DIFFERENCE BETWEEN QUANTUM MECHANICS AND QUANTUM PHYSICS

From “What is the difference between quantum mechanics and quantum physics?” (quantumphysicslady.org):

  • Both “quantum mechanics” and “quantum physics” mean the study of subatomic particles.
  • But “quantum mechanics” is more specific. It’s the term used for the field once it was formulated into mathematical laws. Then, it became a kind of mechanics.
  • Prior to the development of mathematical laws governing subatomic particles, the field was called “quantum theory” or “quantum physics.”
  • When physicists realized that quantum particles do not follow the laws of classical mechanics, they called the new field “quantum theory.” At first, physicists developed quantum laws which were heavily verbal, rather than highly mathematical.
  • In the 1920’s, physicists developed mathematical laws which describe quantum behavior. In particular, Erwin Schrodinger and Werner Heisenberg developed the key mathematical laws governing quantum particles (Schrodinger’s Wave Equation and Heisenberg Matrix Mechanics). At this point, physicists began calling the new field “quantum mechanics” on the model of the phrase “classical mechanics.”
  • The term “quantum mechanics” means the same thing as “quantum physics” though the term “mechanics” emphasizes doing calculations.

BIRTH OF QUANTUM THEORY

From “Einstein’s revolutionary paper” (physicsworld.com):

  • The big idea in Einstein’s March paper [“On a heuristic point of view concerning the production and transformation of light” (Ann. Phys., Lpz 17 132-148)] was his gentle suggestion that light consists of individual, discrete, localized and indivisible quantum particles. This blithely made, audacious claim contradicted a century of compelling empirical evidence, and it challenged the crowning achievement of 19th-century theoretical physics: the electromagnetic theory of light.
  • It can be argued persuasively that Einstein’s March paper was the start of quantum physics.
  • The quantum idea had been introduced by Max Planck in 1900; however, he did this tentatively and under duress (see “Max Planck: the reluctant revolutionary” by Helge Kragh Physics World December 2000 pp31-35).
  • For Planck, the “energy elements” were not physically real, but a mathematical means to his objective. Planck was adamantly opposed to the concept of light quanta.
  • Einstein’s path to the light quantum was not guided by experimental data: there were no data in 1905 that required light to be particulate. Einstein’s starting point was the obvious contradiction between continuity and discontinuity. Physicists were pleased with their electromagnetic wave theory of light, and were intrigued by atoms and the evidence for subatomic particles. But even the cleverest among Einstein’s contemporaries were not troubled by the continuity of light and the discontinuity of atoms. Einstein, however, was concerned. He recognized the fundamental problems that occur when extended light waves and point-like atoms are brought together – for example, when atoms emit or absorb light. It was this juxtaposition of light and atoms that he addressed in his March paper

Further the physicsworld.com article shares the following text from Einstein’s paper (“what the science journalist Albrecht Fölsing has called the most “revolutionary” sentence written by a physicist in the 20th century.”):

“According to the assumption to be contemplated here, when a light ray is spreading from a point, the energy is not distributed continuously over ever-increasing spaces, but consists of a finite number of energy quanta that are localized in points in space, move without dividing, and can be absorbed or generated only as a whole.”

Further the article says:

  • Einstein’s light quantum does not come from a theory that ends with quod est demonstratum. The first two sections of his March paper are tangential to his purpose, and what follows comes from Einstein’s deep well of intuition; specifically, his quantum postulate emerges from an analogy between radiation and an ideal gas.
  • … Einstein extends, by analogy, his results for a sample of an ideal gas to a sample of radiation …
  • Einstein’s “revolutionary” paper has the strange word “heuristic” in the title. This word means that the “point of view” developed – that is, the light particle – is not in itself justified except as it guides thinking in productive ways.
  • Therefore, at the end of his paper, Einstein demonstrated the efficacy of light quanta by applying them to three different phenomena. These were the photoelectric effect, the ionization of gases by ultraviolet light, and Stokes’ rule …
  • It was not until Arthur Compton’s 1923 X-ray scattering experiment, which showed light bouncing off electrons like colliding billiard balls, that physicists finally accepted Einstein’s idea. Bohr, however, continued to reject the light particle until mid-1925, and had even been willing to sacrifice the conservation of energy to keep the light quantum off the stage of physics.
  • In 1926 Einstein’s light particle became the photon, named by Gilbert Lewis. Since then, the photon has become omnipresent in physics. Gone is the electromagnetic field spreading continuously through space, and in its place is a quantized field. Gone is Coulomb’s force continuously filling the space between two charges; in its place are two charges exchanging localized photons.

[to be continued….]