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| Biuletyn PTA nr 17 |
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Biuletyn informacyjny Zarzadu Glownego Polskiego Towarzystwa Astro-
nomicznego (Adres kontaktowy: M. Ostrowski, pta@oa.uj.edu.pl ,
a w bardzo pilnych sprawach: mio@oa.uj.edu.pl )
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Spis tresci:
I. Europejski program ASTROVIRTEL
II. Nowosci naukowe
- LIGHT AT 1 mph
- CANDIDATE DARK MATTER PARTICLES + UPDATE
- STRANGE HALO ORBITS EXPECTED AT SATURN
- BEST MEASUREMENT OF THE GRAVITATIONAL CONSTANT
- MAGNETIC FIELDS ARE EVERYWHERE
- GRAVITY HAS BEEN MEASURED AT THE SUB-MILLIMETER SCALE
- 1997/98 PHYSICS DEMOGRAPHICS
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I. Europejski program ASTROVIRTEL
On behalf of ESA and ESO, we are glad to be able to announce the opening
of the ASTROVIRTEL Programme.
ASTROVIRTEL is an initiative financed by the European Commission under
the scheme "Enhanced Access to Large Infrastructures" of the 5th
Framework Plan and it is operated by the ST-ECF and ESO/DMD.
The aim of ASTROVIRTEL is to give the opportunity to selected groups of
European scientists (from EC Member and Associated States [for a list of
these states, see http://www.stecf.org/astrovirtel/EUStates.html]) to
access and use the ESO/ST-ECF Archive (which currently contains data
obtained with the ESA/NASA HST, with the ESO NTT, VLT and with the Wide
Field Imager on the ESO/MPI 2.2m Telescope) as if it would be a
"virtual" Telescope.
A description of the ASTROVIRTEL Programme, together with the details of
the first Call for Proposals, can be found at
http://www.stecf.org/astrovirtel .
Please note that the deadline for the first Call is June 15th, 2000.
Do not hesitate to contact us if you need further information. The
ASTROVIRTEL email address is astrovirtel@eso.org
Piero Benvenuti
Head / ST-ECF
Peter Quinn
Head / DMD
From: Piero Benvenuti (pbenvenu@eso.org)
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II. Nowosci naukowe
LIGHT AT 1 mph. A year ago Lene Verstergaard Hau used a Bose
Einstein condensate (BEC) as a special nonlinear optical medium for
slowing light from 3 x 10^8 m/sec to a mere 17 m/sec (38 mph; Update
415). This comes about when an incoming light pulse enters the BEC and
experiences an extremely abrupt change in index of refraction (and as for
absorption of the light, this is prevented by applying two laser beams
which induce a transparency at the frequency of the incoming light). In a
talk presented at this week's meeting of the American Association for the
Advancement of Science (AAAS) in Washington, DC, Hau said that she
and her Harvard colleagues had slowed the light further, to a speed of 1
mph. She said that if the velocity could be slowed still more, to a value
of 1 cm/sec, then this would be comparable to the speed of sound in the
condensate and it might be possible to get atoms to surf on the front of the
light pulse. Hau believes that this approach to slowing light, if it can be
simplified, would lead to highly sensitive light switches and to low-power
nonlinear optics (right now high-power laser light is required to produce
nonlinear effects).
CANDIDATE DARK MATTER PARTICLES, specifically thought to be
examples of "weakly interacting massive particles" (WIMPs), have been
indirectly detected by a group operating in the Gran Sasso National Lab
(INFN) in Italy, according to a paper to be delivered by Pierluigi Belli of
the University of Rome (DAMA collaboration) at a dark matter detection
meeting in Marina del Rey, California this week (a meeting sponsored by
UCLA: information from dm20@physics.ucla.edu). Dark matter is a
hypothetical non-luminous substance thought to be lurking in and around
galaxies, influencing the way the galaxies rotate and interact. The dark
matter might consist in part of baryons (particles such as the protons
found in common atoms) or more novel forms such as WIMPs. Because
of the way the Earth orbits the sun and the way our solar system moves
through the galaxy (buffeting the presumed dark matter halo as it goes)
there is reason to think that the flux of WIMP wind we encounter (and the
rate at which WIMPs feebly interact in terrestrial detectors) would be
higher in June than in December. The DAMA experiment reports having
discovered just such an seasonal effect in the frequency of events in which
a presumed incoming WIMP (with masses about 50 times that of the
proton) strikes a shielded sodium-iodine scintillation material, causing
tiny flashes of light deep within the detector (INFN preprint AE-00/01;
www.lngs.infn.it). Dark matter interactions in detectors are expected to
be rare and analysis difficult, so the DAMA interpretation will be subject
to great scrutiny at the California meeting, where other groups searching
for WIMPs will be reporting as well.
