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Tal Cohen writes: "It is said that one of the most important skills a physicist needs is the ability to quickly make "back-of-the-envelope" calculations. For example, Jan Wolitzky (in Jon Bently's "Programming Pearls") tells about Enrico Fermi, Robert Oppenheimer, and the other Manhattan Project brass who were behind a low blast wall awaiting the detonation of the first nuclear device from a few thousand yards away. Fermi was tearing up sheets of paper into little pieces, which he tossed into the air when he saw the flash. After the shock wave passed, he paced off the distance traveled by the paper shreds, performed a quick "back-of-the-envelope" calculation, and arrived at a figure for the explosive yield of the bomb, which was confirmed much later by expensive monitoring equipment." Read on to find out what this has to do with the unusual characteristics of Earth, and how they could influence our search for life elsewhere in the universe. Rare Earth: Why Complex Life is Uncommon in the Universe author Peter D. Ward, Donald Brownlee pages 368 publisher Copernicus rating 7 reviewer Tal Cohen ISBN 0387987010 summary Maybe we are alone, after all.

But expensive monitoring equipment which can confirm the calculation does not always exist, and hence in some fields, our entire knowledge is based on back-of-the-envelope calculations and rough estimates.

Take, for example, the following question: "How many intelligent civilizations, capable of radio communications, currently exist in the Milky Way galaxy?". The worthwhileness of search projects (such as SETI) is closely related to the answer to this question. The number of positively known civilizations is exactly one: the human civilization. And yet, many scientists believe, or at least believed until recently, that the actual number is far, far higher.

This belief was based on various estimates, such as the calculation proposed by Frank Drake, now known as "The Drake Equation." This equation was popularized in Carl Sagan's remarkable TV series, "Cosmos". Sagan himself believed the calculation's result, and was one of the founders of SETI.

Drake's equation is easy to understand. Take the number of stars in the galaxy (about 200 to 300 billion, based on generally accepted estimates), and multiply it by: the percentage of stars that are similar to our Sun in the energy output and stability; the percentage of stars that have planets (since not every star has any); the percentage of planets orbiting their star in a proper distance (so they could hold liquid water, a necessity for maintaining life); the percentage of planets with liquid water on which life actually evolved; and finally, the percentage of life-bearing planets in which intelligent civilizations (i.e., those that can communicate by radio) eventually came to be. All in all, there are five or six factors in this product.

(Note: In my own copy of the book (2nd impression), page 267 states that "a good estimate for the number of stars in our galaxy [is] between 200 and 300 million" - one letter misspelled, and wrong by three orders of magnitude. I do hope the authors' actual calculations were based on the correct value.)

But what values should be used for the various percentages? Drake (and Sagan) chose what they considered to be a conservative approach, and estimated that only about 1 in 10 stars has any planets; only 1 in 10 planets is in the proper orbit, and so forth. Despite the conservative approach, the results were encouraging, indicating that there are thousands of intelligent civilizations in the Milky Way, and probably millions of them in the whole universe. Thus they concluded that there is intelligent life out there, in all likelihood; now we only have to look for it.

In their book Rare Earth, published by Copernicus Press in 2000, Peter Ward and Donald Brownlee point at Drake's (and other physicists') mistakes in a long and depressing discussion, a discussion that took the wind out of more than one SF author's sail.

The book presents what the authors call "the rare Earth hypothesis": simple (bacterial) life is very common in the universe; complex life (multi-cellular life forms, or animals -- let alone intelligent life) is very rare. The first part of the hypothesis is easy to understand, and few scientists will argue with it: indications of simple life were already discovered on rocks originating on Mars, and even here on Earth in conditions that were, until recently, considered completely hostile to life (such as temperatures higher than 100 degrees Celsius, in which 'extremophile' bacteria were found to exist). The second part is the interesting one, and it suggests that the existence of simple life does not necessarily lead to the evolutionary development of complex life, for any number of reasons.

Drake's mistake was basically in the assumption that all it takes for a planet to develop life is being in the proper distance from a proper star. The truth, Ward and Brownlee suggest, is that we have to look at each and every attribute of Earth, and re-estimate its importance for supporting life. Drake's equation is a statistical calculation, but with no other example for life, we're doing statistics with N=1.

