Condensed concepts

 A challenge in understanding and discussing emergence is that it means different things to different people. This is not surprising given emergence involves diverse and complex phenomena, occurs in a wide range of contexts, and is of interest to people in a diverse range of fields. Recently, I have started to be more precise about how emergence might be defined. The subject interests philosophers, but it is hard for physicists to glean insights from (and critique) what philosophers say.  I find the following article from 2011 by Bei Lok Hu, helpful. Emergence: Key physical issues for deeper philosophical inquiries Hu is not a condensed matter physicist. He is more interested in emergence from its relevance to quantum decoherence and the possibility of space-time being emergent leading to a theory of quantum gravity.  Thus, the article is a good entry into literature that I am not familiar with. Hu gives a nice summary and critique of some of the main philosophical discussions and iss

When I read Fundamentals by Frank Wilczek I learnt something that I found beautiful and amazing about general relativity and quantum field theory. Any massless spin-2 field must couple to the stress–energy tensor in the same way that gravitational interactions do. This is an alternative means to derive Einstein's equation. Furthermore, if a massless spin-2 particle is discovered, it must be the graviton. There is a nice discussion of this on physics.forums According to Wikipedia, "For a comparison of the geometric derivation and the (non-geometric) spin-2 field derivation of general relativity, refer to box 18.1 (and also 17.2.5) of Misner, C. W.; Thorne, K. S.; Wheeler, J. A. (1973). Gravitation."  In the 1960s, Steven Weinberg published a series of seminal papers, summarised below by one commenter in the physics.forums discussion. The results are also derived in chapter 5 of Weinberg's Quantum Field Theory text (volume 1). S. Weinberg, “Photons and gravitons in S-