豆瓣正在测试新首页，正如大家看到的那样，新首页将有更多书、电影、音乐、活动的内容，更加显现豆瓣“帮助你发现有用东西”的初衷。

首页将会成为你对兴趣的发现中心。这里的兴趣包括读书、电影、音乐、各种好玩的同城活动以及与这些相关的评论。细心一点的豆子们还会发现，每一本书、电影、音乐下面都有附注。有些是来自全站的热门排行榜，有些是因为你的友邻收藏过并且评分甚高，有些则是我们傻乎乎的“豆瓣猜”用算法猜测出来的。总之，我们希望穷尽一切渠道和可能性，方便你在首页发现可能感兴趣的东西。
 
作为“发现中心”的补充，首页的右侧将会是各种提醒、请求，这里是你的小型办公室，豆瓣上所有需要你关注的事情都会在这里排好队。
 
我们很高兴听到了很多宝贵意见。例如现在书、影、音的数量过多，变化很慢，内容也不对胃口，我们已经在调整了。另外，下一步会尽快补充新的评论内容进来，丰富“发现中心”的内容。至于很喜欢使用友邻广播的豆子们，我们会在“友邻”频道下更好地展现广播，在首页不能看得尽兴的人可以去“友邻”下面与朋友们互动。
 
希望大家在豆瓣的发现之旅从首页开始。多谢支持，希望大家能够给我们更多的意见。

I had a call with Monty the other day and I told him why I think MySQL Server Quality will never be the same again. I’ve been thinking a bit more about it and here is the extended list.

In particular I think MySQL Server will never be able to reach its original quality guidelines (see previous post) and even current release criterias will unlikely be ever reached with any sensible definition of what serious bugs are.

Large Team, Tricky Code During MySQL 3.23 and MySQL 4.0 team the small tight knit team was working on the server and Monty was personally reviewing all the code and knew all the code well to know all the side effects etc. Now MySQL Development is done by different team working on the different components, while MySQL was not designed to be very modular with clear set interfaces and no side effects on the first place to support such development concept easily.

Increasing Complexity Each new MySQL version is getting more and more complicated. MySQL architecture has to be additionally complicated to support modular storage engines with all their interactions and different specifics. Think for example how many DBMS need to do Distributed Transactions (XA) internally just to keep different components in sync. This is multiplied by the fact a lot of MySQL features were implemented in marketing rush being hacks over architecture which was not intended to support it. Remember early MySQL versions knew only about table locks and were not initially designed to support transactions. Most of other databases are designed with these concepts at their core.

Increased User Base I’m not sure if MySQL user base is growing any more but it surely has grown dramatically since MySQL 3.23 or 4.0 times. More users means they naturally find more bugs. This is actually has the good time too. Even though a lot of bugs being found and reported only small fraction of them apply to any given user. The bad thing - in many cases we do not really know which one apply to us

Low Feature Usage Density MySQL 3.23 was simple so Most of its features were used by majority of users. This was even the early Marketing strategy for MySQL. Marten used to frequently say Oracle is Overkill for most users and most users use only 10% of features or less. You know what ? MySQL is becoming overkill for increasingly large amount of users too. A lot of people in the Web world would be as happy with MySQL 4.1 if it would be actively maintained now. And hey this is exactly one if the reasons behind Drizzle project.

Lower usage density means features are less tested by community during release and they also mean they get less attention from developers, because development team (and especially its efficiency) is not growing on the same pace. This also means fewer customers use the same features, and so the revenue associated with feature is not high enough to keep all the bugs reported for this feature. So bugs sometimes stay open for years.

As an example of low usage density features I would mention MySQL 5.1 features. During my talks I often asked people how many of them use 5.1 vs row level replication or events. I think less than 5% are using both features. MySQL 4.1 was out for years and yet I would say no more than 5% of users use prepared statements.