Well then, what are the special attributes of Earth that we have to take into account when attempting to run this calculation?

• Proper distance from the star. If a planet orbits its sun too closely or too far away, liquid water would not exist. There isn't much margin for error here: a change of 5 to 15 percent in Earth's distance from the Sun would lead to the freezing, or boiling, of all water on Earth.
• Proper distance from the center of the galaxy. The density of stars near the center of the galaxy is so high, that the amount of cosmic radiation in that area would prevent the development of life.
• A star of a proper mass. A too-massive star would emit too much ultra-violet energy, preventing the development of life. A star that is too small would require the planet to be closer to it (in order to maintain liquid water). But such a close distance would result in tidal locking (where one face of the planet constantly faces the star, and the other always remains dark -- as with the moon in its orbit around Earth). In this case one side becomes too hot, the other too cold, and the planet's atmosphere escapes.
• A proper mass. A planet that is too small will not be able to maintain any atmosphere. A planet that is too massive would attract a larger number of asteroids, increasing the chances of life-destroying cataclysms.
• Oceans. The ability to maintain liquid water does not automatically imply that there will be any on the planet's surface. It looks like Earth acquired its own water from asteroids made of ice that crashed here billions of years ago. On the other hand, too much water (i.e., a planet with little or no land) will lead to an unstable atmosphere, unfit for maintaining life.
• A constant energy output from the star. If the star's energy output suddenly decreases, even for a relatively short while, all the water on the planet would freeze. This situation is irreversible, since when the star resumes its normal energy output, the planet's now-white surface will reflect most of this energy, and the ice will never melt. Conversely, if the stars energy output increases for a short while, all the oceans will evaporate and the result would be an irreversible greenhouse-effect, preventing the oceans from reforming.
• Successful evolution. Even if all of these conditions hold, and simple life evolves (which probably happens even if some of these conditions aren't met), this still does not imply that the result is animal (multi-cellular) life. The evolution of life on Earth included some surprising leaps; two worth mentioning are the move from simple, single-cellular life to cells which contain internal organs, and the appearance of calcium-based skeletons. It appears like the first of these leaps took more time than the evolution from complex single-celled life to full-blown humans.
• Avoiding disasters. Any number of disasters can lead to the complete extinction of all life on a planet. This include the supernova of a nearby star; a massive asteroid impact (like the one that probably caused the extinction of dinosaurs, and 70% of all other life-forms at the time); drastic changes of climate; and so on.

There are also a few attributes that seem, at first, to be completely unrelated to life and not required for its development. Ward and Brownlee argue strongly for the importance of the following attributes:

• The existence of a Jupiter-like planet in the system. Apparently, Jupiter's large mass attracted many of the asteroids that would have otherwise hit Earth. Could life evolve in a system with no Jovian planet? On the other hand, too many Jovian planets, or one that is too large, could lead to a non-stable solar system, sending the smaller planets into the central sun or ejecting them into the cold of space.
• The existence of a large, nearby moon. Luna, Earth's moon, is atypically large and close. Both of Mars's moons, for example, are minor rocks by comparison. What does this have to do with life? Well, it turns out that Luna kept (and still keeps) Earth's tilt stable. Without Luna, the tilt would have changed drastically over time, and no stable climate could exist. If the tilt would have stabilized on a too-large or too-small value, the results could also be disastrous; Earth's tilt is "just right."
• Plate tectonics. Surprisingly enough, it seems like plate tectonics are required for maintaining a stable atmosphere. Plate tectonics play an important role in a complex feedback system (explained in detail in the book) that prevents too many greenhouse gases from existing in the atmosphere. No other planet (except maybe for Jupiter's moon Europa) is known to have plate tectonics. Is this a rare phenomenon, but required for life?

The bottom line is that many additional factors must be added to Drake's equation. One must keep in mind that as any term in such an equation approaches zero, so too does the final product. For most terms, we have no way of reliably estimating their true value, but it seems like at least some of these values are extremely low.

Two important things should be noted about this book. First, about what it does not contain: although I am sure many people will see the Rare Earth Hypothesis as another proof for the existence of a god, this notion of a proof is completely unrelated to the authors' ideas. The hypothesis claims that the conditions for creating complex life are rare; but we know for a fact that at least in one case, all the required conditions were met. Additionally, anyone who insists on taking the ideas of this book as a proof for god's existence will also have to accept the authors' prepositions about the age of the universe, the age of planet Earth, and more importantly, the theory of evolution.

