When should I use a CSS framework?

Question: What are the best practice scenarios for using a CSS framework? When is it better to "roll your own" CSS from scratch rather than using frameworks?
Answer: CSS frameworks are a bit tricky because are not really frameworks to me. They are simply predefined classes. But, this is not your question.
CSS frameworks are great for mock-ups and wire-frames. I would not recommend using them on a live site because they add meaningless classes to your markup. ".span3" doen't tell me anything about the content, only the design.
However, blueprint does have some cool tools that help your combine its framework classes and your semantic ones once your done.
I always roll my own CSS.
Comment: I am with Thorn007 on this one. CSS is pretty darn simple when you get down to it. Just use a good CSS reset and roll your own.
Answer: Just avoid them, they are nothing but annoying.
Especially this one.
* { margin:0; padding:0; }

My biggest complaint is that if you inherit/maintain a site that uses a reset and/or a framework you must learn that framework before being able to modify the site.
When I add a h1 tag I expect certain things. Most resets take away all the padding, margin, block, etc. so that a h1 tag no longer acts as it should by default. So I have to go back and add everything that was there in the first place.
Generally (in my experience) its people who do not understand css that use the resets and frameworks, and it causes more work for the people who do.
Comment: Eric Meyer and the web developers at Yahoo understand CSS and they use CSS resets. In my experience they save a LOT of time and headache when trying to achieve cross-browser consistency. - CSS frameworks are lame though.
Comment: **When I add a h1 tag I expect certain things**.. unfortunately you **need** to expect different things from each browser.. and, hence, the need for css reset (*not frameworks*)
Comment: h1 is quite a bad example, because browsers tend to use different margins and so on. Personally I prefer a "set" to reset - i.e. instead of resetting everything to 0, just set it in your own stylesheet in the first place!
Comment: `* {margin:0; padding:0;}` is the most important CSS reset of all. It removes `margin-top/maring-bottom` under all tags even the `p` tags so after you won't see a huge space between each line. Even this website is probably using `p {margin:0;}` otherwise the browser would live a huge space between each line. I'm wondering how you can say they are annoying.
Answer: It is my opinion that the only service a CSS Framework provides is a starting point for those who are not very familiar with CSS.
Answer: I also recommend using a reset.css from Eric Meyer, but I think that CSS "frameworks" can be beneficial for layout. Blueprint, YUI grid, and 960-grid can help you achieve some highly complicated layouts without writing the css yourself.
While there are drawbacks that have been mentioned, the same could be said about building a website off a CMS instead of rolling custom code for example, but they can still be useful and for many sites the benefits gained outweigh any extraneous css or performance issues.
Answer: If you're actually implementing a design of your own, you're going to write some of your own CSS (unless someone else happens to have written the exact design you want).
Assuming you are writing some of your own CSS, "frameworks" can be useful when you've got styles that are:

used in more than one place; and
complicated enough to break off into their own classes

For really simple styles, it's better to use classes to indicate what something is (e.g. class="navigation"), and then define its look by applying style rules to that class in your stylesheet.
But for more complex styles that aren't necessarily tied to any one element (e.g. layout-related styles, the kind that frameworks gives names like .span3 to), there's nothing wrong with putting them in a class, and applying that class in the CSS. You can use both approaches together.
In bigger, more complex sites where elements are likely to be combined in unpredictable ways, a framework-like approach can really help keep your code manageable.
I would say that you might as well write your own "framework" though. I put "framework" in quotes because CSS really isn't that big a language. You could read CSS: The Definitive Guide (Eric Meyer) in a day, then read all the code in e.g. Blueprint (a CSS "framework") and pretty much understand what's going on. You might need to research the IE workarounds a bit, but we're talking at least an order of magnitude less complexity than e.g. jQuery here.
Answer: I use blueprint, for CSS.
CSS is, as everyone above says, really straightforward.
However, in reality, you have to consider the platform.
With CSS  the rendering engine  adds to the complexity.
Gecko vs WebKit vs Presto vs Trident... who wants to do all that stuff?
What a 'framework' gives you here, is the ability to write layouts that will most likely work on all browsers, so you wont have to go googling for the 'WTF-am-I-doing=-this-crap' IE 7 quirksmode fix, or something similarly ludicrous.
Frameworks will do 90% of the layout you want, and then you can always add in your own styles later.
So, to answer the question, I'd say pick any framework and see if it works for you.  It just might.  Then you are golden, and can get back to whatever you actually wanted to do.  If not, then extend it, using css and change what did not work. That way you maintain the cross browser benefit, but still can customize.
As a small benefit, if you pick one and use it all the time, and then are taken ill, or get promoted and have to train your successor, you can say:
"Yes, I used framework ABC.  Docs are here.  Code is there.  Training over. Good luck."
Answer: When it solves all your needs. (This is based on the idea of tailoring.)
From the only book on frameworks? http://www.oreilly.com/web-platform/free/book-of-html-css-frameworks.csp. (Only linking because it's free and relevant here.)
Comment: It might be best if you can cut and pasted the relevant parts of the page you linked to. Otherwise, as an answer, this is kind of light. Link only answers are discouraged here because links often break over time. Can you update this answer to help the OP and future users with something they can sink their teeth into?? Thanks in advance!

When should I use a CSS framework?


Will a quad fan arrangement on an aircraft help with pitching moments during VTOL flight transition?

Question: The phenomena of ducted fan assemblies inducing pitching moments whilst in transition from vertical to horizontal flight paths is a problem I have seen mentioned by many designers and engineers of VTOL aircraft. I am wondering if the quadratic arrangement(a fan placed at each of the outermost corners of the aircraft) would help alleviate these issues. It would seem to make sense as the front and rear fan pitching moments about a central CG would cancel each other out, though still causing stresses on the airframe.
Are these pitching moments still a problem in quad fan configurations, and has any research been done into the inlets of ducted fans to stop these pitching moments?
Answer: The Doak VZ-4 twin ducted propulsor flew in 1958, the Bell X-22 took that up to four in 1966. So yes, people have been thinking about it for over half a century.
Analogously to any tailless aircraft, the side-by-side twin is sensitive to CG in vertical mode and trim must be closely maintained. Although not ducted, the Boeing V-22 Osprey convertiplane suffers from this issue. It is not often an operational limitation, but it does need crew/staff training and careful management.
Similarly the quad arrangement brings the same benefits as the tandem wing in allowing a wide range of CG positions. However the quadrotor does not scale well to full-size aircraft, as discussed at Why haven't quadcopters been scaled up yet?


Gif editing in photoshop

Question: I load a gif into photoshop as frames. It is about 200 frames, and I want to add some extra text and drawings to about 100 frames.(The same text and drawing in the same postion on all 100 frames.) How do I do this without drawing and adding text on every single frame seperately?
Answer: howdytom is correct but as you already have 200 frames the easiest way to add extra text/drawings to some of those frames would be to do this on a new layer. Then within the timeline window select the frames (these are shown highlighted -- use [SHIFT] to add to the frames selected if frames are alongside/next to each other; or [Control/Command] if frames are spaced apart.) and either hide or show the new layer.
If you want to make a transition from "no-new content frames" to the "new content frames", then do this after choosing which frames show/hide the new layer by selecting a frame that has layer hidden, allowing a couple of frames between then selecting a frame that has the new layer shown and clicking on the "Animation-Frame Tweening" button (bottom of timeline window). This will cause the new layer content to appear to gradually 'fade-in'.

Gif editing in photoshop


What is the difference between a payload and shellcode?

Question: I am new to penetration testing and I would like to know the differences between these two things:

Payload (reverse TCP vs. bind shell)
Shell vs. Shellcode

Any help?
Answer: When exploiting an application, the payload is the code that the attacker actually wants to execute. It's the part that not just serves the purpose of leveraging the vulnerability itself, but does whatever the attacker considers useful.
When choosing a payload, an attacker typically wants code that starts them a shell. A shell is the program you interact with on the command line to control your OS (on Linux you might be familiar with the Bourne shell or Bash).
A payload that starts a shell is referred to as shellcode. But note that you will eventually find payloads that don't start a shell sloppily called shellcode, too.
A bind shell is a program that binds itself to a port and listens for incoming connections. Since opening a port attracts attention and because the attacker might often not be able to reach the port from outside the network, there is an opposite concept: A reverse shell doesn't wait for incoming connections but connects to a specified address and port itself. That is, the attacker waits for an incoming connection by the compromised server instead of initiating the connection themself.
Answer:
Exploit - An exploit is the means by which an attacker, or penetration tester for that matter, takes advantage of a vulnerability
  within a system, an application, or a service. An attacker uses an
  exploit to attack a system in a way that results in a particular
  desired outcome that the developer never expected. Common exploits
  include buffer overflows, web application vulnerabilities (such as SQL
  injection), and configuration errors.
Payload - A payload is a custom code that attacker want the system to execute and that is to be selected and delivered by the Framework.
  For example, a reverse shell is a payload that creates a connection
  from the target machine back to the attacker as a Windows command
  prompt, whereas a bind shell is a payload that "binds" a command
  prompt to a listening port on the target machine, which the attacker
  can then connect. A payload could also be something as simple as a few
  commands to be executed on the target operating system.
Shellcode - Shellcode is basically a list of carefully crafted commands that can be executed once the code is injected into a running
  application. It's a series of instructions used as a payload when
  exploiting a vulnerability. Shellcode is typically written in assembly
  language. In most cases, a command shell or a Meterpreter shell will
  be provided after the set of instructions have been performed by the
  target machine, hence the name.

Source: https://latesthackingnews.com/2017/07/12/difference-exploit-payload-shellcode/
Answer: Shellcode is injected with the payload or even sometimes referred to as payload in general. Payload on the other hand doesnt necessarily have to be shellcode (code spawning a shell) but could be code performing any action of your choice, i.e. spawning a calculator, reboot the system,... The payload usually not only includes the code to be injected and executed but also encompasses return addresses, NOP sleds, new SEH's, stub data fixing the stack, decoder, egghunter,...


Do I need a general or business visitor visa to take the IELTS in the UK?

