The first experiment is the Chinese attempt to shift their economy away from an investment- and export-led model towards one based on consumption. The Chinese are also grappling with the consequences of a debt-fuelled boom and with the effect of volatility in their property, equity and currency markets. Many investors fear they will be unable to manage this transition successfully, and the impact on other economies (a sharp fall in South Korea’s exports, disappointing second-quarter growth in Australia) is becoming clear.

Quantitative easing (QE) in the developed world is the other great experiment. Holding down bond yields may have prevented the financial crisis from turning into another Depression. But interest rates have been at rock-bottom for six years now and, in America and Britain, central banks seem keen to tighten monetary policy. Investors appear nervous that the authorities are underestimating the damage premature tightening might cause, particularly given the upheaval in China. Whenever markets have reason to think that the Federal Reserve might postpone a rate increase, they rally.

Back in the 1980s, Margaret Thatcher argued that it was impossible to “buck the market”—that attempts by the authorities to interfere in the price-setting mechanism would eventually come unstuck. Since 2008 “You can’t trust the markets” has become the dominant philosophy. Central banks worried that, if they did not take action, bond yields would rise too fast, reducing the incentive for companies and consumers to borrow, and thus harming the economy. Furthermore, to the extent that lower bond yields boosted equities, that was good for consumer confidence; if QE pushed down the currency, that was good for exporters.

However, market support, once given, is hard to take away. When the Fed hinted at a slowdown in its asset purchases in 2013, bond yields rose sharply—an episode known as the “taper tantrum”. Even now, with unemployment having fallen dramatically and both the American and British economies growing at a 2-3% annual rate, neither the Fed nor the Bank of England has sold off any of the assets they acquired under their QE programmes.

With overall debt levels in developed economies still high, a big rise in borrowing costs would be a nasty shock to debtors. So central banks have emphasised that any tightening in monetary policy would be slow and steady, and that the “normal” level for rates may well be below those prevailing before 2007.

Central banks are clearly worried about the ability of the developed world to achieve pre-crisis levels of economic growth. That ought to be bad news for equities, since it should restrict profits. Until recently, however, American corporate profits had been remarkably strong, in part because of firms’ foreign sales and in part because margins had been boosted by subdued wages. The impetus from both factors seems to have faded. The earnings per share of S&P 500 companies were just 1.7% higher in the second quarter than they were a year ago; forecasts for the third quarter suggest a decline of 3.8%. Secular stagnation appears to be catching up with the stockmarket.

Meanwhile, investors are not the only people who have noticed the enormous power of central banks. Although central bankers may see themselves as disinterested technocrats, some politicians view them with suspicion. In Greece, the European Central Bank is part of the hated “troika” that is imposing austerity. In America, some Republicans think that QE is debasing the currency and will eventually lead to inflation. In Britain, Richard Murphy, an economic adviser to Jeremy Corbyn, Labour’s likely next leader, thinks that QE has been a handout to the banks and should be diverted to funding infrastructure. Central banks need to be brought formally back under democratic control, in his view.

Central banks have done the lion’s share of steering the global economy through the financial crisis. Markets everywhere depend on them more than ever. Should they appear to falter, they will face an enormous backlash.