SETI bioastro: Supernovae and Neutrinos

From: Larry Klaes (lklaes@bbn.com)
Date: Wed May 03 2000 - 14:09:56 PDT


SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy-makers.

May 5, 2000 -- Vol. 4 Number 18

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"It took a million years to move from counting
pebbles to the elaborations of quantum mechanics.
Certainly this was an arduous migration of the
multitude -- not a private party of physicists,
but the Long March of the entire human race."

-- Anonymous

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Contents of This Issue:

1. Astrophysics: Supernova Explosions
-------------------------------------
Supernovae are crucial to the dynamical and morphological
development of the Universe, and they are also the focus of many
of the important debates now current among astronomers.

2. History of Physics: The Neutrino
-----------------------------------
The history of particle physics during the first 30 years of the
20th century, and in particular the history of the neutrino,
provides an excellent example of the intimate interplay between
theory and experiment. (Includes related background material.)

1. ASTROPHYSICS: SUPERNOVA EXPLOSIONS

Supernovae are violent explosions marking the terminal stage of
certain stars. They are classified into two broad types, Type I
and Type II. A Type II supernova shows hydrogen in its spectrum,
while a Type I supernova shows no hydrogen in its spectrum. Type
I supernovae are further classified as Type 1a, Type 1b, and Type
Ic. A Type 1a supernova is believed to be due to the explosion of
a *white dwarf star in a binary star system, the result of matter
falling onto it from the companion star. When the mass of the
white dwarf exceeds the *Chandrasekhar limit, the white dwarf
undergoes runaway carbon burning and explodes. Type Ib and Ic
supernovae are thought to result from the collapse of the cores
of massive stars which have lost their hydrogen envelopes. Type
II supernovae arise from the explosion of stars of more than 8
solar masses. In this case, the explosion involves a violent
blow-off of outer-layer material after the massive star has
collapsed into a *neutron star or a *black hole. Despite the
existing classification scheme, Type Ib and Type Ic supernovae
are more closely related to Type II supernovae than to Type Ia
supernovae. Gamma ray bursts are intense flashes of *gamma rays
detected at energies up to 10^(6) *electronvolts. They were
discovered by US Air Force satellites in 1967 but not
declassified until 1973. The detection of these bursts averages
about 1 per day, and measurements indicate the distribution of
bursts is isotropic, i.e., they are uniformly distributed across
the sky. The current consensus is that gamma ray bursts are
produced by the merger of two neutron stars, and up to this
point, the bursts that have been noted apparently originate
outside our own galaxy.

... ... Adam Burrows (University of Arizona Tucson, US) presents
a review of current research on supernova explosions, the author
making the following points:

     1) Supernovae are crucial to the dynamical and morphological
development of the Universe, and they are also the focus of many
of the important debates now current among astronomers. The
author suggests that type 1a supernovae are "now arguably
astronomy's most accurate probe of the scale and geometry of the
Universe." An unknown fraction of another supernova subtype, the
core-collapse supernovae, may be the source of gamma-ray bursts.
The author suggests that as major sources of the chemical
elements of existence, supernovae themselves are primary agents
of stellar and galactic evolution, and supernovae and gamma-ray
bursts share the distinction of being the most powerful
explosions in the Cosmos. Recent observational and theoretical
breakthroughs and a renewed appreciation of the manifold roles of
supernovae have inaugurated a new era in their study.

     2) The light curve and spectra of a supernova reflect more
its progenitor's radius, chemical makeup, and expansion
velocities than the mechanism by which the explosion of the
supernova came into being. To the theorist, the achievement of
the critical Chandrasekhar limiting mass unites the various types
of supernovae: the supernova mechanism is either an implosion to
an object of the density of an atomic nucleus and subsequent
hydrodynamic ejection of material (core-collapse supernovae), or
an explosive incineration produced by a thermonuclear runaway
(type 1a white dwarf supernovae).

     3) There is approximately 1 supernova explosion in the
Universe every second. In our Galaxy, there is one supernova
approximately every 30 to 50 years, and one type 1a supernova
approximately every 300 years. Astronomers using only modest
telescopes can now capture a dozen extragalactic supernovae per
night, mostly the bright type 1a. Approximately 200 supernova
remnant shells are known in our Galaxy, and these are radio,
optical, and x-ray echoes of only the most recent Galactic
supernova explosions. Within the last millennium, humans have
witnessed and recorded 6 supernovas in our Galaxy.

