Physics (from Ancient Greek: φυσική
(ἐπιστήμη), translit.
physikḗ (epistḗmē), lit.
'knowledge of nature', from φύσις phýsis
"nature") is the natural science that studies
matter and its motion and behavior through
space and time and that studies the related
entities of energy and force.
Physics is one of the most fundamental scientific
disciplines, and its main goal is to understand
how the universe behaves.Physics is one of
the oldest academic disciplines and, through
its inclusion of astronomy, perhaps the oldest.
Over the last two millennia, physics, chemistry,
biology, and certain branches of mathematics
were a part of natural philosophy, but during
the scientific revolution in the 17th century,
these natural sciences emerged as unique research
endeavors in their own right.
Physics intersects with many interdisciplinary
areas of research, such as biophysics and
quantum chemistry, and the boundaries of physics
are not rigidly defined.
New ideas in physics often explain the fundamental
mechanisms studied by other sciences and suggest
new avenues of research in academic disciplines
such as mathematics and philosophy.
Advances in physics often enable advances
in new technologies.
For example, advances in the understanding
of electromagnetism and nuclear physics led
directly to the development of new products
that have dramatically transformed modern-day
society, such as television, computers, domestic
appliances, and nuclear weapons; advances
in thermodynamics led to the development of
industrialization; and advances in mechanics
inspired the development of calculus.
== History ==
=== 
Ancient astronomy ===
Astronomy is one of the oldest natural sciences.
Early civilizations dating back to beyond
3000 BCE, such as the Sumerians, ancient Egyptians,
and the Indus Valley Civilization, had a predictive
knowledge and a basic understanding of the
motions of the Sun, Moon, and stars.
The stars and planets were often worshipped,
believed to represent gods.
While the explanations for the observed positions
of the stars were often unscientific and lacking
in evidence, these early observations laid
the foundation for later astronomy, as the
stars were found to traverse great circles
across the sky, which however did not explain
the positions of the planets.
According to Asger Aaboe, the origins of Western
astronomy can be found in Mesopotamia, and
all Western efforts in the exact sciences
are descended from late Babylonian astronomy.
Egyptian astronomers left monuments showing
knowledge of the constellations and the motions
of the celestial bodies, while Greek poet
Homer wrote of various celestial objects in
his Iliad and Odyssey; later Greek astronomers
provided names, which are still used today,
for most constellations visible from the northern
hemisphere.
=== Natural philosophy ===
Natural philosophy has its origins in Greece
during the Archaic period, (650 BCE – 480
BCE), when pre-Socratic philosophers like
Thales rejected non-naturalistic explanations
for natural phenomena and proclaimed that
every event had a natural cause.
They proposed ideas verified by reason and
observation, and many of their hypotheses
proved successful in experiment; for example,
atomism was found to be correct approximately
2000 years after it was proposed by Leucippus
and his pupil Democritus.
=== Physics in the medieval European and Islamic
world ===
The Western Roman Empire fell in the fifth
century, and this resulted in a decline in
intellectual pursuits in the western part
of Europe.
By contrast, the Eastern Roman Empire (also
known as the Byzantine Empire) resisted the
attacks from the barbarians, and continued
to advance various fields of learning, including
physics.In the sixth century Isidore of Miletus
created an important compilation of Archimedes'
works that are copied in the Archimedes Palimpsest.
In sixth century Europe John Philoponus, a
Byzantine scholar, questioned Aristotle's
teaching of physics and noting its flaws.
He introduced the theory of impetus.
Aristotle's physics was not scrutinized until
John Philoponus appeared, and unlike Aristotle
who based his physics on verbal argument,
Philoponus relied on observation.
On Aristotle's physics John Philoponus wrote:
“But this is completely erroneous, and our
view may be corroborated by actual observation
more effectively than by any sort of verbal
argument.
For if you let fall from the same height two
weights of which one is many times as heavy
as the other, you will see that the ratio
of the times required for the motion does
not depend on the ratio of the weights, but
that the difference in time is a very small
one.
And so, if the difference in the weights is
not considerable, that is, of one is, let
us say, double the other, there will be no
difference, or else an imperceptible difference,
in time, though the difference in weight is
by no means negligible, with one body weighing
twice as much as the other”John Philoponus'
criticism of Aristotelian principles of physics
served as an inspiration for Galileo Galilei
ten centuries later, during the Scientific
Revolution.
