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花车乐队的分析报告   

2009-05-01 01:28:10|  分类: 默认分类 |  标签: |举报 |字号 订阅

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关于所谓的Horava-Lifshitz gravity(也许这个gravity存在于多宇宙的一个数学自洽性不被尊重的patch ;) ),我自己不能做详细分析,因为我自己没有研究过,虽然我读了Horava的文章。

今天组会让高显同学做报告,我的印象是:

1、还没有人证明过这个理论是可重正的-所以我前面说数学逻辑可能没有得到足够的尊重。

2、至少出现一个新的标量模,该模与物质耦合没有得到研究,从而我们不知道这个理论是否与已知的实验如太阳系内的实验相容。我个人naive地觉得应该还有一个多出来的模,也许没有。

3、所谓取代inflation的claim,并没有得到认真的研究。

也许有人会说,因为在红外这个理论回到爱因斯坦理论,所以我们不必担心多出来的模会造成什么坏结果。这需要证明,有时取一个极限,我们不一定得到想 要的结果。例如,一个有质量的矢量场,多出一个纵向模,该模不会在质量为0的极限消失。涉及到对称性的问题往往会出现这个情况。在Horava的理论中, 恰恰是少了一个对称性。对称性少了一个而约束没有少,是不是说明这个理论在经典层次上就出了问题?

刚刚看到Motl在新博文中提到NYU的Keith Chan的一篇博文,说NYU的一个小组开会讨论了Horava的“理论”。Chan的博文引用Gruzinov的话说:”Gruzinov even said that life was short, why do we spend time on such a theory.” 这让我想起今天我在组会上的话:“人生苦短,我宁愿做点更有意义的事,例如泡吧”。

我将Keith Chan的博文转贴如下。

Horava-Lifshitz Gravity

Keith Chan

Recently, there are a lot of papers on the a new quantum gravity model, now dubbed Horava-Lifshitz gravity. It has been published in Physical Review. This model appears to be very exciting since it claims to be renormalizable. Many authors have immediately started calculating various things: inflation in this model, bouncing model, Schwarzschild solution, non-Gaussianity, blah, blah, blah.

Beside being renormalizable, another thing that is particularly exciting is that this model seems to be able to generate scale invariant spectrum without inflation. That sounds very cool!

Because of these seemingly attractive features in this model, in last week’s journal club, people in another journal club tried to read this paper. Since the model is novel and complicated, the smart people in our centre failed to understand the model. Gruzinov was very excited, and asked Kleban to talk about it in next week. He even criticized that the high energy people in our centre for their ignoring this interesting model.

So this week, Kleban presented this model. Before the start of the talk, Gruzinov asked Porrati for his comments. It turns out Porrati often makes funny comments. He said that I haven’t really read it, but if it was right, I would change my career to study swine flu. I find his comment very funny.

After all these prelude, I now briefly describe the model. Frankly speaking, I don’t really understand it, so I am not going to comment on it. The model is motivated by the Lifshitz point, which is used to model the triple point in condensed matter physics. By the way, this is another example that ideas in high energy physics originate from condensed matter physics. In this model there is critical exponent z, if it is 1, we get relativistic dispersion relation. This model is z=3 in the UV, and it becomes z=1 in the IR, and Lorentz symmetry is recovered as an accidental symmetry. The model is power counting renormalizable. In this model, space and time scales in differently, so that the the theory is rendered power counting renormalizable. Note that power counting renormalizable is just a first test of renormalizability. One really needs to check in details if it is indeed renormalizable. During the talk, some of the audience, in particular Gruzinov was increasingly impatient as the Lagrangian introduced was rather ad hoc. I don’t really understand it so I am not going to comment on that, just to say that the terms added are mostly spatial, and of high derivatives. Needly to say, it is very different from the usual GR and field theory. Gruzinov even said that life was short, why do we spend time on such a theory. Kleban reminded him that he should blame himself for asking him to present the paper. People were worried about about a parameter introduced in the theory, which start as 1/3 in UV, it has to flow to 1 in order to agree with GR in the IR regime. After browsing the whole paper, Horava did not say (or know ) how to realize this flow. Porrati said that it was almost impossible for any perturbative effect drive the parameter to flow from 1/3 to 1. At the end, nobody was happy about this paper. Gruzinov seemed to be pissed off. I personally find the talk rather amusing. The comments by various people, in particular Porrati and Gruzinov very interesting and illuminating.

So it was pretty funny that last week everybody was excited about this paper and this week everybody loses interest in it. We have to wait for another breakthrough…

Update: I would like to add a few comments on it. Nobody really knows how to come up a workable quantum theory of gravity. So anybody can take strong opinion in this regard. But anybody that asks the right question is much much more likely to succeed than others. So Porrati and Gruzinov think that Horava’s approach is not going to work. Fine.

