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What Is Physics?
Physics is the scientific study of matter and energy and how they interact with each other.
This energy can take the form of motion, light, electricity, radiation, gravity . . . just about
anything, honestly. Physics deals with matter on scales ranging from sub-atomic particles (i.e. the
particles that make up the atom and the particles that make up those particles) to stars and even
entire galaxies.
How Physics Works
As an experimental science, physics utilizes the scientific method to formulate and test hypotheses
that are based on observation of the natural world. The goal of physics is to use the results of
these experiments to formulate scientific laws, usually expressed in the language of mathematics,
which can then be used to predict other phenomena.
The Role of Physics in Science
In a broader sense, physics can be seen as the most fundamental of the natural sciences. Chemistry,
for example, can be viewed as a complex application of physics, as it focuses on the interaction of
energy and matter in chemical systems. We also know that biology is, at its heart, an application of
chemical properties in living things, which means that it is also, ultimately, ruled by the physical
laws.
Because physics covers so much area, it is divided into several specific fields of study, such as
Acoustics - the study of sound & sound waves
Astronomy - the study of space
Astrophysics - the study of the physical properties of objects in space
Atomic Physics - the study of atoms, specifically the electron properties of the atom
Biophysics - the study of physics in living systems
Chemical Physics - the study of physics in chemical systems
Computational Physics - the application of numerical methods to solve physical problems for which a
quantitative theory already exists
Cosmology - the study of the universe as a whole, including its origins and evolution
Cryophysics / Cryogenics / Low Temperature Physics - the study of physical properties in low
temperature situations, far below the freezing point of water
Electromagnetism - the study of electrical and magnetic fields, which are two aspects of the same
phenomenon
Electronics - the study of the flow of electrons, generally in a circuit
Fluid Dynamics / Fluid Mechanics - the study of the physical properties of "fluids," specifically
defined in this case to be liquids and gases
Geophysics - the study of the physical properties of the Earth
High Energy Physics - the study of physics in extremely high energy systems, generally within
particle physics
High Pressure Physics - the study of physics in extremely high pressure systems, generally related
to fluid dynamics
Laser Physics - the study of the physical properties of lasers
Mechanics - the study of the motion of bodies in a frame of reference
Meteorology / Weather Physics - the physics of the weather
Nanotechnology - the science of building circuits and machines from single molecules and atoms
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Nuclear Physics - the study of the physical properties of the atomic nucleus
Optics / Light Physics - the study of the physical properties of light
Particle Physics - the study of fundamental particles and the forces of their interaction
Plasma Physics - the study of matter in the plasma phase
Quantum Mechanics / Quantum Physics - the study of science where the smallest discrete values,
or quanta, of matter and energy become relevant
Relativity - the study of systems displaying the properties of Einstein's theory of relativity, which
generally involves moving at speeds very close to the speed of light
String Theory / Superstring Theory - the study of the theory that all fundamental particles are
vibrations of one-dimensional strings of energy, in a higher-dimensional universe
Thermodynamics - the physics of heat
It should become obvious that there is some overlap. For example, the difference between
astronomy, astrophysics, and cosmology can be virtually meaningless at times ... to everyone, that is,
except the astronomers, astrophysicists, and cosmologists, who can take the distinctions very
seriously.
Physical Laws
Over the years, one thing scientists have discovered is that nature is generally more complex than
we give it credit for. The following laws of physics are considered fundamental, but many of them
refer to idealized, closed systems, which are hard to obtain in the real world. Also, some are
altered slightly in different circumstances. The laws that Newton developed, for example, are
modified by the findings of the theory of relativity, but they are still basically valid in most regular
cases that you'll run into.
Classical Laws of Physics: Newton's Three Laws of Motion and Newton's Law of Gravity
Modern Laws of Physics: Einstein's Theory of Relativity and Quantum Physics
Newton's Three Laws of Motion:Sir Isaac Newton developed the Three Laws of Motion, which
describe basic rules about how the motion of physical objects change. Newton was able to define
the fundamental relationship between the acceleration of an object and the total forces acting upon
it.
"Law" of Gravity: Newton developed his "Law of Gravity" to explain the attractive force between a
pair of masses. In the twentieth century, it became clear that this is not the whole story, as
Einstein's theory of general relativity has provided a more comprehensive explanation for the
phenomenon of gravity. Still, Newton's law of gravity is an accurate low-energy approximation that
works for most of the cases that you'll explore in physics.
Conservation of Mass-Energy: The total energy in a closed or isolated system is constant, no
matter what happens. Another law stated that the mass in an isolated system is constant. When
Einstein discovered the relationship E=mc2 (in other words that mass was a manifestation of energy)
the law was said to refer to the conservation of mass-energy. The total of both mass and energy is
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retained, although some may change forms. The ultimate example of this is a nuclear explosion,
where mass transforms into energy.
Conservation of Momentum: The total momentum in a closed or isolated system remains constant.
