One day, the cat caught the mouse in space. In order to solve the biggest opponent of his life once and for all, the cat decided to throw the mouse into the black hole.

It’s a giant black hole with weaker tidal forces, so the mouse avoids being pulled into noodles. Of course, the rat’s five-short stature also plays a role here. As a result, the mouse accelerated its fall toward the black hole. Finally, it passed through the black hole event horizon, the boundary of no return, and went straight to the singularity at the center of the black hole.

The cat watched with interest the mouse’s falling speed to the black hole was getting slower and slower, and the mouse’s figure was getting darker and darker. Eventually, the mouse disappeared on the surface of the black hole. The unsophisticated cat is not surprised, it knows that this is due to the huge gravitational pull of the black hole. Although it didn’t see the mouse crossing the horizon with its own eyes, it can be sure: the mouse can’t come back (do black holes really swallow objects? – Zhihu)

The cat returned home contentedly and lived a carefree life ever since. But after a while, it felt like life was starting to feel empty. Without the rat’s provocation, even the big dog didn’t bother to beat it. It started to regret throwing the mouse into the black hole.

Therefore, the cat lay beside the black hole every day, hoping to see the traces of mice from the surface of the black hole. Until one day, a peculiar-looking old man floated over in a peculiar-looking aircraft.

After listening to the cat’s story, the old man shook his head and said, “Don’t waste your efforts, you can’t get any information about mice from the black hole.”

“Why?” the cat asked puzzled.

“A black hole has only the characteristics of mass, charge, linear momentum, angular momentum, and position. As for what you’ve thrown into it in the past, that kind of information is long gone.”

“Is a black hole formed by a star the same as a black hole formed by a billion… elephants?” The cat couldn’t believe his ears.

“It’s true. The black hole thing,” the old man pondered for a while, then said, “It’s hairless.”

Thinking of her hairless appearance, the cat couldn’t help shivering. It took a long time to find out which old man had drifted away.

The disheartened cat plans to go home and continue to live a small life of eating and waiting for death. At this time, another old man with immortal style and bones came riding on the auspicious cloud.

With a glimmer of hope, the cat told the old man his story again. After listening, the old man smiled and said, “Actually, the information about the mouse is still there.”

“Where?” the cat asked in surprise and joy, “in the black hole?”

“Yes, inside the black hole, but also on the surface of the black hole.”

“…” The cat felt that his cat brain was not enough.

“It’s not your fault, everyone will be confused when they hear this. Let’s take your time.” The old man took out a small blackboard and began to explain.

“First of all, you have to believe that black holes don’t destroy information. Not only black holes, nothing in the universe can do that.”

“Why?” the cat asked puzzled. “I burn a book, so the information on the book is gone?”

“The information I am talking about here and the information recorded in the book are not the same thing.” The old man shook his head helplessly, wanting to continue explaining, but looking at the cat’s little head, he gave up, “But you can understand it this way.”

The old man continued: “Actually, the information in the book was transferred to other places during the burning process, such as heat, ash, gas and radiation, etc. If you can collect all these objects back, the information can be recovered. Of course you It can’t be done, but you should believe that the information in the book is always there, right?”

The cat nodded, indicating that it was acceptable. But then it asks: “Is this a mathematically proven physical theorem?”

The old man shook his head: “No, but in experiments and observations, it has never gone wrong. Most physicists believe that it must be true. I think it is the most basic theorem in physics, so I call it ‘physics’ Learn the negative first law’.”

The cat said suspiciously: “I have also heard of several laws of physics, but I have never heard of the law of ‘first minus one’.”

The old man smiled and said: “The smaller the number, the more basic it is. I think it is the foundation of the entire physics building. Calling it the first law or even the zeroth law does not show its importance. However, let’s go back to black holes and see Where is the mouse’s information hidden?”

The old man drew a dot on the blackboard and said, “This is a miniature black hole with a mass equivalent to a piece of dust. Now we put a little information in it, that is, throw a photon into it. At this time, the mass of the black hole increases, and it Both volume and surface area will increase.”

The cat said it was understandable. Then the old man continued: “If we continue to throw photons into it, the black hole will continue to grow. Of course, we don’t have to do this. Mathematics can help us calculate how the amount of information we put into the black hole has anything to do with the growth of the black hole. .”

The old man looked at the cat solemnly and said slowly: “The calculation result is: the information that a black hole can hold is proportional to the area of the event horizon of the black hole.”

A minute later, the cat picked up the chin that fell on the ground and shouted, “How is that possible? The information is inside the black hole. The information that the black hole can hold should be proportional to the volume of the black hole, right?”

The old man said blankly, “You can disbelieve me, but you can’t disbelieve in mathematics.”

