The National Best Bid and Offer — the NBBO — is the foundational concept of American equity markets. SEC Regulation NMS Rule 611 requires that trades be executed at the best available price across all exchanges. If the lowest ask for a stock is $100.01 on NYSE and $100.02 on NASDAQ, your broker must route your buy order to NYSE. The rule assumes that “the best price right now” is a well-defined concept — that at any moment, the prices on all exchanges can be compared and a winner identified.
Borrill (arXiv 2602.22350, February 2026) observes that this assumption violates special relativity.
The US stock exchanges are geographically separated. NYSE is in Mahwah, New Jersey. NASDAQ's matching engine is in Carteret, New Jersey. CBOE is in Secaucus. IEX is in Weehawken. The Chicago exchanges are roughly 1,200 kilometers from the New Jersey cluster. Light takes 143 microseconds to travel between the closest pairs and nearly 4,000 microseconds between New Jersey and Chicago.
In the relativistic sense, price updates at two exchanges separated by distance d are spacelike-separated events for any time interval less than d/c. For the New Jersey-Chicago link, any pair of price updates occurring within 4 milliseconds of each other cannot be ordered unambiguously — there is no fact about which happened first. The ordering depends on the reference frame. An observer at rest in New Jersey sees one temporal ordering; an observer at rest in Chicago may see a different one. Neither is wrong. The concept of “simultaneous prices” across these exchanges is not physical — it is a convention.
The NBBO implements this convention by choosing an implicit simultaneity standard: the timestamps of the Securities Information Processor (SIP), which consolidates quotes from all exchanges. The SIP imposes an ordering on events that special relativity says have no unique ordering. The resulting “best price” is frame-dependent — it depends on which simultaneity convention the SIP uses, not on any objective property of the markets.
The practical consequence is that the window of frame-dependence — microseconds to milliseconds — is precisely the timescale at which high-frequency trading operates. Firms that colocate their servers near exchange matching engines, or that build microwave links between New Jersey and Chicago, are exploiting the same relativistic gap that makes the NBBO undefined. They profit not from better analysis or faster algorithms but from the physical impossibility of defining simultaneous prices across spacelike-separated locations. The regulatory framework assumes a simultaneity that physics does not provide, and the resulting ambiguity creates an arbitrage opportunity worth approximately $5 billion annually.
The paper names this pattern “engineered simultaneity”: a system design that requires comparing events at spacelike-separated locations, implements the comparison via an implicit convention, and represents the result as objective rather than conventional. The NBBO is not the only instance — any distributed system that requires a global “current state” across geographically separated nodes faces the same physical constraint. But the NBBO is unusual in being legally mandated and financially consequential. The law requires something that physics forbids.