Ma'aseh Rav-the Rabbi did such and such

Question: Often times in the course of halachic discussions it will be related that a certain Rav did such and such. When do such accounts have halachic significance? How do we safeguard against inaccurate/urban legend stories detrimentally?
Answer: One part of it, I think, is that the Rav himself has to make sure that his actions will not be misconstrued by observers. We thus find, for example, cases where our Sages acted according to the strict letter of the law (or it was suggested that they do so), even though they might have preferred to show deference to a different view, "lest the students see it and [wrongly] decide the halachah accordingly for future generations" (Berachos 11a, Pesachim 100a, and Kesubos 50b).
Answer: The first person recording the Ma'aseh Rav is a talmid of the Rav, and it generally has to be in writing by a well-published talmid of that Rav. For the most part, if we're using a Ma'aseh rav to decide halacha after more than a single generation, that Rav is somebody of extreme importance, like the Ari Z"l or the GR"A. (Or, if you're chassidish, the Rebbe of your branch of chassidus.)

Ma'aseh Rav-the Rabbi did such and such


I need to know if I can ask some homework problems on here?

Question: This is my first post. I need to know if I can ask some homework problems on here? Not full problems but after I provide my attempt at the problem.
Comment: @MikaelFremling Effort is *never* enough to make a homework-like question on-topic for Physics. It *never* has been enough. Please read http://meta.physics.stackexchange.com/questions/714/how-do-i-ask-homework-questions-on-physics-stack-exchange for the policy currently in force, and be aware that there are discussions underway to change that.
Comment: a) this should have  been a meta post. b) Yes, you may ask such problems but it is of utmost importance that you show an effort to solve it yourself first.
Comment: @Mikael Thanks for the info. And do I expect my question to be fully solved after I show my attempt at it?
Comment: Depends on how much of an effort you do,  and how interested people are. Remember,  we're here because we want to,  not because we need to.
Comment: Right right. I'll keep that in mind. And thanks.
Comment: We do not give full solutions.
Comment: @dmckee I see you point. I was merely puching the OP in the direction that he/she should just not post a question without any attempt to solve it him/herself. Thank you for the link.
Comment: @MikaelFremling I appreciate that, but if you tell another user that showing effort is enough and their next question gets closed they often get angry which is something we'd like to avoid.
Answer: The ideal question on this site will ask about some concept in physics. The aim is that after reading the question and its answer people will end up knowing more about physics than when they started.
The reason we don't like homework questions is that in general they are just about solving some specific question so all anyone reading the answer learns is how to solve one question. Unless answering the question illustrates some general principle the answer won't be of much use to anyone but the person asking the question.
The point of all this is that if you're asking a homework question that doesn't help explain some wider concept we won't care whether or not you show how much effort you've put into it - we will close the question anyway. This isn't because we're mean and/or sadistic, it's because by posting questions like this you make the site a worse place for everyone else.
So if you're going to post a homework question make sure you write it to highlight the concept in physics that you don't understand.
Comment: Great piece of info thanks
Comment: could you please tell me how I can attach pictures to my questions?
Comment: John, shouldn't this be migrated to meta before answering?
Comment: @EmilioPisanty: Yeh; we know it would be migrated and that's why John answered it ;P
Comment: @Zlatan: There is a in-build facility for this; look at the toolbox above when you are posting; there is an `image` option, there.
Answer: I disagree with John Rennie's opinion.
I think it is very difficult to learn about physics without discussing and working on specific problems.  That is how I learnt physics.  And judging by the volume of very specific situation-type problems at the end of every chapter of almost every physics textbook, the experts in physics education agree with me.
Keeping everything general and abstract is frustrating for the student, and frustrating for the person trying to answer the student's questions.
My opinion is that there is no difference between a homework problem and any other problem.  The real issue is : What attempt have you made to solve it?  What we are trying to discourage is lazy students who just want an answer without having to think.  So the site policy is to provide hints to help you to answer the question for yourself as much as possible.  That way you get the most benefit from the 'exercise'.


What's the feeling of having more than 10k points of rep?

Question:
Do you still talk to people normally?
Do you plan to donate all after you're gone?
What are some common pitfalls that a normal user has before achieving such status?
Comment: It's just like having 9,999 reputation, except you can **delete stuff**
Comment: It's rather uneventful, to be honest.
Comment: @MikaelBlomkvist WHO TOLD YOU THAT. Now we have to relocate. :(
Comment: I may have garnered a lot of rep, but it hasn't changed me. I still work, for example.
Comment: The best thing about getting to 10K is the [cake](https://twitter.com/balpha/status/240115851259416577)
Comment: Common pitfall? Thinking that when you reach 20K then maybe you'll finally feel differently `:)`
Comment: @Lix The cake is a lie. (PS: Did you marry the girl yet?)
Comment: @Lix That's great!  I had to make my own cake. ;-p
Comment: I'm loving the answers.. you all that are more close to the grand master of SO, i heard that actually is a squad of midget ninjas in a dungeon somewhere in the middle east that actually answers all the questions.. does that holds??
Comment: @AnnaLear I've made a question related to that in the midget.overflow site once and it was **immediately** closed, things started to tick..
Comment: @cod - haha... Not *yet*... :-P
Answer: 10K was a small personal achievement; just an arbitrary number and goal that felt good to meet.
So let me put a twist on the question: How have I changed by spending enough time on SO to garner 10K rep?

I have learned to triple-check my work.
I've learned—though I may not always succeed—to write unambiguously to avoid confusion.
I've learned to keep my mouth shut when I don't have the complete answer (instead, I may post a comment). In other words, if I can't bring a great answer to the table, I refrain.
I've learned to love and to read specifications like never before.
I've been exposed to theories and practices which I had no idea even existed. Part of this comes from reviewing and editing questions (before and after 10K).
My answers are simply better (based on the above criteria) now than when I first joined SO.

After hitting 20K a few weeks ago, I decided that I should become more actively involved in meta. And here I am...
Comment: This answer both speaks to my experience and makes me unaccountably happy.
Comment: This. Not just specifications though; documentation as well.
Comment: I think the best part about hitting 10k was just sitting around and watching the residual 12k+ rep that I've just slowly gotten since then.
Comment: @Chacha102: I don't know you, but it looks like it's been a while since you last posted here.
Answer: I remember my reaction on Superuser when I hit 10K...
Someone else was like "Congratulations" and I was all "Wait, WHAT?". Then I hit 20K and realised there were no more worlds to conquer no more privileges to be gained ...
I'm close to 50K now. Rep is really just a number, and it has not really changed the way I view other users. The stuff I've learned and the people I meet are far more valuable than reputation to me - the communities around the sites I am on matter more to me than reputation.
One change I've taken to since hitting 40K is giving away bounties for questions (maybe 50-100 rep a month, give or take), now that I don't need to worry about privileges.
Comparing it to the other two sites I'm active on, where I have under 3K rep as things are, I do kinda miss the tools available for me to clean things up. Flagging is just inefficient.
Copying Tim Medora's spin on the question I found that there are a few small things that helped me get there.
1 Answer a question a day, at least
Assume you get an upvote for each question. In a year, you hit 3650 rep. You're obviously going to get more than that. This is the secret to really getting lots of rep and not spending hours on the site. If you don't do anything else do this
2 Have fun
The questions you find interesting, others might as well. Some of my most highly voted answers were things I am interested in myself. Post good, clear answers which answer the question, and reflect what you think about them.  This question, now CWified was an epic example of a lot of people solving the problem and having fun
3 clarity is king
If it was simple, people wouldn't be asking. While it isn't as useful on SO, on SU I often post screenshots to help illustrate an answer. I also format things for readability first.
4 Quality is the Emperor!
Post good answers. Improve your answers as you learn more.
5 Be Altruistic
I improve answers I would have posted myself by the same things I'd do. Adding screenshots is one example. This means better answers, and the chance someone notices you do these things.
6 Don't be the fastest gun in the west, find someone who's a bad shot, and shoot better than him
This is sneaky. I get annoyed by bad answers, so I post better answers. It's entirely incidental that this makes your answer look even better and get upvotes.
7 Abuse the hot questions the right way.
I've gotten a ton of rep by posting reasonable quality questions on hot questions. I've learnt to spot these to an extent on SU, and on a good week, I seem to have managed over a kilorep off these.
Don't post shitty answers, Post something that is rule 6 compliant and post a better, well researched answer. Make it clear, make it pop (Relevant graphics are nice here) so it floats up above the rabble , and get a lot of rep
Answer: You assume we talked to people normally in the first place.
I think 10k is a fairly low bar. IMO it's 100-200k that's a real differentiator: The rest of us are just here, doing what we can and/or have actual time for.
That said, I've found that my modest 65k gives people the (largely mistaken) notion that I actually Know What I'm Talking About, leading to general hilarity when the truth is revealed.
I get random emails and tweets from people asking questions. There are surprisingly few people showering me with money for time spent. I have gotten a few legitimate work inquiries, and it has made a difference in my job search.
Honestly, I've found my motivation has decreased somewhat, although that's partially due to just not having time. I'm also in generally low-volume groups meaning that even on a good day it an be a bit of a struggle to get my goal 200 rep so I can earn a Legendary badge, one that I'd actually like.
(My Generalist badge, one I actually cared about, appears to be the result of a glitch.)
So now I'm at the point of not having any more rep-based super-powers goals, running out of badges I care about earning, and the rep milestones I now see as "interesting" will be a long time coming. I now take the long view: I still enjoy answering questions, but it's a rare occasion I have the time to keep up with my earlier ambitions, and 100k+ is a long way off.
Edit Quite by accident, 100k is no longer a long way off. I still feel about the same, along with some explicit negativity these days. I almost never have time to be very active, instead I have time for relatively short comment-only clues-to-an-answer, but some people are offended by those.
Comment: +1 for "That said, I've found that my modest 65k gives people the (largely mistaken) notion that I actually Know What I'm Talking About, leading to general hilarity when the truth is revealed."
Comment: Even with 30k I get emails asking for help. So possibly the bar lies lower.
Answer: To speak for myself, my focus shifted from answering questions to performing more maintenance oriented tasks.
An annoying side effect of having 10k/20k on one site, is that you are going to miss the privileges on the other sites. But that is another incentive to provide lots of quality posts.
Comment: "An annoying side effect of having 10k/20k on one site" so totally true.
Answer: Personally, there was a moment of "cool, I made it to 10,000 points!" followed rather soon by the realization that there is an awful lot of trash (deleted posts) on the site.  This question soon followed:
Hide deleted answers with certain characteristics
Then I came to terms with deleted answers and accepted the responsibility of it (review) rather than worrying about the "pretty" site I was used to before.  In fact I became a community-elected moderator for Mathematica and deal with these things all the more. :-)
I take the second point as jest, but as a matter of fact I have recently started giving points away.
One pitfall is equating "reputation" with actual knowledge.  Dave Newton stated this in his answer most humorously.  It goes both ways; as a new user you may think that the 10k users must know a lot but that's really not the case.  As a high-rep user (not by StackOverflow standards but good for my site) I have to accept the self-deprecation of regularly reminding people that I'm not the most knowledgeable or skilled guy on the site (far from it).
Answer: I participate on small- and medium-sized sites, not the trilogy, so the scale is rather different.  Even though it's just a number, every time I've hit 10k I've felt like I'd reached a real milestone.  Then, as someone else said, I saw how much junk there was that others had been cleaning up before I ever saw it, so I started using my newly-granted delete votes.  I have yet to reach 20k on any site so there is still that to work toward.  But I'm a moderator on some sites, so I've seen and used the custodial powers there.
As I've gained reputation I've tended to do more site-maintenance tasks.  All along I try to leave helpful comments trying to steer newer posters to better-formed questions and answers, but maybe those comments carry more weight with a bigger number in the mouse-hover.  Don't know.
What are common pitfalls that users have before reaching 10k?  I'd say the biggest is not using the powers they do have -- reviews, votes to close and reopen, and flags all kick in well below 10k, and one of the most important of all, the guiding comment, requires only 50 rep.  The first time I hit 10k I felt like I'd gotten a lot of new powers, but in reality I'd had most of them for rather a while.  Don't wait for the rep number to roll over into five digits; there's a lot you can do now.
And, above all, keep doing what brought you to that site in the first place -- asking and answering questions.


