Science has a long-standing tradition of making more powerful microscopes, detectors and other instruments in its quest for greater understanding. From the first suggestion of the existence of atoms, to the high-energy probing taking place at the Large Hadron Collider (LHC) today, it is clear that science aims to unearth the absolute foundations of nature itself. Yet, even with today’s knowledge and technology, there are still some of these ‘foundations’ – including particles – we just can’t see. We know they’re there though, as theory predicts their encounters with things which we can see. In some cases, we haven’t even seen the results of their encounters, but current theory predicts their existence.
The existence of the neutrino was first suggested in 1930, when Wolfgang Pauli found that the beta-decay process (a neutron decaying into a proton and an electron, plus an electron neutrino) required an extra particle in order to balance momentum, but it wasn’t until 1956 that the neutrino’s existence was experimentally proven. The first experiment to confirm the existence of neutrinos was carried out underground, using a massive tank of cadmium chloride in water. The only way to detect a neutrino is to detect the products of its interaction with something else, and use theory to determine whether or not it could have been produced by anything other than a neutrino. Neutrino detection experiments today still use similar methods: large tanks of solution which neutrinos are known to interact with in a unique, ‘signature’ way. The vast majority of neutrinos pass through us, the Earth, and pretty much everything in their path, making them near impossible to see even indirectly – detectors have to be as big as possible in order to get a strong enough signal.
The Higgs boson is an elusive particle, to say the least. First postulated by Peter Higgs in 1964, it has become the most sought-after particle in modern physics. Culminating in the experiments now going ahead at the LHC, the search for the Higgs has been at the heart of modern particle physics for almost half a century. But why is the Higgs so fundamental to particle physics? Physicists are aiming towards a ‘Grand Unifed theory’ (or GUT) in which all the forces – electromagnetic, strong, weak and gravitational – are explained by one theory. They’re one step towards this, having combined the electromagnetic and the weak forces into one explanation; but in order to make this work, the Higgs boson has to exist. And if it doesn’t, well, it’s back to square one, and an even more complex theory: supersymmetry. In a similar way to neutrinos, the Higgs can only be detected through its ‘signature’ – the particles it leaves behind when it decays. The Higgs itself only appears for a fraction of a second (assuming it appears at all) and even if it were possible to detect in this time, the technology just isn’t there yet. So, like a neutrino, we can never actually ‘see’ the Higgs, only its distinctive trail of particle residue.
The Higgs boson and neutrinos aren’t the only particles we can’t see yet. Another significant one – particularly in terms of proportion to everything we can see – is dark matter. No one is sure what dark matter is – it may not be a form of matter at all, or at least not one we know about yet – but we do know it’s there, as we can measure its effects on our universe. There’s a long way to go yet before we’ll see everything, and we’ve got years of bigger and bigger colliders to come before we do.