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When is a constant not a constant?

The fine structure constant, alpha, is defined as e²/c h-bar, where e is the charge on the electron, h-bar is the Planck constant divided by 2 pi, and c is the speed of light, and is about 1/137. If alpha changes with time then at least one of the fundamental constants of physics – e, h-bar or c – must also change with time. Although no evidence has ever been detected for any of these constants changing, such behaviour is predicted by various unified theories of the fundamental forces.

Fine structure can be observed as the splitting of certain energy levels in atoms. So if alpha changes over time, the emission and absorption spectra of these atoms will follow suit. One way to look for any changes in the value of alpha over the history of the universe would be to measure the absorption spectra of distant quasars at different redshifts, and compare the wavelengths of certain spectral lines with present-day values. This is what John Webb of the University of New South Wales (UNSW) in Australia, and colleagues at UNSW, Cambridge University, Pennsylvania State University, the Carnegie Observatories and the University of California at San Diego have done.

Webb and co-workers looked at four independent sets of data – three from optical telescopes and one from a radiotelescope – and found that alpha was smaller in the past than it is now. The result obtained from the optical data was four standard deviations from zero, which means that there is only about a one in 10 000 chance that the result is a statistical fluke. The team also eliminated some 13 potential sources of systematic errors.

In 1999 the same team reported inconclusive evidence for changes in the value of alpha over time based on observations of transitions in iron and magnesium in 17 different quasars. They have now reanalysed those data and combined them with three new sets of data: two sets of observations at higher redshifts taken with the Keck telescope, and radiotelescope observations by Chris Carilli of the National Radio Astronomy Observatory in New Mexico and co-workers. The new data include measurements on transitions in nickel, chromium, zinc, silicon and hydrogen.

Hälg prize rewards neutron scattering in Europe

Brown was recognised for her life-long contribution to the field, from fundamental research to the development of widely used neutron diffraction and polarimetry techniques. She also played a key role in the extensively used Cambridge crystallography subroutine libraries, which help physicists identify crystal structure from diffraction patterns.

After graduating from Cambridge University in the 1960s, Brown became interested in neutron diffraction during a spell at Brookhaven National Laboratory. She subsequently established a programme of neutron diffraction at the UK Atomic Energy Authority’s Harwell laboratory, and was appointed senior scientist at the Institut Laue Langevin in 1972. Although Brown formally retired from Institut Laue Langevin in 1995, she is still active in the field.

Did interplanetary dust seed life on Earth?

The simple organic compounds that exist in interstellar space and the solar system are thought to originate from late-type giant stars. Anthracene is one such compound, and contains 14 carbon atoms and 10 hydrogen atoms arranged into three rings. Micrometre-sized dust grains in the solar system often contain frozen anthracene and water.

To simulate conditions in interplanetary space, Gabla and colleagues cooled a thin layer of anthracene to 150 kelvin and placed it inside a vacuum. The sample was bombarded with 3.5 keV protons, a similar energy to those in the solar wind. A stream of water vapour directed at the sample mimicked the effect of the evaporating water.

After the anthracene had received around 10¹⁷ protons per square centimetre – equivalent to around 100 years of solar wind, under current conditions – Gabla’s team found that a yellowish-brown crust had emerged on the previously white surface. Mass spectrometry revealed that the crust consisted of a wide variety of quinone molecules.

“To our knowledge, this is the first time it has been shown that biological molecules can be formed from the interaction of low-energy protons and non-biological molecules”, Gabla told PhysicsWeb.

Low-energy protons such as those found in the solar wind can only penetrate a few hundred nanometres into the surface of a comet. This means they would have a negligible effect on the overall composition of the comet. But interplanetary dust particles – usually debris from collisions between comets – are typically just a few micrometres across, and these react readily with protons because of their large surface-to-volume ratio. Gabla and colleagues point out that over three thousand tonnes of dust enter the atmosphere every year, and that such small grains of organic matter could enter the Earth’s atmosphere without burning up.

However, astrophysicist Chandra Wickramasinghe of Cardiff University in Wales remains cautious. “This shows that biochemical monomers can form under solar-system conditions. Although this process must occur, its contribution to the origin of life remains conjectural.” Wickramasinghe and colleagues recently suggested that fully fledged microbes may have initiated life on Earth.

