# You Gotta Know These 20th-Century Physicists

- Niels Bohr (1885–1962) Bohr reconciled Rutherford’s results from the gold foil experiment with Max Planck’s quantum theory to create a model of the atom (the Bohr model) in which electrons resided in specific energy levels at specific stable radii. This model was the basis for Johann Balmer’s work with spectroscopy and Johannes Rydberg’s energy formula, which explicitly stated the frequency of light that an electron would emit if it went from a higher energy to a lower energy. Bohr and his son fled to the U.S. in World War II under the pseudonym “Baker,” and contributed to the Manhattan Project.
- Louis de Broglie duh-BROH-lee (1892–1987) De Broglie’s work quantifying the wave-particle duality of quantum mechanics earned him the 1929 Nobel Prize in Physics. His doctoral thesis, which proposed that all particles have a characteristic wavelength dependent on their momentum, was so groundbreaking that the reviewers passed it directly to Albert Einstein, who endorsed it. In opposition to the probabilistic interpretation of quantum mechanics, de Broglie later worked to define a purely causal interpretation, but his work remained unfinished until David Bohm refined it in the 1950s.
- Albert Einstein (1879–1955) In one year — 1905, called his
*annus mirabilis*, or “miracle year” — Albert Einstein authored four papers that revolutionized modern physics. The first explained the photoelectric effect in terms of quantized electromagnetic radiation. The second formed the foundation for modern statistical physics by explaining the seemingly-random motion of particles in a fluid, a behavior called Brownian motion. The third reconciled Maxwellian electrodynamics with classical mechanics by positing a finite, constant speed of light, a theory now known as special relativity. The fourth paper contained his statement that the energy of a body is equal to its mass times the speed of light squared (that is,*E*=*m**c*^{2}). Ten years later, in 1915, Einstein published his theory of general relativity, which generalized special relativity to account for gravitational fields. - Enrico Fermi (1901–1954) Fermi is best known to the public as a main contributor to the Manhattan Project. His work with statistical physics laid the groundwork for modern electronics and solid-state technologies. He applied the Pauli exclusion principle to subatomic particles to create Fermi-Dirac statistics, which accurately predicted the low-temperature behavior of electrons. Particles that obey Fermi-Dirac statistics are called fermions in his honor. Fermi also suggested the existence of the neutrino in order to balance nuclear beta-decay chains.
- Richard Feynman (1918–1988). Feynman developed a mathematical formalism called the path integral formulation of quantum theory that utilized the “sum over histories,” taking into account all possible paths a particle could take. This constituted the creation of quantum electrodynamics and earned him the 1965 Nobel Prize in Physics. He also used the sum over histories in developing Feynman diagrams, which illustrate the interaction of subatomic particles. Aside from being a prolific physicist, Feynman was also an accomplished bongo player and sketch artist.
- George Gamow (1904–1968) Gamow was one of the first to explain the implications of the Big Bang theory of cosmology. He correctly predicted the abundance of hydrogen and helium in the early universe, nicknamed Alpher-Bethe-Gamow theory (an intentional pun on the first three letters of the Greek alphabet, alpha, beta, and gamma, for which the otherwise unrelated physicist Hans Bethe was included), and also theorized that the the heat from the Big Bang would still be visible as the cosmic microwave background radiation. Although Gamow received no Nobel for this prediction, the CMB’s discoverers, Arno Penzias and Robert Wilson, as well as two later observers, John Mather and George Smoot, did receive Nobels.
- Werner Heisenberg (1901–1976) Heisenberg is most known for his matrix interpretation of quantum theory, which constructs observable quantities as operators that act on a system. His famous uncertainty principle (better translated, however, as “indeterminacy principle”) states that the more accurately an object’s position can be observed, the less accurately its momentum can. This is because shorter wavelengths of light (used as a sort of measuring-stick) have higher energies, and disrupt a particle’s momentum more strongly. Heisenberg earned the 1932 Nobel Prize in Physics for discovering the allotropic forms of hydrogen.
- Max Planck (1858–1947) Planck allowed quantum theory to move forward in the early 20th century by correctly modeling how an object radiates heat, solving the ultraviolet catastrophe, which was a predicted unbounded increase in the amount of radiation emitted at high frequencies. Planck’s Law of Radiation superseded the Rayleigh-Jeans Law. He suggested that electromagnetic energy could only be emitted in specific packages, called quanta (singular quantum, from the Latin for “how much”), positing that the energy of this photon was equal to its frequency times a fixed value
*h*, now known as Planck’s constant. - Ernest Rutherford (1871–1937) Rutherford’s gold foil experiment provided the first evidence that each atom is made up of a large, positively-charged nucleus, surrounded by a cloud of negatively-charged electrons. Rutherford won the 1908 Nobel Prize in Chemistry for this work. Rutherford was also an early leader in nuclear fission techniques, having discovered the decay of carbon-14 and providing the impetus for modern carbon dating. As part of this research, he discovered the proton and neutron, the latter in cooperation with James Chadwick. He is also the only native New Zealander with an element named after him (Rutherfordium, atomic number 104).
- Erwin Schrödinger (1887–1961) Schrödinger contributed to the early formulations of quantum theory as a foil to Werner Heisenberg, Niels Bohr, and Paul Dirac, criticizing their Copenhagen interpretation of quantum mechanics with thought experiments like his famous Schrödinger’s Cat argument. He formulated both the time-independent and time-dependent Schrödinger equations, which are partial differential equations that describe how quantum systems behave. Schrödinger’s work was the basis for Heisenberg’s matrix formalism, Feynman’s path-integral formalism, and quantum mechanical perturbation theory, which considers the effects of a small disturbance to a quantum system.

## Honorable mentions:

- Marie (1867–1945) and Pierre (1859–1906) Curie rigorously isolated and experimented on radioactive materials, forming the basis for early nuclear and particle physics.
- Paul Dirac (1902–1984) was one of the first to attempt a generalization of quantum theory to relativistic speeds, the result of which was the Dirac equation.
- Murray Gell-Mann (born 1929) predicted the existence of quarks, which compose protons, neutrons, and other, heavier particles.
- Robert Millikan (1868–1953; not to be confused with Robert Mullikan, a chemist) determined the charge of the electron by meticulously observing oil droplets in an electric field and noting the time it took them to fall a certain distance.
- J. Robert Oppenheimer (1904–1967) oversaw much of the Manhattan Project, but was later stripped of his security clearance during the McCarthy-era Red Scare, as a result of his acquaintance with communists and his enmity with Edward Teller.
- Wolfgang Pauli’s (1900–1958) namesake exclusion principle prohibits most types of particles from occupying the same state, and forms the basis for chemical bonds.

This article was contributed by former NAQT writer Zachary Pace.