博客李淼

奥运还没有结束，但很快就结束了，也许那些选择不看开幕式的人今晚会看闭幕式。

另一个聚会也结束了，这是发生在欧洲核子中心的Strings 08。欧洲即将启动的人类历史上规模最大的实验似乎与我们没有任何关系。奥运伸展人类的身体极限，LHC则伸展人类科学和技术的极限。

22号我们去了水立方看了花样游泳规定动作的比赛，目睹了中国队历史性地拿到第三名，昨天自选动作也拿到了第三名。

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奥运说白了只是一次娱乐大餐。一次成功的奥运首先娱乐了全世界，其次展示了一个国家的实力。但这种展示也是有限的，并不能改变我们需要关闭一些工厂、实行单双号才能使得北京的空气基本达标这个事实，也改变不了大约有五亿人一天的收入在20元人民币左右这个事实（NBC开幕式的解说是一天2个美元）。同样，我们应该忘记仅仅是一百天前发生的那次人类大灾难吗？

一个国家在明显前进的态势中，更多的人需要谨慎地乐观，而不是像我们在网上看到的一些人的狂欢。

奥运公园的确很大很漂亮，不论是规模还是现代化的程度远远超过很多年前我在罗马看到的罗马奥运公园。目睹这个奇观，就像我前些天看到北京的蓝天，总有点觉得不真实，总害怕这是时空中的局部现象。

50块金牌和100块奖牌，也不能调动我在观看亚特兰大奥运时的那种感觉，那时我为中国每一块金牌感动。看到熊倪在三米板上的最后一跳，我高兴得趴在当时我们在East Providence家里的地毯上，半天爬不起来。我将所有的跳水比赛从预赛录到决赛，后来不断地看，甚至看慢镜头。昨天，男子十米跳台决赛，我和老婆两人在看了第一跳之后居然在沙发上齐齐睡着了。到了倒数第二跳，我才醒。

金牌的多少已经没有多大关系了。我觉得这是最大的进步，我们已经不需要少数人的金牌来提高自信，也许这个国家的很多人自己有了独立的自信。

至于展望，我想拷贝一下吕木头那里的David Gross对弦论的展望。奥运不需要去展望，只要人类还没有消失，就会有奥运，同样也会有战争，如同这届奥运开幕时一场战争同时在北方发生了一样。

下面是吕木头总结的Gross的展望（Strings 2008: Friday）：

David Gross looks in the future and he was the optimal choice for a public talk. He starts by thanking the organizers for a wonderful conference, especially for the great review talks in the morning. Gross won’t thank that he was asked to speak.

Gross decided to be completely revolutionary and he won’t use any PowerPoint, blackboard etc. (Also because he had no time to prepare them - but he is still a revolutionary!) Someone else was able to speak about planets without any tools: it was a tour de force. Gross says that others like Elias know Gross as the last grandfather who remembers interactions of theory and experiment.

David recalls that 40 years ago, CERN was a center of theory while the centers of experiments were elsewhere, e.g. SLAC. David wrote one of the first papers to generalize Veneziano’s formula. The paper had a contrived name and it was a failure. It’s nice to see that CERN and string theory survived for 40 years and are both flourishing. It’s amazing and a lot of things have happened.

Both survival stories are fascinating. CERN is pretty much the only lab that survived. Also, only one theoretical umbrella survived in theoretical physics and it has eaten everyone else. So we can see people like string theorist Andrei Linde here, string theorist Lance Dixon, …

The only discipline that string theory has not yet eaten is loop quantum gravity but Gross is not sure whether we want to! Needless to say, crackpots are appalled by Gross’ comments! String theory will of course never be buried because it already includes the Standard Model and it is a working model of quantum gravity and a unifying theory, much like a tool for nuclear physics, condensed matter physics, mathematics. No one expected that 40 years ago when it was a half-baked theory to describe the nuclear force.

Much like other string meetings, it’s been extraordinarily exciting. Unlike other conferences, there have been no great revolutions. There have been a couple of minirevolutions, or as Jeff says, insurrections.

