Physics (from the Ancient Greek φύσις
physis meaning "nature") is the fundamental
branch of science. The primary objects of
study are matter and energy. Physics is, in
one sense, the oldest and most basic academic
pursuit; its discoveries find applications
throughout the natural sciences, since matter
and energy are the basic constituents of the
natural world. The other sciences are generally
more limited in their scope and may be considered
branches that have split off from physics
to become sciences in their own right. Physics
today may be divided loosely into classical
physics and modern physics.
== Ancient history ==
Elements of what became physics were drawn
primarily from the fields of astronomy, optics,
and mechanics, which were methodologically
united through the study of geometry. These
mathematical disciplines began in antiquity
with the Babylonians and with Hellenistic
writers such as Archimedes and Ptolemy. Ancient
philosophy, meanwhile – including what was
called "physics" – focused on explaining
nature through ideas such as Aristotle's four
types of "cause".
=== Ancient Greece ===
The move towards a rational understanding
of nature began at least since the Archaic
period in Greece (650–480 BCE) with the
Pre-Socratic philosophers. The philosopher
Thales of Miletus (7th and 6th centuries BCE),
dubbed "the Father of Science" for refusing
to accept various supernatural, religious
or mythological explanations for natural phenomena,
proclaimed that every event had a natural
cause. Thales also made advancements in 580
BCE by suggesting that water is the basic
element, experimenting with the attraction
between magnets and rubbed amber and formulating
the first recorded cosmologies. Anaximander,
famous for his proto-evolutionary theory,
disputed the Thales' ideas and proposed that
rather than water, a substance called apeiron
was the building block of all matter. Around
500 BCE, Heraclitus proposed that the only
basic law governing the Universe was the principle
of change and that nothing remains in the
same state indefinitely. This observation
made him one of the first scholars in ancient
physics to address the role of time in the
universe, a key and sometimes contentious
concept in modern and present-day physics.
The early physicist Leucippus (fl. first half
of the 5th century BCE) adamantly opposed
the idea of direct divine intervention in
the universe, proposing instead that natural
phenomena had a natural cause. Leucippus and
his student Democritus were the first to develop
the theory of atomism, the idea that everything
is composed entirely of various imperishable,
indivisible elements called atoms.
During the classical period in Greece (6th,
5th and 4th centuries BCE) and in Hellenistic
times, natural philosophy slowly developed
into an exciting and contentious field of
study. Aristotle (Greek: Ἀριστοτέλης,
Aristotélēs) (384 – 322 BCE), a student
of Plato, promoted the concept that observation
of physical phenomena could ultimately lead
to the discovery of the natural laws governing
them. Aristotle's writings cover physics,
metaphysics, poetry, theater, music, logic,
rhetoric, linguistics, politics, government,
ethics, biology and zoology. He wrote the
first work which refers to that line of study
as "Physics" – in the 4th century BCE, Aristotle
founded the system known as Aristotelian physics.
He attempted to explain ideas such as motion
(and gravity) with the theory of four elements.
Aristotle believed that all matter was made
up of aether, or some combination of four
elements: earth, water, air, and fire. According
to Aristotle, these four terrestrial elements
are capable of inter-transformation and move
toward their natural place, so a stone falls
downward toward the center of the cosmos,
but flames rise upward toward the circumference.
Eventually, Aristotelian physics became enormously
popular for many centuries in Europe, informing
the scientific and scholastic developments
of the Middle Ages. It remained the mainstream
scientific paradigm in Europe until the time
of Galileo Galilei and Isaac Newton.
Early in Classical Greece, knowledge that
the Earth is spherical ("round") was common.
Around 240 BCE, as the result a seminal experiment,
Eratosthenes (276–194 BCE) accurately estimated
its circumference. In contrast to Aristotle's
geocentric views, Aristarchus of Samos (Greek:
Ἀρίσταρχος; c.310 – c.230 BCE)
presented an explicit argument for a heliocentric
model of the Solar system, i.e. for placing
the Sun, not the Earth, at its centre. Seleucus
of Seleucia, a follower of Aristarchus' heliocentric
theory, stated that the Earth rotated around
its own axis, which, in turn, revolved around
the Sun. Though the arguments he used were
lost, Plutarch stated that Seleucus was the
first to prove the heliocentric system through
reasoning.
In the 3rd century BCE, the Greek mathematician
Archimedes of Syracuse (Greek: Ἀρχιμήδης
(287–212 BCE) – generally considered to
be the greatest mathematician of antiquity
and one of the greatest of all time – laid
the foundations of hydrostatics, statics and
calculated the underlying mathematics of the
lever. A leading scientist of classical antiquity,
Archimedes also developed elaborate systems
of pulleys to move large objects with a minimum
of effort. The Archimedes' screw underpins
modern hydroengineering, and his machines
of war helped to hold back the armies of Rome
in the First Punic War. Archimedes even tore
apart the arguments of Aristotle and his metaphysics,
pointing out that it was impossible to separate
mathematics and nature and proved it by converting
mathematical theories into practical inventions.
Furthermore, in his work On Floating Bodies,
around 250 BCE, Archimedes developed the law
of buoyancy, also known as Archimedes' principle.
In mathematics, Archimedes used the method
of exhaustion to calculate the area under
the arc of a parabola with the summation of
an infinite series, and gave a remarkably
accurate approximation of pi. He also defined
the spiral bearing his name, formulae for
the volumes of surfaces of revolution and
an ingenious system for expressing very large
numbers. He also developed the principles
of equilibrium states and centers of gravity,
ideas that would influence the well known
scholars, Galileo, and Newton.
Hipparchus (190–120 BCE), focusing on astronomy
and mathematics, used sophisticated geometrical
techniques to map the motion of the stars
and planets, even predicting the times that
Solar eclipses would happen. In addition,
he added calculations of the distance of the
Sun and Moon from the Earth, based upon his
improvements to the observational instruments
used at that time. Another of the most famous
of the early physicists was Ptolemy (90–168
CE), one of the leading minds during the time
of the Roman Empire. Ptolemy was the author
of several scientific treatises, at least
three of which were of continuing importance
to later Islamic and European science. The
first is the astronomical treatise now known
as the Almagest (in Greek, Ἡ Μεγάλη
Σύνταξις, "The Great Treatise", originally
Μαθηματικὴ Σύνταξις, "Mathematical
Treatise"). The second is the Geography, which
is a thorough discussion of the geographic
knowledge of the Greco-Roman world.
Much of the accumulated knowledge of the ancient
world was lost. Even of the works of the better
known thinkers, few fragments survived. Although
he wrote at least fourteen books, almost nothing
of Hipparchus' direct work survived. Of the
150 reputed Aristotelian works, only 30 exist,
and some of those are "little more than lecture
notes".
=== India and China ===
Important physical and mathematical traditions
also existed in ancient Chinese and Indian
sciences.
In Indian philosophy, Maharishi Kanada was
the first to systematically develop a theory
of atomism around 200 BCE though some authors
have allotted him an earlier era in the 6th
century BCE. It was further elaborated by
the Buddhist atomists Dharmakirti and Dignāga
during the 1st millennium CE. Pakudha Kaccayana,
a 6th-century BCE Indian philosopher and contemporary
of Gautama Buddha, had also propounded ideas
about the atomic constitution of the material
world. These philosophers believed that other
elements (except ether) were physically palpable
and hence comprised minuscule particles of
matter. The last minuscule particle of matter
that could not be subdivided further was termed
Parmanu. These philosophers considered the
atom to be indestructible and hence eternal.
The Buddhists thought atoms to be minute objects
unable to be seen to the naked eye that come
into being and vanish in an instant. The Vaisheshika
school of philosophers believed that an atom
was a mere point in space. It was also first
to depict relations between motion and force
applied. Indian theories about the atom are
greatly abstract and enmeshed in philosophy
as they were based on logic and not on personal
experience or experimentation. In Indian astronomy,
Aryabhata's Aryabhatiya (499 CE) proposed
the Earth's rotation, while Nilakantha Somayaji
(1444–1544) of the Kerala school of astronomy
and mathematics proposed a semi-heliocentric
model resembling the Tychonic system.