Galileo cited Philoponus substantially in
his works when arguing that Aristotelian physics
was flawed.
In the 1300s Jean Buridan, a teacher in the
faculty of arts at the University of Paris,
developed the concept of impetus.
It was a step toward the modern ideas of inertia
and momentum.Islamic scholarship inherited
Aristotelian physics from the Greeks and during
the Islamic Golden Age developed it further,
especially placing emphasis on observation
and a priori reasoning, developing early forms
of the scientific method.
The most notable innovations were in the field
of optics and vision, which came from the
works of many scientists like Ibn Sahl, Al-Kindi,
Ibn al-Haytham, Al-Farisi and Avicenna.
The most notable work was The Book of Optics
(also known as Kitāb al-Manāẓir), written
by Ibn al-Haytham, in which he conclusively
disproved the ancient Greek idea about vision,
but also came up with a new theory.
In the book, he presented a study of the phenomenon
of the camera obscura (his thousand-year-old
version of the pinhole camera) and delved
further into the way the eye itself works.
Using dissections and the knowledge of previous
scholars, he was able to begin to explain
how light enters the eye.
He asserted that the light ray is focused,
but the actual explanation of how light projected
to the back of the eye had to wait until 1604.
His Treatise on Light explained the camera
obscura, hundreds of years before the modern
development of photography.
The seven-volume Book of Optics (Kitab al-Manathir)
hugely influenced thinking across disciplines
from the theory of visual perception to the
nature of perspective in medieval art, in
both the East and the West, for more than
600 years.
Many later European scholars and fellow polymaths,
from Robert Grosseteste and Leonardo da Vinci
to René Descartes, Johannes Kepler and Isaac
Newton, were in his debt.
Indeed, the influence of Ibn al-Haytham's
Optics ranks alongside that of Newton's work
of the same title, published 700 years later.
The translation of The Book of Optics had
a huge impact on Europe.
From it, later European scholars were able
to build devices that replicated those Ibn
al-Haytham had built, and understand the way
light works.
From this, such important things as eyeglasses,
magnifying glasses, telescopes, and cameras
were developed.
=== Classical physics ===
Physics became a separate science when early
modern Europeans used experimental and quantitative
methods to discover what are now considered
to be the laws of physics.Major developments
in this period include the replacement of
the geocentric model of the solar system with
the heliocentric Copernican model, the laws
governing the motion of planetary bodies determined
by Johannes Kepler between 1609 and 1619,
pioneering work on telescopes and observational
astronomy by Galileo Galilei in the 16th and
17th Centuries, and Isaac Newton's discovery
and unification of the laws of motion and
universal gravitation that would come to bear
his name.
Newton also developed calculus, the mathematical
study of change, which provided new mathematical
methods for solving physical problems.The
discovery of new laws in thermodynamics, chemistry,
and electromagnetics resulted from greater
research efforts during the Industrial Revolution
as energy needs increased.
The laws comprising classical physics remain
very widely used for objects on everyday scales
travelling at non-relativistic speeds, since
they provide a very close approximation in
such situations, and theories such as quantum
mechanics and the theory of relativity simplify
to their classical equivalents at such scales.
However, inaccuracies in classical mechanics
for very small objects and very high velocities
led to the development of modern physics in
the 20th century.
=== Modern physics ===
Modern physics began in the early 20th century
with the work of Max Planck in quantum theory
and Albert Einstein's theory of relativity.
Both of these theories came about due to inaccuracies
in classical mechanics in certain situations.
Classical mechanics predicted a varying speed
of light, which could not be resolved with
the constant speed predicted by Maxwell's
equations of electromagnetism; this discrepancy
was corrected by Einstein's theory of special
relativity, which replaced classical mechanics
for fast-moving bodies and allowed for a constant
speed of light.
Black body radiation provided another problem
for classical physics, which was corrected
when Planck proposed that the excitation of
material oscillators is possible only in discrete
steps proportional to their frequency; this,
along with the photoelectric effect and a
complete theory predicting discrete energy
levels of electron orbitals, led to the theory
of quantum mechanics taking over from classical
physics at very small scales.Quantum mechanics
would come to be pioneered by Werner Heisenberg,
Erwin Schrödinger and Paul Dirac.
From this early work, and work in related
fields, the Standard Model of particle physics
was derived.