But shouldn’t it be obvious that anything that is obvious is not going to work. Anything that is going to work is not going to be obvious. The reason that those smart people fail to come up with a sensible theory of quantum gravity maybe because they are too strongly biased. I defend this theory does not mean that I believe that this the right theory. But I certainly think that this is an interesting and novel idea. OK, Horava did not lay out the model in details. But I think he has set up a proposal that is interesting enough to pursue it further. One of the reason that so many people have jumped into this theory because they think that it is interesting and worth trying. So my philosophy to these kind of difficult open questions is that we should keep mind opened.

这个世界上,还存在很多有品位的人,虽然我们不时为没有品位的花车乐队打扰了。

Porrati同学说,如果Horava理论是正确的,他宁愿改行去研究猪流感。研究猪流感的确是一个高尚的事情。假如Horava是对的,我该干什么呢?我想我改行研究中国股市去,也许那东西更靠谱一点。

下面转贴Motl的博文。

NYU about Ho?ava-Lifshitz gravity

Motl

Keith Chen wrote a rather illuminating story about the fate of Ho?ava-Lifshitz gravity (click and read this text first!) at the NYU. Journal clubs are apparently alive and well over there. I think that they’re important for the health of science as seen from a broader scientific community’s viewpoint.

Andrei Gruzinov, a professor of theoretical astrophysics, wanted this paper to be analyzed - because of those “cool properties” that have been said about it (including its being a theory of quantum gravity and a replacement for inflation), mostly in dozens of Asian papers. Andrei Gruzinov asked Matt Kleban to present the paper in the journal club.

Their analysis focused on the question whether the heavily nonrelativistic “z=3″ theory in the ultraviolet can flow to the relativistic “z=1″ theory in the infrared. Massimo Porrati gave some arguments why it was not possible. Incidentally, although he hadn’t read the whole paper(s), Porrati claimed that if the paper were right, he would switch his field to swine flu. ;-) At the end, the previously excited people considered the attempt to be a failed one and they have lost interest in it.

So did I. Let me summarize the main problems:

* in the IR, normal GR should be reproduced; but this effective field theory needs all the spacetime diffeomorphisms to be gauged, in order to eliminate all unphysical polarizations of the waves; because the spacetime diffeomorphisms are broken, one can’t get rid of all the unphysical states
* it is questionable whether the very qualitative flow from “z=3″ to “z=1″ is possible, starting from a small perturbative deformation in the ultraviolet
* even if it were possible, one would need a huge amount of fine-tuning to restore the Lorentz symmetry in the IR (so strongly constrained by the observations), especially once all the matter fields are added
* the Lorentz symmetry violation in GR leads to many likely inconsistencies, including the possibility to construct perpetuum mobile gadgets of the second kind, at least in principle
* even the very scale invariance of the hypothetical UV theory doesn’t seem to have been demonstrated beyond the classical level

In my opinion, these are big problems and every single one of them would be enough to kill my interest in a theory. Their combination is even more lethal. If you have something really substantial to say about those technical problems, you’re welcome.

Philosophy of science

Meanwhile, I want to say some important things about the philosophy and sociology. I find the “postmodern” opinions of many young people, including those whom I don’t find personally unpleasant, extremely worrisome. For example, Keith writes:

So it was pretty funny that last week everybody was excited about this paper and this week everybody loses interest in it. We have to wait for another breakthrough.

Well, this is how science works. We can never be sure that we’re right: we can only be sure if we’re wrong. Theories are being abandoned once they disagree with some aspects of the observations. That’s what has apparently happened with this attempt, too. The theories that matter are those that can survive some tests - for some time that can be either long or very long. At the end, more demanding experiments find some problems, too (with the final theory of everything being the only possible exception).

Indeed, it often takes a two-hour journal club to settle such things and kill a hypothesis. Two hours is not a microscopic amount of time. That’s because Petr Ho?ava is a thoughtful physicist. Similarly ambitious, average papers written by less thoughtful physicists than Petr take about 5 minutes to be killed - or 4 minutes, ask Lee Glub Glub Glub Smolin at the bottom of the sea for the most up-to-date timing. :-)

Keith adds a few comments:

Nobody really knows how to come up a workable quantum theory of gravity.

This sentence reveals Keith’s complete misunderstanding of the last 35 years (OK, 34.5) in theoretical high-energy physics. But let’s focus on the philosophy here:

So anybody can take strong opinion in this regard. But anybody that asks the right question is much much more likely to succeed than others.

Except that the journal club has led to the conclusion that the right questions have not been asked in this paper. “Right questions” are not the same things as “any questions”.

But shouldn’t it be obvious that anything that is obvious is not going to work. Anything that is going to work is not going to be obvious.

Well, more precisely, anything that is going to succeed in the future is going to be slightly or very different from the things that have been tried and that have failed in the past. The previous sentence is a tautology.

But whether the new things will be obvious depends on a subjective judgement. For example, I think that the Ho?ava-Lifshitz Lagrangian - or any similar Lorentz-violating attempt to circumvent the failures of Lorentz-invariant local field theories of gravity - is obvious. The only problem is that it seems to be wrong, too.

