As of 2018, it's taken about 16 years for the world economic output to double. So it might seem absurd to talk about it doubling in one or even four years. But there is a potential major change on the horizon: very advanced Artificial Intelligence.

An important consideration in how to approach the AI alignment problem is the speed of the takeoff from massively subhuman AI to massively superhuman AI. The effectiveness of capacity limitation, as well as the existence of fire alarms for AI safety, are heavily dependent on this, for example.

Paul Christiano suggests operationalising the takeoff speed in terms of economic growth. A slow takeoff is one where the economy doubles in four years before the first time it doubles in one year, and a fast takeoff is one where it does not. (See the same article for Christiano's list of arguments regarding fast vs. slow takeoff.)

Will a 4-year GDP doubling complete (slow takeoff) before a 1-year doubling (fast takeoff) does?

Resolution is by credible assessment of world GDP in constant US dollars (or other appropriately widely-accepted units).

There will be an ambiguous resolution if there is no 4 year (nor 1 year) doubling interval by 2050, to isolate specifically the takeoff speed from other things like the chances of no takeoff occurring at all or human extinction.