The Nobel Prize in Physics 2002 - Information for the Public
October 8, 2002
This year's Nobel Prize in Physics is concerned
with the discoveries and detection of cosmic particles and radiation,
from which two new fields of research have emerged, neutrino astronomy
and X-ray astronomy. The Prize is awarded with one half jointly to: Raymond
Davis Jr, Department of Physics and Astronomy, University of Pennsylvania,
Philadelphia, USA, and Masatoshi Koshiba, International Center
for Elementary Particle Physics, University of Tokyo, Japan, "for
pioneering contributions to astrophysics, in particular for the detection
of cosmic neutrinos", and the second half to Riccardo Giacconi,
Associated Universities, Inc., Washington, DC, USA, "for pioneering
contributions to astrophysics, which have led to the discovery of cosmic
X-ray sources". Here is a description of the scientists'
Two New Windows on the Universe
Why does the Sun shine?
In the 19th century there were lively discussions about the source of
the Sun's energy. One theory was that this solar reaction was due
to the release of gravitational energy when the Sun's material contracted.
However, in this case, the calculated life expectancy of the Sun was,
in our eyes, short. It was approximately 20 million years, compared with
the age of the Earth, which we know today is approximately 5 billion years.
In 1920, an experiment showed that a helium atom has less mass than four
hydrogen atoms. The British astrophysicist Sir Arthur Eddington realised
that nuclear reactions in which hydrogen was transformed into helium might
be the basis of the Sun's energy supply, using Albert Einstein's
formula E=m·c2. The transformation
of hydrogen into helium in the Sun gives rise to two neutrinos for each
helium nucleus that is formed by a series of reactions (explained by,
among others, the Nobel Prize Laureate Hans Bethe). The dream of verifying
this theory by detecting neutrinos was considered a practical impossibility
by most scientists. However, in the 1950s the Nobel Prize Laureate Frederick
Reines and his colleagues succeeded in showing that it was possible to
prove the existence of neutrinos. In their experiment they used the reactions
in a nuclear reactor, which generates a large flux of neutrinos.
|Fig. 1 Davis's detector, which for the first
time in history proved the existence of solar neutrinos. The tank,
which was placed in a gold mine, contained more than 600 tonnes of
tetrachloroethylene and was 14.6 metres long, with a diameter of 6.1
The flux of neutrinos from the Sun was estimated to be very large: thousands
of billions of solar neutrinos were reckoned to pass through our bodies
every second without our noticing them. The reason is that these neutrinos
react very weakly with matter, and only one of 1,000 billion solar neutrinos
would be stopped on its way through the Earth.
In the late 1950s Raymond Davis Jr was the only scientist who dared
to try to prove the existence of solar neutrinos, despite these poor odds.
While most reactions in the Sun create neutrinos with energies so low
that they are very difficult to detect, one rare reaction creates a high-energy
neutrino. The Italian physicist Bruno Pontecorvo proposed that it ought
to be possible to detect this neutrino after it had reacted with a nucleus
of chlorine, forming a nucleus of argon and an electron. This argon nucleus
is radioactive and has a life of about 50 days.
Particles captured in mines
In the 1960s Davis placed a tank filled with 615 tonnes of the common
cleaning fluid tetrachloroethylene (Fig. 1) in a gold mine in South Dakota,
USA. Altogether there were some 2·1030
chlorine atoms in the tank. He calculated that every month approximately
20 neutrinos ought to react with the chlorine, or in other words that
20 argon atoms ought to be created. Davis's pioneering approach was
the development of a method for extracting these argon atoms and measuring
their number. He released helium gas through the chlorine fluid and the
argon atoms attached themselves to it - an achievement considerably
more difficult than finding a particular grain of sand in the whole of
the Sahara desert!
This experiment gathered data until 1994 and all in all approximately
2,000 argon atoms were extracted. However, this was fewer than expected.
By means of control experiments Davis was able to show that no argon atoms
were left in the tank of chlorine, so it seemed as though our understanding
of these processes in the Sun was incomplete or that some of the neutrinos
had disappeared on their way to the Earth.
