FANTASTIC BEASTS AND HOW TO DISPROVE THEM
4.1 HIC SUNT DRACONES
One of the chief hallmarks of a fantasy story is the presence of otherworldly creatures. This is true of science fiction as well. In science fiction they are called aliens rather than monsters, but in numerous works the two are treated in a very similar manner. The troll at the bridge judges whether the party is worthy to cross or whether he must eat them; the aliens from Alpha Centauri judge humanity and decide whether to destroy us or reward us with advanced technology.
The chief difference, at least between “hard” science fiction—that based on known scientific principles—and fantasy, is that aliens in science fiction need to “work” somehow; that is, their biology must function along known and understood scientific lines. This does not mean they must be dissimilar to monsters or other creatures in a fantasy period. For example, in Robert Heinlein’s novel Starman Jones, the aliens attacking the human settlers on an unknown world look very much like centaurs, hybrids between humans and horses [122]. Another Heinlein novel, Glory Road, features beasts that are very much like dragons (i.e., large firebreathing lizards) except they cannot fly [116]. Of course, this novel has very strong fantastic elements. There are countless examples from other stories in which alien creatures are based on mythological or fantastic animals.
This raises an interesting question: which fantasy animals are possible ones and which aren’t? That is, if we took a census of fantastic beasts from various fantasy novels, which could we then place in a science fiction story without having to bend the rules too much? Here’s a very small list of fantastic creatures from the Harry Potter novels (not, of course, unique to them): centaurs, giants, unicorns, giant spiders (acromantula), hippogriffs, and dragons. Which of these are possible beasts and which are impossible? To answer the question we need to look at the issue of scaling and how it affects the properties of animals and their metabolism.
4.2 HOW TO BUILD A GIANT
The recent movie Avatar is set on the world of Pandora, a moon of a gas giant circling one of the stars in the Alpha Centauri system. The surface gravity on Pandora is about 80% Earth normal; the Pandorans are portrayed as large, willowy beings standing about eight feet tall and possessed of great physical strength compared to humans. Is this accurate? Given the facts above, how large should a Pandoran be?
There is a lot to this question, and many aspects of biology enter into the answer. However, the field of biomechanics can guide us in a general consideration of this problem. If we want to build a giant, how do we go about it? Can we simply take a human and, say, double him in all dimensions? Or is it more complicated than that?
In the early 1920s, J. B. S. Haldane, the great mathematical biologist, wrote the fascinating essay “On Being the Right Size” [106]. In one part of it he examined whether giants such as Giant Pagan from The Pilgrim’s Progress could stand. This is an interesting point: if we look at organisms from the small (say, insects) to the largest land animals, such as elephants, we realize their shapes are not particularly similar but do change in a systematic way. The legs supporting insects are very narrow and spindly compared to their body size, while the legs supporting an elephant or other giant creature are very thick compared to its body. Also, there are no creatures that stand on two legs, even for short periods of time, that are larger than humans. This is an aspect of a famous law originally formulated by Galileo and usually referred to as the square-cube law: the weight an animal must support is proportional to the cube of the size of the animal, but the supporting structure is proportional only to the square of the size. There are a lot of ramifications to this law, and it is not the only scaling law that applies to biology, so we will go through it slowly and in detail.
Let’s model an upright biped (human, Pandoran, or giant) as a large load standing on two pillars. This will do for a static model; what happens when the thing walks I’ll defer until later. How much weight can be put on the pillars before they collapse? As we will see, this determines the overall structure of the biped.
Let’s model a human as a cylinder (torso and head) standing on two cylindrical pillars (the legs). We’re ignoring the arms and head, but I’m assuming that their weight is not significant compared to the weight of the torso. I’m also going to ignore the weight of the legs. It’s an oversimplified model but should be good to start us out. I’m going to assume that the length of the torso (L) is the same as the length of the legs, and that the radius of the cylinder is about one-sixth the length, meaning that the circumference of the cylinder is very roughly equal to its length; this seems reasonable, given average belt lengths. This is a very rough sketch of a human being, obviously.
The volume of the torso is given by
and the mass is M = ρV, where ρ is the average density of the human body, very roughly equal to the density of water (1,000 kg/m3). This seems a reasonable assumption for the overall shape of a human torso: a simple calculation of the mass for a human whose overall height 2L = 1.8 m gives a torso mass of 60 kg, or 130 lb, which seems about right.
