I didn't expect everyone to be so interested in these topics, so I'll continue writing. I might keep using this title, and if there's nothing left to discuss, I might switch to Greek mythology. At one point, I watched almost 50 episodes in one go, and if I don't write about it now... I might forget everything.
The Multiple Universe, Alles möglich.
Actually, it’s a Multi-Multiverse, meaning there are many versions of this idea. I'll pick two that I'm more familiar with.
Many Worlds Interpretation, this is closest to my heart because it's another explanation of quantum mechanics.
At its core, quantum mechanics, if you ask me, I'd say, Reality is non-local. I know some emphasize that Reality does not exist, but I don't subscribe to that view. My perspective is that Reality exists, but you cannot pinpoint its existence at a specific fixed point.
For example, if my child's father buys a crate of beer, I know there are 20 bottles in total. If he sees a special offer, he might buy two crates, so if I get home a bit late from work, there would be a total of 40 (full + empty) bottles.
That is Reality, but in the micro world of quantum mechanics, it's entirely different.
Let's use the electron, a familiar example. The properties (energy) of an electron are described by a wave function, denoted by the Greek letter psi (ψ). The wave function can't tell you the exact position of an electron at any given moment, nor can it tell you where it will go, but it can calculate the probability of finding it at a particular position.
That's prediction. Observation, however, is when a subject (human) measures a particle at a fixed position at a specific moment. In other words, you reduce the probability of it appearing elsewhere to zero, so you need to update your wave function (Wave-Function Update).
This is the famous Copenhagen Interpretation Postulate, also known as the reduction/collapse of the wave function. The core theory is that once you measure, you have more information, so you need to update your wave function; otherwise, your probabilities would be wrong (the wave function is essentially a probability description).
I want to emphasize that it's not the observation that changes the physical properties. I don't agree with that interpretation (I'll explain why if this doesn't get too lengthy).
Quantum mechanics' instantaneous and non-local nature is due to two reasons:
- A measurement in one place provides information about the measurement outcome in another place.
- It has no variables that can transport the information locally.
Is this hard to understand? Updating the wave function is just to align with our human understanding of the macro physical world. This update, referred to as Spooky Action by Einstein, is very intriguing.
Interestingly, this explanation aligns perfectly with the Many Worlds (a version of the Multiverse) interpretation.
Many Worlds is quite self-explanatory. You see yourself, but because of Many Worlds' existence, there must be many versions of you. The one you see is the one working hard (so pitiful), while another you in another universe might be having a romantic time with a mermaid.
Mathematically, these two interpretations are equivalent, which means:
Multiverse Interpretation = Copenhagen Interpretation
Ninety years ago, a famous paper titled "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" was published by A. Einstein, B. Podolsky, and N. Rosen (known as the EPR paper in the field). A is, of course, Albert; you can Google the other two if interested.
The collapse postulate is not the only hypothesis in quantum mechanics; there are many others (I personally accept this one). If I am not mistaken, another interpretation more widely spread in Chinese is the Update of the wave function as Bayesian update of knowledge, Quantum Bayesianism (QBism). This might be why some insist that observation injects energy into the system, thereby changing its state.
Derived from statistical Bayesian estimation?
Reminds me of a story.
When I was young and foolish (at least more foolish than now), I was discussing a problem with a German professor. He wanted to use Bayesian estimation, and I impatiently blurted out that Bayesian estimation was outdated.
Unexpectedly, a silent expert spoke up, saying, "I always use Bayesian estimation."
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There are, of course, other versions.
But the fundamental point is, if you want to dismiss someone's hypothesis by proposing your own, you're just replacing one hypothesis with another. This applies to both quantum mechanics and Many Worlds.
The Multiverse also includes another concept concerning the boundary of the universe.
The question is, is the universe finite or infinite? If the universe has a boundary, there must be something beyond it, right? What is that?
One theory suggests that our universe is boundless (this is acceptable).
