Two-dimensional Bose-Einstein condensate
1 March 1999
In the last few years, the Bose-Einstein condensates of atoms of
hydrogen and some alkaline metals have been obtained, which form
due to particles gathering together in the zero-momentum zero-
energy state at low temperatures. Scientists from the University
of Turku (Finland) and the Kurchatov Institute (Russia) produced
for the first time the two-dimensional BEC of hydrogen atoms.
After placing hydrogen on the surface of liquid helium-4 at
0.12-0.2 K, a strong magnetic field was applied which aligned the
proton and electron spins of the hydrogen atoms and increased the
density of hydrogen atoms enough to form a condensate. No model
of the two-dimensional BEC is yet available.
Physics News Update, Number 415
Speed of light in a medium
1 March 1999
While the speed of light in a medium is less than in vacuum, a
large speed reduction is problematic because of the increase in
absorption which generally accompanies a significant increase in
the refraction index. Using a new technique developed by a
Harvard University team led by L V Hau, laser light is slowed
down to 17ms-1 (about 20×106 times less than in vacuum) when passed
through the Bose-Einstein condensate of sodium atoms at a
temperature of a few nK; the resulting induced transparency
effect greatly reduces the level of absorption. It is found that
the transparency of the medium varies very strongly with the ray
intensity - a nonlinear effect which can perhaps be used in opto-
electronic components and wavelength conversion devices.
1 March 1999
From the core of a giant elliptic galaxy M87 at the centre of the
Virgo cluster 50 million light-years away, jets of particles
extending for hundreds thousand light-years are spewed out at
relativistic speeds. It is believed that the jets form near a
supermassive (3×109 solar masses) black hole at the galaxy's core
and are powered by the gravitational energy of the matter falling
onto the hole. The most powerful radio source in the Virgo
cluster, M87 is known to emit x-rays from its extended halo. The
x-ray emission from M87 and the central galaxies of other
clusters is usually explained by the cooling flows model, in
which the intergalactic gas in the cluster cools down and falls
inward onto a galaxy. Recent radio images made using the VLA
telescope demand revision of old concepts, however. Observations
at a wavelength of 90 cm have revealed structures about 100,000
light-years across (10 times larger than previously possible with
the same telescope) as well as radio-emitting bubble-like lobes,
about 200,000 light-years across, located in the x-ray-emitting
region. Along the jets from the galaxy's center, huge amounts of
energy exceeding energy lost in the x-ray emission, come outward.
Thus, the energy of the infalling matter is counterbalanced by
that flowing outward along the jets. It is thought that the x-ray
emission has its origin not in the cooling flows toward the
galaxy but rather in the `heating' flows outward.
Optical emission from a gamma ray burst
1 March 1999
Gamma ray bursts, which last only a few seconds and are
distributed isotropically over the sky, keep their origin a
mystery 30 years after their discovery. A BATSE instrument
onboard the Compton Observatory detects one gamma ray burst on
the average every day. Since optical emission from a gamma ray
burst was first detected early in 1997, a number of similar
images, sometimes of remote galaxies, have been obtained. The
observation of optical emission from the gamma ray burst
catalogued as GRB 990123 appears as a breakthrough in the field.
While earlier observations showed optical sources in their
fading, in the present case also the brightening stage was seen.
The important feature of the optical spectrum is the absorption
line corresponding to the redshift z=1.61. GRB 990123 is one of
the brightest gamma ray bursts ever seen. Assuming an isotropic
emission process, the total energy emission exceeds the rest
energy of the Sun. Such energy is obviously unachievable in the
very popular cosmological models in which a gamma ray burst
results from a collision of neutron stars in a binary system in a
remote galaxy. This model remains valid only for a highly
collimated emission. So far no evidence had been found for the
gravitational lensing of a gamma ray burst by any galaxy at the
line of sight. The Hubble and Keck images show that the optical
source is located within a galaxy to which the observed
absorption line presumably belongs. The distance between the
optical source and the assumed center of the galaxy is several
kiloparsek implying that the burst has nothing to do with galaxy
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
Permanent editor is Yu.N. Eroshenko.
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