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OCR for page 11
Progress in Physics
INTRODUCTION
We live in one of the most productive eras in the history of physics.
This chapter highlights some advances and opportunities in physics
that have been culled from the multitude of achievements reported in
the accompanying volumes of the Physics Survey. Our discussion
begins with a brief, nonspecialized summary of the highlights and a
description of the unifying principles that join the different subfields of
physics.
Elementary-Particle Physics
Elementary-particle physics, the science of the ultimate constituents
of matter and their interactions, has undergone a remarkable develop-
ment during the past two decades. A host of experimental observations
made possible by the current generation of particle accelerators and the
accompanying rapid convergence of theoretical ideas have led to a
radically new and simple picture of nature. All matter in its infinite
diversity has been found to be composed of a few basic constituents
called quarks and leptons, which are structureless and indivisible at
current limits of resolution. Great progress has also been made in
understanding the character of fundamental forces that govern-natural
phenomena. The weak and electromagnetic interactions have been
11
OCR for page 12
12 PHYSICS THROUGH THE 1990s: AN OVERVIEW
unified in a theory whose predictions have been verified by many
experiments, culminating in the 1983 discovery of the W and Z
particles, the mediators of the weak interaction The similarity among
quarks and leptons and the mathematical resemblance among the
theories of fundamental interactions spur bold attempts at unification in
which all the fundamental forces are seen as different manifestations of
a single underlying symmetry, a symmetry that is partially hidden. The
new synthesis raises deep questions about family patterns of quarks
and leptons and the origin of particle masses and invites speculation
about the eventual compositeness of quarks and leptons themselves.
These concerns motivate a broad program of experimentation at higher
energies (and, equivalently, on shorter scales of distance and time) to
test the emerging standard model and to uncover clues leading to more
complete understanding.
Nuclear Physics
During the past decades, the building blocks of nuclei were thought
to be protons and neutrons bound together by mesons. Today we know
that protons and neutrons are made of quarks and that the forces
between the quarks are created by particles called gluons. The new
concept is based on theoretical advances in particle physics, but recent
experimental work has demonstrated the importance of this description
for nuclei also. The basic questions facing nuclear physics today
involve detailed exploration of the quark structure in nucleons and
nuclei and the strong many-body forces that confine quarks and gluons.
Finding the answers represents an exciting frontier that may lead to
more basic understanding of the strong forces and of nuclear structure
and dynamics.
Heavy ions have been used to probe nuclear dynamics under
extreme conditions and to create new elements. Theoretical investiga-
tion predicts the existence of a quark-gluon plasma similar to that
which may have existed in the earliest moments of our universe.
Studies are being conducted on new systems made in the laboratory
called hypernuclei, in which a quark has been replaced by a strange
quark. The use of high-energy electron scattering from nuclei is now
revealing unprecedented levels of detail of nuclear structure, probing
the electroweak interactions between nucleons and their underlying
quark components. Finally, nuclear science continues to have great
impact on our understanding of fundamental symmetries in physics,
while also playing an ever increasing role in astrophysics and cosmol-
ogy.
OCR for page 13
PROGRESS IN PHYSICS 13
Condensed-Matter Physics
Condensed-matter physics has continued its historic role as a major
source of new concepts in fundamental science from the explanation of
the behavior of neutron stars, through advances in our understanding
of semiconductors, superconductors, and magnetisms, to prediction
and discovery of a new state of matter, the superfluid phase of liquid
helium atoms of mass 3. This is the area of physics that most directly
fuels advances in technology, from jet engines to computers. Concep-
tual advances abound, increasingly stimulated by the creation of totally
new substances not found in nature. Some of these substances are
produced by novel experimental techniques, such as exceedingly rapid
cooling of liquids to the solid state or controlled deposition of atoms
layer by layer. Others are produced by more conventional means on
the basis of new theoretical hypotheses made possible by deepening
theoretical understanding. Areas of great activity include studies of
systems of one or two dimensions, studies of phenomena at surfaces,
the role of interfaces between different materials, disordered systems,
surprising new forms of ordered systems, the onset of turbulence in
liquids, and the possibility of new forms of superconductivity.
Atomic, Molecular, and Optical Physics
This field has been revolutionized by the laser and modern optics.
New atomic and molecular species have been created using laser light;
spectroscopic resolution has been increased more than a millionfold.
Lasers have made it possible to watch atoms as they collide and
chemical reactions as they take place. Lasers are now being used to
generate femtosecond light pulses and coherent soft x rays and to cool
atoms to the submillikelvin regime. Optical-frequency counting meth-
ods using laser light have become so precise that the meter is no longer
defined in terms of the wavelength of light but as the distance light
travels in a given time interval. Particle-trap techniques have led to
ultraprecise studies of quantum electrodynamics and mass spectra;
they have made it possible to study plasma liquids and to create new
kinds of atomic clocks. Today's research opportunities include
ultrasensitive tests of the properties of space and the symmetries in
nature, studies of relativistic many-body theory and quantum electro-
dynam~cs in heavy ions by advanced x-ray spectroscopy, new exper-
imenta1 and theoretical approaches to the structure and interaction of
atoms and molecules, and the creation of nonlinear optical techniques
and new light sources.
