"
I have long believed that an
experimentalist should not be unduely inhibited by theoretical
untidyness. If he insists on having every last theoretical t
crossed before he starts his research the chances are that he
will never do a significant experiment. And the more significant
and fundamental the experiment the more theoretical uncertainty
may be tolerated. By contrast, the more important and difficult
the experiment the more that experimental care is warranted.
There is no point in attempting a half-hearted experiment with an
inadequate apparatus."1
Bob Dicke contributed to advances in radar, atomic
physics, quantum optics, gravity physics, astrophysics, and
cosmology. The unifying theme is his
application of powerful and scrupulously controlled
experimental methods to issues that really matter.
Though Bob sometimes had to hide his amusement
at theorists he found poorly grounded in phenomenology, he
did not hesitate to speculate where the experimental ground is
thin; the condition was that there had to be the possibility
of a measurement that could teach us something new.
Bob held over 50 patents, from clothes dryers
to lasers. He recognized that two mirrors make a more
effective laser than the traditional closed cavity of microwave
technology. In the company Princeton
Applied Research he and his students packaged his advances in
phase-sensitive detection in the now-ubiquitous "lock-in
amplifier." With its successors this probably has contributed as
much to experimental PhD theses as any device of the past
generation. Bob predicted and experimentally showed that
collisions that restrict the long-range motions
of radiating atoms in a gas can suppress Doppler broadening.
The physics is the same as that of Mössbauer narrowing of
gamma-ray lines; it is used in the atomic clocks of the Global
Positioning System. He contributed to the concept of
adaptive optics in astronomy.
He was among the first to recognize that the accepted gravity
theory, general relativity, could and should be subject to more
thorough tests. His series of gravity experiments mark the
beginning of the present rich network of tests. He set forth
the idea2 of
the anthropic principle that now plays a big part in speculation
on what our universe was doing before it was expanding.
Bob's visualization of an oscillating universe stimulated the
discovery of the cosmic microwave background, the most direct
evidence that our universe really did expand from a dense state.
A key instrument in measurements of this fossil of the Big Bang is
the microwave radiometer he invented.
Bob left us a challenge: discover whether or how laboratory
physics is related to the universe at large. At the turn of the
century Ernst Mach argued for such a relation, that
distant matter determines local inertial frames. Mach's
principle led Einstein to general relativity. In this theory
the mass distribution
does influence inertial motion, but it has no effect on
local laboratory measurements. Bob felt Mach's principle
likely expresses more than this, and he and
Carl Brans3 gave an example, a generalization of
general relativity in which the expansion of the universe causes
the strength of the gravitational interaction to decrease.
Their approach was ruled out by experimental advances in gravity
physics, but the theory reappears in superstring
models. And we are left to wonder what to make of Bob's
belief:1 "the laboratory, earth and solar system could not
be isolated even in principle from the rest of the universe."
"I was born in St. Louis, Missouri in 1916 but my
earliest recollections are of Washington D. C. where my father
worked for the United States Patent Office as a patent examiner.
Later, when my father became a patent attorney for the General
Railway Signal Corp., we moved to Rochester, N. Y. It was there,
at an age of 5, that I had my first contact with the fascination
of science. An old spectacle lens fell into my possession and I
was both fascinated and puzzled by its behavior. Later my
childhood scientific interests ran the usual course -
mechanical gadgets, insect collecting, electricity, chemistry via
a `chemistry set', microscopy via an inexpensive Sears
microscope, astronomy - and I read everything scientific I
could get my hands on."1
Bob entered the University of Rochester intending to major in
engineering, it not having occurred to him that he might make a
living as a physicist. He credits Lee A. DuBridge with attracting
him to physics and Frederic Seitz at Rochester and E. U. Compton
at Princeton University for brokering his transfer there as
a Junior. While at Princeton he published his first research
paper, on a dynamical model of a globular star cluster as an
ideal gas sphere.