What we are interested in here is the width of the support pillars, the legs. The big question is one of scaling: as we increase the mass from that a normal man to that of a giant, will the overall shape stay the same? That is, will the width of the legs with respect to their length stay the same, or should we make them thicker, as Haldane implied? And can we estimate what the maximum height for such a giant could be, both on Earth and on other planets (such as Pandora), with different values of g?
There is one other question one can ask: why did we make the assumptions that we did, that is, that the overall leg length is equal to torso length, and that the torso diameter is proportional to torso length? We’ll address that below as well.
4.2.1 Biological Scaling
The principle of allometric scaling is an extremely important one in biomechanics. Allometry is any relation that can be written between two biological variables in the form of a power law:
Quite often the independent variable is the mass, mainly because it is extremely easy to measure. For example, let’s consider the relation between the mass and the length of the “cylinder torso” we modeled above. We can model the dependence of the length of the cylinder on its mass as
We expect a relation of this sort between body length and mass, as mass is proportional to volume and volume has dimensions of (length)3. Another way to put this is that if we take two objects of the same shape but different overall scale, the ratio of their volumes is the cube of the scaling factor: as the size doubles, the volume and mass increase by a factor of 23, or 8.
This relation holds exactly only for objects that are the same shape. Animals, of course, come in all shapes and sizes. However, studies relating animal mass to overall length show that over a very wide range of sizes and species, the law that overall length is proportional to the cube root of mass holds relatively well. The surface area is related to the mass by the scaling parameter
A ∝ M0.67,
again, consistent with geometrical scaling.
Back to our giant. A column such as a leg bone will buckle if the weight it supports exceeds a critical value. The great and prolific mathematician Leonhard Euler was the first person to investigate this problem, which is consequently referred to as Euler buckling: a column with a circular cross section of radius r and length L will buckle if the weight atop it exceeds a critical value, given by
Here, E is the elastic modulus of the material, which is a measure of how “stretchy” or “bendy” the material is. I am going to assume that the leg proportions are set by this relationship: animal support-bone lengths and widths will be set by the criterion that they need to be able to support the weight on top of them safely and without buckling. Instead of solving the problem exactly, I’ll discuss some scaling issues here. Since the weight is proportional to the volume, which scales as L3, we can write out a proportionality,
Solving this,
r ∝ L5/4.
Another way to put this is that the ratio of radius to length, r/L, is proportional to the fourth root of the length L1/4. As the being increases in size, the width of the legs will grow out of proportion to the rest of the structure. Giants should appear squat compared to normal human beings because they need thicker supports in relation to their height to hold up their weight. One can see that this is true by analogy with quadrupeds: elephants don’t look like scaled-up horses; their legs seem a lot thicker relative to body size than a horse’s legs. Grawp the giant, first introduced in Harry Potter and the Order of the Phoenix, is described as being 16 feet tall, or about three times the height of a normal human, if we round up [204]. His legs should therefore appear 31/4 = 1.3 times thicker than a human’s in relation to their length. To be blunt, this calculation surprised me, as I would have expected the legs to be much thicker. Of course, this model is simple-minded in that I have assumed that the length of the leg is simply proportional to the overall body length, which is questionable.
There’s an interesting supporting piece of evidence for this model. Bone mass is proportional to bone volume, which scales as r2L ∝ L14/4 ∝ M1.17. That is, the bone mass (at least for leg bones) should increase more rapidly than the overall mass of the creature. This is in fact measured: bone mass increases as M1.08. This is less than our estimate indicates, but perhaps not too far out in left field. This may simply indicate a failure of geometrical scaling for our overly simplified model of a biped.
One often sees the assertion that giant humanoids (at least on Earth) are impossible because of square-cube arguments, that is, because the weight increases so much more rapidly than the surface area supporting it. A recent paper in Physics Education states (in discussing the maximum height reached by an animal):
What would be the minimum required thickness of the bones of animals? To answer this question, let us examine the resistance of the bones and the weight-bearing limbs. Compressed solid materials may undergo a maximum tension, Tbr, before breaking. [70].
The authors then go on to state that because of this rule, bone diameter should scale as bone length to the 3/2 power. This is simply untrue: the scaling law quoted above for bone mass implies that bone diameter scales at a lower power (5/4 = 1.2) with bone length, and again, studies of bone mass in relation to body mass bear out a lower scaling power. However, there may be a nugget of truth in the statement. The model we have put together assumes that the scaling between bone length and bone diameter is caused by the need to avoid Euler buckling of the structure. At low masses, the force needed to cause the structure to buckle is less than the force needed to break the bones in compression, but at some critical value of the height/mass ratio, this criterion is reversed. This is worked out in detail in one of the problems on the book’s website (press.princeton.edu/titles/10070.html).
