Tentative evidence of particles travelling faster than light
4 November 2011
Scientists working on the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment presented in September the results of two years of experimental observations at a European Organisation for Nuclear Research (CERN) seminar. OPERA is an international collaboration of about 200 physicists from 36 institutions and 13 countries. The OPERA detector is located in the Gran Sasso National Laboratory in Italy and has been collecting data since 2009.
The results appear to indicate that neutrinos travel at a velocity greater than the speed of light. In its press release CERN cautiously stated: “Given the potential far-reaching consequences of such a result, independent measurements are needed before the effect can either be refuted or firmly established. This is why the OPERA collaboration has decided to open the result to broader scrutiny.”
As the press release indicated, the results, if confirmed, would require a fundamental rethink of our current understanding of physics. Einstein’s theory of special relativity—a cornerstone of modern physics—is based on the conception that nothing with mass can travel faster than the speed of light in a vacuum. Jenny Thomas of University College London said: “The impact of this measurement, were it to be correct, would be huge. In fact it would overturn everything we thought we understood about relativity and the speed of light.”
The experiment conducted by the OPERA collaboration created neutrinos by accelerating protons and colliding them into a graphite target in the particle accelerator at CERN. The process creates mesons, which in turn decay into muon neutrinos with very high energy levels (17 Gev or Giga-electron-volts). The detection apparatus is located 730 km away, deep underground in the Gran Sasso laboratory.
A series of high-precision measurements determined the distance between the CERN facility and the detector in Italy to within an uncertainty of 20 cm and the time of flight to within 10 nanoseconds. The analysis reported at the seminar was based on more than 15,000 neutrino events recorded over the past two years. It showed that the neutrinos arrived at the detector, on average, 60 nanoseconds before they were expected, leading to the conclusion that they were travelling faster than the speed of light.
The researchers were not able to time individual neutrinos as they travelled from the CERN accelerator to the detector at Gran Sasso. Neutrinos, which have been characterised in layman’s terms as “nothing spinning,” are notoriously difficult to detect. Instead, experimenters measured the time distribution of the protons just before they hit the graphite bar, and the time distribution of the corresponding neutrinos as they were detected at the Gran Sasso laboratory.
This is not the first time that results have indicated that neutrinos travel faster than light. In 2007 the MINOS group from Fermilab in the US presented the results of similar experiments. They created an artificial beam of neutrinos that was measured by a detector deep underground about 734 km from the lab where they were created. The results of that experiment also showed that the velocity of neutrinos was greater than the speed of light. However, the experimental uncertainties were much larger, making the result even less reliable. It is expected that the MINOS group will now conduct further tests in an attempt to replicate the results of the OPERA team.
Science has long been interested in the fundamental building blocks of the universe and how they interact. Over the past century, from a rudimentary understanding of protons, neutrons and electrons, far more complex models have emerged involving a large number of subatomic particles. The theory that embodies the current understanding is called the Standard Model of particle physics and is consistent with the results of high energy physics experiments carried out over the past 60 years. It is a theory encompassing three of the four known fundamental forces—weak, strong and electromagnetic—and the elementary particles, including neutrinos, that take part in the corresponding interactions.
Neutrinos are neutrally charged subatomic particles that are created in nuclear reactions. The existence of neutrino particles was first postulated in 1930 by Wolfgang Pauli to explain the apparent loss of energy when electrons were produced by the decay of neutrons (a process known as beta decay). To account for the missing energy, Pauli postulated the presence of neutrally charged particles with no mass. Fermi, who developed the theory of beta decay, called the particle the neutrino. It was not until 1956 that neutrinos were first observed experimentally in what is known as the Cowan-Reines neutrino experiment.
In 1962, when the muon neutrino was detected experimentally, it was confirmed that there was more than one “flavour” or type of neutrino. When the tau particle was discovered in 1975, it was expected to have an associated neutrino. The first detection of tau neutrino interactions was in 2000. Currently there are thought to be three types of neutrinos—the electron, muon and tau neutrinos associated with the electron, muon and tau particles.
Experimental evidence suggests that neutrinos oscillate or change between different types. In 1998, experiments at the Super-Kamiokande detector in Japan found evidence for such oscillations in atmospheric neutrinos—produced when cosmic rays interact with nuclei in the Earth’s atmosphere. Theoretically, the ability to oscillate requires that neutrinos have mass. While the mass of the neutrino is not known, it is believed to be very small, approximately one millionth the mass of an electron.
It is the combined results—that neutrinos have mass and appear to be travelling faster than the speed of light—that, if confirmed, would force a fundamental rethinking of modern physics. According to Einstein’s special relativity theory, no object with mass can attain, let alone exceed, the speed of light in a vacuum—slightly less than 300,000,000 metres/second.
Many physicists are sceptical about the recent findings and cite older results that differ significantly from the new measurements presented by the OPERA team. One contradictory result comes from observations from the 1987 supernova event—the explosion of a large star.
In February 1987, the radiation from a supernova explosion, including a barrage of neutrinos, reached the Earth. Since the neutrinos only weakly interact with other matter, it was expected that they would escape the exploding star almost immediately, whereas photons or light particles would take about three hours to escape. Neutrinos from the supernova were detected three hours before the photons reached earth, in precisely the way that physicists had predicted, based on neutrinos travelling at the speed of light. If the neutrinos had been travelling at the speeds hypothesised by the OPERA team, they should have reached the Earth years earlier.
There is no doubt that the results of the OPERA collaboration will need to be checked. Physicsworld.com reports that a number of the OPERA group leaders were opposed to the release of the paper on the arXiv preprint server. About 10 senior members out of the team of 170 people did not put their names to the arXiv preprint. Their main concerns included the need for independent checks on possible errors in the experimental design, and the statistical treatment of the data collected.
As reported in the Guardian, one of the OPERA members, Luca Stanco, said that while he supported the release of the results, he considered the analysis as preliminary and that it had been presented as more definitive than it was. Laura Patrizii, who did not sign the paper, said: “It is not that people think there is a mistake that is being hidden, but since something going faster than light would kill modern physics as we know it, some researchers would feel more at ease with these independent checks.”
The CERN press release is available here.
The OPERA article is available here.