The results help to confirm the view that other planetary systems form in ways similar to our solar system. The Kuiper belt is a disk of dust, gas and icy comets stretching from 40 to 10000 times the Earth-Sun distance. Recently astronomers observed protoplanetary disks encircling young stars up to 1 million years old and planetary disks surrounding more mature stars around 8 to 20 million years old. The AB Aurigae observations – taken by the Space Telescope Imaging Spectrograph onboard Hubble – are between these two points.
“We don’t see any evidence of unseen large bodies sweeping out dust lanes, ” says Grady. “But we have seen unprecedented structure in the dust clouds, suggesting that material is beginning to clump together in a process which could form planets in the next few million years.”
French cafés have long been a meeting point for intellectuals. In the 18th century Victor Hugo met fellow revolutionaries at the Procope in Paris, and in the 1950s groups of existentialists, including Jean-Paul Sartre and Albert Camus, gathered in the Parisian cafés Flore and Les Deux Magots. Today, the French are discovering what physicists have always known: le café is also an ideal place to discuss science.
The new cafés scientifiques attract scientists and non-scientists alike to talk about the latest research in a convivial atmosphere. The movement is supported by the French Physical Society (SFP), who held the first bar des sciences at their annual congress in 1997. “It is nothing like a conference, ” insists Pablo Jensen, physicist at the University of Lyon and organizer of the local café. Most cafés consist of discussions on a theme, with some invited contributions and much informal comment from table to table.
Yves Sacquin, a physicist at Saclay and also president of the SFP particle-physics group, says the ambience interests people who are not in contact with science on a daily basis. Sacquin participated in a café scientifique at this year’s international Paris book fair and said he was impressed by the range of questions he received. Indeed, both scientists and non-scientists learn from the experience. “It is important for both, ” says Gianni Giardino, café organizer and physicist at the Paris University Jussieu. “Scientists learn about the worries of society and the perception of risk, and non-scientists come both to be informed and to dream. Fundamental science can be a bit like magic to them.”
Giardino says that physicists can have trouble replying to questions posed in the cafés. “They are very, very careful about what they say because they have a colleague sitting just next to them!” But Jensen says that non-scientists learn a lot about the reality of research: “People see that scientists are not devils, but that they are not gods either.” Giardino agrees: “When a scientist does not know the answer, that pleases the public.”
Cafés scientifiques are spreading. There are now 12 in France, one in Geneva, and two in the UK at Leeds and Nottingham. Indeed, the Leeds group has followed the French in more ways than one: they meet each month at the café In Vino Veritas.
The collisions are termed “asymmetric” because the electrons are accelerated to 9 GeV – three times the energy of the positrons. As the two beams collide, they generate B mesons that decay within a trillionth of a second. The newly formed particles fly into the detector at different velocities, which makes it easier to separate the antimatter B mesons from the matter B mesons. In theory, the anti-B mesons should decay much faster than the normal B mesons.
Over 650 scientists and engineers from 73 institutions in the US, Canada, China, France, Germany, UK, Italy, Norway and Russia built the BaBar detector. It joins the KLOE experiment at the DAFNE accelerator in Italy as the second fully operational B factory. A third factory, at the KEK particle physics laboratory in Japan, will come online over the summer. All three teams expect to announce their first results next year.
1 Fluorescence-lifetime imaging A biological sample is illuminated by a series of pulses from an ultrafast laser causing molecules in the sample to fluoresce. The exponential fluorescence decay, l = l0exp(t/τ), is measured from a series of images taken by a gated image intensifier at various time intervals (t1, t2, t3…). Each exposure lasts 90 ps. For every pixel in the image, the decay constant, τ, is calculated and the results are plotted on a fluorescence-lifetime map.
The invention of the silicon microchip and the resulting boom in information technology has shaped the 20th century to a far greater degree than other physics-based developments such as spacecraft and nuclear power stations. In the same way, new techniques aimed at improving the way that biological and medical information is collected and analysed will probably have a far greater impact on medicine in the 21st century than big breakthroughs such as organ transplantation or bionic implants.
Many important discoveries in physics have been rapidly exploited by the medical community for diagnosing and treating a variety of illnesses. Among some of the best known examples are ultrasound, X-rays and magnetic resonance imaging.
Recent research efforts have focused on making medical diagnosis cheaper, faster and more effective by developing new physics-based imaging techniques and by using powerful computers to process and analyse biomedical information. New technological developments are being increasingly driven by the needs of the medical profession, and optics is playing an increasingly important role.
Medical diagnostics
According to some in the medical profession, the “Holy Grail” of medical data acquisition is akin to the Star Trek tricorder: a portable device that delivers a complete diagnosis when simply pointed at a patient. This is some way off and today’s physicians have to collate information obtained using a variety of different techniques, ranging from X-ray and ultrasound imaging to laboratory analysis of samples of tissue removed from patients. Conventional imaging techniques can be used to establish physical damage or abnormality in the structure of various types of tissue, but unfortunately they can not show biological processes within the body or detect the onset of many diseases.
Detecting physiological changes, such as the oxygenation of blood, often relies on distinguishing the different chemical behaviour of the organs or tissue under study.
Optical spectroscopy is a standard tool in laboratory analysis that is used to detect and monitor the concentration of substances, such as oxygen, by measuring the absorption, fluorescence or Raman spectra. But performing spectroscopy in biological tissue is very different from making measurements on a solution in a test tube.
A tissue sample is often made up of many different components – it is said to be heterogeneous. If one simply irradiates a sample with a light beam and observes the optical “signature”, there will typically be many contributions from the different types of tissue that have interacted with the beam and will confuse the picture. Furthermore, quantitative optical measurements in biological tissue are extremely difficult because tissue scatters light strongly, typically within 100 µm. These considerations have prevented the widespread application of optics in clinical medicine.
