A few weeks ago I attended a conference on stars with extremely high magnetic fields in Geneva and while I was there I took the opportunity to visit the European Organization for Nuclear Research (CERN). People who just turn up at its visitor centre in Meyrin can experience the Microcosm - a museum-type exhibition showing both the history of CERN and how everything works - and the Globe - which houses a number of historic items, such as the computer on which the world-wide web was invented, and shows a short film Universe of Particles twice an hour. If you book in advance, you can also go on a guided tour where you get to watch a film about the history of CERN and then visit the control room of ATLAS, a detector on the Large Hadron Collider (LHC), where the discovery of the Higgs Boson was announced. Unfortunately visitors are not able to go underground at the moment and instead can watch a short 3D film showing the Collider being built and in use. All of these things are free to the general public and whilst I did not get to see any science in action, it was a privilege to visit a place that has such an important role in the history and future of science and I would highly recommend a visit if you are ever in the area.
Life before CERN
Before construction begun on CERN in 1954, the atom was known to be composed of electrons (an elementary particle and a type of lepton) and a nucleus containing neutrons and protons (which are hadrons, particles now known to be made of smaller particles called quarks and gluons) and these were all thought to have an anti-matter partner. Fusion and fission reactions had taken place and new particles such as muons (another elementary particle and type of lepton) and pions and kaons (which are also hadrons) had been discovered in cosmic rays - a general name for different types of high energy particles originating from outer space - using particle detectors like cloud chambers and bubble chambers.
The cloud chamber and bubble chamber
The cloud chamber was invented by Scottish physicist Charles Thomson Rees Wilson in 1911 and he received the Nobel Prize in Physics for his invention and help in its development in 1927.
The cloud chamber was made somewhat obsolete in 1952 when American physicist Donald Glaser invented the bubble chamber, for which he was awarded the 1960 Nobel Prize in Physics. Bubble chambers work in the same way as cloud chambers but here energetic charged particles travel through a superheated liquid - a liquid heated by changing pressure - such as liquid hydrogen, instead of a cold gas and the liquid begins to boil around the ionised atoms which form along the path of the charged particle.
In order to for a particle to be detected it must be energetic enough. Cosmic rays are extremely energetic before they enter the atmosphere - reaching energies of up to a hundred billion, billion electron volts (10^20 eV) which is 16 Joules (16 J) - but they could only enter chambers that were taken to the top of mountains or above the atmosphere in balloons and they could not be controlled.
Early particle accelerators
A battery is the simplest particle accelerator. This works because the small voltage between its terminals (produced from having a negative end and a positive end) produces a proportional electric field. A charged particle - an electron - is accelerated in this field and can then travel down a wire.
This is where the unit of the electron volt comes from; it is the unit of energy gained by one electron accelerated by a voltage of 1 volt and is a useful unit to use when the number would be incredibly small if measured in Joules.
Linear accelerators
The first linear particle accelerator was built by Norwegian physicist Rolf Wideroe in 1928. This increases the velocity of charged particles by subjecting them to a series of alternating voltages. Like in a single battery, the particle is accelerated across the gap between differing voltages, but here they meet another gap and travel across this. More and more can be added making the particle travel faster and faster. The longer the accelerator, the faster the particle can travel. Particles were fired at a fixed target and the aftermath could be recorded in cloud or bubble chambers.
Cyclotrons
In 1932 another type of particle accelerator, the cyclotron, was invented by American physicist Ernest Lawrence and his graduate student Milton Stanley Livingston. A cyclotron places charged particles in an alternating electric field, which causes them to accelerate between D shaped electrodes, known as 'dees'. A uniform magnetic field was then placed perpendicular to this. The magnetic field causes the charged particles to move in a spiral. This is because the magnetic field produces a force which is always 90 degrees to the particles velocity, causing it to continually change direction, making it move in a circle. The radius of the circle increases as the particles get faster - due to the electric field - creating a spiral.
Higher velocities are reached the higher the magnetic field and the larger the radius of the cyclotron. Lawrence and Livingston's first cyclotron was about 30 cm in diameter with a field of about half a Tesla and accelerated protons to just over 1 MeV.
Cyclotrons only work up to about 20 MeV and do not take relativistic effects into account, these cause particles to become more difficult to accelerate as they approach the speed of light. This becomes more of a problem the lower the rest mass of the particle. Electrons have particularly small rest masses and so could not be accelerated by cyclotrons.
