Thoughts on the "pseudo-scientific proposition" of "why do people look upside down and not upside down in planar imaging"

Why are people in flat imaging upside down rather than upside down? This problem was once again raised in Tsinghua, and once again ranked first among the top 10 topics.

This is a classic topic of joke. In my memory, there have been no less than three times of this issue, and countless watertrucks have been generous enough to argue over it. From another perspective, this is also a serious scientific topic, because most people will inevitably have doubts when they first come into contact with this question. If you don't think about it carefully, you may not be able to get the correct answer.

The answers of water trucks are varied and representative:

1. People's eyes are left and right, rather than up and down.
2. If the human body's upper and lower body is as symmetrical as the left and right trunk, the upper and lower bodies are reversed.
3. Isn't the mirror upside down when it's under your feet?
4. If you look in the mirror, isn't it "upside down?
5 ....

These answers are incorrect and may be misleading. Some water trucks have also provided the correct answer. Of course, when I write this articleArticleAt that time, the so-called true or false can only be determined according to my personal understanding, but I believe this determination is correct ,:)

The correct answer I think is: this is not a physical topic or a biological topic (the two are involved, but not the root cause), but a mathematical topic. To be exact, is the definition of the coordinate system.

Before answering this question, we must first understand what is "Up and down ". In the process of understanding the world, in order to locate the direction, humans unconsciously establish a three-dimensional coordinate system with themselves as the reference system. In this coordinate system, the orientation of the human face is the positive Z axis, and the orientation of the back is the Negative Z axis, respectively representing the orientation "front" and "back"; and the human trunk (from the brow to the navel) is y axis, the header and the foot are Y axis positive and negative, representing the "up" and "down" of the azimuth, while the direction parallel to the two shoulders is the X axis, then follow the "Right Hand System" rule (here it seems like circular arguments can be defined in other ways, but it will be too tedious to describe it) to determine the positive and negative directions of the X axis, representing the "Left" and "right" directions respectively ". Of course, we can also position this coordinate system as the "left-hand system", but then there must be an axis that will be reversed. Selecting "Right Hand" or "Left Hand" does not affect our subsequent discussions.

Why does humans adopt such a coordinate system? This does have a certain relationship with people's physical characteristics, because the human body is similar to axial symmetry, and the similarity between people's left and right is far greater than the upper and lower similarity. Let's make an interesting assumption here. Assume that a person is a symmetric animal in the center. The human body is like a wheel, the human eye is at the center of the circle, and the walking style of a person is rolling, so in your and my eyes, do people have the upper, lower, and lower points?

This coordinate system also hides another fact: in our real world, the upper and lower, front, and back definitions are absolute (or first defined ), the left and right definitions are relative (or subsequently defined ), the left and right definitions depend on the top, bottom, front, and back definitions (of course, the selection of the left and right hand systems ). For example, in the figure below, can we determine whether B is on the left or right of? No. When we define the s side of a as Front (Front), B is on the left of A. If S is the back of a (back ), then B is on the right of.

With the above preparations, let's take a look at the mirror symmetry. If we define the normal line perpendicular to the mirror as the Z axis, according to the physics knowledge, the direction of the X axis and Y axis in the mirror symmetry remains the same, while the direction of the Z axis is the opposite, the inverse of the Z axis will inevitably lead to the mutual conversion between the right hand and the left hand. At this point, it is not difficult for us to understand the problems in the question. When we say that the mirror looks like the left and right sides are upside down, we are still recognizing the things in the mirror based on the "Right Hand" rules in reality.

The above explains why it is "upside down" and does not seem to explain why it is not "upside down ". This is the previous definition of left and right. If a circular object is parallel to a mirror, can we tell which part of it is reversed? No. If we do not create a coordinate system, we cannot specify the orientation. The reason why we say it is "Inverted right" rather than "upside down" is that we have defined "Up and down" and then "Left and Right". Otherwise, if we set "right and right" first ", so we can also think that it is up and down, but the left and right are not reversed.

I hope I have explained this question clearly ,:). Let's look back at why the previous answer is incorrect.

1. "People's eyes are distributed between left and right, rather than up and down ."

This problem has nothing to do with human eye structure, but is related to the axial symmetry characteristics of people. Even if the human eyes are distributed up and down, even if a person has only one eye and has an axial symmetry of the human body, we will still build a coordinate system like this, and there will still be conclusions in the question.

2. "If the human body's upper and lower body is as symmetrical as the left and right trunk, the upper and lower bodies are reversed ."

Human Body symmetry may only lead to different ways for humans to establish a coordinate system, which is not directly related to "Left and Right" and "Up and down.

3. "isn't the mirror upside down when it's under your feet ?"

This sentence has no question, but it does not answer the question. Because the mirror symmetry is to reverse the normal direction of the mirror. When a person is parallel to the mirror, will the person's front and back still be reversed?

4. "If you look in the mirror, isn't it" upside down ?"

Here we have stolen the concept. The "Up and down" in the question is for the human body's own coordinate system, and the "Up and down" here is for the spatial coordinate system: the gravity direction is "down ", the anti-gravity direction is "up ".

In fact, some seemingly simple things in our daily life actually hide deep learning. If you have time to think about it, it is quite interesting. For example, I searched the internet for the definitions of "Left" and "right" and found that someone was thinking about this problem:
Http://www.blogcn.com/user58/kongzizhizi/index.html