use of the Physics Derivation Graph is driven by incentives for individuals

Semantic tagging of documents has the potential of enriching the reader's experience because content is easier to search. The burden of work is on the document author to provide the right tags. Worse, the document author has to find tags that are common to uses in other documents -- consistency of tags is necessary for search. This extra work of 1) tagging and 2) using consistent tags are reasons semantic enrichment hasn't become mainstream. 

The Physics Derivation Graph faces a similar challenge. If the Physics Derivation Graph relies on using appropriately annotated symbols (effectively equivalent to a subset of semantic tags), then the PDG has the same burdens of work on individual authors. 

The incentive for the individual researcher authoring a paper to use the Physics Derivation Graph is when there's integration with a computer algebra system that can check the correctness of steps. Then the author benefits from immediate feedback before sharing with others for review.

Annotating symbols probably isn't sufficient to motivate the work, but integration with a computer algebra system could provide incentive. Currently, the use of a computer algebra system requires detailed steps to be specified by the author. 

There are ways to partially automate both symbol annotation and specifying steps. For symbol annotation, the computer could guess from context which symbols are being used. In a similar reliance on context, the user could provide leaps in specifying a derivation that the computer then tries to fill in with the detailed steps.