Bob returned to the University of Rochester for graduate work in
nuclear physics. Here he courted Annie Currie; they
were married in Rochester on 6 June 1942.
Bob completed research for his PhD
degree at the University of Rochester in the spring
of 1941. His topic, which he had
selected, was one of the first experimental studies
of inelastic scattering of protons. He recalls that "Professor
DuBridge offered me a position as Instructor in the Department
for the following fall (at the impressive salary of 1800.00 for
the academic year). I was happy to accept but didn't have a
chance to serve. War rumblings were growing louder and Professor
DuBridge had left to establish the Radiation Laboratory at MIT to
develop microwave radar. A few months later he asked me to join
the Laboratory as soon as I could get my thesis finished. I
arrived at MIT in September of 1941."1
A year later Annie joined Bob in Cambridge. She was not
supposed to know about Bob's classified research.
Her first hint came from Bob's cousin Tom
Kuenning, a pilot in the antisubmarine campaign off the New
England coast. A storm during patrol forced Tom to land away from
his base, and since the crew had no money they had to stay with
friends; Tom stayed with the Dickes in Cambridge. Over breakfast
Tom remarked on the marvelous effect of the radar sets from the
Radiation Laboratory.
The Radiation Laboratory also produced a brilliant crop of
physicists, Bob notable among them for his imaginative and subtly
effective approach to physics. Among the results
were his microwave radiometer, which he
took to Florida to demonstrate that humid
air radiates strongly near 1~cm wavelength, and hence that humid
air is a strong absorber at that wavelength. At the time this
limited the push to shorter wavelength radar for
better resolution. Bob found time for a little pure science,
using his radiometer to measure the surface temperature of the
moon and to show that the space between the stars could not
be warmer than 20 degrees above absolute zero.4
After the war Bob returned to the Department of Physics at
Princeton University. He brought his 1.25 cm radiometer, but he
recalls that "As a very junior member of the physics department,
I considered it rash to start doing astronomical research and I
could not develop any interest in the astronomy department. I
realized only later that the physics department was tolerant and
that it would have been proud to have the first radio astronomy
in the country."1 Instead of radio astronomy Bob spent the
next decade on the rich physics of the quantum mechanical
interaction of radiation and matter. The book on quantum
mechanics by him and his former student, James P. Wittke5, was
published in 1960. It was used in many graduate courses, and, we
suspect, consulted by a lot more teachers of quantum mechanics.
Beginning in 1955, Bob turned to gravity physics in a
series of elegant and searching experiments and theoretical
analyses that set the stage for today's active research
community. Bob's students (PJEP) and postdocs (DTW) remember
when his Gravity Group met on Friday evenings; we complained but
attended because the physics was too fascinating to miss. He probably knew we
called ourselves Dicke-birds - it fit his quiet good humour,
which kept us from taking ourselves too seriously while always
remembering that we had better take the physics very seriously.
Bob was among the most imaginative of physicists. One sensed this
in personal interactions by his close attention and support for
work on anything of substance in biology, geology, astronomy,
physics, or any of the other sciences. Discussions
with Bob tended to leave one feeling that science is a wonderful
adventure that one could join.
Bob Dicke was elected to the National Academy of Sciences in 1967.
Among his many prizes and awards are the National Medal of
Science, 1971, the Comstock Prize of the National Academy of
Sciences, 1973, and the NASA Medal for Exceptional Scientific
Achievement, 1973. He was a member of the National
Science Board from 1970 to 1976. Bob was appointed to the
Princeton University Department of Physics in 1946, served as
chair from 1967 to 1970, moved to emeritus in 1984, and kept active in
research until prevented by physical problems including
Parkinson's disease. He and Annie loved and supported each other,
and Bob followed developments in science, until his last moments.
He is survived by Annie and their children Nancy Dicke Rapoport,
John Robert Dicke, and James Howard Dicke.
