PARTICLE NATURE OF FORCES

De Broglie got his idea for the wave nature of the electron from the particle-wave nature of light. The particle of light is the photon which can knock electrons out of a metal surface. The wave nature is the wave of electric and magnetic force that was predicted by Maxwell’s theory. When you combine these two aspects of light, you are led to the conclusion that electric and magnetic forces are ultimately caused by photons. We call any force resulting from electric or magnetic forces as being due to the electric interaction. The photon is the particle responsible for the electric interaction.

Let us see how our picture of the hydrogen atom has evolved as we have learned more about the particles and forces involved. We started with a miniature solar system with the heavy proton at the center and an electron in orbit. The force was the electric force that in many ways resembled the gravitational force that keeps the earth in orbit around the sun. This picture failed, however, when we tried to explain the light radiated by heated hydrogen.
The next real improvement comes with Schrödinger’s wave equation describing the behavior of the electron in hydrogen. Rather than there being allowed orbits as in Bohr’s model, the electron in Schrödinger’s picture has allowed standing wave patterns. The chemical properties of atoms can be deduced from these wave patterns, and Schrödinger’s equation leads to accurate predictions of the wavelengths of light radiated not only by hydrogen but other atoms as well.
There are two limitations to Schrödinger’s equation. One of the limitations we have seen is that it is a non relativistic equation, an equation that neglects any change in the electron’s mass due to motion. While this is a very good approximation for describing the slow speed electron in hydrogen, the wavelengths of light radiated by hydrogen can be measured so accurately that tiny relativistic effects can be seen. Dirac’s relativistic wave equation is required to explain these tiny relativistic corrections.

The second limitation is that neither Schrödinger’s or Dirac’s equations take into account the particle nature of the electric force holding hydrogen together. In the hydrogen atom, the particle nature of the electric force has only the very tiniest effect on the wavelength of the radiated light. But even these effects can be measured and the particle nature must be taken into account. The theory that takes into account both the wave nature of the electron and the particle nature of the electric force is called quantum electrodynamics, a theory finally developed in 1947 by Richard Feynman and Julian Schwinger. Quantum electrodynamics is the most precisely tested theory in all of science.
In our current picture of the hydrogen atom, as described by quantum electrodynamics, the force between the electron and the proton nucleus is caused by the continual exchange of photons between the two charged particles. While being exchanged, the photon can do some subtle things like create a positron electron pair which quickly annihilates. These subtle things have tiny but measurable effects on the radiated wavelengths, effects that correctly predicted by the theory. The development of quantum electrodynamics came nearly 20 years after Dirac’s equation because of certain mathematical problems the theory had to overcome.
In this theory, the electron is treated as a point particle with no size. The accuracy of the predictions of quantum electrodynamics is our best evidence that this is the correct picture. In other words, we have no evidence that the electron has a finite size, and a very accurate theory which assumes that it does not. However, it is not easy to construct a mathematical theory in which a finite amount of mass and energy is crammed into a region of no size. For one thing you are looking at infinite densities of mass and energy.