What determines real interest rates in the long run?

The question of what determines real interest rates in the long run is crucial: if they rise above real growth rates, there will be a debt sustainability problem. There are two schools of thought: Those who believe that monetary policy determines real interest rates even in the long run . If that is the case, central banks have the power to keep real interest rates lower than growth rates and thus prevent a debt crisis; Those who believe that real interest rates depend on the savings-investment equilibrium in the long run . If that is the case, then there is cause for concern. Population ageing may drive down savings and the energy transition will require huge investment with little effect on growth. Real interest rates could then rise without growth rates rising, leading to a risk for debt sustainability.