OCR for page 14
14 PHYSICS THROUGH THE 1990s: AN OVERVIEW
Plasma Physics
Most of the visible matter in the universe is made of plasmas-
neutral gases composed of positive ions and unbounded electrons. Our
understanding of stars, stellar winds, planetary magnetospheres, and
galaxies is being spurred by advances in plasma physics. Spacecraft
have probed the magnetospheres of the planets from Mercury to Saturn
and soon will reach Uranus. The solar wind has been monitored by
many spacecraft, from inside the orbit of Mercury to beyond Pluto. We
may have data on galactic plasma beyond the influence of the Sun
before the year 2000. The Earth's magnetosphere has been measured in
great detail, and we are beginning to understand the complex phenom-
ena seen there- its weather' so to speak.
On Earth, our mastery of high-temperature plasmas has advanced
remarkably. Today, one plasma-confinement approach is expected
soon to achieve breakeven conditions for controlled fusion employing
reactions similar to those that power the Sun and stars. Our under-
standing of plasmas is having an impact on physics in many other ways.
One example is the recent generation of electric fields of tens of
millions of volts per centimeter in plasmas by the excitation of an
electron-plasma oscillation. It is expected that the technique can be
extended to give fields hundreds of times larger, i.e., as large as the
electric field that holds electrons in atoms. The possibility of using
these fields to accelerate particles to high energy is being explored.
Another example is the development of the free-electron laser, which
can generate coherent radiation from microwaves to the ultraviolet.
Cosmology, Gravitation, and Cosmic Rays
The study of the universe is being transformed by new eyes, such as
x-ray and infrared telescopes in space and very-large-array radio
telescopes on the ground. A vivid history of the universe has emerged,
starting with a primordial explosion- the big bang about 15 billion
years ago. Recent discoveries from elementary-particle theory are
offering possible solutions to some of the profound questions in
cosmology (for instance, why the universe appears to be so uniform,
and why there is so much matter relative to antimatter). An intense
search is under way for dark matter in the universe; such matter may
dominate important processes such as the formation of galaxies and the
ultimate fate of the universe. Will the universe expand forever or
collapse to start anew in yet another primordial explosion? Fundamen-
OCR for page 15
PROGRESS IN PHYSICS 15
tal theoretical advances in gravitational physics are leading to a better
understanding of black holes and quantization of gravity, and a
prediction of Einstein's General Theory of Relativity has been verified
to 2 parts in 1000, using the Viking spacecraft on Mars. Basic
theoretical and experimental research has now prepared the way for
major efforts to detect gravitational radiation. This dramatic advance
could take place in the next decade and open an important new window
on the universe. Space-based studies of the enigmatic cosmic rays
suggest that they originate in interstellar space, while ground-based
instruments have discovered localized sources of gamma rays with 10'5
eV of energy.
Interfaces and Applications
Research at the boundaries between physics and neighboring areas
such as chemistry, biology, materials science, and mathematics has
blossomed with new ideas and new approaches. High-vacuum and
surface science, the transition from orderly to chaotic motion, polymer
and macromolecular structure, the origins of biological processes,
microscopic control of structure and function in liquids and solids-
these are but a few examples of physics' rapidly emerging interdisci-
plinary advances, which are enriching all of science.
The applications of physics are broad and affect virtually every area
of society (see Figure 2.14. Our national programs in energy, the
environment, medicine, and security depend critically on physics. Our
industrial posture is linked to the flow of discoveries from physics that
can lead to the creation of industries such as microelectronics and
optical technology.
THE UNITY OF PHYSICS
The highlights in the preceding section hardly begin to portray
current advances in physics and their effects on science and society.
The following sections of this chapter provide a somewhat more
detailed account; but for a comprehensive picture of today's research
and tomorrow's opportunities, the reader is referred to the seven panel
reports of the Physics Survey that accompany this overview volume.
The scope of physics is so broad and its styles of research so diverse
that it is easy to lose sight of the underlying unity that joins even the
most disparate activities into a common enterprise. This unity is a
fundamental source of the strength and vitality of physics: to under-
OCR for page 16
16 PHYSICS THROUGH THE 1990s: AN OVERVIEW
PHYSICS SUBFIELD
ASTROPHYSICS
PLASMA PHYSICS
CONDENSED~MATTER PItYSICS
ATOMIC, MOLECULAR AND
OPTICAL PHYSICS
NUCLEAR PHYSICS
ELEMENTARY-PARTICLE PHYSICS
FIGURE 2.1 Physics subfields and some of their related applications.
APPLICATIONS
LOW-LEVEL X-RAY DETECTION,
X-AAY TOMOGRAPHY
FUSION
PLASMA PROCESSING
MATERIALS DEPOSITION, ETCHING, ETC.
FREE ELECTRON LASERS
SEMICONDUCTOR DEVICES
INTEGRATED CIRCUITS, COMPUTERS.