At the Radiation Laboratory Bob was assigned to the Fundamental Developments
Group under Harvard's Ed Purcell. As one of the young stars of
the Radiation Laboratory, he invented chirped radar, coherent
pulse radar, and monopulse radar, all of which came into
widespread use after the end of World War II. He also invented
the magic tee microwave junction and the microwave radiometer,
devices at the heart of radio telescopes. The flavor of
Dicke's elegant contributions to microwave radar comes through
clearly in Principles of Microwave Circuits6, one of the
classic volumes of the Radiation Laboratory Series.
Characteristically, Bob was the first to make
systematic and potent use of symmetry principles and scattering
matrix ideas from nuclear physics to analyze waveguide
junctions and other microwave devices.
Back at Princeton after the war Bob used the microwave
skills he had acquired at the Radiation Laboratory
to make fundamental measurements in physics. With excellent taste, he started to measure the fine
structure of the n=2 level of hydrogen, but on learning that Willis Lamb
was already working hard on that problem with the resources of
the Columbia Radiation Laboratory, Bob turned to other challenges. Unswayed
by careless assumptions of others that because the g-value of
free electrons could not be measured in an atomic beams machine there
was some fundamental reason the g-value could not be
measured at all, Bob began to generate free electrons by photoionization of
sodium atoms with circularly polarized light. Unaware of Kastler's
work in Paris, Bob and his student Bruce Hawkins7 carried out
one of the first optical pumping experiments - on a beam of sodium atoms.
Bob understood how important narrow spectral linewidths are to
precision spectroscopic measurements. He soon realized that
gas-phase collisions, often a source of line broadening, could be an
advantage in the right circumstances, since sufficiently rapid
randomization of the thermal velocity vector would eliminate the Doppler
broadening of the line8. Bob and his
students showed that this collisional narrowing is particularly effective
for the 0-0 "clock" transitions of hydrogen
and the alkali-metal atoms9. Further development of these
ideas by Tom Carver and others led to fabulously stable atomic clocks. Bob wondered about
applications of these narrowing ideas to other spectral regions,
but it remained for R. Mössbauer to show that at sufficiently low
temperatures the Doppler broadening of certain gamma ray lines could be
eliminated by the same physics.
Fascinated by coherent microwave radiation from pulse-excited
ammonia molecules, Bob conceived of
the phenomenon of superradiance, where properly phased atomic
systems can radiate with great intensity in narrow, pencil-shaped
beams10.
Characteristically, Bob made the concept of superradiance clear
to a large audience with apt and quantitative analogies to the
high-gain antennas he understood so well from his
work at the Radiation Laboratory. Many years later a beautiful series of
experiments in the infrared by Mike Feld and colleagues11
at MIT confirmed the striking properties of superradiant systems
that Bob had forseen.
During sabbatical leave at Harvard in 1954-55 Bob
turned to the experimental and theoretical basis
for gravity physics. At the time
the Eötvös experiment showed that test bodies
of different composition have the same gravitational
acceleration to a few parts in 109.
That was a guide to Einstein's general relativity: a
gravitational acceleration may be transformed away by
going to an accelerating coordinate frame. There were three
tests of Einstein's theory. First, it agrees with
the measured rate of advance of the orbit of the
planet Mercury, 42.56+/- 0.94 arc seconds per century faster
than Newtonian theory.12 This is an impressive
success, but Bob was to emphasize that it depends on the
mass model for the sun. Second, the
relativistic deflection of light by a mass
concentration is twice the Newtonian value. The deflection of
starlight by the sun arguably was detected and consistent with
relativity; the accuracy was at best
10~percent. Third, in a static mass distribution the fractional
shift of the wavelength of light is proportional to the
gravitational potential difference through which the light moves.
The redshift was detected in spectral lines from
surfaces of white dwarf stars, consistent with the theory
to perhaps 30 percent.13 The contrast to the
present range and precision of the experimental basis for
gravity physics is striking.
