v1.0.1 / chapter 18 of 20 / 01 sep 07 / greg goebel / public domain
* The development of integrated theories of quantum field physics has led to a desire to fully integrate all the four forces. This effort has led to various models of unified field theories, such as "string theories" and "loop quantum gravity", which have so far not proven entirely satisfactory. While the theorists pursue the goal of a unified theory, the experimentalists have been busy as well, working to track down neutrinos and other weakly interactive particles.
* One of the great achievements of 19th century physics was James Clerk Maxwell's theory of electromagnetism, which described electricity and magnetism as simply different manifestations of the same basic force, electromagnetism.
The 20th century identified a total of four forces: electromagnetism, the strong force, the weak force, and the gravitational force. In that century, quantum physicists managed to come up with acceptable theories for electromagnetism (QED), the strong force (QCD), and the weak force (the electroweak model, which also incorporates QED). This set of theories is known as the "Standard Model".
The Standard Model is a significant achievement, but it leaves more than a little to be desired. It is clearly complicated, and it also features 18 "adjustable parameters" -- essentially, constants that have to be plugged in to get the model to agree with reality. Even Steven Weinberg once called it "repulsive". Dick Feynman called the Standard Model "three theories", saying the theories were "linked because they seem to have similar characteristics" and asked: "Where does it all go together?"
Nobody thinks the standard model is the last word, or even all that close to the last word. The challenges, though not the solutions, are obvious. Electroweak theory has managed to consolidate electromagnetism and the weak force. The next step is to properly integrate the strong force as well. Such schemes are known as "grand unified theories (GUTs)".
There is a general belief that at the high energies filling the Universe before the separation of the electromagnetic and weak forces, the electromagnetic, weak, and strong forces all acted identically, but nobody has yet figured out the details. One of the hopes for a workable GUT is that it will give some clue as to why the particles are arranged as they are, in three generations of leptons and quarks.
Group theory, having proven so handy for particle physics so far, was an obvious avenue of investigation for physicists working on GUTs. At first, work focused on a group named SU(5) that encompassed the SU(3) x SU(2) x U(1) architecture of the standard model, but the results didn't match observed measurements. Work moved on to a still larger group, SO(10). SO(10) was found to have all sorts of nice properties, for example accounting for why protons and electrons have precisely equal but opposite electric charges, but it had a feature that physicists originally found distressing: it predicted the proton would decay, if over a very long period of time, disintegrating into an electron and pion+. Sakharov had made the same suggestion in the 1960s; proton decay would prove to be a common feature of GUTs, suggesting there was something to the idea.
* The next step beyond GUTs is the consolidation of all four forces, including gravity, or what physicists call a "theory of everything (TOEs)" -- of course, making a GUT a "theory of almost everything". Physicists have also been working on TOEs for a long time -- in fact, much of Einstein's later life was devoted to this end, without much useful result -- and it hasn't been easy, either.
The real problem is that there is no satisfactory quantum theory of gravity. It is taken very much as a given that gravity is mediated by a force carrier named the "graviton", which is a massless spin-2 boson. However, since the graviton only responds to the gravitational force, which is so very weak, gravitons have never been even implicitly observed in experiments. Worse, quantum gravity theories are not consistent with Einstein's theory of General Relativity, which is currently regarded more or less as "law" as far as gravity is concerned. Quantum mechanics as it stands is flatly incompatible with General Relativity.
One approach that has received much attention is an extension of the systems of symmetries that underlie the standard model, of course known as "supersymmetry (SUSY)". The symmetries that exist in the standard model show that, for example, for every particle there is an associated antiparticle. SUSY takes this notion one big step farther, proposing that for every particle there is a "supersymmetric" partner. Particles that are fermions, such as the electron, will have a supersymmetric partner that is a boson, in this case called a "selecton"; quarks will similarly be paired with "squarks".
Of course, particles that are bosons, such as the photon, will have a sypersymmetric partner that is a fermion, in this case called a "photino"; gluons will be similarly paired with "gluinos". At least two Higgs fields are required, with a total of five flavors of Higgs bosons. However, nobody has found any supersymmetric particles yet. Physicists believe they are too massive to be created in contemporary particle accelerators.
The whole subject of GUTs and particularly TOEs is thoroughly booby-trapped, not merely for laymen who become completely confused in listening to the physicists, but for the physicists themselves. In 1976, Murray Gell-Mann was at a physics conference and stood up from the audience to excitedly proclaim that he had developed a theory of everything, his monologue only ending when two persons wearing white coats grabbed him and dragged him out of the lecture hall as he continued to shout out the virtues of his theory.
