"Economies perform badly when this capital becomes blocked up in other things - real estate, stock markets, government bonds."



Real estate will always be important to any economy, but perhaps you are right in cases where capital is tied up in INFLATED real estate.



Stock markets are the means whereby companies raise capital for their investments. Sure, there are also gamblers on the stock market, but it is a very proven and highly liquid way to get capital where it needs to go.



Government bonds serve a critical role for long term institutional investors, for example pension funds. Without them, your pension could be tied up in the stock market during a depression, and not only is that bad for pensioners, but it is also against the law in many countries for pension funds to be "too risky".



"To get the capital to work, there should be an interest rate levied on any passive capital,"



Companies are sitting on massive amounts of idle capital. But if there were any promising projects, wouldn't they have already be doing them? Moreover, in the face of the recent 2008 crisis, where nearly every major corporation in the Western world was faced the possibility of running out of cash and going bankrupt before the crisis was over, I think we can expect that companies will be careful to hold on to lots of cash for a very long time. When the next crisis hits, we will call them wise, but in the meantime, we should not force them to bet on risky projects by taxing "idle capital". That capital sits in banks, bonds, etc., and via financial markets will in any case find its way to one some project where someone needs to borrow money in the hope of earning a profit.