A lot of recent research work has instead focused on magnetic resonance imaging (MRI), a technique that detects the signals generated by the nuclear spins of protons present in water and fat, for instance. However, MRI has limitations. It requires a very high magnetic field, which makes the equipment very expensive. There are also many molecules that it cannot detect with sufficiently useful resolution, and imaging at the cellular level remains a formidable challenge.
If the problems associated with scattering can be overcome, optical imaging could provide a cheaper technology that would allow patients to be screened for diseases such as skin and breast cancer, osteoarthritis and diabetes, as well as revealing much more about the working processes of organs, including the brain.
Fluorescence imaging
One of the most widespread biomedical research tools is the optical microscope, which allows researchers to study biological processes at the cellular level. In recent years microscopy has advanced greatly in terms of resolution and the ability to highlight specific working processes in a living organism. In a spectroscopic technique known as fluorescence microscopy, a sample absorbs incident photons and emits light at different wavelengths depending on the molecules present in the sample. A colour image of the fluorescence radiation therefore corresponds to a map of the distribution of different fluorescence molecules, known as fluorophores, that are present.
Although strong scattering and the heterogeneous nature of samples make fluorescence imaging in tissue difficult, it can be achieved using a technique known as “wavelength-ratiometric imaging” in which the tissue sample typically is doped with fluorophores that have spectra that change in a predictable way. By comparing fluorescence-intensity images at two or more wavelengths, it is often possible to produce a map of the distribution of particular fluorophores, even though there may be a strong background from other naturally fluorescent molecules in the tissue.
The technique can be adapted to image the local chemical or physical environment in a tissue sample, such as the pH or calcium-ion concentration. The sample is stained instead with a fluorescent dye that has an emission spectrum that changes in a predictable way according to the strength of the local environmental perturbation. This method of imaging is used in many areas of science, especially biology, although it is restricted by the availability of dyes that are suitable for a particular application.
2 Evaluating the environment Fluorescent-lifetime imaging (FLIM) can be used to show changes in the physical properties of the medium containing the fluorescing material. The fluorescent lifetime τ = 1/Τ+k depends on both the rate at which the fluorophore decays via light emission, Τ, and its decay rate via other processes that do not emit light, k. For some fluorophores, k is sensitive to the physical and chemical properties of the fluorophore environment. The four pipettes contain solutions of the same fluorophore dissolved in a mixture of ethanol and glycerol. The solutions are identical except that the viscosity has been adjusted by varying the relative amounts of the two solvents. The FLIM map shows that the fluorescent lifetime decreases from left to right, corresponding to the decrease in viscosity from left to right across the samples. FLIM could be equally well applied to image calcium-ion concentration or distributions of other biologically significant molecules.
A complementary approach to fluorescence imaging exploits the lifetime, t, rather than the wavelength dependence of the emission. Typically the fluorescence signal will decay as I = I0exp(-t/t), where t is a characteristic property of the fluorescent molecule. Fluorescence-lifetime imaging (FLIM) can be used to distinguish between different fluorophores in a field of view, or to image changes in the local fluorophore environment that modify t. It is able to make use of a wide range of fluorescent dyes and can often be applied to naturally occurring fluorophores in biological samples.
However, useful changes in the fluorescence lifetimes of biological molecules can be as short as a few picoseconds and many fluorescent dyes have lifetimes in the nanosecond range or shorter. Until very recently most biomedical researchers did not have access to the technology that is required to image on these timescales . Advances in ultrafast lasers and in high-speed imaging detectors, such as CCD cameras and multichannel-plate optical intensifiers, mean that medical researchers are beginning to use FLIM to image living organisms.
Our group at Imperial College in London has developed a system that can image fluorescence-lifetime differences of less than 10 ps. It works by periodically exciting the sample with a series of 10 ps optical pulses from an ultrafast laser and essentially “photographing” the fluorescence-intensity image using a gated optical intensifier and CCD camera with an effective “exposure time” of 90 ps (figure 1). A sequence of fluorescence images is recorded at various time delays after each excitation. So for each pixel in the field of view, the fluorescence decay constant, t, corresponding to that particular part of the sample can then be calculated. The results are plotted in a fluorescence-lifetime image that can serve as a map of the distribution of different fluorophores or as a map of the changes in the local environment (see figures 2 and 3).
This technique also makes it straightforward to analyse samples containing a mixture of fluorophores that have different exponential-decay profiles. In DNA sequencing, for instance, the different bases may be “labelled” with two different fluorophore molecules. By fitting the observed fluorescence-decay profiles to: I = I1exp(-t /t1) + I2exp(-t /t2), one can obtain I1/I2, which is proportional to the concentration ratio.
Optical imaging in 3-D
In 1873 Ernest Abbe recognized that conventional far-field microscopes have a limited resolution in both the transverse (xy) and vertical (z) planes. The resolution in the xy plane varies directly with the wavelength of light, l, and inversely with the numerical aperture, which is a measure of the amount of light gathered by the microscope lens. In addition, conventional microscopes provide a degree of optical “sectioning” in the vertical direction via the depth of focus. The practical resolution limit should be around 0.2 µm for the transverse direction and around 0.7 µm in depth, assuming the maximum practical numerical aperture is 1.4 and the wavelength of the light is 500 nm. Unfortunately, this precision cannot be achieved in practice since the conventional microscope also collects light from outside the depth of focus and this blurs the depth-resolved image, in particular.
In 1955 Marvin Minsky of Harvard University in the US invented the scanning confocal microscope, which overcomes this problem by using a point source of light to interrogate the object pixel-by-pixel and a confocal pinhole at the detector to ensure that only light from a particular pixel is collected. A complete 2-D image is obtained by scanning the illumination across all the pixels. And because out-of-focus light is rejected by the detector pinhole, it is possible to view the object at various depths and build up a high-resolution 3-D image.
