Talk – 11th January, 1995 by S.Oliver
CERN, the European Laboratory for Particle Physics, straddles the French-Swiss border just west of the city of Geneva. Here the Worlds largest particle accelerators are housed in tunnels up to 27 km in circumference and 100m below the surface. In the LEP accelerator particles are accelerated up to very high energy and collide head on with other particles. The energy that is produced in these collisions is so high that conditions equivalent to those existing 1000th of a millionth of a second after the creation of the Universe, 15,000,000,000 years ago, are produced.
This talk will briefly explain the experimental evidence for the Big Bang theory, the fundamental forces and particles of which the Universe is made up of, and describe the giant accelerating machines at CERN.
This afternoon I am gong to talk about CERN, “The European Organisation for Nuclear Research”. Nuclear is a dirty word nowadays so it is now called “The European Laboratory for Particle Physics”. CERN does pure physics research using large particle accelerators to try to understand the make up of the universe. CERN is an acronym for “Conseil European pour la Recherche Nucleaire”. It came into being in 1952 but was only fully inaugurated in 1954 ( so that last year CERN celebrated its 40th anniversary).
S1 Jet d’eau with particles
This slide symbolises the research at CERN where we see the Jet d’eau (water spout driven by 2000HP) at Geneva, on top represents what matter is made of: a molecule, made up the nucleus surrounded by electrons. The nucleus is made up of particles which are themselves made up of Quarks the fundamental building blocks of matter.
Before going into the details of CERN I will try to explain what CERN is trying to do with a brief description of our present understanding of the universe.
Powers of Ten
S2 Powers of Ten
This slide shows how the universe covers the infinitely small to the infinitely large. Man can study with his eyes, objects the size from millimetres to kilometres. With the aid of simple telescopes he can study the planets of our solar system, with more powerful telescopes he can study the nearer galaxies and with the most powerful telescopes on earth and those on satellites, e.g. the Hubble space telescope, he can look far into the universe. As you know the velocity of light is finite at approximately 3.108 m/s and we shall see later that the age of the Universe is about 15,000,000000 years, therefore light, travelling at a velocity of 3.108 m/s for 15 thousand million years has come from a distance of 1026 m. This puts an upper limit to the distance we can see.
Going in the opposite sense, with an optical microscope we can study down to the size of molecules as we are limited by the wavelength of visible light, around 10-8 m. With the electron microscope we can see individual atoms at 10-10 m.
(the wavelength of 100 keV electrons is 4.10-12 m, 105 smaller than visible light).
Thereafter, to study smaller items e.g.the nucleus of the atom, protons and what they are made of, we come to the world of the Accelerator. There particles are accelerated to high energy and collide with other particles to produce new particles.
The Big Bang Theory
S2A Big Bang Theory Red shift – Hubble, Temperature now 2.7 deg K
No contraction under gravitational force
One of the fundamental hinge points of this theory is that the Universe is expanding. In 1929 Edwin Hubble concluded that the universe is expanding from the “observed red shift”. You are familiar with the Doppler effect e.g. the whistle of a train passing through a station appears higher in frequency as it approaches and lower after it has passed, well the same applies to light. It was observed that the spectrum of light from distant stars was shifted towards the red i.e. the lower frequency end of the spectrum, therefore the stars must be receding from the observer. This observation was repeated for many galaxies and it was found that they are rushing away at velocities proportional to their distance from the Milky Way. Many are receding so fast that their radiation is no longer visible but can be detected in the microwave band. This is the reason that the majority of the sky at night appears black, our eyes can not detect the radiation from the most distant galaxies because their radiation is lower than the visible wave band. By playing backwards these observations, it was deduced that the universe originated from a point 15,000,000,000 years ago.
The average temperature of the Universe now has been estimated from the microwave radiation that fills the Universe to be just 2.7 degrees Kelvin – pretty cold. The Kelvin scale starts from absolute zero which is about -273 degrees Centigrade/Celcius. As the Universe expanded from the Big Bang so it cooled. The temperature at creation is estimated at greater than 10 32 degrees Kelvin .
Another way to look at it, is that a static universe would collapse into a single point under the laws of gravity, therefore it must be expanding at a rate that will allow it to go on for ever.
