Einstein once said that if a theory did not offer a physical picture that even a child could understand, then it was probably useless. Fortunately, behind string theory there is a simple physical picture, a picture based on music. According to string theory, if you had a super microscope and could peer into the heart of an electron, you would see not a point particle but a vibrating string. (The string is extremely tiny, at the Planck length, a billion billion times smaller than a proton, so all subatomic particles appear pointlike.) If we were to pluck this string, the vibration would change; the electron might turn into a neutrino. Pluck it again and it might turn into a quark. In fact, if you plucked it hard enough, it could turn into any of the known subatomic particles. In this way, string theory can effortlessly explain why there are so many subatomic particles. They are nothing but different “notes” that one can play on a superstring. To give an analogy, on a violin string the notes A or B or C sharp are not fundamental. By simply plucking the string in different ways, we can generate all the notes of the musical scale. B flat, for example, is not more fundamental than G. All of them are nothing but notes on a violin string. In the same way, electrons and quarks are not fundamental, but the string is. In fact, all the subparticles of the universe can be viewed as nothing but different vibrations of the string. The “harmonies” of the string are the laws of physics.
Strings can interact by splitting and rejoining, thus creating the interactions we see among electrons and protons in atoms. In this way, through string theory, we can reproduce all the laws of atomic and nuclear physics. The “melodies” that can be written on strings correspond to the laws of chemistry. The universe can now be viewed as a vast symphony of strings. Not only does string theory explain the particles of the quantum theory as the musical notes of the universe, it explains Einstein’s relativity theory as well the lowest vibration of the string, a spin two particle with zero mass, can be interpreted as the graviton, a particle or quantum of gravity. If we calculate the interactions of these gravitons, we find precisely Einstein’s old theory of gravity in quantum form. As the string moves and breaks and reforms, it places enormous restrictions on space-time. When we analyze these constraints, we again find Einstein’s old theory of general relativity.Thus, string theory neatly explains Einstein’s theory with no additional work.