BEN BERNANKE, until last year the chairman of America's Federal Reserve, has started blogging. Not just a little bit, either. Mr Bernanke has in the space of a few posts embroiled himself in a weighty online debate with some of the titans of economics, and blogging. Both Paul Krugman and Larry Summers are scrapping with the former chairman on the live and important subject of secular stagnation.

Secular stagnation is an old idea which received an intellectual revival in 2013, when Mr Summers, who not long before was one of Barack Obama's chief economic advisors, began to deliver speeches on the topic. It describes a world in which there are lots of savings and comparatively few attractive places to invest them. The excess of saving over investment represents a shortfall in demand, and weak demand shows up in anaemic growth figures and low inflation.

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Normally a central bank would try to fix the imbalance between saving and investment by reducing interest rates (which should discourage saving and encourage borrowing). But in a weak enough economy with low enough inflation the interest rate needed to balance saving and investment might become negative—maybe even really negative. Given the difficulty of achieving a negative nominal interest rate, the central bank might find it hard to push an economy out of that sort of trap once it fell in.

Indeed, Mr Summers reckons there are generally two ways out, one bad and one good. The bad one occurs when a long period of very low interest rates leads to a bubble in asset prices. The bubble in asset prices can support consumer borrowing and spending, providing an economy with the sensations of a decent recovery while the good times last (after which things often look even worse than before). The good one occurs when governments recognise the problem and take direct action to fix it: by borrowing (to soak up excess saving) and investing the proceeds of that borrowing in demand-boosting investment. Mr Summers is an advocate for this approach; he would like America's government to borrow at historically low interest rates in order to tackle the country's many pressing infrastructure needs.

At his blog, Mr Bernanke worked through these arguments and then made an important, if not entirely original, point. Unless one assumes that the entire world is in secular stagnation, secular stagnation shouldn't really be much of a long-term problem. Old countries with poor growth prospects might accumulate lots of savings and have little need for big investments. But those countries shouldn't suffer from chronic weak demand. Instead, the excess saving should flow abroad in search of better returns. Capital outflows should weaken the domestic currency, and that should lead to rising import demand from abroad. A stagnating economy should be able to export its way to a healthy level of demand.

What it means to suffer from secular stagnation

Of course, Mr Bernanke allowed, the world could get into trouble if big, fast-growing economies like China decided to save huge amounts of money by buying rich-country debt, in order to depress the value of their currencies and boost their exports. If governments were interfering in the market like that then capital might not flow the right way and secular stagnation might stick around. But the way to solve that problem, he argues, is not through deficit-financed demand stimulus but by leaning on the Chinas of the world to knock it off.

Into the discussion, then, plunges Mr Krugman. He points out that China is only part of the problem. Europe is increasingly the source of the demand drain, and Europe is not out there accumulating American debt (as China's government did in the 2000s). Instead, the euro-area economy is falling into a Japan-like trap. It is stagnating secularly, suffering from weak domestic demand while stuck with interest rates near zero. But even though the euro has fallen a lot against the dollar, it probably hasn't fallen by enough to get Europe out of its trouble.

That is because euro-area inflation has also tumbled, raising the real, or inflation-adjusted return on money stashed in Europe. German government bonds don't look like an especially good investment; the return out to 10 years, Mr Krugman points out, is effectively zero. But German bonds are safe, and given the very weak outlook for euro-area inflation they offer a real return that, in this fallen world of rock-bottom yields, is not all that bad. Less money is therefore flowing out of Europe than one might anticipate given the crummy state of euro-zone growth prospects. And the euro zone will therefore putter along in future rather than hum at its economic potential. As the Japanese example demonstrates, this sort of stagnation can last for a long time. We should therefore expect Europe to be a demand suck for a long time, and we should consider a more Summersian policy response: deficit spending.

The debate is inching toward the right question: what sort of imbalance between saving and investment do we have here, anyway? Is the problem that the world has too much saving and too little investment? Or is it that the saving and the investment are stuck in different places?

It seems obvious that the world as a whole does not have too few investment opportunities. Much of the world is very poor relative to America, which suggests that much of the world is overflowing with profitable investment opportunities. It would not cost that much money to build massive new highway networks and ports in Africa, but one could easily imagine such investments paying off handsomely. And yet the savings that could finance such investments are not flowing toward them in anything like the quantities needed to address the world's imbalances.

