To find the cause of radiant energy we have to look deep into the heart of matter. Even though matter appears solid, it's really composed of tiny particles called atoms. If you could shrink down smaller than an atom and look inside, you would find mostly empty space. At the center of the atom is a densely packed nucleus made up of protons and neutrons. If you could stand on the nucleus and look out, very far away you would see electrons encircling the nucleus in orbits arranged in layers called shells. To get to the first shell you would have to travel out about 10,000 times the diameter of the nucleus.