20th century in science

• A much better understanding of the evolution of the universe was achieved, its age (about 13.8 billion years) was determined, and the Big Bang theory on its origin was proposed and generally accepted.
• The age of the Solar System, including Earth, was determined, and it turned out to be much older than believed earlier: more than 4 billion years, rather than the 20 million years suggested by Lord Kelvin in 1862.^[2]
• The planets of the Solar System and their moons were closely observed via numerous
  space probes. Pluto was discovered in 1930 on the edge of the solar system, although in the early 21st century, it was reclassified as a dwarf planet
  (planetoid) instead of a planet proper, leaving eight planets.
• No trace of
  Extrasolar planets
  were observed for the first time.
• In 1969, Apollo 11 was launched towards the Moon and Neil Armstrong and Buzz Aldrin became the first persons from Earth to walk on another celestial body.
• That same year, Soviet astronomer
  Victor Safronov published his book Evolution of the protoplanetary cloud and formation of the Earth and the planets. In this book, almost all major problems of the planetary formation process were formulated and some of them solved. Safronov's ideas were further developed in the works of George Wetherill, who discovered runaway accretion
  .
• The Space Race between the United States and the Soviet Union gave a peaceful outlet to the political and military tensions of the Cold War, leading to the first human spaceflight with the Soviet Union's Vostok 1 mission in 1961, and man's first landing on another world—the Moon—with America's Apollo 11 mission in 1969. Later, the first space station was launched by the Soviet space program. The United States developed the first (and to date only) reusable spacecraft system with the Space Shuttle program, first launched in 1981. As the century ended, a permanent human presence in space was being founded with the ongoing construction of the International Space Station.
• In addition to human spaceflight, uncrewed space probes became a practical and relatively inexpensive form of exploration. The first orbiting space probe, Sputnik 1, was launched by the Soviet Union in 1957. Over time, a massive system of artificial satellites was placed into orbit around Earth. These satellites greatly advanced navigation, communications, military intelligence, geology, climate, and numerous other fields. Also, by the end of the 20th century, uncrewed probes had visited the Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and various asteroids and comets. The Hubble Space Telescope, launched in 1990, greatly expanded our understanding of the Universe and brought brilliant images to TV and computer screens around the world.

Biology and medicine

• James Watson,^[3][4] Francis Crick,^[3][4] Rosalind Franklin^[4] and Maurice Wilkins,^[3][4] following by developing techniques which allow to read DNA sequences and culminating in starting the Human Genome Project (not finished in the 20th century) and cloning the first mammal
  in 1996.
• The role of sexual reproduction in evolution was understood, and bacterial conjugation was discovered.
• The convergence of various sciences for the formulation of the modern evolutionary synthesis (produced between 1936 and 1947), providing a widely accepted account of evolution.
• blinded clinical trials
  became a powerful tool for testing new medicines.
• bacterial diseases and their prevalence
  .
• A vaccine was developed for polio, ending a worldwide epidemic. Effective vaccines were also developed for a number of other serious infectious diseases, including influenza, diphtheria, pertussis (whooping cough), tetanus, measles, mumps, rubella (German measles), chickenpox, hepatitis A, and hepatitis B.
• Epidemiology and vaccination led to the eradication of the smallpox virus in humans.
• sonography and magnetic resonance imaging
  .
• Development of
  vitamins virtually eliminated scurvy
  and other vitamin-deficiency diseases from industrialized societies.
• New psychiatric drugs were developed. These include antipsychotics for treating hallucinations and delusions, and antidepressants for treating depression.
• The role of tobacco smoking in the causation of cancer and other diseases was proven during the 1950s (see British Doctors Study).
• New methods for cancer treatment, including chemotherapy, radiation therapy, and immunotherapy, were developed. As a result, cancer could often be cured or placed in remission.
• The development of blood typing and blood banking made blood transfusion safe and widely available.
• The
  organ and tissue transplantation
  a clinical reality.
• New methods for
  pacemakers and artificial hearts
  .
• drug dealing, and in some countries (e.g. the United States
  ) to prison inmate sentences being 80% related to drug use by the 1990s.
• Contraceptive drugs were developed, which reduced population growth rates in industrialized countries, as well as decreased the taboo of premarital sex
  throughout many western countries.
• The development of medical insulin during the 1920s helped raise the life expectancy of diabetics to three times of what it had been earlier.
• Vaccines, hygiene and clean water improved health and decreased mortality rates, especially among infants and the young.

