How to stop Viber running without uninstalling it?

Question: How do I stop Viber running, without uninstalling the app? When I go Menu→More options→Exit it just comes back. I want it to turn it off completely, so it's not waiting to come on in the background.
Comment: Viber is just bloatware that forces you to use it, you cannot close it, only option is to uninstall.
Comment: Oh this is so annoying. I can exit from Skype, from fb messenger, from hangout, from lync. But not from this. Only hard stop of apps helps. Via settings-apps-running
Answer: This is an official rep. of Viber.
Due to the C2DM feature, you are always available on Viber, there is not way to  "log out" and "log in".
Please note that if you "exit" Viber, it will open again only if you get a msg or call.
Let us know if you have any additional questions.
Thanks!
Viber Team
Comment: Hi @Viber , Can you create an option to disable auto start? It's pointless (and waste battery) to run Viber when there isn't internet. (also I want to use Viber to call others rather than let others call me)
Comment: Funny!God bless telegram.
Answer: No need to root or anything. You can just simply stop it being running by going to Settings>application>viber>force close this will close all the services related to viber. But remind you, you have to force close it every time after you use viber.
Oh there is another way to do that. Start viber, then close it. Then open your multitasking window/history window by pressing the home button. you should see viber there at the bottom. Long press on the screen-shot of viber. You should be prompted with two option, app details/info and clear from list tap on app details/info and click on force close this will do your job.
Hope that helps.
Comment: Just wanted to add that after phone reboot Viber will be running again. So, if you, for example, want to avoid network access by Viber for some reason, then close all network connections before reboot, reboot, disable Viber service and then enable network connections again.
Answer: There is an app for this if you have root: Greenify
Comment: But that'll require root which will void warranty.
Comment: Not really. Well if you are in the EU it doesn't, see this link: http://matija.suklje.name/rooting-and-flashing-your-device-does-not-void-the-warranty-in-eu
Comment: This is the way I do it too, but sometimes it still starts up.
Answer: I have Viber on phone and tablet and it behaves differently. By default, no matter how you stop the service it always turns on when you receive message.
On phone when I go to settings - general - turn off "show Viber status icon", and then I just exit the Viber (via exit or stop service or clean task manager) I am really offline. All messages are received after I turn on Viber myself. I can be offline for days. It behaves like this only when I turn off the Viber icon.
I guess this is a bug but I like it and hope it is not going to be fixed. I don't know if it is bug just for combination of my phone and Android version or if it "works" like this on every phone. On tablet this does not work, Viber cannot be really off.
Answer: Battery saver is the easy option without any additional apps.
Many Mobile manufacturing companies give some inbuilt apps to control background running apps, check if you have one.
If it's a basic stock Android then you need to install 3rd party apps like Greenify or something else..
Answer: AFAIK you can't stop the process in the background without unistalling it. You can logout from your viber account, then it won't display you any notifications
// I must add to here, that once you logged in Viber has no known way to log out


Is it a sin for our son to have "Rahim" as his name on his birth certificate?

Question: After naming our son Rahim we have been told that it is prohibited to say Allah's Names like that. We renamed him Abdul Rahim, but on his birth certificate we still have only Rahim. Should we change it? Is there any sin?
Comment: Related post: http://islam.stackexchange.com/q/2207/18
Answer: Rahim is not one of the names of God.  Al-Rahim, on the other hand, is.  Calling your child's name thus, with the definite article Al-, would be problematic; in such cases you would appropriately go with Abdul-Rahim.
Without the definite article, Rahim is just an adjective, and one not unique to God (As Najeeb describes in his own answer, it's used by God Himself to refer to the messenger).  As long as it's an attribute that is not unique to God (e.g. Muhyi or Mumit), using it as a name is fine.
See also the answers to this related question: Is naming a person using one of Allah's names allowed?
Answer: I don't think there's anything wrong in calling a human Rahim or even Rauf since the Messenger (SAWS) himself has been referred to as such in the Qur'an:
"There has certainly come to you a Messenger from among yourselves. Grievous to him is what you suffer; [he is] concerned over you and to the believers is kind [Rauf] and merciful [Rahim]." [Qur'an 9:128]
(Latter two interpolations mine.)
Answer: Rahim is not the same as Al-Rahim .. and you certainly do not have any intention to match the attributes of God that Al-Rahim was so InsyaAllah it would not be a sin. But it's better that he meant the name Abdul Rahim servants of the Merciful God ..
Wallahu a'lam


How do I get the stars in focus with my astrophotography setup?

Question: Hey  I've problem with blurred stars when I shoot stars with my Canon 700D with EF 50mm f/1.8 STM lens I've set up with 10-20 second shutter speeds and  ISO settings from 400-1600. I try on f/1.8 and in all pictures, the stars are blurry. How do I focus them?
Comment: Possibly related: [How can I avoid star trails?](https://photo.stackexchange.com/questions/93906/how-can-i-avoid-star-trails)
Answer: Getting precise focus is a bit tricky.  Some lenses mark the "infinity" point, but some lenses don't have a focus distance scale on the lens (depending on which Canon EF 50mm f/1.8 lens you are using, you may not have the scale).
But even if you do have the scale on the lens, the "infinity" mark is really just an approximation.  It will not be precise.

Disable auto-focus on the lens (switch the AF/MF switch to the "MF" position)
Find the brightest star you can find in the sky -- even if that's not the area of sky you plan to image.  Sirius is the brightest star (apart from our own sun).  But this time of year, Arcturus or Vega would make good choices.  Point the camera toward one of those stars.
Switch the camera to "live view" mode.
For any camera that supports exposure simulation in live-view mode (such as your Canon DSLR) you should also:

Switch to MANUAL exposure mode
Set the ISO to the maximum value possible (12800)
Set the aperture to the minimum value possible (f/1.8 for your lens)
Set the shutter exposure duration to the maximum standard time available (30 seconds)

This is not the exposure you'll use when imaging the sky... but what it will do is make the stars in live-view much brighter so they will be easier to focus.

Center the star and roughly focus the star.
Use the live-view zoom feature to increase the magnification to 10x (keeping that same star centered)
Very carefully adjust focus.  You are trying to make the star as tiny as possible (a pin-point).
Having done this ... be careful not to bump the focus at this point.  You can point the camera to the area of the sky you want to image.  You should also return the exposure settings to something more sane (e.g. maybe ISO 800 or ISO 1600).
Calculate the exposure duration by:

Multiply the focal length of the lens by the crop factor (1.6x for your camera) which works out to 50 x 1.6 = 80
Divide 500 by the result of the previous step (80 in this case) which works out to 500 ÷ 80 = 6.25.  This is the maximum exposure duration.  (6.25 seconds)

The reason for this maximum exposure duration is to avoid stars elongating due to the rotation of the Earth.  This rule is sometimes referred to as the 500 rule.  It says you can divide 500 by the focal length of the lens.  Except it assumes a full-frame camera (it was originally for 35mm film cameras).  The rule tries to determine how much a star will appear to move over time based on the true angular field of view.  Having a smaller sensor means the true angular field of view is smaller and this is why you have to compensate by factoring in the crop factor of the camera.
Focusing Mask
You can make life a little easier by getting a focusing mask.  The most popular variety is called a Bahtinov Mask.  See:  https://en.wikipedia.org/wiki/Bahtinov_mask  The mask causes any point of light to create a set of diffraction spike.  You get a pattern that appears to be an X shape with a vertical bar through the center if you are close to accurate focus.  But the vertical spike wont be in the center of the X if you are not at perfect focus ... it will be off to one side or the other (depending on if you are inside or outside focus).  You carefully adjust focus until all spikes converge at a common center point and that means you've nailed focus.
Longer Exposures
For longer exposure durations, consider a lens with a shorter focal length.  For example a 20mm lens would allow for a 15 second exposure.  An 8mm fish-eye would allow for nearly 40 seconds.
Much Longer Exposures
If you have a tracking head (properly aligned) then you don't have to worry much about stars elongating in long exposures.  The head is aligned so it's axis of rotation is parallel to Earth's axis of rotation (in other words it's axis is pointed toward the celestial pole).  By doing this, as Earth spins from West to East, the motorized tracking head spins from East to West at precisely the same rate -- cancelling out the motion of the Earth.  This means stars remain stationary in your lens' field of view and you can take very long exposures (I've done 8 minutes ... but could have gone much much longer).

The above example is an image I captured using a Canon 60Da with an EF 135mm f/2L USM lens ... but attached to a tracking head.  This allowed me to use much longer exposure durations.
Another advantage of longer exposures is it allows you to stop down the lens.  While everything is focused at "infinity" you'll find lens optical issues are much less of a problem at higher focal ratios vs. shooting wide-open.  The above image was shot wide-open and if you inspect the brightest stars (the three bright belt-stars in Orion on the left side (this is the lower half of Orion with the "belt" rotated on it's side) have tiny bat-wing shaped spikes.  This is sagittal astigmatism visible because I shot at f/2.  Had I stoped down even a small amount (say f/4) those issues would be much less noticeable.
You can build a tracking device (e.g. a "barn door" tracker) but today there are a few commercial products that are extremely popular.  Namely Sky Watcher makes a device called the "Star Adventurer" head and iOptron makes a device called the "Sky Guider Pro" head.  (I don't own either of these heads.  I have a model made by another vendor which is no longer in production.  But I mention the current models because they are popular.  I have no affiliation or bias toward or against either product.)
Comment: You should probably note that the 500 or 600 rule of thumb applies to images intended to be viewed at 8x10 display size from a distance of 10-12 inches. If one plans to pixel peep or view at much larger sizes, then the number must be proportionally adjusted. If we want to look at an image from a 35mm FF sensor enlarged to 16x20, for example, we need to half the number to 250-300 instead of 500-600.
Comment: Thank you alot mate will post some pictures after i find clear sky and good shot or atleast best shot i get this is the answer i was hopeing for explained in every detail thanks one more time
Answer: We focus our camera to image stars and all other astronomical objects by setting the lens focus to infinity (as far as the eye can see symbol  ∞). This will be the same setting as distant mountains or clouds etc.  When taking pictures of astronomical objects, because most are quite dim, we use a rather long shutter speed. Keep in mind, image sharpness will be compromised unless the camera is solidly mounted. Best use a tripod or other support.
One more thing, these objects have apparent motion. As the earth turns beneath the stars, they appear to move 360⁰ in a day. That works out to 15⁰ an hour. In 1 minute they appear to move ¼ ⁰. Even this tiny movement in one minute is enough to blur the stars. Astronomers typically strap their cameras to an electrically driven telescope. These motorized telescopes counter the rotation of the earth -- thus subject motion is neutralized. Take heart, you can make wonderful star pictures. Use a time exposure (keep the shutter open for hours to capture star trails). Your 10-20 second exposure will do nicely for what you want; just find a way to steady the camera.


