Emergence in physics
Authors: Batterman Robert W. 
Published: September 2018

The concept of emergence is closely connected with the notions of antireductionism, unpredictability, and novelty. In many cases these latter concepts are explicated in mereological terms: very crudely, something is emergent when it (the whole) is greater than the sum of its parts. Alternatively, the behaviour of the emergent whole does not reduce to some function of the behaviour of its components. Or, the behaviour of the emergent whole is unpredictable given knowledge of the nature of its parts. Or finally, the behaviour of the emergent whole is completely different, new, and unexpected, given knowledge of the nature of its parts. In addition, there is often a demand that the emergent feature is not explainable by a theory of the nature of its parts.
Most philosophical discussions focus on the notion of emergence in the context of the mind/body problem, broadly construed: how can the mental, with all of its unique attributes, possibly obtain in a world in which the basic fundamental features are characterized by physical theory. This problem of emergence is intimately connected with the position called non-reductive physicalism.
On the other hand, there have been discussions of emergence (though not necessarily using that term) in the ‘modern’ physics literature at least since the time of the development of kinetic theory and statistical mechanics. Most, though not all, discussions of emergence in physics focus on the question of how the macro-world of our experience can arise out of the behaviour of the micro-constituents of objects of our everyday experience. Thus, most discussions of emergence in the physics literature as well can be seen as focusing on mereological part/whole questions. However, there are, arguably, non-mereological emergent features of the world, so that perhaps emergence should not be characterized in terms of part/whole relations.
There are a number of examples that may naturally be considered to be emergent phenomena in physics. Among these are the early attempts to relate the amazingly successful science of thermodynamics (a phenomenological theory that does not address the ultimate constitution of the systems it describes) to more basic theories such as kinetic theory and statistical mechanics. Another example concerns the nature of measurement in quantum mechanics, since the foundational measurement problem of quantum theory is related to the so-called emergence of the classical world. A third example concerns a debate that came to the boil in the 1970s about reductionism in physics. It addresses the nature of fundamental physics and pits high energy theorists against solid state (condensed matter) theorists. This debate suggests a new attitude toward theories in high energy physics, namely, that all such theories may well be ‘effective’, i.e. restricted in their domains of applicability, and thus that the search for some ultimate theory of everything may be misguided. Some of the mathematical aspects of emergence – in particular, a kind of reasoning often employed in arguing for emergent phenomena – are also philosophically important. It seems that many of our everyday experiences of the physical world are experiences of emergent phenomena.