G20 group of leading economies set to meet in Shanghai over the weekend but that may not be much they can do to boost growth , global interest rates already low and central banks seem to be out of ammunition , but some radical options could be up for consideration , the possibility of other extraordinary measures are old school being rooted in in , but U. S. treasury secretary Jack do you would downplaying hopes were an emergency action plan he said investors shouldn't expect a crisis response until the definite crisis