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Russia sells research time

In the deal NASA gains Russia’s research time during the first four to five years of the space station’s construction, the rights to store experiments onboard a Russian-built module and the help of Russian cosmonauts with any experiments. The deal is worth potentially an additional 5000 hours of cosmonaut time to US researchers. The US and Russia had already agreed that 10000 of the 30000 hours astronauts will spend constructing the space station in orbit would be scheduled for research.

The funds are to be used to finish the stations crew compartment module, which is already a year behind schedule. Without the module the $60 bn project is likely to be delayed another six months. The first components of the space station are due to be launched on a Russian Proton Rocket next month.

But NASA is already coming under criticism in the US for proposing to buy $600 m of equipment from Russia in an attempt to generate some financial stability at the Russian Space Agency. Dan Goldin, NASA’s administrator, told Congress yesterday that the International Space Station should be scrapped if it cannot be properly funded – either by the US or Russia.

To protect the project from any other further economic difficulties, NASA is expected to request another $560 m to construct a station propulsion module which they believe will reduce reliance on some of the Russian modules. If Congress does not agree to the increase, then NASA has to find the money from its existing budget. Space science programmes are likely to take the brunt of any cuts announced in such a move.

Wet weather on Titan

Planetary physicists hope to use atmospheric data from Titan to provide clues to the behaviour of our own atmosphere. The difficulty with studying Earth’s atmosphere is that it is affected by extra variables such as the biosphere, geological activity and large quantities of water on the surface, which can make it difficult to study some of the underlying atmospheric processes. Titan on the other hand, is cooler, atmospherically dense, and believed to be relatively dry.

Griffith and colleagues knew from observations made by the Voyager spacecraft in 1980 that Titan’s atmosphere was quite dynamic. Many scientists claimed that Titan had a weather system based on methane, with methane clouds, rain and oceans. Other data suggested a much ‘drier’ planet because methane would not remain long in the lower atmosphere. But it has been hard to obtain details of the lower atmosphere because radiation is absorbed by the thick concentrations of methane and nitrogen gases on the satellite. However, near-infrared spectroscopy instruments fitted to the United Kingdom Infrared Telescope (UKIRT), Hawaii, were able to peer through some narrow ‘windows’ in the atmosphere that are transparent to infrared radiation.

On occasions the researchers could see bright ‘clumps’ in the methane concentrations at about 15 km up from Titan’s surface. These clumps covered 9 percent of the moon. Unlike the Earth, methane clouds on Titan are expected to be extremely brief events, which would explain their lack of detection in earlier observations. Further information will come to light in 2004 a European Space Agency probe called Huygens will be dropped into Titan’s atmosphere.

Wireless quantum encryption

The technique is based on characteristics of individual photons that are generated randomly between the sender and the receiver. Although researchers have already transmitted quantum encryption keys through optical fibres, atmospheric turbulence had disrupted similar attempts in the open air. William Buttler and colleagues at Los Alamos National Laboratory in the US succeeded by carrying out a series of experiments when atmospheric turbulence was at its lowest. They used a short pulsed laser to transmit the quantum key and a telescope to receive it.

The laser, nicknamed “Alice”, emits millions of single photons per second. Each photon is randomly assigned one of two polarization states that represent a “1” or a “0”. The telescope, called “Bob”, knows what these polarization states are, but randomly collects photons from two separate light paths, each one sensitive to a polarization of Bobs own choosing. Instead of looking for Alice’s polarization states, Bob looks for related polarization states.

This ensures that when Bob is looking for a zero, he will never see a photon if Alice transmitted a one. But when Alice transmits a zero, he will know for a fraction of the time (about 25%) that he is looking for a zero and their two values were in agreement. Bob then indicates to Alice the positions in the sequence where his values match hers. This information is only available to the two players, which allows them to construct the quantum key.

Anyone intercepting the stream of photons will reveal the act by raising the error rate above a threshold value or eliminating the photon stream altogether. The researchers now hope to repeat the experiment during daylight.

PhD theses to go online

The DFG has been experimenting with different ways of digitizing postgraduate dissertations for some time. These documents are usually one of the hardest sets of scientific information to access because copies are held only at national libraries or at the awarding institution, and students tend to only publish 3 or 4 copies.

This new program generates meta data tags so that internet search engines can understand the title tags of the thesis. At the same time it indexes the subheadings of the document so that sections of the report, such as chapters on experimental techniques, are not lost within a wealth of data.

The next stage of the project is to be able to produce a program that can keyword search whole documents. PhD students will also be expected to submit their thesis in a standard suitable for conversion on the web. The first prototype guidelines on dissertation web formats were shown at the fair.

