Special Relativity and Classical Field Theory: The Theoretical Minimum by Leonard Susskind

Author:Leonard Susskind [Susskind, Leonard]
Language: eng
Format: epub
ISBN: 9780465093359
Google: ukjTDQAAQBAJ
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Publisher: Basic Books
Published: 2017-09-26T00:00:00+00:00

Of all the fundamental forces in nature, and there are many of them, few were known before the 1930s. Most are deeply hidden in the microscopic quantum world and only became observable with the advent of modern elementary particle physics. Most of the fundamental forces are what physicists call short-range. This means that they act only on objects that are separated by very small distances. The influence of a short-range force decreases so rapidly when the objects are separated that, for the most part, they are not noticed in the ordinary world. An example is the so-called nuclear force between nucleons (protons and neutrons). It’s a powerful force whose role is to bind these particles into nuclei. But as powerful as that force is, we don’t ordinarily notice it. The reason is that its effects disappear exponentially when the nucleons are separated by more than about 10−15 meters. The forces that we do notice are the long-range forces whose effects fade slowly with distance.

Of all the forces of nature, only three were known to the ancients—electric, magnetic, and gravitational. Thales of Miletos (600 BC) was said to have moved feathers with amber that had been rubbed with cat fur. At about the same time he mentioned lodestone, a naturally occurring magnetic material. Aristotle, who was probably late on the scene, had a theory of gravity, even if it was completely wrong. These three were the only forces that were known until the 1930s.

What makes these easily observed forces special is that they are long-range. Long-range forces fade slowly with distance and can be seen between objects when they are well separated.

Gravitational force is by far the most obvious of the three, but surprisingly it is much weaker than electromagnetic force. The reason is interesting and worth a short digression. It goes back to Newton’s universal law of gravitational attraction: Everything attracts everything else. Every elementary particle in your body is attracted by every particle in the Earth. That’s a lot of particles, all attracting one another, and the result is a significant and noticeable gravitational attraction, but in fact the gravitational attraction between individual particles is far too small to measure.

The electric forces between charged particles is many orders of magnitude stronger than the gravitational force. But unlike gravity, electric force can be either attractive (between opposite charges) or repulsive (between like charges). Both you and the Earth are composed of an equal number of positive charges (protons) and negative charges (electrons), and the result is that the forces cancel. If we imagined getting rid of all the electrons in both you and the Earth, the repulsive electric forces would easily overwhelm gravity and blast you from the Earth’s surface. In fact, it would be enough to blast the Earth and you into smithereens.

In any case, gravity is not the subject of these lectures, and the only other long-range forces are electromagnetic. Electric and magnetic forces are closely related to each other; in a sense they are a single thing, and the unifying link is relativity.

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