and most powerful accelerator in the world. There are even more ambitious projects under consideration, which we will discuss at the end of the day in offering a perspective on the future, but which remain at an embryonic stage for the time being. At the moment, collisions produced in the LHC reach an energy of 13 tera-electronvolts. If this number doesn’t tell you much, just consider that the energy contained in a particle at full speed in the LHC is equivalent to that of a mosquito in flight — except that at CERN, this energy is concentrated in a tiny proton, not distributed among the billions of billions of protons that make up the mosquito!
Collisions occur in the four detectors I mentioned earlier: ATLAS (A Toroidal LHC ApparatuS), CMS (Compact Muon Solenoid), ALICE (A Large Ion Collider Experiment) and LHCb (Large Hadron Collider beauty). CMS research, like that of ATLAS, focuses largely on the study of the Higgs boson. The ALICE experiment is intended for the study of quark–gluon plasmas, a kind of primordial magma that, by cooling, gives rise to other particles of matter. To do this, collisions within ALICE involve heavy ions, i.e., large chemical elements; there are periods when the LHC doesn’t provide collisions between protons, but between much heavier elements, such as lead. The LHCb experiment, for its part, focuses on the quark b, known as “beauty” or “bottom” — a fleeting particle (because its lifetime before decay is very short) that helps physicists to understand the differences between matter and antimatter. All these detectors have different technologies adapted to their research programmes. At the risk of repeating myself later, I would like to stress that ATLAS and CMS, although having very similar programmes, do not use the same technologies for particle detection, or, therefore, the same methods. This is very important, because observing the same phenomenon with two different detectors ensures the reliability of the results and limits errors.
The detectors record the data from the collisions, which are then processed, in comparison with the simulated data, until an experimental result is announced, such as the discovery of a new particle. But one thing at a time: so far, the important thing has been to give you an overview of the path of a proton from the hydrogen bottle to the various detectors of the LHC.
One remark must be made here: this path is in fact only used by less than 0.08% of the protons! It should not be forgotten that many other experiments take place between the Booster and the LHC. For example, some accelerated protons in the PS, instead of being injected into the SPS, leave for the antimatter factory, which we will visit later; others leave for a lead target, producing high-energy neutrons studied by the n-ToF experiment, with applications in both astrophysics and medicine. The same is true for the SPS: some protons, if not injected into the LHC, are directed to the AWAKE experiment, which employs another plasma-based particle acceleration technique; others leave for the COMPASS experiment, which studies quarks and gluons within the atomic nucleus. So, even if we focus today on the ATLAS experiment, there are many others.
Follow me as I move to the other side of this room in the Globe. I would like to show you a little curiosity — which might even be raised to the status of an object of worship. Look into that sphere.
The first Web server
This is the first Web server, which was developed by Tim Berners-Lee in 1989. Take in what I’m about to tell you: the World Wide Web and the HTTP transfer protocol were invented at CERN! If you still had doubts about the usefulness of CERN, keep repeating to yourself: without CERN, no Facebook, no Google, no YouTube, no Wikipedia, no Amazon, no Twitter... Imagine your life today without the Web. Well, that would be a life without CERN.
Next to this server, you can see a document: it is the article that Tim Berners-Lee submitted to his superior in presenting his project. The reaction of the superior: “Vague... but exciting!” Initially, the Web project was intended to allow data sharing between physicists: we know what it is today.
The article at the origin of the Web (Copyright: CERN)
But the Web is not the only contribution of CERN to IT! It is also at CERN that the idea of the touch screen was invented. So, once again, imagine a world without a touch screen: it’s a world without CERN. Last but not least, it is at CERN that the prototype of what can be considered as the ancestor of the computer mouse was developed. That was in 1972, a certain Bent Stumpe had to design a system for the SPS control room — which I was telling you about a moment ago — to move a cursor on a control screen. He then ordered 12 bowling balls and developed the precursor of the ball mouse. The bowling ball was chosen for reasons of stability and fluidity of movement, but fortunately, all this could be miniaturized.
To anyone who would tell you that CERN has never been used for anything and that it is expensive, I now count on you to tell him or her that without CERN there would be no Web, no touch screen, no computer mouse. And if this person persists in his scepticism, sincerely, it is because he or she is acting in bad faith.
Before leaving the Globe, I have two more little surprises. Come on...
The first particle accelerator
Equations which describe the infinitely small
In this sphere, you can contemplate the very first circular particle accelerator, a few centimetres in diameter. Don’t you find the experience moving? And in this other sphere, here, you can admire the beauty of the equations that rule particle physics today. There’s no rule against getting a little bit ecstatic!
Let’s leave the Globe. Now you’re ready for the discussion we’re going to have with Arnaud Marsollier. Arnaud is part of the communication department, and I made an appointment with him to talk about money issues: how much did the LHC cost? And the detectors? What are the various economic and societal benefits? I think it is useful to understand from the beginning of the visit that the money invested in CERN is not lost money, far from it. But first, on the way to his office, let me tell you a little bit about the history of CERN, so that you can feel the weight of history as you walk through its corridors...
A brief history of CERN
If there is one place in the world devoted to science that beats records, it is CERN! Imagine: an international collaboration with 22 Member States in 2016 has built a ring of 27 km in circumference, at an average depth of 100 m, to cause tiny constituents of matter (such as protons) to collide at a vertiginous speed almost equals to that of light. When you work at CERN and repeat this description almost mechanically, you tend to get used to it and forget that... it’s just amazing! But behind such exploits lies a whole history, whose story deserves to be told.
As you may know, the first half of the 20th century deeply changed our view of physics: first with Einstein, who formulated his theories of special relativity and general relativity in 1905 and 1915 respectively, and then with the rise of quantum mechanics, which developed considerably from the 1920s onwards. However, all these conceptual revolutions did not take place just anywhere: they took place in Europe. Heisenberg, Pauli, Dirac, Schrödinger, de Broglie, Born, Bohr, Einstein: almost all the great physicists who founded quantum physics were European.
But the Second World War, which led to a brain drain to America, put an end to this golden age, and fundamental research in Europe came to a standstill. To remedy this, Louis de Broglie — among other personalities such as the Italian Eduardo Amaldi and Pierre Auger and Raoul Dautry
