Last Wednesday, September 10, after 14 years of preparation, scientists at the CERN laboratory switched on the Large Hadron Collider and the world didn't end. To untangle what exactly the LHC is and how it might (or might not) destroy the world, we turned to black hole and dark matter experts David Garfinkle and Richard Garfinkle, author-brothers of the forthcoming Three Steps to the Universe: From the Sun to Black Holes to the Mystery of Dark Matter. They urged calm and offered the following soothing words of wisdom:
Strange as it may sound, scientists are not actually willing to risk destroying the Earth just for a few experimental results. Most of them are fond of the place and would prefer that it still be there after they, as the monster movies say, throw the switch. Yet, somehow, many reports about the startup of the Large Hadron Collider (LHC) at CERN have included the dire warning that it may create a micro black hole which would eat up the entire world.
In medicine such a risk would be described as contra-indicated. The general reaction in the scientific community if such were really possible would be What are you, crazy?
It's frustrating that this has been a main focus of reporting. To be fair, a decent number of reports have treated this idea with humor, but they have done so without talking in depth about the real scientific purpose of the LHC. As a result, much of what anyone has heard is either, Black hole, we're doomed or Black hole, we're doomed. Yeah, right.
Neither story shows what's going on and why it matters. In large part that is because the LHC is a piece of equipment created to test certain theories. While it was many years in the making and most particle physicists have been eager for it to come online so they can find out if certain ideas are right, there is not, in it, any dramatic story. After all, testing the foundations of the universe is dull. Where's the romance? Where's the action?
Let's take a look. The LHC is the world's largest particle accelerator, a 17-mile-long circle of magnets that circulates two beams of protons in opposite directions at almost the speed of light and collides them with a combined energy of 14 trillion electron volts.
That speed and energy are necessary in order to perform experiments that have been on the drawing board for decades, but which were not possible with the punier equipment currently available (sorry, Fermilab). The first such experiment is to see if they can actually discover a particle called the Higgs boson. This is the last missing piece of the so-called standard model of particle physics. That's a pretty dull name for something which outlines all the fundamental components of matter and energy.
The standard model is one of the triumphs of twentieth century physics. It accounts for the strong and weak forces that hold atomic nuclei together, and for the electromagnetic force (which is responsible for almost all of the phenomena of daily life, from radio waves to chemistry), and for all the particles of which atoms are made, as well as several other particles that are so short-lived that they are found only in particle accelerators.
However, the standard model has the odd feature that it offers an explanation for a characteristic of matter which is so basic few people would wonder that it needs an explanation: Mass. Mass is sort of how much stuff there is in matter. But we tend to think of matter as stuff.
The standard model says that the mass of subatomic particles comes about because of an as-yet unobserved particle, the Higgs boson. To test the theory, Higgs bosons need to be produced in a way that can be detected. To produce a particle, an accelerator must have enough energy to make the mass of that particle, but as Einstein famously said, E=mc². Energy is equal to mass times the square of the speed of light. That c² term is pretty large. If you want to use energy to get matter, you need a lot of energy. The Higgs boson is sufficiently massive that up until now no particle accelerator has been powerful enough to produce it. However, hints from existing particle experiments indicate that the LHC will have enough energy to produce the Higgs.
Put more cautiously, if the standard model is not correct, we still know that particles have mass and that the presence of some new particle (or particles) is required to give them mass. Existing experiments indicate that the "mass giving particle" (or particles) can be produced at the LHC energy scale. So we expect the Higgs boson, or something like it, to be found at the LHC. In other words, if the Higgs boson or some other particle or particles that do the job of the Higgs boson can be produced they should be producible by the LHC.
Does that mean that if it can't produce them, then it's been a colossal waste of time, money, Swiss underground, and engineering effort? No. If they can't find them, that means the theory will need to be changed.
Once the Higgs boson is found, what else is left to find? Quite a lot, actually. It turns out that atoms make up only about 5% of the mass of the universe. The rest is dark matter (about 25%) and dark energy (about 70%). As the names imply, neither dark matter nor dark energy give off light. Both are known only through their gravitational effects. By a process of elimination, it has been shown that this dark matter cannot be made of any known particle. Thus some new particle remains to be found, and (by mass) there is about five times as much of this unknown stuff as there is of all the known particles put together.
