Neutron Stars and String Theory in a Lab: Chilled Atoms Give Clues to Deep Space and Particle Physics

Neutron Stars and String Theory in a Lab: Chilled Atoms Give Clues to Deep Space and Particle Physics

  8:22:00 PM    Science Lover

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Using lasers to contain some ultra-chilled atoms, a team of scientists has measured the viscosity or stickiness of a gas often considered to be the sixth state of matter. The measurements verify that this gas can be used as a "scale model" of exotic matter, such as super-high temperature superconductors, the nuclear matter of neutron stars, and even the state of matter created microseconds after the Big Bang.

Artist's rendering of a neutron star. (Credit: NASA/Dana Berry)

The results may also allow experimental tests of string theory in the future.

Duke physicist John Thomas made the viscosity measurements using an ultra-cold Fermi gas of lithium-6 atoms trapped in a millimeter-sized bowl made of laser light. When cooled and placed inside a magnetic field of the correct size, the atoms interact as strongly as the laws of quantum mechanics allow. This strongly interacting gas exhibits "remarkable properties," such as nearly frictionless fluid flow, Thomas said.

The team's report appears in the Dec. 10 issue of Science.

Under the ultra-cold conditions, the properties of the gas are determined by a universal ruler, or natural length scale, much like the scale on an architect's drawing. The ruler for the atomic gas is the average spacing between the atoms. According to quantum physics, this spacing determines all other natural scales, such as the scale for energy, temperature and viscosity, making the ultra-cold gas a scale model for other exotic matter. Thomas said that he and others have verified the gas as a universal scale model for properties such as temperature, but this is the first time they've tested the scaling of viscosity, which happens to be of particular interest to scientists right now.

Thomas first measured the viscosity of the gas at a few billionths of a degree Kelvin, or -459 degrees Fahrenheit. Turning off the trap that confines the gas, and then recapturing it caused the radius of the Fermi gas to vibrate. The oscillation, called a breathing mode, resembles the jiggling of a piece of jelly. The longer the vibrations lasted, the lower the viscosity. At slightly higher temperatures, millionths of a degree Kelvin, the researchers instead observed how fast the gas changed from a cigar shape to a pancake after being released from the trap. A slower change in shape had a higher viscosity.

These results are "extremely important to the field of condensed matter physics and to high temperature superconductivity in particular," said Kathy Levin, a theorist at the University of Chicago, who was not involved in the research. She said that the viscosity of the Fermi gas is similar to the conductivity of a superfluid, which flows with no resistance. This "perfect fluidity" is also observed in the condensed matter world, especially in materials used to make high temperature superconductors. The new data, especially at lower temperatures, "seem quite consistent" with predictions of how superconductors should flow, Levin said.

The Fermi gas as a scale model is also important for studying elements of the cosmos that scientists can't probe in a lab, said Duke physicist Berndt Mueller. Even a very small chunk of a neutron star, a dead star that hasn't become a black hole, would weigh billions of tons on Earth and be much too dense to study. The data showing the universal properties of the Fermi gas, however, let physicists calculate the scale from lithium-6 atomic spacing to the spacing between neutrons in these stars. The measurements made on the Fermi gas can then be used to determine the natural energy and other properties for these stars, which can be compared to theorists' predictions. Similar calculations can be made for the quark-gluon plasma, the state of matter created just microseconds after the Big Bang and being studied in particle accelerators such as the Large Hadron Collider in Geneva.

Thomas said the new results also give experimental insight into predictions made using string theory, the mathematical construct uniting the classical world of gravity with quantum physics. String theorists have provided a lower bound for the ratio of the viscosity or fluid flow to the entropy, or disorder, in a strongly-interacting system. The new experiments measured both properties in the Fermi gas and showed that the gas minimum is between four and five times the string theorists' lower bound.

"The measurements do not test string theory directly," Thomas said, noting a few caveats-- the lower bound is derived for high-energy systems, where Einstein's theory of relativity is essential, while the Fermi gas experiments study low-energy gases. If string theorists create new calculations specifically for a Fermi gas, scientists would be able to make precise experimental tests of the theory with equipment no larger than a desktop.

Disclaimer: Views expressed in this article do not necessarily reflect to us.

