一直比较纳闷：为什么宇宙中的量子效应这么弱？小生我先挖个坑：） 大牛们灌水哈

自从高三了解相对论开始就知道相对论的效应在地球上很弱，因为我在实际生活中几乎感觉不到它的用武之地，但是随着学习深入慢慢的却又知道它在宇宙中的效应很强，只是我所接触的也都是低速运动物体。用不到SR去处理。

后来初学广义相对论时感觉也是如此。因为地球周边是弱场，我能感觉到的它的效应也还是非常的弱。后来又逐步学到黑洞物理学，才知道广义相对论有很强的效应。随着年龄的增大以及学习的深入，之前的观念已经全部抛弃。相对论在宇宙中的效应还是很强的。

慢慢的到了大二，开始自学量子力学，一下子进入微观世界。还是那种感觉：总的来说，体系的量子效应是非常弱的，在日常中我基本不用面对量子力学。后来又逐步的看见物理界的大牛们继续沿着英雄的道路去追寻量子引力。接触到更多的理论：superstring/M theory and loop quantum gravity.作为非物理系的工科学生，这个疑问就越来越强：为什么这么多的高深理论对我们的实际观测修正效应却这么弱？

之所以灌上面那段话，只是说明我浅显的认识历程。诸位一笑而过，下面进入正题。

1.D-branes 动力学（或者说string theory）对可测的哈勃常数修正会有多大？我想一定会很小？但是为什么现在理论对它的修正会这么小？

2.宇宙学常数，一直没有测定过，更不用说理论对它的修正了。但正如polchinski的文章问得：为什么它会这么小？

3.早期的宇宙过程通过CMB以及合理的宇宙模型来刻画。我想问得是，弦宇宙学会对它带来多大的修正效应？可分辨么？

其实上面几个用一句话说：目前的弦论对宇宙学的观测会带来多大的修正效应?或者圈量子引力能带来多大的修正效应？这个问题太笼统，诸位大虾各表己见或引用计算结果皆可。

或许大家更关注的是string对于粒子物理学的观测会带来多大的修正。通常在核物理里头，不到几百个MeV，我是完全不会去考虑相对论或者QCD的修正的，用各种唯象模型就能得到足够的精度了。在航天器屏蔽中，对星际宇宙射线的效应也不用考虑pions或者muons、各种轻子的影响。因为计算结果表明他们的贡献比中子和质子在20GeV以下的贡献小几个数量级。由此类比或许可以让我们相信，string对粒子物理学的效应也是很弱的。尽管大家都在等LHC的结果，我也祈祷能有些可观测得效应，但是很有可能不会达到预期的希望。

乱七八糟的写了这么多，浪费大家的时间了。如果我们发展的理论不能更好的给我们的观测带来实际的可分辨的效应，凭什么让公众相信它？我们是不是要反思一下，为什么理论能带来的量子修正效应这么小？或者找个法子来放大这种修正？

或许你会用能标来搪塞我，说理论要求的能标太高，实验达不到，在低能端带来的可观测的修正的确是很小以至于不可分辨。但是我们也可以换过来想：为什么现有理论在低能端的修正会这么小？不知道现在这方面的工作（计算现有理论的低能区的修正效应）有多少进展。我只是个学生，心里存着这个疑问，想更多的了解一下。

我觉得我简直是在胡思乱想，估计主要是火箭在领先15分的情况下，最后15分钟集体哑火，23投2中，把几乎到手的胜利拱手让人，也让我郁闷到这种程度所致。大家一笑而过阿 呵呵。

这还是你关心的尺度的问题。

You are probably mixing up two questions.

1) whether fundamental theory, such as string theory, if true, determine what we see at our experiments? Of course it will. It determines the basic particle spectrum we are going to see.

2) If we are not convinced that string theory exists, are we going to be able to see direct evidence of it by doing low energy measurements? The answer is obviously no, because strings are just too heavy. Scale determines everything. The basic concept of effective field theory is that the experiments done at large scales (small mass) will not be sensitive to what is going on at the small scale (high mass). Therefore, if you look out into the sky, of course you probably won't see fundamental strings, because they are too too small.

