Physics is the science concerned with the fundamental
laws which govern matter and energy.
·
The
culture of physics research differs from the other sciences in the separation
of theory and experiment. Since the 20th century, most individual physicists have
specialized in either theoretical
physics or experimental physics.
In contrast, almost all the successful theorists in biology and chemistry have
also been experimentalists, though this is changing as of late.
History
·
As
the influence of the Arab Empire expanded to Europe, the works of Aristotle
preserved by the Arabs, and the works of the Indians and Persians, became known
in
·
1543 The Scientific Revolution is held by most
historians to have begun with the first printed copy of Nicolaus
Copernicus's De Revolutionibus
·
1564-1642 Galileo Galilei.
·
1590 Galileo's
experiments with falling objects showed that all objects dropped from the same
height accelerated at the same rate regardless of their weight. This
opposed Aristotle’s doctrine that heavier objects fell faster.
·
1592 Thermometer is invented by Galileo
·
He becomes tutor
to the Medici heirs in
·
1610
Galileo sees the moons of Jupiter (the Medici stars) through his own telescope.
He is the first to use the telescope to investigate the heavens. He publishes his findings in a 24-page
booklet, The Starry Messenger. This
contradicts Aristotle on several points: moons orbiting Jupiter, existence of
sunspots, craters on the Moon, millions more stars than expected, Venus waxing
and waning as it orbited the Sun, supporting Copernicus.
·
1610 He is
appointed the Royal Professor of Mathematics and Philosophy in
·
1632 He
publishes the Dialogue of the Two World
Systems, the first book of popular science.
·
1633 The
Inquisition forces Galileo to recant his belief in Copernican theory – he is
sentenced to house arrest in
·
1609-1618 Johannes Kepler proposes laws of planetary motion.
·
1643-1727 Isaac Newton
·
1687 Isaac
Newton publishes his Philosophiae Naturalis Principia Mathematica,
better known as the "Principia", considered to be one of the greatest
scientific books of all time, in which he gave gravity a mathematical footing.
It contains his Three Laws of Motion and
Law of Universal Gravitation.
- Optics:
- Calculus:
·
After
·
From
the late 17th century onwards, thermodynamics
was developed by physicist and chemist Boyle, Young, and many others.
·
1733 Bernoulli used statistical arguments with classical mechanics
to derive thermodynamic results, initiating the field of statistical mechanics. Ludwig Boltzmann,
in the 19th century, is responsible for the modern form of statistical
mechanics.
·
1821 Michael
Faraday integrated the study of magnetism with the study of electricity.
This was done by demonstrating that a moving magnet induced an electric current
in a conductor. Faraday also formulated a physical conception of electromagnetic fields.
·
1847 Joule stated the law of conservation of energy, in the form of heat as well as
mechanical energy.
·
1864 James Clerk Maxwell formulates a set of equations that
explained the interactions between electric and magnetic fields - electromagnetism.
·
In
addition to other electromagnetic phenomena, Maxwell's equations also can be
used to describe light. Confirmation of this observation was made with the 1888
discovery of radio by Heinrich Hertz and in 1895 when Wilhelm Roentgen detected
X rays.
·
1896 Radioactivity was discovered by Henri Becquerel, and further studied by Marie Curie, Pierre Curie,
and others. This initiated the field of nuclear physics.
·
1905 The ability to describe light in electromagnetic
terms helped serve as a springboard for Albert Einstein's publication of the
theory of special relativity. This
theory combined classical mechanics with Maxwell's equations and unifies space
and time into a single entity, spacetime. Relativity prescribes a different transformation
between reference frames than classical mechanics; this necessitated the
development of relativistic mechanics as a replacement for classical mechanics.
In the regime of low (relative) velocities, the two theories agree. Einstein
built further on the special theory by including gravity into his calculations,
and published his theory of general relativity in 1915
·
1897 J. J. Thomson discovered the electron. In 1904, he proposed the first model of the atom, known
as the plum pudding model.
·
1900 Max Planck published his explanation of blackbody radiation.
This equation assumed that radiators are quantized, which proved to be the
opening argument in the edifice that would become quantum mechanics. By introducing discrete energy elvels, Planck, Einstein, Niels
Bohr, and others developed quantum theories to explain various anomalous
experimental results. Quantum mechanics was formulated in 1925 by Heisenberg
and in 1926 by Schrödinger and Paul Dirac, in two
different ways that both explained the preceding heuristic quantum theories. In
quantum mechanics, the outcomes of physical measurements are inherently
probabilistic; the theory describes the calculation of these probabilities. It
successfully describes the behavior of matter at small distance scales. During
the 1920s Schrödinger, Heisenberg, and Max Born were able to formulate a
consistent picture of the chemical behavior of matter, a complete theory of the
electronic structure of the atom, as a byproduct of the quantum theory.
·
1911 Ernest Rutherford deduced from scattering
experiments the existence of a compact atomic nucleus, with positively charged
constituents dubbed protons. Neutrons, the neutral nuclear constituents, were
discovered in 1932 by Chadwick.
·
The
equivalence of mass and energy (E=mc2) was spectacularly
demonstrated during World War II, as research was conducted by each side into
nuclear physics, for the purpose of creating a nuclear bomb. The German effort,
led by Heisenberg, did not succeed, but the Allied Manhattan Project reached
its goal. In
·
Quantum field theory was formulated in order to extend quantum
mechanics to be consistent with special relativity. It was devised in the late
1940s with work by Richard Feynman, Julian Schwinger,
Sin-Itiro Tomonaga, and
Freeman Dyson. They formulated the theory of quantum electrodynamics, which
describes the electromagnetic interaction, and successfully explained the Lamb
shift. Quantum field theory provided the framework for modern particle physics,
which studies fundamental forces and elementary particles.
·
Chen Ning Yang and Tsung-Dao Lee, in
the 1950s, discovered an unexpected asymmetry in the decay of a subatomic
particle. In 1954, Yang and Robert Mills then developed a class of gauge theories which provided the
framework for understanding the nuclear forces. The theory for the strong nuclear force was first proposed
by Murray Gell-Mann. The electroweak
force, the unification of the weak nuclear force with electromagnetism, was
proposed by Sheldon Lee Glashow, Abdus
Salam and Steven Weinberg and confirmed in 1964 by
James Watson Cronin and Val Fitch. This led to the so-called Standard Model of particle physics in
the 1970s, which successfully describes all the elementary particles observed to
date.
·
Quantum
mechanics also provided the theoretical tools for condensed matter physics, whose largest branch is solid state
physics. It studies the physical behavior of solids and liquids, including
phenomena such as crystal structures, semiconductivity,
and superconductivity. The pioneers of condensed matter physics include Felix
Bloch, who created a quantum mechanical description of the behavior of
electrons in crystal structures in 1928. The transistor was developed by
physicists John Bardeen, Walter Houser Brattain and William Bradford Shockley in 1947 at Bell
Telephone Laboratories.
·
The
two themes of the 20th century, general relativity and quantum mechanics,
appear inconsistent with each other. General relativity describes the universe
on the scale of planets and solar systems while quantum mechanics operates on
sub-atomic scales. This challenge is being attacked by string theory, which
treats spacetime as composed, not of points, but of
one-dimensional objects, strings. Strings have properties like a common string
(e.g., tension and vibration). The theories yield promising, but not yet
testable results. The search for experimental verification of string theory is
in progress.
·
In
condensed matter physics, the biggest unsolved theoretical problem is the
explanation for high-temperature superconductivity. Strong efforts, largely
experimental, are being put into making 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 amongst these are indications
that neutrinos have non-zero mass. These experimental results appear to have
solved the long-standing solar neutrino problem in solar physics. The physics
of massive neutrinos is currently an area of active theoretical and
experimental research. In the next several years, particle accelerators will
begin probing energy scales in the TeV range, in
which experimentalists are hoping to find evidence for the Higgs boson and supersymmetric particles.
·
Theoretical
attempts to unify quantum mechanics and general relativity into a single theory
of quantum gravity, an effort
ongoing for over half a century, have not yet borne fruit. 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 existence of ultra-high energy cosmic rays, the baryon
asymmetry, the acceleration of the universe and the
anomalous rotation rates of galaxies.
·
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, such as the formation of sandpiles, nodes in trickling water, the shape of water
droplets, mechanisms of surface tension catastrophes, or self-sorting in shaken
heterogeneous collections are unsolved. These complex phenomena have received
growing attention since the 1970s for several reasons, not least of which has
been the availability of modern mathematical methods and computers which
enabled complex systems to be modeled in new ways.
Fundamental Interactions (Forces)
·
Fundamental
interactions are
mechanisms by which particles interact with each other, and which cannot be
explained by another more fundamental interaction. Every observed
physical phenomenon, from galaxies to quarks, can be explained by these
interactions
·
There
are four fundamental interactions: gravitation,
electromagnetism, the weak interaction, and the strong interaction. Their magnitude and
behavior vary greatly.
·
It is
strongly believed that three of these interactions are manifestations of a
single, more fundamental, interaction, just as electricity and magnetism are
now understood as two aspects of the electromagnetic interaction.
Electromagnetism and the weak nuclear forces have been shown to be two aspects
of a single electroweak interaction.
·
Grand unified theories seek to unify the electroweak force and
the strong nuclear interaction, but none have passed experimental muster. The
topic of unifying gravitation with the other three into an interaction that is
completely universal is called quantum
gravity.
·
The
modern quantum mechanical view of the three fundamental forces (all except
gravity) is that particles of matter (fermions) do not directly interact with
each other, but rather carry a charge, and exchange virtual particles (gauge
bosons), which are the interaction carriers or interaction mediators. Thus, for
example, photons are the mediators of the interaction of electric charges; and
gluons are the mediators of the interaction of color charges. This coupling of
matter (charged fermions) with force mediating particles (gauge bosons) is the
result of fundamental symmetries of nature. Mathematically, the coupling is
captured by Noether's theorem
|
Interaction |
Current Theory |
Mediators |
Relative
Strength1 |
Long-Distance
Behavior |
|
Strong |
Quantum chromodynamics |
Gluons |
1038 |
1 (see discussion below) |
|
Electromagnetic |
Quantum electrodynamics |
photons |
1036 |
1/r2 |
|
Weak |
Electroweak Theory |
W and Z bosons |
1025 |
e-mw,zr/r |
|
Gravity |
General Relativity |
gravitons |
1 |
1/r2 |
Gravity (Gravitation)
·
Gravitation is by far the weakest fundamental interaction. However, because it has an
infinite range and because all masses are positive, it is nevertheless very
important in the universe.