JUNCTION LASERS, OPTICAL DETECTORS
SUPERCONDUCTORS
HIGH-FIELD MAGNETS
JOSEPHSON TECInINOLOGY
MATERIALS
MAGNETIC MATERIALS, OPTICAL FlaERS,
QUARTZ RESONATORS
LASER-QPTICS
LASER SURGERY, OPTICAL
COMMUNICATIONS, OPTICAL DATA
PROCESSING, INTEGRATED OPTICS, LASER-
ASSISTED PROCESSING, LASER PRINTII`IG
ENVIRONMENTAL MONITORING
SUPERSENSITIVE DETECTION, ANALYSIS,
ATMOSPHERIC CHEMISTRY
M ETROLOGY
MAGNETOMETERS, ATOMIC CLOCKS, RING
GYROSCOPES
MAGNETIC RESONANCE IMAGING
-
NUCLEAR POWER
NUCLEAR MEDICINE
RADIATION THERAPY, ISOTOPE TRACERS
SYNCHROTRON LIGlIT SOURCES, DATA
PROCESSING, PARTICLE DETECTORS
stand physics it must be appreciated. We present some examples
below.
A dramatic confluence of ideas from three diverse subfields illus-
trates the unexpected connections in physics that suddenly occur.
Insights from particle physics based on the quark-gluon model (the
modern theory of the structure of protons, neutrons, and other
subnuclear particles) have been combined with contemporary ideas
from condensed-matter theory to portray the evolution of the universe
in the earliest stages of the primordial explosion the big bang. This
synthesis of thought allows us to understand important features of the
OCR for page 17
PROGRESS IN PHYSICS 17
universe that we can observe today as consequences of elementary
ideas about the structure and organization of matter.
Physicists are drawing on techniques from nuclear, condensed-
matter, and atomic physics to address another cosmological problem-
that of the missing mass in the universe. A critical question is whether
a particle known as the neutrino has a finite mass or whether it is
massless like the photon. The most sensitive laboratory experiments
appear to show that the neutrino's mass must be less than one
ten-thousandth the mass of the electron, but the question of the
neutrino's mass is not yet settled. If it is large enough, general relativity
predicts that the universe will not expand forever but that it will
eventually collapse.
A further example of links between diverse areas is the renewed
interest in the relation between regular and chaotic motion. Abrupt
transitions from regular to chaotic behavior have been discovered in
electrical, acoustical, and optical systems, in fluid flow, in chemical
reactions, and in the behavior of simple differential equations. Cardiac
arrest due to fibrillation of the heart is believed to be due to such an
effect. Recognition of the universal nature of such transitions gives
hope for understanding chaotic motion in more complex systems and,
in particular, of understanding turbulence. This line of research can be
.
expected to have a deep influence on many areas of science and on
problems ranging from aircraft and ship design to weather forecasting.
The subfields of physics are joined by technical as well as conceptual
bonds. Lasers, for example, have had a dramatic effect on science and
technology. They have revolutionized spectroscopy by enormously
increasing its sensitivity and precision and have opened the way to the
creation of new types of atomic and molecular species. Femtosecond (a
millionth of a billionth of a second) laser pulses make it possible to take
"snapshots" of chemical reactions; nonlinear spectroscopy makes it
possible to study reactions as they occur (for instance, in combustion
hames). High-power lasers can create plasmas under unprecedented
conditions and may provide a method for accelerating particles without
the need for gigantic accelerators. The influence of lasers on science is
too broad to summarize; perhaps it is sufficient to point out that lasers
are now ubiquitous in laboratories of physics, materials science,
chemistry, biology, physiology, and many of the other sciences.
One of the most dramatic developments in condensed-matter physics
is the opportunity to carry out spectroscopy from the optical to the
x-ray region by using radiation of unprecedented intensity, many
orders of magnitude brighter than was previously possible. The radia-
tion is provided by a gift from particle physics: synchrotron light
OCR for page 18
18 PHYSICS THROUGH THE 1990s: AN OVERVIEW
sources. The fundamental technology of synchrotron sources is the
technology of electron accelerators. At the same time, this gift has
been reciprocated in that the high-field superconductors created by
materials scientists in the 1960s have made proton accelerators in the
trillion-electron-volt (TeV) range practical. Superconductor technol-
ogy is also having a profound influence on health care, for super-
conducting magnets are an essential element of the magnetic resonance
imaging (MRI) technique.
Countless other examples could be cited of the unifying ideas and
techniques that link even the most disparate subjects in physics and,
indeed, link physics to the other sciences and to the central technical
and industrial needs of society. Appreciation of the unity of physics is
essential in planning research and developing science policy.
PROGRESS IN PARTICLE PHYSICS
Quarks and Leptons as Elementary Particles
The longing to discover the most elementary particles in nature is
deeply rooted in physics. At the beginning of this century, physicists
discovered that the atom is not a single particle but that it consists of
electrons moving rapidly around a central nucleus; in the 1930s, it was
discovered that the nucleus is not a single particle but that it con-
sists of protons and neutrons tightly bound together. Initially, the
protons and neutrons were assumed to be elementary, but during the
1950s and early 1960s a large number of similar particles, called hadrons,
were discovered. Over 100 hadrons are now known. In the 1960s, it was
suggested that the properties of all the hadrons could be explained by
recognizing that they are not elementary particles but are composed of
smaller particles, each with an electric charge of one third or two thirds
that of the electron. These smaller particles are called quarks.
