# A Brief History of Quantum Mechanics – with Sean Carroll

#### What Is Quantum Mechanics ?

Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, quantum field theory, quantum technology, and quantum information science.

Classical physics, the description of physics that existed before the theory of relativity and quantum mechanics, describes many aspects of nature at an ordinary (macroscopic) scale, while quantum mechanics explains the aspects of nature at small (atomic and subatomic) scales, for which classical mechanics is insufficient. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.[3]

Quantum mechanics differs from classical physics in that energy, momentum, angular momentum, and other quantities of a bound system are restricted to discrete values (quantization), objects have characteristics of both particles and waves (wave-particle duality), and there are limits to how accurately the value of a physical quantity can be predicted prior to its measurement, given a complete set of initial conditions (the uncertainty principle).

Quantum mechanics arose gradually from theories to explain observations which could not be reconciled with classical physics, such as Max Planck’s solution in 1900 to the black-body radiation problem, and the correspondence between energy and frequency in Albert Einstein’s 1905 paper which explained the photoelectric effect. These early attempts to understand microscopic phenomena, now known as the “old quantum theory”, led to the full development of quantum mechanics in the mid-1920s by Niels Bohr, Erwin Schrödinger, Werner Heisenberg, Max Born and others. The modern theory is formulated in various specially developed mathematical formalisms. In one of them, a mathematical entity called the wave function provides information, in the form of probability amplitudes, about what measurements of a particle’s energy, momentum, and other physical properties may yield.

The mysterious world of quantum mechanics has mystified scientists for decades. But this mind-bending theory is the best explanation of reality that we have.

Whether it’s Schrödinger’s cat or the many worlds interpretation, the ideas quantum mechanics throws up about the reality we live in can seem incredibly strange and daunting. In this talk, Sean walks you through the history of quantum discoveries, from Einstein and Bohr to the present day, guiding you through the most intriguing subject in science and explaining his own favourite theories along the way. Sean M Carroll is a theoretical physicist, specialising in quantum mechanics, gravitation, cosmology, statistical mechanics, and foundations of physics, with occasional dabblings elsewhere. His official titles are Research Professor of Physics at Caltech and Research Professor at the Santa Fe Institute. He lives in Los Angeles with his wife, writer Jennifer Ouellette.

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