Laws of Thermodynamics: The laws of thermodynamics are actually specific manifestations of the
law of conservation of mass-energy as it relates to thermodynamic processes.
Electrostatic Laws: Coulomb's law and Gauss's law are formulations of the relationship between
electrically charged particles to create electrostatic force and electrostatic fields. The formulas,
it turns out, parallel the laws of universal gravitation in structure. There also exist similar laws
relating to magnetism and electromagnetism as a whole.
Invariance of the Speed of Light: Einstein's major insight, which led him to the Theory of
Relativity, was the realization that the speed of light in a vacuum is constant and is not measured
differently for observers in different inertial frames of reference, unlike all other forms of
motion. Some theoretical physicists have conjectured different variable speed of light (VSL)
possibilities, but these are highly speculative. Most physicists believe that Einstein was right and
the speed of light is constant.
In the realm of relativity and quantum mechanics, scientists have found that these laws still apply,
although their interpretation requires some refinement to be applied, resulting in fields such as
quantum electronics and quantum gravity. Care should be taken in applying them in these situations.
There are a lot of interesting theories in physics. Matter exists as a state of energy, while waves
of probability spread throughout the universe. Existence itself may exist as only the vibrations on
microscopic, trans-dimensional strings. Here are some of the most interesting theories in modern
physics.
Wave Particle Duality
Matter and light have properties of both waves and particles simultaneously. The results of
quantum mechanics make it clear that waves exhibit particle-like properties and particles exhibit
wave-like properties, depending on the specific experiment. Quantum physics is therefore able to
make descriptions of matter and energy based on wave equations that relate to the probability of a
particle existing in a certain spot at a certain time.
Einstein's Theory of Relativity
Einstein's theory of relativity is based upon the principle that the laws of physics are the same for
all observers, regardless of where they are located or how fast they are moving or accelerating.
This seemingly common sense principle predicts localized effects in the form of special relativity
and defines gravitation as a geometric phenomenon in the form of general relativity.
The Big Bang
When Albert Einstein developed the Theory of General Relativity, it predicted a possible expansion
of the universe. Georges Lemaitre thought that this indicated the universe began in a single point.
The name "Big Bang" was given by Fred Hoyle while mocking the theory during a radio broadcast.
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In 1929, Edwin Hubble discovered a redshift in distant galaxies, indicating that they were receding
from Earth. Cosmic background microwave radiation, discovered in 1965, supported Lemaitre's
theory.
Dark Matter & Dark Energy
Across astronomical distances, the only significant fundamental force of physics is gravity.
Astronomers find that their calculations & observations don't quite match up, though.
An undetected form of matter, called dark matter, was theorized to fix this. Recent evidence
supports dark matter.
Other work indicates that there might exist a dark energy, as well.
Current estimates are that the universe is 70% dark energy, 25% dark matter, and only 5% of the
universe is visible matter or energy!
Famous physicists
Sir Isaac Newton (1642-1727)
English physicist and mathematician, considered by many to be the greatest physicist of all time. His most
famous contributions to physics are his law of gravitation and laws of motion. He also invented calculus, and
made important discoveries in the field of optics (for example, the discovery that white light may be split into
the colours of the rainbow by a prism). The SI unit of force is named after him.
André-Marie Ampère (1775-1836)
French physicist most famous for investigating the magnetic fields produced by current-carrying wires. His
work extended that of the Danish physicist Hans Oersted, who discovered in 1819 that a compass needle was
deflected by a current-carrying wire. He also invented the solenoid. Today, the law that governs the magnetic
fields produced by electric currents is called Ampère's Law, and the SI unit of electric current is named in
his honour
Carl Friederich Gauss (1777-1855)
German mathematician who is most famous for his discoveries in pure mathematics. Indeed, he has been
dubbed the 'prince of mathematics'. However, he also made a number of important contributions to physics.
He invented the magnetometer and with the German physicist Wilhelm Weber measured the intensity of
magnetic forces. He also took Coulomb's famous inverse-square law for the electric field of a point charge
and generalized it to an arbitrary charged distribution. This more general law is now known as Gauss's Law.
Michael Faraday (1791-1867)
English physicist who was one of the greatest experimentalists in the history of physics. This is remarkable
as he had no formal training. Instead he learned about physics and chemistry by working as an assistant to Sir
Humphrey
Davy.
Faraday made many important contributions to the study of electricity and magnetism, including the
discovery of electromagnetic induction (now known as Faraday's Law), the invention of the electric motor, and
the laws of electrolysis. The SI unit of capacitance is named after him.
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James Clerk Maxwell (1831-1879)
Scottish mathematician and physicist who, in the 1860s, took the laws of electricity and magnetism that had
been discovered over the previous century or so, and united them into one theory called electromagnetism.
This theory is neatly summarized in 4 simple equations known as Maxwell's equations. One consequence of this
was the demonstration that light is an electromagnetic wave.