During the past decade, the quark model has been experimentally
verified. For example, jets of hadrons discovered in high-energy
experiments have been explained in great detail by viewing the
collision not as a collision of hadrons, but as a collision of the
constituent quarks.
In the earliest version of the quark theory, there were only three
different quarks up, down, and strange. However, the discovery of
the J/¢ particle in 1974 and of the Y particle in 1977 led to the addition
of two new quarks the charm and bottom quarks. Definitive evidence
for a sixth quark, the top quark, is now being sought (Figure 2.24. With
these six quarks, the existence of all the hundred-plus hadrons could be
explained.
OCR for page 19
PROGRESS IN PHYSICS 19
GENERATION
TOP QUARK
QUARKS
LEPTONS I
}3
BOTTOM QUARK
· CHARM QUARK
STRANGE QUARK
- UP QUARK
.-- DOWN QUARK
~ TAU
· TAU NEUTRINO
MUON
—MUON NEUTRINO
| · ELECTRON
ELECTRON
NEUTRINO
~2
}3
2
1900 1940 1980
FIGURE 2.2 Quarks and leptons are the basic particles of matter. Most of them were
discovered in the past two decades. There is not yet definitive evidence for the existence
of the top quark.
A startling feature of quarks is that, as far as we know, there is no
possibility of isolating one of them. Quantum chromodynamics, a
theory of the strong interactions between quarks, accounts for this by
predicting that the energy to separate two quarks grows continuously
as they are separated owing to the creation of a gluon string between
them. Quantum chromodynamics is now so far advanced that theorists
can apply it to calculate numerically the masses of the hadrons. The
gluon-mediated interaction between quarks becomes weaker the closer
the quarks are together. This effect, called asymptotic freedom, makes
it possible to view hadron collisions as a series of collisions between
individual quarks. In addition, it eliminates some of the internal in-
consistencies that plagued previous theories.
The family of quarks was discovered relatively recently. There is a
second family of elementary particles, the leptons, some of whose
members have been known for many decades. The first of these, the
electron, was discovered at the turn of the century; two others- the
neutrino and the muon were predicted in the 1930s and discovered
experimentally in the following two decades. The electron and muon
are charged; the neutrino is electrically neutral. In 1963, it was found
OCR for page 20
20 PHYSICS THROUGH THE 1990s: AN OVERVIEW
that there are at least two kinds of neutrino, one associated with the
electron, the other with the muon. In 1975, a third, very heavy, charged
lepton, the a, was discovered. Soon thereafter, evidence was found for
a third type of neutrino associated with the a. Thus, we now know of
six leptons, which form three groups.
A decade of experimental and theoretical research on the quark and
lepton families has led to the realization that many of their properties
can be explained by two simple ideas. First, the particles can all be
classified into pairs by their properties and interactions. Each charged
lepton is paired to a unique uncharged neutrino. Among the quarks, the
up and down quarks pair together, as do the charm and strange quarks.
Second, the quark family and lepton family are related: each quark pair
is uniquely related to one lepton pair by a simple arrangement called
the generation model.
We do not understand why the generation model works, nor do we
know if there are more generations. In fact, we do not know why the
quarks and leptons are related at all. To find the answers to these
questions, experiments with higher-energy accelerators are being
planned to explore the internal structure and dynamics of the known
particles, to search for new particles, and to provide the data essential
to constructing new theories.
Unification of the Forces of Nature
There are four fundamental forces in nature. Two have been known
for centuries: the force of gravity and the electromagnetic force. In the
period between the World Wars, two other forces were identified: the
strong force, which holds the nucleus together, and the weak force,
which is responsible for many types of radioactivity. Since the days of
Einstein, it has been the dream of physics to develop a unified theory,
a theory that describes all these forces with a single set of equations
and concepts.
In the last two decades, the dream has been partially realized. The
electromagnetic and weak forces have been combined in a single
theory: the photon (a particle of light) carries or mediates the electro-
magnetic interactions, whereas the weak forces are mediated by
massive charged particles called the W+ and W- and by a neutral
particle called the Z°. Interactions mediated by the neutral particle,
called neutral currents, have been discovered in experiments with
high-energy neutrino beams and in studies of how electrons and
positrons annihilate each other. In addition, atomic physicists have
detected minute effects due to neutral currents in the spectra of cesium,
bismuth, and thallium, adding to evidence for the theory. In one of the
OCR for page 21
PROGRESS /N PHYSICS 21
PROG RESS ~ N
UNIFICATION OF
FO RC ES
KNOWN BASIC
FO RC ES
GRAVITATIONAL
FORC E
WEAK FORCE
ELECTROMAGNETIC
FORCE
STRONG OR NUCLEAR
FORCE
1900 1940 1980
PART! CLE
CARRY! NG
THE FORCE _
NOT
DISCOVERED
W AN D Z
PARTICLES I
) - CONF I RM ED
PHOTON ., ~ THEORY
PARTICLES
YEAR WHEN THE PARTICLE CARRYING THE FORCE WAS DISCOVERED
SEVERAL THEORIES, )
NEED HIGHER
ENERGY EXPERIMENT ~
TO TEST J
FIGURE 2.3 The four known basic forces expected to be carried or mediated by an
elementary particle. The particles carrying two of the forces have been discovered in the
past decade. Since the days of Einstein, physicists have wanted to unify the forces so
that they can all be derived from ~ single basic equation. That has been accomplished for
the weak and electromagnetic forces.
most ambitious experiments ever undertaken, the W+, W-, and Z°
particles have recently been observed, eliminating any further doubts
about the origins of the weak force (Figure 2.3~.
