How the Large Hadron Collider Works

Engineers install a giant magnet inside the Large Hadron Collider, an enormous particle accelerator.
Engineers install a giant magnet inside the Large Hadron Collider, an enormous particle accelerator.
Fabrice Coffrini/AFP/Getty Images

One hundred meters (or about 328 feet) underground, beneath the border between France and Switzerland, there's a circular machine that might reveal to us the secrets of the universe. Or, according to some people, it could destroy all life on Earth instead. One way or another, it's the world's largest machine and it will examine the universe's tiniest particles. It's the Large Hadron Collider (LHC).

The LHC is part of a project helmed by the European Organization for Nuclear Research, also known as CERN. The LHC joins CERN's accelerator complex outside of Geneva, Switzerland. Once it's switched on, the LHC will hurl beams of protons and ions at a velocity approaching the speed of light. The LHC will cause the beams to collide with each other, and then record the resulting events caused by the collision. Scientists hope that these events will tell us more about how the universe began and what it's made of.


The LHC is the most ambitious and powerful particle accelerator built to date. Thousands of scientists from hundreds of countries are working together -- and competing with one another -- to make new discoveries. Six sites along the LHC's circumference gather data for different experiments. Some of these experiments overlap, and scientists will be trying to be the first to uncover important new information.

The purpose of the Large Hadron Collider is to increase our knowledge about the universe. While the discoveries scientists will make could lead to practical applications down the road, that's not the reason hundreds of scientists and engineers built the LHC. It's a machine built to further our understanding. Considering the LHC costs billions of dollars and requires the cooperation of numerous countries, the absence of a practical application may be surprising.

­What do scientists hope to find by using the LHC? Keep reading to find out.



Constructing the Large Hadron Collider
Constructing the Large Hadron Collider
Fabrice Coffrini/AFP/Getty Images

In an attempt to understand our universe, including how it works and its actual structure, scientists proposed a theory called the standard model. This theory tries to define and explain the fundamental particles that make the universe what it is. It combines elements from Einstein's theory of relativity with quantum theory. It also deals with three of the four basic forces of the universe: strong nuclear force, weak nuclear force and electromagnetic force. It does not address the effects of gravity, the fourth fundamental force.

The Standard Model makes several predictions about the universe, many of which seem to be true according to various experiments. But there are other aspects of the model that remain unproven. One of those is a theoretical particle called the Higgs boson particle.

The Higgs boson particle may answer questions about mass. Why does matter have mass? Scientists have identified particles that have no mass, such as neutrinos. Why should one kind of particle have mass and another lack it? Scientists have proposed many ideas to explain the existence of mass. The simplest of these is the Higgs mechanism. This theory says that there may be a particle and a corresponding mediating force that would explain why some particles have mass. The theoretical particle has never been observed and may not even exist. Some scientists hope the events created by the LHC will also uncover evidence for the existence of the Higgs boson particle. Others hope that the events will provide hints of new information we haven't even considered yet.

A­nother question scientists have about matter deals with early conditions in the universe. During the earliest moments of the universe, matter and energy were coupled. Just after matter and energy separated, particles of matter and antimatter annihilated each other. If there had been an equal amount of matter and antimatter, the two kinds of particles would have canceled each other out. But fortunately for us, there was a bit more matter than antimatter in the universe. Scientists hope that they'll be able to observe antimatter during LHC events. That might help us understand why there was a miniscule difference in the amount of matter versus antimatter when the universe began.

Dark matter might also play an important role in LHC research. Our current understanding of the universe suggests that the matter we can observe only accounts for about 4 percent of all the matter that must exist. When we look at the movement of galaxies and other celestial bodies, we see that their motions suggest there's much more matter in the universe than we can detect. Scientists named this undetectable material dark matter. Together, observable matter and dark matter could account for about 25 percent of the universe. The other three-quarters would come from a force called dark energy, a hypothetical energy that contributes to the expansion of the universe. Scientists hope that their experiments will either provide further evidence for the existence of dark matter and dark energy or provide evidence that could support an alternate theory.

