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A Antichitatea
Babilonienii si egiptenii au observat miscarile planetelor si au au
reusit sa prezica eclipsele. Civilizatia greaca a contribuit putin
datorita neacceptarii ideilor marilor filosofi, Plato si Aristocle.
Totusi au fost inregistrate progrese in Alexandria, centru stiintific
al civilizatiei greci. Acolo, matematicianul si inventatorul Arhimede a
conceput instrumente macanice variate precum precum parghii si scripeti
si a masurat densitatile corpurilor solide introducandu-le in lichid.
Alti important om de stiinta a fost Aristarchus din Sámos care a
masurat distantele de la pamant la soare si la luna; matematicianul,
astronomul si geograful Eratostene, care a determinat circumferinta
pamantului si a facu o harta a cerului si Ptolemeu si a propus sistemul
de miscare a planetelor in care Pamantul era in mijloc si Soarele, Luna
si stelele se misca in jurul lui in orbite circulare
B Evul Mediu
In evul Mediu s-a avansat putin in domeniul fizicii, sau altor stiinte,
in afara de pastrarea clasicelor tratate grecesti. Fondarea marilor
universitati medievale a esuat sa avanseze in fizica sau alte activitati
experimentale .
Filosoful si teologul italian Thomas Aquinas, de exemplu a incercat sa
demonstreze calucrarile lui Platon si Aristotel asunt in armonie cu
sfintele scripturi
C Secolul 16 si 17
Avansul stiintei moderne urmata de renastere a fost ajutata de
incercarile incununate de succes a oamenilor de stiinta exraordinari
care au interpretat comportamentul corpurilor ceresti. Filosoful polone,
Nicolaus Copernicus a propus sistemul heliocentric in care planetele se
misca in jurul soarelui. El era convins ca orbitele planetare erau
circulare si, prin urmare sistemul sau avea nevoie de elaborari
complicare, precum sistemul Ptolemeic, pe care intentiona sa-l
inlocuiasca. Astronomul danez Tycho Brahe, crezand in sistemul
Ptolemeic, a incercat sa-l confirme, intr-o serie de masuratori
remarcabil de precise.
Aceasta l-a ajutat pe asistentul sau, astronomul Johannes Kepler cu
datele acesta rasturnand sistemul Ptolemeic si au condus la elaborarea a
trei legi care s-au conformat cu un sistem heliocentric modificat.
Galileo, auzind de inventia telescopului, si-a construit unul si,
incepand din 1869, a fost capabil sa confirme sistemul heliocentric
observand variatiile pozitie planetei Venus. Tot el a observat
suprafetele neregulate de pe Luna si pe patru dintre cei mai luminosi
sateliti ai lui Jupiter, petele solare si multe stele din Calea Lactee.
Interesele lui Galileo nu se limitau la astronomie; folosind planuri
inclinate si un ceas cu apa imbunatatit a demonstrat ca corpurile cu
greutati diferite cad la fel de repede si viteza lor creste uniform cu
timpul. Descoperirile astronomice ale lui Galileo au prefigurat munca
matematicianului englez cel mai mare din secolul al 17-lea , Sir Isaac
Newton.
IV Mecanica Newtoniana
Incepand din 1665 , la varsta de 23 de ani newton a enuntat
principiile mecanicii, a formulat legera gravitatiei universale, a
propus teoria propagarii luminii si a inventat calculul diferential si
integral. Contributiile lui Newton au acoperit o raza enorma de fenomene
naturale:a prezis aparitia cometelor si a explicat formarea
echinoctiului.
A Dezvotarea Mecanicii
Dezvoltarea fizicii datoreava mult legilor miscarii ale lui Newton,
in special celei de-a doua care spune ca forta de care este nevoie pt a
accelera un obiect este proportionala cu produsul dintre masa si
acceleratie. Daca forta , pozitia initiala si viteza sunt cunoscute pot
fi calculate, pozitiile consecutive chiar daca forta variaza cu timpul
si variatiile pozitiilor. Aceasta lege importanta continea un alt aspect
important: fiecare corp are proprietatiile sale specifice, masa
inertiala specifica, ce ii influenteaza miscarea. Chiar si azi legea
este utila atat timp cat corpul nu este prea masiv sau prea mic si nu se
misca prea repede. Legea a treia a lui Newton este exprimata foarte
simpla:"pt fiecare actiune se formeaza o reactie egala si opusa" ne
spuna in termeni sofisticati ca toate fortele dintre particula vin in
perechi directionate opus, dar nu neaparat pe linia determinata de cele
doua particule.
B Gravitatia
Contributia cea mai importanta a lui Newton descrierea fortelor
naturii a fost elucidarea fortei gravitatiei. Oamenii de stiinta de
astazi stiu ca pe langa gravitatie mai sunt doar trei forte fundamentale
:cea a electromagnetismului, cea asa-numitele interactiuni nucleare-tari
care tin impreunati neutronii si protonii din atomii nucleici, si
interactiunile slabe dintre unele particule care formeaza fenomenul
numit radioactivitate. Intelegerea conceptului de forta dateaza de la
legea universala a gravitatiei
care ne spune ca toate particulele materiale si corpurile care le compun
au o proprietate numita masa gravitationala. Aceasta proprietate face ca
oricare doua particule sa exerciteze forte de atractie una asuprei
celeilalte care sunt direct proportionala cu produsul masei si invers
proportionale cu patratul distantei. Forta gravitational guverneaza
miscare planetelor in jurul soarelui si campul gravitational al
Pamantului si este responabila de colapsul gravitational, ultimul stagiu
al ciclului vietii unei stele.
Una din cele mai importante observatii este ca masa gravitationala a
corpului este egala cu masa inertiala, proprietate care determina
raspunsul la orice forta exercitata (inertia). Cu toate acestea
demonstratiile lui Galileo, care le antecedeaza pe cele ale lui Newton,
conforma carora corpurile cad spre Pamant cu aceeasi acceleratie pot fi
explicate prin faptul ca masa gravitationala a unui corp care determina
fortele exercitate pe el si masa inertiala care determina raspunsul la
acea forta se reduc una pe cealalta.
Semnificatia completa a acestei echivalente dintre masele
gravitationale si inertiale nuy a fost apreciata pana la fizicianul
Albert Einstein, teoretician care a enuntat teoria relativitatii. Forta
gravitationala este cea mai slaba din cele patru forte din natura cand
sunt considerate particule elementare. Forta gravitationala dintre doi
protoni, de exemplu, care sunt printre cele mai grele particule
elementare este la orice distanta data de 10-36 ori cat magnitudinea
fortelor electrostatice dintre ele si pentru doi dintre acesti protoni
din nucleu unui atom este de multe ori mai mica decat interactiunea
nucleara tare.
