Read more: “Neutrinos: Complete guide to the ghostly particle“

Faster-than-light neutrinos can’t catch a break. If they exist they would not only flout special relativity but also the fundamental tenet that energy is conserved in the universe. This suggests that either the speedy neutrino claim is wrong or that new physics is needed to account for it.

In September, physicists with the OPERA experiment in Gran Sasso, Italy, reported that neutrinos had apparently travelled there from CERN near Geneva, Switzerland, faster than light.

The claim threatened to blow a hole in modern physics – chiefly Einstein’s special theory of relativity, which set the speed of light as the absolute limit for all particles in the universe.


Now a team including Shmuel Nussinov of Tel Aviv University in Israel says it could also put a dent in the principle of the conservation of energy. “This is such a holy principle that has been verified in so many ways,” he says.

The speeding neutrinos were born in a particle accelerator at CERN, when protons slammed into a stationary target and produced a shower of unstable particles called pions. Each pion quickly decayed into both a neutrino and a heavier particle called a muon.

The muons stopped at the end of a tunnel, but the neutrinos, which slip through most matter like ghosts, continued 730 kilometres through the Earth to the OPERA experiment.

Unequal inheritance

The neutrinos apparently outpaced light by 60 nanoseconds. But if energy is conserved – that is, if the daughter muon and neutrino together have the same amount of energy as the pion they decayed from – the neutrinos could not have gone so fast, the team say.

That’s because the rules for inheriting energy treat slow-moving particles differently from those moving close to the speed of light. If the neutrinos did begin their lives moving faster than light, then their slower muon siblings should have gotten the lion’s share of their parents’ energy.

“It’s like a very rich father with a son who wants to go into business and continue by the rules, and a son who’s a black sheep,” Nussinov says. “He bequeaths everything to the good son, and gives the other one literally nothing.”

The energy available to faster-than-light neutrinos from the CERN pions is too small by a factor of 10 to explain the speeds reported, the team say.

Could the neutrinos have started off slow, thereby getting a larger share of the inheritance, and then been accelerated somehow? Possibly, but Nussinov says this is unlikely because neutrinos usually do not interact with anything, making it hard to understand what could be doing the accelerating.

‘Absolute contradiction’

This unequal energy inheritance is even more pronounced if the parent pion has more energy – the more energetic the pion is, the more energy it gives to the muon.

This can be seen in atmospheric pions, which are produced when cosmic rays slam into the atmosphere. Atmospheric pions are 100 to 1000 times more energetic than the pions produced at CERN, and their muon and neutrino progeny have correspondingly high energies.

If neutrinos really can outstrip light as much as the OPERA measurement suggests, the atmospheric pions should decay completely into muons. Curtailing the number of possible decay routes means the pions should decay less often, and the team calculates that they should slam into the Earth before they get a chance to decay. In that case, we should not see any high-energy muons or neutrinos at all.

But we do. Experiments like the IceCube neutrino telescope at the South Pole have seen high-energy neutrinos – and billions of muons to boot.

“We have an absolute contradiction right there,” Nussinov says. “We know there are many, many neutrinos produced with high energy.”

Exotic explanations

The argument adds to a growing list of theoretical strikes against faster-than-light neutrinos. The most popular so far claims that if the particles ever broke the speed of light, they would quickly radiate away their energy and slow down.

“We say energetic neutrinos of this deviant type will never be born; they say if they are born, they will die quickly,” Nussinov says.

Both arguments are worrisome, agrees OPERA team member Luca Stanco of the National Institute of Nuclear Physics in Italy, who did not sign the original version of the paper because he thought it was too preliminary.

“As a result, you may understand that the physics community (and myself, in particular) is still quite embarrassed by the OPERA measurement: it does not fit well in any physics frame now known,” he says. “We urgently require an independent confirmation of the OPERA measurement.”

If the measurement holds up, the only way out may be to break all the rules, Nussinov says. “I would have loved to have [the result be true], but it’s just inconsistent with basic, basic things,” he says. “The only way to avoid this thing is to assume that, well, maybe on the way they went to other dimensions or something.”

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.107.251801