Betatrons
These problems were resolved with the invention of the betatron by American physicist Donald Kerst in 1940. This is like the cyclotron but accelerates electrons with a varying magnetic field. The field is stronger at larger radii and so the electrons are accelerated with a stronger force the more they approach the speed of light. As with the cyclotron, increased speeds could be achieved with larger magnetic fields and larger radii.
Synchrocyclotrons
Another type of improved cyclotron, the synchrocyclotron, was developed by American physicist Edwin McMillan in 1945. This is the same as a cyclotron but only has one 'D' shaped electrode and compensates for relativistic effects by changing the frequency of the electric field instead of keeping it constant.
Synchrotrons
The synchrocyclotron was soon surpassed by the synchrotron which was theorised by Soviet physicist Vladimir Veksler in 1944 and was first constructed by McMillan in 1945, the same year as the synchrocyclotron. Here particles are accelerated by cavities which provide an alternating electric field and move in a circle because of magnetic fields that increase in strength as the particle gets faster. The magnets are placed in the path of the accelerated particles rather than across the whole devise and so synchrotrons can be built with larger radii.
When charged particles are forced to travel in a circle by a magnetic field they emit photons with an energy that is related to the strength of the field and the speed of the particle. The photons emitted by synchrotrons can be used medicine, X-rays and ultra-violet radiation, for example, can be used to treat skin cancer.
CERN, the European Organization for Nuclear Research
CERN was first envisioned by French engineer Raoul Dautry, French physicists Pierre Auger and Lew Kowarski , Italian physicist Edoardo Amaldi and Danish physicist Niels Bohr in 1949.
They wished to create a laboratory to study atomic physics with particle accelerators that would be so large and expensive that they could not be built by a single county alone. The first official proposal was put forward by French physicist Louis de Broglie in December of that year.
In June of 1950 American physicist Isidor Rabi asked the United Nations Educational, Scientific and Cultural Organization (UNESCO) for assistance in the creation of the laboratory in order to encourage the collaboration of scientists from across Europe. Rabi was born in Galicia, which is situated on the border between Poland and the Ukraine and had won the 1944 Nobel Prize in Physics for his work on the magnetic moment and nuclear spin of atoms.
By December of the following year members of UNESCO adopted the first resolution to establish a European Council for Nuclear Research (or Conseil Européen pour la Recherche Nucleaire in French, which is where the acronym CERN came from).
This council came into effect in February 1952 when 11 counties agreed to participate in this council. These were; Belgium, Denmark, France, the Federal Republic of Germany, Greece, Italy, the Netherlands, Norway, Sweden, Switzerland and Yugoslavia. Geneva was selected as a site for the laboratory and the United Kingdom joined the council the following year. Construction began in May 1954 and by September of that year the Council officially became an Organisation.
CERN's first accelerator - the Synchrocyclotron (SC) - was turned on in 1957 and remained in operation for the next 33 years.
The highest energy accelerator in the world
A second accelerator, the Proton Synchrotron (PS) was turned on in 1959 and is still in operation, feeding particles to newer, higher powered accelerators. Reaching energies as high as 28 billion electron Volts (28 GeV) it became the highest energy accelerator in the world - beating the SCs 600 million electron Volts (600 MeV) and Russia's synchrotron, the Synchrophasotron, which had reached 10 GeV. The following year it was surpassed by the Alternating Gradient Synchrotron (AGS) at the Brookhaven National Laboratory in New York which reached energies of 33 GeV.
The discovery of anti-nuclei
In 1965 anti-nuclei were simultaneously created from anti-neutrons and anti-protons in experiments using the PS at CERN and the AGS in New York. Anti-matter had first been proposed in the 1920s with the first anti-particle, the anti-electron (known as the positron), discovered in the 1930s and the anti-proton and anti-neutron discovered in the 1950s. Physicists at CERN would go on to create entire atoms out of anti-matter.
The first computerised detections
In 1968 French physicist Georges Charpak developed the multi-wire proportional chamber, or wire chamber for short, whilst working at CERN. This soon replaced bubble chambers because it could detect particles more quickly and could be linked to a computer so that data did not need to be physically examined in the same way that photographs from bubble chambers were. Charpak was awarded the Nobel Prize in Physics for his invention in 1992.
