The Door 1.1

The following contains spoilers on a fictional work.

In Westworld season 2, last episode, when a person/host X passed through “the door”, he got copied, almost perfectly, into a virtual world. Since the door was adjacent to a cliff, just after passing through it, the original copy (the physical body) fell off the cliff and then died.

Did X still exist after passing through the door?

Existence or non-existence of X is not a property of X itself. So in order for the question “does X exist” to be meaningful, we have to specify “with respect to whom”.

In other words, instead of “does X exist”, we should ask

With respect to the observer Y, does X exist?


There are 3 categories of possible observers (who were observing X passing through the door):

  1. the original person (X1)
    X_1 == X

  2. the copied person (X2) in the virtual world
    For simplicity, assume that X2 is a perfect copy of X.

  3. other people (Y)

— Me@2019-02-09 1:09 PM



2019.02.28 Thursday (c) All rights reserved by ACHK

(反對)開夜車 2.3

Ken Chan 時光機
















  • 第一,你要在未有飛蚊時,防範於未然。

  • 第二,萬一仍然出現飛蚊的話,要立刻看眼科醫生,讓他為你檢查,是否視網膜開始脫落。

    • 如果不是,就不用擔心。

    • 如果是,可以立刻做手術,制止視網膜脫落,保住完整的視力。視網膜開始脫落,而不立刻做手術制止之,有致盲的風險。







— Me@2019-02-25 04:01:37 PM



2019.02.26 Tuesday (c) All rights reserved by ACHK










作者:雅士培 Karl Jaspers















知識論 —— 思

形上學 —— 死

倫理學 —— 生

— Me@2002-01-10

— Me@2019-02-20



2019.02.20 Wednesday (c) All rights reserved by ACHK

Path-distinguishing function, 2

\displaystyle{\gamma(t)} = configuration path function

\displaystyle{\mathcal{F} [\gamma]} = a function of time that measures some local property of the path

……….It may depend upon the value of the function \displaystyle{\gamma} at that time

……….and the value of any derivatives of \displaystyle{\gamma} at that time.


We can decompose \mathcal{F} [\gamma] into two parts:

1. a part that measures some property of a local description


2. a part that extracts a local description of the path from the path function.


— 1.3 The Principle of Stationary Action

— Structure and Interpretation of Classical Mechanics


1. The function that measures the local property of the system depends on the particular physical system;

2. the method of construction of a local description of a path from a path is the same for any system.


\displaystyle{ \begin{aligned}  \mathcal{F} [\gamma] &= \mathcal{L} \circ \mathcal{T}[\gamma] \\  \mathcal{T} [\gamma] &= (t, \gamma (t), \mathcal{D} \gamma (t), ...)  \end{aligned}}


— 1.3 The Principle of Stationary Action

— Structure and Interpretation of Classical Mechanics

— Me@2019-02-22 11:46:50 PM



2019.02.24 Sunday ACHK

Quantum decoherence 9


This is a file from the Wikimedia Commons.

In classical scattering of target body by environmental photons, the motion of the target body will not be changed by the scattered photons on the average. In quantum scattering, the interaction between the scattered photons and the superposed target body will cause them to be entangled, thereby delocalizing the phase coherence from the target body to the whole system, rendering the interference pattern unobservable.

The decohered elements of the system no longer exhibit quantum interference between each other, as in a double-slit experiment. Any elements that decohere from each other via environmental interactions are said to be quantum-entangled with the environment. The converse is not true: not all entangled states are decohered from each other.

— Wikipedia on Quantum decoherence



2019.02.22 Friday ACHK

PhD, 3.3

故事連線 | 碩士 4.3 | On Keeping Your Soul, 2.2.3 | Release early. Release often, 3.3

這段改編自 2010 年 4 月 18 日的對話。






在你不斷做研究的途中,自然會有一刻,累積的研究成果/文章,無論在質素和份量上,都等價甚至優於,一般的博士論文。那時,你就有機會,透過遞交研究成果,向大學申請參與,博士論文答辯試(thesis defense);從而有機會,「直接」獲得一個博士學位。


當年,哲學家維根斯坦(Ludwig Wittgenstein)和物理學家愛因斯坦,都是用這個方法,獲得博士學位的。







— Me@2019-02-21 03:06:38 PM



2019.02.21 Thursday (c) All rights reserved by ACHK

Path-distinguishing function

So we will try to arrange that the path-distinguishing function, constructed as an integral of a local property along the path, assumes a stationary value for any realizable path. Such a path-distinguishing function is traditionally called an action for the system. We use the word “action” to be consistent with common usage. Perhaps it would be clearer to continue to call it “path-distinguishing function,” but then it would be more difficult for others to know what we were talking about.