3 New view on tissue Fluorescent-lifetime imaging (FLIM) may be used to provide structural and functional information, as is required for histopathology. (a) A microscope image of a tissue section from a rat’s ear. The stained image highlights two veins, an artery and cartilage. (b) The fluorescence-intensity image obtained from a similar unstained tissue section immediately after illuminating it with laser light. Only the structure of the veins and the artery can be seen clearly. (c) The corresponding FLIM map of the unstained tissue section shows the different types of tissue present. The two veins appear blue (corresponding to shorter lifetimes) and the red line left-of-centre is the cartilage. (d) A magnified FLIM map of a vein clearly highlights a wall and the tissue structure, and, importantly, provides contrast between different types of tissue with no need for staining.
Indeed, confocal scanning can be used in combination with fluorescence imaging to acquire 3-D spectroscopic images of biological samples, a technique that is becoming a standard clinical research tool. One drawback, however, is that it takes a long time to interrogate all the pixels sequentially and build up the image. Also, it is necessary to use a laser to provide sufficiently bright illumination through the pinholes, which can be both costly and rather limited in terms of the wavelength range. Recently, however, low-cost and short-wavelength solid-state lasers have become commercially available and ever more powerful computing capabilities reduce the time it takes to render an image.
But there is a more serious drawback in using confocal fluorescence microscopy to image biological samples. While the confocal microscope only collects light from the sectioned image plane, the out-of-focus sample in the beam is nevertheless irradiated. This extended exposure to laser radiation can damage or destroy the sample. However, it is possible to avoid this problem by using an ultrafast laser that delivers optical pulses to excite fluorescence in the sample.
Even a laser with a relatively low average power can deliver high-intensity light pulses that last for a fraction of a picosecond. The intensity is sufficiently high for the molecule to absorb two photons simultaneously and produce the same excitation as a single photon with twice the energy. As the rate of two-photon absorption is proportional to the square of the light intensity, it is relatively straightforward to focus the beam to ensure that the excitation only occurs at one point.
Fluorophores with high-energy ultraviolet absorption bands can therefore be excited by much lower energy, and hence less damaging, visible or infrared radiation. In other words two-photon microscopy can permit 3-D imaging of many types of biological tissue for the first time without killing the cells under investigation. This will have a profound impact on biomedical research.
A further advantage of two-photon microscopy is that it permits quantitative fluorescence imaging at deeper depths than conventional confocal microscopy. This feature was exploited by Winfried Denk at AT&T Bell Laboratories in the US, who used the technique to image the concentration of calcium ions in order to study the signal transmission between nerve cells.
There have been many other important advances in optical microscopy and the field continues to evolve rapidly. Stefan Hell’s group at the Max-Planck Institute for Biophysical Chemistry in Göttingen, Germany, has developed a two-photon microscope that provides a confocally sectioned image in real time. And Tony Wilson’s group at Oxford University in the UK has developed a whole-field white-light microscope that has the sectioning capability of confocal microscopy but the speed and much lower cost associated with conventional microscopes.
Currently, the best resolution obtained using a confocal microscope at 543 nm is ~ 150 nm in the xy direction and ~ 420 nm in the z direction. To achieve the resolution needed to probe intracellular structure, Hell and co-workers have combined a novel optical microscope using two objective lenses and sophisticated image processing to yield optical resolutions of less than 100 nm. Optical microscopy is now beginning to approach the resolution that can be achieved with scanning probe microscopy.
Imaging through tissue
The most reliable way to detect disease is to look for characteristic changes in tissue samples that have been taken from a patient during a biopsy. But there is an increasing demand for low-cost medical diagnostic tools that are capable of imaging parts of the body without the need for surgery. “Optical biopsy” appears to be a promising tool for diagnosing and monitoring diseases such as cancer, but the difficulties associated with the scattering of the light need to be resolved. Optical techniques will therefore find their first applications at the surface of tissues – either externally, in the skin, eyes and mouth, for example, or internally via an endoscope. But endoscopy would be much more powerful if it were possible to image below the surface of tissues.
The absorption and scattering of light in biological tissue can be illustrated by shining a torch at one’s hand. It is possible to see a reddish glow, but not the outline of the bones that are in the path of the beam. The bones are not visible because the light is multiply scattered in the tissue. The reddish glow is readily understood from the absorption profile of the most common constituents of biological tissue. There is an absorption minimum in the near infrared around 830 nm that will preferentially transmit the red components of the beam rather than the shorter visible wavelengths. The relatively high-transmission spectral region between 650-1300 nm is often described as the “optical window” of biological tissue. Semiconductor lasers and recently developed solid-state laser technology can now conveniently provide high power, tuneable optical radiation in the near infrared and this has enabled many researchers to tackle the challenge of imaging through biological tissue.
4 Imaging objects in scattering media (a) In the absence of biological tissue, a clear image of the object is formed on the detector. (b) Dense biological tissue, however, scatters the incident photons blurring the resulting image. The intensity of light that does not scatter, known as ballistic light (shown in red), decreases exponentially as the thickness of the tissue sample increases. The signal is usually swamped by the diffuse light (blue). (c) Moderately thick (up to roughly 2 mm) tissue samples can be imaged with high resolution (a few microns) by precisely measuring the arrival time of the transmitted light at a detector and using the ballistic (unscattered) light that arrives at the detector first (red). Up to around 50 ps later, the snake light that has scattered only slightly from its original path (green) arrives at the detector and may be used to form images through thicker (< 2 cm) tissue samples although the resolution is degraded to about 0.5 mm. The diffuse light (blue), arriving up to several nanoseconds later, has scattered randomly and conveys much less information about the image.
When imaging thin (< 2 mm thick) tissue samples, it is possible to use photons that have not been scattered: these are called ballistic photons and they can be used to form high-resolution images. The number of ballistic photons, however, decreases exponentially with propagation distance. So the ballistic signal is usually swamped by multiply scattered photons that obscure any image and saturate most detectors (figure 4). For relatively shallow tissue depths it is possible to use various filtering techniques to block the multiply scattered photons. Indeed, a lot of current research is aimed at extending optical imaging and biopsy to depths of a few millimetres this way. Confocal microscopy, for example, rejects much of the scattered light through its spatial filtering action and has been shown to form useful images at tissue depths of up to 0.3-0.5 mm.
