The original note, as written at the time
ON WARP DRIVES AND THE LIKE OF FASTER THAN LIGHT TRAVEL. In about a century or two we might discover the true limitation of our space travel means after having tried as much as possible to bridge intergalactic space to no reasonable avail. We shall then turn to gravity manipulation, possibly active manipulation of the space-time fabric. I personally think the cost of this kind of space travel will prove to be the deciding factor.
Ongoing development
On warp drives and faster-than-light travel: in a century or two we may discover the true limitation of our means after trying to bridge intergalactic space to no reasonable avail. We would then turn to gravity manipulation, possibly active manipulation of the space-time fabric.
I think the cost of this kind of travel will prove to be the thing that decides it, and that is an unusual position in a field that usually argues about physics.
Every proposed warp scheme requires negative energy density, and negative energy density is not a material you can mine. It is a condition you have to manufacture, and the quantity required scales with the volume of the bubble, which scales with the size of the ship. So the engineering question is not whether a warp drive is possible. It is whether a warp drive can ever be cheaper than the thing it carries.
That reframing is useful because it applies to everything in this collection. Fusion, space elevators, orbital habitats, warp: each is a technology whose viability is decided not by whether it works but by whether the energy budget closes. We tend to treat the physics as the hard part and the economics as an afterthought, and in long-horizon engineering the order is usually reversed.
The other consequence is that the first warp vehicle, if it exists, will be very small. A probe the size of a thimble would require a bubble small enough to be plausible, and a civilisation that can make one of those can make a bigger one later.
So the honest prediction is not starships. It is a very expensive, very small thing that goes nowhere useful, and a century of argument about whether it was worth it.
Check the energy budget before the physics. It is usually the answer.