In many movies. Aliens are depicted as a kind of hairless creature that have a pair of big eyes. Because of this, when we try to think of the image of aliens, an image of a creature with thin fingers, smooth skin, big eyes, and human shape may come into our minds. In addition, their size are believed to be small, about as tall as kids. The appearance of aliens only exists in our imagination because most people have never seen aliens. Although some people claim that they have met one, they have no convincing evidence or don't have any prove at all. Therefore, we can't describe the appearance of aliens precisely. Are they giants that are over three meters tall? Or are they dwarfs that is nearly as tall as kids? Fortunately, we can use science to roughly predict the size and the shape of aliens.
To briefly predict the size of aliens, we have to introduce the "scale law". With this law, we can predict what will happen if an animal is increased or decreased in size. Take an ape as an example. if you increase an ape's size by ten times, its weight will be increased by 10*10*10=1000 times.(because when your size grows, you grow in three dimension) In the meanwhile, the strength of bones and muscles of this ape also increase. The cross-sectional area of bones and muscles goes up by the square of the distance increased, that is, by 10*10=100 times.
It means that if an ape becomes 10 times bigger, it becomes 100 times stronger. But let me remind you, if an animal is increased in size by ten times, its weight will be increased in 1000 times, and the strength of bones and muscles only increases by 100 times. Which mean that if an animal is really big, it may have a hard time moving itself, and may crush its bones when it try to make a step. Therefore, we can assume that the size of intelligent creatures in other planet may not be many times bigger than humans on Earth. What about the small size aliens? Is that possible? Well, let the scale law gives us the answer. According the scale law, if you decrease an animal's size by ten times. its surface area decreases by 10*10=100 times. Since the heat emitted by the animals is decided by the amount of surface area, if an animal become 10 times smaller, its rate of heat lost will be 100 times smaller. However, the heat it can contain will also decreases because it become smaller. The amount of heat it can contain decreases by 1000 times. Since this animal's rate of heat lost decreases by 100 times and the heat it can contain decreases by 1000 times, its body temperature drops easily. Therefore, it have to constantly search for food, gaining enough energy to generate heat. Such an animal will spend most of time searching for food and are impossible to develop a civilization. Using this law, we can theorize that aliens in other planets would resemble us in size.
The prediction I made is based on an assumption that the environment, gravity of the planets that supports intelligent creatures resembles to those on Earth. If the gravity of a planet is weaker than that of Earth, there may be some intelligent lives that is large in size because their bones don't have to induce a lot of pressure. If s planet has higher temperatures, there may be some intelligent lives that are small in size because their lost of heat will be reduced.
The appearance of aliens is still a mystery because we can hardly find one. Nevertheless, we can still use some theories to make predictions. Maybe one day, people on Earth will have opportunities to interact with aliens.
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