·
Because
all masses are positive, large bodies such as planets, stars and galaxies have
large total masses and therefore exert large gravitational forces. In
comparison, the total electric charge of these bodies is zero because half of
all charges are negative.
·
Unlike
the other interactions, gravity works universally on all matter and energy.
There are no objects that lack a gravitational "charge".
·
Because
of its long range, gravity is responsible for such large-scale phenomena as the
structure of galaxies, black holes and the expansion of the universe, as well
as more elementary astronomical phenomena like the orbits of planets, and
everyday experience like objects falling.
·
Gravitation
was the first fundamental interaction which was described by a mathematical
theory. Isaac Newton's law of Universal Gravitation (1687) was a good
approximation of the general behavior of gravity.
·
In
1915, Albert Einstein completed the General Theory of Relativity, a more
accurate description of gravity in terms of the geometry of space-time. Einstein proposed that spacetime
is curved by the presence of matter, and that free-falling objects are
following the geodesics of the spacetime. More
specifically, Einstein discovered the field equations of general relativity,
which relate the presence of matter and the curvature of spacetime.
·
It is
widely believed that in a theory of quantum gravity, gravity would be mediated
by a particle which is known as the
graviton. Gravitons are hypothetical particles not yet observed.
·
The
gravitational field is 9.8 m/s˛ or 32 ft/s˛. This means that, ignoring air
resistance, an object falling freely near the earth's surface increases in
speed by 9.8 m/s (around 22 mph) for each second of its descent.
·
Electromagnetism is the force that acts between electrically
charged particles. This includes the electrostatic force, acting between
charges at rest, and the combined effect of electric and magnetic forces acting
between charges moving relative to each other.
·
Electromagnetism
is a long-ranged force that is relatively strong, and therefore describes
almost all phenomena of our everyday experience—phenomena ranging all the way
from lasers and radios to the structure of atoms and the structure of metals to
friction and rainbows.
·
Electrical
and magnetic forces are involved simultaneously. A changing magnetic field
produces an electric field (this is the phenomenon of electromagnetic
induction). Similarly, a changing electric field generates a magnetic field.
Because of this interdependence of the electric and magnetic fields, it makes
sense to consider them as a single coherent entity — the electromagnetic field
·
Electrical
and magnetic phenomena have been observed since ancient times, but it was only
in the 1800s that it was discovered that these are two aspects of the same
fundamental interaction. One of the first to discover and publish a link
between man-made electric current and magnetism was Romagnosi,
who in 1802 noticed that connecting a wire across a Voltaic pile deflected a
nearby compass needle. However, the effect did not become widely known until
1820, when Řrsted performed a similar experiment. Řrsted's work influenced Ampčre
to produce a theory of electromagnetism that set the subject on a mathematical
foundation.
·
By
1864, James Clerk Maxwell 's equations had quantified
the unified phenomenon into a single theory and discovered the electromagnetic
nature of light. The electromagnetic field obeys Maxwell's equations, and the
electromagnetic force is given by the Lorentz force
law.
·
Starting
around 1927, Paul Dirac unified quantum mechanics
with special relativity; quantum electrodynamics was completed in the 1940s.
·
Theodor Kaluza in 1919 noticed
a curious property of electromagnetism, namely that Maxwell's classical
(non-quantum) theory of electromagnetism arises naturally from the equations of
general relativity with the assumption that there is an extra fourth dimension
of space. This property is the basis of Kaluza-Klein
theories which have been used to formulate a theory of quantum gravity
·
The electromagnetic field encompasses all
of space, and exerts a force on those particles that possess the property of
electric charge, and is in turn affected by the presence and motion of such
particles.
·
Weak Interaction (Weak
Nuclear Force)
·
The weak
interaction is responsible for some phenomena at the scale of the atomic
nucleus, such as beta decay.
·
Electromagnetism
and the weak force are theoretically understood to be two aspects of a unified
electroweak interaction — this realization was the first step toward the
unified theory known as the Standard Model. In electroweak theory, the carriers
of the weak force are massive gauge
bosons called the W and Z bosons.
This unusual feature is explained in the Standard Model by the Higgs mechanism
·
The
weak interaction is the only known interaction in which parity symmetry P
is not conserved; because it only acts on left-handed particles it is
left-right asymmetric. It is the only one which violates CP symmetry. However,
it does conserve CPT
·
The
weak interaction affects all left-handed leptons and quarks. It is the only
force affecting neutrinos (except for gravitation, which is negligible)
·
Due
to the large mass of the weak interaction's carrier particles (about
90 GeV/c2), their mean life is limited to about 3×10−25 seconds
by the uncertainty principle. Even at the speed of light this effectively
limits the range of the weak interaction to 10−18 meters,
about 1000 times smaller than the diameter of an atomic nucleus.
·
Since the weak interaction is both very weak
and very short range, its most noticeable effect is due to its other unique
feature: flavor changing. Consider a neutron (quark content udd;
one up quark, two down quarks). Although the neutron is heavier than the proton
(quark content uud), it cannot decay into a
proton without changing the flavor of one of its down quarks. Neither the
strong interaction nor electromagnetism allow flavor
changing, so this must proceed by weak decay. In this process, a down
quark in the neutron changes into an up quark by emitting a W boson, which then
breaks up into a high-energy electron and an electron antineutrino. Since
high-energy electrons are beta radiation, this is called a
beta decay.
·
There
are three basic types of weak interaction vertices (up to charge conjugation
and crossing symmetry). Two of them involve charged bosons,
they are called "charged current interactions." The third type is
called "neutral current interaction."
o
A
charged lepton (such as an electron or a muon)
emitting or absorbing a W boson and converting into a corresponding neutrino.
o
A
down-type quark (with charge -1/3) emitting or absorbing a W boson and
converting into a superposition of up-type quarks.
o
Either
a lepton or a quark can emit or absorb a Z boson.
·
Strong Interaction (Strong
Nuclear Force)
·
The
Strong Interaction is, as detailed by the theory of quantum chromodynamics (QCD), is the
fundamental force mediated by gluons
acting upon particles that carry "color
charge", quarks, antiquarks, and the gluons
themselves.
·
Although
the strong force only acts upon elementary particles directly, the force
is observed between hadrons as the nuclear force. As has been shown by
many failed free quark searches, the elementary particles affected are
unobservable directly. This phenomenon is called confinement, a theory which allows only hadrons to be seen.
·
Before
the 1970s, when protons and neutrons were thought to be fundamental particles,
the phrase "strong force" was what is today known as the nuclear
force or the residual strong force.
What were being observed were the "residual" effects of the strong
force, which act on hadrons. This force was postulated
to overcome the electric repulsion between protons in the nucleus, and for its
strength (at short distances) it was dubbed the "strong force". After
the discovery of quarks, scientists realized that the force was actually acting
upon the quarks and gluons making up the protons, not the protons themselves.
For some time after this realization, the older notion was referred to as the residual strong force, and the
"new" strong interaction was called colour force.
·
A
unique characteristic of the strong interaction is the fact that these gluons
interact with each other. This causes the strong interaction's strength to be
independent of distance. This can be interpreted to mean that the force has an
infinite range. However, in actuality, since the energy stored in the field
increases with separation between the interacting particles, at large distances
the field contains enough energy to produce particle-antiparticle pairs. When
this occurs, the field lines are cut in half. By this mechanism, strong forces
never act over distances much larger than the proton's radius
·
Hadrons
are held together by the strong force. These include familiar particles such as
protons and neutrons as well as many other baryons and mesons
·
Nucleons
are held to each other in the atomic nucleus by the nuclear force, which
is a residual effect of the strong interaction. This force is unrelated to
electric charge. Because the strong force is so much stronger than the
electromagnetic force, it can easily hold many protons together in the nucleus
despite their tremendous electric repulsion.
·
Classical
mechanics is subdivided into statics (which models
objects at rest), kinematics (which models objects in motion), and dynamics
(which models objects subjected to forces).
·
1553 Giambattista Benedetti, a Venetian
mathematician, determined that falling objects fall at the same rate, a
discovery often credited to Galileo.
·
1590 Galileo's
experiments with falling objects showed that all objects dropped from the same
height accelerated at the same rate regardless of their weight. This
opposed Aristotle’s doctrine that heavier objects fell faster. His work also
showed that if objects were moving at right angles to the ground, the
horizontal and vertical movements were independent of each other. In
other words, there could be a constant speed in the horizontal direction but a
varying speed in the vertical direction. This explained the curved paths that
objects like cannon balls took when fired into the air.
·
1643-1727 Isaac Newton
·
·
1687 Isaac
Newton publishes his Philosophiae Naturalis Principia Mathematica,
better known as the "Principia", considered to be one of the greatest
scientific books of all time, in which he gave gravity a mathematical footing.
·
It contains his Three Laws of Motion:
1.
Law of Inertia: A body at rest remains at rest and a body in motion remains in motion as
long as outside forces are not acting on the body. This removes the need for something to be pushing
planets along. On the Earth, bodies in motion slow down because of forces like
friction or air resistance. In space, where there is no air, no resistance
exists to slow a body down, and it will carry on moving forever.
2.
A force acting on a body causes
it to accelerate in the direction of the force. The acceleration that force imparts on a body is
inversely related to the amount of matter that the body contains, called the
body's mass. If force acts on two objects, the more massive object will
be accellerated less and the less massive object will
be accellerated more. Force = (Mass)(Acceleration).
3.
To every
action there is an equal and opposite reaction. This means that if two bodies collide, they each
impart a force on each other. It also governs the working of jet engines and
rockets: gases escaping from a tube cause the object to move in the opposite
direction.
where F is the force of gravity between
the two masses (m1 and m2). The
distance between them is d. G, is called the Gravitational Constant
and was measured by the English physicist, Henry Cavendish in 1798
with the value 6.673 × 10-11 N m2 kg-2. - .
Energy
Sound
Thermodynamics
·
Thermodynamics
deals with the action of heat and the conversions from one to another of
various forms of energy. Thermodynamics is particularly concerned with how
these affect temperature, pressure, volume, mechanical action, and work.
Historically, it grew out of efforts to construct more efficient heat engines
·
Statistical
mechanics analyzes macroscopic systems by applying statistical principles to
their microscopic constituents and, thus, can be used to calculate the thermodynamic
properties of bulk materials from the spectroscopic data of
individual molecules.