Bug Reporting and Tracking The MySQL 3.23 received quality was at some extent because there were no bugs.mysql.com - bugs were reported to bugs@lists.mysql.com which meant both it was more complicated for users to report a bug - so fewer people did and it was easily to lose the bugs, especially boring and less critical ones. Monty did a very good job keeping track of these but still not as good as when bug tracking system was implemented and so bug would never disappear unless you fix it or call it a feature

MySQL Enterprise Madness For years MySQL QA policy heavily relied on community and I think it is still the case. This means MySQL latest code was instantly downloaded and tested by thousands of users which would find bugs quickly. With MySQL Enterprise it is customers who got the bleeding edge code and for community the binary which is over half a year old was often offered.

Enterprise customers who got the latest and the most bug free/buggy code (less old bugs more new ones) are fractions of percent of MySQL User base and they are also more conservative - few just would grab latest release as soon as it comes out. Many will only upgrade on the schedule or keep on the same release until they run into some issues. This means bugs in Enterprise version have tendency to stay for long time.

The good thing about this item is - it is easily fixable. Just get somebody in his mind to define release policy, and better have him/her understanding engineering not just marketing.

Innodb/Oracle relationship complexities. If the bug happen to affect Innodb things become even more complicated slow. Even though MySQL and Innodb is tight knit system these are two different code bases controlled by different companies. MySQL can’t just do changes to Innodb codebase and Innodb can’t do same for MySQL. Both require other side approval. This is why I guess all new Innodb Development is going with Innodb Plugin which is separate from Innodb included default MySQL. Why is not this part of stock MySQL 5.1 ? I guess because MySQL 5.1 release was closed for any significant changes for years.

Now how it all affects bugs an not features ? Well if you want to use latest Innodb you will need to use plugin which is not part of standard MySQL distribution so it, just as MySQL Enterprise, gets significantly less testing by community than standard MySQL.

Another problem for Innodb Plugin quality is lack of the full disclosure about bugs. Check this change log for example. You would see some bugs fixed have bug numbers and large number does not. This means there are some bugs which are discovered by Innodb/Oracle team but they are not disclosed via searchable public bugs database.

I should note besides all these process issues Innodb remains remarkably high quality component. The initial version of Innodb Plugin was very good in terms of quality for version having first public release as we tested it back in April.

The Good News in the End

The good news are you really do not need general “quality” or “being free from bugs” to run MySQL in production. MySQL as it is now works for a lot of applications, in particular because most of them use only small portion of MySQL features.

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The first thing you’ll notice about 2.7 is its new interface. From the top down, we’ve listened to your feedback and thought deeply about the design and the result is a WordPress that’s just plain faster. Nearly every task you do on your blog will take fewer clicks and be faster in 2.7 than it did in a previous version. (Download it now, or read on for more.)

Next you’ll begin to notice the new features subtly sprinkled through the new interface: the new dashboard that you can arrange with drag and drop to put the things most important to you on top, QuickPress, comment threading, paging, and the ability to reply to comments from your dashboard, the ability to install any plugin directly from WordPress.org with a single click, and sticky posts.

Digging in further you might notice that every screen is customizable. Let’s say you never care about author on your post listings — just click “Screen Options” and uncheck it and it’s instantly gone from the page. The same for any module on the dashboard or write screen. If your screen is narrow and the menu is taking up too much horizontal room, click the arrow to minimize it to be icon-only, and then go to the write page and drag and drop everything from the right column into the main one, so your posting area is full-screen. (For example I like hiding everything except categories, tags, and publish. I put categories and tags on the right, and publish under the post box.)

For a visual introduction to what 2.7 is, check out this video (available in HD, and full screen):

It’s all about you. It’s the next generation of WordPress, which is why we’ve bestowed it with the honor of being named for John Coltrane. And you can download it today.

Last, but certainly not least, this may be the last time you ever have to manually upgrade WordPress again. We heard how tired you were of doing upgrades for yourself and your friends, so now WordPress includes a built-in upgrade that will automatically notify you of new releases, and when you’re ready it will download them, install them, and upgrade your blog with a single click.

(As with any interface change it may take a little bit of time to acclimate yourself but soon you’ll find yourself whizzing through the screens. Even people who have hated it at first tell us after a few days they wonder how they got by before.)

