Maybe this is more quantum, but I'm not sure. I was watching Susskinds first String lecture on youtube, and he was talking about how, within each particle family, you get a straight line called a regge trajectory if you plot spin vs. mass^2. He also mentioned that there was some slight insight as to how it came about, but also hinted that it might have been dumb luck at the time. I'm not even sure what to google, I've looked up regge trajectories, pion-pion scattering, even plot spin vs mass squared, and am not turning anything up. Does anyone have any insight as to why this would be a reasonable thing to plot?