Did the CERN Neutrinos Really Travel Faster Than Light?

Sep 28, 2011
John Austin

Part of the neutrino beam between CERN and Grand Sasso. - CERN and Gran Sasso

The announcement of faster than light neutrinos has captured the imagination of the public. Here, questions are answered, and some explanations are offered.

The Experiments

As discussed here and elsewhere (see references below), faster than light neutrinos have been detected traveling between CERN, near Geneva, Switzerland and Gran Sasso in Italy. The experimental results have been presented in detail.

Public Discussion

During the first week of the announcement of the results, there has been a tremendous amount of internet discussion (see references below). The BBC site alone recorded almost 1200 comments before it was closed to further discussion. Here I choose a number of questions raised by the public and provide some answers based mostly on the physicists' detailed study. The questions are chosen for their teaching quality as well as those that might provide insight into what actually took place.

Questions and Answers

How Can Particles Travel Through Rock?

The journey between the labs. was through the Earth's crust (see photo). Ordinary matter is almost all empty space. For each atom there is a nucleus surrounded by an electron "cloud". It is the cloud that gives us the impression of the solid nature of materials.

Neutrinos are barely affected by matter and can penetrate rock almost entirely without loss of momentum. This is perhaps one of the most difficult ideas to grasp.

Doesn't Common Sense Contradict Relativity Anyway?

Common sense is often not accurate enough to be trusted. If we used our common sense, we would still think the sun revolved around the Earth amongst other things. Common sense is only applicable in our own immediate experience. We have not experienced close to light speed travel, nor can we sense the properties of matter on the sub-atomic scale.

Physics operates by ignoring common sense and using precise instrumentation instead. This has been an extremely successful strategy in providing society with the basis for designing the technology that we take for granted, such as computers and cell phones.

What is the Speed of Light Relative to?

The speed of light is absolute: it is the same for all observers according to the theory of relativity. This assumption was built into the theory to explain early experiments which could not detect the motion of the Earth using beams of light.

Relativity provides predictions for a range of phenomena where high speeds are involved. Some of the predictions are quite unlike what we experience at low speeds. The General Theory of Relativity, which also takes account of gravity, has been confirmed by measurements to high precision. Until now, that is.

Could the Distance Travelled be Wrong?

The experimentalists went to considerable trouble to measure the distance using the global positioning system. The method allows a baseline measurement to a precision of 20 cm (8 inches) in 732 km (454 miles).

What is the Error in Making the Velocity Calculations?

The experimenters quote random and systematic errors of ± 7 ns each, much smaller than the anomalous time advance of 60 ns. These errors are equivalent to velocity errors of 0.9 km/s or 0.0003%.

What Would be the Effect of Earthquakes?

The measurements took place over a period of several years so that the distance between the laboratories could be accurately calibrated. The study shows that there was a steady drift in the distance between the labs at the rate of about 2 cm per year due to earthquakes and continental drift. In addition, the 2009 L'Aquila earthquake caused a 6 cm displacement.

Has There Simply Been a Mistake in the Velocity of Light?

No! The experiments determine the timing difference between the Neutrinos and the velocity of light in a vacuum. The velocity of light is slower in all other mediums. This is measured via the "refractive index". for example the refractive index of glass is 1.5 and the speed of light in glass is 1/1.5 = 2/3 that in a vacuum.

How Were the Neutrinos Timed?

The instruments used atomic clocks accurate to 1 second in 15000 years. They were synchronized at the two labs. to 1 ns precision, much smaller than the 60 ns early arrival found. The time of travel was determined by comparing the pattern of the departing neutrinos with the arriving ones.

One possible area of error will focus in particular on the start of the neutrinos' journey, which depended on the accuracy of the timing from the decay of mesons over a 1 km long vacuum tunnel. However, the mesons were also travelling at close to the speed of light, so errors from this source should be small.

What Other Explanations Are There for the Unexpected Results?

1. One possible explanation relates to the nature of the neutrinos in the experiment. They were high energy particles travelling with 10 billion times the energy of their rest mass. It may be that there are interactions with the gravitational field of the Earth which have not been allowed for, but which might be understood using the Theory of General Relativity.

If General Relativity were important, some dependence on energy (mass) would have been expected. However, the experimentalists found only a weak, not statistically significant, dependence on neutrino energy.

2. Another idea is that the quantum fluctuations of the vacuum might slow down photons. Because of the lack of interaction of neutrinos, they would be unaffected and would be able to travel faster than light. The difference, however, at 0.0025% seems rather large. Solving this problem would require the unification of quantum mechanics with relativity which is known to be a current weakness of relativity theory.

3. Finally, an explanation put forward by Prof. Brian Cox and others is that there may be hidden dimensions which enabled the neutrinos to "take a short cut" through the Earth and arrive at Gran Sasso early. This may be more difficult to test than the other possibilities.

The Future

CERN and Gran Sasso will want to repeat the experiments to try to confirm them, as will other groups. In the meantime the scientific community will likely view the results with excitement tempered with skepticism. Perhaps the reason for the anomalous results will be discovered within a few weeks.