The study of magnetism in Ancient China dates
back to the 4th century BCE. (in the Book
of the Devil Valley Master), A main contributor
to this field was Shen Kuo (1031–1095),
a polymath and statesman who was the first
to describe the magnetic-needle compass used
for navigation, as well as establishing the
concept of true north. In optics, Shen Kuo
independently developed a camera obscura.
=== Islamic world ===
In the 7th to 15th centuries, scientific progress
occurred in the Muslim world. Many classic
works in Indian, Assyrian, Sassanian (Persian)
and Greek, including the works of Aristotle,
were translated into Arabic. Important contributions
were made by Ibn al-Haytham (965–1040),
an Arab scientist, considered to be a founder
of modern optics. Ptolemy and Aristotle theorised
that light either shone from the eye to illuminate
objects or that "forms" emanated from objects
themselves, whereas al-Haytham (known by the
Latin name "Alhazen") suggested that light
travels to the eye in rays from different
points on an object. The works of Ibn al-Haytham
and Abū Rayhān Bīrūnī (973–1050), a
Persian scientist, eventually passed on to
Western Europe where they were studied by
scholars such as Roger Bacon and Witelo.Ibn
al-Haytham and Biruni were early proponents
of the scientific method. Ibn al-Haytham is
considered to be the "father of the modern
scientific method" due to his emphasis on
experimental data and reproducibility of its
results. The earliest methodical approach
to experiments in the modern sense is visible
in the works of Ibn al-Haytham, who introduced
an inductive-experimental method for achieving
results. Bīrūnī introduced early scientific
methods for several different fields of inquiry
during the 1020s and 1030s, including an early
experimental method for mechanics. Biruni's
methodology resembled the modern scientific
method, particularly in his emphasis on repeated
experimentation.Ibn Sīnā (980–1037), known
as "Avicenna", was a polymath from Bukhara
(in present-day Uzbekistan) responsible for
important contributions to physics, optics,
philosophy and medicine. He published his
theory of motion in Book of Healing (1020),
where he argued that an impetus is imparted
to a projectile by the thrower, and believed
that it was a temporary virtue that would
decline even in a vacuum. He viewed it as
persistent, requiring external forces such
as air resistance to dissipate it. Ibn Sina
made a distinction between 'force' and 'inclination'
(called "mayl"), and argued that an object
gained mayl when the object is in opposition
to its natural motion. He concluded that continuation
of motion is attributed to the inclination
that is transferred to the object, and that
object will be in motion until the mayl is
spent. He also claimed that projectile in
a vacuum would not stop unless it is acted
upon. This conception of motion is consistent
with Newton's first law of motion, inertia,
which states that an object in motion will
stay in motion unless it is acted on by an
external force. This idea which dissented
from the Aristotelian view was later described
as "impetus" by John Buridan, who was influenced
by Ibn Sina's Book of Healing.
Omar Khayyám (1048–1131), a Persian scientist,
calculated the length of a solar year and
was only out by a fraction of a second when
compared to our modern day calculations. He
used this to compose a calendar considered
more accurate than the Gregorian calendar
that came along 500 years later. He is classified
as one of the world's first great science
communicators, said, for example to have convinced
a Sufi theologian that the world turns on
an axis.
Hibat Allah Abu'l-Barakat al-Baghdaadi (c.
1080-1165) adopted and modified Ibn Sina's
theory on projectile motion. In his Kitab
al-Mu'tabar, Abu'l-Barakat stated that the
mover imparts a violent inclination (mayl
qasri) on the moved and that this diminishes
as the moving object distances itself from
the mover. He also proposed an explanation
of the acceleration of falling bodies by the
accumulation of successive increments of power
with successive increments of velocity. According
to Shlomo Pines, al-Baghdaadi's theory of
motion was "the oldest negation of Aristotle's
fundamental dynamic law [namely, that a constant
force produces a uniform motion], [and is
thus an] anticipation in a vague fashion of
the fundamental law of classical mechanics
[namely, that a force applied continuously
produces acceleration]." Jean Buridan and
Albert of Saxony later referred to Abu'l-Barakat
in explaining that the acceleration of a falling
body is a result of its increasing impetus.Ibn
Bajjah (c. 1085–1138), known as "Avempace"
in Europe, proposed that for every force there
is always a reaction force. While he did not
specify that these forces be equal, it was
a precursor to Newton's third law of motion
which states that for every action there is
an equal and opposite reaction. Ibn Bajjah
was a critic of Ptolemy and he worked on creating
a new theory of velocity to replace the one
theorized by Aristotle. Two future philosophers
supported the theories Avempace created, known
as Avempacean dynamics. These philosophers
were Thomas Aquinas, a Catholic priest, and
John Duns Scotus. Galileo went on to adopt
Avempace's formula "that the velocity of a
given object is the difference of the motive
power of that object and the resistance of
the medium of motion".Nasir al-Din al-Tusi
(1201–1274), a Persian astronomer and mathematician
who died in Baghdad, authored the Treasury
of Astronomy, a remarkably accurate table
of planetary movements that reformed the existing
planetary model of Roman astronomer Ptolemy
by describing a uniform circular motion of
all planets in their orbits. This work led
to the later discovery, by one of his students,
that planets actually have an elliptical orbit.
Copernicus later drew heavily on the work
of al-Din al-Tusi and his students, but without
acknowledgment. The gradual chipping away
of the Ptolemaic system paved the way for
the revolutionary idea that the Earth actually
orbited the Sun (heliocentrism).
=== Medieval Europe ===
Awareness of ancient works re-entered the
West through translations from Arabic to Latin.
Their re-introduction, combined with Judeo-Islamic
theological commentaries, had a great influence
on Medieval philosophers such as Thomas Aquinas.
Scholastic European scholars, who sought to
reconcile the philosophy of the ancient classical
philosophers with Christian theology, proclaimed
Aristotle the greatest thinker of the ancient
world. In cases where they didn't directly
contradict the Bible, Aristotelian physics
became the foundation for the physical explanations
of the European Churches. Quantification became
a core element of medieval physics.Based on
Aristotelian physics, Scholastic physics described
things as moving according to their essential
nature. Celestial objects were described as
moving in circles, because perfect circular
motion was considered an innate property of
objects that existed in the uncorrupted realm
of the celestial spheres. The theory of impetus,
the ancestor to the concepts of inertia and
momentum, was developed along similar lines
by medieval philosophers such as John Philoponus
and Jean Buridan. Motions below the lunar
sphere were seen as imperfect, and thus could
not be expected to exhibit consistent motion.
More idealized motion in the "sublunary" realm
could only be achieved through artifice, and
prior to the 17th century, many did not view
artificial experiments as a valid means of
learning about the natural world. Physical
explanations in the sublunary realm revolved
around tendencies. Stones contained the element
earth, and earthly objects tended to move
in a straight line toward the centre of the
earth (and the universe in the Aristotelian
geocentric view) unless otherwise prevented
from doing so.
== Scientific revolution ==
During the 16th and 17th centuries, a large
advancement of scientific progress known as
the Scientific revolution took place in Europe.
Dissatisfaction with older philosophical approaches
had begun earlier and had produced other changes
in society, such as the Protestant Reformation,
but the revolution in science began when natural
philosophers began to mount a sustained attack
on the Scholastic philosophical programme
and supposed that mathematical descriptive
schemes adopted from such fields as mechanics
and astronomy could actually yield universally
valid characterizations of motion and other
concepts.
