|MadSci Network: Physics|
Absolute zero has never been obtained and may not even be possible. Absolute zero is defined as the temperature at which all thermally induced motion stops. As objects (matter) get hotter (acquire more energy) they move faster. All matter that is not at absolute zero is in motion. All matter that has a temperature greater than absolute zero also radiates energy. At ambient or room temperature, the radiation is mostly in the infrared part of the electromagnetic spectrum. The heat we feel when we get close to a hot object comes from our bodies absorbing the infrared energy being radiated by the hotter object. Infrared radiation transfers energy from hotter to colder objects. In terms of heat transfer, the inside of a hollow closed sphere is called a blackbody. Any part of the inside surface of the sphere is radiating energy to all the other parts of the surface. Any hot part of the surface will radiate energy to the colder parts of the sphere. The temperature inside the sphere will quickly become the same and the transfer of energy from one location to another will stop. In this condition, when the inside surfaces are at the same temperature, there will be no net gain or loss of energy inside the sphere. This is called thermal equilibrium. The same thing happens if you place an object inside of the blackbody. If the object is hotter than the sphere, it will radiate energy into the sphere until both the sphere and the object are at the same temperature. The opposite will occur if the sphere is hotter than the object. Energy will transfer from the hotter object to the colder object until they are in thermal equilibrium. A closed capsule and objects placed inside the capsule will not spontaneously cool to absolute zero. So how do you cool something to temperatures approaching absolute zero? A gas cools when it expands into a larger volume. This is called adiabatic expansion. Adiabatic expansion is used in most refrigerators and air conditioners to provide cooling. Using adiabatic expansion to remove energy from the gas and lower its temperature can liquefy gasses. Liquid helium at –269.9 degrees K provides the lowest temperature that can be achieved this way. To get even lower temperatures it is necessary to remove more energy than is possible using only adiabatic expansion. It is also necessary to keep the object being cooled from coming into contact with the container. To accomplish this, gasses of elements such as sodium or rubidium are levitated using magnetic fields to form magnetic bottles. Very carefully tuned laser light then collides with the gas atoms in a way that reduces the speed of the gas atom. Light can act like a wave or a particle. Particles of light are called photons. Photons that collide with the atoms of the gas can carry away tiny amounts of energy. The more collisions between the gas and the light, the greater the amount of energy removed from the gas atoms. Less energy equals lower temperature. By manipulating the laser beams and the magnetic fields the rubidium atoms in a small chamber at the National Institute of Standards and Technology (NIST) were cooled to less than 100 nanokelvin. That is 0.00000001 degrees K above absolute zero. Somewhere near this temperature the rubidium atoms that make up the gas stop bouncing off of each other and acting independently and start to all move the same way. The gas becomes something called a Bose-Einstein condensate when all of the atoms begin to move uniformly. The atoms in a Bose-Einstein condensate drop into the lowest possible energy state and the motion is reduced even further. The temperature of the Bose-Einstein condensate drops to 0.5 nanokelvin or less!!! This is the lowest temperature ever observed. It is still not absolute zero, but there are a lot of zeroes after that decimal point! The information on the work at NIST was extracted from a paper by Dr. Eric Cornell which was published in the Journal of Research of the National Institute of Standards and Technology, Volume 101, Number 4, July-August 1996. Dr. Cornell attempted to present the information in a manner that would be understood by general audiences. I attempted to simplify it even more. I hope my interpretation is helpful and correct. The text of the original article can be found at: http://physics.nist.gov/Pubs/Bec/j4cornel.pdf. You will need Acrobat Reader to view it. Bob Novak Sr. Process Engineer Carpenter Technology Corporation
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