I am not saying that souls are neutrinos (but it is an interesting theory), but I do not buy that neutrinos oscillate, either.
The 'discovery' and two more David Jones'
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High-energy physics: Neutrinos reveal split personalities
JOHN N. BAHCALL
John N. Bahcall is at the School of Natural Science, Institute for Advanced Study, Princeton, New Jersey 08540, USA.
sns.ias.edu
For more than 30 years scientists have puzzled over the mystery of the missing neutrinos emitted from the Sun. Data from underground detectors in Canada and Japan combine to provide the answer.
Neutrinos are unique subatomic particles. They have no electrical charge, travel essentially at the speed of light, and come in three types: electron, muon and tau. These particles are so elusive that you do not notice the hundred billion solar neutrinos that pass through your thumbnail every second. The 'solar neutrino mystery' began in 1968, when a pioneering experiment1 found fewer electron-type solar neutrinos than predicted by a detailed model2 of how the Sun shines (and how many neutrinos are detected). Two ideas were widely discussed: either the model of the Sun was wrong, or something happens to the neutrinos on their way to the Earth.
In the 1980s and 1990s, several international teams of scientists performed a range of underground experiments designed to solve this mystery. This work provided circumstantial evidence that some solar neutrinos change en route to the Earth from the electron-type produced in the Sun to another type that is harder to detect. But there was no smoking-gun evidence for such particle personality changes, usually called neutrino oscillations. This situation changed dramatically three weeks ago when scientists working in Canada announced that they had solved the mystery3: "The Sudbury Neutrino Observatory (SNO) finds that the solution lies not with the Sun, but with the neutrinos, which change as they travel from the core of the Sun to the Earth."
How did the SNO scientists — a collaboration of 113 scientists from 11 universities and laboratories in Canada, the United States and the United Kingdom — solve the solar neutrino mystery? Over 2,000 metres below the Earth's surface, within an active copper and nickel mine, the SNO collaboration built a laboratory4 the size of a ten-storey building. Here, their underground detector is shielded from cosmic rays and radioactive contamination from dust — the laboratory is so clean it contains less than a teaspoon of dust. SNO scientists built a spherical detector, 12 metres in diameter, which contains 1,000 tonnes of heavy water (D2O) and is itself immersed in a 30-metre cavity filled with normal water (H2O). Neutrinos from the Sun are occasionally detected by the heavy water (about five per day), producing light that is measured by 10,000 photomultipliers.
In the initial results reported by SNO, only electron neutrinos were detected (by a specific reaction in the heavy water). A Japanese–American experiment, known as Super-Kamiokande5, can detect all three types of neutrinos, but is mostly sensitive to electron neutrinos. But Super-Kamiokande, which uses pure H2O in an underground detector in northern Japan, does not distinguish between electron-type and other solar neutrinos.
If only electron neutrinos travel from the Sun to the Earth, then SNO and Super-Kamiokande would measure the same number of neutrinos. If some solar neutrinos are muon or tau neutrinos, then Super-Kamiokande would measure a larger number. Indeed, the Super-Kamiokande number exceeds the SNO number with a probability of 99.96% (3.3 standard deviations), conservatively calculated. This is a smoking gun.
The Sun emits neutrinos over a wide range of energies, but SNO and Super-Kamiokande are sensitive to a specific energy range. Using data from both measurements, SNO scientists calculated the total number of these solar neutrinos that reach the Earth. The measured number agrees well (within 0.3 standard deviations) with the prediction of the standard solar model6.
What does all this mean? In 1969 two Russian scientists first proposed7 that neutrino oscillations cause the observed discrepancy between the predicted and measured numbers of solar neutrinos. In 1998, experiments at the Super-Kamiokande detector8 provided the first evidence of neutrino oscillations by studying neutrinos produced when cosmic rays interact with the Earth's atmosphere. SNO has now confirmed that solar neutrinos undergo oscillation.
The Sun only produces electron neutrinos, but some muon or tau neutrinos reach us from the Sun. Therefore, solar neutrinos must oscillate from one type to another. This phenomenon, which requires that neutrinos have non-zero mass, is not predicted by the simplest version of the textbook theory of weak particle interactions (called electroweak theory). The standard theory of weak interactions must be modified slightly, which is important but not unexpected. Most importantly, the specific way neutrinos oscillate, which must be determined by future experiments, may help select the correct generalization of existing physical theories.
Neutrinos contribute to the mass density of the Universe. Combining results on neutrino masses from SNO, Super-Kamiokande and nuclear physics experiments, SNO scientists conclude that electron, muon and tau neutrinos contribute between 0.1% and 18% of the critical mass density of the Universe. The most plausible value is 0.1%. A neutrino mass density of 0.1% is probably too small to affect significantly the geometry or fate of the Universe, but it is about one-quarter of the mass density of all the stars we observe. So even though there is an enormous number of neutrinos in the Universe, the small amount of mass they contribute is not going to solve the problem of the Universe's missing 'dark matter'.
Arguably, the most spectacular result from SNO is that the total number of solar neutrinos measured by the observatory and Super-Kamiokande is bang on that predicted by the standard solar model. In appropriate units, the predicted value is 5.05 0.2 and the measured value inferred by comparing the results of SNO and Super-Kamiokande is 5.44 1.0. This is a triumph for the theory of stellar evolution. The predicted number of neutrinos depends on the 25th power of the central temperature of the Sun. Getting the neutrinos correct to 20% implies we can calculate the Sun's central temperature (15.7 million kelvin) to better than 1%. As stellar evolution theory is widely used to interpret observations of stars and galaxies, this agreement is a cause for rejoicing among astronomers.