Claim Status Identifiers

Question: Does a dependent claim's status identifier change to "Currently amended" if amending only the independent claims from which it depends? The dependent claim is not amended.  Examiner is suggesting that I change the status identifier despite no changes to dependent claim (only related independent claim).
Comment: Have you changed the dependent claim number ??
Comment: Does it matter? I haven't seen it handled that way, but I don't think it's a hill worth dying on.
Answer: No need to change status identifiers if any of the independent or dependent claims is unchanged. Status identifiers are needed to be changed in case of amendments (strikethrough of text, insertion of text, or dependency change) only.

Claim Status Identifiers


What does PPS mean/stand for?

Question: On many mining pools/bonds I see things listed as 105% PPS / 110% PPS weekly payout.  What does PPS actually mean?
Answer: Where I've seen it, it refers to the mining payment method "Pay per share".
This question has an answer that describes what a "share" is.
On some mining sites that have existed, the earnings for a share have been essentially paid immediately. However, this method exposes the pool operator to a possibility of loss in the case that, on average, they find less that the expected number of blocks. This can be made much worse by a large miner participating in the pool and performing a block-withholding-attack. To prevent these problems, most pool operators prefer slightly modified pay-per-share methods of payment, such as SMPPS (Shared Maximum Pay Per Share).
A payment method is not strictly Pay Per Share if the earnings aren't available for payout immediately, or if risk is passed on to the miner. Note also that it may not always be clear how much of a fee is being charged by a pool operator; it can be as low as 0% or as high as 7% (but if they tell you the payment per share, you should be able to work it out by dividing the block reward of 50BTC by the current difficulty and comparing it with what the pool pays).
A pool would not payout 105% or 110% except perhaps as a promotion when it is trying to increase its number of participants and offers an increased reward such as 55BTC per block (ie 110% of what is earned) at its own cost.
Update: The discussion you linked to in your comment is regarding the issue of mining "bonds" where the issuer promises to pay 110% of the shares mined. Each bond appears to be selling for around 0.6BTC, and represents 2Mh/s of mining speed. Given that the bonds are "perpetual", presumably the issuer has allowed for equipment replacement and maintenance costs, and power etc. Payouts are advertised as "currently 110% of PPS", so it is presumably a promotional offer. The issuer must be earning a margin somewhere along the way to cover the cost.
Comment: An example would be with a mining bond that can be reviewed at the following [link](https://bitcointalk.org/index.php?topic=75126.msg832718#msg832718).  It states, "weekly payouts are 110% PPS".
Comment: The explanation is that "First, I will be doing merged mining so those coins will be used to increase the pay per share.  Also, I would like to offer a good value for each bondholder", however my understand is that merged mining only increases return by 1% or less, so the promoter appears to be offering coins from his own pocket. Would be enough to make me suspicious.
Comment: @HighlyIrregular: Where mining bonds are concerned, 110% PPS is nothing more than a cheap marketing gimmick, designed to give the illusion of differentiation for what is essentially a commodity. Offering a 100% PPS bond for 0.3 BTC is equivalent to offering a 110% PPS bond for 0.33 BTC. Whatever the issuer needs to pay for the extra has already been priced into the issuing price, which is generally higher than the operating cost of the issuer.
Comment: @HighlyIrregular: PS At the cost of the appearance of shameless self-promotion, I think you're confused about what a mining bond is, and you might want to have a look at the thread where by all accounts the concept of a deterministic mining bond was invented, https://bitcointalk.org/index.php?topic=65569.
Comment: @MeniRosenfeld, I think I figured out how the bonds work somewhat after posting that comment. It's probably worth mentioning that in the case of bonds referred to by this question, one bond is worth 2Mh/s, but the current promotion offering 110% payment makes them temporarily worth 2.2Mh/s for the duration of the promotion. It's not a long term commitment, so wouldn't justify a 10% increase in the bond purchase price though.
Comment: @HighlyIrregular: The way I understand it it should be at least 105% forever. The 110% promotion which is possibly temporary makes things worse, the payments are not determinstic but are rather decided by the issuer's whims.
Answer: PPS is "pay per share", a mining pool reward system where the operator gives a set payment per share submitted proportional to its average worth, regardless of how many valid blocks the pool finds. This way the miner doesn't suffer any variance.
By extension, 100% PPS is the reward that would have been given for a certain hashrate at a PPS pool without a fee. 110% PPS is 110% this amount, etc.
Some pools, for example Chaang-Noi's project, are really pool-hopping proxies so they are able to offer >100% PPS to miners. Things like merged mining, preference for newly generated coins, attacks on Bitcoin or alternative currencies or using the hashrate for things unrelated to Bitcoin - or proxying to gpumax.com, which itself is based on any of the above - can also allow a pool to offer >100% profitably. It could also be a promotional offer.
Some mining bonds state that they pay >100% PPS as a marketing gimmick.

What does PPS mean/stand for?


How does holographic voxel density scale with holographic film metrics?

Question: I'm trying to understand how one can generate bounds on the effective number of voxels (volumetric pixels) in a hologram, or information density, provided various metrics for the two-dimensional holographic film being used.  For example, I often come across film parameters for grain size, resolving power (usually referred to in terms of lines/mm), and emulsion thickness.  Is there a straightforward way to calculate this, or perhaps good rules of thumb?
Answer: Holographic voxels cannot just simply be put into a film(Unless you use a decoder for sendimg information to a computer). It however can put into a file.
The file size will be the same regardless of the density(You meant opacity rigth?) of the voxel, it is saved in the file as a parameter.
About the calculation part, it depends on how the voxel-based 3D image software saves their files.


How does the graphical notation used to denote doubly-controlled gates work?

Question:
$\qquad$
$\qquad$
What is the difference between solid and hollow? How to express the corresponding matrix of these figures? In addition, if they are not adjacent, what should be done in the middle of several qubits (such as in more than 3 qubits)
Answer: The black (white) dot means a condition that the corresponding qubit should be in $|1\rangle$ state ($|0\rangle$ state) in order to apply the gate. The first circuit implements the Hadamard gate only if the first qubit is in $|1\rangle$ state and the second qubit is in $|0 \rangle$ state (similar discussions can be found here). In other words, if the combined state of the first two qubits is $|10\rangle$ the $H$ gate is applied, otherwise, we apply $I$ (nothing). Mathematically it looks like this:
$$\begin{align}C_{B}C_{W}\_H =& |00\rangle\langle 00| \otimes I + |01\rangle\langle 01| \otimes I + |10\rangle\langle 10| \otimes H + |11\rangle\langle 11| \otimes I\\ =& |10\rangle\langle 10| \otimes H + (I-|10\rangle\langle 10|)\otimes I,\end{align}$$
where $C_{B}$ is denoted here as the control with a black dot and $C_{W}$ is the control with a white dot. By taking this into account the corresponding matrix will look like this:
$$C_{B}C_{W}\_H =
\begin{pmatrix}
1&0&0&0&0&0&0&0 \\
0&1&0&0&0&0&0&0 \\
0&0&1&0&0&0&0&0 \\
0&0&0&1&0&0&0&0 \\
0&0&0&0&1/\sqrt{2}&1/\sqrt{2}&0&0 \\
0&0&0&0&1/\sqrt{2}&-1/\sqrt{2}&0&0 \\
0&0&0&0&0&0&1&0 \\
0&0&0&0&0&0&0&1 \\
\end{pmatrix}
$$
The matrixes for the other two gates can be constructed in a similar way. Here is an answer about not adjacent controlled gates. Note that for $n$ qubit gate we will need $2^n \times 2^n$ matrices, so it is not always convenient to use matrices.
Comment: thank you very much! It is clear!
Comment: Hi, I found this question by accident.  The answer is good! Expressing the gate in the form of $C_{B}C_{W}\_H = |00\rangle\langle 00| \otimes I + |01\rangle\langle 01| \otimes I + |10\rangle\langle 10| \otimes H + |11\rangle\langle 11| \otimes I$ is very good! My question, where does this approach come from, can you read it somewhere? Surely there must be something like that in a book or a lecture? Because I thought of this formula: $I \otimes X \otimes H$ which is wrong (I was just thinking about it for a second, I think that would be the answer if the qubits weren't connected...)!
Comment: @Despriobaby, you are welcome :)
Comment: @P_Gate, unfortunately, I don't remember a lecture/book where similar things were discussed.