Now go to some of your detailed questions:


1.D-branes 动力学（或者说string theory）对可测的哈勃常数修正会有多大？我想一
定会很小？

It depends on model. There are many many many models of Dbrane-world cosmology. Some are pretty dramaitic. There are even more models from string theory in general. If you just take a random model, it will predict a very different universe.

但是为什么现在理论对它的修正会这么小？



2.宇宙学常数，一直没有测定过，更不用说理论对它的修正了。但正如polchinski的文章问得：为什么它会这么小？

Nobody knows. If we know anything about field theory, all models give very big corrections.

3.早期的宇宙过程通过CMB以及合理的宇宙模型来刻画。我想问得是，弦宇宙学会对它带来多大的修正效应？可分辨么？

It depends on model. Pretty small, but could be visible if we are lucky.

其实上面几个用一句话说：目前的弦论对宇宙学的观测会带来多大的修正效应?

Model dependent. There is not a unique model in string theory.

或者圈量子引力能带来多大的修正效应？

There is no such a theory call loop quantum gravity. It is a collection of wishful thinking rather than a consistent theory.

这个问题太笼统，诸位大虾各表己见或引用计算结果皆可。

或许大家更关注的是string对于粒子物理学的观测会带来多大的修正。通常在核物理里头，不到几百个MeV，我是完全不会去考虑相对论或者QCD的修正的，用各种唯象模型就能得到足够的精度了。在航天器屏蔽中，对星际宇宙射线的效应也不用考虑pions或者muons、各种轻子的影响。因为计算结果表明他们的贡献比中子和质子在20GeV以下的贡献小几个数量级。由此类比或许可以让我们相信，string对粒子物理学的效应也是很弱的。尽管大家都在等 LHC的结果，我也祈祷能有些可观测得效应，但是很有可能不会达到预期的希望。

乱七八糟的写了这么多，浪费大家的时间了。如果我们发展的理论不能更好的给我们的观测带来实际的可分辨的效应，凭什么让公众相信它？我们是不是要反思一下，为什么理论能带来的量子修正效应这么小？或者找个法子来放大这种修正？


Nobody is asking you to believe in something that is not going to be verfied. Nobody really believes in string theory either.

If you lower the fundamental theory scale, the correction will certainly be larger. There are a lot of models doing that as well.

或许你会用能标来搪塞我，

Yes. I am doing exactly that.

说理论要求的能标太高，实验达不到，在低能端带来的可观测的修正的确是很小以至于不可分辨。但是我们也可以换过来想：为什么现有理论在低能端的修正会这么小？

不知道现在这方面的工作（计算现有理论的低能区的修正效应）有多少进展

Very sophisticated, very detailed. Anything new which is lighter than 10 TeV is pretty strongly constrained.

。我只是个学生，心里存着这个疑问，想更多的了解一下。

我觉得我简直是在胡思乱想，

You probably should study more. Finish Weinberg. It should get you very far.

You probably should study more. Finish Weinberg. It should get you very far.

谢谢，正在一页页的读写。我想会是你说得那样的very far.
其实我昨天突然只是感到困惑，今天该干什么还是干什么。就像每天呆在实验室做事情，却感觉正在浪费国家的钱一样，做的事情毫无意义，却不得不去做，当然也在浪费我的生命。

Anything new which is lighter than 10 TeV is pretty strongly constrained

ok,你这个10TeV是怎么出来的？也想听听你个人的意见，你自己计算的值？
Wati Taylor 在The Joint Meeting of Pacific Region Particle Physics Communities上的报告Can String Theory Make Predictions for Particle Physics?中提到了一个量：10 TeV 。认为在此能标下，理论不会做出什么对粒子物理有什么具体可观的东东（当然也可以放大一些，比如1000TeV）。我想到我死的那一天也未必能在实验室达到理论物理学家要球的能标。难道理论物理学家就只能用能标来搪塞么？也许这正是理论物理学家要做的。
http://www.phys.hawaii.edu/indico/contributionDisplay.py?contribId=96&sessionId=168&confId=3 泰勒的slides

Nobody is asking you to believe in something that is not going to be verfied. Nobody really believes in string theory either.