Maxwell also developed the kinetic theory of
gases, deriving the distribution of molecular speeds in a gas at a given temperature.
James Prescott Joule (1818-1889)
English physicist who made many meticulous experiments that demonstrated that heat and work are
equivalent. Although he was not the first to do this, it was his demonstration that eventually came to be
accepted. The SI unit of work is named in his honour.
William Thomson, Lord Kelvin (1824-1907)
British physicist who published many important papers on the conservation and dissipation of energy.
Kelvin also made contributions to other branches of physics (such as fluid mechanics), and was in charge of
laying the first successful transatlantic cable in 1866. The SI unit of absolute temperature is named after
him.
. Ludwig Boltzmann (1844-1906)
Austrian physicist who founded the branch of physics known as statistical mechanics, which involves
describing large numbers of atoms using averages. He showed that the entropy of a system was a measure of
how disordered it is, and that the amount of disorder in the Universe tends to increase
Albert Einstein (1879-1955)
In 1905, Einstein published a paper on what he called the Special Theory of Relativity, which correctly
describes the motion of particles travelling at speeds close to the speed of light. The theory is based upon
the simple postulates that the laws of physics are the same for all inertial (i.e. non-accelerating) observers,
and that the speed of light is the same for all inertial observers (regardless of their motion relative to the
source of the light). This theory includes the famous formula E = mc2. He subsequently developed the General
Theory of Relativity, which is effectively a theory of gravitation.
Einstein also contributed to the development of quantum theory. In 1905 he published a paper explaining
the photoelectric effect, by postulating that light consists of particles (now known as photons). For this work,
Einstein received the 1921 Nobel Prize for Physics.
Neils Bohr (1885-1962)
Danish physicist who, in 1913, developed a successful quantum theoretical model of the hydrogen atom. It was
an extension of Rutherford's model of the atom, in which the electron orbits the nucleus. In particular,
Bohr's model correctly predicted the frequencies of the spectral lines that had been observed by such men
as Lyman, Balmer and Paschen. Bohr received the 1922 Nobel Prize for Physics for this work. By the late
1920s, Bohr was very much regarded as an elder statesman of quantum theory. Many of the young physicists
who made important discoveries in the early days of quantum mechanics studied under him at Copenhagen.
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Louis de Broglie (1892-1987)
French physicist who, in 1923, proposed that all particles have wave-like properties (just as Einstein had
shown that light has particle-like properties). He came up with this theory while working on his PhD. As it was
such a radical idea, the examiners wrote to Einstein to ask his opinion. Einstein realized what a brilliant idea it
was, and de Broglie got his PhD. De Broglie's theory was instrumental in the development of quantum
mechanics a few years later. For his work, de Broglie received the 1929 Nobel Prize for Physics. The wave-like
nature of electrons was confirmed experimentally in the late 1920s when George Thomson and Clinton
Davisson independently discovered electron diffraction. For this work they shared the 1937 Prize.
Werner Heisenberg (1901-1976)
German physicist who, in 1925, created quantum mechanics. One important aspect of Heisenberg's theory was
that it only dealt with properties of a system that can in theory be measured (for example, the frequency of
the radiation emitted by a hydrogen atom). He said we cannot assign a position in space at a given time to the
electron, nor can we follow an electron in its orbit. This means we cannot assume the orbits postulated by
Bohr actually exist.
Mechanical quantities such as position and velocity cannot be represented by ordinary numbers, but instead
must be represented by matrices. As a result, Heisenberg's version of quantum mechanics is sometimes called
matrix mechanics.
The following year, the Austrian physicist Wolfgang Pauli showed that Heisenberg's theory correctly
predicted the hydrogen spectrum. In 1927 Heisenberg published his famous Uncertainty Principle, which
states one cannot measure the position and momentum of a particle with arbitrary precision.
Heisenberg received the 1932 Nobel Prize for Physics for his work on quantum mechanics
Erwin Schrödinger (1887-1961)
Austrian physicist who, in 1926, created a version of quantum mechanics that involved waves, rather than the
somewhat abstract matrices of Heisenberg's theory. Schrödinger's theory also correctly predicted the
hydrogen spectrum. In the same year, Schrödinger showed that his theory (sometimes called wave mechanics)
is equivalent to Heisenberg's matrix mechanics.
Schrödinger shared the 1933 Nobel Prize for Physics with
Paul Dirac for his work on quantum mechanics
Paul Dirac (1902-1984)
English physicist who created a version of quantum mechanics very similar to Heisenberg's at about the same
time. He also made early contributions to quantum electrodynamics (the study of the interaction of charged
particles with electromagnetic fields). However, he is probably best known for his equation for the electron
that encompassed both quantum mechanics and special relativity, which led to the discovery of antimatter.
Dirac shared the 1933 Nobel Prize for Physics with Erwin Schrödinger for this work.
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