 |
| The Super-Kamiokande Neutrino Detector |
By Geoff Brumfiel, Nature, September 23, 2011
An Italian experiment has unveiled evidence that
fundamental particles known as neutrinos can travel faster than light. Other
researchers are cautious about the result, but if it stands further scrutiny,
the finding would overturn the most fundamental rule of modern physics — that
nothing travels faster than 299,792,458 metres per second.
The experiment is called OPERA (Oscillation Project with
Emulsion-tRacking Apparatus), and lies 1,400 metres underground in the Gran
Sasso National Laboratory in Italy. It is designed to study a beam of neutrinos
coming from CERN, Europe's premier high-energy physics laboratory located 730
kilometres away near Geneva, Switzerland. Neutrinos are fundamental particles
that are electrically neutral, rarely interact with other matter, and have a
vanishingly small mass. But they are all around us — the Sun produces so many
neutrinos as a by-product of nuclear reactions that many billions pass through
your eye every second.
The 1,800-tonne OPERA detector is a complex array of
electronics and photographic emulsion plates, but the new result is simple —
the neutrinos are arriving 60 nanoseconds faster than the speed of light
allows. "We are shocked," says Antonio Ereditato, a physicist at the
University of Bern in Switzerland and OPERA's spokesman.
Breaking the law
The idea that nothing can travel faster than light in a
vacuum is the cornerstone of Albert Einstein's special theory of relativity,
which itself forms the foundation of modern physics. If neutrinos are
travelling faster than light speed, then one of the most fundamental
assumptions of science — that the rules of physics are the same for all
observers — would be invalidated. "If it's true, then it's truly
extraordinary," says John Ellis, a theoretical physicist at CERN.
Ereditato says that he is confident enough in the new
result to make it public. The researchers claim to have measured the
730-kilometre trip between CERN and its detector to within 20 centimetres. They
can measure the time of the trip to within 10 nanoseconds, and they have seen
the effect in more than 16,000 events measured over the past two years. Given
all this, they believe the result has a significance of six-sigma — the
physicists' way of saying it is certainly correct. The group will present their
results tomorrow at CERN, and a preprint of their results will be posted on the
physics website ArXiv.org.
At least one other experiment has seen a similar effect
before, albeit with a much lower confidence level. In 2007, the Main Injector
Neutrino Oscillation Search (MINOS) experiment in Minnesota saw neutrinos from
the particle-physics facility Fermilab in Illinois arriving slightly ahead of
schedule. At the time, the MINOS team downplayed the result, in part because
there was too much uncertainty in the detector's exact position to be sure of
its significance, says Jenny Thomas, a spokeswoman for the experiment. Thomas
says that MINOS was already planning more accurate follow-up experiments before
the latest OPERA result. "I'm hoping that we could get that going and make
a measurement in a year or two," she says.
Reasonable doubt
If MINOS were to confirm OPERA's find, the consequences
would be enormous. "If you give up the speed of light, then the
construction of special relativity falls down," says Antonino Zichichi, a
theoretical physicist and emeritus professor at the University of Bologna,
Italy. Zichichi speculates that the 'superluminal' neutrinos detected by OPERA
could be slipping through extra dimensions in space, as predicted by theories
such as string theory.
Ellis, however, remains sceptical. Many experiments have
looked for particles travelling faster than light speed in the past and have
come up empty-handed, he says. Most troubling for OPERA is a separate analysis
of a pulse of neutrinos from a nearby supernova known as 1987a. If the speeds
seen by OPERA were achievable by all neutrinos, then the pulse from the
supernova would have shown up years earlier than the exploding star's flash of
light; instead, they arrived within hours of each other. "It's difficult
to reconcile with what OPERA is seeing," Ellis says.
Ereditato says that he welcomes scepticism from
outsiders, but adds that the researchers have been unable to find any other
explanation for their remarkable result. "Whenever you are in these
conditions, then you have to go to the community," he says.
No comments:
Post a Comment