Whether or not things look “obvious” to you depends on your definition of “obvious” and your emotions, education, and experience. Moreover, the more experienced the people will become, the more “obvious” those theories will look to them. But that’s not what science is all about. Science is about hypotheses’ being right or wrong, not about hypotheses’ being “obvious”. The latter should be left to the educators who decide how much time they should spend with XY at schools - or to the journalists who try to hype some mind-boggling results. It has nothing to do with the scientific research.

But Keith’s ideas get much worse:

The reason that those smart people fail to come up with a sensible theory of quantum gravity maybe because they are too strongly biased. I defend this theory does not mean that I believe that this the right theory.

Again, the statement that we don’t know the right theory of quantum gravity is a misunderstanding of the last 35 years in theoretical high-energy physics. But the philosophical background behind the two sentences is even sicker.

In science, it is simply not possible to defend a hypothesis or a statement without rational arguments. You may feel compassionate when hypotheses are being killed just like cars that are being made in Henry Ford’s flow production. But you’re not acting as a scientist if you allow these emotions to influence your decisions because the falsification of hypotheses is the most basic and most paramount among all tools that science uses and has to use.

In science, one can’t really defend theories against falsification. The closest thing one can do is to show that the would-be falsification is actually incorrect. For example, chiral superstring theories were believed to be dead in the early 1980s because they were thought to be spoiled by anomalies.

Green and Schwarz believed otherwise. But they actually had to make very extensive, detailed calculations showing that the “incomplete” proof of the nonzero anomalies (accepted by most physicists in the field) was actually incorrect and that all the anomalies canceled when calculated exactly. Without this result, the first superstring revolution of 1984 couldn’t have taken place. And indeed, it would have been profoundly irrational to switch to a theory that was believed, for pretty good (but not quite good) reasons, to be anomalous. You know, it doesn’t matter how beautiful a theory is or what the name of the author is.

The calculation of the anomaly cancellation was no detail. It was one of the key results deciding whether the theory was right or wrong. And by all these criteria (and according to hundreds of other tests that have been successfully made between 1984 and 2009), superstring theory turned out to be right, at the end. But if you have a “similar” hypothesis without such an anomaly cancellation - an argument showing that the “falsification” is actually incorrect - you simply can’t assume that the cancellation is a “formality”. It’s not. A hypothesis is dead unless and until a mistake is found in the falsification.

Scientists have made many mistakes in the past. Many of them have been way too dogmatic. They didn’t want to listen to a certain new type of evidence. They often needed a much longer time to accept a new, demonstrably better theory than the time that the ideal scientists would have needed. But despite all the scientists’ imperfections, it must still be rational arguments - sometimes more extensive and accurate arguments than the ideal scientists would find necessary - that can change the ideal scientists’ opinions as well as the real scientists’ opinions.

If a scientist needs too much time to understand a new theory - and why it is better than the old one - he is an imperfect scientist. But if he is generally deciding according to emotional rather than rational criteria, he is not a scientist at all. That’s much worse. Imperfections slow science down but they are largely inevitable given the finite intelligence and limited integrity of the humans. However, dominant emotional criteria kill science.

Everyone who wants to fill science with emotional arguments and criteria that should have the same impact as technical arguments - but these technical arguments actually don’t exist - is killing science and replacing it with superstitions. Just like the real market imperfections can’t be “cured” by communism, the imperfections of the “market of ideas” can’t be cured by the postmodern feminist pseudoscience. The only thing that these “cures” can do is to kill prosperity and/or science.

So what Keith calls “bias” is actually nothing else than the scientific integrity itself. Keith adds:

OK, Ho?ava did not lay out the model in details.

Well, I actually think that he did. He has made something that is close to the best possible case for this idea.

But I think he has set up a proposal that is interesting enough to pursue it further.

This is exactly what the NYU professors did, isn’t it? This is why they ordered Matt Kleban to present the paper in the journal club. Keith Chen may dislike the outcome. But in science, what matters are the correct outcomes, not outcomes that someone likes. And one simply cannot write papers with pre-determined “signs” of the results.

One of the reason that so many people have jumped into this theory because they think that it is interesting and worth trying.

This sentence shows Keith’s inherent inconsistency and bias. When it comes to the NYU people, he thinks that their opinions are “biased” and something is wrong with the sociology: people can’t be trusted. But when it comes to some Asian authors of not-quite-penetrating followup papers, Keith decides to praise them and to repeat some uncritical hype. Where does the different attitude come from, Keith?

So my philosophy to these kind of difficult open questions is that we should keep mind opened.

Well, the disadvantage of replacing science with an “open-mind philosophy” is that you won’t be able to do any science - because every scientific result and every event in science that matters is closing a small hole in the mind (and, sometimes, opening new holes in a piece of mind that was previously unknown). Every result is filling a hole in our Swiss cheese of knowledge and ignorance. Being constantly open-minded is exactly equivalent to the ban on any progress in science, Keith. Opening one’s mind entirely also makes it possible for the mind to completely evaporate. ;-)

“Fortunately” for Keith, it’s not what he’s doing. He’s keeping his mind about many key issues - such as our knowledge of a consistent theory of quantum gravity - hermetically closed: he has only been closing the holes according to very different criteria than the rational, technical arguments that science is based upon.

And that’s the memo.

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