# Grassmann, 4

In quantum field theory, Grassmann numbers are the “classical analogues” of anticommuting operators. They are used to define the path integrals of fermionic fields.

— Wikipedia on Grassmann number

2013.07.31 Wednesday ACHK

# 十年 2

「十年」或者「一萬小時」這兩個時間長度，雖然未必百分百準確，但是，它們至少可以讓你知道，想要「學有所得」，或者「事業有成」，需要花的時間心神，遠遠大過一般人的想像。

— Me@2013.07.31

# T-duality, 6.2

T-duality interchanges the usual Neumann boundary conditions with Dirichlet boundary conditions …

— Wikipedia on D-brane

2013.07.30 Tuesday ACHK

# Working experience

experience ~ mistakes

earn experience ~ make new mistakes

— Me@2013-07-04 12:05:31 PM

# 無限旅程 4.4

Meaningful 12.4 | 惜此際 4 | A. J. Ayer, 2.2

（安：那我自己的存在呢？為什麼「自己存在」，好過「自己不存在」？）

「性質」的意思是，你可以利用它，來把人群分成兩組。例如，「輕盈」是一個性質。所以，你可以把人群分成兩組：一組是「輕盈」的人，另一組是「不輕盈」的人。

「你的存在」，並不是「你的性質」，而是其他東西的性質。準確一點講，「你的存在」，是「你環境的性質」。例如，你在學校參加課外活動，加入了足球學會，但沒有加入籃球學會。那樣，「你的存在」就是足球學會的性質，而不是籃球學會性質。

「性質」的意思是，你可以利用它，來把人群分成兩組。因為你不可以用「存在」和「不存在」，把人群分成兩組，所以一個人的存在，並不是那個人自己的性質。

」，

— Me@2013.07.28

# T-duality, 6

We can consider the existence of D-branes to be a consequence of the symmetry of T-duality.

— String Theory Demystified, p.165

— David McMahon

2013.07.27 Saturday ACHK

# Interchange

What you are comes to you.

– Ralph Waldo Emerson

Reality and imagination get interchanged during death.

— Me@2011.04.01

HH
HT
TH
TT

HH
HT <
TH <
TT

2
_

4

2
_

4

— Me@2013.07.27

# The Beginning of Time, 2

Brian Greene continues with all the delusions and delusions, infrequently spiced with a correct proposition. The Big Bang created the arrow of time (the latter has nothing to do with the laws of physics), holy cow. “We don’t know why the Universe started in a low-entropy state,” holy cow. We perfectly know why it did. If it started with a state of a high entropy, we could always ask “what was before that”. The only thing that prevents us from going before a moment is that the moment has the minimal mathematically possible value of the entropy, namely zero.

— The Fabric of the Cosmos II

— Lubos Motl

2013.07.25 Thursday ACHK

# 無限旅程 4.3

Meaningful 12.3

（安：但是，我又可以追問，為什麼「存在」好過「不存在」？）

（安：那我自己的存在呢？為什麼「自己存在」，好過「自己不存在」？）

「性質」的意思是，你可以利用它，來把人群分成兩組。例如，「輕盈」是一個性質。所以，你可以把人群分成兩組：一組是「輕盈」的人，另一組是「不輕盈」的人。

— Me@2013.07.24

# T-duality, 5

Open strings and D-branes

T-duality acting on D-branes changes their dimension by +1 or -1.

— Wikipedia on T-duality

2013.07.24 Wednesday ACHK

# 無限旅程 4.2

Meaningful 12.2

「不斷存在」，並不是一個單一事件；「不斷存在」，是一個不斷的過程。

— Me@2013.07.23

# Godel 11

1931: Publication of Gödel’s incompleteness theorems, showing that essential aspects of Hilbert’s program could not be attained.

It showed how to construct, for any sufficiently powerful and consistent recursively axiomatizable system – such as necessary to axiomatize the elementary theory of arithmetic on the (infinite) set of natural numbers – a statement that formally expresses its own unprovability, which he then proved equivalent to the claim of consistency of the theory; so that (assuming the consistency as true), the system is not powerful enough for proving its own consistency, let alone that a simpler system could do the job.

It thus became clear that the notion of mathematical truth [cannot] be completely determined and reduced to a purely formal system as envisaged in Hilbert’s program. This dealt a final blow to the heart of Hilbert’s program, the hope that consistency could be established by finitistic means (it was never made clear exactly what axioms were the “finitistic” ones, but whatever axiomatic system was being referred to, it was a ‘weaker’ system than the system whose consistency it was supposed to prove).

— Wikipedia on Foundations of mathematics

2013.07.21 Sunday ACHK

# Release early. Release often.

— Eric S. Raymond

At any rate, I learned the lesson. When one discovers something significant, it is best to publish it promptly and not wait to incorporate it into some grander scheme.

— John A. Wheeler

2013.07.21 Sunday ACHK

# 無限旅程 4

Meaningful 12

（安：為什麼必須要有一個「無限旅程」，人才會感到「有意義」？

「有限價值」會帶來短暫的開心；「永恆價值」則會引發長久的幸福。

~ （不斷地）有下一步

~ 可保存

~ 可繼續存在

~ 可儲存於時間之中

— Me@2013.07.21

# Logical arrow of time, 3

And as we have explained many times, the results of this inference – the retrodictions – always depend on our priors. So the knowledge of the present is enough to calculate the future (classically) or to predict the unique probabilities of various states in the future (quantum mechanically). But it is simply never enough to calculate the unique state or unique probabilities of various states in the past.

The reason has been explained many times. But we can say that at least in the macroscopic context (when some microscopic detailed information is being omitted, e.g. because it’s unmeasurable), different initial states “A,B” in the past may evolve into the same final state “C” in the future.

— Logical arrow of time and terminology

— Lubos Motl

2013.07.20 Saturday ACHK

# 軟硬智力 11

— Me@2013-07-16 3:00 AM

— Me@2013-07-15 11:08 AM