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Exotic Antimatter: Heaviest Antinucleus Made

Exotic Antimatter: Heaviest Antinucleus Made

  10:28:00 PM    Science Lover

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An international team of scientists studying high-energy collisions of gold ions at the Relativistic Heavy Ion Collider (RHIC), a 2.4-mile-circumference particle accelerator located at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory, has published evidence of the most massive antinucleus discovered to date.

The diagram above is known as the 3-D chart of the nuclides. The familiar Periodic Table arranges the elements according to their atomic number, Z, which determines the chemical properties of each element. Physicists are also concerned with the N axis, which gives the number of neutrons in the nucleus. The third axis represents strangeness, S, which is zero for all naturally occurring matter, but could be non-zero in the core of collapsed stars. Antinuclei lie at negative Z and N in the above chart, and the newly discovered antinucleus (magenta) now extends the 3-D chart into the new region of strange antimatter. (Credit: Image courtesy of DOE/Brookhaven National Laboratory)

The new antinucleus, discovered at RHIC's STAR detector, is a negatively charged state of antimatter containing an antiproton, an antineutron, and an anti-Lambda particle. It is also the first antinucleus containing an anti-strange quark. The results are published online by Science Express on March 4, 2010.

"This experimental discovery may have unprecedented consequences for our view of the world," commented theoretical physicist Horst Stoecker, Vice President of the Helmholtz Association of German National Laboratories. "This antimatter pushes open the door to new dimensions in the nuclear chart -- an idea that just a few years ago, would have been viewed as impossible."

The discovery may help elucidate models of neutron stars and opens up exploration of fundamental asymmetries in the early universe.

New nuclear terrain

All terrestrial nuclei are made of protons and neutrons (which in turn contain only up and down quarks). The standard Periodic Table of Elements is arranged according to the number of protons, which determine each element's chemical properties. Physicists use a more complex, three-dimensional chart to also convey information on the number of neutrons, which may change in different isotopes of the same element, and a quantum number known as "strangeness," which depends on the presence of strange quarks (see diagram). Nuclei containing one or more strange quarks are called hypernuclei.

For all ordinary matter, with no strange quarks, the strangeness value is zero and the chart is flat. Hypernuclei appear above the plane of the chart. The new discovery of strange antimatter with an antistrange quark (an antihypernucleus) marks the first entry below the plane.

This study of the new antihypernucleus also yields a valuable sample of normal hypernuclei, and has implications for our understanding of the structure of collapsed stars.

"The strangeness value could be non-zero in the core of collapsed stars," said Jinhui Chen, one of the lead authors, a postdoctoral researcher at Kent State University and currently a staff scientist at the Shanghai Institute of Applied Physics, "so the present measurements at RHIC will help us distinguish between models that describe these exotic states of matter."

The findings also pave the way towards exploring violations of fundamental symmetries between matter and antimatter that occurred in the early universe, making possible the very existence of our world.

Collisions at RHIC fleetingly produce conditions that existed a few microseconds after the Big Bang, which scientists believe gave birth to the universe as we know it some 13.7 billion years ago. In both nucleus-nucleus collisions at RHIC and in the Big Bang, quarks and antiquarks emerge with equal abundance. At RHIC, among the collision fragments that survive to the final state, matter and antimatter are still close to equally abundant, even in the case of the relatively complex antinucleus and its normal-matter partner featured in the present study. In contrast, antimatter appears to be largely absent from the present-day universe.

"Understanding precisely how and why there's a predominance of matter over antimatter remains a major unsolved problem of physics," said Brookhaven physicist Zhangbu Xu, another one of the lead authors. "A solution will require measurements of subtle deviations from perfect symmetry between matter and antimatter, and there are good prospects for future antimatter measurements at RHIC to address this key issue."

The STAR team has found that the rate at which their heaviest antinucleus is produced is consistent with expectations based on a statistical collection of antiquarks from the soup of quarks and antiquarks generated in RHIC collisions. Extrapolating from this result, the experimenters believe they should be able to discover even heavier antinuclei in upcoming collider running periods. Theoretical physicist Stoecker and his team have predicted that strange nuclei around double the mass of the newly discovered state should be particularly stable.

RHIC's STAR collaboration is now poised to resume antimatter studies with greatly enhanced capabilities. The scientists expect to increase their data by about a factor of 10 in the next few years.

The STAR collaboration is composed of 54 institutions from 13 countries. Research at RHIC is funded primarily by the U.S. Department of Energy's Office of Science and by various national and international collaborating institutions.

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