Question: I'm planning to go to the UK to visit friends and also to take the International English Language Testing System (IELTS) while I'm there. What sort of visa should I apply for? General visitor or business visitor? Also, I was in the UK till December 2013 on a business visitor visa which I extended in the UK twice. Will it pose a problem for my visa application now? It's been 5 months since I've been back home.
Answer: I doubt you'll find an official answer, since this isn't one of the reasons listed on the Check if you need a UK visa page, but since taking a test is a) a perfectly legitimate reason to visit, and b) not work or business, the general visa seems more suitable.  This is doubly so since you're also visiting friends.
Since you have been away for a long period of time and have a good reason to return, I don't see why you would have any problems getting the new visa.  As always, any proof you can offer that you are planning to return back home (job, property, family etc) is useful though.


Where can I find the most comprehensive resource regarding hostels?

Question: Whenever looking for an hostel I mostly used hostelworld. It used to contain most hostels and was a good resouce. Lately, I am starting to get the feeling that I see a lot of hostels, on the spot, that I didn't find on hostelworld.
I heard from a friend working on an hostel, some time ago, that in some cities they were not accepting new units.
Is there a more compreensive resource listing hostels on the Internet?
Answer: There is no complete resource, for good reason: commercial.
Hostelworld has agreements with many hostels to show them (possibly for money).  So that's one collection.
Hostelbookers is similar, and has a slightly different set in certain cities as they have agreements that HW doesn't.
Then there's HIHostels but that obviously only indexes the Hostelling International Hostels.
Google also has a few - if you search 'hostel' on google maps in the city you're in, it'll show some. Again, some, not all.
Long story short - not everyone knows about every hostel. There are some that work on word of mouth. Others are chains. Others only show up in the Lonely Planet books.  There's no one resource, but in my experience, I go to hostelbookers first because I like them more, then hostelworld as it seems to have more.  Unless I'm specifically after an HI hostel, then I go for HiHostels.
In Central Asia (the stans) they only showed one at the time in Kazakhstan and it was more of a guest house, and all the ones I found in Uzbekistan were word of mouth from other travellers. Go figure.


SSH tunnels with putty

Question: Is this setup possible:
isp -> tunnel-1 -> tunnel-2 -> destination
tunnel-2 will be exit IP, but in the "tunnel-1" containter. So ISP should only see traffic from tunnel-1. The programs used should be putty and chrome.
Thanks
Answer: yes it is possible. it seems reasonable to connect a tunnel from the customer premises to the tunnel1 source interface, in which case the isp couldn't see anything at all except a single encrypted tunnel with data passing across it. they can probably tell alot from signal timeing, size, etc, but they couldn't see any data in the tunnel packet.

SSH tunnels with putty


Using Arduino Wifi Shield to Receive Signal

Question: I am trying to use an Arduino Uno with Arduino Wifi Shield 101 to receive a signal from phone or webpage over wifi so it will "initiate" my circuit. I was able to connect the board to wifi but am unsure on how I should continue. My main goal is to be able to send it two different commands that would run different parts of code in my arduino script. I am very new to this stuff! Thanks :)
Comment: use the arduino to serve a web page that has a button
Answer: Look for the Web Server example in the arduino IDE.  That will give you a starting point to allow you to control the arduino via a web page.  This tutorial is also very in depth : https://startingelectronics.org/articles/arduino/switch-and-web-page-button-LED-control/


Installing Tor on network firewall

Question: Can Tor be installed on a network firewall such as Smoothwall? I'd like to have the firewall serve as the relay and free up the PCs behind it.
Answer: I've done that on pfSense, and it's been done on OpenWrt and DD-WRT. I'm sure that it would also work on Smoothwall. Routing and firewall rules for Tor are straightforward. There's no route from LAN to WAN, and LAN can only communicate with localhost on the ports that Tor is using.
In fact, Smoothwall might be one of the best platforms, because it's basically a Linux distro, and may have Tor packages that are more up-to-date. Checking repositories that Smoothwall uses would be a good place to start.

Installing Tor on network firewall


How to improve the efficiency when I need to exhaustively pairwise millions of list in Python?

Question: I have a m*n matrix (m>1000000, n>50000). What I want to do now is to pairwise every two rows of the matrix and do some further computation (e.g computing common elements of two rows, or reveal subsumption relationship between two rows). I wrote applications with naive code (using two for loops) which can work for small matrix, but not for big matrix like the one listed at the beginning. I need some suggestions on how to solve this computation problem in this case.
Comment: The output of computing something pairwise between $m$ vectors is $m^2/2$ quantities, which is ludicrously large in your case. It is better to look at what you're trying to do using those $m^2$ quantities and avoid computing all of them in the first place, than to try to compute them faster. I think this could be an instance of the [X-Y problem](https://meta.stackexchange.com/questions/66377/what-is-the-xy-problem).
Comment: What is a "subsumption relationship" between two vectors?
Comment: @Kirill For "subsumption relationship", I want to see if the set of elements of one row (list) is a superset of another one.
Comment: At the very least, you can reduce the number of comparisons by a factor of two if you treat each row as a bitstring of zeros and ones, depending on whether a column is nonzero in a row. You can then interpret these bitstrings as numbers. A row can then only be subsumed by another if the number that corresponds to the other is larger or equal (because it needs to have at least the same bits set). This is not a sufficient condition, but if you sort rows in ascending order based on their number, then a row $i$ can only be subsumed by row $j$ if $j\ge i$. So you only have to do $n^2/2$ comparisons
Comment: There are likely more clever ways  to arrange all rows in a tree based on these numbers, and to make decisions based on this tree.
Answer: You can't -- if you want to consider all pairs of rows, then your algorithm is inherently ${\cal O}(m^2)$ in the number $m$ of rows. There is fundamentally nothing you can do about this unless you find a way to avoid looking at all pairs of rows.
However, the problem should be easy to parallelize!
Comment: I tried to do this in parallel, but it ran out of memory because there are really too many pairs to check.  I should then find a way to decrease the number of pairs that I need to go through. Thanks.


Normalization of signal against reference

Question: I am trying to figure out if there is any standard way of normalizing a sequence such as a time series against a reference sequence. I am not an expert on signal processing but I was hoping someone would be able to point out a good way of doing this. Specifically, I am working on image analysis where I measure pixel intensities along linescans. This gives me two intensity profiles along space (from 2 channels). I am trying to normalize 1 channel against the 2nd one (reference).
Comment: Normalize to equal energy, maybe?
Answer: I assume your reference image data y_reference is acquired when there is no light on the image sensor - that's the standard thing to do. Then the usual normalized signal is
y_normalized = (y_signal - y_reference) / y_max
where y_signal is the signal image data and y_max is the maximum possible pixel output. y_max can be measured by increasing the light level on the image sensor until the output no longer increases.


In what instances would you want to use path-finding algorithms other than A*?

Question: I am familiar with how the common ones technically work (BFS, DFS, Dijkstra, A*) but as far as their realistic benefits I don't quite see the need for them. Considering that, given the right heuristics, A* is more performant then why bother with any others?
Answer: I'm not very familiar with many of them but I have seen the usage of Dijkstra's algorithm not for pathfinding but for finding the nearest X to you. For example, what powerup is closest to you or what enemy is closest to you out of a group of enemies. So a "target nearest enemy" function often uses that.
Answer: What you are really listing are, rather, graph search algorithms, which are a part of Graph Theory.  A* happens to be one which is optimized for "pathfinding" applications.
Why bother?  Those algorithms are good for much, much more than simple pathfinding.  In fact, there is an entire branch of mathematics devoted to studying graph theory and its properties.
BFS and DFS, in particular, are one of the most basic and powerful tools of graph theory application.
EDIT
It is of use to note that A* is rather weak when the shape and nature of the obstacles to be navigated around is dynamic and changing.  In this case, it is usually a good bet to look into other pathfinding algorithms.
Comment: Graph Theory and game development are inherently tied.  Most AI logics and decisions/choice algorithms are best modeled with graph theory; Physics engines optimize with graph theory, Procedural Generation has a lot intertwined with it, as well as many other aspects of game design that I have not even come to realize.  The other algorithms might not be useful for pathfinding, but they will come up in other parts of your game.
Comment: Good points, but graph theory and game development are not one in the same and my concern is of the latter.
Comment: They may be useful in other parts of the game, but this question was specifically talking about pathfinding.
Answer: I'm not an expert, but I can see Floyd-Warshall being better in some situations where you can afford to just precalculate all best paths up front and then reuse them.
Answer: If you're working with a uniform-weighted grid and don't need to worry about dynamic pathing, Jump-Point Search is an extremely efficient pathfinding algorithm. It's extremely fast, usually ten-to-thirty times as speedy as A*. It achieves this through symmetry reduction, which is a method by which empty spaces are ignored.
Answer: A*, because of the closed and open lists, can be more memory intensive than a well-implemented Dijkstra.  However both are probably processor-bound anyway so you might as well go for the flexibility of A*.
Comment: What do you mean by 'because of the closed and open lists'? Sans-heuristic, A* == Dijkstra
Answer: Zombies!!
Sometimes your AI shouldn't be so "I".  For certain classes of MOBs (see above), the "Face where you want to go and Advance" algorithm is all you need.
Comment: There's another interesting sense in which A* alternatives might be better for zombies. A* is great when you want one path from one start point to the nearest goal. But what if you have a whole horde of zombies all pathing toward the player? In some such situations, it may be more efficient to compute an exhaustive single-source shortest path tree for the area (pathfinding for all zombies at once), rather than re-computing branches of it repeatedly for each zombie tracing a similar route.


Does drinking coffee improve concentration?

Question: Will drinking a cup of coffee before I sit down to work improve my concentration, compared to drinking a cup of water?
Comment: Apparently not - being hydrated and having enough sugar seems to matter more.

