I read this in the 1970s, possibly from an old Gollancz collection of stories - my guess it dates from the 40s-50s, maybe by Henry Kuttner and/or C.L. Moore.
Some scientists are told that an invisibility machine (or it might have been for teleportation or time travel) has been created, but was accidentally destroyed and they have to recreate it from some sketchy plans.
At the end they find out that the plans were a ruse to give them the confidence to build something which was theoretically impossible.
Some scientists are told that an invisibility machine (or it might have been for teleportation or time travel) has been created, but was accidentally destroyed and they have to recreate it from some sketchy plans.
At the end they find out that the plans were a ruse to give them the confidence to build something which was theoretically impossible.