     3) The brightness of supernovae suggests their use in
surveying the Universe. If supernovae were *standard candles, a
comparison between their apparent brightness and their intrinsic
(absolute) brightness would yield their distance. A spectrum
taken with a large-aperture telescope capable of precision
measurements of dim objects made dim by distance would yield the
spectral *redshift (z) of the host galaxy of the supernova in the
*Hubble flow of the expanding Universe. A selection of these
measurements would provide redshift-distance and redshift-
magnitude relations which can be used to distinguish different
models of the Cosmos, to determine the geometry and mass-energy
content of the Cosmos, and to help determine the ultimate fate of
the Cosmos.

     4) The author concludes: "In important ways, the histories
of star and galaxy formation and of supernovae are inextricably
linked. Progress in understanding one demands progress in
understanding the other. Today, as we attempt to fathom the
mechanisms of supernova explosions, the origin of the elements,
the death of stars, and the birth of neutron stars and black
holes, we are simultaneously advancing the means by which we can
comprehend our origins. Crucial to the development of the
Universe, supernovae tell a story that goes beyond the exotic
physics, the state of the art numerical technique, and their role
in surveying the Universe, to the heart of mankind's ability to
comprehend its home."

-----------
Adam Burrows: Supernova explosions in the Universe.
(Nature 17 Jan 00 403:727)
QY: Adam Burrows [aburrows@as.arizona.edu]

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Text Notes:

... ... *white dwarf star: White dwarf stars are extremely dense
and compact stars that have undergone gravitational collapse.
They are the final stage in the evolution of low-mass stars after
they have lost their outer layers. White dwarf stars are about
the size of Earth, but with a mass about that of the Sun.

... ... *Chandrasekhar limit: The remnant mass after the blow-off
during the terminal stage of the life of a star determines the
ultimate fate of the star. If the remnant mass is less than 1.44
solar masses (the Chandrasekhar limit for a star with no hydrogen
content), the star collapses into a white dwarf. If the remnant
mass is greater than 1.44 solar masses, depending on the remnant
mass, the star collapses into either a neutron star or a black
hole. Named after Subrahmanyan Chandrasekhar (1910-1995), who
first proposed the modern theory of stellar gravitational
collapse, and who received the Nobel Prize in Physics 1983.

... ... *neutron star: If, following its terminal stages, the
remnant mass of a star is between 1.4 and 2 to 3 solar masses,
the star will collapse into a neutron star, a body with a radius
of 10 to 15 kilometers, with a core so dense that its component
protons and electrons have merged into neutrons. The average
density of a neutron star is 10^(15) grams per cubic centimeter,
and the weight of an object on the surface of a neutron star
would be 10^(11) its weight on the surface of the Earth. Neutron
stars apparently have an outer shell of iron, but it is iron like
no Earth iron, an iron of 4 orders of magnitude greater density.

... ... *black hole: If the terminal stages of star death leave
a remnant star mass greater than 3 solar-masses, the ultimate
gravitational collapse will produce a black hole, a relativistic
singularity. A black hole is a localized region of space from
which neither matter nor radiation can escape. The "trapping"
occurs because the requisite escape velocity, which can be
calculated from the relevant equations, exceeds the velocity of
light and is therefore unattainable. Another view of a black hole

... ... *gamma rays: Gamma rays are radiation of high energy,
from about 10^(5) electronvolts to more than 10^(14)
electronvolts -- radiation with the shortest wavelengths and
highest frequencies, the gamma ray region of the electromagnetic
spectrum merging into the adjacent lower energy x-ray region.

... ... *electronvolts: (eV) A unit of energy defined as the
energy acquired by an electron in falling through a potential
difference of 1 volt. 1 electronvolt = 1.602 x 10^(-19) joule.

... ... *standard candles: In general, in this context, the term
"standard candles" refers to astronomical objects whose intrinsic
brightness is known and whose distance can therefore be
calculated from apparent brightness.

... ... *redshift (z): Redshift (symbol: z) is a lengthening of
the wavelengths of electromagnetic radiation from a source caused
either by the movement of the source (Doppler effect) or by the
expansion of the universe (cosmological redshift). Redshift is
defined as the change in wavelength of a particular spectral line
divided by the unshifted wavelength of that line. Large redshifts
imply large radial velocities (which imply large distances,
according to current cosmological theory), but at redshifts
greater than about 0.2 there is a relativistic divergence from a
linear relation. A redshift of 4.0 corresponds to an object
receding with a radial velocity 92% that of the velocity of
light. The largest astrophysical redshifts so far observed are of
the order of z = 4.9.