Although caffeine does change your susceptibility to weak arguments (link to paper broken) -
And it certainly doesn't help if you are trying to [build webs](http://lumiere.ens.fr/~alphapsy/blog/?2006/11/10/113-how-to-stop-academics-from-drinking-coffee)
Comment: @NobodySpecial I think I found the paper [here](http://espace.library.uq.edu.au/view/UQ:78113).
Comment: I can tell you it sure doesn't work for me.  Makes me all twitchy and results in me getting easily distracted.  :-)
Comment: @JeromyAnglim wrote [an interesting summary of some psychopharmacological research literature reviews on Cognitive Sciences](http://cogsci.stackexchange.com/q/111/4086). Check it out!
Comment: If you drink a lot of coffee you will have to pee a lot, which dehydrates you an makes the subsequent urine more concentrated.
Answer: Caffeine is a stimulant which makes you feel less tired and, for athletes, delays exhaustion.  So if the alternative is being asleep, caffeine cetainly helps.
The other issue is that the regular use of caffeine leads, as with other psychoactive substances, to dependency (tolerance or addiction are other words used).  In such cases, lack of caffeine leads to withdrawal symptoms such as drowsiness and  irratability.  For dependent individuals, a cup of coffee will remove these symptoms in the short term, aiding concentration, though it is less clear whether this is an improvement on never having taken caffeine.
Answer: Yes
There seem to be a lot of scientific literature around this question. If you agree that higher productivity at work should be a consequence of higher concentration, we have a lot of evidences.
A. Smith: Effects of caffeine on human behavior
The paper provides us with an extensive literature review of the past researches done on the effects of caffeine. It shows that many studies has demonstrated that regular coffee consumers show better performance at work. Caffeine seems to stop the decrease of performance as time past at work. However, that is only true for moderate consumption of caffeine. When people drink a large amount, the exact opposite happens, lost of concentration and lower productivity.
As a side note, I also came across this paper about the effect of Red Bull (which compare to coffee):
C. Alford, H. Cox and R. Wescott: The effects of Red Bull Energy Drink on human performance and mood


In three spatial dimensions, would it be possible to have a force that decreases with the inverse of the distance?

Question: I know that in our universe, electromagnetism and gravity decrease with the inverse of the distance squared. Would it be possible though for there to be a force in another universe with three spatial dimensions that decreases with the inverse of the distance instead of with the distance? If so, could such a force have an infinite range?
Comment: Seems that would contravene the law of conservation of energy...
Comment: You mean it _increases_ with distance (not necessarily linearly)? If such force had infinite range, would the pull become infinitely strong, too?
Answer: In our universe, electric and gravitational force decrease with $r^{-2}$ because of the inverse-square law.
Imagine a spherical shell around some point source. The area of a shell at some radius $R$ is $4\pi R^2$; thus, the flux through that shell is reduced by a factor of $r^{-2}$. This is true in three dimensions, and can be proved mathematically. In $N$ dimensions, a force will always decrease by $r^{N-1}$. This can be proven in a similar fashion.
So, unfortunately, your scenario is impossible in three dimensions.

If so, could such a force have an infinite range?

One of the great things about having a force that decreases by $r^{N-1}$ is that no matter how far away an object is, there will always be some non-zero force between two particles. Always. It might be very small, but it will be non-zero. The force will always have an infinite range.

Proof of the coefficient of surface area (see these notes):
Technically, the term "sphere" refers only to the $N-1$-dimensional surface; "ball" refers to the $N$-dimensional region. I will use those terms in the following proof.
The volume of an $n$-ball is proportional to some power of its radius:
$$V_N=C_Nr^N\tag{1}$$
Let us imagine an $n-1$-dimensional spherical shell. The volume-area relation is well known:
$$\mathrm{d}V_n=A_n\mathrm{d}r\to A_n=\frac{\mathrm{d}V_N}{\mathrm{d}r}\tag{2}$$
We can differentiate $(1)$ by the power rule to obtain
$$\frac{\mathrm{d}V_N}{\mathrm{d}r}=C_NNr^{N-1}\tag{3}$$
Take, for the sake of conjecture, the integral
$$\int_{-\infty}^{\infty}e^{-r^2}\mathrm{d}V_N\tag{4}$$
We can re-write this as
$$\int_{-\infty}^{\infty}\mathrm{d}x_1\cdots\int_{-\infty}^{\infty}\mathrm{d}x_Ne^{-x_1^2-x_2^2-\cdots-x_N^2}\tag{5}$$
This last step is done because
$$V_N=x_1x_2\cdots x_N$$
We then have
$$\left(\int_{-\infty}^{\infty}\mathrm{d}xe^{-x^2}\right)^N=\pi^{N/2}\tag{6}$$
Again, this is because of the equality of all $x_k$s.
Substituting in $\mathrm{d}V_N=A_N\mathrm{d}r$, we have
$$\int_0^{\infty}e^{-r^2}A_N\mathrm{d}r=C_NN\int_0^{\infty}e^{-r^2}r^{N-1}\mathrm{d}r=\frac{C_NN}{2}\int_0^{\infty}e^{-s}s^{N/2-1}\tag{7}$$
Solving this integral using the gamma function, we get
$$\frac{C_NN}{2}\Gamma(N/2)=C_N(N/2)!\to C_N=\frac{\pi^{N/2}}{(N/2)!}\tag{8}$$
Finally, this gives us
$$A_N=N\frac{\pi^{N/2}}{(N/2)!}r^{N-1}\to A_N\propto r^{N-1}\tag{9}$$
Why did we use
$$\int_{\infty}^{\infty}e^{-r^2}\mathrm{d}V\text{ ?}$$
Well, it turns out that this becomes $\sqrt{\pi}^N$, a factor that we know will arise when finding the surface area or volume of any sphere. We're simply using the integrals to calculate that factor and the rest of the coefficient $C_N$.

A much simpler proof (that gets to the point!)
This is actually a really simple method, but you might not think of it right away. An answer by Colin Pratt on Mathematics Stack Exchange gives a good 30-second introduction on the $N=3$ case.
First, let's calculate the volume of an $N$-sphere. We can reference what we did above and note that the volume of region in $N$-dimensional space can be found by
$$\int\cdots\int_V\mathrm{d}x_1\cdots\mathrm{d}x_N$$
However, we can then translate this into hyperspherical coordinates. We find that the volume element is
$$\mathrm{d}V=r^{N-1}\left(\prod_{i=2}^{N-1}\sin^{N-i}(\phi_i)\right)\mathrm{d}r\mathrm{d}\phi_1\cdots\mathrm{d}\phi_{n-1}$$
We then integrate this to find the volume of the sphere. Be careful about choosing your bounds, though.
Next, we can find surface area through volume. Interestingly enough,
$$\frac{\mathrm{d}V}{\mathrm{d}r}=A$$
for all dimensions of $N$. It's not a coincidence, either. Therefore, if we know the volume, we can find the surface area.
Comment: @ConfusedMerlin: That's a "proportional to" sign. It summarises the complex equation on the left hand side of the arrow (which helpfully removes most of the letters).
Comment: Wonderful explanation!  Got my vote...
Comment: Jus tread your bio, how old are you broham?
Comment: Dear HDE 2268686, while I'm glad that there are people out there who can solve such stuff with ease, I usually skip those answers, because... seriously, when I reach (9), all I see is that there is... well... there is a inompleate infinite-sign? Or a fish? Maybe most of the worldbuilders do see this and are exited by that proof, but I tend to skip answers that contain such stuff, because my lasting impression of this is "wat?". So does it mean, that numbers can be cut in half infinite times without "becomming zero" or is this something else?
Comment: I have no idea why the answer throws in the integral on stages 4 through 6 though? Unless it is to show that this arbitrary radial distribution of static "charge" has no impact on the per-area flux at the end . . . but that seems tangential to the question, as if lifted from some other related proof?
Comment: @NeilSlater I don't have the time to fully explain it at the moment, but I did make an edit to try to make that a bit clearer. I will, however, be adding more information later.
Comment: @ConfusedMerlin What Neil said. Also, I find that https://en.wikipedia.org/wiki/List_of_mathematical_symbols is typically helpful, in a pinch.
Comment: @ConfusedMerlin I've added in a proof that I think is much simpler, and perhaps more intuitive.
Comment: I appreciate your second approach... well, as long as the questioner does understand your answer, its good. I keep considering myself as a hopeless case ;) Now don't waste any more time for me on that.
Comment: Your arguments assume that this fictional force must obey Gauss's law; that is true for gravity and electromagnetism, but not for the nuclear forces. The strong nuclear force actually *increases* linearly in strength with distance over its range of action.
Answer: If you assume that the natural state of the universe is for everything to be clumped together in one cosmic singularity then you can redefine gravity as 'the force that keeps that from happening', which increases with distance from an object.
Take the following statements:
1: The natural state of the universe if for everything to be in one place, and the universe exerts a force to cause this. This is the statement that must be assumed.
2: Not everything is in one place, and things are at rest.
3: Therefore there is a force keeping everything from being in one place that is balanced against the universal collapse. We know by experiment that we feel this force less strongly at the surface of the planet, and more strongly as we move away from it.
4: This force therefore increases as the inverse square of the distance from an object.
It's all a matter of perspective. Sadly that particular  perspective is rather contrived, counterintuitive and really not that useful, as it's simpler to assume that the natural state of the universe is to be at rest and the force is pulling things in, which decreases as the square of distance (aka gravity). At this point HDE 226868's excellent answer on the nature of spheres and flux applies.
On the other hand if you are building a whole other universe the residents of that universe don't have to have the same idea of what defines simple. Maybe it's easier for them to assume that the universe is constantly trying to collapse. Who knows?


What are the benefits of MailChimp over EEs native Mailing Lists?