=== Nicolaus Copernicus ===
A breakthrough in astronomy was made by Polish
astronomer Nicolaus Copernicus (1473–1543)
when, in 1543, he gave strong arguments for
the heliocentric model of the Solar system,
ostensibly as a means to render tables charting
planetary motion more accurate and to simplify
their production. In heliocentric models of
the Solar system, the Earth orbits the Sun
along with other bodies in Earth's galaxy,
a contradiction according to the Greek-Egyptian
astronomer Ptolemy (2nd century CE; see above),
whose system placed the Earth at the center
of the Universe and had been accepted for
over 1,400 years. The Greek astronomer Aristarchus
of Samos (c.310 – c.230 BCE) had suggested
that the Earth revolves around the Sun, but
Copernicus' reasoning led to lasting general
acceptance of this "revolutionary" idea. Copernicus'
book presenting the theory (De revolutionibus
orbium coelestium, "On the Revolutions of
the Celestial Spheres") was published just
before his death in 1543 and, as it is now
generally considered to mark the beginning
of modern astronomy, is also considered to
mark the beginning of the Scientific revolution.
Copernicus' new perspective, along with the
accurate observations made by Tycho Brahe,
enabled German astronomer Johannes Kepler
(1571–1630) to formulate his laws regarding
planetary motion that remain in use today.
=== Galileo Galilei ===
The Italian mathematician, astronomer, and
physicist Galileo Galilei (1564–1642) was
the central figure in the Scientific revolution
and famous for his support for Copernicanism,
his astronomical discoveries, empirical experiments
and his improvement of the telescope. As a
mathematician, Galileo's role in the university
culture of his era was subordinated to the
three major topics of study: law, medicine,
and theology (which was closely allied to
philosophy). Galileo, however, felt that the
descriptive content of the technical disciplines
warranted philosophical interest, particularly
because mathematical analysis of astronomical
observations – notably, Copernicus' analysis
of the relative motions of the Sun, Earth,
Moon, and planets – indicated that philosophers'
statements about the nature of the universe
could be shown to be in error. Galileo also
performed mechanical experiments, insisting
that motion itself – regardless of whether
it was produced "naturally" or "artificially"
(i.e. deliberately) – had universally consistent
characteristics that could be described mathematically.
Galileo's early studies at the University
of Pisa were in medicine, but he was soon
drawn to mathematics and physics. At 19, he
discovered (and, subsequently, verified) the
isochronal nature of the pendulum when, using
his pulse, he timed the oscillations of a
swinging lamp in Pisa's cathedral and found
that it remained the same for each swing regardless
of the swing's amplitude. He soon became known
through his invention of a hydrostatic balance
and for his treatise on the center of gravity
of solid bodies. While teaching at the University
of Pisa (1589–92), he initiated his experiments
concerning the laws of bodies in motion that
brought results so contradictory to the accepted
teachings of Aristotle that strong antagonism
was aroused. He found that bodies do not fall
with velocities proportional to their weights.
The famous story in which Galileo is said
to have dropped weights from the Leaning Tower
of Pisa is apocryphal, but he did find that
the path of a projectile is a parabola and
is credited with conclusions that anticipated
Newton's laws of motion (e.g. the notion of
inertia). Among these is what is now called
Galilean relativity, the first precisely formulated
statement about properties of space and time
outside three-dimensional geometry.
Galileo has been called the "father of modern
observational astronomy", the "father of modern
physics", the "father of science", and "the
father of modern science". According to Stephen
Hawking, "Galileo, perhaps more than any other
single person, was responsible for the birth
of modern science." As religious orthodoxy
decreed a geocentric or Tychonic understanding
of the Solar system, Galileo's support for
heliocentrism provoked controversy and he
was tried by the Inquisition. Found "vehemently
suspect of heresy", he was forced to recant
and spent the rest of his life under house
arrest.
The contributions that Galileo made to observational
astronomy include the telescopic confirmation
of the phases of Venus; his discovery, in
1609, of Jupiter's four largest moons (subsequently
given the collective name of the "Galilean
moons"); and the observation and analysis
of sunspots. Galileo also pursued applied
science and technology, inventing, among other
instruments, a military compass. His discovery
of the Jovian moons was published in 1610
and enabled him to obtain the position of
mathematician and philosopher to the Medici
court. As such, he was expected to engage
in debates with philosophers in the Aristotelian
tradition and received a large audience for
his own publications such as the Discourses
and Mathematical Demonstrations Concerning
Two New Sciences (published abroad following
his arrest for the publication of Dialogue
Concerning the Two Chief World Systems) and
The Assayer. Galileo's interest in experimenting
with and formulating mathematical descriptions
of motion established experimentation as an
integral part of natural philosophy. This
tradition, combining with the non-mathematical
emphasis on the collection of "experimental
histories" by philosophical reformists such
as William Gilbert and Francis Bacon, drew
a significant following in the years leading
up to and following Galileo's death, including
Evangelista Torricelli and the participants
in the Accademia del Cimento in Italy; Marin
Mersenne and Blaise Pascal in France; Christiaan
Huygens in the Netherlands; and Robert Hooke
and Robert Boyle in England.
=== René Descartes ===
The French philosopher René Descartes (1596–1650)
was well-connected to, and influential within,
the experimental philosophy networks of the
day. Descartes had a more ambitious agenda,
however, which was geared toward replacing
the Scholastic philosophical tradition altogether.
Questioning the reality interpreted through
the senses, Descartes sought to re-establish
philosophical explanatory schemes by reducing
all perceived phenomena to being attributable
to the motion of an invisible sea of "corpuscles".
(Notably, he reserved human thought and God
from his scheme, holding these to be separate
from the physical universe). In proposing
this philosophical framework, Descartes supposed
that different kinds of motion, such as that
of planets versus that of terrestrial objects,
were not fundamentally different, but were
merely different manifestations of an endless
chain of corpuscular motions obeying universal
principles. Particularly influential were
his explanations for circular astronomical
motions in terms of the vortex motion of corpuscles
in space (Descartes argued, in accord with
the beliefs, if not the methods, of the Scholastics,
that a vacuum could not exist), and his explanation
of gravity in terms of corpuscles pushing
objects downward.Descartes, like Galileo,
was convinced of the importance of mathematical
explanation, and he and his followers were
key figures in the development of mathematics
and geometry in the 17th century. Cartesian
mathematical descriptions of motion held that
all mathematical formulations had to be justifiable
in terms of direct physical action, a position
held by Huygens and the German philosopher
Gottfried Leibniz, who, while following in
the Cartesian tradition, developed his own
philosophical alternative to Scholasticism,
which he outlined in his 1714 work, The Monadology.
Descartes has been dubbed the 'Father of Modern
Philosophy', and much subsequent Western philosophy
is a response to his writings, which are studied
closely to this day. In particular, his Meditations
on First Philosophy continues to be a standard
text at most university philosophy departments.
Descartes' influence in mathematics is equally
apparent; the Cartesian coordinate system
— allowing algebraic equations to be expressed
as geometric shapes in a two-dimensional coordinate
system — was named after him. He is credited
as the father of analytical geometry, the
bridge between algebra and geometry, important
to the discovery of calculus and analysis.
=== Isaac Newton ===
The late 17th and early 18th centuries saw
the achievements of the greatest figure of
the Scientific revolution: Cambridge University
physicist and mathematician Sir Isaac Newton
(1642-1727), considered by many to be the
greatest and most influential scientist who
ever lived. Newton, a fellow of the Royal
Society of England, combined his own discoveries
in mechanics and astronomy to earlier ones
to create a single system for describing the
workings of the universe. Newton formulated
three laws of motion which formulated the
relationship between motion and objects and
also the law of universal gravitation, the
latter of which could be used to explain the
behavior not only of falling bodies on the
earth but also planets and other celestial
bodies. To arrive at his results, Newton invented
one form of an entirely new branch of mathematics:
calculus (also invented independently by Gottfried
Leibniz), which was to become an essential
tool in much of the later development in most
branches of physics. Newton's findings were
set forth in his Philosophiæ Naturalis Principia
Mathematica ("Mathematical Principles of Natural
Philosophy"), the publication of which in
1687 marked the beginning of the modern period
of mechanics and astronomy.
