CERN has recently approved further work on the Future Circular Collider (FCC) – a successor to the Large Hadron Collider (LHC). The 100km long particle collider will be built in Geneva, Switzerland, next to the current one. Particle accelerators and colliders are machines that can use electromagnetic fields to bring particles up to massive speeds and crash them into each other, creating extremely high-energy environments that tell us about the fundamentals of the Universe.
Physicists hope to use the FCC to continue the experiments done by the LHC once it begins reaching its limits.
The LHC began construction in 1998, being finished and turned on 11 years later in 2009. Prior to this, it attracted a storm of media attention after scientists theorised the collider could potentially lead to a “doomsday scenario”. The leading fear was that the machine could create micro black holes that would grow out of control and destroy the Earth. Dramatic headlines about the end of the world were quick to appear, with certain scientists coming out and expressing fears that the Earth could be consumed. Obviously, in the end, the fears were unfounded – the Earth still stands to this day.
History
The first kind of accelerators were known as electrostatic nuclear accelerators. They were relatively simple and couldn’t reach very high energies by today’s standards. The basic principle was to use a static electric field and accelerate a charged particle towards a high voltage electrode. The particle’s acceleration was limited by the maximum voltage that could be achieved by the device and it was due to this, that they were not appropriate for high energy experiments. Despite this limitation, electrostatic accelerators are still in use today in research labs and in medical equipment where lower energy particles are needed.
Due to the maximum acceleration imposed by the electrostatic accelerator, different methods were used to reach higher energy collisions.
The first type of electrodynamic particle accelerators were called linear accelerators (or a linacs). Instead of using a static electric field, electrodynamic accelerators use a changing one instead. This means the charge of the electrodes is constantly swapping. Linear accelerators are made up of a long, straight tube and a series of cylindrical electrodes surrounding it, each with alternating charges. An oscillating voltage is applied, causing the electrodes to flip between the two charges repeatedly. As the particle travels through the pipe, it is always being dragged forward and gets faster because as it reaches the end of one electrode, the charges flip, and it begins accelerating towards the next one.
Linacs are limited by how long you can make them – the longer the pipe, the faster the particle. The largest one currently in use is in California, being operated by Stanford University. It is 3.2km long and used to accelerate electrons and positrons.
To remedy this limitation, different types of particle accelerators were developed. Cyclotrons make the particles travel in a spiral path using a magnetic field. In these accelerators, two large D shaped electrodes (or ‘Dees’) shield the particle’s path. As it travels in between each electrode, the polarity of the dees flips like in the linac accelerators, accelerating the particle across the gap. As the particle gets faster, it’s momentum increases and so the radius of the curvature it travels increases. It’s for this reason that a spiral is used.
However, these machines are limited – again – by space. Whilst the spiral path is more ‘space effective’ than a straight tube, the path of the particle still gets wider and wider. To reach truly high speeds, a synchrotron is used.
Synchrotrons
Modern-day accelerators and colliders, for research purposes, are usually synchrotrons (Both the LHC and proposed FCC are synchrotrons). They can accelerate the particles to such an extent that they begin to approach the speed of light. Like the cyclotrons, these machines are circular, however instead of a constant magnetic field that lets the particle spiral outwards, the magnetic field is synchronised to the particle speed and gets stronger as it gets faster. This means the particle always follows the same circular path and doesn’t ‘spin out’ in a spiral.
The LHC can bring particles up to dizzying speeds. Its current record is 11 km/h under the speed of light.
These massive feats of engineering help us learn exactly what the Universe is comprised of. All matter in the Universe is made of atoms and these atoms comprise of protons, neutrons and electrons. Protons and neutrons are further comprised of even smaller particles, known as quarks and gluons.
The only way to detect these extremely small particles is through intensely high-energy collisions. Particle colliders, or ‘atomsmashers’ as they were sometimes called, were used throughout the mid-20th century to probe these fundamental particles. A massive array of subatomic particles was discovered – known as the ‘particle zoo’ – which eventually came to form the standard model.