Heat
·
1643 Torricelli develops the mercury barometer
·
1714 Fahrenheit develops the temperature scale and in 1718 the mercury thermometer.
·
Absolute Zero is 0°K, -460°F, -273°C. At this temperature all substances have zero
thermal energy.
o
In
1848, William Thomson, 1st Baron Kelvin proposed an absolute thermodynamic
temperature scale in which equal reduction in measured temperature gave rise to
equal reduction in the heat of a body. This freed the concept from the
constraints of the gas laws and established absolute zero as the temperature at
which no further heat could be removed from a body.
o
At
very low temperatures in the vicinity of absolute zero, matter exhibits many
unusual properties including superconductivity, superfluidity,
and Bose-Einstein condensation.
o
The lowest
temperature actually reached is 450 pK (0.45 billionths of a Kelvin or
4.5x10-10K) by researchers at MIT in 2003. It
may be possible that scientists have already reached Absolute Zero, but when
the system's temperature is measured, entropy is introduced to the system, thus
we may never know for sure.
Electricity
·
1740s Cathode Ray tubes were investigated in the time of Benjamin Franklin.
Electric discharges were passed through rarefied gases in a tube from which the
air was gradually removed by a pump. The tube had two small metal plates inside
a glass container. The plate connected to the negative side of the electricity
supply was called the cathode, and that to the positive side was the anode. As
the pressure was lowered, a spark which fattened into a purplish glowing,
writhing snake appeared going from the cathode to the anode.
·
1745 The Leyden Jar is
described by Georg Van Kleist
at the
·
1790 Aloisio Galvani
describes contact electricity
·
1790s The
·
1811 Sir Humphrey Davy discovered the arc
lamp, an electrical arc passing between two poles produces light
·
1823 The electromagnet is invented in
·
1831 Michael Faraday’s Law of Electrical Induction. Faraday builds an electric dynamo.
·
Faraday formulated the laws of electromagnetic induction
and did the groundwork necessary to make dynamos, electric motors and
transformers. It was Faraday who devised the laws of electrolysis and laid the
foundation for the electroplating industry. Faraday has the international unit
for capacitance named after him, the Farad, marking his distinguished work with
dielectrics; and also a physical constant, the Faraday Constant. He developed
the concept of magnetic and electrical fields, and also showed that the
electrical phenomena exhibited by lightning, electric eels and voltaic cells
are all related. The `Faraday dark space', observed with electrical
discharges in gases (for example, as in fluorescent tubes), pays tribute to
him, and the `Faraday effect' in magneto-optics was one of his triumphs
later in his career.
·
1839 The fuel cell is invented in
·
1841 Arc
lights were installed as public lighting along Place de la Concorde in
·
1879 Light bulb invented simultaneously by Thomas
Edison and Sir Joseph Wilson Swan
·
1882 In
·
1909 The tungsten filament is developed by William Coolidge of the
Superconductivity
·
Superconductivity
occurs in certain metals and alloys at low temperatures (4-90°K) which exhibit
zero electric resistance and diamagnetism.
·
In ordinary conductors such as copper and silver,
impurities and other defects impose a lower limit. Even near absolute zero a
real sample of copper shows a non-zero resistance. The resistance of a
superconductor, on the other hand, drops abruptly to zero when the material is
cooled below its "critical temperature", typically 20K or less. An
electrical current flowing in a loop of superconducting wire can persist
indefinitely with no power source.
·
Like ferromagnetism and atomic spectral lines,
superconductivity is a quantum mechanical phenomenon. It cannot be understood simply
as the idealization of "perfect 
conductivity" in classical
physics.
·
Superconductivity occurs in a wide variety of materials,
including simple elements like tin and aluminum, various metallic alloys, some
heavily-doped semiconductors and a family of cuprate-perovskite
ceramic materials known as high-temperature superconductors. Superconductivity
does not occur in noble metals like gold and silver, nor
in most ferromagnetic metals.
·
The Meissner Effect
will levitate a magnet as long as the magnetic field does not exceed the
critical magnetic field. A magnet that is suspended by the superconductor has
two interesting properties; it does not move, and it can spin without friction.
The ability for the magnet to stay perfectly still is due to flux pinning, in
which the magnetic field lines become trapped within the superconductor at
sites of impurity in the crystal structure.
·
Superconductors are used to make some of the most
powerful electromagnets, including in MRI machines and the beam-steering
magnets used in particle accelerators. They can also be used for magnetic
separation, where weakly magnetic particles are extracted from a background of
less or non-magnetic particles, as in the pigment industries. Superconductors
have also been used to make digital circuits (e.g. based on the Rapid Single
Flux Quantum technology) and microwave filters for mobile phone base stations.
Superconductors are used to build Josephson junctions
which are the building blocks of SQUIDs
(superconducting quantum interference devices), the most sensitive
magnetometers known. Other early markets are arising where the relative
efficiency, size and weight advantages of devices based on HTS outweigh the
additional costs involved.
·
Promising future applications include high-performance
transformers, power storage devices, electric power transmission, electric
motors (e.g. for vehicle propulsion, as in vactrains
or maglev trains), magnetic levitation devices, and Fault Current Limiters.
However superconductivity is sensitive to moving magnetic fields so
applications that use alternating current (e.g. transformers) will be more difficult
to develop than those that rely upon direct current.
·
1911
Superconductivity was discovered by Heike Onnes, who was studying the
resistance of mercury at cryogenic temperatures using the recently-discovered
liquid helium as a refrigerant. At the temperature of 4.2 K, he observed that
the resistance abruptly disappeared. For this discovery, he was awarded the
Nobel Prize in Physics in 1913. In subsequent decades, superconductivity was
found in several other materials.
·
1933 Meissner
and Ochsenfeld discovered that superconductors
expelled applied magnetic fields, a phenomenon which has come to be known as
the Meissner effect.
·
1957 The complete
microscopic theory of superconductivity was proposed by Bardeen,
Cooper, and Schrieffer. This BCS theory explained the
superconducting current as a superfluid of Cooper
pairs, pairs of electrons interacting through the exchange of phonons. For this
work, the authors were awarded the Nobel Prize in 1972.
·
1986 The discovery of
high-temperature superconductors, with critical temperatures in excess of 90 kelvins, spurred renewed interest in superconductivity.
These materials represented a new phenomenon not explained by the current
theory. And, because the superconducting state persists up to more manageable
temperatures, more commercial applications are feasible.
·
SQUIDs, or Superconducting
Quantum Interference Devices, are used to measure extremely small magnetic
fields; they are currently the most sensitive such devices (magnetometers)
known
·
While a typical fridge magnet is ~0.01 tesla (10−2 T), some processes in animals
produce magnetic fields sized between a microtesla
(10−6 T) and a nanotesla (10−9
T).
·
The DC SQUID was invented in 1964 by Ford Research Labs
after B. D. Josephson postulated the Josephson effect in 1962 and the
first Josephson Junction was made at Bell Labs in
1963. The RF SQUID was invented in 1965 at Ford.
·
There are two main types of SQUID: DC and RF (or AC). RF SQUIDs have only one Josephson
junction whereas DC SQUIDs have two or more junctions.
This makes DC SQUIDs more difficult and expensive to
produce, but DC SQUIDs are much more sensitive.
·
Most SQUIDs are fabricated from
lead or pure niobium. To achieve the necessary superconducting characteristics,
the device is cooled to within a few degrees of absolute zero with liquid
helium.
·
Magnetoencephalography (MEG) uses measurements
from an array of SQUIDs to make inferences about
neural activity inside brains.
·
Probably the most common use of SQUIDs
is in magnetic property measurement systems. These are turn-key systems, made
by several manufacturers, that measure the magnetic
properties of a material sample.
·
Magnetism is one of the phenomena by which
materials exert an attractive or repulsive force on other materials.
·
Some
well known materials that exhibit easily detectable magnetic properties are
iron and iron-containing materials (some steels, and the mineral lodestone);
however, all materials are influenced to greater or lesser degree by the
presence of a magnetic field
·
Magnetism
is seen whenever electrically charged particles are in motion. This can arise
either from movement of electrons in an electric current, resulting in an
"electromagnet", or from
spin of electrons, resulting in a "permanent
magnet".
o
Electron
spin is the dominant effect within atoms. The so-called 'orbital motion' of
electrons around the nucleus is a secondary effect.
o
The
overall magnetic moment of the atom is the net sum of all of the magnetic
moments of the individual electrons. The differences in configuration of the
electrons in various elements thus determine the magnetic properties of various
materials.
·
Maxwell's
equations and the Biot-Savart law describe the origin
and behavior of magnetic fields.
·
Magnetic
fields are dipoles, having a
"South pole" and a "North pole"; terms dating back to the
use of magnets as compasses.
o
When
a magnet, that is, an object conventionally described as having a north and a
south pole, is cut in half across the axis joining those "poles", the
resulting pieces are two normal (albeit smaller) magnets each with its own
north pole and south pole, rather than two separate north-only and south-only
pieces.
o
A
magnetic field contains energy, and physical systems stabilize into the
configuration with the lowest energy. Therefore, when placed in a magnetic
field, a magnet tends to align
itself in opposed polarity to that field, thereby canceling the net field
strength as much as possible and lowering the energy stored in that field to a
minimum. For instance, two identical bar magnets normally line up North to South (as opposed to North-North or South-South)
resulting in no net magnetic field.
o
Magnetic monopoles have been posited to exist by some
particle theories, notably Grand Unified Theories and superstring theories, but have not been observed in nature.
·
Maxwell
did much to unify static electricity and magnetism, producing a set of four
equations relating the two fields. However, under Maxwell's formulation, there
were still two distinct fields describing different phenomena. It was Albert
Einstein who showed, using special relativity, that electric and magnetic
fields are two aspects of the same thing, and that one observer may perceive a
magnetic force where a moving observer perceives only an electrostatic force.
Thus, using special relativity, magnetic forces are a manifestation of
electrostatic forces of charges in motion and may be predicted from knowledge
of the electrostatic forces and the velocity of movement (relative to some
observer) of the charges.
·
Einstein
explained in 1905 that a magnetic field
is the relativistic part of an electric field. When an electric charge is
moving from the perspective of an observer, the electric field of this charge
due to space contraction is no longer seen by the observer as spherically
symmetric due to non-radial time dilation, and it must be computed using the Lorentz transformations. One of the products of these
transformations is the part of the electric field which only acts on moving
charges - and we call it the "magnetic field".
·
The
magnetic force is actually due to the finite speed of a disturbance of the
electric field, the speed of light, which gives rise to forces that appear to
be acting along a line at right angles to the charges. In effect, the magnetic
force is the portion of the electric force directed to where the charge used to
be.