The Story Behind 2.7

The real reason Coltrane is such a huge leap forward is because the community was so involved with every step of the process. Over 150 people contributed code directly to the release, our highest ever, with many tens of thousands more participating in the polls, surveys, tests, mailing lists, and other feedback mechanisms the WordPress dev team used in putting this release together.

For some of the back story in the development of 2.7, check out these blog posts (thanks to WeblogToolsCollection for the list):

• Usability Testing Report: 2.5 and CrazyHorse
• The New 2.7 Dashboard
• The Visual Design of 2.7
• WordPress 2.7 wireframes
• Comprehensive Codex Article on WordPress 2.7
• WordPress Development Updates from Jane Wells
• WordPress Usability Testing in New York
• WordPress 2.7 UI Configurability from Ryan Boren
• Customizable Post Editing Screen from Mark Jaquith
• WordPress 2.7 Navigation Survey from Jane Wells
• Shortcuts/Favorites Menu from Jane Wells
• CrazyHorse Presentation at WordCamp SF
• WordPress 2.7 Walkthrough by Matt
• WordPress 2.7 and beyond from Matt at WordCamp Utah

This was interesting to us, a blogging software release we actually blogged about, but the process was hugely informative. Prior to its release today Crazyhorse and 2.7 had been tested by tens of thousands of people on their blogs, hundreds of thousands of you count .com. The volume of feedback was so high that we decided to push back the release date a month to take time to incorporate it all and do more revisions based on what you guys said.

For those of you wondering why we didn’t call this release 3.0, it’s because we abhor version number inflation. 3.0 will just be the next release after 2.9. The major features in new point releases approach also works well for products like OS X, with huge changes between a 10.3 and 10.4.

The Future

Those of you following along at home might have noticed this was our second major redesign of WordPress this year. Whoa nelly! While that wasn’t ideal, and I especially sympathize with those of you creating books or tutorials around WordPress, there’s good news. The changes to WordPress in 2.5 and 2.7 were necessary for us to break free of much of the legacy cruft and interface bloat that had built up over the years (gradually) and more importantly provide us with a UI framework and interface language we can use at the foundation to build tomorrow’s WordPress on, to express ideas we haven’t been able to before. So at the end of 2009 I expect, interface-wise, WordPress to look largely the same as it does now.

That said, we couldn’t be more excited about the future with regards to features. Now that we’ve cleared out more basic things, we are looking forward in the coming year to really tackling media handling including audio and video, better tools for plugin and theme developers, widgets, theme updates, more integrated and contextual help, and easier integration with projects like BuddyPress and bbPress.

Thank Yous

We would like to take a moment to thank the following WordPress.org users for being a part of 2.7: 082net, _ck_, Aaron Brazell, Aaron Campbell, Aaron Harp, aaron_guitar, abackstrom, Alex Rabe, Alex Shiels, anderswc, andr, Andrew Ozz, andy, Andy Peatling, Austin Matzko, axelseaa, bendalton, Benedict Eastaugh, betsyk, Björn Wijers, bobrik, brianwhite, bubel, Byrne Reese, caesarsgrunt, capripot, Casey Bisson, Charles E. Frees-Melvin, Chris Johnston, codestyling, corischlegel, count_0, Daniel Jalkut, Daniel Torreblanca, David McFarlane, dbuser123, Demetris Kikizas, Dion Hulse, docwhat, Donncha O Caoimh, Doug Stewart, Dougal Campbell, dsader, dtsn, dwc, g30rg3x, guillep2k, Hailin Wu, Hans Engel, Jacob Santos, Jamie Rumbelow, Jan Brasna, Jane Wells, Jean-LucfromBrussels, Jennifer Hodgdon, Jeremy Clarke, Jérémie Bresson, jick, Joe Taiabjee, John Blackbourn, John Conners, John Lamansky, johnhennmacc, Joost de Valk, Joseph Scott, kashani, Kim Parsell, Lloyd Budd, Lutz Schröer, Malaiac, Mark Jaquith, Mark Steel, Matt Freedman, Matt Mullenweg, Matt Thomas, matthewh84, mattyrob, mcs_trekkie, Michael Adams, Michael Hampton, MichaelH, mictasm, Mike Schinkel, msi08, msw0418, mtekk, Nick Momrik, Nikolay Bachiyski, Noel Jackson, Otto, Ozh, paddya, paul, pedrop, pishmishy, Po0ky, RanYanivHartstein, raychampagne, rdworth, reinkim, rickoman, rm53, rnt, Robert Accettura, roganty, Ryan Boren, Ryan McCue, Sam Bauers, Sam_a, schiller, Scott Houst, sekundek, Shane, Simek, Simon Wheatley, sivel, st_falcon, stefano, strider72, tai, takayukister, techcookies, Terragg, thinlight, tott, Trevor Fitzgerald, tschai, Txanny, Valiallah (Mani) Monajjemi, Viper007Bond, Vladimir Kolesnikov, wasp, wet, wfrantz, x11tech, xknown, xorax, ydekproductions, yoavf, yonosoytu, yoshi, zedlander