* There is a more ambitious approach to understanding the elementary particle zoo known as "string theory". The field traced its origins, somewhat indirectly, back to 1919, when a mathematician named Theodor Kaluza (1885:1954) at the University of Konigsberg began to tinker with Einstein's theory of general relativity. General relativity is based on the concept that the three dimensions of space are intimately related to time, forming a four-dimensional "spacetime". Kaluza was curious as to how Einstein's equations might look if a fifth dimension were thrown in.
Somewhat surprisingly, Kaluza's modified equations could be seen to encompass both Einstein's equations of general relativity and James Clerk Maxwell's classic equations for electromagnetism. The fifth dimension could "ripple" and this ripple turned out to model the electromagnetic field. This was more in the nature of an interesting curiosity than a grand revelation, in particular because Kaluza had no idea of what his fifth dimension really signified.
In 1926, the Swedish mathematician Oskar Klein (1894:1977) suggested that Kaluza's fifth dimension was actually another spatial dimension, one that couldn't be perceived because it was on a subatomic scale. Klein pointed out that a tube seen from a distance might seem to be a two-dimensional line; the fact that any point on the line really amounted to a cross-section of the tube, a simple circle, couldn't be noticed from a distance. The whole matter was somewhat academic, and this was the time of the distracting emergence of the new quantum physics, so the "Kaluza-Klein" theory was shelved as a footnote in the physics literature and more or less forgotten for decades.
In the late 1960s, an Italian physicist named Gabriele Veneziano (born 1942) was looking for a new approach to the description of the strong force, and came across a set of equations created by the 19th-century Swiss mathematician Leonard Euler that seemed to be remarkably convenient for describing particle interactions. The only problem was that rationalizing the equations for such an application meant invoking 26 spatial dimensions, which seemed like a bit much.
Veneziano's work might have ended up in the same pile of footnotes as the Kaluza-Klein theory, but the Japanese-American physicist Yoichiro Nambu (born 1921), then at the University of Chicago, noticed that the equations described by Veneziano seemed to match the mathematics of vibrating strings. Nambu suggested that the various types of hadrons could be explained as being different resonant modes of a stringlike entity about the size of a proton.
The physics community found Nambu's "string theory" exciting and there was a rash of papers on string theory. There was one serious limitation of Nambu's theory in that it only covered bosons -- in effect, it was a "mesons only" theory -- but in 1971 Pierre Ramond of Yale modified it to cover fermions as well. John H. Schwartz of Princeton and Andre Neveu in France were covering similar ground, with the end result being the "Ramond-Neveu-Schwartz" theory, which dealt with all the hadrons, and also reduced the embarrassing 26 dimensions to a more manageable if still somewhat cumbersome ten dimensions -- nine in space, one in time.
* However, that was about as far as it went at the time. Particle physics was in its own era of great discoveries, such as the electroweak force and the quark model, and so string theory, like the Kaluza-Klein theory before it, was sidelined. String theory was a sideshow at best, hobbled by its postulate of ten dimensions. Worse, the various string theories gave inconsistent results. The flow of papers all but ceased.
John Schwartz, who moved to Caltech in 1972, worked with French physicist Joel Scherk (1946:1979) to try to keep the string flame burning, though it seemed a bit like a rearguard action at the time, Schwartz saying: "We still thought that string theory deserved a last look before being abandoned." If it was to be reduced to a footnote, as had the Kaluza-Klein theory, some of the loose ends might as well be tied up in case somebody decided to pick up the trail later.
In the course of their efforts, the two physicists came to an interesting revelation: string theory could encompass the graviton, the mysterious undiscovered messenger particle of gravity, and so string theory stood a chance of uniting gravity with the other three forces, the grand unification that physicists had been after for years. This required adjusting the scale of the strings downward by 20 orders of magnitude, from about 10^-13 centimeter, the scale of the proton, to 10^-33 centimeter.
This miniscule measure is called the "Planck length", which in a sense defined by the uncertainty principle is effectively the minimum possible meaningful length that anything could have. Incidentally, the concept of the Planck length also leads to the related concept of the "Planck mass", which corresponds to a particle with a wavelength given by the Planck length, which by the de Broglie relation gives a value of 10^-5 grams. This sounds like a large value by particle standards, and it is, corresponding to an incredible density in the small space defined by the Planck length. By the Einstein mass-energy relationship, the Planck mass corresponds to about 10^19 GeV -- an energy whose scale can be appreciated when it is remembered that the most powerful particle accelerators on Earth achieve energies of less than 10^4 GeV.