Denmark and Schengen - border controls

Question: As I know, Denmark is going to re-impose the border controls. I want to know, what exactly it means for the tourists who are on a Schengen visa? Can I visit Denmark with the Schengen visa, or I need some additional documents?  I'm a Russian citizen.
Answer: From the linked article:

Because Denmark is a Schengen member, it cannot reinstate full frontier controls and the minister said the new measures would "take place within the limits of Schengen".

So yes, you can still visit Denmark with your Schengen visa and you don't need additional documents. The only thing that's extra is that you can be checked by customs at some particular border crossings. But as long as you have a valid visa, and you are not in possession of anything illegal, you should be fine.
Answer: This sad measure is a populist political one. The plan is to selectively check people crossing the border in order to reduce the amount of illegal immigrants.
As a tourist, you will probably not notice the difference, unless you happen to come from an Eastern European or North African country.
Comment: So racial profiling may be at play?
Comment: @hippietrail, I think this question is hard to answer without becoming political; something I feel we should avoid in our Travel community.
Comment: It's definitely a good straightforward objective on-topic question. He's not asking why just what. It doesn't hurt to mention the why to a degree though as you did.
Comment: The question is good. It's just hard to answer the *why* in detail.
Answer: The Schengen rules have two different aspects:
Unified visa and travel rules
Most residents or visa holders from one Schengen country can visit any other Schengen country (note that this is a generalization, check your specific case). You are allowed to cross with your existing papers.
Lack of internal border controls
Internal borders within the Schengen area are looking like strange road signs or lines on the road. For all practical purposes, you can cross without waiting in line or showing your papers. But technically you are still required to have an ID and/or a visa.
At times border controls are reintroduced. On a temporary basis, this is allowed by the Schengen rules. But that wouldn't change who is allowed to cross, just the likelihood of having to show papers.

Denmark and Schengen - border controls


Error While Connecting Mobile to ad hoc WiFi network

Question: I am able to create an ad hoc WiFi network on my laptop and it is waiting for users.
Now when I try to connect from my windows phone 7.5 I can find this WiFi, but when I supply the password it says "INCORRECT PASSWORD" and this happens repeatedly even though I am entering the correct password.
Comment: I think Windows Phone does not support ad hoc connection.
Answer: Might be useful if you supply more details, but I will assume that you are using internet connection sharing on your Windows 7 laptop.
I had a similar problem with my iPad and it was due to the security protocol Windows was using for the ad hoc network (for me it was WPA2-Personal). Try changing the encryption to WEP and see if this works.
WEP is not very secure and can easily be cracked, so I would advise using caution.


virtual machine just like another terminal window in Linux?

Question: I would like to have a 64bit Linux virtual machine running just like another terminal window in my 32bit Linux Vx-enabled laptop. How can I do that? I don't need a GUI for the virtual machine, just a bog-standard terminal-based Linux VM.
Answer: You'd need a VM host that will let you run a 64 bit os on a 32 bit system - virtualbox will do fine with a VT enabled system. Do a basic installation including a ssh server. Run it in Headless mode after that. Then you can open up a terminal window and ssh into your guest.
Your instructions may vary based on the VM software you choose to use, but for most part this should work fine.


Bad first year marks?

Question: How are bad first year marks (i.e., ranked around average among my peers) but outstanding 2nd-4th year marks (i.e., top 2%) perceived by PhD admissions committees in the U.S.? What emphasis is placed onto first year marks?
Answer: There's no absolute universal standard as to how this will be perceived, but it's definitely better to be in the situation you describe than the reverse (good in general courses, mediocre in the major)!
One of the issues is where the first-year grades are bad. If you have some poor grades in "general education" classes unrelated to your major (for example, an engineer who gets a "C" in American history and English literature), that hopefully won't matter too much if the admissions committee is actually looking at the transcript, rather than just the overall GPA.
Ultimately, it comes down to how rigid the admissions committee is about reviewing things, as you can see in this thread.
Answer: As long as you are able to justify what went wrong during the first year in the personal statement,for example, you had a major illness that caused you to miss many classes and thus resulted in average performance, I think adcom will not take it too seriously.
Comment: Well any evidence of what happened is no longer existing. Surely, given the lack of evidence, claiming something like this is unacceptable?
Comment: The reason should be convincing,however, to adcom.

Bad first year marks?


Could we implement a spam filter?

Question: We've been seeing a lot of spam like this lately. I've been flagging all of them, but it's still annoying to see them on the front page. Is there some way to put in regex filters to combat this sort of thing?
Answer: We have one implemented, and we've adjusted the black list accordingly. The issue is that this is coming from a vast number of large networks, so it's taking a while to stop.
I'm going to be modifying Bitcoin's setting in our global spam shield so that your flags (and moderators destroying these obvious spammers) block these even faster. I raised it a bit before the weekend, but I can take it a bit higher comfortably.
Now, FAIR WARNING ... (for all)
If someone leaves an answer like this:

The best I've seen is FooCoin (http://foocoin.com)

... that consists of barely more than a link, it's probably going to be deleted very quickly. If you're a new user leaving such answers, you might find your account removed.
Additionally, questions that seek recommendations for third-party tools or services that do not provide a very narrow criteria that limits the number of answers are probably going to be closed very quickly, because those are precisely the kinds of answers that they attract.
This is exactly the kind of thing that attracts this sort of spam, so we need our default quality to be much higher than what the spammers want to contribute in order for it to stand out.

Could we implement a spam filter?


How much cycling distance is needed to travel every day for good muscular exercise?

Question: As a bicycler I used to travel on cycle daily for about half an hour to maintain fitness of my legs and body.
But I want to know how much time and distance is needed for a good exercise.
Comment: Quality matters more than quantity. Cycling for half an hour at walking pace gives you barely any exercise at all -- much less than walking for half an hour, since bikes are ridiculously efficient.
Comment: "half an hour" a day of activity is enough to maintain basic health and is generally recommended for all healthy adults.  How far do you ride in that half-hour and at what relative intensity?  IE are you cruising about, or going hard?  Are there a lot of stops at lights (ie recovery time) or are you moving most of the time?
Comment: The fitter you get, the more kilometers you'll *want* to ride. When I restarted riding after an eight year pause, I started with something like eight kilometers a day. That was all I could do. Now, I'm doing at least 25km a day, but usually I take some detours bringing the figure up to 35km. Just because the longer route is more fun. At some point of fitness, the extra cost of additional kilometers just becomes negative...
Comment: @Criggie someday I cruise and someday go hard, and their are 2 stops at lights.But this recovery time for me is very short and I keep moving most of the time.
Comment: Yeah I agree with u @cmaster as it's all about stamina of a person.
Comment: @DavidRicherby Coasting could be the issue since you're not really exercising for the time that you're coasting. One could use a heart rate monitor or a power meter, or pay attention to perceived exertion.
Answer: As individual speeds vary depending on fitness, bike and conditions, most information on this subject discusses intensity and time spend exercising, rather than distance. Most leisure cyclists ride between 10-18mph (16-30kph) on the road, a bit less off-road. As you can see it's a fairly wide range, so time and intensity are better measures.They can also more easily be applied to other sports.
For good health, the WHO recommends:

Adults aged 18–64 should do at least 150 minutes of moderate-intensity
  aerobic physical activity throughout the week or do at least 75
  minutes of vigorous-intensity aerobic physical activity throughout the
  week.

If you'd like to improve your cycling fitness as well as your health, then British Cycling has a number of beginners training plans, depending on what you would like to achieve.
However, not everyone responds the same way to exercise, so the improvements are not guaranteed.
Comment: I'd modify your first sentence to say that most information discusses time spent exercising _and intensity_.
Answer: There is no real answer to this, as it totally depends on what you mean by 'good exercise'.
For a professional racing cyclist it means 30 hours a week of structured training including effort above an below that required for an actual race.
For adults the American Heart Association recommends as a minimum:

at least 150 minutes per week of moderate-intensity aerobic activity or 75
  minutes per week of vigorous aerobic activity, or a combination of both,

How many km and at what level of effort you can ride depends on your current level of fitness. Work up to a level that is sustainable and enjoyable for you.
Answer: I have no hard data to answer this, just my experience: I commute every day, 13 km one way, practically flat, and I do my best to do the route in 30-35 minutes with a city bike.
My legs are pretty toned, and when I have to step up on the travelled distance it's not a big deal.
But a more precise answer strongly depends on the single individual.
Comment: This is anecdotal but a fine answer. I also travel circa 10km one way, to work and back 5 days a week. This works very well to keep muscular strength and overall fitness top notch. (as long as you eat sensibly as well). TLDR: 3-4 hours / 100 km / 60 miles per week
Answer: The distance covered is only one measure of your trip.  The efforts and rewards of an exercise session will be related to both the length and the intensity.
Your 30 minutes could be 5 km at a casual dawdle, or 20 km at a hard pace.
Slower sustained riding builds the "slow twitch" fibres which can fire repeatedly for long times but aren't that strong.  These are "endurance" muscles and help you ride for a long time.
Short, High intensity efforts build the "fast twitch" fibres, which are  the "sprint" muscles.  These get tired real quick and once that happens you have to slow down.
Riders with high endurance can ride all day at a moderate pace.  Riders with fast twitch muscles can burst into fast sprints but may not have as much endurance.   Ideally you want both.
Relevance?   You need to mix it up - for example: From a red traffic light, go hard in a bigger gear than normal, right up to your fastest speed for as long as you can, then relax.
See if you can vary your route to get in a hill or slope to climb.   Move your position on the bike a little, to recruit and train other muscles - you'll feel this.
Answer:
As a bicycler I used to travel on cycle daily for about half an hour to maintain fitness of my legs and body. But I want to know how much time and distance is needed for a good exercise.