We can do a few other interesting calculations. For example, the walking speed of a biped is set by the pendular frequency of the swing of its legs. Detailed arguments can be found in Steven Vogel’s textbook, Comparative Biomechanics, but essentially, the period of a pendulum is proportional to 
, where L is the length of the pendulum and g is the acceleration of gravity [243]. Viewing the moving leg as a swinging pendulum of length L, the stride length will be proportional to L, so the speed will be proportional to 
. If we assume that a typical human walking speed is about 4 mph (about 1.8 m/s) and that L is about 1 m, we arrive at the formula
A few things to note:
• Larger animals have greater walking speeds than smaller ones. Interestingly, the time it takes to make a stride increases with size, but this is more than compensated for by the increased length of each stride.
• Higher-gravity planets will have faster walkers because of the factor g in the equation.
Grawp, being three times the size of a human, should be able to move 70% faster than one, although the movements will seem labored because of the time it takes for each stride.
Now, back to the Pandorans: they are giants compared to humans. The males’ average height is 3 m, or roughly 10 feet. By comparison, a typical human height of 6 feet is about 1.9 m. Their planet has only 80% of the surface gravity of Earth. Because of this, human walking speed on Pandora should be about 10% less than on Earth because of the lower gravity. This should be noticeable, but the movie doesn’t show people walking more slowly. However, Pandorans on Pandora should move 30% faster than humans because of the size difference. One should note that these results are dictated by simple physics and by the fact that Pandorans seem to walk the same way humans do.
Are these scaling laws correct? One must be a bit careful when applying them. The results should be taken with a grain of salt, for several reasons:
• First, these considerations are dictated by general physical principles, and may not be valid under local, specific conditions.
• Second, technically speaking, these scaling laws should apply only when comparing different species with similar shapes. That is, we might expect these laws to apply when comparing humans to (hypothetical) giants (i.e., two different species of bipeds), but not when comparing humans to horses or horses to eagles. However, no two species are ever exactly scaled versions of one another, so the results above have to be taken with a grain of salt.
• Finally, all of these scaling laws have to be assessed against actual experimental data, which can be tricky. It is hard to measure these scaling parameters with precision; usually they are quoted with error bars of about 10% or more.
Actual scaling parameters from studies done on real animals indicate that the arguments given above are within the bounds of possibility, but not proven. There is simply too much smear in the actual data. In particular, body surface area scales with mass to a power somewhere between 0.63 and 0.67. The higher exponent is what one would expect if geometric scaling were the only issue involved, and we did not need to worry about the support forces on the structure.
4.3 KLEIBER’S LAW, PART 1: MERMAIDS
“The whale is not a fish, you know—it’s an INSECT.”
—MONTY PYTHON: THE SECRET POLICEMAN’S BALL
4.3.1 Mermaids Aren’t Fish!
I have made perhaps one of the most important discoveries in the field of cryptozoology in the last century: mermaids aren’t fish. Perhaps I should say “merpeople” instead.… This actually seems a fairly obvious conclusion, at least when one looks at almost all depictions of mermaids in popular culture. The big clue is that merpeople, in almost any depiction you see them in, have horizontal flukes. This means they are actually oceangoing mammals, like whales, and not fish at all. Perhaps I should explain.
Herman Melville is best known for his massive fantasy novel, Moby-Dick. There are no mermaids in the story, but boy are there plenty of descriptions of whales. In the chapter “Cetology” he enters into a long discussion of whether the whale is a fish:
To be short, then, a whale is A SPOUTING FISH WITH A HORIZONTAL TAIL. There you have him.… A walrus spouts much like a whale, but the walrus is not a fish because he is amphibious. But the last term of the definition is still more cogent, as coupled with the first. Almost any one must have noticed that all the fish familiar to landsmen have not a flat, but a vertical, or up-and-down tail. Whereas, among spouting fish the tail, though it may be similarly shaped, invariably assumes a horizontal position.
