IT HAS been described as the world's most charming police state. Oman's ruler, Sultan Qaboos, who overthrew his father in 1970, now stands out as easily the longest-serving ruler in the Middle East, and perhaps as the world's only absolute monarch not to have a publicly designated successor. A few reforms have got under way since strikes and protests hit the country last year. But with most power still in the sultan's hands, questions about the future have begun to loom. The calm is now being challenged, albeit still a lot less fiercely than elsewhere in the Arab world. Some of Oman's more hopeful political activists say it is better that no ambitious crown prince is waiting in the wings. In a country where it remains utterly taboo to criticise the sultan openly, reformists instead argue that they need to create checks and balances not against him but against his unknown successor. Reform even of a gradual kind - evolution, not revolution, as Gulf officials often like to say - has barely begun in earnest. But tension has risen since late last month. The oil industry, the economy's backbone, has been hit by strikes and sit-ins as workers call for better pay. Pro-democracy campaigners, starting with three from a human-rights group who had tried to visit a sit-in at an oilfield, have been arrested. On June 11th 22 more were detained for protesting outside the central police station in Muscat, the capital, calling for the first lot to be freed. The treatment of political prisoners is proving a rallying point for opposition groups across the ideological spectrum in Bahrain, Saudi Arabia and the United Arab Emirates (UAE); now usually placid Oman has joined the club. Whether out of genuine respect or pragmatism, Omani protesters have generally avoided criticising the sultan himself. But a few Twitter users have now begun to point the finger at him directly. In response, the prosecutor-general has threatened legal action against anyone who "incites" people with "disrespectful speech": a vague prohibition that reverses a recent easing of curbs on free speech. Oman, a sturdy ally of Western governments, especially Britain, has been good at keeping out of the news. The protests and strikes that took place during the early stages of the Arab spring were more extensive than is often realised in the West. Demonstrators called for jobs and better pay in the less well-off parts of the country. Protesting workers also briefly barricaded some five-star hotels on a smart seafront. But after an initial crackdown, in which two protesters were shot dead by police who had probably never seen a demonstration before, the sultan took a cannier approach. In contrast to the ruthless rulers of Bahrain, he swept a clutch of powerful ministers out of office, replaced senior members of the security services, freed political prisoners and promised potentially far-reaching reforms, including freer media and tentative steps towards more independent courts. Since then, Omanis have become a bit more interested in the bicameral parliament, hitherto weak and often uninspiring, stuffed with mandarins and former ministers. The number of Omanis who registered to vote in the general election late last year was one-third higher than the time before, in 2007, though still only two-thirds of Omanis bothered to register and political parties remain banned. But more Omanis are warily beginning to discuss social and political issues online; a web forum called Sablat Oman, which claims to be visited by a tenth of the adult population every day, has become required reading for ministers and democracy campaigners alike. Some young Omanis are quietly organising political and social debates. But the recent arrests remind everyone that the "red lines" persist. Even reform-minded Omanis censor themselves, are wary of talking too openly and look over their shoulder when meeting in public. The sultan has felt obliged to increase handouts and pay and to create 50,000 new jobs, mostly in the army and security services. But such responses may not do the trick for ever, since Oman's oil production is in long-term decline. Unlike Bahrain, Oman turned down an offer from the rest of the six-country Gulf Co-operation Council, led by Saudi Arabia, for $10 billion in aid, since it wants to remain resolutely independent. Moreover, with half the population following the Ibadi school of Islam, Oman is keen to protect its religious diversity from the cultural encroachment of Saudi Salafists, some of whom think Ibadis are heretics and magicians. Omanis are still generally content. But the twin problems of the succession and the transition to a post-oil economy mean that even this tranquil sultanate cannot ignore the Arab spring.