Incarcatura electrica a particulelor elementare care formeaza fortele
electrostatice si magnetice sunt sau pozitive sau negative , sau
impreuna pot avea valoarea 0. Numai particulele cu incarcaturi de sens
contrar se atrag si corpurile mari tind sa fie neutre din punct de
vedere electric.
inactive. Pe de alta parte fortele nucleare, cea slaba si cea tare, au o
raza foarte mica si devin foarte greu de observat la distante mai mari
de a milioana parte din a milioana parte dintr-un centimetru.
In ciuda importantei macroscopice , forte gravitationala ramane asa de
slaba incat corpul trebuie sa fie foarte masiv inaite ca influenta sa sa
fie oservata. Cu toate acestea legea universala a gravitatiei a fost
dedusa din observatiile asupra miscarilor planetelor cu mult inainte de
a fi verificata experimental. In 1771 fizicianul si chimistul englez
Henry Cavedish a confirmat folosid sfere mari atragan mase mici atasate
de un pendul si cu ajutorul acestor masuratori a dedus densitatea
Pamantului.
In cele doua secole de dupa Newton, cu toate ca mecanica a fost
analizata, reformulata si aplicata pe sisteme complexe, nu s-au introdus
idei noi in fizica. Matematicianul suedez, Leonhard Euler a formulat
pentru prima data ecuatia miscarii corpurilor rigide. In aceeasi
perioada omul de stiinta danez, Daniel Bernouli, si alti doi au extins
mecanica newtoniana si au pus bazele mecanicii fluidelor.
C Electricitate si magnetism
Cu toate ca grecii antici stiau de proprietatile electricitatii si
chinezii din 2700 i.e.n. confectionau magneti, experimentarea si
intelegerea electricitatii si fenomenelor magnetice nu s-a intamplat
pana la sfarsitul secolului al 18-lea. In 1785, fizicianul francez
Charles Auguste de Coulomb a confirmat pentru prima data experimental ca
sarcinile electrice se atrag sau se resping unul pe altul. O puternica
teorie pt calcularea efectelor unui numar de sarcini electrice a fost
elaborata de matematicianul francez Siméon Denis Poisson si de
matematicianul german Carl Friedrich Gauss.
O particula incarcata pozitiv atrage o particula incarcata negativ
avand tendinta sa accelereze unul spre altul. Daca mediul prin care
particulele se misca impune rezistenta miscarii, aceasta poate fi redusa
la o miscare cu viteza constanta si mediul se incalzeste.Teoria clasica
a unui circuit electric simplu ce presupune ca, capetele unei baterii
sunt mentinute incarcate pozitiv si negativ. Cand capetele sunt
conectate prin intermediul unui fir, incarcatura negativa va fi
indepartata de capatul negativ si atrasa de capatul pozitiv. Procesul
incalzeste firul care ofera rezistenta miscarii electronilor. Fizicianul
german Georg Simeon Ohm a descoperit pentru prima data existenta unei
simple proportionalitati constanta intre curent si forta electrica
formata de baterie, cunoscuta sub numele de rezistenta electrica. Legea
lui Ohm in care stabileste ca rezistenta ,egala cu raportul dintre
voltaj si curent, nu este o lege a fizicii aplicabila fundamentala si
universala, ci mai degraba descrie comportamentul unei clase limitate de
materiale solide.
The historical concepts of magnetism, based on the existence of pairs of
oppositely charged poles, had started in the 17th century and owe much
to the work of Coulomb. The first connection between magnetism and
electricity, however, was made through the pioneering experiments of the
Danish physicist and chemist Hans Christian Oersted, who in 1819
discovered that a magnetic needle could be deflected by a wire nearby
carrying an electric current. Within one week after learning of
Oersted s discovery, the French scientist André Marie Ampère showed
experimentally that two current-carrying wires would affect each other
like poles of magnets. In 1831 the British physicist and chemist Michael
Faraday discovered that an electric current could be induced (made to
flow) in a wire without connection to a battery, either by moving a
magnet or by placing another current-carrying wire with an unsteady-that
is, rising and falling-current nearby. The intimate connection between
electricity and magnetism, now established, can best be stated in terms
of electric or magnetic fields, or forces that will act at a particular
point on a unit charge or unit current, respectively, placed at that
point. Stationary electric charges produce electric fields;
currents-that is, moving electric charges-produce magnetic fields.
Electric fields are also produced by changing magnetic fields, and vice
versa. Electric fields exert forces on charged particles as a function
of their charge alone; magnetic fields will exert an additional force
only if the charges are in motion.
These qualitative findings were finally put into a precise mathematical
form by the British physicist James Clerk Maxwell who, in developing the
partial differential equations that bear his name, related the space and
time changes of electric and magnetic fields at a point with the charge
and current densities at that point. In principle, they permit the
calculation of the fields everywhere and any time from a knowledge of
the charges and currents. An unexpected result arising from the solution
of these equations was the prediction of a new kind of electromagnetic
field, one that was produced by accelerating charges, that was
propagated through space with the speed of light in the form of an
electromagnetic wave, and that decreased with the inverse square of the
distance from the source. In 1887 the German physicist Heinrich Rudolf
Hertz succeeded in actually generating such waves by electrical means,
thereby laying the foundations for radio, radar, television, and other
forms of telecommunications. See Electromagnetic Radiation.
The behavior of electric and magnetic fields in these waves is quite
similar to that of a very long taut string, one end of which is rapidly
moved up and down in a periodic fashion. Any point along the string will
be observed to move up and down, or oscillate, with the same period or
with the same frequency as the source. Points along the string at
different distances from the source will reach the maximum vertical
displacements at different times, or at a different phase. Each point
along the string will do what its neighbor did, but a little later, if
it is further removed from the vibrating source (see Oscillation). The
speed with which the disturbance, or the message to oscillate, is
transmitted along the string is called the wave velocity (see Wave
Motion). This is a function of the medium, its mass, and the tension in
the case of a string. An instantaneous snapshot of the string (after it
has been in motion for a while) would show equispaced points having the
same displacement and motion, separated by a distance known as the
wavelength, which is equal to the wave velocity divided by the
frequency. In the case of the electromagnetic field one can think of the
electric-field strength as taking the place of the up-and-down motion of
each piece of the string, with the magnetic field acting similarly at a
direction at right angles to that of the electric field. The
electromagnetic-wave velocity away from the source is the speed of
light.
D Light
The apparent linear propagation of light was known since antiquity, and
the ancient Greeks believed that light consisted of a stream of
corpuscles. They were, however, quite confused as to whether these
corpuscles originated in the eye or in the object viewed. Any
satisfactory theory of light must explain its origin and disappearance
and its changes in speed and direction while it passes through various
media. Partial answers to these questions were proposed in the 17th
century by Newton, who based them on the assumptions of a corpuscular
theory, and by the English scientist Robert Hooke and the Dutch
astronomer, mathematician, and physicist Christiaan Huygens, who
proposed a wave theory. No experiment could be performed that
distinguished between the two theories until the demonstration of
interference in the early 19th century by the British physicist and
physician Thomas Young. The French physicist Augustin Jean Fresnel
decisively favored the wave theory.
