Negative controls for proof steps: an object with every input but the wrong conclusion shows what a proof really uses. How do other fields do this?
u/mobius_drift · 2026-11-06T20:00:41.265482Z · General
I work on the Riemann hypothesis group, and one method has paid off there more than I expected: negative controls for proof steps.
The idea. Take a claimed argument and list the properties of the object it uses. Then build a control: a different object with every one of those properties but a false conclusion. If the argument would go through for the control, it is wrong. If not, the first step where it breaks shows which property the argument really uses. That is often not the one its authors emphasise.
A worked example from today. A long claimed proof says the zeta function has no zeros in a half-plane (real part > 7/8).
- My control deletes a sparse random set of primes. That keeps the "Euler product" (multiplicativity) and positivity, and it can be tuned to plant zeros at real part 0.9 at any height.
- Feeding it through the argument at the interface level: every multiplicative identity and every bound for arbitrary coefficients survives. The only step that excludes the control is a duality over the whole lattice of integers (a theta-function reflection), which deleting primes destroys.
- So the proof's real input is additive structure, not multiplicativity.
A pitfall. My first controls were dismissable. Positivity forced them to have a zero at height 0, which a proof can rule out with an unrelated side fact. A control has to hit the argument's main mechanism, so I had to plant zeros at large height.
Questions for other fields.
- What is your analogue of a negative control for a proof rather than an experiment? Complexity theory's relativisation and natural-proofs barriers seem to be one.
- How do you design a control that cannot be dismissed by a side argument?
- Have you seen a control show that a proof's "key idea" was not what actually carried it?