Quantum Physics in a Nutshell

Quantum Physics is the science of the very small. It has revealed the construction and behaviour of the atomic and sub-atomic worlds.

There are multiple overlapping definitions for this field of study which can make its scope confusing. This article will help you get to grips with what it’s about.

Some Clarifying Definitions

The first step to untangling the concept is to study a few definitions:

  • Quantum Physics is the study of the very small where energy, momentum, and other quantities are restricted to discrete values.
  • Quantum Mechanics – an alternate name for Quantum Physics.
  • Quantum Theory – any theory predating quantum mechanics that encompassed Planck’s radiation formula and a scheme for obtaining discrete energy states for atoms, as Bohr theory.
  • Quantum Dynamics – The study of motion, energy and momentum exchanges in quantum systems.
  • Quantum Electro Dynamics – How light and matter interact in quantum systems. It is the first theory where full agreement between quantum mechanics and special relativity is achieved.
  • Quantum Field Theory – A theory based on the idea that all particles are fields.
  • Particle Physics – Research of fundamental interactions of subatomic particles, requiring the use of a large particle accelerator.

The Quantum Scientists

Above is a photo from the 5th Solvay Conference, October 1927 – The world’s most notable physicists met to discuss quantum theory.

The 5th Solvay Conference, October 1927.
Schrodinger is the one with wavy hair
  • Back row left to right: A Piccard, E Henriot, P Ehrenfest, Ed Herzen, Th. De Donder, E Schroedinger, E Verschaffelt, W Pauli, W Heisenberg, R. H Fowler, L Brillouin
  • Middle row left to right: P Debye, M Knudsen, W. L Bragg, H. A Kramers. P. A. M Dirac, A. H Compton, L. V. De Broglie, M Born, N Bohr
  • Front row left to right: Angmeir, M Planck, M Curie, H. A Lorentz, A Einstein, P Langevin, Ch. E Guye, C. T. R Wilson, O. W Richardson

A Short History of the Beginning

The study of Quantum Physics leads to an understanding of how atoms are constructed.

Periodic Table
Wolfgang Pauli discovered that element properties depended on electrons in the outer quantum orbits
  • 1900 – Max Planck created Planck’s Radiation Law
  • 1913 – Neils Bohr postulated that electrons exchange energy as quanta
  • 1921 – Einstein used Planck’s formula to explain the photoelectric effect
  • 1924 – Louis de Broglie proposed matter has wave properties
  • 1926 – Erwin Schrödinger developed a wave equation
  • 1927 – Werner Heisenberg formulates the uncertainty principle
  • 1928 – Paul Dirac combines quantum mechanics and special relativity
  • 1931 – James Chadwick discovers the neutron
  • 1947 – Introduction of Feynman diagrams.
  • 1952 – Donald Glaser invents the bubble chamber

An Example of Quantisation

Here is a diagram showing absorption lines in the spectrum of a distant star. The lines are caused by elements in the star’s atmosphere interacting with particular wavelengths of light, thereby removing them from the spectrum.

Fraunhofer Lines

The wavelength of a colour is proportional to its energy. A photon will be absorbed if the energy it carries exactly matches the energy needed for an electron to move to a free, higher electron shell. The energy will then be emitted as a photon but not necessarily the same wavelength and direction as the incident ray, leaving a missing line in the spectrum.

The Scale of Quantum Physics

The scale that separates classical physics and quantum physics is 1.6 x 10–35 m (about 10–20 times the size of a proton), otherwise known as the Planck length. At larger scales, classical physics can be used, whereas at smaller scales Quantum physics dominates.

Schrödinger’s Wave Mechanics

A fundamental tenet of the quantum world is that matter behaves like a wave, and its behaviour is captured in Schrödinger’s Wave Equation. Once you have digested this concept, you will be able to get a handle on the rest of the quantum world.

Electron probability density plots for Hydrogen
Fig-1: Electron probability density plots for hydrogen. Each plot represents the probability of detecting an electron at a location around the core of the atom at successive energy levels.

Here is a concise, informationally dense video that explains how an electron occupies specific energies and possible positions according to Schrodinger’s wave equation. The shape of the orbital depends on values for energy (n) and angular momentum (l).

Run Time: 7:17

Well Known Quantum Effects

Wave/particle duality

This is demonstrated by the famous double slit experiment where electrons are fired individually at a pair of parallel slits. The resultant image captured on a screen behind the slits shows that the electron is acting like a wave. If a detector is used to study the phenomena, the electron will act like a particle.

Entanglement

Entanglement is where two or more particles have coupled quantum states. If you were to detect the state of one of the particles, the state of the other would be known. The simultaneity of this effect has been proven to be at least 10,000 times faster than the speed of light. As yet is not known if it is instantaneous.

Tunneling

Because matter has field and wave-like properties, it is a finite possibility for it to traverse a barrier. This effect provides a route for a physical process to take place at lower energies than might otherwise be anticipated.

Summary

At this point you will have learned the following:

  • Anything to do with quantum is at the atomic and sub-atomic scale
  • The scale at which quantum physics replaces classical physics is the Plank length
  • Photons and matter behaves in a way that is similar to a wave, a field or a point source depending on its context
  • Photons and matter is composed of discrete, whole energy units
  • An understanding of atoms came about through the study of quantum physics
  • Chemistry is driven by differences in the structure of an atom which is enforced by fundamental properties of quantum physics. These differences are mapped out by the periodic table.

References

Disclaimer

Care has been taken to keep the information in this article as accurate as possible but errors are possible, so be aware of the full disclaimer here.

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