科学与翻译

A new model of proteins seeks to explain how enzymes extract energy form their vicinity and put it to use in regulating cell chemistry. Enzymes are huge protein molecules that play a crucial role in catalyzing chemical reactions among other molecules or atoms by lowering the energy barrier that would otherwise keep the reaction from happening. Enzymes can therefore be considered as energy-processing chemical-reaction-facilitating machines.

They are usually large, typically containing thousands of heavy (non-hydrogen) atoms, but of these only a few dozen atoms actually participate in the catalytic process. Addressing this important issue, a team of scientists at the Ecole Normale Superieure (Lyon, France) and the Ecole Polytechnique Federale de Lausanne (Switzerland) have concentrated on modeling the behavior of the stiff parts of the enzyme since they believe that some of the energy used in carrying out the catalytic task is stored not just as chemical energy (in the form of adenosine triphosphate, or ATP, the all-purpose “food” of cells) but also as mechanical energy in the form of a waggling or “breathing” motion in the stiffer parts of the enzyme.

Extending this research to proteins in general, Yves-Henri Sanejouand says that he and his colleagues would like to scrutinize in more detail the nonlinear process by which some proteins catch and store thermal energy from their environment and also how chemical energy can be turned into mechanical energy, such as in muscle contraction.

原文：http://www.aip.org/pnu/2007/split/846-2.html

这是Nature上最近的一篇科学新闻。有理论家认为暗能量是宇宙虚空的体现，或者说暗能量是不存在的。

As an alternative hypothesis, he suggests that our Galaxy might lie in a huge bubble of comparatively empty space. The denser area of space outside this bubble would then pull material towards it. To an observer inside the bubble, it would seem as though a dark-energy-like force was pulling the Universe apart.

研究文章：

1. Is the evidence for dark energy secure?
2. Extragalactic Radio Sources and the WMAP Cold Spot

A chart of the density of our Universe. Different colours have been used to show different types of galaxies; the black spaces show low-density gaps.Sloan Digital Sky Survey Team, NASA, NSF, DOE

(more…)

The human eye is very sensitive but can we see a single photon? The answer is that the sensors in the retina can respond to a single photon. However, neural filters only allow a signal to pass to the brain to trigger a conscious response when at least about five to nine arrive within less than 100 ms. If we could consciously see single photons we would experience too much visual “noise” in very low light, so this filter is a necessary adaptation, not a weakness.

Some people have said that single photons can be seen and quote the fact that faint flashes from radioactive materials (for example) can be seen. This is an incorrect argument. Such flashes produce a large number of photons. It is also not possible to determine sensitivity from the ability of amateur astronomers to see faint stars with the naked eye. They are limited by background light before the true limits are reached. To test visual sensitivity a more careful experiment must be performed.

The human retina at the back of the eye has two types of receptors known as cones and rods. The cones are responsible for colour vision but are much less sensitive to low light than the rods. In bright light the cones are active and the iris is stopped down. This is called photopic vision. When we enter a dark room the eyes first adapt by opening up the iris to allow more light in. Over a period of about 30 minutes there are other chemical adaptations which make the rods become sensitive to light at about a 10,000th of the level needed for the cones to work. After this time we see much better in the dark but we have very little colour vision. This is known as scotopic vision.

The active substance in the rods is rhodopsin. A single photon can be absorbed by a single molecule which changes shape and chemically triggers a signal which is transmitted to the optic nerve. Vitamin A aldehyde also plays an essential role as a light-absorbing pigment. A symptom of vitamin A deficiency is night blindness because of the failure of scotopic vision.

It is possible to test our visual sensitivity by using a very low level light source in a dark room. The experiment was first done successfully by Hecht, Schlaer and Pirenne in 1942. They concluded that the rods can respond to single quanta during scotopic vision.

In their experiment they allowed human subjects to have 30 minutes to get used to the dark. They positioned a controlled light source 20 degrees to the left of the point on which the subjects eyes were fixed so that the light would fall on the region of the retina with the highest concentration of rods. The light source was a disk which subtended an angle of 10 minutes of arc and emitted a faint flash of 1 millisecond to avoid too much spatial or temporal spreading of the light. The wavelength used was about 510 nm (green light). The subjects were asked to respond “yes” or “no” to say whether or not they thought they had seen a flash. The light was gradually reduced in intensity until the subjects could only guess the answer.

They found that about 90 quanta had to enter the eye for a 60% success rate in responding. Since only about 10% of photons which arrive at the eye actually reach the retina this means that about 9 photons were actually required at the receptors. Since the photons would have been spread over about 350 rods they were able to conclude statistically that the rods must be responding to single photons even if the subjects were not able to see them when they arrived too infrequently.

In 1979 Baylor, Lamb and Yau were able to use rods from toads placed into electrodes to show directly that they respond to single photons.