Depends on what your goal is.
If your goal is to get strong muscles, I suspect half an hour of hard riding, riding on a hilly terrain, sprinting from stoplights, climbing hills standing is enough. A 3-hour ride won't develop your muscles more than a half-hour ride would do, because the last hours you are anyway so tired that your average speed drops and you aren't using the full strength of your muscles anymore.
If your goal is to reduce weight by burning fat, I would suggest to allocate much more than half an hour for your ride. Start to ride to some direction. When you start to feel exhausted and your average speed drops, start to ride back using the same route. This way, half of your cycling time is done when you are exhausted. This is important: fat releases energy slower than carbohydrates do. To burn fat, you need to exhaust your supply of carbohydrates first. The exhaustion occurs because your supply of carbohydrates stops and energy comes from burning fat.
If you carefully calibrate the length of your trip so that you will be back at the moment you start to feel exhausted, you won't lose much weight. The important part is that you have to ride when feeling exhausted, to lose weight.


Are there any poker websites that accept bitcoins?

Question: I would think that bitcoins would be a great fit for websites that allow for gambling, and with Texas Hold 'em being such a big card game it seems that a Hold 'em site might allow players to use bitcoins to fund their accounts. Does anyone know of a reputable poker site that accepts bitcoins?
Comment: OK, I just found a new site called Bet with Bitcoin (http://betwithbtc.com) and there are poker forums there with some info as well (http://betwithbtc.com/simplemachinesforum/index.php?PHPSESSID=3ptrgpqp3d5t970tehokb48g97&board=30.0)
Comment: In case anyone cares, I've settle on sealswithclubs.org as a BTC hold'em site.  Very friendly.
Answer: The best resource to look for a poker site is the gambling section on bitcointalk.org.  There are a lot of smaller sites advertised in that section of the forums trying to cater to the poker players who want to use their bitcoins.
I think I've also seen people will to trade bitcoins for money on bigger poker sites, but I do not believe any major sites accept bitcoins directly at this time.
Answer: SwitchPoker.com, Sealswithclubs.org, btcontilt.com, and betco.in have Bitcoin Poker.
Comment: The site that I like most right now out of those is Seals With Clubs.
Answer: I have used BTC On Tilt a few times and I like it. They have a nice interface and constant free rolls during the day.
Comment: I just went over and created an account at BTC On Tilt and I like it so far.  Thanks.
Answer: Check out the GAMES section on http://stuffexists.com/
It shows:
BtcOnTilt.com ,
BitJack21.com ,
StrikeSapphire.com ,
BitWorldPoker.com ,
Betco.in ,
SealsWithClubs.org
Answer: as of this writing there is only one site to play bitcoin poker on, and that's sealswithclubs.eu - they have games running almost all the time / very fast cashouts / no contest
Answer: Switch Poker is a real money poker site, and it accepts bitcoins:
Switch Poker
Answer: I'd second http://betwithbtc.com/ as a good resource for gambling related bitcoin sites.
Along with the bitcoin centric poker sites mentioned in other answers switchpoker.com takes bitcoin as a payment method along with other more traditional payment methods.


Whatever happened to adding a 'shopping request' close reason?

Question: Came here to ask why there's no close reason to deal with tool and service requests.
Found it'd already been asked.
Do we need to add an additional close reason for Shopping Request questions?
Answer: Whilst I still believe it would be useful on this site, it's not really something that has enough necessitity for it to be enabled here.
I've posted about it on the main Stack Exchange Meta site and general opinion was that "Primarily opinion-based' is a close reason that generally encapsulates that issue.
In order to set up a specific one for Shopping Requests we would have to sacrifice one of the three custom ones we already have:

Questions about Implementation are off-topic because this site is for User Experience design questions, not questions around how to implement these designs. Therefore, questions around the use of programs like Photoshop or languages such as CSS or JavaScript are off topic.
Questions about Site Reviews are off-topic because questions here are expected to be relevant for a variety of people in the same situation. Reviewing a site, flow or interface only helps one person at a specific time. Instead try to ask a focused question about a particular aspect of the design that solicits solutions, not opinions.
Questions requesting Icon Suggestions are off topic. While the subject of icons is on topic, there's very little value in soliciting suggestions for a specific icon in a specific context. See this meta post for more information about this topic.

And these closures get used pretty often and can't really be sacrificed when the 'Primarily Opinion-Based' closure will probably suffice.


Do artificial sweeteners cause insulin spikes?

Question: Which artificial sweeteners can cause insulin spikes?
Comment: What do you mean by "the body acts"? They don't cause insulin spikes (unless you are eating sweeteners "filled" with glucose), but other hormones react as with real sugar. Also, do you consider the gut flora to be part of the acting body or not?
Comment: I understand that an insulin spike is a reaction to carbs, which have calories. Artificial sweeteners, since they have no calories, should thus cause no insulin spike, in theory.
Comment: Insulin secretion is usually a reaction to carbs or fat (with the carbs causing a larger spike because of high GI) but this does not mean that it doesn't increase in other cases. Anyway, artificial sweeteners interfere with leptin signalling, I don't remember if they also do something about insulin or not. If you want to keep the question small and answerable, it's better to ask about insulin effects only. But like everything else we consume, they have tons of other effects, and the net effect on weight (which I assume you are interested in) tends to be weight gain.
Comment: Jiniyt, As @rumtscho stated, it's better to limit yourself on the scope of your questions if you hope to have an answer. Please ask only one question at a time. Other content should be explanatory. Thanks.
Answer: There is a good article on that in Nature, where in a study it was found that non-caloric artificial sweeteners (NAS) in fact do lead to a rise of serum insuline levels, possibly via a induced change of the gut flora, proven by the fact that the effect could be reproduced after fecal transplantation in mice. (Full article here)
And this is about the status of scientific knowledge about it. There is a good review on the NAS topic in childrens' diets.
Also, Aspartame exposure during pregnancy has been found to reduce insulin sensitivity in male mice, thereby showing a gender difference and other side effects.
So, to answer the question: By today's knowledge it's yes, they may cause insulin spikes, but rather indirectly. But nonetheless, they produce measurable adverse effects on your glucose homeostasis, the pathways of which are still not quite clear but may - amongst others - have to do with your gut flora, your insulin receptor sensitivity or behaviourial mechanisms of the brain reacting to sweet taste.
Comment: From an anonymous suggested edit: "The [article on that in Nature](http://www.nature.com/nature/journal/v514/n7521/full/nature13793.html),  is shown to exhibit signs of p-hacking and should not be taken as absolute proof."


Does character creation dictate my role?

Question: During character creation, you select race and gender, birthsign and class (or create your own) in addition to cosmetic appearance. No matter what I select, will I be able to play different roles ("fighter", "mage" and "thief" are the character types commonly associated with the game) efficiently, as long as I'm willing to develop role-specific skills?
Comment: Sorry, I haven't yet been able to choose the best answer.
Answer: PapaStan was right that Race and Birthsign can provide you with some unique bonus that you cannot get otherwise, like the bonus of the Atronach (allows building the strongest possible mage and mage killer at the same time), or that is hard to get otherwise, even though possible: for example, Thief gives you an initial boost of +10 Luck: this attribute is extremely hard to level up, because there are no skills tied to it, so the best you can get is +1 each level. And Luck is very important, determining loot you get in the dungeons and providing a bonus to all other attributes.
Class also really does provide you a boost into some attributes and skills, but the important thing is that the skills that get the inital bonus are also leveled a lot faster, especially in the cases like Restoration.
So you will be able to take any role while playing your character, but some roles can be harder or easier to fullfil depending on your build.
There are some builds based on skills from different groups that are possibly weaker at the beginning, but allow you to develop all of the attributes relatively quickly, thus making your character a stronger "generalist". I strongly suggest you to read an article on efficient leveling to understand what am I talking about.
Comment: If might be worthwhile to note that your major skills also determine when you level up- which can be good or bad (since monsters are leveled with you, but NPC allies usually aren't, so going through [spoiler town] at level 1 is a bit easier than doing it at level 50).
Comment: @Delioth Feel free to directly edit your phrase into the post, it would be very helpful!
Answer: I've been playing Oblivion for three years now with different characters so I can share my experiences.
Race: Gives you a boost on various skills, also a great power (Adrenaline Rush for Redguards) or an ability (Resist Magick for Bretons)
Class: More boosts on skills
Birthsign: It can boost a couple of skills or grant you with a great power
Take a look at this page for more information, also previous links will help you to understand the leveling system.
I do recommend playing with a custom class because default classes are not well balanced, for example the warrior has three combat skills (Blade, Blunt and Hand to Hand) when you will be using mainly only one. But you should play at least a couple of times in order to understand how skills work, with different characters. Then, you should start your roleplaying adventure ;)
But once you get used to the game you will realize you can play any class with any character. It will be hard early in the game, for example, to play a warrior with a Breton (due to low Endurance and Strengh) or a mage with a Nord (average Willpower and Intelligence), but in the long run you can maximize your skills to 100 (it can be a little challenging but is possible).
Comment: "you can maximize your skills to 100" - this is the most important point. I'm not yet familiar enough with the game to know if there are major stat-enhancing abilities outside of "skills".
Answer: Race and Birthsign can give you unique abilities or attribute boosts.
Class determines the starting level of each attribute.
So yes, you will be able to play every roles with your character.
Answer: It doesn't really matter, the game just isn't that difficult.
However it is far, far easier to take a beefy race and make them a competent mage than it is to take a magey race and make them a competent fighter. There are several reasons, the main being the health-leveling mechanic.
When you level you gain (IIRC) 1/10 your Endurance rounded down. Starting with higher Endurance and maxing it first confers not just an initial boost to your starting health but a cumulative advantage in max health over the course of the game.
Magic by itself as your main DPS isn't really viable unless you've got very high Willpower, lots of potions, or both. You'll run out of magicka long before the enemies are dead. Meaning your mage will be doing some melee, especially in the earlier stages of the game.
Nords start with a Journeyman level Destruction spell, Shield, and a bonus to Endurance. Orcs have Berserk, and Female Orcs in particular have acceptable magey stats in addition to their combat prowess. Redguards don't really get any magic-related cool stuff, but magic (and therefor related stats) is easily leveled with cantrips, even a few of the non-custom vanilla spells are usable for that purpose (lowest level light, detect life, etc.).
Same reasoning applies to stealth, its just easier to take a Nord or Orc and level stealth skills by taping your controller down and sneaking all night (IRL) or what have you.
If you want the most powerful possible build, then the arguably best race is the Breton but you'll have lower max health, the early game will be harder (it's still not that hard), etc.
But 50pts Shield and magicka resistance make them awesome. Add in one of the power signs like Atronach, or if using magic frequently perhaps the Mage.
Comment: @user598527 I think that Reddit post overstates the issue. It's more likely that you'll make choices that will make your character overpowered than underpowered. If you're really worried about it though you can just not level up, since monsters level up with you there's actually no need to level up your character. http://en.uesp.net/wiki/Oblivion:Under_Leveling
Comment: The game can get difficult if you make bad choices during leveling, to the point that killing an enemy takes several minutes: https://www.reddit.com/r/oblivion/comments/2w3qek/to_all_the_new_players_and_people_considering/
Comment: @user598527 That's part of the reason I recommend taking a beefy race: the combat skill and stat bonuses make it harder to shoot yourself in the foot by power-leveling magic or stealth.
Answer: To answer your question in short, yes.
The character creation won't affect your gameplay too much. Only look out for the main altering factors such as race and birthsign.
Otherwise you can pretty much change who you want to be throughout the game.
For example, different clothing and weapons can have different effects on you.
You can also choose which skills you want to maximize to 100, others can be left alone, this will also steer your character towards what you want.


