Changes

Absolute zero is the practical minimum of [[Temperature|temperature]], defined as 0 degrees [[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 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.

It is worthy of note that absolute zero has never actually been attained. The (possibly biased) scientific community claim that this is for "physical reasons," but many are prone to doubt the existence of this unsettling "as cold as you can get" temperature because of the lack of tangible proof.

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]].

 

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.

 

 

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