Nearly 100000 times smaller than an atom itself is the small grain at its center: the atomic nucleus. By its mass, but even more by its electrical charge, the nucleus determines all properties of the atom of which it makes part. Because of the sturdiness of the nucleus, it seems as if the atoms that give shape to our daily world are themselves unchangeable, even if they interact with each other to form chemical substances. But sturdy as it is, a nucleus can be torn apart. If atoms are smashed against each other with dazzling velocities, it may happen that two nuclei hit each other, and then they can either break into pieces or coalesce. At the same time, subnuclear particles are released. The new physics of the first half of the twentieth century was dominated by the new riddles these particles presented.

But we have quantum mechanics, you will say, and is that not applicable everywhere? What's the difficulty? Indeed, quantum mechanics does apply to the subatomic particles, but there is more to it than that. The forces these particles exert on one other, and which keep the atomic nucleus together, are so strong that the velocities with which they swirl around each other, inside and outside the nuclei, come close to the speed of light, which is about 300000 kilometers/second, or 186000 miles/second. When such velocities come into play, a second modification is needed to the nineteenth century laws of physics: we have to take account of Einstein's special theory of relativity.