Absolute zero, 0K (−273.15°C), is the lowest possible temperature. Achieving temperatures close to absolute zero is challenging, as energy must be transferred to warmer surroundings. In 2015 scientists at the Massachusetts Institute of Technology (MIT) in the USA reached a record low of just 500 nK (500 × 10−9K) by cooling sodium–potassium molecules (NaK, Figure 1), which can exist only at low temperatures.
Perhaps surprisingly, there is also an ‘absolute hot’. At 1.42 × 1032K, known as the Planck temperature, this is the highest possible temperature according to the standard model of particle physics. For anything to reach this temperature, the entire universe would need to be in thermal equilibrium (everything at the same temperature). It is thought that this occurred about 10 −42s after the Big Bang, but cannot happen again. In 2010, in an experiment at the Brookhaven National Laboratory, USA, collisions between gold nuclei reached 4 × 10 12K (Figure 2) and set a record for the highest temperature yet produced on Earth.
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