Second, about what the book does contain: the book discusses at length all the issues I've listed above, and more. The problem is that sometimes one gets the feeling that these issues are discussed in too much detail, and the authors tend to repeat themselves, or to delve too deep into some of the less-important aspects of their theory. This is certainly not your common popular-science book; it relies on very up-to-date research results (including some results that were not even published when the book went to press). The writing gets technical on many points in astrophysics, biology, chemistry, and geology (as well as the new field of astrobiology, of course). Over 25 pages of bibliography and references are included.

The theory's weakest point, however, is obvious. The authors admit (after 281 pages of discussion) that their base assumption was that every complex life-form would be similar in many ways to life on Earth: "We assume in this book that animal life will be somehow Earth-like. We take the perhaps jingoistic stance that Earth-life is every-life, that lessons from Earth are not only guides but also rules. We assume that DNA is the only way, rather than only one way" (p. 282).

For me, reading this book was a fascinating and awe-inspiring experience. The most important conclusion (apart from SETI being a huge waste of resources) is an unavoidable cliché, which the authors avoided presenting directly, even though it stares into the reader's face from every page and each paragraph: What we have here is rare, maybe even unique. We should try a little harder to make sure it survives.

Post Scriptum: A news item in the November/December 2001 issue of the Skeptical Inquirer (Vol. 25, No. 6) states that "David Darling, an astronomer who is a critic of the Rare Earth hypothesis, has revealed that one of the strongest influences on the authors, a young [...] astronomer who they acknowledge in their preface 'changed many of our views about planets and habitable zones', has a hidden, Earth-is-unique agenda motivated by strong 'intelligent design' religious views." That astronomer, Guillermo Gonzalez, published several articles in Connections, a quarterly newsletter published by Reasons to Believe, Inc. In one of these articles, co-authored with the creationist scientist Hugh Ross, Gonzalez writes: "The fact that the Sun's location is fine-tuned to permit the possibility of life [...] powerfully suggests divine design."

Darling published these findings, along with a detailed point-by-point scientific critique of the Rare Earth hypothesis, in his book Life Everywhere: The Maverick Science of Astrobiology . Skeptical Inquirer quotes Darling as saying, "What matters is not whether there's anything unusual about the Earth; there's going to be something idiosyncratic about every planet in space. What matters is whether any of Earth's circumstances are not only unusual but also essential for complex life. So far we've seen nothing to suggest there is."

For more about this book, please see this page. For additional book reviews, please visit Tal's bookshelf. You can purchase Rare Earth from bn.com. Want to see your own review here? Just read the book review guidelines, then use Slashdot's handy submission form.

linuxator writes: "While scientists were looking at stuff that Pathfinder collected from the red planet, they discovered that they may be looking at chlorophyll. What does it prove if it really is chlorophyll? Well, chlorophyll is that green stuff in plants... So figure it out yourself :) "

1%warren writes "The Register has posted a report on AOL's purchase of many of the more popular Blogs. It quotes an AOL executive as saying: 'You can't really put figures on this, but we think we have 78 per cent of the libertarian news blogs, and 91 per cent of the ClueTrain Manifesto fan sites.'" As happens with these things, I am not allowed to comment.

An anonymous reader writes: "Anyone ever had to set up virtual hosting on a server that allows CGI execution, etc? This seems to be simple, until you want to keep users out of each other's data. The Apache config seems straightforward enough, but I still haven't figured out the best way to set up the user groups on the box to keep them trapped in their areas and out of each other's business. I thought I could put each user in his own group to block prying eyes on the system side, then add the web user to all the other user's groups allowing him to get to their files, using suexec to prevent one user from using the web server to look at another user's files. This works well, but there seems to be a limit on the number of secondary groups a user can be a member of. So, the web user hits a wall at roughly 16 "customers" or user accounts. Any suggestions on how to improve on this and get beyond the limit? Or is there a better way to approach this than the group/suexec thing? Any pointers to online resources dealing with this type of config would be great..."

