You Gotta Know These Scientific Experiments

  1. Thomas Young’s double-slit experiment (circa 1800), which predated the development of quantum mechanics by over a century, demonstrated that light can behave as either a wave or a particle. After passing a beam of light through two narrowly spaced slits, Young observed the characteristic light and dark fringes of interference seen when light acts as a wave. More modern versions that add detectors showing which slit the light passes through, however, show that the light passes through one slit or the other, acting as photons; moreover, the interference pattern disappears. A modern variant, called a quantum eraser, demonstrates quantum entanglement (the ability to exchange information over large distances instantaneously).

  2. The Michelson-Morley experiment (1887), conducted by Albert Michelson and Edward Morley at what is now Case Western Reserve University, disproved the existence of the luminiferous aether, a hypothetical medium through which light waves supposedly moved. (The aether is sometimes called simply “ether,” but is not to be confused with ethers from organic chemistry.) The experiment used an interferometer, a device that splits a beam of light and aims it using mirrors to allow the beam to interfere with itself; the interferometer was mounted on a slab of marble floating in a pool of mercury so that it could turn without friction, to eliminate the possibility that the interferometer was misoriented. While Michelson and Morley expected to find a shift in the interference pattern’s fringes as a result of the ether, the experiment showed that the Earth had no motion relative to the ether, suggesting that the ether did not exist. Often called the most famous failed experiment in science, the Michelson-Morley experiment is a fundamental test of special relativity.

  3. The Millikan oil-drop experiment (1909), performed by Robert A. Millikan and Harvey Fletcher to measure the charge of the electron. In the first step, the terminal velocity of an oil drop was measured, which means that the drag force (which can be calculated using Stokes’ law) equals the force of gravity. From this, the mass can be calculated. Then, by turning on an electric field, the particle starts to move upward with a terminal velocity when the electric force balances out the forces of gravity and drag. Using this, and the mass of the drop, the total charge on the drop can be calculated. Millikan and Fletcher found that the total charge on the drops were always quantized—that is, always an integer multiple of some constant; specifically, the constant they found is about 1.59 × 10–19 coulombs, within 1% of the currently accepted value.

  4. The Rutherford gold foil experiment (1908–1913), sometimes named after Rutherford’s assistants Hans Geiger and Ernest Marsden, discovered the positively-charged nucleus of the atom; as a result, it disproved J. J. Thomson’s plum pudding model. The experimenters fired alpha particles (helium nuclei) at a sheet of gold foil. (They also used other elements, including silver.) The scattered particles were detected by a screen containing zinc sulfide, which fluoresced when the alpha particles hit it. While most of the alpha particles went straight through the foil with minimal scattering, a small fraction of alpha particles were reflected back at the source. This result was unexpected, as backscattering could only occur if the alpha particles were colliding with a particle massive enough to reverse their momentum. Rutherford said “It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”

  5. The Stern-Gerlach experiment (1922), conducted by Otto Stern and Walther Gerlach, demonstrated that the angular momentum of an atom is quantized. A beam of silver atoms was fired through an inhomogeneous magnetic field (one that varies through space). Instead of hitting a screen in a continuous distribution, they hit at discrete points, demonstrating the quantized nature of angular momentum. This experiment was actually performed several years before the concept of electron spin was even proposed. A variant on this experiment was used to create an energy source for the first hydrogen maser.

  6. The Davisson-Germer experiment (1923–1927), performed by Clinton Davisson and Lester Germer, confirmed the de Broglie hypothesis by showing that electrons can exhibit wave-like behavior. The experimenters fired electrons at a nickel crystal, and measured the diffraction patterns using an electron counter called a Faraday box (or Faraday cup) mounted on an arc so that it could detect electrons emitted at various angles. The peak intensity was observed at 50 degrees and 54 electronvolts, corresponding to the diffraction predicted for X-rays by Bragg’s law. (Note that diffraction is a property of waves, not particles, and thus could only be observed if electrons can act as waves.)

  7. Gregor Mendel’s experiments with pea plants (1860s) pioneered the studio of genetics. Mendel, an Austrian monk, proposed the law of segregation, which holds that each organism has two alleles for each trait, which are segregated into gametes so that each gamete inherits one copy, as well as the law of independent assortment, which says that genes for individual traits are inherited independently. He supported both of those laws with experimental evidence by growing and counting pea plants. He worked with seven characteristics including plant height, seed shape, and color. His experiments primarily consisted of hybridizing plants with certain characteristics, and observing what fraction of the next generation had certain traits. His results were remarkably close to the values that would be predicted from modern genetics—close enough, in fact, that Mendel has been accused of manipulating his data.

  8. The Hershey-Chase experiment (1952), by Alfred Hershey and Martha Chase, demonstrated that the material responsible for inheritance of traits was DNA rather than protein. The experiment was carried out by creating radiolabeled T2 bacteriophages (viruses that infect bacteria). In one population, the phages’ DNA contained phosphorus-32 in its backbone; in the other population, the phages’ proteins contained sulfur-35. The phages were then allowed to infect E. coli. After using a centrifuge to remove the viral coats from the bacteria, Hershey and Chase found that the viruses labeled with sulfur did not transfer their radioactivity to the cells, while the viruses labeled with phosphorus did. This result, combined with other experiments, demonstrated that the genetic material was DNA, not protein.

  9. The Miller-Urey experiment (1952) was an attempt to demonstrate a possible mechanism—proposed by John Haldane and Alexander Oparin—for how life could form from inorganic chemicals. Stanley Miller and Harold Urey modeled Earth’s prebiotic atmosphere as a mixture of water, methane (CH4), ammonia (NH3), and hydrogen. They allowed those four substances to react in an apparatus over a one-week period; the apparatus included a heater to convert the water to water vapor and an electrode to simulate lightning strikes. The resulting mixture contained more than 20 distinct amino acids that formed spontaneously; a more modern "volcanic" version of the experiment produced even more amino acids by including sulfur compounds.

  10. The Meselson-Stahl experiment (1958) proved that DNA replication is semiconservative, meaning that when a double-helix strand of DNA is duplicated, the result is two double-helix strands, each of which has one helix from the parent molecule and one newly-synthesized helix. Matthew Meselson and Franklin Stahl used E. coli grown in a medium containing only nitrogen-15; they were then allowed to synthesize DNA in an environment containing only nitrogen-14 over two generations. The net result was that in the second generation, half the DNA molecules contained nitrogen-15 in one strand and nitrogen-14 in the other, while the other half of the molecules contained only nitrogen-14.

This article was contributed by former NAQT writer Aryaman Tummalapalli.

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