To better understand GTR, consider that wherever a mass exists in space, it warps space. Imagine a bowling ball sinking into a rubber sheet. This is a convenient way to visualize what stars do to the fabric of the universe. If you were to place a marble into the depression formed by the stretched rubber sheet, and give the marble a sideways push, it would orbit the bowling ball for a while, like a planet orbiting the Sun. The warping of the rubber sheet by the bowling ball is a metaphor for a star warping space.
GTR can be used to understand how gravity warps and slows time. In a number of circumstances, GTR also appears to permit time travel.
Einstein additionally suggested that gravitational effects move at the speed of light. Thus, if the Sun were suddenly plucked from the Solar System, the Earth would not leave its orbit about the Sun until about eight minutes later, the time required for light to travel from the Sun to the Earth. Many physicists today believe that gravitation must be quantized and take the form of particles called gravitons, just as light takes the form of photons, which are tiny quantum packets of electromagnetism.
SEE ALSO Newton’s Laws of Motion and Gravitation (1687), Black Holes (1783), Special Theory of Relativity (1905), Noether’s Idealtheorie (1921), Einstein as Inspiration (1921), Time Travel (1949), Gravitational Waves (2016).
Einstein suggested that gravity results from the curvature of space-time caused by masses in space-time. Gravity distorts both time and space.
1919
String Theory • Clifford A. Pickover
Theodor Franz Eduard Kaluza (1885–1954), John Henry Schwarz (b. 1941), Michael Boris Green (b. 1946)
“The mathematics involved in string theory . . . ,” writes mathematician Michael Atiyah, “in subtlety and sophistication . . . vastly exceeds previous uses of mathematics in physical theories. String theory has led to a whole host of amazing results in mathematics in areas that seem far removed from physics. To many this indicates that string theory must be on the right track. . . .” Physicist Edward Witten writes, “String theory is twenty-first century physics that fell accidentally into the twentieth century.”
Various modern theories of “hyperspace” suggest that dimensions exist beyond the commonly accepted dimensions of space and time. For example, the Kaluza-Klein theory of 1919 made use of higher spatial dimensions in an attempt to explain electromagnetism and gravitation. Among the most recent formulations of these kinds of concepts is superstring theory, which predicts a universe of ten or eleven dimensions—three dimensions of space, one dimension of time, and six or seven more spatial dimensions. In many theories of hyperspace, the laws of nature become simpler and more elegant when expressed with these several extra spatial dimensions.
In string theory, some of the most basic particles, like quarks and electrons, can be modeled by inconceivably tiny, essentially one-dimensional entities called strings. Although strings may seem to be mathematical abstractions, remember that atoms were once regarded as “unreal” mathematical abstractions that eventually became observables. However, strings are so tiny that there is no current way to directly observe them.
In some string theories, the loops of string move about in ordinary 3-space, but they also vibrate in higher spatial dimensions. As a simple metaphor, think of a vibrating guitar string whose “notes” correspond to different particles such as quarks and electrons or the hypothetical graviton, which may convey the force of gravity.
String theorists claim that a variety of higher spatial dimensions are “compactified”—tightly curled up (in structures known as Calabi-Yau spaces) so that the extra dimensions are essentially invisible. In1984, Michael Green and John H. Schwarz made additional breakthroughs in string theory.
SEE ALSO Standard Model (1961), Theory of Everything (1984), Large Hadron Collider (2009).
In string theory, the vibrational pattern of the string determines what kind of particle the string is. For a metaphor, consider a violin. Pluck the A string and an electron is formed. Pluck the E string and you create a quark.
1920
Hydrogen Bonding • Derek B. Lowe
Worth Huff Rodebush (1887–1959), Wendell Mitchell Latimer (1893–1955), Maurice Loyal Huggins (1897–1981)
Hydrogen bonds are the secret adhesive of the living world. They hold together the strands of DNA, help to determine the shapes of proteins, and occur in every kind of carbohydrate molecule. The active sites of receptors and enzymes, for example, almost invariably feature key hydrogen bonds in the protein’s own structure and with the substrate molecules that bind there.
American chemist Maurice Loyal Huggins was the first to suggest the concept of hydrogen bonds, and his work inspired his colleagues Wendell Mitchell Latimer and Worth Huff Rodebush to publish a 1920 paper that used hydrogen bonds to explain properties of certain liquids. Almost one hundred years of work since then have still not revealed all their secrets, though.
So what’s a hydrogen bond? That’s not such an easy question to answer. Even the best minds (such as American chemist Linus Pauling) have found plenty to occupy them here. Hydrogen bonding is partly just the attraction between a positively charged hydrogen atom and a negatively charged atom, such as nitrogen or oxygen, on a nearby molecule. These don’t have to be full charges. Oxygen and nitrogen atoms usually have extra electron density, making them clumps of partial negative charge. But this isn’t just an ionic bond, because hydrogen bonds are directional—if they’re not pointed in the right way, the attraction mostly disappears. It’s like a ghostly form of a standard single bond, and it’s strongest when the hydrogen itself is attached to an electron-rich atom like oxygen as well. Such oxygen-hydrogen and nitrogen-hydrogen compounds are found over a huge range of chemistry, and they’re especially crucial in the behavior of many biomolecules.