Newton was able to refute the Cartesian mechanical
tradition that all motions should be explained
with respect to the immediate force exerted
by corpuscles. Using his three laws of motion
and law of universal gravitation, Newton removed
the idea that objects followed paths determined
by natural shapes and instead demonstrated
that not only regularly observed paths, but
all the future motions of any body could be
deduced mathematically based on knowledge
of their existing motion, their mass, and
the forces acting upon them. However, observed
celestial motions did not precisely conform
to a Newtonian treatment, and Newton, who
was also deeply interested in theology, imagined
that God intervened to ensure the continued
stability of the solar system.
Newton's principles (but not his mathematical
treatments) proved controversial with Continental
philosophers, who found his lack of metaphysical
explanation for movement and gravitation philosophically
unacceptable. Beginning around 1700, a bitter
rift opened between the Continental and British
philosophical traditions, which were stoked
by heated, ongoing, and viciously personal
disputes between the followers of Newton and
Leibniz concerning priority over the analytical
techniques of calculus, which each had developed
independently. Initially, the Cartesian and
Leibnizian traditions prevailed on the Continent
(leading to the dominance of the Leibnizian
calculus notation everywhere except Britain).
Newton himself remained privately disturbed
at the lack of a philosophical understanding
of gravitation while insisting in his writings
that none was necessary to infer its reality.
As the 18th century progressed, Continental
natural philosophers increasingly accepted
the Newtonians' willingness to forgo ontological
metaphysical explanations for mathematically
described motions.Newton built the first functioning
reflecting telescope and developed a theory
of color, published in Opticks, based on the
observation that a prism decomposes white
light into the many colours forming the visible
spectrum. While Newton explained light as
being composed of tiny particles, a rival
theory of light which explained its behavior
in terms of waves was presented in 1690 by
Christiaan Huygens. However, the belief in
the mechanistic philosophy coupled with Newton's
reputation meant that the wave theory saw
relatively little support until the 19th century.
Newton also formulated an empirical law of
cooling, studied the speed of sound, investigated
power series, demonstrated the generalised
binomial theorem and developed a method for
approximating the roots of a function. His
work on infinite series was inspired by Simon
Stevin's decimals. Most importantly, Newton
showed that the motions of objects on Earth
and of celestial bodies are governed by the
same set of natural laws, which were neither
capricious nor malevolent. By demonstrating
the consistency between Kepler's laws of planetary
motion and his own theory of gravitation,
Newton also removed the last doubts about
heliocentrism. By bringing together all the
ideas set forth during the Scientific revolution,
Newton effectively established the foundation
for modern society in mathematics and science.
=== Other achievements ===
Other branches of physics also received attention
during the period of the Scientific revolution.
William Gilbert, court physician to Queen
Elizabeth I, published an important work on
magnetism in 1600, describing how the earth
itself behaves like a giant magnet. Robert
Boyle (1627–91) studied the behavior of
gases enclosed in a chamber and formulated
the gas law named for him; he also contributed
to physiology and to the founding of modern
chemistry. Another important factor in the
scientific revolution was the rise of learned
societies and academies in various countries.
The earliest of these were in Italy and Germany
and were short-lived. More influential were
the Royal Society of England (1660) and the
Academy of Sciences in France (1666). The
former was a private institution in London
and included such scientists as John Wallis,
William Brouncker, Thomas Sydenham, John Mayow,
and Christopher Wren (who contributed not
only to architecture but also to astronomy
and anatomy); the latter, in Paris, was a
government institution and included as a foreign
member the Dutchman Huygens. In the 18th century,
important royal academies were established
at Berlin (1700) and at St. Petersburg (1724).
The societies and academies provided the principal
opportunities for the publication and discussion
of scientific results during and after the
scientific revolution. In 1690, James Bernoulli
showed that the cycloid is the solution to
the tautochrone problem; and the following
year, in 1691, Johann Bernoulli showed that
a chain freely suspended from two points will
form a catenary, the curve with the lowest
possible center of gravity available to any
chain hung between two fixed points. He then
showed, in 1696, that the cycloid is the solution
to the brachistochrone problem.
==== Early thermodynamics ====
A 
precursor of the engine was designed by the
German scientist Otto von Guericke who, in
1650, designed and built the world's first
vacuum pump and created the world's first
ever vacuum known as the Magdeburg hemispheres
experiment. He was driven to make a vacuum
to disprove Aristotle's long-held supposition
that 'Nature abhors a vacuum'. Shortly thereafter,
Irish physicist and chemist Boyle had learned
of Guericke's designs and in 1656, in coordination
with English scientist Robert Hooke, built
an air pump. Using this pump, Boyle and Hooke
noticed the pressure-volume correlation for
a gas: PV = k, where P is pressure, V is volume
and k is a constant: this relationship is
known as Boyle's Law. In that time, air was
assumed to be a system of motionless particles,
and not interpreted as a system of moving
molecules. The concept of thermal motion came
two centuries later. Therefore, Boyle's publication
in 1660 speaks about a mechanical concept:
the air spring. Later, after the invention
of the thermometer, the property temperature
could be quantified. This tool gave Gay-Lussac
the opportunity to derive his law, which led
shortly later to the ideal gas law. But, already
before the establishment of the ideal gas
law, an associate of Boyle's named Denis Papin
built in 1679 a bone digester, which is a
closed vessel with a tightly fitting lid that
confines steam until a high pressure is generated.
Later designs implemented a steam release
valve to keep the machine from exploding.
By watching the valve rhythmically move up
and down, Papin conceived of the idea of a
piston and cylinder engine. He did not however
follow through with his design. Nevertheless,
in 1697, based on Papin's designs, engineer
Thomas Savery built the first engine. Although
these early engines were crude and inefficient,
they attracted the attention of the leading
scientists of the time. Hence, prior to 1698
and the invention of the Savery Engine, horses
were used to power pulleys, attached to buckets,
which lifted water out of flooded salt mines
in England. In the years to follow, more variations
of steam engines were built, such as the Newcomen
Engine, and later the Watt Engine. In time,
these early engines would eventually be utilized
in place of horses. Thus, each engine began
to be associated with a certain amount of
"horse power" depending upon how many horses
it had replaced. The main problem with these
first engines was that they were slow and
clumsy, converting less than 2% of the input
fuel into useful work. In other words, large
quantities of coal (or wood) had to be burned
to yield only a small fraction of work output.
Hence the need for a new science of engine
dynamics was born.
== 18th-century developments ==
During the 18th century, the mechanics founded
by Newton was developed by several scientists
as more mathematicians learned calculus and
elaborated upon its initial formulation. The
application of mathematical analysis to problems
of motion was known as rational mechanics,
or mixed mathematics (and was later termed
classical mechanics).
=== Mechanics ===
In 1714, Brook Taylor derived the fundamental
frequency of a stretched vibrating string
in terms of its tension and mass per unit
length by solving a differential equation.
The Swiss mathematician Daniel Bernoulli (1700–1782)
made important mathematical studies of the
behavior of gases, anticipating the kinetic
theory of gases developed more than a century
later, and has been referred to as the first
mathematical physicist. In 1733, Daniel Bernoulli
derived the fundamental frequency and harmonics
of a hanging chain by solving a differential
equation. In 1734, Bernoulli solved the differential
equation for the vibrations of an elastic
bar clamped at one end. Bernoulli's treatment
of fluid dynamics and his examination of fluid
flow was introduced in his 1738 work Hydrodynamica.
Rational mechanics dealt primarily with the
development of elaborate mathematical treatments
of observed motions, using Newtonian principles
as a basis, and emphasized improving the tractability
of complex calculations and developing of
legitimate means of analytical approximation.
