Have you ever wondered why the masses of the fundamental particles have the small values that they do, compared to, say, the Planck scale? Whether the fundamental forces all unify at some high energy? And whether there's a natural, compelling particle candidate for dark matter? Well, in theory supersymmetry (or SUSY, for short) could have solve all three of these problems. In fact, if it solves the first one alone, there will be definitive experimental signatures for it at the Large Hadron Collider. Well, the LHC has completed its first run, and found nothing. What does this mean for theoretical physics, for SUSY in particular, and what are the implications for string theory?Supersymmetry started around 1970, along with various grand unified field theories. Theoretical physics started to diverge from what was observed, to how theorists thought that the universe ought to be. String theory followed, and it was almost entirely dependent on SUSY.
Physicists praised SUSY as correct in the same way that they say the Copernican model should have been accepted in the 16th century. It solved several independent theoretical conundrums, and just seemed more natural.
The trouble with SUSY is that it predicts new particles pairs with the known quarks, leptons, and bosons. None of these has been found. It is possible that they are all of such high energy that we will never find them, but then they are not so natural. To really solve the problems, we should have seen them. A lot of physicists have invested their whole careers on a wrong idea.
The above story is from May 2013, but is is only more true today:
For some insight into the current concerns of particle theorists, you can watch some of the videos at last week’s KITP conference. In particular, there’s Matt Strassler’s talk, where he got all Peter Woit and argued that “one could make the argument” that not seeing SUSY (or anything else stringy) at the LHC “would be significant circumstantial evidence against string theory as a description of nature” and that just seeing the SM at the LHC would be “circumstantial evidence against effective quantum field theory as a complete description of known particle physics”. This got him an argument from Gross about his insufficient enthusiasm for a 100 TeV collider. Gross then also got all Peter Woit, arguing that the failure of the “naturalness” argument for new physics was no big deal since it wasn’t a very good argument to begin with (I get all sorts of grief when I do this..).You can be sure that physicists will find arguments for building a 100 TeV collider, at a cost of $20B or more.