人择原理（5）
人择原理重新成为大家关注的话题，自然是因为1998年暗能量的发现。超弦界一直认为宇宙学常数为零，虽然大家努力了许多年，没有找到一个公认的机制，即使如此，一个等于零的值总比一个不为零但非常小的值相对容易解释。
人择原理堂而皇之地出现在弦论的文章中，大概自Bousso和Polchinski的文章开始，他们重新将1988年Brown和Teitelboim的机制找了出来。那个减小宇宙学常数的机制很类似2维时空中电场背景下正负电子的产生减小电场一样。在4维时空中，正负电子对换成了球状2维膜，2维膜的产生改变宇宙学常数和4阶反对称场。但由于2维膜对应的张力往往很大，每次宇宙学常数改变太大。Bousso和Polchinski指出弦论中很容易就会有不同的2维膜，这样宇宙学常数的改变可以变得很小。
KKLT等人后来构造了很复杂的模型，利用内部空间中的不同反对称场的通量使得一些标量场固定下来（通过生成superpotential），再通过一些instanton或gluion condensation效应固定整个内部空间的体积，经过这些手术后得到一个有超对称的anti-de Sitter space。最后，引进一些反D3膜破坏超对称，并生成一个正宇宙学常数，所得到的时空是de Sitter时空，这个时空不稳定，因为当内部空间的体积变成无限大时，生成的正能趋于零，de Sitter 空间只是一个局域极小。这就是著名的KKLT模型。
在KKLT之后，Douglas提出要研究这类模型的统计，Susskind提出了string landscape概念，并从此不遗余力地鼓吹我们进入了超弦第三次革命，也许是自量子力学以来最大的革命，并为此写了一本科普书。
我不喜欢landscape有如下几个理由：
（1） KKLT等人的方法是半经典方法，我们没有任何技术来控制这类构造的可信性。
（2） 这样的构造too complex to be true－当然这不是一个科学的判据。
（3）最后要用人择原理在超过这样的“亚稳”态中选出我们的宇宙，恰如上帝在6000年前用相对比较容易的办法构造出我们的宇宙。
按照现在的走势，如果很快有一天有人出来证明landscape中有无限个局部极小，人择原理的计算需要人为地引进某种正规化以逃避无限大的困难，我一点也不会惊讶。那个时候，就是我们如同上帝一样，想要什么就有什么的时候了。
2005年12月14日 11:19:00
why you say that we can do whatever we want, if we invent certain regulization technic to overcome the infinity? we always do this in quantum field theory.
2005年12月14日 21:32:43
In quantum field theory, there is almost unique way to do renormalization, while in the case of landscape, we can hardly come up with a method using first principle-there is no first principle available so far. Thus people will resort to their imagination to invent a variety of regularizations.
I imagine that figuring out a first principle to fix the problem of an infinite landscape is like working out some selection principle, which is great of course, but this is far beyond the scope of the current anthropic landscape philosophy.
By the way, people already encountered a problem of infinity in the eternal inflation scenario.
2005年12月16日 20:19:45
Do you mean that the number of vacua is infinite? There are a large number of vacuas in some string models, such as flux compactification. But in the present low energy effective theory frame, the number is very huge, and I donot know whether the number is finite, the counting is rather complicated. Although we have some constraints like supersymmetry, tadpole cancellation, etc, it doesnot reduces the number greatlty, I am wondering is there any other constaints ariseing from the string theory that will reduce the number of vacua, like the finitness of moduli volume, the rank of gauge group as Vafa states in his recent paper on swampland.
Personally, I donot think there will be any selection principle to select the unique vacua if the present vacuas we consider are consistent. Do you have any opinion on the classical or quantum consistency of the flux vacua?
2005年12月16日 20:39:35
I suspect that the number will become infinite along the line of current strategy of kklt construction.
I have no idea how to constrain the construction, I feel we need to develop tools.
As the selection principle is concerned, it appears that the only availabe one is due to Banks and Johnson.
2005年12月30日 23:31:39
I am not a big fan of landscape either. On the other hand, I have a few comments about your statement about the landscape.
1) I don’t think the idea of landscape in general relies on the validity of a KKLT like construction. The real question is whether there are a large number of vacuum in nature. To me, it does not seem to be such a outrageous idea. Afterall, string theory (not only KKLT type of construction), does seem to have a lot of possible vacua. Of course, I undersatnd all the reservations people have, such as the ones you have against KKLT. On the other hand, I think the claim that string theory has an unique vacuum is at least as impossible.
2) I am not sure about the meaning of ‘too complex to be true’. as you said, it is not scientific.
3) actually, multi-universe is the most anti-religious approach to the why we are here problem. we are here with absolutely no purpose, nothing is beautiful about our existence. We are here because of a freaking accident. There is no accident that the Church burnt Bruno, because he believe in multi-verse.
2005年12月31日 1:40:36
Sage:
1) There is certainly no proof that there are not many vacua in string theory. For the moment, I think the both sides of the debate lack strong
evidence for their cases.
2) Simplicity and beauty is the best guidance when we don’t have any experimental clues. again it is just soemthing like faith.
3) You are right, the anthropic idea and the cosmic landscape are not religious, but are just as meta-physical as any religion. I don’t see any chance to prove the idea experimentally. This is a quite different situation than what Copernicus and Bruno presumed. Many anti-string people argue that string theory is not science and we can fight back with the argument that it can be falsified and we had a case. Now with the landscape even a string theorist will have to give in.
2005年12月31日 8:27:43
I am not so sure that there is no way to prove the existence of many vacua. I think this is something one should work on in order to prove or disprove such a scenario. It is not a faith, rather a statement about nature.
2006年4月18日 13:29:57
[…] 所以如果有 1 googol 颗谷子的话，我们所处的宇宙远远容不下它们啊。需要多宇宙，不过超弦的landscape概念给了足够多的宇宙，可以从中选择个来存放丰收的谷子。这被称为谷择原理。 […]
2006年4月19日 22:52:47
[…] 所以如果有 1 googol 颗谷子的话，我们所处的宇宙远远容不下它们。需要多个宇宙，不过超弦的 landscape 理论给了足够多的宇宙，我们可以从中选择个来存放丰收的谷子。这被称为谷择原理。 […]
2007年8月31日 22:45:26
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