jeffy124 writes: "It appears marketers are finding that there's little value in stockpiling the personal info of web users. Either that or they're responding to the negative feedback of users. In a survey of 400 commercial websites, when compared with figures from a 2000 FTC study, more sites have more prominent and explanatory privacy policies, less carry third-party cookies, less collect personal info, and more use opt-in collection. The study was performed by the Progress & Freedom Foundation, and the full report can be found here." Note that the Progress & Freedom Foundation is an industry-sponsored group which generally favors a non-regulated online marketplace.

danny writes: "'Free as in Freedom: Richard Stallman's Crusade for Free Software' is an insightful biography of a figure whose mere name tends to start flame wars on Slashdot ..." Stallman may be one of the most interesting people alive right now: read on to see how well the biography is up to the task of describing him and his movement -- acccording to Danny, that may depend on the reader. Free as in Freedom: Richard Stallman's Crusade author Sam Williams pages 225 publisher O'Reilly rating 9 reviewer Danny Yee ISBN 0-596-00287-4 summary Life of Stallman

Free as in Freedom is a generally sympathetic but far from hagiographic biography of Richard Stallman, inspiration of the free software movement. While much of the material in it will be familiar to anyone actively involved with free software, there are, as Williams claims, "facts and quotes in here that one won't find in any Slashdot story or Google search." It is also an entertaining and accessible study, which I finished within a day of my review copy arriving.

Williams begins with the famous jamming printer and Stallman's encounter with a non-disclosure agreement that prevented him writing reporting software for it. He then jumps forwards to a speech given by Stallman in 2001, responding to attacks by Microsoft on the GNU GPL. Having used these episodes to introduce Stallman and explain the basic idea of free software, the rest of the work continues in a similar vein, mixing historical chapters with ones describing Williams' own meetings with Stallman.

Chapter three describes Stallman's childhood as a prodigy; chapter four his experiences at Harvard and MIT; chapter six the MIT AI Lab and the Emacs "commune"; chapter seven the death of the MIT hacker community and the first announcement of the GNU Project; chapter nine the GNU GPL; chapter ten the appearance of Linux and debates over GNU/Linux; and chapter eleven the coining of the term "open source" and the arguments over that. These contain quotes by everyone from Stallman's mother to the leading lights of free software, as well as plenty by Stallman himself. The narrative never strays too far from its subject, but becomes inextricably interwoven with the broader history and politics of free software and sometimes digresses to cover key figures and events with which Stallman wasn't directly involved.

Williams' first-hand accounts help give Stallman a human face: chapter five recounts a meeting in 1999 LinuxWorld, chapter eight a meeting in Hawaii, and chapter twelve a frustrating car trip with Stallman at the wheel. These give a feel for Stallman's personality and presence, his forthrightness and emotional intensity, his steadfastness and his abrasiveness, and his ability to unsettle. Chapter thirteen attempts to predict Stallman's status "in 100 years," quoting opinions from from Eben Moglen, John Gilmore, Eric Raymond, and Lawrence Lessig; it also suggests that Stallman's personality may be inseparable from his achievements.

Although I was already involved with free software advocacy, my first encounter with Richard Stallman came when he turned up to a rehearsal of my gamelan group; afterwards I tried without much success to explain to my fellow musicians just how important the strange bearded man they'd just met was. I don't think Free as in Freedom would help much with that: it jumps around too much and assumes too much general knowledge of the computer industry to be a good introduction for complete outsiders. Those already interested in the history and politics of free software and hacker culture, however, should relish it.

In an epilogue Williams talks about the writing of Free as in Freedom and the choice of copyright license. Despite the big fuss made about it being released under the GNU Free Documentation License, however, only a sample chapter is available online now and the rest will not, apparently, be put online until June. (This is not a violation of the OFDL, because Williams as copyright holder can allow O'Reilly to distribute the book in any way they like.) So if you don't want to buy a printed copy, you can either wait three months or hope someone OCRs the book sooner.