Melville places whales among the fishes, but he cites a number of reasons why biologists don’t: the whale has lungs, not gills; the whale lactates and gives birth to its young, rather than laying eggs; and the whale spouts. And, as in almost all depictions of mermaids, it has a horizontal tail (or fluke), whereas all true fish have vertical tails. This implies a vastly different method of swimming for them: sharks, for example, swim by wriggling their fins and bodies back and forth, in a motion almost impossible to emulate by a swimming person. A whale, on the other hand, moves its tail up and down, which can easily be done by any swimming human. This is because the whale is a seagoing mammal. Millions of years ago the ancestors of the whale left the land and went back into the oceans. Their hind legs fused together to form the tail, which kept its skeletal structure more or less intact, meaning that the joints in it are hinged for up-and-down motion. Indeed, the cartoonist Chuck Jones, when trying to teach animators how to animate seals swimming, took one of his grandsons and tied his forearms to his torso and his two legs together, then put flippers on hands and feet and tossed him into the pool [131]. The natural motions the boy made in keeping afloat were very similar to how seals and all other sea mammals swim.
It seems clear from these arguments that merpeople must be a species of hominids that left the land thousands or perhaps millions of years ago and adapted to the waters. This seems even clearer when we consider other…ahem… “interactions” that sailors have claimed to have had with these creatures. The best story detailing a liaison between a human and a mermaid is “The Professor and the Mermaid” by Giuseppe di Lampedusa, author of the novel The Leopard. In the story, the eponymous professor meets a mermaid while a graduate student studying the classics; the mermaid Lighea is described as a combination of goddess and animal, and the aging professor is implied to have returned to her by jumping off a ship at the end of the story. Other similar stories exist, the best known being “The Little Mermaid” by Hans Christian Andersen, in which a mermaid falls in love with a handsome sailor. This doesn’t seem too plausible if she were some species of fish. In the Disney version of the story, Ariel is just a girl with a tail, though a tail with clearly horizontal flukes.
Interestingly enough, the mermaids in the Harry Potter stories appear to be fish. In the book Harry Potter and the Goblet of Fire, they are described as having gills; in the movie, although it is a little hard to tell, careful examination shows that they have vertical tails that move back and forth rather than up and down [203]. It makes one suspect there are two different unrelated species out there that, by chance or convergent evolution, look remarkably similar. The picture of the mermaid in the Prefect’s bathroom in the same film shows a much more hominid-looking mermaid, complete with horizontal flukes.
4.3.2 Kleiber’s Law, Metabolic Rate, and Lung Capacity
If merpeople are really seagoing mammals, how long can they stay underwater? All seagoing mammals, such as whales and dolphins, must come up for air from time to time. Let’s try to calculate this by making two assumptions:
• The metabolic rate of any animal is proportional to the rate at which the animal can breathe in air (technically, the mass or [equivalently] volume flow rate.
• The metabolic rate is proportional to M3/4.
The first is easy to understand: metabolic processes need oxygen to proceed. The metabolic rate, or the rate at which energy is burned by the body, must be proportional to the amount of air taken in, divided by the time between breaths. The second assumption is known in biology as Kleiber’s law, after the biologist who first formulated the rule. There is a good deal of evidence to support this rule over a very large range of body masses, although there is some debate over the exact scaling exponent [243, pp. 62–63]. To the best of my knowledge the fundamental reason for Kleiber’s law is unknown.
The metabolic rate is proportional to lung volume multiplied by the average rate at which the animal breathes:
where M is the average metabolic rate, VL is lung volume, and R is the average number of breaths the animal takes per second. One tricky part: when whales breach the surface, after they blow water out their blowholes, they typically take a large number of breaths before diving again. This allows them to stay underwater longer.
Studies show, unsurprisingly, that lung volume is proportional to body volume, that is, mass; assuming this, and Kleiber’s law, we find that, according to Walter Stahl [224],
As the animal gets larger, the number of breaths per second it needs to take gets smaller, which explains why whales can stay underwater for long periods of time. Experimental data support this relationship. Stahl’s paper gives the formula [224]:
for the average respiration rate as a function of mass (in kg). This formula gives a respiration rate of 17 per minute for an 80 kg adult male, which seems right: one breath every three seconds or so. Because of the water’s buoyancy, merpeople could be significantly more massive than people. If we assume a mass five times that of a human, or 300 kg, the respiration rate works out to be 12 per minute. That is, respiration rate scales only slowly with body mass. We therefore expect merpeople to stay under the surface of the water only about 40% longer than the average human (if my assumptions about mass are correct). Adaptations to the water may enable them to stay underwater for longer times, just as long as they hyperventilate (for want of a better word) when they surface. However, there seems to be no way they can stay underwater for the extended periods of time indicated in movies and fairy tales. An analogy: U-boats were German submarines of World War II. They were small craft with diesel engines and could remain submerged for only a few hours at a time. Today’s nuclear submarines are huge craft and can stay submerged for weeks at a time. Merpeople are to whales as U-boats are to today’s nuclear submarines.