Following the discovery of a particle with
properties consistent with the Higgs boson
at CERN in 2012, all fundamental particles
predicted by the standard model, and no others,
appear to exist; however, physics beyond the
Standard Model, with theories such as supersymmetry,
is an active area of research.
Areas of mathematics in general are important
to this field, such as the study of probabilities
and groups.
== Philosophy ==
In many ways, physics stems from ancient Greek
philosophy.
From Thales' first attempt to characterise
matter, to Democritus' deduction that matter
ought to reduce to an invariant state, the
Ptolemaic astronomy of a crystalline firmament,
and Aristotle's book Physics (an early book
on physics, which attempted to analyze and
define motion from a philosophical point of
view), various Greek philosophers advanced
their own theories of nature.
Physics was known as natural philosophy until
the late 18th century.By the 19th century,
physics was realised as a discipline distinct
from philosophy and the other sciences.
Physics, as with the rest of science, relies
on philosophy of science and its "scientific
method" to advance our knowledge of the physical
world.
The scientific method employs a priori reasoning
as well as a posteriori reasoning and the
use of Bayesian inference to measure the validity
of a given theory.The development of physics
has answered many questions of early philosophers,
but has also raised new questions.
Study of the philosophical issues surrounding
physics, the philosophy of physics, involves
issues such as the nature of space and time,
determinism, and metaphysical outlooks such
as empiricism, naturalism and realism.Many
physicists have written about the philosophical
implications of their work, for instance Laplace,
who championed causal determinism, and Erwin
Schrödinger, who wrote on quantum mechanics.
The mathematical physicist Roger Penrose had
been called a Platonist by Stephen Hawking,
a view Penrose discusses in his book, The
Road to Reality.
Hawking referred to himself as an "unashamed
reductionist" and took issue with Penrose's
views.
== Core theories ==
Though physics deals with a wide variety of
systems, certain theories are used by all
physicists.
Each of these theories were experimentally
tested numerous times and found to be an adequate
approximation of nature.
For instance, the theory of classical mechanics
accurately describes the motion of objects,
provided they are much larger than atoms and
moving at much less than the speed of light.
These theories continue to be areas of active
research today.
Chaos theory, a remarkable aspect of classical
mechanics was discovered in the 20th century,
three centuries after the original formulation
of classical mechanics by Isaac Newton (1642–1727).
These central theories are important tools
for research into more specialised topics,
and any physicist, regardless of their specialisation,
is expected to be literate in them.
These include classical mechanics, quantum
mechanics, thermodynamics and statistical
mechanics, electromagnetism, and special relativity.
=== Classical physics ===
Classical physics includes the traditional
branches and topics that were recognised and
well-developed before the beginning of the
20th century—classical mechanics, acoustics,
optics, thermodynamics, and electromagnetism.
Classical mechanics is concerned with bodies
acted on by forces and bodies in motion and
may be divided into statics (study of the
forces on a body or bodies not subject to
an acceleration), kinematics (study of motion
without regard to its causes), and dynamics
(study of motion and the forces that affect
it); mechanics may also be divided into solid
mechanics and fluid mechanics (known together
as continuum mechanics), the latter include
such branches as hydrostatics, hydrodynamics,
aerodynamics, and pneumatics.
Acoustics is the study of how sound is produced,
controlled, transmitted and received.
Important modern branches of acoustics include
ultrasonics, the study of sound waves of very
high frequency beyond the range of human hearing;
bioacoustics, the physics of animal calls
and hearing, and electroacoustics, the manipulation
of audible sound waves using electronics.Optics,
the study of light, is concerned not only
with visible light but also with infrared
and ultraviolet radiation, which exhibit all
of the phenomena of visible light except visibility,
e.g., reflection, refraction, interference,
diffraction, dispersion, and polarization
of light.
Heat is a form of energy, the internal energy
possessed by the particles of which a substance
is composed; thermodynamics deals with the
relationships between heat and other forms
of energy.
Electricity and magnetism have been studied
as a single branch of physics since the intimate
connection between them was discovered in
the early 19th century; an electric current
gives rise to a magnetic field, and a changing
magnetic field induces an electric current.
Electrostatics deals with electric charges
at rest, electrodynamics with moving charges,
and magnetostatics with magnetic poles at
rest.
=== Modern physics ===
Classical physics is generally concerned with
matter and energy on the normal scale of observation,
while much of modern physics is concerned
with the behavior of matter and energy under
extreme conditions or on a very large or very
small scale.