作者 AdSense 优化小组

您是不是在为不知道怎样提高网站上的广告单价而烦恼？那么就好好学习一下我们今天的这篇文章吧！

在Google AdSense 系统投放广告时，网页HTML代码中最前面的广告单元总是最先展示竞价最高的广告。同时，如果没有足够的广告在您网站上展示时，第一个广告单元也会优先展示广告。所以如果您的网页上投放了两个广告单元，在HTML 代码中靠前的广告单元的广告单价总是比另一个要高一些。

根据这个原理，您可以使用自定义渠道跟踪各个广告单元效果，找到点击率最高的广告单元，并在 HTML 代码中把这个广告单元放在最前面。这样，点击率最高的广告单元广告单价也最高，这样就您网站的整体点击的单价也就提高了。

请注意，这里我们所说的“第一个广告单元”是指在网页 HTML 代码中最先出现的广告代码，而源代码中的第一个广告单元并不一定是用户在浏览器中看到的第一个广告单元。您可以在代码中使用 DIV 标签进行设置，把页面底部的广告单元作为HTML代码的第一个广告单元。

马上检查一下，您的点击率最高的广告单元现在是不是“第一个广告单元”。

MySQL 5.1 release as “GA” seems to be the most controversial to date. It had very negative response from Monty, original MySQL Founder and controversial responses in community including another beating by Kevin Burton.

There is also very interesting reading on MySQL 5.1 open bugs

So how do I take it and where do I think MySQL 5.1 Quality Stands ?

I’ve been MySQL user for years since version 3.22 and when MySQL 3.23 came out we were using it in production since 3.23.3 or so, when it was in the Alpha stage still. A lot of bugs were fixed after we started using it… we just made sure the bugs which are important for us are fixed. Later while working for MySQL I’ve seen big and famous companies shipping MySQL 4.1 beta build into their hardware solutions. This is to tell stability and amount of bugs is not about number but about the things which affect you.

So when people are writing hey, MySQL 5.1 works for them I do not think this has anything to do with MySQL 5.1 stability. I know people who are happy with MySQL 6.0 and Falcon stability for months now - it works for their workload.

So what is Quality Guidelines for Release (GA) when ? The current policies are here but a history of how they evolved over time is much more interesting and allows to much better understand why Monty is upset with release.

MySQL Release Quality Guildines devaluated during last years probably worse than dollar in the last century. In early days it was “no known bugs in release” when it changed to “no known bugs in production release” when it changed to “no crashing bugs in production release” and now it changed to be to “believed to be reliable, free of serious bugs”. I believe this is already below the standards of what qualities “GA” release should have.

But guess what? MySQL 5.1 did not really meet even these guidelines when it was released as GA. Check bugs Monty mentions and even official MySQL 5.1 bug list. These are number of bugs which I would consider serious which are not fixed in 5.1.30. But who cares about me ? “serious” is not well defined term and you can pick whatever you like to call serious.

On the bright side MySQL 5.1 “GA” quality was much better than MySQL 5.0 quality when it was released.

Though the question remains, was MySQL 5.1 release as GA now a good move ?

