It has become a pleasant habit for me to visit Bassano del Grappa every February for a conference on particle physics aimed at high-school students. Thanks to the efforts and the skill of dr. Sergio Lucisano, the schools of Bassano organize every year several conferences on physics and cosmology. These conferences are connected with the european program of the Masterclasses, but they extend the scope considerably into the history of physics and other topics of interest for the students.
This year I was a bit frightened by a change of format on which dr. Lucisano had insisted: I needed to merge into a single 2-hour presentation the material I used to discuss in two one-hour lessons given one week apart. My concern was that 17- and 18-year-old students would not have the stamina to follow my extended exposition, with the accompanying 100 crowded powerpoint slides. I had foreseen that they would soon lose interest, doze off, or leave. But Sergio assured me that they were excellent students, and that they were motivated to follow the entire lesson: I should not remove material to lessen the burden. I followed his advice, and was later glad I did.
As soon as about forty students gathered in the early afternoon in a conference room of Liceo Brocchi, and Sergio started a highly adulatory introduction of the speaker, I realized that those students were quite mature for their age. I could have heard a mosquito flying in the room if there had been one: they kept incredibly quiet, focused, and vigil during my talk, as if they had been nailed to the chairs. I venture to bet a dollar that among them there is at least one future Ph.D. graduate in Padova.
My presentation was indeed quite thick. Below I offer a summary of what I discussed.
I introduced the lesson by discussing the very early ideas on the structure of matter, the multiple meaning of "elementary", and the explanation that even understanding what the world is made of is useless until we figure out how things are held together and interact.
Then I explained that I would be using three fundamental concepts in the description of the progress in our understanding of particle physics: classification, spectroscopy, and technological advances.
Classification a powerful method in scientific investigation. I made a few specific examples -like the classification of animals which allowed to see the structure and the evolution of the species, the classification of galaxies, the Hertzprung-Russell diagrams for star evolution, and the Mendeleev table. I pointed out how the russian scientist could predict the existence of new elements by a simple compilation of elements according to their weight and valence. I would be later using this image to explain how the "Omega-minus" particle withstood the very same destiny as eka-aluminum and eka-silicon.
The second idea -the importance of spectroscopy- was discussed starting from the Balmer series of the Hydrogen atom, explaining how energy spectra provide a footprint with which to identify composite systems. This concept was to become handy forty slides later, when I convinced the audience of the reality of quarksby comparing the charmonium energy spectra to those of excited atoms.
A third point I made was that there can be progress in particle physics only with the development of complex technologies, and with the support of funding agencies. In our history we observe that science always had to fight with hindrances such as religion, superstition, or other factors that stopped fundamental research or strongly slowed it down. I mentioned Giordano Bruno. I then made recent examples of the US budget, which devotes to science a tenth of what it devotes to defense; and the italian INFN budget, which has shrunk by 20% in purchase power since 2001. A connected problem is that science is not perceived as a priority, and scientists do too little to explain to the public the importance of what they do.
And then, I gave a bonus theme -a fourth important element that would underline much of the discussion: Ockham's Razor. I explained that on his lex parsimoniae ("entia non sunt multiplicanda praeter necessitatem") is funded our methodology of scientific investigation.
After this introduction, I explained that to look deep inside matter we need to invent a new vision. Microscopes -even the most sophisticated electron ones- are limited by diffraction. We owe the alternative to Lord Rutherford: if you want to know how something is made up but you cannot see it, throw something at it!
After discussing how the neutron, the neutrino, and the muon came about, I gave a short excursus in the discovery of the first particles at accelerators, the formulation of a few selection rules (baryonic number, strangeness) for particle reactions, and discussed the classification of hadrons in multiplets, and the prediction of the existence of the Omega particle.
The classification scheme of hadrons pointed to the discovery of quarks: deep inelastic scattering was understood to be the analogue of Rutherford's experiment, and then the discovery of the J/psi meson clarified the picture.
The standard model was introduced by discussing the four forces of nature, and their observed phenomenology: strong interactions bind quarks inside hadrons, electromagnetic ones at the subnuclear level are responsible for some important decays, and weak interactions are carried by massive bosons which would take a new accelerator and a new concept to discover. As for gravity, I pointed out its irrelevance for particles, except that the evidence of dark matter in rotational velocity distributions of galaxies points to the existence of dark matter, which could be due to an as-of-yet undiscovered new particle.
I then discussed the discoveries of bottom and top quarks, and the searches for the Higgs boson. This forced me to explain in some detail the techniques we use to search these particles at modern-day collider experiments.
I concluded by discussing the shortcomings of the standard model, and gave a brief mention of some proposed extensions: supersymmetry, large extra dimensions, string theory. I could only offer some examples of how supersymmetric particles -in particular, a neutralino- could be observed by the LHC experiments, and how missing transverse energy may be a catch-all signature for many other exotic processes, like the escape of a graviton in an extra dimension of space.
My last slide was about the future of physics: I explained that the theoretical landscape has not made much progress in the last thirty years or so, and that experimentally we are also not breaking much ground: we need fresh minds with new ideas, and I do believe that the future of science lays in a new generation of scientists. I also explained that undertaking a career in physics is not economically advantageous, especially in Italy; but it is extremely rewarding from many other standpoints, and I encouraged them to consider studying physics at the university.
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