Higgs search loses a fortnight

With a collision energy of 189 GeV, LEP is currently the world’s highest energy electron-positron collider. It was originally due to be shut down at the end of this year, but physicists working at the facility successfully lobbied to have its life extended and its energy increased to 200 GeV. The extension is to be funded by a series of ‘special contributions’ from CERN’s member states. The collider is scheduled to run for 169 days in 1999 (the same as this year) and 156 days in 2000.

However, some member states – such as the UK – have not provided these extra funds, while there are concerns about the ability of Greece and Poland to pay their regular subscriptions. Sweden’s subscription also remains uncertain in the run-up to the Swedish national elections. “If all the money is not found from further contributions then a further strain will come on CERN finances, ” says Roger Cashmore, the Oxford University physicist who will be director of collider programmes at CERN from next year.

India and Pakistan lied about nuclear tests

These latest results, according to Barker et al., indicate “high confidence” that the network being set up to monitor the CTBT can indeed verify small nuclear explosions. Concerns over the effectiveness of the monitoring network have made it extremely difficult for supporters of the treaty to have it ratified by the US congress. Only two of the five ‘official’ nuclear states – France and the UK- and 19 other countries have ratified the CTBT^*. Another 129 countries have signed but have yet to ratify the treaty.

When it is completed, the monitoring network will consist of 321 stations that will be capable of registering shock waves emanating from a nuclear explosion underground, in the sea or in the air, as well as detecting the radioactive particles released into the atmosphere. Only 61% of the seismic stations were operational at the time of the Indian and Pakistani tests. However, Barker et al. point out that the system automatically recognised the nuclear explosions on May 11, 28 and 30 among a total of 70000 seismic events. Later refinements of the data enabled them to produce a map pinpointing the locations of the explosions to within 10 km.

As the network was unable to detect India’s sub-kiloton tests on May 13, the scientists suggest that the explosions must have been 500 times smaller than the estimated 16 kiloton event on May 11. Such small explosions are unlikely to be nuclear denotations. And although the monitoring stations did not pick up two explosions on May 11, the fact that the seismic data clearly demonstrated a nuclear denotation signal shows, according to Barker et al., that the CTBT monitoring network was “remarkably successful”

*The 21 States that have ratified the Treaty are: Australia, Austria, Brazil, Czech Republic, El Salvador, Fiji, France, Germany, Grenada, Japan, Jordan, Micronesia (Federated States of), Mongolia, Peru, Qatar, Slovakia, Spain, Tajikistan, Turkmenistan, United Kingdom, and Uzbekistan.

International projects face flak in Congress

Sensenbrenner has long campaigned against ITER, an international collaboration between fusion labs in Europe, Japan, Russia and the US. He claims that ITER has soaked up valuable funds that could be used for cheaper, more effective fusion programmes, and called last week’s extension “irresponsible.” He argues that the original goal set by the project – a sustainable fusion reactor – is impossible to achieve with the current reactor design and that ITER is a “dead end” project. He was particularly incensed by the extension as the Department of Energy had previously agreed to seek “congressional concurrence” for any future extension to the programme.

Upon hearing that NASA was seeking an additional $600 m to keep the space station on target for habitation next year, Sensenbrenner called the Russian involvement in the programme an “expensive mistake”. He also suggested that Russia could no longer be treated as an equal in the project, and that the station had been turned into “Russian foreign aid” instead of a science programme. Gingrich was even more scathing about NASA itself. The agency had “done a spectacular job of slowing down the rate which we’ve gotten into space, ” he said. If it were not for NASA’s bureaucracy, and the “slow, cumbersome and extraordinarily expensive” nature of American space launch systems, the US could have had a permanent presence on the moon, said Gingrich.

Positrons found in quasar jets

Quasar jets are streams of plasma that travel at near the speed of light from supermassive black holes at the centre of galaxies. The jets appear at radio wavelengths, but can also be detected in the optical, X-ray and gamma-ray regions of the spectrum. The difficulty for astronomers has been distinguishing between electrons and positrons in the jets. Both particles emit synchrotron radiation with the same wavelength and linear polarization when they travel at relativistic speeds through a magnetic field.

However, linear polarization can be converted into circular polarization in a magnetized plasma by two processes: Faraday conversion and Faraday rotation. Both processes occur in an electron-proton plasma, whereas only Faraday conversion can take place in an electron-positron plasma. By comparing the measured circular polarization with computer predictions, Wardle et al. conclude that the plasma in a quasar jet is made of electron-positron pairs.