That's just the tip of the particle physics iceberg, though. There are even more exotic and counterintuitive things the LHC might turn up. Like what? Find out in the next section.

This building houses the research facility 100 meters above the Compact Muon Solenoid (CMS) detector.
This building houses the research facility 100 meters above the Compact Muon Solenoid (CMS) detector.
Johannes Simon/Getty Images

If theoretical particles, antimatter and dark energy aren't unusual enough, some scientists believe that the LHC could uncover evidence of other dimensions. We're used to living in a world of four dimensions -- three spatial dimensions and time. But some physicists theorize that there may be other dimensions we can't perceive. Some theories only make sense if there are several more dimensions in the universe. For example, one version of string theory requires the existence of no fewer than 11 dimensions.

String theorists hope the LHC will provide evidence to support their proposed model of the universe. String theory states that the fundamental building block of the universe isn't a particle, but a string. Strings can either be open ended or closed. They also can vibrate, similar to the way the strings on a guitar vibrate when plucked. Different vibrations make the strings appear to be different things. A string vibrating one way would appear as an electron. A different string vibrating another way would be a neutrino.

Some scientists have criticized string theory, saying that there's no evidence to support the theory itself. String theory incorporates gravity into the standard model -- something scientists can't do without an additional theory. It reconciles Einstein's theory of general relativity with the Quantum Field Theory. But there's still no proof these strings exist. They are far too small to observe and currently there's no way to test for them. That has lead to some scientists to dismiss string theory as more of a philosophy than a science.

­String theorists hope that the LHC will change critics' minds. They are looking for signs of supersymmetry. According to the standard model, every particle has an anti-particle. For example, the anti-particle for an electron (a particle with a negative charge) is a positron. Supersymmetry proposes that particles also have superpartners, which in turn have their own counterparts. That means every particle has three counter-particles. Although we've not seen any indication of these superpartners in nature, theorists hope that the LHC will prove they actually exist. Potentially, superparticles c­ould explain dark matter or help fit gravity into the overall standard model.


­How big is th­e LHC? How much power will it use? How much did it cost to build? Find out in the next section.

The magnet core of the Large Hadron Collider
The magnet core of the Large Hadron Collider
Fabrice Coffrini/AFP/Getty Images

The Large Hadron Collider is a massive and powerful machine. It consists of eight sectors. Each sector is an arc bounded on each end by a section called an insertion. The LHC's circumference measures 27 kilometers (16.8 miles) around. The accelerator tubes and collision chambers are 100 meters (328 feet) underground. Scientists and engineers can access the service tunnel the machinery sits in by descending in elevators and stairways located at several points along the circumference of the LHC. CERN is building structures above ground where scientists can collect and analyze the data LHC generates.

The LHC uses magnets to steer beams of protons as they travel at 99.99 percent the speed of light. The magnets are very large, many weighing several tons. There are about 9,600 magnets in the LHC. The magnets are cooled to a chilly 1.9 degrees Kelvin (-271.25 Celsius or -456.25 Fahrenheit). That's colder than the vacuum of outer space.

Speaking of vacuums, the proton beams inside the LHC travel through pipes in what CERN calls an "ultra-high vacuum." The reason for creating such a vacuum is to avoid introducing particles the protons could collide with before they reach the proper collision points. Even a single molecule of gas could cause an experiment to fail.

There are six areas along the circumference of the LHC where engineers will be able to perform experiments. Think of each area as if it were a microscope with a digital camera. Some of these microscopes are huge -- the ATLAS experiment is a device that is 45 meters (147.6 feet) long, 25 meters (82 feet) tall and weighs 7,000 tons (5,443 metric tons) [source: ATLAS].

