The Large Hadron Collider
The Large Hadron Collider (or LHC) is the largest machine ever built by humans to study the fundamental particles and their interactions. More than 10,000 people all around the world are working on the LHC at CERN in Switzerland. These people have designed altogether the most complicated experiments, the fastest circuit, and the coolest machine, creating the hottest collisions in the world. The LHC started in September 2008 and will very probably change the way we see the Universe.
The Standard Model of particle physics describes the fundamental particles and their interactions. Although the model has been thoroughly tested and verified with data, it still contains some serious gaps.
One of the biggest problems for physics today is the origin of mass. What is mass? Why are some particles so light and others so heavy? Why do some particles not have any mass at all? At present, there are no answers to these questions. Theoretical physicists have some ideas, which are going to be tested with the LHC. They are hunting the Higgs boson, a particle that appears in the most likely theoretical mechanism we have to explain the origin of mass.
Another issue is understanding what the Universe is made of. Everything we can see in the Universe is made of ordinary particles, which we call matter. This matter is actually only 4% of the composition of the Universe. Now the following question arises: what is the invisible 96% made of? Physicists think it could be made of very weakly interacting particles, which could be produced in the collisions at LHC.
Finally, physicists want to understand why nature prefers matter to antimatter. Antimatter is identical to matter, only with the opposite electric charge. During the Big Bang, the birth of the Universe, equal amounts of matter and antimatter have been created. We know that when matter and antimatter meet, they annihilate each other. But today, as we live in a world made of matter, it seems that some matter has survived the annihilations.
The Large Hadron Collider is a circular particle accelerator, in which two beams of particles, called hadrons, travel in opposite directions and collide. Each collision in the LHC dissipates a huge amount of energy, converted into mass and, in this way, physicists are recreating the conditions just after the Big Bang. Physicists from all over the world are studying theses collisions using different detectors designed for the LHC.
Figure 1. The accelerator in the tunnel 100 metres underground. Image courtesy of CERN.
The LHC is the largest scientific machine ever built. It is 27 km long, and is located 100 metres underground near Geneva in Switzerland. The hadrons in the LHC travel at more than 99.99% the speed of light and will race around the LHC accelerator 11,245 times a second.
The conditions in the LHC are extraordinary. The temperature is -271.3°C, which is reached using 10,080 tonnes of liquid nitrogen and 60 tonnes of liquid helium. This temperature is lower than the temperature of the Universe. The LHC needs to be as empty as possible, to avoid interactions with gas molecules. The pressure inside the LHC is 10 atm, ten times less than the pressure on the moon. Each collision dissipates a lot of energy, creating matter and particles. The temperature right after the collision is more than 100,000 times higher than the heart of the sun.
To realise the physics program of the LHC, physicists have designed several experiments. These experiments will study in detail the collisions at the LHC. This will generate 600 million collisions per second. Recording the data produced in the LHC would require 100,000 dual layer DVDs every year. Tens of thousands of computers distributed all around the world will be used and connected to each other.
The main LHC experiments take place in four huge underground caverns located around the LHC. The two largest detectors are called ATLAS and CMS. They are general-purpose detectors and are designed to investigate the widest range of physics possible. A third detector, called LHCb, is dedicated to the understanding of the difference between matter and antimatter. The fourth one, called ALICE, is dedicated to heavy ion collisions.
Figure 2. The detectors ATLAS (top left), CMS (top right), ALICE (bottom right), and LHCb (bottom left). Image courtesy of CERN.
Despite the huge amount of enthusiasm surrounding the LHC, some fears surfaced few years ago; this phenomenon was amplified when newspapers spoke about LHC and possible dangers a few months before the start of the LHC. One of the most important fears was the potential creation of black holes in the LHC’s collisions that could destroy the Earth. Some theories predict the creation of micro black holes at the LHC. This fear built up over the months and some instances of legal action were initiated. In the region around Geneva a number of people sold their houses and decided to move away from the LHC.
Even if it were possible for black holes to be created at the LHC, they will not be dangerous for many reasons. Firstly, collisions with energy much higher than the ones that are going to occur at the LHC have been naturally produced in the Earth’s atmosphere for billions of years and, yet, the Earth is still there. Secondly, to be stable a black hole needs to be six times as heavy as the mass of the sun, which obviously can’t be reached at the LHC.
Another fear was the creation of strangelets, particles that could change the nature of the matter of which everything is composed. If strangelets were to be produced at LHC they would be so heavy that they would decay before they could interact with anything. There were other more or less serious fears, like that of a possible disturbance of the space–time continuum that might permit aliens to invade Earth…
The most interesting aspect is that, no matter the scientific proof, some people do not trust scientists, and that the phantasm of the Earth’s destruction spoke so strongly to people’s imaginations that most people really were worried. This fear was very much amplified by newspaper reports about the imminent demise of the Earth.