“Which is more likely? That the universe was designed just for us, or that we see the universe as having been designed just for us?” -Michael Shermer
One of the problems with the Standard Model of particle physics is known as the hierarchy problem. If you were to calculate the masses of the fundamental particles from first principles, you’d get something on the order of the only natural mass scale that nature provides: the Planck mass, at 10^19 GeV or so. With a lower mass value, the Higgs can account for the smallness of the other masses, but at just 125 GeV, there needs to be some explanation for the Higgs.
The Standard Model particles and their supersymmetric counterparts. Exactly 50% of these particles have been discovered, and 50% have never showed a trace that they exist. Image credit: Claire David, of http://davidc.web.cern.ch/davidc/index.php?id=research.
Physicists were expecting to find some clues to this at the LHC. Perhaps it would have been supersymmetry, whose particle/superparticle symmetry would have protected the Higgs mass down to a low energy scale. Perhaps it could’ve been extra dimensions, technicolor, or any type of new physics. But the LHC found nothing else at all; nothing beyond the Standard Model. What, then does that mean?
The particle tracks emanating from a high energy collision at the LHC in 2014. Image credit: Wikimedia Commons user Pcharito, under a c.c.a.-by-s.a.-3.0 license.
Does it mean that “naturalness” isn’t as natural as we think? That there’s something undiscovered just beyond our reach? Or that we need to appeal to the multiverse to avoid catastrophe? Sabine Hossenfelder has a careful consideration of what the answer might be… and isn’t!