Notable diseases

• An
  Spanish Flu
  , killed anywhere from 20 to 100 million people between 1918 and 1919.
• A new
  developing countries
  , and a cure has yet to be discovered.
• Because of increased life spans, the prevalence of cancer, Alzheimer's disease, Parkinson's disease, and other diseases of old age increased slightly.
• Sedentary lifestyles, due to labor-saving devices and technology, along with the increase in home entertainment and technology such as television, video games, and the internet contributed to an "epidemic" of obesity, at first in the rich countries, but by the end of the 20th century spreading to the developing world.

Chemistry

In 1903, Mikhail Tsvet invented chromatography, an important analytic technique. In 1904, Hantaro Nagaoka proposed an early nuclear model of the atom, where electrons orbit a dense massive nucleus. In 1905, Fritz Haber and Carl Bosch developed the Haber process for making ammonia, a milestone in industrial chemistry with deep consequences in agriculture. The Haber process, or Haber-Bosch process, combined nitrogen and hydrogen to form ammonia in industrial quantities for production of fertilizer and munitions. The food production for half the world's current population depends on this method for producing fertilizer. Haber, along with Max Born, proposed the Born–Haber cycle as a method for evaluating the lattice energy of an ionic solid. Haber has also been described as the "father of chemical warfare" for his work developing and deploying chlorine and other poisonous gases during World War I.

In 1905,

In 1909,

In 1913, Niels Bohr, a Danish physicist, introduced the concepts of quantum mechanics to atomic structure by proposing what is now known as the Bohr model of the atom, where electrons exist only in strictly defined circular orbits around the nucleus similar to rungs on a ladder. The Bohr Model is a planetary model in which the negatively charged electrons orbit a small, positively charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar) - the gravitational force of the solar system is mathematically akin to the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons.

In 1913, Henry Moseley, working from Van den Broek's earlier idea, introduces concept of atomic number to fix inadequacies of Mendeleev's periodic table, which had been based on atomic weight. The peak of Frederick Soddy's career in radiochemistry was in 1913 with his formulation of the concept of isotopes, which stated that certain elements exist in two or more forms which have different atomic weights but which are indistinguishable chemically. He is remembered for proving the existence of isotopes of certain radioactive elements, and is also credited, along with others, with the discovery of the element protactinium in 1917. In 1913, J. J. Thomson expanded on the work of Wien by showing that charged subatomic particles can be separated by their mass-to-charge ratio, a technique known as mass spectrometry.

In 1916,

Some view the birth of quantum chemistry in the discovery of the

Still, skepticism remained as to the general power of quantum mechanics applied to complex chemical systems.^[citation needed] The situation around 1930 is described by Paul Dirac:^[6]

The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that the exact application of these laws leads to equations much too complicated to be soluble. It therefore becomes desirable that approximate practical methods of applying quantum mechanics should be developed, which can lead to an explanation of the main features of complex atomic systems without too much computation.

Hence the quantum mechanical methods developed in the 1930s and 1940s are often referred to as theoretical

In the 1940s many physicists turned from molecular or atomic physics to nuclear physics (like J. Robert Oppenheimer or Edward Teller). Glenn T. Seaborg was an American nuclear chemist best known for his work on isolating and identifying transuranium elements (those heavier than uranium). He shared the 1951 Nobel Prize for Chemistry with Edwin Mattison McMillan for their independent discoveries of transuranium elements. Seaborgium was named in his honour, making him one of three people, along Albert Einstein and Yuri Oganessian, for whom a chemical element was named during his lifetime.

Molecular biology and biochemistry

By the mid 20th century, in principle, the integration of physics and chemistry was extensive, with chemical properties explained as the result of the

This heuristic approach triumphed in 1953 when

In 1970, John Pople developed the Gaussian program greatly easing computational chemistry calculations.^[10] In 1971, Yves Chauvin offered an explanation of the reaction mechanism of olefin metathesis reactions.^[11] In 1975, Karl Barry Sharpless and his group discovered a stereoselective oxidation reactions including Sharpless epoxidation,^[12][13] Sharpless asymmetric dihydroxylation,^[14][15][16] and Sharpless oxyamination.^[17][18][19] In 1985,

