The hottest event since the Big Bang has now been delayed till spring. No, not a Chinese nuclear test: the Large Hadron Collider, the world’s most powerful particle accelerator, buried deep beneath the Franco-Swiss border near Geneva. One of its super-cold superconducting magnets overheated in all the fuss surrounding its much-discussed initial trial run.
Now, a little learning is a dangerous thing, and not a few observers around the world are convinced that this reprieve is the only thing saving the world from immediate annihilation by the fallout from the collider’s coming collisions. We’ll come back to why this concern is unfounded in a minute. First, let’s go over why it is that the world’s physicists are grumbling at the delay.
The last couple of decades have been something of a drought for elementary particle physics. The W and Z bosons — the particles that allow radioactive elements, like radium or plutonium, to decay — were found in the early 80s, and the second-to-last missing hole in the Standard Model of particle physics, the super-heavy top quark, was found at Fermilab way back in 1995, when Bill Clinton was in the Oval Office and eBay was a brand-new start-up: an eternity ago in the usually fast-paced world of theoretical physics. Accelerators have been running in the years since, but have had to content themselves with incremental improvements; and no one typically wins a Nobel Prize for adding one extra digit of precision to a measurement, no matter how consequential that digit appears to the doyens of the field.
So now, at the cost of upwards of US$10 billion, the Large Hadron Collider — or LHC for short — is coming on the scene. What’s different about this new machine that makes it possible, or even likely, that it will finally relieve the long drought?
The main thing all that money buys is power: the 14 million million electron-volts of energy that goes into each particle collision there is more than ten times as much as the previous record holder — Fermilab’s Tevatron — ever produced. What that extra factor of ten buys is three things: a final up or down determination of whether the Standard Model lives up to its name, a good gambler’s chance at opening up a brand new zoo of elementary particles, and a hairsbreadth possibility of demonstrating the existence of extra spatial dimensions.
The missing piece in the universe’s jigsaw puzzle
The Standard Model is complete but for one particle, the Higgs boson. But this missing link is vitally important one: according to this fantastically successful model — it has passed all previous experimental tests — the Higgs is the reason that all the other particles have mass. This may sound strange — after all, it seems common sense that particles should just have a mass by themselves, by virtue of being there. However, common sense is an uncommonly poor guide in the realm of quantum fields. Without the Higgs particle treating them each differently, all the particles we know and love would look much more similar.
The easiest way to picture its effect is as follows. Imagine a road laid down with hundreds of evenly spaced speed bumps on it. In a car with a very large wheel — a monster truck, say — those speed bumps won’t slow you down very much. However, if you were facing such a road in a Geo Metro, you’d have to go very slow indeed if you wanted to make it through. The Higgs field — for in the quantum world, every particle is really part of a space-pervading energy field — provides those speed bumps for all the other particles. Contrary to our driving intuition, though, the particles with the biggest “wheels” that are slowed down the least are the very lightest ones, like the electron. The ones most affected by the Higgs are the heaviest, like the top quark.
Beating the bushes
“Ok, ok,” you say, “I’m convinced: this Higgs thing is a big deal! But why do you expect the LHC to see it?” First of all, because the Higgs plays such a supporting role in the interactions between elementary particles, it’s a bit of a trick to flush it out into the open, all by itself. In fact, when it was first discussed theoretically, some people thought it would be far too heavy ever to be isolated. This is where all those relatively dull experiments from the last decade or so come in.
Although smaller accelerators, like Stanford’s Linear Accelerator, can’t make the Higgs directly, they can make a lot of the other particles in the Standard Model that we know are bumping into the Higgs a lot. By studying them very precisely, with hundreds of computers and reams of analysis, it turns out we can figure out a lot about the Higgs particle indirectly. This is where the first known surprise comes in: the mass of the Higgs particle that they guess from these detailed, careful experiments is so low that it should have already been discovered at the much-less-powerful Tevatron! So something fishy is going on. Some people are starting to worry that there’s no Higgs particle there at all — but there must be something there playing the same role, or all our calculations wouldn’t work out so nicely. The LHC is a machine built with the purpose of ferreting out what is going on here: if there’s a particle like the Higgs to be seen, it will see it.
As exciting as finding the Higgs would be, many scientists are really hoping for much more to come out of the LHC. The fishy business I mentioned above is part of a long series of clues present in the data we already have that something new and different is happening at the energy scale that will be seen by this new collider.