How to encourage a child to learn to swim instead of playing at pool?

Question: My 5 years old boy is not listening to his instructors at all, he is confident inside the pool and likes it, but he thinks it is a playground. During last 2 years (once a week lesson)he made some progress in the beginning, but now because his foot reaches to the pool ground he prefers just walking or pushing the ground and not kicking. I tried with different instructors, tried to push him hard tried to cheer up him, but it doesn't work. Should I stop his learning and start it again couple years later?
Comment: Is it your son who asked to take swimming lessons or did you push it on him? In any case, there seems to be little point in continuing...
Comment: Did you try to let him play with quite big, floating, inflattable objects, like a [swim_ring](https://en.wikipedia.org/wiki/Swim_ring)? Once ge grabs a toy, he may discover a pleasure of floating by himself pretty soon. Then you'll show him how to move. Finally he will do good enough to abandon the ring... In short: instead of pushing, make a possibility and let the child to explore it.
Comment: I was a competitive swimmer.  A love of the pool must be encouraged so playtime is good!  I started a 5 years old in a "short" lane (~10yds) for <1hr a few times a week.  I think it's important to have him be part of a group (to follow, foster a little competitiveness).  Swimming by himself will always just be play time.  You can't expect a 5 year old to just swim laps if he's by himself!
Comment: I had a swim teacher who would give me candy if I did a good job. I learned to swim pretty well!
Comment: @CiaPan Inflatable toys as swim helps are disputed among swimming instructors. Some recommend them for making children more comfortable with water. Others think they lead to developing bad swimming techniques or even consider them dangerous (the argument being that parents and children get careless, the children can lose them while in deep water and drown). The truth might be somewhere in the middle.
Comment: @Philipp Well, parents just need to use their brains and not let children to a deep water until kids swim well, despite they having any arm bands, vest or ring with them or not. All those devices make playing in water more attractive and more fun by making it a bit safer, but they are **not** for switching judgment off.
Answer: I made the experience that it is easiest to get children to focus by giving them a goal they want to achieve.
I taught my niece (6years at that time) in a small local pool where the depth of the pool was adjustable. They had times where it was very shallow so they could stand (this was the time I was practicing with her) and times later when it was very deep. Only children who could swim where allowed inside at the "deep times".
So I would show her the clock each day and said: If you can swim by 4PM we will stay here 2more hours in the deep water and play.
In the beginning it was difficult because she would see other children play but she began focusing as soon as she looked back at the clock.
She quickly tried her best to learn how to swim and after a week she got her badge (a thing in Germany that says you can swim safely).
Altogehter:

Set a fixed goal, make sure he can (and wants to) achieve it!
Show him the upsides of staying focused!
Make sure to show that you care about his progress! (Maybe by rewarding him with a small thing to symbolize his achievement.)
Answer: In swim classes kids start to actually learn about age 4. Before that is mostly group fun and play and just getting water sense and comfort.
Your son is 5, just one year off of average learning age for swimming. I would not worry much. If he enjoys his time in the pool, that's a good sign he will pick up the swimming. Be sure you are not forcing him to it. Also, be sure you're not distracting him in trying to tell him what to do during his class. It's okay if he still exhibits playfulness. Being in a peer group is important at this age all the way up to about 12. Basically, adjust your expectation a little because he seems fine (from experience as both parent of two accomplished swimmers & a swim instructor).
If you're still anxious, you may want to talk to the swim instructors & encourage them to add more group games that force the kids to swim for them. There are toys and tools such as dive toys that can be used and creative rewards that will encourage the kids to go get them by swimming there. Even without the toys, there are plenty of games the classes can do where part of the rule requires swimming at least a few strokes.
In the end, your goals should align with what is normal to expect. We do know that most classes before age 4 are for social fun and getting comfortable in water because kids normally don't take instructions well enough for swim techniques. This doesn't negate the fact that the classes with their peers help them enjoy their time in the water and feed on to their further exploration of swimming at later ages.
If it's not a burden to you, and more importantly, if he enjoys his time, do continue his class.
Swimming is both science and art, and then discipline and individuality and power of will at a later stage. For your son, it's still fun, and that's a healthy way to enter the water!
Answer: If it was my child and I felt that he needed to learn to swim soon, I'd forgo lessons and take him swimming myself. I would spend time playing games. Here are some suggestions:

Talking and singing or blowing bubbles underwater teaches breath control. Keep it fun.
Grabbing a toy from your hand, make it easy to start and increase difficulty until your son is putting his face underwater to reach the toy.
Give him 'rides' on your back. In the beginning it gets him used to moving in the water. After a while, suggest that he kicks his feet to 'help' you. Keep it fun.
Use a flotation device like a flutter board. He holds one end and you hold the other. He kicks and you help him move along the edge of the pool. Do not let go until he wants you to. Keep it fun! (I think you get this part! ;))
Kick while sitting on the edge of the pool. "Make the biggest splash!"
Catch him when he jumps in. Try not to put his face in the water if that is a problem, in time, his time -- it won't matter.
If he can stand with his head out of the water, let him walk the width of the pool. Knowing he can reach is a big boon to the process, but is not necessary.
Jump! Show him he can jump in the water, even when the water is over his head. Learning to swim in a pool makes this easy. This teaches him he can reach the bottom and the air on his own.
Play games in the water. Basketball, catch, tag, splash me/splash you -- this last game only works when everyone is happy being splashed.
Model swimming. You go jump or dive into the deep end. You swim and show him you are happy and confident in the water. If he asks to join you in the deep water, it is a sign of trust.
After he is comfortable and showing an interest in swimming, go to swim lessons to watch and encourage your son when he's participating. BUT leave the teaching and discipline to the instructor.

You pick and choose the items that will work for you both.
If you are positive and happy, whether it is swimming or riding a bike or skiing, your child will catch on and probably enjoy the activity.
Answer: Two years ago (now he 4 years old) I did teach my boy to swim once every week. He did think that pool is playground until he wears a LifeVest and follows me to a deeper pool. He is freak out, somehow, he enjoys it. I know this is a dangerous method until I let him know for not going to a deeper pool if there is no guidance at all (without me or my wife). With my guidance, I show him what happen if he doesn't have a LifeVest on a deeper pool.
I continue to teach him that way and after two months, I buy him Inflatable Arm Bands. He more flexible and aware of every action he does but still with my guidance. On year one, he already swims by side, circle me, and swim around 10 meters (32.8084 feet) to reach me in the middle of the pool.
I teach my boy how to avoid fear by giving him a little fear on when swimming and he can be more focus on how his hands and legs follow my instructions. And then he can let the fear goes by time and keep aware after all (But don't let him swimming alone, after all, even he can swimming far, they still a child).
My advice is to get involve with the instructors for a couple of days and spend your time to teaching him by your hands.
Remember

Always swim with a buddy (older person); do not allow anyone to swim
  alone.
  Never leave a young child unattended near water.
  Do not trust a child's life to another child; teach children to always
  ask permission to go near water.
  Have young children or inexperienced swimmers wear inflatable, life
  jackets, life-vest, but do not rely on life jackets alone.
  Maintain constant supervision.
  Make sure you learns to swim well.
Answer: I had the same problem when I was a child. I was taken to a pool with very shallow water, I could swim already, but I could stand there. I only started to learn to swim when I was taken to a pool with water deep enough. I suggest to do the same.
Maybe you should start with a pool with parts of different depth, just in case your child needs some rest.
If your child is with a trainer, it's unlikely that something bad happens even if things go very wrong -- trainers are taught for those purposes. Maybe you may stay with your son for some time to watch him.
Also remember that group games in a pool will actually force him to swim to win if the pool is deep enough. That was probably the only thing that could keep me motivated to learn when I was small.


