Good design 13 others.Anyway,whether or not the reason is the energy expense,it seems to be the case that,with the possible exception of some weird deep-sea fish,no animal apart from man uses manufactured light to find its way about. What else might the engineer think of?Well,blind humans sometimes seem to have an uncanny sense of obstacles in their path.It has been given the name 'facial vision',because blind people have reported that it feels a bit like the sense of touch,on the face.One report tells of a totally blind boy who could ride his tricycle at a good speed round the block near his home,using 'facial vision'.Experiments showed that,in fact,'facial vision'is nothing to do with touch or the front of the face,although the sensation may be referred to the front of the face,like the referred pain in a phantom (severed)limb.The sensation of'facial vision',it turns out,really goes in through the ears. The blind people,without even being aware of the fact,are actually using echoes,of their own footsteps and other sounds,to sense the presence of obstacles.Before this was discovered,engineers had already built instruments to exploit the principle,for example to measure the depth of the sea under a ship.After this technique had been invented,it was only a matter of time before weapons designers adapted it for the detection of submarines.Both sides in the Second World War relied heavily on these devices,under such code names as Asdic(British)and Sonar (American),as well as the similar technology of Radar (American)or RDF(British),which uses radio echoes rather than sound echoes. The Sonar and Radar pioneers didn't know it then,but all the world now knows that bats,or rather natural selection working on bats,had perfected the system tens of millions of years earlier,and their 'radar' achieves feats of detection and navigation that would strike an en- gineer dumb with admiration.It is technically incorrect to talk about bat 'radar',since they do not use radio waves.It is sonar.But the underlying mathematical theories of radar and sonar are very similar, and much of our scientific understanding of the details of what bats are doing has come from applying radar theory to them.The American zoologist Donald Griffin,who was largely responsible for the discovery of sonar in bats,coined the term 'echolocation'to cover both sonar and radar,whether used by animals or by human instruments.In practice, the word seems to be used mostly to refer to animal sonar. It is misleading to speak of bats as though they were all the same.It is as though we were to speak of dogs,lions,weasels,bears,hyenas, pandas and otters all in one breath,just because they are all carnivores. Different groups of bats use sonar in radically different ways,and they
Good design 13 others. Anyway, whether or not the reason is the energy expense, it seems to be the case that, with the possible exception of some weird deep-sea fish, no animal apart from man uses manufactured light to find its way about. What else might the engineer think of? Well, blind humans sometimes seem to have an uncanny sense of obstacles in their path. It has been given the name 'facial vision', because blind people have reported that it feels a bit like the sense of touch, on the face. One report tells of a totally blind boy who could ride his tricycle at a good speed round the block near his home, using 'facial vision'. Experiments showed that, in fact, 'facial vision' is nothing to do with touch or the front of the face, although the sensation may be referred to the front of the face, like the referred pain in a phantom (severed) limb. The sensation of 'facial vision', it turns out, really goes in through the ears. The blind people, without even being aware of the fact, are actually using echoes, of their own footsteps and other sounds, to sense the presence of obstacles. Before this was discovered, engineers had already built instruments to exploit the principle, for example to measure the depth of the sea under a ship. After this technique had been invented, it was only a matter of time before weapons designers adapted it for the detection of submarines. Both sides in the Second World War relied heavily on these devices, under such code names as Asdic (British) and Sonar (American), as well as the similar technology of Radar (American) or RDF (British), which uses radio echoes rather than sound echoes. The Sonar and Radar pioneers didn't know it then, but all the world now knows that bats, or rather natural selection working on bats, had perfected the system tens of millions of years earlier, and their 'radar' achieves feats of detection and navigation that would strike an engineer dumb with admiration. It is technically incorrect to talk about bat 'radar', since they do not use radio waves. It is sonar. But the underlying mathematical theories of radar and sonar are very similar, and much of our scientific understanding of the details of what bats are doing has come from applying radar theory to them. The American zoologist Donald Griffin, who was largely responsible for the discovery of sonar in bats, coined the term 'echolocation' to cover both sonar and radar, whether used by animals or by human instruments. In practice, the word seems to be used mostly to refer to animal sonar. It is misleading to speak of bats as though they were all the same. It is as though we were to speak of dogs, lions, weasels, bears, hyenas, pandas and otters all in one breath, just because they are all carnivores. Different groups of bats use sonar in radically different ways, and they