How do you protect your camera when taking pictures in inclement weather?

Question: Title basically speaks for itself: How do you protect your equipment when you are shooting in severe conditions? Rain? Severe heat/cold? Snow? Fog? ... Other? What sorts of techniques do you use to produce great-looking photography when your camera (and all the expensive lenses, too!) are exposed to the elements?
Comment: Possible duplicate: http://photo.stackexchange.com/q/2386/21
Answer: Using weather sealed camera bodies and lenses helps, but each condition has its own issues.

Rain: in light rain you can probably get away without doing anything special, but in heavy rain, you'll need to wrap your camera in a plastic bag with an opening for the lens.  Dry it off well with a towel when you get inside.
Heat: Two separate issues... static and air conditioning.  In very dry climates (like the desert) you can have static buildup.  If you are using film, you might experience triboluminescent sparks which will partially expose the film.  Wind slowly.  Also when going in or out of air conditioning keep the lens cap on until the camera comes to ambient temperature, particularly in high humidity areas.
Cold: Your battery life will be measured in minutes, so you might have to take along an outboard battery for longer shooting sprees.  If it gets cold enough, your digital camera won't even function.  If you have film, it will become brittle so wind slowly and compensate for exposure.
Snow: Same as rain.  As long as your camera is dry there is nothing to worry about.  If you drop it in the snow, wipe it off with a dry towel
Fog: Nothing really needs to be done.  You'll experience the same issue as taking an air conditioned camera out in hot and humid air: condensation on the lens.  Fog can produce some fantastic pictures.  Wipe down the camera when you get inside, and clean the lens.
Underwater: Get a water tight case to put your camera in.  They're expensive, but open up the world of underwater photography.  NOTE: test the case with paper towels on the inside first.  You don't want to find out you didn't put the thing together properly with your expensive camera inside.
Sand storm: Stay inside or put a "throwaway" UV filter on the lens.  Nothing like pitted lenses due to the natural sandblasting going on.

The good news is that the professional line of cameras and lenses tend to be sealed against the weather.  They can withstand dust, rain, and minor contact with water (not underwater photography though).
Use common sense.  All electronics have operating conditions they are designed to work within.  If you exceed those conditions (extreme hot or cold) then the sensor won't respond or will be filled with lots of noise.  The good news is that most cameras have a wider range of use than you do.  All mechanical and film cameras tend to be able to survive even more extreme environments (hot and cold here) than the digital counterparts, but not without their own trials.  Too hot and the film will melt.  Too cold and it will become brittle and break in your camera.  Too dry and you have tiboluminescent streaks.  Too many XRays and you have weird sine waves across your film.
Comment: My favorite quote from that post: "Sand storm: stay inside."
Comment: I'd hate to have it happen to my face
Comment: Actually, I got sand blasted while on the world's highest sand dune (Cerro Blanco in Peru, 2.5km tall). I had a polarizer on and I had sand in it for months after that! My camera was a K-7 with weather-sealed DA* 16-50 lens so all I did back at the hotel was keep it under the tap about 30s. Another photographer had a Canon Rebel (not sure what model) and it died while ON TOP OF THE DUNE in roughly 5 seconds! I heard Canon Peru charged $150 to take it apart and put it back together minus the sand.
Comment: I got snow blasted years ago (sand blasting with snow...). My glasses were horribly pitted and had to be replaced (to say nothing of my face). I'd hate to have this happen to a lens.
Answer: If you are shooting in inclement weather, the best thing you can do is use a camera that's built for it. Canon's 1D and Nikon's D3 have weather sealing that protects the camera's circuitry. This sealing explains part of their cost. The sealing doesn't make them submersible, just resistant.
For inclement weather of the sort you often get when shooting soccer, I have a rain cover for the camera. It has three ports. One which fits with a velcro strap and vinyl sleeve to the lens, and two with drawstring vinyl covers for my hands. It makes shooting more difficult but shots of players with falling rain and flying mud are absolutely worth it. My rain hood cost AUD$100.
There is no shortage of waterproof accessories available from camera stores. Head on in armed with a fat wallet and let the sales staff fit you up.
Comment: yes, weather sealed cameras are your best bet. To your list you can add the Pentax K5 and K7 which have effective weather sealing.
Answer: Moderate dry Coldness is not such a problem anymore ... for the camera:
I carried my D90 in Val Thorens/Valloire (skiing resorts in France) several hours on the slopes  (-20/-10 degress Celsius). Carry a spare battery, change if the one in camera becomes too cold and weak.
The problems starts when changing temperature/humidity. Enter a restaurant and you'll have your ice-cold camera covered in condensation in seconds. If you don't have a towel ready the camera won't dry before you leave, resulting in ice-frosted lens or filter. Bonus for a filter: you can remove the frozen thingy. Maybe the ziplock bag will help there.
The other problem is the one you'll have handling the camera with numb fingers. DSLR can be handled with thick gloves, but not always gripped tight enough. Be sure to carry thin undergloves.
Don't know if there were some good-looking pictures though ;)
Comment: +1 on the battery issue in the cold. My old 5D ate batteries like it was candy in -15 to -20 deg C. Carry spares (multiple) and keep them warm. The 1Ds series batteries may be more robust in this respect (if not, the required extra batteries are going to cost an arm and a leg!)
Answer: I use a large ziplock bag. Cut a hole in the bottom and poke the end of the lens out. Secure it with rubberband. Alternatively you can use a small umbrella.
Answer: A quick and free way is to use - wait for it - a shower cap!  Use the clear ones that you get in most hotel rooms.  The elastic rim keeps the plastic securely around the base of the camera so that any water falls off when it gets to the bottom rather than get onto the equipment.  These things are small enough to squish into your back pocket, ready to be whipped out and put into action at the first sign of rain!  If you're brave enough, shoot through the plastic for a different look!
Got me a good collection of the little boxes from hotel rooms whenever I travel and never leave home without one/two always in my kit bag! :-)
Answer: Just to add to the above -- keep a stash of silica gel dessicants in your bag, next to your lenses and the camera body. Very handy to absorb moisture.
Answer: Mostly I don't bother too much unless it's a particularly inclement environment.  I use good quality bodies and lenses that are designed to resist some moisture/dust/shock/heat/cold etc.
For particularly wet environments, I use a Dicapac waterproof case, although quite often it's more for transport purposes than for use while actually shooting.
Answer: For shooting in the rain, I bought an extra lens hood for each of my lenses. Then I glued a plastic bag to each lens hood, so that I could cover the entire camera with the bag, without having to futz about with getting the lens hole just right. When I'm not using the modified lens hoods, I fold the plastic up inside them and keep them in my bag, ready for quick deployment.
Answer: If it's just lightly raining or if it is snow I don't worry about it too much.  If I'm going to be out for a long time I'll use a small garbage bag with a hole cut in one end & a rubber band to hold it in place.  I have also used these cheap rainsleeves; they fit better than the garbage bag & look slightly less ghetto.
Also, don't forget that cloth diapers are a great way to dry your lenses!  It's probably more important to keep the front element dry than the camera body... Not because it'll cause damage but because water on the lens will distort your pictures!
Answer: If you're into DIY, maybe try this for example...
Answer: As stage photog for a outdoor music fest, I've run into a lot of inclement weather. Tyler's answer is great if you've planned ahead. Similarly, I've used the plastic bag (gallon freezer bags are excellent, as are grocery store bags) approach in an emergency by tearing a hole in the bottom and sliding it over the lens and holding it place with the lens hood. The bag opening allows space to hold the camera while keeping it dry. If the bag extends slightly into the edge of the frame. I zoom out a shade to avoid vignetting the subject.
Answer: I made a harsh-environment cover for my cameras with a baggie and an old UV filter cemented into the side of it. Works well. It even completely protected my camera when it accidentally slid off the seat of an old and leaky crab-fishing boat into a foot of water one time. When it comes time to replace the baggie from wear-'n-tear, there's no huge cost involved. When making one for yourself, put your largest camera and lens in the baggie to discern the best location to mount the filter so that all controls and viewfinder options are readily accessible through the plastic. You don't want, for example, any printing on the baggie being right over your LCD or EVF. Word of advice, few sealants and cements will adhere to the poly plastics used in sealable baggies. The only one I found to work well is a product by Loctite called Stick'n Seal.


Demo Reel Etiquette?