S2B Universe created 15,000,000,000 years ago, Temperature at Creation 10 32 degrees Kelvin, Temperature now 2.7 degrees Kelvin
S3 The Big Bang
This slide depicts the history of the Universe from the Big Bang with a temperature greater than 1032 degrees K to the present day where the average temperature of the Universe is just below 3 degrees K. As you can see, initially, things took place rapidly but it took 1000 years before stable atoms eg Hydrogen and Helium were formed. The particles existing just after creation were very short lived, existing for fractions of a second, 10-12 s. It is this era that we can recreate in the accelerators at CERN. At present we can recreate the conditions that existed 10-9 s after the Big Bang, the era when the W and Z bosons were formed which we shall hear more of later. With the next generation accelerators we shall be able to go back to 10-10 s after the Big Bang.
In 1905 Albert Einstein published his famous equation E=mc2.
The velocity of light has been measured to be constant and is about 3.108 m/s. This equation tells us that 25 Gw-hr of power weighs 1 gm.
S3B E=mc2, 25 Gw-hr of power weighs 1 gm.etc
It is apparent that a large amount of energy is required to produce a small amount of mass. This conversion of energy to mass is utilised in the particle accelerators at CERN to produce new particles; those that existed just after the creation of the universe. It is nothing to do with Nuclear Bombs.
This conversion of mass to energy also gives us the atomic bomb and the nuclear reactors of power stations. This is nuclear fission, the splitting of the nucleus of an atom into smaller parts and the release of energy.
S3C Fision, U + n = U’ +n + energy, Fusion, 4H = He + energy
The Nuclear fusion people are trying to copy the process of the Sun where nuclei of Hydrogen smash together to form Helium with the release of enormous amounts energy.
One day perhaps here on earth we shall be able to perform this process in a controlled way to produce cheap power for ever. As we saw on the previous slide: 1 kg of iron contains enough energy to supply the United Kingdoms power requirements for 1 week.
Another result of Einstein’s theory was that nothing can go faster than the speed of light and that as things approach the speed of light so their mass increases.
This is important in the functioning of accelerators.
S4 Table of forces showing transmission of a force by a particle
This slide demonstrates how forces can be transmitted by the exchange of particles, the same analogy applies to all forces.
There are 4 forces in nature:
The Strong force which holds the Quarks together in the nucleus of the atom. Here Gluons are the exchange particles.
The Electromagnetic force, which you are all familiar with in Electricity and Magnetism where photons are the exchange particles.
The Weak force which holds atoms together, where Bosons are the exchange particles.
and the Gravitational force, again a force that you are all familiar with. Here Gravitons are postulated to be the exchange particles. As you can see from this table of relative strengths of the fundamental forces of nature, Gravity is an extremely weak force, making it very difficult to measure. To observe an effect, very large masses are needed – the size of planets. Gravity is the most known force but the least understood. So far gravitational waves have not been detected in the Universe so the postulate of the graviton has not been confirmed.
One of the objects of science is to unify the above forces into a theory which describes them with one set of formulae.
S5 Unification of the Forces
In 1831 Michael Faraday demonstrated that a moving magnetic field could produce an electric current, laying the foundations for modern electrical engineering. Following on from this, in 1864, James Clerk Maxwell formulated a set of 4 elegant equations describing the whole of electricity and magnetism thus unifying the electric and magnetic forces. The carrier of the electromagnetic force is the photon.
Until the 1930’s just two fundamental forces of nature were known;
With the discovery of radioactivity and in 1932 “splitting of the atom” by John Cockcroft and Ernest Walton two new forces were postulated:
[using their famous diode capacitor ladder network which produced 800 kV to accelerate protons (ripped out of Hydrogen gas) to bombard a Lithium target and produce Helium]
The Weak force which holds atoms together
The Strong force which holds quarks together to form the nucleus
In 1979 Sheldon Glashow, Abdus Salaam and Steven Weinberg received the Nobel Prize for Physics for their theoretical work in unifying electromagnetism with the weak nuclear force now called the electroweak force. It had to wait until 1973 at CERN with the discovery of neutral currents. Up until 1973 the Americans were the world leaders in this branch of physics but from then on CERN has never looked back
In 1983 at CERN, the W and Z particles were confirmed to exist. These are the exchange particles, Bosons, for the weak force. The Nobel Prize for Physics was awarded for this discovery thus associating the Weak force with the Electromagnetic force . So we are still left with 3 forces to unify: Gravity, Eletroweak ( electricity, magnetism and the weak force) and the Strong (which holds the nucleus together).
