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Humans are the cause of climate change

The team looked in particular for ‘lag time’ effects in the data. For example, the eruption of Mt Pinatubo in the Philippines in 1991 lowered the global temperature by 0.3 C in the following year as fine aerosols of volcanic ash and sulphuric acid caused a cooling effect. However, the team believes that volcanic eruptions have a short lived effect on the long-term temperature. They therefore looked at two possible variables that could adjust climate – greenhouse gases and changes in the sun’s output.

Wigley and colleagues ran the two independent computer models four times, each time with a different set of parameters. In the first data set, they created a world in which greenhouse gases remained constant over 100 years and changes in solar output do not affect the climate. In the second set they assumed that the Earth’s climate can be affected by solar output – called solar forcing – but greenhouse gases remain constant. The third set had no solar output, but greenhouse gases rise over time. Finally the fourth set had a mixture of both solar forcing and rising levels of greenhouse gases.

They first found that there was wide discrepancies between the output of both computer models and the experimental data in the first data set. This indicated that some mechanism was increasing the global temperature this century. Then they found that if solar heating was solely responsible for climate change, the Earth’s climate would have to be six times more sensitive to solar heating than realistic estimates suggest. Wigley, Smith and Santer concluded that unlike a paper published in Physical Review Letters this week (see CERN plans global-warming experiment), the only model that matched both simulation and experimental data sets was the one containing a strong greenhouse gas effect and a small amount of solar heating.

According to Wigley: “These results provide another important piece in the jigsaw puzzle of climate change, strengthening yet further our confidence that there has been a discernible human influence on climate. Furthermore, they provide additional evidence that the models used to make projections of future climate change are realistic.”

Laser emits multi-wavelength light

Semiconductor lasers are usually monocromatic because the electrons stored in energy bands do not emit any photons unless they move out of the semiconductor’s conduction band. Federico Capasso and colleagues have created the multi-wavlength laser by modifying the conduction band by building 25 different sandwich layers of material into the device. Each sandwich consists of four atomic layers of the semiconductor alloy aluminum indium arsenide, alternated with 18 atomic layers of another alloy, gallium indium arsenide. This creates “sub-energy” bands – discrete electronic states – in the conduction band. These sub-bands form quantum wells in which the electrons fall. The electrons do not have enough energy to escape the quantum well, but can move through quantum tunelling into lower energy bands. As they cascade down through the energy bands, they emit photons as they go. By precisely tailoring the layers’ thickness, the electrons have energy levels designed to emit light at several pre-selected wavelengths

The maximum power output of the new system has so far been 1 Watt, which occurs when it is cooled to 10 Kevlin. The device only emits two wavelenths if operated above 250K, and optical powers of no more than a few hundred milliwatts are available at room temperature.

Uniform universe

The 1995 image was taken in a small patch near the Big Dipper constellation, which astronomers previously thought was completely blank. This new image taken by the Space Telescope Science Institute and NASA’s Goddard Space Flight Center last month, was of the constellation Tucana, near the south celestial pole. “We have eagerly awaited this new set of images ever since the first deep field image, which had a dramatic impact on the entire science of astronomy, ” says Robert Williams, who was the principal investigator on the latest observation.

Astronomers are also benefiting from new instruments that have been recently fitted to the telescope, which are allowing them to make more detailed measurements. For example, they will be studying the light from a quasar in the image which travelled through numerous intervening gas clouds in galaxies and intergalactic space. These clouds of primeval hydrogen subtract specific colours from the beam. The resulting absorption spectrum, recorded by an Imaging Spectrograph on the Hubble telescope, allows astronomers to probe the hydrogen clouds to find out what they are made of and where they are. “This is going to provide an extremely important way to test our ideas of how the intergalactic medium turned into galaxies, ” says Williams. Meanwhile, a new infrared camera called NICMOS captured some of the galaxies whose light had been ‘redshifted’ because of the expansion of the universe.

