What make system different from environment?

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When I was reading about QM, I always saw the word system and environment. What differentiate them?
 
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The word "system" is among the most used in pysics.QM couldn't make an exception...
The word "environment" relates to "everything which is outside a closed system"...I woudn't use it in connection with QM...

Daniel.
 
I think we just choose to call whatever tiny part of the universe we are studying "the system" and the other part of the universe interacting with or surrounding the part we are studying "the environment".

The only closed system is the universe itself because particle interactions are too common even in deep space to allow any system within the universe to be truly isolated and separate from the environment.

We can still treat a small system as if it is truly isolated from its environment to investigate quantum theory, though. Schrodinger's cat is an example of, theoretically, making the box a tiny "universe" for the cat. Experimentally, however, the cat always interacts with the environment surrounding the box and this destroys the weird effects for the cat a truly isolated "universe" would have.
 
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