DARK MATTER UPDATE. At the dark matter detection meeting in
Marina del Rey, California last week (Update 473) a group form Gran
Sasso, Italy reported detecting evidence for dark matter particles. The
Cryogenic Dark Matter Search collaboration (10 US institutions), using a
different detection scheme, reported finding no evidence for such particles,
and asserted that their results were incompatible with the Gran Sasso
finding. (Stanford press release, 2/24. see preprint at
http://arXiv.org/abs/astro-ph/?0002471.)
STRANGE HALO ORBITS EXPECTED AT SATURN. Consider particles in orbit
above a planet. If the particles are uncharged or have a very low
charge-to-mass ratio, they will follow a conventional ("Keplerian")
trajectory centered about the axis of the planet at the equator
(Saturn's rings are an example of such particles). If, however, the
particles are highly charged, their motions are dominated by an
electromagnetic interaction with the planet's magnetic dipole (Earth's
van Allen belts are an example). If the charge is somewhere in between
these two cases, and gravity and electromagnetic forces are comparable,
then strange orbits are possible. Scientists at the University of
Colorado (Mihaly Horanyi and Jim Howard, 303-492-6903) and Loughborough
University (Holger Dullin) in the UK estimate that if conditions are
just right some particles could race around a planet in orbits (stable
for as long as 10 years) that never cross the planet's equatorial plane
(see figure at www.aip.org/physnews/graphics). The dust analyzer on the
Cassini craft now gliding toward Saturn might be able to detect
particles in these novel orbits. (Howard et al., upcoming article in
Physical Review Letters; Select Article.)
BEST MEASUREMENT OF THE GRAVITATIONAL CONSTANT. At this week's American
Physical Society Meeting in Long Beach, Jens H. Gundlach of the
University of Washington (paper P11.3) reported a long-awaited higher
precision measurement of the gravitational constant, usually denoted by
the letter G. Although G has been of fundamental importance to physics
and astronomy ever since it was introduced by Isaac Newton in the
seventeenth century (the gravitational force between two objects equals
G times the masses of the two objects and divided by their distance
apart squared), it has been relatively hard to measure, owing to the
weakness of gravity. Now a group at the University of Washington has
reduced the uncertainty in the value of G by almost a factor of ten.
Their preliminary value is G=6.67390 x 10^-11 m^3/kg/s^2 with an
uncertainty of 0.0014%. Combining this new value of G with measurements
made with the Lageos satellite (which uses laser ranging to keep track
of its orbital position to within a millimeter) permits the calculation
of a brand new, highest precision mass for the earth: 5.97223 (+/-
.00008) x 10^24 kg. Similarly the new mass of the sun becomes 1.98843
(+/- .00003) x 10^30 kg. Gundlach's (206-543-4080,
jens@phys.washington.edu) setup is not unlike Cavendish's venerable
torsion balance of two hundred years ago: a hanging pendulum is obliged
to twist under the influence of some nearby test weights. But in the
Washington experiment measurement uncertainties are greatly reduced by
using a feedback mechanism to move the test weights, keeping pendulum
twisting to a minimum. (See Gundlach's written summary at
http://www.aps.org/meet/APR00/baps/vpr/layp11-03.html; figures at
www.aip.org/physnews/graphics.)
MAGNETIC FIELDS ARE EVERYWHERE. The history of the universe is usually
described in terms of the distribution of matter: first primordial
knots, then clouds, galaxies, stars, and clusters. A parallel, and
perhaps not unrelated, saga can be written for magnetic fields.
Basically, Philipp Kronberg (416-978-4971) of the University of Toronto
finds magnetic fields every place he has looked in the cosmos: within
the Milky Way (where the fields are typically about 5 microgauss), in
intergalactic areas within galaxy clusters (1-2 microgauss for the Coma
cluster, 350 million light years away), and even outside clusters. The
latter observations are brand new and were reported by Kronberg at the
APS meeting (http://www.aps.org/meet/APR00/baps/vpr/layb7-02.html).
