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Void Filler

There is a game called Nihilumbra: You are born from the void, and the void keeps chasing you, trying to engulf you. To live, you travel, you learn all kinds of "colors" as tools to fight back. After a long journey, however, the void still catches you. You are in despair. Unexpectedly, however, the void expels you - all those colors that you collected during your journey have filled your void inside. You are no longer empty. You are no longer part of the void.

You are free at last.

But that's not the end. The world has been destroyed. That will prevail in your conscience unless you fix it. 

I like this game a lot. I had always been empty since childhood. “从童年起,我便独自一人照顾着,历代的星辰。” That emptiness reached its maximum during my undergraduate study. I wouldn't have felt sorry at all, had I died at some random accident. Well, yes, there were people who loved me, there were interesting things that were nice to experience, the world was in general good. But I had no passion for anything.

Not until I finally made my way to physics.

But that's not the end. I was and still am lacking way too much knowledge. I hope to use the rest of my life to fill this new, lovely, adorable void. 

I can never fail - at least I can fill a very small part of it.


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Basics: dimensional analysis

I decide to begin writing some back-of-the-envelope stuff in this blog; mostly very basic things. Consider Dirac fermion coupled to $U(1)$ field. $$\mathcal{L}=\bar{\psi}\gamma^\mu (\partial_\mu-iA_\mu) \psi - \frac{1}{e^2} F_{\mu\nu}F^{\mu\nu}. $$ In d spacetime dimensions, the first part in the kinetic term implies $[\partial]+2[\psi]=d$, leading to $[\psi]=(d-1)/2$. The second part leads to $2[\psi]+[A]=d$. Plugging in the value for $[\psi]$, we have $[A]=1$, then $[F]=2$. Using this and plug back in the Maxwell term $2[F]-[e^2]=d$, we arrive at $[e^2]=4-d$. When we are in 3+1d, the gauge coupling is thus dimensionless. For 2+1d, the gauge coupling has dimension $[e^2]=1$, so this theory is strongly coupled in the infrared. Also, the potential energy between two charges increases logarithmically with respect to the distance between them, which is a very mild form of confinement. References:  [1] David Tong, Lectures on gauge theory, chapter 8.


哥哥, 生日快乐,我来夸你! 1. 你是我认识的所有人里面坚持duolingo时间最久的。 2. 你是我认识的所有爸爸里面最好的爸爸。 3. 你是我认识的所有人里面最爱小动物的。很多人说自己喜欢动物,但其实不是那么的喜欢。你是真的。你和小动物一样可爱。 4. 你是我认识的所有人里面最能改变自己的人。要你下定决心很难,但你一旦决定要改变,行动力和持久度让人震惊。 5. 你的声音真好听。You almost had me at "hello".  6. 你一点儿也不迷茫,对自己的大目标一清二楚。 7. 你的笑点非常低。每次看你魔性大笑,我都会被感染,也很开心。 8.  你在我和宝宝兔面前语气千变万化抑扬顿挫,但在给报告以及和别人讨论的时候就是严肃脸低音炮,再紧张也看不出来。深沉极了,性感极了。 9. 你爱学习,什么talk都能听,非常open-minded。 10. 你坐得住,真的能废寝忘食。(还是不鼓励废寝忘食!) 11. 你很节省,其实节省辛苦,我不想你辛苦。 12. 你太能走路了。 13. 你英语词汇量好大。 14. 衣服架子。我觉得好看的衣服,买给你你穿起来都好看。 15. 你非常勇敢,和我一样勇敢。 16. 你吹口哨音很准,听歌过耳不忘,怎么不去搞音乐。 17. 你毛发重。年纪再大也不可能秃头的! 18. 你对脸部肌肉的控制力超强!宝宝兔也像你。 19. 你懂各种各样的trivia。 20. 哥哥……很厉害。 21. 你抓猫一流!我以为我运动神经已经很好了,可是远不如你。 22. 你去超市买泡面,会给我带花。 23. 你摄影天赋非常可以。以后多拍。 24. 你俄罗斯方块太强了吧。 25. 你手好暖,手指长得又好看。 26. 你和我一样更喜欢狗。 27. 你睫毛好长,好可爱。 28. 几何大师。我做空间变换总要找你帮忙。 29. 你总是很温柔。 30. 你爱我,你眼光真好。 能陪你度过三十年里的六分之一,小企鹅很幸福。前路还是会有很多困难,但是我们可以一起waddle过去。有你,我什么都不怕。 希望年年有今日,岁岁有今朝。 期待你的racing car experience吧!!