Someone can’t understand how they can do experiments with 2,000 collaborators, and Gross is not certain either. But it’s because it’s not one experiment but one world of experiments. And string theory is kind of similar.

Gross was also excited to see the LHC that will run in two weeks and feed us with new data. When it comes to string theory, the progress in the calculation of gauge-theory quantities is remarkable. They were beautifully reviewed and discussed and the progress both for QFT and string theory was enormous. Gross was really impressed by fermionic T-duality explanation for the dual superconformal symmetry. Ah! It’s so obvious when we see it.

Together with the unbelievable advances in integrability of the closed sector of the N=4 theory, we are gaining evidence that the planar limit will be solved. We may be close to it. Janik’s calculation of the Konishi operator anomaly is just stunning.

And the membrane BLG-ABJM is just beautiful and will lead to hundreds of papers in the two years. A lot of tools and applications. Progress in string phenomenology is beautiful. It’s great to see Cumrun transformed into a phenomenologist - a bottom-up one.

Things were not discussed here: there was no talk on string field theory and the only anthropic talk was moved after a dinner. So he doesn’t have to talk about them and he won’t. But let’s ask: what do we really want to learn from the LHC? We will discover SUSY because much like your humble correspondent, Gross has a lot of bets on it, too. SUSY will be extremely important to learn about but it won’t be enough to crack the deep questions or to prove that string theory is right as the “candidate”.

We might be lucky to produce black holes (whispering) but it is very unlikely. Nature owes us something - something totally unexpected. Of course, we will abruptly realize that the unexpected thing was an obvious prediction of string theory. Condensed matter physicists found the Hall effect “obvious” once it was seen but even at that time, they were not visionary enough to predict the fractional one.

Gross is going to be critical. String theorists in Hiroši’s 1,2,3,4 were following the lamppost strategy. There are thousands of lampposts. They’re really lasers, not old-fashioned lampposts, and not enough work is dedicated to connecting the lasers and build new lampposts. Gross was disappointed not to hear about three kinds of things. (Young people, go to the darkness!)

Gross wanted to hear an alternative to the anthropic principle, which we know is wrong, right. (Yes, of course, David!) But to do better, we must explain the cosmological constant differently. Gross wanted to hear more about time dependence, besides Simeon, including string cosmology. Stationary/BPS lampposts are safer and good cosmological lampposts are not available.

Third, what is string theory? Everyone is testing AdS/CFT. Maybe it’s time to admit that it’s true. There’s no value of further tests. No one has any doubt that it is true. When someone says “this is another test”, people should ask “what is it good for?” Maybe, can you use the tests to answer What is string theory?

AdS/CFT is the equivalence between two rich, but not impossible, structures. That makes it priceless. We have other, less useful dualities where one side is trivial and the other is hard. For example, there’s a duality between perturbative string theory (for 4 decades) which is easy and classically solved (Veneziano) and a theory - Matrix theory - that’s impossibly hard to calculate.

If we fully solve the planar limit in a few years, have we solved string theory? No, only classical string theory. If you manage to go to finite N, is it a solution to string theory? Well, if you made it independent of the background and go e.g. to the flat space, by adding higher-dimension operators (moving you away from the horizon). It seems impossible right now to allow the motion in all directions. Non-commutative theories perhaps give us hints of some safe directions where you can move.

This was about the question What is string theory? What fundamental equations govern all of it? These are the future topics he would love to hear. But Gross doesn’t want to end up with a pessimistic mood. It’s, in fact, fantastic that we have such great questions. The history of the field assures us that it will remain rich and dynamic for many years. And we should be assured to get some hints, and maybe crucial ones, from Nature.

这篇文章发表于 2008年8月24日, 星期天 13:26:51，属于分类 闲聊, 理论物理。 你可以下载本文的 PDF版本和LaTeX源文件，查看 HTML打印版 或通过 新闻聚合(RSS) 来追踪这篇文章的最新评论。