·
The
quantum-mechanical motion of electrons in atoms produces the magnetic fields of
permanent ferromagnets. Spinning charged particles
also have magnetic moment. Some electrically neutral particles (like the
neutron) with non-zero spin also have magnetic moment due to the charge
distribution in their inner structure. Particles with zero spin never have
magnetic moment.
History
·
Although known
to the ancients, William Gilbert (1544-1603) pioneered the study of
magnetism. He investigated the earth’s
magnetic field in standard terms of magnetism.
·
John Van Vleck
(1899-1980) is one of the fathers of modern magnetism. He explained the
magnetic, electric, and optical properties of many elements and compounds with
the ligand field theory, and the 
effects
of temperature of paramagnetic materials (Van Vleck paramagnetism).
Electromagnets
· Electromagnets are useful in cases where a magnet must be switched on or off; for instance, large cranes to lift junked automobiles.
·
For
the case of electric current moving through a wire, the resulting field is
directed according to the "right hand rule." If the right hand is
used as a model, and the thumb of the right hand points along the wire from
positive towards the negative side ("conventional current", the
reverse of the direction of actual movement of electrons), then the magnetic
field will wrap around the wire in the direction indicated by the fingers of
the right hand.
·
If a
loop of wire is formed such that the current is traveling in a circle, then all
of the field lines in the center of the loop are directed in the same
direction, resulting in a magnetic
dipole whose strength depends on the current around the loop multiplied
by the number of turns of wire.
· When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as "magnetic"). Due to the way their regular crystalline atomic structure causes their spins to interact, some metals are (ferro)magnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt, nickel, gadolinium, and dysprosium (when at a very low temperature).
·
Technology
has expanded the availability of magnetic materials to include various manmade
products, all based, however, on naturally magnetic elements.
·
Many
materials have unpaired electron spins, but the majority of these materials are
paramagnetic. Paramagnetism is a form of
magnetism which only occurs in the presence of an externally applied magnetic
field. Paramagnetic materials are attracted to magnetic fields, hence have a
magnetic susceptibility. However, unlike ferromagnets,
paramagnets do not retain any magnetisation in the
absence of an externally applied magnetic field. Examples are aluminum, barium,
calcium, oxygen, sodium, uranium, and others.
The
Electromagnetic Spectrum
·
1800 William Herschel discovers infrared
rays. He measured the temperature of different colors of the spectrum by
placing a thermometer on each color. The hottest part of the spectrum was a
spot beyond the red end of the spectrum where there was no color. This light became known as infrared and for the first
time it was possible to talk about invisible
light.
·
Infrared radiation is produced by the vibrations of
molecules. Human skin feels this radiation as heat. Microwave ovens work by
using Infra Red radiation of the correct frequency to make the water molecule
vibrate faster. InfraRed is used as an analytical
tool for molecules in Chemistry. Cool, proto-stars are studied with Infra Red
detectors.
·
1801 Johann William Ritter discovers ultraviolet
light. Ritter was famous as the discoverer of electroplating. He was
experimenting with silver chloride
(AgCl), which is decomposed by light, which
liberates silver and makes the colorless substance turn black. This reaction is
the basis of photography. Ritter measured the speed at which silver chloride
broke down with different colours. Blue light was
indeed more efficient that red light, but he was amazed that the most vigorous
reactions took place in the region beyond the violet where nothing could be
seen. This new radiation was originally called Chemical Rays but is now called Ultra Violet.
o
Visible and Ultraviolet Light is produced by chemical
reactions and ionizations of outer electrons in atoms and molecules. There are
many chemical reactions that are instigated by this radiation: the chemical
retinal in animal eyes, chlorophyll in plants, silver chloride in photography, the
chemical melanin in human skin, silicon converts light to electricity. Light is
the most familiar electromagnetic radiation because the Earth's atmosphere is
transparent to it. Light (and a little of InfraRed
and UltraViolet) can pass through the atmosphere.
Living organisms have evolved to use these waves. Visible Light is simply the
part of the electromagnetic spectrum that reacts with the chemicals in our
eyes. Bees can see more UltraViolet
than we can, and snakes can detect InfraRed.
·
1802 Thomas Young proves the wave theory of light.
The Greeks thought that light was a stream of particles. Isaac Newton thought this also. Young
first demonstrated a simple proof of the wave theory of light. He forced the
light from a single light source to pass through a narrow slit and then through
two more narrow slits within a fraction of an inch of each other. The light
from the two slits fell on a screen and in areas of overlap bands of bright
light alternated with bands of darkness, demonstrating the interference of light. He used his theory to explain the colours of thin films (such as soap bubbles). He also
worked out the wavelength of light to be approximately 1μm, and, relating colour to wavelength, he calculated the approximate
wavelengths of the seven colours recognized by
·
1842 Christian Doppler describes the Doppler Effect – the change in apparent
wavelength of radiation (sound, light etc) emitted by moving the source or the
receiver. The wavelength shortens as the
source and receiver become nearer, and it lengths as they move away from each
other. This can be appreciated by an
approaching car, train etc. Applications
include Doppler radar, medical Doppler ultrasound, and determining the
direction and speed of movement of astronomical objects.
·
1864-1873 James Clerk Maxwell, a Scottish physicist, develops The Electromagnetic Theory.
o
Maxwell examined
the way electricity and magnetism worked and found many similarities between
them (negative / positive, north / south, etc). Mathematically, he devised a
few simple equations that explained all the varied phenomena of electricity and
magnetism and bound them together.
o
He showed that
the oscillation of an electric charge produced an electromagnetic field moving
outward from its source at a constant speed, c, which he calculated to be 300,000 km/s, the same as the speed of light. This was too much of
a coincidence for Maxwell. He suggested that light itself was an
electromagnetic wave. At the time it was not known what oscillating electric
charge was involved. Whatever it was it had to be oscillating at 300 million
MHz to produce the correct type of waves. The oscillations were later found
when the properties of the atom were better understood. Maxwell also suggested
that since electric charges could oscillate with any frequency, there should be
a whole family of electromagnetic radiation of which visible light was only a
small part. Maxwell's work was so important that it survived the Physics
revolution that occurred at the beginning of the 1900s.
·
1885-1889 Heinrich Rudolf Hertz discovers radio
waves. Hertz clarified and expanded the electromagnetic theory of light
that had been put forth by the James Clerk Maxwell in 1884. Hertz proved
that electricity can be transmitted in electromagnetic waves, which travel at
the speed of light and which possess many other properties of light. Hertz
became the first person to broadcast and receive radio waves, and to establish
the fact that light was a form of electromagnetic radiation. He set up electric
circuits that produced oscillations and managed to produce electromagnetic
radiation with a wavelength of 66cm (over a million times longer than light).
This radiation could be picked up by other circuits set up quite a distance
away. His experiments with these electromagnetic waves, known as Hertzian waves and later became known as radio waves, led
to the development of the wireless telegraph and the radio.
o
Radio Waves are produced when free electrons are forced to move
in a magnetic field, or when electrons change their spin in a molecule. They
are used for communication and to study low energy motions in atoms. All electrical
goods generate Radio Waves. Radio Waves from space can be used to study cool
interstellar gases.
·
1895 William Roentgen, a German physicist, discovers X-rays. He was working on Cathode Ray Tubes and
noticed a glow coming from a chemical called barium platinocyanide. This chemical
glowed whenever the tube was on, even if he put cardboard between it and the
tube. Roentgen went on to show that the glow was caused by a highly penetrating
but invisible radiation given off by the tube.
o
He found that the
x-rays traveled in straight lines, and (unlike the cathode rays) were not
deflected by magnetic fields. On the other hand, they didn't exhibit any
diffraction phenomena, so did not seem to be waves. Roentgen, and independently
J. J. Thomson, found that the x-rays were ionizing radiation - as they passed through air, ions were
created. A gold leaf electroscope
exposed to the x-rays would lose its charge, as the newly created ions
were attracted to the charged leaves.
o
It passed
through paper and thin sheets of metal, and flesh almost unimpeded, but bone
cast a shadow. By having his wife place her hand between the point source of
x-rays on the Crookes' tube and some unexposed film
in a box, then developing the film, he took a picture of the bones in her hand.
o
He found lead
was a good shield, and, by using it to shield different parts of the Crookes' tube, established that the x-rays originated in the part of the glass that
fluoresced where the cathode rays hit it. It could
ionize gases and had wave properties like light but only much shorter
wavelengths. The new radiation was called X-Rays because of their mysterious properties. Roentgen refused to
patent the discovery or make any financial gain out of it but he was awarded
the first ever Nobel Prize for Physics.
o
X-Rays are produced by fast electrons stopping suddenly, or
by ionization of the inner electrons of an atom. They are produced by high
energy processes in space: gases being sucked in to a black hole and becoming
compressed; exploding stars. They are used in medicine to look through flesh.
In Physics the waves are small enough to pass between atoms and molecules so
they can be used to determine molecular structures
o
In 1899, Haga and Wind noticed a slight broadening of an x-ray beam
after it passed through a slit a few thousandths of a millimeter wide. This
could be from diffraction if the wavelength were of order 10-10
meters. This problem was not resolved conclusively until 1912, when Laue made the observation that since the wavelength of
x-rays was apparently similar to the distances between planes of atoms in a
crystal, perhaps a crystal would act as a diffraction grating for x-rays. This
turned out to be correct, and in fact is now the standard way of finding
crystal structure.
o
Once the
usefulness of x-rays was established, techniques for producing them evolved
rapidly. It was found that they were produced far more copiously if the cathode
rays impinged on a piece of heavy metal, such as Molybdenum, rather than glass.
The physical picture of x-ray production was that the electrons radiated as
they suddenly decelerated on hitting the target, unloading their kinetic energy
as radiation (plus some heating of the target). This deceleration radiation is
called bremsstrahlung in German, and this word
is sometimes used to describe it.
·
1900 Paul Villard discovers Gamma Rays. Paul Villard's main interest was in chemistry, which guided him
into his studies of cathode rays, x rays, and "radium rays." His
experiments in radioactivity led to the unexpected discovery of gamma rays in
1900. Villard recognized them as being different from
x rays because the gamma rays had a much greater penetrating depth. He had
discovered they were emitted from radioactive substances and were not affected
by electric or magnetic fields, thus were waves not particles. These came to be
called gamma rays by another scientist, Ernest Rutherford. It wasn't until 1914
that Rutherford showed that they were a form of electromagnetic (EM) like light
only with a much shorter wavelength than x rays when
o
Gamma Rays were
found to be very short wave electromagnetic radiation, more penetrating and of
shorter wavelength than even X-Rays. Gamma
Rays are produced by very high energy processes, usually involved with
the nucleus of atoms. Radioactivity and exploding stars produce Gamma Rays.