A representative contemporary textbook was
published by Johann Baptiste Horvath. By the
end of the century analytical treatments were
rigorous enough to verify the stability of
the solar system solely on the basis of Newton's
laws without reference to divine intervention—even
as deterministic treatments of systems as
simple as the three body problem in gravitation
remained intractable. In 1705, Edmond Halley
predicted the periodicity of Halley's Comet,
William Herschel discovered Uranus in 1781,
and Henry Cavendish measured the gravitational
constant and determined the mass of the Earth
in 1798. In 1783, John Michell suggested that
some objects might be so massive that not
even light could escape from them.
In 1739, Leonhard Euler solved the ordinary
differential equation for a forced harmonic
oscillator and noticed the resonance phenomenon.
In 1742, Colin Maclaurin discovered his uniformly
rotating self-gravitating spheroids. In 1742,
Benjamin Robins published his New Principles
in Gunnery, establishing the science of aerodynamics.
British work, carried on by mathematicians
such as Taylor and Maclaurin, fell behind
Continental developments as the century progressed.
Meanwhile, work flourished at scientific academies
on the Continent, led by such mathematicians
as Bernoulli, Euler, Lagrange, Laplace, and
Legendre. In 1743, Jean le Rond d'Alembert
published his Traite de Dynamique, in which
he introduced the concept of generalized forces
for accelerating systems and systems with
constraints, and applied the new idea of virtual
work to solve dynamical problem, now known
as D'Alembert's principle, as a rival to Newton's
second law of motion. In 1747, Pierre Louis
Maupertuis applied minimum principles to mechanics.
In 1759, Euler solved the partial differential
equation for the vibration of a rectangular
drum. In 1764, Euler examined the partial
differential equation for the vibration of
a circular drum and found one of the Bessel
function solutions. In 1776, John Smeaton
published a paper on experiments relating
power, work, momentum and kinetic energy,
and supporting the conservation of energy.
In 1788, Joseph Louis Lagrange presented Lagrange's
equations of motion in Mécanique Analytique,
in which the whole of mechanics was organized
around the principle of virtual work. In 1789,
Antoine Lavoisier states the law of conservation
of mass. The rational mechanics developed
in the 18th century received a brilliant exposition
in both Lagrange's 1788 work and the Celestial
Mechanics (1799–1825) of Pierre-Simon Laplace.
=== Thermodynamics ===
During the 18th century, thermodynamics was
developed through the theories of weightless
"imponderable fluids", such as heat ("caloric"),
electricity, and phlogiston (which was rapidly
overthrown as a concept following Lavoisier's
identification of oxygen gas late in the century).
Assuming that these concepts were real fluids,
their flow could be traced through a mechanical
apparatus or chemical reactions. This tradition
of experimentation led to the development
of new kinds of experimental apparatus, such
as the Leyden Jar; and new kinds of measuring
instruments, such as the calorimeter, and
improved versions of old ones, such as the
thermometer. Experiments also produced new
concepts, such as the University of Glasgow
experimenter Joseph Black's notion of latent
heat and Philadelphia intellectual Benjamin
Franklin's characterization of electrical
fluid as flowing between places of excess
and deficit (a concept later reinterpreted
in terms of positive and negative charges).
Franklin also showed that lightning is electricity
in 1752.
The accepted theory of heat in the 18th century
viewed it as a kind of fluid, called caloric;
although this theory was later shown to be
erroneous, a number of scientists adhering
to it nevertheless made important discoveries
useful in developing the modern theory, including
Joseph Black (1728–99) and Henry Cavendish
(1731–1810). Opposed to this caloric theory,
which had been developed mainly by the chemists,
was the less accepted theory dating from Newton's
time that heat is due to the motions of the
particles of a substance. This mechanical
theory gained support in 1798 from the cannon-boring
experiments of Count Rumford (Benjamin Thompson),
who found a direct relationship between heat
and mechanical energy.
While it was recognized early in the 18th
century that finding absolute theories of
electrostatic and magnetic force akin to Newton's
principles of motion would be an important
achievement, none were forthcoming. This impossibility
only slowly disappeared as experimental practice
became more widespread and more refined in
the early years of the 19th century in places
such as the newly established Royal Institution
in London. Meanwhile, the analytical methods
of rational mechanics began to be applied
to experimental phenomena, most influentially
with the French mathematician Joseph Fourier's
analytical treatment of the flow of heat,
as published in 1822. Joseph Priestley proposed
an electrical inverse-square law in 1767,
and Charles-Augustin de Coulomb introduced
the inverse-square law of electrostatics in
1798.
At the end of the century, the members of
the French Academy of Sciences had attained
clear dominance in the field. At the same
time, the experimental tradition established
by Galileo and his followers persisted. The
Royal Society and the French Academy of Sciences
were major centers for the performance and
reporting of experimental work. Experiments
in mechanics, optics, magnetism, static electricity,
chemistry, and physiology were not clearly
distinguished from each other during the 18th
century, but significant differences in explanatory
schemes and, thus, experiment design were
emerging. Chemical experimenters, for instance,
defied attempts to enforce a scheme of abstract
Newtonian forces onto chemical affiliations,
and instead focused on the isolation and classification
of chemical substances and reactions.
== 19th century ==
In 1800, Alessandro Volta invented the electric
battery (known of the voltaic pile) and thus
improved the way electric currents could also
be studied. A year later, Thomas Young demonstrated
the wave nature of light—which received
strong experimental support from the work
of Augustin-Jean Fresnel—and the principle
of interference. In 1813, Peter Ewart supported
the idea of the conservation of energy in
his paper On the measure of moving force.
In 1820, Hans Christian Ørsted found that
a current-carrying conductor gives rise to
a magnetic force surrounding it, and within
a week after Ørsted's discovery reached France,
André-Marie Ampère discovered that two parallel
electric currents will exert forces on each
other. In 1821, William Hamilton began his
analysis of Hamilton's characteristic function.
In 1821, Michael Faraday built an electricity-powered
motor, while Georg Ohm stated his law of electrical
resistance in 1826, expressing the relationship
between voltage, current, and resistance in
an electric circuit. A year later, botanist
Robert Brown discovered Brownian motion: pollen
grains in water undergoing movement resulting
from their bombardment by the fast-moving
atoms or molecules in the liquid. In 1829,
Gaspard Coriolis introduced the terms of work
(force times distance) and kinetic energy
with the meanings they have today.
In 1831, Faraday (and independently Joseph
Henry) discovered the reverse effect, the
production of an electric potential or current
through magnetism – known as electromagnetic
induction; these two discoveries are the basis
of the electric motor and the electric generator,
respectively. In 1834, Carl Jacobi discovered
his uniformly rotating self-gravitating ellipsoids
(the Jacobi ellipsoid). In 1834, John Russell
observed a nondecaying solitary water wave
(soliton) in the Union Canal near Edinburgh
and used a water tank to study the dependence
of solitary water wave velocities on wave
amplitude and water depth. In 1835, William
Hamilton stated Hamilton's canonical equations
of motion. In the same year, Gaspard Coriolis
examined theoretically the mechanical efficiency
of waterwheels, and deduced the Coriolis effect.
In 1841, Julius Robert von Mayer, an amateur
scientist, wrote a paper on the conservation
of energy but his lack of academic training
led to its rejection. In 1842, Christian Doppler
proposed the Doppler effect. In 1847, Hermann
von Helmholtz formally stated the law of conservation
of energy. In 1851, Léon Foucault showed
the Earth's rotation with a huge pendulum
(Foucault pendulum).
There were important advances in continuum
mechanics in the first half of the century,
namely formulation of laws of elasticity for
solids and discovery of Navier–Stokes equations
for fluids.
=== Laws of thermodynamics ===
In the 19th century, the connection between
heat and mechanical energy was established
quantitatively by Julius Robert von Mayer
and James Prescott Joule, who measured the
mechanical equivalent of heat in the 1840s.
