[neilep (OP)]

Absolute zero is the lowest temperature possible. At a temperature of absolute zero there is no motion and no heat. Absolute zero occurs at a temperature of 0 degrees Kelvin, or -273.15 degrees Celsius, or at -460 degrees Fahrenheit.

How do we know this if Absolute Zero Is unattainable ?

[chiralSPO]

Extrapolation!

If you put a gas in a sealed piston (which will keep the internal pressure constant by allowing the volume to change), and start cooling it down (and/or heating it up) you will notice that the volume varies with temperature. After collection of much data it will become apparent that the volume is proportional to temperature, but only if you use a really odd temperature scale, that has a zero point at –273.15 °C.

There are other ways to get there, but I believe that historically, this is how it came about.

[alancalverd]

Absolute zero is the lowest temperature possible........
How do we know this if Absolute Zero Is unattainable ?

Make up your mind!

Anyway, the concept is derived from the Carnot efficiency equation for a perfect heat engine, η = 1 - T_cold/T_hot.
For any nonzero value of T_cold, η < 1, and we can measure the efficiency of real heat engines and show that they follow this law.

So we can hypothesise a zero value of temperature and an absolute scale such that any heat engine with a cold source at T =0  would have η = 1

The problem is one of "thermal relativity". If we have a hypothetical heat sink at T = 0 and a heat source at 0 + δT,  the efficiency with which we can extract energy from it decreases toward zero as δT → 0  so we need to expend an infinite amount of energy to cool it to T = 0.

The idea that there is no motion at T = 0 contradicts Heisenberg, as we would then be able to predict the momentum (zero) and position (right here) of a particle, with zero indeterminacy.

[evan_au]

Experiment shows that you have to work harder and harder to get to extremely low temperatures.
- The current record-holder is 100 picoKelvins, set in 1999
- A current experiment on the ISS takes advantage of microgravity, and (it is hoped) might reach 1 picoKelvin

There are some tricks you can play to get a hot gas to act as if it has a negative temperature on the Kelvin scale, but that is playing with the definitions, not a real temperature.

See: https://www.jpl.nasa.gov/news/news.php?feature=7677
https://en.wikipedia.org/wiki/Absolute_zero#Very_low_temperatures

[neilep (OP)]

Thank you all for your wonderful and informative answers. Can you hypothesise what would happen (if anything) to the surrounding locality if Absolute Zero was attained ?