Question: Hey guys, I was just wondering while I'm starting my demo reel for video games if I am allowed to cut up different trailers for my sound design demo purpose without running into legal problems?
Answer: Well, if you mean cutting up trailers and doing a sound re-design on them, you'll want to tag them as a sound re-design so as to not be confused with the real deal.
If it's trailer's that you've actually done, I believe it should be fine since it is your own work and being shown for demonstration purposes.  In my reel I make sure to tag the production company and post sound house that actually secured the film (since I'm an independent contractor, not all the films I work on are strictly through my own company).  That way I'm properly crediting the involved studios and not making an ill "political sidestep, however unintended it may be - something as seemingly mundane as giving credit where credit is due can actually go a long way.
Answer: Ideally you want to showcase original work (i.e. product originally had your sound, not someone else's). It's then far easier to assume you have the right to showcase the work if you haven't explicitly requested so. Realise this is a catch 22 but it also means keeping a pretty tight focus on what's included in your reel. Over time, with some luck, i'm guessing you will have less and less time to devote updating it...
Always credit whoever you worked with, and whom it was for. Be very clear about labelling the clips where you substituted the original sound with yours. Seeing that the reel is to showcase your abilities, i doubt anyone in any legal department will lift a finger. That said, asking for permission is common sense, and so is assuming you won't get it, especially on a AAA excerpt. Basically, find an indie developer and do a project. Like, right now.
Finally, trailers are slightly on the cinematic side. I suspect more often than not you will be judged on your potential to contribute to the interactive and way more programmatic in-game experience.
Answer: Interesting,
When I posted an audio show reel a few months ago for feedback this matter was raised by one of our SSD mates. The issue was because I had included a clip of re-designed sound for the Ratatouille animation. According to one of the answers I got, it is not permitted to use any type of film material with out the express authorization of its owner in publc forums. How ever the more I ask around its seems to me that copy-right law is a pretty subjective matter in many situations.
I think this is an interesting one...
Answer: I recommend MTV style lower 3rds with your title/role in the production and credits to the co-mixer and/or co-designer/editor for the beginning of each scene included.  I have also included a credit list with this info to accompany my reel as well.
I have actually gotten a gig or two based on someone who saw my reel and knew someone I had worked with.
Note: I always ask everyone I credit.  I have never had anyone say "no."  They almost always are happy to hear that I am giving them the respect and advertisement.

Demo Reel Etiquette?


Why were the first Universities created?

Question: I have read through my world history book and did some research online, but I have been unable to learn why the first Universities were created?
Comment: https://en.wikipedia.org/wiki/University
Comment: I researched this for a module at university and my conclusion was that they were for the sons of the middle classes. In order to have a middle class you need to get away from a feudal landowners + peasants framework first. Italy was the first European country to move towards a city-state model, creating a middle class, and that's why many of the earliest medieval universities appeared there.
Answer: University: from Latin universitas magistrorum et scholarium ("community of teachers and scholars)"
The university (as we know it today) was not created ex nihilo but rather grew out of something else, which was the pursuit of learning in urban centers among those who could afford it.
European higher education as far back as the 6th century was often found in Christian cathedral schools or monastic schools.  Monks or nuns were the usual instructors.  An example of one such school that later transformed into a university is the University of Paris.  But the Church was not the only source of higher education.  There had been a tradition for centuries of private tutors for the children of those with the means to afford them, and of course the opportunities for a select few to study with famous scholars from which "the Socratic method" has been passed down to us.
The University of Bologna was founded in 1088 and is arguably the oldest university in Europe.  It was established as a guild/group of students who were learning civil and canon law, and found it in their interest to form a society/guild for other reasons.

The University arose around mutual aid societies of foreign students
  called "nations" (as they were grouped by nationality) for protection
  against city laws which imposed collective punishment on foreigners
  for the crimes and debts of their countrymen. These students then
  hired scholars from the city to teach them. In time the various
  "nations" decided to form a larger association, or universitas — thus,
  the university.

The "why" in this case was as much protection from political bullying as advancement of education.
In contrast we can look at the University of Naples, founded in 1232.  It is the most ancient state-supported institution of higher education and research in the world.
A university founded by a head of State was slightly different from the guild/corporate model with the same general purpose.  The Emperor Frederick II created this university with the hope of curbing or opposing universities of Northern Italy (such as Bologna and Padua) which he felt were either too independent or under the strong influence of the Pope.
The "why" in this case was politics as much as it was the advancement of education.

The independence was granted by the Charter, which gave the Emperor the highest authority. He hired professors, who would become royal employees paid through royal funds. Moreover, the Emperor himself examined candidates and conferred degrees.

(It is somewhat ironic that Thomas Aquinas was one of the early distinguished graduates of the University of Naples, given the ideological tension between the clergy and that Emperor.)
Answer: During medieval times, the only thing close to what we would call education was carried out by the church. Usually by monks and nuns, studying religious texts and similar items. Some of these schools tended to be much better at this than others, and essentially this gave the opportunity to those more gifted in academia to study further and it became more than just learning to read and write, it became more about study of the texts and what they could mean. The exceptional schools then slowly transformed into what were known as universities.
This all grew in Europe, and eventually they started coming out of the hands of the church. They started to be created by royalty. With things like The rediscovery of Aristotle's work, there were plenty of young men who yearned to learn about things like mathematics, astronomy and literature. These students didn't always study religious texts or similar (although it may have played a part), it was more about the study of new and exciting things like science and engineering. This is arguably where the modern university culture started.
if you want more info, the wikipedia page on universities has some good examples of universities created around this time, but obviously remember it is wikipedia, and its just to get the idea of the world at that time: https://en.wikipedia.org/wiki/University#Medieval_universities
Answer: The USA modelled it's education system on Germany's (started with Kindergarten of course) and then ending with your "Gymnasium" or PhD.
The first Universities were in Upstate New York (Cornell, Colgate) and were "land grant Universities" dedicated to the study of Agriculture.
This they did very well...
Comment: Note the presence of the "middle ages" tag. Note the absence of any "...in the USA" qualifier.
Comment: William and Mary, and Harvard, seem to have arrived before Cornell and Colgate ...
Comment: You got me on that. "The first practical schools of learning were in Upstate NY." Everything else was about establishing a Social Hierarchy.


How bitcoin gets monetary value

Question: This looks like a complex thing but I am trying hard to understand it. Why would any one in the world honor a crypto currency till it is backed by a government with monetary value? How does a crypto currency get its monetary value? Who will back it?
Comment: What do you mean "backed by a government"? Can you give an example?
Comment: Spelling note: "monetary", not "monitory".  I edited for you.
Comment: @JBaczuk I am trying to get the answer for the following example. Assume I have a bit coin and want to buy a product from a marchent. Why would he sell that product to me for a bit coin when he bought it from the supplier  for  dollars.
Answer: Currencies get value from demand and supply not from governments backing them. Currencies have existed long before government's started stamping them out in their own name. The demand for a currency comes from the utility that it offers as a medium of exchange. Even pre-historic humans went out of their way to create currencies because they were useful.
Bitcoin has value because it is useful as a global, irreversible electronic medium of exchange and store of value. That is why it is in demand.
Answer: Bitcoin's value is given by the merchant who is willing to exchange it for something of economic value to you (goods, services, etc). These persons who are willing to trade with you for Bitcoin are considered the "economic majority", as they decide which Cryptocurrency definition they are willing to trade value for. The economic majority is the group of persons backing Bitcoin or other accepted Cryptocurrencies.
The US dollar value in contrast  is guaranteed by legal-tender laws: Dollars are always accepted for paying taxes, and that reflects a value that will always remain, even if the entire economy decides to stop accepting the US dollar. Dollars are NOT backed by gold.
Comment: Neither Bitcoin or USD are backed by anything. That term has a specific meaning, namely another valuable held in reserve with a guarantee of exchange. Just because certain activity results in a value does not imply it is backed by it
Comment: I should write "guaranteed value" instead of "backed". Clearly there is no reserve nor guarantee of exchange for reserves held. However, state guarantees value of USD by accepting taxes paid with such. That is a guarantee of value, though not backed by reserve. Bitcoin's only has a known present/past value. Future value is not guaranteed and unpredictable.

How bitcoin gets monetary value


Is the presence of a rabbi required for an orthodox wedding?

Question: Is it possible for two Orthodox Jews to be married by anyone other than a rabbi?
Answer: Yes, but it's not recommended-
The "orchestrator" of the wedding makes sure nothing goes wrong.
The sages say that anyone who does not know the marriage laws well should not get involved with them (kiddushin 6a), as many mistakes can result.

פרש"י : בטיב גיטין. בהלכותיהן : לא יהא לו עסק עמהם. להיות דיין בדבר שמא יתיר איסור ערוה וזהו עיוות שאינו יכול לתקן

Rashi there: Shouldn't have any involvement, as he may come to allow a prohibited relationship
Although R' Yaakov Kaminetzky in Emes L'Yaakov reportedly (it is published מפי השמועה) allowed a grandparent or close relative to conduct the wedding

מותר לסבא או קרוב שכיבדוהו לסידור קדושין לקבלה אע"פ שאינו בקי בעניני קדושין שאם אירע איזה ספק בדין אפשר להתקשר למורה הוראה בקלות

this is usually understood in light of the Shvus Yaakov (3:121)

ובודאי לא בחינם הורגלו דורות הללו שאין מסדרין קדושין בלתי התרת הרב כי יש לחוש להרבה מכשולים שיבא לקדש ח"ו איסור ערוה או שניה כאשר קרה בזמנינו
It was not for no reason that the generations have not assumed the role of marriage conductor without the allowance of the Rabbi, because there are many issues that can arise of forbidden unions (translation mine)

that this is specifically for a relative who knows the family and knows if there are any forbidden or prior relationships, and if anything "strange" happens a Rabbi could be called.
As an aside, the "orchestrator" does not have to have the title "Rabbi" and does not have to have a piece of paper saying he received ordination.  He just has to know all of the laws, which is possible without being titled "Rabbi."  However, as a general rule, today those who would qualify as having the prerequisite knowledge are Rabbis.
Comment: In the UK, they have historically had a sort of lay-clergy, styled "reverend" (such as Simeon Singer, of siddur fame) who may serve in the position typically applied to a pulpit rabbi, such as at weddings and funerals, or to make a drasha on shabbat. On the other hand, he cannot serve as a dayan or a posek.
Comment: A teenage friend of mine was asked to be part of a minyan at a small wedding...he declined to be an eid, but was made mesader kiddushin instead!
Comment: @Shokhet Yeah, that's not a good idea
Comment: @YEZ Right...forgot to mention that the rabbi in charge put him there...since in any event there was rabbinical oversight from someone else, mesader kiddushin there was just a glorified MC ;)
Answer: In a nutshell: in U.S. state laws, a couple is "married" by a clergyman. (Alternatively, they could go to a justice of the peace or the like.) The laws vary from state to state as to what's called a clergyman; for instance, in New York City the congregation would sign a letter stating that he is "a pastor or associate pastor."
From the perspective of Jewish law (Orthodox), the marriage happens when he gives her something valuable that he owns (usually a ring) in front of two "kosher" witnesses. The witnesses need to be Orthodox-observant men, so often at a wedding with few Orthodox attendees, the rabbi is also one of the witnesses.
About a thousand years ago people were making all sorts of mistakes regarding weddings (e.g. using a borrowed ring, or related witnesses), so a policy was enacted to have a local rabbi present -- just to make sure everything is being done by the book. Thus the practice today to have an Orthodox rabbi "officiating" -- just making sure everything is done correctly. If a couple got it all right and no formal rabbi was there, the wedding would still be valid. (Though from a US state perspective, they'd have to go to a judge -- or a formal rabbi -- to sign their papers afterwards.)
Answer: I've attended many Orthodox weddings where, though rabbis may or may not have been present, they did not conduct the service.
I've personally never heard of a wedding requiring anything but any one of the following with two kosher witnesses present:

exchange of an article of estimable monetary worth
exchange of a contract
some time alone for the bride and groom

