Sociotechnical Systems Analysis

A reverse salient is a component of an expanding technological system that has fallen behind the advancing front of the whole, holding back its growth—Hughes borrows the metaphor from a military line that bulges backward. It resembles a bottleneck but is richer: a reverse salient becomes actionable only when system builders translate it into a well-defined 'critical problem' that engineers and inventors can attack. The act of defining the salient as a solvable problem is itself a central part of how systems advance.

Technological momentum is the directional inertia a system acquires as it matures and accumulates invested capital, trained personnel, supporting organisations, and committed users. Hughes proposed it as a middle position between technological determinism (technology drives society) and social constructivism (society shapes technology): young systems are highly open to social shaping, while mature, high-momentum systems increasingly shape and constrain their social environment. It explains why large infrastructures are so hard to redirect.

Both treat technology as heterogeneous and refuse a strict social/technical divide, and both emerged in the same period. But Hughes works with a system that has a boundary, a goal, and identifiable builders, and he retains analytic categories like the social and the technical even as he weaves them together. ANT grants more thoroughgoing symmetry to non-human actors, dissolves stable groups into traceable associations, and avoids the strong notion of a goal-directed system. Hughesian analysis is more historical and structural; ANT is more relational and descriptive.