Efforts to unify the electroweak forces and the strong force are
leading to exciting new challenges in theoretical and experimental
physics. Theories have been proposed predicting that the proton is not
stable but will decay and that magnetic monopoles exist. Testing these
predictions is one of the many opportunities for particles physics in the
coming decade.
PROGRESS IN NUCLEAR PHYSICS
The challenge to understand the diverse arrangements of protons and
neutrons in the nuclei of atoms has fascinated physicists since the
1930s. In the 1960s, the model of the nucleus as a simple collection of
protons and neutrons evolved into a more complex picture in which the
strong nucleon-nucleon interactions arose from the exchange of me-
sons; now this picture is being replaced by the rich portrait that
emerges from recognition of the underlying quark-gluon nature of
nucleons. One day it should be possible to explain the entire nucleus as
a many-body system of interacting quarks and gluons. The experimen-
tal and theoretical challenge is enormous, but so is the reward of
understanding nuclear matter (Figure 2.41.
OCR for page 33
PROGRESS IN PHYSICS 33
Alternatives to toroidal confinement systems are also being devel-
oped. One approach is the mirror configuration, which employs a linear
magnetic-field geometry that pinches the ends to form mirrors for the
plasma particles. The tandem mirror concept, in which the electric
fields are generated Flora magnetic field lines, has been introduced to
suppress plasma leakage through the ends. Results point toward the
possibility of mirror confinement systems adequate for fusion-reactor
applications. Multimegawatt neutral-beam sources have been devel-
oped to fuel the mirror machine and heat plasmas to fusion tempera-
tures. Two-hundred-million-degree plasmas at fusion-plasma densities
have been achieved. In toroidal devices, 80-million-degree tempera-
tures have been obtained (Figure 2.7~.
_ 1 \ ~ 1 ' ' ' ~ '
- 3/85 \ \ Ignition j
lol4
a) 1013
llJ
c lol2
A Icator C
( 1983)
Alcator A ~ JET
( 1978) ( 1 9842.
-~e TFTR
AIcator A ~ (1985)
( 1976) D111
_ ~1 984 ) ~ ( 1~484 )
( 1 979)
T 3
(1968)
0.1
Thermalized
\ Breakeven
-
-
TFR
(1975)
ST
~ (1971)
. , , , , ,,,,1 , , ,
POX
· (1981)
· PLT -
(1978)
,,, ,,1 , . . .....
Tj(O)(keV)
10 100
FIGURE 2.7 Plot of Lawson confinement parameter n(O)~,: versus central ion temper-
ature The) for several tokamaks. Here' n(O) is the central density and ~~ is the energy
confinement time. The year of the result is indicated in parentheses. The JET (Joint
European Torus) and TFTR (Tokamak Fusion Test Reactor) tokamaks, with auxiliary
heating, are expected to operate in the 10- to 15-keV range during 1985-1987. The
thermalized breakeven and ignition curves refer to an equidensity fuel mixture of
deuterium and tritium plasma with Maxwellian ions.
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34 PHYSICS THROUGH THE 1990s: AN OVERVIEW
Space Plasmas
The interactions of the Sun's wind and its magnetic field with the
magnetic fields of the planets are but one example of the plasma
phenomena that occur throughout our solar system. The magnetic field
and plasma surrounding each planet define a region known as the
magnetosphere. The Earth has a relatively quiet magnetosphere, which
we are understanding in increasing detail. For example, we are
beginning to understand how the reconnection of the Earth's magnetic
field lines is related to auroral activity. Data from the Pioneer and
Voyager space missions have yielded a detailed picture of planetary
magnetospheres and the electromagnetic activity in the solar system
(Figure 2.8~. Today, space probes are providing data on collective
oscillations, shocks, particle acceleration, and instabilities. Outside the
solar system, plasma behavior in extreme astrophysical environments
can give rise to such bizarre phenomena as the jets of particles in
opposite directions that have been observed to be ejected from pulsars.
FIGURE 2.8 The magnetosphere of Jupiter. The insert shows a Voyager photograph of
lo's sulfur plasma torus. (Courtesy of NASA.)
OCR for page 35
PROGRESS IN PHYSICS 35
Fluid Physics
The physics of fluids is far from understood because fluid motion
involves many degrees of freedom and is inherently nonlinear. Under-
standing fluid flow, whether the fluid is a gas or a liquid, is essential for
applications such as weather prediction, flight and transportation, plate
tectonics, combustion and chemical reactions in flames, and biological
problems such as blood flow in cardiovascular systems. Thus, any
advance in our understanding of flows, particularly turbulent and
unsteady fluid flows, can be expected to have enormous technological
impact. For example, recent advances in the theory of acoustic
damping and turbulent flow, applied to jet noise, led to a thousandfold
reduction in the acoustic energy emitted by aircraft, providing a major
reduction in perceived noise levels.