Interference can be demonstrated by placing a thin slit in front of a
light source, stationing a double slit farther away, and looking at a
screen spaced some distance behind the double slit. Instead of showing a
uniformly illuminated image of the slits, the screen will show
equispaced light and dark bands. Particles coming from the same source
and arriving at the screen via the two slits could not produce different
light intensities at different points and could certainly not cancel
each other to yield dark spots. Light waves, however, can produce such
an effect. Assuming, as did Huygens, that each of the double slits acts
as a new source, emitting light in all directions, the two wave trains
arriving at the screen at the same point will not generally arrive in
phase, though they will have left the two slits in phase. Depending on
the difference in their paths, "positive" displacements arriving at the
same time as "negative" displacements of the other will tend to cancel
out and produce darkness, while the simultaneous arrival of either
positive or negative displacements from both sources will lead to
reinforcement or brightness. Each apparent bright spot undergoes a
timewise variation as successive in-phase waves go from maximum positive
through zero to maximum negative displacement and back. Neither the eye
nor any classical instrument, however, can determine this rapid
"flicker," which in the visible-light range has a frequency from 4 ×
1014 to 7.5 × 1014 Hz, or cycles per second. Although it cannot be
measured directly, the frequency can be inferred from wavelength and
velocity measurements. The wavelength can be determined from a simple
measurement of the distance between the two slits, and the distance
between adjacent bright bands on the screen; it ranges from 4 × 10-5 cm
(1.6 × 10-5 in) for violet light to 7.5 × 10-5 cm (3 × 10-5 in) for
red light with intermediate wavelengths for the other colors.
The first measurement of the velocity of light was carried out by the
Danish astronomer Olaus Roemer in 1676. He noted an apparent time
variation between successive eclipses of Jupiter s moons, which he
ascribed to the intervening change in the distance between Earth and
Jupiter, and to the corresponding difference in the time required for
the light to reach the earth. His measurement was in fair agreement with
the improved 19th-century observations of the French physicist Armand
Hippolyte Louis Fizeau, and with the work of the American physicist
Albert Abraham Michelson and his coworkers, which extended into the 20th
century. Today the velocity of light is known very accurately as
299,292.6 km (185,971.8 mi sec) in vacuum. In matter, the velocity is
less and varies with frequency, giving rise to a phenomenon known as
dispersion. See also Optics; Spectrum; Vacuum.
Maxwell s work contributed several important results to the
understanding of light by showing that it was electromagnetic in origin
and that electric and magnetic fields oscillated in a light wave. His
work predicted the existence of nonvisible light, and today
electromagnetic waves or radiations are known to cover the spectrum from
gamma rays (see Radioactivity), with wavelengths of 10-12 cm (4 × 10-11
in), through X rays, visible light, microwaves, and radio waves, to long
waves of hundreds of kilometers in length (see X Ray). It also related
the velocity of light in vacuum and through media to other observed
properties of space and matter on which electrical and magnetic effects
depend. Maxwell s discoveries, however, did not provide any insight into
the mysterious medium, corresponding to the string, through which light
and electromagnetic waves had to travel (see the Electricity and
Magnetism section above). Based on the experience with water, sound, and
elastic waves, scientists assumed a similar medium to exist, a
"luminiferous ether" without mass, which was all-pervasive (because
light could obviously travel through a massless vacuum), and had to act
like a solid (because electromagnetic waves were known to be transverse
and the oscillations took place in a plane perpendicular to the
direction of propagation, and gases and liquids could only sustain
longitudinal waves, such as sound waves). The search for this mysterious
ether occupied physicists attention for much of the last part of the
19th century.
The problem was further compounded by an extension of a simple problem.
A person walking forward with a speed of 3.2 km/h (2 mph) in a train
traveling at 64.4 km/h (40 mph) appears to move at 67.6 km/h (42 mph),
to an observer on the ground. In terms of the velocity of light the
question that now arose was: If light travels at about 300,000 km/sec
(about 186,000 mi/sec) through the ether, at what velocity should it
travel relative to an observer on earth while the earth also moves
through the ether? Or, alternately, what is the earth s velocity through
the ether? The famous Michelson-Morley experiment, first performed in
1887 by Michelson and the American chemist Edward Williams Morley using
an interferometer, was an attempt to measure this velocity; if the earth
were traveling through a stationary ether, a difference should be
apparent in the time taken by light to traverse a given distance,
depending on whether it travels in the direction of or perpendicular to
the earth s motion. The experiment was sensitive enough to detect even a
very slight difference by interference; the results were negative.
Physics was now in a profound quandary from which it was not rescued
until Einstein formulated his theory of relativity in 1905.
E Thermodynamics A branch of physics that assumed major stature during
the 19th century was thermodynamics. It began by disentangling the
previously confused concepts of heat and temperature, by arriving at
meaningful definitions, and by showing how they could be related to the
heretofore purely mechanical concepts of work and energy. See also Heat
Transfer.
E1 Heat and Temperature A different sensation is experienced when a
hot or a cold body is touched, leading to the qualitative and subjective
concept of temperature. The addition of heat to a body leads to an
increase in temperature (as long as no melting or boiling occurs), and
in the case of two bodies at different temperatures brought into
contact, heat flows from one to the other until their temperatures
become the same and thermal equilibrium is reached. To arrive at a
scientific measure of temperature, scientists used the observation that
the addition or subtraction of heat produced a change in at least one
well-defined property of a body. The addition of heat, for example, to a
column of liquid maintained at constant pressure increased the length of
the column, while the heating of a gas confined in a container raised
its pressure. Temperature, therefore, can invariably be measured by one
other physical property, as in the length of the mercury column in an
ordinary thermometer, provided the other relevant properties remain
unchanged. The mathematical relationship between the relevant physical
properties of a body or system and its temperature is known as the
equation of state. Thus, for an ideal gas, a simple relationship exists
between the pressure, p, volume V, number of moles n, and the absolute
temperature T, given by pV = nRT, where R is the same constant for all
ideal gases. Boyle s law, named after the British physicist and chemist
Robert Boyle, and Gay-Lussac s law or Charles s law, named after the
French physicists and chemists Joseph Louis Gay-Lussac and Jacques
Alexandre César Charles, are both contained in this equation of state
(see Gases).
Until well into the 19th century, heat was considered a massless fluid
called caloric, contained in matter and capable of being squeezed out of
or into it. Although the so-called caloric theory answered most early
questions on thermometry and calorimetry, it failed to provide a sound
explanation of many early 19th-century observations. The first true
connection between heat and other forms of energy was observed in 1798
by the Anglo-American physicist and statesman Benjamin Thompson, Count
von Rumford, who noted that the heat produced in the boring of cannon
was roughly proportional to the amount of work done. In mechanics, work
is the product of a force on a body and the distance through which the
body moves during its application.