Detecting weak magnetic fields outside clusters was difficult and
required the use of new low-frequency receivers mounted on the Very
Large Array (VLA) radio telescope. The radio range employed, around 75
MHZ, is normally problematic owing to scattering in the Earth's
ionosphere, but new image processing techniques have allowed a sharp VLA
"deep field" image to be formed. From the intensity of the radio glow,
Kronberg deduced a magnetic field of about 10^-8 to 10^-7 gauss for a
distant region outside any galaxy cluster, a place (near the "Great
Wall") where fields had not been mapped before. Where did such fields
come from? Kronberg suggests that huge shock waves, formed where two
large streams of weakly magnetized gas come together, could amplify
existing fields to much higher levels, as well as playing a part in the
acceleration of cosmic rays. Angela Olinto (paper B7.1) of the
University of Chicago (773-702-8206) discussed the idea of primordial
magnetism, fields that might have existed at or shortly after the time
of the big bang. Such fields, she speculated, might have come about
through the development of some asymmetry (just as matter came to
predominate over antimatter) in the infant universe. Early magnetism
might then have influenced subsequent galaxy formation or even the
distribution of matter now seen imprinted in the cosmic microwave
background (CMB). She said that the surprising absence of subsidiary
peaks in the CMB spectrum (see Update 481) might be attributable to
magnetic effects. This hypothesis could be addressed, Olinto said, by
the Planck satellite (launch date several years from now; see Update
342), dedicated to mapping the CMB with unprecedented precision.
GRAVITY HAS BEEN MEASURED AT THE SUB-MILLIMETER SCALE for the first
time. Gravity has of course long been studied over planetary distances
but is more difficult to gauge at the terrestrial scale, where intrusive
electric and magnetic fields, many orders of magnitude stronger than
gravity fields, can be overwhelming. Nevertheless, Eric Adelberger and
his colleagues at the University of Washington have managed to measure
the force of gravity over distances as small as 150 microns using a
disk-shaped pendulum carefully suspended above another disk, with a
copper membrane stretched between them to help isolate electrical
forces. (This experiment should not be confused with another University
of Washington effort in which the gravitational constant is measured
with higher precision see Update 482). Adelberger (206-543- 4294,
eric@gluon.npl.washington.edu) presented one of several talks at this
week's APS meeting in Long Beach, California devoted to short-range
gravity, a subject which has suddenly attracted much theoretical and
experimental interest owing to a relatively new model which supposes the
existence of extra spatial dimensions in which gravity, but not other
forces, might be operating. According to Nima Arkani-Hamed of LBL
(arkani@thsrv.lbl.gov, 510-486- 4665) this is why gravity is so weak: it
dilutes itself in the extra dimensions. In other words, ordinary
particles are tethered to our conventional spacetime, or "brane," while
gravitons are free to roam into otherwise unseeable dimensions. One
implication of the model, testable with tabletop experiments such as
Adelberger's, is that the gravitational force might depart from Newton's
inverse square law (gravity inversely proportional to the square of the
distance between two objects) at close range. Adelberger did not observe
such a departure at distances down to tenths of a millimeter and will
continue to explore even shorter distances. For a list of tabletop
experiments underway, see
http://gravity.phys.psu.edu/mog/mog15/node12.html. Another interesting
implication of the model introduced by Arkani-Hamed (and others; see
preprint hep-th 9803315) two years ago is that the unification of the
four known forces would not necessarily occur at energies as high as
10^19 GeV but possibly at energies as low as 10^4 GeV, an energy scale
within reach of the Large Hadron Collider under construction at CERN.
Extra dimensions could, for example, manifest themselves in proton-
proton smashups as an apparent disappearance of energy, implying that
some of the collision energy had been converted into gravitons (the
particle form of gravity) which then disappear into the extra
dimensions. The gravitons produced in this way might come back into our
conventional world of 3 spatial dimensions and decay into two photons.
Physicists have already looked for this kind of event. Gregory Landsberg
of Brown University (401-863-1464; landsberg@hep.brown.edu) reported
that at the D0 experiment at Fermilab some energetic two-photon events
have been observed (including one in which the energy of the photons
added up to 574 GeV, representing the highest composite mass ever seen
in the D0 experiment) but not often enough to constitute evidence for
extra dimensions. In fact this shortage of events has been translated
into a lower limit of 1300 GeV for the energy at which a prospective
unification of the forces could take place.
1997/98 PHYSICS DEMOGRAPHICS for US institutions from a
recent AIP report: 1323 physics PhDs were granted, a 4% drop
from the previous year. Of these 13% were to women and 46% to
foreign citizens. Among first-year physics graduate students,
foreign citizens now exceed their US counterparts. Physics PhDs
were awarded to a total of 9 African-Americans and 9 Hispanic-
Americans during the academic year. Over the three year period
1996-1998 the leading institutions for African-American physics
bachelors were Xavier U (LA), Southern U & A&M Coll (LA), and
Lincoln U (PA). In astronomy 116 PhDs were granted: 19% went
to women and 30% to foreign citizens. (Enrollment and Degrees
Report, AIP Education and Employment Statistics Div;
www.aip.org/statistics/trends/undtrends.htm)
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