Unfortunately, Wegener provided no convincing mechanism for this drift, and his ideas were not generally accepted during his lifetime. Arthur Homes accepted Wegener's theory and provided a mechanism: mantle convection, to cause the continents to move.^[27] However, it was not until after the Second World War that new evidence started to accumulate that supported continental drift. There followed a period of 20 extremely exciting years where the Theory of Continental Drift developed from being believed by a few to being the cornerstone of modern Geology. Beginning in 1947 research found new evidence about the ocean floor, and in 1960 Bruce C. Heezen published the concept of mid-ocean ridges. Soon after this, Robert S. Dietz and Harry H. Hess proposed that the oceanic crust forms as the seafloor spreads apart along mid-ocean ridges in seafloor spreading.^[28] This was seen as confirmation of mantle convection and so the major stumbling block to the theory was removed. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypothesis of paleomagnetism, the record of the orientation of the Earth's magnetic field recorded in magnetic minerals. British geophysicist S. K. Runcorn suggested the concept of paleomagnetism from his finding that the continents had moved relative to the Earth's magnetic poles. Tuzo Wilson, who was a promoter of the sea floor spreading hypothesis and continental drift from the very beginning,^[29] added the concept of transform faults to the model, completing the classes of fault types necessary to make the mobility of the plates on the globe function.^[30] A symposium on continental drift^[31] was held at the Royal Society of London in 1965 must be regarded as the official start of the acceptance of plate tectonics by the scientific community. The abstracts from the symposium are issued as Blacket, Bullard, Runcorn;1965.In this symposium, Edward Bullard and co-workers showed with a computer calculation how the continents along both sides of the Atlantic would best fit to close the ocean, which became known as the famous "Bullard's Fit". By the late 1960s the weight of the evidence available saw Continental Drift as the generally accepted theory.

Other theories of the causes of climate change fared no better. The principal advances were in observational

Another clue to the nature of climate change came in the mid-1960s from analysis of deep-sea cores by

In 1969, NATO was the first candidate to deal with climate change on an international level. It was planned then to establish a hub of research and initiatives of the organization in the civil area, dealing with environmental topics^[41] as acid rain and the greenhouse effect. The suggestion of US President Richard Nixon was not very successful with the administration of German Chancellor Kurt Georg Kiesinger. But the topics and the preparation work done on the NATO proposal by the German authorities gained international momentum, (see e.g. the Stockholm United Nations Conference on the Human Environment 1970) as the government of Willy Brandt started to apply them on the civil sphere instead.^{[41][clarification needed]}

Also in 1969,

In the early 1970s, evidence that aerosols were increasing worldwide encouraged Reid Bryson and some others to warn of the possibility of severe cooling. Meanwhile, the new evidence that the timing of ice ages was set by predictable orbital cycles suggested that the climate would gradually cool, over thousands of years. For the century ahead, however, a survey of the scientific literature from 1965 to 1979 found 7 articles predicting cooling and 44 predicting warming (many other articles on climate made no prediction); the warming articles were cited much more often in subsequent scientific literature.^[45] Several scientific panels from this time period concluded that more research was needed to determine whether warming or cooling was likely, indicating that the trend in the scientific literature had not yet become a consensus.^[46][47][48]

John Sawyer published the study Man-made Carbon Dioxide and the “Greenhouse” Effect in 1972.^[49] He summarized the knowledge of the science at the time, the anthropogenic attribution of the carbon dioxide greenhouse gas, distribution and exponential rise, findings which still hold today. Additionally he accurately predicted the rate of global warming for the period between 1972 and 2000.^[50][51]

The increase of 25% CO2 expected by the end of the century therefore corresponds to an increase of 0.6°C in the world temperature – an amount somewhat greater than the climatic variation of recent centuries. – John Sawyer, 1972

The mainstream news media at the time exaggerated the warnings of the minority who expected imminent cooling. For example, in 1975, Newsweek magazine published a story that warned of "ominous signs that the Earth's weather patterns have begun to change."^[52] The article continued by stating that evidence of global cooling was so strong that meteorologists were having "a hard time keeping up with it."^[52] On 23 October 2006, Newsweek issued an update stating that it had been "spectacularly wrong about the near-term future".^[53]

In the first two "Reports for the Club of Rome" in 1972^[54] and 1974,^[55] the anthropogenic climate changes by CO₂ increase as well as by waste heat were mentioned. About the latter John Holdren wrote in a study^[56] cited in the 1st report, “… that global thermal pollution is hardly our most immediate environmental threat. It could prove to be the most inexorable, however, if we are fortunate enough to evade all the rest.” Simple global-scale estimates^[57] that recently have been actualized^[58] and confirmed by more refined model calculations^[59][60] show noticeable contributions from waste heat to global warming after the year 2100, if its growth rates are not strongly reduced (below the averaged 2% p.a. which occurred since 1973).