They are very dangerous because if they strike atoms and molecules they will do
lots of damage. If the molecules are the long and complex molecules of life,
death and mutation could occur. Now we know that gamma rays are a form of EM
radiation similar to x rays. Gamma rays tend to have a higher energy and a
shorter wavelength than x rays do. However, the dividing line between these two
forms of radiation is not clearly defined. Scientists typically apply the term
gamma ray to EM radiation with energies above several hundred thousand electron
volts. One electron volt is the amount of energy gained by an electron as it
moves freely between two points with a potential difference of 1 volt. What I
like to try to think of is that an unstable nucleus or nuclear process (like
annihilation, isometric transitions, etc.) gives off gamma rays, and x rays are
involved in energy transformation of electrons
·
Discovery of electrons
o
1830s
Faraday found that lowering the pressure in a cathode ray tube causes a dark
space to open up near the cathode, now called the Faraday Dark Space.
o
1850 Geissler invented a much better (mercury) pump for cathode
ray tubes. In 1869, Hittorf found that in a very good
vacuum (0.01 mm Hg), the Faraday dark space expanded to fill the whole tube,
and the cathode emitted rays that caused the glass to glow where they hit. In
1876, Goldstein called them "cathode rays".
o
1879 William Crookes, an Englishman, declared that cathode rays must be
particles of some sort, and demonstrated that they traveled in straight lines
by inserting a Maltese cross in the tube, which cast a sharp shadow on the end
of the tube. Heinrich Hertz, with his student Lenard,
discovered in 1891 that the rays could penetrate a thin aluminum plate, and
detected them in the air just outside the tube. Hertz found experimentally that
he could not deflect the rays with an electrostatic field applied from outside
the tube, and they didn't affect a compass, so he concluded (wrongly) that they
must be waves, not particles. Afterwards Germans physicists thought the cathode
rays were waves, the English thought they were
particles (presumably ions of some kind).
o
1898 J. J. Thomson discovers that electrons negatively charged particles, and measured e/m, the ratio of charge
to mass.
§
He placed
cathode tubes in an electric field and the cathode rays bent to one side. Since
these fields will deflect particles but not waves, Thomson proposed that cathode
rays must be made of small particles, which he dubbed "corpuscles”
§
Thomson added a
magnetic field, and by adjusting this magnetic field strength to cancel the
deflection of the rays caused by the electric field he was able to measure the
speed of the rays and the e/m. The value of e/m was a complete
surprise: it was 2,000 times the value for the hydrogen ion. Cathode rays were
particles smaller than the smallest atom.
§
Thomson found
this ratio e/m to be independent of the material the
cathode was made of. The cathode rays could have been fragments of atoms which
were different for different atoms, but evidently this was not the case.
Furthermore, he found in 1899 that photoelectrically
produced particles had the same e/m, so were probably the same particles. For the photoelectrically produced particles, Thomson was able to
find the charge e, in a cloud chamber experiment. Their results
suggested that the particles had the same magnitude charge as the hydrogen ion.
The emerging picture was that the atom, known of course to be electrically
neutral, contained negatively charged particles, the electrons, which could be
removed in various ways, leaving a positively charged ion which contained
almost all the mass of the original atom.
·
1690 Christian Huygen proposes the
wave theory of light
·
1728 James Bradley, an English physicist, estimated the speed of light in a vacuum to be around 186,282 miles (300,000 km)
per second. He used stellar aberration: the apparent position of stars to
change due to the motion of the Earth around the sun and is approximately the
ratio of the speed the earth orbits the sun to the speed of light. He knew the
speed of the earth around the sun and could measure this stellar aberration
angle, enabling him to calculate the speed of light.
o
1676 Olaus Roemer, Danish
astronomer, measures the speed of light
using Jupiter's moons. Roemer was able to predict eclipses of Io by Jupiter.
However, over a period of months, Roemer's predictions were steadily off by
longer and longer intervals of time, then they became
more accurate, till they were correct again. This strange cycle repeated itself
and Roemer realized that this was caused by the differences between the
distance between the earth and Jupiter. Roemer calculated the distance between
the earth and Jupiter and used these inaccurate distances to calculate the
speed of light to be around 200 000 km/sec
·
Color is
determined by the wavelength of light.
Optics
·
1282 Eyeglasses invented by Alessandro di Spina
in
·
1608 Telescope invented by Hans Lippershey, who made glasses
in
·
1783 Bifocal glasses are invented by Benjamin Franklin
Microscopes
·
1590 Simple microscope is developed in
·
1674 Multiple-lens microscope developed by Dutch scientist Antonie van Leeuwenhoek (Single-lens microscopes were used as early as the
mid-1400s).
Atomic
Structure
·
Molecules are the smallest particles into which a
non-elemental substance can be divided while maintaining the physical
properties of the substance.
·
Each
type of molecule corresponds to a specific chemical compound. Molecules are
composites of one or more atoms.
·
Atoms are the smallest neutral particles into which
matter can be divided by chemical reactions.
·
An
atom consists of a small, heavy nucleus surrounded by a relatively large, light
cloud of electrons.
·
Each
type of atom corresponds to a specific chemical element, of which 111 have been
officially named.
·
Atomic nuclei consist of protons and neutrons.
·
Each
type of nucleus contains a specific number of protons and a specific number of
neutrons, and is called a nuclide or isotope.
·
Nuclear
reactions can change one nuclide into another
·
Nuclei
are held together by the strong interaction
(mostly exchanging pions), but electromagnetic
repulsion between the positively charged protons tends to push each other
apart, according to Coulomb's law.
·
Stable nuclei have the lowest energy ratio of protons to
neutron for their atomic weight.
·
Nuclei
near enough to this ratio to be bound but not close enough to be stable, give
off electrons or positrons (beta decay) or take in electrons (and also give off neutrinos), to move closer to that
ratio. This is the main place where the weak
interaction comes in.
·
Nuclei
that are unstable are pulled apart
by the coulomb repulsion of their protons and either fission or give off alpha
particles
History
·
By at
least the 6th century BC the philosophical doctrine of atomism was studied by ancient Greek philosophers such as
Leucippus, Democritus, and Epicurus.
·
1803 John
1.
All matter is
composed of small indivisible particles termed atoms
2.
Each element has
an atom that differs in mass to other atoms
3.
Three types of
atoms exist: simple (elements), compound (simple molecules), and complex
(complex molecules). Chemical combinations of different
elements occurs in simple numerical ratios by weight, which led to the
development of the laws of definite and multiple proportions.
·
1896 Radioactivity was discovered by Henri Becquerel, and further studied by Marie Curie, Pierre Curie,
and others.
·
1905 Einstein proved the existence of atoms from direct observations (an effect called
Brownian Motion).
·
1897 Joseph J. Thomson discovered the electron. In 1904, he proposed the first model of the atom, known
as the plum pudding model.
·
1911 Ernest Rutherford propose the existence of the atomic nucleus as a central
concentration of protons and,
incorrectly, electrons. This was confirmed by the experiments of Geiger and Marsden
·
In
the 1940s and 1950s, it was discovered that the nucleus was composed of protons and neutrons.
·
Throughout
the 1950s and 1960s, a bewildering variety of subatomic particles was found in
scattering experiments. This was referred to as the "particle zoo".
·
In
the 1970s the Standard Model is
formulated in which the large number of particles was explained as combinations
of a (relatively) small number of fundamental particles.
Binding
Energy
·
When
a nucleon such as a proton or neutron is added to a nucleus, the strong force
attracts it to other nucleons, but primarily to its immediate neighbors due to
the short range of the force. The nucleons in the interior of a nucleus have
more neighboring nucleons than those on the surface. Since smaller nuclei have
a larger surface-to-volume ratio, the binding energy per nucleon due to the
strong force generally increases with the size of the nucleus.
·
The
electrostatic force, on the other hand, is an inverse-square force, so a proton
added to a nucleus will feel an electrostatic repulsion from all the
other protons in the nucleus. The electrostatic energy per nucleon due to the
electrostatic force thus increases without limit as nuclei get larger.
·
The
net result of these opposing forces is that the binding energy per nucleon
generally increases with increasing size, up to the elements iron and nickel,
and then decreases for heavier nuclei. Eventually, the binding energy becomes
negative and very heavy nuclei are not stable.
·
The
energy released in most nuclear reactions is much larger than that for chemical
reactions, because the binding energy that holds a nucleus together is far
greater than the energy that holds electrons to a nucleus. For example, the
ionization energy gained by adding an electron to a hydrogen nucleus is 13.6
electron volts—less than one-millionth of the 17 MeV
released in the D-T (deuterium-tritium) reaction
Subatomic
Particles
·
Many
elementary particles do not occur under normal circumstances in nature, but can
be created and detected during energetic collisions of other particles, as is
done in particle accelerators
·
Subatomic
particles include atomic constituents such as electrons, protons, and neutrons
(protons and neutrons are actually composite particles, made up of quarks),
particles produced by radiative and scattering
processes, such as photons, neutrinos, and muons, as
well as a wide range of exotic particles.
·
The
term particle is a misnomer because the dynamics of particle physics are
governed by quantum mechanics. As such, they exhibit wave-particle duality, displaying particle-like behavior under
certain experimental conditions and wave-like behavior in others (more
technically they are described by state vectors in a Hilbert space; see quantum
field theory).
The Standard Model
·
All
the particles and their interactions observed to date can be described by a
quantum field theory called the Standard
Model.
·
The
Standard Model has been found to agree with almost all the experimental tests
conducted to date. However, most particle physicists believe that it is an
incomplete description of Nature, and that a more fundamental theory awaits
discovery. In recent years, measurements of neutrino mass have provided the first experimental deviations from
the Standard Model.
·
The
Standard Model has 40 species of elementary particles (24 fermions, 12 vector bosons,
and 4 scalars), which can combine to
form composite particles, accounting for the hundreds of other species of
particles discovered since the 1960s.
·
It
describes the strong, weak, and electromagnetic fundamental forces, using
mediating gauge bosons. The species
of gauge bosons are the gluons, W and Z bosons, and photons, respectively.
·
The
model contains 24 fundamental particles, which are the constituents of matter.
·
It
predicts the existence of a type of boson known as the Higgs boson, which has
yet to be discovered.
Particle Accelerators
·
Particle
accelerators typically cost several billion US dollars and require large
amounts of government funding. In 1993, the US Congress stopped the
Superconducting Super Collider (SSC) after US$2
billion had already been spent on its construction. Many believe that the
decision to stop construction of the SSC was due in part to the end of the Cold
War.
