藍色情懷

弦論造景新解：膜的穿隧

Brane trust: tunneling and stringy physics

October 30, 2007  By Miranda Marquit

"這事常在科學當中發生：每個人都會貢獻出他們的一小部份，然後在多年的艱苦研究後，終於形成一幅完整的圖像，" Amanda Weltman 表示。Weltman，一位在 Cape Town 大學與劍橋大學的科學家，相信她與她的同僚發現謎題的另一部份，尤其是關於弦論（string theory）的那個部份。"我們採用量子場論（quantum field theory）中所開發的一種工具，並用它來研究弦的物理學。"

“As is often the case in science, everybody contributes their piece, forming a complete picture only after years hard work,” Amanda Weltman tells PhysOrg.com. Weltman, a scientist at the University of Cape Town and at Cambridge University, believes that she and her collaborators have found another piece of that puzzle, especially with regard to string theory. “We took a tool developed in quantum field theory and adapted it to study stringy physics.”

Weltman 與她的同事，在劍橋大學的 Adam Brown 與 Saswat Sarangi，以及在科羅拉多大學的 Benjamin Shlaer 以一篇在 Physical Review Letters 上發表，題為「Enhanced Brane Tunneling and Instanton Wrinkles」的論文來解釋她們的研究。

Weltman and her colleagues, Adam Brown and Saswat Sarangi at Columbia University, and Benjamin Shlaer at the University of Colorado, explain their work in a piece titled “Enhanced Brane Tunneling and Instanton Wrinkles,” published in Physical Review Letters.

Weltman 表示，她對於了解力學，即在弦論中所謂的造景（或『景繪』；landscape：用視覺化的(地理、)幾何景觀描繪方式，來闡釋弦論），最感興趣。

Weltman says she is most interested in understanding dynamics in what is known in string theory as the landscape.

"在弦論早期，人們希望只有唯一一種基礎狀態（ground state，譯註：ground 也指地面）來描述我們的宇宙。" 她描述一個在二山丘中間的山谷，並指出還有其他擁有最低點，或稱為真空（vacua），的山谷存在。

 “In the early years of string theory, people hoped that there would be a unique ground state describing our universe.” She describes a valley between two hills, and then points out that there are other valleys with similar low points, or vacua.

"那裡可能有像真空這樣的造景，而且有各種不同的方式可以到達那裡，" 她繼續說。"對可能值全般造景的領悟，為這個領域開啟了新的問題。"

“There may be a landscape of such vacua with many different ways that each can be reached,” she continues. “This realization of a whole landscape of possible values has opened new questions for the field.”

Weltman 表示，她們 "問了一個相當基本的問題：若宇宙是屬於無數個這種真空的其中一個，那麼它會在那停留多久？" 為了回答這個問題，她們 "研究不同真空之間的穿隧（tunneling），包含弦的自由度。"

Weltman says that she and her collaborators “asked a very basic question: If the universe were in one of the myriad of such vacua then how long would it stay there?” To answer this question they “studied tunneling between different vacua, including stringy degrees of freedom.”

"穿隧概念在量子理論中存在好一陣子了，" 她指出。"這個想法是，若你有個量子粒子撞到一面牆，那麼該粒子出現在牆另一邊的機率，不像其古典的對應理論，並不會為零。我們正依著弦論裡，弦與膜（brane）之中的上下脈絡研究這樣的穿隧，而非只是量子理論當中的粒子。"

“The concept of tunneling has been around in quantum theory for a while,” she points out. “The notion is that if you have a quantum particle hitting a wall, the probability of it appearing on the other side of the wall is non-zero, unlike its classical counterpart. We are now studying such tunneling in the context of strings and branes in string theory rather than just particles in quantum theory.”

秉持著山谷這樣的構想，Weltman 解釋山谷當中一個場，會試著到達山頭的另一邊 -- 然後進入另一個山谷中。"你可能會認為當藩籬愈高，愈難到達那裡，就好像你要繞過一座山一樣。" 她停頓一下。"不過它不是以那種方式運作的。而是當障礙愈高，穿隧愈容易。"

Keeping with the idea of the valley, Weltman explains that a field in a valley would try to get to the other side of the hill – and into another valley. “You would think that the higher the barrier, the harder it would be, much as if you tried to cycle over a hill.” She pauses. “It didn’t work that way. The higher the barrier, the easier the tunneling was.”

Weltman 說，那使用量子的老技巧來研究更加複雜的弦論的造景，結果，獲得大家意料之外的答案。她所談到的是 D-branes 的穿隧，那位於弦的末端。這些膜代表弦論中弦的邊界。 Weltman 將其視覺化，說那是一個膜在一段歐亞甘草（licorice）的任一個末端。

Weltman says that using the old quantum techniques to study the more complicated landscape of string theory brought out quite a different answer than many would expect. She speaks of tunneling in terms of D-branes, which are located at the ends of strings. These branes represent the boundaries of the strings in string theory. Weltman visualizes it as one brane at either end of a length of licorice.

在一封 e-mail 中，Weltman 解釋膜的穿隧如何作用："我們對於這種結果的詮釋並非場穿隧過去，而是一個新的膜-反膜對（brane-antibrane pair）在另一邊被創造出來，然後這個「反膜」穿隧回來，消滅掉原來的膜。藉由將障礙提高，這種對的集結（nucleation）會被加強，也因此穿隧會跟著加強。你會得到一個比你原先所預期還要更快的穿隧。"

In an email, Weltman expounds on how the brane tunneling works: “Our interpretation of this result is that rather than the field tunneling through, a new brane-antibrane pair is created on the other side, and the antibrane tunnels back and annihilates the original brane. By raising the barrier height, the nucleation of such pairs, and consequently tunneling, is enhanced. You get faster tunneling than you would naively expect.”

現在，如同許多弦論，膜穿隧概念依然在早期階段。"下一件事是要研究具體模型。" Weltman 說她們已經在進行了。"我們要儘可能的明確，並看看它如何改變人們之前所抱持的觀點。"

Right now, like much of string theory, this brane tunneling concept is in its early stages. “The next thing is to study concrete models.” Weltman says it is already being worked on. “We want to be as specific as possible, and see how it changes what people have looked at before.”

她繼續說："在其他研究領域中，人們正在研究這些真空，並尋求真空的分類，那能賦予我們在標準模型中所預期的特徵 -- 粒子的質量、它們的耦合以及世代的正確數目（譯註：目前實驗結果強烈指出只有三個世代）。並解釋為何我們處在這樣一種真空狀態，我們必須了解弦論的力學。"

She continues: “In other areas of research, people are studying these vacua and looking for classes of vacua that would give us the features we expect of the standard model – the particle masses, their couplings and the correct number of generations. To explain why we are in such a vacuum state, we must understand the dynamics of string theory.

困難之處是要讓它正確。"在同一時間得到每樣東西並不容易，特別是當我們將宇宙論的常數納入其中時，" Weltman 承認。"任何驅使我們到達宇宙論常數的小值，卻不讓我們成為零的任何機制將令人信服。"

The difficulty lies in getting it right. “It’s not easy getting everything at the same time, especially when we include the cosmological constant,” Weltman admits. “Any mechanism which drives us to small values of the cosmological constant, but keeps us from zero, would be compelling.”

Copyright 2007 PhysOrg.com. 

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