This table differs slightly from Table 13-2 on page 247 because I adopt L M 4 . We shall see later that the age of the Universe is most probably about 15 billion years old. Each of these forces is carried out by a different kind of virtual particle. Of the four fundamental forces, we are most familiar with gravity and electromagnetism, because their efforts are most obvious in the world around us. The other two forces act only within atomic nuclei, so they are less noticeable.
Gravity is the natural force of attraction that acts between objects with mass.
The greater the mass of two objects, the greater the force of their mutual attraction. The farther the objects are from each other, however, the weaker is the force of gravity between them. Gravity is the weakest of the fundamental forces, yet it has an unlimited range. It shapes the Universe on its largest and most dramatic scales, because it acts over such vast distances throughout space. The electromagnetic force acts between all particles with an electric charge, such as electrons, protons and ions.