There is increasing interest in the so-called nonideal fluids. New
constitutive models for fluids, based on molecular structure, have led
to a better understanding of the striking flow properties of polymer
solutions and drag-reducing agents. These fluids have an interesting
application in fire-fighting equipment; the addition of minute quantities
of very long macromolecules greatly increases water flow and reduces
steam backup.
Progress toward understanding the onset of turbulent flow is encour-
aging. Developments include an increased understanding of chaotic
behavior in simple systems, new methods for observing fluid behavior
near onset, and new techniques for analyzing the data. The advances
are due in large part to modern large-scale computational capabilities.
Among the potential applications of the work is the prediction of
global-scale flows for both short- and long-term weather forecasting.
PROGRESS IN GRAVITATION, COSMOLOGY,
AND COSMIC-RAY PHYSICS
Gravitational Physics
The best-known prediction of Einstein's General Theory of Relativ-
ity is that gravity bends light, but the theory predicts other equally
startling ejects. They are all so small, however, or so hard to observe,
that testing general relativity presents a formidable challenge. During
the past decade, there has been a breakthrough due to space tech-
OCR for page 36
36 PHYSICS THROUGH THE /990.s: AN OVERVIEW
piques. For example, the propagation time for light passing the Sun has
been monitored using a spacecraft. General relativity predicts that light
is not only bent by the Sun's gravity but also slowed by it; a beam
grazing the Sun is delayed by 250 microseconds. By using the Viking
Lander spacecraft on Mars, this tiny delay has been measured to an
accuracy of 0.1 percent.
Space techniques have provided other tests of gravitational theory.
A hydrogen-maser atomic clock in a rocket was compared with a
similar maser on the ground, allowing the minute effect of the Earth's
gravity on clock rate (the gravitational redshift) to be measured to an
accuracy greater than 1 part in 10,000. The results agreed with the
value predicted by the equivalence principle, which relates the effects
of acceleration and gravity. Cosmological arguments have opened the
possibility that Newton's gravitational constant G may not really be a
constant but that it changes as the universe ages. Ranging measure-
ments using the Viking Lander on Mars have been combined with other
solar-system data to set a limit on the possible change: it is no more
than I part in 10" per year.
One of the most dramatic quests in gravitational physics today is the
search for gravitational radiation. The radiation from known sources is
predicted to be so weak that detecting it requires highly innovative
experimental techniques. One method attempts to sense the passing of
gravitational waves by their eject on the length of a large aluminum bar
that is cooled to liquid-helium temperature and carefully isolated from
vibration. A strain level (fractional change in length) of 10-'8 can be
detected. The extreme sensitivity of this measurement can be appre-
ciated by noting that a strain of 10-'8 in a 1-meter-long bar is a change
of length by 0.1 percent of the diameter of an atomic nucleus. Further
improvements are under way. Another approach uses laser inter-
ferometers whose mirrors are mounted on inertial platforms. These
detectors are expected to reach strain levels of 10-23 when baselines of
several kilometers are achieved. interest in gravitational radiation goes
beyond its role In gravitational theory; the waves can reveal sources,
like black-hole formation, that are invisible to us now. The discovery
of gravitational radiation would truly open a new window on our
universe.
A compelling demonstration of the reality of gravitational radiation
has been provided by careful observations of a system of two compact
objects, one of which is a pulsar that emits regular pulsed signals. The
8-hour orbit has been studied in exquisite detail by clocking the radio
pulses. Since 1975, the orbit has decayed owing to the loss of energy by
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PROGRESS IN PHYSICS 37
gravitational radiation; theory and observation agree to within 1
percent.
Fundamental advances in relativity theory have accompanied these
experimental advances. The positive energy theorem has proved that,
in general relativity, any isolated system must have positive total
energy. This is by no means obvious, because gravitational binding
energy is negative. In another milestone discovery, theoretical relativ-
ists have shown that black holes evaporate by emitting thermal
radiation, the temperature being inversely proportional to the mass.
They have also shown that black holes have well-defined entropy and
that a generalized form of the Second Law of Thermodynamics is valid
even for systems containing black holes.
Cosmology
Confidence in the theory of the primordial explosion the big bang-
continues to increase. New measurements of the spectrum of the 3-K
radiation that fills the universe, as well as recent measurements of the
cosmic abundances of the light elements, match the big-bang predic-
tions. Equally important, whatever direction one looks in, the 3-K
radiation is found to be remarkably uniform: the universe is apparently
isotropic to better than 0.01 percent. This isotropy confirms Einstein's
assumption of cosmic homogeneity (the cosmological principle), but it
presents a puzzle in causality. The regions of space being viewed had
not yet been connected by light signals at the time of emission; so
lacking any possibility of communication, how could these regions
"know" the temperature elsewhere?
