Laws and principles of physics that were developed before about 1900 are considered “classical” physics.
Classical physics is the physics of everyday objects—tennis balls and squeaky toys, stoves and ice cubes, magnets and electrical wiring. Classical laws of motion govern the motion of anything large enough to see with the naked eye. Classical thermodynamics explains the physics of heating and cooling objects, and the operation of engines and refrigerators. Classical electromagnetism explains the behavior of lightbulbs, radios, and magnets.
Modern physics describes the stranger world that we see when we go beyond the everyday. This world was first revealed in experiments done in the late 1800s and early 1900s, which cannot be explained with classical laws of physics. New fields with different rules needed to be developed.
Modern physics is divided into two parts, each representing a radical departure from classical rules. One part, relativity, deals with objects that move very fast, or are in the presence of strong gravitational forces. Albert Einstein introduced relativity in 1905, and it’s a fascinating subject in its own right, but beyond the scope of this book.
The other part of modern physics is what I talk to my dog about.
Quantum physics or quantum mechanics
is the name given to the part of modern physics dealing with light and things that are very small—molecules, single atoms, subatomic particles. Max Planck coined the word “quantum” in 1900, and Einstein won the Nobel Prize for presenting the first quantum theory of light. The full theory of quantum mechanics was developed over the next thirty years or so.
The people who made the theory, from early pioneers like Planck and Niels Bohr, who made the first quantum model of the hydrogen atom, to later visionaries like Richard Feynman and Julian Schwinger, who each independently worked out what we now call quantum electrodynamics (QED), are rightly regarded as titans of physics. Some elements of quantum theory have even escaped the realm of physics and captured the popular imagination, like Werner Heisenberg’s uncertainty principle, Erwin Schrödinger’s cat paradox, and the parallel universes of Hugh Everett’s many-worlds interpretation.
Without an understanding of the quantum nature of the electron, it would be impossible to make the semiconductor chips that run our computers. Without an understanding of the quantum nature of light and atoms, it would be impossible to make the lasers we use to send messages over fiber-optic communication lines.
The terms “quantum physics,” “quantum theory,” and “quantum mechanics” are more or less interchangeable.
Material particles have wave nature and can diffract around objects. If you send a beam of electrons at a barrier, they’ll go around it to the left and to the right, at the same time.
Particle-wave duality - the fact that both light and matter have particle-like and wavelike properties at the same time. A beam of light, which is generally thought of as a wave, turns out to behave like a stream of particles in some experiments. At the same time, a beam of electrons, which is generally thought of as a stream of particles, turns out to behave like a wave in some experiments. Particle and wave properties seem to be contradictory, and yet everything in the universe somehow manages to be both a particle and a wave. The discovery in the early 1900s that light behaves like a particle is the launching point for all of quantum mechanics
Light waves travel only in straight lines, while sound waves seem to bend around obstacles.