S6 Einstein’s last dream
Einstein’s dream was to unify all of Physics and produce one simple theory of everything. He was still working on this when he died. His quest has carried on and more unification has taken place.
S7 Table of particles
This slide lists the fundamental particles. On the left we have matter, made up of atoms, which have a nucleus with electrons around the outside. There are in fact 6 electron type particles residing under the common name of Leptons. The Electron is the smallest particle that can travel alone. The Nucleus is made up of protons and neutrons which in turn are made up of Quarks. In the giant atom smashers at CERN and elsewhere, particles have been reduced to their constituent parts until finally we are left with 6 quarks that has made up all matter in the Universe. All ordinary matter is made up of: the Electron, the Electron Neutrino, the Up quark and the Down quark. The other particles existed in the early moments after the Big Bang and now they are only found in accelerators and cosmic rays. So far the Top quark has not yet been discovered, it is hoped to find this in the next generation of accelerators and is one of the main reasons for constructing the Large Hadron Collider which we shall cover later.
To summarise our present understanding of the Universe.
S7A The Big Bang- History of the Universe
The Universe was created 15,000,000,000 years ago from a single point as a Big Bang.
S8 Summary slide of particles and forces
All matter is made up from the Up quark, the Down quark, the Electron neutrino and the Electron. The Top quark has not yet been discovered.
There are three main forces: Gravity, Electroweak and Strong and it is hoped to unify these forces one day into a single force.
Nearly everyone has a linear particle accelerator in their home, the cathode ray tube in your television set.
S8A schematic of TV tube
In this electrons are emitted from a heated cathode and accelerated by the anode voltage – typically 30 kV- this accelerates the electrons to a high velocity/energy which is released as photons when it hits the phosphor on the inside of the tube. The electrons are guided by a magnetic field to produce a picture on the screen. The tube is evacuated as the electrons would be stopped by any molecules of gas left in the tube.
The standard of energy measurement is the electron volt which is the energy attained by one electron passing across a potential difference of one volt. The energy of the electron beam in the above example is defined as thirty thousand electrron volts or 30 keV and are actually travelling at one third of the velocity of light. The same units are used in accelerators although of course they are much more powerful, usually in the hundred thousand million electron volt range, 100.109 eV or 100 GeV. Although this sounds a lot of energy, a pencil falling onto a table has an energy of 100 GeV, but in an accelerator this energy is concentrated into a point smaller than an atom.
S8B schematic of LEP accelerator with RF cavities and magnets
In a circular accelerator, particles are constrained into a ring by a magnetic field produced by the white bending magnets. The blue quadrupole magnets act as lenses to keep the beam small and focussed. On each revolution energy is given to the particles as they cross an electric field provided by the brown Radio Frequency cavities. As the velocity/energy of the particles increases, so the magnetic field is increased to keep the particles in the ring. This process lasts for a few seconds and then the particles are kicked out of the ring and directed to collide with a stationary target. This process is repeated and gives rise to the name pulsed accelerator. The Super Proton Synchrotron SPS at CERN is this type of accelerator. It has a top energy of 450 GeV, 14 million times more powerful than your 30 keV television. During acceleration the particles quickly achieve a velocity very close to the speed of light and, as we mentioned before, nothing can go faster than the speed of light, so the particles increase their energy by increasing their mass according to Einstein’s theory.
By using two beams of particles travelling in opposite directions it is clear that much higher collision energies can be achieved. In this type of accelerator two beams of contra rotating particles are accelerated to the top energy and allowed to collide head on. This lasts for several hours until the beams are exhausted. This is called a Collider and the Lepton Positron Collider LEP at CERN is this type of machine.
In both of these machines particles are accelerated to very high energies and are allowed to collide with other stationary or moving particles. With this release of energy, matter is formed according to Einstein’s formula. The concentration of energy is so high that temperatures of the order 1020 degrees are achieved and conditions 10-9 seconds after the Big Bang produced.
After tea I will describe the accelerators at CERN but if there are are any questions so far I shall be pleased to answer them.
1510 hr 2630 words should take 25 mins
We shall now stop for tea . On the table is an example of one of the largest thyristors that exists, rated at 3350 A and 1700 V. This is the silicon chip which is inside. Twelve of these units provide 5400 A at 1800 V to energise the LEP main ring bending magnets.I hope that the second half of the talk will be more interesting.
Now we come to details of CERN