However, imaging to tissue depths beyond ~0.5 mm does not appear to be practical using optical microscopy and spatial filtering alone. Therefore, more sophisticated techniques have been developed to favour the detection of the ballistic photons. These techniques either exploit the fact that the ballistic-signal photons retain their coherence with the incident light, or that the ballistic photons arrive at the detector earlier than any diffuse photons that have been scattered back into their original path.
5 Optical coherence tomography Optical coherence tomography is a technique to detect the ballistic-light signal at different depths in a tissue sample. The input laser light pulse is divided at a beam splitter and the reflected component is directed onto a tissue sample via an optical fibre. The different layers within the tissue sample reflect the light back down the fibre and back to the beam splitter. Here the ballistic light will interfere with light that has been reflected from a mirror at the end of the adjustable reference arm, but only if the two light signals have travelled the same optical distance. By adjusting the length of reference arm, it is therefore possible to detect the ballistic-light signal from specific depths within the sample and thus build up a 3-D image.
James Fujimoto and co-workers from the Massachusetts Institute of Technology in the US have successfully developed a ballistic-light imaging technique, known as optical coherence tomography (OCT), which combines confocal microscopy with heterodyne detection using low-coherence light (figure 5). The weak ballistic-light signal reflected from various layers within the tissue sample is combined with a powerful, coherent reference beam. The resulting interference or “beat” signal is detected with high sensitivity using a lock-in amplifier. Using low-coherence-length radiation means that interference will only occur when the reference signal and ballistic-light signal have travelled the same optical distance (to within the coherence length). By adjusting the pathlength of the reference beam, one can therefore detect the ballistic signal from specific depths in the sample and so build up a depth-resolved image. This has proved clinically useful in acquiring depth-resolved images in the eye and many groups are now developing the technique for imaging in strongly scattering biological tissue.
Although the image acquisition time is relatively slow (due to having to scan pixel-by-pixel) the use of ultrafast lasers to provide high average power, low-coherence radiation permits OCT systems to provide real-time depth-resolved images. This technique can readily be used in conjunction with an arthroscope – a type of endoscope specially designed to examine joints. Mark Brezinski at Harvard Medical School in the US has worked with Fujimoto’s group to apply OCT to detect changes in the orientation of the tissue surrounding joints that can signal the advent of osteoarthritis.
Deep imaging
The techniques that rely on ballistic-light detection can only image tissue to depths of a few millimetres. But how do we image through many centimetres of tissue when the ballistic-light signal is not detectable? Can we extract useful information from the multiply scattered photons?
For moderate tissue depths we can exploit the fact that biological tissue tends to scatter light in the forward direction, meaning that most photons will only deviate slightly from their original direction. After a few centimetres of tissue, there can be a significant number of photons that have followed a reasonably well defined “snake-like” path about the original direction through the tissue (figure 4c). The transmitted light therefore comprises three components: ballistic, “snake” and diffuse photons. The “snake light” will arrive at the detector after the ballistic light but before the fully diffuse photons, and can still retain some coherence with the incident light. Images formed using the least-scattered snake light have a poorer resolution compared with the ballistic-light images, but sub-millimetre spatial resolution is still possible. Humio Inaba from Tohoku Institute of Technology in Japan has exploited the technique to image through thicker biological tissue samples, such as human fingers and teeth. And Robert Alfano’s group at the City University of New York has demonstrated improved imaging using polarization to preferentially select the “snake light”.
Another approach is to divide the light arriving at the detector into time windows using ultrafast lasers and a variety of high-speed cameras or photon-counting systems that provide picosecond time gates ranging from ~ 1-100 ps. This permits the earliest arriving, least scattered, light to be selected and provides information about the tissue structures that produced the scattered-light distribution.
Inverse problem images deeper
For many important biomedical applications, such as mammography or functional imaging of the brain, it is necessary to penetrate through several centimetres of tissue, after which almost all the detected signal is diffuse and the ballistic-light signal is negligible. But it is possible to get round this by tackling the “inverse problem”, which entails measuring the scattered-light signal as comprehensively as possible and calculating what distribution of material would have produced this measured signal. The calculations exploit statistical models of photon transport and can have varying degrees of accuracy.
By considering the most probable paths that the photons will take from a given source to a detector, one can probe a volume of tissue and quantify changes in the optical properties within that volume. In general, there will be least uncertainty in the paths of the photons that are detected earliest and so the volume through which they might have travelled can be more precisely defined. This approach provides a means to quantify the average optical properties of biological tissue and to form relatively low-resolution tomographic images.
Optical fibres are often used to conveniently deliver light from a laser source to the area of tissue under investigation and also to collect the scattered light (figure 6a ). The arrival time of the scattered photons can be measured in various time windows using high-speed detectors in conjunction with ultrafast laser sources (the time-domain approach) or from the phase delay of the collected light with respect to an incoming sinusoidally modulated beam (the frequency domain). In theory the two approaches are equivalent, but in practice the time-domain approach has superior resolution compared with the frequency-domain approach because more complex (and expensive) apparatus has been used.
6 Deep-tissue imaging (a) Fibre optics can be used in conjunction with ultrafast lasers and statistical physics to build up an image through many centimetres of tissue. The most probable path taken by the photons is computed from the arrival time of the light at the detector. Through thick (> 5 cm) tissues, inverse scattering calculations are required to reconstruct images that may be limited to around centimetre resolution. (b) A more sophisticated “tomographic” approach is to use several sources and detectors arranged around a sample.
David Delpy and co-workers at University College, London, have developed a clinical instrument that uses the frequency-domain approach to measure the degree of oxygenation of the blood in the brains of premature babies. The instrument comprises a single detector and tunable laser source to determine the mean time-of-flight of photons between source and detector, and therefore the average optical pathlength. Measurements of the amplitude of the detected signal can then quantify changes in absorption. If this measurement is repeated at a number of different wavelengths, one can obtain useful medical data.
