# Pointer state, 2

Eigenstates 3.2

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Microscopically, a state can be definite or indefinite. Even if it is indefinite, the overlapping of superpositions of states of a lot of particles, or the superposition of a lot of system-microstates gives a definite macrostate.

If a state is definite, it is corresponding to one single system-macrostate directly.

I am referring to the physical definition, not the mathematical definition.

— Me@2012-12-31 09:28:08 AM

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If a microstate is definite, it is called an “eigenstate”. It is corresponding to one single system-macrostate directly.

However, the microstate is NOT the macrostate. The microstate is just corresponding to that macrostate.

— Me@2019-09-20 07:02:10 AM

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In quantum Darwinism and similar theories, pointer states are quantum states, sometimes of a measuring apparatus, if present, that are less perturbed by decoherence than other states, and are the quantum equivalents of the classical states of the system after decoherence has occurred through interaction with the environment. ‘Pointer’ refers to the reading of a recording or measuring device, which in old analog versions would often have a gauge or pointer display.

— Wikipedia on Pointer state

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In quantum mechanics, einselections, short for environment-induced superselection, is a name coined by Wojciech H. Zurek for a process which is claimed to explain the appearance of wavefunction collapse and the emergence of classical descriptions of reality from quantum descriptions.

In this approach, classicality is described as an emergent property induced in open quantum systems by their environments. Due to the interaction with the environment, the vast majority of states in the Hilbert space of a quantum open system become highly unstable due to entangling interaction with the environment, which in effect monitors selected observables of the system.

After a decoherence time, which for macroscopic objects is typically many orders of magnitude shorter than any other dynamical timescale, a generic quantum state decays into an uncertain [in the sense of classical probability] state which can be decomposed into a mixture of simple pointer states. In this way the environment induces effective superselection rules. Thus, einselection precludes stable existence of pure superpositions of pointer states. These ‘pointer states’ are stable despite environmental interaction. The einselected states lack coherence, and therefore do not exhibit the quantum behaviours of entanglement and superposition.

Advocates of this approach argue that since only quasi-local, essentially classical states survive the decoherence process, einselection can in many ways explain the emergence of a (seemingly) classical reality in a fundamentally quantum universe (at least to local observers). However, the basic program has been criticized as relying on a circular argument (e.g. R. E. Kastner). So the question of whether the ‘einselection’ account can really explain the phenomenon of wave function collapse remains unsettled.

— Wikipedia on Einselection

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Here I simply review the basic approach to ‘deriving’ einselection via decoherence, and point to a key step in the derivation that makes it a circular one.

— Ruth E. Kastner

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We should not derive einselection via decoherence. Instead, they should be regarded as different parts or different presentations of the same theory.

In other words, “einselection” and “decoherence” are synonyms.

— Me@2019-09-21 05:53:53 PM

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There has been significant work on correctly identifying the pointer states in the case of a massive particle decohered by collisions with a fluid environment, often known as collisional decoherence. In particular, Busse and Hornberger have identified certain solitonic wavepackets as being unusually stable in the presence of such decoherence.

— Wikipedia on Einselection

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# 無限旅程 3.2

The main use of doing academic works is to enlarge your personal world.

Just like travel: travel in itself is useless, unless you are going to live there.

— Me@2011.10.11

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… or the travel experience travels with you.

— Me@2019-09-15 10:53:15 PM

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# 財政自由 1.2

（問：那就即是話，要在還年青時，就賺到一生夠用的金錢？

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（問：但是，財政自由，又可以如何實現呢？）

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（問：依靠「售賣時間」而來的收入，就為之「主動收入」？

（問：有自動生成收入的話，就當然十分高興。但是，那些「自動收入」，又從何而來呢？

— Me@2019-09-10 08:33:38 PM

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# 事件實在論，更正

Event Realism | 事件實在論 6.1

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exist = can be found

— Me@2013.09.25

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If the consequences of an event cannot be found anymore, that event no longer exists.

— Me@2019.09.05

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The surprising implications of the original delayed-choice experiment led Wheeler to the conclusion that “no phenomenon is a phenomenon until it is an observed phenomenon”, which is a very radical position. Wheeler famously said that the “past has no existence except as recorded in the present“, and that the Universe does not “exist, out there independent of all acts of observation”.