First module of international space station launched

The 24 ton Zarya (meaning “daybreak”) module is based on the long experience the Russian Space Agency has had with the Mir space station. Unlike the US space station Skylab, which was until Mir the largest object in orbit, the module can stay in space for 430 days without re-fuelling. The Russian space company Khrunichev built Zarya under contract to Boeing Aerospace.

After launch Zarya will move into a circular orbit to make it easier for the space shuttle Endeavour to rendezvous with it on 3 December. Astronauts will then dock the US-built module ‘Unity’ with Zarya. The new 500 ton station is to be built over the next six years by 44 flights, making it the largest and most complex manmade object in space.

The International Space Station is made up by five partners, United States, Russia, Japan, Canada and the European Space Agency. Only ten of ESA’s fourteen member states Belgium, Denmark, France, Germany, Italy, the Netherlands, Norway, Spain, Sweden and Switzerland are involved in the project. However, a sixth partner may soon join the list. NASA has approached the UK government to discuss using some UK technologies, such as Matra Marconi’s ion thrusters, on the project. “We’re in the early stage of negotiations” said a spokesperson from the British National Space Centre.

New look for optical fibres

Fibre-optic cables usually consist of an inner core of highly refractive glass sheathed inside glass with a lower refractive index. Light travels along the inner core fibre by total internal reflection. However, despite their popularity, standard optical fibers have a number of limitations. They are difficult to manufacture, light can leak from the inner core, and it is only possible to change the optical properties of the fibre at one wavelength.

In the approach developed at Bath, solid silica rods and hollow silica capillary tubes are stacked in hexagonal arrangement and heated to 2000 Celsius. The silica is then stretched into a fibre that has a pattern of submicron air holes over its cross sectional area. Instead of travelling along a region with a high refractive index, light travels through a region of the fibre containing an extra air hole. Moreover, these holes create a photonic band-gap effect which means that only certain wavelengths can propagate in the fibre, as predicted by researchers from the Technical University of Denmark.

The photonic waveguide has a number of advantages for optoelectronics applications. For example, whereas normal optical fibres rely on doping with impurities such as krypton to change their refractive index, the wavelength sensitivity of the new fibre can be changed by adjusting the spacing, temperature or pattern of the rods in the manufacturing process. Doping is not necessary.

The photonic band-gap fibre also handles the polarization of light beams in a completely different way to normal fibers. According to Knight, it allows light to travel much more slowly along its length, which can lead to increased nonlinear effects. This could lead to a reduction in the size of devices that rely on optical fibres, such as gyroscopes.

“In the near term I suspect it will be used in niche markets” says Knight. Some companies have already approached the Bath team about using the fibres made in a similar way for laser welding equipment. The challenge for commercial manufacturers will be to make long lengths of the fibre. The Bath team have only just succeeded in making fibres many metres long with only small fluctuations in the fibre width.

Time’s arrow seen in particle decays

The CPLEAR collaboration studied the decays of neutral kaons produced in proton- antiproton collisions. The collisions produce both neutral kaons – which contain a down quark and an anti-strange quark – and their antiparticles. These particles can decay in many ways. The CPLEAR team study so-called semileptonic decays in which the neutral kaons decay into a positron, negative pion and a neutrino. They find that this is different to the rate at which the antiparticles decay in an electron, positive pion and an anti- neutrino. If the decay process was time-symmetric the two rates would be the same. This is the first time that the violation of time-reversal (T) symmetry has been observed directly in an experiment.

The KTeV collaboration at Fermilab saw evidence for the violation of T symmetry in very rare decays of neutral kaons into pairs of pions and electrons. Their technique relies on the fact that reversing the direction of time for a particle is equivalent to reversing its momentum. The KTeV team detect the violation of T symmetry by comparing the rates of certain decays with other decays in which the particles emerge in the “time-reversed” direction.

The violation of T symmetry has been expected ever since 1964 when a violation of charge-parity (CP) symmetry in neutral kaons was detected in experiments at the Brookhaven National Laboratory in the US. This discovery led to a Nobel prize for James Cronin and Val Fitch. The violation of both CP and T symmetry is predicted by theory, but their combined effect – so-called CPT symmetry – is thought to be conserved in all quantum field theories.