By scanning the source and detector across a sample, one could, in principle, build up a 3-D image, but this would be prohibitively slow. A more sophisticated approach is to use an array of sources and detectors simultaneously to acquire as much information as possible about the scattered light (figure 6b).
At present the computation required to perform the iterative inverse-scattering calculations for 3-D imaging can take hours and the achievable image resolution is of the order of 0.5-1 cm for mammography, which is rather poor compared with other imaging methods such as MRI. However, the power of computer processors is increasing month by month and the resolution will improve with the sophistication of the data acquisition and the optical-image-reconstruction algorithms.
The current state-of-the-art in thick-tissue imaging is probably an instrument being developed by Jeremy Hebden and co-workers at University College, London, which boasts 32 time-gated detector channels with 50 ps resolution and 32 sources derived from a tunable ultrafast laser. This is able to rapidly characterize the scattered-light signal with unprecedented precision and provides some indication of what may be achieved in the next decade.
Although the spatial resolution is not likely to improve much beyond ~0.5 mm for breast or brain imaging, this does not matter too much – it is the accuracy and speed that are critical. The goal of thick-tissue optical imaging is usually to obtain spectroscopic information with a view to detecting the presence of a specific type of tissue, such as a tumour, rather than to provide a detailed map of tissue structure. The latter may be better left to MRI, with optical techniques providing complementary information, such as monitoring changes or detecting abnormalities in tissue properties. One can envisage using expensive MRI technology to provide an initial high-resolution map of tissue distribution and applying this information to aid the computational reconstruction of the optical image. A low-cost optical-imaging instrument could then be left in place on the patient to monitor changes in critical parameters, or to facilitate ongoing diagnosis and research.
Functional imaging may well provide the ability to detect tissue abnormalities that are smaller than the resolution limit and this will be an important tool for screening against disease or diagnosing complications following injury. Possibly the most exciting prospect for thick-tissue imaging, however, is the real-time imaging of brain activity.
As advances in information processing and optoelectronics make the technology cheaper and more powerful, progress in biomedicine will grow exponentially. The advances that have been seen in the 20th century may seem incremental and predictable in comparison with the advances that will be made in the next century.
Advances in microscopy will lead to breakthroughs in microbiology and genetic engineering, and new optical techniques will permit “optical biopsies” without the need for surgery. This in turn may increase the reliability of screening programmes and of treatments. The superior understanding of disease and its effects on tissue will allow new therapies and surgical procedures to be developed that can be tuned to the specific needs of the patient. Finally, thick-tissue imaging will lead to breathtaking insights into the working mechanisms of organs. In particular, imaging brain activity will be fascinating and it may even become possible to watch people thinking. Let us hope there is something good to think about.
To many sports fans, the month of June is synonymous with the Wimbledon tennis tournament. Thousands of people will flock to the All England Lawn Tennis Club to watch the players battle it out on the grass courts, and millions more will watch the tournament on television. But many tennis officials, players and spectators complain that the speed of the players’ serve on “fast” courts, like those at Wimbledon, has simply become too quick. Indeed, most games at Wimbledon are won on the strength of a player’s serve alone rather than following a long rally. And for some players over 30% of their sets end in tie-breaks.
Possible solutions to this situation include changing the surface or the dimensions of the tennis court, limiting the power of the racket or changing the tennis balls. Getting rid of the grass and replacing it with a slower surface would clearly solve the problem. However, the grass courts at Wimbledon are the heart and soul of lawn tennis and this will never be a viable option. A more popular suggestion is either for the players to use wooden rackets or to limit the power of the racket in some other way. The International Tennis Federation (ITF) is currently investigating whether there is indeed a practical way to limit the power of the racket.
One of the most popular suggestions for slowing down the game is to change the balls. One approach would be to make the balls bigger. This would influence the interaction between ball and the racket, the ball’s flight through the air, and its bounce on the ground. The ITF, the world ruling body of the game, has thus embarked on a challenging series of projects to study the physics of tennis.
Currently the rules of the game state that a tennis ball should have a mass between 56.7 g and 58.5 g, and a diameter between 65.41 mm and 68.58 mm. The ITF proposes that larger tennis balls (up to 71 mm) could be used on fast courts such as those at Wimbledon. But before such a rule change is made it is important to study its effect on the game.
To simplify the problem, the first stage of the investigation has concentrated on non-spinning tennis balls. Simon Goodwill at the ITF fired both standard-sized balls (65 mm in diameter) and balls that were 8% larger (69 mm in diameter) at velocities of up to 60 m s-1 (135 mph) from a compressed-air gun at a tennis racket. Using light guides he measured the impact and rebound velocities for both freely suspended and rigidly clamped rackets.
Only small differences were found in the velocities with which the different-sized balls rebounded from the racket and these were attributed to slight variations in the stiffness and mass of the balls. Balls that have either a higher mass or stiffness were found to rebound faster from the racket. This is because the racket strings deform more than they would with a lighter or less stiff ball. More energy will therefore be returned to the heavier or stiffer ball, causing it to rebound at a higher speed.
At the University of Sheffield, we carried out an aerodynamic study on the standard and larger-sized balls by mounting non-spinning balls on a force platform in a closed-circuit wind tunnel. We found that the drag coefficients, which relate the drag force on the ball to its velocity, diameter and density, remained constant over a large range of velocities for each size of tennis ball. Rod Cross of the University of Sydney carried out a similar study confirming that the airflow around tennis balls is unlike the airflow around other types of ball.