24 The Blind Watchmaker seem to have 'invented'it separately and independently,just as the British,Germans and Americans all independently developed radar. Not all bats use echolocation.The Old World tropical fruit bats have good vision,and most of them use only their eyes for finding their way around.One or two species of fruit bats,however,for instance Rousettus,are capable of finding their way around in total darkness where eyes,however good,must be powerless.They are using sonar, but it is a cruder kind of sonar than is used by the smaller bats with which we,in temperate regions,are familiar.Rousettus clicks its tongue loudly and rhythmically as it flies,and navigates by measuring the time interval between each click and its echo.A good proportion of Rousettus's clicks are clearly audible to us (which by definition makes them sound rather than ultrasound:ultrasound is just the same'as sound except that it is too high for humans to hear). In theory,the higher the pitch of a sound,the better it is for accurate sonar.This is because low-pitched sounds have long wavelengths which cannot resolve the difference between closely spaced objects.All other things being equal therefore,a missile that used echoes for its guidance system would ideally produce very high-pitched sounds. Most bats do,indeed,use extremely high-pitched sounds,far too high for humans to hear-ultrasound.Unlike Rousettus,which can see very well and which uses unmodified relatively low-pitched sounds to do a modest amount of echolocation to supplement its good vision,the smaller bats appear to be technically highly advanced echo-machines. They have tiny eyes which,in most cases,probably can't see much. They live in a world of echoes,and probably their brains can use echoes to do something akin to 'seeing'images,although it is next to imposs- ible for us to 'visualize'what those images might be like.The noises that they produce are not just slightly too high for humans to hear,like a kind of super dog whistle.In many cases they are vastly higher than the highest note anybody has heard or can imagine.It is fortunate that we can't hear them,incidentally,for they are immensely powerful and would be deafeningly loud if we could hear them,and impossible to sleep through. These bats are like miniature spy planes,bristling with sophisticated instrumentation.Their brains are delicately tuned packages of miniaturized electronic wizardry,programmed with the elaborate software necessary to decode a world of echoes in real time. Their faces are often distorted into gargoyle shapes that appear hideous to us until we see them for what they are,exquisitely fashioned instruments for beaming ultrasound in desired directions. Although we can't hear the ultrasound pulses of these bats directly
24 The Blind Watchmaker seem to have 'invented' it separately and independently, just as the British, Germans and Americans all independently developed radar. Not all bats use echolocation. The Old World tropical fruit bats have good vision, and most of them use only their eyes for finding their way around. One or two species of fruit bats, however, for instance Rousettus, are capable of finding their way around in total darkness where eyes, however good, must be powerless. They are using sonar, but it is a cruder kind of sonar than is used by the smaller bats with which we, in temperate regions, are familiar. Rousettus clicks its tongue loudly and rhythmically as it flies, and navigates by measuring the time interval between each click and its echo. A good proportion of Rousettus's clicks are clearly audible to us (which by definition makes them sound rather than ultrasound: ultrasound is just the same'as sound except that it is too high for humans to hear). In theory, the higher the pitch of a sound, the better it is for accurate sonar. This is because low-pitched sounds have long wavelengths which cannot resolve the difference between closely spaced objects. All other things being equal therefore, a missile that used echoes for its guidance system would ideally produce very high-pitched sounds. Most bats do, indeed, use extremely high-pitched sounds, far too high for humans to hear - ultrasound. Unlike Rousettus, which can see very well and which uses unmodified relatively low-pitched sounds to do a modest amount of echolocation to supplement its good vision, the smaller bats appear to be technically highly advanced echo-machines. They have tiny eyes which, in most cases, probably can't see much. They live in a world of echoes, and probably their brains can use echoes to do something akin to 'seeing' images, although it is next to impossible for us to 'visualize' what those images might be like. The noises that they produce are not just slightly too high for humans to hear, like a kind of super dog whistle. In many cases they are vastly higher than the highest note anybody has heard or can imagine. It is fortunate that we can't hear them, incidentally, for they are immensely powerful and would be deafeningly loud if we could hear them, and impossible to sleep through. These bats are like miniature spy planes, bristling with sophisticated instrumentation. Their brains are delicately tuned packages of miniaturized electronic wizardry, programmed with the elaborate software necessary to decode a world of echoes in real time. Their faces are often distorted into gargoyle shapes that appear hideous to us until we see them for what they are, exquisitely fashioned instruments for beaming ultrasound in desired directions. Although we can't hear the ultrasound pulses of these bats directly