Charpak's invention involved using a proportional counter. Proportional counters were invented by British physicist Samuel Curran in 1948 and combine a Geiger Müller tube - the sensor used in Geiger counters which were developed in the 1920s - with an ionisation chamber - which measures the charge of ions created by high energy charged particles and were developed in the 1800s.
The first hadron collider
By the 1950s physicists had realised that they would be able to create higher energy collisions if they fired two moving targets at each other instead of firing accelerated particles at a fixed target. The first collider accelerator was developed by Austrian-Italian physicist Bruno Touschek for the National Institute of Nuclear Physics in Italy and was turned on in 1961. This collided electrons and positrons (leptons), positrons have the same energy as electrons but they are positively charged. This means that they travel in the opposite direction to elections when placed in a particle accelerator and so electrons and positrons can be placed in the same tube.
The beams of particles must be lined up very precisely in order for them to collide. This is easier to do in lepton collisions since leptons are elementary particles, all of which have the same energy. Collisions between hadrons are less precise because they are made of smaller particles - quarks - and the total energy is shared between them, with some having more energy than others. Hadron collisions may be more difficult but they can collide at a wide range of energies and so are more useful than leptons for discovering new particles.
Physicists at CERN suggested that protons (which are hadrons) could be made to collide by using the PS to feed two rings - known as the Intersecting Storage Rings (ISR) - where beams of protons could be fired in different directions. This project was approved in 1965, construction on the ISR began the following year and it became operational in January of 1971, becoming the first hadron collider in the world.
The following month the Super Proton Synchrotron (SPS) - a synchrotron collider - was commissioned, this was to be CERN's largest accelerator yet built about 40 meters below the ground with a circumference of 7 km. The SPS crossed the border into France becoming the first accelerator to cross an international border. It became operational in 1976, operating at energies of hundreds of GeV.
The discovery of neutral currents
All of the detectors discussed so far detect charged particles and cannot be used to detect neutral particles like neutrons or neutrinos. Neutrinos are elementary particles and a type of lepton, they were first theorised in the 1930s and detected in the 1940s.
Physicists at CERN first suggested creating a bubble chamber that could detect neutral particles, known as Gargamelle, in 1964 and Gargamelle became operational in 1970. It was attached to the PS until 1976 when it was moved to the SPS.
Gargamelle first showed evidence of neutral currents via the detection of neutrinos in 1973. The discovery of W and Z bosons was not possible, however, until they could be produced in powerful collisions.
The discovery of the W and Z bosons
The SPS was converted into a proton-antiproton collider in 1979, the first proton-antiproton collisions occurred in 1981 and in 1983 W and Z bosons were discovered in these collisions. Italian physicist Carlo Rubbia and Dutch physicist Simon van der Meer were awarded the 1984 Nobel Prize in Physics for their role in this discovery.
Now that W and Z bosons had been discovered using a hadron collider, more precise collisions using a lepton collider were needed in order to determine their mass. The Large Electron-Positron Collider (LEP) was first proposed in order to measure the mass of the Z boson produced in collisions between electrons and positrons.
The LEP would be the largest lepton collider in the world, located up to 100 meters below the ground with a circumference of 27 km. It had been approved in 1981 and became operational in 1989 at energies of about 100 GeV. Millions of Z bosons were produced and it was shown that they produced only three generations of particles of matter. W bosons required more energy and these were not produced until the 1990s when the LEP was improved with more cavities added so that the collisions doubled in energy.
The development of the World Wide Web
The world's first website and server - Info.cern.ch - went public in 1991, before this invention the internet - which was developed in the late 1960s and introduced to the public in 1969 - was mainly used by scientists to send information to each other in plan black and white text. It was not possible to send the vast amounts of complex data that were dealt with at CERN.
By 1990 British computer scientist Tim Berners-Lee had developed the URL, http, html and the first browser and server software, while working at CERN. The world's first web page http://info.cern.ch/hypertext/WWW/TheProject.html gave information on how others could create their own websites and search the web for information. In 1993 CERN made the World Wide Web free for anyone who wanted to use it.
Creation of the first anti-atoms
The Low Energy Anti-Proton Ring (LEAR) was constructed at CERN in 1982 in order to store antimatter which would be used to create antihydrogen atoms in the PS and nine atoms of antihydrogen were created in collisions between antiprotons and xenon atoms in 1995.
