In the story, it's the "Overdrive" - an FTL drive. Earth has a kingdom spanning a few stars, maintained with slower than light ships. These outworlds are always behind earth in technology because ships bringing them earth's information take decades or centuries. (Why don't the outworlds develop their own tech?)
Now someone is onto a theory that can lead to FTL, ending earth's hegemony. So earth has sent a military to nip the technology in the bud...
Fact sheet.First published: Analog, March 1964.
Download full text from Baen Books.
Among the stories from Astounding/Analog issues edited by John Campbell.
Related: Stories of Norman Spinrad.