In a Jewish context, the officiant doesn't "marry" the bride and groom - the groom "buys" the bride and the bride accepts.  Two witnesses also sign a document stating that they saw it happen.  Then the bride and groom go off to the yichud room to spend some time alone -- their aloneness witnessed, usually, by two different witnesses.


How exactly, or whats the process, rather, of energy changing into matter?

Question: $E=mc^2$ this is the equation by Einstein claiming energy can change from energy to mass. this would happened at the big bang I assume, when electrons and protons were made to create hydrogen and some helium.
Comment: Related: http://physics.stackexchange.com/q/43879/11053, http://physics.stackexchange.com/q/10889/11053
Answer: The law of conservation of energy states that the amount of energy doesn't change.
This is basically a definition of energy. That is, if according to your current definition of energy, there is some disappearing or appearing , that means your definition is not complete enough.
Einstein equation $E=mc^2$ means that mass is a form of energy. It's not energy changing into mass, it's a given form of energy changing in another form that is mass.
It can be observed when two particles with high velocity collides. Particles are created during collision, whose mass includes the former kinetic energy.
Comment: but wasn't the big bang made of just energy? because ik that atoms came from stars except for hydrogen, it was made when the the universe expanded and got cool enough for protons and electrons to combine... by the way, im only 14 so im not as smart.... >.<
Answer: The equation $E =mc^2$ describes the mass–energy equivalence in relativity. It indicates that mass and energy are indeed the same thing, they are convertible.
One way to convert matter into usable energy is to annihilate matter with antimatter. And the energy released is proportional to the mass as the equation $E =mc^2$ shows, which means it is very efficient.
The common but less efficient ways to convert matter into energy are nuclear fusion and nuclear fission.
Nuclear fusion is a type of nuclear reaction. It is the process of colliding two or more atomic nuclei at very high speed and joining them to form a new type of atomic nucleus. The first man-made nuclear fusion on a large scale was carried out on November 1, 1952, in the Ivy Mike hydrogen bomb test.
Sun generates its energy through a fusion process called proton–proton chain reaction, which converts hydrogen to helium at a very high temperature.
Nuclear fission is the process in which the nucleus of an atom is split into smaller parts. It can be either a nuclear reaction or a radioactive decay process. It is commonly used in nuclear power plant to produce nuclear power.
You can read more about the mass–energy equivalence here if you like.


Is it possible to select the root partition at boot time using GPIO pins

Question: My Raspberry PI has 2 identical (kind of) root partitions. I would like to be able to select the other root partition at boot time using a GPIO pin in case I brick it.
Answer: This answer does not address directly your question but it may solve your problem. After reading all suggested solutions it seems not to be simple task to have a save second boot option. A broken system may also affect the second boot option.
If you really want to have a fail safe system I simply would provide a second Raspberry Pi. This can be full tested and ready to run. It should not be a big problem to advise a person by phone to just swap the RasPis.
Comment: In that case I'd throw in a second power supply and install the second system in a different location. That would cover failures affecting the first location, such as power grid blackouts and problems with the network provider.
Answer: The odds of having a "bricked" system will be a product of the probability of a critical problem (P1) and the probability of the recovery mechanism not working as planned (P2). Designing a recovery mechanism lowers P2 (P2=1 if no recovery mechanism is in place), but at the same time it could increase P1 as your setup becomes more complex. As such, a recovery mechanism is only beneficial if the product P1*P2 becomes lower in the end.
As such, you should try to minimize P1 as much as possible before you even start thinking about recovery. Using a good SD card with enough spare space (I'd go for as much as 100% reserve, i.e. if your system takes up 10 GB, I'd use at least a 32 GB SD card). Make sure journaling is enabled on the root partition. Make sure there are no optional boottime activities which could prevent the system from booting (e.g. entries in fstab without the nofail option). If the network connection is via WiFi, I'd consider having two NICs (e.g. built-in adapter + USB dongle). If there are multiple wireless networks to connect to, I'd configure all of them (with priorities) in your wpa_supplicant.conf. Only then I would consider working on the recovery features.
If there's someone around your Pi who you could ask to to "fix" is as long as the fix doesn't involve anything complicated, then a spare SD card could be an option. Asking someone to swap it with the one in the Pi over the phone is not much more complicated than pushing a button. This has the advantage of lowering P2 without increasing P1.
Another idea: you could write a pair of SystemD units: the first one runs as early as possible (right after local-fs.target) and edits cmdline.txt to use the spare root, and the second one runs at the end (after multiuser.target, or perhaps even inside a "graceful shutdown" script you'd have to call manually) and rolls back the changes. That way, if a critical problem appears between the two, you'll get into your spare system on the next reboot. Enabling the watchdog could facilitate the transition, or you could just ask your remote proxy person to reset the system. However, I would only implement something like this after an extensive testing proving that the system works like a clock, in both normal scenarios and in cases of failure.
Answer: AFAIK it is not possible "to be able to select the other root partition at boot time using a GPIO pin" BUT it is simple to change the boot partition by editing cmdline.txt.
Comment: Thanks, but I'm gonna be hundreds of miles away when I need it to boot from a different partition. I can ask someone to push a button over the phone, not to edit `cmdline.txt`.
Answer: This doesn't answer your question directly but may be a good workaround for you. A Raspberry pi 4B can easily be made to boot from a flash drive. Once setup that way, if you leave the original SD card in, it will boot to the SD in the event that booting from the flash drive fails. I've only tested this on a 4B but some literature clams that a 3B will do it as well.
How to Boot Raspberry 4 from USB
Comment: Thanks, but adding a flash drive is not an option.
Answer: Its been possible for a while to configure the boot order of the PI.. So you can tell it the order of SD, USB and Network boot.
In the case of boot order you could remotely change the boot order than then reboot.
In the case of network boot what the PI boots is decided by server your connecting to over the network.   At which point its up to you what root filesystem you mount.
Which means in either case you could control what gets to be boot and root.
Bootloader Config
The network boot instructions canoe found here..
Network boot
As a bonus I notice there is a 'Wake on GPIO' boot loader option so might be a segway into your original ask.
Comment: I've also added a new link so both the official Raspberry PI instructions for network and boot order are detailed..


How do French people chant for sports, like how the English say, "Go! Go! Go!" ?

Question: What do the French say when they are chanting at a sporting event? In English, people might say, "Go! Go! Go!", or chant the MVP's name/number while clapping rhythmically. What's a substitute for,
"Go! Go! Go!"?
Answer: In France, chanting is usual at soccer games. There's a lot of different songs depending on the club and players.
Here are some classics:

"Qui ne saute pas n'est pas français" (for the French national team, but the end can be changed to whatever team you're supporting, eg "qui ne saute pas n'est pas marseillais") - meant to encourage people to jump in place to produce a giant rumble
"Allez les Bleus" (again this would be for the national team based on the jersey color, but can be used for other teams by matching their jersey colors)
Victory chants: "On a gagné" (for any victory), "On est en finale" (for a qualification for the finals of a tournament) and "On est les champions" for a championship win; "Et 1, et 2, et 3-0" for a large goal difference.

But really there are many club specific chants too, and sometimes the name of a particularly outstanding player will be chanted.
Here you can listen to some of them, as well as more recent, less popular creations just for this year's world cup: https://youtu.be/NdLRKQpdgUU
Comment: Soccer ? What's soccer ? Hmmm... very common in rugby too. However, most of them being... bawdy songs... Any non rugby-enthusiastic mod would blame me for singing them here! ;) No kidding! there is such a huge collection that the ASM sups are compiling them into an encyclopaedia.
Comment: @aCOSwt I'm less aware of the rugby ones. There aren't any chants at tennis games...
Comment: lol! In tennis ? Hmm... singing is not the fact of the jet set. Even when the jet set is made of singers ); Well more seriously, you are correct, no songs in individual games.
Comment: Some other common soccer chants in France and other French-speaking countries: chanting the name or the nickname of a player (but not the number - at least I have never heard that),eg "ZI-ZOU, ZI-ZOU" (Zidane's nickname), chanting "et 1, et 2 et 3-0" if that is the score (this became popular after France's 3-0 victory over Brazil in 1998), chanting "on est chez nous" (esp. sung by visiting supporters, as some sort of provoking the home supporters).
Comment: And let's not forget "I will survive"... :-)


How to add Newsfeed webpart to SharePoint 2013?