... ... *Hubble flow: In general, the outward motion of galaxies
resulting from the uniform expansion of the Universe, with all
motions lying in a radial direction from the observer, and with
velocities proportional to the distance of the galaxies. (Because
of mutual gravitational interactions between galaxies, the actual
pattern of galaxy motions is not precisely of this form.)

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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 5May00
For more information: http://scienceweek.com/swfr.htm

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2. HISTORY OF PHYSICS: THE NEUTRINO

     The history of particle physics during the first 30 years of
the 20th century is an excellent example of the intimate
interplay between theory and experiment. One of the central
problems in the physics of matter during this period was to
understand the emissions of radioactive substances first
discovered in 1896 by Henri Becquerel (1852-1908). Spontaneous
radioactive decay is essentially a spontaneous transmutation of
an unstable atomic nucleus (nuclide) A into nuclide B, with
nuclide A initially in a higher energy state and losing energy to
transmute into the "daughter" nuclide B. During the early years
of particle physics, the energy loss was considered to be
accomplished by emission of one of three types, depending on the
nature of nuclide A: positively charged alpha particles (helium
nuclei), negatively charged beta particles (electrons), or
neutral gamma rays (high energy electromagnetic radiation). Since
the energies of decaying nuclides and daughter nuclides are fixed
according to nuclide identity, one would expect the observed
energies of the 3 types of particles to also be fixed for each
species of decaying nuclide. During the period before 1927, this
was known to be true for alpha particles and gamma rays, but
there was intense controversy about whether it was true for beta
particles. Indeed, some early experiments indicated that it was
not true for beta particles, and this posed a problem, since
conservation laws require an accounting for all the energy and
the numbers for beta decay did not add up. The controversy
continued for nearly 30 years, particularly among
experimentalists who disagreed concerning experimental methods
and interpretations of experimental results, until finally in the
late 1920s it was conclusively demonstrated by experiment that
during the beta-decay process high-speed electrons of various
energies are emitted with a continuous beta-emission energy
distribution spectrum (i.e., a plot of the number of electrons
vs. energy of these electrons) over the range of energies.

     Given the experimental evidence of a continuous beta-decay
spectrum, theoreticians tackled the problem of accounting for
beta decay without violating conservation laws. In 1930,
Wolfgang Pauli (1900-1958) proposed that when a beta particle was
emitted, another particle, without charge, and perhaps without
mass, was also emitted, and that this second particle carried off
the missing energy. Enrico Fermi (1901-1954) suggested the
particle carrying the missing energy be called "neutrino", which
is Italian for "little neutral one", and in 1934 Fermi
incorporated the neutrino into his theory of beta decay.

     Most theoretical and experimental physicists immediately
accepted the proposed existence of the neutrino as the best
solution to an important puzzle, but it was not until 1956 that
Frederick Reines (1918-1998) and Clyde Cowan (1919-1974) managed
to finally obtain experimental evidence for the existence of the
elusive neutrino by means of experiments involving emission beams
from a fission reactor. Enrico Fermi received the Nobel Prize in
Physics in 1938; Wolfgang Pauli received the Nobel Prize in
Physics in 1945; and Frederick Reines received the Nobel Prize in
Physics in 1995. (Clyde Cowan was not eligible for the Nobel
Prize at the time it was awarded to Reines, since the Nobel Prize
is not awarded posthumously.)

... ... Allan Franklin (University of Colorado Boulder, US)
presents an essay on the history of beta decay and the neutrino
1900-1930. The author points out there were two major responses
to the establishment of the continuous energy spectrum of beta
decay. One idea, favored by Niels Bohr (1885-1962), was that
energy might not be conserved in beta decay. But work on the
*Compton effect provided evidence against this view. The second
major response was Pauli's "desperate way out", Pauli suggesting
that a very light, neutral particle was also emitted in the beta
decay. Pauli originally called this particle the "neutron", but
Fermi christened the particle the "neutrino" and quickly
incorporated the neutrino into a successful theory of beta decay.
During the next few decades, Fermi's theory was strongly
supported by experimental observations, and that success provided
most physicists with sufficient evidence for the existence of the
neutrino. As stated by Frederick Reines, for 26 years before the
existence of the neutrino was experimentally demonstrated, "the
[Fermi] theory was so attractive in its explanation of beta decay
that belief in the neutrino as a 'real' entity was general."

