Neutrinos: speed record?
written by Simona Romaniello
Apparently the sensational discovery made by CERN researchers of Geneva, which claims that neutrinos travel faster than light, seems to have been the result of an instrumental error. In fact, an accurate measurement of the distance between the CERN and the LNGS has highlighted a small error in the initial measurement, which caused the miscalculation of the speed of neutrinos. The confutation, published in the important journal Science, makes the debate start all over again.
On 22 September, the newspaper “Il Giornale” published an interview in which Prof. Antonino Zichichi declared that researchers of the Gran Sasso National Laboratories (LNGS) in Abruzzo had discovered that neutrinos travelled faster than light. At once the news ricochetted through the media like a rubber ball and, in defiance of friction, got greater speed and momentum with each rebound. For the world of research it was a bombshell because the news was on the point of being broadcast. The researchers of the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment, the international project lead by Italian scientists, whose headquarters are in the Gran Sasso National Laboratories and who detected the neutrino beam sent from the CERN in Geneva, would have wanted to make the announcement that night in an article posted on arxiv.org and with a public conference at CERN the following day.
The properties of neutrinos
Let us take a step back and get to know neutrinos better. Do they have mass? How can they travel faster than light?
Neutrinos are elementary particles, just like electrons, but without an electric charge. They were detected for the first time in 1956 by the physicists C.Cowan and F. Reines, even though their existence had been postulated years before, in 1930, by the physicist Wolfgang Pauli. The latter needed to explain an energetic anomaly of the beta decay of a neutron into a proton, electron and antineutrino. Although Pauli had not yet considered the presence of a neutrino, he noticed a violation of the law of conservation of energy. However, in physics, existing conservation laws are difficult to modify, so Pauli tried to maintain it, postulating the existence of a neutral particle, invisible to instruments, that could add the “missing”energy in order to balance the energy equation. Neutrinos come from different sources; they are produced as a result of the interaction of cosmic rays in the Earth's atmosphere, in nuclear fusion reactions in the Sun and in supernova explosions. There are three known types (flavours) of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos. It has been observed that neutrinos seems to oscillate between these three flavours during their propagation. Moreover, the discovery of their “evanescent” nature has proven that neutrinos have a very small mass: 100,000 or maybe even one million times less than that of an electron. Hence it can be understood why they rarely interact, which means that they can travel through enormous layers of matter without being affected by them at all. In order to understand this better, it would take a lead wall one light-year thick (over 63,000 times the distance between the Earth and the Sun) to manage to stop half the neutrinos that pass through it. After this preamble, it seems logical to ask how the OPERA experiment detects the neutrinos travelling from the CERN to the Gran Sasso Laboratories and how it can measure their speed.
How is the neutrino beam in the OPERA experiment produced?
In order to have a neutrino, you must wait for a particle to produce one in a reaction involving decay. The neutrinos used in the OPERA experiment are muon neutrinos, obtained when a positive pion, a subatomic particle with a positive charge, decays (or is transformed) into an elementary particle (anti-muon) and a muon neutrino. Just like the Italian nursery rhyme which says “ To make a tree you need a seed...” to make a neutrino you need positively-charged pions. This can be done quite easily: it is sufficient that a proton beam hits a target made of protons. The impact produces positive pions and other particles such as neutrons and photons. Having solved one problem, we are faced with another one: how can a neutrino beam be obtained? It is as if we had been given a mixed pulses soup when we had ordered lentil soup. You might think a solution is to collect them one by one. There's the rub! Any action you undertake to deviate them, neutrinos will always give you the slip and will continue to follow the direction of the initial beam. Going back to our soup analogy, we would have to use a strainer, which can be exemplified by the combined action of magnetic fields and walls, that stops the other particles but allows neutrinos to pass. First of all a magnetic field is utilised to separate the positively-charged pion from the other neutral particles that we are not interested in. These are then stopped by a wall. Subsequently we wait for the pions to decay into muons and neutrinos and lastly a magnetic field is activated once again in order to deviate the muons. The neutrinos can now proceed undisturbed on their 730-km-long journey to the Gran Sasso Laboratories.