Jupiter helps to explain planets around multiple stars

It has long been thought that the rocky cores of all the planets in the solar system formed by a slow process of dust aggregation followed by mutual gravitation and collisions. This theory suggests that the cores formed at roughly the same time, and that the giant planets attracted their gas exteriors later. But it does not account for the strange orbits of many of the planets discovered beyond our solar system.

Astronomers recently adapted this model, suggesting that huge clouds of gas could have merged in the early solar system to form the gas planets long before the terrestrial planets took shape. This means that the giant planets would have disturbed the orbits of the young planets as they circled the central star. As these planetesimals gained mass, this effect would have intensified. Gas drag would also have slowed down the planetesimals, and these effects could have led to unusual orbits.

Kortenkamp’s team developed a computer simulation that accounted for these effects, but ignored the weak gravity between the planetesimals, which was central to the original theory. “We were very surprised to find that the influence of the giant planet led to runaway growth of planetesimals bigger than a large asteroid”, Kortenkamp told PhysicsWeb. The group has dubbed the phenomenon type-II runaway growth. Type I occurs in the earlier stages of aggregation.

The adapted model describes both the solar system and systems in which planets orbit more than one star, because the orbital disturbances can be caused by any massive object – giant planets, brown dwarfs or other stars. “Our earlier work focused on the solar system, and we made the connection with many-star systems later”, Kortenkamp says.

The new results suggest that the process of planet formation is even more robust than astronomers previously thought, at least for terrestrial planets. ‘To use a phrase from Jurassic Park‘, says Kortenkamp, ‘give planet formation a chance and it will find a way’.

First snapshot of superconductor twisters

Physicists have long suspected that column-shaped defects attract magnetic vortices – thin tubes of magnetic flux that can penetrate some superconductors. Previous attempts to study these vortices measured their effect on electrons fired through the superconductor by an electron microscope. But existing electron microscopes were only powerful enough to penetrate very thin samples, and gave little information about the positions of the vortices.

Tonomura and colleagues used a similar technique, but built a one-megavolt electron microscope capable of penetrating a superconductor sample to twice the depth of existing devices. They bombarded a crystal of bismuth-2212 with an angled beam of high-energy argon ions to create column-shaped defects at an angle of 70 degrees to the crystal normal. Bismuth-2212 has a superconducting transition temperature of 85 kelvin.

A magnetic field was applied to create vortices inside the sample, and the powerful electron microscope was switched on. The electrons entered the crystal in the direction of the normal, that is, at an angle to the defects. Tonomura and colleagues then studied the pattern of the electron beam exiting the sample, which revealed the positions and orientations of the vortices by their projected shapes. These so-called Lorentz images showed that between 19 kelvin and the transition temperature, the individual vortices coincided with the column-shaped defects – irrespective of the magnetic field direction.

Below 12 kelvin, however, the vortices did not line up with the defects. Tonomura and colleagues believe that a different pinning mechanism could be responsible for this effect.

“This is the first direct observation of vortex pinning by columnar defects”, team member Ken Harada told PhysicsWeb. Although the team set out to achieve this result, they are still excited by their findings. “Our results show that superconducting magnets with high critical currents can be manufactured by using these defects”, says Harada.

Most superconductors expel all magnetic flux from their interiors – this is known as the Meissner effect and is often considered to be the acid test of superconductivity.

Biophysicist scoops 2001 Dirac medal

Hopfield’s research into the behaviour of neurons in the brain – based on the time evolution of various mathematical algorithms – led to his recent discovery of a new principle of olfaction, the way in which the brain processes smells. The well-known Hopfield model of neural processing highlighted the differences between computation in a computer and in the brain, and his early research into light-emitting diodes earned him the Buckley Prize for condensed matter physics in 1969.

The Dirac medal is awarded every year on Paul Dirac’s birthday – 8 August – and is accompanied by a cash prize of $US 5000.

Counting photons in a flash

Infrared photons are invisible to photon counters based on semiconductors because they have less energy than the energy gap: photons must supply at least this much energy to change the electrical characteristics of the semiconductor. Sobolewski and colleagues realised that superconductors could be more sensitive to lower-energy photons because their energy gaps are thousands of times smaller.

Conventional superconductors lose their electrical resistance when electrons pair up and flow through the superconductor crystal, aided by lattice vibrations known as phonons. The energy gap in such materials is the energy needed to split the electron pairs.

