guardian.co.uk,
Alok Jha, science correspondent, Friday 18 November 2011
 |
| Scientists from Cern have repeated their finding of neutrinos travelling faster than the speed of light. Photograph: Cern/Science Photo Library |
The
scientists who appeared to have found in September that certain subatomic particles can travel faster than light have ruled out one potential source of
error in their measurements after completing a second, fine-tuned version of
their experiment.
Their
results, posted on the ArXiv preprint server on Friday morning and submitted
for peer review in the Journal of High Energy Physics, confirmed earlier
measurements that neutrinos, sent through the ground from Cern near Geneva to
the Gran Sasso lab in Italy 450 miles (720km) away seemed to travel faster than
light.
The finding
that neutrinos might break one of the most fundamental laws of physics sent
scientists into a frenzy when it was first reported in September. Not only
because it appeared to go against Albert Einstein's theory of special
relativity but, if correct, the finding opened up the troubling possibility of
being able to send information back in time, blurring the line between past and
present and wreaking havoc with the fundamental principle of cause and effect.
The
physicist and TV presenter Professor Jim Al-Khalili of the University of Surrey
expressed the incredulity of many in the field when he said that if the
findings "prove to be correct and neutrinos have broken the speed of
light, I will eat my boxer shorts on live TV".
In their
original experiment scientists fired beams of neutrinos from Cern to the Gran
Sasso lab and the neutrinos seemed to arrive sixty billionths of a second
earlier than they should if travelling at the speed of light in a vacuum.
One
potential source of error pointed out by other scientists was that the pulses
of neutrinos sent by Cern were relatively long, around 10 microseconds each, so
measuring the exact arrival time of the particles at Gran Sasso could have
relatively large errors. To account for this potential problem in the latest
version of the test, the beams sent by Cern were thousands of times shorter –
around three nanoseconds – with large gaps of 524 nanoseconds between them.
This allowed scientists to time the arrival of the neutrinos at Gran Sasso with
greater accuracy.
Writing on his blog when the fine-tuned experiment started last month, Matt Strassler, a
theoretical physicist at Rutgers University, said the shorter pulses of
neutrinos being sent from Cern to Gran Sasso would remove the need to measure
the shape and duration of the beam. "It's like sending a series of loud
and isolated clicks instead of a long blast on a horn," he said. "In
the latter case you have to figure out exactly when the horn starts and stops,
but in the former you just hear each click and then it's already over. In other
words, with the short pulses you don't need to know the pulse shape, just the
pulse time."
"And
you also don't need to measure thousands of neutrinos in order to reproduce the
pulse shape, getting the leading and trailing edges just right; you just need a
small number – maybe even as few as 10 or so – to check the timing of just
those few pulses for which a neutrino makes a splash in Opera."
Around 20
neutrino events have been eventually measured at the Gran Sasso lab in the
fine-tuned version of the experiment in the past few weeks, each one precisely
associated with a pulse leaving Cern. The scientists concluded from the new
measurements that the neutrinos still appeared to be arriving earlier than they
should.
"With
the new type of beam produced by Cern's accelerators we've been able to to
measure with accuracy the time of flight of neutrinos one by one," said
Dario Autiero of the French National Centre for Scientific Research (CNRS).
"The 20 neutrinos we recorded provide comparable accuracy to the 15,000 on
which our original measurement was based. In addition their analysis is simpler
and less dependent on the measurement of the time structure of the proton
pulses and its relation to the neutrinos production mechanism."
In a
statement released on Friday, Fernando Ferroni, president of the Italian
Institute for Nuclear Physics, said: "A measurement so delicate and
carrying a profound implication on physics requires an extraordinary level of
scrutiny. The experiment Opera, thanks to a specially adapted Cern beam, has
made an important test of consistency of its result. The positive outcome of
the test makes us more confident in the result, although a final word can only
be said by analogous measurements performed elsewhere in the world".
Since the
Opera (Oscillation Project with Emulsion-tRacking Apparatus) team at Gran Sasso
announced its results, physicists around the world have published scores of
online papers trying to explain the strange finding as either the result of a
trivial mistake or evidence for new physics.
Dr Carlo Contaldi of Imperial College London suggested that different gravitational
effects at Cern and Gran Sasso could have affected the clocks used to measure
the neutrinos. Others have come up with ideas about new physics that modify
special relativity by taking the unexpected effects of higher dimensions into
account.
Despite the
latest result, said Autiero, the observed faster-than-light anomaly in the
neutrinos' speed from Cern to Gran Sasso needed further scrutiny and
independent tests before it could be refuted or confirmed definitively. The
Opera experiment will continue to take data with a new muon detector well into
next year, to improve the accuracy of the results.
The search
for errors is not yet over, according to Jacques Martino, director of the
National Institute of Nuclear and Particle Physics at CNRS. He said that more
checks would be under way in future, including ensuring that the clocks at Cern
and Gran Sasso were properly synchronised, perhaps by using an optical fibre as
opposed to the GPS system used at the moment. This would remove any potential
errors that might occur due to the effects of Einstein's theory of general
relativity, which says that clocks tick at different rates depending on the
amount of gravitational force they experience – clocks closer to the surface of
the Earth tick slower than those further away.
Even a tiny
discrepancy between the clocks at Cern and Gran Sasso could be at the root of
the faster-than-light results seen in September.
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