原始链接：Can a Human See a Single Photon?

Personal prejudices and a lack of understanding by the Nobel-prize committee left the pioneers of quantum mechanics unrewarded until the discovery of antimatter in 1932.

In 1933 the Nobel prizes seemed of little importance compared with the global economic depression and the rise to power of the Nazis, but many physicists still kept a watchful eye on Stockholm. Their bewilderment and chagrin over the most recent decisions by the Royal Swedish Academy of Sciences had fuelled anticipation. No prize had been awarded in physics since 1930, yet recent theoretical and experimental achievements had led to a revolutionary new quantum-mechanical depiction of the atom. Would the Academy finally acknowledge these accomplishments?

When the Academy eventually announced its decision in November, the results pleased some, angered others and puzzled many. The prize reserved from 1932 went to Werner Heisenberg alone for “the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of allotropic forms of hydrogen”. Meanwhile, the 1933 prize was shared by Erwin Schrödinger and Paul Dirac for “the discovery of new productive forms of atomic theory”.

The prizes for quantum mechanics have long been the subject of speculation and gossip. Why were these men the only ones to be rewarded, why were the prizes divided so awkwardly, and why was the official rationale for the awards so odd? More generally, the 1933 decisions point to the broader question that peppers both popular and scholarly histories of modern physics: why were so few Nobel prizes awarded for theoretical contributions? Was this the result of Alfred Nobel’s testament, which specifies that the prize is awarded for “discovery or invention in the field of physics”? Is it inherently more difficult to define a theoretical breakthrough as a discovery?

I have studied the Nobel archives, and the correspondence of former committee members, in an effort to clarify the reasons for the traditional neglect of theory, as well as to make sense of the 1933 prizes. These activities have provided an insight into the committee’s treatment of theoretical accomplishments prior to 1933, which helps us to understand the significance of the awards that year, including the last-minute inclusion of Paul Dirac among the winners.

Academy rewards

The Nobel prizes may well be international in scope but from the start the Royal Swedish Academy of Sciences based its decisions on the recommendations of the five members of the Nobel committees for physics and chemistry. The Swedish committee members’ own judgement, their understanding of science and their interests have been critical to the outcome. Those scientists invited to submit nominations rarely provided the committees with a clear consensus. And even when a single strong candidate did emerge - such as Albert Einstein for relativity theory or Henri Poincaré for various contributions to mathematical physics - the committees often ignored the mandate. A simple change in the composition of the committee could, on occasion, decide the fate of a candidate.

Although the five committee members evaluated the candidates and proposed who should receive a prize, their recommendation still had to be approved by the ten members of the Academy’s Physics Section, and then by the 100 members of the full Academy. Usually the committee’s authority prevailed, but not always. Sometimes the Academy of Sciences rebelled against its committees. In the cases of Gustaf Dalén (1912) and Jean Perrin (1926), members of the Academy successfully rallied their colleagues to overturn the committee’s declaration that these candidates did not merit prizes.

Although formal statutes govern all aspects of the Nobel system, they by no means provide unambiguous guidelines for the committees to go about their business. Such crucial phrases as “most significant discovery or invention in the field of physics” or “recent” or “benefit on mankind” are not defined. Interpretive traditions have arisen and changed over time. But even when everyone involved has tried to rise above pettiness and partiality, the task of selecting winners has always been - and remains - exceedingly difficult. Occasionally committee members have confessed privately that, at times, there have been several equally deserving candidates.

摘自http://physicsweb.org/articles/world/15/8/7，这篇文章较长，只需翻译以上部分。

A preliminary analysis of data from the Gravity Probe B satellite has confirmed that the Earth’s mass distorts the fabric of space and time as predicted by Einstein’s theory of general relativity. Although this “geodetic effect” has already been proven with greater accuracy through other measurements, the Gravity Probe team claim that their successful analysis paves the way for using data from the satellite to make a very accurate measurement of a second, much subtler consequence of general relativity called “frame-dragging”. However, some physicists are questioning this claim and asking if the final results will be worth the probe’s $700 million price tag.

The Gravity Probe B (GP-B) satellite is a collaboration between NASA and Stanford University and was launched in 2004 with an aim to study two effects predicted by general relativity, a theory first put forth by Einstein in 1915. In addition to the geodetic effect, the theory also predicts that massive bodies will pull space and time along with them as they rotate — an effect called frame dragging.

Now analysis of the data from GP-B has confirmed the geodetic effect with an accuracy of better than one percent. Although the same effect has already been measured by NASA’s Cassini mission, the results indicate that the much subtler frame-dragging effect should be confirmed by further data analysis by the end of this year. Frame-dragging has also been measured before by NASA’s LAGEOS satellites with an accuracy of ten percent, and it is currently unclear whether GP-B data will yield a more accurate result.

