# If the earth is only one centimeter in diameter, how big is the Milky way? How big is the observable universe?

This paper takes part in the series essay competition of “great science” of Recordunkown.

After the big bang, countless celestial bodies and materials were born, and these materials all have sizes. Human beings live on the beautiful blue planet of the earth. Since they were born, they have begun to understand the world. Our understanding of the world is often based on the comparison of the size of objects and materials.

We all know that motion is relative, but in fact, the size of objects is also relative. If a grain of sand is compared with an apple, the apple is a giant. An apple is a huge object compared with a watermelon. But compared with the whole earth, watermelon can only be regarded as a grain of dust.

When we walk out of the earth and see the vast universe, we also understand that if the earth is compared with the Milky way, it is only a grain of dust, and the Milky way is also a grain of dust compared with the whole universe. At this time, if there are infinite things, there may be only two, one is human stupidity, the other is the vastness of the universe.

If we want to study and explore the size of galaxies and the size of the universe, it is difficult to intuitively feel the huge gap between them from the existing scale, and it is also difficult to really understand how incredible the vastness of the universe is. So someone put forward such a method to explore the universe: if the diameter of the earth is only 1 cm, how much is the Milky way and how big is the observable universe?

The diameter of the earth is about 12756 km, which is about 1.275 billion cm in centimeter. If the diameter is reduced to 1 cm, other celestial bodies and the universe will be reduced by more than 1 billion times. At this time, we can feel the wonderful contrast in the universe.

At this time, the diameter of the earth’s moon was only 0.27 cm, and Mars, the most explored by human beings, was only 0.53 cm. Jupiter, the largest planet in the solar system, is 318 times the diameter of the earth. When it is reduced to 11 cm in proportion to the earth, it is the size of two ping-pong balls. Of course, it’s not interesting to compare with these planets. Let’s compare with the sun. At this time, the huge sun in our eyes is only 110cm, which is the contrast in the solar system family.

In front of the whole universe, the solar system is just like dust. In the universe, there are many objects that are bigger than our imagination. For example, uy scutellus, which is about 45 million light-years away from the earth, is a red supergiant. Its diameter is about 1700 times that of the sun, and its volume is 4.5 billion times that of the sun. Such a huge star, when calculated according to the scale that the earth shrinks to one centimeter, has a diameter of 1900 meters. Compared with 1900 meters, the difference between one centimeter and 1900 meters is still very huge. In front of it, the earth is really as small as a grain of dust.

Is shield uy big? Of course not. Compared with the largest black hole in the universe, it’s like a grain of sand. At present, the largest black hole observed in the universe is SDSS j140821.67 + 025733.2. Its mass is about 196 billion times that of the sun. In our proportion, its diameter is about 920000 meters. The earth in front of it may not even count as dust.

After comparing these objects, let’s put the galaxy in our proportion and see how big it is. As we all know, the latest data of the diameter of the Milky way is about 200000 light-years. For this data, many people may only know that it is huge and vast, but they have no idea how huge it is. Now in our proportion, its diameter is about 400 million kilometers.

400 million kilometers is equivalent to the distance between the earth and Jupiter. According to this ratio, we will find that the Milky way is not so huge. Indeed, compared with the solar system and the earth, the Milky way is too big to imagine. The earth is not even dust in front of it. But in the whole universe, the Milky way is really nothing, it is still just a dust in the vast universe.

Looking at the whole universe, one might wonder how big the observable universe is in our proportion? Although we don’t know the specific size of the universe, there are general data about the observable universe of human beings, which is about 93 billion light years. If it is reduced to our proportion, it will be about 9.3 billion light years. This range is still an unimaginable huge number for us.

Through such scaling and comparison, we can more clearly recognize the smallness of the earth and the human beings. The vast universe of the earth may be as big as a tiny cell in an organism. It has been speculated that the universe may be a huge organism, and the various celestial bodies in the universe are all kinds of cells.

This analogy seems absurd, but after careful consideration, it is not totally unreasonable. If the universe is reduced to the size of human body, then our body is the whole universe, and those organs in the body may be the huge galaxy clusters, black holes and so on in the universe. Stars, Xingde and other celestial bodies are all kinds of smaller cells in the human body. Different cells correspond to different celestial bodies.

And the human body has about 80 trillion cells, which seems to mean that the number of celestial bodies in our universe may be similar to this number? The more we know about the universe, the more we fear, the more we divide ourselves, and finally discover the truth that we don’t want to accept. The great physicist Einstein burned his manuscripts in his later years before he died. Why did Einstein do this?

For this, people have a variety of conjectures, one of which is that Einstein may have found a trace of the truth of the universe in his later years of universe research, and this truth may not be good, once let mankind know, it may be afraid. That’s why the manuscript was burned, leaving mankind with an unknown bright future.

Sometimes, the unknown is a kind of happiness for human beings. If you know too much, it may not be a good thing. What is the ultimate mystery of the universe? Why do great scientists like to study theology in their later years? It may not be as simple as we think. Perhaps with the rapid development of human science and technology, one day in the future, we will find the truth of the universe. At that time, we don’t know whether it is human’s luck or misfortune.

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