We have all been hearing plenty of things in the last decade about the development of carbon free cars. Although someone impractical right now, it is certainly doable. However, I have not really heard anything about alternative fuels for aircraft. It seems reasonable to expect that we could develop carbon free subsonic aircraft, since we could use electrically powered props. But is there any research out there on either carbon free or low carbon propulsion systems that would enable supersonic air travel? Is this a feasible possibility at some point in the future or just wishful thinking?