domingo, 25 de março de 2018

Mysteriously enough, not only is there a large number of particles in the visible universe, but the basic laws themselves display large numbers. According to modern physics, there are four fundamental interactions between particles: the electromagnetic, the gravitational, the strong, and the weak.

The electromagnet interaction holds atoms together, governs the propagation of light and radio waves, causes chemical reactions, and prevents us from walking through walls and sinking through the floor. In an atom, electrons, with their negative electric charges, are prevented from flying off because of their attraction to the positive charges carried by protons located in the nucleus. The gravitational interaction keeps us from flying off into space, holds planetary systems and galaxies together, and controls the expansion of the universe. The strong interaction holds the nucleus of the atom together, the weak causes certain radioactive nuclei to disintegrate. Although of fundamental importance in nature's design, the strong and weak interactions do not appear to play a role in any phenomenon at the human scale. As we saw earlier, all four interactions play crucial roles in stellar burning.

As the names strong and weak suggest, one interaction is considerably stronger than the electromagnetic interaction, the other considerably weaker. But most dramatically, the gravitational force is far, farf weaker than the other three. The electric force between two protons is stronger then the gravitational attraction by the enormous ratio of 1 to about 10^38, another absurdly large number.

ironically, we are normally most aware of gravity, by far the most feeble force in nature. Although the gravitational attraction between any two atoms is fantastically small, every atom in our bodies is attracted to every atom in the earth, and the force adds up. In this example, the incredibly large number of particles involved compensates for the incredible weakness of gravity. In contrast, the electric force between two particles is attractive or repulsive according to the signs of the electric charges involved. A lump of everyday matter contains almost exactly an equal number of electron and protons, so the electric force between two such lumps almost cancels out.

Anthony Zee, "Fearful Symmetry: The Search for Beauty in Modern Physics"
Princeton University Press, 2016