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Biological lessons for physicists

Interdisciplinary research is currently in fashion, and no interface is more in vogue than that between physics and biology. But whereas the subject matter of, say, chemical physics or materials physics is relatively well defined, there is no such agreement on what constitutes biological physics, and no broad research programme to follow.

Even the name is in doubt. “Biophysics” is one option, but that name is already in use for something slightly different – a thriving subdiscipline of biochemistry, with professional societies and journals already in place. And biophysics in this sense is really about biologists using physics, rather than physicists solving problems in biology.

To convey a sense of the latter, one sometimes sees the cumbersome phrase “biologically inspired physics”. This suggests how we should carry out such activities, namely by looking at biology and searching for inspiration from it. But although such an approach puts physics back in the driving seat, it hardly defines a coherent research programme.

The claim made by Konstantin Bogdanov in Biology in Physics is that the book “covers all the important topics in modern biophysics”. However, I think that this is very far from being true. One should instead see the book as a series of anecdotes, each telling the story of one specific application of physics to biology. The approach is unsystematic, but the book is nonetheless appealing and full of interest. On one page there is a description of how bats hunt using echolocation, while a few pages later we follow an order-of-magnitude calculation explaining how a karate expert can break a concrete block with a bare hand.

Sometimes the information veers towards the sort of interesting fact useful only for particularly recondite pub quizzes, as we learn, for example, how the Romans recommended the use of electricity-producing fishes for the treatment of gout. Nevertheless, despite the breezy and attractive writing style, and the relatively simple science, there are fairly comprehensive recent references so one could follow up a particularly interesting subject in more depth.

The bias of the author is clearly towards biomechanics and physiology, rather than molecular biophysics. In terms of the style and coverage, I am reminded of the books of the zoologist Steven Vogel, and particularly Life’s Devices (Princeton University Press, 1989), which covers quite a lot of the same ground at a similar level.

What is different about Biology in Physics is its extensive coverage of the physics underlying the way the senses work. There is up-to-date material on the physics of hearing, and a readable chapter describing the physics of vision, which touches on the differences and similarities between the eyes of vertebrates, insects and molluscs. Towards the end of the book is a short but fascinating chapter on how magnetism helps animals to sense direction.

The kind of physics that the author relies on is largely classical mechanics and fluid dynamics – rather than statistical mechanics and quantum mechanics. There is also little mention of molecules. This leads to the complete absence of many important biological problems currently being studied by physicists. There is, for example, no mention of the folding of proteins, the coiling of DNA or the fluctuations of membranes.

Also missing from the discussion is the whole, though rather vaguely defined, area of the application of systems theory and non-linear science to biology. For example, ideas of complexity and self-organization are finding tentative applications in areas of biology as diverse as the evolution of species, sensory-information processing and morphogenesis. Of course, these ideas are still imperfectly formed – even half-baked. And while they are certainly controversial, their absence from the book makes one feel that the author has missed the chance to take on some of the really big ideas in biology.

Put another way, we should note that no biologist would have a problem with this book. There is certainly a virtue in rehearsing those areas of biology in which physics can make an unquestionable and uncontroversial contribution. But the creative tension that could produce big advances at the interface of physics and biology will only come from those areas where the philosophical gulf between physicists and biologists is deepest. These are the areas where biologists question whether physicists have anything to offer the subject at all, and where physicists wonder whether the biologists are completely missing the point. These debates will, however, have to wait for another author.

I certainly enjoyed reading this book, although I am not completely sure at whom it is aimed. Those who teach undergraduate physics will find some rather interesting applications of subjects like mechanics and optics that demonstrate the widespread applicability of physics. The book should certainly encourage physicists to believe that there is a place for their way of thinking in biology, and that in itself is a worthy achievement – even if some bigger themes are passed by.

Could Maxwell have deciphered quantum mechanics?

There is no doubt that quantum theory has been remarkably successful and has passed every experimental test it has been subject to. But the interpretation of quantum theory – in particular the meaning of the wavefunction, the role of observers, and the question of what happens to the state of a system during a measurement – have remained a topic of debate and discussion among theoretical physicists and philosophers of science.

Gottfried writes that prior to quantum mechanics, physics was a cumulative pursuit in which new concepts such as thermodynamics and electrodynamics could be related to pre-existing notions such as space and time. Even the special and general theories of relativity can be understood in these terms, albeit with major conceptual innovations. Quantum mechanics, however, is different. “In retrospect,” writes Gottfried, “the successful developments of physics followed a clear conceptual path beginning in the Principia, but in 1925 [when Heisenberg discovered “matrix” mechanics] this path entered a no-man’s-land from which it has not yet emerged.”

The problem is that although the Schrödinger equation describes the time evolution of the wavefunction, the actual meaning of the wavefunction must be added as an extra axiom to quantum theory. In the orthodox statistical interpretation of quantum mechanics, the wavefunction contains the maximal knowledge that is available about the state of a system, and this wavefunction determines the probabilities that various results will be obtained when measurements are made on the dynamical variables of the system. Finally, it is not possible to assign values to these variables before the measurement is made.