Gravity Probe B used superconducting quantum interference devices (SQUIDs) to measure tiny changes in the orientations of four perfectly-spherical, quartz gyroscopes as the experiment orbited the Earth for one year. The gyroscopes were housed inside a vacuum chamber and were maintained at 1.8 Kelvin during the measurements using liquid helium. The probe also includes a telescope that was trained on a distant “guide star” to provide a reference direction for measurements on the gyroscopes. General relativity predicts that the frame-dragging effect will cause the direction of the gyroscopes to change by a tiny 0.041 of an arc second.

Prior to launch, however, the satellite suffered numerous delays, and now there is the possibility that the accuracy of its data will not surpass that of other experiments performed before now. “On one level one can say that [Gravity Probe B] is a fantastic triumph of engineering — nobody has ever done an experiment like this before,” Clive Speake, a physicist from the University of Birmingham, told Physics Web. “On the other hand, one can’t do these experiments for fun. We have to wait until the frame-dragging result comes out.”

原文：Gravity Probe B backs general relativity

参考翻译：

对引力探测器B卫星（Gravity Probe B）发送回来数据进行的初步分析表明：地球本身的质量造成了时空结构的扭曲，这和爱因斯坦的广义相对论所预测的一致。在短程线效应（geodetic effect）已经由其它高精度测量手段确认存在后，该项目小组声称对另一广义相对论的预测——坐标系拖曳效应（frame-dragging），也将因此次探测计划的成功而可以进行精确的测量。但仍有学者对此存有疑问，那么最终的实验结果和探测仪器的7亿美元身价是否相称呢？

引力探测器B卫星(GP-B)是美国宇航局和斯坦福大学合作的产物，于2004年发射，其目的就是研究1915年爱因斯坦广义相对论预言的两种效应。除了短程线效应以外，另一个就是坐标系拖曳效应，其表现就是大质量物体会在旋转的同时拖着周围的时空一起旋转。

现在从GP-B所得数据做出的分析可以使短程线效应的测量精度控制在1%以下。虽然美国宇航局的卡西尼计划已经对此进行过测量，此次的结果表明更难探测到的坐标系拖曳效应还需要进行分析，这个工作要持续到年底。之前LAGEOS卫星对坐标系拖曳效应的测量精度为10%，不知道GP-B能获得何种精度的结果。

引力探测器B采用超导量子干涉仪(SQUID)对其搭载在卫星上的四个完美球面石英陀螺仪在一年绕地运行过程中所发生的微小变动进行测量。陀螺仪被安置在真空室内，由液氦处理使温度保持在1.8K。探测器还安装一台望远镜，将其一直对准一颗远距离的“基准恒星”作为测量参照方向。广义相对论预测坐标系拖曳效应会使这些陀螺仪的指向（自转轴）发生0.041弧秒的偏转。

虽然这次发射是优先进行的，不过该项目的实施已经多次延后。现在看起来，测量数据的精确性可能不会超过此前的实验。伯明翰大学的物理学家Clive Speake在接受物理网（Physics Web）采访时表示：“一方面，‘重力探测器B’是一项工程技术领域的梦幻之作，之前从来没有人会想到还可以做这样的实验。不过另一方面，实验不是游戏，我们还需要等待坐标系拖曳效应的最终结果。”

Some physicists are uncomfortable with the idea that all individual quantum events are innately random. This is why many have proposed more complete theories, which suggest that events are at least partially governed by extra “hidden variables”. Now physicists from Austria claim to have performed an experiment that rules out a broad class of hidden-variables theories that focus on realism — giving the uneasy consequence that reality does not exist when we are not observing it (Nature 446 871).

Some 40 years ago the physicist John Bell predicted that many hidden-variables theories would be ruled out if a certain experimental inequality were violated – known as “Bell’s inequality”. In his thought experiment, a source fires entangled pairs of linearly-polarized photons in opposite directions towards two polarizers, which can be changed in orientation. Quantum mechanics says that there should be a high correlation between results at the polarizers because the photons instantaneously “decide” together which polarization to assume at the moment of measurement, even though they are separated in space. Hidden variables, however, says that such instantaneous decisions are not necessary, because the same strong correlation could be achieved if the photons were somehow informed of the orientation of the polarizers beforehand.

Bell’s trick, therefore, was to decide how to orient the polarizers only after the photons have left the source. If hidden variables did exist, they would be unable to know the orientation, and so the results would only be correlated half of the time. On the other hand, if quantum mechanics was right, the results would be much more correlated – in other words, Bell’s inequality would be violated.

Many realizations of the thought experiment have indeed verified the violation of Bell’s inequality. These have ruled out all hidden-variables theories based on joint assumptions of realism, meaning that reality exists when we are not observing it; and locality, meaning that separated events cannot influence one another instantaneously. But a violation of Bell’s inequality does not tell specifically which assumption – realism, locality or both – is discordant with quantum mechanics.

