Nico Gubernari

The idea that matter is made up by basic building blocks goes back a long way. It was first proposed by the ancient Greek philosophers Leucippus of Miletus and Democritus in the fifth century BC. That is why the word atom comes from the ancient Greek word atomos, meaning indivisible. However, these were mere speculations and the modern (scientific) understanding of the atom began to emerge in the 19th century. In 1803, John Dalton proposed his atomic theory, supported by strong experimental evidence. According to this theory, matter is made up of tiny, indivisible particles (the atoms). He also suggested that not all atoms are the same, but that there are different types with different properties.

To date, 118 different types of atoms (or elements) have been identified. This is more or less the final number of existing elements, due to the problem of nuclear stability. The most common elements on Earth include oxygen, aluminium and iron. Atoms are extremely small compared to the objects we usually deal with in our everyday lives. In fact, the typical diameter of an atom is less than a billionth of a millimetre. To give you an idea of how small atoms are, a grain of salt contains about a quintillion atoms, or if you prefer, a million million million (=1,000,000,000,000,000,000) atoms. Please take this information with a grain of salt.

Atoms can combine to form molecules through chemical bonds. Molecules in turn assemble into complex structures like cells, which are the basic units of life. Within cells, intricate processes take place to carry out the functions necessary for living beings. In the case of humans, billions of cells, each with specific roles and functions, come together to form tissues, organs and ultimately an entire organism.

Geiger and Marsden's experiment explains one of the reasons why colliding particles can be useful. By shooting particles at a fixed target or making them collide, we can understand both their "shape" and whether they are themselves made up of even smaller particles. (Shape refers to the geometry of the potentials through which the particles interact. In the experiment considered, the shape in question is that of the electromagnetic potential of the nucleus of a gold atom.) There is no other way to understand what is inside an atom, or even worse what happens inside the nucleus of an atom, as it is impossible to observe such phenomena with any optical microscope. In fact, an atom is hundreds of times smaller than the wavelength of light visible to the human eye. Such experiments can therefore be seen as a natural evolution of the optical microscope concept.

Let me mention something that a physicist might consider trivial, but not everyone is familiar with. Whenever we touch an object, like a table, what happens at the microscopic level is that the electrons in the atoms of our fingertips repel the electrons in the atoms of the table. This is due to the electromagnetic force, which states that two negative particles (like electrons) repel each other. As a result, we never actually "touch" anything in the sense of direct contact at the atomic level. Instead, it's the electromagnetic force between the electrons that we perceive as the sensation of touch. Furthermore, all fundamental particles are considered to be point-like, i.e. they have no size. Therefore, there is never any direct contact between particles, but only interactions through the fundamental forces (electromagnetic, weak, strong and gravitational).

Synchrotrons have a circular (or rather polygonal) shape and consist of two pipes with a diameter of several tens of centimetres, through each of which particle beams are passed. The particle beams in the two different pipes travel in opposite directions (see figure). Clearly, air is removed from these pipes, i.e. a vacuum is created so that the beams can travel unimpeded. In a synchrotron, beams of particles are accelerated along the two pipes by alternating electric fields, while magnetic fields guide and concentrate the beams, ensuring they maintain a circular trajectory and avoid dispersion.

Let us consider, for example, a billiard table. Classical mechanics tells us that by making different billiard balls collide with each other, the balls will take different directions after the collision. It is possible to calculate the direction and speed of the balls after the collision if the direction and speed of the balls before the collision are known. Imagine increasing the speed of the balls disproportionately, causing them to jump off the table or even break on impact, but nothing more can happen.

That is pretty intuitive to understand, right? Now forget it, because collisions in quantum (relativistic) mechanics can have very different outcomes from those in classical mechanics. I already have Einstein's famous formula above: \(E=mc^2\). This formula implies that mass and energy are equivalent, i.e. they are two sides of the same coin. Hence, mass can be converted into energy and vice versa. The case of matter antimatter annihilation is an example of the conversion of mass into energy. A high-energy particle collision is an example of the conversion of energy into mass. The mass-energy equivalence, which may seem strange at first, has surprising consequences. The energy released in a collision generates new particles, which can be completely different from the particles that collided. For instance, by colliding two electrons, it is possible to create every particle in the universe whose mass is less than or equal to the energy of the collision. So, in the quantum world, the collision of two billiard balls at sufficient speed could create a rugby ball, a cake, or even a house. Strange right? Let me point out something about this example. While it's true that in theory, particles and billiard balls at sufficient energy levels could potentially form macroscopic objects through various interactions and combinations, in practice, this doesn't occur for mainly reasons. First, it is not possible in practice to achieve the extremely high energies required to trigger such phenomena. Second, the likelihood of the products of a collision arranging into recognizable macroscopic objects like rugby balls, bedside tables, or houses is incredibly low due to the complexity of such arrangements and the dominance of statistical processes at the quantum level. Nevertheless, quantum effects are observable in collisions between subatomic particles.

Therefore, by using particle accelerators and colliding electrons and/or protons with each other it was possible to create the fundamental particles (or what we believe to be such) that constitute the entire visible universe. To date, 17 (types of) fundamental particles have been identified: the electron and two of its heavier brothers (muons and tauons), three neutrinos, six quarks, the photon, the W and Z bosons , the gluon, and the Higgs boson. These particles combine, interact with each other, and interact to form the objects we see and use in everyday life.

I hope that I have convinced you that colliding subatomic particles is much more fun and interesting than it might seem. For instance, every collision at the LHC between two protons creates thousands of particles. This can give an idea of how complicated both the work of experimental physicists is, given that around 600 million collisions per second occur at the LHC, and that of theorists like me, who have to calculate the outcome of these collisions.