Physicists are happy because they have an interesting phenomenon to study; astronomers are happy because their solar theory has been proven correct. But the work has only just begun. Scientists have so far made detailed measurements of only 0.005% of the neutrinos astronomers believe are emitted by the Sun. The remaining neutrinos are at lower energies and so are more difficult to detect. Until these lower-energy neutrinos are observed and compared with theory, we cannot be sure we really understand the intricacies of the mystery of the missing neutrinos. In the meantime, SNO and Super-Kamiokande have crucial additional measurements to make. It is a great time to be involved in neutrino research.
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Well, I can't bring up Nature.
I will post these articles tomorrow:
Daedalus: Neutrinos in orbit
David Jones
Solar neutrinos are now known to have some mass. So solid objects, like the planets, must slow them down, perhaps revealing information about their interiors....
Nature 412, 784 (23 August 2001)
Full Text | PDF
6. Daedalus: A cosmic background
David Jones
A weak cosmic background of neutrinos, similar to the cosmic microwave background, could exist, although its faintness will require sensitive telescopes to detect it....
Nature 412, 872 (30 August 2001)
Full Text | PDF
edit: they are coming in now:
Daedalus: Neutrinos in orbit
DAVID JONES
The neutrino telescope, essentially 100,000 gallons of cleaning fluid at the bottom of a gold mine, arouses Daedalus's strong admiration: he would so like to have invented something like that himself. He now wants to make it directional, like a normal telescope. He plans a tube many metres long, underground as before, and full of dry-cleaning fluid, or another particle-detecting liquid. Only in the direction of the steerable tube, when neutrinos traverse it from end to end, is it at all sensitive. Several parallel tubes even permit energy-dispersion studies.
Solar neutrinos do not quite ignore the Sun and planets. Unlike photons, they are now known to have mass. They must be slowed by solid objects, perhaps even down to Solar or planetary orbital speeds. They could even be deviated by asymmetric encounters with matter, and so enter orbit. There could be quite a lot of neutrinos in nearby space, slowed by deep encounters with Mercury, Venus, the Sun, Earth and the Moon. They will be orbiting these solid bodies, entering them with little loss per orbit, and be topped up by new arrivals. Their equilibrium concentration will reflect the deep neutrino absorption of these objects.
Neutrinos slowed and deviated by the Moon, for example, should enter an Earth orbit from which a few of them would penetrate below the surface once per orbit, and traverse the telescope. Neutrinos slowed by the nearer planets but little deviated by them should enter a narrow elliptical Solar orbit. Some of them should traverse the telescope twice per orbit. At the other extreme, they would swing round the outer layers of the Sun, sampling its own neutrino-absorbing and -deviating properties. Daedalus's new telescope will look for these wayward particles. He hopes to calculate those orbits best configured to encounter new orbital neutrinos.
Of course, the neutrino flux of any orbit must carry the 'signature' of the whole of that orbit. Neutrinos deviated by the Moon, and passing through it once per orbit, will have an orbital equilibrium concentration reflecting its deep neutrino-absorbing and -deviating properties. These could be calculated, and would slowly give reliable information about the deep geology of the Moon. Similarly, the deep geology of the planets, and even of the Sun, would gradually become clear. It should reveal how opaque, or how deviating, these bodies are to entering neutrinos.
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Daedalus: A cosmic background
DAVID JONES
One of the most significant discoveries in cosmology is the cosmic microwave background, or 'relic radiation' as the Russians call it. Originally, in the early Universe, it was visible light. But the Universe has expanded so much since then that its frequency has been shifted down all the way to the microwave band, a frequency drop of perhaps 105 or so.
Last week Daedalus proposed a directional neutrino telescope, underground as usual to filter out competing particles, but consisting of a tube many metres long. It would be filled with a liquid such as dry-cleaning fluid or heavy water, as used in dedicated neutrino detectors. For some liquids it would be covered with photomultipliers. There would be several telescopes. Giving each some spectroscopic resolution, they could even estimate neutrino energies. Each tube would be sensitive to neutrinos mainly in its long direction, in which they would traverse it from end to end, and so had the best chance of interacting with its contents.
Seeking new uses for his device, Daedalus now reckons that the early Universe should also have created plentiful neutrinos, by the combination of protons or the commonest helium nuclei. As the Universe expanded, these 'relic neutrinos' will have lost energy. By now they should have formed a fairly uniform cosmic background. His directional neutrino telescope will therefore slowly establish that background, and distinguish it from that of the Sun.
It will take many years to sample this very weak neutrino background. But there are very few pieces of information in cosmology (the microwave background is one of them), so the slow effort seems worthwhile. It might even identify a neutrino flux from the nearer stars — Sirius is perhaps the best bet, although Alpha Centauri is nearer. But Daedalus will aim his device in the stellar gaps, especially out of the galactic plane, so as to acquire a true background. There might be surprises, of course. All previous neutrino astronomy has used detectors with no directionality, so it would be intriguing to discover that many neutrinos were coming from some concentrated source.
Indeed, Daedalus would like his telescope to found a whole new branch of astronomy. But he freely admits that the very low rate of data acquisition will greatly restrict the rate at which DREADCO can transform cosmology. |