LEAR was shut down in 1996 and replaced with the lower energy Antiproton Decelerator (AD) which was approved in 1997 and became operational in 2000. In 2011 the ALPHA experiment used the AD to trap 300 antihydrogen atoms and study them in detail for over 16 minutes.
The creation of quark-gluon plasma
Quark-gluon plasma is a state of matter which occurred in the very early universe when quarks and gluons existed as single particles rather than within atoms like they do now. This state can be re-created by increasing the temperature or density of hadrons. Physicists at CERN first tried to create this plasma in 1986 by colliding heavy nuclei - nuclei containing many neutrons and protons - in the SPS. They hoped that in doing this the quarks and gluons that make up protons and neutrons would separate. At first oxygen and sulphur nuclei were used, heavier lead nuclei were used in experiments first conducted in 1994 and by 2000 they were able to prove that they had created a quark-gluon plasma.
The Large Hadron Collider and the Higgs boson
Even before the LEP became operational in 1989, physicists considered how they could create higher energy collisions if it were converted to a hadron collider. Experiments which could be conducted in such a machine were first considered in 1984, although the construction of the Large Hadron Collider (LHC) was not approved until 1994. It was initially planned to be developed in two stages but donations from non-European counties such as Japan, India, Russia, the United States and Canada meant that it was completed in one, becoming operational in 2009.
The PS and SPS are used to accelerate particles before they are injected into the LHC. The first attempt to circulate a beam of protons was conducted in 2008 but it failed due to a faulty magnet connection and spent the next year being repaired.
In November 2009 two beams successfully circulated the LHC and by March of 2010 physicists were able to make two beams collide with an energy of 7 trillion eV (7 TeV), beating Fermilab's Tevatron which reached about 2 KeV and making the LHC the highest energy accelerator in the world. The LHC produced so much data that it took years to analyse.
The LHC was designed to run a number of experiments, all with their own detectors. These detectors are much more complicated than bubble or wire chambers and are much larger, the largest, ATLAS, is the size of a 5 story building.
In 2012 it was shown that the Higgs boson had been created in the LHC and detected by both CMS and ATLAS. This was confirmed in 2013.
The ALICE detector
The ALICE experiment was designed to study the quark-gluon plasma which existed in the early universe and had been re-created in the SPS in the 1990s. Unlike ATLAS and the CMS, which measure the effects of collisions between protons, ALICE was designed to study the collisions of iron nuclei. It has a tracking systems and muon detectors but unlike ATLAS and the CMS its main detector is a time projection chamber. This is a particle detector similar to a wire chamber.
The LHCb detector
The LHCb detector was designed to detect antimatter, particularly the anti-beauty quark (which is another name for the anti-bottom quark), so that physicist can study CP violation. Instead of surrounding the entire collision point it uses a series of sub-detectors to detect particles thrown forwards in the collision.
Other experiments
The three other experiments at the LHC are the Monopole and Exotics Detector At the LHC (MoEDAL) experiment, the TOTal Elastic and diffractive cross section Measurement (TOTEM) experiment and the Large Hadron Collider forward (LHCf) experiment. MoEDAL has been designed to detect magnetic monopoles. TOTEM has been designed to measures the size of the proton with unprecedented precision and the LHCf was designed to measure the energy of neutral pions and explain the origin of the highest energy cosmic rays.
The future of CERN
CERN currently has 20 member states, though Israel, the Republic of Serbia and Romania are in the process of becoming members, and has co-operation agreements, scientific contacts and observers from 59 counties around the world.
Earlier this year the LHC was shut down so that it could be upgraded and it is expected to become operational again in 2015 when it should be able to achieve energies of 14 TeV. This will allow physicists to explore states of matter that haven't existed since just after the big bang, to test theories of quantum gravity and hopefully to detect even more elusive particles than the Higgs boson, such as those responsible for dark matter.
Possibilities for future accelerators include the Very Large Hadron Collider, which could have a circumference of over 200 km and reach energies of about 30 TeV, although this is not being seriously considered at the moment.
Another possibility is building an accelerator to collide muons and antimuons. Muons are about 200 times as massive as electrons but they are unstable and soon decay into electrons and neutrinos so there are still many technical difficulties with this at the moment.
A more likely option is building a more powerful linear accelerator. CERN is currently considering a proposal for a Compact Linear Collider (CLIC) to accelerate electrons and positrons at energies of up to 3 TeV, which would make it the highest energy lepton collider in the world.