·
The
completion of the Large Hadron Collider
(LHC) in 2007 will continue the search for the Higgs boson, supersymmetric
particles, and other new physics.
·
An
International Linear Collider (ILC) was proposed in
2004, but the site has still to be agreed upon.
In particle physics, the major international collaborations are:
·
Brookhaven
National Laboratory on
·
Budker Institute of Nuclear Physics (
·
CERN,
near
·
DESY
in
·
Fermilab, near
·
KEK
in
·
SLAC,
near
Fermions (half-integer spin)
· Fermions are the basic building blocks of all matter.
·
Fermions
have half-integer spin; for all known elementary fermions this is ˝.
·
Each fermion has its own distinct antiparticle.
·
They
are classified according to whether they interact via the color force or not.
·
According
to the Standard Model, there are 12 flavors of elementary fermions: six quarks
and six leptons.
·
Quarks interact via the color force. Their respective
antiparticles are known as antiquarks. Quarks exist
in six flavors:
|
Generation |
Name/Flavor |
Electric charge
(e) |
Mass (MeV) |
Antiquark |
||
|
1 |
Up |
(u) |
+2/3 |
1.5 to 4 |
antiup quark |
|
|
Down |
(d) |
−1/3 |
4 to 8 |
antidown quark |
|
|
|
2 |
Strange |
(s) |
−1/3 |
80 to 130 |
antistrange quark |
|
|
Charm |
(c) |
+2/3 |
1,150 to 1,350 |
anticharm quark |
|
|
|
3 |
Bottom |
(b) |
−1/3 |
4,100 to 4,400 |
antibottom quark |
|
|
Top |
(t) |
+2/3 |
171,400 ± 2,100[1] |
antitop quark |
|
|
·
Leptons do not interact via the color force. Their
respective antiparticles are known as antileptons (although
the antiparticle of the electron is called the positron for historical
reasons). Leptons also exist in six flavors:
|
Charged lepton /
antiparticle |
|
Neutrino /
antineutrino |
||||||
|
Name |
Symbol |
Electric charge
(e) |
Mass (MeV) |
|
Name |
Symbol |
Electric charge
(e) |
Mass (MeV) |
|
Electron/ Positron |
|
−1 / +1 |
0.511 |
|
Electron neutrino / Electron antineutrino |
|
0 |
< 0.0000022 |
|
Muon |
|
−1 / +1 |
105.7 |
|
Muon neutrino / Muon antineutrino |
|
0 |
< 0.17 |
|
Tau lepton |
|
−1 / +1 |
1,777 |
|
Tau neutrino / Tau antineutrino |
|
0 |
< 15.5 |
·
Note that the neutrino masses are known to be non-zero
because of neutrino oscillation, but their masses are sufficiently light that
they have not been measured directly as of 2006.
Bosons (integer spin)
· Bosons have whole number spins. The fundamental forces of nature are mediated by gauge bosons, and mass is hypothesized to be created by the Higgs boson. According to the Standard Model the elementary bosons are:
|
Name |
Charge (e) |
Spin |
Mass (GeV) |
Force mediated |
|
Photon |
0 |
1 |
0 |
Electromagnetism |
|
W± |
±1 |
1 |
80.4 |
Weak nuclear |
|
Z0 |
0 |
1 |
91.2 |
Weak nuclear |
|
Gluon |
0 |
1 |
0 |
Strong nuclear |
|
Higgs |
0 |
0 |
>112 |
See below |
·
The
Higgs boson (spin-0) is predicted by electroweak theory, and is the only
Standard Model particle not yet observed. In the Higgs mechanism of the
Standard Model, the massive Higgs boson is created by spontaneous symmetry
breaking of the Higgs field. The intrinsic masses of the elementary particles
(particularly the massive W± and Z0 bosons) would be
explained by their interactions with this field. Many physicists expect the
Higgs to be discovered at the Large Hadron Collider (LHC) particle accelerator now under construction
at CERN.
·
Supersymmetric theories predict the existence of more particles,
none of which have been confirmed experimentally as of 2006.
Hadrons
·
Hadrons
are defined as composite particles which are strongly interacting. Hadrons are
either composites of:
o
Fermions,
in which case they are called baryons.
o
Bosons,
in which case they are called mesons.
·
Quark
models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe the
known hadrons as composed of valence quarks and/or antiquarks,
tightly bound by the color force, which is mediated by gluons. A
"sea" of virtual quark-antiquark pairs is
also present in each hadron.
Mesons
·
Ordinary
mesons contain a valence quark and a valence antiquark,
and include the pion, kaon,
the J/ψ, and many other types of mesons
Baryons
·
Ordinary
baryons (fermions) contain three valence (up/down)
quarks or three valence antiquarks each.
·
Nucleons are the fermionic
constituents of normal atomic nuclei:
o
Protons
o
Neutrons
·
Hyperons such as the Λ, Σ, Ξ, and Ω
particles, which contain one or more strange quarks, are short-lived and
heavier than nucleons. Although not normally present in atomic nuclei, they can
appear in short-lived hypernuclei.
·
A
number of charmed and bottom baryons have also been observed.
Nuclear
Reactions
·
A nuclear
reaction is a process in which two nuclei or nuclear particles collide, to
produce different products than the initial products. For example: 63Li
+ 21H → 42He + 42He
·
Many
particles appear in reactions so often that they are usually abbreviated. Thus,
a helium nucleus (also known as an alpha
particle) is written with the Greek letter "α". Deuterons
(heavy hydrogen, 21H) are written simply as
"d".
·
The
energy released in a nuclear reaction can appear mainly in one of three ways:
kinetic energy of the product particles , emission of
very high energy photons, called gamma rays, or some energy may remain in the
nucleus, as a metastable energy level. When the
product nucleus is metastable, this energy is
eventually released through nuclear decay.
·
Energy
may be released during the course of a reaction or energy may have to be
supplied for the reaction to take place. This can be calculated by adding the
atomic weights on each side of the reaction. Using Einstein's famous
"E=mc˛" formula, we can work out how much energy has been released or
required, as one atomic mass unit is equivalent to 931 MeV
·
In
the initial collision which begins the reaction, the particles must approach
closely enough so that the short range strong force can affect them. As most
common nuclear particles are positively charged, this means they must overcome
considerable electrostatic repulsion before the reaction can begin. Thus, such
particles must be first accelerated to high energy, for example by:
·
Particle
accelerators
·
Nuclear
decay (alpha particles are the main type of interest here, since beta and gamma
rays are rarely involved in nuclear reactions)
·
Thermonuclear
reactions producing very high temperatures, on the order of millions of degrees
·
Cosmic
rays
·
Reactions
between heavy nuclei require higher initiating energy than light nuclei.
·
Neutrons,
on the other hand, have no electric charge to cause repulsion, and are able to effect a nuclear reaction at very low energies.
·
While
the number of possible nuclear reactions is immense, there are several types
which are more common, or otherwise notable. Some examples include:
·
Fusion reactions: Two light nuclei join to form a heavier one,
with additional particles (usually protons or neutrons) thrown off to conserve
momentum.
·
Fission reactions: A heavy nucleus, spontaneously or after
absorbing additional particles (usually neutrons), splits into two or sometimes
three pieces. (α decay is not usually called
fission.)
·
Spallation: A nucleus is hit by a particle with
sufficient energy and momentum to knock out several small fragments or, smash
it into many fragments
Nuclear Fission
·
Nuclear fission is a process in which the nucleus of an atom
splits into two or more smaller nuclei and some by-product particles. The
by-products include free neutrons, photons usually in the form gamma rays, and
other nuclear fragments such as beta particles and alpha particles. Fission is
an exothermic reaction and can release substantial amounts of useful energy
both as gamma rays and as kinetic energy of the fragments (heating the material
where fission takes place).
·
Many
heavy elements, such as uranium, thorium, and plutonium, undergo both spontaneous fission, a form of
radioactive decay and induced fission, a form of nuclear reaction.
·
Nuclear
fission differs from other forms of radioactive decay in that it can be
harnessed and controlled via a chain
reaction: free neutrons released by each fission event can trigger yet more
events, which in turn release more neutrons and cause more fissions.
·
Isotopes that can sustain a fission chain
reaction are called nuclear fuels,
and are said to be fissile.
·
The
most common nuclear fuels are 235U (Uranium-235) and 239Pu
(Plutonium-239).
·
Nuclear
fuels break apart into a range of chemical elements with atomic masses near 100
(fission products).
·
Most
nuclear fuels undergo spontaneous
fission only very slowly, decaying mainly via an alpha/beta decay chain
over periods of millennia to eons.
·
Until the early 1970s there was little reason to think that nuclear
fission had ever occurred naturally on Earth. In 1972, however, French
scientists found something odd in a group of uranium ore samples from a mine in
·
Nuclear
fission produces energy for nuclear
power and to drive explosion of nuclear
weapons.
·
In a
nuclear reactor or nuclear weapon, most fission events are induced by
bombardment with another particle such as a neutron.
·
Chain reactions release energy at a controlled rate in a nuclear reactor or
at a very rapid uncontrolled rate in a nuclear weapon
· The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline. However, the waste products of nuclear fission are highly radioactive and remain so for millennia, giving rise to a nuclear waste problem.
·
The
energy of nuclear fission is released as kinetic energy of the fission products
and fragments, and as electromagnetic radiation in the form of gamma rays; in a
nuclear reactor, the energy is converted to heat as the particles and gamma
rays collide with the atoms that make up the reactor and its working fluid,
usually water or occasionally heavy water.
History
·
1934 The first studies by Enrico Fermi of the bombardment of uranium by neutrons had proved
puzzling and were not properly interpreted until several years later.
·
1939 Danish physicist Niels Bohr arrived in the
·
The
occasion for this hypothesis was the important discovery of Otto Hahn and Fritz
Strassmann in
·
A
major focus of early fission research was on producing a controllable nuclear
chain reaction, which would mark the first harnessing of nuclear power. This
led to the development of Chicago Pile-1, the world's first man-made critical
nuclear reactor (which used uranium, the only natural nuclear fuel available in
macroscopic quantities), and then to the
Nuclear Reactors
·
Power
reactors are intended to
produce heat for nuclear power, either as part of a generating station or a
local power system such as a nuclear submarine.
·
Research
reactors are intended to
produce neutrons and/or activate radioactive sources for scientific, medical,
engineering, or other research purposes.