The extreme temperatures predicted for the earliest moments of the
big bang correspond to energies far beyond the wildest possibilities for
elementary-particle accelerators, but they can nevertheless be con-
ceived in the imagination of theorists. If we calculate the cosmological
consequences of theories like the Grand Unification Theory (Figure
2.9) and compare them with observations, both particle physics and
cosmology are advanced. Possible explanations are being found not
only for the large-scale causality puzzle but for problems such as why
the ratio of baryons to photons in the universe, only 10-9, iS SO small.
Theoretical cosmology is advancing rapidly on many fronts. Impor-
tant progress has been made toward understanding the formation and
evolution of large-scale structure galaxies and clusters of galaxies, for
example. Such studies may lead to an estimate of mass distribution
early in the life of the universe. Theoretical ideas for possible dark-
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Representative terms from entire chapter:
cosmic rays
38 PHYSICS THROUGH THE 1990s: AN OVERVIEW
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PROGRESS IN PHYSICS 39
to, but significantly different in detail from, either that found in the
solar system or that resulting directly from nucleosynthesis in super-
novae. Information from deep-space probes has increased our under-
standing of the interaction of galactic cosmic rays with the local
interplanetary environment.
The highest-energy cosmic rays are observed using ground-based
and underground detectors. Cosmic rays with energies of as much as a
joule have been observed, and the isotropy in the arrival direction of
these highest energy particles has indicated that they have an extra-
galactic origin. In addition, discrete sources of gamma rays with
energies up to 10~5 eV have been observed, and they indicate a few
localized cosmic-ray accelerators with enormous power. Understand-
ing the acceleration mechanisms for such energetic particles is a major
goal of current research.
INTERFACES AND APPLICATIONS
Physics is woven so tightly into the fabric of science and society that
the boundaries separating it from neighboring sciences are hardly
discernible. In the wider context of society as a whole, hardly any
aspect of modern life remains unaffected by the discoveries of physics.
Interface Activities
The basic concepts of physics are incorporated in virtually every
area of science. In addition, physics interacts directly with many
sciences by exchanging theoretical approaches and experimental tech-
niques.
CHEMISTRY
The interface between physics and chemistry is among the best
developed interdisciplinary areas in science. Advances in spectros-
copy, including laser spectroscopy, nonlinear optics, extended x-ray
absorption fine structure (EXAFS), the advent of synchrotron light
sources, and molecular beams are playing increasing roles in chemis-
try. These advances have already had a significant influence on
molecular physics and surface chemistry. They are also finding appli-
cations in other areas of growing physical-chemical interest (such as
the study of polymers and liquids) and in photochemistry and photo-
chemical processing. Order-disorder transitions a central problem in
physics—are being studied in the chemical arena using media such as
40 PHYSICS THROUGH THE 1990s: AN OVERVIEW
micellar and colloidal crystals, microemulsions, and liquid crystals.
Condensed-matter physics is benefiting from the new classes of mate-
rials with unique physical properties that are being created by organic
chemistry. For example, the discovery of organic conductors of
electricity in the early 1970s has led to the creation of effectively
one-dimensional conductors that display the drastic fluctuation effects
predicted by one-dimensional statistical mechanics. By manipulating
the molecular architecture, one can pass systematically from insulators
to semiconductors to metals. We can look forward to numerous
applications of these discoveries, particularly in materials science and
electronics.
BIOPHYSICS
Physics encompasses increasingly complex problems as its experi-
mental and theoretical powers expand. In biophysics, for example,
new frontiers have been opened by the development of methods to
observe the conductance of single molecular channels through biolog-
ical membranes. Hundreds of different channels have been identified,
with more surely to be discovered. Each channel is switched in a
quasi-random process while it is simultaneously modulated by mech-
anisms such as binding of neurotransmitter molecules. One can look
forward to working out the finest details at the molecular level in the
next few years. Molecular genetic technology will assist this research
by creating mutants that are specially engineered to probe the molec-
ular mechanisms. The secrets of protein structure and enzymatic
processes can be expected to unfold as they are probed with synchro-
tron radiation, x-ray and electron-beam crystallography, laser Raman
spectroscopy, magnetic resonance, and other tools from physics. The
new, powerful techniques of biophysics and molecular genetics prom-
ise to deepen our understanding of molecular processes in biology at all
levels.
GEOPHYSICS
Experimental and theoretical methods of physics are contributing to
geophysical understanding of the Earth's three phases atmosphere,
oceans, and solid planet. Today, solid-earth geophysics is dominated
by the concepts of plate tectonics. Modern seismology and innovative
techniques of metrology, particularly those using lasers and atomic
clocks, have made it possible to monitor terrestrial motions with
PROGRESS IN PH YSICS 41
unprecedented precision. Extended movements of the solid-earth crust
can now be interpreted with such high precision that one can begin to
account for fine details like mineralization. In the atmosphere, the
analysis of the turbulent fluid flow associated with global weather
patterns is being revolutionized by advances in large-scale computing.