MySQL 5.1 spent in RC stage well too long so people already started to use it in production (because they started with 5.1RC during their development) without clear understanding how it is going to be supported from MySQL. It also needed more Community Testing to fix those rare bugs which only become found with very wide use. Finally it was helpful for corporate users who are stuck with 5.0 because of policies of using only “GA” versions of the product.

Here is actually there another challenge comes from - MySQL does not make a difference between which release is recommended (offered by default for download) supported and later GA release.

Decoupling these terms would allow to get users to the Recommended release, which is also supported while have still high and clear GA guidelines which could be met months after release is getting Recommended status.

Though in this case MySQL of course minds its commercial interests more than follows community sentiments - in commercial software market there is usually no such difference. So it is complex to make people to understand and sell. Plus people assume first GA release is crap anyway. I’ve seen a lot of policies like waiting at least 3 minor releases or waiting 6 months after release people use for Commercial databases this gets transfered to MySQL too.

As a Summary I think this was inevitable MySQL 5.1 will be released for commercial/marketing reasons, not because it really meets quality guidelines. At the same time I would like to send my warm congratulations to MySQL 5.1 development team which made this release better quality than MySQL 5.0 initial release.

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Ken Croswell 文 Shea 编译

版本信息：本文最初于2002年5月首次刊载于“火流星”网站，此版本为译者再编辑版。

有时，天文学中最简单的问题却是最难回答的……

太阳又下山了。玫瑰色的云在头顶上浮动，朱红与金黄又一次出现在地平线上。家里都点上了灯，餐桌上蜡烛隐隐作亮，在晚霞的映衬下，街边的路灯就像是项链上的珍珠闪闪发光。黄昏越来越暗，星星出现了，它们一如既往地出现在这墨黑的穹顶之上。但是为什么夜空是黑的呢？

这是一个简单的也许只有孩子才会问的问题，父母对此也许会不屑一顾，但是宇宙学家爱德华·哈里森（Edward Harrison）却花了几十年的时间来研究这个看似简单的问题。“许多年前我开始对夜空的黑暗之谜产生了兴趣，”在《夜的黑》（Darkness at Night）这本书中他写道，“我常常思考宇宙为什么没有充满光，即使在我解决了这一问题之后也是如此。这个古老的谜题仍旧萦绕在我周围。有时几个小时，有时几天，我会重新思考这个问题，为大自然的力量和精妙所深深打动。”

夜空为什么是黑的，乍一看很明显，因为太阳下山了。但是还有恒星在闪耀啊。如果宇宙是无限的，充满着无数颗恒星，那么夜晚将和白天一样的明亮。这种理论和观测之间的矛盾被称为奥伯斯佯谬，奥伯斯是19世纪的医生和天文学家。

 

 

图片版权：Wojciech Gaczek。

从彗星到宇宙学

海因里·奥伯斯（Heinrich Wilhelm Olbers）每晚只睡4个小时。白天医治德国北部城市布莱梅（Bremen）的病人，为此他赢得了治愈霍乱的荣誉。晚上他观测天空寻找彗星，发现了前4颗小行星中的2颗。

[右图]：海因里·奥伯斯。

1823年，奥伯斯写了一篇宇宙学论文。他说，想象一下，如果恒星均匀的分布在宇宙空间里。离地球近的恒星，自然看起来就大，但远处的恒星数量多，因为在一个固定的视角下，距离越远空间就越大。奥伯斯证明数量巨大的远距离恒星弥补了它们较小的视尺寸，在给定半径的球壳中也能覆盖整个天空。如果宇宙是无限的，这样的球壳也将有无限多个，就意味着天空中将充满星星，夜空将会和白天一样明亮。

奥伯斯写道：

多幸运，大自然并没有这样安排事物。多幸运，地球并没有沉浸在耀眼的星光之中。否则，天文学仍将处于蒙昧的阶段。我们将无法看到星星，只有通过观测黑子来推断太阳的行踪，而月亮和行星仅仅是明亮背景上移动着的暗斑。