Question: Why should I use a AddOn to let Members opt-in a Mailing-List managed by MailChimp, when there is a native EE-Mailing-List-Module? What are the benefits of using MailChimp instead of EEs own/native/local Mailing-List-Module?
Answer: MailChimp's Email Delivery feature page is essentially a list of all the things MailChimp does that EE's mailing list (which is basically a dumb interface to your server's sendmail function) does not.
In short: do not rely on EE's Mailing List feature.
Comment: Absolutely true. Be sure to checkout Postmaster for advanced integrations with MailChimp and ExpressionEngine.
Answer: It should be noted that you can very easily hook up EE's email sending methods to a Transactional Email Service such as Mandrill (http://mandrill.com/) this is actually what Mailchimp is using, Mailgun, Postmark and others.
This means that all email sent by your EE installation goes through that service and can be monitored for delivery success/failure etc.
You can use Mandrill for free up to a point so it's worth giving it a go. Just sign up and enter the SMTP connection info into EE's email setup.
I wrote up a little guide to set up EE to use Mandrill a while ago which might be helpful
http://www.foamcow.com/general/configure-expressionengine-to-use-mandrill-transactional-email-services/
Comment: Though I would check with the Mandrill folks first to make sure that using their service to send bulk email is acceptable, as it is designed as a *transactional* email service.
Answer: MailChimp give you a lot of option and stats on your mail!
You can see the % of people that open it and track them with your google analytic.
You can easily customize the design of the email your sending to get your members attention!
If you use EE's mailing list feature you have more chance of getting your Server IP banned if you get report for mail spamming.. That not nice!


I dislike the current head of my dynasty, can I arrange an accident for him?

Question: The current ruler of my dynasty is a bit of an idiot. His heir shows much more promise; is there a way to get rid of my current ruler?
Comment: +1 for the wording of the title, because 'Accidents' do happen... all the time. >:)
Comment: RIP [tag:murder]
Comment: several options...  have a very negative opinion of some family member, like your wife, hopefully they'll start a plot to kill you.  Another thing, put yourself at the head of every battle (and try to lose them), like rebellions
Comment: Nowhere near surefire, but you could keep organizing hunts, as a lot of those events give you some minor chance of dying. And, of course, to the computer, it seems 1% chance of death == 80% chance of death...
Answer: ways I do that (legally):

Really mistreat someone (like your wife), hopefully they'll start a plot to kill you.
Put yourself in every battle you do, try to lose them ;)

In a more 'cheaty' way, you probably can load the game as one of your counts/dukes and select the plot to kill the liege :)  then go back to yourself and wait until they succeed
Answer: One of the most reliable ways is to try and assassinate someone with very high intrigue. They will discover your assassin, and then they are very likely to send an assassin after you. If they fail, or don't send an assassin, just try again. Conveniently, after you die the reputation hit from assassinations disappear.
Answer: Just keep throwing him in harms way. Hunts, get into as many fights as possible, piss off the natives, go crusading at every chance. That should up the odds nicely. There's gamey ways to do it like Rodolfo stated, but who would do that? ;)
Answer: You can also commit suicide. This option is available in your decision tab, if your character has the "depressed" trait. I'm uncertain when this was introduced; I don't think it was possible in 1.0.
Comment: Is having the depressed trait the only way you get the suicide option? How does a character get depressed?
Comment: There might be other ways, I don't know. When I hover over it, it simply lists "(*) is depressed" as the only requirement. My character got depressed after his first born son died, but I believe there are random triggers for it.
Answer: Another way to get your heir on the throne is for your current character to be forced to give up the throne as a result of surrendering in an excommunication war or a war to dispose of a tyrannical liege.
In the case of the latter here are some ways to get your vassals motivated to stand up against your rule:

revoke their titles
imprison them
execute said prisoners

I'm not sure what the odds are they won't start a war for independence as opposed to a war to depose of their liege, but if your current ruler is a bit of a douche to start with it's another in-character way to get what you're after.
Answer: tryusing console command.

Hit tilde ("~") key
type "charinfo 1"and hit enter (this is to enable the game tooltip to show more information)
Hover to your character, memorize his character ID (strings of numbers, like 543524)
type "kill -yourcharacterID-", hit enter. Voila!
Comment: While this does kill the character, it doesn't do so via arranging an in-game accident. Captain Kirk would approve, though.


How secure are self decrypting HTML files for data storage?

Question: I recently came across a JavaScript self decrypting archive. Is it secure enough to be used as a portable password storage tool? The author has even challenged it to be cracked.
Answer: At least the author wrote a rather clear page on how his encryption works. Notwithstanding, this looks like a rather old-style homemade stream cipher, which is not good news, since most of such systems have been thoroughly broken. It appears to consist of a basic LFSR subsystem (two LFSR with key-dependent polynomials; the bit about the polynomials operating in "reverse direction" is a red herring, because this subsystem is still wholly linear) and another LFSR which acts as a non-linear selection engine. There are a few select designs which roughly look like that and appear to offer a non-negligible cryptanalytic resistance (e.g. the Alternate Step Generator), but this comes at a price, namely that you need quite much more than n internal bits of state if you want to achieve a 2n security level. Also, many such designs turned out to be weak.
We can trust in the resistance of a cryptographic algorithm based on the combination of the following properties:

The designer is known to be very good at designing algorithms (e.g. his name is Rivest).
The design got wide exposure, such as being published in some high-profile conference and/or being deployed at a large scale.
Several years have elapsed, and nobody found any issue.

This design is a bit lacking on the three points.
Edit: I had not noticed it (I need more coffee in the morning) but the generated stream is deterministic for a given password. This means that if you encrypt two archives with the same password, you are in the infamous "two times pad" situation, and you have basically lost.

Another point is that the password is derived into a key with only a pair of invocations of MD5. MD5 is very fast; a good, but off-the-shelf and not utterly expensive GPU can evaluate MD5 at a rate of roughly one billions per second. If the first bytes of the encrypted data are guessed by the attacker (since cleartext is supposed to be Javascript, the first bytes are probably "var" or "function"), then the whole thing is vulnerable to dictionary attacks, aka "exhaustive search on password" (trying out potential passwords until a match is found). It is possible to have a big fat random password which defeats dictionary attacks, and a human brain could still remember it with some effort. But most passwords are weak in that respect. Also, no salt: for a given password, you always get the same stream. Therefore, one could precompute the first bytes of the stream for each potential password, and afterward cracking any instance would be a matter of a few lookups in the table (the table could take the form of a rainbow table for more compact storage).
For the password-handling part, this scheme is definitely weak.

The use case is also dubious. Since this is Javascript code in which the user enters his password, this is vulnerable to active attacks: an active attacker could simply alter the Javascript code, so that when the user enters his password, a copy of the password is also recorded somewhere (e.g. the decrypted data -- which is supposed to be Javascript itself -- gets an additional code snippet which prints out a hidden  tag with a URL pointing to an attacker-controlled server and encoding the password).
To defeat active attacks of that kind, the solution is known, and that's SSL. The "self-decrypting archive" should be served only over HTTPS. But if HTTPS is used, why would self-decryption be needed ? Instead, have the server give the sensitive piece of Javascript only after having duly authenticated the user, with the password. This is a very common model, existing Web servers implement it out of the box, and it is stronger since it thwarts offline dictionary attacks (from the outside, the attacker cannot "try passwords" at the maximum speed of his GPU; he has to ask to the server for each guess, and the server can answer as slowly as it wishes).

Summary: I recommend against using this piece of code. I still note that, at least, the designer of SDA appears to have made commendable efforts at documentation, and to be sane -- sanity alone puts him in a minority, among the crowd of cipher designers on the Internet.
Comment: Wow, thanks for that! Although you identify the password handling as the weakest part in the code, wouldn't it be okay to carry it around without deploying it over HTTP?
Comment: @pewfly: I am still reserved about the quality of the encryption itself. Even abstracting away the password, it is quite similar to many weak systems.
Answer: The problem with all of these types of things is that you can't really determine how secure they are. What we do instead is place some trust in ones that have been tested over time by a large number of experience people (@ThomasPornin has an excellent answer on this, which I will try and find a link to) - this is probably why the author has placed the challenge: He wants it tested.
This one may use a secure algorithm, but if the implementation has weaknesses then it may be insecure.
In summary - if you have a real need for a secure SDA, I would advise going with a tried and tested one, as your risk is likely to be lower. If however you can review the code behind this one and assess the risk to you for your uses to be sufficiently low, then this may be fine for your purposes.
(Sorry that's a bit uncertain, but a lot of this is...)
Comment: Thanks for the advice. But I'm looking for a portable SDA and this once seems to be backed by good people (Paul Johnston for instance). Any other suggestions?
Answer: I general would stay away from anything where you can't review the code and you have to do encryption online. Maybe it's not the case here, but every time in the past I've seen new encryption or steganography algorithms which were available only online, it was a honeypot collecting data on what people out there might want to encrypt/hide.


Why is C still in the category of High Level Language?

