All of the available experimental data that have been collected are now well described by a theory called the Standard Model, which has been verified experimentally to great precision in an extraordinarily diverse set of measurements. It has proven to be frustratingly accurate. Every experimental result so far either has agreed with the Standard Model prediction or has turned out to be wrong! Some experiments made startling new discoveries, which helped to develop the model, and others made measurements of unprecedented precision in order to test it.” — National Research Council (U.S.), Elementary particle physics, National Academic Press, 1998.
Nothing much has changed. Here is how Chad Orzel of Uncertain Principles recently contemplated the Standard Model:
It occurs to me that the Standard Model is in kind of a unique position among scientific theories. I know of lots of examples of theories that everybody thought were right that turned out to be wrong, and there are plenty of examples of theories that at least some people think are right but that they can’t prove right. The Standard Model is the only theory I can think of that everybody knows is wrong, but nobody can prove is wrong.
OK, “wrong” may be a little too strong– “incomplete” is probably a better word. The Standard Model consists of a set of twelve material particles: six quarks (up, down, strange, charm, top, and bottom) and six leptons (electron, muon, tau, and electron, muon and tau neutrinos) with their associated antiparticles. It also includes four forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force, plus their associated force carriers. Taken together, these particles and forces describe everything about the structure and organization of ordinary matter.
The problem is, they don’t explain everything. Most of the universe is made up of “dark matter” that we see only indirectly through its gravitational interactions with stars and galaxies. For various reasons, we know that this matter, whatever it is, can’t be made up of quarks, but beyond that, we have no idea what it is. There are lots of proposals of different sorts of particles not included in the Standard Model that could account for this extra mass, but nobody has ever conclusively seen one.
There’s also the question of mass: The Standard Model enumerates the particles and their masses, but doesn’t say why they have those masses. There’s a proposed mechanism by which fundamental particles could acquire their masses from interactions with another sort of particle– the interaction is called the “Higgs mechanism” and the particles are “Higgs bosons,” and there ought to be one for every type of material particle. Nobody has ever seen conclusive evidence of a Higgs boson, though, despite many active searches for them.
This is really a strange and awkward position to be in. The Standard Model works extremely well for those things that it describes, but we know it can’t be the whole story.
And yet, every attempt to find physics beyond the Standard Model has come up empty. Nobody has yet found a particle or force that isn’t accounted for in the theory, despite a couple of decades’ worth of searching. In a certain sense, it’s a theory that works too well. We’ve got excellent indirect evidence that says it can’t be the whole story, but we can’t find any direct evidence of anything that doesn’t fit the theory.
There has to be a message here…
