Number 231, June 23, 1995 by Phillip F. Schewe and Ben Stein
PRIMORDIAL HELIUM HAS BEEN DETECTED by an ultraviolet telescope on board
the Astro-2 observatory. According to the big bang model, the nuclei of
primarily two elements--- hydrogen and helium---would have been created
in the aftermath of the big bang. All heavier elements had to be incubated
in the interiors of stars that formed in a later epoch. Arthur Davidsen
of Johns Hopkins reported at last week's meeting of the American Astronomical
Society in Pittsburgh on how the helium detection was made. The telescope
looked at the ultraviolet rays issuing from the quasar HS1700+64. Coming
from so far away (10 billion light years) and so far back in time (when
the universe was but a fraction of its current age) the UV rays had to
traverse some of the primordial helium, which left its mark on the UV spectrum
by subtracting energy at a characteristic wavelength. The detection of
primordial helium, but not primordial hydrogen, is explained in this way:
the hydrogen was entirely ionized by the intense radiation of later stars.
Without its lone electron, the hydrogen nucleus was no longer an atom,
and no longer capable of absorbing the radiation. Helium atoms, with two
electrons, were at times able to retain at least one electron, making it
possible for them to absorb some of the quasar's UV photons.
A "MOLECULE" OF LIGHT, a group of photons acting as a single
bound object, can effectively be created with a device proposed by Joseph
Jacobson and his colleagues at Stanford University. All objects, whether
atoms or photons, can be thought of as waves spreading out in space, with
a "deBroglie wavelength" proportional to the object's momentum.
For instance, if one binds together two atoms having the same momentum,
the deBroglie wavelength of the resulting molecule is half that of the
individual atoms. By contrast, if one combines two photons each having
a wavelength corresponding to blue light, the wavelength normally remains
that corresponding to blue light. But Jacobson (415-725-7699) and colleagues
hope to get around this dilemma by proposing the development of an interferometer
(a device in which a wavetrain is split into two parts and later recombined
to form an interference pattern) using an optical element known as a quantum
switch, consisting of a specially prepared atom contained in a tiny resonant
cavity. When illuminated by the light beam, the switch would permit either
all or none of the photons to pass through. Jacobson suggests that if an
ensemble of 100 photons were sent into the switch, the photons would take
on a dual personality: considered as a form of light, the blue photons
would indeed retain their blue wavelengths; but, if the photon wave train
were made to interfere with itself, the group of photons would behave as
if it were a composite object with an effective wavelength only 1/100th
that of individual blue photons. (Think of a busy restaurant where a fussy
maitre'd, addressing a waiting list of 10 unrelated strangers, tells them
that they will be admitted only if they all sit together.) Why enforce
such a togetherliness on photons? The much smaller wavelength of the photon
"molecule" would result in a greater sensitivity than is possible
with current interferometers which, exquisitely sensitive to small rotations
or changes in the path lengths over which the light waves propagate, are
used in such devices as accelerometers and gyroscopes. In addition, this
sort of interferometry will permit the study of the extent to which the
deBroglie wavelength depends on the internal structure of an object. (Joseph
Jacobson et al., Physical Review Letters, 12 June 1995. Science journalists
can obtain copies of the article or related figures by contacting AIP Public
Information, 301-209-3091 or physnews@aip.org)
|
