A huge, extraordinary machine will soon begin to study an elusive particle in a bid to reveal some of the deep secrets of the cosmos
On the outskirts of Karlsruhe, in south-west Germany, engineers have buried a giant, stainless steel device, bigger than a blue whale, inside the towns institute of technology. The machine looks for all the world like a grounded zeppelin or a buried blimp.
In fact, the apparatus is one of the worlds biggest vacuum chambers. Air pressure inside it is lower than that on the surface of the moon and it has been installed to help solve a single, intricate problem: finding the mass of the universes most insignificant entity, the neutrino.
Every second, billions of neutrinos pass through our bodies. The sun sends trillions streaming across space every minute. Uncountable numbers have been left over from the Big Bang birth of the cosmos 13.8 billion years ago.
In fact, there are more neutrinos in the universe than any other type of particle of matter, though hardly anything can stop these cosmological lightweights in their paths. And this inability to interact with other matter has made them a source of considerable frustration for scientists who believe neutrinos could bring new understandings to major cosmological problems, including the nature of dark matter and the fate of our expanding universe. Unfortunately, the unbearable lightness of their being makes them very difficult to study.
Hence the decision to build the Karlsruhe Tritium Neutrino Experiment, or Katrin. It is designed to measure the behaviour of neutrinos and electrons that are emitted by the hydrogen isotope, tritium, in order to uncover slight variations in their paths as they fly through the experiments vacuum chamber. These variations should reveal precise details about the neutrinos physical properties, in particular its mass.
We have pushed technology to the limit in building Katrin, says the projects leader, Guido Drexlin. Apart from creating a near perfect vacuum inside its huge chamber, we also have to keep the temperature of the tritium, which is the machines source of neutrinos, inside the device to a constant 30C above absolute zero. We have also had to take incredible care about the magnetic fields inside the machines. Essentially, we have had to demagnetise the whole building.
It has taken more than a decade of planning and construction to put Katrin together. Its price tag, just over 60m, has been met by the German taxpayer via the countrys state-funded Helmholtz Association, with a further 6m chipped in by US, Russian, Czech and Spanish scientists who will have a minor involvement with the project.
Final trials are now being completed and full operations are set to begin in June, though it will take a further five years of gathering data before scientists can expect to have enough information to make an accurate assessment of the neutrinos mass.
Even then, we may have to go to a second phase of operations to get our answer, says Drexlin. We are moving into unknown territory here.
The neutrino was first postulated in 1930, by the Nobel physics laureate Wolfgang Pauli, to explain the behaviour of other subatomic particles during radioactive decay. It took a further 26 years of search before neutrinos were first pinpointed in detectors and they remain maddeningly elusive.
An illustration of their insubstantial nature is provided by Canadas Sudbury Neutrino Observatory, where a 1,000-tonne tank of heavy water is used to stop some of the 10 million million neutrinos that pass through it each second. Of these, only about 30 are actually detected in an average day.
Three different forms of the particle are now known to exist: the electron neutrino, the muon neutrino and the tau neutrino and until relatively recently it was thought that none of them had any mass at all. They were the ultimate in ephemeral ghostliness, a bizarre situation that was celebrated by John Updike in his poem, Cosmic Gall.
Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
However, in the late 20th and early 21st century, scientists started to uncover evidence that suggested Updike was not entirely correct in his claims about the neutrino and that it did have some mass after all. This work culminated in experiments, carried out separately by Takaaki Kajita, from Japan, and Arthur McDonald, from Canada, which showed that neutrinos switch form as they travel across space. For example, some of the electron neutrinos emitted by the sun are transformed into muon and tau neutrinos as they hurtle towards the Earth. The process is known as neutrino oscillation.
The discovery was crucial, Drexlin insists. There is a straightforward constraint in cosmological theory that states that only objects with mass can oscillate between different forms in this way. Massless particles could not change in this way. So the inference is clear: neutrinos must have mass.
Drexlin recalls attending the physics conference where the results of these first experiments were unveiled. It was like a rock concert. People were cheering and stamping their feet for a good reason. We knew the universe would never be the same again. For revealing the neutrinos massive secret, Kajita and McDonald were awarded the Nobel prize in physics in 2015.
But if neutrinos have mass, exactly how much do they possess? It is not a trivial question, for as Mark Thomson of Cambridges Cavendish Laboratory points out, the precise result of such a measurement could have critical consequences. Neutrinos have mass but they remain staggeringly insubstantial. They are still a billion times smaller than any other type of known subatomic particle. On the other hand, there so many of them that their combined masses could give them cosmological significance. We badly need to know what that mass is in order to figure out how they might affect the future of the universe.
For example, if neutrinos prove to be on the heavy side of current estimates, then their combined gravitation pull would effect the expansion of the cosmos and slow it down. However, if their mass is on the light side, neutrinos, despite their cosmological ubiquity, will be unable to act as any kind of meaningful brake to the universes expansion.
Nor is this the only reason that scientists are fascinated by the fact that neutrinos have mass. What is so intriguing is that their mass is just so much less than that of any other particle by a factor of a billion which suggests they must get their mass from some other mechanism, adds Thomson. All other particles get their mass by attaching themselves to Higgs bosons but the neutrino must do it by a very different route. So there is some other basic force that seems to be involved and uncovering that would be a real prize.
It is for these reasons that scientists have struggled, over the decades, to find the exact mass of the neutrino. First efforts, made after the second world war, placed an upper limit on its mass at around 500 electron volts (ev). This figure is about 1/500th of the mass of the electron, itself a relatively tiny particle. (Using a unit of energy to describe the mass of an object may seem strange but all subatomic particles are measured in electron volts, which can also be used as a unit of mass because energy and mass are convertible concepts according to Einsteins E=mc equation.)