Markus Aspelmeyer, Anton Zeilinger and colleagues from the University of Vienna, however, have now shown that realism is more of a problem than locality in the quantum world. They devised an experiment that violates a different inequality proposed by physicist Anthony Leggett in 2003 that relies only on realism, and relaxes the reliance on locality. To do this, rather than taking measurements along just one plane of polarization, the Austrian team took measurements in additional, perpendicular planes to check for elliptical polarization.

They found that, just as in the realizations of Bell’s thought experiment, Leggett’s inequality is violated – thus stressing the quantum-mechanical assertion that reality does not exist when we’re not observing it. “Our study shows that ‘just’ giving up the concept of locality would not be enough to obtain a more complete description of quantum mechanics,” Aspelmeyer told Physics Web. “You would also have to give up certain intuitive features of realism.”

However, Alain Aspect, a physicist who performed the first Bell-type experiment in the 1980s, thinks the team’s philosophical conclusions are subjective. “There are other types of non-local models that are not addressed by either Leggett’s inequalities or the experiment,” he said. “But I rather share the view that such debates, and accompanying experiments such as those by [the Austrian team], allow us to look deeper into the mysteries of quantum mechanics.”

原文： Quantum physics says goodbye to reality

The harder you push a sled across a frozen pond, the faster it accelerates. In 1687, Isaac Newton quantified this most basic bit of physics with his second law, which states that the force applied to an object equals its mass times its acceleration (F=ma). Now scientists have verified this law with unprecedented precision, challenging critics who have suggested the rule somehow bends for very small accelerations.For almost 300 years, F=ma was the law of the land. Then some physicists began to have their doubts. About 30 years ago, astronomers noticed that stars swirl around the outer edges of galaxies so fast that they ought to fly into space. Most believe some unseen “dark matter” provides the extra gravity that keeps the fast-moving stars from escaping. But in the 1980s, a few researchers noted that the observations could be explained without dark matter if Newton’s second law didn’t quite hold for very small accelerations, a scheme known as Modified Newtonian Dynamics or MOND. Also, NASA’s Pioneer 10 and 11 spacecraft have shown a small, unexplained acceleration toward the center of the galaxy that might also hint at a breakdown in second law.

Those possibilities seem less likely, now that physicists Jens Gundlach and Stephan Schlamminger of the University of Washington in Seattle and colleagues have tested F=ma with unprecedented accuracy. To make the test, the team employed a type of pendulum. Unlike the standard pendulum that swings back and forth, however, this torsion pendulum consisted of a small cylinder that twisted back and forth on the end of a tungsten wire one meter long and 20 micrometers thick. If Newton’s second law holds, the frequency of the twisting should be the same regardless of its amplitude. Smaller twists meant smaller accelerations, and the researchers found that the frequency remained constant–verifying Newton’s law–for accelerations as small as 0.00005 nanometers per second per second, as they report in a paper to be published in Physical Review Letters.

The new result doesn’t quite rule out MOND because the twisting pendulum is still subject to other accelerations, such as the downward pull of gravity, which might hide any deviation because of MOND, Gundlach says. But as for the mysterious effect on the space craft, “It’s not a simple violation of F=ma,” Gundlach says. “That we can rule out.” If Newton’s law is still valid, then there must be some as-yet-unidentified force pulling on the spacecraft to create the mysterious acceleration.

“It’s a very powerful, very useful result,” says Riley Newman, a physicist at the University of California, Irvine. Newman has one quibble, however. The researchers simply assume that the force with which the tungsten fiber tries to unwind is exactly proportional to the amount that it twists, a relation known as Hooke’s law. It’s possible that this relationship is violated in some way that masks a violation of Newton’s law, Newman says. Although such a conspiracy is unlikely, he says, the results “would have been more powerful if they had done this check.”

原文网址：No Twisting Out of Newton’s Law

相关网址：Testing the gravitational inverse-square law

参考译文：

在一个结冰的湖面上，你越用力推一个雪橇，它加速得越快。1687年，艾萨克·牛顿在他的第二定律中将这个物理学中最基本的现象量化了，按照这个定律施加在物体上的力等于它的质量乘以它的加速度（F=ma）。现在科学家以“前所未有”的精度验证了这个定律，给了那些认为牛顿第二定律可能在很小加速度下不成立的理论家一个巨大的挑战。

（自牛顿以后）300年来，F=ma被认为是绝对正确的。但是也有一些科学家开始怀疑。大约30年前，天文学家注意到星系边缘的恒星围绕星系的转动速度过快，快到应该被甩（出现有轨道）到宇宙空间中去，但实际上却没有。大多数科学家认为存在某种看不见的“暗物质”提供了额外的向心力以维持快速转动的恒星，使其不会被甩出去。但在20世纪80年代，少数研究者发现这些现象也能够在没有暗物质的假设下解释得通，只要假设牛顿第二定律在很小的加速度下不完全成立即可，这就是修正的牛顿动力学方案（MOND，Modified Newtonian Dynamics）。美国宇航局的先驱者10号，11号宇宙飞船在飞出太阳系的时候具有一个额外的指向银河系中心的很小的，难以解释的加速度，这也许意味着牛顿第二定律的倒塌。