In 1849, Joule published results from his
series of experiments (including the paddlewheel
experiment) which show that heat is a form
of energy, a fact that was accepted in the
1850s. The relation between heat and energy
was important for the development of steam
engines, and in 1824 the experimental and
theoretical work of Sadi Carnot was published.
Carnot captured some of the ideas of thermodynamics
in his discussion of the efficiency of an
idealized engine. Sadi Carnot's work provided
a basis for the formulation of the first law
of thermodynamics—a restatement of the law
of conservation of energy—which was stated
around 1850 by William Thomson, later known
as Lord Kelvin, and Rudolf Clausius. Lord
Kelvin, who had extended the concept of absolute
zero from gases to all substances in 1848,
drew upon the engineering theory of Lazare
Carnot, Sadi Carnot, and Émile Clapeyron–as
well as the experimentation of James Prescott
Joule on the interchangeability of mechanical,
chemical, thermal, and electrical forms of
work—to formulate the first law.
Kelvin and Clausius also stated the second
law of thermodynamics, which was originally
formulated in terms of the fact that heat
does not spontaneously flow from a colder
body to a hotter. Other formulations followed
quickly (for example, the second law was expounded
in Thomson and Peter Guthrie Tait's influential
work Treatise on Natural Philosophy) and Kelvin
in particular understood some of the law's
general implications. The second Law was the
idea that gases consist of molecules in motion
had been discussed in some detail by Daniel
Bernoulli in 1738, but had fallen out of favor,
and was revived by Clausius in 1857. In 1850,
Hippolyte Fizeau and Léon Foucault measured
the speed of light in water and find that
it is slower than in air, in support of the
wave model of light. In 1852, Joule and Thomson
demonstrated that a rapidly expanding gas
cools, later named the Joule–Thomson effect
or Joule–Kelvin effect. Hermann von Helmholtz
puts forward the idea of the heat death of
the universe in 1854, the same year that Clausius
established the importance of dQ/T (Clausius's
theorem) (though he did not yet name the quantity).
=== Statistical mechanics (a fundamentally
new approach to science) ===
In 1859, James Clerk Maxwell discovered the
distribution law of molecular velocities.
Maxwell showed that electric and magnetic
fields are propagated outward from their source
at a speed equal to that of light and that
light is one of several kinds of electromagnetic
radiation, differing only in frequency and
wavelength from the others. In 1859, Maxwell
worked out the mathematics of the distribution
of velocities of the molecules of a gas. The
wave theory of light was widely accepted by
the time of Maxwell's work on the electromagnetic
field, and afterward the study of light and
that of electricity and magnetism were closely
related. In 1864 James Maxwell published his
papers on a dynamical theory of the electromagnetic
field, and stated that light is an electromagnetic
phenomenon in the 1873 publication of Maxwell's
Treatise on Electricity and Magnetism. This
work drew upon theoretical work by German
theoreticians such as Carl Friedrich Gauss
and Wilhelm Weber. The encapsulation of heat
in particulate motion, and the addition of
electromagnetic forces to Newtonian dynamics
established an enormously robust theoretical
underpinning to physical observations.
The prediction that light represented a transmission
of energy in wave form through a "luminiferous
ether", and the seeming confirmation of that
prediction with Helmholtz student Heinrich
Hertz's 1888 detection of electromagnetic
radiation, was a major triumph for physical
theory and raised the possibility that even
more fundamental theories based on the field
could soon be developed. Experimental confirmation
of Maxwell's theory was provided by Hertz,
who generated and detected electric waves
in 1886 and verified their properties, at
the same time foreshadowing their application
in radio, television, and other devices. In
1887, Heinrich Hertz discovered the photoelectric
effect. Research on the electromagnetic waves
began soon after, with many scientists and
inventors conducting experiments on their
properties. In the mid to late 1890s Guglielmo
Marconi developed a radio wave based wireless
telegraphy system (see invention of radio).
The atomic theory of matter had been proposed
again in the early 19th century by the chemist
John Dalton and became one of the hypotheses
of the kinetic-molecular theory of gases developed
by Clausius and James Clerk Maxwell to explain
the laws of thermodynamics.
The kinetic theory in turn led to a revolutionary
approach to science, the statistical mechanics
of Ludwig Boltzmann (1844–1906) and Josiah
Willard Gibbs (1839–1903), which studies
the statistics of microstates of a system
and uses statistics to determine the state
of a physical system. Interrelating the statistical
likelihood of certain states of organization
of these particles with the energy of those
states, Clausius reinterpreted the dissipation
of energy to be the statistical tendency of
molecular configurations to pass toward increasingly
likely, increasingly disorganized states (coining
the term "entropy" to describe the disorganization
of a state). The statistical versus absolute
interpretations of the second law of thermodynamics
set up a dispute that would last for several
decades (producing arguments such as "Maxwell's
demon"), and that would not be held to be
definitively resolved until the behavior of
atoms was firmly established in the early
20th century. In 1902, James Jeans found the
length scale required for gravitational perturbations
to grow in a static nearly homogeneous medium.
== 20th century: birth of modern physics ==
At the end of the 19th century, physics had
evolved to the point at which classical mechanics
could cope with highly complex problems involving
macroscopic situations; thermodynamics and
kinetic theory were well established; geometrical
and physical optics could be understood in
terms of electromagnetic waves; and the conservation
laws for energy and momentum (and mass) were
widely accepted. So profound were these and
other developments that it was generally accepted
that all the important laws of physics had
been discovered and that, henceforth, research
would be concerned with clearing up minor
problems and particularly with improvements
of method and measurement. However, around
1900 serious doubts arose about the completeness
of the classical theories—the triumph of
Maxwell's theories, for example, was undermined
by inadequacies that had already begun to
appear—and their inability to explain certain
physical phenomena, such as the energy distribution
in blackbody radiation and the photoelectric
effect, while some of the theoretical formulations
led to paradoxes when pushed to the limit.
Prominent physicists such as Hendrik Lorentz,
Emil Cohn, Ernst Wiechert and Wilhelm Wien
believed that some modification of Maxwell's
equations might provide the basis for all
physical laws. These shortcomings of classical
physics were never to be resolved and new
ideas were required. At the beginning of the
20th century a major revolution shook the
world of physics, which led to a new era,
generally referred to as modern physics.
=== Radiation experiments ===
In the 19th century, experimenters began to
detect unexpected forms of radiation: Wilhelm
Röntgen caused a sensation with his discovery
of X-rays in 1895; in 1896 Henri Becquerel
discovered that certain kinds of matter emit
radiation on their own accord. In 1897, J.
J. Thomson discovered the electron, and new
radioactive elements found by Marie and Pierre
Curie raised questions about the supposedly
indestructible atom and the nature of matter.
Marie and Pierre coined the term "radioactivity"
to describe this property of matter, and isolated
the radioactive elements radium and polonium.
Ernest Rutherford and Frederick Soddy identified
two of Becquerel's forms of radiation with
electrons and the element helium. Rutherford
identified and named two types of radioactivity
and in 1911 interpreted experimental evidence
as showing that the atom consists of a dense,
positively charged nucleus surrounded by negatively
charged electrons. Classical theory, however,
predicted that this structure should be unstable.
Classical theory had also failed to explain
successfully two other experimental results
that appeared in the late 19th century. One
of these was the demonstration by Albert A.
Michelson and Edward W. Morley—known as
the Michelson–Morley experiment—which
showed there did not seem to be a preferred
frame of reference, at rest with respect to
the hypothetical luminiferous ether, for describing
electromagnetic phenomena. Studies of radiation
and radioactive decay continued to be a preeminent
focus for physical and chemical research through
the 1930s, when the discovery of nuclear fission
opened the way to the practical exploitation
of what came to be called "atomic" energy.
=== Albert Einstein's theory of relativity
===
In 1905 a young, 26-year-old German physicist
(then a Bern patent clerk) named Albert Einstein
(1879–1955), showed how measurements of
time and space are affected by motion between
an observer and what is being observed. To
say that Einstein's radical theory of relativity
revolutionized science is no exaggeration.
