A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction.
It's kind of like a snowball that's rolled downhill. Everyone who's tried it knows that what they show in cartoons doesn't really happen. You don't start with a tiny snowball and it rolls on it's own, picking up snow as it goes along and getting bigger and rolling faster. You have to have a certain mass of snow in the snowball to begin with—snowman-segment-sized—to get the rolling started. That's kind of like critical mass.
I use the term quite a bit in a non-nuclear meaning, for the minimum amount of effort needed to get something going and keep it going. When we're working on a project at work, early efforts are typically slow, or seem slow. Little progress is seen. Then, at some point, the amount of work done reaches "critical mass"; all other activities on the project can be done in rapid succession, and the project zooms to completion. In the workplace training program I'm trying to build, reaching a critical mass of people able and willing to prepare and teach noon hour courses is something I'm looking for. I'd define that as enough people trained and prepared that we could have a class every week without a lot of convincing and cajoling on my part.
"Critical mass" is a valid concept in church growth. You need a certain number people to attract more people. You need a certain number of programs to attract people. You need a certain amount of assimilation going on to keep the people who come to take part in the programs you do to attract more people. Yes, critical mass is a valid concept here.
In terms of writing, I think critical mass applies in the writing of books. They normally start out slowly, build momentum, critical mass kicks in, and are finished in a rush. I know most of my books have been that way, both novels and non-fiction.
It's also true about the writers groups I've been involved with, especially the one I started at church and the one I'm in now. Neither one had or has critical mass. I'd define that as having enough active members (meaning writing something regularly, sharing it, and attending meetings) that meeting makes sense. At church, I learned of several others who either wrote or had a desire to write. I mentioned this to the pastor and offered to start a writing ministry. He said go for it, so I did. A couple of writers in the congregation also had friends in the city who wrote, and they started coming. It got to the point where we had nine or ten on our roster, and six of us were attending either regularly or close enough to regularly that we anticipated they would attend a meeting and weren't surprised to see them. I thought we were at critical mass, and would a sustaining group.
Alas, I was wrong. One person dropped out, a regular became irregular, and suddenly it was two of us meeting, sometimes just me. We weren't at critical mass after all, we were just below it. After two or three "meetings" of being the only person present, I folded the group.
My current writing group is headed in the same direction, I'm afraid. We have eight people on the roster, plus one man's name who a member says is intending to come. We had nine, but it turned out one of those names was a woman who came only once, sometime before I started, and never came back. So I deleted her from the roster. Am I the leader of the group? Not really. Another woman was, but she quit coming shortly after I started. We floundered for a while without someone to facilitate discussions of meeting days, times, and places, so I stepped up and just started being the leader. No one objected. We settled on a meeting day, time, and I found us a place. We had four people coming regularly, one coming some, and everyone was talking to each other.
Now, evening work has taken two of our members away on meeting nights. One does taxes, and won't come back till after tax season, sometime later in May. Another may be back when the school semester is over. Another, a retired woman, no longer replies to e-mails. I don't know what's up with her, though I know her husband has some health problems. Two meetings ago I knew two members were going to be away, but not having heard from two others I showed up. I was the only one there. The last meeting I had an evening meeting. I alerted the group that I wouldn't be there, and they didn't meet. I'm going to miss the next meeting also, due to travel. Alas, we don't have critical mass to keep the meeting going when one or two people are gone.
I suspect this group will die as well. We might limp on through the summer, depending on how many are traveling, but I don't really anticipate it will survive. Critical mass is just too hard to come by with writers groups.