The beauty of Lego is that an infinite variety of things can be built by simply combining a finite number of basic building bricks. But say one day the Lego corporation launched a version of its bricks in which each was hundreds of times bigger than its standard bricks. And say it then launched another version in which the bricks were thousands of times bigger. You would think that the company had gone completely bonkers. But this is exactly what nature has done with its fundamental building blocks: quarks and leptons.
All normal matter is made of just four building blocks: two leptons and two quarks. The two leptons are the electron and the electron-neutrino. The electron is familiar because it commonly orbits in atoms, but the neutrino is less well-known, mainly because it is so mind-bogglingly unsociable. Although neutrinos are generated in prodigious quantities by the sunlight-generating nuclear reactions in the heart of the Sun, they “interact” with normal matter extremely rarely.
The two leptons of normal matter are accompanied by two quarks: the up-quark and the down-quark. These clump together in threes to make the proton and neutron, the principal components of the cores, or nuclei, of atoms. The proton is composed of two up-quarks and one down-quark, and the neutron of two down-quarks and one up-quark. The existence of quarks was demonstrated in the late 1960s and early 1970s when particle physicists in California fired high-speed electrons into protons and witnessed the electrons ricocheting, exactly as if they were bouncing off three point-like grains buried deep inside.
The discovery that all the ordinary matter in the Universe is made of just four building blocks – the electron, neutrino, up-quark and down-quark – is a truly astounding scientific achievement. But there is a peculiar twist. Because there also exists a second generation of quarks and leptons, in which all the particles are identical to the first, apart from being hundreds of times heavier, and a third generation in which they are identical but thousands of times heavier. The second generation consists of the heavier muon, muon-neutrino, strange-quark and charm-quark. And the third generation is made of the even heavier tau, tau-neutrino, bottom-quark and top-quark.
The heavier generations take a lot of energy to create so they are rarely seen today. However, they were common in the super-energetic conditions in the first split-second of the Big Bang. It is therefore possible that they played some crucial role in creating the Universe we see around us.
In 1936, when the muon – a heavier version of the electron – was discovered, the American physicist I. I. Rabi famously said: “Who ordered that?” The same could be said today of all the duplicates of nature’s four basic building blocks. Who ordered them? Why has nature decided to triplicate its basic building blocks? The answer is: nobody knows. However, an important step on the road to finding an answer would be understanding why the particles in each generation have such wildly different masses. And, recently, the American Nobel Prize winner Steven Weinberg, now 87, has hinted at an explanation.
The basic building blocks of matter gain their masses by interacting with the Higgs field, an invisible fluid that fills all of space. You can think of them interacting with the Higgs particle, a localised hummock in this energy field, first seen in 2012 at CERN, the European centre for particle physics. Weinberg points out that particles that interact most strongly with the Higgs field end up with masses close to that of the Higgs particle, and these are the particles not of the first but of the third generation. Maybe, then, Weinberg speculates, the particles of the third generation are the only ones that interact directly with the Higgs. Maybe the second generation get their masses by interacting indirectly – by interacting with an as yet undiscovered particle that interacts directly with the Higgs. And maybe the first generation get their masses even more indirectly – by interacting with a second undiscovered particle that interacts with the first.
Weinberg’s proposed mechanism is similar to Chinese whispers, in which the message relayed gets ever further removed from what was originally said. Perhaps with each lower generation of quarks and leptons, the Higgs field becomes ever more distant and its mass-generating effect ever more diluted. Weinberg does not yet know in detail how such a mechanism could work. But other physicists feel he may at least have shown the way to solve the puzzle of nature’s mysteriously triplicated building blocks.
This article is from the New Humanist spring 2021 edition. Subscribe today.