-----------
Editor's note: Although modern views of beta decay and the
neutrino (see related background material below) are more complex
than the views held in the early years of the 20th century, a
remarkable group of early experimental and theoretical particle
physicists (only some of whom are mentioned in this SW brief)
provided the foundation that still supports our understanding of
the atomic nucleus and radioactive decay.

-----------
Allan Franklin: The road to the neutrino.
(Physics Today February 2000)
QY: Allan Franklin, Univ. of Colorado Boulder 303-492-6694.

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Text Notes:

... ... *Compton effect: (Compton scattering) In general, the
reduction in the energy of high energy photons when the photons
are scattered by free electrons, the electrons thereby gaining
energy, with total energy conserved. The effect was discovered in
1923 by A.H. Compton (1892-1962). Compton received the Nobel
Prize in Physics in 1927.

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Summary & Notes by SCIENCE-WEEK http://scienceweek.com 5May00
For more information: http://scienceweek.com/swfr.htm

-------------------
Related Background:

ON NEUTRINO OSCILLATIONS

The fundamental particles of 20th century physics came into
existence as theoretical constructions designed to explain
certain specific experimental observations. In some cases, the
existence of a particular particle has been verified by direct
experiment; in other cases, the required verification experiments
are extremely difficult to accomplish, and the particles related
to these experiments have remained theoretical constructions.
The neutrino was first theoretically postulated by Wolfgang Pauli
(1900-1958) in 1930 in order to maintain the conservation of
energy principle in the analysis of the results of certain *beta-
decay experiments. The Pauli neutrino was a particle with no
charge and zero rest mass. Experimentally, the particle was
tentatively identified by F. Reines and C. Cowan in 1953 and more
definitely in 1956. Neutrinos are "leptons", which are a group of
point-like particles with *spin of 1/2 that are not affected by
so-called "*strong interactions" and that are not constructed of
*quarks. In the *Standard Model in particle physics, there are 6
particle types categorized as leptons: the electron, the *muon,
the massive *tau lepton, and a neutrino associated with each of
these (denoted as 3 neutrino "flavors" or "generations").
Neutrinos are produced in great numbers by the Sun, but they
almost never interact with atoms, and an estimated 10^(12) solar
neutrinos flow through our bodies each second without any
consequence. Measurements of solar neutrinos, however, have
produced a mystery: the neutrino density measured by detectors is
approximately one-third that expected from theoretical
calculations of solar neutrino emission. Two kinds of solutions
have been proposed to resolve this mystery, one solution
involving revisions to the theory of stellar structure, and the
other solution involving revisions to nuclear particle theory. In
the latter case, the proposal is that the neutrino may oscillate
among the 3 different flavors (states), with the result that
neutrino detectors detect only one flavor or one-third of the
solar emission. The existence of such neutrino oscillation would
have important implications, since it has been believed that
neutrinos, like photons, have zero mass. But theory indicates
that if neutrinos oscillate they must have mass, and neutrinos
are so numerous that even an extremely small mass would
theoretically be sufficient to affect the future of the Universe
as a whole. The question of neutrino oscillation, therefore, is a
critical problem affecting a good deal of fundamental physics and
cosmology, and there is recent evidence interpreted to indicate
that such oscillation does indeed occur and that neutrinos do
indeed have nonzero mass.

... ... K. Kaneyuki and K. Scholberg (2 installations, JP US)
present a detailed review of current research concerning neutrino
oscillations, the authors making the following points:

     1) The basic strategy for measuring neutrino oscillations is
simple. Given a source of neutrinos, either natural or
artificial, one allows the neutrinos to propagate for a known
distance, and then one obtains as much quantitative information
as possible concerning their energy and flavor. If the amount of
a given flavor, as a function of energy and distance, is that
expected from the quantum mechanical predictions arising from the
oscillation hypothesis, then neutrino oscillation has been
discovered.

     2) Three neutrino sources are currently used in research:
The Sun, atmospheric *cosmic-ray showers, and particle
accelerators. At present, the clearest neutrino oscillation
evidence from atmospheric neutrinos comes from the "Super-
Kamiokande" experiment, which observes neutrino interactions by
detecting *Cherenkov (Cerenkov) radiation. The Super-Kamiokande
experiment has been built and operated by a collaboration of
approximately 130 scientists from Japan and the US, the project
headed by Y. Totsuka (University of Tokyo, JP). The apparatus
consists of 50 kilotons of ultrapure water housed approximately
one kilometer underground in the Kamioka mine in Japan. The
detector consists of 2 concentric cylinders 40 meters high and
with an outer radius of 20 meters. The inner cylinder contains
11,146 inward-facing photomultiplier tubes, each 50 centimeters
in diameter. These photomultiplier tubes detect Cherenkov
radiation from particle interactions inside the inner cylinder
(which contains the ultrapure water). The outer cylinder has 1885
20-centimeter-diameter photomultiplier tubes facing outward to
check for non-neutrino related Cherenkov radiation from entering
charged particles (cosmic-ray muons and radioactivity). Super-
Kamiokande began operation on April 1, 1996.

