# The Naked Scientists Forum

### Author Topic: How is absolute zero degrees determined?  (Read 1049 times)

#### annie123

• Sr. Member
• Posts: 247
##### How is absolute zero degrees determined?
« on: 03/04/2014 22:16:46 »
I was at a lecture where a physicist - very well known - was describing how an atomic clock is made, and in the process he mentioned measurements he had done with various things to establish their temperature relative to absolute zero, which apparently no one has reached, although come close to by the billionth of a degree. But how was the original temp. decided upon? how do you know what is absolute zero? And if it reached according to the present belief in value what happens if someone goes lower?

#### dlorde

• Neilep Level Member
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• ex human-biologist & software developer
##### Re: How is absolute zero degrees determined?
« Reply #1 on: 04/04/2014 00:36:15 »
It's not as simple as it might seem. This article may help explain - it also tells you about negative temperatures, below absolute zero (but warmer  ).

#### evan_au

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##### Re: How is absolute zero degrees determined?
« Reply #2 on: 04/04/2014 10:36:21 »
One of the first hints was obtained by graphing the relationship of temperature and volume of a gas, see Charles Law.

If you extrapolate this straight line to very low temperatures, you estimate that the volume of the gas will drop to zero at around -273C.

Now, most real gases turn into liquids long before they reach this temperature, and display other non-ideal behaviours (after all, their atoms have a finite volume, and can't disappear into nothing by just cooling them). But gases like Helium can come very close, allowing a good estimate of absolute zero.

#### The Naked Scientists Forum

##### Re: How is absolute zero degrees determined?
« Reply #2 on: 04/04/2014 10:36:21 »