It is the beginning of the academic year and once again I am teaching introductory thermodynamics to engineering undergraduate students and my MOOC entitled ‘Energy: Thermodynamics in Everyday Life‘ is running in parallel. Last week after my lecture on thermodynamic systems, a student approached me to ask whether the universe is a closed and isolated system. It’s an interesting question and the answer is depends on the definition of universe. In thermodynamics, we usually define a boundary to delineate the system of interest as everything inside the boundary and everything else are the surroundings. The system and surroundings taken together are the universe (see my post ‘No beginning or end‘ on February 24th, 2016). If the universe is defined as the system then there are no surroundings; hence the system cannot exchange energy or matter with anything which is the definition of a closed and isolated system.
Physicists often refer to the observable universe, or define the universe as everything we can observe. We are aware that we cannot observe everything. Hence, it is reasonable to suppose that the observable universe exchanges energy and matter with the unobservable space beyond it, in which case the observable universe is an open system. We could also consider the concept that we are part of multiverse and our universe is only one of many, in which case it seems likely that is not isolated, i.e. it can exchange energy, and perhaps it is open, i.e. it can exchange both energy and matter with other parts of the multiverse.
This is not really thermodynamics in everyday life. However, the occurrence of closed systems in nature appears to interest a lot of people to judge from the visits to my previous posts on this topic. See ‘Closed Systems in Nature?‘ on December 12th, 2012; Is Earth a closed system? Does it matter? on December 10th, 2014; and ‘No Closed Systems in Nature‘ on August 12th, 2015. For more about system boundaries, see my post entitled ‘Drawing Boundaries‘ on December 19th, 2012.