the now infinite mass, which cannot happen. So the speed of light (*c = 300,000 km/sec*) is the speed limit for our universe.

Electrons can now be accelerated to 0.999999999+ the speed of light. This gives to each electron a mass (relative to the earth’s inertial frame) that is about forty thousand times its mass at rest! Relativistic changes of time are also observable.

It also gave a great explanation of time dilation, whereby a space traveler ages slower than his counterpart remaining on Earth.

Back in Einstein’s time, scientists didn’t have the technology to prove their theories in the lab, so they ran “thought experiments.” Particle accelerators and colliders which have been developed since then have validated many of their thought experiments. However…

There is no experiment of any sort, the general theory says, by which an observer in any sort of motion, uniform or nonuniform, can prove whether he is moving or at rest.

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This means that regardless of how an observer is moving, he can describe all the laws of nature (as he sees them) by the same mathematical equations. He may be a scientist working in a laboratory on the earth, or on the moon, or inside a giant spaceship that is slowly accelerating on its way to a distant star. The general theory of relativity provides him with a set of equations by which he can describe all the natural laws involved in any experiment he can perform. These equations will be exactly the same regardless of whether he is at rest, moving uniformly, or moving with acceleration with respect to any other object.

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…suppose a boy comes along and kicks it with his bare foot. He shouts in pain because the kick hurts his toes. A Newtonian would say that the apple’s inertia resisted his kick. An Einsteinian can say the same thing, but he can also say, if he prefers, that the boy’s toes caused the entire cosmos (including the toes) to accelerate backward, setting up a gravitational field that pulled the apple with great force against his toes. It is all a matter of words.

Einstein’s relativity theory (technically there are two theories: the general theory and the special theory) also introduces a fourth dimension that we are unable to perceive. This is why regular people have such a hard time using their 3D oriented brain to understand Einstein’s theories. How can you understand something your brain was not naturally designed to understand? Answer: lots of thinking.

There are three space dimensions and one time dimension, united in a way that is specified by the equations of relativity. This structure is such that a geodesic, although still the straightest possible path in spacetime, is the longest instead of the shortest distance. This concept is impossible to explain without going into complicated mathematics…

Some additional passages I enjoyed:

Experiments using the Mossbauer effect have shown that time near the bottom of a building (where gravity is stronger) is a bit slower than time near the top of the same building. “A typist working on the first floor of the Empire State Building,” Gamow observed, “will age slower than her twin sister working on the top floor.” The difference in aging is, of course, infinitesimal; nevertheless, it is real and can be measured.

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Galileo and Newton made experiments, but the extraordinary thing about Einstein is that he made no experiments. Moreover, he was often unaware of significant tests that had strong bearings on his speculations. He just sat alone, thinking deeply about the secrets of the Old One, as he liked to call the universe. Newton was a devout Anglican who spent half his life struggling to unravel the mysteries of biblical prophecy. Einstein had no interest in any religion except in the sense that Spinoza, whose secular pantheism he admired, was religious. Yet he and Newton, in addition to their giant intellects and creative intuitions, shared a strong sense of wonder toward the Old One and of humility before the unanswerable riddle of existence. Both were Platonists in their conviction that what science knows is an infinitesimal portion of what it does not know.

Tons of diagrams and artwork are provided to help explain and conceptualize the topics. You will still need to re-read many portions for full understanding, but unlike with Russell’s book, you won’t become lost for good if you happen not to understand a paragraph or page.

Einstein’s theories are the most important scientific developments since Darwin’s theory of evolution, but most people can’t describe it. Why is this? Because it’s a very hard concept. It does take time and deep concentration to even begin to imagine the fourth dimension that is necessary for understanding space-time, and basic physics knowledge is essential. In spite of this difficulty, relativity was a massive leap forward in the history of science, and I believe all men curious of his place in the universe should take a couple hours to understanding this important concept.

**Read More: “Relativity Simply Explained” on Amazon**