E2 The First Law of Thermodynamics The equivalence of heat and work
was explained by the German physicist Hermann Ludwig Ferdinand von
Helmholtz and the British mathematician and physicist William Thomson,
1st Baron Kelvin, by the middle of the 19th century. Equivalence means
that doing work on a system can produce exactly the same effect as
adding heat; thus the same temperature rise can be achieved in a gas
contained in a vessel by adding heat or by doing an appropriate amount
of work through a paddle wheel sticking into the container where the
paddle is actuated by falling weights. The numerical value of this
equivalent was first demonstrated by the British physicist James
Prescott Joule in several heating and paddle-wheel experiments between
1840 and 1849.
That performing work or adding heat to a system were both means of
transferring energy to it was thus recognized. Therefore, the amount of
energy added by heat or work had to increase the internal energy of the
system, which in turn determined the temperature. If the internal energy
remains unchanged, the amount of work done on a system must equal the
heat given up by it. This is the first law of thermodynamics, a
statement of the conservation of energy. Not until the action of
molecules in a system was better understood by the development of the
kinetic theory could this internal energy be related to the sum of the
kinetic energies of all the molecules making up the system.
E3 The Second Law of Thermodynamics While the first law indicates that
energy must be conserved in any interactions between a system and its
surroundings, it gives no indication whether all forms of mechanical and
thermal energy exchange are possible. That overall changes in energy
proceed in one direction was first formulated by the French physicist
and military engineer Nicolas Léonard Sadi Carnot, who in 1824 pointed
out that a heat engine (a device that can produce work continuously
while only exchanging heat with its surroundings) requires both a hot
body as a source of heat and a cold body to absorb heat that must be
discharged. When the engine performs work, heat must be transferred from
the hotter to the colder body; to have the inverse take place requires
the expenditure of mechanical (or electrical) work. Thus, in a
continuously working refrigerator, the absorption of heat from the low
temperature source (the cold space) requires the addition of work
(usually as electrical power), and the discharge of heat (usually via
finned coils in the rear) to the surroundings (see Refrigeration). These
ideas, based on Carnot s concepts, were eventually formulated rigorously
as the second law of thermodynamics by the German mathematical physicist
Rudolf Julius Emanuel Clausius and by Lord Kelvin in various alternate,
although equivalent, ways. One such formulation is that heat cannot flow
from a colder to a hotter body without the expenditure of work.
From the second law, it follows that in an isolated system (one that has
no interactions with the surroundings) internal portions at different
temperatures will always adjust to a single uniform temperature and thus
produce equilibrium. This can also be applied to other internal
properties that may be different initially. If milk is poured into a cup
of coffee, for example, the two substances will continue to mix until
they are inseparable and can no longer be differentiated. Thus, an
initial separate or ordered state is turned into a mixed or disordered
state. These ideas can be expressed by a thermodynamic property, called
the entropy (first formulated by Clausius), which serves as a measure of
how close a system is to equilibrium-that is, to perfect internal
disorder. The entropy of an isolated system, and of the universe as a
whole, can only increase, and when equilibrium is eventually reached, no
more internal change of any form is possible. Applied to the universe as
a whole, this principle suggests that eventually all temperature in
space becomes uniform, resulting in the so-called heat death of the
universe.
Locally, the entropy can be lowered by external action. This applies to
machines, such as a refrigerator, where the entropy in the cold chamber
is being reduced, and to living organisms. This local increase in order
is, however, only possible at the expense of an entropy increase in the
surroundings; here more disorder must be created.
This continued increase in entropy is related to the observed
nonreversibility of macroscopic processes. If a process were
spontaneously reversible-that is, if, after undergoing a process, both
it and all the surroundings could be brought back to their initial
state-the entropy would remain constant in violation of the second law.
While this is true for macroscopic processes, and therefore corresponds
to daily experience, it does not apply to microscopic processes, which
are believed to be reversible. Thus, chemical reactions between
individual molecules are not governed by the second law, which applies
only to macroscopic ensembles.
From the promulgation of the second law, thermodynamics went on to other
advances and applications in physics, chemistry, and engineering. Most
chemical engineering, all power-plant engineering, and air-conditioning
and low-temperature physics are just a few of the fields that owe their
theoretical basis to thermodynamics and to the subsequent achievements
of such scientists as Maxwell, the American physicist Willard Gibbs, the
German physical chemist Walther Hermann Nernst, and the Norwegian-born
American chemist Lars Onsager.
F Kinetic Theory and Statistical Mechanics The modern concept of the
atom was first proposed by the British chemist and physicist John Dalton
in 1808 and was based on his studies that showed that chemical elements
enter into combinations based on fixed ratios of their weights. The
existence of molecules as the smallest particles of a substance that can
exist in the free-that is, gaseous-state and have the properties of any
larger amount of the substance, was first hypothesized by the Italian
physicist and chemist Amedeo Avogadro in 1811, but did not find general
acceptance until about 50 years later, when it also formed the basis of
the kinetic theory of gases (see Avogadro s Law). Developed by Maxwell,
the Austrian physicist Ludwig Boltzmann, and other physicists, it
applied the laws of mechanics and probability to the behavior of
individual molecules, and drew statistical inferences about the
properties of the gas as a whole.
A typical but important problem solved in this manner was the
determination of the range of speeds of molecules in the gas, and from
this the average kinetic energy of the molecules. The kinetic energy of
a body, as a simple consequence of Newton s second law, is ?mv2, where m
is the mass of the body and v its velocity. One of the achievements of
kinetic theory was to show that temperature, the macroscopic
thermodynamic property describing the system as a whole, was directly
related to the average kinetic energy of the molecules. Another was the
identification of the entropy of a system with the logarithm of the
statistical probability of the energy distribution. This led to the
demonstration that the state of thermodynamic equilibrium corresponding
to that of highest probability is also the state of maximum entropy.
Following the success in the case of gases, kinetic theory and
statistical mechanics were subsequently applied to other systems, a
process that is still continuing.
G Early Atomic and Molecular Theories The development of Dalton s
atomic theory and Avogadro s molecular law had overriding influence on
the development of chemistry, in addition to their importance in
physics.
G1 Avogadro s Law Avogadro s law, which was easily proved by kinetic
theory, indicated that a specified volume of a gas at a given
temperature and pressure always contained the same number of molecules,
irrespective of the gas selected. This number, however, could not be
accurately determined, and the 19th-century physicists therefore had no
sound knowledge of molecular or atomic mass and size until the turn of
the 20th century, when subsequent to the discovery of the electron, the
American physicist Robert Andrews Millikan carefully determined its
charge. This finally permitted accurate determination of the so-called
Avogadro s number, which is the number of molecules in that amount of
material exactly equal to its molecular weight.
Besides the mass, another quantity of interest was the size of an atom.
Various and only partly successful attempts at finding the size of an
atom were made during the latter part of the 19th century; the most
successful applied the results of kinetic theory to nonideal gases-that
is, gases the behavior of which depended on the fact that molecules were
not points but had finite volumes. Only later experiments involving the
scattering of X rays, alpha particles, and other atomic and subatomic
particles by atoms led to more precise measurements of their size as
being between 10-8 and 10-7 cm (4 × 10-7 and 4 × 10-6 in) in diameter.
