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Absolute zero is the practical minimum of [[Temperature|temperature]], defined as 0 [[Kelvin]] on the absolute temperature scale (the zero on the Kelvin scale has had to adjust, slightly, as measurements of absolute zero are refined). This corresponds to -273.15 degrees [[Celsius]] and -459.67 degrees [[Fahrenheit]]. At absolute zero, all thermal motion (i.e. the movement of [[Molecule|molecules]] in a [[States of matter#gaseous|gas]], [[Atom|atoms]] in a [[Crystal|crystal]]) ceases.
Absolute zero is the practical minimum of [[Temperature|temperature]], defined as 0 [[Kelvin]] on the absolute temperature scale (the zero on the Kelvin scale has had to adjust, slightly, as measurements of absolute zero are refined). This corresponds to -273.15 degrees [[Celsius]] and -459.67 degrees [[Fahrenheit]]. At absolute zero, all thermal motion (i.e. the movement of [[Molecule|molecules]] in a [[States of matter#gaseous|gas]], [[Atom|atoms]] in a [[Crystal|crystal]]) ceases.
== Examples of Absolute Zero ==
No place in our universe can reach absolute zero, but there are places that get close. [[Space]] is approximately 2.73 Kelvin. This is not because heat from [[star|stars]] in the universe keep the temperature up, but because of [[Black-Body Radiation]], which occupies the [[vacuum]] of space. This [[radiation]] keeps the universe at this temperature. The [[Cosmic Backround Explorer]] measured this value. It has since been theorized that this risidual heat energy is leftover from the [[Big Bang]], the theorized beginning of our universe, although this has not been proven.
== Reaching Absolute Zero ==
Scientists have not yet reached Absolute Zero, but have come quite close. The first example of this occured in [[1908]] when [[Kammerlingh Onnes]] liquified [[Helium]]. The temperature he reached was approximately four Kelivn. Lower temperatures have been reached with different [[isotope|isotopes]] of Helium. A temperature of 0.3 Kelvin has been reached with <sup>3</sup>He. The lowest temperature ever reached by any substance was 280 picoKelvin.
== Sources ==
[http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html]