Questions of sustainability imply the need to take complex systems into account: from socio-technical systems to earth systems. Paradoxically, knowledge and approaches based on complex systems are still underrepresented in sustainability research. Here, I will examine the matter-energy balance of the Earth system, arguing that we don't have an energy (J) problem, but fundamental questions concerning power (W) and materials. I will invoke the notion of metabolism to show the unsustainability of the technosphere as it currently exists.  For example, I will present the questions raised by a long-term status quo scenario. Are we heading for the end of metals? The metabolism of living beings and of the earth system will serve as a guide to asking the right research questions for the invention of strong sustainability. In conclusion, I will stress the need to move away from today's zombie technologies towards living technologies.