那时，奥伯斯的工作并没有引起注意，同样夜晚的黑暗之谜也无人问津。英国人托马斯·迪格斯（Thomas Digges）赢得了这一荣誉。1576年，在其父所写的一本书（Prognostication Euerlastinge）的附录中，迪格斯提到了这个问题。这本书采用了以地球为中心的宇宙体系，但在附录中迪格斯却是哥白尼的支持者。而且他比哥白尼更进一步，他认为恒星散布于宇宙空间中。之后，他尝试解释为什么夜晚没有被星光照亮：

无穷天球上的恒星发出无限的光芒。但是它们互相遮挡，而且距离相当遥远，于是星光就越来越弱。所以我们就看到了现在的景象。

因此，迪格斯认为夜晚是黑的原因是遥远的恒星太暗弱以致于看不见了。尽管这个解释看起来是合理的，但它是错的。把所有看不见的恒星的光结合起来，可以达到能被看见的程度。事实上，肉眼可以看到仙女座星系，但是仙女星系中没有一颗恒星亮到足以能让肉眼看到。

 

 

[图片说明]：肉眼可以看到仙女座星系，但是仙女中没有一颗恒星亮到足以能让肉眼看到。图片版权：Jerry Lodriguss。

迪格斯认为宇宙是无限的，但是伟大的德国天文学家约翰内斯·开普勒（Johannes Kepler）却对此表示不满。开普勒认为无限的宇宙会使太阳迷失在茫茫的星海之中。他说，夜空之所以是黑的，是因为在我们居住的星际空间周围有一堵暗墙。

一百多年后，爱德华·哈雷（Edward Halley）也思考了这一问题。1721年，在皇家科学院的一次演讲中，他提出了两个方法来调和无限而又充满恒星的宇宙，使它有一个黑暗的夜晚。第一，由于数学上的错误，他错误的认为远处大量恒星的光无法等效为近处少量恒星的光。第二，他重复了迪格斯的观点，看不见的恒星对于光没有贡献，“它们的光线……太弱，无法使我们感觉到。”所以，哈雷也没有给出夜晚为什么是黑暗的的答案。

另一个思考这一问题的人是瑞士天文学家让-菲利普·卢瓦斯·德·谢诺（Jean-Phillippe Loys de Cheseaux）。1744年，在有关当年一颗有着6条彗尾的彗星的书的附录里，他也讨论了这个问题。与迪格斯和哈雷不同，谢诺认识到远处看不见的恒星对于整个天空中的光是有贡献的。他说，夜空之所以是黑的，原因是宇宙空间并不是透明的。相反，它充满着物质，它们会“吸收”光线，产生一个黑暗的夜空。奥伯斯在1823年的论文中也提到了相同的观点。但是奇怪的是，奥伯斯有谢诺写的这本书，但是他却没有引用它，可是却提到了哈雷。

然而，谢诺和奥伯斯也都错了。空间中的消光物质无法使夜空变暗。不久，天文学家认识到，消光物质在遮挡光线的同时，也会被光线所加热，进而发光，它们将会和恒星一样的明亮。这就像大雨中的树。起先叶子还能保护地面不受雨淋，可是不久雨水便会从叶子上滴落下来，最终地面还是会湿透。

渡鸦的述说

令人惊讶的是，第一个给出奥伯斯佯谬正确解释的人不是来自欧洲装备齐全天文台的天文学家，而是一个美国诗人。在他短暂的一生中，爱伦·坡（Edgar Allan Poe）以其带有恐怖和超自然色彩的小说、诗歌享誉世界。黑暗笼罩着爱伦·坡的一生。在他两岁时，他的母亲死了。在大学时，因为赌博和酗酒而生活拮据，之后他便生活在贫困之中。他妻子二十几岁时便离开了人世。他40岁时，在一阵狂饮之后，结束了自己的生命。

[右图]：爱伦·坡。

黑暗也同样萦绕着爱伦·坡的作品。在《渡鸦》（Raven）中他写道：

Deep into that darkness peering, long
I stood there wondering, fearing,
Doubting, dreaming dreams no mortal
ever dared to dream before;
But the silence was unbroken, and
the darkness gave no token...