Part of the weirdness of the quantum world is this rule: anything that can happen, does happen, at some level of probability. In practice, this is encoded in the calculations physicists do by including in them every possible intermediate step between what goes in and what comes out. It’s when we start counting up these intermediate steps that things gets interesting.
Not just symmetry, but supersymmetry
Several calculations that work out in a slightly inelegant way using just the intermediate steps possible within the Standard Model work out perfectly if there are (at least) two times as many particles in the theory as the Standard Model has. That is, where there’s an electron in the known theory, we add a selectron; the photon gets a partner known as a photino. This doubling has a name: supersymmetry, or SUSY for short.
These doppelgangers of the known elementary particles are merely hypothetical for now; if they exist, they would have been hidden from us so far because they are all much heavier than the particles we already have, and Nature loves for heavy particles to decay into lighter ones. However, there’s an extra twist to how nicely dovetailed SUSY is to the current puzzles of physics as we understand it.
Detailing this dovetailing takes us far from Geneva to the world of astronomy and cosmology. For decades, there has been a mystery surrounding observations of spinning galaxies, like our own Milky Way: why do their outermost stars rotate as fast as stars in the middle? If you recall your astronomy lessons, you’ll remember that the outermost planets in the solar system move much more slowly than the inner ones do. Galaxies are in many ways very large versions of the same thing, so we might expect the outermost stars in galaxies to orbit the centers of the galaxies more slowly than those in the middle. In point of fact, they almost never do: the outer stars move every bit as fast as those in the middle. The only way to account for this is to assume that most of the mass in galaxies isn’t in the stars themselves, but in some hidden, or dark, form of matter that is spread out over lengths considerably larger than the orbits of the outermost stars. This would make a galaxy very different from the solar system, and would naturally explain the rotation rates.
But what in the world is this dark matter? This is where SUSY comes in. It turns out that the very lightest supersymmetric partner particle is always stable — ie, it doesn’t decay into other things. And it would be generated in very great numbers in the very early universe. In fact, in a remarkable numerical coincidence, the number of leftover particles one predicts, if one assumes SUSY, matches almost exactly the number needed to explain the galaxy data.
This fit is so beautiful that some cosmologists don’t even doubt that SUSY exists: their only question is whether the LHC will be able to get to high enough energies to prove it. That is an open question: we can calculate till our pens run dry, but we won’t know for sure until we go and have a look.
Speaking of having a look, there is one last possibility that’s the craziest of all: that the high energy collisions at the LHC will actually demonstrate the existence of more than three spatial dimensions. These extra dimensions would be curled up, like the plastic hooks on the grabbing side of velcro, with a size much tinier than we could ever test through normal means. But the collisions at the LHC constitute the most powerful microscope ever made, giving us a possibility of seeing these tiny new dimensions if they are there.
The clearest way this could show up is if we found, among the things that spray out of the LHC collisions, miniscule black holes — microscopic versions of the big bodies whose gravitational strength is so great that not even light can escape them. Without extra dimensions, the LHC has no hope of generating black holes: the force of gravity is far too weak in our known theories for this to happen. However, when there are extra dimensions around, gravity can get dramatically stronger when you look at length scales smaller than the size of those extra dimensions. Unlike their big astrophysical brethren, these mini-black holes would rapidly decay into other things, through a process first described by Stephen Hawking.
This last bit has never actually been tested, which makes some people nervous. However, there’s no cause for alarm: collisions as strong as those at the LHC happen on the moon all the time, and in the atmosphere nearly as often. This is because very fast moving cosmic rays steadily rain down on us from who knows where, some with energies far greater than even the LHC can achieve. Because none of these collisions has ended the Earth, we know that any tiny black holes made in that way are safe.
But is this all really worth it? I know this is the question behind some of the quizzical looks I get when I describe these things over the dinner-table at Thanksgiving, or from the curious border guard who decides to ask what I do for a living.
Is it worthwhile?
The main answer I have for this question is this. Man has ever wanted to understand the world he finds himself living in, and poetry, philosophy, art, and science are among the ways he has attempted to answer that question. And in a world that spends US$50 billion a year on movie tickets, $10 billion for a many-year search for some of the most fundamental facts that are knowable by humankind doesn’t strike me as too outrageous.
To unlock the Universe’s secrets is an ineradicable drive of humanity: the LHC is but another tool, albeit a big one, in this quest. But when the possibilities of what may learn are so fundamental, the need to know grows all the more voracious. How could we not want to know, once we know that we can?
Mark Wyman is a postdoctoral researcher in cosmology at the Perimeter Institute in Ontario.