Daily Archives: January 5, 2022

2.10 A spacetime orbifold in two dimensions, 6

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A First Course in String Theory

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(d) Consider the two curves \displaystyle{x^+ x^- = a^2} for some fixed \displaystyle{a}. The identification (2) makes each of these curves into a circle. Find the invariant circumference of this circle by integrating the appropriate root of \displaystyle{ds^2} between two neighboring identified points. Give your answers in terms of \displaystyle{a} and \displaystyle{\lambda}. Answer: \displaystyle{\sqrt{2} a \lambda}.

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Identification (2):

\displaystyle{(x^+, x^-) \sim \left( e^{-\lambda} x^+, e^{\lambda} x^- \right)}, where \displaystyle{e^\lambda \equiv \sqrt{\frac{1+\beta}{1-\beta}}}.

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\displaystyle{ \begin{aligned} ds &= \sqrt{- ds^2} \\ \int_{(x^+, x^-)}^{(e^{-\lambda} x^+, e^{\lambda} x^-)} ds &= \int_{x^+}^{e^{-\lambda} x^+} \sqrt{2 \frac{a^2}{(x^+)^2} (dx^+)^2} \\ &= \sqrt{2} a \int_{x^+}^{e^{-\lambda} x^+} \sqrt{\frac{1}{(x^+)^2}} dx^+ \\ \end{aligned}}

Choose a segment on which \displaystyle{x^+ > 0} .

\displaystyle{ \begin{aligned} \int_{(x^+, x^-)}^{(e^{-\lambda} x^+, e^{\lambda} x^-)} ds &= \sqrt{2} a \int_{x^+}^{e^{-\lambda} x^+} \frac{1}{x^+} dx^+ \\ &= \sqrt{2} a \ln \left|\frac{e^{-\lambda} x^+}{x^+} \right| \\ &= - \sqrt{2} a \lambda \\ \end{aligned}}

The lower limit should be smaller than the upper limit.

\displaystyle{ \begin{aligned} \left| \int_{(x^+, x^-)}^{(e^{-\lambda} x^+, e^{\lambda} x^-)} ds \right| &= \sqrt{2} a \int_{e^{-\lambda} x^+}^{x^+} \frac{1}{x^+} dx^+ \\ &= \sqrt{2} a \lambda \\ \end{aligned}}

— Me@2022-01-05 02:40:45 PM

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2022.01.05 Wednesday (c) All rights reserved by ACHK

時光起源

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The Origin of Time

這段改編自 2021 年 12 月 5 日的對話。

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昨日的那段影片說，個別的粒子，其實都是質量零；以光速行走，感受不到時間。

但是，把一堆粒子圈成一件東西的話，那件東西整體而言，就會有質量；就會低於光速行走；就會感受到時間。

變相來說，時間的來源就是，你將一堆粒子組成一個物件、一個身份、一個自我時，那個自我就會，感受到時間。

— Me@2022-01-04 12:23:25 PM

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To form an object (an observer), the component particles cannot all always move at light speed in the same direction, for that would prevent the object as a whole from feeling time.

Anything moving at the speed of light cannot feel the passage of time. If a set of particles all moves at light speed in the same direction all the time, they cannot feel time either as individuals or as a whole; so they cannot form an “object”.

To form an object (an observer), the component particles need to interact. So some component particles need to move in other directions sometimes.

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An object requires an internal structure to exist and evolve. The component particles need to interact in order to evolve as a single identity. So different particles need to move in different directions sometimes. As a result, the component particles as a whole, aka “the object”, will move slower than light.

— Me@2021-12-08 08:09:17 AM

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2022.01.05 Wednesday (c) All rights reserved by ACHK