Physics News Update no. 572
This physics news bulleting reports on the intervals of ice ages and the phases of atomic nuclei.
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The American Institute of Physics Bulletin of Physics News
Number 572 8 January 2002
RANDOM "NOISE" COULD HAVE TRIGGERED A CLIMATIC ROLLER COASTER during the last Ice Age, research suggests. Under certin conditions, random noise, such as electrical static, can paradoxically increase a weak signal's detectability, and in general amplify the signal's influence on its surroundings. This phenomenon, called "stochastic resonance" (SR), has been observed in settings as diverse as chaotic lasers and human reflex systems (Updates 121, 293, 509). Interestingly, researchers originally proposed the concept of SR in 1982, to explain how random climate events may have helped generate a regularly repeating interval of approximately 100,000 years between Ice Ages. However, subsequent evidence did not support this idea.
Now, SR is coming back home to climate: Researchers(Andrey Ganopolski and Stefan Rahmstorf, Potsdam Institute for Climate Impact Research in Germany, andrey@pik-potsdam.de) have shown that stochastic resonance may have played a role in triggering Dansgaard-Oeschger (D/O) events, abrupt and dramatic climate shifts during the last great Ice Age, which lasted from about 120,000 to 10,000 years ago. These events started with sudden warmings of at least 10 degrees Celsius over the north part of the Northern Atlantic, taking place over approximately a decade and lasting for centuries.
Curiously, the D/O events most often occurred 1,500 years apart, but sometimes they "missed a beat" and occurred after 3,000 or 4,500 years. This suggests they were caused, at least in part, by a weak underlying cycle, such as a periodic, but slight, fluctuation in the sun's intensity.
Furthermore, using a sophisticated computer model of the world's climate, the researchers found that North Atlantic ocean currents during the Ice Age could flip between two different states, one in which warm Gulf Stream waters reached only to mid-latitudes and another in which warm waters penetrated much farther north. As the researchers explain, these climate-altering circulation patterns might have switched from one state to another through the influence of a weak 1,500 year cycle, whose effects were amplified by environmental noise, such as random changes in the amount of precipitation and meltwater (melted ice and snow) entering the Nordic Seas.
While the exact source of the regular cycle remains unspecified, a SR-based explanation reproduces key features of the D/O events and North Atlantic ocean circulation during the last Ice Age. If confirmed, this mechanism may help to explain why the Ice Age climate was so much less stable compared to that of the past 10,000 years, in which human civilization was able to thrive. (Ganopolski and Rahmstorf, Physical Review Letters, 21 January 2002; text available at www.aip.org/physnews/select)
NUCLEI GO THROUGH PHASES. For the first time, scientists have staked out, in the form of a diagram, how nuclear matter goes from the liquid phase to the gas phase. Liquid-gas phase diagrams are a staple of chemistry, where they anatomize the energy frontier between, say, liquid water and water vapor. Altering the pressure or the temperature can send one back and forth across the two forms of existence. Do the protons and neutrons sheltering together inside a nucleus act like molecules in an ordinary gas or liquid? Theorists have thought as much, but it's been hard to prove owing to the extreme finiteness of a nucleus (with perhaps 100-200 constituent protons and neutrons) compared to a macroscopic liquid (with 10^24 or more molecules). In an experiment at Brookhaven 8 GeV pions are slammed into gold nuclei.
What happens next can be compared to the evaporation or boiling processes in chemistry. First, some nucleons are ejected, leaving behind an agitated nucleus; it now casts off more fragments of various sizes and can be said to possess a virtual "vapor pressure."
By looking at collisions of various degrees of violence, and by counting the number and size of fragments thrown off, an equivalent nuclear "pressure" and "temperature" can be calculated for these events.
Such an experiment has been carried out at Brookhaven with the Indiana Silicon Sphere (ISiS) detector as the thermometer and pressure gauge. The ISiS scientists (Indiana/Laval/Los Alamos/Simon Fraser/Texas A&M/Maryland; contact Vic Viola, viola@indiana.edu, 812-855-6537) have collaborated with two different teams of scientists, one at LBNL (contact James Elliott, jbelliott@lbl.gov, 510-486-7962,) and one at Michigan State University (Wolfgang Bauer, bauer@pa.msu.edu, 517-353-8662) to survey, for the first time, an experimentally based Mason-Dixon line between nuclear liquid and vapor on a previously uncharted pressure-vs-temperature plot. Indeed this represents the first time an experimentally-derived phase diagram has ever been made for a system of particles that wasn't held together by the electromagnetic force. It is interesting to note that the vapor from an excited nucleus, if you take into account the sticky interactions among nucleons, behaves approximately like an ideal gas (loosely conforming to Boyle's law: PV=nRT).
While the absolute scales of the nuclear and atomic forces are quite different, the shape of these two types of interactions (repulsive at very short range, attractive at longer range) are qualitatively similar.
Just to appreciate the difference in scales being compared here, take the case of a group of krypton atoms and a krypton nucleus. For the atoms, the critical temperature (boiling point) is 209 K and the critical density about 0.01 moles per cubic cm. For the nucleus, the critical temperature would be about 7 MeV, or 8 x 10^10 K, and the critical density about .05 nucleons per cubic fermi, or 8 x 10^13 moles/cubic cm. Finally, the experiment is germane to astrophysics since the opposite of nuclear boiling namely nuclear condensation is what happens during a supernova when a neutron star forms. (Two papers in Physical Review Letters for: Elliott et al. (LBNL) in the next few weeks; and Berkenbusch et al. (MSU) in the 14 Jan 2002 issue; for the ISiS experimental results see Lefort et al., Physical Review C, 1 December 2001; texts at http://www.aip.org/physnews/select.)
