Architect Dan Jay has been checking in with a widening circle of thinkers around town inviting them to participate in and contribute to a thought experiment:

What would the City of St. Louis look like with a population of 500,000?

More specifically:

Which city districts or neighborhoods most likely would become centers for new residents? In which order and by what increment would they add population? Over what period of time? Through what mix of amenities, service improvements, and housing types? By what logic?

More concretely:

If St. Louis' Central Business District - the square mile bounded by Chouteau, Cole, the Riverfront and Tucker - grew in population from 872 residents, in the 2000 Census, to 3,721 residents, in the 2010 Census, what might its housing and residential profile look like if, over time, the city's overall population grew from the 2012 estimate of 318,294 to 500,000? What portion of so significant a population expansion might Downtown and its environs reasonably be expected to bear - compared, say, to areas with high vacancy, or to neighborhoods with stable housing stock, or to those that are in transition?

More minutely:

How might YOUR block, or neighborhood, or neighborhoods adjacent to you (even extending out to inner-ring suburbs) change and look if they accommodated some logical part of a steady citywide population growth of up to 200,000 additional residents?

Most broadly:

How might various population growth scenarios look on a time-lapse aerial map?

Some, no doubt, would be skeptical about such an exercise. They may want to know:

What's a "thought experiment?"

It's more than a parlor game, but less than a planning process. Think of it this way:

Any formal civic goal of St. Louis attaining a city population of 500,000 would be sure to set skeptics' eyes rolling. St. Louis, after all, has lost population over each of six consecutive decennial censuses. The rate of loss has slowed dramatically. But St. Louis is part of a region that collectively, over four decades, has achieved virtually no population growth.

Consider the scale of the undertaking. To add through internal migration the 185,000 new residents needed for St. Louis to become a 500,000 population city, for example, would require almost every soul counted in the 2010 census in Florissant, University City, Webster Groves, Kirkwood, Clayton, Jennings, Normandy, Richmond Heights, Brentwood and Maplewood to move into the city.

On the other hand, St. Charles County grew by more than 200,000 people over the 30 years between 1980 and 2010. Chicago, Jay points out, staged its great Columbian Exposition of 1893 less than 22 years after Catherine O'Leary's cow reportedly kicked over a lantern in a small barn behind 137 DeKoven Street.

St. Louis, itself, has twice before reached a population of 500,000 - ahead of the 1900 census, on the way up; and sometime before the 1980 census, on the way down.

What does St. Louis today have in common with the St. Louis that grew meteorically to 500,000 around the start of the last century? Foreign-born immigrants.

We had lots of them, then. About half of the city's population in 1910 was foreign born or first-generation American.

We want lots of them, today. St. Louis' Mosaic Project has set as its goal St. Louis becoming, by 2020, the metro area with the fastest growing immigrant population.

It's not hard to imagine 200,000 productive and educated people out of a world population of more than 7 billion being attracted to this city. St. Louis' status as a world center in agricultural and food science alone could be such a draw.

Immigration of course isn't the only path to population growth. There's the old fashioned way.

The St. Louis that reached 500,000 people more than a century ago no doubt differed from today's St. Louis in this respect: It had larger families.

But consider this: According to 2012 census estimates, St. Louis City and St. Charles County have virtually the same number of total households, 139,840 and 134,693, respectively.

St. Louis City has an average household size of 2.20. St. Charles County's average household is 2.64. (The national average in 2010 was 2.58).

If St. Louis grew its average household by just the .44 people needed to match St. Charles County's average - Presto! We add more than 60,000 people to the city's population.

Why 500,000?

Good question. Here's the thinking: St. Louis, at its 800,000 population peak, from the 1930s to the 1950s, had much higher than ideal density. Today, a city population of less than 40 percent of the all-time high comes with density that is far less than ideal, at least in parts of the city.

A half-million people citywide is within shouting distance of the St. Louis City's density sweet spot. It is a reasonable point to start a conversation. It represents, generally, a higher density than our suburban neighbors. It encourages urban amenities such as street retail, public transit, shared roads, and housing diversity that a region's urban core should have.

Here's what can be said with certainty:

As recently as 20 years ago, the following barely qualified as science fiction concepts in the City of St. Louis: Cortex, the Grove, Botanical Heights, CityGarden, loft living downtown, Northside Regeneration, the CityArchRiver project, Great Rivers Greenway, a growing number of charter schools offering quality educational choices, and a citywide crime reduction of more than 50 percent.

A growing St. Louis that steadily moves toward a stable population of 500,000 would yield a more sustainable, diverse and dynamic city -- if we think things through.

So, hats off to Dan Jay and a growing cadre of intrepid analysts for imagining tomorrow's possibilities, taking stock of its opportunities, and having fun along the way - and for understanding that rolling eyes, by definition, are incapable of looking ahead.

(Photo Credit: Adam Crane. Used with his permission. )