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Understanding what is absolute zero requires an initial understanding of the second law of thermodynamics. The basic definition is that absolute zero is the theoretical temperature at which entropy would reach its minimum value. Simple and straightforward until you find out that the laws of thermodynamics state that absolute zero cannot be reached because it would require a thermodynamic system to be fully removed from the rest of the universe. That is a little more interesting isn’t it. International agreement says that absolute zero is defined as 0 kelvins, ?273.15°C, or ?459.67°F.
No place in the universe is known to be at absolute zero and it can not be achieved by man, but(there is always a but). The average temperature of the universe due is 2.73 K. Cosmic microwave background radiation prevents 0 K. Absolute zero has never been achieved artificially; however, temperatures close to it have been achieved with cryocoolers. Laser cooling has been used to take temperatures to within a billionth of a degree of 0 K. Such low temperatures cause matter to exhibit many unusual properties like superconductivity, superfluidity, and Bose-Einstein condensation. Scientists have conducted many experiments to obtain lower and lower temperatures in order to further study these phenomena. The world record for man-made cold was set in 1999 at a temperature of 100 picokelvins. The coldest place in the universe is thought to be the Boomerang Nebula at 1 K.
Absolute temperature is conventionally measured in kelvins and occasionally Rankines. The other scales(Celsius and Fahrenheit) express temperatures in negative numbers because they can achieve truly negative temperatures; that is to say that their thermodynamic temperature can be of a negative quantity. A system with a negative temperature is not actually colder than absolute zero. In truth, a system with a negative temperature is hotter than the kelvin scale to begin with.
Science is striving to reach absolute zero temperature as quickly as possible. If it could be achieved, many avenues of study would be opened. The actions of Bose-Einstein condensate could be further studied or the reaction of bosons in an absolute zero environment might be able to provide interesting data.
We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.