Experimental studies of boson fields in solids This book provides a new understanding of the large amount of experimental results gained in solid state physics during the last seven decades. For more than 160 different materials data analyses shown in terms of atomistic models (Hamiltonians) have not provided a quantitatively satisfactory description of either excitation spectra or dynamic properties. Instead the experimental evidences have elaborated that field theories are necessary. However most experimentalists are not familiar with field theories and realistic field theories of magnetism are absent. The book illustrates in an empirical way the elements of future field theories of solid state physics with special emphasis on magnetic materials. In contrast to the many available textbooks on quantum field theories that emphasize more on algorithmic formalities rather than referring to the experimental facts the approach in this book is pragmatic instead of abstract theoretic. This methodical concept considerably facilitates experimentalists to get acquainted with the basic ideas of field theories even if a ready field theory is not provided by this experimental study History of conventional spin wave theory Basic issues of renormalization group (RG) theory Universality Microscopic processes Relevant and non relevant magnons Crossover phenomena Metastability of universality classes Relevant and non relevant interactions Temperature dependence of the magnon excitation spectra Magnetic heat capacity Experimental verification of GSW bosons Magnets with and without magnon gap Microscopic details: spin structure site disorder two order parameters The boson fields at the magnetic ordering transition Thermal lattice expansion and magnetostriction Boson fields near melting transition Boson fields in superconductors Summary