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© Copyright 1997, Jim Loy
Absolute Zero is the coldest possible temperature. How do they know what this temperature is? Do they just keep trying to produce colder and colder temperatures, and keep coming against this barrier. Not exactly.
Gasses follow certain laws, which physicists have deduced. Double the pressure, and the volume of gas becomes half. Well, lower the temperature of a gas, and the volume decreases (or the pressure drops, if you keep the volume the same). This reduction of volume happens in a very predictable way. And you can deduce how cold you would have to make the gas, in order for the volume to be zero. This is the same temperature for all quantities of all gasses. It is -273°C (actually it is -273.15°C) or -493°F (approximately).
A gas cannot have a zero volume. So, absolute zero is an unattainable limit.
Incidentally, all gasses liquify, and most gasses solidify (helium remains a liquid) at very very low temperatures. I hear that there are theoretical reasons why helium must remain a liquid. And experiments have verified this.