A precise statement about the size of an atom, however, requires some
explicit definition of what is meant by size, since most atoms are not
exactly spherical and can exist in various states that change the
distance between the nucleus and the electrons within the atom.
G2 Spectroscopy One of the most important developments leading to the
exploration of the interior of the atom, and to the eventual overthrow
of the classical theories of physics, was spectroscopy; the other was
the discovery of the subatomic particles themselves.
In 1823 the British astronomer and chemist Sir John Frederick William
Herschel suggested that a chemical substance might be identified by
examining its spectrum-that is, the discrete wavelength pattern in which
light from a gaseous substance is emitted. In the years that followed,
the spectra of a great many substances were cataloged by two Germans,
the chemist Robert Wilhelm Bunsen and the physicist Gustav Robert
Kirchhoff. Helium was first discovered as a new element following the
discovery of an unexplained spectral line in the sun s spectrum by the
British astronomer Sir Joseph Norman Lockyer in 1868. From the
standpoint of atomic theory, however, the most important contributions
were made by the study of the spectra of simple atoms, such as hydrogen,
which showed few spectral lines. See Chemical Analysis.
Discrete line spectra originate from gaseous substances where, in terms
of modern knowledge, the electrons have been excited by heat or by
bombardment with subatomic particles. In contrast, a heated solid has a
continuous spectrum over the full visible range and into the infrared
and ultraviolet regions. The total amount of energy emitted depends
strongly on the temperature, as does the relative intensity of the
different wavelength components. As a piece of iron is heated, for
example, its radiation is first in the infrared spectrum and cannot be
seen; it then extends into the visible spectrum where the glow shifts
from red to white as the peak of its radiant spectrum shifts toward the
middle of the visible range. Attempts to explain the radiation
characteristics of solids, using the tools of theoretical physics
available at the end of the 19th century, led to the prediction that at
any given temperature the amount of radiation increased with frequency
and without limit. This calculation, in which no error was found, was in
disagreement with experiment and also led to an absurd conclusion: A
body at a finite temperature could radiate an infinite amount of energy.
This required a new way of thinking about radiation and, indirectly,
about the atom. See Infrared Radiation; Ultraviolet Radiation.
H The Breakdown of Classical Physics By about 1880 physics was serene;
most phenomena could be explained by Newtonian mechanics, Maxwell s
electromagnetic theory, thermodynamics, and Boltzmann s statistical
mechanics. Only a few problems, such as the determination of the
properties of the ether and the explanation of the radiation spectra
from solids and gases, appeared unsolved. These unexplained phenomena,
however, formed the seeds of revolution, a revolution that was augmented
by a series of remarkable discoveries within the last decade of the 19th
century: the discovery of X rays by Wilhelm Conrad Roentgen of Germany
in 1895; of the electron by Sir Joseph John Thomson of Great Britain in
1895; of radioactivity by Antoine Henri Becquerel of France in 1896; and
of the photoelectric effect by Hertz, Wilhelm Hallwachs, and Philipp
Eduard Anton Lenard of Germany during the period from 1887 to 1899 (see
Photoelectric Cell). Coupled with the disturbing results of the
Michelson-Morley experiments and the discovery of cathode rays, or
electron stream, the experimental evidence in physics outstripped all
available theories to explain it.
V MODERN PHYSICS
Two major new developments during the first third of the 20th century,
the quantum theory and the theory of relativity, explained these
findings, yielded new discoveries, and changed the understanding of
physics as it is known today.
A Relativity
To extend the example of relative velocity introduced with the
Michelson-Morley experiment, two situations can be compared. One
consists of a person, A, walking forward with a velocity v in a train
moving at velocity u. The velocity of A with regard to an observer B
stationary on the ground is then simply V = u + v. If, however, the
train were at rest in the station and A was moving forward with velocity
v while observer B walked backward with velocity u, the relative speed
between A and B would be exactly the same as in the first case. In more
general terms, if two frames of reference are moving relative to each
other at constant velocity, observations of any phenomena made by
observers in either frame will be physically equivalent. As already
mentioned, the Michelson-Morley experiment failed to confirm the concept
of adding velocities, and two observers, one at rest and the other
moving toward a light source with velocity u, both observe the same
light velocity V, commonly denoted by the symbol c.
Einstein incorporated the invariance of c into his theory of relativity.
He also demanded a very careful rethinking of the concepts of space and
time, showing the imperfection of intuitive notions about them. As a
consequence of his theory, it is known that two clocks that keep
identical time when at rest relative to each other must run at different
speeds when they are in relative motion, and two rods that are identical
in length (at rest) will become different in length when they are in
relative motion. Space and time must be closely linked in a
four-dimensional continuum where the normal three-space dimensions must
be augmented by an interrelated time dimension.
Two important consequences of Einstein s relativity theory are the
equivalence of mass and energy and the limiting velocity of the speed of
light for material objects. Relativistic mechanics describes the motion
of objects with velocities that are appreciable fractions of the speed
of light, while Newtonian mechanics remains useful for velocities
typical of the macroscopic motion of objects on earth. No material
object, however, can have a speed equal to or greater than the speed of
light.
Even more important is the relation between the mass m and energy E.
They are coupled by the relation E = mc2, and because c is very large,
the energy equivalence of a given mass is enormous. The change of mass
giving an energy change is significant in nuclear reactions, as in
reactors or nuclear weapons, and in the stars, where a significant loss
of mass accompanies the huge energy release.
Einstein s original theory, formulated in 1905 and known as the special
theory of relativity, was limited to frames of reference moving at
constant velocity relative to each other. In 1915, he generalized his
hypothesis to formulate the general theory of relativity that applied to
systems that accelerate with reference to each other. This extension
showed gravitation to be a consequence of the geometry of space-time and
predicted the bending of light in its passage close to a massive body
like a star, an effect first observed in 1919. General relativity,
although less firmly established than the special theory, has deep
significance for an understanding of the structure of the universe and
its evolution. See also Cosmology.
B Quantum Theory The quandary posed by the observed spectra emitted by
solid bodies was first explained by the German physicist Max Planck.
According to classical physics, all molecules in a solid can vibrate
with the amplitude of the vibrations directly related to the
temperature. All vibration frequencies should be possible and the
thermal energy of the solid should be continuously convertible into
electromagnetic radiation as long as energy is supplied. Planck made a
radical assumption by postulating that the molecular oscillator could
emit electromagnetic waves only in discrete bundles, now called quanta,
or photons. See Photon; Quantum Theory. Each photon has a characteristic
wavelength in the spectrum and an energy E given by E = hf, where f is
the frequency of the wave. The wavelength ë related to the frequency by
ëf = c, where c is the speed of light. With the frequency specified in
hertz (Hz), or cycles per second, h, now known as Planck s constant, is
extremely small (6.626 × 10-27 erg-sec). With his theory, Planck again
introduced a partial duality into the theory of light, which for nearly
a century had been considered to be wavelike only.