但现在看来这种可能性很小，西雅图华盛顿大学的物理学家Jens Gundlach和Stephan Schlamminger及其同事已经以空前的精度验证了F=ma的正确。为了做这个试验，他们使用了扭摆。扭摆不像普通的钟摆那样前后摆动，这个扭摆是由一个系在1米长20微米粗的钨线末端的小圆柱组成的，这个小圆柱可以来回扭动。如果牛顿第二定律正确，无论振幅如何变化，扭动的频率将会保持不变。小的扭动意味着小的加速度，研究者发现在小到0.00005nm/s^2的加速度下频率值仍然保持为一个常数，这证实了牛顿第二定律的正确，这些结果将发表在他们最近出版的物理学评论快报（Physical Review Letters）的论文中。

这个新的实验结果并没有完全排除掉修正的牛顿动力学（MOND），因为扭摆本身有其他的加速度，比如受到竖直向下的重力的作用，这可能会掩盖掉一些由于MOND所导致的偏差。但对于作用在飞船上的神秘效应而言，“这并非仅仅是违反了F=ma那么简单，”Gundlach说。“这我们可以排除了。”如果承认牛顿定律依然正确，那么一定还存在我们不知道的力作用在飞船上从而导致神秘的加速度。

“这是一个极有力，极有用的结果，”加州大学欧文分校的物理学家Riley Newman说，但Newman仍有所保留。研究者们可能简单地假定钨线试图恢复原状的力精确地和它扭转的角度成正比，即假设胡克定律可以描述钨线的扭动。但这个关系可能不完全成立，以致在某种程度上掩盖了牛顿定律的不适用。虽然发生这种巧合情况的可能性非常小，Newman说，“如果研究人员把这个因素也排除掉的话将会更有说服力”。

A spontaneous aggregation of proteins randomly determines which of the two X chromosomes in a woman’s cell will remain active, and which one will stay silenced, according to a new physical model.

In all placental mammals, the females of the species have two versions of the X chromosomes while males have just one X, plus a Y chromosome. To avoid overexpression of X-chromosome genes, female cells must virtually shut down one of their X’s. X chromosomes are able to wrap themselves up in a goo of RNA — produced by one of their genes, called XIST — inhibiting the expression of all of their genes.

But until recently, it was not known how a female’s cells know that they have two X’s, how they choose which one to shut down, or how they keep exactly one active. Experiments in mice — the results presumably apply to other mammals — have shown that during early development, each embryo cell has a 50-50 chance of shutting down one X or the other.

Recently it has been proposed that an X remains active when certain proteins aggregate at a specific spot on the chromosome, shutting down its “suicide gene” XIST. But it remained unclear why proteins floating in the nucleus would aggregate around one of the chromosome, but not around the other — an example of what physicists call spontaneous symmetry breaking.

Now an upcoming paper in Physical Review Letters describes a statistical-mechanics model for the proteins’ aggregation that would explain this phenomenon. The model relies on a key discovery published last year, namely that in females the two X chromosomes line up next to each other right at the time when one of them is due to be silenced. For a critical value of the protein’s binding energies, the authors show, there is a high probability that exactly one aggregate will form in the vicinity of the two chromosomes. The aggregate will quickly bind to one of the X’s, shutting down its XIST gene and thus preventing the chromosome from silencing itself.

The model also explains how cells would “count” their X’s. In males, the protein complex would only have one chromosome to bind to, so it would save the single X from self-silencing. On non-sexual chromosomes, a similar mechanism could also determine which of two versions of certain genes is expressed and which one is silenced.

原文：Spontaneous Symmetry Breaking in Women’s Genes

参考译文

根据一个新的物理学模型：自发聚集的蛋白质可以随机地决定两个X染色体中的哪一条在女性的细胞中仍将活跃，哪条将保持沉默。

在所有胎盘哺乳动物中，物种中的雌性个体有两条X染色体,而男性只有一条X, 加一条Y染色体。为避免X染色体上基因的过度表达，雌性个体的细胞要“关闭”它们的一条X染色体。X染色体能够把自己包裹在由它的一个基因产生的RNA粘性物质（称为XIST）中，从而抑制其上所有基因的活化表达。

但直到最近，人们才知道一个女性的细胞是如何知道它们自己有两个X染色体的,并且如何选择哪一个该关闭，或如何仅仅保持一个活跃。小鼠实验结果（也适用于其它哺乳类动物）表明，在早期发展过程中，每个胚胎细胞都有50%的机会关闭某一条X染色体或者是另一条.

