What does Absolute zero mean

What a strange notion – that there is a temperature below which it does not make sense to talk about temperature.  Why can a body not have lower and lower temperature, and what sets the temperature we call ‘absolute zero’?  This is a story of the power of the scientific method to lead us to the truth about the universe. 

To start with, what in the world do we mean by temperature?  Does it identify anything fundamental about a physical system?  After all, the Centigrade system of temperature measurement was determined by two fixed points – where water froze (0°C) and where water boiled (100° C).  As the properties of water do not seem fundamental in our universe, it is clear that the way we describe the temperature of a material is more or less arbitrary. 

How do you tell if one material is colder than another?  One material is colder than another simply when energy can be transferred from the first material to a second material without irreversibly expending energy in the transfer. 

The absolute zero of temperature is the condition where all classical kinetic motion of the particles comprising matter ceases, so that they are at complete rest in the “classic” (non-quantum mechanical) sense.  At absolute zero, matter contains no thermal energy.  Matter does still possess a residuum of ‘zero point energy’, which creates movement of the particles, but this zero-point energy cannot be extracted from the matter.  It is a purely quantum mechanical effect, which does not appear at present to provide energy which can be extracted for applications.    

Why is there an absolute zero of temperature in the first place?  The definition of matter at absolute zero is matter that cannot share its energy with any external material.  But why does such a temperature exist? 

Absolute zero is actually the temperature of a vacuum in which no matter exists.  To define such a temperature requires rather sophisticated quantum mechanics, but these details don’t matter.  A vacuum with no material sources of energy will be colder than any potentially existing physical system.  The proof of this statement involves rather sophisticated quantum statistical mechanics, but nonetheless appears to be a limitation of the universe in which we find ourselves. 

Absolute zero turns out to be about -273.15 °C, but that specific number is an artifact of how we set up our method of comparing the temperature between two objects.  The real point here is that our universe is set up so that a physical system at the absolute zero of temperature cannot share its energy with any other physical system in the universe.  This need not have been true, and as a result represents a profound truth about our universe.