4.4 KLEIBER’S LAW, PART 2: OWLS, DRAGONS, HIPPOGRIFFS, AND OTHER FLYING BEASTS
What is your name? What is your quest? What is the airspeed velocity of an unladen swallow?
—OLD MAN, MONTY PYTHON AND THE HOLY GRAIL
To quote J. B. S. Haldane:
It is an elementary principle of aeronautics that the minimum speed needed to keep an aeroplane of a given shape in the air varies as the square root of its length. If its linear dimensions are increased four times, it must fly twice as fast. Now the power needed for the minimum speed increases more rapidly than the weight of the machine. [106, p.18]
To explore this point in detail, let’s consider air flowing over a wing with area A at speed v; this can represent any object flying at speed v through still air. Because of the shape of the wing, a lift force is generated upward on the wing (fig. 4.1). The lift is due to the fact that air is deflected downward by the wing, implying a force that pushes the air downward. By Newton’s third law, there must be an equal but oppositely directed force pushing up on the wing. The expression for this force is
where ρ is the density of the fluid the wing is moving through (1 km/m3 for air), A is the area of the wing surface, and CL is a dimensionless lift coefficient that is usually about equal to 1. It varies with the shape of the wing and the speed, but not enormously. We can take it as 1 for our purposes here. In steady flight, neither rising nor descending, the lift must be equal to the weight. We can equate the two expressions and solve for the speed required:
where M is the mass of the creature or plane and g is the acceleration of gravity. The speed therefore depends on the ratio of the mass to the surface area of the creature. If L represents some measure of the length of the beast, then 
. Bigger beasts must fly faster, proportional to the square root of body length. A Roc, a bird larger than an elephant, would have a flight speed of more than 100 mph just to keep it aloft.
These formulas can be used for science fiction stories as well. For example, in Childhood’s End, the Overlords are a race of winged humanoids who live on an artificially low-gravity world [52]. They are somewhat larger than humans, about 7 feet tall, and presumably their mass follows the same scaling laws. A human visiting their world wasn’t too discomfited by the their world’s acceleration of gravity. Let’s assume that it’s about 80% of the acceleration of gravity on Earth. I estimate that their flight speed would need to be about 35 m/s (76 mph) to stay in the air, but could they fly at all? This is the question for the next section.
4.4.1 Metabolic Rates and Flying
The biggest problem with flight in large animals is that the metabolic rate for a large animal cannot be great enough to sustain the power needs for flying. This is indirectly related to the speed needed for flight: higher speeds are needed for larger animals, but higher speed means higher power requirements.
The power requirement for flying can be found from the resistive forces acting on the animal as it flies. The drag force acting on the flying animal is
This looks almost identical to the formula for lift force except that CD is a different coefficient, typically about 0.1 for streamlined fliers, and A′ is the effective projected forward area for the animal. This is a dissipative force, so power must be provided to keep the flier moving forward. This is an estimate; in particular, it may overestimate the power needs for larger fliers, which mostly fly by soaring, using thermals for their lift. However, it is a useful rule of thumb.
The power requirement can be found by using the formula P = F v. So the power requirement for flying is given by
We are interested in how this scales as we increase the size or mass of the flier: in this formula, the two parameters of interest are the effective surface area (A′) and speed (v). As seen above, v ∝ L1/2; surface area scales as L2, so:
A delightful book everyone should read is Bird Flight Performance: A Practical Calculation Manual by C. J. Pennycuick [191]. It is concerned with the computer modeling of avian flight. In it are formulas for estimating almost any quantity one would wish for when modeling flying creatures, on this world or any other. It is the only book I know that has a picture of a goose mounted in a wind tunnel [191, p. 51, fig. 5.6]. On p. 101 the author also mentions that metabolic rate scales as M3/4, while flying power scales, in principle, as M7/6, the derivation I gave above [191, p. 101]. However, his calculations are more detailed than mine, and he includes a graph of the ratio of flying power to (basal) metabolic rate derived from his computer simulations. If my very simple estimate were correct, the ratio should increase as mass increases:
In fact, when plotted on a log-log graph, the slope is 0.35, reasonably close to our estimate. Whatever the true scaling, it is clear that the metabolic demands of flight increase more rapidly than metabolic rate as mass increases. This is the real reason why no massive biological fliers exist; Pennycuick estimates an upper mass limit between 15 and 20 kg for birds, which is close to the average mass of the California condor, the largest North American bird. The power required for flying is just too high for larger animals.
