The works of Ptolemy (Astronomy) and Aristotle (Physics) were
also found to not always match everyday observations. An example
of this is an arrow flying through the air after leaving a bow
contradicts with Aristotle's assertion that the natural state of
all objects is at rest.
The Scientific Revolution and Isaac Newton
The willingness to question previously held truths and search
for new answers resulted in a period of major scientific advancements,
now known as the Scientific Revolution. Its origins can be found
in the European re-discovery of Aristotle in the twelfth and
thirteenth centuries. This period culminated with the publication
of the Philosophiae Naturalis Principia Mathematica in 1687 by
Isaac Newton (dates disputed).
The Scientific Revolution is held by most historians (e.g., Howard
Margolis) to have begun in 1543, when there was brought to the
Polish astronomer Nicolaus Copernicus the first printed copy of
the book De Revolutionibus he had written about a dozen years earlier.
The thesis of this book is that the Earth moves around the Sun.
Other significant scientific advances were made during this time
by Galileo Galilei, Christiaan Huygens, Johannes Kepler, and Blaise
During the early 17th century, Galileo pioneered the use of experimentation
to validate physical theories, which is the key idea in the scientific
method. Galileo formulated and successfully tested several results
in dynamics, in particular the Law of Inertia. In 1687, Newton
published the Principia Mathematica, detailing two comprehensive
and successful physical theories: Newton's laws of motion, from
which arise classical mechanics; and Newton's Law of Gravitation,
which describes the fundamental force of gravity. Both theories
agreed well with experiment. The Principia also included several
theories in fluid dynamics. Classical mechanics was exhaustively
extended by Lagrange, Hamilton, and others, who produced new formulations,
principles, and results. The Law of Gravitation initiated the field
of astrophysics, which describes astronomical phenomena using physical
After Newton defined classical mechanics, the next great field
of inquiry within physics was the nature of electricity. Observations
in the 17th and 18th century by scientists such as Robert Boyle,
Stephen Gray, and Benjamin Franklin created a foundation for later
work. These observations also established our basic understanding
of electrical charge and current.
James Clerk Maxwell
In 1821, Michael Faraday integrated the study of magnetism with
the study of electricity. This was done by demonstrating that
a moving magnet induced an electric current in a conductor. Faraday
also formulated a physical conception of (what are now called)
electromagnetic fields. James Clerk Maxwell built upon this conception,
in 1864, with an interlinked set of 20 equations that explained
the interactions between electric and magnetic field. These 20
equations were later reduced, using vector calculus, to a set
of four equations.
In addition to other electromagnetic phenomena, Maxwell's equations
also can be used to describe light. Confirmation of this observation
was made with the 1888 discovery of radio by Heinrich Hertz and
in 1895 when Wilhelm Roentgen detected X rays. The ability to
describe light in electromagnetic terms helped serve as a springboard
for Albert Einstein's publication of his theory of special relativity.
This theory combined classical mechanics with Maxwell's equations.
The theory of special relativity unifies space and time into
a single entity, spacetime. Relativity prescribes a different
transformation between reference frames than classical mechanics;
this necessitated the development of relativistic mechanics as
a replacement for classical mechanics. In the regime of low (relative)
velocities, the two theories agree. Einstein built further on
the special theory by including gravity into his calculations,
and published his theory of general relativity in 1915.
One part of the theory of general relativity is Einstein's field
equation. This describes how the mass-energy tensor creates a curvature
in spacetime, and when combined with the geodesic equation forms
the basis of general relativity. Further work on Einstein's field
equation produced results which predicted the Big Bang2,3 black
holes, and the expanding universe. Einstein believed in a static
universe and attempted to fix his equation to allow for this, but
by 1927, the expanding universe was sought for by astronomers,
and in 1929 evidence was found by Edwin Hubble.
From the 18th century onwards, thermodynamics was developed by
Boyle, Young, and many others. In 1733, Bernoulli used statistical
arguments with classical mechanics to derive thermodynamic results,
initiating the field of statistical mechanics. In 1798, Thompson
demonstrated the conversion of mechanical work into heat, and in
1847 Joule stated the law of conservation of energy, in the form
of heat as well as mechanical energy.
