For more than two thousand years, it was believed that atoms were the ultimate constituents of matter. They were pictured as tiny particles, indivisible and solid. But modern physics shows that nothing could be further from the truth.
Early in the twentieth century, physicists realized that atoms are composed of even smaller„subatomic„particles. An atom may be small, a mere billionth of an inch across, but these subatomic particles are a hundred-thousand times smaller still. Imagine the nucleus of an atom magnified to the size of a tennis ball. The electrons would spinning around it in orbits several miles across, making the atom itself the size of London or Manhatten. As the early twentieth-century British physicist Sir Arthur Eddington put it, matter is mostly ghostly empty space„99.9999999999999% empty space to be a little more exact. If you could take away the empty space then all the subatomic particles in all the six billion people on planet earth would pack into a volume only a little larger than a grain of rice.
With the advent of quantum theory, it was found that electrons, protons, neutrons, and the other subatomic particles were themselves far from solid„and far from even being particles. On closer examination they appear to be just waves of energy, with no exact location in space„just a probability of being around at certain point in space and time. Solid matter had, literally, disappeared into empty space.
Why then does the world seem so solid? Why doesn't the 99.99999...% empty space of my hand simply pass through straight through the 99.99999...% empty space of the table it is resting upon? The answer has to do with the forces that bind the atoms together. When my hand meets the table, the force fields in the atoms of my hand come up against the equally strong fields in the atoms of the table. The mutual repulsion of these billions of tiny, but immensely strong, force fields prevents my hand penetrating the table, giving rise to the appearance of solidness. But however real it may seem, this solidness is only how things appear to us; it is not an intrinsic part of matter.