That isn't that hard to understand. Should capital flow to China? Given the massive investment the country has experienced over the last 15 years the return on new capital might not be especially high; meanwhile, controls on capital flows into and out of the country still make investing there more of a challenge than investing in rich economies. Many of the emerging economies with financial markets that are developed enough to handle portfolio inflows are not obviously in a position to deliver high returns: think of Brazil or Russia, for example. In much of the emerging world large-scale portfolio investment is basically impossible; foreign-direct investment is the only real option, and FDI cannot be done well at scale. That is in some ways a good thing. In countries with weak institutions a tidal wave of foreign capital could cause more harm than good. And even the limited investment that is made is illiquid, and while returns may be good on average the variance is high.

So in one sense there is a geographic imbalance between savings and investment. But this imbalance is persistent and structural. It also isn't new. It is hard to rate this as the cause of secular stagnation. The transitory geographic imbalance of the 2000s—the China glut—is fading, and yet secular stagnation remains an apparent feature of the rich world. Perhaps worst of all, China may be slipping toward stagnation as its population ages and inflation rates tumble.

The imbalance is a global one, in other words. Its effects are more or less acute depending on which big economy one focuses on. But the problem, it seems to me, is that the industrialised world has a much greater capacity to produce things than an interest in buying things. The world is stuck with too little demand. So what ought to be done about it?

Mr Bernanke's solution, to lean on currency manipulators, is probably not going to do the trick. It isn't clear that Chinese consumers are capable of driving forward the global economy. America could ask Europe and Japan to halt their quantitative easing programmes, which should push their currencies higher relative to the dollar. But that move would almost certainly lead to galloping deflation in both economies and a serious recession (and possibly a break-up) in Europe. That seems a poor way to get Europeans splashing out on new goods.

Another option, which Mr Bernanke does not consider, is for America to do more monetary easing rather than less. One way of looking at the global economy is that it is in need of a source of excess demand and upward price pressure. Rich-world central banks with interest rates close to zero can only do so much to create their own demand. They will attempt to recover by siphoning off spending from the relative well-positioned American and British economies. If those economies—which also remain stuck with low interest rates—are not allowed to run hot then the demand drain and disinflationary pressure from abroad will slowly drag them back to general crumminess.

This can be put more simply: the world as a whole is not spending enough money. Large parts of the world economy are short-run incapable of spending more for political and economic reasons. Unless other parts take up the slack, then the too-little-spending problem will grow more serious and ever more of the world will slip into this monetary trap.

What else is there? The solutions that remain are all politically tricky to implement. There is the Summersian option of deficit spending in rich countries. This could be effective in closing the savings-investment imbalance but has two big problems. The first is that doing it adequately is almost certainly beyond the capability of the American political system. As Mr Summers repeatedly points out, the government has failed manifestly to tackle even the highest-return infrastructure projects available: like improvements to America's crumbling but critical JFK airport. Republican presidents have historically had more luck running massive deficits, so circumstances might change after 2016.

Yet even then, there is a second concern: that while there are lots of positive return investments not being made in America there probably aren't that many positive return investments not being made in America. American public infrastructure needs are enormous but probably amount to about $200 billion a year at the absolute most, which is less than the euro-zone current-account surplus. Directing money to uses beyond those would probably yield relatively low returns, at least in narrow financial terms. (If America wanted to borrow heavily to build a moon base that's its business.)

Congress could just hand the money out. There is much to recommend that solution (though not its politics). It has its downsides. Much of the proceeds would go toward consumption, on goods and services produced by companies that are already investing at close to optimal levels. Lots might go toward residential investment. But the spending should also create new labour demand in low-productivity sectors, adding to employment, adding to wage pressure and kicking the economy back into a better equlibrium.

Still, this is probably not the first best solution. We would not say that savings, in that case, were being mobilised to their most productive uses. Not while so much of the world is so poor.

There is another option available. The rich world could address the imbalance within its economies while simultaneously addressing the geographic imbalance. It could allow much more immigration. Investing in people in developing countries in hard and risky. But if those people wanted to come to America and were allowed to, then lots of things change. Investing in those people would not then require that money be sent abroad, to a different financial system in a different currency overseen by a different government. If the savings are in rich countries and the most productive investments are in poor ones, then the savings can move or the investments can move.

What this discussion should make clear is that secular stagnation isn't much of a puzzle. Rather, it is a dilemma. The ageing societies of the rich world want rapid income growth and low inflation and a decent return on safe investments and limited redistribution and low levels of immigration. Well you can't have all of that. And what they have decided is that what they're prepared to sacrifice is the rapid income growth. In aggregate that decision looks somewhat reasonable if not entirely right. But it is a choice with pretty significant distributional consequences. And the second era of secular stagnation will come to an end when political and demographic shifts allow the losers from this arrangement to say: enough.