近来有人提出当某种蛋白质聚集在一个X染色体特定部位时，这条染色体将保持活跃，并抑制其“自杀基因”XIST。但目前仍不清楚为什么漂浮在细胞核内的这种蛋白质会聚集在一条X染色体上， 而不是聚集在另一条染色体上，这就是物理学家所说的自发对称性破缺。

现在，一篇发表在物理学评论快报上的论文，发展了一个蛋白质聚集的统计力学模型可以解释这一现象。这个模型是建立在去年发表的一项重要发现基础上的，即当女性的两条X染色体中的一条即将被抑制时，染色体会彼此相邻地排列。科学家证明存在着一个蛋白质结合能的临界值，使相邻两条X染色体中仅有一条 聚集蛋白质的几率很高。蛋白质会迅速地聚集在其中一条X染色体上，关闭XIST基因，从而阻止了染色体抑制自身。

这一模型也解释了细胞是如何“计算”它们的X染色体的。在男性中，蛋白质复合物将只与一条染色体结合，这样就使一条X染色体免于自我抑制。对于非性染色体，类似的机制也能确定两个版本的特定基因哪些基因应该表达而哪些应该受抑制而保持沉默。

An ancient theatre filters out low-frequency background noise.

The wonderful acoustics for which the ancient Greek theatre of Epidaurus is renowned may come from exploiting complex acoustic physics, new research shows.

The theatre, discovered under a layer of earth on the Peloponnese peninsula in 1881 and excavated, has the classic semicircular shape of a Greek amphitheatre, with 34 rows of stone seats (to which the Romans added a further 21).

Its acoustics are extraordinary: a performer standing on the open-air stage can be heard in the back rows almost 60 metres away. Architects and archaeologists have long speculated about what makes the sound transmit so well.

Now Nico Declercq and Cindy Dekeyser of the Georgia Institute of Technology in Atlanta say that the key is the arrangement of the stepped rows of seats. They calculate that this structure is perfectly shaped to act as an acoustic filter, suppressing low-frequency sound — the major component of background noise — while passing on the high frequencies of performers’ voices.

It’s not clear whether this property comes from chance or design, Declercq says. But either way, he thinks that the Greeks and Romans appreciated that the acoustics at Epidaurus were something special, and copied them elsewhere.

Sound steps

In the first century BC the Roman authority on architecture, Vitruvius, implied that his predecessors knew very well how to design a theatre to emphasize the human voice. “By the rules of mathematics and the method of music,” he wrote, “they sought to make the voices from the stage rise more clearly and sweetly to the spectators’ ears… by the arrangement of theatres in accordance with the science of harmony, the ancients increased the power of the voice.”

Later writers have speculated that the excellent acoustics of Epidaurus, built in the fourth century BC, might be due to the prevailing direction of the wind (which blows mainly from the stage to the audience), or might be a general effect of Greek theatre owing to the speech rhythms or the use of masks acting as loudspeakers. But none of this explains why a modern performer at Epidaurus, which is still sometimes used for performances, can be heard so well even on a windless day.

Declercq and Dekeyser suspected that the answer might be connected to the way sound reflects off corrugated surfaces. It has been known for several years now that these can filter sound waves to emphasize certain frequencies, just as microscopic corrugations on a butterfly wing reflect particular wavelengths of light. The sound-suppressing pads of ridged foam that can plastered on the walls of noisy rooms also take advantage of this effect.

Declercq has shown previously that the stepped surface of a Mayan ziggurat in Mexico can make handclaps or footsteps sound like bird chirps or rainfall. Now he and Dekeyser have calculated how the rows of stone benches at Epidaurus affect sound bouncing off them, and find that frequencies lower than 500 hertz are more damped than higher ones.

Murmur murmur

“Most of the noise produced in and around the theatre was probably low-frequency noise,” the researchers say: rustling trees and murmuring theatre-goers, for instance. So filtering out the low frequencies improves the audibility of the performers’ voices, which are rich in higher frequencies, at the expense of the noise. “The cut-off frequency is right where you would want it if you wanted to remove noise coming from sources that were there in ancient times,” says Declercq.

Declercq cautions that the presence of a seated audience would alter the effect, however, in ways that are hard to gauge. “For human beings the calculations would be very difficult because the human body is not homogeneous and has a very complicated shape,” he says.

Filtering out the low frequencies means that these are less audible in the spoken voice as well as in background noise. But that needn’t be a problem, because the human auditory system can ‘put back’ some of the missing low frequencies in high-frequency sound.

“There is a neurological phenomenon called virtual pitch that enables the human brain to reconstruct a sound source even in the absence of the lower tones,” Declercq says. “This effect causes small loudspeakers to produce apparently better sound quality than you’d expect.”

