1902
Chromosomal Theory of Inheritance • Clifford A. Pickover
Theodor Heinrich Boveri (1862–1915), Walter Stanborough Sutton (1877–1916)
Chromosomes are threadlike structures, each made of a long coiled DNA molecule wrapped around a protein scaffold. Chromosomes are visible under a microscope during cell division. Human body cells contain 23 pairs of chromosomes—one member of the pair contributed by the mother and one by the father. Sperm and egg each contain 23 unpaired chromosomes. When the egg is fertilized, the number of chromosomes is restored to 46.
Around 1865, Austrian priest Gregor Mendel observed that organisms inherit traits via discrete units that we now refer to as genes, but it was not until 1902 that German biologist Theodor Boveri and American geneticist and physician Walter Sutton independently identified chromosomes as the carrier of this genetic information.
While studying sea urchins, Boveri concluded that sperm and egg each had a half set of chromosomes. However, if sperm and egg united to create sea urchin embryos with abnormal numbers of chromosomes, the embryos developed abnormally. Boveri concluded that different chromosomes affected different aspects of the creatures’ development. Sutton’s studies of grasshoppers demonstrated that matched pairs of chromosomes separate during the generation of sex cells. Not only did Boveri and Sutton suggest that the chromosomes carry parental genetic information, they also showed that chromosomes were independent entities that persisted even when they were not visible, during various stages of a cell’s life, which was counter to one prevailing belief that the chromosomes simply “dissolved” during the course of cell division and reformed in the daughter cells. Their work provided a foundation for the new field of cytogenetics, the combination of cytology (the study of cells) and genetics (the science of heredity).
Today we know that during the creation of sperm and egg, matching chromosomes of parents can exchange small parts in a “crossover” process, so that new chromosomes are not inherited solely from either parent. Incorrect numbers of chromosomes can lead to genetic disorders. People with Down syndrome have 47 chromosomes.
SEE ALSO Discovery of Sperm (1678), Cell Division (1855), Mendel’s Genetics (1865), HeLa Cells (1951), DNA Structure (1953), Epigenetics (1983), Human Genome Project (2003).
LEFT: Artist’s representation of a chromosome. RIGHT: Within each chromosome, strands of DNA are coiled around proteins to form a nucleosome (shown here). Nucleosomes, in turn, are folded into even more complex structures within the chromosome, thus providing additional regulatory control of gene expression.
1903
Wright Brothers’ Airplane • Marshall Brain
Wilbur Wright (1867–1912), Orville Wright (1871–1948)
Airplanes are so familiar today that it’s hard to imagine life without them. But at the start of the twentieth century, there was not a single airplane in existence. Many believed that humans would never fly.
The Wright brothers made the dream of flight a reality in North Carolina in 1903. They certainly were engineers, but they were also scientists and inventors. There were so many problems and fundamental questions that they needed to resolve: How to create lift? How to generate sufficient thrust? How to control flight? How to make the plane light enough? How to combine it all together?
For example, they built a wind tunnel and did fundamental research to discover wing shapes that provided maximum lift. Then they had to render those shapes as strong, lightweight structures—the bi-wing arrangement of the original Wright flyer using wood, fabric, and wire. Then they had to bend those structures during flight to control the plane. We look at their solution today as slightly bizarre—the entire wing warped, and they controlled warping with their hips. We consider their forward-mounted control surfaces to be strange as well. That’s because the Wright Brothers started with a blank sheet of paper, with everything unprecedented and unknown. The conventions of rudder, elevator, and ailerons would evolve quickly once the brothers unlocked the core secrets of flight.
The entire aircraft weighed 605 pounds (275 kg) empty. How to get it off the ground? The engine seems primitive by today’s standards. At 200 cubic inches (3.3 liters) and roughly 200 pounds (91 kg), it produced just 12 horsepower (9,000 watts). A small pan of gasoline in the engine’s air intake served as a carburetor, contact breakers in the cylinders created the spark, and evaporating water cooled the engine. A man named Charles Taylor built it from scratch from a three-way dialog with the brothers. But it reliably produced its 12 horsepower to spin two counter-rotating, hand-carved wooden propellers.
