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.
To explain further, in the micro world, a particle should theoretically appear in both the left and right regions simultaneously, but once observed, the probability of it not appearing must be reduced to zero, while the observed part's probability becomes 100%.
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.
This theory posits that each measurement can only occur in its own universe space, and we can only observe the one in our universe space, while the ones in other universe spaces are invisible to us (note, not non-existent). Isn't this akin to the collapse postulate of the wave function in the quantum world from the Copenhagen Interpretation?
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.
陶三來上課（4）——Multiple-Universe (Multiverse)
來源:
donau 於 2024-06-27 05:54:27
[檔案] [博客] [舊帖] [給我悄悄話] 本文已被閱讀： 5790 次 (11978 bytes)之前沒想到大家對這些話題感興趣，那我接著寫好了。題目大概我會一直用下去，如果沒得聊了那麽也許會轉到希臘神話裏的小毛片。當時一口氣看了差不多50集，再不寫出來......不知道轉身還是忘幹淨先發生。
Multiple Universe 啊，Alles möglich。
其實是Multi-Multiverse，也就是說，這件事有很多版本。我挑兩個我知道的多一點的好了。
Many Wrolds版，這個對我最親切，因為完全是量子力學的另外一個解釋。
量子力學的最本質的本質，你問我的話，我會說，Reality non-local。我知道有一派強調的是Reality does not exist，我自己不是這樣認為的。我的觀點是，Reality存在，隻是你不能在某一個固定的點確定它的存在。
比如，我們孩子爸爸買了一筐啤酒，我知道那總共是20瓶，如果他看到特價在再特價，常常會貪便宜買兩筐，那麽我知道如果我下班晚一點到家，會總共有40（滿+空）瓶。
那個叫Reality，但是在微觀世界量子領域，完全不是這樣。
用大家最熟悉的電子舉例好了。描述電子性質（能量）用的是波函數，希臘字母psi,
.。波函數不能告訴你電子在某一時刻具體在什麽位置，也不能告訴你它會到哪一個位置去，但是可以用來計算它在某一位置能測量到的概率。
以上是預測。而觀測呢，是主體（人）在某一時刻測量到粒子在某一個固定位置，換句話說，你把它在其他位置出現的概率減小到零了，所以這時候，你要對你的波函數進行修正 （Wave-Function Update）。
這是著名的Copenhagen Interpretation Postulate，也叫reduction /collapse of the wave function。核心的理論是，一旦完成測量，那麽你已經獲得了更多的信息，因此需要對你的波函數進行修正，不然你的概率就錯了啊（波函數實際上是概率描述）。
我想再強調的是，不是觀測改變物理性質，我本人不同意那個解釋（如果我看這次字數不多，大概下麵會講原因）。
量子力學的instaneous and non-local，原因有兩個
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
再多解釋一句，微觀世界的粒子，按照理論它應該在左部和右部區域同時出現，而一旦觀測確定，那麽沒有出現的那一部分概率必須修正為0也就是消失，觀測到的部分概率修正為100%。
這很難理解嗎？Update波函數隻是為了符合人類對宏觀物理世界的認知。而這個對波函數的Update, 被愛因斯坦稱為，Spooky Action。
很玄妙的是，這個解釋恰恰和Many Worlds（Multiverse的一個版本）的解釋吻合。
Many Worlds從字麵也可以很好理解。你看見的是你，但是因為Many Worlds的存在，那必然有很多個你，你看見的是在苦哈哈上班的那個（太可憐了），另一個時空的你，正在和Mermaidia浪漫。
這個理論的假設是，每一個測量隻能在它自己的宇宙空間出現，而我們也隻能觀測到在我們這個宇宙時空中出現的那個，出現在其他宇宙時空的，你是看不見的（注意，不是不存在）。對照前麵的Copenhagen Interpretation Postulate，不完全是量子世界對波函數的Collapse Postulate？
僅從數學上，這兩個解釋也是相等的，即可以理解成
Multiverse Interpretation=Copenhagen Interpretation
90年前，有一篇著名的論文，作者是A. Einstein, B. Podolsky. N. Rossen (也被稱為EPR paper提起來業界都知道的)，題目是，Can Quantum-Mechanical Description of Physical Really Be considered complete?