Evidence for warming accumulated. By 1975, Manabe and Wetherald had developed a three-dimensional

The 1979 World Climate Conference (12 to 23 February) of the World Meteorological Organization concluded "it appears plausible that an increased amount of carbon dioxide in the atmosphere can contribute to a gradual warming of the lower atmosphere, especially at higher latitudes....It is possible that some effects on a regional and global scale may be detectable before the end of this century and become significant before the middle of the next century."^[62]

In July 1979 the

By the early 1980s, the slight cooling trend from 1945 to 1975 had stopped. Aerosol pollution had decreased in many areas due to environmental legislation and changes in fuel use, and it became clear that the cooling effect from aerosols was not going to increase substantially while carbon dioxide levels were progressively increasing.

Hansen and others published the 1981 study Climate impact of increasing atmospheric carbon dioxide, and noted:

It is shown that the anthropogenic carbon dioxide warming should emerge from the noise level of natural climate variability by the end of the century, and there is a high probability of warming in the 1980s. Potential effects on climate in the 21st century include the creation of drought-prone regions in North America and central Asia as part of a shifting of climatic zones, erosion of the West Antarctic ice sheet with a consequent worldwide rise in sea level, and opening of the fabled Northwest Passage.^[64]

In 1982, Greenland ice cores drilled by Hans Oeschger, Willi Dansgaard, and collaborators revealed dramatic temperature oscillations in the space of a century in the distant past.^[65] The most prominent of the changes in their record corresponded to the violent Younger Dryas climate oscillation seen in shifts in types of pollen in lake beds all over Europe. Evidently drastic climate changes were possible within a human lifetime.

In 1985 a joint UNEP/WMO/ICSU Conference on the "Assessment of the Role of Carbon Dioxide and Other Greenhouse Gases in Climate Variations and Associated Impacts" concluded that greenhouse gases "are expected" to cause significant warming in the next century and that some warming is inevitable.^[66]

Meanwhile, ice cores drilled by a Franco-Soviet team at the Vostok Station in Antarctica showed that CO₂ and temperature had gone up and down together in wide swings through past ice ages. This confirmed the CO₂-temperature relationship in a manner entirely independent of computer climate models, strongly reinforcing the emerging scientific consensus. The findings also pointed to powerful biological and geochemical feedbacks.^[67]

In June 1988,

The 1980s saw important breakthroughs with regard to global environmental challenges. Ozone depletion was mitigated by the Vienna Convention (1985) and the Montreal Protocol (1987). Acid rain was mainly regulated on national and regional levels.

Colors indicate temperature anomalies (

Since the 1990s, research on climate change has expanded and grown, linking many fields such as atmospheric sciences, numerical modeling, behavioral sciences, geology and economics, or security.

Engineering and technology

One of the prominent traits of the 20th century was the dramatic growth of technology. Organized research and practice of science led to advancement in the fields of communication, engineering, travel, medicine, and war.

• The number and types of
  record players These were in turn replaced by the CD during the late 1980s and 1990s. The proliferation of the Internet in the mid-to-late 1990s made digital distribution of music (mp3s) possible. VCRs
  were popularized in the 1970s, but by the end of the 20th century, DVD players were beginning to replace them, making the VHS obsolete by the end of the first decade of the 21st century.
• The first airplane was flown in 1903. With the engineering of the faster jet engine in the 1940s, mass air travel became commercially viable.
• The
  divided highways
  reduced the death rate.
• The triode tube, transistor and integrated circuit successively revolutionized electronics and computers, leading to the proliferation of the personal computer in the 1980s and cell phones and the public-use Internet in the 1990s.
• New materials, most notably stainless steel, Velcro, silicone, teflon, and plastics such as polystyrene, PVC, polyethylene, and nylon came into widespread use for many various applications. These materials typically have tremendous performance gains in strength, temperature, chemical resistance, or mechanical properties over those known prior to the 20th century.
• Aluminium became an inexpensive metal and became second only to iron in use.
• Semiconductor materials were discovered, and methods of production and purification developed for use in electronic devices. Silicon became one of the purest substances ever produced.
• Thousands of
  chemicals
  were developed for industrial processing and home use.

