Seeing has always been a trigger for curiosity – the desire to know reality – and light is a means for bridging reality with our minds. It is not the only means, but probably the most important. Sight conveys the most information, the most detail about the world around us. Think, for example, of the richness of detail in today’s high-definition (HD) or 3D images. Now, to remind us of light’s importance and how useful it is in our lives, the UN has declared 2015 as the International Year of Light.
From Euclid, who first put down the principles of geometric optics in 300 BC, to Alhazen, whose first real theory of light and sight around 1000 AD was so influential in Europe, to Francesco Maurolico who in the 16th century developed a modern theory of sight and the functioning of the eyes – light and sight have long fascinated scientists. Indeed, light is fundamentally linked to the birth of modern science. In 1609–1610, Galileo Galilei was able to perfect the lens and telescope, making the first modern scientific instrument. The “canone occhiale” or “spectacles cannon” – the words at the root of the Italian for telescope – allowed him to see “things never seen beforehand”, as he wrote in his “instant book” Sidereus Nuncius. Thanks to an instrument based on light, he was able to discover the moons of Jupiter and make the Empyrean Heaven a place where change happens, and therefore worthy of investigation by physicists.
Later in the 17th century, Francesco Grimaldi first observed diffraction – soon formalized by Christiaan Huygens in a complete physics theory – and in 1873 Ernst Abbe showed that this limits the detail of what we can see. The resolution of our vision depends on the wavelength of the light or any other wave used for detection, such as sound waves, as in bats, or electromagnetic waves of different wavelengths. So, if we use millimetre-range infrared waves, the image is inevitably less well resolved than with submicrometre visible light. That is why our vision is so good and we can appreciate the splendour of HD images.
For more than a century, physicists have been able to see with finer wavelength “light” – for example, X-rays with wavelengths 100–1000 times shorter – and today, being able to “see” atoms at the nanometre scale, daily life is invaded by “nanotechnology”. Nevertheless, we can peer down to much smaller scales. Just 90 years ago, Louis de Broglie put forward the unimaginable idea that a particle can behave like a wave, with a wavelength inversely proportional to its momentum. This completed the particle–wave duality initiated by Albert Einstein in his annus mirabilis, when he realized that waves behave like particles and introduced the concept of light quanta, the photons.
In this way, particle accelerators can generate the finest “light”. The cyclotrons and synchrotrons of the 1950s and 1960s were capable of illuminating entities such as protons, but were limited by diffraction in the femtometre range. Each new, more powerful accelerator joined the race for the finest light, allowing the best resolving power. Most recently, with the LHC, the simple relation λ = h/p tells us that at 1 TeV (the average collision energy of a quark–quark interaction) we can resolve the attometre, or 10–18 m, scale. However, thanks to higher energy in some collisions and to sophisticated experimental techniques, the LHC has shown that quarks are point-like at the level of 5 × 10–20 m, or 50 zeptometres.
But light is only a means, a bridge between reality and our minds, where the image is formed and vision occurs. Indeed the light generated by the LHC would be useless without “eyes” – the LHC detectors that collect the collision events to record the detail illuminated by the light. As with the eyes, the collected information is then transmitted to the mind for image formation. At the LHC, the computers, the physics theory, the brains of the experimentalists and theoretical physicists – all of these form the “mind” where the wonderful images of, for example, the Higgs boson, are formed and, finally, known. Exactly as with sight, some signals (most of them, in fact) are first treated “unconsciously” (by the trigger) and only a selected part is treated consciously on a longer time scale.
Now the LHC is restarting and we will be able to generate light almost as twice as fine, thanks to the 13 TeV collision energy. Moreover, the High-Luminosity LHC project is already on the starting blocks to be ready 10 years from now (see A Luminous future for the LHC). Why high luminosity? Just as in a room where we might ask for more light to investigate finer details and measure the properties of objects more precisely, with the LHC we are planning to increase luminosity by a factor of five (instantaneous) or 10 (integrated) to make more precise measurements and so extend our sight, i.e., the physics reach of the collider and the detectors.
With our accelerators, detectors, computing facilities, physics analysis and theory, we really do reproduce the act of sight, generating the finest light and therefore perceiving a reality that is unimaginable to our normal senses: the frontier of the infinitely small.
• Lucio Rossi in high resolution at CERN.