Can Neutrinos Travel Faster than Light?
The results picked up by the Opera team, in Italy, were so shocking because they raised the question not just about Einstein’s Theory but all the evidence that’s been gathered to support it. That said, we shouldn’t be so shocked by the results because the Opera scientists were studying one of the strangest and least understood particle there is - the neutrinos. And if there was one particle that was going to break the rules, it was this one.
Professor Joao Magueijo, Imperial College London
Professor Joao Magueijo, Imperial College London “The neutrino’s been the bad boy of physics and, basically, putting physicists out of their comfort zone. A lot of unusual things have been revealed by the neutrino.”
There are 16 types of fundamental particle, that are the smallest, simplest building blocks in the universe. Together they explain the world and what holds it together. Three of these elementary particles are neutrinos.
Their existence was first predicted in 1930 by Austrian physicist Wolfgang Pauli. But, Pauli didn’t think it would ever be possible to find one, because their properties make them incredibly difficult to spot.
Professor Magueijo “Its a very antisocial particle. It doesn’t like to talk to the world in general. Right now, you are being crossed by billions of neutrinos every second. They go through you, the Earth, everything without interacting at all. The universe is pervaded by them, there are many more neutrinos than particles of light or atoms.”
In order to understand how neutrinos are able to travel straight through matter without being noticed, we need to think about what matter is made of. Every physical thing in the world around us from mountains and buildings to you and me, is made of atoms and atoms are made up of a nucleus at the centre surrounded by orbiting electrons a bit like the solar system with a Sun and orbiting planets.
If the atom is the size of the solar system then the neutrino is the size of a golf ball.
The mind boggling thing about matter is that although it looks and feels very solid, it’s actually mostly empty space. There are vast swathes of nothingness between the tiny nucleus and the orbiting electrons. The neutrino is so small and without any charge that it can pass through this space very easily. In fact, the neutrino is so tiny that if the atom is the size of the solar system then the neutrino is the size of a golf ball.
These tiny particles existed in theory for a quarter of a century without anybody being able to see them. But then, something happened to change that - The Nuclear Bomb!
The power of a nuclear bomb comes from a chain reaction, splitting atomic nuclei. In the 1950s, a young researcher, Fred Reines, realised that this chain reaction produced an intense burst of neutrinos and so, be the perfect place to hunt for the elusive particle. But, detecting neutrinos from a nuclear explosion wasn’t practical. So, Reines turned his attention to the much more controlled reaction in a nuclear reactor.
Although most neutrinos produced by the reactor pass through the gaps inside atoms, so many neutrinos were produced that, every now and then, one would collide with an atom’s nucleus. When it did, a charge particle would be ejected. He set up his experiment which he called Project Poltergeist and waited for the characteristic signal of this interaction. A distinctive double-pulse of energy. In June 1956 Reines announced that he had detected the neutrino.
Since that first discovery, we’ve become a bit more adept at creating and observing this most elusive of particles. We’ve created neutrinos in man-made particle accelerators like the ones in CERN in Geneva as well as detecting them naturally in cosmic rays and from the Sun.
We now know they are essential to our existence. All of the elements are made by nuclear reactions that would be impossible without neutrinos. We also know, that despite their tiny size, they do still have a small mass. Which means, according to Einstein, that they can’t travel faster than the speed of light.
But that theory has now been challenged by a small group of scientists working in one of the most unusual science labs in the world. Assergi is a sleepy town nestled beneath Gran Sasso, a 3,000 metre peak in The Apennines of central Italy. In the early 1980s, a new road was planned here and cut right through the mountain. Italian scientists had a brilliant idea. They realised that the road would give them a unique opportunity to create a physics lab like no other. It would give them easy access to the heart of the mountain, the perfect place to build a neutrino detector.
Dr. Chiara Sirignano
Dr. Chiara Sirignano “Here we have 15 different experiments, and there are, roughly speaking, 100 physicists per day working here”
Neutrinos so rarely interact with matter that it is easy for an experiment to be swamped by false readings. Readings triggered by naturally occurring radiation and charged particles such as cosmic rays hitting the experiment. The only way to study neutrinos is to find some way to weed out many of these interfering particles as possible.
Dr. Chiara Sirignano “Now we are in the middle of the gallery. On top of us we have 1400 meteres of rock, the top of Gran Sasso mountain. Here the cosmic rays are very few, because outside there are 200 per square meter per secod. Here there is just one per square metre per hour”
Thanks to the mountain above it, this vast chamber is a natural laboratory for neutrino research. It was here in 2008 that scientists began work on a sophisticated experiment designed to study nature of neutrinos, it was called the Oscillation Project with Emulsion tRacking Apparatus or OPERA for short. At this stage they had no idea of the impact that OPERA would have.
To begin with measuring the speed of neutrinos was not at the forefront of the scientists minds. They were trying to understand how the three different types of neutrinos formed and how they behaved. The first step of the experiment was to create some neutrinos. For this they turned to another underground lab. CERN in Switzerland. CERN is most famous for The Large Hadron Collider, but is was two much less heralded particle accelerators that began the OPERA experiment.
The scientists began by generating a beam of protons which they accelerated around CERN’s Proton synchrotron. The proton beam was then passed into the Super Proton synchrotron to accelerate them even further. The resulting high-energy beam of protons was slammed into a graphite target. This produced a cocktail of exotic sub-atomic particles, including neutrinos, which then, flew off through the Earth in the direction of Gran Sassa. The 730km journey took them 2.4 milliseconds.
Dr. Sirignano “Several billions of neutrinos are produces every day at the CERN accelerators , they go through the Earth’s crust and reach the OPERA Detector.”
Even with billions of neutrinos streaming into the laboratory detecting them still wasn’t easy The key was the huge detector at the heart of the Gran Sasso lab. It’s made from 150,000 bricks of lead and wighs 4,500 tonnes . Lead is particularly dense which increases the chances of a neutrino encountering a nucleus. As the neutrinos smashed into the lead nucleus they created charge particles which are detected as tiny flashes of light.
The process generated around 30 flashes of light a day, and provided a chance to test more than just the type of neutrino arriving.
Dr. Tara Shears
Dr. Tara Shears “The nice thing about this experiment is, although it was really set-up to study the behaviour of neutrinos in a very fundamental sense, you can study more basic properties of them such as the speed that neutrino’s travel.”
Nobody had anticipated what happenrd when they started measuring how long it took the neutrinos to arrive. They seemed to arrive early, earlier than the laws of physics allow. 60 nanoseconds sooner than a beam of light would if it were to cover the same distance. That meant the neutrinos had travelled at just over 2,000ths of 1% faster than the speed of light. Now, if I was on the motorway, I wouldn’t expect to get into trouble for exceeding the speed limit by that small amount but, not in physics. The thing about an absolute speed limit is that it is absolute it can’t be exceeded in any circumstances by however small an amount. Under our current understanding of the universe this just isn’t possible.
The researchers themselves were pretty shocked by the results. They spent many months looking for mistakes. They brought in outside experts. They pored over the figures hundreds of times searching for an error. They even made sure they’d factored the movement continents that changes the distance between Italy and Switzerland by small amounts. But, they couldn’t find any mistakes. So, they decided to publish. When the news broke it caused a sensation.
For me this is a great example of science in action . The OPERA team found some data that they can’t explain, then for months they’ve been questioning it, doubting it and repeating it. Only after intense scrutiny did they publish it.
External Links
Faster than Light Neutrinos - Wikipedia Page