In 1895, Roentgen discovered X-rays, which turned out to be high-frequency
electromagnetic radiation. Radioactivity was discovered in 1896
by Henri Becquerel, and further studied by Marie Curie, Pierre
Curie, and others. This initiated the field of nuclear physics.
In 1897, Thomson discovered the electron, the elementary particle
which carries electrical current in circuits. In 1904, he proposed
the first model of the atom, known as the plum pudding model. (The
existence of the atom had been proposed in 1808 by Dalton.)
Henri Becquerel accidentally discovered radioactivity in 1896.
The next year Joseph J. Thomson discovered the electron. These
discoveries revealed that the assumption of many physicists that
atoms were the basic unit of matter was flawed, and prompted
further study into the structure of atoms.
In 1900, Max Planck published his explanation of blackbody radiation.
This equation assumed that radiators are quantized in nature, which
proved to be the opening argument in the edifice that would become
The Chemical Behavior of Matter
During the 1920s Erwin Schrödinger, Werner Heisenberg,
and Max Born were able to formulate a consistent picture of
behavior of matter, a complete theory of the electronic structure
of the atom, as a byproduct of the quantum theory. Schwinger, Tomonaga,
and Richard Feynman were able to explain the Lamb shift using a
quantum field theory and quantum electrodynamics by the 1940s.
In 1959, Feynman presented the hypothesis that it is possible to
manipulate matter at the level of atoms, starting the field of
In 1911, Rutherford deduced from scattering experiments the existence
of a compact atomic nucleus, with positively charged constituents
dubbed protons. Neutrons, the neutral nuclear constituents, were
discovered in 1932 by Chadwick.
The equivalence of mass and energy (Einstein, 1905) was spectacularly
demonstrated during World War II, as research was conducted by
each side into nuclear physics, for the purpose of creating a nuclear
bomb. The German effort, led by Heisenberg, did not succeed, but
the Allied Manhattan Project reached its goal. In America, a team
led by Fermi achieved the first man-made nuclear chain reaction
in 1942, and in 1945 the world's first nuclear explosive was detonated
at Trinity site, near Alamogordo, New Mexico.
Beginning in 1900, Planck, Einstein, Bohr, and others developed
quantum theories to explain various anomalous experimental results
by introducing discrete energy levels. In 1925, Heisenberg and
1926, Schrödinger and Dirac formulated quantum mechanics,
which explained the preceding quantum theories. In quantum mechanics,
the outcomes of physical measurements are inherently probabilistic;
the theory describes the calculation of these probabilities.
It successfully describes the behavior of matter at small distance
Quantum mechanics also provided the theoretical tools for condensed
matter physics, which studies the physical behavior of solids and
liquids, including phenomena such as crystal structures, semiconductivity,
and superconductivity. The pioneers of condensed matter physics
include Bloch, who created a quantum mechanical description of
the behavior of electrons in crystal structures in 1928.
Quantum field theory was formulated in order to extend quantum
mechanics to be consistent with special relativity. It achieved
its modern form in the late 1940s with work by Feynman, Schwinger,
Tomonaga, and Dyson. They formulated the theory of quantum electrodynamics,
which describes the electromagnetic interaction. Quantum field
theory provided the framework for modern particle physics, which
studies fundamental forces and elementary particles. C. N. Yang
and T. D. Lee, in the 1950s, discovered an unexpected asymmetry
in the decay of a subatomic particle4. In 1954, Yang and Mills
developed a class of gauge theories5,6 which provided the framework
for the Standard Model. The Standard Model, which was completed
in the 1970s, successfully describes almost all elementary particles
observed to date.
The two themes of the 20th century, general relativity and quantum
mechanics, are not currently consistent with each other. General
relativity describes the universe on the scale of planets and solar
systems while quantum mechanics operates on sub-atomic scales.
This challenge is being attacked by string theory, which treats
spacetime as a manifold, not of points, but of one-dimensional
objects, strings. Strings have properties like a common string
(e.g., tension and vibration). The theories yield promising, but
not yet testable results. The search for experimental verification
of string theory is in progress.
The United Nations have declared the year 2005 as the World Year