Science advanced dramatically during the 20th
century. There were new and radical developments
in the physical, life and human sciences,
building on the progress made in the 19th
century.The development of post-Newtonian
theories in physics, such as special relativity,
general relativity, and quantum mechanics
led to the development of nuclear weapons.
New models of the structure of the atom led
to developments in theories of chemistry and
the development of new materials such as nylon
and plastics. Advances in biology led to large
increases in food production, as well as the
elimination of diseases such as polio.
A massive amount of new technologies were
developed in the 20th century. Technologies
such as electricity, the incandescent light
bulb, the automobile and the phonograph, first
developed at the end of the 19th century,
were perfected and universally deployed. The
first airplane flight occurred in 1903, and
by the end of the century large airplanes
such as the Boeing 777 and Airbus A330 flew
thousands of miles in a matter of hours. The
development of the television and computers
caused massive changes in the dissemination
of information.
== Astronomy and space exploration ==
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.
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 plutoid instead of a planet proper, leaving
eight planets.
No trace of life was discovered on any of
the other planets in our solar system (or
elsewhere in the universe), although it remained
undetermined whether some forms of primitive
life might exist, or might have existed, somewhere.
Extrasolar planets were observed for the first
time.
In 1969, Apollo 11 was launched towards the
Moon and Neil Armstrong became the first person
from Earth to walk on another celestial body.
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 manned presence in space was being
founded with the ongoing construction of the
International Space Station.
In addition to human spaceflight, unmanned
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, unmanned 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 ==
Genetics was unanimously accepted and significantly
developed. The structure of DNA was determined
in 1953 by James Watson, Francis Crick, Rosalind
Franklin and Maurice Wilkins, 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.
Placebo-controlled, randomized, blinded clinical
trials became a powerful tool for testing
new medicines.
Antibiotics drastically reduced mortality
from 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.
X-rays became powerful diagnostic tool for
wide spectrum of diseases, from bone fractures
to cancer. In the 1960s, computerized tomography
was invented. Other important diagnostic tools
developed were 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 invention and development of immunosuppressive
drugs and tissue typing made organ and tissue
transplantation a clinical reality.
New methods for heart surgery were developed,
including pacemakers and artificial hearts.
Cocaine/crack and heroin were found to be
dangerous addictive drugs, and their wide
usage had been outlawed; mind-altering drugs
such as LSD and MDMA were discovered and later
outlawed. In many countries, a war on drugs
caused prices to soar 10–20 times higher,
leading to profitable black market drugdealing,
and 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 influenza pandemic, Spanish Flu, killed
anywhere from 20 to 100 million people between
1918 and 1919.
A new viral disease, called the Human Immunodeficiency
Virus, or HIV, arose in Africa and subsequently
killed millions of people throughout the world.
HIV leads to a syndrome called Acquired Immunodeficiency
Syndrome, or AIDS. Treatments for HIV remained
inaccessible to many people living with AIDS
and HIV in 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, Albert Einstein explained Brownian
motion in a way that definitively proved atomic
theory. Leo Baekeland invented bakelite, one
of the first commercially successful plastics.
In 1909, American physicist Robert Andrews
Millikan - who had studied in Europe under
Walther Nernst and Max Planck - measured the
charge of individual electrons with unprecedented
accuracy through the oil drop experiment,
in which he measured the electric charges
on tiny falling water (and later oil) droplets.
His study established that any particular
droplet's electrical charge is a multiple
of a definite, fundamental value — the electron's
charge — and thus a confirmation that all
electrons have the same charge and mass. Beginning
in 1912, he spent several years investigating
and finally proving Albert Einstein's proposed
linear relationship between energy and frequency,
and providing the first direct photoelectric
support for Planck's constant. In 1923 Millikan
was awarded the Nobel Prize for Physics.
In 1909, S. P. L. Sørensen invented the pH
concept and develops methods for measuring
acidity. In 1911, Antonius Van den Broek proposed
the idea that the elements on the periodic
table are more properly organized by positive
nuclear charge rather than atomic weight.