In any case, Schwartz and Scherk published a number of papers on their ideas in the mid-1970s, and though the papers were politely received -- as Schwartz put it, "no one accused us of being crackpots" -- almost nobody paid any attention. In 1979, however, Schwartz was on a visit to CERN and ran into a British physicist, Michael B. Green (born 1946) of the University of London, who was a fellow string-theory diehard. They chatted a bit and parted company with a general idea that they might get together later and join forces. When they met again in 1980 at a physics meeting at Aspen Center for Physics in Colorado -- a summer retreat for the physics community -- they began to work together, coming up with a series of "superstring" theories that merged string theory with SUSY. Schwartz and Green published a series of papers; they were not surprised when the papers were generally ignored.
There were in fact some noticeable holes in their work, but when the two physicists met once more at Aspen in 1984, they worked things out and found to their surprise the holes magically disappearing, with the end result being a tight, mathematically consistent theory. In a state of excitement, they published a paper on their modified superstring theory, though they realized it was likely to be ignored like all the others.
It wasn't. One of the people who hadn't been ignoring their work was the highly regarded Edward Witten of Princeton (born 1951), who found the paper exciting as well and enthusiastically promoted their ideas. Hundreds of other theoretical physicists, excited by a theory that seemed to provide a reasonable shot at grand unification, jumped in as well.
A group of four other Princeton superstring enthusiasts -- David Gross (who would share the Nobel prize for his work on asymptotic freedom), Jeffrey Harvey, Emil Martinec (born 1958), and Ryan Rohm, who called themselves the "Princeton String Quartet" -- published a refined model of the Schwartz-Green theory that allowed the strings to be seen as closed loops. This new theory incorporated elements of Nambu's earlier work and so the quartet called it the "heterotic" theory, the term coming from genetics and more or less meaning "crossbred".
In early 1985, Witten and a few of his colleagues extended the notion of closed strings by suggesting, as in the old Kaluza-Klein theory, that the six additional spatial dimensions required were curled up on such a small scale that they were not noticeable. Two mathematicians, Eugenio Calabi (born 1923) of the University of Pennsylvania and Shing-Tung Yau (born 1949) of the University of California at San Diego, had already considered the possible configurations that such closed strings might take. Witten contacted Yau to see which of these "Calabi-Yau manifolds" might prove useful for heterotic superstring theory, and Yau came back with literally thousands of possibilities.
In superstring theory, the configurations of these manifolds determine the modes of vibration of the strings, creating leptons or quarks as observed. The number of possible string configurations may well be infinite, meaning that the number of possible particles is infinite as well. Only a finite number of particles is observed because the rest imply higher and higher energies that are not observed, or even attainable with, our technologies or in our cosmic era.
* The attractions of this theory were obvious to many physicists. One was that it gave a consistent model for all elementary particles; the other was that it reconciled Einstein's theory of General Relativity with quantum physics, a reconciliation that has long evaded physicists.
General relativity claims that gravity is caused by a curvature of spacetime due to the presence of a mass. However, at the Planck length the uncertainty principle claims that spacetime becomes chaotic and unpredictable, becoming a "quantum foam" that makes calculations go mad, introducing infinities that cause results to explode. String theory got around the mad behavior of spacetime below the Planck length because the strings were long relative to the scale of the quantum foam, allowing the foam to be ignored. String theory imposed a minimum level of detail to the Universe that was greater than the scale of the quantum foam.
Witten called heterotic superstring theory "a miracle" and insisted it was the way of the future. Such grand claims provoked critics who were not so impressed by the theory, the most prominent being Dick Feynman. Feynman apologized for coming on like an old fogey, but still claimed that string theory didn't feel right; more caustically, he commented: "String theorists don't make predictions, they make excuses." The loudest critic was Sheldon Glashow, who insistently hammered on the weaknesses of string theory. The whole concept seemed too theoretical, particularly with its postulate of ten dimensions, and not very closely tied to observations.
Glashow was particularly troubled by the fact that the string structures were
so incredibly small. If an atom were the size of a galaxy, a string
structure would be about the size of a beachball. There was no prospect that
any experiment could detect them directly and prove or disprove the theory.
Glashow put it: "The theory is safe, permanently safe. Is that a theory of
physics or a philosophy?" Glashow came up with some verse to express his
suspicion:
The Theory of Everything, if you dare to be bold
Might be something more than a string orbifold.
While some of your leaders have got old and sclerotic,
Not to be trusted alone with things heterotic,
Please heed our advice that you too are not smitten --
The Book is not finished ...
... the Last Word is not Witten.
* A particular embarrassment to string theory was the fact that the work of
the horde of physicists investigating the matter resulted in five
distinct string theories. It was as though they were all after the "one true
religion" and ended up with five of them, which only aggravated the assertion
of the critics that string theory was more theology than physics. The field
began to bog down again.