Although many modern theatres improve audibility with loudspeakers, Declercq says that the filtering idea might still be relevant: “In certain situations such as sports stadiums or open-air theatres, I believe the right choice of the seat row periodicity or of the steps underneath the chairs may be important.”

原文：Why the Greeks could hear plays from the back row

参考译文：

一项新的研究表明，古希腊埃皮达罗斯（Epidaurus）圆形剧场优异的声学效果也许是缘自对复杂声学物理机制的探索。

埃皮达罗斯剧场于1881年在伯罗奔尼撒半岛地下发现然后被挖掘出来，它拥有一个希腊圆形剧场经典的半圆形外形，有34排石制座位（后来罗马人又增加了21排）。

埃皮达罗斯剧场的声学效果很特别：站在露天舞台中间表演者的声音在大约60米之外的最后一排也能被听到，建筑学家和考古学家一直在推测声音传播效果这么好的原因。

现在，亚特兰大乔治亚理工大学的Nico Declercq和Cindy Dekeyser认为，声音传播效果好的根本原因在于这些座位的排列方式。他们经过计算认为这个结构正好可以作为一个完美的声学滤波器，抑制低频部分——这也是主要的背景噪声——是讲话者高频声音传播时的噪音。

Declercq说，还不清楚这个特点是出于偶然还是人为的设计。但是不管是哪一个原因，他认为希腊人和罗马人都非常欣赏埃皮达罗斯剧场的声学效果，并且到处模仿建造。

声阶

公元前一世纪，古罗马在建筑方面的权威，维特鲁威斯（Vitruvius）认为他们的先人，对如何设计一个能够增强人声音效果的剧院很内行，他写道：“通过数学和音乐的法则，他们寻求一种方法，使得从舞台上发出来的声音能够更加清楚、悦耳地传到听众的耳朵里。通过和谐科学地排列剧院的结构，古人增强了声音的效果。”

后来的作家们推测，建造于公元前四世纪埃皮达罗斯剧场杰出的声学结构，也许是由于风向（从舞台吹到观众），或者是由于古希腊剧场里演讲者的韵律及利用面具作为扩声器的效果。但是任何一种都无法解释为什么今天的讲演者站在埃皮达罗斯剧场的舞台（至今这个剧场仍被用于演出），在一个无风的日子里仍然可以听得很清楚。

Declercq 和 Dekeyser认为答案与声音从波纹状曲面反射回来有关。几年来人们认为波纹曲面可以过滤声波并增强某个特定的频段，正象蝴蝶翅膀上细密的波纹可以反射特定波长的光一样，（剧院）墙上用来抑制声波反射的泡沫板也是利用的同样原理。

Declercq以前曾经证明，墨西哥玛雅金字塔的台阶形表面可以使鼓掌声或者是脚步声听起来像鸟唧唧叫的声音或者是下雨声。现在他和Dekeyser已经计算出埃皮达罗斯剧场的石凳是如何反射声音的。他们发现低于500Hz的低频声音比高频部分更容易衰减。

咕哝

研究者说：“剧院里大多数噪声是低频噪声，比如树叶的沙沙声和看戏人的咕哝声”， 因为表演者声音主要集中在高频，所以过滤掉低频部分抑制了噪声，增强了表演者声音的效果。“截至频率则正好是你所需要的从声源分离出的噪声频率。”，Delclercq解释说。

Declercq提醒说有观众坐在座位上时会改变这种效果，但这种改变却很难计算。“对人体而言，这个计算太难了，因为不同人的身体各不一样并有着复杂的形状，”他说。

过滤掉低频部分意味着说话的声音跟背景噪声频率差不多时不能够被听到。但是这并不是难题，因为人类的听觉系统能够自动补偿高频声音中损失的低频部分。

“这是一种叫做虚拟音调（virtual pitch）的神经生理现象，它能够使人的大脑在没有低音的时候重建声源。”Declercq解释说：“这种效应使得小扬声器的声音质量比你想象的要好得多。

尽管很多现代剧院是使用扩音器来增强可听度的，Declercq说滤波的思想仍然有用：“在特定的环境下如运动馆或者露天剧场，我认为如何选择座位的排列方式和主席台下的台阶数是很重要的。”

A computer model of the way opinions evolve in social networks has shown why two groups holding opposing views can quickly become reconciled or remain at odds.

The key, say European researchers, is how strongly the groups communicate with each other. The work could explain how language differences persist across geographic boundaries and how political thought can quickly become polarized.

To model the evolution of opinions, researchers led by physicist Renaud Lambiotte of the University of Liege in Belgium imagined two groups, initially isolated, whose members gradually begin to talk to members of the other group.

They supposed for simplicity that individuals hold one of two opinions, assigned randomly at the start. People then change their views by a “majority rule” – each person tends to adopt the opinion that is held by a majority of those with whom they are linked in the social network.