·
Breeder reactors are intended to produce nuclear fuels in bulk
from more abundant isotopes. The most common type makes 239Pu (a
nuclear fuel) from the naturally very abundant 238U (not a nuclear
fuel).
Nuclear
Fusion
·
Nuclear
fusion is the process by
which multiple nuclei join together to form a heavier nucleus. It is
accompanied by the release or absorption of energy depending on the masses of
the nuclei involved. Iron and nickel nuclei have the largest binding energies
per nucleon of all nuclei and therefore are the most stable. The fusion of two
nuclei lighter than iron or nickel generally releases energy while the fusion
of nuclei heavier than iron or nickel absorbs energy; vice-versa for the
reverse process, nuclear fission.
·
Building
upon the nuclear transmutation experiments of Ernest Rutherford done a few
years earlier, fusion of light nuclei (hydrogen isotopes) was first observed by
Mark Oliphant in 1932, and the steps of the main cycle of nuclear fusion in
stars were subsequently worked out by Hans Bethe
throughout the remainder of that decade.
·
Nuclear
fusion of light elements releases the energy that causes stars to shine and hydrogen
bombs to explode.
o
The
fusion process which powers the stars is the fusion of four protons into one
alpha particle, with the release of two positrons, two neutrinos (which changes
two of the protons into neutrons), and energy, but several individual reactions
are involved, depending on the mass of the star. For stars the size of the sun
or smaller, the proton-proton chain dominates. In heavier stars, the CNO cycle
is more important. Both types of processes are responsible for the creation of
new elements as part of stellar nucleosynthesis.
o
Nuclear
fusion of heavy elements (absorbing energy) occurs in the extremely high-energy
conditions of supernova explosions. Nuclear fusion in stars and supernovae is
the primary process by which new natural elements are created.
o
The
only fusion reactions thus far produced by humans to achieve ignition are those
which have been created in hydrogen bombs.
o
Controlled
nuclear fusion as a source of usable energy is still being investigated.
·
It
takes considerable energy to force nuclei to fuse, even those of the lightest
element, hydrogen. But the fusion of lighter nuclei, which creates a heavier
nucleus and a free neutron, will generally release more energy than it took to
force them together—an exothermic process that can produce self-sustaining
reactions
·
A
very substantial energy barrier must be overcome before fusion can occur. As
two nuclei approach each other, all the protons in one nucleus repel all the
protons in the other. Not until the two nuclei actually come in contact can the
strong nuclear force overcome the electrostatic repulsion. Consequently, even
when the final energy state is lower, there is a large energy barrier that must
first be overcome, called the Coulomb
barrier.
o
The
Coulomb barrier is smallest for isotopes of hydrogen - they contain only a
single positive charge in the nucleus.
o
Using
deuterium-tritium fuel, the resulting energy barrier is about 0.1 MeV.
o
If
the energy to initiate the reaction comes from accelerating one of the nuclei,
the process is called beam-target fusion; if both nuclei are
accelerated, it is beam-beam fusion.
o
If
the nuclei are part of a plasma near thermal
equilibrium, one speaks of thermonuclear fusion. Temperature is a
measure of the average kinetic energy of particles, so by heating the nuclei
they will gain energy and eventually have enough to overcome this 0.1 MeV barrier. Converting the units between electronvolts and kelvins shows
that the barrier would be overcome at a temperature in excess of 1 GK,
obviously a very high temperature.
o
There
are two effects that lower the actual temperature needed. One is the fact that
temperature is the average kinetic energy, implying that some nuclei at
this temperature would actually have much higher energy than 0.1 MeV, while others would be much lower. The other effect is
quantum tunneling. The nuclei do not actually have to have enough energy to
overcome the Coulomb barrier completely. If they have nearly enough energy,
they can tunnel through the remaining barrier. For this reason fuel at lower
temperatures will still undergo fusion events, at a lower rate.
·
The
fusion reaction can sustain itself if enough of the energy produced goes into
keeping the fuel hot. This is done by concentrating the fuel, known as
confinement
o
In
the case of stars, the gravitational force produces the energy necessary for
confinement
o
Magnetic
confinement: Since
plasmas are very good electrical conductors, magnetic fields can also confine
fusion fuel. A variety of magnetic configurations can be used, the most basic
distinction being between mirror confinement and toroidal
confinement, especially tokamaks and stellarators
o
Inertial
confinement: Applying a
rapid pulse of explosive energy to the surface of a pellet of fusion fuel will
cause it to simultaneously "implode" and heat to very high pressure
and temperature. Inertial confinement is used in the hydrogen bomb, where the
driver is x-rays created by a fission bomb. Inertial confinement is also
attempted in "controlled" nuclear fusion, where the driver is a
laser, ion, or electron beam, or a Z-pinch
·
Radioactivity was found to be caused by atoms breaking down and emitting three
different types of rays: Alpha, Beta and Gamma Rays.
·
Alpha Rays
were shown to be a Helium nucleus.
·
Beta Rays
were shown to be very fast electrons.
·
Gamma Rays were found to be very short wave electromagnetic
radiation (photons), more penetrating and of shorter wavelength than even
X-Rays.
·
Uranium and
other radioactive elements emit alpha particles or beta particles from their
nuclei when they transform into new elements. An instant later, these nuclei
may give off gamma rays.
·
A nucleus may
also emit a gamma ray alone in an isomeric transition in which the composition
of the nucleus does not change but the nucleus merely loses a certain amount of
energy
·
Most
radioactive nuclei are therefore relatively young, having formed in stars
(particularly supernovae) and during ongoing interactions between stable
isotopes and energetic particles.
Carbon-14, a radioactive nuclide with a half-life of only 5730 years, is
constantly produced in Earth's upper atmosphere due to interactions between
cosmic rays and Nitrogen.
·
The decay
rate, or activity, of a radioactive substance is characterized by
its half life (t1 / 2) - the time
for half of a substance to decay
·
History
·
1896 Henri Becquerel discovers radioactivity
in uranium salts. Following the discovery of X-rays which was accompanied by a
type of phosphorescence in the vacuum tube, Becquerel decided to investigate
whether there was any connection between X-rays and naturally occurring
phosphorescence. Phosphorescent
materials glow in the dark after exposure to light, and he thought that the
glow produced in cathode ray tubes by X-rays might somehow be connected with
phosphorescence. So he tried wrapping a photographic plate in black paper and
placing various phosphorescent minerals on it. All results were negative until
he tried using uranium salts. The result with these compounds was a deep
blackening of the plate. There was some new form of radiation that could pass
through paper that was causing the plate to blacken. After experimenting with various substances, he
concluded that the radiation came from any substances containing
uranium, whether they fluoresced or not, and was a property of the uranium
atom. He found the intensity of the radiation increased with the amount of
uranium present, and did not appear to change in intensity with time, or
temperature, or chemical action. Like x-rays, the radiation was ionizing.
If a piece of uranium is held near an electroscope, the electroscope
discharges. They differed from X-rays in that they could be deflected by
electric or magnetic fields but couldn't be turned off. Becquerel was awarded
half of the Nobel Prize for Physics in 1903, the other
half was given to Pierre and Marie Curie.
·
It
was found that an electric or magnetic field could split radioactive emissions
into three beams. The rays were given the names alpha, beta, and gamma. It was
obvious from the direction of electromagnetic forces that alpha rays carried a
positive charge, beta rays carried a negative charge, and gamma rays were
neutral. From the magnitude of deflection, it was also clear that alpha
particles were much more massive than beta particles. Passing alpha rays
through a thin glass membrane and trapping them in a discharge tube allowed
researchers to study the emission spectrum of the resulting gas, and ultimately
prove that alpha particles are in fact helium nuclei. Other experiments showed
the similarity between beta radiation and cathode rays; they are both streams
of electrons, and between gamma radiation and X-rays. Alpha particles are
completely stopped by a sheet of paper, beta particles by an aluminum plate.
Gamma rays however, can only be reduced by much more substantial obstacles,
such as a very thick piece of lead.
·
1898 Marie Curie discovers the radioactive elements radium and polonium. The mineral pitchblende,
rich in uranium oxide, was found to be more radioactive than pure
uranium metal. Marie concluded that it must contain some other element that was
more radioactive than uranium. In 1898, the Curies chemically separated out a
new element they called polonium, after Marie's country of birth,
·
1899 Ernest Rutherford studied absorption of the Becquerel rays,
and found that one component, which he called the alpha-rays, could be absorbed by a sheet of writing paper,
or a few centimeters of air. In fact, Becquerel had not detected these alphas,
because they were absorbed by the box containing the film. A second component,
the beta rays,
·
By 1900 because
of their deflection by a magnetic field it was found that the beta rays were
negatively charged particles. Becquerel found e/m for beta rays
to be quite close to that for cathode rays, suggesting that they also were
electrons. One difference between the beta rays and cathode rays was that the
beta rays could be much faster
- up to 95% of the speed of light. The interesting historical point here is
that physicists already expected that mass might change with speed. In 1908, a
German experimenter, Bucherer, claimed that the best
fit to the e/m data was given by Einstein's much simpler recent
formula for mass variation with speed.
·
Alpha radiation
proved more difficult to identify. At first the alphas were thought to be electrically
neutral, because there was no observed deflection as they shot through a
magnetic field. It became clear later that this lack of deflection was because
the alphas were much more massive than the betas (electrons). In 1903
·
1904
General theory of radioactivity is proposed by Rutherford and Soddy.
·
1908 The Geiger counter is invented in
·
1911 Georg von Hevesy conceives the
idea of using radioactive tracers. This idea is later applied to, among other
things, medical diagnosis. Von Hevesy wins the Nobel
Prize in 1943.
·
Acute
effects of radiation were first observed in the use of X-rays when an Serbo-Croatian-American
electric engineer Nikola Tesla intentionally
subjected his fingers to X-rays in 1896. He published his observations
concerning the burns that developed, though he attributed them to ozone rather
than to the X-rays. Fortunately his injuries healed later. The genetic effects
of radiation, including the effects on cancer risk, were recognized much later.
It was only in 1927 that Hermann Joseph Muller published his research that
showed the genetic effects. Before the biological effects of radiation were
known, many physicians and corporations had begun marketing radioactive
substances as patent medicine and Radioactive
quackery; particularly alarming examples were radium enema treatments, and
radium-containing waters to be drunk as tonics. Marie Curie spoke out against
this sort of treatment, warning that the effects of radiation on the human body
were not well understood (Curie later died from aplastic
anemia assumed due to her own work with radium, but a better candidate for her
disease was her long exposure to unshielded X-ray tubes while a volunteer
medical worker in WW I). By the 1930's, after a number of cases of
bone-necrosis and death in enthusiasts, radium-containing medical products had
all but vanished from the market.