It wasn't until 1995 that Witten delivered a talk in which he reconciled the five competing string theories, showing they were all aspects of a single unifying theory, which he called "M-theory". Witten never insisted on the definition of the "M" in the name, saying that it could be interpreted according to taste as "membrane", "master", "matrix", "mother", or "mystery". Witten also honestly added that some interpreted it as "murky". M-theory did resolve the tangle of five string theories, but to no surprise at the price of increased complexity -- most significantly in adding another spatial dimension, giving a total of eleven dimensions in the scheme. The additional spatial dimension allowed the strings to be aspects of sheets or "branes" that could span Universes.
The strings then became projections of a brane; one string theorist compared the concept to that of a grove of aspen trees, in which the trees share a common root network but appear to be independent above the ground. The branes provided a very direct link between quantum mechanics, which concerns itself with the world of the very tiny, and cosmology, which concerns itself with the Universe as a whole.
* That seemed to put string theory on a more solid basis and the critics quieted down, all the more so because string theorists were finally acknowledging that they hadn't really quite solved all the final problems of the Universe and had some more proving to do. However, all this turned out to be was a lull in the fighting.
In the late 1990s, astronomers measuring the expansion of the Universe determined to everyone's surprise that the rate of expansion was accelerating. It implied that a significant portion of the Universe was bound up in what for lack of a better term was called "dark energy", which provided a repulsive force to drive the galaxies apart. The only way string theorists could figure out how to account for dark energy was to modify their theories to accommodate a boundless range of alternative universes, each with its own form of dark energy, with our particular Universe just happening to fit the description of ours. There was considerable skepticism over this idea, with even some string theorists suggesting it was on the wrong track.
In addition, the variations in string theories appear to be diverging once again instead of converging. The suspicion is that any scheme that permits such a wide range of interpretations isn't really all that well grounded in reality, and the attacks have ramping up again. The critics sometimes refer to Witten as the "Pope" -- which, incidentally, was also Enrico Fermi's nickname, though it was meant much more respectfully. On 1 April 2005, a physics blogger published a dry and very authentic-sounding mock press release announcing the creation of a combined string theory / theology studies institute at Stanford.
* There is a competing approach to a TOE that seems to be gaining ground these days at the expense of string theory. As the string theorists had discovered, one way to get around the conflict between quantum physics and general relativity was to quantize spacetime, ensuring that nasty infinities didn't arise. The string theorists dealt with this problem by creating manifolds in additional spatial dimensions, but another faction simply bit the bullet and said that the unified spacetime described by Einstein was quantized. Not too surprisingly, the level of quantization is very small, on the order of the "Planck volume", which is the Planck length cubed: 10E-33 * 10E-33 * 10E-33 = 10E-99. Notice that this a spacetime quanta, implying not only that space is divided into irreducible small elements, but time is as well. The scheme was somewhat confusingly called "loop quantum gravity (LQG)", though the "loops" had little to do with strings, instead simply referring to a calculational approach.
LQG immediately brings up a vision of spacetime being chopped into a matrix of tiny, really tiny little Lego blocks, but to no surprise it's not quite that simple. Instead of representing spacetime as made up of a mass of cubes of spacetime, the LQC model visualizes spacetime as made up of "nodes" connected by "links", a "connect-the-dots" scheme called simply a "graph". The graphs defined by LQG are specifically known as "spin networks" because their properties are related to quantum-mechanical spin. Different types of particles are represented as nodes in the network, with their mediating force particles making up the lines in the network. A succession or "movie" of a spin network going through changes is called a "spin foam".
Analysis of a spin foam shows that it can be used to calculate all four force interactions. LQC does seem to have one of the big problems of string theory in that the Planck volume LQC is based on is so very small, too small to be directly detected. However, LQC advocates say that indirect effects can be calculated and measured. One of the most important is that high-energy gamma-ray photons will have a slightly smaller speed than low-energy light photons. The difference is so slight that it could only be measured from the emissions of objects in the far distant Universe, billions of light-years away, but such a measurement still seems practical. This discrepancy does cause some trouble for Einstein's theory of special relativity, which states that the speed of light is an invariant constant, but work has been done that shows special relativity still works fine if a few tweaks are added.
* The squabbling of physicists over the theories of grand unification exists in the context of a more general skepticism. Some physicists have suggested that the entire push towards the TOEs are a waste of time. Dick Feynman once stated in his outspoken way that he was tired of journalists asking physicists about their search for ultimate particles and a TOE. Feynman wondered if these weren't the wrong questions to ask. Relativistic and quantum physics were created because experiments had produced results that couldn't be explained by existing physics, meaning new physics had to be invented. The push toward TOEs is basically driven by a sense of aesthetics, not because there are ghastly problems with existing physics that are in need of explanation: there's nothing seriously broken, so there's no need to fix it. In addition, as these critics point out, without something broken there's no clear roadsign to point out the direction of a fix -- there's no Lamb shift to help determine which way to go -- and so such efforts are doomed to failure.
