I've been hearing about recent progress in amplitudes, which, as I understand, uses unitarity, locality, and Lorentz invariance to find scattering amplitudes (I often hear buzz words like BCFW recursion). I was wondering what the relationship between this research program and the S-matrix theory of the 1960s? I'm not too familiar with either subject, so any historical context and broad-stroke overviews are appreciated.

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    $\begingroup$ An Introduction to On-shell Recursion Relations. $\endgroup$ Commented Feb 2, 2018 at 21:59
  • $\begingroup$ in short, in the 60's they expected too much by consistency conditions for scattering massive spin-0 particles, the less constrained of all particles, wanting even to catch non-perturbative physics. Today, much progress have been made by focusing instead on particles with zero mass but non-zero spin, and focusing on perturbative results almost exclusively. The new strategy has been extremely rewarding and real progress has come. $\endgroup$
    – TwoBs
    Commented Feb 2, 2018 at 23:29
  • $\begingroup$ Are the techniques used similar? Are modern methods based off the results found in the 60s? $\endgroup$
    – pianyon
    Commented Feb 3, 2018 at 1:32


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