— 1.3 The Principle of Stationary Action

— Structure and Interpretation of Classical Mechanics



2019.02.17 Sunday ACHK

Logical arrow of time, 7

When we imagine that we know and keep track of all the exact information about the physical system – which, in practice, we can only do for small microscopic physical systems – the microscopic laws are time-reversal-symmetric (or at least CPT-symmetric) and we don’t see any arrow. There is a one-to-one unitary map between the states at times “t1” and “t2” and it doesn’t matter which of them is the past and which of them is the future.

A problem is that with this microscopic description where everything is exact, no thermodynamic concepts such as the entropy “emerge” at all. You might say that the entropy is zero if the pure state is exactly known all the time – at any rate, a definition of the entropy that would make it identically zero would be completely useless, too. By “entropy”, I never mean a quantity that is allowed to be zero for macroscopic systems at room temperature.

But whenever we deal with incomplete information, this one-to-one map inevitably disappears and the simple rules break down. Macroscopic laws of physics are irreversible. If friction brings your car to a halt and you wait for days, you won’t be able to say when the car stopped. The information disappears: it dissipates.

— The arrow of time: understood for 100 years

— Lubos Motl


If there is a god-view, there is no time arrow.

Time arrow only exists from a macroscopic point of view. Microscopically, there is no time arrow.

If there is a god-view that can observe all the pieces of the exact information, including the microscopic ones, there is no time arrow.

Also, if there is a god-view, there will be paradoxes, such as the black hole information paradox.

Black hole complementarity is a conjectured solution to the black hole information paradox, proposed by Leonard Susskind, Larus Thorlacius, and Gerard ‘t Hooft.

Leonard Susskind proposed a radical resolution to this problem by claiming that the information is both reflected at the event horizon and passes through the event horizon and cannot escape, with the catch being no observer can confirm both stories simultaneously.

— Wikipedia on Black hole complementarity

The spirit of black hole complementarity is that there is no god-view. Instead, physics is always about what an observer can observe.

— Me@2018-06-21 01:09:05 PM



2019.02.11 Monday (c) All rights reserved by ACHK


E: Can you just, you know, like, just tell me the answer?

J: Sorry?

E: You know, the answer. To everything.

What’s the point of love if it’s just disappear?

There has to be meaning to existence, otherwise the universe is made of pain and I don’t like the thought of that.

So, tell me the answer!

J: The more human I become, the less things make sense.

But that’s part of the fun. Right?

E: What do you mean?

J: If there were an answer I can give you to, how the universe works, it wouldn’t be special. It would be just a machinery fulfilling its cosmic design. It would be just a big, dumb food processor.

But, since nothing seems to make sense, when you find something or someone that does, it’s euphoria.


In all of this randomness, in this pandemonium, you and Chidi found each other and you had a life together.

Isn’t that remarkable?

E: Pandemonium is from Paradise Lost. Milton called the center of hell “pandemonium”, meaning “place of all demons”.

I guess all I can do is to embrace the Pandemonium.

Find happiness in the unique insanity of being here, now.

— The Good Place



2019.01.27 Sunday ACHK

(反對)開夜車 2.2

Ken Chan 時光機


The light that burns twice as bright burns half as long, …

— Blade Runner (1982)


我現在的記憶,暫時未能確定,他在課堂中,有沒有明示推介過,用這個方法。但是,我記得在他派發的筆記中,其中一頁,有一個正在深夜讀書的漫畫。而在漫畫下面,有一句「study to 3 a.m.」(讀書至深夜三點鐘)。





在 Ken Chan 的部分課堂,特別是假期的補課中,我就見到他時常咳嗽。可能,他在工作時代,仍然過著,極度忙碌的生活。


還記得在九月,在課程的第一課,他就已經遲到,遲了大概 15 至 30 分鐘之間。那課原定於早上八時半開始。上課中途,他有點咳嗽。他說,因為當天早上之前,吸入了太多的氮氣。他懷疑一位同事,作弄了他。

話說,他工作的地方,有一部儀器壞了。他的同事跟他說:「部機好似壞咗,漏緊啲 nitrogen 喎,你聞下係唔係?」(那儀器疑似壞了,正在洩漏氮氣。你聞一聞,看看是不是。)

Ken Chan 事後回心一想,才醒起:「Nitrogen 又點會有味咖?」(氮氣又何來會有味道呢?)




— Me@2019-02-03 07:02:42 AM



2019.02.03 Sunday (c) All rights reserved by ACHK



Structure and Interpretation of Classical Mechanics (SICM) is a classical mechanics textbook written by Gerald Jay Sussman and Jack Wisdom with Meinhard E. Mayer. The first edition was published by MIT Press in 2001, and [the] second edition was released in 2015. The book is used at the Massachusetts Institute of Technology to teach a class in advanced classical mechanics, starting with Lagrange’s equations and proceeding through canonical perturbation theory.

— Wikipedia on Structure and Interpretation of Classical Mechanics



2019.02.02 Saturday ACHK