Among his gravity experiments Bob was most proud of the
modern Eötvös experiment and the solar oblateness measurement
as a probe of the solar interior.
The Eötvös experiment monitored the difference of
gravitational accelerations of test masses in a
torsion balance. The balance is triangular, to suppress
tidal torques, with two aluminum weights and one gold. The
orientation of the balance is measured by a light beam reflected
by an optical flat to intersect a wire vibrating at 3000 Hz. A
servo system electrostatically torques the balance to null
the fundamental period in the light passing the wire. A
difference of gravitational accelerations of aluminum and gold
toward the sun would cause the feedback voltage to the electrodes
to vary with the orientation of the balance
relative to the sun. This elegant experiment showed the
fractional difference of gravitational accelerations of
aluminum and gold is (1.3 +/- 1.0) x 10-11, an
improvement of two orders of magnitude. It is no slight
to Eötvös' magnificent achievement to say the
modern error budget is more reliable.
The oblateness experiment is another memorable example of
effective design of an experiment to test a bold hypothesis:
the test of general relativity theory from the rate of
precession of the perihelion of the orbit of the planet
Mercury may be compromised
by the departure from a spherical mass distribution in the
Sun.14 This would be reflected in the shape of the
solar surface. By the time of the first oblateness measurements
the experimental tests of gravity theories were much
improved, in large measure because of Bob's work and example, and
they favored general relativity (as they still do). But it was
characteristic that, having set out to make this important
test, Bob pushed it to the limit for a ground-based
observation. With his former students Jeffrey
R. Kuhn and Kenneth G. Libbrecht the experiment was improved
and moved from Princeton to Mount Wilson (above
Pasadena). Observations there suggested the oblateness
varies from year to year.15 Now measurements
from the SOHO satellite, above the blurring of the atmosphere,
show the oblateness is close to what would be expected from
the mean rotation of the solar surface,16 indicating the
departure from a spherical
mass distribution is not a serious
factor in the precession of Mercury's perihelion. Bob's former
colleagues, Henry Hill, Kuhn, and Libbrecht, are among those who
have established that the Solar interior indeed is a dynamic
system, but not in the way Bob imagined.
While Bob was involved in the demanding Eötvös and oblateness
experiments he and his students were producing many
other tests of gravity physics. Here are examples with dates of
PhD. James W. Brault (1962) showed that the gravitational
redshift of the solar spectrum is 1.05 +/- 0.05 times the
predicted value. The Doppler shifts that compromised previous
measurements were suppressed by the use of
a strong line that originates high in the atmosphere. Kenneth C.
Turner (1962) improved the
Kennedy-Thorndike bound on the variation of
an oscillator frequency with velocity relative to a preferred
frame, perhaps one defined by distant matter. The fractional
difference of frequencies of two oscillators with relative
velocity  u that are otherwise identical may be
expressed as f/f = 2u ·
u/c2,
where u is the velocity relative to the preferred frame.
The Kennedy-Thorndike bound is 17u=10 +/- 10
km~s-1. The Mössbauer effect with the gamma-ray source and
absorber on opposite sides of a centrifuge gave
u < 900 cm~s-1. James E. Faller (1963) obtained an absolute
measurement of the local acceleration of gravity, to an accuracy
of 7 parts in 107, by using one element of an optical
interferometer as the freely
falling object. Lloyd Kreuzer (1966) tested the universality of
the ratio r of active to passive gravitational masses for
a solid floating in a liquid. At neutral buoyancy the passive
mass distribution is independent of the position of the solid in
the liquid. If the ratio r were different in the solid and
liquid the gravitational field produced by the active mass
distribution would depend on the position of the solid.
Kreutzer's limit (for teflon floating in a mixture of methyl
bromide and trichloroethelene) is |r1-r2|<4 x 10-5 at
68% confidence. With the discovery of the thermal
background radiation it became of great interest to improve the
measurements of the helium abundance, to
compare to the predicted production in the early universe.