Good design 25 we can get some idea of what is going on by means of a translating machine or'bat-detector'.This receives the pulses through a special ultrasonic microphone,and turns each pulse into an audible click or tone which we can hear through headphones.If we take such a 'bat- detector'out to a clearing where a bat is feeding,we shall hear when each bat pulse is emitted,although we cannot hear what the pulses really 'sound'like.If our bat is Mvotis,one of the common little brown bats,we shall hear a chuntering of clicks at a rate of about 10 per second as the bat cruises about on a routine mission.This is about the rate of a standard teleprinter,or a Bren machine gun. Presumably the bat's image of the world in which it is cruising is being updated 10 times per second.Our own visual image appears to be continuously updated as long as our eyes are open.We can see what it might be like to have an intermittently updated world image,by using a stroboscope at night.This is sometimes done at discotheques,and it produces some dramatic effects.A dancing person appears as a suc- cession of frozen statuesque attitudes.Obviously,the faster we set the strobe,the more the image corresponds to normal 'continuous'vision. Stroboscopic vision 'sampling'at the bat's cruising rate of about 10 samples per second would be nearly as good as normal 'continuous' vision for some ordinary purposes,though not for catching a ball or an insect. This is just the sampling rate of a bat on a routine cruising flight. When a little brown bat detects an insect and starts to move in on an interception course,its click rate goes up.Faster than a machine gun,it can reach peak rates of 200 pulses per second as the bat finally closes in on the moving target.To mimic this,we should have to speed up our stroboscope so that its flashes came twice as fast as the cycles of mains electricity,which are not noticed in a fluorescent strip light Obviously we have no trouble in performing all our normal visual functions,even playing squash or ping-pong,in a visual world 'pulsed' at such a high frequency.If we may imagine bat brains as building up an image of the world analogous to our visual images,the pulse rate alone seems to suggest that the bat's echo image might be at least as detailed and 'continuous'as our visual image.Of course,there may be other reasons why it is not so detailed as our visual image. If bats are capable of boosting their sampling rates to 200 pulses per second,why don't they keep this up all the time?Since they evidently have a rate control 'knob'on their 'stroboscope',why don't they turn it permanently to maximum,thereby keeping their perception of the world at its most acute,all the time,to meet any emergency?One reason is that these high rates are suitable only for near targets.If a pulse
Good design 25 we can get some idea of what is going on by means of a translating machine or 'bat-detector'. This receives the pulses through a special ultrasonic microphone, and turns each pulse into an audible click or tone which we can hear through headphones. If we take such a 'batdetector' out to a clearing where a bat is feeding, we shall hear when each bat pulse is emitted, although we cannot hear what the pulses really 'sound' like. If our bat is Myotis, one of the common little brown bats, we shall hear a chuntering of clicks at a rate of about 10 per second as the bat cruises about on a routine mission. This is about the rate of a standard teleprinter, or a Bren machine gun. Presumably the bat's image of the world in which it is cruising is being updated 10 times per second. Our own visual image appears to be continuously updated as long as our eyes are open. We can see what it might be like to have an intermittently updated world image, by using a stroboscope at night. This is sometimes done at discotheques, and it produces some dramatic effects. A dancing person appears as a succession of frozen statuesque attitudes. Obviously, the faster we set the strobe, the more the image corresponds to normal 'continuous' vision. Stroboscopic vision 'sampling' at the bat's cruising rate of about 10 samples per second would be nearly as good as normal 'continuous' vision for some ordinary purposes, though not for catching a ball or an insect. This is just the sampling rate of a bat on a routine cruising flight. When a little brown bat detects an insect and starts to move in on an interception course, its click rate goes up. Faster than a machine gun, it can reach peak rates of 200 pulses per second as the bat finally closes in on the moving target. To mimic this, we should have to speed up our stroboscope so that its flashes came twice as fast as the cycles of mains electricity, which are not noticed in a fluorescent strip light. Obviously we have no trouble in performing all our normal visual functions, even playing squash or ping-pong, in a visual world 'pulsed' at such a high frequency. If we may imagine bat brains as building up an image of the world analogous to our visual images, the pulse rate alone seems to suggest that the bat's echo image might be at least as detailed and 'continuous' as our visual image. Of course, there may be other reasons why it is not so detailed as our visual image. If bats are capable of boosting their sampling rates to 200 pulses per second, why don't they keep this up all the time? Since they evidently have a rate control 'knob' on their 'stroboscope', why don't they turn it permanently to maximum, thereby keeping their perception of the world at its most acute, all the time, to meet any emergency? One reason is that these high rates are suitable only for near targets. If a pulse
