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Physics News Update
Number 843, October 19, 2007 by Phil Schewe

Relativistic Thermodynamics

Einstein’s special theory of relativity has formulas, called Lorentz transformations, that convert time or distance intervals from a resting frame of reference to a frame zooming by at nearly the speed of light. But how about temperature? That is, if a speeding observer, carrying her thermometer with her, tries to measure the temperature of a gas in a stationary bottle,what temperature will she measure? A new look at this contentious subject suggests that the temperature will be the same as that measured in the rest frame. In other words, moving bodies will not appear hotter or colder.

You’d think that such an issue would have been settled decades ago, but this is not the case. Einstein and Planck thought, at one time,that the speeding thermometer would measure a lower temperature,while others thought the temperature would be higher. One problem is how to define or measure a gas temperature in the first place.

James Clerk Maxwell in 1866 enunciated his famous formula predicting that the distribution of gas particle velocities would look like a Gaussian-shaped curve. But how would this curve appear to be for someone flying past? What would the equivalent average gas temperature be to this other observer? Jorn Dunkel and his colleagues at the Universitat Augsburg (Germany) and the Universidad de Sevilla (Spain) could not exactly make direct measurements (no one has figured out how to maintain a contained gas at relativistic speeds in a terrestrial lab), but they performed extensive simulations of the matter.

Dunkel (joern.dunkel@physik.uni-augsburg.de ) says that some astrophysical systems might eventually offer a chance to experimentally judge the issue. In general the effort to marry thermodynamics with special relativity is still at an early stage. It is not exactly known how several thermodynamic parameters change at high speeds. Absolute zero, Dunkel says, will always be absolute zero, even for quickly-moving observers. But producing proper Lorentz transformations for other quantities such as entropy will be trickier to do. (Cubero et al., Physical Review Letters, 26 October 2007)

Nuclear Syrup

A new measurement of how long it takes certain nuclei to fission into large fragments suggests that the “liquid-drop” model of the nucleus should be replaced with a “nuclear syrup”model. Fission is the most dramatic form of radioactivity, when a nucleus loses not merely a small fragment-such as an electron, gamma ray, or an alpha particle-but actually splits in half. The fission of many nuclei has been studied through the years, most famously uranium-235.

As early as 1939 Niels Bohr and John Wheeler tried to model the nature of fission by saying that the nucleus is like a drop of water in which the tendency of the drop to fly apart is checked by the force of surface tension; something like this, they said, kept a nucleus intact until such time as the rapid oscillations of an unstable nucleus became so large that the “surface tension” normally keeping the nucleus together was overcome.

Sometimes as a prelude to fission, the nucleus relieves some of its instability and effectively reduces its internal “nuclear temperature” by flinging out neutrons or gamma rays. In fact, the lifetime for fission has been indirectly measured by observing those cast-off neutrons. The results suggest that the old liquid-drop model was off by a factor of ten or so in predicting lifetimes. Some scientists have begun to think that an additional stickiness in the nuclear substance is at work, which slows up the fission process.

An experiment at Oak Ridge National Laboratory has probed this proposition by creating several fissionable nuclei artificially with heavy-ion beams bombarding a tungsten target; the projectile and target nuclei temporarily fuse together, travel a short distance through the tungsten crystal, and then fission. The spacing of the atoms in the crystal is used as a reference to measure the recoil of the composite nucleus before fission.

According to team member Jens Andersen of the University of Aarhus in Denmark (jua@phys.au.dk, 45-8942-3713), the Oak Ridge experiment suggests that the fission lifetimes are even longer (an additional factor of ten to one hundred) than those derived with the more indirect neutron-emission method. This could imply that the nuclear shape does not oscillate as rapidly as a water droplet would but instead deforms very slowly like a drop of syrup. (Andersen et al., Physical Review Letters, 19 October 2007)

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