How can the speed of neutrinos be measured?
In the 100 metres world record, the Jamaican athlete Usain Bolt ran the distance in 9.58 seconds, running at an average speed of 10.44 m/s equivalent to 37.58 km/h. Hence the speed of any object can be calculated by measuring the time it takes for that object to cover a distance whose length is known.
It may seem an easy process but actually problems arise at this very point which cast doubts on the result obtained. Let us consider the distance of 730 km that separates the CERN from the LNGS: how has it been measured?
How precise is the measurement? The distance, measured using topographic surveys (those used in road construction sites), was calculated with a precision of 20 cm, hence the distance the neutrinos travel is 730534.61 ± 0.20 m. However, there are uncertainties as to how these measurements were taken, especially as far as the last part of the journey is concerned, because it passes through a cavern in the mountain. In addition to this, errors due to tidal forces must be taken into account.
In fact, the attraction of the Moon can deform the Earth's crust and modify the above-mentioned distance. If the measurement of the distance seems questionable, the measurement of the time represents the main topic of debate and consequently even the calculation of the speed. The emission and arrival times of the neutrino beam are measured independently by the two laboratories; this requires perfect clock synchronisation. This is achieved thanks to two GPS receivers that have approximately a 100 ns accuracy. The OPERA researchers, however, claim that they have considerably reduced uncertainty. Once the clocks are synchronised, we are ready to carry out the measurement. The first problem that arises is that only part of the neutrinos generated at CERN actually reach the Gran Sasso. In fact, the conically-shaped neutrino beam widens along the way and hence some neutrinos fall beyond the area marked off by OPERA. It is therefore impossible to calculate the travelling time of a single neutrino. However, one can determine the average time taken by the 'packet' of neutrinos generated at CERN to reach the OPERA detector. The result is that the neutrino packet should have covered the distance in about 2.4 milliseconds (2.4 thousandths of a second), but it was detected 61 nanoseconds (61 billionths of a second) sooner, i.e. to put it in sports terminology, when the neutrinos crossed the finishing line, light was 18 metres behind.
On 23 February 1987 the supernova SN1987A was observed in the Large Magellanic Cloud at a distance of about 168,000 light-years from Earth.
The supernova originated from the explosion of a massive star, a blue supergiant, in which large quantities of neutrinos were produced. Observations of this supernova have allowed an extremely precise measurement of neutrino speed. In fact, the measurements of the flight time of the neutrinos generated by the supernova SN1987A are definitely more accurate than those of the OPERA experiment since the discrepancy between expected and effective time is of several years and not of just a few nanoseconds, like in the OPERA experiment.
The results obtained show a speed value inferior to that of light; however, it must be highlighted that the neutrinos observed in that occasion were electron neutrinos, while those of the OPERA experiment are muon neutrinos, that have one thousand times more energy than electron neutrinos. If what OPERA scientists discovered should be true, and considering that the supernova is 168,000 light-years away, it can be deduced that the neutrinos that originated from the explosion should have arrived 3.36 years before the light, but this was not recorded. Hence the two measurements appear to be inconsistent.
Whether the fastest particles are neutrinos or photons, the laws of physics would not change. The second postulate of the theory of relativity claims that no object with mass can travel in a vacuum at a speed equal to or greater than light. In other words, particles that travel slower than light, known as bradions, cannot be accelerated beyond that limit because to reach that speed they would require infinite energy. Also, the theory postulates the existence of photons, particles with no mass that travel at the speed of light. Moreover, it does not exclude the existence of particles (called tachyons) that travel faster than light. For the latter, the speed of light represents an insurmountable inferior limit. The theory of relativity considers light as a divide between the world of subliminal speed (slower than light) and that of superluminal speed (faster than light). Scientists await further verification.
If neutrinos can really travel faster than light this would not defy the theory of relativity as many claim, but rather it would contribute to extending it without violating fundamental laws, but with extraordinary effects on the world of cosmology research.
written by Simona Romaniello
she is an astrophysicist and science populariser; she is concerned with formation and development and the mounting of museum displays for the Planetarium in Turin.