THE LONG AND THE SHORT OF IT
“And there were giants in those days…” – Genesis 6:4
There is a simple principle of physics that has changed the way I look at the universe. It has also prevented me from being taken in by a huge array of common legends and misconceptions. Anyone who has ever played with blocks as a child has been exposed to it, but most people don’t grasp the ramifications. Personally, I need to thank Stephen Jay Gould for writing the article that turned on the light bulb for me. I am referring to the principle of scale. Let me begin with an illustration.
Imagine that you have a set of children’s blocks. They are cubes of one inch per side, and, to make things easy, they each weigh one pound. Now, draw yourself a chart with three columns – length, surface area, and weight (mass). With one block in front of you, it is easy to see that the length is one inch, the surface area is six square inches, and the mass is one pound.
Now, double the length without changing any of the proportions. You now have a cube that is two inches long. But, the surface area is now 24 square inches and the mass is eight pounds. Double it again. Here is the result:
Length.........Surface Area...............Mass
1 Inch ..........6 Square Inches.......... 1 Pound
2 Inches.... 24 Square Inches......... 8 Pounds
4 Inches.... 96 Square Inches........ 64 Pounds
This is important. Don’t proceed until you understand and can visualize these numbers.
About this time, you are asking what I’m smoking to make me think this is important. Well, notice how the numbers change. If you double the length, the surface area increases four times (or 2 squared), and the mass increases by eight times (or 2 cubed). This holds true regardless of the shape of the object or its composition. Surface area increases as the square of the length, and mass increases as the cube of the length. When you think about this, it makes perfect sense. Length is one dimensional, surface area is two dimensional, and mass is three dimensional. And, if I can add another wrinkle, in some cases, kinetic energy increases 32 times faster than the length (that’s 2 to the fifth power).
So, what does this have to do with anything? Well, let’s work through an example. Many years ago, I saw a remake of the movie Attack of the 50 Foot Woman. It wasn’t particularly terrifying. Being attacked by Daryl Hannah is not my idea of horror.
Imagine that our original woman was five feet tall and weighed one hundred pounds (round figures make everything easier). I am told that the average human has 6.5 square feet of skin. If we stretch her to 50 feet, she would now have 650 square feet of skin, and she would weigh 100,000 pounds.
The human body requires a certain amount of surface area to function. We cool ourselves by radiating heat through our skin. We absorb oxygen through the surface area of our lung sacs. Our normal woman had a weight to skin ratio of around 15 pounds to 1 square foot. The huge woman would have a ratio of 150 to 1. Poor Daryl would have been gasping for breath, and seriously overheating. But, that wouldn’t be her only problem. Bone strength is a function of the area of the bone cross section. It is proportional to area. But, her ankle bones (now with 100 times greater cross section) would be expected to carry a disproportionately heavy mass (1000 times heavier). It would be like trying to walk with nine additional people on her back. In all likelihood, she would never have been able to stand up. And, if she did, her heel bones would probably have squished their way through the flesh of her feet. That kind of spoils the effect of the movie.
It’s no surprise that all large animals have similar proportions. Elephants, mammoths, and dinosaurs all have powerful legs, and aren’t exactly known for their leaping ability. Huge creatures live in a world dominated by the forces of gravity.
But, things actually get worse as you get smaller. Imagine that we now shrink Daryl to the size of a fly (around a quarter of an inch). To aid in visualizing this, let’s substitute Rick Moranis for Daryl. We begin with a five foot, 100 pound human, and shrink him down to ¼ inch – a factor of 240. Rick’s weight to skin ratio drops from 15 to 1, down to 0.06 to 1. Instead of having 15 pounds of mass for every square foot of skin, he now has one ounce of mass per square foot of surface area. Insects live in a world where the forces of gravity are negligible. Their world is dominated by the forces of friction.
Go to the wall, and try to climb it, using only your bare hands and feet. You will find that you do not have anywhere near enough adhesion to pull your weight. But, imagine that the surface area of your hands and feet was 240 times larger, without increasing your weight. Your ability to climb walls would increase dramatically. So, forget about spider man clinging to the side of a building. The only way he could do it would be to dramatically deform his body. I bet Kirsten Dunst wouldn’t kiss him then, even if he was upside down.
This also explains why insects can easily walk on water. Their proportionally huge surface area is more than enough to take advantage of enough surface tension to carry their tiny weight.
Finally, kinetic energy also fluctuates with size. When a child falls on its butt, it rarely gets hurt, because it only hits the ground with 1/32 of the energy of an adult. Likewise, Goliath could likely knock your head off, assuming that he’s not in the hospital on life support.
Each living organism has evolved to function in its own world. Insects don’t require a strong internal skeleton like humans. Neither do they have lungs. They breath through their thoracic and abdominal spiracles. Insects can’t function at human size, because they don’t have the infrastructure.
Similarly, humans are optimized for a particular size. If they deviate too much, they are in for a host of problems. The tallest recorded human was Robert Wadlow of Illinois, who reached a height of 8 feet 11 inches. He died at age 22. Someday, someone may grow even taller. But, he won’t live long.
And, huge animals are built with a heavy duty chassis. Anything less would fail to hold them up.
This principle destroys many of our beloved children’s characters, as well as a host of folk wisdom. Just take a look at some of the casualties:
Jack and the Beanstalk – The giant would be crumpled in a corner, gasping for breath.
Goliath – David could just jog away. Goliath would never be able to keep up. But, it wouldn’t matter anyway, since Goliath would have long since expired from organ failure.
The Fly – Upon emerging from his transmutation chamber, Jeff Goldblum’s exoskeleton would immediately collapse under his weight, and his oxygen starved body would quickly die.
Honey, I shrunk the kids – In addition to an array of physical problems, Rick Moranis’s brain would not contain enough neurons to provide any form of intelligence.
Snow White and the Seven Dwarfs – Because of their short arm length, dwarfs make spectacularly poor miners.
The 50 Foot Woman – This one has already been covered.
In addition to these characters, there is a plethora of credulous folk anecdotes, that were hoisted on me to prove unusual and unrelated points. I was always told that the wonders of the animal kingdom declare the glory of God’s creation. Behold, the mighty ant, that can carry several times its weight on its back. The bee is a marvel of aeronautics – even today scientists cannot determine why it can fly. The cat can jump from 10 times it’s height without being hurt. None of these things are anywhere near as amazing as I was lead to believe.
When I first learned this simple principle, I was astounded at how easy it was to dismantle common misconceptions. I was also aghast at how gullible the average person can be, when the refutation is so simple.
I was particularly disappointed by how easily the bible was taken in. In Deuteronomy 3:11, it tells us about Og, king of the giants, who was so tall that he required a bed nine cubits long (that’s 13.5 feet). You would think that God, the greatest physicist of them all, would have been able to fabricate a story with a little more authenticity than Jack and the Beanstalk.