Sobolewski’s team deposited a strip of niobium nitrite just a few atoms thick onto a sapphire substrate. The strip was 0.2 micrometres thick and 1 micrometre long, and became a superconductor when it was cooled to 4.2 kelvin. When a photon with more energy than the superconducting energy gap is absorbed by this strip, it breaks apart the electron pairs and creates a local pool of energetic electrons – or a ‘hotspot’ – in which superconductivity breaks down. Since the hotspot is as wide as the strip, it blocks superconductivity and a voltage is registered.

After about 30 picoseconds – that is, 30 x 10^-12 seconds – the energetic electrons in the hotspot spread out and lose their energy through collisions with phonons. This restores superconductivity and the device is ready to detect the next photon.

This gigahertz repetition rate enables the device to detect extremely brief bursts of photons, such as the fleeting pulse of infrared light emitted by a transistor when it switches. This light reveals whether the transistor is switching at the correct time. According to Sobolewski and colleagues, their device could identify a single misbehaving component among the billions in a modern computer.

Operating at cryogenic temperatures, the new device can detect single photons because it is less prone to thermal ‘noise’. This could make the detector useful in communications between Earth and Mars, and has attracted the interest of NASA. “When you’re dealing with such incredible distances, you may only be able to catch a few photons from a transmitter on Mars”, explains Sobolewski.

Spotting craziness in cloud-cuckoo-land

Few things in life are utterly black or white. Even scientific ideas come in shades of “truth”, which is arrived at differently in different kinds of science. There are other gradations too, starting with theories that we are pretty damn sure are right and that we accept without question, such as Newton’s three laws of motion. Our attitude to these differs subtly from those that we also think are correct but might wish were not, if only to make a richer universe, such as the relativistic limitation on faster-than-light travel. At the opposite extreme are ideas that seem wrong to most scientists but are espoused by a minority (sometimes a minority of one) and may be accepted by non-scientists too, like the belief that AIDS is not caused by the HIV virus.

All of these issues enter into Robert Ehrlich’s Nine Crazy Ideas in Science. The title could imply that the book provides no more than a series of chuckles at laughably inane theories, but Ehrlich is serious. In critically examining more-or-less marginal ideas drawn from several fields of science, he contemplates the meaning of scientific truth, how emotions and personal agendas influence scientists, and how unproven ideas can do harm. He also notes that some valid theories, such as quantum physics, sound plenty weird, and that we need to learn how to recognize those seemingly crazed ideas that just happen to be true breakthroughs.

Ehrlich already does some filtering, for he makes “crazy” a technical term. According to him, a “crazy” idea violates no known fundamental laws of nature, whereas a “nutty” one blithely disregards those laws. So you’ll find no perpetual motion machines here. Instead, you’ll read about the idea that there was no big bang, that our solar system has two suns, and that the Earth’s coal and oil did not arise from biological sources. You’ll also weigh up the possibilities that time travel can be realized, and that faster-than-light particles exist (Ehrlich’s own field of research). In addition to the contrary opinion about AIDS, you’ll consider the premises that low doses of nuclear radiation are good for people, that exposure to the Sun is beneficial and that more guns lead to fewer crimes.

For each notion, Ehrlich gives the accepted and minority views, or the differing views where there is no overwhelming consensus. He then presents the background of the relevant scientists, before giving point-by-point comparisons showing what evidence supports which view, and how strongly. Ehrlich writes clearly and simply, backed up by figures and ample references. Except for a few difficult moments as he gets into details, readers who are not experts in the areas will still find themselves grasping the essential issues.

It’s instructive to see the “tools of truth” that Ehrlich uses. Research on sunlight, nuclear radiation and the human effects of guns necessarily relies on statistical correlations, rather than direct cause-and-effect relationships. Ehrlich’s approach to these statistics reminds us of how important it is to go back to original sources. He shows how data change remarkably “in translation”; that is as interpreted and displayed by succeeding layers of analysts. Of course, the original investigator may also consciously or unconsciously put spin on the data, which is one reason that experiments are supposed to be capable of replication.

Ehrlich’s crazy ideas in the physical sciences tend to arise differently. Here they start with a theoretical basis that leads to predictions that may be experimentally verifiable. For instance, Einstein’s equations show that supposed faster-than-light particles (dubbed “tachyons”) would have a mass m that would be an imaginary number, so that m² would be negative. You might think this pretty definitively eliminates all known elementary particles, but here too statistics steps in. According to Ehrlich, the statistical uncertainty in measurements of the supposed zero mass of neutrinos allows the possibility that m² is actually negative – and so keeps the idea alive.