A當然是那個Albert， 後兩個你有興趣可以自己google。
Collapse postulate 的解釋當然不是量子力學唯一的假設，其實還有很多種（我自己最接受這個而已）。看一下網上的討論之後，如果我沒猜錯，大概另一個在中文裏傳的更廣一點，Update of wave function be interpretated as Bayesian update of knowledge，Quantum Bayesiansum (QBism)。這應該是有人堅持的，觀測會為係統注入能量進而改變狀態的原因。
從統計學上的貝葉斯估計來的？
想起來個故事。
我年輕的時候，很傻（起碼比現在傻）。和一個德國教授討論問題，對方想用貝葉斯估計，我不耐煩沒忍住說出來，貝葉斯估計早就不流行了。
沒想到一直沒吭聲的專專家說話了，我一直是做貝葉斯估計啊。
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當然還有別的各種版本。
但是最根本的一點是，如果你想拿掉別人的（一個）假設說自己的對，也不過是補一個新的進去。這一點，對量子力學和Many Worlds都適用。
Multiverse還包含了另外一個概念，關於宇宙的邊界問題。
原問題是，宇宙是有限的嗎，還是無限的？如果宇宙有邊界，那麽邊界之外應該還有什麽吧，那是什麽？
有一派的理論說，我們生存的宇宙的是無邊無界的（這個沒問題的）。
但是更被學術界接受的觀點是，我們的宇宙沒有邊的，但是有一個界。那個界是光的起點，光從那裏以每秒30萬公裏的速度返回到我們可觀測距離。如果從時間的角度解釋是，宇宙從誕生也已經有了130億年，從那個時間點起，光回到我們這裏，那個點就是宇宙的界。
整個這一時空範圍，也被稱為可觀測區,observed zone（類似於舞台上聚光燈下那個概念吧）
其實我更傾向於拿我中學時候看的科普畫報來理解這個邊界問題，你可以想象有一個球體，一直瓢蟲在上麵爬，它爬啊爬的永遠沒有盡頭，但是它爬的總的空間是有界的。
如果宇宙大到無邊無界這個假設，那麽有一件事很有可能發生（無論概率多小，anything that possible that maybe real）——Many Worlds。而且，每個人都（至少）有一個COPY在他的平行空間。
這是很有哲學味道的一個假設（所以很多物理學家寫哲學書的，下次介紹一個我的偶像）,Infinite monkey theorem。
我不多解釋這個理論有興趣的可以自己去查，它是說，給定無限長時間或無限大的宇宙，任何具有非零發生概率的事件一定會發生。
回到我們的問題，就是Many Worlds一定還會出現。
另外一個支持Multiverse的是string theory（大概近10年熱度已經下去了），代表人物是Brian Greene，他寫了一本書，the Hidden Reality，就是講這些。核心內容是說不同宇宙中有不同類型物質，適應各自自然規律，Gravity的不同導致Radio Decay很不一樣。
結論是一個宇宙時空中的觀測者不可能觀測到另外一個宇宙時空中的世界。
而當下更流行的另外一個觀點是, 我們的宇宙是由一種稱為“膨脹”的場的量子漲落產生的，這個場填滿了一個無限大的空間，我們的宇宙隻是其中一小塊，也就是發生fluctuation的地方。但是這個場的fluctuation是不會停止的，所以有無數其他的宇宙其中誕生，這個過程持續造成了宇宙的永恒膨脹。
這些當然今天都還僅僅是理論，也許有一天會被證實或者證偽，也許永遠也不能。
但是我想那也沒什麽。
