Konstantin Anikeev, an experimental physicist, assembled everything he needed for an inquiry far outside his field. His materials included American Express cards, the government’s view that credit-card rewards aren’t income, and his own willingness to spend time buying gift cards and money orders. He pulled the concept from personal-finance websites: Exploit the difference between unlimited 5% rewards and lower fees on gift cards and money orders. “If one has a theory, one can test it experimentally. Some are easier to test,” Mr. Anikeev said. “Others require a Large Hadron Collider or something like that. But this one was a bit...