The December issue of Physics World will contain an article by Nick Mavromatos of Oxford University on the significance of the CPLEAR and KTeV experiments.

Measuring the conductivity of blood

Doctors need to measure blood conductivity to calculate the internal pump volume of the heart and the blood circulation rate. However, when patients receive additional fluids – for example, water, drips or drugs during surgery – the conductivity can fluctuate. The new system is able to measure these changing conductance levels in real time.

The device works by inserting a catheter (a flexible tube) containing a series of electrodes into the main pump chamber of the heart. By applying a small constant current to the electrodes and measuring the potential difference between each pair of electrodes, it is possible to calculate the conductivity using a simple formula. The first tests of the technique are now being carried out at the Cleveland Clinic Foundation.

Microlasers get even smaller

Attempts to scale down current laser designs have been made before, but the devices were not efficient enough for real applications. Now a group of researchers from Darmstadt University of Technology, the University of Frankfurt and IMM in Mainz have found a zeolite that can trap the dye molecules with the correct alignment and orientation for lasing. The team trap Pyridine 2 molecules, which have a width of 0.6 nm, inside the zeolite known as AIPO₄-5, which has cavities 0.73 nm wide.

The researchers used light from a standard Nd:YAG laser to excite the dye. When the energy went above a certain threshold, they detected a series of strong fluorescent energy spikes, indicating that the zeolite was acting as a laser. The zeolite dye laser is three times smaller than the smallest dye lasers constructed to date.

Hawking and Rotblat choose their favourites

Hawking was one of ten famous names asked by the National Portrait Galley to pick photographs that defined the 20^th century. He was the only scientist in a group that included the film producer David Puttnam, the fashion designer Vivienne Westwood, and David Bowie. “I have concentrated on scientists and women, the important members of society, ” said Hawking, who revealed that one of his choices would be a photograph of Francis Crick and James Watson, the discoverers of the double helix.

In between choosing his favourite pieces of music for Desert Island Discs, Rotblat talked about how he learnt from a early age of the hardship caused by war. He was five when his family became destitute as a result of the First World War. These experiences convinced him that only science could improve poverty. In 1939 Rotblat received a scholarship to work with James Chadwick at Liverpool University while his wife remained in Poland. Despite frantic efforts to get her out of the country at the start of the Second World War, she was turned back at the Polish border and later died at Auschwitz. Rotblat was the only physicist to quit the Manhattan atomic bomb project during the war, and later went on to set up the Pugwash movement and receive the Nobel peace prize in 1995.

Rotblat said that his favourite record was Beethoven’s ‘Ode to Joy’, and that if he was allowed one book on a desert island it would be the CD-Rom version of the Encyclopaedia Britannica, along with a solar-powered portable computer. His other choices included “Where have all the flowers gone”, “Ol man river” and works by Chopin, Dukas and Bruch.

How the Nobel prize was won

The Nobel Foundation chooses Nobel prize winners with the utmost secrecy. But in 1974 it agreed to allow historians access to any of its archives over fifty year old. Since then Crawford has documented how the earliest awards were chosen. She found that scientists who were nominated for an award, but failed to receive one, include Thomas Edison, the Wright brothers, and the astronomers George Hale and Arthur Eddington. Indeed, Arnold Sommerfeld, one of the pioneers of quantum theory, was nominated 74 times without success. The largest mistake made by the foundation, says Crawford, was the fact that Lise Meitner did not receive a share of the 1944 chemistry prize, which was awarded to Otto Hahn for the discovery of fission.

Over 4000 people were nominated for a Nobel prize during 1901 and 1939. According to Crawford the large number of nominees meant that the awarding committee’s own ideas about what type of the scientific work should be honoured had an overall influence on the final result. In physical sciences, basic research always won over applied research, experiment over theory, and atomic and nuclear physics over astronomy and geophysics.

It was for these reasons that Albert Einstein won the 1921 prize for the photo-electric effect, rather than any of his more famous work. The committee was, according to Crawford, reluctant to give him the award for ‘speculations’ such as relativity.

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