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The motion of particles at different points on the Kelvin scale.<\/p>\n
Image credit: Watthana Tirahimonchan\/Shutterstock.com<\/p>\n
An infinite road<\/p>\n
As with the first two laws of thermodynamics, the third law can be formulated in many ways. The original may be the easiest to understand. Walther Nernst presented his law by saying, \u201cIt is impossible for any procedure to lead to the isotherm T=0 in a finite number of steps.\u201d In other words, to reach absolute zero, energy must be removed from a system an infinite number of times. That, of course, cannot be done.<\/p>\n
Nernst reached this conclusion through experiments, where he successively cooled substances, but always had some heat left to remove. No cooling process could take all the heat out of a sample in a single go, and the heat left behind kept the temperature above absolute zero. Statistical mechanics subsequently proved that this conclusion can be derived from the other laws of thermodynamics.<\/p>\n
More recently, scientists showed it is also impossible to reach absolute zero in a finite amount of time<\/a>, so it would take an infinitely old universe for absolute zero to exist.<\/p>\n How do we get near absolute zero?<\/p>\n If you want to get temperatures in your freezer\/icebox to 0 \u00b0C, you move the heat to the outside world using a refrigeration cycle. Gas is compressed, heating it up. Much of this heat is drawn off to the surroundings. The gas is then allowed to expand, cooling it, in isolation so that it can\u2019t heat up again. The product can then be passed near whatever needs cooling, drawing the heat out.<\/p>\n Repeated often enough, the process will cool helium to -269 \u00b0C, or just 4 degrees above absolute zero. A substance immersed in enough liquid helium will release its heat until it reaches the same temperature.<\/p>\n This approach allows us to achieve temperatures not much higher than the cosmic background radiation, which at 2.7 K, left over from the Big Bang, is the temperature of the universe where nothing warms it. We can go a degree lower using helium–<\/strong>3<\/a>, the rarer isotope with just one neutron.<\/p>\n If we want to get colder still, things get harder. Processes like nuclear demagnetization, where magnetic fields are used instead of changes in pressure, alternately aligning the magnetic poles of atomic nuclei and releasing them, are sometimes used.<\/p>\n Laser cooling allows us to bring microscopic amounts of material to temperatures less than a billionth of a degree above absolute zero. The process, which won the 1997 Nobel Prize for Physics<\/a>, has lasers focused in three dimensions on a cloud of atoms to resist their motion in any direction. The lasers act like a frictional force, slowing the atoms down and therefore making them lose energy.\u00a0<\/p>\n Effective as this is, it doesn\u2019t get around the third law; there is still some heat left behind when the lasers have done their thing. A more exotic approach, known as a matter-wave lens, has cooled rubidium atoms to temperatures 10 times lower still<\/a>. This is great if you want to study Bose-Einstein condensates<\/a> in brief moments of microgravity, but still won\u2019t get you to absolute zero, unless you pretend by rounding off at ten decimal places.<\/p>\n While you’re here: Don\u2019t be fooled by \u201cnegative temperatures\u201d<\/p>\n