Although Einstein made many other important
contributions to science, the theory of relativity
alone represents one of the greatest intellectual
achievements of all time. Although the concept
of relativity was not introduced by Einstein,
his major contribution was the recognition
that the speed of light in a vacuum is constant,
i.e. the same for all observers, and an absolute
physical boundary for motion. This does not
impact a person's day-to-day life since most
objects travel at speeds much slower than
light speed. For objects travelling near light
speed, however, the theory of relativity shows
that clocks associated with those objects
will run more slowly and that the objects
shorten in length according to measurements
of an observer on Earth. Einstein also derived
the famous equation, E = mc2, which expresses
the equivalence of mass and energy.
==== Special relativity ====
Einstein argued that the speed of light was
a constant in all inertial reference frames
and that electromagnetic laws should remain
valid independent of reference frame—assertions
which rendered the ether "superfluous" to
physical theory, and that held that observations
of time and length varied relative to how
the observer was moving with respect to the
object being measured (what came to be called
the "special theory of relativity"). It also
followed that mass and energy were interchangeable
quantities according to the equation E=mc2.
In another paper published the same year,
Einstein asserted that electromagnetic radiation
was transmitted in discrete quantities ("quanta"),
according to a constant that the theoretical
physicist Max Planck had posited in 1900 to
arrive at an accurate theory for the distribution
of blackbody radiation—an assumption that
explained the strange properties of the photoelectric
effect.
The special theory of relativity is a formulation
of the relationship between physical observations
and the concepts of space and time. The theory
arose out of contradictions between electromagnetism
and Newtonian mechanics and had great impact
on both those areas. The original historical
issue was whether it was meaningful to discuss
the electromagnetic wave-carrying "ether"
and motion relative to it and also whether
one could detect such motion, as was unsuccessfully
attempted in the Michelson–Morley experiment.
Einstein demolished these questions and the
ether concept in his special theory of relativity.
However, his basic formulation does not involve
detailed electromagnetic theory. It arises
out of the question: "What is time?" Newton,
in the Principia (1686), had given an unambiguous
answer: "Absolute, true, and mathematical
time, of itself, and from its own nature,
flows equably without relation to anything
external, and by another name is called duration."
This definition is basic to all classical
physics.
Einstein had the genius to question it, and
found that it was incomplete. Instead, each
"observer" necessarily makes use of his or
her own scale of time, and for two observers
in relative motion, their time-scales will
differ. This induces a related effect on position
measurements. Space and time become intertwined
concepts, fundamentally dependent on the observer.
Each observer presides over his or her own
space-time framework or coordinate system.
There being no absolute frame of reference,
all observers of given events make different
but equally valid (and reconcilable) measurements.
What remains absolute is stated in Einstein's
relativity postulate: "The basic laws of physics
are identical for two observers who have a
constant relative velocity with respect to
each other."
Special Relativity had a profound effect on
physics: started as a rethinking of the theory
of electromagnetism, it found a new symmetry
law of nature, now called Poincaré symmetry,
that replaced the old Galilean (see above)
symmetry.
Special Relativity exerted another long-lasting
effect on dynamics. Although initially it
was credited with the "unification of mass
and energy", it became evident that relativistic
dynamics established a firm distinction between
rest mass, which is an invariant (observer
independent) property of a particle or system
of particles, and the energy and momentum
of a system. The latter two are separately
conserved in all situations but not invariant
with respect to different observers. The term
mass in particle physics underwent a semantic
change, and since the late 20th century it
almost exclusively denotes the rest (or invariant)
mass. See mass in special relativity for additional
discussion.
==== General relativity ====
By 1916, Einstein was able to generalize this
further, to deal with all states of motion
including non-uniform acceleration, which
became the general theory of relativity. In
this theory Einstein also specified a new
concept, the curvature of space-time, which
described the gravitational effect at every
point in space. In fact, the curvature of
space-time completely replaced Newton's universal
law of gravitation. According to Einstein,
gravitational force in the normal sense is
a kind of illusion caused by the geometry
of space. The presence of a mass causes a
curvature of space-time in the vicinity of
the mass, and this curvature dictates the
space-time path that all freely-moving objects
must follow. It was also predicted from this
theory that light should be subject to gravity
- all of which was verified experimentally.
This aspect of relativity explained the phenomena
of light bending around the sun, predicted
black holes as well as properties of the Cosmic
microwave background radiation — a discovery
rendering fundamental anomalies in the classic
Steady-State hypothesis. For his work on relativity,
the photoelectric effect and blackbody radiation,
Einstein received the Nobel Prize in 1921.
The gradual acceptance of Einstein's theories
of relativity and the quantized nature of
light transmission, and of Niels Bohr's model
of the atom created as many problems as they
solved, leading to a full-scale effort to
reestablish physics on new fundamental principles.
Expanding relativity to cases of accelerating
reference frames (the "general theory of relativity")
in the 1910s, Einstein posited an equivalence
between the inertial force of acceleration
and the force of gravity, leading to the conclusion
that space is curved and finite in size, and
the prediction of such phenomena as gravitational
lensing and the distortion of time in gravitational
fields.
=== Quantum mechanics ===
Although relativity resolved the electromagnetic
phenomena conflict demonstrated by Michelson
and Morley, a second theoretical problem was
the explanation of the distribution of electromagnetic
radiation emitted by a black body; experiment
showed that at shorter wavelengths, toward
the ultraviolet end of the spectrum, the energy
approached zero, but classical theory predicted
it should become infinite. This glaring discrepancy,
known as the ultraviolet catastrophe, was
solved by the new theory of quantum mechanics.
Quantum mechanics is the theory of atoms and
subatomic systems. Approximately the first
30 years of the 20th century represent the
time of the conception and evolution of the
theory. The basic ideas of quantum theory
were introduced in 1900 by Max Planck (1858–1947),
who was awarded the Nobel Prize for Physics
in 1918 for his discovery of the quantified
nature of energy. The quantum theory (which
previously relied in the "correspondence"
at large scales between the quantized world
of the atom and the continuities of the "classical"
world) was accepted when the Compton Effect
established that light carries momentum and
can scatter off particles, and when Louis
de Broglie asserted that matter can be seen
as behaving as a wave in much the same way
as electromagnetic waves behave like particles
(wave–particle duality).
In 1905, Einstein used the quantum theory
to explain the photoelectric effect, and in
1913 the Danish physicist Niels Bohr used
the same constant to explain the stability
of Rutherford's atom as well as the frequencies
of light emitted by hydrogen gas. The quantized
theory of the atom gave way to a full-scale
quantum mechanics in the 1920s. New principles
of a "quantum" rather than a "classical" mechanics,
formulated in matrix-form by Werner Heisenberg,
Max Born, and Pascual Jordan in 1925, were
based on the probabilistic relationship between
discrete "states" and denied the possibility
of causality. Quantum mechanics was extensively
developed by Heisenberg, Wolfgang Pauli, Paul
Dirac, and Erwin Schrödinger, who established
an equivalent theory based on waves in 1926;
but Heisenberg's 1927 "uncertainty principle"
(indicating the impossibility of precisely
and simultaneously measuring position and
momentum) and the "Copenhagen interpretation"
of quantum mechanics (named after Bohr's home
city) continued to deny the possibility of
fundamental causality, though opponents such
as Einstein would metaphorically assert that
"God does not play dice with the universe".
The new quantum mechanics became an indispensable
tool in the investigation and explanation
of phenomena at the atomic level. Also in
the 1920s, the Indian scientist Satyendra
Nath Bose's work on photons and quantum mechanics
provided the foundation for Bose–Einstein
statistics, the theory of the Bose–Einstein
condensate.
The spin–statistics theorem established
that any particle in quantum mechanics may
be either a boson (statistically Bose–Einstein)
or a fermion (statistically Fermi–Dirac).