Question: Many of the textbooks on the C programming language tell that C is a high-level programming language, but many of the tutors online say that C is also a middle-level programming language. Why is it like that?
Comment: I empathize with your confusion.  This is the sort of classification that would aggravate me to see on an exam because even a "wrong" answer is correct.  Pray you never have this as a true/false or multiple-choice exam question.  If you do, answer based on what the textbook said, not what you feel is correct (I feel evil recommending going against your gut).
Comment: recommended reading: **[Discuss this ${blog}](http://meta.programmers.stackexchange.com/questions/6417/discuss-this-blog)**
Comment: Historically 'high[er]-level languages' were those that, unlike assembly language, allowed the programmer to abstract from the specific machine and write code in a more problem-domain way. Fortran and Cobol were 'high-level' because of that in early days. C is but a thin veil above the machine-specific stuff, but its very purpose is to be portable, machine-unspecific. So by 1960s standards C was high-level. Compared to C++14 or Haskell or OCaml or Rust, C is not-so-high level, of course.
Comment: Most of these "modern high level "languages"" are really just scripting languages. The difference is if it's compiled or interpreted.
Comment: Asm is high level. The abstraction provided by asm over hardware is enormous.
Answer: Historically, everything that abstracts over assembly code was called high-level. C certainly does that. This definition is also relatively clear-cut, in contrast to what follows.
Over time, we created more and more programming languages and invented more and more abstractions and tools. Compared to, say, Python, the C language is positively primitive in semantic richness and level of abstraction over the hardware. With that in mind, many people find it misleading to call C high-level, when there is a wealth of languages that are far higher above the hardware.
So now "high-level" usually means "abstracts a lot over hardware" and "low-level" means "abstracts little". This is the definition your tutors use.
Not everyone agrees though, and old texts don't magically adopt the new terminology, so you still see the old use of "high-level" (under which C is high-level) floating around. Keep in mind that many good C books are basically newer editions of books released twenty years ago.
Comment: For something to be considered a proper low-level programming language, it must be allow a programmer to easily operate at the same level of abstraction as the underlying hardware.  While the C Standard would allow implementations to behave in such fashion, there is no formally-recognized distinction between implementations that operate on that level of abstraction versus those that do not.
Answer: It is a higher level language than machine code (assembly), which is the point of view that C programming books and tutorials come from.
In that respect it is a high level programming language.
However, it is still very close to the hardware - much more so than other, more modern languages (Java, C# and such) - when viewed from this point of view, it is a middle level programming language.
Comment: @manutd - that's a question to book authors ;) - but that's the thing. From their point of view, C is a high level language. That's what they deal with. The concept of "high level language" is fuzzy - there is no single definition agreed by all.
Comment: Then why can't we clearly say that C is a middle level programming language.In textbooks we don't find that.
Comment: Unfortunately agreeing opinions in a community isn't how defining works though.  It's creators and it's concepts are higher level than the assembly language of the processor - this is the defining line.  This is what defines it as high level.  Any book you pick up on computer science will always define it in this way.  Tutors may not and peoples opinions may not, but the actual science of how computers work and how it has been defined in the science of computers and processors is very clear.
Comment: C# allows programmers to access a range of memory as a sequence of 64-bit, 32-bit, 16-bit, or 8-bit values as convenient; if memory holds 16-bit pixels, for example, code can process pixels individually or in groups of 2 or 4, depending upon what's best for the algorithm.  C as defined by the Standard doesn't define a practical means by which programmers can request similar semantics.  Many C dialects are lower level than C#, of course, but the language defined by the Standard isn't.


Where can I get a fork that will fit a 7 speed gear and a disc brake? Or how could I make one for said specification?

Question: Im trying to insert a 7 speed gear and disc brake on the front wheel of a recumbent cycle that I'm trying to make. However its difficult to find or almost impossible to get forks with widths that accommodate such components. My aim is to make a FWD cycle. Can it be fabricated if not available as such? If so, can someone tell me how?
Comment: @Criggie I've thought of that but its difficult to get a geared donor bike. That was our teams first idea, to cut off the back end and attach a BB housing. Now what I'm trying to do it make a BB housing out of some other pipes, attach the whole setup to the fork and steering handle by welding probably. But can't get forks that wide to fit a gear set and disc brake.
Comment: Doesn't it need to be recumbent specific or custom fab anyway to have a place to mount the derailleur?  Or are you using an IGH?
Comment: Do you distinguish between FWD bike and MBB for Moving Bottom Bracket?  I've seen some that have an entire rear triangle carved off a donor bike in order to give a triangle of power with a BB housing etc.
Comment: @Affe Yes custom fabrication is what I'm thinking of, IGH is a possibilty but I've only seen 3 speed internal gears available around me. But that too at the expense of disc brakes.
Answer: If you are looking to re-purpose off the shelf components: Fat bike forks use dropout spacings of 135mm and 150mm, I believe. Road disc brake bikes and some mountain bikes use 135mm rear spacing, so you should be able to combine a fat bike fork and road rear disc hub. A 135mm rear hub will have space for an 11 speed cassette, but you can use spacers to fit a  7 speed cassette.
If you are building a front wheel drive recumbent, you'll have to at least do some fabricating for the frame attached to the fork that holds the bottom bracket bearings, cranks, derailleur etc. That would include welding steel or aluminium tube. If you can do that you can also design a fabricate a custom fork.
Comment: Yeah I'm actually gonna try that now. Need to source the parts. I'm trying to minimise the amount of fabrication and also budget, otherwise would've gone straight to fabricating the entire setup. Also could could you possibly tell how much width a standard 7 set gear would need?
Answer: Atomic Zombie is a website with a lot of DIY bike designs, including this FWD recumbent, which may give you some ideas. There are a few FWD recumbents you can buy off the peg, including the Cruzbike and some models from Performer. No idea if they would sell you just a fork.
Comment: This cycle kind of looks like what I'm trying to do. Thanks.


Why can't I select perks other than increasing SPECIAL skills at level up?

Question: I've now hit level four but the only perks available to me are still stat increases.  All the perks at L01 and below are greyed out, and unselectable, even if I meet the pre-requistes.  Is this expected behaviour?  Is there something else I need to do to get other perks?
Answer: The perks screen uses fading to indicate a skill you don't have, and outlining to indicate a skill you can't have (due to not meeting the requirements), see this screenshot here:

If you've got skills that are faded out, you should be able to get these by clicking on them and then pressing the confirmation key as prompted at the bottom of the screen:

If the perk is outlined entirely, hovering over the perk will still tell you what it does but also inform you of why it's outlined (not enough strength in my case).
Comment: @FeralChimp If you already have a point in a perk, you can tell if you can put another point into it by looking at the stars above it. If you have level 2 in a perk and there's only 2 stars there, none flashing, you can't put in another point yet. If there's the 2 stars indicating your current level and a 3rd flashing star, you can put another point into it.
Comment: Good answer. There's one exception to these general rules: if you already have one or more levels of a skill, that icon looks the same whether or not you meet the requirements for the next level of that skill (these are usually character level requirements). Once you have at least one level in a skill there's no way to differentiate from the icon whether you can or can't upgrade it. So unfortunately you need to hover over every one of these skills to see whether you can upgrade it. Unless I'm missing something--which surely I hope I am.
Comment: @VanBuzzKill +1 Nice, I missed that. Thank you.
Comment: OMG. I thought I had to do ALL the first row. Im at level 43 and havent even used anything below the first row! I guess I have a strong base to work with, but geez, what a terribly designed system to not explain this to users. Thanks for the great info.


Printing or Private Use

Question: I want to find a Print on Demand (POD) outfit that will print my book without an ISBN, as I do not want to sell my work through any outlet, print or electronic.  I just want copies to have and give to friends and family.  What do I do?
Comment: Are you set on a PoD service or would you be happy to branch out? I mean, some stationers will print and bind your work but I'm not sure if that comes under PoD.
Comment: How many copies do you want at a time? We had a small run of books printed very cheaply by just supplying a printer with a colour cover and the book as a Word file.
Answer: Do you want to print without an ISBN just because you don't want to pay for one? Or is there some reason why you really don't want to have an ISBN on the cover?
If the issue is the cost, then Create Space and Lulu will give you an ISBN for free, making that a moot point.
Answer: I've worked in numerous printing companies and every single one of them would gladly print your book any way you want it.
ISBN's are not required by printers or authors (at least not if you are simply giving them away to friends and family); you'd simply supply your artwork/copy however you like.
If it's a small run (read: qty. of books produced) and a small page count (less than 100 pages), you could conceivably get Kinko's or the UPS Store to print and bind using Plastic Coil or Wire-O binding.
If it is a larger run (>500 books?) and/or a larger page count, you'd most likely need to go to a commercial printer for Stitch, Perfect or Case binding. Mind you, they'll also likely be able to do what you could get done at a retailer as mentioned above with better quality, but at a greater cost.
It may be beneficial to quote your project both ways: through a retail establishment and a commercial printer.
Hope that helps!

Printing or Private Use


Linux: Measuring network consumption per remote host

Question: Is there a way to measure the network consumption to a specific IP port.
I am running 2-3 process with multiple threads which connect to a remote server and I need to know the exact used network bandwidth from my server to the remote server.
My server runs under Linux - unfortunately I have no access to the involved network components.
Answer: As far, as I know, you can measure bandwidth out of the box only on specific interface.
To measure traffic by ports or IP's you have to use accounting software. For example ntop
Answer: I'm not 100% sure whether you need to know active flow or cumulative usage.  For the latter and other more involved data gathering, the aforementioned ntop is an excellent choice.
If you just need something quick and dirty to see the active connection flows on a NIC, you can try the iftop command.
Answer: My experience is that ntop doesn't scale well (this was a couple of years ago).  I am now using a combination of fprobe and nfsen to do bandwith usage monitoring.  See my notes here.
Comment: You are generating netflow, it's not needed. And one more, wire-speed packet capturing: http://www.ntop.org/PF_RING.html
Comment: My problem wasn't so much that ntop wouldn't capture, it was that after a while it started generating links to itself that wouldn't work.  Also my setup above lets me have the listener (fprobe) be on a different computer than the usage calculator (nfsen) as well as do other analysis to the traffic activity.
Answer: I ended up using pktstat as ntop looked good, but was a hazzle to install. it gives exactly the required breakdown.