Question: I cannot see it anywhere: neither in Edit -> Insert -> Web Part, nor Edit -> Insert -> App Part. Do I have to enable a certain Feature to be able to use Newsfeed?
Comment: Here it is: Insert > WebPart > Social Collboration > Site Feed.
Answer: Yes. You need to activate Site Feed feature in Site Settings --> Manage site features. Then, when adding web part, under Site Collaboration group you will have the Site Feed WebPart
UPDATED
Yo must activate this feature in _layouts/15/ManageFeatures.aspx
Comment: See the answer from Anders Austad- the Site Feed feature needs to be activated in the site collection root web as well, for some reason. Doing this got the Site Feed web part to show up for me in my sub site.
Comment: I still dont see the option to add news feed. I have already activated the site feature.
Comment: The feature is called "Site Feed"
Comment: I don't see it either. I'm on a Team site and the Site feed webpart is on the default home page. But I cannot see it under Social Collaboration or any other category
Comment: I updated the post with a picture
Comment: Hmm, I am at the top level and enabled the feature, still the part is not showing up. I am just going to have to put something else there. But I am on SharePoint.com.
Answer: As stated by others: The "Site feed" feature needs to be activated on the site where you are trying to add the Newsfeed webpart.
But there is one very important gotcha: The "Site feed" feature also needs to be activated on the site collection root site if you are working on a sub-site. I found this out by accident a while ago, went back and checked it now on SP 2013 SP1, and the issue has not been fixed. You still have to activate the feature on the root site.
Comment: Current SP-Online as of today -> still need to activate "SiteFeed" on Site & rootSite !
Answer: Just a clarification - This is NOT an issue but just the way SP is designed.
Essentially allowing for more granular control across your site.
Features need to be enabled top down (Application all the way to the subsite level) Only then the feature is available. Still needs to be turned on - does NOT get turned on by default on all sites (understandably as you would not want features turning on by default across the few hundred sites/subsites you may have).
Answer: Just download the sitefeed webpart: site settings\ under web designer galleries go to web parts. if you do not see this go to top level site collection and go to web parts\ look to see if you can find site feed web part, if you do download it to desktop, go to site you want site feed webpart, edit the page, click on the zone where you want to add a webpart, insert a webpart, under categories upload a web part and browse and upload, you can add now. if you do not see the site feed web part in the gallery click files, new document, new, now you can check the box for site feed web part to add site feed webpart to the gallery and you can do rest by following above instructions.
Answer: Go to Insert, under categories scroll to Social Collaboration, under Parts click Newsfeed and add.
Comment: It clearly states in the question that the webpart is not visible here


Impact of a non-uniform distribution in a secret sharing scheme

Question: When it comes to the secret sharing schemes, it seems that the random numbers need to be drawn uniformly. Is there any difference between the random numbers from uniform distribution and the non-uniform distribution? Do different distributions affect the security of the scheme?
Answer: Just to add to the other answers, you may also want to take a look at leakage resistance secret sharing, for example this work.
The idea is to study the following question:

What is leaked about the underlying secret when the adversary gets some leakage from the shares?

This turns out to be a very interesting question, and in some sense, changing the distribution can be seen as "leaking" information since some shares may become more likely than others.
Interestingly, in some cases, even a 1-bit leakage can completely break security.
For example, if you use Shamir's secret sharing over $GF(2^n)$, then leaking the first bit of a large enough amount of shares will leak the first bit of the secret! So if you have a distribution that generates shares for which it is more likely that the first bit is, say, $1$, then you will know something about the secret.
Answer: The distribution for random values is extremely important, and using a non-uniform distribution can certainly affect the security of the secret sharing scheme (or, really, any cryptographic operation).  How a random number from a non-uniform distribution affects the security depends on the specific algorithm/scheme being used.
A non-uniform distribution always implies a bias so that some values are more likely than others.  At a minimum this would help an attacker that knows the bias by helping them to optimize a search for the more likely values first.  (e.g. If half the time the value comes up all zeros, I'll guess try that first.)
Depending on the underlying operations, the biases in the non-uniform distribution could also allow for specialized attacks on the underlying cryptographically hard problem that wouldn't apply in the general case.  The biases might not lead to an practically exploitable, but would only undermine some proof of security.  (However, holes in proofs that are "only theoretical" today often turn into practical attacks later.)
Comment: Thanks very much for your detailed explanation. Regarding to the additive secret sharing scheme, it simply splits a number into several shares. If the random values are drawn from the uniform distribution while the original numbers have some distribution, e.g., normally distributed, will the original distribution not be leaked?
Answer: You ask in a comment to the first answer: "If the random values are drawn from the uniform distribution while the original numbers have some distribution, e.g., normally distributed, will the original distribution not be leaked?"
Consider $GF(q)$ and let $X$ have an arbitrary distribution while $U$ has a uniform distribution.
If you add randomness by means of an independent variable $U$, uniformity is preserved.
This is because for any $a\in GF(q)$
$$
\mathbb{P}[X+U=a]=\sum_{b\in GF(q)}
\mathbb{P}[X=a-b]\mathbb{P}[U=b]
$$
$$=
\frac{1}{q} \sum_{b\in GF(q)}
\mathbb{P}[X=a-b]=\frac{1}{q}.
$$
In $GF(p)= \mathbb{Z}_p$ the map $x\rightarrow ax$ is a permutation for $a\neq 0.$ So, in the Shamir setup, one can derive uniformity for monomials, and thus for polynomials by adding monomials using the result above, if the coefficients are chosen uniformly from
$\mathbb{Z}_p^{\ast}$. Since the missing monomials (when $a=0$ is chosen) don't appear in the sum defining the polynomial, this is not a problem.


Comparing values of indicator between different stocks

Question: I would like to ask whether there is a good way of analysing stock indicators that have no value limitations. For indicators like RSI we have a closed range ( 0 - 100 ) but in case of indicators like for example OBV we may have two stocks that have 10000000 and 1000 value of this indicator respectively in the same period of time. Does it make sense to discretize values of such indicators, by creating the same range (for example from 0 to 100) for each stock or it is completely senseless?
Comment: Thinking about this concern from a more statistical POV may help. For example your statement about RSI having "a closed range" can be expressed in statistical terms as "RSI is standardardized to fall in a range between 0 and 100." Similarly for the OBV values of the two stocks -- expressing their OBVs as a *rate per* something would *normalize* their values, thereby making them comparable between stocks. @michaelhartman (below) has suggestions regarding how to best do that.
Comment: A common trick in Statistics (actually the foundation of Nonparametric Statistics) is to replace the observations by their rank. So you might look at the cross-sectional rank of OBV rather than OBV iitself, and fr example you might buy stocks with "OBV in top quartile" rather than OBV above a fixed value.
Answer: Technical analysis is not quite in my wheelhouse, but it's been an interesting topic to me, so hopefully I can lend a hand. Let's start with some basic assumptions: Because OBV is based on volume, there is obviously a huge range as you've pointed out. This makes comparison straight across companies impossible.
To compare companies, you need to take out certain factors that could affect volume. My first inclination would be to adjust for the size of a company. Common markers of size would be revenue (REV) and market cap (MKCAP). I wouldn't have much hesitancy in using size as an adjustment for any company data.
Thinking through it a bit more, because OBV is often seen as an indicator of how "smart money" is moving in an economy, you might want to adjust for price, as small investors will be slow to buy really expensive stocks and quicker to buy cheaper stocks. You may want to rethink this for other indicators, but it is an option.
Looking at these two options I'd be more inclined to adjust an indicator for size. This way you can look at companies across a range of sizes and sectors. This might work better for some indicators and worse for other. I don't know how well these would work but it has sparked some interest for me, hopefully it helps you find what you're looking for!


No DNS Resolution

Question: I am told by our server admins, that our server has no DNS resolutions because they are under tight PCI DSS regulations for outbound data. I don't quite understand this because the server is used to host a website which all can access from the URL, but when I logon via Remote Desktop onto the server, I can't use the internet from the server i.e it can serve data but can't send outbound data. How is this possible? How can it have access to the internet by sending http data to and fro, but can't resolve DNS names?
Answer: Generally in this situation the firewall is configured to do a couple things:

Only allow inbound connections on specific service ports (HTTP and HTTPS).
Only allow outbound connections that are related to inbound connections (ie. the response to clients' HTTP requests).  In many cases the DNS requests are explicitly blocked.

This prevents the server from making its own connections to other IP addresses, including preventing connections to DNS servers for the purpose of DNS lookup.  This is one way of stopping malicious code on the server from transferring data elsewhere and from downloading other data.  It also stops server administrators from being able to 'browse the web' on the server, which they definitely shouldn't be doing anyway.
Comment: thanks so much guys, voted this one as it was a bit more clearer
Answer: What you describe can be achieved by stateful firewalling.
When an inbound TCP connection arrives the header of the initial packet has the SYN bit set. This indicates that it is the first packet in a new TCP stream. Now should the firewall permit it in, for instance to port 80, then a "state" is created and applied to all further packets in the same stream. Any packets matching this state will be further permitted by the firewall which includes responses being sent from the server to the client.
The tracking of UDP and ICMP connections is a little more fuzzy because fundamentally they are connectionless. They don't contain any of the stream information found in a TCP session. But it works more or less the same way.
With that in mind, your outbound DNS lookups are being firewalled off because the connections are initiating from the inside. Whereas HTTP traffic is initiating from the outside and being permitted in by policy.

No DNS Resolution


Add Bitbucket button to webpage

Question: It's easy to add GitHub buttons to webpages as a way to link to your profile.  Is there an analogous button for Bitbucket accounts?
Answer: The short answer is not exactly.
It highly depends on what you want you want to do. If you need the current numbers like shown in https://ghbtns.com/ or you want the current user to create a fork without being redirected to Bitbucket website, you could send a POST request like explained in the fork api reference.
If you just want a button that looks and works approximately the same that you linked to, just place a normal bootstrap button and link it to your Bitbucket repo. For example if you want the "fork" link, get it by right clicking on "Fork this repository" menu item shown below and selecting "Copy link location/address".
 .
After the button works you'll probably want to style the button and the icon to look similar to a GitHub button. Note that Bitbucket has no "Star" function like GitHub.

Add Bitbucket button to webpage


How thin/worn out can you allow road bike rims to get?