Shouldn't the diameter of the moon with a 600mm lens be double what it is with a 300mm lens?

Question: I used a 300 mm lens and recently bought a 600 mm lens, thinking that the area in the frame with the 600 mm lens will be exactly half of the area covered by the 300 mm lens. Practically I am not getting this; in fact very little difference. Should the diameter of the moon captured by a 600 mm lens be double the diameter of the moon captured by a 300 mm lens?
Comment: Can you post examples? And, out of curiosity, which lenses, exactly?
Comment: What's the actual change? Are you using the same camera body with both lenses?
Comment: Objects seen at 600mm should be twice the size (in half of the overall view) of same seen at 300 mm.  How are you viewing these images?  To compare size, you MUST see the full frame of both.  My suspicion is that if you are just zooming them so the moon is about same size in both, then of course it will appear about same size.
Answer: Assuming both lenses are being used on the same size sensor, the area in the frame with a 600mm lens should be one quarter the area in the frame with a 300mm lens. The linear dimensions should change by a factor of two, the areal dimensions should change by a factor of the square of two, which is four.
If you are not seeing the same object shot from the same distance appear to be twice as wide using the 600mm lens as with the 300mm lens then something else is going on here.

One or both of the two lenses may not be labeled properly. The chances are probably greater that the 600mm lens is not really 600mm than that the 300mm lens is appreciably longer than 300mm.
You are using a Nikon full frame (FX) camera and the 300mm lens is a Dx lens. When a Dx lens is attached to an FX camera the camera will automatically crop the image to only use the center of the FX sensor - the part that is completely covered by the smaller light circle of the Dx lens. This will give a field of view with the 300mm lens that is closer to the FoV of a 450mm lens with an image circle large enough to cover the entire sensor. Look at the resolutions of the photos using the two lenses. Are they the same? Or is the image with the 300mm lens much smaller in terms of pixels than the image with the 600mm lens?
Comment: Good observation on the DX->FX automatic cropping!
Answer: The camera lens acts just like a movie or slide projector lens in that it projects an image on film or sensor. We can use the geometry associated with tringles to calculate the projected image size of the moon. The moon's mean distance is 384,400 kilometers and its diameter is 3,474 kilometers. The ratio of distance to diameter is 384400 ÷ 3474  = 0.0090.  In other words, our view of the moon traces out a triangle with a ratio of 0.0090 diameter to distance.
The camera lens projects an image of the moon and the image and the image forming rays trace out a triangle that tracks with the same ratio of distance to diameter.  If a 300mm lens is mounted, this will be the distance lens to image. The diameter of the image will have the same ratio as the actual view. Thus we multiply 300 x 0.0090 = 2.71. We have calculated the diameter of the moon's image, it is 2.71mm.
If we mount a 600mm lens we can recalculate 600 x 0.0090 = 5.40. The moon's image using the 600mm is 5.40mm in diameter which is twice the size of the image formed by a 300mm.
By the way, the focal length of a lens is measured when the lens is imaging a far distant object (an object said to be a an infinite distance symbol ∞). This measure is taken from a point called the rear nodal.  If the lens has a super simple design, the rear nodal is likely near the center of the lens array. A true telephoto differs from a simple long lens. It is constructed using a array of positive and negative lens elements and likely the barrel is artificially shortened by a clever element arrangement. Commonly, to make the lens barrel less unwieldy, the rear nodal is shifted far forward of center. In many cases it falls in air ahead of the lens. Nevertheless it preforms, as to magnification, exactly as if it were a simple lens. In other words a 600mm lens is a 600mm lens and a 300mm is a 300mm. The 600mm delivers twice the magnification.


Google sheets spreadsheet will not format date correctly

Question: I have a Google spreadsheet with a date column formatted to dd mmm yyyy. Many of the dates are formatted this way, but some cells will not format this way, and no matter what I select under "Format", "Number", I cannot get these cells to change at all. It seems to remain the text that I entered, but I want it to switch to my preferred format selection.
Comment: share a copy of your sheet please
Answer: If the cell values don't change the way that they are displayed then those values are text, not dates.
This occurs because Google Sheets internally manage dates and duration as numbers, to be more precise as serialized numbers.
This commonly occurs on ambiguos "dates", those that use 2 digits for months instead of three letters and that have day equal or greater than 13.
There are several workarounds, one is to split the "date" parts and use date(year, month, day) function,


ERA Interim problem with Waves

Question: Has anyone had a problem with significant wave height (swh) and mean wave period (mwp) from ERA-Interim not matching very well with either a time series of measurements or a different model? Or does anyone know if the formulas in the model (the way they made it) have changed from one period to another? (In my case they seem to correlate better/worse at either side of 1991).
Answer: I haven't looked into ERA-Interim wave data a huge amount but I know around 1991 it started to assimilate ocean wave data from satellite altimeters (ERS-1).
See the first figure here
https://climatedataguide.ucar.edu/climate-data/era-interim
I remember Jean-Raymond Bidlot who runs WAM at ECWMF showing me so experiments with and without the assimilation into the wave. I think the Hs differed by ~1m in large events so not insignificant.
Answer: This doesn't answer your question but I can't comment; It is worth noting that there are several changes in SST packages which drive the models. The most significant is the difference from before December 2001 and after January 2002 - it could be that there is a difference from before 1991 and after for similar reasons.
It might also be worth checking if SWH or MWP are data packages used to drive the model or if they are produced at the output.
You can find model formulas here: https://www.ecmwf.int/sites/default/files/elibrary/2011/8174-era-interim-archive-version-20.pdf
and the reanalysis process is described here;
http://onlinelibrary.wiley.com/doi/10.1002/qj.828/pdf
Hope this helps! :)

ERA Interim problem with Waves


How can you check if you are in a particular page in the WP Admin section? For example how can I check if I am in the Users > Your Profile page?

Question: I'm building a plugin and I want to add bits of javascript in the admin head but only for certain admin pages. I don't mean pages as in a WordPress page that you create yourself but rather existing admin section pages like 'Your Profile', 'Users', etc. Is there a wp function specifically for this task? I've been looking and I can only find the boolean function is_admin and action hooks but not a boolean function that just checks.
Comment: Gentle nudge to remind you an answer has not been accepted. If none the answers provided answer your question sufficiently or you're struggling to understand the information provided please comment to let us know.
Answer: There is a global variable in wp-admin called $pagenow which holds name of the current page, ie edit.php, post.php, etc.
You can also check the $_GET request to narrow your location down further, for example:
global $pagenow;
if (( $pagenow == 'post.php' ) && ($_GET['post_type'] == 'page')) {

    // editing a page

    }

if ($pagenow == 'users.php') {

    // user listing page

    }

if ($pagenow == 'profile.php') {

    // editing user profile page

    }

Comment: `global $pagenow; if ( ( 'admin.php' === $pagenow ) && ( 'prefix-theme-settings' === $_GET['page'] ) ) { logic.. }` for example checks if you are on a custom theme settings page `admin.php?page=prefix-theme-settings`.
Comment: thank you . This is a great answer and helped it.
Comment: This is perfect. Thanks a lot! I tried the `$wp` global variable, but the request property would remain empty there.
Answer: The most comprehensive method is get_current_screen added in WordPress 3.1
$screen = get_current_screen();

returns
WP_Screen Object (
    [action] =>
    [base] => post
    [id] => post
    [is_network] =>
    [is_user] =>
    [parent_base] => edit
    [parent_file] => edit.php
    [post_type] => post
    [taxonomy] =>
)

Comment: "Usage Restrictions --

This function is defined on most admin pages, but not all. Thus there are cases where is_admin() will return true, but attempting to call get_current_screen() will result in a fatal error because it is not defined. One known example is wp-admin/customize.php.

The function returns null if called from the admin_init hook. It should be OK to use in a later hook such as current_screen. " -- [Codex](https://codex.wordpress.org/Function_Reference/get_current_screen).
Comment: @shahar this is still the case, according to the documentation linked above and my experience.
Comment: @ThatBrazilianGuy A quick check suggests this is no longer the case, as far as I can see `get_current_screen()` is safe to use on all admin pages.
Comment: it is still the case. You can reproduce it when creating a plugin and then activating it. This will throw a fatal error on activation since you are on a page where the function is not defined :(
Comment: you can always check&bail `if (!function_exists('get_current_screen')) return ;` if your logic depends on it :)
Answer: The way to do this is to use the 'admin_enqueue_scripts' hook to en-queue the files you need.  This hook will get passed a $hook_suffix that relates to the current page that is loaded:
function my_admin_enqueue($hook_suffix) {
    if($hook_suffix == 'appearance_page_theme-options') {
        wp_enqueue_script('my-theme-settings', get_template_directory_uri() . '/js/theme-settings.js', array('jquery'));
        wp_enqueue_style('my-theme-settings', get_template_directory_uri() . '/styles/theme-settings.css');
        ?>

        <?php
    }
}
add_action('admin_enqueue_scripts', 'my_admin_enqueue');

Answer: To offer an alternative method/approach to the above question.
// When you are viewing the users list or your editing another user's profile
add_action( 'admin_print_scripts-users.php', 'your_enqueue_callback' );

// When you are editing your own profile
add_action( 'admin_print_scripts-profile.php', 'your_enqueue_callback' );

function your_enqueue_callback() {
    wp_enqueue_script( .. YOUR ENQUEUE ARGS .. );
}

This method targets the specific pages more directly and avoids needing conditional logic inside your callback(because you've already made that distinction in the selected hook).
Comment: Wish i knew why this answer didn't get more votes, covers the exact use case in the question, and provides the solution in a minimal amount of code. Feedback would be nice..
Answer: I find it weird that no one has mentioned the fact that the add_menu_page function returns an action hook which you can use to do certain actions only on those pages
$hook = add_menu_page($menu_title, $page_title, $capability, $slug, $function, $icon_url, $position);
add_action( 'load-' . $hook, 'my_admin_enqueue_scripts' );
function my_admin_enqueue_scripts() {
    wp_enqueue_script(/*...*/);
    wp_enqueue_style(/*...*/);
}