However, the more academically accepted view is that our universe has no edge but does have a boundary. That boundary is the starting point of light, where light travels back to us at 300,000 kilometers per second. From a time perspective, the universe has existed for 13 billion years. From that point, light travels back to us, and that point is the boundary of the universe.
This entire time-space range is also called the observable zone (similar to the concept of a spotlight on stage).
I prefer to understand this boundary issue using the science comic I read in middle school. Imagine a sphere with a bug crawling on it; it can crawl endlessly without ever reaching an end, but the total space it covers is finite.
If the universe is infinitely large, then one thing is highly likely to happen (no matter how small the probability, anything possible may be real)—Many Worlds. And each person has at least one COPY in their parallel space.
This is a very philosophical hypothesis (which is why many physicists write philosophy books; I'll introduce one of my idols next time), the Infinite Monkey Theorem.
I won't explain this theory much; you can look it up if interested. It states that given infinite time or space, any event with a non-zero probability will eventually happen.
Back to our topic, Many Worlds will certainly appear.
Another support for the Multiverse is string theory (which has waned in popularity over the past decade), represented by Brian Greene. He wrote a book called "The Hidden Reality," which discusses these ideas. The core content suggests that different universes have different types of matter, adapting to their respective natural laws, with gravity differences leading to very different rates of radio decay.
The conclusion is that an observer in one universe cannot observe the world in another universe.
A more current popular view is that our universe was generated by the quantum fluctuations of a field called "inflation," which fills an infinitely large space. Our universe is just a small part of it, where fluctuations occurred. However, these fluctuations never stop, so countless other universes are born from them, continuously causing the eternal expansion of the universe.
Of course, these are all theories today. They might be proven or disproven someday, or maybe never.
But I think that’s okay.
陶三来上课（4）——Multiple-Universe (Multiverse)
Multiple Universe 啊，Alles möglich。
其实是Multi-Multiverse，也就是说，这件事有很多版本。我挑两个我知道的多一点的好了。
Many Wrolds版，这个对我最亲切，因为完全是量子力学的另外一个解释。
量子力学的最本质的本质，你问我的话，我会说，Reality non-local。我知道有一派强调的是Reality does not exist，我自己不是这样认为的。我的观点是，Reality存在，只是你不能在某一个固定的点确定它的存在。
比如，我们孩子爸爸买了一筐啤酒，我知道那总共是20瓶，如果他看到特价在再特价，常常会贪便宜买两筐，那么我知道如果我下班晚一点到家，会总共有40（满+空）瓶。
那个叫Reality，但是在微观世界量子领域，完全不是这样。
用大家最熟悉的电子举例好了。描述电子性质（能量）用的是波函数，希腊字母psi,
.。波函数不能告诉你电子在某一时刻具体在什么位置，也不能告诉你它会到哪一个位置去，但是可以用来计算它在某一位置能测量到的概率。以上是预测。而观测呢，是主体（人）在某一时刻测量到粒子在某一个固定位置，换句话说，你把它在其他位置出现的概率减小到零了，所以这时候，你要对你的波函数进行修正 （Wave-Function Update）。
这是著名的Copenhagen Interpretation Postulate，也叫reduction /collapse of the wave function。核心的理论是，一旦完成测量，那么你已经获得了更多的信息，因此需要对你的波函数进行修正，不然你的概率就错了啊（波函数实际上是概率描述）。
我想再强调的是，不是观测改变物理性质，我本人不同意那个解释（如果我看这次字数不多，大概下面会讲原因）。
量子力学的instaneous and non-local，原因有两个
1）A measurement in one place provides information about the measurement outcome in another place
2）It has no variables that can transport the information locally
再多解释一句，微观世界的粒子，按照理论它应该在左部和右部区域同时出现，而一旦观测确定，那么没有出现的那一部分概率必须修正为0也就是消失，观测到的部分概率修正为100%。
这很难理解吗？Update波函数只是为了符合人类对宏观物理世界的认知。而这个对波函数的Update, 被爱因斯坦称为，Spooky Action。
很玄妙的是，这个解释恰恰和Many Worlds（Multiverse的一个版本）的解释吻合。
Many Worlds从字面也可以很好理解。你看见的是你，但是因为Many Worlds的存在，那必然有很多个你，你看见的是在苦哈哈上班的那个（太可怜了），另一个时空的你，正在和Mermaidia浪漫。
仅从数学上，这两个解释也是相等的，即可以理解成
Multiverse Interpretation=Copenhagen Interpretation
90年前，有一篇著名的论文，作者是A. Einstein, B. Podolsky. N. Rossen (也被称为EPR paper提起来业界都知道的)，题目是，Can Quantum-Mechanical Description of Physical Really Be considered complete?
