Eric Drexler describes how you can apply scientific methods to assess the lower bounds of the capabilities of future technologies:
A subset of the potential capabilities of future levels of technology can be understood by means of a design process that can be described as exploratory engineering. This process resembles the first phase of standard design engineering (termed conceptual engineering, or conceptual design), but it serves a different purpose
In the early 20th century, a missing fabrication technology was the combination of engineering expertise and metalworking techniques (among others) that were required to build large aerospace vehicles. The physics of rocket propulsion, however, were well understood, and the strength and weight of large, well-made aluminum structures could be estimated with reasonable accuracy.
On the basis of exploratory engineering applied to this kind of knowledge, engineers who studied the matter were confident that orbital flight could be achieved by means of multistage chemically fueled rockets.
This was an element of Drexler’s Engines of Creation I found especially compelling: that we should base our ideas of future technologies not on what we already have, but what lies within the bounds of what is possible by physical laws as we understand them.