As a physicist, I want to calculate things. I want to calculate how fast a rocket is travelling, I want to calculate where a quantum mechanical particle will end up and I want to calculate what the environmental impact is of street lighting. But then there is this term sustainability. I very much like the definition of Brundtland: "Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs".
This is a definition of sustainability in such a way that you could calculate whether you live sustainable! A measure for this could be the ecological footprint. This footprint shows if everyone would live the same way you do, how many earths would be needed to sustain humanity. If everyone would live like someone in Bangladesh, we could easily do with just the one earth. But if we all lived like an American, we would need a bit over 4 Earths. I did a short calculation; for me it would be about 2 Earths. However, for this calculation you need to know exactly in which ways you use Earths' natural resources. This is a very difficult thing to keep track of, so there are a lot of calculations to be done. And as a physicist, this makes me happy.