It was later found that all fundamental bosons
transmit forces, such as the photon that transmits
electromagnetism.
Fermions are particles "like electrons and
nucleons" and are the usual constituents of
matter. Fermi–Dirac statistics later found
numerous other uses, from astrophysics (see
Degenerate matter) to semiconductor design.
== Contemporary and particle physics ==
=== Quantum field theory ===
As the philosophically inclined continued
to debate the fundamental nature of the universe,
quantum theories continued to be produced,
beginning with Paul Dirac's formulation of
a relativistic quantum theory in 1928. However,
attempts to quantize electromagnetic theory
entirely were stymied throughout the 1930s
by theoretical formulations yielding infinite
energies. This situation was not considered
adequately resolved until after World War
II ended, when Julian Schwinger, Richard Feynman
and Sin-Itiro Tomonaga independently posited
the technique of renormalization, which allowed
for an establishment of a robust quantum electrodynamics
(QED).Meanwhile, new theories of fundamental
particles proliferated with the rise of the
idea of the quantization of fields through
"exchange forces" regulated by an exchange
of short-lived "virtual" particles, which
were allowed to exist according to the laws
governing the uncertainties inherent in the
quantum world. Notably, Hideki Yukawa proposed
that the positive charges of the nucleus were
kept together courtesy of a powerful but short-range
force mediated by a particle with a mass between
that of the electron and proton. This particle,
the "pion", was identified in 1947 as part
of what became a slew of particles discovered
after World War II. Initially, such particles
were found as ionizing radiation left by cosmic
rays, but increasingly came to be produced
in newer and more powerful particle accelerators.Outside
particle physics, significant advances of
the time were:
the invention of the laser (1964 Nobel Prize
in Physics);
the theoretical and experimental research
of superconductivity, especially the invention
of a quantum theory of superconductivity by
Vitaly Ginzburg and Lev Landau (1962 Nobel
Prize in Physics) and, later, its explanation
via Cooper pairs (1972 Nobel Prize in Physics).
The Cooper pair was an early example of quasiparticles.
=== Unified field theories ===
Einstein deemed that all fundamental interactions
in nature can be explained in a single theory.
Unified field theories were numerous attempts
to "merge" several interactions. One of formulations
of such theories (as well as field theories
in general) is a gauge theory, a generalization
of the idea of symmetry. Eventually the Standard
Model (see below) succeeded in unification
of strong, weak, and electromagnetic interactions.
All attempts to unify gravitation with something
else failed.
=== Standard Model ===
The interaction of these particles by scattering
and decay provided a key to new fundamental
quantum theories. Murray Gell-Mann and Yuval
Ne'eman brought some order to these new particles
by classifying them according to certain qualities,
beginning with what Gell-Mann referred to
as the "Eightfold Way". While its further
development, the quark model, at first seemed
inadequate to describe strong nuclear forces,
allowing the temporary rise of competing theories
such as the S-Matrix, the establishment of
quantum chromodynamics in the 1970s finalized
a set of fundamental and exchange particles,
which allowed for the establishment of a "standard
model" based on the mathematics of gauge invariance,
which successfully described all forces except
for gravitation, and which remains generally
accepted within its domain of application.The
Standard Model groups the electroweak interaction
theory and quantum chromodynamics into a structure
denoted by the gauge group SU(3)×SU(2)×U(1).
The formulation of the unification of the
electromagnetic and weak interactions in the
standard model is due to Abdus Salam, Steven
Weinberg and, subsequently, Sheldon Glashow.
Electroweak theory was later confirmed experimentally
(by observation of neutral weak currents),
and distinguished by the 1979 Nobel Prize
in Physics.Since the 1970s, fundamental particle
physics has provided insights into early universe
cosmology, particularly the Big Bang theory
proposed as a consequence of Einstein's general
theory of relativity. However, starting in
the 1990s, astronomical observations have
also provided new challenges, such as the
need for new explanations of galactic stability
("dark matter") and the apparent acceleration
in the expansion of the universe ("dark energy").
While accelerators have confirmed most aspects
of the Standard Model by detecting expected
particle interactions at various collision
energies, no theory reconciling general relativity
with the Standard Model has yet been found,
although supersymmetry and string theory were
believed by many theorists to be a promising
avenue forward. The Large Hadron Collider,
however, which began operating in 2008, has
failed to find any evidence whatsoever that
is supportive of supersymmetry and string
theory.
=== Cosmology ===
Cosmology may be said to have become a serious
research question with the publication of
Einstein's General Theory of Relativity in
1915 although it did not enter the scientific
mainstream until the period known as the "Golden
age of general relativity".
About a decade later, in the midst of what
was dubbed the "Great Debate", Hubble and
Slipher discovered the expansion of universe
in the 1920s measuring the redshifts of Doppler
spectra from galactic nebulae. Using Einstein's
general relativity, Lemaître and Gamow formulated
what would become known as the big bang theory.
A rival, called the steady state theory was
devised by Hoyle, Gold, Narlikar and Bondi.
Cosmic background radiation was verified in
the 1960s by Penzias and Wilson, and this
discovery favoured the big bang at the expense
of the steady state scenario. Later work was
by Smoot et al. (1989), among other contributors,
using data from the Cosmic Background explorer
(CoBE) and the Wilkinson Microwave Anisotropy
Probe (WMAP) satellites that refined these
observations. The 1980s (the same decade of
the COBE measurements) also saw the proposal
of inflation theory by Guth.
Recently the problems of dark matter and dark
energy have risen to the top of the cosmology
agenda.
=== Higgs boson ===
On July 4, 2012, physicists working at CERN's
Large Hadron Collider announced that they
had discovered a new subatomic particle greatly
resembling the Higgs boson, a potential key
to an understanding of why elementary particles
have mass and indeed to the existence of diversity
and life in the universe. For now, some physicists
are calling it a "Higgslike" particle. Joe
Incandela, of the University of California,
Santa Barbara, said, "It's something that
may, in the end, be one of the biggest observations
of any new phenomena in our field in the last
30 or 40 years, going way back to the discovery
of quarks, for example." Michael Turner, a
cosmologist at the University of Chicago and
the chairman of the physics center board,
said:
This is a big moment for particle physics
and a crossroads — will this be the high
water mark or will it be the first of many
discoveries that point us toward solving the
really big questions that we have posed?
Peter Higgs was one of six physicists, working
in three independent groups, who, in 1964,
invented the notion of the Higgs field ("cosmic
molasses"). The others were Tom Kibble of
Imperial College, London; Carl Hagen of the
University of Rochester; Gerald Guralnik of
Brown University; and François Englert and
Robert Brout, both of Université libre de
Bruxelles.Although they have never been seen,
Higgslike fields play an important role in
theories of the universe and in string theory.
Under certain conditions, according to the
strange accounting of Einsteinian physics,
they can become suffused with energy that
exerts an antigravitational force. Such fields
have been proposed as the source of an enormous
burst of expansion, known as inflation, early
in the universe and, possibly, as the secret
of the dark energy that now seems to be speeding
up the expansion of the universe.
== Physical sciences ==
With increased accessibility to and elaboration
upon advanced analytical techniques in the
19th century, physics was defined as much,
if not more, by those techniques than by the
search for universal principles of motion
and energy, and the fundamental nature of
matter. Fields such as acoustics, geophysics,
astrophysics, aerodynamics, plasma physics,
low-temperature physics, and solid-state physics
joined optics, fluid dynamics, electromagnetism,
and mechanics as areas of physical research.
In the 20th century, physics also became closely
allied with such fields as electrical, aerospace
and materials engineering, and physicists
began to work in government and industrial
laboratories as much as in academic settings.
Following World War II, the population of
physicists increased dramatically, and came
to be centered on the United States, while,
in more recent decades, physics has become
a more international pursuit than at any time
in its previous history.
== Seminal physics publications ==
== See also ==
== Notes