— Wikipedia on Wheeler’s delayed choice experiment

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「事件」並不完全「實在」。

— Me@2019-09-05 09:08:41 PM

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# A Tale of Two L’s

Lagrange’s equations are traditionally written in the form

$\displaystyle{\frac{\mathrm{d}}{\mathrm{d}t} \left ( \frac {\partial L}{\partial \dot{q}} \right ) = \frac {\partial L}{\partial q}}$

or, if we write a separate equation for each component of $\displaystyle{q}$, as

$\displaystyle{\frac{\mathrm{d}}{\mathrm{d}t} \left ( \frac {\partial L}{\partial \dot{q^i}} \right ) = \frac {\partial L}{\partial q^i}}$

In this way of writing Lagrange’s equations the notation does not distinguish between $\displaystyle{L}$, which is a real-valued function of three variables $\displaystyle{(t, q, \dot q)}$, and $\displaystyle{L \circ \Gamma[q]}$, which is a real-valued function of one real variable $\displaystyle{t}$.

— Structure and Interpretation of Classical Mechanics

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2019.09.04 Wednesday ACHK

# Quantum observer 2

Consistent histories, 6.2 | Relational quantum mechanics, 2 | Eigenstates 2.3.2.2

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Would an observer see itself being in a superposition?

In a sense, tautologically, an observer is not a superposition of itself, because “an observer” can be defined as “a consistent history”.

an observer ~ a consistent history

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Because “state” is expressed in RQM as the correlation between two systems, there can be no meaning to “self-measurement”.

— Wikipedia on Relational quantum mechanics

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Would an observer see itself being in a superposition?

When we say that “before observation, observable B is in a superposition of some eigenstates”, you have to specify

1. it is a superposition of what?

2. it is a superposition with respect to what apparatuses or experimental setups?

— Me@2018-02-05 12:45 AM

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# The Time Machine

This is the place.

So it is.

_But there’s nothing here.

Well, it was different then. My laboratory was all around here. The kitchen was up there where that tree is. Not that Mrs. Watchit ever let me go in there.

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I don’t know what to tell you, sir. He’s been gone this whole week.

_And you’ve no idea where he went?

No, sir.

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_[Alexander] This would be my greenhouse. There was a garden outside.

Gren’tormar’tas?

Yes.

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_Sir?

Maybe he’s finally found some place where he can be happy.

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This was my home.

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His home.

— The Time Machine (2002 film)

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2019.09.01 Sunday ACHK

# Ken Chan 時光機 2.1

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1.1 除了是物理補習天王外，

2.1 他宣稱有在大學教書。是教授、講師，還是其他，我就不知道。我忘記了，他有沒有講過。

2.2 他在大學做研究。據我理解，他當時研究的是有關激光的實驗物理。

2.3 他有時要往大陸作學術演講。

3.1 然後，他亦是某某什麼工程學會的主席。

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— Me@2019-08-30 09:31:52 PM

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# The Hong Kong Way

The Hong Kong Way (Chinese: 香港之路) was a peaceful political campaign held in Hong Kong on the 30th anniversary of the Baltic Way. Organisers estimated that 210,000 people participated, to draw attention to the 2019 anti-extradition bill movement and the five demands for government accountability and expanded democratic freedoms. In the early night time hours on 23 August 2019, Hongkongers joined hands to create a human chain 50 kilometres long, stretching across both sides of Hong Kong harbour and over the top of Lion Rock.

The action was inspired by the Baltic Way demonstration of 1989, which involved 2 million people, and contributed to the collapse of Soviet Russian control of the region. The Hong Kong Way event assembled participants along routes that corresponded to the three main MTR lines, forming a chain of people that stretched nearly 50 kilometres through Hong Kong Island, Kowloon and the New Territories, and without any disruptions to traffic.

— Wikipedia on Hong Kong Way

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2019.08.28 Wednesday ACHK

# Problem 13.6

A First Course in String Theory

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13.6 Orientifold Op-planes

(a) For an O23-plane the two normal directions $\displaystyle{x^{24}, x^{25}}$ can be represented by a plane. A closed string at a fixed $\tau$ appears as a parameterized closed curve $\displaystyle{X^a(\tau, \sigma)}$ in this plane. Draw such an oriented closed string that lies fully in the first quadrant of the $\displaystyle{(x^{24}, x^{25})}$ plane. Draw also the string $\displaystyle{\tilde{X}^a(\tau, \sigma) = -X^a(\tau, 2\pi - \sigma)}$.

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This one is wrong.

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— Me@2019-08-26 10:31:07 PM

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# Quantum Computing, 3

Instead of requiring deterministic calculation, you allow (quantum) probabilistic calculation. What you gain is the extra speed.

