Particles and Waves and the Uncertainty Principle

Abstract

As we know, the values of the mass and charge of electrons, protons, neutrons, alpha-particles, etc., are known with tremendous—accuracy approximately one part in a billion. Their velocities can also be changed by application of electric and magnetic fields. Thus, we usually tend to visualize them as tiny particles. However, they also exhibit diffraction and other effects which can only be explained if we assume them to be waves. Similarly, using light beams, there are experiments on interference, diffraction, etc., which can only be explained if we assume a wave model for light. There are also phenomena, like photoelectric effect, which can only be explained if we assume a particle model of light. Thus, answers to questions like what is an electron or what is light are very difficult. Indeed electrons, protons, neutrons, photons, alpha-particles, etc. are neither particles nor waves. The modern quantum theory describes them in a very abstract way which cannot be connected with everyday experience. To quote Feynman [Ref.1 ]:

Newton thought that light was made up of particles, but then it was discovered that it behaves like a wave. Later, however (in the beginning of the twentieth century), it was found that light did indeed sometimes behave like a particle. Historically, the electron, for example, was thought to behave like a particle, and then it was found that in many respects it behaved like a wave. So it really behaves like neither. Now we have given up. We say: ‘It is like neither’. There is one lucky break, however—electrons behave just like light. The quantum behaviour of atomic objects (electrons, protons, neutrons, photons, and so on) is the same for all, they are all ‘particle-waves’, or whatever you want to call them.

For the rest of my life, I will reflect on what light is.

Albert Einstein, ca. 1917.

All the fifty years of conscious brooding have brought me no closer to the answer to the question, ‘What are light quanta?’ Of course today every rascal thinks he knows the answer, but he is deluding himself.

Albert Einstein, 1951.