It seems nearly impossible that three people could bring so many ideas and engineering disciplines together to create a flying machine that worked. Inspiration, curiosity, persistence, and the thrill of discovery powered them through.
SEE ALSO Internal Combustion Engine (1908), First Humans in Space (1961), Saturn V Rocket (1967).
Pictured: The first sustained flight of the Wright Brothers’ airplane.
1903
Classical Conditioning • Wade E. Pickren
Ivan Pavlov (1849–1936)
Russian physiologist Ivan Pavlov insisted that the scientific study of the nervous system and its expressions must be objective, mechanistic, and materialistic in orientation. Pavlov was born and reared in central Russia, the son of a village priest. Initially, it seemed as though Pavlov would follow in his father’s footsteps, but a growing personal interest in science led him to the University of Saint Petersburg, where he earned a degree in physiology.
By 1890, Pavlov was the director of the department of physiology at the university’s Institute of Experimental Medicine. Pavlov’s specialty was the study of digestion, for which he was awarded the Nobel Prize in physiology or medicine in 1904. Dogs were Pavlov’s subject of choice for his experiments. When his work on digestion in the stomach came to an end, Pavlov began to study salivation as a necessary part of the digestive processes. In 1903, one of the dog handlers in Pavlov’s laboratory observed that the dogs began salivation even before they were fed. When this came to Pavlov’s attention, he experimentally investigated what he called the “psychic processes” involved in the phenomenon.
Pavlov explored how external stimuli could be manipulated to control behavior. His most famous example came to be called classical conditioning in English. It was a convincing demonstration that when a ringing bell is presented in association with the offering of food, dogs will become conditioned, or learn, to salivate even without food being offered. Pavlov claimed that such conditioning was a matter of processes in the nervous system itself and not a matter of the mind. Thus learning in the dog, and by extension in humans and other animals, was a matter of forming elementary associations that then led to the formation of chains of associations. For many years, Pavlov and his team explored the implications of his model of learning, including how it might explain mental disorders.
SEE ALSO The Principles of Psychology (1890), Psychoanalysis (1899), Placebo Effect (1955), Cognitive Behavior Therapy (1963), Theory of Mind (1978).
ABOVE: Ivan Pavlov in his laboratory, 1922. RIGHT: Bronze statue of Pavlov and one of his dogs, located on the grounds of his laboratory in Koltushi, Russia.
1905
E = mc2 • Clifford A. Pickover
Albert Einstein (1879–1955)
“Generations have grown up knowing that the equation E = mc2 changed the shape of our world,” writes author David Bodanis, “. . . governing, as it does, everything from the atomic bomb to a television’s cathode ray tube to the carbon dating of prehistoric paintings.” Of course, part of the equation’s appeal, independent from its meaning, is its simplicity. Physicist Graham Farmelo writes, “Great equations also share with the finest poetry an extraordinary power—poetry is the most concise and highly charged form of language, just as the great equations of science are the most succinct form of understanding of the aspect of physical reality they describe.”
In a short article published in 1905,
Einstein derived his famous E = mc2, sometimes called the law of mass-energy equivalence, from the principles of Special Relativity. In essence, the formula indicates that the mass of a body is a “measure” of its energy content. c is the speed of light in vacuum, which is about 186,000 miles per second (299,792,468 meters per second).
Radioactive elements are constantly converting part of their masses to energy as governed by E = mc2, and the formula was also used in the development of the atomic bomb to better understand the nuclear binding energy that holds the atomic nucleus together, which can be used to determine the energy released in a nuclear reaction.
E = mc2 explains why the Sun shines. In the Sun, four hydrogen nuclei (four protons) are fused into a helium nucleus, which is less massive than the hydrogen nuclei that combine to make it. The fusion reaction converts the missing mass into energy that allows the Sun to heat the Earth and permits the formation of life. The mass loss, m, during the fusion supplies an energy, E, according to E = mc2. Every second, fusion reactions convert about 700 million metric tons of hydrogen into helium within the Sun’s core, thereby releasing tremendous energy.