你打错了verified.
是，的确是没有人要我相信它。但是总的有点信仰，就像我信仰标准模型（也许可以称为stardard theory了)一样。当然很多时候会发现自己的确学了一些你很长时间不知道有没有意义，也不知道对不对得东东。但是还是会去认真学，呵呵。

If you lower the fundamental theory scale, the correction will certainly be larger. There are a lot of models doing that as well.

我也知道降低能标会带来大的修正，我只是想知道一些具体的进展。因为我搜索信息的能力还是非常有限。我想各位前辈在这方面没准会有很多现成的资料，大发慈悲一下，省却了我们学生小辈的许多功夫，也长见识的说。谢谢

There is no such a theory call loop quantum gravity. It is a collection of wishful thinking rather than a consistent theory.

对这个我不清楚，只听他们谈论LQG。自洽不自洽我没有能力去辨别，当然也不会轻易去相信某一个人的论断。:-）

It depends on model. There are many many many models of Dbrane-world cosmology. Some are pretty dramaitic. There are even more models from string theory in general. If you just take a random model, it will predict a very different universe.

Nobody knows. If we know anything about field theory, all models give very big corrections.

It depends on model. Pretty small, but could be visible if we are lucky.

Model dependent. There is not a unique model in string theory

全是Model，但不是theory.当然我们要从Model走向theory。一点一滴的，一步步地走。

You are probably mixing up two questions.

1) whether fundamental theory, such as string theory, if true, determine what we see at our experiments? Of course it will. It determines the basic particle spectrum we are going to see.

2) If we are not convinced that string theory exists, are we going to be able to see direct evidence of it by doing low energy measurements? The answer is obviously no, because strings are just too heavy. Scale determines everything. The basic concept of effective field theory is that the experiments done at large scales (small mass) will not be sensitive to what is going on at the small scale (high mass). Therefore, if you look out into the sky, of course you probably won't see fundamental strings, because they are too too small.

关于1），你是信奉爱因斯坦：理论决定我们能观测到什么？我们需要一个什么样的尺度来处理理论预言与实验之间的关系。很有可能好几代人都无法在实验上（完全或部分)确证某个理论，但是我们要以什么样的方式来对待这个理论？全盘否定？全盘肯定？中庸之道？或许就像各国几千年来对于中央与地方的关系一样难以处理，这个结怎样才能解开？

关于2），我相信它，所以才在实验上有所期待。我也知道这很难，在面临抉择的时候往往需要勇气和魄力。你后面所提到的基本概念我都懂。最后一句话说得很幽默：）

谢谢sage兄（我看他们这么叫你，我也就这么跟着叫了，所以也没有称呼您。但我敬重才华横溢的人！）的答复
顺祝好
hawk

把泰勒报告的总结贴在这：

5. Summary I
² String theory need not make predictions for particle physics
below 100 TeV
² We can't de¯ne string theory yet
² The number of suspected solutions is enormous, and growing fast
² Nonetheless, constraints on low-energy physics correlated between
calculable corners of the landscape may lead to predictions
² If not, probably need major conceptual breakthrough to have any
possibility of predictivity for low-energy particle physics.
² Raison d'etre for string theory: quantum gravity.
Remarkably, also gives new insight into gauge theory.
Suggests interesting new structures for phenomenological models.
Speci¯c low-energy physics prediction would be unexpected bonus.

他选取了100 TeV，不知道为何。

I am going to only reply to the part of your message which is actually a question.

You asked about the 10 TeV. The reason for that is if we have random new physics below that scale, their effect would have been already visible, for example, through the so called electroweak precision tests. There is nice review of it on the webpage of particle data group, which is also a good source for a lot of information.

There are some observables, such as Kaon mixing, which is sensitive a particular kind of new particles if they are at more than 100 TeV.

I don't know why Wati is talking about 100 TeV. I think he is probably just refering to some low scale and pick up that number.