You can purchase Free as in Freedom from Barnes & Noble, read chapter three online, or check out Danny's 600 other book reviews. Want to see your own review here? Just read the book review guidelines, then use Slashdot's handy submission form.

tu-tone writes "The LA times has done an article titled "Simpsons analysts show how math figures into episodes" based off of work done by two professors Andrew Nestler and Sarah J. Greenwald. The work is a Guide to Appearances of Mathematics and Mathematicians on "The Simpsons" . They even gave a talk on it at Harvey Mudd College. It's a fun read." There's a transcript of one of their presentations available.

racerx509 writes: "I just did something thats probably going to cost me much. I was reinstalling my hdd after trying to ghost the partition for another pc, when I jammed the cable in wrong. The cable is keyed and the keyed side was in correctlty, so I figured that it was going in right. However, I have severly bent several of the pins. The HDD will no longer detect even after I attempted to straighten the pins. Pin #11 has receded into the aperature and pins #15 and #16 are seriously bent. I've straightened them out with tweezers as much as I can, but it looks like this drive is gone. I would replace it, but i have some very important data on it. Does anyone know of a way I could straigten the pins and pull receded ones? If not, what about a good data recovery service in the Atlanta area?"

mattsimerson writes "Looking for a more secure and reliable alternative to BIND for serving DNS on Mac OS X? Look no further than the article I wrote up on the topic. I use djbdns extensively in a large network environment (hundreds of thousands of zones and servers) and it works just as well on my G4 laptop. If you're a heavy duty DNS guy, you might enjoy my other DNS links." Excellent. I am just a dumb programmer, not a sysadmin. It takes me hours to figure out how to configure something relatively simple in NetInfo, so this HOWTO is very welcome.

jimmi writes: "A couple of months ago cgisecurity.com released a paper called 'Fingerprinting Port80 Attacks.' Today they released Part 2, which is even bigger then the first. Part two can be found here. This paper deals with web application attacks and how to detect them, along with figuring out what the data means."

gwernol writes "Jef Raskin, one of the original Mac design team and a distinguished figure in the world of user interface design has given an interesting interview over on OS Opinion. He talks about the tradeoff between interface consistency and customizability, and particularly the impact of skinnable applications on usability. Interesting reading, including some harsh words for "guru UI designers" like Steve Jobs..."

cabbey writes: "Say the name MegaTokyo and most people, if they recognize it, think 'one of the best manga/comics on the net today. (ignoring the recent 'stick figure dom' days while Piro was moving).' But few people think about the social, economic and philosophic issues the authors' rants can delve into. This morning Piro put up a rather long 'rant' that's really a catching insight into why the dot-com world didn't have a snowball's chance in hell of surviving. (archive link to the rant in question, it's below the comic. ;) "

An anonymous reader sent in: "In this post to the Cryptography Mailing List, someone who knows more about math than I do claimed "effectively all PGP RSA keys shorter than 2k bits are insecure, and the 2kbit keys are not nearly as secure as we thought they were." Apparently Dan Bernstein of qmail fame figured out how to factor integers faster on the same cost hardware. Should we be revoking our keys and creating larger ones? Is this "the biggest news in crypto in the last decade," as the original poster claims, or only ginger-scale big?"

SomeoneYouDontKnow writes: "The New York Times (free registration reguired, blah blah blah) has a fascinating article about a former Iranian rock star who has launched a pirate TV station broadcasting back into Iran from, of all places, L.A. From reading the article, I can't help but compare Narional Iranian Television to U-62 from the movie "UHF" because of its ultra-low-budget operations and programming, but, like the fictional station, it's wildly popular. OK, I know this is a little off the beaten track for Slashdot articles, but it's nice to see that there's a broadcaster out there more interested in providing a meaningful service than figuring out ways to squeeze more and more money out of viewers."

NaturePhotog asks: "IT clearly isn't "it", at least as far as personal transport goes--price and weight rule that out. I live in the SF Bay Area, and while regional transport such as BART is pretty good, there are a lot of gaps and annoyances in local transport. I'm looking for something like the RAD2GO E10 or Zappy to help fill those gaps. It doesn't need to be fast, but it needs to fold and be light enough to carry on BART, be able to handle some hills, and ideally have a range of about 10-15 miles on a charge. There are hundreds of electric scooters and dozens of sites, so figuring out what would be best is difficult to say the least. Any in the Slashdot crowd have experiences (good or bad) with something like this? Or suggestions for other alternatives?"

gilroy writes: "According to a New York Times article (free registration, yadda yadda), despite taking the moral high ground (that they want to see artists compensated, as opposed to all those evil downloaders), the record companies have actually set up pay schedules so as to -- wait for it -- rip off the artists who record the music. Some figure they will earn less than $0.0023 per download -- yes, that's hundredths of a penny. Best quote from the article: 'For many acts, suddenly there appears to be little difference between the illicit file-sharing system and record-label services.' Good to see they're fighting for the artists, n'est-ce pas?"