An overview of the Large Hadron Collider experiments
An overview of the Large Hadron Collider experiments
Image courtesy CERN

The LHC and the experiments connected to it contain about 150 million sensors. Those sensors will collect data and send it to various computing systems. According to CERN, the amount of data collected during experiments will be about 700 megabytes per second (MB/s). On a yearly basis, this means the LHC will gather about 15 petabytes of data. A petabyte is a million gigabytes. That much data could fill 100,000 DVDs [source: CERN].

It takes a lot of energy to run the LHC. CERN estimates that the annual power consumption for the collider will be about 800,000 megawatt hours (MWh). It could have been much higher, but the facility will not operate during the winter months. According to CERN, the price for all this energy will be a cool 19 million Euros. That's almost $30 million per year in electricity bills for a facility that cost more than $6 billion to build [source: CERN]

­What exactly happens during an experiment? Keep reading to find out.

A model of the Large Hadron Collider in the CERN visitor's center in Geneva.
A model of the Large Hadron Collider in the CERN visitor's center in Geneva.
Johannes Simon/Getty Images

The principle behind the LHC is pretty simple. First, you fire two beams of particles along two pathways, one going clockwise and the other going counterclockwise. You accelerate both beams to near the speed of light. Then, you direct both beams toward each other and watch what happens.

The equipment necessary to achieve that goal is far more complex. The LHC is just one part of the overall CERN particle accelerator facility. Before any protons or ions enter the LHC, they've already gone through a series of steps.

­Let's take a look at the life of a proton as it goes through the LHC process. First, scientists must strip electrons from hydrogen atoms to produce protons. Then, the protons enter the LINAC2, a machine that fires beams of protons into an accelerator called the PS Booster. These machines use devices called radio frequency cavities to accelerate the protons. The cavities contain a radio-frequency electric field that pushes the proton beams to higher speeds. Giant magnets produce the magnetic fields necessary to keep the proton beams on track. In car terms, think of the radio frequency cavities as an accelerator and the magnets as a steering wheel

­Once a beam of protons reaches the right energy level, the PS Booster injects it into another accelerator called the Super Proton Synchotron (SPS). The beams continue to pick up speed. By now, beams have divided into bunches. Each bunch contains 1.1 x 1011 protons, and there are 2,808 bunches per beam [source: CERN]. The SPS injects beams into the LHC, with one beam traveling clockwise and the other going counterclockwise.

Inside the LHC, the beams continue to accelerate. This takes about 20 minutes. At top speed, the beams make 11,245 trips around the LHC every second. The two beams converge at one of the six detector sites positioned along the LHC. At that position, there will be 600 million collisions per second [source: CERN].

When two protons collide, they break apart into even smaller particles. That includes subatomic particles called quarks and a mitigating force called gluon. Quarks are very unstable and will decay in a fraction of a second. The detectors collect information by tracking the path of subatomic particles. Then the detectors send data to a grid of computer systems.

Not every proton will collide with another proton. Even with a machine as advanced as the LHC, it's impossible to direct beams of particles as small as protons so that every particle will collide with another one. Protons that fail to collide will continue in the beam to a beam dumping section. There, a section made of graphite will absorb the beam. The beam dumping sections are able to absorb beams if something goes wrong inside the LHC. To learn more about the mechanics behind particle accelerators, take a look at How Atom Smashers Work.

­The LHC has six detectors positioned along its circumference. What do these detectors do and how do they work? Find out in the next section.

Peter Higgs, the man for whom the Higgs boson particle was named, tours the LHC.
Peter Higgs, the man for whom the Higgs boson particle was named, tours the LHC.
Alan Walker/AFP/Getty Images

The six areas along the circumference of the LHC that will gather data and conduct experiments are simply known as detectors. Some of them will search for the same kind of information, though not in the same way. There are four major detector sites and two smaller ones.

­The ­detector known as A Toroidal LHC ApparatuS (ATLAS) is the largest of the bunch. It measures 46 meters (150.9 feet) long by 25 meters (82 feet) tall and 25 meters wide. At its core is a device called the inner tracker. The inner tracker detects and analyzes the momentum of particles passing through the ATLAS detector. Surrounding the inner tracker is a calorimeter. Calorimeters measure the energy of particles by absorbing them. Scientists can look at the path the particles took and extrapolate information about them.