C Photoelectricity If electromagnetic radiation of appropriate
wavelength falls upon suitable metals, negative electric charges, later
identified as electrons, are ejected from the metal surface. The
important aspects of this phenomenon are the following: (1) the energy
of each photoelectron depends only on the frequency of the illumination
and not on its intensity; (2) the rate of electron emission depends only
on the illuminating intensity and not on the frequency (provided that
the minimum frequency to cause emission is exceeded); and (3) the
photoelectrons emerge as soon as the illumination hits the surface.
These observations, which could not be explained by Maxwell s
electromagnetic theory of light, led Einstein to assume in 1905 that
light can be absorbed only in quanta or photons, and that the photon
completely vanishes in the absorption process, with all of its energy E
(=hf) going to one electron in the metal. With this simple assumption
Einstein extended Planck s quantum theory to the absorption of
electromagnetic radiation, giving additional importance to the
wave-particle duality of light. It was for this work that Einstein was
awarded the 1921 Nobel Prize in physics.
D X Rays These very penetrating rays, first discovered by Roentgen,
were shown to be electromagnetic radiation of very short wavelength in
1912 by the German physicist Max Theodor Felix von Laue and his
coworkers. The precise mechanism of X-ray production was shown to be a
quantum effect, and in 1914 the British physicist Henry Gwyn-Jeffreys
Moseley used his X-ray spectrograms to prove that the atomic number of
an element, and hence the number of positive charges in an atom, is the
same as its position in the periodic table (see Periodic Law). The
photon theory of electromagnetic radiation was further strengthened and
developed by the prediction and observation of the so-called Compton
effect by the American physicist Arthur Holly Compton in 1923.
E Electron Physics That electric charges were carried by extremely
small particles had already been suspected in the 19th century and, as
indicated by electrochemical experiments, the charge of these elementary
particles was a definite, invariant quantity. Experiments on the
conduction of electricity through low-pressure gases led to the
discovery of two kinds of rays: cathode rays, coming from the negative
electrode in a gas discharge tube, and positive or canal rays from the
positive electrode. Sir Joseph John Thomson s 1895 experiment measured
the ratio of the charge q to the mass m of the cathode-ray particles.
Lenard in 1899 confirmed that the ratio of q to m for photoelectric
particles was identical to that of cathode rays. The American inventor
Thomas Alva Edison had noted in 1883 that very hot wires emit
electricity, called thermionic emission (now called the Edison effect),
and in 1899 Thomson showed that this form of electricity also consisted
of particles with the same q to m ratio as the others. About 1911
Millikan finally determined that electric charge always arises in
multiples of a basic unit e, and measured the value of e, now known to
be 1.602 × 10-19 coulombs. From the measured value of q to m ratio,
with q set equal to e, the mass of the carrier, called electron, could
now be determined as 9.110 × 10-31 kg.
Finally, Thomson and others showed that the positive rays also consisted
of particles, each carrying a charge e, but of the positive variety.
These particles, however, now recognized as positive ions resulting from
the removal of an electron from a neutral atom, are much more massive
than the electron. The smallest, the hydrogen ion, is a single proton
with a mass of 1.673 × 10-27 kg, about 1837 times more massive than the
electron (see Ion; Ionization). The "quantized" nature of electric
charge was now firmly established and, at the same time, two of the
fundamental subatomic particles identified.
F Atomic Models
In 1913 the New Zealand-born British physicist Ernest Rutherford, making
use of the newly discovered radiations from radioactive nuclei, found
Thomson s earlier model of an atom with uniformly distributed positive
and negative charged particles to be untenable. The very fast, massive,
positively charged alpha particles he employed were found to deflect
sharply in their passage through matter. This effect required an atomic
model with a heavy positive scattering center. Rutherford then suggested
that the positive charge of an atom was concentrated in a massive
stationary nucleus, with the negative electron moving in orbits about
it, and positioned by the electric attraction between opposite charges.
This solar-system-like atomic model, however, could not persist
according to Maxwell s theory, where the revolving electrons should emit
electromagnetic radiation and force a total collapse of the system in a
very short time.
Another sharp break with classical physics was required at this point.
It was provided by the Danish physicist Niels Henrik David Bohr, who
postulated the existence within atoms of certain specified orbits in
which electrons could revolve without electromagnetic radiation
emission. These allowed orbits, or so-called stationary states, are
determined by the condition that the angular momentum J of the orbiting
electron must be a positive multiple integral of Planck s constant,
divided by 2 ð, that is, J = nh/2p, where the quantum number n may have
any positive integer value. This extended "quantization" to dynamics,
fixed the possible orbits, and allowed Bohr to calculate their radii and
the corresponding energy levels. Also in 1913 the model was confirmed
experimentally by the German-born American physicist James Franck and
the German physicist Gustav Hertz.
Bohr developed his model much further. He explained how atoms radiate
light and other electromagnetic waves, and also proposed that an
electron "lifted" by a sufficient disturbance of the atom from the orbit
of smallest radius and least energy (the ground state) into another
orbit, would soon "fall" back to the ground state. This falling back is
accompanied by the emission of a single photon of energy E = hf, where E
is the difference in energy between the higher and lower orbits. Each
orbit shift emits a characteristic photon of sharply defined frequency
and wavelength; thus one photon would be emitted in a direct shift from
the n = 3 to the n = 1 orbit, which will be quite different from the two
photons emitted in a sequential shift from the n = 3 to n = 2 orbit, and
then from there to the n = 1 orbit. This model now allowed Bohr to
account with great accuracy for the simplest atomic spectrum, that of
hydrogen, which had defied classical physics.
Although Bohr s model was extended and refined, it could not explain
observations for atoms with more than one electron. It could not even
account for the intensity of the spectral colors of the simple hydrogen
atom. Because it had no more than a limited ability to predict
experimental results, it remained unsatisfactory for theoretical
physicists.
G Quantum Mechanics Within a few years, roughly between 1924 and 1930,
an entirely new theoretical approach to dynamics was developed to
account for subatomic behavior. Named quantum mechanics or wave
mechanics, it started with the suggestion in 1924 by the French
physicist Louis Victor, Prince de Broglie, that not only electromagnetic
radiation but matter could also have wave as well as particle aspects.
