From time to time we encounter questions of the form:
What is the earliest time X could be discovered or built?
- What is the earliest time a pulsejet could be built?
- How early can transparent glass windows be easily available?
- How early can nuclear reactors be built?
- Earliest Possible Gun?
- What is the earliest that a civilization could develop airships?
These are fun questions because they ask us to look backward with 20/20 hindsight and consider what shifts in history would have permitted an earlier discovery. In this regard, they are alternate-history questions.
They're also prone to being primarily opinion-based due to the scope of the problem. Any one aspect of technology stands atop a mountain of experience, education, and innovation that has been influenced heavily by societal developments including philosophy/religion and government, as well as resource availability. The development of any technology or scientific discovery is quite complex and determining how much of that mountain need not have existed to permit earlier discovery is a challenge.
Because the question type is relatively unique and represents a quality challenge for Worldbuilders, I would like to create an earliest-time tag that defines the nature of this question, warns of its weaknesses (to avoid closure), and helps guide the development of good questions with (hopefully) a modicum of research.
Question: what guidance should we give to users of this tag? (Alternatively, "what's the best way to ask this type of question?")