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quantumaniac: Physicists Explain the Collective Motion of Fermions Some people like company. Others prefer to be alone. The same holds true for the particles that constitute the matter around us: Some, called bosons, like to act in unison with others. Others, called fermions, have a mind of their own. Different as they are, both species can show “collective” behaviour — an effect similar to the wave at a baseball game, where all spectators carry out the same motion regardless of whether they like each other. Scientists generally believed that such collective behavior, while commonplace for bosons, only appeared in fermions moving in unison at very long wavelengths. Now, however, collective behavior has been discovered at short wavelengths in one Fermi system, helium-3. A team led by Professor Eckhard Krotscheck — a physicist who recently joined the University at Buffalo from the Johannes Kepler University in Linz, Austria — predicted the existence of the behaviour using theoretical tools. Independently, but practically at the same time, a French team observed the collective behaviour. “Knowing how nature ticks at a microscopic scale, we set out to develop a robust theory that was capable of dealing with a wide range of situations and systems,” Krotscheck said. “We demanded that our mathematical description is accurate for both fermions and bosons, in different dimensions, and for both coherent and incoherent excitations. Only after we were done, we looked at experiments.”

quantumaniac:

Physicists Explain the Collective Motion of Fermions

Some people like company. Others prefer to be alone. The same holds true for the particles that constitute the matter around us: Some, called bosons, like to act in unison with others. Others, called fermions, have a mind of their own.

Different as they are, both species can show “collective” behaviour — an effect similar to the wave at a baseball game, where all spectators carry out the same motion regardless of whether they like each other.

Scientists generally believed that such collective behavior, while commonplace for bosons, only appeared in fermions moving in unison at very long wavelengths. Now, however, collective behavior has been discovered at short wavelengths in one Fermi system, helium-3.

A team led by Professor Eckhard Krotscheck — a physicist who recently joined the University at Buffalo from the Johannes Kepler University in Linz, Austria — predicted the existence of the behaviour using theoretical tools. Independently, but practically at the same time, a French team observed the collective behaviour.

“Knowing how nature ticks at a microscopic scale, we set out to develop a robust theory that was capable of dealing with a wide range of situations and systems,” Krotscheck said. “We demanded that our mathematical description is accurate for both fermions and bosons, in different dimensions, and for both coherent and incoherent excitations. Only after we were done, we looked at experiments.”

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ikenbot: Experiment Investigates How to Fight Fire in Space Image: Color image of a fuel droplet burning in space during NASA’s Flame Extinguishment Experiment aboard the International Space Station. Credit: NASA/GRC New experiments on the International Space Station may reveal tips on fighting fires in space, and back here on Earth, too. In space, where there is no up or down, and the atmosphere and pressure within the closed confines of a spacecraft are tightly controlled, fire burns differently. The Flame Extinguishment Experiment, known as FLEX, aims to better understand how. For example, in the weightless environment of orbit, flames can burn at a lower temperature and require less oxygen to ignite than they do in normal gravity. Thus, fire extinguishers in space must use more-concentrated material. Another difference is that a flame around a droplet of fuel in space can be completely spherical, while that is very different to achieve on the ground. And this spherical symmetry makes it easier to observe droplets’ behavior and to craft the calculations that explain it, the study’s leader, University of California, San Diego, aerospace engineering professor Forman Williams, said in a statement.

ikenbot:

Experiment Investigates How to Fight Fire in Space

Image: Color image of a fuel droplet burning in space during NASA’s Flame Extinguishment Experiment aboard the International Space Station. Credit: NASA/GRC

New experiments on the International Space Station may reveal tips on fighting fires in space, and back here on Earth, too.

In space, where there is no up or down, and the atmosphere and pressure within the closed confines of a spacecraft are tightly controlled, fire burns differently. The Flame Extinguishment Experiment, known as FLEX, aims to better understand how.

For example, in the weightless environment of orbit, flames can burn at a lower temperature and require less oxygen to ignite than they do in normal gravity. Thus, fire extinguishers in space must use more-concentrated material.

Another difference is that a flame around a droplet of fuel in space can be completely spherical, while that is very different to achieve on the ground. And this spherical symmetry makes it easier to observe droplets’ behavior and to craft the calculations that explain it, the study’s leader, University of California, San Diego, aerospace engineering professor Forman Williams, said in a statement.

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The pitch drop experiment is a long-term experiment which measures the flow of a piece of pitch over many years. Pitch is the name for any of a number of highly viscous liquids which appear solid, most commonly bitumen. At room temperature, tar pitch flows at a very slow rate, taking several years to form a single drop. The most famous version of the experiment was started in 1927 by Professor Thomas Parnell of the University of Queensland in Brisbane, Australia, to demonstrate to students that some substances that appear to be solid are in fact very-high-viscosity fluids. Parnell poured a heated sample of pitch into a sealed funnel and allowed it to settle for three years. In 1930, the seal at the neck of the funnel was cut, allowing the pitch to start flowing. Large droplets form and fall over the period of about a decade. The eighth drop fell on 28 November 2000, allowing experimenters to calculate that the pitch has a viscosity approximately 230 billion times that of water. This is recorded in the Guinness Book of Records as the world’s longest continuously running laboratory experiment, and it is expected that there is enough pitch in the funnel to allow it to continue for at least another hundred years. To date, no one has ever actually witnessed a drop fall. The image above features The University of Queensland pitch drop experiment with its current custodian, Professor John Mainstone (taken in 1990, two years into the eighth drop). Credit: John Mainstone and The University of Queensland

The pitch drop experiment is a long-term experiment which measures the flow of a piece of pitch over many years. Pitch is the name for any of a number of highly viscous liquids which appear solid, most commonly bitumen. At room temperature, tar pitch flows at a very slow rate, taking several years to form a single drop.

The most famous version of the experiment was started in 1927 by Professor Thomas Parnell of the University of Queensland in Brisbane, Australia, to demonstrate to students that some substances that appear to be solid are in fact very-high-viscosity fluids. Parnell poured a heated sample of pitch into a sealed funnel and allowed it to settle for three years. In 1930, the seal at the neck of the funnel was cut, allowing the pitch to start flowing. Large droplets form and fall over the period of about a decade. The eighth drop fell on 28 November 2000, allowing experimenters to calculate that the pitch has a viscosity approximately 230 billion times that of water.

This is recorded in the Guinness Book of Records as the world’s longest continuously running laboratory experiment, and it is expected that there is enough pitch in the funnel to allow it to continue for at least another hundred years. To date, no one has ever actually witnessed a drop fall.

The image above features The University of Queensland pitch drop experiment with its current custodian, Professor John Mainstone (taken in 1990, two years into the eighth drop).

Credit: John Mainstone and The University of Queensland

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