Data analysis also drives the notion that our Sun has a binary companion star so distant that astronomers have yet to spot it. The companion periodically affects the Earth: each time it orbits through the Oort cloud – the swarm of comets surrounding the solar system – it shakes loose comets. These rain down meteorites that extinguish earthly life, as in the demise of the dinosaurs 65 million years ago. Analysis of the geological record suggests that similar extinctions occur every 26 million years, supposedly correlated with the motion of our second sun. But, as Ehrlich notes, finding periodicity in a set of imperfect data is a notoriously tricky endeavour that is hard to view with total confidence.

Lord Rutherford once said that “if your experiment needs statistics, you ought to have done a better experiment”. But often in matters of health and social policy, we can’t do the better experiment, and statistics are all that we have. As science journalists know to their despair, however, it is very hard to convey to non-experts the line between unthinking acceptance of all “scientific” studies and understanding that even valid results carry uncertainty. Ehrlich’s book can help practising scientists, students and ordinary citizens to think critically about scientific evidence that may affect their lives.

Are there any gems among the nine ideas? Ehrlich rates the believability of each on a scale from zero cuckoos – meaning it has sufficient merit to make us say “why not?” – to four cuckoos, meaning “certainly false”. I won’t reveal his ratings except for his evaluation of the potentially harmful belief that AIDS is not caused by the HIV virus, which could lead people to abandon effective treatment and has influenced the politics of AIDS in Africa. This notion receives short shrift. Ehrlich gives it three cuckoos: “Almost certainly not true.”

I’m delighted to report, though, that the average cuckoo rating for the crazy ideas lies between “probably not true, but who knows” and “very likely not true”. This leaves leeway for science to continue as a “work in progress”, as Ehrlich calls it, and for some lovely future surprises. Myself, I’m holding out for evidence that gives the myth of Atlantis a rating of 3.9 – anything but that deadly, definitive four cuckoos.

Helping ‘big G’ get back on track

Ever since Henry Cavendish first measured Newton’s gravitational constant in 1798, the value of “big G” has remained the least accurate of all the fundamental physical constants. Indeed, when the most recent list of recommended values for the fundamental constants was published in 1998, the uncertainty in G had increased by a factor of 10 from the previous list.

Last year, however, two US physicists reported the most accurate measurement ever, and now a team of researchers from the Bureau International des Poids et Mesures (BIPM) in France and Birmingham University in the UK has reported the results of another experimental tour de force. Moreover, they claim to have discovered the reasons for the errant measurement that led to the factor of 10 increase in the uncertainty surrounding the constant.

Big G is difficult to measure because gravity is by far the weakest of the four fundamental forces and because it is impossible to shield experiments from the gravitational influences of their environment. Today the official value is G = 6.673 x 10^-11 in units of metres cubed per kilogram per second squared, with a relative uncertainty of 1500 parts per million (ppm). By contrast, the mass of the electron is known with an accuracy of 0.08 ppm.

In 1994, when Winfried Michaelis and co-workers at the German standards lab, the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, began an experiment to measure G, the accepted value was 6.672 59 x 10^-11 with an uncertainty of 128 ppm. The PTB result shocked the big G community because it was more than 5000 ppm larger than the accepted value at the time, placing it well outside the experimental error bars (Metrologia 1995/6 32 267). Metrologists have been puzzling over the PTB results ever since.

Back to basics

In a traditional Cavendish “torsion balance” experiment, test masses are suspended on a wire that rotates in response to a gravitational torque created by source masses placed nearby. This rotation is opposed by the torsion of the wire. By measuring the amount of the twist in the wire, which is proportional to the gravitational attraction between the masses, the value of G can then be determined.

Last year Jens Gundlach and Stephen Merkowitz of the University of Washington in Seattle used a highly modified version of Cavendish’s torsion balance in which both the source and test masses rotated on turntables. The key feature of their set-up was that the torsion fibre did not twist, which, according to Gundlach, enabled them “to avoid most of the systematic uncertainties”. Last year Gundlach and Merkowitz reported a value of 6.674 215 x 10^-11 with an uncertainty of just 14 ppm, surpassing all previous measurements in accuracy and setting a whole new standard for measuring G (Phys. Rev. Lett. 2000 85 2869).