Solving their model mathematically, the authors found that when the two groups were isolated or nearly isolated, people within each group quickly came to share one opinion but the groups were as likely to agree as disagree with each other.

Tipping point

As Lambiotte and colleagues began adding social links between the groups. But they found no change, at first. The two groups continued to form opinions independently.

But rather than a gradual increase in the way opinions “leak” from one group to the other as more connections are added, the researchers found a surprise when the number of links between the groups reached a precise threshold. Suddenly, the final opinions of the two groups were always identical. Even a few extra links between groups were enough to “tip” their final opinions from a state of full polarization to full agreement.

“We didn’t expect such an abrupt transition,” says Lambiotte. “It implies that even a small change in network structure can lead to drastic consequences.”

Political poles

Lambiotte and colleagues suggest the results may help to explain why polarized communities can suddenly emerge rather than gradually appear. In the US, for example, researchers have noted an extreme and persisting polarization among bloggers expressing either Democratic or Republican political views. If most bloggers tend to read only those who agree with them, Lambiotte points out, groups associated with these differing positions can easily persist.

The work may also help explain why languages remain distinct across geographic borders rather than merging into a common tongue. The researchers argue that the same dynamic may preserve differences across boundaries where communications links are weak.

It may also help to account for the existence of small communities who use products different from the majority, such as Mac users in the graphic arts, Lambiotte suggests. But the model hints that the existence of these communities may be tenuous because even a small amount of interconnection could lead to their blending back into the majority.

The researchers hope to make more realistic models in the future, going beyond binary opinions and the strict majority rule. “There are still plenty of open questions,” says Marcel Ausloos, Lambiotte’s colleague at the University of Liege. “But we think the basic process is likely to remain the same.”

以下是参考译文（根据材料0605 40631143的翻译修改而成）：

社会性网络解释观点的演化

为何持对立观点的两群人可以在短时间内意见统一或依然保持分立？一个基于观点在社会性网络中演化的计算机模型解释了其中的原因。

欧洲研究人员称，这里的关键是人群间交流的强度。这项成果可以解释语言差异是如何由地理界线而保持的，以及政治思想是如何在短时间内变得两极分立。

为了模拟观念的转变过程，由比利时国王大学（the University of Liege）物理学家Renaud Lambiotte领导的研究人员设想了最初相互隔离的两群人，其中的成员逐渐地开始进行和另一群成员的交流。

为了简化实验，他们假设每人起初均随机地被指定持两种观点之一。接下来人们按一个“多数决定原则”交换意见——每人都倾向于接受他们在社交网络中接触的人们中多数人的意见。

用数学方法分析模型时，研究人员发现当两群人是隔离的或近乎隔离时，每群人会很快地持有共同的观点，但是两组之间观点的一致与否却不一定。

临界点

随后Lambiotte及其同事开始在两群人之间增加社交联系。起初，他们并没有发生变化，这两群人依然互相独立地形成观点。

但令研究人员惊讶的是，两群人的观点并未随着其间联系的增多（观点的渗透）而逐渐趋同，而是在最终联系的数量达到某一特定的极限后，突然达成共识。甚至仅在两组间增加几个联系就足以将他们的最终观点由完全分立的状态颠覆至完全统一。

Lambiotte称：“我们没料到会有如此突然的转变。这表明社交网络中哪怕很小的改变都能得到极其不同的结果。”

政治民意

Lambiotte及其同事认为这一结果可以解释对立的政体为何能突然产生而非逐渐出现的原因。以美国为例，研究者们注意到网志作者们中代表民主党或共和 党观点的极端、持续两极对立的现象。Lambiotte指出，如果大多数网志作者倾向于仅阅读与其观点相同的文章时，与此两种不同观点相关的群体很容易持 续存在下去。

此项成果还可以解释为何在地理边缘的两侧语言可以保持截然不同的状态而非融为相同的语言。研究者们推断，同样的原因可以使交流沟通较弱的地区边界两侧保持差异。

Lambiott指出这项研究还有助于解释那些与大多数人使用不同物品的小团体的存在，比如创意工作者中Mac（苹果电脑）的使用者。但模型也揭示，这类团体的存在可能是十分脆弱的，因为仅仅很少的相互联系就能导致他们融合到大多数中去。

研究人员希望能在将来作出超越二元观点和严格“少数服从多数原则”的更加具有现实意义的模型。“仍有许多悬而未决的问题，”Lambiotte在国王大学的同事Marcel Ausloos说，“但我们认为基本的研究步骤将保持不变。”

注：在互联网上力挺百度和鄙视百度的博客们也是这样的例子，他们都倾向于阅读和自己观点相同博主的文章。

作者：Mark Buchanan, 原文网址
该研究发表在: Physical Review E (DOI: 10.1103/PhysRevE.75.030101)