For example, atomic and nuclear physics studies
matter on the smallest scale at which chemical
elements can be identified.
The physics of elementary particles is on
an even smaller scale since it is concerned
with the most basic units of matter; this
branch of physics is also known as high-energy
physics because of the extremely high energies
necessary to produce many types of particles
in particle accelerators.
On this scale, ordinary, commonsense notions
of space, time, matter, and energy are no
longer valid.The two chief theories of modern
physics present a different picture of the
concepts of space, time, and matter from that
presented by classical physics.
Classical mechanics approximates nature as
continuous, while quantum theory is concerned
with the discrete nature of many phenomena
at the atomic and subatomic level and with
the complementary aspects of particles and
waves in the description of such phenomena.
The theory of relativity is concerned with
the description of phenomena that take place
in a frame of reference that is in motion
with respect to an observer; the special theory
of relativity is concerned with motion in
the absence of gravitational fields and the
general theory of relativity with motion and
its connection with gravitation.
Both quantum theory and the theory of relativity
find applications in all areas of modern physics.
=== Difference between classical and modern
physics ===
While physics aims to discover universal laws,
its theories lie in explicit domains of applicability.
Loosely speaking, the laws of classical physics
accurately describe systems whose important
length scales are greater than the atomic
scale and whose motions are much slower than
the speed of light.
Outside of this domain, observations do not
match predictions provided by classical mechanics.
Albert Einstein contributed the framework
of special relativity, which replaced notions
of absolute time and space with spacetime
and allowed an accurate description of systems
whose components have speeds approaching the
speed of light.
Max Planck, Erwin Schrödinger, and others
introduced quantum mechanics, a probabilistic
notion of particles and interactions that
allowed an accurate description of atomic
and subatomic scales.
Later, quantum field theory unified quantum
mechanics and special relativity.
General relativity allowed for a dynamical,
curved spacetime, with which highly massive
systems and the large-scale structure of the
universe can be well-described.
General relativity has not yet been unified
with the other fundamental descriptions; several
candidate theories of quantum gravity are
being developed.
== Relation to other fields ==
=== Prerequisites ===
Mathematics provides a compact and exact language
used to describe the order in nature.
This was noted and advocated by Pythagoras,
Plato, Galileo, and Newton.
Physics uses mathematics to organise and formulate
experimental results.
From those results, precise or estimated solutions
are obtained, quantitative results from which
new predictions can be made and experimentally
confirmed or negated.
The results from physics experiments are numerical
data, with their units of measure and estimates
of the errors in the measurements.
Technologies based on mathematics, like computation
have made computational physics an active
area of research.
Ontology is a prerequisite for physics, but
not for mathematics.
It means physics is ultimately concerned with
descriptions of the real world, while mathematics
is concerned with abstract patterns, even
beyond the real world.
Thus physics statements are synthetic, while
mathematical statements are analytic.
Mathematics contains hypotheses, while physics
contains theories.
Mathematics statements have to be only logically
true, while predictions of physics statements
must match observed and experimental data.
The distinction is clear-cut, but not always
obvious.
For example, mathematical physics is the application
of mathematics in physics.
Its methods are mathematical, but its subject
is physical.
The problems in this field start with a "mathematical
model of a physical situation" (system) and
a "mathematical description of a physical
law" that will be applied to that system.
Every mathematical statement used for solving
has a hard-to-find physical meaning.
The final mathematical solution has an easier-to-find
meaning, because it is what the solver is
looking for.Physics is a branch of fundamental
science, not practical science.
Physics is also called "the fundamental science"
because the subject of study of all branches
of natural science like chemistry, astronomy,
geology, and biology are constrained by laws
of physics, similar to how chemistry is often
called the central science because of its
role in linking the physical sciences.
For example, chemistry studies properties,
structures, and reactions of matter (chemistry's
focus on the atomic scale distinguishes it
from physics).
Structures are formed because particles exert
electrical forces on each other, properties
include physical characteristics of given
substances, and reactions are bound by laws
of physics, like conservation of energy, mass,
and charge.
Physics is applied in industries like engineering
and medicine.
=== Application and influence ===
Applied physics is a general term for physics
research which is intended for a particular
use.
An applied physics curriculum usually contains
a few classes in an applied discipline, like
geology or electrical engineering.
It usually differs from engineering in that
an applied physicist may not be designing
something in particular, but rather is using
physics or conducting physics research with
the aim of developing new technologies or
solving a problem.