— Me@2018-02-08 01:50:06 PM

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# 年年齡 10.2

We don’t stop playing because we grow old; we grow old because we stop playing.

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— Me@2011.10.11

— Me@2019.08.24

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# 財政自由 1.1

（問：根據你的講法，好像大部分情況下，都不應該讀研究院似的。）

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（問：那樣，你心目中的理想情況是什麼？）

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（問：那就即是話，要在還年青時，就賺到一生夠用的金錢？

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（問：但是，財政自由，又可以如何實現呢？）

「售賣時間」的致命傷是，收入不可倍增放大。那就是所謂的「一分耕耘，一分收穫」。

（問：「一分耕耘，一分收穫」有什麼問題？）

— Me@2019-08-21 07:40:35 PM

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# Physical laws are low-energy approximations to reality, 1.6

QM GR

too many particles

when you cool down the system, you see something that your equations cannot predict

only by experiment, you discover that you can go to there

the system state goes from one local minimum to another

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Since each theory is valid only when near a particular energy local minimum, we cannot use it to predict other local minima (other physical realities). That’s why we need to keep doing experiments that are designed for stress-testing existing theories. For example, one experiment can put a system in extreme high pressure. Another experiment can put a system in extreme high temperature.

Once a theory breaks down in one of those experiments, we see phenomena that we have never seen before. From there, we construct new theories to explain those phenomena.

Once a theory breaks down in one of those experiments, we see phenomena that we have never seen before. In this sense, experimental physics is much richer.

Computational physics goes further in only one direction. Only experiments let you try randomly.

— Me@2019-08-18 07:51:05 PM

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# Literal numbers

All primitive mathematical procedures are extended to be generic over
symbolic arguments. When given symbolic arguments, these procedures
construct a symbolic representation of the required answer. There are
primitive literal numbers. We can make a literal number that is
represented as an expression by the symbol “a” as follows:

(literal-number 'a)        ==>  (*number* (expression a))


The literal number is an object that has the type of a number, but its
representation as an expression is the symbol “a”.

(type (literal-number 'a))          ==>  *number*

(expression (literal-number 'a))    ==>  a


— SCMUTILS Reference Manual

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2019.08.17 Saturday ACHK

# Multiple dimensions of time

What would be the implications of multiple dimensions of time?

That means the (past) history itself can change, as commonly seen in time travel stories.

But wouldn’t that be the case with one dimension also?

In reality, there is only one dimension of time, meaning that the state of a system keeps changing, forming the timeline. But the timeline itself cannot be changed once formed. In other words, (past) history cannot be changed.

— Me@2019-08-11 04:07:48 PM

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# （反對）開夜車 4.2

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— Me@2019-07-30 11:11:42 PM

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# Hong Kong

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2019.07.29 Monday ACHK

# Quick Calculation 15.1.2

A First Course in String Theory

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Recall that a group is a set which is closed under an associative multiplication; it contains an identity element, and each element has a multiplicative inverse. Verify that $\displaystyle{U(1)}$ and $\displaystyle{U(N)}$, as described above, are groups.

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# Definition

A group is a set, G, together with an operation $\displaystyle{\bullet}$ (called the group law of G) that combines any two elements a and b to form another element, denoted $\displaystyle{a \bullet b}$ or $\displaystyle{ab}$. To qualify as a group, the set and operation, $\displaystyle{(G, \bullet)}$, must satisfy four requirements known as the group axioms:

Closure

For all a, b in G, the result of the operation, $\displaystyle{a \bullet b}$, is also in G.

Associativity

For all a, b and c in G, $\displaystyle{(a \bullet b) \bullet c = a \bullet (b \bullet c)}$.

Identity element

There exists an element e in G such that, for every element a in G, the equation $\displaystyle{e \bullet a = a \bullet e = a}$ holds. Such an element is unique, and thus one speaks of the identity element.

Inverse element

For each a in G, there exists an element b in G, commonly denoted $\displaystyle{a^{-1}}$ (or $\displaystyle{-a}$, if the operation is denoted “+”), such that $\displaystyle{a \bullet b = b \bullet a = e}$, where e is the identity element.

— Wikipedia on Group (mathematics)

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The axioms for a group are short and natural… Yet somehow hidden behind these axioms is the monster simple group, a huge and extraordinary mathematical object, which appears to rely on numerous bizarre coincidences to exist. The axioms for groups give no obvious hint that anything like this exists.

— Richard Borcherds in Mathematicians: An Outer View of the Inner World

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2019.07.28 Sunday ACHK