Explored is the role of thought experiments in the development of physical theories. Drawing examples from quantum theory it asks how a method that generates no new empirical data can nonetheless exert an influence upon a subject defined by its empiricism. (Author/KR)

Many properties of new particle field theories can only be tested by comparing their predictions about the physical conditions immediately after the big bang with what can be reconstructed about this event from astronomical data. Facts/questions about big bang, unified field theories, and universe epochs/mass are among the topics discussed. (JN)