The approach is similar to that of applied
mathematics.
Applied physicists use physics in scientific
research.
For instance, people working on accelerator
physics might seek to build better particle
detectors for research in theoretical physics.
Physics is used heavily in engineering.
For example, statics, a subfield of mechanics,
is used in the building of bridges and other
static structures.
The understanding and use of acoustics results
in sound control and better concert halls;
similarly, the use of optics creates better
optical devices.
An understanding of physics makes for more
realistic flight simulators, video games,
and movies, and is often critical in forensic
investigations.
With the standard consensus that the laws
of physics are universal and do not change
with time, physics can be used to study things
that would ordinarily be mired in uncertainty.
For example, in the study of the origin of
the earth, one can reasonably model earth's
mass, temperature, and rate of rotation, as
a function of time allowing one to extrapolate
forward or backward in time and so predict
future or prior events.
It also allows for simulations in engineering
which drastically speed up the development
of a new technology.
But there is also considerable interdisciplinarity
in the physicist's methods, so many other
important fields are influenced by physics
(e.g., the fields of econophysics and sociophysics).
== Research ==
=== 
Scientific method ===
Physicists use the scientific method to test
the validity of a physical theory.
By using a methodical approach to compare
the implications of a theory with the conclusions
drawn from its related experiments and observations,
physicists are better able to test the validity
of a theory in a logical, unbiased, and repeatable
way.
To that end, experiments are performed and
observations are made in order to determine
the validity or invalidity of the theory.A
scientific law is a concise verbal or mathematical
statement of a relation which expresses a
fundamental principle of some theory, such
as Newton's law of universal gravitation.
=== Theory and experiment ===
Theorists seek to develop mathematical models
that both agree with existing experiments
and successfully predict future experimental
results, while experimentalists devise and
perform experiments to test theoretical predictions
and explore new phenomena.
Although theory and experiment are developed
separately, they are strongly dependent upon
each other.
Progress in physics frequently comes about
when experimentalists make a discovery that
existing theories cannot explain, or when
new theories generate experimentally testable
predictions, which inspire new experiments.Physicists
who work at the interplay of theory and experiment
are called phenomenologists, who study complex
phenomena observed in experiment and work
to relate them to a fundamental theory.Theoretical
physics has historically taken inspiration
from philosophy; electromagnetism was unified
this way.
Beyond the known universe, the field of theoretical
physics also deals with hypothetical issues,
such as parallel universes, a multiverse,
and higher dimensions.
Theorists invoke these ideas in hopes of solving
particular problems with existing theories.
They then explore the consequences of these
ideas and work toward making testable predictions.
Experimental physics expands, and is expanded
by, engineering and technology.
Experimental physicists involved in basic
research design and perform experiments with
equipment such as particle accelerators and
lasers, whereas those involved in applied
research often work in industry developing
technologies such as magnetic resonance imaging
(MRI) and transistors.
Feynman has noted that experimentalists may
seek areas which are not well-explored by
theorists.
=== Scope and aims ===
Physics covers a wide range of phenomena,
from elementary particles (such as quarks,
neutrinos, and electrons) to the largest superclusters
of galaxies.
Included in these phenomena are the most basic
objects composing all other things.
Therefore, physics is sometimes called the
"fundamental science".
Physics aims to describe the various phenomena
that occur in nature in terms of simpler phenomena.
Thus, physics aims to both connect the things
observable to humans to root causes, and then
connect these causes together.
For example, the ancient Chinese observed
that certain rocks (lodestone and magnetite)
were attracted to one another by an invisible
force.
This effect was later called magnetism, which
was first rigorously studied in the 17th century.
But even before the Chinese discovered magnetism,
the ancient Greeks knew of other objects such
as amber, that when rubbed with fur would
cause a similar invisible attraction between
the two.
This was also first studied rigorously in
the 17th century and came to be called electricity.
Thus, physics had come to understand two observations
of nature in terms of some root cause (electricity
and magnetism).
However, further work in the 19th century
revealed that these two forces were just two
different aspects of one force—electromagnetism.
This process of "unifying" forces continues
today, and electromagnetism and the weak nuclear
force are now considered to be two aspects
of the electroweak interaction.
Physics hopes to find an ultimate reason (Theory
of Everything) for why nature is as it is
(see section Current research below for more
information).
