It was Niven ("Rotating Cylinders and the Possibility of Global Causality Violation"), but I misremembered. The mathematician was telling his own emperor about it, to get him to use agents to convince the enemy to start up one of the machines, leading to *their* doom. The nova leads to their own doom.

On Wed, Dec 20, 2017 at 12:32 PM, Jerry Barrington <xxxxxx@gmail.com> wrote:

On Wed, Dec 20, 2017 at 11:21 AM, C. Berry <xxxxxx@gmail.com> wrote:
On Wed, Dec 20, 2017 at 1:07 AM, Rob O'Connor <xxxxxx@ozemail.com.au> wrote:

FTL information signalling is only problematic if it allows useful causality violations. Misjumps in Traveller are the cosmic censorship hypothesis writ large ;-)

Traveller jumps (or anything else carrying information FTL) can be made to produce causality violations; it's all about picking the right observer reference frame. And I like that formulation for misjumps. God to characters: "Hey, knock that off!" :)

I forget the author (maybe Niven?), but a great story has these time machines that are huge, massive spindles. None operational, and all associated with the deaths of the civilizations that were making them. A researcher figures out it's the universe protecting itself from causality violation, so he goes and tries to sell the idea of spinning one up to the (enemy) emperor, hoping to wipe out the emperor's civilization.

During the interview, the universe pre-empts the idea with a nova.