If you need the $hook and you didn't add the menu page yourself the doc is here
For instance the hook for a top level menu page is

load-toplevel_page_$MenuSlug

The hook for a submenu page is

load-$MenuSlug_page_$SubMenuSlug

Following that logic, the hook for the user's profile page is

load-users_page_profile


None of the outlets in the kitchen are working

Question: The other day I switched on the garbage disposal and now none of the outlets in the kitchen work. This circuit is GFCI protected from the panel but the GFCI breaker in the panel is not tripping. When I test the outlets with the breaker on they show hot on the hot and neutral sides. I have replaced the outlet feeding the garbage disposal but this did not work. I have also replaced the switch controlling the disposal but still nothing works. I tested the wires at the breaker and they appear normal. This condition exists from the first line on the circuit to the last. Does anybody know what could be going on here?
Comment: What are you using to test voltage with? I don't understand how you tested hot on the neutral (white) wire: did you test for voltage between the neutral (white) and the ground wire? Since the problem started when you switched on the disposal, I would think there's something wrong there. Maybe the disposal is shorting out. Try disconnecting the disposal wires and connecting everything else.
Answer: If you are showing hot on neutral this is an indication of an open neutral on the circuit with a load attached. You are reading the voltage on the neutral through the load.
Find the open neutral and you'll find your solution. It could be anywhere in the circuit, from the panel through any box or outlet.
Answer: Sounds like the GFCI breaker in the panel is bad. You should 120V from hot to the neutral terminals on the breaker. If you have 120V from the neutral/ground bus to the neutral terminal on the GFCI breaker with the disposal on then there is an open in the neutral side of the breaker or the neutral wire from the breaker to the neutral bus is broken or loose.


Can I buy stocks with Bitcoin?

Question: I have got some USD balance with a Bitcoin exchange. I intend to buy stocks for these USDs. Is there an online broker that accepts Bitcoin as a deposit option for stock market trading? I mean the usual stock market where items like Google, Microsoft etc. are traded. Or do I have to go the usual way of withdrawing my USDs from the exchange to my bank account and then invest from there?
Answer: I am not aware of any. Whilst I like to think of such not-yet-existing services as simply waiting to be provided soon, there is a significant barrier to creating such a service. A broker wanting to offer taking bitcoin deposits as collateral for your stock holdings would need a reliable and classical-finance-compatible way of converting this collateral into USD. This would be needed in case stock price or bitcoin price movements come close enough to using up all the available collateral. With stock exchanges, the broker can do such a service whilst not taking any risk not somehow addressed or knowingly tolerated in the world of finance. Existing bitcoin exchanges do not fit into this because they operate slightly differently from regular stock or foreign currency exchanges. Hence there is additional risk that, whilst apparently acceptable to customers of existing bitcoin exchanges, would probably be seen as utterly unacceptable on many levels by a broker (regulatory, reputational, own risk assessment, ...). Basically, I think such a broker would have to find a bitcoin exchange operating the same way and regulated in the same way as a stock exchange, to be able to make it square with the rest of his business. There may possibly be exceptions among brokers only catering to accredited investors---those are people rich enough that the law automatically trusts them to enter financial business otherwise prohibited for customer protection reasons. But if you qualified as such, I doubt you'd go to a website for advice about finding a broker...
Answer: I've looked into 1broker and while there is quite a bit of good feedback about the site and the GUI is very clean the 4 guys who run it are students. I ran a hedge fund for many years and know a fair bit about what they do which is CFD trading. The short answer is, and it sucks in a way because eventually they will tank. A black swan event will come along and take them out.
They don't understand hedging (which the main guy says he uses "when he feels it is necessary") That is not hedging. Hedging is a considered and consistent strategy you put in place before you start running your trading business. Not something you do when the mood strikes you. However, it is my belief that eventually a professional group will build a professional platform that supports digital currency as security. The world has to catch up with the idea and i am certain it is coming.
I've got a few BTC I wouldn't mind putting to work. Anyone else?
Answer: pyramids provided a really good answer. I would like to add though that as time passes and Bitcoin prevails as an alternative viable virtual currency due to ever growing acceptance/usage then Investment/Financial Firms and Banks may well consider accepting Bitcoin. They would have to remain in business. But you can bet there will likely be BIG FEES for accepting it and stringent margin requirements with minimum 50% in all likelihood and doubtful will allow lending on margin over and above this.
Answer: No services for mainstream companies, but there are a couple of sites I've seen for stocks and IPO's however these are mostly in Crypto related companies.
https://cryptostocks.com/
https://www.havelockinvestments.com/
Answer: Yes, you can fund a brokerage account with Bitcoin, at least at DriveWealth.
Disclaimer: I have not tried it.
How do I deposit funds?

Last Updated: Aug 11, 2015 05:44PM EDT
  DriveWealth has multiple safe and secure ways to deposit funds into your Live Investing Account:

<code>Charge Cards - We accept Visa, Mastercard, Discover and American Express
Wire Transfer
Check
Dwolla - U.S. Only
TransferWise - For European Currencies
*BitPay*
China Union Pay
Safety Pay - Select Latin American and European countries
</code>
How do I deposit funds to my account with Bitpay?

...
  Please Note: Bitpay deposits can take up to 3 business days. When the funds are deposited in your DriveWealth account, you will be notified via e-mail.
There is a minimum $20 deposit. DriveWealth is not responsible for the Bitcoin exchange rates you receive.
Answer: 1broker is a brokerage that facilitates Bitcoin purchases of stocks of Google, Microsoft, Apple and Facebook. They also offer Bitcoin investment in commodities and forex.
The service is free to use, as in they don't have an up front charge, but it costs 0.001 BTC per withdrawal. Quoting from their website, "We profit from the spread, the difference between the bid and ask price. This means you will start with a very small loss, when a new position is opened."
Answer: You may want to give First Global Credit a try (www.firstglobalcredit.com). They provide stock trading facilities and provide a DMA trading platform (all trades are executed directly on the relevant exchange) with all positions secured using Bitcoin as collateral.
I've tried their simulated trade platform and it seems to do what it says on the tin.

Can I buy stocks with Bitcoin?


Is it OK to ask question about non-GIS products like Power ON in GIS community?

Question: I proposed to start a new community for Magik programmers but Robert Cartaino closed it with a announcement that GE Smallworld questions are welcome in GIS community but GENS is a much wider area and gis does not covers it completely.
Will other GIS members downvote questions about OMS, SW Schematics, ......? I think they will since it would be considered irrelevancy
Comment: "some of them have not a map at all, so they can not be considered a GIS(suppose everything that has map is GIS and this community is for its Q/A" I would say this is flat-out wrong. GIS != maps.
Comment: I wouldn't down vote a [well written](http://meta.gis.stackexchange.com/questions/3349/how-to-frame-a-good-question) question about any of those topics. Is anyone familiar with Magik not going to be familiar with Smallworld as well? Is an OSM not a specialized GIS, that automates GIS analyses (network tracing, routing)?
Comment: Its nice of you, but most of the GENS can not be considered a GIS only and they are far more of that, also some of them have not a map at all, so they can not be considered a GIS(suppose everything that has map is GIS and this community is for its Q/A) Just take look at the questions taged Smallworld ( only 6 ) I dont say Smallworld user find this community relevant to their activities)
Comment: I am agree with you, we cant say everything with a map is a GIS, Robert closed the proposal as it could be addressed in GIS community, And I think he assumed such a hypothesis.
Comment: I note that the proposal has now been renamed to [GE Network Solutions](http://area51.stackexchange.com/proposals/90692/magik).
Comment: Yes, the new name supports and covers a much wider range,however the old one was the name of a very specific programming language and since GE is about to switch from Magik to Java ( This October ), I changed the name and I wish the new term covers better.
Comment: This question has just been asked to prove that GIS community does not and can not cover GENS, but thanks for your answer.
Answer: It is my perception that in the last few days we have started to see quite a few questions that have been further from "mainstream" GIS than usual.
GIS is a very broad topic as you can see from the discussion we are having at How should we describe GIS Stack Exchange on its Tour page?
I am only superficially familiar with Smallworld GIS which can make it hard to say whether I think the type of questions you seek to ask are on-topic or not, but I think that at least some of them may be.
As you and others start to ask questions further from the "mainstream" of GIS, I strongly recommend that you take the time to ensure that terms/abbreviations that you use are already in use here (just search for them), and if not, always define or link to definitions of them.
You are likely to find poorly framed questions on unfamiliar topics get downvoted more often than those with which we are more familiar, but I think the onus is on you to convince the community that your questions have relevance to the broad topic of GIS by asking very well framed questions.
Comment: I used to work for a GIS company that provided software, services and support for the OMS of ArcFM, and that support was provided by GIS Professionals who had no/little Electrical Distribution experience prior to taking on that product suite.  GIS Professionals work with far more than maps in computers.  My advice would be to proceed with your Magik site proposal in Area 51 but work with the Stack Exchange Network to try and find a welcoming home for your questions until it gets up, or in case it does not.
Comment: I was about to prove that GENS community is not a duplicate one and its question can not be addressed here, I have a master degree in GIS and I believe I know the different definitions and aspects of GIS, as you said you are not familiar with GENS, To picture what I have asked above, Consider an outage Managment System in a Electricity Distribution Company, Do you think the questions about this product could be framed into gis community? I am hearing your NO from here
Comment: thanks for the advise, Acctually thats the plan, I am a simple member of Smallworld community and I like to hand over the community to the products owner and those who are moderator in our current yahoo group, I dont have that time and I woudn't work for free! about your OMS, I shall say some of the softwares are standalone but support location based services and some are completely based on GIS I assume yours are like that, both in most of the time are supported by GIS professionals of course, however the fact that they both are supported by GIS experts doesn't mean that they do the same job.
Comment: Also consider that GE is an energy company not a GIS application Provider like ESRi, this shows itself in their products too, believe me Smallworld products are much more end-user oriented so the core spatial technology (CST) is not a menu driven software like ArcGIS


