Physicists urge caution over apparent speed of light violation By Alok Jha and Ian Sample

 Scientists react with disbelief and call for repeat experiments after results suggest particles can travel faster than light.

Photo: Graeme Robertson/Guardian

 Professor Brian Cox has said that if the Italian discovery is proved correct, it would require 'a complete rewriting of our understanding of the universe'.

 Scientists around the world reacted with shock yesterday to results from an Italian laboratory that seemed to show certain subatomic particles can travel faster than light. If true, the finding breaks one of the most fundamental laws of physics and raises bizarre possibilities including time travel and shortcuts via hidden extra dimensions.

 Scientists at the Opera (Oscillation Project with Emulsion-tRacking Apparatus) experiment in Gran Sasso, Italy, found that neutrinos sent through the Earth to its detectors from Cern, 450 miles (730km) away in Geneva, arrived earlier than they should have. The journey would take a beam of light around 2.4 milliseconds to complete, but after running the Opera experiment for three years and timing the arrival of 15,000 neutrinos, the scientists have calculated the particles arrived at Gran Sasso 60 billionths of a second earlier, with an error margin of plus or minus 10 billionths of a second. The speed of light in a vacuum is 299,792,458 metres per second, so the neutrinos were apparently travelling at 299,798,454 metres per second.

 A cornerstone of modern physics is the idea that nothing can travel faster than light does in a vacuum. At the turn of the 20th century Albert Einstein encapsulated this idea in his theory of special relativity, which proposes that the laws of physics are the same for all observers and led to the famous equation E=mc², indicating that mass and energy are equivalent.

 Brian Cox, a professor of particle physics at the University of Manchester, urged caution. "If you've got something travelling faster than light, then it's the most profound discovery of the last 100 years or more in physics. It's a very, very big deal," he said on BBC 6 Music on Friday. "It requires a complete rewriting of our understanding of the universe."

 Professor Jim Al-Khalili at the University of Surrey said it was most likely that something was skewing the results. "If the neutrinos have broken the speed of light, it would overturn a keystone theory from the last century of physics. That's possible, but it's far more likely that there is an error in the data. So let me put my money where my mouth is: if the Cern experiment proves to be correct and neutrinos have broken the speed of light, I will eat my boxer shorts on live TV."

 Opera co-ordinator Antonio Ereditato said his team was "recovering from the shock" of the discovery and would leave the physics community to explain the result. "We made a measurement and we believe our measurement is sound," he said. "Now it is up to the community to scrutinise it. We are not in a hurry. We are saying, tell us what we did wrong, redo the measurement if you can." He added: "There will be all sorts of science fiction writers who will give their own opinions on what this means, but we don't want to enter that game."

 If the measurements are shown to be correct, physicists will have to modify their understanding of special relativity. There are several theories that could help explain the results.

 Heinrich Paes at Dortmund University and colleagues believe it might be possible for neutrinos to move through hidden extra dimensions of space and effectively take shortcuts through space-time.

 "The extra dimension is warped in a way that particles moving through it can travel faster than particles that go through the known three dimensions of space. It's like a shortcut through this extra dimension. So it looks like particles are going faster than light, but actually they don't."

 Another potential explanation for the observation was given by Alan Kostelecky at Indiana University. He proposed in 1985 that an energy field that lies unseen in the vacuum could allow neutrinos to move faster through space than photons, the particles that make up light.

 "This is a field that sits in the vacuum and as a result, things travelling, in the vacuum will have unconventional properties," he said. "It may very well be that neutrinos travel faster than light does in that medium. It is not at all unreasonable that that would be the case."

 Professor Dave Wark, leader of the UK group on the T2K neutrino experiment in Japan, cautioned that scientists would "require a very high standard of proof and confirmation from other neutrino experiments around the world".

 Susan Cartwright, senior lecturer in particle astrophysics at Sheffield University, said there were many potential sources of error in the Opera experiment. "The sort of thing you might worry about is have they correctly accounted for the time delay of actually reading out the signals? Whatever you are using as a timing signal, that has to travel down the cables to your computer and when you are talking about nanoseconds, you have to know exactly how quickly the current travels, and it is not instantaneous."

 Cartwright works on T2K, which sends neutrinos over a 295km distance. "We could certainly check this, but MINOS [the neutrino experiment at Fermilab in the US] are in a better position because we are still doing repairs after the earthquake that struck Japan."

 Professor Jenny Thomas of University College London, and a spokesperson for the MINOS neutrino experiment, said if the discovery was proved correct, it "would overturn everything we thought we understood about relativity and the speed of light".

 Ereditato said the Opera team was going through a mix of feelings. "There is excitement, adrenaline, because you feel you have hit something hot. Another feeling is exhaustion. A third feeling is let's look again and again and think of other checks we have not yet done."

Q&A

What has been discovered?

 A fundamental subatomic particle, the neutrino, seems to be capable of travelling faster than the speed of light.

Where on the scale of amazing/ surprising is this finding?

 If the Gran Sasso results are correct, scientists would have reason to believe that Einstein's of special relativity is wrong. This is troubling, as the theory has been tested countless times in experiments and never disproved.

 The trip would take a beam of light around 2.4 milliseconds to complete, but after running the experiment for three years and timing the arrival of 15,000 neutrinos, the scientists discovered that the particles arrived at Gran Sasso 60 billionths of a second earlier, with an error margin of plus or minus 10 billionths of a second.

 Since the speed of light in a vaccum is 299,792,458 metres per second, the neutrinos were apparently travelling at 299,798,454 metres per second.

What are neutrinos?

 Neutrinos are electrically neutral particles that have a tiny (but non-zero) mass. They interact very weakly with normal matter, making them almost impossible to detect. Tens of billions of neutrinos pass through your fingertip every second. They are created in certain types of radioactive decay, during collisions between atoms and cosmic rays and during nuclear reactions such as those that occur at the heart of the Sun.

Are there any theories that might explain the result?

 If the result is proved correct – and that is still a big if – you have to go into some relatively uncharted areas of theoretical physics to start explaining the it, if they are proved correct - and that is still a bit if. One idea is that the neutrinos are able to access some new, hidden dimension of space, which means they can take shortcuts. Joe Lykken of Fermilab told the New York Times: "Special relativity only holds in flat space, so if there is a warped fifth dimension, it is possible that on other slices of it, the speed of light is different."

 Alan Kostelecky, an expert in the possibility of faster-than-light processes at Indiana University, put forward an idea in 1985 predicting that neutrinos could travel faster than the speed of light by interacting with an unknown field that lurks in the vacuum. "With this kind of background, it is not necessarily the case that the limiting speed in nature is the speed of light," he told the Guardian. "It might actually be the speed of neutrinos and light goes more slowly."

Does this mean that time travel is possible?

 Don't hold your breath – we won't be routinely jumping into the past in DeLoreans any time soon. If particles could travel faster than light, special relativity suggests travelling backwards through time is a possibility, but how anyone harnesses that to do anything useful is beyond the reach of any technology or material we have today.

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