26 The Blind Watchmaker follows too hard on the heels of its predecessor it gets mixed up with the echo of its predecessor returning from a distant target.Even if this weren't so,there would probably be good economic reasons for not keeping up the maximum pulse rate all the time.It must be costly producing loud ultrasonic pulses,costly in energy,costly in wear and tear on voice and ears,perhaps costly in computer time.A brain that is processing 200 distinct echoes per second might not find surplus capacity for thinking about anything else.Even the ticking-over rate of about 10 pulses per second is probably quite costly,but much less so than the maximum rate of 200 per second.An individual bat that boosted its tickover rate would pay an additional price in energy,etc., which would not be justified by the increased sonar acuity.When the only moving object in the immediate vicinity is the bat itself,the apparent world is sufficiently similar in successive tenths of seconds that it need not be sampled more frequently than this.When the salient vicinity includes another moving object,particularly a flying insect twisting and turning and diving in a desperate attempt to shake off its pursuer,the extra benefit to the bat of increasing its sample rate more than justifies the increased cost.Of course,the considerations of cost and benefit in this paragraph are all surmise,but something like this almost certainly must be going on. The engineer who sets about designing an efficient sonar or radar device soon comes up against a problem resulting from the need to make the pulses extremely loud.They have to be loud because when a sound is broadcast its wavefront advances as an ever-expanding sphere. The intensity of the sound is distributed and,in a sense,'diluted'over the whole surface of the sphere.The surface area of any sphere is proportional to the radius squared.The intensity of the sound at any particular point on the sphere therefore decreases,not in proportion to the distance (the radius)but in proportion to the square of the distance from the sound source,as the wavefront advances and the sphere swells.This means that the sound gets quieter pretty fast,as it travels away from its source,in this case the bat. When this diluted sound hits an object,say a fly,it bounces off the fly.This reflected sound now,in its turn,radiates away from the fly in an expanding spherical wavefront.For the same reason as in the case of the original sound,it decays as the square of the distance from the fly. By the time the echo reaches the bat again,the decay in its intensity is proportional,not to the distance of the fly from the bat,not even to the square of that distance,but to something more like the square of the square-the fourth power,of the distance.This means that it is very very quiet indeed.The problem can be partially overcome if the bat
26 The Blind Watchmaker follows too hard on the heels of its predecessor it gets mixed up with the echo of its predecessor returning from a distant target. Even if this weren't so, there would probably be good economic reasons for not keeping up the maximum pulse rate all the time. It must be costly producing loud ultrasonic pulses, costly in energy, costly in wear and tear on voice and ears, perhaps costly in computer time. A brain that is processing 200 distinct echoes per second might not find surplus capacity for thinking about anything else. Even the ticking-over rate of about 10 pulses per second is probably quite costly, but much less so than the maximum rate of 200 per second. An individual bat that boosted its tickover rate would pay an additional price in energy, etc., which would not be justified by the increased sonar acuity. When the only moving object in the immediate vicinity is the bat itself, the apparent world is sufficiently similar in successive tenths of seconds that it need not be sampled more frequently than this. When the salient vicinity includes another moving object, particularly a flying insect twisting and turning and diving in a desperate attempt to shake off its pursuer, the extra benefit to the bat of increasing its sample rate more than justifies the increased cost. Of course, the considerations of cost and benefit in this paragraph are all surmise, but something like this almost certainly must be going on. The engineer who sets about designing an efficient sonar or radar device soon comes up against a problem resulting from the need to make the pulses extremely loud. They have to be loud because when a sound is broadcast its wavefront advances as an ever-expanding sphere. The intensity of the sound is distributed and, in a sense, 'diluted' over the whole surface of the sphere. The surface area of any sphere is proportional to the radius squared. The intensity of the sound at any particular point on the sphere therefore decreases, not in proportion to the distance (the radius) but in proportion to the square of the distance from the sound source, as the wavefront advances and the sphere swells. This means that the sound gets quieter pretty fast, as it travels away from its source, in this case the bat. When this diluted sound hits an object, say a fly, it bounces off the fly. This reflected sound now, in its turn, radiates away from the fly in an expanding spherical wavefront. For the same reason as in the case of the original sound, it decays as the square of the distance from the fly. By the time the echo reaches the bat again, the decay in its intensity is proportional, not to the distance of the fly from the bat, not even to the square of that distance, but to something more like the square of the square - the fourth power, of the distance. This means that it is very very quiet indeed. The problem can be partially overcome if the bat