Hearing the horrible word “math”, the cat gave in and chose to accept the result. It thought about it for a long time and said hesitantly: “Does this result imply that the information is on the surface of the black hole?”

The old man nodded and said, “It’s not a hint, I believe it is. Moreover, we can also calculate how much area each bit has on the surface of the black hole.”

“Does this count?” the cat asked in surprise, “How big is it?”

“The area of each bit is [the formula]. We call it the Planck region.”

Cats who are less sensitive to numbers don’t think about how small the area is. It suddenly thought of another question: “I heard that the black hole event horizon is just a sphere around the black hole, and there is no special place. According to you, it seems that there are a lot of things gathered there. And they are crowded together and stay at the event horizon. Nearby, but refuses to fall in. It’s hard to understand: what is storing that information?”

“This is a good question,” said the old man. “Current research is not enough to answer this question. However, string theory provides an answer, it’s up to you to accept it or not.”

After speaking, the old man drew a squiggly line on the blackboard, and then said: “All elementary particles, electrons, photons, etc., in fact, they are all energy that vibrates in different ways, which is what we call String. It’s a low-energy state string.”

The old man drew a longer, more twisted line and said, “If you give it energy, it becomes a string in a high-energy state, or, rather, a higher-energy elementary particle.”

After putting down the chalk, the old man concluded: “So, the surface of the black hole’s event horizon may be covered with various high-energy strings. They occupy all the area of the event horizon. The temperature outside the black hole is very high. Any object that passes through the event horizon will be melted. to ashes. Then leave its information on the surface of the event horizon.”

Hearing this, the cat realized what a terrible thing he had done, and broke out in a cold sweat. However, it quickly found a reason to refute: “No, I heard that the mass of the black hole is concentrated in the central singularity, and the event horizon is just a sphere on the periphery of the black hole, and there is nothing. The mouse should be able to pass through it unharmed. Vision?”

The old man smiled and said, “Yes, it’s true.”

“Then what you just said was turned to ashes by the horizon?”

The old man smiled and said, “This is also true.”

The cat was completely confused: “So are the mice distributed on the surface of the black hole or have they entered the black hole?”

The old man smiled and said: “From your point of view, the mouse turned to ashes at the event horizon, and its information remained on the surface of the black hole. From the mouse’s point of view, it passed through the event horizon unharmed and continued to fall toward the black hole singularity. .”

After a while, the old man continued: “So, the mouse exists in two places at the same time: 1) the three-dimensional mouse inside the black hole and 2) all the information of the mice on the surface of the black hole encoded in a two-dimensional manner.

Hearing that a two-dimensional plane can encode information about a three-dimensional object, the cat asked, “Is it like a hologram?”

The old man nodded approvingly and said, “It is similar to a holographic image. A holographic image encodes the information of a three-dimensional image on a two-dimensional plane, and then displays the three-dimensional image in a special way. You can directly look at a pair of two-dimensional images. The holographic image of 2D, certainly can’t see anything. The information on the surface of the black hole is the same, it is a chaotic code.”

The cat suddenly thought of a surprising fact: “If the surface of a black hole is similar to a holographic image, does it mean that the two-dimensional information on the surface of the black hole is real, and the three-dimensional mouse in the black hole is just some kind of projection?”

The old man said: “You can understand it like this, and you can also understand that they are all real. There is only one mouse, but its information exists in two places at the same time. These are two mathematical descriptions of a phenomenon, and neither is more real. .”

The cat felt that his intelligence could only understand this point, so he began to change the subject: “I heard that our observable universe also has an event horizon. Is the surface of that event horizon also covered with information about all the objects in the universe? ?”

The old man was a little surprised by the cat’s imagination. He explained: “If we generalize this theory, maybe we can make such an inference. However, the horizon of the observable universe is very different from the horizon of a black hole. The horizon of a black hole is a sphere from which not even light can escape; while The horizon of the observable universe is just the opposite. It is a spherical surface where light from the outside cannot reach us. So some people say that the observable universe is a black hole turned upside down.”

The cat felt like he had made an amazing discovery. It said excitedly: “So, the three-dimensional objects around us can also be regarded as the projection of two-dimensional information on the surface of the cosmic event horizon?”

The old man said noncommittally: “The research in this area has just begun, and it is still in a very preliminary stage, so a decisive judgment cannot be made yet. If it can be confirmed in the future, it is understandable for you to understand this.”

The cat thought for a while, and when he looked up again, the old man had disappeared. It struggles to recall what it has learned today. It felt as though it had heard a lot of bizarre talk, but couldn’t think of a single detail. The only thing that comes to mind in the end is that the mouse’s information still exists in the universe and may one day be slowly released as the black hole evaporates. Although there is no mouse in life, maybe you can still figure out how to play tricks on that big dog. Thinking of this, the cat went home contentedly.

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