Some critics from outside the particle physics community take an even dimmer view of the matter. If quantum physics tends toward the obscure, particle physics takes the obscurity to a new level, and some haven't hesitated to take jabs at it. One of the loudest critics, Rustum Roy, a prominent materials researcher at the University of Pennsylvania, responded to the discovery of the top quark with the curt remark: "Who gives a damn?"
Hundreds of millions of dollars are spent on particle physics each year, and Roy has claimed the money is not well spent, basing his judgement on a rule of thumb called the "Weinberg criterion", named for nuclear engineer Alvin Weinberg, not particle physicist Steven Weinberg: How relevant is the work to other fields? Roy has argued that particle physics is not only irrelevant to fields outside of physics, but of little importance to rest of physics; it is relevant only to itself. The same could be said of, say, Renaissance poetry or other highly specialized academic research fields that nobody has much serious objection to, but the difference is that scholars of Renaissance poetry are not seeking huge grants to support their work.
While proponents of particle physics claim to seek a "theory of everything", Roy has countered that what they are pursuing is a "theory of nothing". His views are extreme, but the question he raises is not going to go away, and if particle physicists want to construct a TOE, they had better do it in a hurry.
* The hunt for the TOE is not the only game in town for particle physicists. One of the interesting alternative lines of research has been the investigation of the spooky neutrino and other ghostly particles.
As mentioned, the first neutrino detector was the Poltergeist experiment build by Cowan and Reines in the 1950s, followed by the oversized tank of cleaning fluid built by Raymond Davis and his colleagues in the 1960s. Theoretical analysis by astrophysicist John Bahcall (1934:2005), who was working closely with Davis, indicated that the detector should acquire one radioactive argon atom, created by a collision with a neutrino, per day -- but in reality, Davis got one atom every 2.5 days. There were three possibilities for the discrepancy:
Although there was considerable skepticism at the outset that Davis's experiment could actually give useful results, the experiment was checked and rechecked, and if there was a flaw there nobody could figure out what it was. Models of fusion processes were similarly cross-examined, and nothing seemed to be wrong. That left misunderstandings about the neutrino as the prime suspect. The trick was to get better data.
* By the 1980s, the same or similar technology was being considered for another purpose. In that decade, physicists were tinkering with GUTs, and in general the GUTs had a prediction in common: that the proton should decay. Obviously, since matter has shown no tendency to disappear from the Universe, the decay rate had to be slow, but if it were too slow, there would be no way to confirm it experimentally. However, analysis suggested that the proton had, at the very least, a half life of about 10^31 years. That meant that if somebody observed 10^31 protons for a year, one was likely to decay. 1,000 tonnes of water, a perfectly manageable mass for an experiment, contains 10^33 protons, meaning that 100 decay events should occur in a year.
A number of deep-underground detector systems were developed to search for proton decay. Some were built as what were called "swimming pools", big tanks of ultrapure water -- impurities would have caused false events from the decay of radioactive isotopes -- surrounded by arrays of photomultiplier tubes to pick up Cerenkov radiation, which would be emitted as a "downstream" consequence of proton decay. One possible decay mode for a proton was breakdown into a positron and a pion0, with the pion0 then breaking down further into two gamma rays. The positron and the pion0 would be emitted in opposite directions, with the positron producing one "cone" of Cerenkov radiation in one direction, and the gamma rays produced by the decay of the pion0 producing two adjacent cones in the other direction.
Swimming pool detectors were built in a gold mine in the Kolar gold fields in India; in a side gallery to the Mount Frejus tunnel in the French Alps; in the Kamioka zinc mine in Japan; and in the Morton Thiokol salt mine in the US state of Ohio, with this system known as the "Irvine-Michigan-Brookhaven (IMB)" experiment after its backers.
An alternative detector system, more like a spark chamber, was set up under Mount Blanc in France. The detector, known as the "Nucleon Stability Experiment (NUSEX)", was a 150 tonne cube 3.5 meters on a side, composed of stacks of iron plates a centimeter thick, separated by "rafts" of "streamer tubes". The streamer tubes were effectively long proportional counters, with a filling of argon, carbon dioxide, and pentane, a positively-charged wire running down the center of the tube, and a negatively-charged tube wall. A charged particle passing through a tube would ionize the gas, causing electricity to flow. A measurement system connected to all the tubes could trace the passage of a charged particle through detector. A roughly similar detector was build in the Soudan iron mine in the US state of Minnesota, though in this case the detector was a big block of pure concrete threaded with detector tubes.