Number 572 8 January 2002
RANDOM "NOISE" COULD HAVE TRIGGERED A CLIMATIC ROLLER COASTER during the last Ice Age, research suggests. Under certin conditions, random noise, such as electrical static, can paradoxically increase a weak signal's detectability, and in general amplify the signal's influence on its surroundings. This phenomenon, called "stochastic resonance" (SR), has been observed in settings as diverse as chaotic lasers and human reflex systems (Updates 121, 293, 509). Interestingly, researchers originally proposed the concept of SR in 1982, to explain how random climate events may have helped generate a regularly repeating interval of approximately 100,000 years between Ice Ages. However, subsequent evidence did not support this idea.
Now, SR is coming back home to climate: Researchers(Andrey Ganopolski and Stefan Rahmstorf, Potsdam Institute for Climate Impact Research in Germany, andrey@pik-potsdam.de) have shown that stochastic resonance may have played a role in triggering Dansgaard-Oeschger (D/O) events, abrupt and dramatic climate shifts during the last great Ice Age, which lasted from about 120,000 to 10,000 years ago. These events started with sudden warmings of at least 10 degrees Celsius over the north part of the Northern Atlantic, taking place over approximately a decade and lasting for centuries.
Curiously, the D/O events most often occurred 1,500 years apart, but sometimes they "missed a beat" and occurred after 3,000 or 4,500 years. This suggests they were caused, at least in part, by a weak underlying cycle, such as a periodic, but slight, fluctuation in the sun's intensity.
Furthermore, using a sophisticated computer model of the world's climate, the researchers found that North Atlantic ocean currents during the Ice Age could flip between two different states, one in which warm Gulf Stream waters reached only to mid-latitudes and another in which warm waters penetrated much farther north. As the researchers explain, these climate-altering circulation patterns might have switched from one state to another through the influence of a weak 1,500 year cycle, whose effects were amplified by environmental noise, such as random changes in the amount of precipitation and meltwater (melted ice and snow) entering the Nordic Seas.
While the exact source of the regular cycle remains unspecified, a SR-based explanation reproduces key features of the D/O events and North Atlantic ocean circulation during the last Ice Age. If confirmed, this mechanism may help to explain why the Ice Age climate was so much less stable compared to that of the past 10,000 years, in which human civilization was able to thrive. (Ganopolski and Rahmstorf, Physical Review Letters, 21 January 2002; text available at www.aip.org/physnews/select)
NUCLEI GO THROUGH PHASES. For the first time, scientists have staked out, in the form of a diagram, how nuclear matter goes from the liquid phase to the gas phase. Liquid-gas phase diagrams are a staple of chemistry, where they anatomize the energy frontier between, say, liquid water and water vapor. Altering the pressure or the temperature can send one back and forth across the two forms of existence. Do the protons and neutrons sheltering together inside a nucleus act like molecules in an ordinary gas or liquid? Theorists have thought as much, but it's been hard to prove owing to the extreme finiteness of a nucleus (with perhaps 100-200 constituent protons and neutrons) compared to a macroscopic liquid (with 10^24 or more molecules). In an experiment at Brookhaven 8 GeV pions are slammed into gold nuclei.
What happens next can be compared to the evaporation or boiling processes in chemistry. First, some nucleons are ejected, leaving behind an agitated nucleus; it now casts off more fragments of various sizes and can be said to possess a virtual "vapor pressure."
 (see sequence of figures at http://www.aip.org/mgr/png) |
Such an experiment has been carried out at Brookhaven with the Indiana Silicon Sphere (ISiS) detector as the thermometer and pressure gauge. The ISiS scientists (Indiana/Laval/Los Alamos/Simon Fraser/Texas A&M/Maryland; contact Vic Viola, viola@indiana.edu, 812-855-6537) have collaborated with two different teams of scientists, one at LBNL (contact James Elliott, jbelliott@lbl.gov, 510-486-7962,) and one at Michigan State University (Wolfgang Bauer, bauer@pa.msu.edu, 517-353-8662) to survey, for the first time, an experimentally based Mason-Dixon line between nuclear liquid and vapor on a previously uncharted pressure-vs-temperature plot. Indeed this represents the first time an experimentally-derived phase diagram has ever been made for a system of particles that wasn't held together by the electromagnetic force. It is interesting to note that the vapor from an excited nucleus, if you take into account the sticky interactions among nucleons, behaves approximately like an ideal gas (loosely conforming to Boyle's law: PV=nRT).
While the absolute scales of the nuclear and atomic forces are quite different, the shape of these two types of interactions (repulsive at very short range, attractive at longer range) are qualitatively similar.
Just to appreciate the difference in scales being compared here, take the case of a group of krypton atoms and a krypton nucleus. For the atoms, the critical temperature (boiling point) is 209 K and the critical density about 0.01 moles per cubic cm. For the nucleus, the critical temperature would be about 7 MeV, or 8 x 10^10 K, and the critical density about .05 nucleons per cubic fermi, or 8 x 10^13 moles/cubic cm. Finally, the experiment is germane to astrophysics since the opposite of nuclear boiling namely nuclear condensation is what happens during a supernova when a neutron star forms. (Two papers in Physical Review Letters for: Elliott et al. (LBNL) in the next few weeks; and Berkenbusch et al. (MSU) in the 14 Jan 2002 issue; for the ISiS experimental results see Lefort et al., Physical Review C, 1 December 2001; texts at http://www.aip.org/physnews/select.)
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