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| Number 563, October 31, 2001 by Phil Schewe, James Riordon, and Ben Stein
What Is Intelligence? This may seem to be more of a question for psychologists than physicists. But two researchers (Joseph Wakeling, jwakeling@webdrake.net, now at the University of Fribourg, Switzerland, and Per Bak, Imperial College, 011-44-20-7594-8528, p.bak@ic.ac.uk) argue that intelligence is not an abstract concept, but must be considered as a physical phenomenon. Any definition of intelligence, they say, cannot ignore a living being's environment, including its very own body. In their view, an organism is only intelligent relative to how well it solves the problems that its surroundings throw at it. This runs counter to many historical ideas, including the concept that the mind is separate from the body, or that it is possible to build a desktop computer that thinks like a human without having the same physical environment or body. To explore the idea of intelligence, the researchers ran computer simulations of artificial neural networks called "minibrains." In the simulations, 251 minibrains each attempted to pick the less popular of two choices, 0 and 1, analogous to 251 motorists all trying to pick the less congested road. This "Minority Game" would be repeated over many successive rounds. Each minibrain consisted of three layers of "neurons": "input neurons," which dictated how many past rounds it could remember, leading to an intermediary layer, which then led into an "output" layer that determined what choice was made. If the minibrain ending up making an incorrect choice, it would reduce the strength of the connections between neurons supplying the "wrong answer." The researchers were in for a surprise when they endowed all of the minibrains with equal abilities, which would be analogous to a bunch of motorists having the same amount of decision-making skill. In this situation, no minibrains correctly guessed the minority choice with even a 50 percent success rate, which is what you'd get by making the choice with a random flip of a coin. Even an E. coli bacterium, which searches for glucose by moving in random directions in its environment, is seemingly more intelligent than this. Only when the researchers introduced a "rogue" minibrain with more intermediate neurons to analyze the past rounds did it attain more than a 50 percent success rate. Their simulations suggest that intelligence often hinges on how much one can make use of the data in its physical environment. (Wakeling and Bak, Physical Review E, November 2001.) X-ray Flash Fly Photography Researchers at Cornell University have created striking images of tiny subjects, including houseflies and fruit flies, illuminated by the brilliant burst of x rays emitted by vaporizing wires. The radiographs (x-ray photographs) help to demonstrate the characteristics of the flash that erupts as 100,000 amps of current are rammed through the crossed wires of an "X-pinch" machine. When a current courses through X-pinch wires, the metal vaporizes and leaves trails of plasma behind. In the absence of solid wires, the current continues to flow through the plasma, leading to a magnetic field that in turn pinches down on the plasma. With increasing current, the magnetic field grows and ultimately causes the plasma to implode, typically resulting in one or two dense plasma points less than a thousandth of an inch across with temperatures as high as 10 million K. The unstable plasma points emit bursts of x-rays that last less than a billionth of a second, and then the plasma points explode. Bright, point-source x-ray bursts generated by the X-pinch machine are ideal illumination for x-ray radiographs of thin objects. Details on the order of a few millionths of a meter, such as the hairs on a fly's wing, would be impossible to discern with larger x-ray sources, but are clearly visible in images created with X-pinch flashes (see figures). Sergei Pikuz of Cornell will discuss X-pinch photography at the annual meeting of the American Physical Society's Division of Plasma Physics (Long Beach, California, Oct 29-Nov 2), while papers by T. A. Shelkovenko and D. B. Sinars will address detailed studies of the X-pinch plasma itself. (For additional information, please visit the conference's Virtual Pressroom; see a Cornell press release; or contact David Hammer, dah5@cornell.edu, 607-255-3916)
All-Optical Electron Injector Conventional electron acceleration at a place like SLAC needs miles to boost particles up to 50 GeV energies by feeding them microwaves in a succession of special cavities. In recent years physicists have been developing alternative acceleration concepts that might someday do the job in a much smaller space. Their near-term goal is to produce a first stage accelerator that outputs electron beams with lower energy but with properties that are more suitable for x-ray sources, such as those based on Compton scattering or the proposed linear synchrotrons at SLAC and DESY. In the plasma wakefield approach, for example, a terawatt laser beam bites into a plasma-filled cell, setting up fast waves in the plasma. If timed just right, electrons in the plasma can surf the plasma waves to high speeds, as high as 100 MeV in the space of only a millimeter. One problem with this concept is the mismatch between the electron source (sometimes an external photocathode, sometimes a haphazard and uncontrolled cloud of electrons from the plasma itself) and the incoming laser pulse. At the APS plasma meeting, Donald Umstadter of the University of Michigan (Paper BO1.1) has reported a new means of generating electrons in a controllable way, namely the use of a pair of crossed laser beams which position, heat, and synchronize the insertion of electrons into the plasma wave. This dramatically increased the number of energetic electrons as compared with use of only one of their laser beams. Besides potential applications to particle physics and x-ray lasers, high gradient acceleration schemes are also expected to benefit the production of medical radioisotopes and the ignition of thermonuclear fusion reactions. |