Water is the best example, with two hydrogen atoms attached to a single oxygen. Water molecules are very good hydrogen bond donors and acceptors at the same time, which is what makes it such a weird substance. It has a much higher boiling point than such a tiny molecule should, and it freezes into a hydrogen-bonded crystal lattice that’s actually less dense than the liquid. (Most liquids don’t have ice that floats.)
SEE ALSO Periodic Table (1869), Electron (1897), DNA Structure (1953).
Hydrogen bonding is essential to the properties of water, which are essential to life on Earth.
1920
Radio Station • Marshall Brain
If we could get in our time machine, go back to 1912, and stand on the deck of the sinking Titanic, there is something we would see overhead that marked the beginning of a new era in communication. The Titanic had two masts, one at either end of the ship, and a long wire stretching between them. This was the antenna for a 5,000-watt spark-gap radio, and the Titanic was using it to send out Morse code distress signals.
The Titanic put radio on the map. Because of that disaster, the Radio Act of 1912 required ships to monitor for distress calls 24 hours a day and set up a system for the US government to license radio stations.
By 1920, the first AM radio station was broadcasting in the United States: KDKA in Pittsburgh, PA. What had happened between 1912 and 1920 was the mass production of vacuum tubes, accelerated by World War I. Vacuum tubes gave electrical engineers the ability to create amplifiers for radio transmitters and receivers. Once engineers created the equipment, radio exploded in popularity. Everyone had to have a radio. By 1922 there were more than a million radio receivers in the US. Hundreds of organizations—newspapers, colleges, department stores, and individuals—had created radio stations. The Golden Age of Radio was born.
NBC started in 1926 and CBS started in 1927. Government regulation changed to make the advertising model possible in radio. With a revenue stream in place, there was a good reason for broadcasters to expand and plenty of money to pay for content.
This whole story is fascinating. The war led to tubes, which led to radios. The result was an entirely new way of thinking—instantaneous, electronic, free mass m
edia to millions of people through nationwide networks funded by advertising. None of that existed in 1920. By 1930 nearly half of United States homes had radios. With the Great Depression starting, radio provided an inexpensive form of news and entertainment. Electrical engineering had created a massive societal change.
SEE ALSO Telegraph System (1837), Fiber Optics (1841), Telephone (1876), ARPANET (1969).
Turning on an early model radio. During the 1920s, amplifying vacuum tubes led to advancements in radio transmitters and receivers.
1921
Noether’s Idealtheorie • Clifford A. Pickover
Amalie Emmy Noether (1882–1935)
Despite the horrible prejudice they faced, several women have fought against the establishment and persevered in mathematics. German mathematician Emmy Noether was described by Albert Einstein as “the most significant creative mathematical genius thus far produced since the higher education of women began.”
In 1915, while at the University of Göttingen, Germany, Noether’s first significant mathematical breakthrough was in theoretical physics. In particular, Noether’s theorem dealt with symmetry relationships in physics and their relationship to conservation laws. This and related work was an aid to Einstein when he developed his general theory of relativity, which focused on the nature of gravity, space, and time.
After Noether had received her Ph.D., she attempted to teach at Göttingen, but her opponents said that men could not expect to learn “at the feet of a woman.” Her colleague David Hilbert replied to her detractors, “I do not see that the sex of the candidate is against her admission as a privatdozent [licensed lecturer]. After all, the university senate is not a bathhouse.”
Noether is also known for her contributions to noncommutative algebras, where the order in which terms are multiplied affects the results. She is most famous for her study of “chain conditions on ideals of rings,” and, in 1921, Noether published Idealtheorie in Ringbereichen, which is of major importance in the development of modern abstract algebra. This area of mathematics examines the general properties of operations and often unifies logic and number theory with applied mathematics. Alas, in 1933, her mathematical achievements were utterly dismissed when the Nazis terminated her from the University of Göttingen because she was Jewish.
She fled Germany and joined the faculty at Bryn Mawr College in Pennsylvania. According to journalist Siobhan Roberts, Noether “made weekly trips to lecture at Princeton’s institute, and to visit her friends Einstein and Herman Weyl.” Her influence was far and wide, and many of her ideas appeared in papers written by students and colleagues.
SEE ALSO Hilbert’s 23 Problems (1900), General Theory of Relativity (1915), Einstein as Inspiration (1921).
Amalie Emmy Noether, author of Idealtheorie in Ringbereichen (Theory of Ideals in Ring Domains), which was of major importance in the development of modern abstract algebra. Noether also developed some of the mathematics of general relativity but often toiled without pay.
