Physics Tip Sheet #67, May 29, 2007

Contact:
James Riordon
riordon@aps.org
301-209-3238
American Physical Society

Highlights in this Issue: a living memory chip, black holes on the loose, and a clearer picture of ocean currents.

Learning, Memory, and Progress toward a Living Chip

Itay Baruchi and Eshel Ben-Jacob
Physical Review E
available online

A new experiment has shown that it's possible to store multiple rudimentary memories in an artificial culture of live neurons. The ability to record information in a manmade network of neurons is a step toward a cyborg-like integration of living material into memory chips. The advance also may help neurologists to understand how our brains learn and store information.

Itay Baruchi and Eshel Ben-Jacob of Tel-Aviv University used an array of electrodes to monitor the firing patterns in a network of linked neurons. As previous studies have shown, simply linking the neurons together leads them to spontaneously fire in coordinated patterns. In the study published this month in the journal Physical Review E the researchers found that they could deliberately create additional firing patterns that coexist with the spontaneous patterns. They claim that these new firing patterns essentially represent simple memories stored in the neuron network.

To create a new memory in the neurons, the researchers introduced minute amounts of a chemical stimulant into the culture at a selected location.  The stimulant induced a second firing pattern, starting at that location. The new firing pattern in the culture along coexisted with the original pattern. Twenty-four hours later, they injected another round of stimulants at a new location, and a third firing pattern emerged. The three memory patterns persisted, without interfering with each other, for over forty hours.

In addition to producing the first chemically operated neuro-memory chip, the researchers propose that their work implies that chemical stimulation may be crucial to learning and memory formation in living organisms. -KM

Black Holes on the Loose

Manuela Campanelli, Carlos O. Lousto, Yosef Zlochower, and David Merritt
Physical Review Letters, forthcoming

JoseA. Gonzalez, Mark Mannam, Ulrich Sperhake, Bernd Brugmann, and Sascha Husa
Physical Review Letters, forthcoming (preprint available)

Abraham Loeb
Physical Review Letters, forthcoming (preprint available)

Two merging black holes can generate gravitational waves so powerful that the merged hole shoots out of its host galaxy at a speed of up to 2,500 miles per second, according to new simulations by two separate research groups. A pair of papers (Campanelli et al. and Gonzalez et al.) describing nearly identical examinations of the catastrophic events will be published back to back in an upcoming issue of Physical Review Letters.

The studies show that the violent recoil following a merger is capable of ejecting the supermassive black holes known to lie at the heart of most light-emitting galaxies. These black holes may be cruising through the universe, virtually undetectable unless they should crash into something and gain matter.

The groups both considered the optimal conditions for producing recoil speeds high enough to free a supermassive black hole from its host galaxy. In this scenario, the two black holes orbit around one another. They have equal masses and spin at the highest possible rate. They must be tilted onto their sides, with their axes of rotation lying in the plane of their orbit, and they must spin in opposite directions. They spiral toward one another, and when they merge, they are kicked in a direction perpendicular to the orbital plane.  

Some astrophysicists have argued that such conditions are rather unlikely. The probability that black hole ejection will occur remains an open question for future research. Even if supermassive black holes have been removed from galactic cores, the odds that one of them will streak through our solar system are small enough that we need not fear a sudden obliteration.

A third study, conducted by Abraham Loeb of Harvard University, examines the possibility of detecting a black hole that has been kicked by gravitational recoil.  If the black hole is surrounded by a ring of gas, it will emit light and resemble a star-like object known as a quasar.  

A quasar exists when the supermassive black hole at the center of a galaxy rapidly acquires gas.  As a result, the gas near the black hole heats up and radiates several times as much energy as the Milky Way.  A quasar that is displaced from galactic core may well be a kicked black hole. Unfortunately, it would require a real stroke of luck to catch one in action - the gas fueling the light would only last about ten million years, so an ejected black hole would be dark by the time it left its galaxy. -KM

A Clearer Picture of Ocean Currents

G. Froyland, K. Padberg, M. H. England, A. M. Treguier
Physical Review Letters, forthcoming

A novel analysis of water flow in the Southern Ocean surrounding the Antarctic is revealing previously hidden structures that are crucial in controlling the transport of drifting plants and animals as well as the distribution of nutrients and pollutants that affect ocean life. Researchers at the University of New South Wales in Australia and the Universitat Paderborn in Germany discovered that barriers to currents, which can lead to swirling gyres and eddies that trap material for long periods, may escape detection with traditional analyses that concentrate on monitoring average water flow or sea surface height.
 
Rather than tracking flow in the ocean point by point, as is typical of most ocean studies, the researchers applied a more holistic approach based on a mathematical technique known as Lagrangian analysis. In effect, the method allows them to simultaneously consider all the possible ways that currents can move in the ocean, and then pick out the most likely solution.
 
When the team tested their approach on a simulated model of ocean current flow, they found that regions where drifting material might be trapped in seas near the Antarctic were clearly identified with the Lagrangian approach. Traditional analyses, however, can only hint at the regions' locales. The researchers plan to extend their study to encompass current flow on a global scale. The work should help to provide a clearer picture of the currents that are vital to the health of our planet's oceans. -JR

Katherine McAlpine and James Riordon contributed to this Tip Sheet.

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