Neutrinos from space
While Davis's experiment was running, the Japanese physicist Masatoshi
Koshiba and his team constructed another detector, which was given
the name Kamiokande. It was placed in a mine in Japan and consisted of
an enormous tank filled with water. When neutrinos pass through this tank,
they may interact with atomic nuclei in the water. This reaction leads
to the release of an electron, creating small flashes of light. The tank
was surrounded by photomultipliers that can capture these flashes. By
adjusting the sensitivity of the detectors the presence of neutrinos could
be proved and Davis's result was confirmed. Decisive differences
between Davis's and Koshiba's experiments were that the latter
registered the time for events and was sensitive to direction. It was
therefore possible for the first time to prove that neutrinos come from
the Sun (Fig. 2a).
|Fig. 2 a) Solar-neutrino observations in the Kamiokande
experiment. A clear peak is visible at the angle corresponding to
the direction of the Sun. The flat background comes from cosmic radiation
and radioactivity around the detector. b) Observation of the burst
of neutrinos from SN1987A. This figure shows the number of photomultipliers
hit in a 17-minute interval beginning at 07.33 UT. The burst of neutrinos
came at 07.35.35 UT on 23 February, 1987.
The Kamiokande detector was hit in February 1987 by a burst of neutrinos
from a supernova explosion, named 1987A, in a neighbouring galaxy to the
Milky Way called the Large Magellanic Cloud (Fig. 2b). This lies at about
170,000 light years from the Earth (one light year corresponds to 1016
metres). If a neutron star is formed when a supernova explosion takes
place, most of the enormous amount of energy released will be emitted
as neutrinos. A total of about 1058
neutrinos is estimated to have been emitted from supernova 1987A, of which
Koshiba's research group observed twelve of the approximately 1016
that passed through the detector. A similar experiment in the United States
confirmed this discovery.
Do neutrinos change?
In order to increase sensitivity to cosmic neutrinos, Koshiba constructed
a larger detector, Super Kamiokande, which came into operation in 1996.
This experiment has recently observed effects of neutrinos produced within
the atmosphere, indicating a completely new phenomenon, neutrino oscillations,
in which one kind of neutrino can change to another type. This implies
that neutrinos have a non-zero mass, which is of great significance for
the Standard Model of elementary particles and also for the role that
neutrinos play in the universe. It could also explain why Davis did not
detect as many neutrinos as he had expected.
Davis's and Koshiba's discoveries and their development of instruments
have created the foundation for a new field, neutrino astronomy, which
is of great importance for elementary particle physics, astrophysics and
cosmology. The Standard Model for elementary particles will have to be
modified if neutrinos have mass, and this mass can be highly significant
for the collected mass of the universe. Studies designed to confirm or
disprove the neutrino oscillation theory are in progress at many laboratories
around the world.
An invisible firmament
The X-rays Wilhelm Röntgen discovered in 1895 were quickly put to
use by physicists and doctors at laboratories and clinics all over the
world. In contrast it took half a century for astronomers to study this
type of radiation. The main reason was that X-ray radiation, which can
so easily penetrate human tissue and other solid material, is almost entirely
absorbed by the air in the Earth's thick atmosphere. It was not until
the 1940s that rockets had been developed that could send instruments
high enough up in the atmosphere.
The first X-ray radiation outside the Earth was recorded in 1949 by instruments
placed on a rocket by the late Herbert Friedman and his colleagues. It
was shown that this radiation came from areas on the surface of the Sun
with sunspots and eruptions and from the surrounding corona, which has
a temperature of several million degrees Celsius. But this type of radiation
would have been very difficult to record if the Sun had been as far away
as other stars in the Milky Way.
||Fig. 3 The instrument in the nose of
the Aerobee rocket that was launched in June 1962 by Giacconi and
his group and which was the first to record a source of X-rays outside
the solar system. The instrument, about one metre long, contained
three Geiger counters (indicated by arrows), provided with windows
of varying thickness so that the energy of the radiation could be
In 1959 the then 28-year-old Riccardo Giacconi was recruited to
build up a space-research program for a company that was to make it easier
for young researchers to get commissions from e.g. NASA. Together with
the man who took this initiative, the late Bruno Rossi, Giacconi worked
out principles for how an X-ray telescope should be constructed. This
construction collected radiation with cone-shaped, curved mirrors onto
which the radiation falls very obliquely and is totally reflected. This
is the same phenomenon as when a landscape is reflected in the air above
an asphalt road on a hot summer's day.
Giacconi and his newly-formed group also carried out rocket experiments
to try to prove the presence of X-ray radiation from the universe, primarily
to see whether the moon could emit X-ray radiation under the influence
of the Sun. In one experiment a rocket flew at a high altitude for six
minutes. No radiation from the moon could be detected, but a surprisingly
strong source at a greater distance was recorded since the rocket was
rotating and its detectors (Fig. 3) swept the sky. In addition, a background
of X-ray radiation was discovered evenly distributed across the sky.