The helium abundance in a star affects its
luminosity for given mass; the mass measurement requires improved
orbital elements in older binary stars that are likely to have
closer to primeval abundances. Work on improving measurements of
the angular separations of close binary stars
commenced with David R. Curott (1965) and Dennis J. Hegyi (1968),
and concluded with William Wickes' (1972)
interferometer that is capable of measuring separations as small
as 0.2'' with experimental error of about
0.008''.
Bob's largest experimental collaboration grew in part from his
remark (and later independently that of Kenneth L. Nordtvedt)
that if the strength of the gravitational interaction were a
function of position the gravitational acceleration of a
body massive enough to have a significant gravitational
self-energy would differ from that of a nearly massless test
particle. Nordtvedt18 analyzed the effect in an extension
of the parametrized formulation of metric gravity theory. In
1960 Bob with William F. Hoffman and Robert Krotkov showed that
an optical corner reflector offers a good way to get precision
distances to an
artificial satellite. In 1969, at the first moon landing,
the astronauts set out a rack of 100 corner reflectors designed
to reflect a pulsed laser beam from earth. The time delay gives a
precision distance, which can be used to test the Nordtvedt
effect, among other things. The results, under the early
leadership of Bob's former student Carroll Alley, now
significantly
restrict the spatial variation of the gravitational interaction.
Bob's role in the discovery of the thermal background
radiation is legendary, and legends tend to beguile even those
personally involved. Bob wrote:1
"There is one unfortunate and embarrassing
aspect of our work on
the fire-ball radiation. We failed to make an adequate literature
search and missed the more important papers of Gamow, Alpher and
Herman. I must take the major blame for this, for the others in
our group were too young to know these old papers. In ancient
times I had heard Gamow talk at Princeton but I had remembered
his model universe as cold and initially filled only with
neutrons."
Many have wondered how Bob could have forgotten Gamow's
work. The last sentence in this quote agrees with all we know;
memory can fail. For example, when work began at Princeton on
a Dicke radiometer to search for thermal cosmic radiation we had
to remind Bob that he had measured a significant bound on its
temperature two decades earlier. In the second sentence of this
quote Bob may be referring to an unpublished paper by
one of us on light element production in a hot Big Bang
cosmology, written before we knew Gamow already had worked out
the key physics. Our paper19 interpreting the radiation as a
fossil of the Big Bang referred to the
"
 "
paper. This was inappropriate; Gamow had not
yet taken account of the effect of
the mass density in thermal radiation
on the rate of expansion of the universe through the
epoch of light element production.
We also referred to a later paper by Alpher, Follin, and
Hermann20 that gives a close to modern treatment of the
centrally important evolution of the neutron-proton ratio, and
notes that the predicted hydrogen-to-helium
ratio is in the range 1:7 to 1:10 by number, in line
with astronomical data. This certainly was one of the most
important of the earlier papers. We did miss the most important
of all, by Gamow,21 that set forth the now standard picture
for light element production. By 1971 we had the story
straight.22
Bob arrived at the idea of a hot Big Bang by considering element
destruction rather than formation. He favored an oscillating
universe as a way to understand what the universe was doing
before it was expanding. There has to be a provision for
removal of stars and the heavy elements they produce from the
last cycle. Bob noted
that if the bounce were deep enough to blueshift starlight from
the last cycle to above MeV energies the radiation would
thermalize
and could evaporate stars and heavy elements. He persuaded
P. G. Roll and DTW to build a Dicke radiometer to look for the
thermalized starlight, which would be adiabatically cooled by a
large factor since elimination of the heavy elements. News of
this experiment led Arno A. Penzias and Robert W. Wilson to
realize the excess noise temperature in a radio
telescope at the Bell Laboratories might be extraterrestrial.
And cosmology dramatically advanced.
|