This puts the kibosh on a large number of mythological beasts: flying dragons, hippogriffs, wyverns, and the like seem ruled out on the grounds that their metabolism simply can’t handle the power needed to fly. Perhaps this is why it is Mr. Weasley’s fondest desire, in Harry Potter and the Half-Blood Prince, to learn what keeps airplanes up [205]. This also seems to imply that flying humanoid aliens such as the Overlords are impossible. While human-powered flight in ultralight aircraft or hang gliders is possible, the wingspans required (of order 5 m or so) dwarf the size of the people being carried, and also require elaborate means of getting up in the air: being powered by bicycle train gears, or launched from hilltops or from behind airplanes or something similar.
Poul Anderson thought up a good workaround to these problems in his stories concerning the Ythrians. These were flying intelligent aliens whose total mass was about 20–30 kg. To get around the mass restrictions, he postulated a very high metabolic rate for them: they were extreme carnivores with a special added digestive system to make their metabolism more efficient. With all of this, there were still issues with how they adapted to their environment: the population on their original home world was low because of the need for families to be separated by about 20 miles so that they could have enough hunting grounds to satisfy their enormous hunger. Before humans came to their world, they were limited to a Bronze Age culture because of their low population, which effectively forbade the growth of cities or even animal husbandry [23].1
When I was a child, I had a coffee-table book called, I think, Dragons! The authors tried to provide a rational explanation for dragon “flight” by postulating that dragons were large blimps. Dragons, the book said, generated large amounts of … ahem … methane and other gases in their digestive tracts, giving them lift. They combusted the methane when breathing fire. This is ingenious, but I have no idea whether it is possible. There may be some reason that this mechanism is fundamentally impossible, as I can’t think of any animals that actually use it. I leave it as an exercise for the reader to work out the details.
One of the most pervasive aspects of life in the Harry Potter universe is the delivery of mail by owls, to the point that it is mocked by a character in Lev Grossman’s book The Magicians [103]. Ignoring the issue of whether owls can be trained to deliver mail, it’s worth considering what the upper mass limit for an owl parcel is (delivered by a single owl, that is).2
Anyone who has read T. H. White’s The Once and Future King knows that owls eat mice, and that an owl can carry mice in its claws or beak [251]. So the mass of a mouse (call it 50 grams) is a lower bound. Estimating the greatest mass they can carry from first principles is probably an exercise in futility, so let’s assume that the mass of the largest prey they hunt represents an upper bound. Since Hedwig is a snowy owl, let’s consider them snowy owls. These large owls, with masses up to about 3 kg, will occasionally hunt hares, with mass up to perhaps 1 kg. So perhaps owl post isn’t a crazy idea.
What about speed? In Harry Potter and the Goblet of Fire, Harry’s owl Hedwig delivers messages to the fugitive Sirius Black [203]. Black is implied to be hiding out on a tropical beach somewhere. I’m going to assume that his beach hideaway is at least 1,000 km away from chilly England. Owl flight speed is somewhere around 5 m/s, or 18 km/hr [191]. I’ll round up to 20 km/hr to make the math easy: to fly 1,000 km would take about 50 hours’ total flight time. Assuming that the owl flew eight hours each day, this implies a total of six days’ flying time to reach Black and six days to return. This is actually pretty consistent with times given in the books for exchanges of letters, so a point goes to Ms. Rowling for realism.
4.4.3 God Makes Power, but Man Makes Engines
The theme of this chapter is that animal behavior is limited by power generation. Humans as animals are capable of creating a few hundred watts of power, only a fraction of which is useful for work. Societies that run on human and animal power, common in fantasy settings, are extremely limited in scope compared to industrialized societies. Perhaps the great distinction between fantasy and science fiction is in the nature of the societies they portray. In fantasy, magic is a substitute for technology, a means of controlling “great powers” without the need for machines. In science fiction, men work through machines to control great powers.
NOTES
1. Anderson was a master at examining how biology and physics influence and limit science fiction. He was also very knowledgeable about history and sociology, all of which play a strong role in his stories. His essay “How to Build a Planet” was a direct inspiration for this book.
2. As an aside, why do characters in the Harry Potter novels write with quill pens and ink on parchment? Try it; it’s not easy. Cutting a nib from a feather is highly skilled work, and writing with one is not much less so. Quill pens are three complete revolutions behind current technology, maybe more. And parchment is an expensive substitute for paper.