| Chronology | Current Month | Current Thread | Current Date |
| [Year List] [Month List (current year)] | [Date Index] [Thread Index] | [Thread Prev] [Thread Next] | [Date Prev] [Date Next] |
stand
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 454 October 26, 1999 by Phillip F. Schewe and Ben Stein
GRAVITY'S GRAVITY. A new experiment at the University of
Washington seeks to determine whether the gravitational binding
energy of an object generates gravity of its own. As formulated by
Albert Einstein, the Equivalence Principle (EP) states that if we
in a closed room we cannot tell whether the weight we feel is theviolated
result of gravity pulling down or the force of a rocket carrying us
forward through otherwise empty space. All of this gets complicated
in some theories of gravity, which predict that the EP will be
to a small degree since in addition to the usual gravity, carried fromthe
place to place by spin-two particles called gravitons, there should
exist another, fainter kind of gravity carried by spin-zero particles
(sometimes called dilatons). For this reason, and because recent
observations of supernovas suggest that some repulsive gravitational
effects might be at work in the cosmos, scientists want to explore
possibility of EP violations. Three decades of lunar laser rangingit
(bouncing light off reflectors placed on the Moon) show that the
Moon and the Earth fall toward the Sun with the same acceleration to
within half a part in a trillion (10^12). What the Washington
physicists (Eric Adelberger, 206-543-4294,
eric@gluon.npl.washington.edu) have done is focus attention on the
subject of gravitational binding energy, or self-energy, and whether
too obeys the EP. To illustrate the concept of binding energy,in a
consider that the mass of an alpha particle is actually about 28 MeV
less than the sum of its constituents. This energy (about 7.6 parts
thousand of the alpha mass) represents the energy (vested in theself-energy
strong nuclear force) needed to hold two protons and two neutrons
together inside the alpha. Gravity being very much weaker than the
strong nuclear force, the gravitational binding energy, the
of gravity attraction, is almost infinitesimal. For example, self-the
energy effectively reduces the mass energy of the Earth by a factor of
only about 4.6 parts in 10^10. Is this tiny "mass" also subject to
EP? Supplementing existing lunar laser ranging results with new dataPhysical
from special test masses mounted on a sensitive torsion balance (see
www.aip.org/physnews/graphics) to take into account the different
compositions of the Earth and Moon, the Washington physicists show
that gravitational self energy does obey the equivalence principle at
the level of at least one part in a thousand. Thus gravitational self
energy does indeed generate its own gravity. (Baessler et al.,
Review Letters, 1 November; see also Clifford Will's article, Physics
Today, Oct 1999.)
VACUUM TUBES ATTEMPT A COMEBACK. Vacuum tubes were
the backbone of the electronics industry until the 1960s, when their
large size, excessive power dissipation, and lack of integration
allowed solid-state technology to win out. Now forests of 100-nm
sized nanotriodes might bring vacuum designs back, at least for niche
applications. Researchers at the University of Cambridge (Alexander
Driskill-Smith, David Hasko, and Haroon Ahmed,
aagd100@cus.cam.ac.uk, 011-44-1223-337556) have made an anode-
gate-cathode device in which the cathode consists of multiple
nanopillars which can be crowded together in a dense formation. This
will eventually enable nanotriode densities of 10^9 per cm^2
(including interconnects) to be reached, comparable with the best
packing densities for metal-oxide-semiconductor (MOS) transistors,
the electronics industry workhorse. Shooting electrons through
vacuum rather than a semiconductor not only makes switching fast
(the ballistic electrons always travel without scattering), but gives
nanotriodes a few advantages over MOS technology: the nanotriodes
are radiation resistant, operate well at high and low temperatures,
and, because they are vertically-oriented, will permit integration in
the third dimension, allowing even greater packing densities.
Electrons (or, more accurately, the electron waves) issuing from the
nanopillars are coherent and highly focused, and might be useful for
doing holography or nanolithography. Remaining problems with this
vacuum design include a relatively high operating voltage (10 V) for
large scale integration applications and the reproducibility and
longevity of the nanotriodes. (Applied Physics Letters, 1 November
1999.)
CORRECTIONS: 1. Diamonds precipitated from methane in an anvil
press (Update 451) squeezed up to pressures of 50 GPa, equivalent to
0.5 (not 10) million atm. 2. One can contemplate, at least in
principle, the wave behavior of bowling balls (not bawls; Update
453).