By now, just about everyone has heard of string theory. Even those who don’t really understand it-which is to say, just about everyone-know that it’s the hottest thing in theoretical physics.

But despite its extraordinary popularity among some of the smartest people on the planet, string theory hasn’t been embraced by everyone-and now, nearly 30 years after it made its initial splash, some of the doubters are becoming more vocal. Skeptical bloggers have become increasingly critical of the theory. (1)Mathematician Peter Woit and Professor Lee Smolin both argue that string theory (or superstring theory, as it is also know) is largely a fad propped up by practitioners who tend to be arrogantly dismissive of anyone who dare suggest that the emperor has no clothes.

There were good reasons for the theory’s appeal when it first emerged in the late 1970s and early 1980s. At the time, physicists found themselves facing a crisis： the two most important ideas of 20th century physics, relativity and quantum theory, were known to be fundamentally incompatible. Quantum theory describes the universe as intrinsically discontinuous: energy, for example, can come in bits just so small, but no smaller. Relativity treats time and space and gravity as a smooth, unbroken continuum. Each theory has its purpose, and the usually don’t overlap.(2)But when dealing with large masses or time periods that are infinitesimally small, like the core of a black hole or the first moments after the Big Bang, neither quite works. So lots of physicists began working on string theory.

Since then, however, superstrings have proved a lot more complex than anyone expected. The mathematics is extremely tough, and when problems arise, the solutions often introduce yet another layer of complexity. (3)Complexity isn’t necessarily the kiss of death in physics, but in this case the new, improved theory posits a nearly infinite number of different possible universes, with no way of showing that ours is more likely than any of the others.

That lack of specificity hasn’t slowed down the string folks. Maybe, they’ve argued, there really are an infinite number of universes-an idea that’s currently in fashion among some astronomers as well-and some version of the theory describes each of them. That means any prediction, however outlandish, has chance of being valid for at least one universe, and no prediction, however sensible, might be valid for all of them.

However, it is that absence of proof that is perhaps most damning. (4)Physicists have a tolerance for theory;indeed， unless you were there to witness a phenomenon yourself-the Big Bang, say-it will always be, at some level, hypothetical.But the slow accretion of data and evidence eventually eliminates reasonable doubt. Not so-or at least not yet-with strings.

It’s true that nobody has any good idea of how to test string theory, but who’s to say someone won’t wake up tomorrow morning and think of one? (5)The reason so many people keep working on it is that, whatever its flaws, the theory is still more promising than any other approach we have.

迄今为止，几乎每个人都听说过弦理论。即使那些并不真正了解这个问题的人--也就是说，几乎每个人--都知道这一理论是理论物理学中最热门话题。

尽管弦理论在地球上一部分最聪明的人中极为流行，但它并未为所有人接受--如今距其问世之初的轰动已近30年，怀疑论者也日益直言。持怀疑态度的博客们越来越多地批评这一理论。(1)数学家彼得·沃特和李·斯莫林教授都认为，弦理论(有时也被称为超弦理论)很大程度上是被一群自命不凡、总是鄙视那些敢于怀疑皇帝新衣的人们鼓吹起来的流行时尚。

弦理论在20世纪70年代末至80年代初刚刚面世时具有吸引力，有其充足的理由。当时，物理学家们发现自己面临着一个危机：相对论和量子论这两个20世纪物理学最重要的理论，当时被认为是从根本上不相容的。量子论把宇宙描述为本质是不连续的，比如能量可以以很小的单位元出现，但不能更小。相对论则将时间、空间及重力看作光滑、不可分割的连续统一体。两个理论各有其目的，通常它们互不交迭。(2)但是在研究大质量物质或无限小的时间段时，比如，对黑洞的核或者是宇宙大爆炸后的最初时刻，二者都不能完美地加以解释。于是很多物理学家便开始了对弦理论的研究。

然而，从那时起，超弦理论被证明比人们预料的更为复杂。数学上的求证极其艰难，而当问题出现时，研究结论又往往引出另一层复杂的问题。(3)在物理学中，复杂不一定就不好;但对于超弦理论而言，这种新改进的理论假定存在着几乎无限多的、各种可能的宇宙，但是，又没有办法证明我们的这个宇宙比其他的更为真实可信。

虽然缺乏特色，但却并没有使那些"弦人"(弦理论家及其追随者)的步伐放慢。他们认为，也许真有无限多个宇宙存在--这一观点在部分天文学家中也同样流行--并且弦理论的某种说法阐述了其中的每个宇宙。这意味着，无论怎样怪异，任何一种预测，都有可能对至少一个宇宙是正确的;与此同时，无论怎样合理，没有一种预测对所有宇宙都是正确的。

缺乏实验证明也许是弦理论最致命之处。(4)物理学家们对于学说有相当的容忍度;事实上，除非你亲临目击了一个独特事件--比如说，宇宙大爆炸--它在一定程度上永远是一个假设。然而，随着资料和证据的逐渐增多，合理的怀疑最终将被消除。但弦理论却不是如此，或者说至少现在不是这样。

确实，怎样检验弦理论，现在没有什么好主意，但是谁又能说某人明早醒来时不会想出一个好主意呢?(5)之所以有如此众多的人从事其研究，其原因在于，无论弦理论有着怎样的瑕疵，比起我们所有的任何其他探索途径，弦理论仍然是目前最有前景的学说。