What is quantum physics?
The word quantum refers to discreetness. In Newtonian physics, all quantities are allowed to be continuous. For instance, particles can have any momentum, light can have any frequency. A quanta is a discreet packet of energy, charge, or any other quantity. For instance, one might say that electric charge is quantized in units of e. (Although with quarks, we will learn that it comes in units of e/3)
In the next few pages, we will discuss the fact that all exchanges of energy come in discreet amounts. For instance, when light is absorbed by some material, the energy of the material does not rise continuously, but in discreet jumps. One says that the material has absorbed a light quanta. We will also learn that energy levels (orbits) of an electron in an atom do not have a continuous range of possible energies, but instead that only discreet orbits are possible. This rather bizarre behavior will be linked to the concept of wave-particle duality. We will see that particles can be discribed by wave functions that describe the probability of finding the particle. But, we will also see that these particles interact at specific points, like particles.
A new fundamental constant must be introduced to account for all these new phenomena, Planck's constant. It is denoted by h.
The constant will be used to relate wave-like quantities to particle-like quantities. For instance, a particle's energy E is related to the frequency f of its wave function, and a particle's momentum p is related to the wavelength l of its wave function.
Quantum physics is necessary to explain properties of solids, atoms, nuclei and light. Aside from providing the basis for our understanding of natural phenomenon, quantum principles represent a fundamental change in how humans view nature. To many philosophers the probabilistic interpretation of quantum mechanics is very uncomfortable to accept, as it affects all aspects of determinism. The development of quantum physics in this century represents the greatest success of both mankind's science and mankind's philosophy.