In 1911, the first Solvay Conference was held
in Brussels, bringing together most of the
most prominent scientists of the day. In 1912,
William Henry Bragg and William Lawrence Bragg
proposed Bragg's law and established the field
of X-ray crystallography, an important tool
for elucidating the crystal structure of substances.
In 1912, Peter Debye develops the concept
of molecular dipole to describe asymmetric
charge distribution in some molecules.
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, Gilbert N. Lewis published his seminal
article "The Atom of the Molecule", which
suggested that a chemical bond is a pair of
electrons shared by two atoms. Lewis's model
equated the classical chemical bond with the
sharing of a pair of electrons between the
two bonded atoms. Lewis introduced the "electron
dot diagrams" in this paper to symbolize the
electronic structures of atoms and molecules.
Now known as Lewis structures, they are discussed
in virtually every introductory chemistry
book. Lewis in 1923 developed the electron
pair theory of acids and base: Lewis redefined
an acid as any atom or molecule with an incomplete
octet that was thus capable of accepting electrons
from another atom; bases were, of course,
electron donors. His theory is known as the
concept of Lewis acids and bases. In 1923,
G. N. Lewis and Merle Randall published Thermodynamics
and the Free Energy of Chemical Substances,
first modern treatise on chemical thermodynamics.
The 1920s saw a rapid adoption and application
of Lewis's model of the electron-pair bond
in the fields of organic and coordination
chemistry. In organic chemistry, this was
primarily due to the efforts of the British
chemists Arthur Lapworth, Robert Robinson,
Thomas Lowry, and Christopher Ingold; while
in coordination chemistry, Lewis's bonding
model was promoted through the efforts of
the American chemist Maurice Huggins and the
British chemist Nevil Sidgwick.
=== Quantum chemistry ===
Some view the birth of quantum chemistry in
the discovery of the Schrödinger equation
and its application to the hydrogen atom in
1926. However, the 1927 article of Walter
Heitler and Fritz London is often recognised
as the first milestone in the history of quantum
chemistry. This is the first application of
quantum mechanics to the diatomic hydrogen
molecule, and thus to the phenomenon of the
chemical bond. In the following years much
progress was accomplished by Edward Teller,
Robert S. Mulliken, Max Born, J. Robert Oppenheimer,
Linus Pauling, Erich Hückel, Douglas Hartree,
Vladimir Aleksandrovich Fock, to cite a few.Still,
skepticism remained as to the general power
of quantum mechanics applied to complex chemical
systems. The situation around 1930 is described
by Paul Dirac:
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 molecular or atomic physics
to underline the fact that they were more
the application of quantum mechanics to chemistry
and spectroscopy than answers to chemically
relevant questions. In 1951, a milestone article
in quantum chemistry is the seminal paper
of Clemens C. J. Roothaan on Roothaan equations.
It opened the avenue to the solution of the
self-consistent field equations for small
molecules like hydrogen or nitrogen. Those
computations were performed with the help
of tables of integrals which were computed
on the most advanced computers of the time.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 the only person, 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 electronic structure of the
atom; Linus Pauling's book on The Nature of
the Chemical Bond used the principles of quantum
mechanics to deduce bond angles in ever-more
complicated molecules. However, though some
principles deduced from quantum mechanics
were able to predict qualitatively some chemical
features for biologically relevant molecules,
they were, till the end of the 20th century,
more a collection of rules, observations,
and recipes than rigorous ab initio quantitative
methods.
This heuristic approach triumphed in 1953
when James Watson and Francis Crick deduced
the double helical structure of DNA by constructing
models constrained by and informed by the
knowledge of the chemistry of the constituent
parts and the X-ray diffraction patterns obtained
by Rosalind Franklin. This discovery lead
to an explosion of research into the biochemistry
of life.
In the same year, the Miller–Urey experiment
demonstrated that basic constituents of protein,
simple amino acids, could themselves be built
up from simpler molecules in a simulation
of primordial processes on Earth. Though many
questions remain about the true nature of
the origin of life, this was the first attempt
by chemists to study hypothetical processes
in the laboratory under controlled conditions.In
1983 Kary Mullis devised a method for the
in-vitro amplification of DNA, known as the
polymerase chain reaction (PCR), which revolutionized
the chemical processes used in the laboratory
to manipulate it. PCR could be used to synthesize
specific pieces of DNA and made possible the
sequencing of DNA of organisms, which culminated
in the huge human genome project.