The ATLAS detector also has a muon spectrometer. Muons are negatively charged particles 200 times heavier than electrons. Muons can travel through a calorimeter without stopping -- it's the only kind of particle that can do that. The spectrometer measures the momentum of each muon with charged particle sensors. These sensors can detect fluctuations in the ATLAS detector's magnetic field.

The Compact Muon Solenoid (CMS) is another large detector. Like the ATLAS detector, the CMS is a general-purpose detector that will detect and measure the subparticles released during collisions. The detector is inside in a giant solenoid magnet that can create a magnetic field nearly 100,000 times stronger than the Earth's magnetic field [source: CMS].

Then there's ALICE, which stands for A Large Ion Collider Experiment. Engineers designed ALICE to study collisions between ions of iron. By colliding iron ions at high energy, scientists hope to recreate conditions similar to those just after the big bang. They expect to see the ions break apart into a quark and gluon mixture. A main component of ALICE is the Time Projection Chamber (TPC), which will examine and reconstruct particle trajectories. Like the ATLAS and CMS detectors, ALICE also has a muon spectrometer.

Next is the Large Hadron Collider beauty (LHCb) detector site. The purpose of the LHCb is to search for evidence of antimatter. It does this by searching for a particle called the beauty quark. A series of sub-detectors surrounding the collision point stretch 20 meters (65.6 feet) in length. The detectors can move in tiny, precise ways to catch beauty quark particles, which are very unstable and rapidly decay.

The TOTal Elastic and diffractive cross section Measurement (TOTEM) experiment is one of the two smaller detectors in the LHC. It will measure the size of protons and the LHC's luminosity. In particle physics, luminosity refers to how precisely a particle accelerator produces collisions.

Finally, there's the Large Hadron Collider forward (LHCf) detector site. This experiment simulates cosmic rays within a controlled environment. The goal of the experiment is to help scientists come up with ways to devise wide-area experiments to study naturally occurring cosmic ray collisions.

Each detector site has a team of researchers ranging from a few dozen to more than a thousand scientists. In some cases, these scientists will be searching for the same information. For them, it's a race to make the next revolutionary discovery in physics.

How will scientists handle all the data these detectors will gather? More on that in the next section.

Angela Merkel, Chancellor of Germany, tours the LHC with a group of engineers.
Angela Merkel, Chancellor of Germany, tours the LHC with a group of engineers.
Jean-Pierre Clatot/AFP/Getty Images

With 15 petabytes of data (that's 15,000,000 gigabytes) gathered by the LHC detectors every year, scientists have an enormous task ahead of them. How do you process that much information? How do you know you're looking at something significant within such a large data set? Even using a supercomputer, processing that much information could take thousands of hours. Meanwhile, the LHC would continue accumulating even more data. 

CERN's solution to this problem is the LHC Computing Grid. The grid is a network of computers, each of which can analyze a chunk of data on its own. Once a computer completes its analysis, it can send the findings on to a centralized computer and accept a new chunk of data. As long as scientists can divide the data up into chunks, the system works well. Within the computer industry this approach is called grid computing.

The scientists at CERN decided to focus on using relatively inexpensive equipment to perform their calculations. Instead of purchasing cutting-edge data servers and processors, CERN concentrates on off-the-shelf hardware that can work well in a network. Their approach is very similar to the strategy Google employs. It's more cost efficient to purchase lots of average hardware than a few advanced pieces of equipment.

Using a special kind of software called midware, the network of computers will be able to store and analyze data for every experiment conducted at the LHC. The structure for the system is organized into tiers:

  • Tier 0 is CERN's computing system, which will first process information and divide it into chunks for the other tiers.
  • Twelve Tier 1 sites located in several countries will accept data from CERN over dedicated computer connections. These connections will be able to transmit data at 10 gigabytes per second. The Tier 1 sites will further process data and divide it up to send further down the grid.
  • More than 100 Tier 2 sites will connect with the Tier 1 sites. Most of these sites are universities or scientific institutions. Each site will have multiple computers available to process and analyze data. As each processing job completes, the sites will push data back up the tier system. The connection between Tier 1 and Tier 2 is a standard network connection.

