I came back from a business trip this morning, and then I had a lot of things going on in the company again, and now I feel tired. So I can't update it today, so please be considerate.

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[Extended material: What does the theory of parallel worlds look like?] 】

The following is a hypothesis. Many scientists also believe that these questions are more philosophical than scientific. Of course, we cannot rule out that there may be experimental methods to test these hypotheses in the future. However, it is undeniable that with our current knowledge framework, these concepts can neither be proved nor falsified. So, no matter how many scientific terms appear in the answers below, remember that they are not scientific conclusions!

There are many versions of "Multiverse or parallel universe or alternative universe":

In general, they are all hypotheses: in addition to the universe we live in, there are many universes that are "on the same level" as ours.

Reference: Multiverse

https://./atch?v=aU7patpm9s...

There are several levels to this concept:

Level 1:

According to the general theory of relativity and the measured expansion and acceleration of the universe, there is a part of space-time that is far away from us because its relative speed exceeds the speed of light (*without violating the theory of relativity, no information and matter travels in space faster than the speed of light)--- we cannot observe this part of space-time, and even never have any causal relationship with it. The sum of the observable universe and the sum of the universe that can have a causal relationship with us are defined by the event horizon and the particle horizon, respectively. Answer:

If the light from the edge of the universe comes to Earth, then can we see how the universe was formed? - Loves the answer of the little smelly

Conjectures and questions after the Big Bang was faster than the speed of light? - Loves the answer of the little smelly

Based on the available observations, it is quite possible that the universe beyond the above conceptual part is infinite. Note the concept of "infinity", which means that no matter how small the probability of something happening, as long as the probability exists, it will happen---- because "infinite" can exhaust all possibilities. Therefore, far enough away from our observable universe there must be Hubble volumes that are close to our initial conditions, or even identical. Recall the concept of "infinity" again--- there must be one existence, there must be two existences---- there must be countless universes like us, and at the same time, there must be countless universes that are slightly different from our universe, very different.

It is important to note that at this level, since the entire space-time is continuous and one, the laws of physics that govern us also rule all these countless parallel universes. The theory of general relativity dictates that they cannot interact with each other.

Level 2:

According to the bubble universe model, a model provided by the Eternal inflation theory, the expansion of the universe is uneven everywhere: fast in some places, slow in others--- which causes some places to degenerate into a "true vacuum" – and some parts of space-time are stripped away. Note that in the first level, space-time is always continuous, and in this level, the space-time of such a "bubble universe" is discontinuous.

In different bubble sets, some elements may spontaneous symmetry breaking and produce different physical constants (speed of light, Planck's constant), resulting in different physical laws.

Level 3:

This hierarchy comes from the Copenhagen elaboration of quantum mechanics and is the most popular version of the "parallel universe". Answer:

Is the stochastic nature of events explained by chaos theory or quantum theory? - Loves the answer of the little smelly

Why can't I understand wave-particle duality, is it because I don't have enough IQ? - Loves the answer of the little smelly

The above two answers show the most counterintuitive understanding that quantum mechanics has brought me----- true randomness: the effect of no cause, and the observer effect: the power of consciousness.

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

Scientists don't like the "special": we are not the firstborn of God. So the implication of this hypothesis is to compensate for this "speciality". The core concept is that our observations do not cause the wave function to collapse to a single position in the true sense of the word: every position of the wave function accesses ---- what is possible happen, and our observations simply mean that we experience one of the infinite possibilities. These fundamental quantum events give rise to the infinite dimension of Hilbert space.

We found that although the concept is very different. But there seems to be no difference between the results of Level 1 and Level 3, and the core is that "what can happen will happen". In fact, the only difference between them is that, in the first class, we are connected to other parallel universe brothers by a finite three-dimensional space; In the third level, we live on a different quantum branch than the universe in the first level.

Similarly, spontaneous symmetry disruption in Level 2 is possible at "forks in the road".

From this point of view: in fact, the concept of parallel universes at these three levels can be unified.

Level 4:

This grade comes from Tegmark. The Theory of everything, which can describe the sum total of all "beings", must be detached from the vague terms and cognition of human beings, and can only be described by one language: abstract mathematics.

Tegmark argues that all versions of the universe are merely descriptions of mathematical structures. All grades can also be described in a mathematical structure. That's the final level! There are no later ranks.

Some scientists say: nonsense! What mathematical structure describes the universe, this concept is not good! shit, did I say something was missing?

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

"Hidden Reality" by string theorist Brian Green (in popular science, he seems to be better known for "The Strings of the Universe").

The book has been translated into Chinese. The theme is the parallel universe hypothesis that appears in various physics.

Or, if you still have the energy, try reading Max Tegmark's bizarre original paper

This includes the division and reasons for the four levels of parallel universes mentioned by the respondent upstairs

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

“ I hypothesize that only computable and decidable (in Godel's sense) structures exist”

Originally, he proposed that only computable (Turing-decidable) mathematical structures correspond to real physical existence, and that this "mathematical structure" is well-defined, and mathematical logic can talk about what computable mathematical structures (mathematical logic) are. Each computable mathematical structure corresponds, of course, to an infinite number of computer programs.

Of course, whether this is a physical reality or not is another matter.