Searches for the decay of the proton proved frustrating, with the lower limit on half-life of proton decay stretching out to 10^33 years, but the detectors could be used to also hunt for neutrinos, as well as muons, which have a considerable penetrating ability. The Kamioka detector system was upgraded to the "Kamioka II" configuration, and was able to pick up a burst of neutrinos associated with the explosion of Supernova 1987 in that year.
Proton decay detectors used for neutrino hunts also turned up the same solar neutrino deficit observed by Raymond Davis. It was for real, not the result of some defect in the instrument. Davis's work, incidentally, won him the 2002 Nobel Prize in physics, which he shared with Japanese neutrino researcher Masatoshi Koshiba (born 1926) of the University of Tokyo.
* Neutrino searches became fashionable again, and improved experiments were set up. One of the problems with Davis's detector was that it was limited in the energy spectrum of neutrinos that it could pick up. Theoretical studies of solar fusion processes indicated that the neutrinos emitted early in the chain of fusion reactions wouldn't be energetic enough to be picked up by Davis's detector.
In 1990, the "Gallium Experiment (GALLEX)", run by an international collaboration, went online at the Gran Sasso laboratory in the Italian Appenines. Gran Sasso is the tallest peak in the chain; when a highway tunnel was bored through mountain, Italian researchers managed to lobby successfully for digging a side gallery to the tunnel to perform research on neutrinos and other weakly interacting particles. Other particles would be screened out by the mass of granite above the lab.
The GALLEX experiment used 30 tonnes of gallium, which interacted with
neutrinos as follows:
neutrino_e + Ga<71/31> --> Ge<71/32> + electron
The gallium was in the form of GaCl3, dissolved in hydrochloric acid. The
neutrino reaction resulted in the formation of GeCl4, which was purged out
the tank of hydrochloric acid periodically by running nitrogen through the
tank. The nitrogen and GeCl4 were passed into a water-based scrubber system,
where the GeCl4 turned into GeH4, with the radioactive Ge<71/32> detected by
a proportional counter system.
GALLEX ran from 1991 to 1997 and confirmed the solar neutrino deficit. A follow-on experiment, the "Gallium Neutrino Observatory (GNO)", began in 1998, using the same system after overhaul and installation of improved systems. At last notice, GNO was still in operation.
A parallel experiment based on the same neutrino reaction with gallium was established as the "Soviet-American Gallium Experiment (SAGE)", installed inside Mount Andyrchi in the Caucusus. The Soviet Union now being history, the experiment is now formally called the "Russian-American Gallium Experiment", but the SAGE acronym has been retained, nobody wanting to use RAGE instead. It appears to be still performing test runs.
* In the meantime, astrophysicists refined their models of the workings of the Sun and everything seemed to fit the data except for the neutrino deficit. By simple process of elimination, suspicion focused on the possibility that we really didn't have an accurate understanding of the neutrino. Of course, the neutrino hunters knew all along that there were three distinct flavors of neutrino -- the electron neutrino, muon neutrino, and tau neutrino. Neutrino detector systems like Davis's tank of chloroethylene could only pick up electron neutrinos. What if, so the line of thinking went, electron neutrinos somehow changed their flavor to muon or tau neutrinos, "oscillating" as they travelled the distance from the Sun's core to the Earth?
This idea was given greater weight in 1998, when the underground Super-Kamiokande experiment in Japan found that muon neutrinos produced by cosmic rays tended to disappear at a rate proportional to the distance of the cosmic-ray collision to the detector. The theorists went back to the blackboard and came up with several scenarios, one in which the neutrino oscillated at a uniform rate along its flight path, and another in which it oscillated at a greater rate as it passed through the upper layers of the Sun on the way to space.
* Such theoretical analyses were all well and good, but theory was all that they would be until somebody conducted an experiment to actually see what was going on. Experiments with neutrino beams shot over long distances through the Earth to neutrino detector systems seemed to show that neutrinos oscillated, but what was really needed was a tool to probe the neutrino emissions of the Sun more directly. That tool was built, in the form of the "Sudbury Neutrino Observatory (SNO)", buried two kilometers under the ground in a nickel mine in Sudbury, Ontario, Canada.
While the Homestake detector could only pick up neutrinos through a single type of interaction, SNO could detect neutrinos through two different classes of interactions. One of these interactions was only with electron neutrinos, while the other two were with all flavors of neutrinos. Analysis showed that if neutrinos weren't changing flavor, the count of interactions with electron neutrinos would be basically the same as the count of interactions with all three flavors of neutrinos. If neutrinos did change flavor, the count of interactions with electron neutrinos would be an obviously lower proportion.