When the airflow around a smooth sphere is even and laminar, the drag coefficient is high. But at high velocities, the airflow undergoes a transition from laminar to turbulent flow and the drag coefficient drops by more that a factor of two. Indeed, over the years golf balls have acquired dimples to induce this transition and consequently lower the drag at much lower velocities. This is why dimpled golf balls travel greater distances than smooth ones. Tennis balls, on the other hand, do not seem to experience this transition and the drag coefficient remains constant regardless of velocity. We found that the dominant factor affecting the drag coefficient was the height of the nap above the surface of the tennis ball – in other words, “fluffier” balls have a higher drag coefficient. This might explain why tennis players comment that a ball travels faster as the nap is worn off during a game.
Another major factor governing the ball’s flight through the air is its diameter. A larger ball experiences a higher drag force due to its increased cross-sectional area, which causes it to slow down more than a standard-sized ball. The knock-on effect is that the larger ball lands at a steeper angle, causing it to rebound off the court at a steeper angle.
This brings us to the impact of the balls with the court. Standard and larger-sized balls were projected at both clay and acrylic courts using a bowling machine that can fire the balls at speeds of up to 50 m s-1. The impact and rebound of each ball were recorded using a high-speed video camera that recorded 9000 frames per second. We found that both balls rebounded with a slightly higher velocity from an acrylic court that they did from a clay court. The larger ball, however, rebounded at a slightly steeper angle than the standard-sized ball, especially on the clay surface. Although this angle amounted to less than 2°, when added to the fact that the larger ball also hits the court at a steeper angle, a player will notice a significant difference in the way the larger ball “plays”.
We also analysed the complete trajectories of balls that were served with a velocity of 50 m s-1 and bounced just inside the service line. The larger balls travel 15 cm less than the standard balls before landing on the court, and arrive at the baseline approximately 30 ms later on both acrylic and clay surfaces.
This may not sound significant but the difference between the standard and larger balls may be as large as the difference between clay and acrylic courts. Players know that clay is a much slower surface than acrylic and therefore the effect of a larger ball will be significant.
This year the ITF will propose a rule change that will allow larger tennis balls to be used to assess players’ reactions in tournaments. Meanwhile, the next stage for the physics investigation is to include the effects of spin and racket design on the ball’s impact and trajectory.
There is no doubt that the world has an increasingly intense love-hate relationship with science. Physics certainly does not escape this deep ambivalence, and we naturally wonder if there is anything that might make “them” love us a little more and hate us a little less? This question can be formulated seriously, and will be among the issues concerning science and its relations with society that will be addressed at the World Conference on Science in Budapest, Hungary, later this month. Most scientists will be represented in some way at the conference, since it is sponsored by both the United Nations Educational, Scientific and Cultural Organization (UNESCO) and the International Council for Science (ICSU), which represents national scientific bodies.
However, most world-weary physicists will see the conference, which will be attended by some 2000 delegates nominated by governments, non-governmental agencies and other organizations, as unlikely to make much difference. Nevertheless, the chance of actually influencing the governments of the world should not be passed up lightly. Indeed, the conference could prove to be influential, and there are plans for it to adopt two documents: World Declaration on Science and the Use of Scientific Knowledge and Science Agenda – Framework for Action.
So what can physicists add to the UNESCO discussions? In preparation, a workshop on “the future of physics and society” was held in Debrecen, Hungary, in March, attended by over 40 physicists from Europe and beyond. Many branches of physics were represented at the workshop, and a number of those attending had a specialist interest in physics education. A statement prepared at the workshop has now been forwarded to UNESCO, and the hope is that its recommendations will find their way into the conference’s two documents. Many other points, which are relevant to the wider physics community rather than the UNESCO conference itself, were also raised at the workshop.
Submission of hope
The document submitted to UNESCO begins with a three-point declaration. It states that:
The contribution of physics to all aspects of life, material and non-material, will be essential for the foreseeable future;
Physics currently faces serious problems in the world. Many of these problems affect science in general, but a number are specific to physics;
Actions are needed to assure the continued health of physics research, teaching and cultural influence. Some form of “contract” between physicists and the rest of society will be required.
The first, positive point was supported by a range of arguments. Physical science was affirmed to be of immense cultural importance, not least as a bulwark against irrationality. And it was pointed out that physical science will make a vital contribution to solving environmental and energy problems in the coming century. There was also a feeling that the educational value of physics is immense, and that this value is widely under-appreciated. (An interesting fact that emerged at the workshop was that trained physicists have won Nobel prizes in all categories – physics, chemistry, medicine (Allan Cormack and Godfrey Hounsfield), peace (Joseph Rotblat), economics (Robert Merton) and literature (Alexander Solzhenitsyn).)
Before turning to the problems that physics faces with the world, let us mention some of the actions that the workshop recommended in the third point, concerning the support for physics and physics education. The statement calls for UNESCO to “explore ways of establishing a recognized authoritative and impartial international body, set up under the auspices of UN or UNESCO, to adjudicate damaging disputes involving scientific issues”. This new body would investigate the extent to which claims are based on established science or are simply ungrounded opinion, perhaps influenced by pressure groups. It would therefore provide an authoritative scientific basis for important political decisions.
The “cold fusion” affair of the late 1980s is an example of the sort of dispute that such a body could tackle. But there are also serious scientific issues, many relating to the environment, that such a body could tackle. For example, the question of whether the controversial Gab�cikovo-Nagymaros dam on the River Danube between Slovakia and Hungary should be built involves disputed scientific issues. These could, and should, be disentangled from the political issues and be settled by an independent body.
Two very positive findings also emerged at the workshop, and we must learn to exploit these. The first is the high national prestige that flows from scientific achievement. Only sporting accomplishments, it was felt, are rated as highly. Indeed, this scientific prestige is beginning to be exploited in international negotiations at the highest level. The other positive development is the burgeoning public thirst for popular science. Never have there been as many popular-science books, and the increasing public interest is certainly something we must build on. Many speakers also emphasized our responsibility to communicate with the public and professionalize the way we do so. But there is a downside to scientific publicity – too often the media, and sometimes even scientists, behave irresponsibly.
