Why must the measurements of length be done over an instantaneous moment?

Question: With respect to length contraction in special relativity, in order to measure the length of something (a meter stick say) in motion with uniform velocity to the right with respect to our stationary frame, it must be done in an instantaneous moment. My only guess is the ends of the stick must be measured (located on coordinates) at the same time.  By "locate on coordinates," suppose we are standing on the ground facing a large sheet of graph paper in metric.  Suppose the meter stick mentioned above were to fly horizontally to the right past us.  We would need to measure both ends at the same time on the graph paper by marking them on the paper.  If the right end is measured first and then the left end a few moments later, the length of the stick will measure shorter than it should be; this is due to error and not relativistic length contraction. Likewise if the left end is measured first and then the right end, it will measure longer than it should be and there is no such thing as length dilation.
So why must the length of something moving in another frame of reference be done in an instantaneous moment if my reasoning is wrong?
Comment: I thought I answered my own question too but I worry I don't understand something.
Answer: Well, you answered the question on your own. If you are in rest reference farme, it doesn't matter what time you measure the position of both edges. However, if the object is moving, of course you must measure both at the same time; if you don't, one end has "escaped" in the meantime and you'll measure a different lenght. It's okay, if you do know the velocity te object is moving with, you can also measure it in different times and then apply the correction, as the common sense says. There's nothing abour relativity here yet.
Special relativity enters here, not before. What Sp. Rel. tells you is the relation between those two measurements: $L_{rest}=\gamma L_{mov}$. Classically, both measurements (at rest, and moving but taken at the same time) should be the same, but in special Relativity they are different.
Comment: You're right!  When the stick is at rest, you can measure the ends in any order and over any time interval whereas if its in motion, the ends must be located simultaneously!  Thank you so much!


Change display language to Arabic

Question: how can I change the display language of the site for all users to Arabic?  I mean, I want the user interface to be for all users from right to left. I want users to see the right-to-left interface when they enter the site. As in the attached picture.
Thank you
Comment: Did you check the regional settings in site settings?
Comment: Check [this](https://support.microsoft.com/en-us/office/change-regional-settings-for-a-site-e9e189c7-16e3-45d3-a090-770be6e83c1a) and [this](https://docs.microsoft.com/en-us/answers/questions/439914/communication-site-in-arabic-tempate-not-working.html).
Comment: let me know if above suggestions work for you.
Comment: Brother, the default language of the site is Arabic, but the site appears from left to right and in English. When I change the display language from my personal account, the interface changes and becomes Arabic from right to left, but it does not appear to other users, it is the same as it is in English. How can I make the site interface appear to all users from right to left?
Answer: The surest way to force the use of a language is to create the site in that language.  You are prompted for the site language when you create the site

To prevent users from having a different language, the go to Site Settings / Language settings / Show advanced settings and make sure that there are no other "available languages",also called Alternate languages.
When a user visits a site, SharePoint will check the language(s) in the user's profile.  If one of them matches the alternate languages or the site's base language, that is the language that is used for the interface.  For Arabic, Hebrew, or Dari, that sets the right-to-left direction of the layout. If there is no alternate language, then displaying in other languages is not an option.
Comment: thank you very much that's really helpful
Answer: You must be signed in as a site owner or site collection administrator to choose language settings for a site. In addition, if you are using SharePoint Server, the SharePoint administrator must first deploy language packs for the languages you want to use. There is no need to install language packs for SharePoint in Microsoft 365.
1.From a site page, click Settings  Settings button from SharePoint Online, click Site Settings. If you don't see Site settings, click Site information and then click View all site settings. Some pages might require you to select Site contents, and then Site settings.
2.On the settings page, in the Site Administration section, click Language Settings.
Note: If the Language settings link doesn't appear on the Site Settings page of your SharePoint Server site or site collection, then language packs haven't been installed. See your SharePoint Server administrator.
3.Select the languages you want to make available:

If you are using a communication site and you have page translations
enabled, select languages by typing them or using the dropdown for
each language. For more information on how to do this, see Enable the
multilingual feature for communication sites and choose languages.
If you're using a communication site without page translations
enabled, or using a Team site or a classic site, select the check
boxes next to the languages you want to make available.

