Defending the "Multi-Universe" Theory

Our earth is located in a galaxy, which is located in a universe, which is located in a first-level multiverse full of incarnations, which is located in a more diverse second-level multiverse, which is located in a third-level multiverse of quantum mechanics, which is located in a fourth-level multiverse of mathematical structure.

The first parallel universe is simply a distant area, and light doesn't have enough time to catch up with us; The area covered by the second parallel universe is always out of reach because of the cosmological expansion of the space between us and them; The third parallel universe, Hugh Everett's Many Worlds, discusses the parts of Hilbert space in quantum mechanics that can't communicate. No matter the first, second or third parallel universes, they all follow the same basic mathematical formula (perhaps a standard "equation of everything", describing quantum mechanics, inflation and so on. ), and the fourth parallel universe is like a dancer shuttling in different modes, involving different "standard equations" and corresponding to different mathematical structures.

Personally, I think this is really beautiful, amazing, magnificent and inspiring. But is it true? Is it possible that this picture is just misleading us, and those glories are just a mirage? Do you really live in a multiverse? Is it possible that the whole question is stupid and beyond the scope of science? The multiverse has always been ignored by the authorities: 1600, Bruno was burned to death at the stake for insisting on the multiverse of infinite space; From 65438 to 0957, Hugh Everett and his quantum multiverse were "burned down" by the job market in physics. But in recent years, things have changed a lot. Parallel universes became popular, appearing in books, movies and even jokes: "You passed the exam in many parallel universes, but this one didn't."

The popularity of this idea has not made scientists understand * * *, but I think it has made the debate on the multiverse more subtle and interesting, because scientists no longer yell at each other, but sincerely try to understand different views from themselves. An anti-multiverse article recently published in Scientific American is an excellent example. We use "our universe" to indicate that the light emitted in 654.38+0.4 billion years after the Big Bang can reach our spherical region. When it comes to parallel universes, we use four levels to distinguish them: the first level is other similar areas in space, but they are very far away from us, where the physical laws are the same as ours, but the history of the universe is different because their initial States are different; The second layer is other areas in space, where the physical laws of the surface are different from ours; The third layer is other parallel worlds in Hilbert space where quantum reality plays a role; The fourth layer is a completely separated reality, which is dominated by different mathematical equations. In Ellis's criticism, he classified the arguments supporting the classification and thought that they were all problematic. I summarize his argument against the multiverse as follows:

Inflation may be wrong (or not eternal).

Quantum mechanics may or may not be wrong.

String theory may be wrong (or lack multiple solutions).

The multiverse theory may not be falsified.

Some evidence claiming to be a multiverse is suspicious.

Fine-tuning arguments may assume too many premises.

This is a landslide fallacy, which will slide into a larger multiverse.

What can we refute about this?

Let's start with the first four. Inflation will naturally produce the first layer of the multiverse; If you add the image of string theory and its possible solutions, you will get the second multiverse. The form of the third multiverse brought by quantum mechanics is the simplest in mathematics and will never collapse. Therefore, if these theories are excluded, the key evidence of the multiverse will collapse. Remember: parallel universes are not a theory, they are predictions of some theories.

I think it is very important that if a theory is scientific, all the results of studying and discussing it are reasonable, even if they involve unobservable entities. For a theory with falsifiability, we don't need to be able to observe and test all its predictions, we just need to test at least one of them. Therefore, my answer to the above argument 4 is that our mathematical theories are testable in science, but their inferences do not need to meet the requirements of testability, even if they are not testable. For example, because Einstein's general theory of relativity successfully predicted many observable things, we also believe its prediction of those unobservable things, such as what happened inside a black hole. Similarly, if inflation or quantum mechanics have made impressive and correct predictions at present, we should treat their other predictions, including the first and third multiverses, with the same attitude. Ellis even mentioned that the theory of eternal expansion may be ruled out one day; And I think this just proves that eternal expansion is a scientific theory.

At present, string theory has not developed to a perfect stage like inflation theory and quantum mechanics, nor has it become a testable scientific theory. However, I doubt that even if string theory doesn't succeed in the end, we can't get rid of the second multiverse. It is common for a mathematical equation to have multiple solutions. As long as there are multiple solutions to the basic equations describing reality, the huge space created by eternal expansion will realize these solutions one by one. For example, the governing equation of water molecules has nothing to do with chord theory. It can get three different solutions, corresponding to gaseous, liquid and solid ice respectively. Similarly, if there are different phases in space itself, inflation will be realized one by one.

Ellis cited some suspicious observation data, which claimed to support the multiverse theory. For example, some evidence claims to prove that some constants in nature are not always constants, and some evidence from cosmic microwave background radiation claims to prove that the universe collided with other universes or strangely connected spaces. I totally agree with his doubts about the evidence. But in these examples, the controversial part is the analysis of the data, just like the cold nuclear fusion scam of 1989. In my opinion, scientists' measurement and debate on the details of data further prove that this belongs to the category of science, which is the difference between scientific debate and non-scientific debate.

Fine-tuning means that our universe seems to have made amazing fine-tuning for the emergence of life, because if you change the constants of our nature even a little, the life forms we know will cease to exist. Why is this? If there is a second multiverse in which the "constant" can take any possible value, it is not surprising that we are in a rare and livable universe, just as it is not surprising that we were born on the earth instead of Mercury or Neptune. Ellis opposes this, because he doesn't think it is necessary to assume a multiverse theory as the premise to reach this conclusion, but this is the way we test any scientific theory: we assume it is true, calculate the result, and if the prediction doesn't match the observation, we give up the theory. Some fine-tuning seems to be enough. For example, in order to generate livable galaxies, the number of dark energy bits that need to be fine-tuned is as high as 123. In my opinion, this unexplained coincidence may indicate that there is still a gap in scientific understanding. If someone belittles such a coincidence and says, "We are just lucky and don't look for an explanation", it is not only unsatisfactory, but also may overlook a very crucial clue.

Ellis believes that if we really believe that all possible things will happen, then we are like stepping on a landslide, and below it is a larger multiverse, such as the fourth multiverse. The fourth multiverse is my favorite layer, so, as one of the few loyal supporters of this layer, I will be happy to sit on this landslide and slide all the way down! Ellis also mentioned that the multiverse may violate Occam's razor principle because it introduces too much unnecessary complexity. As a theoretical physicist, when I evaluate the elegance and simplicity of a theory, I don't look at the theory itself, but at its mathematical equations-to my surprise, the simplest theory in mathematics always tends to produce multiple universes. Facts have proved that it is quite difficult to write down a theory that will only produce the universe we see and nothing else.

Finally, there is an anti-multiverse view, which I suggest Ellis try to avoid, but I think this is the general view of most people, that is, parallel universes look too strange to be true. However, as we discussed in the introduction, this is exactly what we should expect-evolution has given us intuition, but this intuition can only focus on the daily affairs that are of survival value to our ancient ancestors, so every time we use technology to spy on the real category outside the human scale, the evolved intuition will collapse. This happens again and again before our eyes, including counterintuitive relativity and quantum mechanics. Therefore, we should also expect that the ultimate theory of physics (whatever it is in the end) should make people feel weird.