# Quantum - Planck's Paper Of 1900, Einstein's Light Quantum, Neils Bohr And The "old Quantum Theory"

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The German physicist Max Planck (1858–1947) introduced the quantum of action *h* (Planck's constant) into physics in his successful theory of blackbody radiation in 1900 and thus started quantum theory, one of several revolutionary discoveries that occurred in physics at the turn of the twentieth century. Others were Albert Einstein's (1879–1955) special theory of relativity, his theory of the light quantum, and the experimental proof of the existence of atoms and molecules, based on Einstein's theory of Brownian motion, all published in the single year 1905. These theories and the general theory of relativity distinguish modern physics from the classical physics that was dominant in 1900.

The desire to formulate physics in terms of observable quantities motivated Einstein's special theory of relativity, which eliminated the unobservable ether assumed by James Clerk Maxwell (1831–1879) and his followers. The "new quantum theory" of the 1920s was also supposed to be a theory of observables, as it eliminated the visualizable orbits of the "old quantum theory" that preceded it, but the new version introduced concepts such as Erwin Schrödinger's (1887–1961) wave function (psi-function) that involves the imaginary number *i* 1. Quantum mechanics and quantum field theory employ other mathematical concepts that are far from the observable realm.

While other developments of modern physics, such as relativity, can be seen as generalizing classical physics, quantum mechanics makes a much stronger break with the past. Even the basic notion of measurement is problematic in quantum theory. One of its consequences is a fundamental indeterminacy that prevents, for example, the simultaneous determination of the position and momentum of a particle. That brings into question the entire epistemology on which classical physics is based. The psi-function of an atom, for example, is the solution of a differential equation that is linear in the time variable, and once given, psi is determined for all time. The psi-function thus provides a deterministic description of the system, but from this complete description one can predict with certainty the outcome of only special "compatible" measurements; for most measurements, only the probability of a given result can be calculated.

The implications of quantum theory are so profound that even its creators, such as Planck and Einstein, found it difficult to accept and wrestled with its concepts all their lives. Skeptical physicists devised ways to avoid the apparent contradictions, and these proposals have led to experimental tests. All of the tests performed so far have confirmed the predictions of quantum theory, although its laws are subject to differing physical interpretations.

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