Experiments That Changed the World
Certain key experiments were so pivotal that they fundamentally altered our understanding of biology, physics, and chemistry.
The Miller-Urey Experiment (1952)
Question: Could the organic molecules necessary for life have formed under the conditions of the early Earth?
Setup: Stanley Miller and Harold Urey created a closed system containing a heated pool of water (the 'ocean'), and a mixture of gases thought to be present in the early atmosphere (methane, ammonia, hydrogen). They introduced electrical sparks to simulate lightning.
Result: After a week, they found that various amino acids—the building blocks of proteins—and other organic molecules had formed spontaneously.
Significance: This experiment demonstrated that complex organic molecules could arise from simpler inorganic precursors under plausible early-Earth conditions, lending support to the theory of abiogenesis (life from non-life).
The Michelson-Morley Experiment (1887)
Question: Does the Earth move through a substance called the 'luminiferous aether', which was thought to be the medium through which light waves propagated?
Setup: Albert Michelson and Edward Morley used an interferometer, an extremely sensitive optical device. They split a beam of light, sent the two beams on perpendicular paths of equal length, and then recombined them. If the Earth were moving through an aether, there should be an 'aether wind', and the light beam traveling into the wind should be slightly slower than the one traveling across it, causing a detectable shift in the interference pattern when the apparatus was rotated.
Result: They detected no shift. The speed of light was the same in all directions.
Significance: This famous 'failed' experiment was a crucial piece of evidence against the aether theory and helped pave the way for Einstein's theory of special relativity, which postulates that the speed of light is constant for all observers.
Griffith's Experiment (1928)
Question: How do bacteria make people sick? Can genetic traits be transferred between bacteria?
Setup: Frederick Griffith worked with two strains of Streptococcus pneumoniae: a pathogenic (disease-causing) 'S' strain with a protective capsule, and a non-pathogenic 'R' strain without a capsule.
1.He injected mice with the S strain → mice died.
2.He injected mice with the R strain → mice lived.
3.He injected mice with heat-killed S strain → mice lived.
4.He injected mice with a mixture of heat-killed S strain and live R strain → mice died.
Result: He found live S strain bacteria in the blood of the dead mice from the fourth group.
Significance: Griffith concluded that some 'transforming principle' had been transferred from the dead S strain bacteria to the live R strain bacteria, transforming them into the pathogenic S strain. This work set the stage for Avery, MacLeod, and McCarty, who later identified this transforming principle as DNA.