At the core of the SNO detector was a 12-meter-wide tank made of acrylic plastic that contained the 1,000 tonnes of heavy water. The tank was in turn encapsulated inside a geodesic spheroid 18 meters in diameters that was studded with more than 9,500 PMTs, each capable of picking up a single photon. The entire assembly was then submerged into a cavity dug into the bottom of the Sudbury mine and filled with ultrapure ordinary water,
The three interactions captured by SNO included:
Interactions were expected to occur about ten times a day. There was no way to assign any single detection event to any one of these three possibilities, but there were indirect clues that can be used to perform a valid statistical analysis to sort out the events:
Muons from cosmic rays could also produce Cerenkov light, with about three such events an hour at the underground depth of SNO, but they produced events in both the normal water outside the detector core and in the heavy water inside the core. A worse problem was traces of radioactive materials in the immediate environment; all was done to minimize this problem, and the experiment was "calibrated" to determine the level of background radiation.
The first test run began in late 1999 and went into May 2001, with a half billion events detected, with just under 3,000 validated after screening. The detected electron neutrino flux was about a third of the total rate, which strongly suggested that neutrinos were oscillating in flavor as they passed from the Sun. SNO has since been upgraded. with ultrapure table salt dissolved in the core to improve neutron capture, and has been fitted with new detector systems.
The fact that the neutrino oscillated clearly indicated that it has mass. If it were massless, it would travel at the speed of light; if it traveled at the speed of light, time would not exist for it; if time did not exist for it, it would not be able to change in any way, ruling out oscillations. Neutrinos had to have at least a very small mass, and they didn't travel at the speed of light. Since the Standard Model didn't take neutrino mass into account, some changes were obviously in order.
* Muon and neutrino detectors have been built in the deep ocean and deep under Antarctic ice that consist of strings of photomultiplier tubes strung through the (liquid or solid) water. They detect the direction and energy of the particle by tracking the streak of Cerenkov radiation it produces. The large volume of the water or ice bracketed by the strings of detectors gives a higher probability of picking up events. Placing the strings well below the surface screens out more common particles that could confound the measurements. The water-based detector arrays are simpler to set up than ice-based arrays. However, deep ice is pure and has no bubbles, so it makes a better detector medium than fluid water.
The first attempt to built a water array was the "Deep Underwater Muon And Neutrino Detector (DUMAND)", with a prototype string of detectors placed in the deep ocean the Hawaiian Islands beginning in 1993. In completion, it was to have a "footprint" of 20,000 square meters, but the system proved highly unreliable and the project was cancelled in 1995. A similar deep water experiment was set up under Lake Baikal in Russia. The project, simply known as "Baikal" was a German-Russian collaboration; inspection of the Baikal website shows the effort was either abandoned or is in a low state of activity.
However, nobody has given up on water arrays by any means. Work is now underway on a big array set up in the Mediterranean, at a depth of four kilometers off the coast of Greece. The array is named the "Neutrino Extended Submarine Telescope with Oceanographic Research (NESTOR)", also rendered as "Neutrinos from Supernovas & TeV sources Ocean Range". NESTOR is an international collaboration. In completion, NESTOR will consist of detectors mounted on seven towers, each 330 meters tall. The towers will have 12 hexagonal floors each, with PMTs at the corners of the floors. ANTARES is being built by a collaboration of several European countries.
Yet another array, "Astronomy with a Neutrino Telescope & Abyss Environmental Studies (ANTARES), is being set up at a depth of 2.5 kilometers (8,200 feet) near Toulon, on the coast of Southern France. Instead of using rigid towers, the ANTARES detectors are mounted on ten cables, each 400 meters long, with a float on one end and anchored to the seabed. Completion is currently scheduled for 2007.
In both NESTOR and ANTARES, the detectors are arranged over a patch of seafloor about a 100,000 square meters (121,000 square yards) in size, with the arrangement helping to determine the direction of the event. The Europeans are talking about collaborating to build a larger second-generation undersea array.
* The first deep-ice detector array, the "Antarctic Muon And Neutrino Detector Array (AMANDA)", was set up in the mid-1990s, with strings containing a total of 80 PMT modules dropped into Antarctic ice holes bored using a jet of hot water. It had a "footprint" of 10,000 square meters; it has since been upgraded to "AMANADA II", with about 650 modules.
So far results have not been encouraging, with the neutrinos detected apparently being produced by cosmic-ray showers. It is believed that the cosmic neutrino flux is lower than expected and a bigger detector is needed, and so the AMANDA II array is being further updated to "IceCube", using much the same technology, with 5,000 detector modules in a cubic kilometer of ice. IceCube is scheduled for completion in 2011.
AMANDA is complemented by another experiment, the "Radio Ice Cerenkov Experiment (RICE)", which involves radio receivers buried in the Antarctic ice to detect neutrino emissions from supernovas through radio waves emitted when neutrinos interact with the ice.