1921
Einstein as Inspiration • Clifford A. Pickover
Albert Einstein (1879–1955)
Nobel-prize winner Albert Einstein is recognized as one of the greatest physicists of all time and the most important scientist of the twentieth century. He proposed the Special and General Theories of Relativity, which revolutionized our understanding of space and time. He also made major contributions to the science of quantum mechanics, statistical mechanics, and cosmology.
“Physics has come to dwell at such a deep remove from everyday experiences,” writes Thomas Levenson, author of Einstein in Berlin, “that it’s hard to say whether most of us would be able to recognize an Einstein-like accomplishment should it occur [today]. When Einstein first came to New York in 1921, thousands lined the street for a motorcade. . . . Try to imagine any theoretician today getting such a response. It’s impossible. The emotional connections between the physicist’s conception of reality and the popular imagination has weakened greatly since Einstein.”
According to many scholars I consulted, there will never be another individual on par with Einstein. Levenson suggested, “It seems unlikely that [science] will produce another Einstein in the sense of a broadly recognized emblem of genius. The sheer complexity of models being explored [today] confines almost all practitioners to parts of the problem.” Unlike today’s scientists, Einstein required little or no collaboration. Einstein’s paper on special relativity contained no references to others or to prior work.
Bran Ferren, cochairman and chief creative officer of the Applied Minds technology company, affirms that “the idea of Einstein is perhaps more important than Einstein himself.” Not only was Einstein the greatest physicist of the modern world, he was an “inspirational role model whose life and work ignited the lives of countless other great thinkers. The total of their contributions to society, and the contributions of the thinkers whom they will in turn inspire, will greatly exceed those of Einstein himself.”
Einstein created an unstoppable “intellectual chain reaction,” an avalanche of pulsing, chattering neurons and memes that will ring for an eternity.
SEE ALSO Newton as Inspiration (1687), E = mc2 (1905), Special Theory of Relativity (1905), Photoelectric Effect (1905), General Theory of Relativity (1915).
Photo of Albert Einstein, while attending a lecture in Vienna in 1921 at the age of 42.
1924
De Broglie Relation • Clifford A. Pickover
Louis-Victor-Pierre-Raymond, 7th duc de Broglie (1892–1987), Clinton Joseph Davisson (1881–1958), Lester Halbert Germer (1896–1971)
Numerous studies of the subatomic world have demonstrated that particles like electrons or photons (packets of light) are not like objects with which we interact in our everyday lives. These entities appear to possess characteristics of both waves and particles, depending on the experiment or phenomena being observed. Welcome to the strange realm of quantum mechanics.
In 1924, French physicist Louis-Victor de Broglie suggested that particles of matter could also be considered as waves and would possess properties commonly associated with waves, including a wavelength (the distance between successive crests of wave). In fact, all bodies have a wavelength. In 1927, American physicists Clinton Davisson and Lester Germer demonstrated the wave nature of electrons by showing that they could be made to diffract and interfere as if they were light.
De Broglie’s famous relationship showed that the wavelength of a matter wave is inversely proportional to the particle’s momentum (generally speaking, mass times velocity), and, in particular, λ = h/p. Here, λ is the wavelength, p is the momentum, and h is Planck’s constant. According to author Joanne Baker, using this equation, it is possible to show that “Bigger objects, like ball bearings and badgers, have minuscule wavelengths, too small to see, so we cannot spot them behaving like waves. A tennis ball flying across a court has a wavelength of 10−34 meters, much smaller than a proton’s width (10−15 m).” The wavelength of an ant is larger than for a human.
Since the original Davisson-Germer experiment for electrons, the de Broglie hypothesis has been confirmed for other particles like neutrons and protons and, in 1999, even for entire molecules such a buckyballs, soccer-ball-shaped molecules made of carbon atoms.
De Broglie had advanced his idea in his PhD thesis, but the idea was so radical that his thesis examiners were, at first, not sure if they should approve the thesis. He later won the Nobel Prize for this work.
SEE ALSO Wave Nature of Light (1801), Electron (1897), Schrödinger’s Wave Equation (1926).
In 1999, University of Vienna researchers demonstrated the wavelike behavior of buckminsterfullerene molecules formed of 60 carbon atoms (shown here). A beam of molecules (with velocities of around 200 m/sec, or 656 ft/sec) were sent through a grating, yielding an interference pattern characteristic of waves.
1925
Pauli Exclusion Principle • Clifford A. Pickover
Wolfgang Ernst Pauli (1900–1958)
Pauli’s Exclusion Principle (PEP) explains why matter is rigid and why two objects cannot occupy the same space. It’s why we don’t fall through the floor and why neutron stars resist collapsing under their own incredible mass.
More specifically, PEP states that no pair of identical fermions (such as electrons, protons, or neutrons) can simultaneously occupy the same quantum state, which includes the spin of a fermion. For example, electrons occupying the same atomic orbital must have opposite spins. Once an orbital is occupied by a pair of electrons of opposite spin, no more electrons may enter the orbital until one leaves the orbital.
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