Absolute Zero

Absolute Zero

Heat pumps use energy to take away heat from one object, which gets cooler, and deposit it in another object, which gets warmer. But it is not possible to keep doing this forever, for there is a limit to how much heat can be taken away from an object.

The coldest possible temperature, absolute zero, represents this limit. This temperature, which is designated 0 K in the absolute scale, -459.69°F in the Fahrenheit scale, and –273.15°C in the Celsius scale, is the lowest possible temperature for any object. As mentioned earlier, this is the temperature in which the motion of the atoms and molecules is at a minimum.

Another way of looking at absolute zero is that it is the point at which no further heat energy can be extracted from the body. No matter how hard the pump tries, no more heat will flow.

Absolute zero is the subject of the third law of thermodynam- ics. This law, like the other laws of thermodynamics, is restrictive, because the third law describes something people will never be able to do. The third law of thermodynamics says that not only is

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The Spitzer Space Telescope, launched on August 25, 2003, is seen here in preparation. The orbiting telescope detects and images infrared radiation, and some of its instruments must be cooled to within a few degrees of absolute zero to avoid interfering signals from their own radiation. Liquid helium is the coolant. (NASA)

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absolute zero the coldest possible temperature, but no one can ever get an object to even reach this temperature. Nature will simply not allow this to happen. Physicists can cool an object to a tem- perature that is extremely close to absolute zero, and there is no limit to how close they can come. But absolute zero can never be attained.

Cooling objects to within a few billionths of a degree above absolute zero is possible, but is difficult to do. Heat flows from high to low temperatures, and anything that is close to absolute zero is a lot colder than everything else in the environment. Scien- tists who study very cold temperatures must do their research in special laboratories. Even with complex equipment, it is hard to stop heat from seeping into a frigid object and raising its tempera- ture. Conduction and convection are heat transfer mechanisms that can be controlled to a great extent, but radiation is not easy to stop. Materials cooled to temperatures close to absolute zero are usually tiny in size and must be placed in special containers.

Despite the problems, there is a lot of incentive for physicists to study objects with temperatures near absolute zero. The prop- erties of matter only a few degrees above absolute zero can be remarkably strange and enormously useful. Dutch physicist Heike Kamerlingh Onnes (1853–1926) discovered superconductivity— the loss of all resistance to the flow of electrical current—in a 1911 experiment in which he cooled mercury to about four degrees above absolute zero. Physicists have since discovered materials that become superconductors at higher temperatures, although still not quite warm enough for these superconductors to be practical for everyday usage.

Another cold and unusual object is space itself. It might be surprising that empty space has a temperature. Space is mostly a vacuum; there are extremely few atoms and molecules, and there- fore little atomic and molecular motion. There is, however, a lot of radiation, and space has a well-defined temperature. That temper- ature is not absolute zero but is close, about –458.8°F (–270.45°C or 2.7 K).

Why does space have this particular temperature? Physicists believe that this energy—the radiation that fills space—is leftover

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from the cataclysmic “big bang” in which the universe was born,

14 billion years ago. The energy is like the warmth in a chimney after the fire has gone out; this temperature, though only a few degrees above absolute zero, is the remainder of the explosion that created the universe so long ago.

An object placed in space far away from any star or planet would eventually come to a thermal equilibrium with space, attain- ing a temperature of –458.8°F (–270.45°C or 2.7 K). But cooling an object on Earth to this temperature requires a great deal of effort, moving heat against its natural flow and preventing it from returning. Refrigerators and air conditioners can do the job, but as the temperature of the object drops, the cooling equipment must work against an increasing temperature difference—this means that the “hill” gets steeper. Powerful equipment and special mate- rial that can handle such cold temperatures must be used.

Moving heat from cold objects to warm ones takes a great deal of ingenuity. The laws of thermodynamics are not much help, since they enforce strict constraints on what is possible. But knowledge of these laws prevents society from wasting time and money on equipment that has no hope of ever working. People employ tech- nology to maintain a comfortable temperature, even in extreme environments. This technology works well, but the physics of ther- modynamics indicates there will always be a cost involved.