FASTER THAN LIGHT?

An article by Warwick particle physicist, Daniel Scully

Could Einstein be wrong? In a special Christmas lecture, particle physicist Daniel Scully, who works on the T2K neutrino oscillation experiment in Japan, looks at an experiment that may contradict Einstein's theory of special relativity: the foundation on which most of modern physics is built.

Over the past few years, particle physics has made the headlines more than a few times, most often because of the Large Hadron Collider (LHC) at CERN. This is probably the largest, most complex machine that humanity has ever built. Recently much has been said about the search for the elusive Higgs boson particle, but in September 2011 it was another experiment which made the news, with a claim that could revolutionise physics as we know it. It claimed that particles, known as neutrinos, were travelling faster than light – something forbidden by Einstein's theory of relativity.

Particle physics is the study of the smallest building blocks of our universe. Every day we encounter thousands of different materials with different colours, textures and behaviours. Some of them are hard, some are soft; some conduct electricity, some won't. These materials are all constructed from the 100 or so chemical elements.

The discovery of the elements was a triumph – the discovery that the construction of our world was in fact much simpler than we had first thought. As these new elements were studied further, patterns began to emerge in their behaviours. We discovered that they could be grouped by these behaviours and arranged into the ordered table which now graces the wall of probably every science class in the country: the periodic table. This pattern was, in fact, a hint that underneath this collection of elements there was something more fundamental going on.

It was at the turn of the 20th century that we discovered atoms, which are the more fundamental building blocks from which all elements are made. Digging deeper inside these atoms revealed that they were all composed of just three building blocks: protons, neutrons and electrons. These three particles could now explain all of the millions of materials and properties that we see around us. A fourth particle, the neutrino, was discovered soon after, in the radioactive decays of atoms.

More recently, our ability to accelerate particles to higher and higher energies has allowed us to discover more exotic states of matter that are too short-lived to find in everyday life, but were crucial to how the universe began, shortly after the Big Bang. In these exotic states, too, we looked for the more simple underlying structure with which to explain them.

If relativity is found to be broken there will be few areas that will escape unscathed.

This is particle physics: the study of the underlying structure of matter to find the fundamental building blocks from which the universe is built. Today, particle physics contains 12 particles which - combined - describe every piece of matter we have encountered so far. The behaviours and interactions of these 12 particles are explained by just four forces: gravity, electro-magnetism, the weak nuclear force and the strong nuclear force. Everything that happens in our universe is a consequence of 12 particles and four forces – well, almost everything.

This all sounds very impressive, and it is. Today we understand the universe, its history and how it works at an incredible level of detail, particularly considering that we're confined to a small planet orbiting an unremarkable star in an unremarkable galaxy. But there are still holes in what we know - still questions left to answer - and so we try to solve them by building experiments.

In addition to answering those questions, these experiments often give us the opportunity to test other aspects of physics. Part of being a scientist is constantly checking and challenging the assumptions we've made and the rules we've written, even though we expect them to be correct and we've checked them before. It's when we find something unexpected that things really get interesting.

In this way, the OPERA experiment was built to investigate “neutrino oscillations”: a behaviour of neutrinos that we know could answer some of those questions in particle physics. But the design of the experiment also gave its creators the opportunity to check the well-verified rule of Einstein's relativity that nothing can travel faster than the speed of light. Einstein wrote his theory over 100 years ago and it has been proven correct countless times since.

The result of the OPERA experiment wasn't what we were expecting.

Its measurement claimed that the neutrinos were travelling faster than light: arriving 60-billionths-of-a-second earlier than they should have done. Measuring something that precisely is a difficult task. A mistake would have been easy to make, but after repeatedly checking every part of their measurement, the neutrinos were still arriving 60-billionths-of-a-second early.

If you want to make an extraordinary claim, you need extraordinary evidence to back it up.

Sixty-billionths-of-a-second may not seem like much, but according to Einstein's theory, the speed of light is an absolute upper limit which nothing should exceed, not even slightly, not even by 60-billionths-of-a-second. If neutrinos are indeed travelling faster than light, then Einstein's relativity has been broken.

Relativity would not be the only casualty either. In the 100 years of physics which have followed its discovery, relativity has become one of the foundations upon which everything else was constructed. From particle physics to astrophysics – the small to the large – almost every theory in physics has been built upon this foundation. If relativity is found to be broken there will be few areas that will escape unscathed.

If you want to make an extraordinary claim, you need extraordinary evidence to back it up. The measurement OPERA made was a difficult one. One measurement, in which a small mistake could easily have been made, is not extraordinary evidence. If we are to abandon the foundations on which 100 years of physics has been built and tested, we are going to need something more convincing. We are going to need another experiment to make the same determination, but this will take time. None of the other suitable experiments will be able to make a precise enough measurement in the immediate future: much investment of time and money will be required.

Depending upon whether OPERA is correct, we could be seeing one of the strangest, but ultimately misleading, measurements which litter the history of science. Or we could be on the verge of a revolution in our understanding of the universe and how it works, greater than at any time in the history of physics. Only time will tell.

Daniel Scully is a particle physicist at the University of Warwick studying the smallest most fundamental building blocks of our universe, to understand how it all works.

His work focuses on the weak and mysterious particles, known as neutrinos, which could hold the answers to some of the biggest questions in particle physics. This includes working on the T2K experiment, based in Japan, which fires a beam of neutrinos 300km through the Earth to investigate how they behave.

When not doing research, Daniel attempts to pass on his enthusiasm for particle physics to anyone he can by organising or participating in events for school students, science groups or society in general.