“Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe?” -Stephen Hawking
After a long search spanning more than my entire lifetime (so far), the Higgs boson has finally been discovered at both detectors — CMS and ATLAS — at the Large Hadron Collider at CERN.
For a little more on this, check out the earlier posts here celebrating Higgs week:
- The Biggest Firework of them all: The Higgs
- How the Higgs gives Mass to the Universe
- Explaining the Higgs: on TV last night!
Our standard model of elementary particles and interactions is now complete, with every single particle that’s a part of it having been discovered.
Combined with our knowledge of gravitation through general relativity, and the framework of quantum field theory describing all the standard model particles and how they interact with one another, it’s no stretch to say we’re sitting pretty, and that we’ve come an incredibly long way to arrive at our present understanding, especially considering practically none of this was known a century ago.
But it isn’t like physics is over now, not by a long shot. There are still a great many mysteries to uncover, and a great many things we don’t fully understand about the Universe. In addition, there are also a great many hypotheses in physics that could lead us towards solving some of these questions. All of them are speculative, none of them are certain, and — unfortunately — a few of them have been (and are being) grossly oversold to the general public.
In no particular order, here are the questions we have on our plate at the moment.
Why do the fundamental particles have the masses that they do? The standard model allows the Higgs to give rest mass to all of the particles (or leave them massless), but the reason they have these values is unknown.
What’s more, is that these values are not what we would have expected based on our current understanding of how the laws of physics work. Based on that, the masses of these particles should be on the order of 1019 GeV, some quadrillion times heavier than even the top quark, which is the heaviest known particle. This problem is known in physics as the hierarchy problem, and the standard model has no answer for it.
There’s also the (much newer) problem of neutrino masses: why are they so mind-bogglingly light, and yet of non-zero mass? Do they get their mass from the Higgs? If so, why is it so much smaller than all the other particles, and if not, why not?
We know that the electromagnetic force and the weak force are, at very high energies, different manifestations of the same fundamental force: the electroweak force. It’s only in our low-energy Universe that they appear so different from one another. Another way of saying this is that “the electroweak symmetry is broken,” and the separate electromagnetic force and weak force are how that broken symmetry is manifested.
Well, it’s possible that at very high energies, the strong force unifies with the electroweak force, giving us what’s known as a Grand Unified Theory. Extending even beyond that, it’s possible that at even higher energies, gravity unifies with the other three forces; this is the basic idea behind string theory, the most promising framework for unifying all of the known forces and interactions into one theory.
Where does the origin of the matter-antimatter asymmetry come from? In other words, why do we have a Universe full of matter and only a tiny amount of antimatter, rather than equal amounts of both. We know how this is possible, of course, but we do not yet know how it actually happened. This problem of baryogenesis is yet another great unsolved problem of physics, and finding the Higgs and completing the Standard Model sheds no light on it.
If you look at the fundamental interactions of matter, particles and antiparticles have a great deal in common. (Known as C-symmetry.) Particles spinning (or with angular momentum) in one direction have a lot in common with those spinning in the opposite direction. (Known as P-symmetry.) But in the weak interactions, not only do particles and antiparticles behave differently, and not only do particles spinning in one orientation behave differently that particles spinning in the opposite one, but particles spinning in one direction have slightly different physics than antiparticles spinning in the opposite direction! In other words, not only are C and P symmetries violated, they’re both violated together.
But for some unknown reason, even though there’s nothing in the standard model that prevents it, the CP-symmetry is not violated in the strong interactions. Known as the strong-CP problem, this is yet another physical reality that is unexplained by our current understanding of the Universe.
And finally, there’s dark matter and dark energy. They are both required for the Universe to look the way it does, but our current understanding of things does not explain where either one of them comes from.
Don’t let anyone tell you that “Physics is over now that we’ve found the Higgs.” On the contrary, it’s only the physics that we had every right to expect was correct that’s over. Now’s where the fun begins.
And I say “fun” fully recognizing that even our best ideas for what comes next have severe problems, and they may all be wrong.
Supersymmetry — SUSY for short — is the best candidate theory to solve the hierarchy problem. If it’s correct, it can also conceivably provide a dark matter candidate, give evidence for the potential unification of the strong force, and give circumstantial evidence for superstring theory (which requires SUSY).
Unfortunately, in order to actually solve the hierarchy problem, the masses of the superparticles need to be of the same magnitude as the masses of the normal, known, standard model particles. If the masses of the superparticles are beyond the reach of the LHC, then SUSY, even if it exists, no longer solves the hierarchy problem, which was the original motivation for SUSY in the first place! (In fact, even the best-case scenario for SUSY already has a little hierarchy problem.)
The longer the LHC goes without finding any of them, the more disfavored SUSY is going to become. Now that we’ve discovered the Higgs and we know its mass, it’s conceivable (although it would be a true nightmare scenario) that there are no new particles to be found until we get up to a monstrous 1010 GeV in energy! (It could’ve been even worse if the Higgs were a few GeV heavier!)
(And good luck getting a particle accelerator the size of Saturn’s orbit to find it!)
There could have been multiple Higgs particles (a Higgs multiplet), as predicted by practically all grand unified theories.
But the data very convincingly shows that there is just one, singlet, spin=0 Higgs, which is what the standard model alone predicts. I have written before about how the lack of proton decay disfavors nearly all GUTs, but the lone standard model Higgs may be an even more damning observation.
So while I may not be impressed with string theory, SUSY, or grand unification, the cracks and unsolved problems are where the new, exciting (and probably surprising) discoveries are bound to happen. The first thing to check — when sufficient data comes in (and not before) — is whether the “particle-consistent-with-the-Higgs” that we’re producing at CERN does, in fact, behave like the Higgs is supposed to!
Because if it doesn’t, you can add that to the list of unsolved problems in physics, too!
It may not be what you want to hear, that the leading attempts to extend the standard model have severe problems with them, but in physics, as always, data must be the ultimate decider of the veracity of our theories. There are some great possibilities for this Universe, and — just like you — I can’t wait until we find out more. Hope you’ll continue to join me as we keep looking!