Voltage across short circuit

Question: We know that current is passed through a circuit if there is a potential difference between the two terminals of the conductor. But in the case of a short circuit, we say that there is no potential difference between the two terminals and a large amount of current is passed through it. It's a violation of Ohm's law. Isn't it wrong to say that there is no potential difference between the terminals?
Comment: This is a good example of the difference between "zero", "nearly zero", and "limit as variable tends towards zero". Also that current is a bulk approximation to the motion of individual electrons.
Comment: If you're powering the circuit, then you will certainly have a potential difference, and by having a short circuit, you create a circuit path by which the current can flow. I'm having a hard time understanding your question. Could you provide a circuit schematic of a situation where you think Ohm's Law is being violated?
Comment: FWIW, I answered more or less the same question at physics.se with a longish addendum to distinguish the role of ideal circuit theory in modelling physical circuits.  http://physics.stackexchange.com/questions/86677/how-can-there-be-a-current-and-an-electric-field-in-an-idealized-wire-with-no-vo/86683#86683
Answer:
It's a violation of Ohm's law

Why do you think so?  I don't understand where the idea that Ohm's Law is "violated" by an ideal wire (or ideal short-circuit) comes from.
Ohm's Law:
$$V = IR$$
Now, if \$R=0\$, as is the case for an ideal wire, there is zero voltage across for any current through.
Consider the I-V characteristic for an ideal resistor with a large resistance:

Note that the slope of the characteristic is \$\frac{1}{R}\$ and thus, as \$R \rightarrow \infty\$, the slope approaches zero, i.e., the I-V characteristic becomes horizontal through the origin.  This is an ideal open circuit; the current is zero for any voltage across.
Now, consider the I-V characteristic for an ideal resistor with a small resistance:

As \$R \rightarrow 0\$, the slope approaches infinity, i.e., the I-V characteristic becomes vertical through the origin.  This is an ideal short circuit; the voltage is zero for any current through.
There is no violation of Ohm's Law - the open circuit and short circuit are simply the limits of \$R \rightarrow \infty\$ and \$R \rightarrow 0\$ respectively.
Comment: @user34606, no we don't *know* "that potential difference must be needed to flow of current".  Just as *force* is not required to keep an object moving in the absence of *friction*, an electric field is not required to maintain a current in the absence of resistance.
Comment: Here's the logical answer. Your assumption that there must be a voltage difference to make current flow is wrong. That assumption is only true if a non-zero resistance exists between two points. If you accept that there can exist a resistance of zero ohms then you must accept that there can be a voltage of zero volts across it regardless of current flowing through it.
Comment: I don't mean that..I am just trying to say that why we say that there is a 0 volt across an ideal short circuit as current is passing through it.Because,we know that potential difference must be needed to flow of current..So,if there is no potential difference between a short circuit then how can current flow?It may seem odd to you..But I am talking with logic.Please give me a logical answer.
Comment: Any normal conductor, including the one forming your short circuit, does have _some_ resistance, although the resistance may be very small, it is still there, so there will in fact be some small voltage across the short circuit.  Although this voltage may be small, the resistance of the short is also very small, so quite large currents can flow.
Comment: So,what is that term which is responsible for flowing of current through the circuit where electric field is not required?@Alfred Centauri
Comment: @user34606, what is that term that keeps a mass moving in the absence of friction?
Comment: To be honest I don't know the reason.Tell me please.@Alfred
Comment: @user34606, it's not a term, it's a principle:  conservation of energy and momentum
Comment: So,according to the principle you mentioned can I get the answer of flowing current through the circuit without requiring electric field?@alfred
Comment: @user34606 Yes, you can get a flow of current without requiring an electric field. Electrons traveling through the vacuum of space are a current without an electric field. If you define a "short circuit" as zero ohms, then by definition it must be possible to have current flow without a voltage through a short circuit. Stop mixing _ideal_ circuits with _physical_ circuits.
Comment: As Alfred already said, you need to remember that real wires always have a real resistance, even if it is negligibly small, just like a satellite in the atmosphere encounters negligible air resistance which would eventually cause its' orbit to decay and the satellite to crash to Earth.  Superconducting wires have a perfect *zero* resistance.  In any case though, when you are analyzing circuits on paper, the resistance of wires isn't taken into account, and short circuits are assumed to be conditions where far too much current will flow.
Comment: That's the separation from theory and reality.  If you look at a circuit simulator for instance, you can't short circuit a power source without causing some sort of error and the simulator to stop.
Comment: @krb686, that's a good point and one I touched on in a longish addendum to an answer to a more or less identical question here at physics.se:  http://physics.stackexchange.com/questions/86677/how-can-there-be-a-current-and-an-electric-field-in-an-idealized-wire-with-no-vo/86683#86683
Answer: Ohm's Law says that
\$V = I \times R \$
so when \$R\$ is zero, as is the case for a short circuit, then \$V\$ will be zero as well, no matter how high the current \$I\$ is.
\$ I \times 0\Omega = 0V \$
The current is caused by a potential difference in the circuit as a whole, it doesn't have to be between any two points. In a loop there's only 1 current, which is the same everywhere, even between points at the same voltage.
Answer: Consider the following circuit

simulate this circuit – Schematic created using CircuitLab
What is the current flowing from node1 to node2?
What is the potential difference from node1 to node2?
With copper wires at room temperature, there is a non-zero resistance, but for most purposes it is perfectly valid to ignore this.
Comment: @AlfredCentauri if you know where to buy some 1 ppb resistors, please let me know.
Comment: OK, and correct me if I'm wrong, and this is a subtle point, but if this is an ideal circuit schematic, there are just two nodes in this circuit, the "top" node and the "bottom" node.  I suppose one could argue that the "top" node is actually a super node enclosing the ideal wire (in essence, treating the wire as a 0V ideal voltage source) and that might be interesting but it would be unconventional.
Comment: For OP, here's another way to look at it. Imagine in Gritty's diagram there is 1 micro-ohm of resistance between node 1 and node 2. Now the voltage across R1 is only 11.999998 V or so instead of 12 V, and the current through R1 is only 0.11999998 A instead of 0.12 A. The errors due to this effect are much smaller than the errors we'd have if we built this circuit due to the resistor not really being exactly 100 Ohm and the source not really being exactly 12 V, so we simply say "the drop between node1 and node2 is negligible", and we solve the circuit as if it were 0 V.
Comment: @Alfred: I'll remove the mention of "ideal" elements - I hoped to persuade the OP that many real circuits can usefully be analyzed by ignoring resistance in wires, without tripping up over ohms law.
Comment: @ThePhoton, what you've touched on is, in fact, *the* connection between ideal circuit theory and physical circuit elements.  If, in your example, we're working to, say, three significant figures, then the voltage across R1 *is* 12.0V and, thus, the ideal wire approximation is valid.  On the other hand, if we're working to 9 significant figures, the physical wire must be modelled as an ideal resistor with resistance \$R = 16.6666694 \mu \Omega\$.
Comment: @AlfredCentauri, well there's also the lumped-element approximation, but yes.
Comment: @ThePhoton, correct but let's not go there for this DC circuit.  :)
Answer: If short circuited , it doesn't means two nodes. It will be single node only. As per @RedGrittyBrick image shown, both (node1 and 2) will be representing single node only.
And potential difference is measured across two nodes. So if you connect battery or DC source in your case, current will flow from high potential to low potential.

Voltage across short circuit


Symantec Backup Exec runing DBCC CheckDB and Backup on my SQL server

Question: What is Symantec Backup Exec (TM)?
I noticed that it is running in one the database server i am maintaining.
I am a newbie in db administration and i am not sure what it is.
It is running DBCC CheckDB on different databases and running Backup on different databases.
Can anybody give some info on this?Is this something windows server team does to protect data?
I did some research and all i can see is information that says it is some kind of data protection program.
Answer: I'm not sure if Symantec still owns it, but Backup Exec is Netbackup's little brother. I don't remember the feature differences but Netbackup is more for Enterprise environments and Backup exec for smaller ones.
Are you sure it's Backup Exec that's running DBCC? I've used Netbackup for SQL for many years and it's always done checksum checking during backup. I've never seen it run a DBCC check directly from the Netbackup server or via the client.
Any third party SQL backup software rides on top of the MS written SQL backup. What you're paying for is much better compression, AlwaysOn support, centralized job scheduling and backup image storage. SQL backup has a checksum option and most of the third party products support it as well. But the only time i've seen something not-SQL run dbcc or alter index on a database is Sharepoint doing it.
I've used both and prefer the third party products to native SQL backup any day.
Comment: https://www.veritas.com/support/en_US/article.TECH53367. I checked Veritas support and apparently BE can run DBCC. Symantec spun Veritas off a year or two ago which is why it still says Symantec. I would not make changes or run your own processes until you figure out company policy and who is running it. I've personally seen personal backups mess up those taken by Netbackup and if the situation is serious enough it can get you fired.
Comment: When i ran `sp_whoisactive` on that particular DB server , i saw DBCC checkDB for dataabases and then BACKUP query running for databases.I created maintenanace plan for backups and i know that it runs during night time ,so i checked and i can see program name as Symantec Backup Exec(TM).
Comment: Was it the same SPID or hostname running DBCC and backup? You should also be careful about running your own backups if Backup Exec is running since you can break the log chain for log backups or cause differential backups to use your backup as a full instead of the Backup Exec. Short story is if you need to restore and someone tries to do it via backup exec and fails, it will be your fault. And this can be months down the road. I've personally seen situations like this.
Comment: DBCC is over and i didn't notice the SPID for that.But the login and hostname was same for Backup and DBCC.Login is `domain\sbe` and prgram name is `Symantec Backup Exec(TM)` I think someone is running it at server location.But i was not informed about this.
Comment: okay.@Alen.Thanks for the info.it was really helpful.I have enquired windows server team to if someone turned it on.
Answer:
What is Symantec Backup Exec (TM)?

Symantec Backup Exec is a backup software that works on many types of platforms in your environment. One of those features is to back up SQL Server Databases.

It is running DBCC CheckDB on different databases and running Backup
  on different databases. Can anybody give some info on this?

Best practice dictates that you would run DBCC CHECKDB on your database before backing it up. This checks for corruption or other irregularities detrimental to the integrity of the database. After all, would you want to take a backup of a bad database, pay to store the backup, and then try to use the backup in a disaster recovery scenario?
So I'm not surprised it runs the check, there are other back up software platforms out there that do things similarly.

Is this something windows server team does to protect data?

There's a good chance that someone on your windows team configured the software to do this. There's a good chance that someone who was in the database role prior to you did this. I would not consider this "protecting" data, but "preserving" data.
I would recommend to do some reading regarding backups. I'm also a strong believer in the "Ozar" hierarchy. If you are familiar with psychology and Maslow's hierarchy of needs, databases are very similar. The core block for database administration is having data to administrate. If you cannot recover or use the data you are administrating, this is how most DBA's lose their jobs. This is the most important piece of database administration. (Shy of security. Some scenarios, you may prefer to have no data than let someone else get a hold of your data. Both are terrible things and I hope you never need to deal with either.)
Philosophy aside, here's a great article to read to get your feet wet. Grant Fritchey of Redgate on backups.
Ozar also has some great questions to think about regarding backups.
I would reach out to whomever at your company configured the software or knows about it. Possibly even the person who purchases / renews the software and see if you have support with Symantec. If you have support with them, you may be able to call and receive a demo on how to use the product and basic instructions on how to configure and adjust it.

