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Nothing travels faster than light but gossip Печать
24.10.2011 07:29

This phrase of one lecturer of physics was quoted by Pauline Gagnon, a Canadian physicist of the European Organisation for Nuclear Research, on her website at CERN. The news that neutrinos could beat light in a race was spread by all world news agencies, radio and television channels, quality newspapers and tabloids at the rate of rumours on the 23th of September. The hypothesis has turned the whole world on its head.

On the same day, September 23, the OPERA experiment presented sensational results in a special seminar at CERN, Geneva, (the detectors of this construction were developed with the assistance of Dubna physicists). There was no place enough for all comers willing to take part in the discussion. The debate was webcast throughout the world.

Three days later the same seminar was held in Dzhelepov Laboratory of Nuclear Problems, JINR, Dubna. The conference-hall couldn’t seat all people who wanted to deal with the sensation as well. The head of DLNP Yuri Gornushkin, the researcher who actively assisted in the experiment, did a solo. Oh no, I beg your pardon, he made a detailed report devoted to the OPERA results.

What’s the matter, indeed?

The matter is that the modern laws of physics which describe life on Earth and in the Universe have definitely set that the velocity of moving objects (including elementary particles) can’t exceed the velocity of light. The meaning of the formulas of relativity which affirm the present fact can be briefly described in the following way: the velocity of light couldn’t be beaten because every effect is preceded by its cause. Should some objects travel faster than light in the world we live, they would break the cause-effect relation. In other words, if there were superluminal objects, we would observe impossible things to occur: for instance, a cat would shout at the top of its voice before its tail was really pinched. The prospect of such a fantastic turnabout was revealed when the collaboration of physicists working on the OPERA experiment processed the information that was obtained from the OPERA detector.

What does the opera have to do with it?

At the very beginning the opera considered as a musical performance had nothing to do with physics. It’s due to the physicists’ tradition to invent such abbreviations for their experiments that compose bright words. The name OPERA was formed in the following way:  the full name of the experiment consists of five English words Oscillation Project with Emulsion-tRacking Apparatus that give five capitalized letters for composing a word with a sense. So the meaning of the word ‘opera’ seems to have defined the development of the OPERA experiment as a colourful spectacle. We get convinced of its musicality owing to ‘fanfares’ of mass media that circulate the hypothesis proving that the velocity of light could be exceeded.

OPERA has been designed as the Bolshoy Theatre

The reconstruction of the Bolshoy Theatre began in 2005 in order to turn this impressive historic building into a high-tech one even in the smallest details that gave the opportunity to discover new facets of this musical art.

The OPERA experiment has been designed to present the most detailed transformation of one type of neutrinos (muons) into another (taus). This transformation has been to reveal new facets of our knowledge of nature.

The SPS accelerator generates a beam of muon neutrinos at CERN.  Through the Earth’s crust the beam (though being not an audible wave as in the real piece of music, but practically a light one) travels underground 730 km away and reaches the detector at a depth of 1,5 km at Gran Sasso, a mountain resort in Central Italia. Doesn’t it remind you of an outstanding performance with some spectacular scenery?

In 2003 the international collaboration made of 200 scientists from 39 institutes and 12 countries set to work on building the OPERA detector at the Gran Sasso National Laboratory. So called scintillation strips of this detector were developed in Kharkov Institute for Scintillation Materials of Scientific Technology with the assistance of a group of physicists from LNP JINR.

As for plastic strips

The phrase ‘a scintillation strip’ seems to be a real tongue-twister for a person who has little to do with physics. Say ‘a scintillation strip’ and be sure –a person is likely to lose interest. However, the phrase doesn’t mean anything terrible or boring. Quite the contrary, these two words indicate a beautifully flickering strip (to scintillate means to produce short flashes of bright light). A strip of special plastic flickers at the very moment when it gets a charge. Thereby, a scintillation strip informs that it has fixed some elementary particles.

Neutrinos don’t carry electric charge. That’s the reason why a neutron travelling through a scintillation strip can’t cause luminescence. But the collision between neutrinos and protons (nuclei of atoms of any substance) stimulates the formation of other particles, including charged ones. A scintillator reacts   to them and ‘illuminates’ neutrinos that are unlikely to reveal.

