Federal Reserve Chairman Jerome Powell.Andrew Harnik/Getty ImagesThe Fed probably can't take rates as low as it would like to, according to Larry Summers.Summers pointed to the Fed's target of 2.9% for the so-called neutral rate.But taking the neutral lower than 4% risks reigniting inflation, he said.The Fed doesn't have the wiggle room to cut interest rates as low as it hopes to, according to ex-Treasury Secretary Larry Summers.The top economist issued a word of caution for investors hoping for steep rate cuts from the Fed.