4.+Fundamental+Forces




 * Basic Forces** As best we can tell, the behavior of normal matter on all scales—from elementary particles to clusters of galaxies—is ruled by just a few basic forces. Forces and the fields and energies they engender are the root cause of changes everywhere; they are //fundamental// to //everything// in the cosmos. In a sense, the search to understand the nature of the Universe is synonymous with the quest to understand the nature of these forces. Forces, fields, and energies are among the essential keys needed to unlock some of the most concealed secrets of the Universe.


 * Gravity** is perhaps the best known force. Gravity binds galaxies, stars, and planets, and it also of course holds us on Earth. Like other forces, its strength decreases with distance from any object; in fact, it decreases as the square of the distance, and is said to obey the “.” However, that’s only half of the law of gravity, as its strength is also proportional to mass. Thus, gravity is terribly weak near, for example, a puny atom, but enormously powerful near a huge galaxy. In fact, although gravity is by far the weakest of all of Nature’s known forces, its effect can accumulate impressively over large volumes of space that contain mass. Nor can anything cancel the attractive pull of gravity; there is no such thing as antigravity that repels objects—at least not for normal matter. Even the peculiar stuff known as antimatter (discussed shortly) has gravity, not antigravity. Consequently, the gravitational forces of all objects—including our own bodies—extend to the outer limits of the Universe, hence the reason why gravity is known as a “long-range” force. To be sure, on scales larger than Earth, gravity is the dominant force in the Universe. [|Video]

The **electromagnetic force** is another of Nature’s basic agents. Any particle having a net electric charge, like an atom’s electron and proton, exerts an electromagnetic force. This force acts as the cement for most ordinary materials, including virtually everything in our homes, such as tables, chairs, books, even the kitchen sink. Because the electromagnetic force also binds the atoms within all life forms, some biologists call it the “life force”—which, unfortunately, leads some to think that life is governed by some special “vitalism,” which is wrong. Like gravity, the strength of the electromagnetic force decreases with distance according to the same inverse-square law. But unlike gravity, it can repel (between like charges) as well as attract (between opposite charges). Such forces can then sometimes cancel one another, as when similar numbers of positive and negative charges neutralize the electromagnetic force, thereby diminishing its influence. For example, although a human body is made of ~1029 charged particles, it comprises almost equal mixtures of positive and negative charges; our bodies therefore exert hardly any net electromagnetic force. Overall, is much stronger than gravity on microscopic scales and smaller, but is much less influential on macroscopic scales where gravity rules. [|Video B] [|Video A] [|VIDEO]

A third fundamental force is termed the **weak force**, as its effective range is less than the size of an atomic nucleus and its influence on matter much more subtle than any of the other forces. We shall not encounter it much in the course of describing cosmic evolution, except to note that the weak force helps to change one kind of elementary particle into another (such as the arcane particles released during nuclear reactions at the Sun’s core). The weak force also governs the emission of radiation from radioactive atoms, which are useful in establishing dates that, in turn, reveal the tempo of cosmic evolution. Most scientists now agree that the weak force is not really a separate force at all; rather, it’s probably another form of the electromagnetic force acting under peculiar circumstances. As such, we now often speak of the “electroweak force,” an idea to which we shall return in the next section.

A stronger force than any of these is the **strong force**, mediated by the gluon particle that holds the quarks together. It glues—hence its name—protons and neutrons within atomic nuclei and, in effect, serves as the source of energy in the Sun and stars. Like the weak force, yet unlike the forces of gravity and electromagnetism, the nuclear force operates only at very close range; it’s useless when matter is separated by more than ~10-12 cm. But within this range, as for all atomic nuclei, it binds particles with enormous strength—stronger, in fact, than any other force known. Numerically, and in absolute terms independent of their most potent ranges, the nuclear force is 137 times stronger than the electromagnetic force, 100,000 times stronger than the weak force, and 1039 times stronger than gravity. Ironically and despite its extraordinary weakness, gravity is the only force that affects all things at all times on all scales.

Eric Chaisson [|Particle Epoch]