A当然是那个Albert， 后两个你有兴趣可以自己google。
Collapse postulate 的解释当然不是量子力学唯一的假设，其实还有很多种（我自己最接受这个而已）。看一下网上的讨论之后，如果我没猜错，大概另一个在中文里传的更广一点，Update of wave function be interpretated as Bayesian update of knowledge，Quantum Bayesiansum (QBism)。这应该是有人坚持的，观测会为系统注入能量进而改变状态的原因。
从统计学上的贝叶斯估计来的？
想起来个故事。
我年轻的时候，很傻（起码比现在傻）。和一个德国教授讨论问题，对方想用贝叶斯估计，我不耐烦没忍住说出来，贝叶斯估计早就不流行了。
没想到一直没吭声的专专家说话了，我一直是做贝叶斯估计啊。
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当然还有别的各种版本。
但是最根本的一点是，如果你想拿掉别人的（一个）假设说自己的对，也不过是补一个新的进去。这一点，对量子力学和Many Worlds都适用。
Multiverse还包含了另外一个概念，关于宇宙的边界问题。
原问题是，宇宙是有限的吗，还是无限的？如果宇宙有边界，那么边界之外应该还有什么吧，那是什么？
有一派的理论说，我们生存的宇宙的是无边无界的（这个没问题的）。
但是更被学术界接受的观点是，我们的宇宙没有边的，但是有一个界。那个界是光的起点，光从那里以每秒30万公里的速度返回到我们可观测距离。如果从时间的角度解释是，宇宙从诞生也已经有了130亿年，从那个时间点起，光回到我们这里，那个点就是宇宙的界。
整个这一时空范围，也被称为可观测区,observed zone（类似于舞台上聚光灯下那个概念吧）
其实我更倾向于拿我中学时候看的科普画报来理解这个边界问题，你可以想象有一个球体，一直瓢虫在上面爬，它爬啊爬的永远没有尽头，但是它爬的总的空间是有界的。
如果宇宙大到无边无界这个假设，那么有一件事很有可能发生（无论概率多小，anything that possible that maybe real）——Many Worlds。而且，每个人都（至少）有一个COPY在他的平行空间。
这是很有哲学味道的一个假设（所以很多物理学家写哲学书的，下次介绍一个我的偶像）,Infinite monkey theorem。
我不多解释这个理论有兴趣的可以自己去查，它是说，给定无限长时间或无限大的宇宙，任何具有非零发生概率的事件一定会发生。
回到我们的问题，就是Many Worlds一定还会出现。
另外一个支持Multiverse的是string theory（大概近10年热度已经下去了），代表人物是Brian Greene，他写了一本书，the Hidden Reality，就是讲这些。核心内容是说不同宇宙中有不同类型物质，适应各自自然规律，Gravity的不同导致Radio Decay很不一样。
结论是一个宇宙时空中的观测者不可能观测到另外一个宇宙时空中的世界。
而当下更流行的另外一个观点是, 我们的宇宙是由一种称为“膨胀”的场的量子涨落产生的，这个场填满了一个无限大的空间，我们的宇宙只是其中一小块，也就是发生fluctuation的地方。但是这个场的fluctuation是不会停止的，所以有无数其他的宇宙其中诞生，这个过程持续造成了宇宙的永恒膨胀。
这些当然今天都还仅仅是理论，也许有一天会被证实或者证伪，也许永远也不能。
但是我想那也没什么。