Mathematics

The 20th century saw mathematics become a major profession. As in most areas of study, the explosion of knowledge in the scientific age has led to specialization: by the end of the century there were hundreds of specialized areas in mathematics and the

In a 1900 speech to the International Congress of Mathematicians, David Hilbert set out a list of 23 unsolved problems in mathematics. These problems, spanning many areas of mathematics, formed a central focus for much of 20th-century mathematics. Today, 10 have been solved, 7 are partially solved, and 2 are still open. The remaining 4 are too loosely formulated to be stated as solved or not.

In 1929 and 1930, it was proved the truth or falsity of all statements formulated about the

• New areas of physics, like special relativity, general relativity, and quantum mechanics, were developed during the first half of the century. In the process, the internal structure of atoms came to be clearly understood, followed by the discovery of elementary particles.
• It was found that all the known forces can be traced to only four fundamental interactions. It was discovered further that two forces, electromagnetism and weak interaction, can be merged in the electroweak interaction, leaving only three different fundamental interactions.
• Discovery of nuclear reactions, in particular nuclear fusion, finally revealed the source of solar energy.
• Radiocarbon dating was invented, and became a powerful technique for determining the age of prehistoric animals and plants as well as historical objects.
• chemical elements were created by nuclear fusion
  reactions within stars.

Quantum mechanics

Quantum mechanics in the 1920s

From left to right, top row: Louis de Broglie (1892–1987) and Wolfgang Pauli (1900–58); second row: Erwin Schrödinger (1887–1961) and Werner Heisenberg (1901–76)

This section is an excerpt from History of chemistry § Quantum mechanics.[edit]

In 1924, French quantum physicist Louis de Broglie published his thesis, in which he introduced a revolutionary theory of electron waves based on wave–particle duality. In his time, the wave and particle interpretations of light and matter were seen as being at odds with one another, but de Broglie suggested that these seemingly different characteristics were instead the same behavior observed from different perspectives — that particles can behave like waves, and waves (radiation) can behave like particles. Broglie's proposal offered an explanation of the restricted motion of electrons within the atom. The first publications of Broglie's idea of "matter waves" had drawn little attention from other physicists, but a copy of his doctoral thesis chanced to reach Einstein, whose response was enthusiastic. Einstein stressed the importance of Broglie's work both explicitly and by building further on it.

In 1925, Austrian-born physicist

quantum numbers. Pauli made major contributions to quantum mechanics and quantum field theory – he was awarded the 1945 Nobel Prize for Physics for his discovery of the Pauli exclusion principle – as well as solid-state physics, and he successfully hypothesized the existence of the neutrino

. In addition to his original work, he wrote masterful syntheses of several areas of physical theory that are considered classics of scientific literature.

${\displaystyle H(t)|\psi (t)\rangle =i\hbar {\frac {d}{dt}}|\psi (t)\rangle }$

The Schrödinger equation

In 1926 at the age of 39, Austrian theoretical physicist Erwin Schrödinger produced the papers that gave the foundations of quantum wave mechanics. In those papers he described his partial differential equation that is the basic equation of quantum mechanics and bears the same relation to the mechanics of the atom as Newton's equations of motion bear to planetary astronomy. Adopting a proposal made by Louis de Broglie in 1924 that particles of matter have a dual nature and in some situations act like waves, Schrödinger introduced a theory describing the behaviour of such a system by a wave equation that is now known as the Schrödinger equation. The solutions to Schrödinger's equation, unlike the solutions to Newton's equations, are wave functions that can only be related to the probable occurrence of physical events. The readily visualized sequence of events of the planetary orbits of Newton is, in quantum mechanics, replaced by the more abstract notion of probability. (This aspect of the quantum theory made Schrödinger and several other physicists profoundly unhappy, and he devoted much of his later life to formulating philosophical objections to the generally accepted interpretation of the theory that he had done so much to create.)

German theoretical physicist

cosmic rays, and subatomic particles, and he was instrumental in planning the first West German nuclear reactor at Karlsruhe, together with a research reactor

in Munich, in 1957. Considerable controversy surrounds his work on atomic research during World War II.

• Ivan Pavlov developed the theory of classical conditioning.
• The
  economic theory
  gained in prominence.

References