Radioactive Decay
·
Radioactive
decay is the set of
various processes by which unstable atomic
nuclei emit subatomic particles (radiation).
This is a random process, i.e. it is impossible to predict the decay of
individual atoms
·
Quantum-mechanical
particles are never at rest; they are in continuous random motion. If its
constituent particles move in concert, the nucleus can spontaneously destabilize. The resulting transformation alters
the structure of the nucleus
Relativity
·
In the 1800s the procession of Mercury was described: as Mercury
went around its orbit, its entire orbit rotated as well. People thought that another planet was causing it to
deviate from its predicted (Newtonian) path, but this hypothetical planet
(originally called Vulcan) was
never seen.
·
1889 Michelson-Morley Experiment.
o
After the
development of Maxwell's theory of electromagnetism, it was postulated that
since light traveled as a wave, it would have to travel through some substance
(like waves in water). Since there was no logical answer to what this was, an
ancient idea was revived: the aether. Several
experiments were performed to prove the existence of aether
(through which light would travel as a wave) and its motion relative to the
Earth.
o
Albert
Michelson and Edward Morley
set up an experiment to measure the “drag” of the aether
on the Earth and thus prove its existence. They measured the speed of a light
beam traveling parallel to the direction of the Earth's motion and compared
that to a light beam traveling perpendicular to the Earth's motion. The aether idea predicted that light would have a slightly
different speed in the two directions. Michelson and Morley found no
difference in the speed of light in two perpendicular directions. They repeated
the experiment many times but eventually they gave up and published their
experiments describing them as a failure.
This leads to Einstein’s two basic assumptions below.
·
1905 Albert Einstein develops The
Special Theory of Relativity.
o
The theory of
relativity did away with the following assumption of Newtonian physics:
o
Time is constant
and the same for all observers
o
There is no
limit to how fast something can go
o
Matter and energy are two separate phenomena
o
The mass and length of a body are fixed.
o
Einstein made two basic assumptions:
o
Aether does not
exist. Light is an oscillation of
magnetic and electric fields, not a vibration in a medium. This actually means
that you cannot measure absolute speed. Speed is relative between two objects.
You can measure your speed relative to the Earth, the Earth's speed relative to
the Sun, etc., but you cannot measure your speed absolutely.
o
The velocity
of light is constant for all observers. If an
object travelling at 100 km/s sends out a beam of
light to us and we measure its speed, common sense tells us that we should
measure the speed of light to be 300,100 km/s. But no, Einstein says the speed
of light will be measured at 300,000 km/s regardless of how the source or the
observer is moving. Modern laser techniques cannot measure any difference in
the speed of light no matter what speed the source or observer have.
o
Predictions made by the Theory:
o
The mass
of an object increases with its velocity. As a body approaches the speed of
light its mass approaches infinity. This is where the dictum that nothing can
travel faster than light came from. It has been tested with sub-atomic
particles and close binary stars and in both cases the mass change agrees with
Einstein's predictions. Mercury’s velocity is high enough to increase its mass
enough to cause the discrepancy with its orbit observed over half a century
before
o
The length
of an object decreases with its velocity. Again, Einstein's equations predict
that an object's length would become zero at the speed of light. This has been
tested indirectly by an experiment depending on something called the Mossbauer Effect.
o
Time slows down for a body that is moving and for one in
a gravitational field. At the speed of light all time slows down to zero.
Sub-atomic particles last longer before decaying when they are moving close to
the speed of light. Energy from Pulsars
and White Dwarfs (stars with
huge gravitational fields) slow down the vibration of atoms which can be
detected. Using Atomic clocks it is possible now to measure these effects on
the Earth.
o
Light should bend around a
gravitational field. This one is difficult to explain but is to do with time
varying near a strong gravitational field. This was first measured during a
total eclipse of the sun in 1919. The effect is similar to refraction where a
stick appears to bend where it enters water. These so-called gravitational
lenses have since been observed amongst distant galaxies. For Cosmology, if
there is enough matter in the Universe, a beam of light will eventually return
to its starting point.
o
Matter and Energy are the same phenomenon, E=mc2. Matter can thus be
converted into a very large amount of energy, i.e. the atomic bomb. This
prediction also indicates that if the Universe were static it would be
unstable, and a few years after this prediction was made the Universe was found
to be expanding.
·
1915 Einstein’s General Theory of Relativity.
Quantum
Mechanics
·
The Newtonian
(or Classical) description of the atom, with protons and neutrons in a nucleus
surrounded by electrons, violated Maxwell's Laws of Electromagnetism. Under
those laws, an electrically charged object (like an electron) that was changing
direction (in orbit around the nucleus of an atom) should be radiating energy
away until it spiraled into the nucleus. Atoms only absorb specific wavelengths
of energy: sodium radiated yellow light (hence its use in street lamps),
potassium radiated lilac (hence the color of fireworks). This was a major flaw
in the physics of the turn of the century. Physics had other phenomena that
didn't work as predicted: a hot, glowing body radiated energy at a given
temperature (the Black Body Problem); metals produced electricity when
light shone on them (the Photoelectric Effect); the way atoms decayed
when they were radioactive.
·
1900 Max Planck’s Quantum Theory. Most people at the turn of the century thought of
light as a wave. Planck was
studying the “black box problem”: the laws of physics predicted that if you
heat up a box in such a way that no light can get out (known as a "black
box"), it should produce an infinite amount of
ultraviolet radiation. But in real life the box radiated different
colors, red, blue, white, just as heated metal does, but there was no infinite
amount of anything. Planck presumed that the light wasn't a continuous wave,
but radiated only specific amounts, or quanta,
of energy. Planck didn't really believe this was true about light, in fact he later referred to this mathmatical
gimmick as "an act of desperation." But with this adjustment,
the equations worked, accurately describing the box's radiation.
·
1905 Einstein applied Planck's quantum idea to explain the Photoelectric Effect. It
was as if in some experiments (refraction, diffraction) light was clearly a
wave; in others (black body radiation, the photoelectric effect) it was a
particle. This effect was known as wave-particle duality.
·
1912 Louise
de Broglie, suggested that if energy could behave as both
particles and waves, perhaps matter could also. He predicted that under the
right conditions a beam of electrons (clearly matter made of particles) might
show wave properties. When the experiment was performed, a beam of electrons was found to diffract just like a
wave. It looked like energy and matter could both exhibit wave-particle duality
·
1913 Ernest Rutherford proposed that all matter consisted of these three
particles referred to as elementary
particles: protons, neutrons, and electrons
·
1913 Niels Bohr published a theory about the structure of the atom
based on an earlier theory of
·
1919 Protons
are discovered
·
1925 Erwin Schrodinger
and Werner Heisenberg separately
describe quantum mechanics.
The 'position' of a particle like an electron is given by a probability.
Electrons exist in energy states. When they absorb energy, they absorb a whole
number of quanta, disappear, appearing at a different energy state. Gone is the
idea of little ball-like particles. The orbit of an electron is a cloud of
probability around the nucleus.
·
The
Heisenberg Uncertainty Principle
states on a subatomic level it is impossible to simultaneously measure the
speed and the position of an electron. If the speed is well-established then
there simply does not exist a well-established position (the electron is
smeared out like a wave) and vice versa.
·
A
zero energy is impossible since
it would be a precise state. This is the reason that nothing can be cooled
below -273 degrees C (Absolute Zero). An atom must retain at least one quantum
of energy and this keeps it from cooling below Absolute Zero. This means that
nothing can ever be at rest.
·
Albert Einstein
disliked this idea. When confronted with the notion that the laws of
physics left room for such vagueness he announced: "God does not play dice
with the universe."
·
Quantum
Mechanics is used to understand phenomena like radioactivity, chemical bonding,
semi-conductors, solid-state micro-chips, electronics, sub-atomic physics,
radiation from black holes, and many others.
·
1926
·
1928 Paul Dirac
predicted that all particles should have opposites called anti-particles.
·
1930 The Cyclotron is developed by Ernest O. Lawrence, U.S. physicist
·
1931
Harold C. Urey discovers heavy hydrogen
·
1932 Chadwick and Carl Anderson describe neutrons and
positrons. A positron is identical
in every respect to the electron apart from its positive electric charge. When
an electron and positron come into contact, they mutually annihilate each other
producing a flood of energy in accordance with Einstein's equation E=mc2.
Both the proton and the neutron have anti-particles. These also destroy each
other if they meet with their particle.
·
The Universe is
made up of ordinary matter which is made up from particles. Matter composed of
anti-particles is known as anti-matter. Anti-matter can be created in
the laboratory but does not last long as it quickly comes into contact with
ordinary matter and is destroyed.
·
1938
Two German scientists, Otto Hahn and
Fritz Strassman, demonstrate nuclear fission.
·
1942 Enrico Fermi
achieves a controlled nuclear chain
reaction at the
Positrons
·
When an
oxygen-15, nitrogen-13 or carbon-11 atom undergoes radioactive decay, it emits
a positron—a positively charged particle that is the antimatter counterpart of
the electron. The positron does not get far before it collides with an
electron, and the two particles annihilate each other. Their mass is converted
into energy in the form of two photons that travel in opposite directions from
the site of the collision. Using detectors to look out for the
near-simultaneous arrival of pairs of photons, it is possible to work out where
the positrons are being emitted and form an image of the tissues where the
radioactive atoms have accumulated
·
One of the first
medical studies that attempted to take advantage of the unique physics of
positron emitters was reported in the early 1950s by Gordon Brownell and
William Sweet of the
·
Two obstacles,
however, hampered the use of biologically important positron emitters for some
time. The first was that the radioactive elements in question decay very
quickly. This is a good thing from the patient's point of view, since it
minimizes the dose of radiation, but it means that the radiotracers must be
manufactured very close to the imaging system. And that highlights the second
obstacle: such positron emitters must be made in a
expensive cyclotron (a type of particle accelerator). Clinical PET would
probably not have succeeded without the development of a new radiotracer, a
sugar molecule tagged with the radioactive isotope fluorine-18. Fluorodeoxyglucose, or FDG for short, turned out to be
exceptionally useful for a variety of reasons.
·
In the late
1990s
·
Around 2.7m
scans will take place worldwide this year, up from 2m scans in 2005. Although
PET accounts for only a small fraction of all medical imaging, its use is
expected to grow by 30% a year over the next few years. (By comparison, 45m CT
scans were performed in
Structural
Engineering
·
2004 The
opening of
Revised: 1/8/07