Question: My mechanic measured the rim off my front wheel to be 0.8mm thick and recommended to get a new wheel. How thin can a rim get until it gets very dangerous to keep riding it?
Answer: 0.8mm is very thin and dangerous. The mechanic is right to tell you to replace the rim! Keep it only if you want to play with your life because it may fail in a catastrophic manner! More dangerous even on a front wheel.
Many rims have wear indicators, usually a couple of holes along the braking surfaces, that tell you that the rim is over the limit once one of the holes becomes invisible.
Comment: Gotta be honest, an indentation that is _no longer there_ is a terrible way to indicate wear! It would be much easier to spot (though harder to manufacture) if they machined the hole from the inside so that it became _visible_ once you've hit the wear limit. (Note: not mad at you, just amazed at this methodology!)
Comment: @FreeMan: That would be a very dangerous thing to do. There would be a hole opening on the braking surface while you are riding (even braking) which would certainly cause an explosive blow-out of the inner tube and with tubeless tyres an instant flat, making the bike uncontrollable. Especially if it happened at the front.
Answer: I don't know exact numbers, but you will know when you need to replace your wheel when the rim starts to have a concave feel to it when you run your fingers over it (I'm talking about aluminum rims here). An extremely bad case will look something like this:

Don't let your rim ever get here, but this is a good example of what your rim will do if you let it go. Letting it go this far can lead to a pretty nasty crash, especially if it's your front tire. It's basically like riding on a ticking time bomb, as the wheel could give out any second.
You can definitely feel when your rim starts to become concave before you see it, and when you do feel that, it is time to replace your wheel.
On some wheels, there is a little hole called a wear indicator hole. I'm not sure if you ever noticed it on your rim, but if that gets worn away and you can no longer see it, it is also time to replace your wheel.
Comment: You didn't mention explicitly but it's a time bomb both because the rim could collapse and because, if the surface cracks it could catch the brake pad and cause an instant deceleration. *OUCH*.


What does hedyotot mean?

Question: I was recently discussing the prohibition of studying "shitrei hedyotot" (loosely translated as secular studies). But when looking at a debate of the Rabbis in Talmud Bavli Sanhedrin 21:b-22:a the word hedyotot is there but it's referring to Samaritans, who are definitely not secular. So what does hedyotot actually mean? Is it a derogatory word?
Comment: I wouldn't translate Shitrei as "studies". Documents seems more like it.
Comment: "secular" is relative.
Comment: http://cal.huc.edu/oneentry.php?lemma=hdyw%2B%20N&cits=all
Answer: Rashi (Shabbat 116b s.v. shtar hedyotot) describes it as page of [business] calculations or a letter regarding a lost item:

שטר הדיוטות - כגון של חשבונות, או איגרות השלוחות למצא חפץ.

Elsewhere (Shabbat 149a) he describes it as business documents:

כל הני - שמא יקרא בשטרי הדיוטות של מקח וממכר קאמר.

The Yerushalmi Megillah (3:4) apparently understood it to even include Scripture (cf. Tosefta Kifshuttah to Shabbat (13:2-3 p. 202)).
Jastrow writes that the etymology of "הדיוט" is from the Greek ἰδιώτης (person lacking professional skill, a private citizen, individual) the precursor of the English word idiot.
He defines it as:

commoner, ignoble, ignorant

He also notes that it can refer to Samaritans (presumably in contradistinction to Pharisees).
Evidently, the term can have a range of meanings depending on context (see the Jastrow entry below for some examples.)

(Image source)
Answer: I always was under the impression that it means, ordinary or regular.
Like we have a Kohein Gadol, and a Kohein Hedyot.
See for example how it is translated here, in the Oxford Dictionary of the Jewish Religion.
Comment: *ordinary* is good but *regular* might not the best word here: there is a traditional though confusing distinction in English (from Latin) between *religious priests* and *secular priests*, with the former following a specific rule (so also called *regular*) while the latter serve the wider world

What does hedyotot mean?


Why should plants transform glucose into sucrose before transporting it to other parts?

Question: I've learned that plants transform glucose into sucrose before sending it into phloem. But the process seems to be complex and energy comsuming. Why should plants do it? Is it really necessary?
Answer: Glucose, fructose and galactose are the three dietary monosaccharides. Glucose and Fructose are  simple monosaccharides found in plants. A monosaccharide is the basic unit of carbohydrate and the simplest form of sugar, glucose are aldose and Fructose are ketose.

If the carbonyl is at position 1 (that is, n or m is zero), the molecule begins with a formyl group H(C=O)-, and is technically an aldehyde. In that case, the compound is termed an aldose. Otherwise, the molecule has a keto group, a carbonyl -(C=O)- between two carbons; then it is formally a ketone, and is termed a ketose. Ketoses of biological interest usually have the carbonyl at position 2.
  http://en.wikipedia.org/wiki/Monosaccharide

Whereas Sucrose is a disaccharide composed of glucose and fructose. A disaccharide is more complex than monosaccharide, more complex compounds like oligosaccharides and polysaccharides exists. Sucrose synthesised within the cytosol of photosynthesizing cells is then available for general distribution and is commonly trans located to other carbon-demanding centers via the phloem.
Sucrose and starch are more efficient in energy storage when compared to glucose and fructose, but starch is insoluble in water. So it can't be transported via phloem and the next choice is sucrose, being water soluble and energy efficient sucrose is chosen to be the carrier of energy from leaves to different part of the tree. Another problem exists,   glucose is highly reactive and this may result in some intermediate reactions while transporting glucose. Being a complex structure, sucrose is not as much reactive as glucose. So plants uses the sucrose as a medium to transfer energy. Inside the cells, sucrose is converted back to glucose and fructose. Energy is yielded when it is needed. So plants transfer glucose and fructose in the form of sucrose in order to:

Increase energy storage
Efficient energy transfer
Removing in between reactions

References

Carbohydrates
Sucrose & Starch Biosynthesis
Sucrose Metabolism
Sucrose and starch synthesis
Disaccharide
Answer: there is no free glucose in the photosynthesis. Stop to spread that myth. The net product is G3P. The end products of photosynthesis are sucrose and starch, but never glucose. Do you test glucose in the leaves? No... it is always for starch. ;) The G3P is converted to sucrose and other molecules, for example, thiamine. Glyceraldehyde 3-phosphate occurs as a reactant in the biosynthesis pathway of thiamine (Vitamin B1), another substance that cannot be produced by the human body. Part of sucrose is then translocated to the phloem. Starch is stored in the stroma of chloroplasts. It is also stored in the amyloplasts in the roots, stems cells after sucrose suffers a conversion to starch.
Comment: Welcome to Bio. Could you post a reference to support your statement?
Comment: See very first statement in https://en.wikipedia.org/wiki/Calvin_cycle
Answer: During Calvin cycle, 3 molecules of CO2 combine with RuBP acceptors to form 6 molecules of G3P. 1 G3P molecule exits the cycle and goes towards making glucose while the other 5 are recycled, regenerating 3 RuBP acceptor molecules.
Plants do synthesise glucose.
Comment: This seems more like a comment to the foregoing answer. Some link would come in handy. Thank you!


How does one cite a MO post with an anonymous author?