These unexpected discoveries gave an impetus to the development of X-ray
astronomy. In time the way in which the direction of the radiation could
be determined was improved and the sources could be identified with observations
made in normal light. The source discovered in the first successful experiment
was a distant ultraviolet star in the Scorpio constellation, Scorpius
X-1 (X for X-ray, 1 for the first). Other important sources were stars
in the Swan constellation (Cygnus X-1, X-2 and X-3). Most of the newly-discovered
sources were double stars, in which one star circles in a narrow orbit
around another object which is very compact - a neutron star or perhaps
a black hole (Fig. 4). However, it was difficult to carry out these studies
because the possible observation times from the balloons and rockets were
X-ray satellites broadened our horizons
In order to extend observation times, Giacconi initiated the construction
of a satellite to survey the sky for X-ray radiation. This satellite was
launched in 1970 from a base in Kenya and was given the name UHURU ("freedom"
in Swahili). It was ten times more sensitive than the rocket experiments
and every week it was in orbit it produced more results than all the previous
experiments put together.
However, so far no high-definition X-ray telescope had been sent into
space that could provide sharp images. Giacconi constructed one, which
was ready for use in 1978. It was called the Einstein X-ray Observatory
and was able to provide relatively sharp images of the universe at X-ray
wavelengths. Its sensitivity had been improved and objects a million times
weaker than Scorpius X-1 (see above) could be recorded.
|Fig. 4 A double star that generates X-rays. Gas
streams out of the star down towards the compact object and accelerates
in its strong gravitational field up to very high speeds. When the
gas atoms collide with each other and are decelerated at the surface
of the neutron star and by its magnetic field, intensive X-ray radiation
This telescope made a large number of discoveries. Many X-ray double
stars were studied in detail, not least a number of objects that were
thought to contain black holes. More normal stars could also be studied
for the first time in X-ray radiation. Remnants of supernovas were analysed,
X-ray stars in galaxies outside the Milky Way were discovered and eruptions
of X-ray radiation from distant active galaxies could be examined more
closely. The X-ray radiation from the gas between galaxies in galaxy
groups helped scientists draw conclusions about the dark matter content
of the universe.
In 1976 Giacconi initiated the construction of an improved, even larger
X-ray observatory. It was not launched until 1999, and was named Chandra
after the astrophysicist and Nobel Prize Laureate Subrahmanyan Chandrasekhar.
Chandra has provided extraordinarily detailed images of celestial bodies
in X-ray radiation (Fig. 5) corresponding to those from the Hubble Space
telescope or the new Earth-based telescopes using visible light.
|Fig. 5 Remnants of the supernova -
an exploding star - in the Cassiopeia constellation which Tycho
Brahe discovered in 1572 from Herrevadskloster and described in
detail. The supernova lies at a distance of 7,500 light years from
Earth and is 20 light years wide (one light year corresponds to
1016 metres). This image was taken
by the Chandra satellite in X-ray radiation. NASA/CXC/SAO.
New light thrown on black holes
Thanks to X-ray astronomy and its pioneers, in particular Giacconi, our
picture of the universe has been changed in decisive ways. Fifty years
ago our viewpoint was dominated by a picture of stars and star constellations
in equilibrium, where any developments were very slow and gradual. Today
we know that the universe is also the scene of extremely rapid developments
in which enormous amounts of energy are released in processes lasting
less than a second, in connection with objects that are not much larger
than the Earth, but incredibly compact. Studies of processes at these
objects and in the central parts of active galaxy cores are largely based
on data from X-ray astronomy. A new, fantastic zoo of important and strange
celestial bodies has been discovered and studied. Today the universe seems
much more remarkable than we believed 50 years ago - in no small
part thanks to X-ray astronomy.
Links and further reading >>
|Raymond Davis Jr
University of Pennsylvania
Dept. of Physics and Astronomy
School of Arts and Sciences
University of Pennsylvania
116 College Hall
Philadelphia, PA 19104-6377
Born 1914 (87 years) in Washington, DC, USA. PhD in Chemistry 1942
at Yale University, Connecticut, USA. Professor Emeritus at the Department
of Physics and Astronomy, University of Pennsylvania, Philadelphia,
International Center for Elementary Particle Physics
University of Tokyo
7-3-1 Hongo, Bunkyo-ku
|Japanese citizen. Born 1926 (76 years),
in Toyohashi, Aichi, Japan. PhD 1955 at the University of Rochester,
New York, USA. Professor Emeritus at the University of Tokyo, Japan.
Associated Universities, Inc.
1400 16 th St., NW
Washington, DC 20036
|US citizen. Born 1931 (71 years),
in Genoa, Italy. PhD 1954 at the University of Milan. Director of
Associated Universities, Inc., Washington, DC, USA.