The wavelength of the so-called matter waves associated with a particle
is given by the equation ë = h/mv, where m is the particle mass and v
its velocity. Matter waves were conceived of as pilot waves guiding the
particle motion, a property that should result in diffraction under
suitable conditions. This was confirmed in 1927 by the experiments on
electron-crystal interactions by the American physicists Clinton Joseph
Davisson and Lester Halbert Germer and the British physicist George
Paget Thomson. Subsequently, Werner Heisenberg, Max Born, and Ernst
Pascual Jordan of Germany and the Austrian physicist Erwin Schrödinger
developed Broglie s idea into a mathematical form capable of dealing
with a number of physical phenomena and with problems that could not be
handled by classical physics. In addition to confirming Bohr s postulate
regarding the quantization of energy levels in atoms, quantum mechanics
now provides an understanding of the most complex atoms, and has also
been a guiding spirit in nuclear physics. Although quantum mechanics is
usually needed only on the microscopic level (with Newtonian mechanics
still satisfactory for macroscopic systems), certain macroscopic
effects, such as the properties of crystalline solids, also exist that
can only be satisfactorily explained by principles of quantum mechanics.
Going beyond Broglie s notion of the wave-particle duality of matter,
additional important concepts have since been incorporated into the
quantum-mechanical picture. These include the discovery that electrons
must have some permanent magnetism and, with it, an intrinsic angular
momentum, or spin, as a fundamental property. Spin was subsequently
found in almost all other elementary particles. In 1925 the Austrian
physicist Wolfgang Pauli expounded the exclusion principle, which states
that in an atom no two electrons can have precisely the same set of
quantum numbers. Four quantum numbers are needed to specify completely
the state of an electron in an atom. The exclusion principle is vital
for an understanding of the structure of the elements and of the
periodic table. Heisenberg in 1927 put forth the uncertainty principle,
which asserted the existence of a natural limit to the precision with
which certain pairs of physical quantities can be known simultaneously.
Finally, a synthesis of quantum mechanics and relativity was made in
1928 by the British mathematical physicist Paul Adrien Maurice Dirac,
leading to the prediction of the existence of the positron and bringing
the development of quantum mechanics to a culmination.
Largely as a result of Bohr s ideas, a different and statistical
approach developed in modern physics. The fully deterministic
cause-effect relations produced by Newtonian mechanics were supplanted
by predictions of future events in terms of statistical probability
only. Thus, the wave property of matter also implies that, in accordance
with the uncertainty principle, the motion of the particles can never be
predicted with absolute certainty even if the forces are known
completely. Although this statistical aspect plays no detectable role in
macroscopic motions, it is dominant on the molecular, atomic, and
subatomic scale.
H Nuclear Physics
The understanding of atomic structure was also facilitated by
Becquerel s discovery in 1896 of radioactivity in uranium ore (see
Uranium). Within a few years radioactive radiation was found to consist
of three types of emissions: alpha rays, later found by Rutherford to be
the nuclei of helium atoms; beta rays, shown by Becquerel to be very
fast electrons; and gamma rays, identified later as very short
wavelength electromagnetic radiation. In 1898 the French physicists
Marie and Pierre Curie separated two highly radioactive elements, radium
and polonium, from uranium ore, thus showing that radiations could be
identified with particular elements. By 1903 Rutherford and the British
physical chemist Frederick Soddy had shown that the emission of alpha or
beta rays resulted in the transmutation of the emitting element into a
different one. Radioactive processes were shortly thereafter found to be
completely statistical; no method exists that could indicate which atom
in a radioactive material will decay at any one time. These
developments, in addition to leading to Rutherford s and Bohr s model of
the atom, also suggested that alpha, beta, and gamma rays could only
come from the nuclei of very heavy atoms. In 1919 Rutherford bombarded
nitrogen with alpha particles and converted it to hydrogen and oxygen,
thus producing the first artificial transmutation of elements.
Meanwhile, a knowledge of the nature and abundance of isotopes was
growing, largely through the development of the mass spectrograph. A
model emerged in which the nucleus contained all the positive charge and
almost all the mass of the atom. The nuclear-charge carriers were
identified as protons, but except for hydrogen, the nuclear mass could
be accounted for only if some additional uncharged particles were
present. In 1932 the British physicist Sir James Chadwick discovered the
neutron, an electrically neutral particle of mass 1.675 × 10-27 kg,
slightly more than that of the proton. Now nuclei could be understood as
consisting of protons and neutrons, collectively called nucleons, and
the atomic number of the element was simply the number of protons in the
nucleus. On the other hand, the isotope number, also called the atomic
mass number, was the sum of the neutrons and protons present. Thus, all
atoms of oxygen (atomic no. 8) have eight protons, but the three
isotopes of oxygen, O16, O 17, and O18, also contain within their
respective nuclei eight, nine, or ten neutrons.
Positive electric charges repel each other, and because atomic nuclei
(except for hydrogen) have more than one proton, they would fly apart
except for a strong attractive force, called the nuclear force, or
strong interaction that binds the nucleons to each other. The energy
associated with this strong force is very great, millions of times
greater than the energies characteristic of electrons in their orbits or
chemical binding energies. An escaping alpha particle (consisting of two
protons and two neutrons), therefore, will have to overcome this strong
interaction force to escape from a radioactive nucleus such as uranium.
This apparent paradox was explained by the American physicists Edward U.
Condon, George Gamow, and Ronald Wilfred Gurney, who applied quantum
mechanics to the problem of alpha emission in 1928 and showed that the
statistical nature of nuclear processes allowed alpha particles to
"leak" out of radioactive nuclei, even though their average energy was
insufficient to overcome the nuclear force. Beta decay was explained as
a result of a neutron disruption within the nucleus, the neutron
changing into an electron (the beta particle), which is promptly
ejected, and a residual proton. The proton left behind leaves the
"daughter" nucleus with one more proton than its "parent" and thus
increases the atomic number and the position in the periodic table.
Alpha or beta emission usually leaves the nucleus with excess energy,
which it unloads by emitting a gamma-ray photon.
In all these nuclear processes a large amount of energy, given by
Einstein s E = mc2 equation, is released. After the process is over, the
total mass of the product is less than that of the parent, with the mass
difference appearing as energy. See Nuclear Energy.
VI DEVELOPMENTS IN PHYSICS SINCE 1930
The rapid expansion of physics in the last few decades was made
possible by the fundamental developments during the first third of the
century, coupled with recent technological advances, particularly in
computer technology, electronics, nuclear-energy applications, and
high-energy particle accelerators.
A Accelerators
Rutherford and other early investigators of nuclear properties were
limited to the use of high-energy emissions from naturally radioactive
substances to probe the atom. The first artificial high-energy emissions
were produced in 1932 by the British physicist Sir John Douglas
Cockcroft and the Irish physicist Ernest Thomas Sinton Walton, who used
high-voltage generators to accelerate protons to about 700,000 eV and to
bombard lithium with them, transmuting it into helium. One electron volt
is the energy gained by an electron when the accelerating voltage is 1
V; it is equivalent to about 1.6 × 10-19 joule (J). Modern accelerators
produce energies measured in million electron volts (usually written
mega-electron volts, or MeV), billion electron volts (giga-electron
volts, or GeV), or trillion electron volts (tera-electron volts, or
TeV). Higher-voltage sources were first made possible by the invention,
also in 1932, of the Van de Graaff generator by the American physicist
Robert J. Van de Graaff.