Secret files reveal plan to kidnap Bohr

Bohr had escaped from German-occupied Denmark in September 1943 and crossed the Atlantic to work with British and American physicists on the Manhattan atomic-bomb project at Los Alamos. However, he returned to Copenhagen in August 1945, partly on the advice of his close contact, Sir John Anderson, who was chancellor of the exchequer and political head of the British effort on the bomb project.

It was then that the first report of the Russian plan to kidnap Bohr surfaced. An initial telegram from the Foreign Office on 12 September 1945 says that the report was “low category and unconfirmed”. However, a second telegram sent on 17 September states that the “conversation on which [the] report was founded took place a month ago in a country neighbouring Denmark and [in front of an] official employed by [the] government in question”. According to the telegram, the idea was to get Bohr to Bornholm – the Danish island in the Baltic Sea that the Russians had occupied since the German capitulation after the war in May 1945 – and to abduct him from there with the help of Danish “comrades”.

Bohr was then contacted by the British envoy, who told the Foreign Office that Bohr’s first reaction had been to discount the report on the grounds that nothing would be gained by kidnapping him. “The recent developments in use of atomic energy, ” wrote the envoy, quoting Bohr, “were not so secret as was generally thought…His own part in the discoveries had been modest and his knowledge was incomplete…He did not therefore believe that any responsible Russian with any knowledge of the recent experiments would want to seize his person.” Bohr promised “not to run unnecessary risks or do anything foolish”, and told the envoy that he would not on any account accept an invitation to visit Bornholm.

In the end, no invitation came, but a Soviet physicist – Jakov Petrovich Terletsky – did visit Bohr in Copenhagen in December 1945. The meeting was arranged by the author Martin Andersen Nexø – a Communist who lived in Bornholm – and staged by Lavrentii Beriya, the head of the KGB. At the meeting Bohr managed to satisfy the Soviet authorities by giving them nothing more than “some atomic generalities”.

Niels Bohr’s son Aage, who is also a physicist, confirms that precautions were taken against his father’s kidnapping. “[My father] would have done everything not to be kidnapped and he knew that he should do everything to avoid a trap, ” he said. “He would not dream of letting himself be tempted to give information.”

Finn Aaserud, head of the Niels Bohr archive at the Niels Bohr Institute in Copenhagen, says that it is unlikely that Bohr would have given information about the atom bomb that had not yet been cleared.

Superconductivity debate gets ugly

However,it is the last paragraph that makes the paper unique: “The tragedy of beautiful theories, ” write Chakraverty et al., “is that they are often destroyed by ugly facts. One perhaps can add that the comedy of not so beautiful theories is that they cannot even be destroyed; like figures in a cartoon they continue to enjoy the most charming existence until the celluloid runs out.” Provocative and unusual stuff for the world’s leading physics journal.

The paper provoked an immediate response from Sasha Alexandrov of Loughborough University in the UK, one of the champions of the bipolaron theory. On the same day that the “ugly facts” paper appeared in PRL, Alexandrov submitted a “comment” on the paper to PRL and to the Los Alamos e-print server, in which he argued that the objections in the paper were “the result of an incorrect approximation… and the misuse of the bipolaron theory”. He concluded by stating: “What is clear, however, is that any theory, beautiful or not, cannot be destroyed by ‘ugly’ artefacts as those in Chakraverty et al.”

So what is a bipolaron? And why has it polarized the HTS community so strongly?

The term polaron was coined by the late Lev Landau to describe the lattice distortion or polarization caused by the charge on the electron, and which follows the electron as it moves through the solid. One side effect is that the electron’s effective mass is increased. Bipolarons are bound pairs of polarons, mutually attracted by the lattice distortion. In the 1970s various physicists, including Chakraverty, used bipolarons to explain certain properties of different solids.

In 1981, before HTS was discovered, Ranninger and Alexandrov, both then working in Grenoble, published a paper in which they suggested that “bipolarons might be superconducting”. Ranninger later abandoned this theory, but Alexandrov continued to champion the bipolaronic approach in collaboration with the late Nevill Mott at Cambridge.

Fast forward to 1998. Physicists now have lots of theories of HTS but they are not sure which, if any, of them is correct. They know that HTS involves pairs of charge carriers, but the precise nature of these carriers and the pairing mechanism holding them together remain unclear. Chakraverty, Ranninger and Feinberg, however, are sure that the bipolaron theory is wrong.

According to Ranninger, their paper points out two main flaws in the bipolaron approach: the superconducting transition temperature, T _c, is inversely proportional to the effective mass of the bipolarons, but the effective mass of the bipolarons in HTS materials is so high – at least 210 electron masses – that T _c cannot be higher than 10 K, which is clearly too low to explain HTS. The paper also points out: “The existence of a Fermi surface in the high-T _c materials has now been established experimentally beyond doubt. Bipolarons being bosons do not have a Fermi surface.”

