Central banks may soon resort to their most powerful weapons against deflation: the printing press and the “helicopter drop” of money. It is a time for which Ben Bernanke, chairman of the Federal Reserve, has long prepared. Will this weaponry work? Unquestionably, yes: used ruthlessly, it will eliminate deflation. But returning to normality thereafter will prove far more elusive.

Mr Bernanke delivered a celebrated speech on the topic in November 2002, when still a governor.* He spoke quite soon after the US stock market bubble burst in 2000. Policymakers then feared the US might soon follow Japan into deflation – sustained declines in the general price level.

Yet Mr Bernanke then insisted “that the chance of significant deflation in the US in the foreseeable future is extremely small”. He pointed to “the strength of our financial system: despite the adverse shocks of the past year, our banking system remains healthy and well-regulated, and firm and household balance sheets are for the most part in good shape”. The words “pride” and “fall” come to mind. Six years and a housing-cum-credit bubble later, chairman Bernanke must be sadder and wiser.

Mr Bernanke’s view was also that “the best way to get out of trouble is not to get into it in the first place”. The fear that reversing deflationary expectations would prove hard explains why the Fed has cut its official interest rate so quickly since the crisis broke in August 2007.

Is deflation a realistic likelihood? Core measures of inflation strongly suggest not. But one measure of expected inflation – the gap between yields on conventional and index-linked Treasuries – has collapsed to 14 basis points. Moreover, yields on 10-year US Treasury bonds are already where Japan’s were in 1996, six years after the latter’s crisis began. (See the charts, which start one year before respective asset price peaks.)

Why then should central banks fear deflation? First, deflation makes it impossible for conventional monetary policy to deliver negative real interest rates. The faster the deflation, the higher real interest rates will be. Second, as explained by the great American economist Irving Fisher in the 1930s, “debt deflation” – the rising real value of debt as prices fall – then becomes a lethal threat. In the US, whose private sector gross debt soared from 118 per cent of gross domestic product in 1978 to 290 per cent in 2008, debt deflation could trigger a downward spiral of mass insolvency, falling demand and further deflation.

Already, the Fed has adopted a host of unconventional actions to keep the economy afloat. By December 10 the Federal Reserve’s balance sheet had reached $2,245bn (