What if we have too much of capital, and what would happen if we had less?

Right now, 20 percent of capital in the world's two biggest economies may well be sitting idle.

That's a lot of stuff -- with a global capital stock of close to $200 trillion just in regular financial markets. As the economy changes to produce different goods and services with new technologies, the composition of the capital in use changes as well.

Some capital might be obsolete, just like workers with skills that are no longer needed.

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Central banks have been pouring money into the markets, in the hope that lenders would open their wallets, companies would make new investments, and more workers would get new jobs. Yet even in boom times, the capital stock is apparently overgrown; the utilization rate in the United States hasn't hit 81 percent in the industries followed by the Federal Reserve since the third quarter of 2000.

When interest rates fall, capital becomes cheaper for reasons completely unrelated to the productive capacity of the capital itself – lower opportunity costs for capital allocation

Corporate bosses may buy capital just to use up the cash flow inside their firms. Even if they have no profitable opportunities to increase production, they may prefer speculative purchases of capital to returning the cash to investors via dividends.

Even when the companies have no profitable investments, they have ways of paying the investors a return; they can dilute the return to existing investors or cut into workers' share of revenue. An excess of capital is the result.

Until recently, Japan was locked in a high-saving, low-spending, deflationary doldrums. Some of the reasons for high saving rates were likely demographic.

George W. Bush's tax cuts were directed at increasing the return on saving, expanding the capital stock, and spurring a higher level of capital-based innovation. In an economy that's growing at a healthier pace, raising taxes on wealth might do the trick. Such taxes might speed economic growth by helping to allocate opportunities more efficiently.

Economists should verify whether the capital stock is indeed too large. Why is so much capital sitting unused?  Are the Fed and its counterparts around the world measuring the right thing? Once we know the answers to these questions, we'll be able to confront what could be an enormous and yet broadly ignored inefficiency in the global economy.