Thursday, 14 March 2019

Plasma as state of matter


A plasma is a hot ionized gas consisting of approximately equal numbers of positively charged ions and negatively charged electrons. The characteristics of plasmas are significantly different from those of ordinary neutral gases so that plasmas are considered a distinct "fourth state of matter." For example, because plasmas are made up of electrically charged particles, they are strongly influenced by electric and magnetic fields (see figure) while neutral gases are not. An example of such influence is the trapping of energetic charged particles along geomagnetic field lines to form the Van Allen radiation belts.



In addition to externally imposed fields, such as the Earth's magnetic field or the interplanetary magnetic field, the plasma is acted upon by electric and magnetic fields created within the plasma itself through localized charge concentrations and electric currents that result from the differential motion of the ions and electrons. The forces exerted by these fields on the charged particles that make up the plasma act over long distances and impart to the particles' behavior a coherent, collective quality that neutral gases do not display. (Despite the existence of localized charge concentrations and electric potentials, a plasma is electrically "quasi-neutral," because, in aggregate, there are approximately equal numbers of positively and negatively charged particles distributed so that their charges cancel.).
The plasma universe

It is estimated that 99% of the matter in the observable universe is in the plasma state...hence the expression "plasma universe." (The phrase "observable universe" is an important qualifier: roughly 90% of the mass of the universe is thought to be contained in "dark matter," the composition and state of which are unknown.) Stars, stellar and extragalactic jets, and the interstellar medium are examples of astrophysical plasmas (see figure). In our solar system, the Sun, the interplanetary medium, the magnetospheres and/or ionospheres of the Earth and other planets, as well as the ionospheres of comets and certain planetary moons all consist of plasmas.
The plasmas of interest to space physicists are extremely tenuous, with densities dramatically lower than those achieved in laboratory vacuums. The density of the best laboratory vacuum is about 10 billion particles per cubic centimeter. In comparison, the density of the densest magnetospheric plasma region, the inner plasmasphere, is only 1000 particles per cubic centimeter, while that of the plasma sheet is less than 1 particle per cubic centimeter.
The temperatures of space plasmas are very high, ranging from several thousand degrees Celsius in the plasmasphere to several million degrees in the ring current. While the temperatures of the "cooler" plasmas of the ionosphere and plasmasphere are typically given in degrees Kelvin, those of the "hotter" magnetospheric plasmas are more commonly expressed in terms of the average kinetic energies of their constitutent particles measured in "electron volts." An electron volt (eV) is the energy that an electron acquires as it is accelerated through a potential difference of one volt and is equivalent to 11,600 degrees Kelvin. Magnetospheric plasmas are often characterized as being "cold" or "hot." Although these labels are quite subjective, they are widely used in the space physics literature. As a rule of thumb, plasmas with temperatures less than about 100 eV are "cold," while those with temperatures ranging from 100 eV to 30 keV can be considered "hot." (Particles with higher energies--such as those that populate the radiation belt--are termed "energetic.")
A Bose-Einstein condensate is a group of atoms cooled to within a hair of absolute zero . When they reach that temperature the atoms are hardly moving relative to each other; they have almost no free energy to do so. At that point, the atoms begin to clump together, and enter the same energy states. They become identical, from a physical point of view, and the whole group starts behaving as though it were a single atom.
To make a Bose-Einstein condensate, you start with a cloud of diffuse gas. Many experiments start with atoms of rubidium. Then you cool it with lasers, using the beams to take energy away from the atoms. After that, to cool them further, scientists use evaporative cooling. "With a [Bose-Einstein condensate], you start from a disordered state, where kinetic energy is greater than potential energy," said Xuedong Hu, a professor of physics at the University at Buffalo. "You cool it down, but it doesn't form a lattice like a solid."
Instead, the atoms fall into the same quantum states, and can't be distinguished from one another. At that point the atoms start obeying what are called Bose-Einstein statistics, which are usually applied to particles you can't tell apart, such as photons.
Theory & discovery
Bose-Einstein condensates were first predicted theoretically by Satyendra Nath Bose (1894-1974), an Indian physicist who also discovered the subatomic particle named for him, the boson. Bose was working on statistical problems in quantum mechanics, and sent his ideas to Albert Einstein. Einstein thought them important enough to get them published. As importantly, Einstein saw that Bose's mathematics — later known as Bose-Einstein statistics — could be applied to atoms as well as light.
What the two found was that ordinarily, atoms have to have certain energies — in fact one of the fundamentals of quantum mechanics is that the energy of an atom or other subatomic particle can't be arbitrary. This is why electrons, for example, have discrete "orbitals" that they have to occupy, and why they give off photons of specific wavelengths when they drop from one orbital, or energy level, to another. But cool the atoms to within billionths of a degree of absolute zero and some atoms begin to fall into the same energy level, becoming indistinguishable.
That's why the atoms in a Bose-Einstein condensate behave like "super atoms." When one tries to measure where they are, instead of seeing discrete atoms one sees more of a fuzzy ball.
Other states of matter all follow the Pauli Exclusion Principle, named for physicist Wolfgang Pauli. Pauli (1900-1958) was an Austrian-born Swiss and American theoretical physicist and one of the pioneers of quantum says that fermions — the kinds of particles that make up matter — can't be in identical quantum states. This is why when two electrons are in the same orbital, their spins have to be opposite so they add up to zero. That in turn is one reason why chemistry works the way it does and one reason atoms can't occupy the same space at the same time. Bose-Einstein condensates break that rule. Though the theory said such states of matter should exist, it wasn't until 1995 that Eric A. Cornell and Carl E. Wieman, both of the Joint Institute for Lab Astrophysics (JILA) in Boulder, Colorado, and Wolfgang Ketterle, of the Massachusetts Institute of Technology, managed to make one, for which they got the 2001 Nobel Prize in Physics.
In July 2018, an experiment aboard the International Space Station cooled a cloud of rubidium atoms to ten-millionth of a degree above absolute zero.

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