I already have database back job and DBCC checkdb for the databases as
  sql jobs which runs as per maintenanace plan.So this Symantic Backup
  Exec is required or not?

There is no correct answer I can give you regarding this question. What I will say though is that if you are running a DBCC CHECKDB and a FULL backup on the same server with two different processes, you are duplicating computing effort and time. I typically recommend (have not found that edge case yet) to have only one backup "process" per server.
(Process meaning, you use the Symantec software, or perhaps maintenance jobs, or personal made scripts, etc.)
Questions I would ask:

Does this software give you extra capabilities?
Does this software have any kind of guarantees?
Why did your company decide to use this software, were the SQL Servers just thrown into the mix?

Maintenance plans are typically frowned upon because they are not highly configurable or robust. I don't think there is anything wrong with using them, but this goes into the discussion about the right tool for the right job. Don't hammer a screw into place if you have a screwdriver.
What your action plan probably should be (and in order):

Make sure you have a complete set of backups that have had DBCC CHECKDB ran on them and verify they work and can be restored from.
Verify your RPO / RTO / SLA of the databases and verify your current configuration.

Does this match what your company and employer needs and do they understand the ramifications? Typically the less lost data, the higher the monetary cost.

Reduce redundancy.
Monitor, report, and test your backups.

This goes in hand with practicing disaster recovery.

Continue to improve and innovate your backup and disaster recovery process.

Apart from that, make sure to do a lot of reading. Backups are a very important subject to know well if this is part of your job.
Comment: Thanks for the info.I will do the readings as mentioned.One more questions I already have database back job and DBCC checkdb for the databases as sql jobs which runs as per maintenanace plan.So this Symantic Backup Exec is required or not?
Comment: I've edited my answer to include this question too. @user9516827


Sharing rules on Community Users

Question: I need to create a Account and have a lookup from account to Account as (Partner). Lookup will determine if related Account, related Contacts and Opportunities are visible to these Partner Community Users. I have to create Sharing Rules that enable Partner Account Contacts (as Community Users) to access their related Account and Contact's records.
Please guide me how to create a Sharing rule for this scenario.
Comment: Which account has the users that need CRUD access, the parent account or child account?
Comment: in parent account i need CRUD access.
Answer: Quick caveat to my answer, there could be a way to accomplish this with sharing sets but I don't have enough knowledge of that particular feature to say one way or another. The best way I would go about accomplishing this is by managing sharing with apex. You would need a trigger on account and user that calls a helper class that has method to accept both a list of users and list of accounts passed in from the trigger.
The user method, needed only on insert, would find all child accounts associated with the user's community account and insert a sharing row for each.
The account method is trickier, as it would need to operate in all contexts (update, delete, insert, etc.). This method would need to fire when the parent account is not null (and on update probably be isolated to only when the parent account changes), and you would then find all users associated with the parent account and grant access to the account, contacts, etc. being listed as a new child. This is a relatively complex trigger, as you will need to make it bulk safe, which will require a few maps in your apex method(s). You'll also want to make sure to clean up and delete sharing rows when a parent/child relationship is changed or severed.
Comment: Bulkify for sure.

Sharing rules on Community Users


Difference in rent listed on company's website and the lease they sent me

Question: I'm trying to rent an apartment for next year. The rent listed on the company's website is $150 less than the rental amount contained in the lease they've emailed me. I haven't signed anything yet, but I'm wondering what my options are as far as trying to get the rental price down to what is listed on the website.
Comment: This may be bait-and-switch (https://en.wikipedia.org/wiki/Bait-and-switch) which is illegal in most locations. Or it could be rent is more expensive because you are renting for e.g. one year instead of five.
Comment: Did you call them and ask that question?
Comment: I've emailed them, but haven't received a response. I just wanted to know what all I could do so I could be as knowledgeable as possible when speaking with them.
Comment: Did you check that the website prices aren't "Starting with X..." or "X and up..."? That's the usual trick they're playing. Otherwise, consumer protection agency in your jurisdiction can definitely shake them up.
Answer: You can ask them for the cause behind the difference.
They probably list the cheapest version of each apartment size.  Apartments may rent for more if they have superior views, higher floors, different exposures, different layouts, better installed appliances, or better construction materials.  It could also be out of date if they increase their rent regularly.  I'm used to seeing it expressed as a range, but sometimes they give you the headline cheapest price.
Your negotiating position depends on the occupancy rate of the building, the neighborhood, and the city/region, as well as the particular market niche of that building.  If you can get a better deal somewhere else, then you can say that to them if they are intransigent on price (only say it if you mean it).
Some landlords negotiate, but a lot of corporate managers are not accustomed to regular negotiation.  You may be able to instead ask for an accent wall (painted a striking color) or free parking, if they won't move on price.  But if they have almost all their units filled, then negotiating is more a courtesy than a necessity.
Comment: Ya it's a corporate manager and there aren't many units to begin with, but I'm pretty sure most of them are filled, though they are all the same version. Thank you!
Comment: Fewer units may marginally improve your negotiating position, unless they are in high demand.  It makes a vacancy  a higher proportionate drag on the property's income potential.  But realistically the market is pretty decent and they probably have multiple locations to spread exactly this risk, so even if they are open to talking they probably have a better BATNA than you.  Especially if the starting date is close.
Answer: If you haven't signed the lease yet, you have one very powerful negotiating tactics at your disposal: Going someplace else. In a free market, your most powerful weapon as a consumer is the ability to walk away.
As others have noted, the first question is whether this is a mistake, if the advertisement was actually deceitful, or if you were fooled by a vague advertisement.
It's certainly worth asking about the discrepancy. Maybe they will reply, "Oh, we're sorry, that was a mistake", and that will be the end of it.
If the company actually lied in an advertisement -- if they said this particular unit with this size, this location, etc costs $X per month but then when you go to sign the papers it says $X+150 -- you might be able to report them to a consumer protection agency or sue them for false advertising. But if you haven't actually signed anything or paid anything yet, I wouldn't bother. Unless they're very stupid -- which is possible, of course -- they have some fine print somewhere that says that what they did is legal, you'll get yourself tied up in legal proceedings, and for what? Just find a different apartment and save all the trouble. I wouldn't get into legal proceedings unless they already have your money and now are trying to change the terms of the contract.
BTW, I would not want to sign a contract committing myself to a long-term relationship, like a lease, with someone that I was in the process of suing. You can't imagine that they will WANT to provide you with good service on Tuesday if your lawyer was just badgering them in court on Monday.
It's fairly common for advertisers to say "prices starting at $X" or "as low as $X". In that case you should take it for granted that the units at that price are going to be very low quality or otherwise undesirable, or why would they be the cheapest? In the case of an apartment, it's going to be the one that's smallest, or has the most inconvenient location, or has old barely-working appliances, or some such. On the one hand, if a company has a wide range of products to offer, you can't expect them to list the price of every one in an ad. On the other hand, companies often make one item that they know is so shabby that almost no one will actually want it, slap some super low price on it, and then use that in the advertisements. You could debate how honest or dishonest this is endlessly. If I was looking for an apartment and a complex advertised "$800 and up" and when I look at the apartments the one I want is $900, I'd probably say okay fine, it's the "and up". If it turned out they really had nothing decent for less than $2000 and the $800 unit is a closet with privileges to use the outhouse out back, I'd say it's a scam and go somewhere else.
I guess the other question is what other options you have. If this place is advertising apartments for $X and, whatever the story is about the advertisements, the unit you want is really $X+150, and in fact there are plenty of other apartments available that are $X or less, then just go somewhere else. If all other comparable apartments are $X+300, then this place is still the best deal and you may just have to swallow it.


Why they collect coffee beans that are eaten by wild civets? Why don't they use fresh coffee beans instead of collecting these unhygienic beans.

Question: How is this possible to collect coffee beans that were eaten by wild civets in forest? Is it  hygienic? Why so expensive?
Answer: The coffee bean we brew our coffee from is the seed of a fruit and the flesh of that fruit must first be removed. There are a number of industrial / cottage industry means for processing the fruit for the bean, but the civet's digestive system also does the job. :) The beans are raised well above the 140F needed to kill bacteria during the roasting process before we brew coffee from them, so no worries about the hygienics unless you're brewing green beans.
Answer: The reason to collect these beans is flavor. Passing through the digestive system changes the flavor of the beans. I have never personally tasted it, but I have heard that the beans have a sweet caramel like flavor. As long as the beans are roasted, it is perfectly hygienic.


Imaginary line on the Earth is straight or not?

Question: I was watching Google map. Saw a line which is joinging north to south pole. It is going between middle of China and Mexico. But, the line is not straight. Tried to search in net but not able to get anything. Can anyone check and let me know https://www.google.com/maps/@-19.6123649,-178.5132229,3z or http://pichost.org/image/UJKHD
Answer: I think you're referring to the International Dateline: https://en.wikipedia.org/wiki/International_Date_Line. If so, no, it isn't straight; there are a few irregularities that have been put in place for various political reasons.
Comment: I read. It tells there are some islands around International Dateline(assuming International dateline is straight. There will be some around both side).  So, it will create confusion having to use different Date in different islands. To avoid that, the line is drawn to cover all island in one side


How to verify that a surge protector actually works?

Question: I need surge protectors to prevent expensive equipment from exploding. Manufacturers of these devices obviously make a lot of fantastic claims about features and performance, but how can I trust them when there appears to be no independent tests available on the internet? I know of at least one device that's designed for these kinds of tests, but it's expensive, and I don't need to do this regularly. Is there some sort of lab that I can send samples for them to test or any alternatives besides waiting for a catastrophic failure?
Answer: Check your surge protector and you will most likely see marks from Underwriters Laboratories (UL), ANSI (American National Standards Institute) /IEEE (Institute of Electrical and Electronics Engineers) and IEC (International Electrotechnical Commission). These groups define standards for how products should functions as well as some also do testing and certification.
Check the surge protector for stamps from these groups in particular look for these codes:

IEC 61643-1
ANSI/IEEE C62.xx
UL 1449

These indicate that the device is designed to function to specs for surge supression.
You cannot verify that the devices you buy actually perform to spec since the only way to actually test it is to push it to the point it fails, thus breaking the device. Good manufactures test random samples and verify that they meet the specs and so can make a good faith claim that the devices they sell will also meet the specs.
Comment: "You cannot verify that the devices you buy actually perform to spec .....".
Comment: I guess this is a good as it's going to get at the moment
Answer: The best bet is probably to go with a reliable company that insures their devices.  APC for example has devices with hundreds of thousands of dollars or more of insurance coverage if the device fails to protect your hardware.  That's usually a pretty good indicator that whoever was underwriting the insurance wanted some proof that it actually does it's job reliably, though it isn't necessarily fool proof.