=== Research fields ===
Contemporary research in physics can be broadly
divided into nuclear and particle physics;
condensed matter physics; atomic, molecular,
and optical physics; astrophysics; and applied
physics.
Some physics departments also support physics
education research and physics outreach.Since
the 20th century, the individual fields of
physics have become increasingly specialised,
and today most physicists work in a single
field for their entire careers.
"Universalists" such as Albert Einstein (1879–1955)
and Lev Landau (1908–1968), who worked in
multiple fields of physics, are now very rare.The
major fields of physics, along with their
subfields and the theories and concepts they
employ, are shown in the following table.
==== Nuclear and particle physics ====
Particle physics is the study of the elementary
constituents of matter and energy and the
interactions between them.
In addition, particle physicists design and
develop the high energy accelerators, detectors,
and computer programs necessary for this research.
The field is also called "high-energy physics"
because many elementary particles do not occur
naturally but are created only during high-energy
collisions of other particles.Currently, the
interactions of elementary particles and fields
are described by the Standard Model.
The model accounts for the 12 known particles
of matter (quarks and leptons) that interact
via the strong, weak, and electromagnetic
fundamental forces.
Dynamics are described in terms of matter
particles exchanging gauge bosons (gluons,
W and Z bosons, and photons, respectively).
The Standard Model also predicts a particle
known as the Higgs boson.
In July 2012 CERN, the European laboratory
for particle physics, announced the detection
of a particle consistent with the Higgs boson,
an integral part of a Higgs mechanism.
Nuclear physics is the field of physics that
studies the constituents and interactions
of atomic nuclei.
The most commonly known applications of nuclear
physics are nuclear power generation and nuclear
weapons technology, but the research has provided
application in many fields, including those
in nuclear medicine and magnetic resonance
imaging, ion implantation in materials engineering,
and radiocarbon dating in geology and archaeology.
==== Atomic, molecular, and optical physics
====
Atomic, molecular, and optical physics (AMO)
is the study of matter–matter and light–matter
interactions on the scale of single atoms
and molecules.
The three areas are grouped together because
of their interrelationships, the similarity
of methods used, and the commonality of their
relevant energy scales.
All three areas include both classical, semi-classical
and quantum treatments; they can treat their
subject from a microscopic view (in contrast
to a macroscopic view).
Atomic physics studies the electron shells
of atoms.
Current research focuses on activities in
quantum control, cooling and trapping of atoms
and ions, low-temperature collision dynamics
and the effects of electron correlation on
structure and dynamics.
Atomic physics is influenced by the nucleus
(see, e.g., hyperfine splitting), but intra-nuclear
phenomena such as fission and fusion are considered
part of nuclear physics.
Molecular physics focuses on multi-atomic
structures and their internal and external
interactions with matter and light.
Optical physics is distinct from optics in
that it tends to focus not on the control
of classical light fields by macroscopic objects
but on the fundamental properties of optical
fields and their interactions with matter
in the microscopic realm.
==== Condensed matter physics ====
Condensed matter physics is the field of physics
that deals with the macroscopic physical properties
of matter.
In particular, it is concerned with the "condensed"
phases that appear whenever the number of
particles in a system is extremely large and
the interactions between them are strong.The
most familiar examples of condensed phases
are solids and liquids, which arise from the
bonding by way of the electromagnetic force
between atoms.
More exotic condensed phases include the superfluid
and the Bose–Einstein condensate found in
certain atomic systems at very low temperature,
the superconducting phase exhibited by conduction
electrons in certain materials, and the ferromagnetic
and antiferromagnetic phases of spins on atomic
lattices.Condensed matter physics is the largest
field of contemporary physics.
Historically, condensed matter physics grew
out of solid-state physics, which is now considered
one of its main subfields.
The term condensed matter physics was apparently
coined by Philip Anderson when he renamed
his research group—previously solid-state
theory—in 1967.
In 1978, the Division of Solid State Physics
of the American Physical Society was renamed
as the Division of Condensed Matter Physics.
Condensed matter physics has a large overlap
with chemistry, materials science, nanotechnology
and engineering.
==== Astrophysics ====
Astrophysics and astronomy are the application
of the theories and methods of physics to
the study of stellar structure, stellar evolution,
the origin of the Solar System, and related
problems of cosmology.
