I came back from a business trip this morning, and then the company had a lot of things. I feel very tired now. So I can't update today, so please understand.

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[Expand information: What is the theory of parallel worlds?]

The following content is hypothesis. Many scientists also believe that these problems have more philosophical attributes than scientific attributes. Of course, we cannot rule out that there may be experimental methods to verify these hypotheses in the future. However, it is undeniable that with our current knowledge framework, these concepts cannot be proved or falsified. Therefore, no matter how many scientific terms appear in the answer below, please remember that they are not scientific conclusions!

There are many versions of "parallel world" or "parallel universe":

In general, they are all hypotheses: In addition to the universe we live in, there are many universes that are "aligned" with our universe.

Reference: multive

https:/wath?v=auw7patpm9s

There are several levels of this concept:

Level 1:

According to general relativity and the calculated expansion speed and acceleration of the universe, some space-time and space-time exceed the speed of light due to its relative speed away from us (* does not violate the theory of relativity, and no information and matter propagate in space exceeds the speed of light) - we cannot observe this part of space-time, and even can never have any causal relationship with it. The sum of the observable universe and the sum of the universe that can have causal relationship with us are defined by the event horizon and the particle horizon respectively. Reference answer:

If light at the edge of the universe comes to the earth, can we see the formation process of the universe? - Ai Xiaochao's answer

The guesses and questions after the speed of the Big Bang exceeds the speed of light? - Ai Xiaozhao's answer

According to the existing observations, the universe that exceeds the above conceptual part is likely to be infinite. Pay attention to the concept of "infinity", infinity in time means: no matter how small the probability of an event happening, as long as the probability exists, it will definitely happen - because "infinity" can exhaust all possibilities. Therefore, outside our observable universe far enough, there will definitely be Hubble volumes that are similar to our initial conditions, or even exactly the same as ours. Recalling the concept of "infinity" - there is one existence, and there must be two existences - there must be countless universes that are the same as ours, and at the same time, there must be countless universes with slight differences and great differences from our universe.

It should be noted that at this level, since the entire time and space are continuous and integrated, the laws of physics that dominate us also dominate all these countless parallel universes. General relativity stipulates that they cannot interact.

Level 2:

According to a model provided by the expansion theory: the bubble univee model, the expansion of the universe everywhere is uneven: some places are fast and some places are slow - which leads to some places degenerating into "true vauum" - and some time and space are stripped away. Note that in the first level, time and space are always continuous, and in this level, the time and space of such a "bubble universe" are discontinuous.

In different sets of bubbles, some elements may undergo spontaneous symmetry breaking, resulting in different physical constants (speed of light, Planck constant), resulting in different physical laws.

Level 3:

This level comes from the Copenhagen explanation of quantum mechanics and is also the most popular version of "parallel universe". Reference answer:

Is the random performance of events explained by chaos theory or quantum theory? - Ai Xiaochao's answer

Why can't I understand the wave-particle duality? Is it because IQ is not enough? - Ai Xiaozhao's answer

The above two answers show that quantum mechanics brings me the most abnormal knowledge - true randomness: the result of no cause, the observer effect: the power of consciousness.

Observations lead to the collapse of the fundamental particle "wave function" - what makes our "observations" affect the behavior of the universe at such a basic level? Are we really that special?

Scientists do not like "special": we are not the eldest sons of God. So the meaning of this hypothesis is to compensate for this "special". The core concept is: our observations do not cause the wave function to collapse to a position in the true sense: every position of the wave function will be accessed - what an happens, will happen. Our observations just mean that we have experienced one of the countless possibilities. These basic quantum events produce Hilbert space with infinite dimensions.

We found that although the concepts are very different, there seems to be no difference in the results of Level 1 and Level 3, and the core is "what may happen will happen." This is true. The only difference between them is: in Level 1, we are connected with other parallel universe brothers by a finite three-dimensional space; in Level 3, we live on different quantum branches from the universe in the first level.

Similarly, spontaneous symmetry damage in Level 2 is also possible at a "fork in the road".

From this perspective: In fact, these three levels of parallel universe concepts can be unified.

Level 4:

This level comes from tegmark. Theory of everything can describe the sum of all "existence", which must be separated from the fuzzy terms and cognition of human beings. It can only be described by one language: abstract mathematics.

Tegmark believes that all versions of the universe are just a description of mathematical structures. All levels can also be described by a mathematical structure. This is the final level! There is no subsequent level.

Some scientists said: Nonsense! What kind of mathematical structure describes the universe, this concept is not a good idea! The only universe that can be simply described by mathematical structure is a computer program. oh shit, did I say something missing?

ps: There is also a reference book that you can read.

"Hidden Reality" is written by string theorist Brian Green (in terms of popular science, it seems that he is more famous for "The Strings of the Universe").

This book has a Chinese translation. The theme is the parallel universe hypothesis that appears in various physics.

Or, if you still have the energy, you can try to read the original work of the weird essay max tegmark

arxiv/abs/0704.0646

This includes the division and reasons for the four levels of parallel universe mentioned by the answerer above.

In addition, the so-called "what mathematical structure describes the universe, this concept is not a good idea! The only universe that can be simply described by mathematical structure is a computer program" has long been clearly stated by Tegmark itself:

“ i hypothesize that only putable and deidable (in god's sense) structures exist”

Originally, he proposed that only mathematical structures that can be calculated (Turing can determine) correspond to real physical existence, and this "mathematical structure" is a good decision. In mathematical logic, we can talk about what a mathematical structure (mathematical logi) is. Each mathematical structure that can be calculated certainly corresponds to an infinite number of computer programs.
Chapter completed!