The fundamental theory of turbulence, however, has yet to be created;
the lack of such a theory continues to limit progress. In the ocean,
underwater seismology, sound-propagation tomography, and satellite
sensing are providing data on the ocean's temperature, its level, the
state of the sea, and the strength of the currents, all of which are
essential for basic understanding of this intricate system. The problems
of geophysics present a formidable challenge because of their inherent
complexity. However, it is vital that progress continue, because
geophysics is involved in the utilization of every type of energy
resource and is an essential element of any attack on the global energy
problem.
MATERIALS SCIENCE
The systematic study of materials and the development of techniques
to control, modify, and create materials are central goals of this
interdisciplinary subject. To this effort, physics contributes fundamen-
tal theory, such as the rapidly developing theory of disordered mate-
rials. It also contributes new experimental tools, such as synchrotron
light sources and the free-electron laser. Microelectronics would not
have been possible without the techniques for making ultrapure
semiconductors developed by materials scientists; a strong underpin-
ning of physics and chemistry was essential to this advance. Con-
versely, the precisely controlled materials fabricated by materials
scientists constitute an invaluable resource for condensed-matter phys-
ics. The development of methods for making artificially structured
materials is one of the many areas in which condensed-matter physics
and materials science are moving forward together. These materials
provide a testing ground for basic physical theory and a likely arena for
discovering novel physical behavior. At the same time, they offer the
possibility of creating materials with unique properties for applications
such as ultrahigh-speed microelectronics. The development of low-loss
optical fibers and the creation of the fiber-optics industry are examples
of the dramatic advances made possible by the confluence in materials
science of basic science, applied science, and technology.
42 PHYSICS THROUGH THE 1990s: AN OVERVIEW
Applications
The most profound technological advances from scientific research
have often been unanticipated: no one could predict that experiments
on amber and lodestone would lead to the replacement of water wheels
by electrical power or that research on the magnetic interactions of
nuclei in matter would lead to a revolutionary technique for medical
imaging. Nevertheless, much of today's scientific research in the
United States is motivated by the need to solve specific problems
facing the nation. Physics plays an important role in this task. The
applications of physics research to societal needs are too broad even to
summarize, but the following are a few examples.
ENERGY AND THE ENVIRONMENT
Society's welfare depends on its energy supply. Physics is involved
in almost every aspect of research on the generation and efficient use
of energy. Progress in fusion energy has been described; fission and
other technologies from physics (such as photovoltaics) are expected
to play increasingly significant roles in the future. For the present,
combustion remains our major source of energy. New diagnostic tools
and new materials can make combustion more efficient, and even a
small increase in efficiency will have an enormous economic impact.
Every manufacturing process and every technology for producing
energy somehow affects the environment. Understanding and control-
ling these effects are crucial to our future well-being. To this task
physics brings essential data, analytical techniques, and theoretical
tools for monitoring the earth, the oceans, and the atmosphere.
MEDICINE
Physics is addressing increasingly complex problems of biophysics
and physiology. In addition to basic research, however, it contributes
directly to the quality of medical care by providing new analytical
tools, diagnostic techniques, and therapies. X-ray tomography has had
a major impact on x-ray diagnostics; magnetic resonance imaging
(described in Chapter 1) is widely regarded as a revolutionary advance
in medical diagnostics. Ultrasonic imaging is yet another of the
noninvasive diagnostic tools from physics. Lasers are finding increas-
ingly widespread applications in medicine. Laser surgery replaces
some highly delicate or traumatic operations with simple and straight-
PROGRESS IN PHYSICS 43
forward procedures. Fiber-optic endoscopes exemplify the many new
diagnostic instruments. Combined with lasers, the endoscopes can be
used to provide new therapies that may replace elaborate surgical
procedures. These examples represent but a few of the many new
instruments and technologies from physics that are today enhancing
the quality of health care in this nation and elsewhere.
NATIONAL SECURITY
Physics is without peer as a source of discoveries that have an
impact on national security strategies and tactics. The profound effects
of physics are apparent in the development of weapons systems and
strategic defense systems and in the complex process of arms control.
Lasers, for example, are now widely used for communications, guid-
ance, and surveillance. The free-electron laser and cyclotron-reso-
nance maser are capable of providing intense coherent radiation from
microwave through ultraviolet wavelengths, with a potentially major
influence on radar technology, particularly "stealth" technology, and
on optical countermeasures. The ability to respond appropriately to a
rapidly changing situation can depend on the real-time ability to
acquire and interpret vast amounts of data: new optical and informa-
tion-processing techniques are at the heart of this effort. In these and
countless other ways, the discoveries from basic physics in former
years, combined with the efforts of scientists and engineers working
today, are helping to assure that the nation can meet its national
security goals.
INDUSTRY
Physics contributes broadly to industry through the creation of new
materials, instruments, and technologies. Beyond these, discoveries in
basic research can lead to the creation of industries such as microelec-
tronics and laser optics. Microelectronics has made possible the
information revolution that is transforming society. Laser optics is
revolutionizing communications and printing. Laser-assisted manufac-
turing, particularly when applied to robotics, is spreading through
every kind of industry. As described in Global Competition, the Report
of the President's Commission on Industrial Competitiveness (U.S.
Government Printing Office, Washington, D.C., January 1985), ". . .
basic research in the Nation today is a critical factor in our long-term
preeminence."