哈里森发现，爱伦·坡在其死前一年所写的《我发现了》（Eureka）中正确揭示了奥伯斯佯谬。哈里森说：“当我第一次读爱伦·坡的作品时，我大吃一惊。一个诗人，不，一个最了不起的业余科学家在140年前就领悟到了问题的本质，而在我们的学校中仍旧在宣扬错误的观点。”

1848年，爱伦·坡出版了《我发现了》。他写道，“当我写完〈我发现了〉之后，我便丧失了对生的渴望。我已无法再写作了。”爱伦·坡本希望他的出版商能出版5万本，结果只有5百本。评价也是各不相同，有人说它新颖、骇俗；另一些人则对此不屑一顾。在近代，当哈里森欢呼，认为它是人类思想的杰作时，英国天文学家爱丁顿（Eddington）爵士则认为它是怪人的理论。

对爱伦·坡来说，上帝就是一个诗人，宇宙就是“最卓越的诗”。他这样解释奥伯斯佯谬：

星星无穷无尽，天空的背景就会呈现出明亮，就象是银河——它们不会呈现点状，在背景中也不会出现一颗星星。因此，只有一种可能，由于恒星的距离实在是太远了，它们发出的光还没来得及到达地球。

简要地说，爱伦·坡认为，之所以遥远恒星的光没有照亮星空是因为它们还没来得及到达地球；我们无法看到比宇宙更远的地方。用现在的话来讲，我们无法看到137亿光年之外的东西。所以，黑暗的夜空是宇宙诞生的证据。

但是，立刻爱伦·坡又开始怀疑自己的解释。“也许是这样的吧，有谁会冒险来质疑它呢？”他写道，“我相信它是正确的。”

 

 

[图片说明]：宇宙诞生和演化的时间线。图片版权：NASA。

在《我发现了》中，爱伦·坡数次提到了德国天文学家约翰·马德勒（Johann Madler），由于在他十几岁时出现的大彗星使他对天文学产生了兴趣。马德勒绘制月面图，并且出版了一本书《大众天文学》，这本书再版了6次。在早期的版本中，马德勒和奥伯斯一样，认为是星际物质吸收星光而使夜空变得黑暗。但在1858年的另一本书（《我发现了》出版后10年）以及畅销的1861年版的《大众天文学》中，他则给出了和爱伦·坡相似的解释：

光速是有限的；它传播需要时间，因此，我们看到的光是经过有限的时间才到这我们这儿的。这样夜空黑暗之谜就可以得到合理而充分的解释，星际消光的限制就可以被排除了。更确切的讲，远处的星光还没有到达我们这里。

1901年，苏格兰数学家、物理学家开尔文（Kelvin）对这一解释进行了量化。开尔文的计算表明，若要夜空变得明亮，我们至少要能看到数百万亿光年远的范围。由于宇宙的年龄现在远小于1万亿年，所以夜空是黑的。

爱伦·坡、马德勒和开尔文都认识到了天文学家所能看到的宇宙是过去的样子而不是现在的样子。看得越远，就越深入过去——哈里森认为宗教的信条延缓了奥伯斯佯谬的解决。“我们回顾历史，为什么人们对整件事无动于衷，这个问题有着文化背景，”哈里森说，“从一个层面上讲，人们已认识到了光速的重要性，但是真正的矛盾来自根深蒂固的宇宙观。这涉及到宇宙的年龄。如果你意识到这将与《圣经》相悖，你最好保持沉默。”《圣经》是这样说的，宇宙将近有6000年的历史，也就是说天文学家仅能看到6000光年远的范围。

深入奥伯斯佯谬

尽管奥伯斯佯谬已有几个世纪的历史，但是直到20世纪50年代它才开始引人注目。的确，如果你在奥伯斯的年代，提起奥伯斯佯谬，人们根本不知道你在谈论什么。

1952年，亨曼·邦迪（Hermann Bondi）的《宇宙学》（Cosmology）一书首次提到了奥伯斯佯谬。邦迪是稳恒态宇宙学的支持者。与大爆炸宇宙学不同，稳恒态宇宙学认为宇宙不是创生于150亿年前的大爆炸，相反宇宙永远存在着。在一个永存的宇宙中，爱伦·坡对奥伯斯佯谬的解释——遥远的星光还没有抵达地球——就行不通了。如果宇宙的年龄是无限的，则天文学家能看到无限远处。