­A­ny Tier 2 site can access any Tier 1 site. The reason for that is to allow research institutions and universities the chan­ce to focus on spe­cific information and research.­

One challenge with such a large network is data security. CERN determined that the network couldn't rely on firewalls because of the amount of data traffic on the system. Instead, the system relies on identification and authorization procedures to prevent unauthorized access to LHC data.

Some people say that worrying about data security is a moot point. That's because they think the LHC will end up destroying the entire world.

Is it really possible? Find out in the next section.

CERN engineers lower a large dipole magnet into the LHC tunnel.
CERN engineers lower a large dipole magnet into the LHC tunnel.
CERN/AFP/Getty Images

The LHC will allow scientists to observe particle collisions at an energy level far higher than any previous experiment. Some people worry that such powerful reactions could cause serious trouble for the Earth. In fact, a few people are so concerned that they filed a lawsuit against CERN in an attempt to delay the LHC's activation. In March 2008, former nuclear safety officer Walter Wagner and Luis Sancho spearheaded a lawsuit filed in Hawaii's U.S. District Court. They claim the LHC could potentially destroy the world [source: MSNBC].

What is the basis for their concerns? Could the LHC create something that could end all life as we know it? What exactly might happen?

One fear is that the LHC could produce black holes. Black holes are regions in which matter collapses into a point of infinite density. CERN scientists admit that the LHC could produce black holes, but they also say those black holes would be on a subatomic scale and would collapse almost instantly. In contrast, the black holes astronomers study result from an entire star collapsing in on itself. There's a big difference between the mass of a star and that of a proton.

Another concern is that the LHC will produce an exotic (and so far hypothetical) material called strangelets. One possible trait of strangelets is particularly worrisome. Cosmologists theorize that strangelets could possess a powerful gravitational field that might allow them to convert the entire planet into a lifeless hulk.

Scientists at LHC dismiss this concern using multiple counterpoints. First, they point out that strangelets are hypothetical. No one has observed such material in the universe. Second, they say that the electromagnetic field around such material would repel normal matter rather than change it into something else. Third, they say that even if such matter exists, it would be highly unstable and would decay almost instantaneously. Fourth, the scientists say that high-energy cosmic rays should produce such material naturally. Since the Earth is still around, they theorize that strangelets are a non-issue.

Another theoretical particle the LHC might generate is a magnetic monopole. Theorized by P.A.M. Dirac, a monopole is a particle that holds a single magnetic charge (north or south) instead of two. The concern Wagner and Sancho cited is that such particles could pull matter apart with their lopsided magnetic charges. CERN scientists disagree, saying that if monopoles exist, there's no reason to fear that such particles would cause such destruction. In fact, at least one team of researchers is actively looking for evidence of monopoles with the hopes that the LHC will produce some.

Other concerns about the LHC include fears of radiation and the fact that it will produce the highest energy collisions of particles on Earth. CERN states that the LHC is extremely safe, with thick shielding that includes 100 meters (328 feet) of earth on top of it. In addition, personnel are not allowed underground during experiments. As for the concern about collisions, scientists point out that high-energy cosmic ray collisions happen all the time in nature. Rays collide with the sun, moon and other planets, all of which are still around with no sign of harm. With the LHC, those collisions will happen within a controlled environment. Otherwise, there's really no difference.

Will the LHC succeed in furthering our knowledge about the universe? Will the data collected raise more questions than it answers? If past experiments are any indication, it's probably a safe bet to assume the answer to both of these questions is yes.

To learn more about the Large Hadron Collider, particle accelerators and related topics, accelerate over to the links on the next page.

Related HowStuffWorks Articles

More Great Links


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