Because astrophysics is a broad subject, astrophysicists
typically apply many disciplines of physics,
including mechanics, electromagnetism, statistical
mechanics, thermodynamics, quantum mechanics,
relativity, nuclear and particle physics,
and atomic and molecular physics.The discovery
by Karl Jansky in 1931 that radio signals
were emitted by celestial bodies initiated
the science of radio astronomy.
Most recently, the frontiers of astronomy
have been expanded by space exploration.
Perturbations and interference from the earth's
atmosphere make space-based observations necessary
for infrared, ultraviolet, gamma-ray, and
X-ray astronomy.
Physical cosmology is the study of the formation
and evolution of the universe on its largest
scales.
Albert Einstein's theory of relativity plays
a central role in all modern cosmological
theories.
In the early 20th century, Hubble's discovery
that the universe is expanding, as shown by
the Hubble diagram, prompted rival explanations
known as the steady state universe and the
Big Bang.
The Big Bang was confirmed by the success
of Big Bang nucleosynthesis and the discovery
of the cosmic microwave background in 1964.
The Big Bang model rests on two theoretical
pillars: Albert Einstein's general relativity
and the cosmological principle.
Cosmologists have recently established the
ΛCDM model of the evolution of the universe,
which includes cosmic inflation, dark energy,
and dark matter.
Numerous possibilities and discoveries are
anticipated to emerge from new data from the
Fermi Gamma-ray Space Telescope over the upcoming
decade and vastly revise or clarify existing
models of the universe.
In particular, the potential for a tremendous
discovery surrounding dark matter is possible
over the next several years.
Fermi will search for evidence that dark matter
is composed of weakly interacting massive
particles, complementing similar experiments
with the Large Hadron Collider and other underground
detectors.
IBEX is already yielding new astrophysical
discoveries: "No one knows what is creating
the ENA (energetic neutral atoms) ribbon"
along the termination shock of the solar wind,
"but everyone agrees that it means the textbook
picture of the heliosphere—in which the
Solar System's enveloping pocket filled with
the solar wind's charged particles is plowing
through the onrushing 'galactic wind' of the
interstellar medium in the shape of a comet—is
wrong."
== 
Current research ==
Research in physics is continually progressing
on a large number of fronts.
In condensed matter physics, an important
unsolved theoretical problem is that of high-temperature
superconductivity.
Many condensed matter experiments are aiming
to fabricate workable spintronics and quantum
computers.In particle physics, the first pieces
of experimental evidence for physics beyond
the Standard Model have begun to appear.
Foremost among these are indications that
neutrinos have non-zero mass.
These experimental results appear to have
solved the long-standing solar neutrino problem,
and the physics of massive neutrinos remains
an area of active theoretical and experimental
research.
The Large Hadron Collider has already found
the Higgs Boson, but future research aims
to prove or disprove the supersymmetry, which
extends the Standard Model of particle physics.
Research on the nature of the major mysteries
of dark matter and dark energy is also currently
ongoing.Theoretical attempts to unify quantum
mechanics and general relativity into a single
theory of quantum gravity, a program ongoing
for over half a century, have not yet been
decisively resolved.
The current leading candidates are M-theory,
superstring theory and loop quantum gravity.
Many astronomical and cosmological phenomena
have yet to be satisfactorily explained, including
the origin of ultra-high energy cosmic rays,
the baryon asymmetry, the acceleration of
the universe and the anomalous rotation rates
of galaxies.
Although much progress has been made in high-energy,
quantum, and astronomical physics, many everyday
phenomena involving complexity, chaos, or
turbulence are still poorly understood.
Complex problems that seem like they could
be solved by a clever application of dynamics
and mechanics remain unsolved; examples include
the formation of sandpiles, nodes in trickling
water, the shape of water droplets, mechanisms
of surface tension catastrophes, and self-sorting
in shaken heterogeneous collections.These
complex phenomena have received growing attention
since the 1970s for several reasons, including
the availability of modern mathematical methods
and computers, which enabled complex systems
to be modeled in new ways.
Complex physics has become part of increasingly
interdisciplinary research, as exemplified
by the study of turbulence in aerodynamics
and the observation of pattern formation in
biological systems.
In the 1932 Annual Review of Fluid Mechanics,
Horace Lamb said:
I am an old man now, and when I die and go
to heaven there are two matters on which I
hope for enlightenment.
One is quantum electrodynamics, and the other
is the turbulent motion of fluids.
And about the former I am rather optimistic.
== See also ==
== Notes