The BIPM-Birmingham experiment also uses a modified Cavendish set-up, which rests on a marble block that is bonded to the local bedrock in a temperature-controlled laboratory. The four 1.2 kg test masses are mounted on a disk, which is suspended from a copper-beryllium ribbon in a vacuum chamber. There are four 12 kg source masses outside the chamber, resting on a carousel that is driven by a stepping motor. When the source masses are radially aligned with the test masses, there is no gravitational torque on the balance. When the source masses are rotated by 18.7° in either direction, the torque is at a maximum.

“A unique feature of our experiment,” says Terry Quinn of the BIPM, “is that we can carry out both a Cavendish experiment and a servo-controlled experiment with the same apparatus.” Using the servo-controlled method, the gravitational torque generated by the source masses is balanced by an electrostatic torque that acts directly on the test masses. This electrostatic torque is generated by applying an AC voltage between the test masses and a pair of thin copper electrodes near each mass. G can be calculated by measuring the electrostatic torque needed to balance the gravitational torque due to the source masses.

Following some 38 Cavendish experiments lasting four hours each and 25 servo experiments, all 10 hours long, the BIPM team arrived at a value of G = 6.675 59 x 10^-11, with an uncertainty of 41 ppm. “The close agreement of the results from the two methods is evidence for the absence of many of the systematic errors to which a G measurement is subject,” says Quinn. “Our result is close to, but not exactly equal to, the value of Gundlach and Merkowitz.”

Indeed, the BIPM and Seattle results differ by more than four times their combined uncertainty. “The difference is likely to be due to systematic errors, at the level of one or two parts in 10⁴, hidden in one or both of the measurements,” the BIPM team writes in a paper accepted by Physical Review Letters.

PTB or not PTB

The BIPM team also believes it knows the reason for the PTB discrepancy. “It is very subtle,” says Quinn, explaining that the problem is related to the frequency dependencies of various capacitances in the experiment and the use of AC and DC servo-control systems. Put simply, Quinn claims that the PTB team calibrated its apparatus at one frequency and made its measurements at another. The BIPM team encountered similar problems in the early days of its experiments when it was using a DC servo system. “The problems were eliminated when we started using an AC servo working at the same frequency,” says Quinn.

However, Winfried Michaelis has visited the BIPM experiment and does not believe that Quinn’s theory can explain the errant PTB measurement. “The BIPM experiment has a relatively open arrangement of electrostatic components with insulating materials in it,” he says, “whereas we had a nearly closed electrometer box without any insulators.” Nonetheless, the PTB team will soon be performing a new experiment together with specialists in the measurement of capacitance.

The big G community now awaits the results of other experiments. In July a task group from CODATA, the international body that controls the values of the fundamental constants, decided not to change either the value of G or the uncertainty given in the 1998 recommended values.

“What is most likely to happen when other results come out,” says Quinn, “is that the value of G may be changed towards where we are now, and the uncertainty is going to be reduced, probably by a factor of 10.”

Can phonons help explain high-temperature superconductivity?

Superconductivity – the absence of electrical resistance in a material – is observed when the material is cooled below the superconducting transition temperature. In 1957 John Bardeen, Leon Cooper and Robert Schrieffer explained that interactions between electrons and phonons allowed the electrons to overcome their mutual electrostatic repulsion and bind together to form Cooper pairs, which lead to superconductivity.

By the mid-1980s the highest known transition temperature was 23 Kelvin. In 1986, however, materials that became superconducting at much higher temperatures were discovered, and the record transition temperature is now around 130 Kelvin. Most of these materials contain copper oxide layers and are known as cuprates. However, the BCS theory could not explain their behaviour, and ever since both theorists and experimentalists have been looking for an alternative mechanism to explain the formation of Cooper pairs in the cuprates.

Zhi-xun Shen of Stanford University in California, and co-workers at Stanford, the Lawrence Berkeley National Laboratory, also in California, and the University of Tokyo have used angle-resolved photoemission spectroscopy (ARPES) to study the electron dynamics in three different families of cuprate superconductors. Shen and co-workers used the ARPES technique – in which synchrotron radiation causes electrons to be ejected from a sample – to measure the electron velocity and scattering rate as a function of energy.

In all three families of material they observed kinks in the electron distributions which, they claim, cannot be explained by any known process other than interactions between electrons and phonons. “This suggests,” they write, “that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity.”

In an accompanying article Philip Allen of the State University of New York at Stony Brook writes that this “interpretation will be controversial.”

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