Gottfried asks and then answers a rhetorical question: “Could Maxwell have figured out what the wavefunction means had he been handed Schrödinger’s equation? It would seem that Maxwell would have needed help from the wonder rabbis of Copenhagen and Göttingen.” Gottfried contrasts quantum theory with general relativity: in the latter “you do not need him [Einstein] whispering in your ear.”

Gottfried then imagines that he is Maxwell and armed only with the Schrödinger equation, the knowledge that it correctly describes all phenomena at the atomic scale (and in non-relativistic cases), and the value of the Planck constant, he tries to derive the familiar statistical interpretation of quantum mechanics. He makes some progress and discovers that, in the classical limit, the Schrödinger equation does not describe a single system “but a population of replicas of such a system moving along a set of trajectories.” Later Maxwell is told about the results of the Stern-Gerlach experiment – that is, that the magnetic moment or “spin” of an atom can only have discrete values – from which he derives something very close to the uncertainty principle.

In the end Maxwell is able to derive the statistical interpretation of quantum mechanics for discrete degrees of freedom, such as spin, but not for continuous degrees of freedom, such as position or momentum. “[However, the] portions of the quantum mechanical formalism that are being used in the arguments come in through the front door, and not so surreptitiously that even the author is confused about what is assumed and what is derived,” he writes, citing his own book on quantum mechanics.

Gottfried also claims that decoherence – put simply, the process by which a quantum system, which can be in two or more states at the same time, produces a classical probability distribution – occurs naturally and that “there is no need for an external environment not included in the Schrödinger equation, nor a pyramid of devices which make laboratory demonstration of coherence effective impossible.”

Gottfried wrote the article as a “belated” response to an article entitled “Against ‘measurement'” by the late John Bell that was published in Physics World in 1990. “I suspect that the extension of the argument to degrees of freedom with a continuous spectrum is not difficult,” Gottfried told PhysicsWeb, “but I have not figured out how to do that.”

New insights into surfaces

Berndt and co-workers prepared gold, silver and copper surfaces with standard techniques and then used a scanning tunnelling microscope with a specially prepared tungsten tip to identify regions on the surface that were free from defects. The advantage of this technique over others, such as photoelectron spectroscopy, is that the same instrument that is used to check the surface can also measure the lifetime of the “holes”. (These holes are left behind on metal surfaces when electrons are excited to higher energies.) The lifetime of the holes can be determined by carefully measuring how the current through the tip changes as the voltage difference between the tip and the surface is varied.

However, the theory also had to be modified to resolve the discrepancy with experiment – which ranged from a factor of two in copper, four in gold and seven in silver. Berndt and co-workers extended previous treatments to calculate in detail how the hole lifetime is determined by electron-electron interactions and electron-phonon coupling. (Phonons are quantized vibrations of the metal lattice). They find that the lifetime is essentially determined by the two-dimensional electron gas near the surface, but that the interactions between these electrons are modified by the three-dimensional electron gas of the bulk crystal.

Not all clusters are the same

Smaller clusters tend to have elongated shapes, and the energy of formation is essentially independent of the number of atoms in the cluster. Larger clusters are spherical and the formation energy is inversely proportional to the cube root of the number of atoms. Such detailed knowledge of the energetics of cluster formation is needed to understand how the condensed phase of a material can form from the vapour phase.

Guntherodt and co-workers used a molecular beam apparatus in their experiment, and a “shutter” technique allowed the average size of the clusters in the beam to be varied between 95 and 975 atoms. The team then used a combination of micromechanical and pyroelectric calorimeters to measure the heat released when the clusters were deposited on a polymer foil. The heat released can be related to the formation energy of the cluster.

The Swiss team conclude that “the existence of elongated clusters with significantly reduced formation energies compared to the compact spherical clusters can strongly influence the kinetics of the formation of the condensed phases.”

Phenomenology success for Durham

The institute has been established for 10 years in the first instance, with the Particle Physics and Astronomy Research Council (PPARC) investing £7m and the University of Durham investing £5m. Durham already has a strong track record in phenomenology lead by James Stirling, who is director-designate of the new institute. To begin with the institute will have four new permanent positions and seven postdoctoral positions.

“This is a major opportunity to strengthen and expand UK phenomenology,” said PPARC chief executive, Ian Halliday. “I believe this is an opportunity for the University and PPARC to have a major international impact; relations between the Institute and the experimental community will be all important.”

The many shapes of lead-186

Mark Huyse from the University of Leuven in Belgium and co-workers in Germany, Slovakia, the UK, Finland, Russia, Sweden and Belgium studied lead-186, which contains 82 protons and 104 neutrons. 82 is a “magic number” in nuclear physics, which confers special properties on nuclei with this number of neutrons or protons. Moreover, 104 is mid-way between 82 and the next highest magic number, 126, which should make this isotope of lead particularly special. Theory predicts that the first excited state in lead-186 will have an oblate shape in which two protons are promoted to higher energy levels within the nucleus, leaving two “holes” behind. The next highest state should be a prolate shape in which four protons have been promoted.

