What if Rome had never fallen?

Over the past few weeks, I've been reading a book by science fiction author Robert Silverberg called 'Roma Eterna' (Note: Spoilers below). I remember it coming out in the 1980s, and was intrigued by the concept ("the glory of the Roman Empire lives on," with a cover illustration of Roman centurians moving through a forest with some giant armored machine) but I never had a chance to read it. It quickly disappeared from store shelves, and I assumed it had been a dud -- good science fiction usually enters reprints.

But I spotted the original hardcover version a few weeks ago at the local library. One of the reason it disappeared from the sci-fi sections of bookstores is it's not science fiction -- it's "alternate reality" set entirely in the past, with the exception of the last chapter, which takes place at the dawn of the Roman world's rocket age.

I found it to be very interesting, especially since I had just read a history of the late Republic covering the reigns of Sulla to Augustus (Rubicon: The Last Years of the Roman Republic). In grad school, I was introduced to a qualitative research technique called counterfactual reasoning, which basically involves making "what if" scenarios for important historical events (what if JFK had not been assassinated?) in order to ascertain the importance of individual historical factors or answer other questions (would the U.S. have fought the Vietnam War?). This is the same stuff alternate reality fiction is made of, but in this book, Silverberg takes the scenario to its long-term extremes. In Roma Eterna, montheistic religious movements are either hobbled (Judaism) or never exist (Christianity and Islam) and the Roman Empire is able to survive far beyond the lifespans of both the Western Roman Empire and the Byzantine Empire. This leads to some odd twists of history. Spain, Portugal, Britain, and the Italian city states never rise as independent naval powers, or claim parts of the New World. Rather, Latin America is discovered by Viking explorers, and Roman legions attempt to conquer it, but are thwarted. The first circumnavigation of the globe is carried out centuries later by a brutal Roman emperor, who brings back with him gunpowder and the printing press from "Cipangu" and "Khitan" (China and Japan). The eastern and western halves of the Roman Empire are reunited in violent fashion, and the Republic is eventually restored.

Silverberg is a talented author, but I wish that the dynamics of the Roman/Chinese relationship were explored in more depth. There was limited trade between the Roman and Chinese empires, but never formal contacts between the two civilizations. I recall reading about Roman entertainers, or people claiming to be Roman entertainers, showing up in the court of the Tang or Song emperors, and there are suggestions that Roman soldiers may have been captured and traded as slaves to the Chinese at an earlier period. But imagine of these two powerful and completely self-centered empires had formal contact? How would that have changed the Chinese worldview of it being the center of civilization? What types of knowledge would have been exchanged, and how would that have influenced history?