Judebert writes: "Watching your stainless steel silverware rust is enough to drive a geek to apoplexy. Not that you care, just that it is stainless, after all. Well, some clever Brits at the Imperial College of Science, Technology and Medicine have figured out Why Stainless Steel Corrodes and published a like-named article in Nature. Science Daily, as usual, mirrored their press release. When stainless steel goes, the results can be catastrophic. Especially considering that the stuff is used in Formula 1 engines, industrial equipment, and thousands of other places. Turns out the problem is sulfur in the steel / chromium alloy. But they've also figured out ways to fix the problem without resorting to very expensive low-sulphur steels."

Kathleen, I wanted to do this in this most potentially embarassing way possible, and I figured doing it here and now, in front of a quarter of a million strangers was as good a way as any. I love you more then I can describe within the limits of this tiny little story. We've been together for many years now, and I've known for most of that time that I wanted to spend my life with you. Enough rambling. Will you marry me? Update 15 minutes 30 seconds later: Subj: "Yes", message body: "Dork. You made me cry. :)" Hazah! I'm getting married! :)

This edition of Slashback has updates and clarifications on the official release of Sony's PS2 Linux, relative security among various operating systems, dirty output on power-boosted Linksys wireless access points, and more, flying hardware you might have figured was no more, and more.

Maybe a bad day at the factory? An anonymous reader submits: "I'm not sure where the other fellow got his WAP11, but mine don't show the dirty output his does." See this diagram for a much more desireable outcome, if you care to play with (a little bit of) fire.

First application should be a GPL'd AIBO obedience school. gonz writes: "An update to the previous reported linux on ps2 kit has been submitted by Sony Computer Entertainment Europe (SCEE) to the people previously registering interest on their technology sites. The update consist of that it will be released in May on both SCEA (us) and SCEE (pal areas, including Europe and Australia) territories. A website has been set up at this place. On a side note, registering for notification when pre-ordering can apparently be done too: 'Finally, although sales haven't yet started, if you send an e-mail with the message "subscribe" to ps2linux-request@technology.scee.net we'll let you know when pre-ordering starts.'"

Lessons in obviousness. John Kozubik writes: "I have written an article describing, in a manner I have not yet seen, why the court decision by the U.S. appeals court in SF that claimed in-line linking was not fair use was inherently flawed. It is a short piece written for both the technical and the non-technical, and I think it raises a strong point concerning the arbitrary nature of browser behavior."

If they'd launch some pigs, perhaps global phones would be affordable. Guppy06 writes: "Many of you may be surprised to learn that Iridium (famous for trying to compete with cell phones and failing miserably) is still throwing up satellites (I sure was). The article on CNN tells of the technical woes of getting this particular Delta II off the pad in Vandenberg as Iridium tries to put five more spares into orbit."

Couldn't they have spayed or neutered them instead? Speaking of old hardware, Anonymous Radio Shack Employee writes: "RadioShack has sent a notice to all of its employees to destroy all CueCats (preferably with a hammer). Apparently the CueCat is among a couple of dozen items that RadioShack has given up on, and wants destroyed. The memo says that store employee's can not benefit from the items on the list. Which sucks because my store has over a hundred of these things just sitting in the back room." This week's Linux Weekly News has a great, detailed followup to the recent flap over relative OS security sparked by a post in Windows Informant.

jonr writes: "Well, the holy grail of digital photography is finally found. A company named Foveon have developed a sensor that captures RGB colours on each pixel. So what you say? Well, for the past 30 years (or since the CCD was invented) we have been using CCD with with red, green & blue sensors (or cyan/magneta/yellow) and then used software to figure out the real colour. But Foveon is the first company to deliver RGB-in-each-pixel sensor. For those of you who are not into digital imaging, this makes a lot of difference, it's would be just as revelutionary if somebody would make a flatscreen with a real colour pixels, instead of the RGB dots. dpreview.com has the scoop. (No, it won't mean the death of film, but I suspect we'll see dramatic improvement in quality)."