
6 Acknowledgements
I wish to thank the editors of Living
Reviews for the invitation to write this article, and for
their great patience in waiting for its delivery. I am also
grateful to John Donoghue and Ted Jacobson for their many useful
comments on an early draft of this review. My own ideas on this
topic are heavily indebted to Steven Weinberg, as conveyed through
his graduate lectures on quantum field theory (and now by his
textbooks on the subject).
