Looking at the little and large
30 July 2002
The microscope and telescope are two of the most potent symbols of science. Both became central to the Scientific Revolution that occurred during the seventeenth century, but the instruments in use today would hardly be recognised by those early pioneers.
The magnifying and diminishing properties of convex and concave glass objects have been known for over 2000 years. Lenses, however, were not introduced into the West until the end of the thirteenth century when glass of reasonable quality became relatively cheap, as grinding and polishing techniques reached a high state of development. Once spectacles became widespread, it was only a matter of time before it was discovered that optical instruments could be made by combining lenses.
Despite history’s desire to ascribe named ‘inventors’ to such instruments, much controversy concerns their invention. It is thought that the microscope came first and, since a microscope can simply be reversed to make a telescope, this could be how the latter originated. What is clear is that both were in use in Holland by the end of the sixteenth century and that Galileo purchased his first telescope around 1607. With it he was able to make out mountains and craters on the moon and that the ribbon of diffuse light arching across the sky – the Milky Way – was composed of stars.
Microscopes and telescopes now ‘see’ without lenses
Since that time, both instruments have continued to advance to such an extent that we now have the ability to image the largest and smallest objects in the Universe. And although the devices now in use in the Physics Department at Bristol University are still called microscopes and telescopes, it seems unlikely that those early Dutch pioneers would distinguish anything familiar about them. For example, Professor Mervyn Miles, head of the Scanning Probe Microscopy (SPM) group, uses a scanning force microscope that has no lenses. Nevertheless, it is so powerful it can image biological systems down to the molecular scale, and even follow molecular processes occurring on the surface of DNA in situ in real-time.
Scanning force microscopes are members of the larger family of ‘local’ probe microscopes. They image by ‘feeling’ the surface of the object being imaged, in much the same way as the stylus on the spring cantilever at the end of a record player arm ‘feels’ the variations in the groove of a record. However, the force microscope must do this with 100 million times less force than a stylus, if it is not to destroy the biomolecules that it is imaging. The image is built up by scanning the force-sensing tip in a raster – a predetermined pattern of scanning lines that uniformly covers the specimen. The resulting image is a three dimensional surface that can be viewed from any direction using 3-D rendering software.
Another mode of imaging is similar to the way that a blind person taps the ground in front of them with a stick in a sweeping action. ‘Tapping’ mode is used in force microscopy of particularly delicate objects as it causes less damage. The cantilever is usually made to tap at a characteristic frequency but the problem is that when in liquid, its motion will be heavily damped so that much greater forces need to be applied to oscillate it the same amount. It’s like trying to move a plank of wood through the water in a swimming pool compared to through the air. This greater force means that when the probe does touch the specimen the forces are much higher than they are when in air.
DNA processes can be followed in situ in real time
Bristol’s SPM group has invented a way of decreasing the effective damping on the cantilever. Using a feedback system, the viscous damping of the water environment can be backed off, so that the forces are greatly decreased and the sensitivity of the force-sensing probe is restored. The result is higher resolution images of less distorted biomolecules. The Bristol SPM group has focused on the invention and development of such scanning probe microscopes with the aim of imaging biomolecular systems at ever higher resolution and speeds. A university spin-out company, Infinitesima Ltd, has been formed to develop and commercialise these appliances.
Despite the remarkable sensitivity of these instruments, it would be better still if the force probe could ‘feel’ the specimen surface without touching it at all! The group has therefore developed another member of the force microscopy family where the probe does not actually come in contact with the biomolecule, but senses it through a thin (~1nm) layer of water. This microscope has obtained the highest resolution images of DNA molecules ever seen so far and will allow the biochemists to watch exactly how DNA enzyme interaction takes place. The benefits of these techniques to life scientists such as Dr. Mark Szczelkun, Professor Miles collaborator in the Department of Biochemistry, are immense. Previously, the models for such interactions were based on indirect biochemical tests which could give anomalous results. In many cases, the information available using standard biological methods is limited by the organism’s complexity, but these new techniques have made these processes much easier to study.
At the other end of the scale, Professor Mark Birkinshaw’s astrophysics group, also in the Physics Department, similarly does not uses lenses to view their objectives. Instead, a suite of ground-based and satellite telescopes captures unfamiliar consequences of cosmic processes using wavelengths not visible to the eye. Their research aims to better understand huge and exotic objects far outside our Solar System by looking in detail at the faint traces of the radiation they emit.
The group has been working on an H-alpha study of the Milky Way Galaxy, which contains our own Solar System. This images our Galaxy in the light emitted by excited hydrogen atoms as they lose energy after being hit by an explosion, or after irradiation by a bright nearby star. H-alpha is one of the most sensitive optical tracers of the excitation process. It is relatively bright since there is a lot of hydrogen around, and its spectral line has a high relative strength so it is easy to identify. The study has produced images of supernova remnants, such as the Veil Nebula in Vela, in exquisite detail.
The strange tracery of ‘excited’ hydrogen is produced by the collision between the blast wave from a star that exploded about 100,000 years ago and the diffuse gas between the stars. Odd corrugated filaments in the picture may be something to do with magnetic fields in the Galaxy. In the hundreds of such pictures made, there is so much to discover about violent processes in the Galaxy.
Supernovae form when large stars explode at the end of their lives and are characterised by short-lived, but extreme brilliance. Stars with a lower mass than those that die as supernovae end their lives throwing off a gaseous shell to form a planetary nebula, just before they settle down to become shrunken white dwarves. The H-alpha survey has found more planetary nebulae in the past three years than had previously been found in a century of studying the Galaxy.
Professor Birkinshaw’s group has also been studying the light currently seen from galaxy 3C351 that was emitted at about the time that our Sun was born, approximately six billion years ago. Using the orbiting X-ray telescope Chandra and the ground-based Very Large Array of radio telescopes situated in New Mexico, his group is trying to discover what causes galaxy 3C351 to be one of the brightest radio sources in the sky.
The yellow contour lines on the picture of 3C351 show increasing brightness of its radio emission. These radio signals are produced when very fast electrons spiral around magnetic fields with velocities just below (0.1%) the speed of light. The super-massive black hole originally responsible for this energy output is seen in the centre of the image, where the contours are closely spaced. But even brighter radio emission is seen from hot-spots many thousands of light-years from the black hole (area to the north west). These mark the point where immense jets of ultra-hot plasma, ejected from near the black hole, splash into gas around the galaxy. Mysterious processes at these points suddenly cause the plasma to light up as bright radio emitters. One of the group's interests is to understand how this happens.
The colours in the picture show the brightness of X-rays from 3C351. Intense X-rays come from near the super-massive black hole, signalling the infall of gas into oblivion. We also see bright X-rays from the radio hot-spots, and fainter, but still important X-ray emission, from the peculiar plume seen towards the top right of the picture. These X-rays come from collisions between fast electrons and radio waves. Some of the radio waves are those of the source itself, shown by the contours, and some are left over from the Big Bang. The brightness of these X-rays tells us about the amount of plasma that 3C351's black hole has emitted over the past hundred million years, and it also gives clues about the processes that energise electrons in hot-spots and make them radio and X-ray bright.
At the beginning of the twentieth century, the beauty and craftsmanship of the microscope and telescope were being replaced by high-volume, low-cost, mass produced instruments, which paved the way for the progress of modern science. A hundred years later, twenty-first century scientists are still developing bigger and better instruments. These will allow us to peer up to the distant reaches of space, almost to the beginning of time, and down to the very molecules of life. Who knows what we shall find next – or where we shall find it?