Debt is a claim on future Tau-flow. The borrower commits future directed Tau-flow (principal + interest) to the lender's address in exchange for present tokens. Debt is a Tau-flow mortgage on the borrower's future.
Interest rate r is the Tau-flow premium per unit time — the cost of bridging the temporal distance between present and future Tau-addresses. Negative real rates imply Tau-flow destruction at system level.
D(t) = D(0)×ert. Since individual Tau-flow capacity grows linearly, compound debt inevitably outpaces ability to service it. Debt spirals are arithmetic certainty, not policy failure.
Sovereign debt mortgages the collective Tau-flow of all future citizens — Tau-addresses that cannot consent. Borrowing for Tau-productive investment is self-liquidating; borrowing for consumption is pure Tau-mortgaging.
When debt exceeds realistic Tau-flow capacity of borrowers, it is already economically fictitious. The Jubilee tradition and Solon's seisachtheia are Tau-liberation events — recognition of physical reality.
Equity and revenue-linked lending are Tau-conserving — repayment is linked to actual Tau-flow generated. Compound interest debt is not. Islamic prohibition of riba is Tau-conservation in traditional form.