Where do I find things to do in Firefall?

Question: After the first missions about the water I've sort of run out of things to do with my accord recon character. Are there more missions available somewhere? Ive heard about the staged content phenomenon where you can push back the melding but how do I actually do that?
Comment: Simon & Lewis from the yogscast did a few videos on this game. Link is down at the moment.
Answer: There isn't much in the game at the moment beyond the first mission and endless PvP, for the simple reason that it's still in beta.
The developers are slowly adding things to do, but at the moment it's fairly empty.
Answer: do /gf or /givefun to have Aero or Oilspill tell you about events nearby that you can do.


Effect of noise on high and low frequency components

Question: It is given that noise is present in the noisy image in the form of high frequency components. Can any one please explain this concept. Also please tell me that whether it is correct to say that "effect of white Gaussian noise is more on high frequency part of the image as compared to low frequency components" or not.
Comment: I think there is a language problem here: "In theory it is given that noise is present in the noisy image in the form of high frequency components."  Can you rephrase this and explain "it is given" ?  I think the first answer below is _probably_ the answer you want; but since I don't know the technical meaning of the question I can't be sure.  For instance, you might have meant broadband, or out of,  band noise leaking in; alternatively, you might have meant temporal edge time measurement.
Answer: This is actually due to the characteristics of the actual signal, rather than noise.
You might have heard of pink noise and its spectral characteristics $$S(f)\propto \frac{1}{f^\alpha}$$
Now forget about the term "noise" in here, and imagine a stochastic process whose spectral density vanishes following a $\frac{1}{f^\alpha}$ law, in which $\alpha$ is a constant.
Interestingly, this is the law that nature follows a lot. More specifically, many natural signals follow this law. It has been observed and actually been verified to be valid for instance in human voice, natural images, music, quasar light emissions, heart beat, firings of neurons, and even in financial markets. Note that natural is a key term here and by that, we refer to attributes of a natural environment. For a more detailed discussion see here and here. For 2D (image) signals we should expect $\alpha=2$.
Since according to the $\frac{1}{f}$ law the contribution of the actual [natural] signal will diminish at higher frequencies, we would expect more contribution of noise at these frequencies. Consequently, noise is usually dominant at high frequencies and the signal is more dominant at lower frequencies.
Comment: [Schaaf and Hateren found](http://www.sciencedirect.com/science/article/pii/0042698996000028) that the spatial power spectra of their linear-intensity outdoor photos had on average $\alpha = 2$. They had almost always $1 < \alpha < 3$. Images that are composed of solid-color areas with well-defined edges will have $\alpha = 2$, because of the color discontinuity at the edges of the areas.
Comment: Yes, $\alpha=2$ is just an approximation for natural images and is indeed case-specific, as you pointed out.
Answer: effect of noise is more considerable in high frequency because it contain information(high) so any small noise is considerable...small noise even can change the whole information....on the other hand low frequency component contain small information over a large period and a small noise can't give much effect on it...
Answer: Additive noise (important to mention) will be added to the Fourier spectrum because the transform in question has the property that it is linear. And as msm mentions many signals in different fields have much more energy in the lower frequency bands than in the higher frequency bands, so the relative effect of the noise as compared to the signal energy will likely be larger for the higher frequencies.


Why do NHL referees relax their penalty calls in the playoffs?

Question: It seems that in NHL playoffs, referees tend to be a lot less likely to call a penalty. What is the reason and is this an official rule in the referee handbook or is it more of an unspoken rule?
Answer: I think there is some kind of unwritten rule that refs do not want to change the issue of a match with a bad call in any direction. That rule hits even harder when in playoffs, and even harder when its a win or go home game, especially game 7.
There is no official directive that I know of, and there is probably some studies that have been made to confirm or deny what you are advancing.
My opinion is that they still officiate the same, but those missed calls, that happen ever so often, are magnified because of the importance of the match.
Answer: I believe that actual penalty rates are not particularly different between the NHL regular season and NHL postseason. However, the heightened nature of the playoffs means every missed call is magnified. Also, common refereeing complaints of "reffing to the score" and "not wanting to make a game changing call late" are just as common in the regular season and the playoffs. There is obviously no rule in the rulebook that states different standards for the playoffs.
Comment: It's possible that penalty rates do stay the same in playoffs, but that still doesn't mean referees are not relaxing their penalty calls.  Playoffs (at least to me) seem much more rough and violent.  So if refs were consistent between the regular season and playoffs I would expect there to be a higher rate of penalties in the playoffs.


How close humans are for interstellar travel?

Question: I have heard many times about interstellar travels. Are centrifuge space craft (artificial gravity) and warp drives technologies being built or still it is just concepts ? Nearest start system is alpha centauri so is it possible to reach there by mentioned technologies ? and if yes then how.
Comment: *"How close humans are for interstellar travel?"* About 4.2 light years away.. ;) *"centrifuge space craft"* this can be useful for **space stations,** and while it might be used in an interstellar ship, it's not useful to *propel* the ship. Warp drives are sci-fi and not an actual 'technology' as such. We are far (far, far) from achieving technologies that could get a ship to the closest star within a human life time.
Answer: Manned interstellar space flight is not going to happen anytime soon.  Definitely not within the next 50 years, most likely not within the next 100.  There are too many basic issues that are hard to work around.
The biggest issue (apart from politics and funding) is propulsion.  So far, nobody's figured out how to build an engine that doesn't require some form of reaction mass (propellant) to accelerate, meaning your total ΔV (change in velocity) is limited by how much reaction mass you can carry along, and the amount of reaction mass you need grows exponentially as ΔV increases (i.e., for N times more ΔV, you need eN times more reaction mass).  Conventional chemical and ion engines are simply not up to the task of interstellar travel; either the amount of propellant necessary is many orders of magnitude larger than the spacecraft itself1, or you accept travel times of thousands to tens of thousands of years (which brings up issues with power, life support, spacecraft maintenance, etc.).
There are ideas for engines that use nuclear fission or fusion that provide better performance, but the best of them would still require on the order of a century or so to reach the nearest star.  And again, you're limited by the amount of reaction mass you can carry.
The logical step up from that is to not carry your reaction mass with you; instead, you either gather reaction mass from the interstellar medium as you travel (like the Bussard ramjet), or you use a sail to exchange momentum with the solar wind or a powerful EM source, like a laser array.  There's a project called Breakthrough Starshot that intends to use a massive bank of ground-based lasers to accelerate a gram-scale spacecraft with a large sail to something like 20% c.
The problem with ramjets is that there just isn't that much material to gather, and the drag from the scoops would probably cancel out whatever thrust you could generate.  And the problem with sails in general (and Starshot in particular) is that you have no way to slow down once you reach your destination.
Basically, interstellar travel with reaction drives is not going to happen.  We need to develop some kind of reactionless drive, one that doesn't require you to exhange momentum with some kind of reaction mass in order to accelerate.
There's the EmDrive, which allegedly provides thrust without reaction mass.  There's a lot of skepticism associated with this, with good reason.  At first blush, it seems to violate conservation of momentum.  Also, people who've tried to replicate the results using sane test setups have not been able to reproduce the effect.
So, that pretty much leaves us with warp drives, which...aren't going to happen anytime soon, if ever.  They're mathematically possible, but that doesn't mean that they will ever be physically possible, and even if they're physically possible, the energy requirements are such that we could probably never build one.

To reach 1% c (3000000 m/s) using the ion engine on the Dawn spacecraft, you'd need on the order of 1088 kg of propellant for every kg of spacecraft mass.  To reach that same velocity using an RL-10 chemical (hydrogen/oxygen) engine, you'd need on the order of 10295 kg of propellant for every kg of spacecraft mass.
Answer: A sucessful flight of humans to mars and back might be possible in some years or some decades. But we don't have the technology for a human flight to the outer planets and their moons. Before we did not manage to send an orbiter to pluto and land a rover on pluto we are not able to leave our solar system with human passengers. We should send some spacecrafts to the oort cloud before. There is so much work to be done before. We did not even try a space station in an earth orbit with artifical gravity in a centrifuge yet and we would need many years of experience with humans living there.
Answer: Humans will NEVER travel to another star!
It requires either so much time, or so radically new technology, that humans cannot exist under such conditions. We are not created to be star travelers, we're deeply rooted in Earth. We're just the guys in the post office. We'll send Earth life out there, but not ourselves.
Comment: To say that the difficulties (both theoretical and engineering-wise) are enormous is reasonable. But to go from there to that humans "never" will travel to another star seems excessive and ultimately unsupported. We don't know what humans of 10,000 years in the future might be able to do. If you told a human of 10,000 years ago that we would be *regularly* sending spacecraft out into the solar system, they probably wouldn't believe you in the slightest. 1,000 years ago, still highly implausible. 100 years ago, science-fiction but not unimaginable. 10 years ago, already doing it.
Comment: @MichaelKjörling None of our agricultural plants and livestocks existed 10,000 years ago, humans created them all by breeding. When we can substantially re-engineer our DNA molecules directly, which we certainly will do way before we can travel light years as we are, humankind will quickly cease to exist. We'll get rid of everything that makes us human, such as mortality, sexual reproduction, ape bodies, limited senses, individuality and inefficient brains. Life from Earth will likely go to other stars, but it will not be human, that is impossible.
Comment: NEVER ?
Damn, too bad i cant downvote.