SEE ALSO Radioactivity (1896), Conservation of Energy (1843), Special Theory of Relativity (1905), Atomic Nucleus (1911), Einstein as Inspiration (1921), Energy from the Nucleus (1942), Stellar Nucleosynthesis (1946).
A USSR stamp from 1979, dedicated to Albert Einstein and E = mc2.
1905
Photoelectric Effect • Clifford A. Pickover
Albert Einstein (1879–1955)
Of all of Albert Einstein’s masterful achievements, including the Special Theory of Relativity and the General Theory of Relativity, the achievement for which he won the Nobel Prize, was his explanation of the workings of the photoelectric effect (PE), in which certain frequencies of light shined on a copper plate cause the plate to eject electrons. In particular, he suggested that packets of light (now called photons) could explain the PE. For example, it had been noted that high-frequency light, such as blue or ultraviolet light, could cause electrons to be ejected—but not low-frequency red light. Surprisingly, even intense red light did not lead to electron ejection. In fact, the energy of individual emitted electrons increases with the frequency (and, hence, the color) of the light.
How could the frequency of light be the key to the PE? Rather than light exerting its effect as a classical wave, Einstein suggested that the energy of light came in the form of packets, or quanta, and that this energy was equal to the light frequency multiplied by a constant (later called Planck’s constant). If the photon was below a threshold frequency, it just did not have the energy to kick out an electron. As a very rough metaphor for low-energy red quanta, imagine the impossibility of chipping a fragment from a bowling ball by tossing peas at it. It just won’t work, even if you toss lots of peas! Einstein’s explanation for the energy of the photons seemed to account for many observations, such as for a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. Today, numerous devices, such as solar cells, rely on conversion of light to electric current in order to generate power.
In 1969, American physicists suggested that one could actually account for the PE without the concept of photons; thus, the PE did not provide definitive proof of photon existence. However, studies of the statistical properties of the photons in the 1970s provided experimental verification of the manifestly quantum (nonclassical) nature of the electromagnetic field.
SEE ALSO Wave Nature of Light (1801), Atomic Theory (1808), Electron (1897), Special Theory of Relativity (1905), General Theory of Relativity (1915), Einstein as Inspiration (1921), Quantum Electrodynamics (1948).
Photograph through night-vision device. U.S. Army paratroopers train using infrared lasers and night-vision optics in Camp Ramadi, Iraq. Night-vision goggles make use of the ejection of photoelectrons due to the photoelectric effect to amplify the presence of individual photons.
1905
Special Theory of Relativity • Clifford A. Pickover
Albert Einstein (1879–1955)
Albert Einstein’s Special Theory of Relativity (STR) is one of humankind’s greatest intellectual triumphs. When Albert Einstein was only 26, he made use of one of the key foundations of STR—namely, that the speed of light in a vacuum is independent of the motion of the light source and the same for all observers, no matter how they are moving. This is in contrast to the speed of sound, which changes for an observer who moves, for example, with respect to the sound source. This property of light led Einstein to deduce the relativity of simultaneity: Two events occurring at the same time as measured by one observer sitting in a laboratory frame of reference occur at different times for an observer moving relative to this frame.
Because time is relative to the speed one is traveling at, there can never be a clock at the center of the Universe to which everyone can set their watches. Your entire life can be the blink of an eye to an alien who leaves the Earth traveling close to the speed of light and then returns an hour later to find that you have been dead for centuries. (The word “relativity” partly derives from the fact that the appearance of the world depends on our relative states of motion—the appearance is “relative.”)
Although the strange consequences of STR have been understood for over a century, students still learn of them with a sense of awe and bewilderment. Nevertheless, from tiny subatomic particles to galaxies, STR appears to accurately describe nature.
