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S16i. What is a system (e.g., an ideal gas)?
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Theories of physics do not say what a system (such as an
electron, a star, an ideal gas, a crystal) is in reality.
Nevertheless, it is possible to check the reality contents
of a physical theory. How does this come about?

Let us consider thermodynamics. Thermodynamics does not say which 
system is an ideal gas, which is only a van-der-Waals gas,
which is a liquid, or a solid. 

Indeed, such questions need not be answered by the theory.
Instead, they are answered by checking how a system behaves:


If a real system behaves as the theory for an ideal gas (a solid,
a crystal, an electron) requires, a physisict will say it 'is' 
an ideal gas (a solid, a crystal, an electron); if not, it is not.

While this definition may seem circular, it isn't once it is recognized
that one can check some characterizing properties of systems that have 
a particular label (such as 'ideal gas') by a small amount 
of measurements, and then deduce many more properties from the theory 
that can be checked subsequently.

Engineers call this process 'system identification'.

Thus the task of theory is to provide models with just enough 
flexibility that they cover the range of relevant possibilities,
while being still restricted enough so that one can identify 
the system with a limited amount of data. Exactly in this case
a theory has predictive value.