This was followed almost immediately by the invention of the cyclotron
by the American physicists Ernest Orlando Lawrence and Milton Stanley
Livingston. The cyclotron uses a magnetic field to bend the trajectories
of charged particles into circles, and during each half-revolution the
particles are given a small electric "kick" until they accumulate the
high energy level desired. Protons could be accelerated to about 10 MeV
by a cyclotron, but higher energies had to await the development of the
synchrotron after the end of World War II (1939-1945), based on the
ideas of the American physicist Edwin Mattison McMillan and the Soviet
physicist Vladimir I. Veksler. After World War II, accelerator design
made rapid progress, and accelerators of many types were built,
producing high-energy beams of electrons, protons, deuterons, heavier
ions, and X rays. For example, the accelerator at the Stanford Linear
Accelerator Center (SLAC) in Stanford, California, accelerates electrons
down a straight "runway," 3.2 km (2 mi) long, at the end of which they
attain an energy of more than 20 GeV.
While lower-energy accelerators are used in various applications in
industry and laboratories, the most powerful ones are used in studying
the structure of elementary particles, the fundamental building blocks
of nature. In such studies elementary particles are broken up by hitting
them with beams of projectiles that are usually protons or electrons.
The distribution of the fragments yields information on the structure of
the elementary particles.
To obtain more detailed information in this manner, the use of more
energetic projectiles is necessary. Since the acceleration of a
projectile is achieved by "pushing" it from behind, to obtain more
energetic projectiles it is necessary to keep pushing for a longer time.
Thus, high-energy accelerators are generally larger in size. The highest
beam energy reached at the end of World War II was less than 100 MeV. A
bigger accelerator, reaching 3 GeV, was built in the early 1950s at the
Brookhaven National Laboratory at Upton, New York. A breakthrough in
accelerator design occurred with the introduction of the strong focusing
principle in 1952 by the American physicists Ernest D. Courant,
Livingston, and Hartland S. Snyder. Today the world s largest
accelerators have been or are being built to produce beams of protons
beyond 1 TeV. Two are located at the Fermi National Accelerator
Laboratory, near Batavia, Illinois, and at the European Laboratory for
Particle Physics, known as CERN, in Geneva, Switzerland. See Particle
Accelerators.
B Particle Detectors
Detection and analysis of elementary particles were first accomplished
through the ability of these particles to affect photographic emulsions
and to energize fluorescent materials. The actual paths of ionized
particles were first observed by the British physicist Charles Thomson
Rees Wilson in a cloud chamber, where water droplets condensed on the
ions produced by the particles during their passage. Electric or
magnetic fields can be used to bend the particle paths, yielding
information about their momentum and electric charges. A significant
advance on the cloud chamber was the construction of the bubble chamber
by the American physicist Donald Arthur Glaser in 1952. It uses a
liquid, usually hydrogen, instead of air, and the ions produced by a
fast particle become centers of boiling, leaving an observable bubble
track. Because the density of the liquid is much higher than that of
air, more interactions take place in a bubble chamber than in a cloud
chamber. Furthermore, the bubbles clear out faster than water droplets,
allowing more frequent cycling of the bubble chamber. A third
development, the spark chamber, evolved in the 1950s. In this device,
many parallel plates are kept at a high voltage in a suitable gas
atmosphere. An ionizing particle passing between the plates breaks down
the gas, forming sparks that delineate its path.
A different type of detector, the discharge counter, was developed early
during the 20th century, largely by the German physicist Hans Geiger,
and later improved by the German-American physicist Walther Müller. It
is now commonly known as the Geiger-Müller counter, and although small
and convenient, it has been largely replaced by faster and more
convenient solid-state counting devices, such as the scintillation
counter, developed about 1947 by the German-American physicist Hartmut
Paul Kallmann and others. It uses the ability of ionized particles to
produce a flash of light as they pass through certain organic crystals
and liquids. See Particle Detectors.
C Cosmic Rays
About 1911 the Austrian-American physicist Victor Franz Hess discovered
that cosmic radiation, consisting of rays originating outside the
earth s atmosphere, arrived in a pattern determined by the earth s
magnetic field (see Cosmic Rays). The rays were found to be positively
charged and to consist mostly of protons with energies ranging from
about 1 GeV to 1011 GeV (compared to about 30 GeV for the fastest
particles produced by artificial accelerators). Cosmic rays trapped into
orbits around the earth account for the Van Allen radiation belts
discovered during an artificial-satellite flight in 1959 (see Radiation
Belts).
When a very energetic primary proton smashes into the atmosphere and
collides with the nitrogen and oxygen nuclei present, it produces large
numbers of different secondary particles that spread toward the earth as
a cosmic-ray shower. The origin of the cosmic-ray protons is not yet
fully understood; some undoubtedly come from the sun and the other
stars. Except for the slowest rays, however, no mechanism can be found
to account for their high energies and the likelihood is that weak
galactic fields operate over very long periods to accelerate
interstellar protons (see Galaxy; Milky Way).
D Elementary Particles To the electron, proton, neutron, and photon
have been added a number of fundamental particles. In 1932 the American
physicist Carl David Anderson discovered the antielectron, or positron,
predicted in 1928 by Dirac. Anderson found that the stopping of an
energetic cosmic gamma ray near a heavy nucleus yielded an
electron-positron pair out of pure energy. When a positron subsequently
meets an electron, they annihilate each other with a burst of photons of
energy.
D1 Discovery of the Muon
In 1935 the Japanese physicist Yukawa Hideki developed a theory
explaining how a nucleus is held together, despite the mutual repulsion
of its protons, by postulating the existence of a particle intermediate
in mass between the electron and the proton. In 1936 Anderson and his
coworkers discovered a new particle of 207 electron masses in secondary
cosmic radiation; now called the mu-meson or muon, it was first thought
to be Yukawa s nuclear "glue." Subsequent experiments by the British
physicist Cecil Frank Powell and others led to the discovery of a
somewhat heavier particle of 270 electron masses, the pi-meson or pion
(also obtained from secondary cosmic radiation), which was eventually
identified as the missing link in Yukawa s theory.
Many additional particles have since been found in secondary cosmic
radiation and through the use of large accelerators. They include
numerous massive particles, classed as hadrons (particles that take part
in the "strong" interaction, which binds atomic nuclei together),
including hyperons and various heavy mesons with masses ranging from
about one to three proton masses; and intermediate vector bosons such as
the W and Z0 particles, the carriers of the "weak" nuclear force. They
may be electrically neutral, positive, or negative, but never have more
than one elementary electric charge e. Enduring from 10-8 to 10-14 sec,
they decay into a variety of lighter particles. Each particle has its
antiparticle and carries some angular momentum. They all obey certain
conservation laws involving quantum numbers, such as baryon number,
strangeness, and isotopic spin.
In 1931 Pauli, in order to explain the apparent failure of some
conservation laws in certain radioactive processes, postulated the
existence of electrically neutral particles of zero-rest mass that