To help understand another aspect of STR, imagine yourself in an airplane traveling at constant speed relative to the ground. This may be called the moving frame of reference. The principle of relativity makes us realize that without looking out the window, you cannot tell how fast you are moving. Because you cannot see the scenery moving by, for all you know, you could be in an airplane on the ground in a stationary frame of reference with respect to the ground.
SEE ALSO Michelson-Morley Experiment (1887), E = mc2 (1905), General Theory of Relativity (1915), Einstein as Inspiration (1921), Dirac Equation (1928), Time Travel (1949).
There can never be a clock at the center of the Universe to which everyone can set their watches. Your entire life can be the blink of an eye to an alien who leaves earth traveling at high speed and then returns.
1908
Internal Combustion Engine • Marshall Brain
The first widely adopted internal combustion engine was found in the Model T Ford starting in 1908. The Model T engine was based on the Otto cycle engine, also known as the four-stroke engine, patented in 1861 by Alphonse Beau de Rochas. An incredible amount of engineering went into making the Model T engine cheap, reliable, and long-lasting, given the materials and manufacturing processes available at the time. Over fifteen million Model Ts had been manufactured when production ended in 1927.
The engine contained a number of engineering marvels. Materials engineers improved on the Bessemer Process to create vanadium steel, which is so strong that some Model T engines still run today. Electrical engineers created the trembler coil ignition system that helped the engine run on gasoline, kerosene, or ethanol. Engineers also created the thermosiphon system, which moved water through the radiator without a water pump. But the real heroes were the manufacturing engineers, who made it possible to eventually produce two million cars per year with amazing efficiency, keeping prices low.
But if you compare the Model T engine with today’s engines, you can see that engineers since have been able to achieve a galaxy of improvements. The Model T engine had four cylinders displacing 2.9 liters, yet it produced only 20 horsepower. There are motorcycles today that produce 200 horsepower from one-liter engines. How is this possible? Engineers created overhead valve trains and high compression ratios to replace the flathead design on the Model T. They increased the redline and created fuel injection systems to replace carburetors. They designed much more powerful and precise ignition systems. They created tuned intake and exhaust systems.
Few
technologies are in such widespread use and have been so highly refined without a reconceptualization. After over one hundred years of change, all of the core principles of the Model T engine—pistons, valves, spark plugs, water cooling, gasoline—are still in place. But each one has been fine-tuned and highly optimized by engineers to create the compact, reliable, high-power engines of today.
SEE ALSO High-Pressure Steam Engine (1800), Carnot Engine (1824), Wright Brothers’ Airplane (1903).
Technician working on an internal combustion engine, possibly at Ford Motor Company, in 1949.
1910
Chlorination of Water • Clifford A. Pickover
Carl Rogers Darnall (1867–1941), William J. L. Lyster (1869–1947)
In 1997, LIFE magazine declared, “The filtration of drinking water plus the use of chlorine is probably the most significant public health advancement of the millennium.” Adding chlorine, a chemical element, to water is usually an effective treatment against bacteria, viruses, and amoebas, and has played a large role in the dramatic increase in life expectancy in developed countries during the 1900s. For example, in the United States, waterborne bacterial diseases such as typhoid and cholera became rare after drinking water was chlorinated. Because chlorine remains in the water supply after initial treatment, it continues to fight contamination from possible pipe leaks.
Chlorination was known to be an effective disinfectant in the 1800s, but water chlorination systems for public water supplies were not in continuous use until the early 1900s. Around 1903, the community in Middelkerke, Belgium, used chlorine gas for disinfection of drinking water, and in 1908 a water utility in Jersey City, New Jersey, used sodium hypochlorite for water chlorination. In 1910, Brigadier General Carl Rogers Darnall, a U.S. Army chemist and surgeon, used compressed liquefied chlorine gas to purify water for troops on the battlefield. The basic ideas in his invention of a mechanical liquid-chlorine purifier are now used throughout the developed world. Army scientist Major William Lyster subsequently invented the Lyster cloth bag, containing sodium hypochlorite and used to conveniently treat water by troops in the field.
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