To create the lead-186 nuclei, Huyse and co-workers collided chromium-52 ions with a target containing neodymium-142. This created polonium-190 nuclei which then decayed by alpha-particle emission to create lead-186 nuclei. The Separator for Heavy Ion Production (SHIP) at the GSI laboratory in Darmstadt, Germany, was used to separate the 300 polonium nuclei produced every hour from the much higher background. By measuring the energies of the alpha particles – along with the energies of the electrons and X-ray photons emitted at the same time – the team was able to confirm the three different shapes of the lead nucleus.

Experiments like this should allow nuclear physicists to refine their models of the nucleus. One of the biggest challenges in theoretical nuclear physics is that heavy nuclei contain too many nucleons (over 100) for microscopic approaches to work, but too few for statistical approaches to be valid.

PPARC starts to identify its priorities

The particle physics panel identified three projects related to the Large Hadron Collider – a 14 TeV proton-proton collider that is due to start in 2005 at CERN – as top priority. These were a computational grid to handle the data from the LHC, an increase in human resources for experiments, and increased support for theory. Accelerator R&D for a proposed neutrino factory was the other top priority. The second group of priorities included accelerator R&D for a future linear collider and further R&D for new technologies and detectors.

In the field of particle astrophysics, a series of missions and experiments to study the cosmic microwave background, gravitational waves, dark matter, cosmic and gamma-rays, neutrinos and black holes have been identified as top priority.

In astronomy the top priorities included the Atacama Large Millimetre Array, participation in the European Southern Observatory (of which the UK is not a member), involvement in the Next-Generation Space Telescope, further development of the SCUBA detector, R&D for new technologies and facilities, and increased human resources. Ian Halliday is due to fly to Munich next week to start official discussions about the UK joining ESO. He said he felt there was “serious chance of success”.

Priorities in solar system science fell under two headings: understanding the origins and evolution of the solar system, and understanding the influence of the space environment on Earth.

US astronomers look to the future

The other priorities established by the panel are the ground-based 30-metre Giant Segmented Mirror Telescope, the space-based Constellation-X Observatory, expansion of the Very Large Array radio telescope in New Mexico, a ground-based 6.5 metre survey telescope, and the Terrestrial Planet Finder. The panel describe this last mission as “the most ambitious space mission ever attempted by NASA.”

Other priorities include completion of existing telescopes and missions, a large telescope to detect gamma-rays, an instrument to measure gravitational waves, and a powerful telescope to study the Sun. Among the smaller projects given priority is a “virtual observatory” that would make large sets of astronomical data available to scientists and the public over the Internet.

Hendrik Casimir and John Ward

Hendrik Brugt Gerhard Casimir was born in the Hague in the Netherlands in 1909 and received his PhD from the University of Leiden in 1931. After working with Bohr in Copenhagen and Pauli in Zurich, he joined Philips Research Laboratories in Eindhoven, where he spent the rest of his career. Casimir is best known for his prediction of the Casimir effect in 1948. He predicted that two parallel conducting plates in a vacuum would experience an attractive force due to their influence on the electromagnetic vacuum. The force was detected in experiments 10 years later. Casimir received many honours in his life including an honorary fellowship of the Institute of Physics, fellowship of the Royal Society, and the George E Pake Prize of the American Physical Society. He was also president of the European Physical Society.

John Clive Ward was born in 1924 and educated at Oxford University. After spells at Princeton and Johns Hopkins University in the US, in 1967 he joined Macquarie University in Sydney, Australia, where he spent most of his career. Ward is best known for the “Ward identities” which greatly simplify calculations in quantum field theory and which played an important role in the development of the standard model of particle physics. Ward was elected to the Royal Society in 1965 and received its Hughes Medal in 1983.

Single molecules demonstrate resolving power

However, the resolution possible with scanning near-field optical microscopy (NSOM), as this approach is called, is essentially limited by the size of the aperture. To improve the resolution it is necessary to use ever smaller apertures, or to replace the aperture with an extremely small light source. A team of physicists at the University of Konstanz in Germany has now taken this latter approach to the extreme and used the light from a single molecule instead of an aperture (J Michaelis et al. 2000 Nature 405 325). The technique was used to obtain images of a hexagonal lattice of 25 nanometre high triangles on an aluminium surface.

Vahid Sandoghdar and colleagues started by growing micron-sized crystals of p-terphenyl that contained about one molecule of terrylyne for every ten million molecules of the host. A suitable microcrystal was then glued to the end of an optical fibre. The low doping concentration of the crystal and operation at 1.4 kelvin meant that a single terrylene molecule could be selectively excited by laser radiation. The photons emitted from the molecule then passed through the sample and various optics before being detected by an avalanche photodiode.

By moving the sample relative to the fibre, it was possible to build up an image. Moreover, the quality of the image improved as the sample-probe separation was reduced from 350 nm to 20 nm. Although the images are not yet as good as those obtained with the best optical near-field microscopes, the Konstanz team hope to improve the resolution of their technique by using smaller microcrystals and different ways to excite the molecules near the end of the crystals.

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