Number 538, May 7, 2001 by Phil Schewe,
James Riordon, and Ben Stein
First Direct Evidence of Black Hole Rotation
The first direct evidence of black hole rotation arrives in the form
of the telltale dimming of x rays coming from a microquasar about 10,000
light years from Earth. The object in question, GRO J1655-40, consists
of a black hole devouring a nearby normal-star companion. The pillage
is not direct. Instead matter from the star collects on an accretion
disk orbiting the black hole before taking the final plunge through
the event horizon. This jumping-off platform is so hot that matter there
glows at x-ray wavelengths.
Seeing this glow and measuring how the glow changes over short time
intervals requires the use of a special telescope—the Rossi X Ray
Timing Explorer (RXTE), which takes snapshots at a rate of 1000 per
second. A common type of x-ray modulation seen in x-ray binary systems,
called a quasi-periodic oscillation (QPO), is thought to occur because
the hottest x-ray emitting part of the disk, in its swift orbit around
the black hole, is periodically occluded by the black hole itself. The
gravitational fields at work are enormous—after all, the inner
edge of the accretion disk is only tens of kilometers or so from a black
hole of about 7 solar masses. The specific orbital radius can be deduced
from the laws of general relativity which predict a fixed "innermost
stable orbit" for matter circling a black hole. In this case the
predicted orbit is about 64 km.
Many theorists believe, however, that a black hole that spins would
have a much smaller event horizon and this would permit orbiting matter
to attain a much tighter innermost stable position, and a correspondingly
faster orbital rate.
At last week's APS meeting in Washington DC, Tod Strohmayer of the
Goddard Space Flight Center (301-286-1256) reported a previously undiscovered
QPO pattern in x rays from GRO J16550-40. The frequency of this QPO,
450 Hz, is the highest ever seen for x rays coming from a black hole
system, implying an orbital radius of only 49 km—a value consistent,
Strohmayer says, with a spinning black hole. (Preprint
on Los Alamos server: astro-ph/0104487); video
here)
Chaotic Two Photon Laser
In many lasers an ensemble of coherent photons is amplified through
the process of stimulated emission: a photon with just the right energy
can strike an excited atom, causing it to jump down one quantum level,
in the process emitting another photon just like the one that stimulated
the atom. The total number of photons thereby goes from one up to two.
In principle two-photon stimulation can also be achieved. In this
process a population of atoms can be excited in such a way that stimulation
requires the presence of two incoming photons, causing the atom to jump
down two quantum levels, which results in two new photons. The net effect:
two photons are amplified into four photons (see figure at Physics
News Graphics).
The main obstacle to such a scheme, finding a suitable amplifying medium,
is overcome by physicists at Duke University. Daniel Gauthier and his
colleagues (919-660-2511, gauthier@phy.duke.edu)
use as their medium a beam of potassium atoms which have been oriented
(polarized) in a single direction by the fields of a separate control
laser beam and triggered by yet another laser pulse.
The first continuous two-photon laser was first demonstrated in
1992 (Update
75), but only now are their properties being explored in detail.
For example, a 2-photon laser not only produces double the photons per
atom, but has another property that sets it apart from other lasers.
The amount of amplification is not only proportional to the number of
atoms involved but also to the number of photons present; this creates
a nonlinear effect which can lead to an unusual laser output as time
goes by.
The Duke physicists have now driven their laser to this nonlinear regime,
and have found, unexpectedly, that the output is chaotic. Next the researchers
hope to tame the chaos and exploit the nonlinear properties of their
two-photon laser to create ultrabright entangled twin laser beams which
would display correlations at the photon-by-photon level, a feature
expected to benefit precision measurement and quantum cryptography.
(Pfister et
al., Physical Review Letters, 14 May; text at Physics
News Select)
Correction
Concerning the recent RHIC results (Update
537), the antiproton to proton ratio measured by the BRAHMS collaboration
is approximately 2:3, not 3:2.
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