Physics and the public
Not all the views expressed at the workshop were positive. Some of the problems faced by physics arise from the public’s increasing “anti-science” feelings and their seemingly growing irrationality. While the character of such sentiments varies from country to country, there are common features, such as the fashion for politicians to make decisions on advice from their astrologers. We also heard rumours that the Ukrainian “mafia” have cornered the market for quack medicines in a certain east European country; we feel that the belief in such products embodies an outright rejection of science. Although these examples might not seem to have much to do with physics, they affect all of science.
But physics itself does seem particularly malign in the eyes of many people. These feelings go deep. There is, for example, a profound fear of radioactivity, based on misconceptions, of course, which seems to provide a focus for the alienation. Science, including physical science, is seen to be “reductive” and “cold”, and it is felt to have eliminated meaning from human life. Particularly worrying developments in anti-science include the growing belief that imagined conspiracies, both malign and covert, control the media – for example, to deceive us about the truth about unidentified flying objects or alien abductions. Recent debates in biological science, such as the issue of genetically modified food, have highlighted the public’s deeply confused attitude to authority. In such cases people are not sure whether to believe the scientific establishment, which is seen to be in the pocket of multinational companies, or a particular pressure group, whose Web site seems to offer all the right answers.
This problem of how the public sees science is inseparable from related tendencies in academia and the wider intellectual world, where, in certain quarters, science is not accepted as producing “truths” about the world. It is ironic that this growth of “relativism” has apparently been nurtured by misinterpreted physics – self-indulgent popularizers please note! How, for example, has Heisenberg’s uncertainty principle, interpreted as “everything is uncertain”, gained such widespread and damaging currency? And how did Einstein’s theory of relativity ever come to give aid and comfort to relativism? The gut feelings of most working physicists against extreme cultural relativism is enshrined in our statement for the UNESCO conference, where we state that: “physics is global and constitutes our best ‘anti-Babel’. Generations of physicists of the most diverse political and cultural backgrounds have collaborated on the basis of shared understanding and shared ideals”.
But one contributor at the workshop suggested that some physicists have gone too far in their blanket criticism of post-modern writings. Maybe physicists have not done justice to the context and form of the discourse of some of the statements that they have criticized. Out of these thoughts, and through the realization that the world outside science is not monolithic, arose a new idea that physicists should try to reach a rapprochement with at least part of the world of sociology. In particular, we should enlist their expert aid in analysing some of the problems faced by physics in its relations with the world. We urge the UNESCO conference to consider this suggestion.
Economics and education
What UNESCO might do for science or physics remains to be seen, although we hope that it comes forward with measures that will be helpful in particular countries. We recommend, for example, that UNESCO should establish a committee that can advise on national science policies and can set up guidelines linking a country’s expenditure on research and development to its gross national product, at a level appropriate to the economic state of that particular country. Such guidelines would certainly help the science communities of the “new democracies” of eastern Europe. They would also benefit if they could point to an affirmation by the World Conference of the importance of both basic physical science and curiosity-led research.
UNESCO might also do something for physics education, which is a matter of concern in both affluent and less affluent countries, albeit in different ways. The fact that so many members of the public – even in wealthy countries – seem quite unable to make sound judgements about things such as alien abductions suggests that the veneer of scientific understanding is thin indeed. This is a serious point, since this ignorance seems to go hand in hand with extreme distrust in authorities.
To what extent is this a failure of education? The workshop heard examples from many nations of a whole range of problems in physics education, especially at the secondary level. In many countries, it seems, physics education has simply failed. Common problems are outdated educational policies, and a lack of financial and political support for the subject. Another theme is the lack of response of science teaching to the changing social and scientific environment. We heard eloquent pleas at the workshop for the introduction of more exciting and relevant topics, such as cosmology, the environment, radioactivity and energy, into the physics curriculum.
Get set for Budapest
So what could UNESCO do? We have put forward recommendations to set standards on the content and practice of physics education. It would be of immense help to governments and national organizations of physics teachers if the World Conference adopted these recommendations. Teachers and others would then have standards and norms – rising above the politics of their own country – to which they could refer. UNESCO must also establish and make available the data that would make it possible to monitor and defend these standards.
It is up to us physicists to improve our relationship with society. Our recommendations therefore involve our relations with the public, with other physicists, with government, with other academic disciplines and with educational establishments. It is essential that we ourselves understand the social roots of the current situation. We must also be careful not to provide opportunities for those who would divide and rule.
A frequent complaint at gatherings of senior physicists is that that everyone with a PhD in theoretical physics abandons research to follow a lucrative career as a “rocket scientist” in the City. This is good, some senior figures argue, because it shows that theoretical physics can create wealth, which is important when applying for research grants. The exodus to the City is bad, others argue, because it means that too many bright people are leaving the subject. A recent report from the Centre for the Study of Financial Innovation (CSFI) in London confirms that there is a genuine and growing demand for physicists in the City, but that the numbers involved are relatively small. Would-be rocket scientists should also be aware that the City is only interested in the “top two per cent” of talent, and that interpersonal skills are more important than in-depth knowledge of finance.
The CSFI was commissioned by the UK’s Engineering and Physical Sciences Research Council to investigate the City’s requirements for postgraduate research and skills in financial engineering. Its report, the first to look at this subject, estimates that no more than about 100 PhDs go to work in the City as quantitative analysts – “quants” or “rocket scientists” – every year. This number is small but it is not insignificant: for instance, the UK’s research councils fund around 1500 new post-docs in science and engineering every year. However, significant numbers of physicists also find employment as IT specialists in the financial sector, and there is a growing band of physicists who work part-time in finance while retaining their academic positions (see Physics World January 1999 pp25-29).
Needless to say the financial rewards in the City are high. Whereas a post-doc in a university can expect to start on a salary of around £20 000, a quant “with a good PhD and the right personal skills can expect a starting salary of £35 000-£50 000, plus the same again in bonuses and perks such as a car and subsidised mortgage”. Nice work if you can get it.
