Arguably the two greatest problems facing particle physicists, cosmologists, string theorists, and the like are both associated with an apparent failure of a notion called “naturalness”. Until now, I’ve mostly avoided this term on this site, because to utter the word demands an extended explanation. After all, how could nature be unnatural, by definition?
Well, the answer is that the word “natural” has multiple meanings. The one that scientists are using in this context isn’t “having to do with nature” but rather “typical” or “as expected” or “generic”, as in, “naturally the baby started screaming when she bumped her head”, or “naturally it costs more to live near the city center”, or “I hadn’t worn those glasses in months, so naturally they were dusty.” And unnatural is when the baby doesn’t scream, when the city center is cheap, and when the glasses are pristine. Usually, when something unnatural happens, there’s a good reason.
I’ve started writing an article about naturalness and unnaturalness, and how there are two great mysteries about how unnatural our universe is, one of which lies at the heart of the Large Hadron Collider‘s [LHC's] research program. What I’ve written so far explains what naturalness means and (in part) how it applies to the Standard Model (the equations we use to describe the known elementary particles and forces). I’ll be extending the article to explain this in more detail, and to explain the scientific argument as to why it is so unnatural to have a Higgs particle that is “lonely” — with no other associated particles (beyond the ones we already know) of roughly similar mass. This in turn is why so many particle physicists have long expected the LHC to discover more than just a single Higgs particle and nothing else… more than just the Standard Model’s one and only missing piece… and why it will be a profound discovery with far-reaching implications if, during the next five years or so, the LHC experts sweep the floor clean and find nothing more in the LHC’s data than the Higgs particle that was found in 2012.