Question: I asked the question "Averaging $2^{\omega(n)}$ over a region" because this is a necessary step in a research paper I am writing. The answer is detailed and does exactly what I need, and it would be convenient to directly cite the result. However, the author of the answer is anonymous... how would one deal with such a situation? I could of course very easily just reproduce the argument in my paper, but that would be academically dishonest.
Comment: Once, I left a comment indicating I would like to give the author proper credit, and asked them to contact me (they did). Another time, I asked the moderators to contact the author and ask them to contact me so proper credit could be given (they did).
Comment: If you want the member to "out" themselves to you, while still preserving their anonymity, you should not tell us what question title is involved.  I might be curious enough to read all your answered questions and corresponding papers to find out,  but probably not, whereas you make it too easy by specifying which question.  While Chris Godsil has a good suggestion, you should still give the poster the opportunity (ask them in a comment or through moderators) to unmask for this paper.  Gerhard "Maybe 'Reveal' Is The Word" Paseman, 2017.11.04.
Comment: Just to add minor clarification to @AndrésE.Caicedo comment, moderators might have access to more personal information about a user than regular users do. But they are bound by [moderator agreement](https://mathoverflow.net/legal/moderator-agreement) not to disclose this information to other users. (Of course, this is different from what Andrés E. Caicedo describes in his comment.)
Comment: One perfectly ethical way is to simply give credit in the prose of the paper, just as you would for an unpublished result that someone told you out loud at a conference.
Comment: Did you try to email the user? There is an email address at the user page.
Comment: A related (recent) post on Academia Stack Exchange: [How to acknowledge a MathOverflow user?](https://academia.stackexchange.com/q/107963)
Answer: I think you should just reproduce the argument in your paper while attributing it to the user in question (with a link to the question).  As long as you give the correct attribution, there is nothing academically dishonest about this.
I had to do this in one of my papers; see the top of page 20 of this.
I don't think it is any different from including an argument that a non-anonymous person told you.  People do this all the time, and there is nothing wrong with it as long as you indicate who told you the argument (assuming it isn't really standard, in which case you can just thank them in the acknowledgements).  For example, I did this in the "Proof of Theorem B" on page 3 of the paper I linked to above (which was explained to me by Eduard Looijenga).
Comment: @StefanKohl: Not sure that is such a big deal.  I think that readers of my papers are sophisticated enough to realize that the internet is not a static thing.  The important things are that 1. the argument is correct, and 2. it is given in my paper (in case it is deleted from the internet at some future date), and 3. I did the best job I could at attributing it to the right person.
Comment: @StefanKohl the citation could/should list their MO user page, which is identified by a unique number. There are a lot of "what ifs" that prevent readers down the track looking up stuff; but maths is a lot better than subjects where an unjustified statement is made with a citation to "Private communication".
Comment: Well -- imagine the pseudonymous user changes their pseudonym, or even chooses to switch to their real name at some point. -- How does a citation by pseudonym look like then?
Comment: It has probably something to do with versions, but it is the bottom page 22 now instead of the top of page 20
Comment: It would be helpful to update the online PDF to add proper links to the text. Thanks to a ligature, the link to the relevant answer doesn't copy/paste properly.
Comment: Reproducing is always better than just referencing, but I'm not sure if this answers the question about the citation format :)
Answer: You seem to view a nickname in the bibliography as embarrassing or unprofessional. There is no reason this should be the case. If you use the built-in "cite" feature of the stackexchange network, the author's nickname will immediately be followed by the URL of their profile, which should allay any doubts about what kind of name it is:
<code>@MISC {496728,
    TITLE = {Decomposition of a tensor product of Lie algebra representations into irreducibles},
    AUTHOR = {Balerion_the_black (https://math.stackexchange.com/users/88863/balerion-the-black)},
    HOWPUBLISHED = {Mathematics Stack Exchange},
    NOTE = {URL:https://math.stackexchange.com/q/496728 (version: 2013-09-17)},
    EPRINT = {https://math.stackexchange.com/q/496728},
    URL = {https://math.stackexchange.com/q/496728}
}
</code>
The profile link also survives name changes and is a unique identifier (and in this case, also a disambiguator).
In the unlikely case the editors object to such a reference, let them suggest an alternative. It is ultimately not your fault if they mutilate your bibliography; this is what arXiv is for :)
Answer: You attribute it to an anonymous author and give the link. So you are not claiming credit and your readers can check the source.
I would see no harm in reproducing the argument as well, especially if it's short.
Comment: It's extra information, and it's no effort.
Comment: I don't think the link is needed; this is not really different than a personal communication.
Answer: This blog posting by me was cited in this article on page 12, footnote 2.
Comment: This is nice, but it's an URL in a footnote, not a proper citation. I think the OP wants the latter, as he wants to rely on the proof.
Comment: @CarlMummert: Modern paper isn't like that. But even for the more immediate future, the survival of paper journals is far from guaranteed. When I was at MIT, it was in a process of throwing away journals that had been scanned by the publishers (e.g., Elsevier journals). Even barring this, nowadays most universities only subscribe electronically. When I publish in a "paper" journal, I cannot rule out that my paper never ends up seeing a printer.
Comment: @darij grinberg: for something that is not published in a permanent format, a footnote may indeed be the most appropriate citation. I would generally not put ephemeral things like emails, letters, or anonymous website comments in the references section. They can be acknowledged in the prose of the article. The same would go if someone told you the result orally at a departmental tea - MO is not really different than that, in my opinion.
Comment: @CarlMummert: MO is not ephemeral; it is archived on [the Internet Archive](https://archive.org/details/stackexchange) under a license that makes mirroring easy. More likely that various currently existing journals will disappear.
Comment: @darij grinberg: I suppose it depends on what we mean by ephemeral. For printed journals on paper, we have a good sense that they should be readable in thousands of years. For MO, I'm not sure we have that sense; I do view it more as the equivalent of a letter than the equivalent of a paper.  (Of course, I doubt people will want to read most current math papers in thousands of years...  )


Which country visa should i apply to? - Schengen

Question: My port of entry and exit is Frankfurt, Germany. However i will only be spending a day or two there. I am visiting Spain and Portugal for a week each. So should I be applying to Spain embassy or German embassy? I am an Indian citizen.
Answer: The rule is to apply to the country that will be your main destination and if there is no main destination, apply to the country you will enter first. The problem here is that the way you describe your plan, there is no obvious main destination (you would be staying the same time in Spain and Portugal) but Germany is clearly a secondary destination.
How the rule applies to this situation seems quite clear to me: You should apply to Germany. But alas I have heard about consulates making difficulties in such cases. (Effectively they seem to consider that if you are staying for a longer time in several countries, you should apply to one of them, but this "rule" is nowhere to be found in the regulation, see also the discussion in this answer to another question).
Alternatively, you could change your plan a little bit to stay longer in Spain or in Portugal (depending on what you want to see or perhaps on the most convenient consulate) and make one of them the main destination.
In any case, just try to apply as soon as possible and don't worry too much. If the consulate declines to process your application for that reason, it won't count as a refusal and you should get back the visa fee (not the "processing" or "service" fee from outsourcing companies, however) and all the documents quickly, which would allow you to reapply easily to another consulate (do keep copies in any case, it's always useful).


Capitalizing of Titles

Question: In  English, several types of words are capitalized in titles that are not normally capitalized. In German, are words capitalized in the same way? And if not, how are they capitalized?
Comment: well, regarding the Fact all Nouns are already capitalized )))
Answer: I am not aware of such extra capitalization for titles in Germany.
As far as I know, titles are capitalized like any normal sentence. Or they are ALL CAPS, of course.
Comment: All CAPS in a title is more a question of *typography* rather than *orthography* - But basically, your answer is correct. Note that punctuation is different in titles, normally, but that is the same in English.
Comment: @tofro: Correct, it is more a typographical issue, but the result is the same and then you can't judge the capitalization. Anyway, all caps are not so usual. And indeed, you don't punctuate titles the same way. You omit the final period, for instance.
Answer: In German there are no extra rules for capitalization of titles. This means: Titles are capitalized exactly according to the same rules as any other German text.
Rules of thumb:

Standard: All words are written with lower case characters only.
Exceptions from that standard:
  2.1. The first letter of all nouns is always written as an uppercase letter.
  2.2. The first letter of the first word in a sentence is always written as an uppercase letter.

There are also other rules, that clarify which word takes the role of a noun in a sentence, and which one doesn't, which can be quite tricky sometimes, and of course you can also use CAPS LOCK, which means: write everything in uppercase letters only, but you can do this in titles as well as in normal texts.
I give you an example:
There was a movie with this English title:

  Those Magnificent Men in their Flying Machines

As you can see, all words except "in" and "their" have been capitalized. The German title of this movie is:

  Die tollkühnen Männer in ihren fliegenden Kisten

You can see: Only three words are capitalized:

Die
Because it is the first word
Männer, Kisten
Because they are nouns

All other words are neither nouns nor the first word, and therefore written all in lowercase letters.
Comment: @DewiMorgan: German has lots of compound nouns. The longest ever used in an official document was the title of a law: [Grundstücksverkehrsgenehmigungszuständigkeitsübertragungsverordnung](https://de.wiktionary.org/wiki/Grundstücksverkehrsgenehmigungszuständigkeitsübertragungsverordnung). But "flying machine" translates to "fliegende Maschine" and in the movie title *Kiste* (engl: *box*) was used as funny sounding synonym. A compound German word would be *Flugmaschine*, but *Flugmaschine* is not *"flying machine"* in English. *"Flugmaschine"*is *"flight machine"*.
Comment: @DanLeveille *"would it look very strange to German speakers?"* Yes, because it is a wrong as anY oTher WroNG uSagE of upPerCasE LetterS. *"Is the concept understood?"* Yes, because a very high percentage of German native speakers also speaks English and everybody has seen Engish titles before. But it's still wrong. *"Error or stylization?"* It not only would look like an error, it is an error. And therefore nobody who speaks German writes German titles in such a manner.
Comment: To me, this seems weird, as in English, "Flying Machine" is a compound noun meaning "Aeroplane", rather than being treated as an adjective and a noun. Compound nouns cause headaches for people trying to write automatic title-case scripts for English, as it should be "The In Crowd" but "Alice in Wonderland". Is treating them as separate words a translation error, or does German not have the concept of compound nouns? Knowing nothing about German, I'd have expected Fliegendenkisten or Fleiderkisten or somesuch.
Comment: Interesting, thank you! So it is a good translation, which deviates from using identical grammar, in order to far better depict the *feeling* of the English title :)
Comment: @HubertSchölnast If a title were put in title case in German like it would in English, would it look very strange to German speakers? Or is the concept generally understood? Like in your movie title example, if it were in title case, would it look like an error or just a stylization?

