How much a civilization throws away and what it throws away is a very rough gauge of its technological and scientific level. At the far-future side of the spect

How much a civilization throws away and what it throws away is a very rough gauge of its technological and scientific level. At the far-future side of the spectrum, you only detect very low levels of heat energy close to zero Kelvin, and some similarly low leakage of EM. They’ve mastered nanotechnology say, and what we throw out today is high-energy, rich feedstocks to them.