The planes of scintillators were assembled in Strasbourg, France, from 7-metre-long plastic scintillation strips produced in Kharkov (70 tons for 2 years).    A group of JINR physicists arrived in Strasbourg for the assembly of planes. The international team tested and calibrated the assembled scintillation detector, after that it was sent to Gran Sasso.

Lead sandwiches

In addition to electronic detectors which include scintillation strips, in the OPERA experiment scientists make the use of 155 000 emulsion detectors – ‘bricks’ that consist of 9 million lead plates covered with a thick layer of photoemulsion. This combination of detectors makes it possible to consider the point of interaction between neutrinos and protons in details.

Lead is chosen because of its high density. The denser the material is the greater proportion of protons it has, hence, the higher probability of interaction between this substance and neutrinos appears (the particle is likely to elude, so it is not always ‘easy to get in touch with’).

The traces of particles which pass through the thick layer of photoemulsion are imprinted as in the picture. Therefore, while studying a plate for a plate under a microscope, we can detect the particular place where a neutrino collides with a proton and ‘a fountain’ of other particles squirts with micron-level precision.

In general, the OPERA detector looks like a sandwich of lead ‘bricks’ made by scintillator planes. As soon as the certain strip lights up, a robot-manipulator drives up and takes out ‘a brick’ lying in front of the strip. This lead stuffing is delivered for a development and scan of each photoplate in order to detect the endpoint of neutrinos produced at CERN.

How to measure the velocity of neutrinos

Flickering strips only state the fact that ‘there has been a neutrino’. To measure neutrinos’ velocity, it’s necessary to indicate the exact coordinates of their travel path. The starting point is a target of the SPS accelerator in the tunnel under Geneva. This target is fixed. Every neutrino has its own finishing point that is distinguished by the OPERA detector.To be precisely, a neutrino occurs there where strips flicker. The following points define neutrinos’ exact time of arrival and its location.

Everything is functioning in a proper way, isn’t it?

The precision of the OPERA experiment is enormous. The special geodesy campaign in Germany and Switzerland performed a series of high precision measurements of the distance between the source and the detector with an uncertainty of 20 cm over the 720 km travel path. The scientists even paid attention to the rock shift that was resulted from an earthquake in Acqueville. The synchronization of neutrinos’ origin at CERN and its arrival at Gran Sasso was determined with an accuracy of 10 nanoseconds by using GPS satellite navigation systems. And here is an unbelievable result which was analyzed for three years because the scientists  didn’t believe their eyes: on the observation of the coordinates and the flight time from Geneva to Gran Sasso it appears to indicate that neutrinos travel at a speed 20 parts per million above the velocity of light (0,002 %). The exceeding seems to be insignificant, however, it’s not a figure of crucial importance here, it’s a matter of principle. That’s the reason why the collaboration of the OPERA has decided to open this special case to broader scrutiny on the 23th of September. The role of discovery is so important for the modern science that there’s a great need for a joint effort of all scientists throughout the world to clarify the point whether it’s a mistake of the experiment or it’s a breakthrough of the physical theory which will lead to a branch of knowledge that’s still undefined.

 

During both seminars on the 23th of September at CERN and on the 27th September in Dubna the scientists discussed probable causes which could satisfactorily explain the sensational deviation. They might be instrumental, technical or methodological errors of processing the experimental data. The similar assumptions and hypotheses were expressed in both seminars. The scientists invite broader scrutiny from other physical centers in Russia as well as all over the world to study the unexpected result. There’s still an intention to take the situation over the additional control. In doing so, physicists are likely to use the monitor of muons at CERN that is the source of the neutrinos’ beam and to change the structure of the beam itself. Simultaneously,the data of OPERA will be checked by other independent measurements. One of them is MINOS that is designed to detect the beam of neutrinos from the American Tevatron at the Fermi National Accelerator Laboratory.

A neutrino is an electrically neutral, weakly interacting elementary particle of low energy. About 60 billion solar neutrinos per second pass through every square centimeter in the region of the Earth; however, they have no effect on a human body.  High-energy neutrinos can be detected by the interaction with targets. Wolfgang Pauli, a Swiss scientist, postulated the existence of this particle and Enrico Fermi, an Italian physicist, coined the term ‘neutrino’. The neutrino means ‘small neutral one’.

Natalia Teryaeva

 

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