When does Gurobi add cuts from callback

Question: A while ago I used Gurobi and I added user cuts from within a callback. However, I got the feeling that my user cuts were not directly added to the model. Is it right that Gurobi can decide to postpone adding my inequalities? If yes, is there a way to force Gurobi to immediately add them?
Answer: Assuming you are using more than a single thread, then Gurobi performs all work you code in the callbacks with a single thread. All remaining threads process the branch and bound nodes (without doing the callbacks). When your user cuts are added is then determined by some  black box magic of Gurobi.
To be specific, suppose we have 4 threads. Then, 3 of the threads are processing branch&bound nodes. If a particular branch&bound node is processed and there are valid inequalities (i.e. user cuts) to be separated, it is put in a pool of such nodes. This pool is processed by the 4th thread. Now suppose that some valid inequalities are separated by this thread for some particular branch & bound node. As these processes (of processing callbacks on the one hand and branch&bounding on the other hand) are more or less independent (which is what I presume, but please correct me if I am wrong), it is left to Gurobi to when/how to add the separated inequalities.
Luckily, they are good in that. If you want more control you can use CPLEX or if you want all control I suggest you the nice framework of SCIP :)
Comment: I did a quite thorough attempt but I could not find a way to make it happen. But, please  anybody, prove me wrong :)
Comment: Thus it is not possible to tell Gurobi to do it immediately after the callback? That their black-box magic works like a charm is good for business-related problems, however, for my academic purposes, I would prefer a bit more control.
Answer: While lazy constraints must be added each time a node is expanded at a branch-and-bound tree, a solver is not obligated to add user cuts at any point in time; it chooses when to add user cuts.
If you want Gurobi to add them immediately, you are better off adding lazy constraints. See here: https://orinanobworld.blogspot.com/2012/08/user-cuts-versus-lazy-constraints.html for more details on the differences between the two approaches (note that CPLEX and Gurobi actually implement callbacks quite differently, so the blog post won't be entirely accurate for Gurobi).

When does Gurobi add cuts from callback


Automatically hide Linux Mint notification bar after timeout

Question: It seems like the notification bar on the bottom of the desktop in Linux Mint 12 never hides unless I click N+1 times on it first (N being the number of messages). Since it's overlaid over all windows (even fullscreen video) it's a bit annoying. Right- and left-clicking the notification bar does not bring up any settings dialog, so I'm wondering how to either automatically hide messages after a timeout, disable the display of certain messages (I don't need to know about external disks which are plugged in at boot time), or (if all else fails) disable the notification bar altogether.
Comment: I'm also annoyed by this and have found no solution so far :(. Wifi disconnect messages pop up and then must be clicked to go away.
Answer: On Arch Linux I removed exec permission on notification-daemon lib.
I found this temporary quick fix when I was searching for solution to this same problem.

$ sudo chmod -x /usr/lib/notification-daemon-1.0/notification-daemon

Some of annoying notifications you can disable here:

$ gnome-session-properties

For example you can disable "Print Queue applet" and you wont get notification about every printed document. Or "Disk Notifications" and this should resolve your "Your USB disk is plugged in" notification problem.
Comment: The only things shown in `gnome-session-properties` are "Caribou - No description", "GNOME Login Sound", "mintUpdate", "mintUpload" and "mintWelcome".
Answer: I've decided to turn them completely off: in advanced settings, shell extensions, disable notifications extension
Comment: It still shows notifications after disabling notifications, even after logging in again.


How to change the "W" (English) to "O"(French) on the North Arrow direction using ArcMAP 10.0?

Question: How to change the "W" (English) to "O"(French) on the North Arrow direction using ArcMAP 10.0?
I tried to convert the North Arrow sign to graphic but it didn't work.
Answer: ESRI North Arrows are stored in a Font file on Windows in C:\Windows\Fonts, 'esri_40.ttf' or 'ESRI North Regular' as labelled.  You'd have to modify or adapt the file there to change the font file for your purposes, though I am not sure as to the licensing behind the north arrows and whether or not you can use them as modified.
You may consider contacting ESRI support to see if they have something already made up somewhere, or using a different set of North Arrows.
Comment: I guess it is kinda obvious, but you could use another style of North Arrow that only labels the 'N' and not the other cardinal directions!
Answer: The fonts are images that need to be edited outside of ArcGIS, or you can use fonts without letters and add the letters yoursefl as text elements.
French fonts for the North arrows are availale on the Georezo forum after registration. http://georezo.net/forum/viewtopic.php?id=41480 . Maybe their author is on this site too.
Answer: If your map is oriented with north at the top you could create an emf graphic using ArcMap as your drawing/design tool.  You can add the image of the north arrow you want as a raster and using it as a guide digitize (draw) the north arrow using polygon, polyline and point shapefiles. Add annotations for font and move as needed. EXPORT final product to EMF file and add graphic to map.
I use this procedure to create complex north arrow that are set to a certain magnetic declination.  So although this is more cumbersome then simply adding a north arrow (which is a font as ESRI is yet to figure out how to allow users to use vector graphics) is is possible to design your arrows at a specific azimuth by setting the the rotation of your data frame.
Here is an example of my typical "custom" north arrow:
Answer: A quick fix would be just to add a white graphic box in your layout over the "W" and put a "O" (same size and font style) text element over that spot.  Ideally, you could export out this image and make your own picture to insert for other maps, however I'm not sure how that would work on the licensing side of things.
Comment: I already did the white graphic box and placed the letter "O"  over the white box... Thanks. I was just wondering if it was possible to "politically correct" modify the Esri North Arrow.


Why is liquid water considered a requirement for life?

Question: I`ve heard about different extremofiles living on earth. Is water a requirement for life because we have not seen life survive on earth without it, is it a chemical  reason which makes life without water seem impossible? Could life exist with water, but in gas form? What makes the liquid part so important? Can life be based on other  liquids on potential exoplanets?
Comment: This question appears to be off-topic because it is about biology and as such it should be asked on [Biology.stackExchange](http://biology.stackexchange.com/). [Chemistry.stackexchange](http://chemistry.stackexchange.com/) also seems useful.
Comment: This is a question frequently discussed in astronomy more specifically astrobiology. I believe this is on topic for this site
Comment: There are probably two separate questions to be answered here. 1) The requirement of liquid water for the genesis of life on a planet, and 2) how necessary a supply of liquid water is to the maintenance of life once it gets going.
Comment: The definition of life is probably hard to pin down, since we only know terrestrian life. A hypothetical life form based on silicon, diamond chips or fullerenes as kind of advanced computers, if it ever could occur, wouldn't necessarily need water as basis.
Comment: @EduardoSerra I disagree with that assessment in this case. It is equally on topic here as a matter of astrobiology.
Comment: @called2voyage I agree with you, but the question may have more atention (or probability of be answered) on Biology SE.
Comment: @MagnoC That's true, but the OP may be more interested in leaving this question here for future answer by qualified people who later come to this site.
Answer: The reason for which water is needed for Life as we know it (and we have not been able to successfuly imagine a different model from that of DNA that has ability to self-replicate, react to the ambient, and evolve), is that water is the "universal solvent".
That means that most chemical reactions needed for life happen on a liquid environment, more specifically a water-based environment for most of them.
Comment: I'm okay with that. What I said is 'for now' is just what we know. All our premise is based on our observation of the surrounding world. As we are talking about the universe, may have things we do not know yet. But you're right, lets keep talking about what we know or soon we will philosophising about spiritual and digital life.
Comment: I ask you to define `life`.
Comment: Schrödinger did it way better than I could: http://en.wikipedia.org/wiki/What_Is_Life%3F (I recommend reading this book. It's brilliant)
Comment: `.living matter, while not eluding the "laws of physics" as established up to date, is likely to involve "other laws of physics" hitherto unknown, which however, once they have been revealed, will form just as integral a part of science as the former.` We may have life without water, just only don't know at this moment how that can be. Not implies in affirm that does not exist.
Comment: @MagnoC Schrödinger defines DNA (the "aperiodic crystal) as a requisite for life, and DNA-driven reactions can happen only in a water-based medium.
Answer: "Life" seems to be an emergent phenomenon, a complex result of a very large number of simple parts or interactions. The whole is more than the sum of it's parts, unpredicted by the properties of the individual parts. If those individual parts are atoms and molecules, they have to move to be able to interact, to build larger, more complex structures. To move, they need to be suspended in something that permits movement and allows chemical reactions. On Earth, that would be liquid water.
Other liquids might work, but they'd need to allow for the movement and reactions life requires. It's hard to imagine something other than a liquid doing the job.
Comment: Do you have some sort of argument for why a gas couldn't work because I don't see it.   It would be slower because molecules would interact more rarely, but that just means it would evolve over longer timescales.
Answer: Water molecules are bipolar, a bit like a bar magnet. For this reason they form non-covalent bonds with other molecules. This makes both the breaking up and making of strong covalent chemical bonds easier. Water is not only a fluid in the meaning that things float in it. Water also increases the chemical turnover and this is important for biological processes where different molecules need to find each other.
The bipolarity of water helps make the right amino acids find their place along an RNA molecule, simply by facilitating more tests per millisecond until the right one comes along and sticks. Methane, for example, is not a bipolar molecule. It does not facilitate chemical recombinations in the way water does. Life would have a hard time to originate and survive in liquid methane. There are of course other bipolar volatile molecules than water.
I don't really know what I'm talking about here, but I heard someone who seems to do so... And animations like this help my impression that bipolarity is important in order to make the right stuff come together sooner rather than later.
Answer: Water contains oxygen and hydrogen which are suitable for growth of life so yes it is necessary for life...at least now... We may find something else on different parts of universe which is necessary for growth of life.