頭痛と共にLeggett本

Leggett, Quantum Liquids, Oxford Univ. Press をぺらぺらめくってるのだが，真面目に序文に戻ってみたらトニーの英語が難し過ぎてびびった...

It will become clear to the reader from an early stage that I have at least two rather strong convictions about the theory of quantum condensation which are not necessarily shared by a mojority of the relevant theoretical community. The first is that it is neither necessary nor desirable to introduce the idea of "spontaneously broken U(1) symmetry," that is to consider (alleged) quantum superpositions of statess containing different total numbers of particles; rather, I take from the start the viewpoint first enunciated explicitly by C. N. Yang, namely that one should simply think, in non-technical terms, about the behavior of single particles, or pairs of particles, averaged over the behavior of all the others, or more technically about the one- or two-particle density matrix. At the risk of possibly seeming a bit obsessive about this, I have tried to derive all the standard results not only for Bose but for Fermi systems using this picture; the idea of spontaneous U(1) symmetry breaking is mentioned only to make contact with the bulk of the literature.1 My second strong conviction is that many existing texts on superconductivity and/or superfluidity do not adequately emphasize the distinction, which to my mind is absolutely crucial, between the equilibrium phenomenon which in the context of a neutral superfluid is known as nonclassical rotational inertia (or the Hess-Fairback effect) and in a charged system underlies the Meissner effect, and the metastable phenomenon of persistent currents in a neutral or charged system; again, I have tried to place some emphasis on this, see in particular Chapter 1, Section 1.5.
1 And in Appendix 5C as a purely formal device to streamline an otherwise cumbrous algebraic step.

本文はそうでもないが．