An important piece in the double helix puzzle
was solved by one of Pauling's students Matthew
Meselson and Frank Stahl, the result of their
collaboration (Meselson–Stahl experiment)
has been called as "the most beautiful experiment
in biology".
They used a centrifugation technique that
sorted molecules according to differences
in weight. Because nitrogen atoms are a component
of DNA, they were labelled and therefore tracked
in replication in bacteria.
=== Late 20th century ===
In 1970, John Pople developed the Gaussian
program greatly easing computational chemistry
calculations. In 1971, Yves Chauvin offered
an explanation of the reaction mechanism of
olefin metathesis reactions. In 1975, Karl
Barry Sharpless and his group discovered a
stereoselective oxidation reactions including
Sharpless epoxidation, Sharpless asymmetric
dihydroxylation, and Sharpless oxyamination.
In 1985, Harold Kroto, Robert Curl and Richard
Smalley discovered fullerenes, a class of
large carbon molecules superficially resembling
the geodesic dome designed by architect R.
Buckminster Fuller. In 1991, Sumio Iijima
used electron microscopy to discover a type
of cylindrical fullerene known as a carbon
nanotube, though earlier work had been done
in the field as early as 1951. This material
is an important component in the field of
nanotechnology. In 1994, Robert A. Holton
and his group achieved the first total synthesis
of Taxol. In 1995, Eric Cornell and Carl Wieman
produced the first Bose–Einstein condensate,
a substance that displays quantum mechanical
properties on the macroscopic scale.
== 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 home appliances increased
dramatically due to advancements in technology,
electricity availability, and increases in
wealth and leisure time. Such basic appliances
as washing machines, clothes dryers, furnaces,
exercise machines, refrigerators, freezers,
electric stoves, and vacuum cleaners all became
popular from the 1920s through the 1950s.
The microwave oven became popular during the
1980s and have become a standard in all homes
by the 1990s. Radios were popularized as a
form of entertainment during the 1920s, which
extended to television during the 1950s. Cable
and satellite television spread rapidly during
the 1980s and 1990s. Personal computers began
to enter the home during the 1970s–1980s
as well. The age of the portable music player
grew during the 1960s with the development
of the transistor radio, 8-track and cassette
tapes, which slowly began to replace 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 assembly line made mass production of
the automobile viable. By the end of the 20th
century, billions of people had automobiles
for personal transportation. The combination
of the automobile, motor boats and air travel
allowed for unprecedented personal mobility.
In western nations, motor vehicle accidents
became the greatest cause of death for young
people. However, expansion of 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.
Aluminum 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 Mathematics Subject
Classification was dozens of pages long. Every
year, thousands of new Ph.D.s in mathematics
were awarded, and jobs were available in both
teaching and industry. More and more mathematical
journals were published and, by the end of
the century, the development of the World
Wide Web led to online publishing. Mathematical
collaborations of unprecedented size and scope
took place. An example is the classification
of finite simple groups (also called the "enormous
theorem"), whose proof between 1955 and 1983
required 500-odd journal articles by about
100 authors, and filling tens of thousands
of pages.
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 natural numbers plus one of addition and
multiplication, was decidable, i.e. could
be determined by some algorithm. In 1931,
Kurt Gödel found that this was not the case
for the natural numbers plus both addition
and multiplication; this system, known as
Peano arithmetic, was in fact incompletable.
(Peano arithmetic is adequate for a good deal
of number theory, including the notion of
prime number.) A consequence of Gödel's two
incompleteness theorems is that in any mathematical
system that includes Peano arithmetic (including
all of analysis and geometry), truth necessarily
outruns proof, i.e. there are true statements
that cannot be proved within the system. Hence
mathematics cannot be reduced to mathematical
logic, and David Hilbert's dream of making
all of mathematics complete and consistent
needed to be reformulated.