The report has good news for postgraduates in science and engineering: “There is little doubt that the City will continue to demand PhD-level quants.” Moreover, the strongest demand will be for “people who have had a rigorous training in applied sciences (physics, engineering, etc), where the emphasis is on problem solving” rather than people who have specifically trained in financial mathematics. The report has even better news for physicists, especially those with skills in probability theory, stochastic calculus and partial differential equations: “Even though mathematical skills are sought, we found a strong preference for physicists over mathematicians. As one bank explained it: ‘Physicists want to find the answers to problems. Mathematicians have all the answers and want problems to solve.’ ”
One surprising fact to emerge is that “the process of placing PhD quants seems terribly wasteful. Banks report hundreds of applications for a single position – only to discover that the attrition rate among successful applicants is 50% or more”. The report recommends a programme of internships so that both sides know what they are letting themselves in for.
The report is refreshingly direct in places: “Put bluntly, the City is depressingly unenthusiastic about university research.” Or, as one US banker explained: “Academics are looking for market perfection, practitioners are trying to make a living out of imperfections.” One reason for the lack of interest in funding external research is that most banks are interested in the short-term development of new financial products based on existing knowledge.
The report makes clear, however, that there is a need for long-term research: “Despite its importance in the financial world, quantitative finance as a science is still in its infancy, and an enormous amount of research is required simply to underpin present developments, let alone those coming over the horizon.” The whole area of risk, for instance, is ripe for new discoveries, and offers pickings for the physicist, or anyone else, who can develop reliable systems to manage it.
Helbing and co-workers have identified six different traffic phases: homogeneous congestion, oscillatory congestion, triggered stop-and-go traffic, moving localised clusters, pinned localised clusters, and free traffic. One axis of their phase diagram represents the flow of vehicles already on the motorway, while the other represents vehicles joining the motorway. Transitions between the states are triggered by localised event – such as a convoy of slow moving lorries passing the intersection. The boundaries between the different phases are related to various properties of the traffic flow such as the density of vehicles.
According to group’s simulations, the occurrences of traffic jams can be reduced by adjusting various properties of the intersections where vehicles join the motorway.
The report has already led to security being tightened at all laboratories run the Department of Energy, including those at Los Alamos and Livermore. Fears over the new security regime – visitors from “sensitive” countries such as India, China and Russia now require special permission from Bill Richardson, head of the DOE, to attend the laboratories – have already led several foreign-born researchers to turn down post-doctoral positions at Los Alamos. More than half the current post-docs at Los Alamos were born outside the US.
The Cox report also reveals that security leaks allowed China to build and test a so-called neutron bomb – a device which kills people but leaves buildings intact – in 1988. As well as acquiring plans to all the main US warheads, China has also obtained the simulation software used to model nuclear explosions.
During 1996 China declared it would stop nuclear testing and sign the Comprehensive Test Ban Treaty. But signing the treaty would hinder China’s efforts to develop more sophisticated and smaller nuclear weapons, hence its interest in stealing secrets from the US. “It would have been virtually impossible for the People’s Republic of China to fabricate and test successfully small nuclear warheads prior to its 1996 pledge to adhere to the Comprehensive Test Ban Treaty, ” says the report.
Two physicists, Wen Ho Lee and Peter Lee, are accused in the report of leaking details of nuclear weapons and advanced radar systems to the Chinese. Peter Lee was arrested and sentenced to 12 months in prison for providing details of synthetic-aperture radar systems. Such systems can be used to detect nuclear submarines. Meanwhile, Wen Ho Lee transferred thousands of classified documents on the US nuclear weapons programme from a secure computer to one accessible from the Internet. China is believed to have downloaded files from this system. The case against Wen Ho Lee was dropped due to lack of evidence, although he was later fired by Los Alamos for breaching security precautions and failing a lie detector test.
The Cox report highlights how China not only relied on spies at the US laboratories, but also on gathering information piecemeal by reviewing unclassified publications and by interacting extensively with nuclear weapon scientists outside the laboratories. China also managed to obtain a large amount of ‘dual-use’ technology – which could be adapted from civilian to military use – by using over 3000 so-called front companies. And two US companies, Loral Space Systems and Hughes Electronics, are heavily criticised in the report for providing the Chinese space programme with information that was subsequently used to improved the reliability of China’s missile programme.
The Hubble team, led by Wendy Freedman of the Observatories of the Carnegie Institution of Washington, has spent the last eight years using the telescope to make more accurate measurements of the Hubble constant – the rate at which the universes expands. The team’s final result this is that the Hubble constant is 70 kilometres per second per megaparsec. (A megaparsec is 3.26 million light years). The uncertainty in the measurement is 10%. “After all these years, we are finally entering an era of precision cosmology, ” said Freedman.
To calculate the age of the universe from the Hubble constant it is necessary to know both the density of matter in the universe and the value of the cosmological constant. The Hubble team assumed that the density was critical – that is, high enough to eventually stop the expansion of the universe, but not enough to cause the universe to collapse back in on itself. If the cosmological constant is non-zero – as suggested by recent observations which suggest that the expansion of the universe is increasing with time – then the age would be greater.
Freedman and colleagues used the Hubble telescope to measure the brightness of Cepheid variables, well-known astronomical objects whose brightness waxes and wanes on a regular timescale. Astronomers have been able to associate this time variation with the total luminosity of the star. Once they have calculated the brightness of the star, the luminosity measured from the Earth is depends only on how far away it is (assuming that the star is not obscured by dust).
Lineweaver, on the other hand, combined recent independent measurements of the cosmic microwave background – the radiation left over from the Big Bang – with six other cosmological measurements to determine the Hubble constant, the density of matter in the universe and the cosmological constant. Whereas matter slows down the expanding universe, the cosmological constant represents a kind of antigravity force that speeds up the expansion. Einstein once famously called the cosmological constant his greatest blunder but recent observations by two international teams of astronomers suggest that it is non-zero.
