Physics news Update 771

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Number 771, March 29, 2006 by Phil Schewe and Ben Stein

Black Hole Merger Movie

Accurate calculations of the gravitational waveforms emitted during the collision of black holes can now be made. A new computer study of how a pair of black holes, circling each other, disturbs the surrounding space and sends huge gusts of gravitational waves outwards, should greatly benefit the experimental search for those waves with detectors such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the planned Laser Interferometer Space Antenna (LISA).

The relative difficulty of computer modeling of complicated physical behavior depends partly on the system in question and on the equations that describe the forces at work. To describe the complicated configuration of charges and currents, one uses Maxwell's equations to determine the forces at work. In the case of black-hole binaries, the equations are those from Albert Einstein's theory of general relativity.
Black holes encapsulate the ultimate in gravitational forces, and this presents difficulties for computations attempting to model behavior nearby. Nevertheless, some physicists at the University of Texas at Brownsville have now derived an algorithm that not only produces accurate estimates of the gravity waves of the inspiraling black holes, even over the short time intervals leading up to the final merger, but also is easily implemented on computers (see figures and movie at Physics News Graphics).

"The importance of this work," says Carlos Lousto, one of the authors of the new study, "is that it gives an accurate prediction to the gravitational wave observatories, such as LIGO, of what they are going to observe." The new results are part of a larger study of numerical relativity carried out at the University of Texas, work referred to as the Lazarus Project.
Campanelli, Lousto, Marronetti, and Zlochower, Physical Review Letters, 24 March 2006
Contact Carlos Oscar Lousto, lousto@phys.utb.edu, 956-882-6651
Figures and movie at Physics News Graphics

A Submersible Holographic Microscope

A new device allows scientists to form 3D images of tiny marine organisms at depths as great as 100 m. The device allows the recording of behavioral characteristics of zooplankton and other marine organisms in their natural environment without having to bring specimens to the surface for examination.
Scientists at Dalhousie University in Halifax, Canada, used the hologram arrangement originally invented by Denis Gabor: light from a laser is focused on a pinhole that acts as a point source of light if the size of the hole is comparable to the wavelength of light. The spherical waves that emanate from the pinhole illuminate a sample of sea water. Waves scattered by objects in the sea water then combine at the chip of a CCD camera with un-scattered waves (the reference wave) from the pin hole to form a digitized interference pattern or hologram. The digital holograms are then sent to a computer where they are digitally reconstructed with specially developed software to provide images of the objects.
The Dalhousie researchers packaged their holography apparatus in such a way that the laser and digital camera parts are in separate watertight containers, while the object plane is left open (see images at Physics News Graphics).
One difficulty was to get container windows of optical quality that are thin enough for high resolution imaging but thick enough to resist sea pressure. The new submersible microscope can also record the trajectories of organisms in the sample volume so that movies of the swimming characteristics of micron-sized marine organisms can easily be produced.
Holograms with 1024x1024 pixels can be recorded at 7 to 10 frames per second. This requires a large bandwidth for data transmission to a surface vessel and was accomplished with water tight Ethernet cables. Imaging volumes can be several cubic centimeters depending on the desired resolution.
The Gabor geometry allowed the Dalhousie researchers to design a very simple instrument capable of wavelength limited resolution of marine organisms in their natural environment. Past generations of submersible holographic microscopes had lower resolution, weighed several tons, had to be deployed from large ships, and used high-resolution film as the hologram recording medium. This meant that only a small number of holograms could be recorded. In contrast, the Dalhousie instrument only weighs 20 kilograms, or 44 pounds, can be deployed from small boats or even pleasure vessels, and can record thousands of holograms in a few minutes so that the motion of aquatic organisms can be captured in detail.
Jericho et al., Review of Scientific Instruments, upcoming article
Contact M.H. Jericho, jericho@fizz.phys.dal.ca
Images at Physics News Graphics

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