Dark energy was found by applying the idea of using speed to measure gravity to the universe as a whole. By measuring the speeds of distant galaxies (and by accurately measuring the distance of those galaxies using exploding stars) astronomers were able to find the rate of the expansion of the universe both now and at earlier times. The result is that the rate of expansion of the universe is speeding up. The universe is accelerating. Since gravity is usually an attractive force, and since an attractive gravity would tend to slow down the expansion of the universe, the acceleration of the universe requires the presence of some very exotic substance whose gravity is repulsive rather than attractive. This exotic substance is called dark energy, and the measurements of the universe's expansion show that it makes up about 70% of the mass of the universe.
Unfortunately, the LHC is unlikely to tell us much about dark energy; but it could tell us a lot about dark matter. Since there is a lot of dark matter in the universe, one might be able to find it by making a dark matter detector and then just waiting for it to be hit by some dark matter particles. Indeed, several such dark matter detectors are in operation right now, though at this point they haven't had a definitive detection of dark matter. However, instead of just waiting for the dark matter, one could take a more active approach and try to produce the dark matter. Up until now, no particle accelerator has produced dark matter particles, which means that their mass is too large for those accelerators to produce. But since the LHC has more energy, it might well succeed in producing dark matter.
So where does the black hole idea come from?
At first the idea of black holes being produced at a particle accelerator seems reasonable. Black holes are made when a large enough energy is concentrated in a small enough space. Accelerators produce high energy particles and probe very small length scales. Maybe they could produce conditions extreme enough to form a black hole.
How much energy is needed? About 1,000,000,000,000,000 times the energy of the protons at the LHC. That's a big number! So why are people talking about black holes at the LHC? Well, maybe our current understanding of gravity isn't right. Maybe our three dimensions of space are just some three dimensional membrane in some higher dimensional space with all forces except gravity confined to this membrane, so that gravity is a very weak force except when we get to the right energy scale, and maybe that energy scale is the one we are about to probe with the LHC, and maybe the parameters of this model turn out just right so that when the LHC collides protons, black holes will form.
That's a huge pile of maybes. The point is that there is no experimental evidence to support any of these suppositions. If you pile supposition upon supposition with no evidence that any of it is correct, what you end up with is less likely than each member of a person's immediate family winning the lottery.
Even so, it can be argued that the risk is so vast (world destroyed, game over), that no one should even whisper about taking the gamble. We're talking about black holes. Everyone who has seen the right movies and TV shows knows that means (drum roll) the Earth is doomed.
Actually, no. Suppose all these dubious maybes turn out to be yeses, and black holes do form at the LHC. What happens then? In the 1970s Stephen Hawking did a calculation that showed that black holes have a temperature, and therefore give off radiation and eventually evaporate. The smaller the black hole, the higher the temperature, the faster the evaporation. If a black hole did form at the LHC, its temperature would be so large that it would completely evaporate in a tiny fraction of a second. Thus, far from being a world-swallowing monster, a black hole formed at the LHC would be an extremely ephemeral event. Frustratingly, from the perspective of those who believe in all those maybes, the event that proved them right might not even be noticed.
OK, but suppose, just suppose, that these farfetched theories of black hole formation at the LHC are right and Hawking's calculation of black hole evaporation is wrong. After all, we have never turned on an LHC before. Can we really be confident that no world-swallowing monster will be produced?
Yes. Switzerland is not the highest energy source near Earth. There are cosmic rays, high energy particles from outer space that are bombarding the Earth all the time. Some of them, not many, but some, have energies higher than those produced in the LHC. The Earth has been around for billions of years. In that time some of these high energy cosmic rays have already struck the Earth. Anything that the LHC will do has already happened many times and in many places on the Earth in its several-billion-year history. If there were any chance of CERN producing an Earth-swallowing black hole, cosmic rays would have done so already, and we wouldn't be here.
So nothing is going on in Switzerland except investigation into the fundamental nature of matter. Whatever the results, three things will be true: The experiments will be immensely valuable to particle physics. Particle physicists will be very busy interpreting the data. The Earth will not be destroyed by a black hole, and particle physicists—and everyone else—can go about their normal lives.
If, however, anyone is still worried about black holes at the LHC, we suggest periodically checking this website.
—David and Richard Garfinkle
Three Steps to the Universe: From the Sun to Black Holes to the Mystery of Dark Matter will be published in November.
In 1988, during a presidential campaign of yore, election coverage quoted Presidents Creating the Presidency coauthor Kathleen Hall Jamieson so frequently that the New York Times ran a story about it. "In every Presidential campaign," the Times noted, "a handful of people become 'hot sources' of information, quoted seemingly everywhere only to fade from view the day after the election."
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