Standard Model
The Standard Model is the prevailing theory that categorises subatomic particles and explains how they relate to the fundamental forces of the Universe.
There are many different types of quark, but only the ‘up’ and ‘down’ quarks are stable, these are known as the first-generation quarks. The ‘charm’ and ‘strange’ (second generation) and ‘top’ and ‘bottom’ (third generation) quarks existed at the dawn of the Universe but quickly decayed into up and down quarks. This is why we use particle accelerators; they help recreate the conditions during the first moments of the Universe, giving us the ability to detect these less-stable particles.
Leptons, alongside the quarks, are part of the basic building blocks of matter but use different forces. They do not show internal structure and, like the quarks, are split into three generations. The electron and its neutral counterpart, the electron neutrino, are the first generation leptons. Muons, tau leptons (and their corresponding neutrinos) are the second and third generation leptons respectively.
The Standard Model also contains the force carrier particles, responsible for the forces felt between other particles. These four fundamental forces – gravity, the electromagnetic force, the strong nuclear force and the weak nuclear force – are what keeps the Universe and atoms together. 3 out of 4 of these forces operate using known bosons. The W and Z bosons help generate the weak force, gluons generate the strong force and the photon generates the electromagnetic force. The particle responsible for gravity, the ‘graviton’, remains elusive and is yet to be discovered.
Finally, there is the Higgs Boson (sometimes dramatically referred to as ‘the God particle’), the first evidence of which was only found in 2012. This unique particle generates the ‘Higgs field’, a field of energy that exists across the Universe, giving every particle their mass. Whilst evidence was only found recently using the LHC, the existence of the Higgs Boson was first predicted in 1964. The theorised Higgs Boson helped make predictions that were later confirmed during the 20th century, including the existence of the top quark.
Not just physics
As well as shedding light onto some of the deepest mysteries in particle physics, accelerators have many other, more practical uses. The synchrotron’s ability to create high energy particle beams is utilised in many scientific fields.
Synchrotrons are commonly used as sources of electromagnetic radiation. When a charged particle moves in a curved path or accelerates in a straight line, it emits electromagnetic radiation. Beams of light that are more than a million times brighter than the Sun are created. This phenomenon is typically done by accelerating electrons and can be used to produce special, high-intensity, focused X-rays.
In chemistry, these X-rays can be used to probe the structure of certain molecules. Through a process known as X-ray crystallography, where the way the radiation interacts with a material is investigated. By carefully analysing the ‘scattering pattern’, researchers can determine the intricate structures of molecules. In 2009, the Nobel Prize in Chemistry was awarded to a group of scientists that used this method to solve the structure of the ribosome – a complex molecular ‘machine’ found in living cells.
Means of analysis like this are used in agriculture for soil analysis. By analysing how the X-ray radiation is absorbed by a sample, scientists determine the elemental makeup of it. This is called spectroscopy and can be used to reveal environmental pollutants or clarify soil compositions. Characterisation of these things is important when developing fertilisers for more efficient farming.
The FCC
The team behind the new collider are doing so with the hope that it will help us understand even more of the mysteries that plague the scientific community. For example, scientists hope that it will maybe find dark matter particles, a form of matter that has yet to be directly observed. This is among several aspects of the Universe that remain unexplained.
The proposed project has come under criticism, however. It’s projected to cost over 21 billion Euros and some prominent physicists claim that there is no evidence it will make notable discoveries. Sabine Hossenfilder, a theoretical physicist at Frankfurt Institute for Advanced Studies, said ‘A bigger particle collider is one of the most expensive experiments you can think of, and we do not currently have a reason to think it would discover anything new’. Jared Kaplan, from John Hopkins University, said ‘There are a lot of other experiments that are proposed and ongoing that are much cheaper’.
So, whilst the new FCC work has been approved, CERN has held off from making a firm decision as to if the entire project will go ahead. So for now, expect the debate to continue.