Capitalizing of Titles


medieval technology vs today's technology

Question: In my writing there are people in a high fantasy setting with medieval technology, although there are times when they are involved with our world.
My question is, they are aware of our technology and some are familiar with its  workings, so why wouldn't they use it?
(In short why do they prefer not to use our's?)
Comment: What do you mean by "prefer not to use ours"? Obviously, mobile phones won't work in medieval Prague, but they would be silly not to import pocket knives, hunting knives, cheap copper and brass, files and saw blades, synthetic dyes, bales of cheap fabrics, spools of thread, reams of dirt-cheap paper, pencils etc. There are countless items of trade which are very cheap in the modern world and would be of immediate use in the Middle Ages. Consider for example ordinary buttons, the kind used to fasten clothes. The price differential of buttons between now and then is *huge*.
Comment: [Luddism and neoluddism](https://en.m.wikipedia.org/wiki/Neo-Luddism) are actually a thing.
Comment: @AlexP your comment has a good point. i guess i was meaning the power (mechanical) and warfare side of things.
Comment: Charlie Stross examines this question quite a lot in his *Merchant Princes* books - there is limited teleportation between the present day and a medieval world, where they *do* end up using as much tech as they can get their hands on. But it's a very small amount for the upper class, limited by the physical ability to import it.
Comment: What about putting magic out of the equation?
Comment: @OlegLobachev magic is engrained quite deeply as it is their only way to get here (earth).
Answer: Most likely because they cannot use it due to the lack of infrastructure.
The creation of modern machinery and weaponry requires a vast basis of developments and knowledge from metallurgy to chemistry, mathematics and physics, given that most of our modern world relies of electricity as a source of power. The creation of a modern day gun is impossible with even the best medieval smithy as they cannot work steel fine enough. (You can compare some other posts on the topic: Can medieval people make a potato gun? or Why no firearms? or Anachronistic things built with medieval technology)
Additionally, lack of actual knowledge of how the modern day things work or are created would prohibit your average medieval guy who has seen them exist and being used from actually reproducing them in a functioning way. Probably not even your average modern day person would be able rebuild most things having only medieval technology at hand, but they have years of experience and schooling which systematically taught them about science and technology behind them, your medieval people have no such base of knowledge.
Comment: You probably could make a crude but workable gun in a medieval workshop.  What you couldn't do is make ammunition for it.  Likewise for many things: you could import battery-powered devices, for instance, but then you become dependent on the battery supplier.
Answer:
What does it cost to interact with our world, and who can do it? Say it takes a genuine wizard to open the interplanar portal, and it will deplete the spellcasting ability of the wizard (some kind of "mana point" game mechanic). The wizard might open the interplanar portal to get aspirins and toilet tissue for himself, and perhaps for the king if the king asks pointed questions, but not for the general population.
How can they act in our world, and will we learn about it? Say the interplanar portal cannot be opened just anywhere, or perhaps that it is permanently open and cannot be moved. Smart visitors might realize that some three-letter agency from our world would built a top-secret base on the site if they knew, and put any new arrival into quarantine/interrogation. So they can't just sell magical love potions from an alchemist's store to raise money, the FDA would wonder. And they can't sell too much gold from the dragon's hoard, the DHS could suspect them of money laundering. So the visitors are mostly pennyless, and can't buy technology over here. Perhaps the travelers will bring "really neat" cold weather clothing for rangers from a charity, and the plastic dishes and spoons from the soup kitchen are amazingly lightweight, but society as a whole won't change.
Answer: Look at this in reverse.
Why WOULD they use it?
Why do WE use it?
The assumption is that they are like us, created in our image.
We use technology because that is what we do. We have evolved evolutionarily (if that is a word) to be technology users. In fact, we are so ingrained in this orientation that we take it for granted that ANY extremely sentient being would have as its main focus, the development and use of technology.
But this is just our bias.
An extremely sentient being could, alternatively, be interested in music, dance, writing, and other creative expression.
They might consider culinary development the highest form of achievement. They would look at our society and consider US the barbarian society, with a total lack of culture. A land full of toys, uncouth and trivially childish. What is the usefulness of an iPhone if you can not eat it? What is the purpose of a computer if you can not physically turn it into a sculpture? All of these are mere distractions from what is REALLY important. Aesthetic perfection. This would probably be a very physical, 'earthly' society that would abhor virtual reality.
Dancing in very exquisite choreography would rank far above talking to someone siting next door on Facebook.
Singing together in a large group, perfecting and exploring harmony and tone, rhythm and melody, personally experiencing its creation, would be existentially more rewarding than listening to the performance of someone else over mass media.
Doing is far more rewarding than spectating.
They would aspire to the great, hand-written, carefully illustrated hand replicated literary tomes of the ancient monks that you can heft in your hands, feel the texture, smell the paper, than the modern, sterile, cold ebooks of today.
Being a poet would have much higher social status than being a lowly engineer.
Scientists would only be useful if they developed new paint pigments or such.
In that sense, so much of our technology would simply be utterly useless to them.
Answer:
Because almost anything we can do with technology, they can do
with magic.      Why create a cell phone when you have a
hand-held magic mirror which allows you to contact anyone with
another magic mirror ... even a demon or djinn who can answer
questions for you (who needs Google)
Because it is against one or more of their religions or forbidden by
a God/ruler/overlord.
Because they are more in tune with nature and see the devastating
effects producing our advanced technology has created in our environments ... so they fear creating advanced technology would eventually destroy their planet.
Comment: for three do you mean they use us as an example of what not to do?
Comment: Yes @ajnatorixzersolar that is what I meant ... clarified a little.
Answer: Many lamented here the absence of the modern tools. Yes, but!
Mindset and knowledge
What we have and our historic-medieval folks did not, is the mindset of a modern world person and an accompanying knowledge. Arguably, there is no way to throw a modern society further back than to Renaissance tech-wise. Medieval folks are before that, both in tech and in mentality.
We know much better than medieval folks how the world works and tics, applying this knowledge even in the absence of modern technology is much, much easier than to invent it from scratch.
Here were mentioned already such things as threads and pencils and paper. If the modern-society group is large enough, following options might also work:

Building a primitive steam engine.
Reaching out to electricity.
Building a glider plane.
"Inventing" or using at a much larger scale the black powder.
"Inventing" modern explosives.
Compromise between the knowledge of modern firearm design and available technology.
Building better roads.
Building better ships.
Using primitive optics.
Signaling and coding (Morse ist just a begininng!)
More general: information retrieval and processing.

To highlight the ship example: If we manage a steam engine: wonderful, welcome to the 18th century. If not: we still know much more about hydrodynamics for ship shape, aerodynamics for sail form and configuration, general engineering for better masts and what not.
For coding: Even a quite simple Vigenere chiffre was unbreakable for 300 years. This alone will turn over the world. Imagine a group than has a monopole on the secure communications...
Well, in fantasy the medieval folks might compensate all that with magic. But historic medieval people? No chance.


Electron shell, how do they know it?

Question: I started to learn Chemistry with something like Hydrogen having 1 electron in the first shell, Oxygen having 2 electron in the first shell and 6 electron in the second shell... But I don't really understand how scientists know it? How do they know there are how many shells and how many electron per shell? Is that something visible? Or just an imagination theory or they using a method to measure?
Answer: Electrons are not actually separated in shells. That is a simplified model to explain the atom's structure when people first start approaching Chemistry or Physics.
In reality, electrons' movements around the nucleus are governed by complex equations (some of them not even resolvable so approximations are needed) to form what is called an electron cloud. This cloud is confined in proximity of the nucleus by the electromagnetic force. Every electron is defined by its wave equation, which tells us the probability that the electron appears to be at a particular location when its position is measured. These equations allows us to define regions in space where the electron spend the most part of its time around the nucleus, and they can be visualised as atomic orbitals, as shown below. Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation.

Each orbital (which is defined by a different equation) has different associated parameters, called quantum numbers, and a particular energy. These properties allow us to divide them into levels and sublevels, to form a structure that resembles multiple shells one inside each other. But as you can see from what I explained the structure is in fact far more complex.
This knowledge has been gained over time both from the mathematical and physical analysis of different equations (like Schröndiger equation) and by experimental data collected by the means of different techniques. Starting from Thomson and Rutherford the model of the atomic structure has seen many changes (Bohr, Moseley, Van der Broek, de Broglie, etc.) and it has come closer to a realistic representation of the actual atom. Take a look at their experiments to understand how it has been studied. However, they often involve particle physics and are not really straightforward to understand.
Comment: "This knowledge has been gaining over time both from the mathematical and physical analysis of different equations (like Schröndiger equation) and by experimental data collected by the means of different techniques.". Can you give me some example of these techniques ? I really want to focus on which techniques and how they knowledge it.
Comment: As I said above, you should look at their experiments. Just Google any of the aforementioned scientists plus 'experiment' (e.g. Rutherford's experiment, https://www.google.co.uk/search?q=rutherford+experiment&rlz=1CDGOYI_enGB608GB608&oq=rutherford+ex&aqs=chrome.0.0j69i57j0l2&sourceid=chrome-mobile&espv=1&ie=UTF-8&hl=en-GB) and you'll find more on these techniques. They're mostly single and specifically designed experiments rather than particular instruments used (however, nowadays some of them could be used to have a better understanding of the theory).
Answer: One way to think of this is to think of the total number of bonds that atoms can make. You can then use experimentation to look at the structure of the compounds, and that will tell you a lot about how the electrons are configured around the nucleus. For instance, Carbon compounds tend to take on tetrahedral shape if the bonds are saturated (single covalent), this means that carbon bonds most favourably by forming 4 bonds. If the moieties that are bound are all the same such as hydrogen, the angle of the bonds should be the same, and using x-ray crystallography for instance shows us that this is the case. We understand chemical bonding quite well, and we can deduce that each hydrogen, donates its single valence electron to a single valence electron from the carbon to form a covalent bond, the total number of valence electrons for carbon must therefore be 4. This is because the 2p shell is higher energy than the 2s shell and only the 2p electrons are free to bond. (This isn't really the case, the bonds are hybrid sp bonds but I won't get in to that right now)

Electron shell, how do they know it?


How to change dell edge gateway 3001 gpio pin to analog

Question: In dell edge gateway how we can change the gpio pin from digital to analog? It has Ubuntu core as OS. It has 8 gpio pins only some of them I want to make digital and some of them analog. I have checked the Dell edge gateway 3001 manual but they have not given anything about it though they mentioned we can make a gpio pin either digital or analog.
Answer: Not sure if you found the answer you were looking for, but thought I'd actually sign up and give you some pointers since I've just been down this path myself.
The tool you need is 'dcc.cctk' which can be used to view and change the mode of each GPIO.
For example, the following command would show the current mode of GPIO0
    dcc.cctk --adimodechannel1

You can also change the mode with one of the following options (unused|adcinput|dacoutput|dacandadc|gpio), e.g.:
    dcc.cctk --adimodechannel1=adcinput

Note: You must use 'sudo' or login as root to issue these commands, and you must also reboot the Dell once you have changed the mode before it takes effect.
To change the mode of other GPIO pins, you need to use 'adimodechannelX' where X is the channel number between 1-8. GPIO0 is 'adimodechannel1', GPIO1 is 'adimodechannel2', etc.
If you need to print help and usage instructions to screen, then use:
    dcc.cctk -h

or for more specific help with this method:
    dcc.cctk -h --adimodechannel1

Hope this helps.
Answer: Without Dell giving you the full documentation, it's going to be difficult, but still we can have a shot.
As per the Dell Manual for 3001 Edge Gateway, they are using AD5593R IC for GPIO. This is an Analog Devices' IC with 8 GPIOs which can act as an ADC, DAC and Digital I/O. As per the AD5593R Docs, the IC is configured using an I2C interface. You can read the AD5593R to understand the configuration in full. Beyond this, you will have to refer to the Linux Kernel's official Docs on GPIO to communicate with and configure the GPIO.