Number 253, January 4, 1996 by Phillip F. Schewe and Ben Stein
ANTIHYDROGEN ATOMS HAVE BEEN CREATED at CERN. Physicists from a Julich/Erlangen-Nuernberg/GSI/Genoa
collaboration passed a beam of antiprotons through a jet of xenon gas.
Occasionally, some of the antiproton's own energy can be converted into
electron-positron pairs. In the case of nine events, the newly created
positron's motion was well matched to that of the antiproton and they formed
an atom, in effect an atom of antihydrogen. Antimatter has been produced
in the lab artificially for decades; antiprotons, for example, were first
produced in the 1950s. The positron was first discovered in the 1930s.
But not until now have anti-atoms been made and detected. The Low Energy
Antiproton Ring (LEAR) at CERN makes it possible to slow antiprotons down
sufficiently to perform a variety of physics experiments, including the
insertion of antiprotons into ordinary atoms and the creation of anti-atoms.
In the present experiment, the anti-hydrogen atoms were not trapped, and
very quickly annihilated with ordinary matter in the vicinity. Scientists
at CERN hope soon to actually capture and study the new exotic atoms. First
of all, one wants to be sure that all the physical laws that pertain to
atoms---such as quantum mechanics---also govern the behavior of antimatter
as well. (CERN press release, 4 Jan.)
A FEYNMAN THOUGHT EXPERIMENT HAS BEEN DEMONSTRATED IN AN ATOM INTERFEROMETER .
In his lectures, Richard Feynman imagined an experiment that explored what
would happen when you shine light at an object passing through an interferometer,
a device that can split the object into a pair of wavelets which are later
recombined to produce an interference pattern. This experiment has been
unrealizable in electron and neutron interferometers because neither type
of particle interacts strongly with light. Now, using their atom interferometer,
an MIT team has shone single photons on atoms inside the interferometer.
As Feynman correctly described, scattering a photon from an object inside
the interferometer can destroy its wave properties. If one can in principle
determine a pathway for the atom by detecting the position of the scattered
photon, then the object acts as a particle. On the other hand, if the two
atom-wave paths are separated by less than about one-half the wavelength
of the light, a scattered photon can no longer provide information on which
path the atom traversed, so the wave properties are not destroyed. In a
variant of the experiment, the group detected only those atoms that scattered
photons in a narrow range of directions. As it turns out, the once-washed-out
interference patterns reappear at the sacrifice of information about the
paths the atoms took. (Michael Chapman et al, Phys. Rev. Lett., 20 November
1995.)
A GIGANTIC CELESTIAL MASER provides evidence for the presence of a supermassive
black hole in an active galaxy. Theorists believe that in galactic masers
coherent microwaves are produced and amplified within gas clouds; the energy
supply would come from a nearby black hole. The new maser is ten times
more powerful than any previous specimen. Furthermore, the astronomers
at the Max Planck Institute (Germany) who discovered the new water-vapor
giga-maser believe that it hints at the existence of yet more powerful
masers at higher red shifts and that the study of such distant objects
may facilitate an alternative measurement of the Hubble constant. (Koekemoer
et al., Nature, 14 December 1995.)
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