Alexandrov replies that Chakraverty et al. have “misused” the theory by using onsite bipolarons, in which both electrons are near the same lattice site, rather than intersite bipolarons, in which the electrons are near neighbouring sites. He says that the effective mass of the bipolarons is about 12 electron masses and that this is consistent with experiments in HTS materials. “I am pretty sure we are correct, ” says Alexandrov. “There are no experimental facts that destroy our theory at present.”

He also disputes that experiments have shown that HTS materials have Fermi surfaces, pointing out that experiments have shown that the Fermi surface is destroyed in underdoped superconductors. (Doping refers to the addition of oxygen or some other element to provide charge carriers for the superconducting material. Optimal doping is the amount of doping which gives the highest T _c.) And he claims that other properties of bipolarons – in particular the fact that they are mobile – can explain some of the other experimental results cited by Chakraverty et al . However, Alexandrov admits that not all theorists agree that bipolarons are mobile.

Chakraverty et al. have submitted a response to Alexandrov’s “comment”, and were reluctant to discuss this before PRL had reached a decision on publication. However, Ranninger said that their paper had already addressed the situation of intersite bipolarons and found that T_c was still only 5 K.

But why did they conclude with such a provocative paragraph? Ranninger says the last paragraph was written specifically to “calm the situation” and does not think that it was provocative. “We could have had a devastating statement at the end and that would have been a lot worse.” But Alexandrov, for one, says that he found the final paragraph “unhealthy and not motivated by any reason”.

Reaction to the paper in the HTS community has been mixed. In a letter to Ranninger, Alexei Abrikosov of the Argonne National Laboratory wrote: “I would like to express my pleasure upon reading your paper about bipolaronic superconductivity. I completely agree with it, and I appreciated the last two sentences.”

Philip Anderson of Princeton University, a long-term critic of the bipolaron theory, also welcomed the paper. “It had worried me that two theorists as competent as Ranninger and Chakraverty kept on with some support for the bipolaron theory of high-T _c [when] the rest of the serious many-body community had long since rejected it.” The tone of the paper also went down well with Anderson: “I rather like physicists to express these kinds of sociological and methodological ideas, when the editors let them get away with it, and when it is done as eloquently as this.”

Gene Wells, managing editor of Physical Review Letters, agrees that the last paragraph of the paper was unusual. “I am not surprised that the letter caused a few raised eyebrows, ” says Wells, “but in context, I do not find it misleading or wrong. I doubt that an author who had never positively contributed to bipolaron theories would have been ‘allowed’ such a conclusion. Ranninger is criticized nearly as much as Alexandrov in the letter.”

However, the tone was “unhelpfully polemic” according to Alan Bishop of the Los Alamos National Laboratory. “I might comment in the same vein [that] ‘beauty is in the eye of the beholder’. In this case there are several beholders!”

The reaction among HTS experimentalists has also been mixed. Besides the question of the Fermi surface, there are “many other severe disagreements” between the bipolaron theory and experiment according to Juan-Carlos Campuzano of the University of Illinois at Chicago. For instance, he says, experiments find an effective mass of 2-3 free electron masses, compared with 12 electron masses predicted by the bipolaron theory. Campuzano adds that he found the PRL paper “almost charming” and that the original authors of the polaron theory had shown “a wonderful sense of humour” in pointing out that their own theory was wrong. “Would science not be a much more pleasant enterprise if more of us were willing to admit our faux pas ?” he asks.

Guo-meng Zhao and Hugo Keller of the University of Zurich put the lack of experimental support for the bipolaron theory down to the fact that most experiments have been carried out on optimally doped cuprates. Experiments have shown, they say, that two types of charge carriers – Fermi-liquid-like carriers and polarons or bipolarons – co-exist in optimally doped cuprates. Since the bipolaron theory is based on only one type of carrier, it will not be consistent with these experiments.

However, the situation is different for deeply underdoped cuprates. “To our knowledge, ” say Zhao and Keller, “no other theory can explain the physical properties of deeply underdoped cuprates in a more consistent way than the bipolaron theory.”

And Alan Bishop, for one, does not agree that Chakraverty et al. have demolished the bipolaron theory. But he does not think that Alexandrov and co-workers have proven it to be “the” theory either. “The situation remains experimentally and theoretically complicated, ” he says. “In my opinion, there is no such thing as ‘the theory of HTS’ presently available.”

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