     3) The essential basis of the Super-Kamiokande experiment is
as follows: When a high-energy cosmic-ray particle (e.g., a
proton) hits an atomic nucleus in the upper atmosphere, the
collision produces a shower of secondary particles. Some of these
particles decay to other particles, some of which are neutrinos.
Most of the charged particles produced in the shower lose energy
as they move through the atmosphere and into the Earth's surface.
Neutrinos, however, because of their extremely small rate of
interaction, pass through the atmosphere and the ground, the vast
majority penetrating to the other side of the Earth. But a few
neutrinos do interact (e.g., with the ultrapure water in the
Super-Kamiokande reservoir), and the Super-Kamiokande apparatus
can detect the interaction of approximately 8 neutrinos per day
in its inner volume.

     4) The authors conclude: "These are exciting times for
neutrino physics, and for elementary particle physics as a whole.
The atmospheric neutrino data fit the neutrino-oscillation
hypothesis beautifully, and this verification that at least some
neutrinos have mass is an enormous step forward: It is the first
clear indication of physics beyond the Standard Model."

-----------
K. Kaneyuki and K. Scholberg: Neutrino oscillations.
(American Scientist May-Jun 99 87:222)
QY: Kenji Kaneyuki, Dept. of Physics, Tokyo Institute of
Technology, JP.

-----------
Text Notes:

... ... *beta-decay: A type of interaction in which an unstable
atomic nucleus changes into a nucleus of the same mass number but
different proton number. The change involves the conversion of a
neutron into a proton with the emission of an electron and an
electron *antineutrino, or of a proton into a neutron with the
emission of a positron and an electron neutrino. The electrons or
positrons emitted are called "beta particles". (Positrons are
electron antiparticles. See *antineutrino.)

... ... *antineutrino: An antiparticle (antimatter) is a
subatomic particle that has the same mass as another particle and
equal but opposite values of some other property or properties.
For example, the antiparticle of the electron is the positron. An
antineutrino, the antiparticle to the neutrino, has zero mass,
*spin 1/2, and positive helicity. There are 2 antineutrinos, one
associated with the electron and one associated with the *muon.

... ... *spin: In quantum mechanics, "spin" is the intrinsic
angular momentum of a subatomic particle. Spin states are
quantized, multiples of h/2, where h = Planck's constant, and
each particle is characterized by a quantum spin number which is
the multiple factor.

... ... *strong interactions: According to the *Standard Model,
the fundamental forces comprise the gravitational force, the
electromagnetic force, the nuclear strong force, and the nuclear
weak force.

... ... *quarks: A quark is a hypothetical fundamental particle,
having charges whose magnitudes are one-third or two-thirds of
the electron charge, and from which the elementary particles may
in theory be constructed.

... ... *Standard Model: In particle physics, the Standard Model
is a theoretical framework whose basic idea is that all the
visible matter in the universe can be described in terms of the
elementary particles leptons and quarks and the forces acting
between them.

... ... *muon: The 3 leptons (electron, muon, tau) differ from
each other only in mass. The muon is 200 times more massive than
the electron.

... ... *tau: (tauon) The mass of the tau particle is 3560 times
the mass of the electron.

... ... *cosmic-ray: Cosmic rays are highly energetic particles
moving at close to the speed of light and continuously bombarding
the Earth's atmosphere from all directions. The energies of the
particles are enormous and range from 10^(8) to over 10^(19)
electronvolts.

... ... *Cherenkov (Cerenkov) radiation: Discovered in 1934 by
Cerenkov (1904-1990), Cerenkov radiation is electromagnetic
radiation, usually bluish light, emitted by a beam of high-energy
charged particles passing through a transparent medium at a speed
greater than the speed of light in that medium. The radiation is
essentially a shock wave, the effect analogous to that of a sonic
boom.

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Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 25Jun99
For more information: http://scienceweek.com/swfr.htm



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