* There have been attempts to find other exotic particles using ingenious detector systems. One of the other consequences of many GUTs is that they predict the existence of "magnetic monopoles". Although in classical terms, a magnet will always have a north and south pole, in quantum-mechanical terms it is possible to have an isolated north or south pole, or monopole.
A monopole would be a very massive particle, about 10^16 GeV, roughly the mass of a bacterium. In the early 1980s, a Stanford researcher named Blas Cabrera built a monopole detector around a loop of superconducting niobium wire; if a monopole passed through the loop, it would set up a persistent magnetic field that could be detected. Cabrera did record a single event in 1982 that might have been a monopole, but nobody was able to duplicate his results, and he ultimately recanted.
Current theory suggests that it is in fact impossible to observe a free monopole, just as it is impossible to observe a free quark. The focus on exotic particle searches has moved on to "dark matter". Surveys of the overall mass of the Universe suggest that a good portion of it is hidden away in some sort of "dark matter", not like ordinary matter locked up in protons, neutrons, and electrons.
The fact that the neutrino seems to have mass provides one useful candidate for dark matter. Although only an upper limit on neutrino mass is known at present, if the neutrino mass is close to that upper limit, neutrinos could explain all the dark matter in the Universe. There are two other proposed forms of dark matter, known as "weakly interacting massive particles (WIMPs)" -- particularly the "neutralino", the lightest particle described in the SUSY model -- and "axions". Since dark matter should sleet through ordinary matter in much the same way that neutrinos do, most dark matter searches are performed in deep underground sites where the bulk of confounding events can be screened out.
Early WIMP searches included the "UK Dark Matter Collaboration (UKDMC)", the Italian-Chinese "Dark Mater (DAMA)" collaboration, and the "Cryogenic Dark Matter Search (CDMS)" in the US. UKDMC was placed in the Boulby salt mine in the UK at a depth of 1,100 meters, while DAMA was set up at the Gran Sasso lab; CDMS uses a cosmic-ray veto scheme that allows the experiment to only be placed ten meters under the ground at Stanford University in California.
WIMPs by definition don't interact with matter much, but given enough of them, every now and then one will hit an atomic nucleus, with the recoil of impact observed to give an indication of the presence of the WIMP. The most noticeable recoil occurs when the WIMP and the nucleus have the same mass; since nobody's exactly sure how massive a WIMP is, it is useful to use a range of nuclei of different masses. The expected rate of interaction is about one per day per every ten kilograms of target material.
The recoiling nucleus can be observed by the light emission of a scintillating material; or the ionization of the target material; or a slight rise in temperature in a cryogenic material. The experimental setup must include highly pure materials that have minimal traces of radioactive impurities.
The UKDMC and DAMA both use sodium iodide scintillation detector arrays, with the mass and sensitivity of the arrays increased in steps. The later "Heidelberg Dark Matter Search (HDMS)", a German-Russian collaboration at Gran Sasso, uses germanium ionization detectors. However, germanium ionization detectors don't provide useful clues on whether their ionization was due to a WIMP or other background event, and so the Stanford CDMS experiment uses a arrangement of cryogenic germanium and silicon detectors to pick up both ionization and heating. A later dark-matter search, the Anglo-German "Cryogenic Rare Event Search With Superconducting Thermometers (CRESST)" uses a heating detection setup based on superconducting detectors.
Axion search experiments have also been conducted, likely the best-known being the "CERN Axion Solar Telescope (CAST)". Unlike the WIMP instruments, CAST actually looked a little bit like a telescope. According to theory, an axion should be converted to X-rays in a strong magnetic field, and so CAST consisted of an X-ray detector telescope peering through a strong magnetic field. CAST ran for six months in 2003, with negative results.
One later Italian hunt for axions, the "Polarization of the Vacuum by Laser (PVLAS)" experiment, involved bouncing a laser beam back and forth through a strong magnetic field tens of thousands of times and then measuring a polarization of the laser beam. It shifted by a few millionths of a degree, which was predicted by axion theory. There has been some skepticism over the finding because of the negative results of CAST. A more sophisticated hunt is now being planned, involving sending a laser beam through a strong magnetic field -- not too surprisingly, in theory the conversion reaction is two-way, with photons converted to axions in a magnetic field or the reverse -- and into a solid block that absorbs light but of course not the elusive axions. A strong magnetic field on the other side of the block was to convert axions back into photons that were to be picked up by a light detector system.
So far, results of all dark matter experiments have been negative or inconclusive, though given the difficulty of picking rare dark matter events out of the noise, nobody ever expected progress to be easy. Incidentally, the physicists who coined the name "axion" were unaware that this was also the name of a laundry detergent, and they were soon told that the axion had been discovered -- with the proof obtained from a local supermarket.