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'''Absolute zero''' is the practical minimum of [[Temperature|temperature]], defined as 0 [[Kelvin]] on the [[absolute temperature scale]]s (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]]) would cease.

'''Absolute zero''' is the minimum [[temperature]], defined as 0 [[Kelvin]] on the [[absolute temperature scale]]s (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 motion (i.e. the movement of [[molecules]] in a [[gas]], [[atom]]s in a [[crystal]]) would cease.

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 Background Explorer]], (CBE), measured this value. The [[Big Bang]] theory predicted a leftover background radiation and that such radiation would have a relative uniformity, which is reinforced by the evidence presented by the CBE's observations.

No place in our universe is at absolute zero, but there are places that get close. [[Outer space|Space]] is approximately 2.73 kelvin. This is not because heat from [[star]]s in the universe keep the temperature up, but because of cosmic background radiation. The [[COBE|Cosmic Background Explorer]] (CBE) measured this value. The [[Big Bang theory]] predicts a leftover background radiation and that such radiation would have a relative uniformity, which is reinforced by the evidence presented by the CBE's observations.  However, these observations cannot account for the [[horizon problem]].

== Reaching Towards Absolute Zero ==

== Reaching Towards Absolute Zero ==

It is considered theoretically impossible to achieve absolute Zero, but scientists have come quite close.  The first example of this occured in [[1908]] when [[Kammerlingh Onnes]] liquefied [[Helium]].  The temperature he reached was approximately four Kelvin.  Lower temperatures have been reached with different [[isotope|isotopes]] of Helium.  A temperature of 0.3 Kelvin has been reached with ³He.  The lowest temperature ever reached by any substance was 280 picoKelvin.

It is considered theoretically impossible to achieve absolute zero, but scientists have come quite close.  The first example of this occurred in 1908 when Kammerlingh Onnes liquefied [[Helium]].  The temperature he reached was approximately four kelvin.  Lower temperatures have been reached with different [[isotope]]s of Helium.  A temperature of 0.3 kelvin has been reached with ³He.  The lowest temperature ever reached by any substance was 280 picokelvins.

== Sources ==

== Sources ==

[http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html]

[http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html http://www.ph.rhul.ac.uk/schools/ZeroT/Absolute.html]

[[category:physics]]

[[Category:Thermodynamics]]