Quantum Physics (or Mechanics) is a broad collection of theories in modern physics, with over 100 years of history. The word “quantum” explains the important difference between QM and classical physics: in quantum theories, energy levels and other qualities of the particles being studied don’t have continuous possible values, and instead, these quantities will “jump” from one discrete level to another. For example, if you’re driving your car 10 mph, you don’t have to jump straight to 20 mph; you can also go all the speeds in-between, like 11 or even 11.0001 mph. Particles at the atomic scale don’t have this luxury, and physicists spent a long time figuring out why.
19th-century discoveries that couldn’t be explained
In the 1800s, scientists kept finding results that didn’t make sense in classical physics. Faraday, Kirchhoff, Boltzmann, and Hertz all studied aspects of chemistry and radiation that seemed to suggest energy states were discrete. In 1900, Max Planck created his famous formula showing that the frequency of radiation was proportional to its energy — cementing the idea that radiation comes in waves.
Although he’s more famous for his work on grand, cosmic scales — the Theory of Relativity — Einstein actually won his Nobel Prize for quantum physics by explaining the photovoltaic effect in 1905. (This is the relationship between light and electricity that makes solar panels and LED lights work.) He decided that electromagnetic radiation, (i.e., light) was actually composed of individual particles called “photons” — which still have the odd property of behaving like waves in some circumstances.
Schrödinger’s wave equation
Today, possibly the most important equation in Quantum Physics is the wave equation formulated by Erwin Schrödinger. This equation describes the “probability amplitude” for a quantum system. Multiple interpretations for the wave function exist and some people still argue today about what it means, and how a particle can act like a wave and still be a particle at the same time. Some physicists even believe that the wave function implies the existence of a multiverse — science fiction fans get a lot of mileage out of this “many worlds” hypothesis.
Heisenberg’s uncertainty principle
Another cornerstone of Quantum Physics is that there is fundamental uncertainty built into the system. The more precisely you attempt to measure a quantum system, the more you interfere with it — changing the values you’re attempting to measure. At first, this was interpreted to mean that we just needed more careful methods to measure these values, but now we believe it really is a package deal in the underlying nature of reality and not just clumsy science on our parts. This was discovered by Werner Heisenberg in 1927 and was controversial for quite a while (Einstein hated ideas like this, but couldn’t find anything to disprove them).
Quantum theory continues to develop beyond the basic rules of Quantum Mechanics. Quantum Chromodynamics (QCD) studies quarks and gluons, the fundamental building blocks of all hadronic matter, and Electroweak Theory (EWT) studies interactions involving the weak nuclear force and its odd properties of symmetry-breaking and radioactive decay. Quantum Field Theory (QFT) combines elements of almost all known physics with QM, including both Special Relativity and classical mechanics — but nobody’s yet figured out how to fold gravity and General Relativity into QFT. “Quantum Gravity” is still considered a holy grail of physics and several contenders, like String Theory, are popular fields of study today.