为此，稳恒态理论用宇宙膨胀来解决这个问题。膨胀的空间会使穿行其中光的波长变长，或者红化，因此光传播的越远，红移就越大。红光的光子能量比黄光或是蓝光来得低，红移会减弱来自遥远星系星光的能量，进而夜空是暗的。1955年，稳恒态宇宙学家福雷德·霍伊尔（Fred Hoyle）在他的《天文学前沿》（Frontiers of Astronomy）一书中写道，“因为宇宙膨胀，所以夜晚是黑的。这是一个意料之外的解释——以致于19世纪的天文学从来没有想到过。”

 

 

[图片说明]：膨胀的宇宙就好像是不断往里吹气的气球，随着气球变大，波长也在增大。图片版权：Addison Wesley。

虽然，这是一种进步，但是红移无法解释奥伯斯佯谬。它仅仅在稳恒态宇宙学中适用，而这一宇宙学模型并没有被天文学家广为接受。在大爆炸宇宙学中，膨胀的空间对夜晚的黑暗不起什么作用，即使宇宙停止膨胀夜晚仍将是黑的。然而大多数的书中却不是这么写的。1987年的调查显示，只有30%的天文学书籍正确解释了为什么夜晚是黑的。

哈里森认为夜晚的黑暗和膨胀的宇宙之间的联系很清楚——第一，微不足道；第二，过于深奥——这得归咎于50年代的宇宙学普及。“晚上走到户外，天文学家这样吸引他们的听众，抬头仰望繁星点点的夜空，”他的《夜的黑》一书中写道，“黑暗的夜空证明宇宙在膨胀。这就象是一本书的主题，扉页上则写着多普勒效应，以此来吸引大量的听众。”

宇宙的能量危机

从爱伦·坡开始，天文学家已对黑暗的夜空有了一个正确的解释：宇宙还太年轻。1964年，哈里森发现了另一个正确的解释：宇宙拥有的能量太少。

在邦迪的宇宙学书中，哈里森首次知道了奥伯斯佯谬。哈里森决定计算，若是夜空要被无数的星星照亮要多少能量。“起先，我的计算结果简直我法令人相信，”他写道，“但之后，事情就清楚了，我们一直沿用错误的角度来看待整个问题。”

 

 

[图片说明]：射电波段的天空。

[图片说明]：近红外波段的天空。

[图片说明]：可见光波段的天空。

[图片说明]：X射线波段的天空。

[图片说明]：γ射线波段的天空。

 

在可观测的宇宙中，所有恒星所产生的能量是非常小的。哈里森的计算表明，若要照亮夜空，可观测宇宙需要的能量为现今的10万亿倍——每颗恒星的发光度要上升10万亿倍，或者恒星的数目要增加10万亿倍。另外，恒星不可能永生，就算宇宙无限老，夜空仍旧是黑暗的，原因是恒星总是会死亡的。

就像太阳，恒星通过核反应将质量转化成能量。哈里森证明，就算宇宙中的所有质量都转化成能量，夜晚也不会比一个有月亮的晚上亮。因此，现在有了双保险——宇宙还太年轻而且能量不足。点亮整个宇宙就像是用一根蜡烛花上一个小时来加热一幢房子：一个小时太短了，即使你能等更长的时间，可是在完成这项任务前，蜡烛也已燃尽了。

所以，天文学家终于能回答为什么夜空是黑暗的了。一百多年前科学家解释了为什么天空是蓝的，但是看起来更简单的问题——夜空为什么是黑暗的，却直到20世纪才有了完整的答案。它印证了，被黑暗迷住的人才是第一个揭开它神秘面纱的人。

[Sky & Telescope 2001年12月]