Good design 27 beams the sound by means of the equivalent of a megaphone,but only if it already knows the direction of the target.In any case,if the bat is to receive any reasonable echo at all from a distant target,the out- going squeak as it leaves the bat must be very loud indeed,and the instrument that detects the echo,the ear,must be highly sensitive to very quiet sounds-the echoes.Bat cries,as we have seen,are indeed often very loud,and their ears are very sensitive. Now here is the problem that would strike the engineer trying to design a bat-like machine.If the microphone,or ear,is as sensitive as all that,it is in grave danger of being seriously damaged by its own enormously loud outgoing pulse of sound.It is no good trying to combat the problem by making the sounds quieter,for then the echoes would be too quiet to hear.And it is no good trying to combat that by making the microphone ('ear')more sensitive,since this would only make it more vulnerable to being damaged by the,albeit now slightly quieter,outgoing sounds!It is a dilemma inherent in the dramatic difference in intensity between outgoing sound and returning echo,a difference that is inexorably imposed by the laws of physics. What other solution might occur to the engineer?When an analogous problem struck the designers of radar in the Second World War,they hit upon a solution which they called 'send/receive'radar The radar signals were sent out in necessarily very powerful pulses, which might have damaged the highly sensitive aerials (American antennas')waiting for the faint returning echoes.The 'send/receive circuit temporarily disconnected the receiving aerial just before the outgoing pulse was about to be emitted,then switched the aerial on again in time to receive the echo. Bats developed 'send/receive'switching technology long long ago, probably millions of years before our ancestors came down from the trees.It works as follows.In bat ears,as in ours,sound is transmitted from the eardrum to the microphonic,sound-sensitive cells by means of a bridge of three tiny bones known (in Latin)as the hammer,the anvil and the stirrup,because of their shape.The mounting and hinging of these three bones,by the way,is exactly as a hi-fi engineer might have designed it to serve a necessary 'impedance-matching' function,but that is another story.What matters here is that some bats have well-developed muscles attached to the stirrup and to the hammer.When these muscles are contracted the bones don't transmit sound so efficiently it is as though you muted a microphone by jamming your thumb against the vibrating diaphragm.The bat is able to use these muscles to switch its ears off temporarily.The muscles contract immediately before the bat emits each outgoing pulse
Good design 27 beams the sound by means of the equivalent of a megaphone, but only if it already knows the direction of the target. In any case, if the bat is to receive any reasonable echo at all from a distant target, the outgoing squeak as it leaves the bat must be very loud indeed, and the instrument that detects the echo, the ear, must be highly sensitive to very quiet sounds - the echoes. Bat cries, as we have seen, are indeed often very loud, and their ears are very sensitive. Now here is the problem that would strike the engineer trying to design a bat-like machine. If the microphone, or ear, is as sensitive as all that, it is in grave danger of being seriously damaged by its own enormously loud outgoing pulse of sound. It is no good trying to combat the problem by making the sounds quieter, for then the echoes would be too quiet to hear. And it is no good trying to combat that by making the microphone ('ear') more sensitive, since this would only make it more vulnerable to being damaged by the, albeit now slightly quieter, outgoing sounds! It is a dilemma inherent in the dramatic difference in intensity between outgoing sound and returning echo, a difference that is inexorably imposed by the laws of physics. What other solution might occur to the engineer? When an analogous problem struck the designers of radar in the Second World War, they hit upon a solution which they called 'send/receive' radar. The radar signals were sent out in necessarily very powerful pulses, which might have damaged the highly sensitive aerials (American 'antennas') waiting for the faint returning echoes. The 'send/receive' circuit temporarily disconnected the receiving aerial just before the outgoing pulse was about to be emitted, then switched the aerial on again in time to receive the echo. Bats developed 'send/receive' switching technology long long ago, probably millions of years before our ancestors came down from the trees. It works as follows. In bat ears, as in ours, sound is transmitted from the eardrum to the microphonic, sound-sensitive cells by means of a bridge of three tiny bones known (in Latin) as the hammer, the anvil and the stirrup, because of their shape. The mounting and hinging of these three bones, by the way, is exactly as a hi-fi engineer might have designed it to serve a necessary 'impedance-matching' function, but that is another story. What matters here is that some bats have well-developed muscles attached to the stirrup and to the hammer. When these muscles are contracted the bones don't transmit sound so efficiently - it is as though you muted a microphone by jamming your thumb against the vibrating diaphragm. The bat is able to use these muscles to switch its ears off temporarily. The muscles contract immediately before the bat emits each outgoing pulse