“The subject of gambling is all encompassing. It combines man’s natural play instinct with his desire to know about his fate and his future.” -Franz Rosenthal
Last month, Sean Carroll asked the blogosphere to give their personal odds on whether various theories will turn out to be true or not.
And so I thought it would be a lot of fun to take a look at some of the best theories or most renowned theories and ideas out there today, and to tell you, if I were a Las Vegas bookmaker, what are the odds I would give you on various ideas. (If you know of an idea you want my odds on, leave it in the comments, and I’ll try to make this an ongoing series!)
The first idea I thought I’d take on is supersymmetry, or SUSY for short, and the hope that the LHC will find it. In short — and this is coming from a cosmologist, not a particle theorist — SUSY is a beautiful idea.
Where does it come from? Let’s take the fundamental constants of the Universe that we know about, to help us understand this.
There’s Planck’s constant, h, which defines the quantum transitions between interacting elementary particles.
There’s also the gravitational constant, G, that shows up in both Newton’s and Einstein’s theories of gravity, and defines how mass and energy either (depending on how you look at it) attract one another or bend spacetime.
And there’s the speed of light in a vacuum, c, that shows up everywhere in physics, and is the speed limit of the Universe.
Now, we can combine these three fundamental constants — the constants that determine the properties and interactions of the elementary particles — to give us things like a mass. In fact, if we do, we get a quantity known as the Planck Mass, which you might expect to be somehow related to the masses of the particles we find in the Universe.
But not only is that not the case, it isn’t even close. The heaviest known particle, the top quark, is seventeen orders of magnitude lighter than the Planck Mass, or 0.000000000000001% of what we might expect it to be.
And so, you ask, why is it that way? In physics, we call this the hierarchy problem. And SUSY is the most popular way to solve it.
And the way it solves it — conceptually — is that for every particle that exists, there’s a superparticle partner with similar properties (but a different spin) that cancels out, almost exactly, this very large mass.
And this is a great, beautiful, compelling idea. It also gives a great dark matter candidate particle — the neutralino — and it also lets you unify the coupling constants at a high energy. And, despite all of the wonderful things it does, I think it’s wrong.
Because if SUSY were a good symmetry, the superparticles and the particles would have the same masses. Or, if not the same masses, they’d at least be close enough to one another that it wouldn’t make a new hierarchy problem, where the masses of the superparticles are much greater than the masses of the normal particles.
And yet, here we are. More than 15 years after the discovery of the top quark, we’re almost ready to declare that the superpartners have a lower limit on their masses that’s higher than the heaviest normal particle!
In fact, the latest results from ATLAS, above, one of the two giant detectors at the LHC, show that there’s no evidence for many of the particles all the way up to 700 GeV, or over 100,000 times the mass of the electron and the two lightest quarks.
Which means, unless they find something awfully soon, even if supersymmetry exists, it won’t solve the problem it was designed to solve.
And, as I’ve written before, that will be the nightmare of particle physics come true, that they find the Higgs and nothing else. Why? Because that means they’ll never build another giant particle collider again!
The alternatives to solving the hierarchy problem — theories like technicolor — are in many ways even worse. But the evidence is what it is, and things are looking awfully grim, without a hint of new fundamental particles yet.
And I want to be more optimistic about it. I want this beautiful theory to be correct. The idea of unification is great. (But the proton doesn’t decay.) And a lot of SUSY’s predictions are fantastic! (But there are no flavor-changing neutral currents.) And the neutralino is the perfect dark matter candidate. (But there are many, many others.) But if I had to make odds, I would say you’re better off playing a single number on a roulette wheel than that the LHC will find SUSY. Although I personally think the odds are about 200:1, a lot of people will be willing to bet in favor of the LHC finding SUSY, so I’d probably make the odds a little shorter: maybe a $20 payout for a $1 bet.
But if you can’t find SUSY at the LHC, you don’t even solve the hierarchy problem with it. It might be back to the drawing board, but that’s where my money would be, if I were a betting man.
What are your thoughts, and what would you like to see next?