In 1963, Paul Cohen proved that the continuum
hypothesis is independent of (could neither
be proved nor disproved from) the standard
axioms of set theory. In 1976, Wolfgang Haken
and Kenneth Appel used a computer to prove
the four color theorem. Andrew Wiles, building
on the work of others, proved Fermat's Last
Theorem in 1995. In 1998 Thomas Callister
Hales proved the Kepler conjecture.
Differential geometry came into its own when
Einstein used it in general relativity. Entirely
new areas of mathematics such as mathematical
logic, topology, and John von Neumann's game
theory changed the kinds of questions that
could be answered by mathematical methods.
All kinds of structures were abstracted using
axioms and given names like metric spaces,
topological spaces etc. As mathematicians
do, the concept of an abstract structure was
itself abstracted and led to category theory.
Grothendieck and Serre recast algebraic geometry
using sheaf theory. Large advances were made
in the qualitative study of dynamical systems
that Poincaré had begun in the 1890s.
Measure theory was developed in the late 19th
and early 20th centuries. Applications of
measures include the Lebesgue integral, Kolmogorov's
axiomatisation of probability theory, and
ergodic theory. Knot theory greatly expanded.
Quantum mechanics led to the development of
functional analysis. Other new areas include
Laurent Schwartz's distribution theory, fixed
point theory, singularity theory and René
Thom's catastrophe theory, model theory, and
Mandelbrot's fractals. Lie theory with its
Lie groups and Lie algebras became one of
the major areas of study.
Non-standard analysis, introduced by Abraham
Robinson, rehabilitated the infinitesimal
approach to calculus, which had fallen into
disrepute in favour of the theory of limits,
by extending the field of real numbers to
the Hyperreal numbers which include infinitesimal
and infinite quantities. An even larger number
system, the surreal numbers were discovered
by John Horton Conway in connection with combinatorial
games.
The development and continual improvement
of computers, at first mechanical analog machines
and then digital electronic machines, allowed
industry to deal with larger and larger amounts
of data to facilitate mass production and
distribution and communication, and new areas
of mathematics were developed to deal with
this: Alan Turing's computability theory;
complexity theory; Derrick Henry Lehmer's
use of ENIAC to further number theory and
the Lucas-Lehmer test; Rózsa Péter's recursive
function theory; Claude Shannon's information
theory; signal processing; data analysis;
optimization and other areas of operations
research. In the preceding centuries much
mathematical focus was on calculus and continuous
functions, but the rise of computing and communication
networks led to an increasing importance of
discrete concepts and the expansion of combinatorics
including graph theory. The speed and data
processing abilities of computers also enabled
the handling of mathematical problems that
were too time-consuming to deal with by pencil
and paper calculations, leading to areas such
as numerical analysis and symbolic computation.
Some of the most important methods and algorithms
of the 20th century are: the simplex algorithm,
the Fast Fourier Transform, error-correcting
codes, the Kalman filter from control theory
and the RSA algorithm of public-key cryptography.
== Physics ==
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.
=== Quantum mechanics ===
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 thesis. 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 restriction
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 Wolfgang
Pauli developed the Pauli exclusion principle,
which states that no two electrons around
a single nucleus in an atom can occupy the
same quantum state simultaneously, as described
by four 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.
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 Werner Heisenberg
was one of the key creators of quantum mechanics.
In 1925, Heisenberg discovered a way to formulate
quantum mechanics in terms of matrices. For
that discovery, he was awarded the Nobel Prize
for Physics for 1932. In 1927 he published
his uncertainty principle, upon which he built
his philosophy and for which he is best known.
Heisenberg was able to demonstrate that if
you were studying an electron in an atom you
could say where it was (the electron's location)
or where it was going (the electron's velocity),
but it was impossible to express both at the
same time. He also made important contributions
to the theories of the hydrodynamics of turbulent
flows, the atomic nucleus, ferromagnetism,
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.
== Social sciences ==
Ivan Pavlov developed the theory of classical
conditioning.
The Austrian School of economic theory gained
in prominence