4.If you want to overwrite site translations in the site interface, select Yes under Overwrite Translations. If you're on a communication site with page translations enabled, select Advanced and then select Yes under Overwrite Translations.
5.Select OK.
Now your users can change the display language for their personal use of the user interface. Learn how users can change their preferred language in Change your language and region settings.
After you turn on alternate languages for your site, you can create new lists or libraries in one language, then display the site in a different language, and modify the list or library name for that language. The same process works for list and library columns, and for navigation links.

If the answer is helpful, please click "√" on the left panel of the answer and kindly upvote it.
Comment: thank you very much that's really helpful

Change display language to Arabic


How can I help my daughter to realize advantages of books?

Question: My daughter is 9, she is very clever, but I want her to work harder, read more as after doing the lessons quickly she takes her cell phone and doesn't notice anything else around. How can I help her to love reading as she says all you want me to read I can watch? Or does anyone know some helpful kids book club which will help her not to get bored?
Comment: Related, possibly a duplicate: https://parenting.stackexchange.com/questions/21980/suggestions-on-making-reading-fun-for-a-10-year-old-girl
Answer: Do you read to her? Maybe at bed time? 9 is a bit late to start, but its still worth it.
You say "I want her to work harder, read more". She says "all you want me to read I can watch". That sounds like the wrong conversation to be having. Reading should be a pleasure, not a chore. It sounds like you want her to read books you choose because it will be good for her education, but to a child "education" is the boring stuff that adults make you do. By defining it as a chore you set yourself up to fail before you start.
Pick some trashy books aimed at 9 year old girls. They should be about things she knows and likes (sports? horses? magic?), and not have movie or TV versions. Read them aloud to her. Make it a daily thing; just a special time for the two of you. Encourage her to pick the books she would like to hear. Finish reading half way through a chapter ("My throat is getting dry; I need to stop now") and leave the book for her to carry on with if she wants. If one book doesn't catch her interest, try another.
Comment: I was reading for her when she was younger and it was just that daily thing for us about what you wrote. Is it my omission that now when I have no time to read with her, to take part in some activities related to her, she prefers an easy way to spend and enjoy the time? As when I am reading, explaining, we speak about what we read, she listens carefully but after some time when I ask her to do it herself, she can't concentrate.
Comment: I was at her age and I know that it is difficult to take control of your day but we are too different as I liked to do my best to get that discipline and tried to put things in their places. but it is not a priority for her and should I accept that? But it's a permanent process, ya? to explore and recognize your child to help find her own way in life, not yours.  I'll agree with you I should find what will interest her. Thanks!
Comment: @AngCalv Like I say, you need to stop thinking of this as something you are trying to *make* her do, and start thinking of this as something you can enjoy together. If you have to stop to explain then you are reading the wrong thing. Never mind all the "improving" stuff, find stuff she *enjoys*. Once she wants to hear more, the rest will follow. You are systematically teaching her that reading is a boring chore. Stop that!
Answer: At nine, I wouldn't recommend permitting unlimited usage of cell phones/tablets/screens for entertainment.  Not to encourage reading per se, just because it's still quite hard at that age to have the level of self control needed to manage screentime appropriately.
That said, aside from limiting screentime, the best thing we've found is very simple: make sure books are readily available, make sure the books that are readily available are interesting to the child, and make sure you are modeling the desired behavior.
Also, don't be too picky about what you consider "good literature".  My 9 year old was unwilling to read "real books" when he was 6-7, but he did really enjoy graphic novels.  So, we got him a good library of graphic novels, and he'd spend time reading them.
Guess what - now, two years later, he reads all the time.  Sometimes graphic novels, sometimes written fiction, sometimes written nonfiction.  He got in the habit of reading, realized it was fun, found things he likes, and now he reads for hours a day!
Another thing that is worth considering, is a library app, like Overdrive or Libby, if one is available in your area.  Having the ability to download any book at the moment they want to is amazing for a kid. My kids use Libby and Epic (a subscription library with more stuff readily available), and love being able to browse for things they like.


(Ghidra) How to get the parameter name given a Function object and the storage location as a register string?

Question: Given an instance of something that implements ghidra.program.model.listing.Function, and the register as a string (e.g. x0) how can I get the name of the argument (e.g. param1)?
Answer: The first way I found is (as Kotlin code)
function.parameters.first { it.variableStorage.register.toString() == "x0" }.name

i.e. go through the parameters and find the first (and hopefully only one), that has a storage location with the string representation x0.
This feels somewhat hacky, but should at least work. The whole expression type checks, so it should cover basically all cases, and I am not sure if there can ever be a case where somehow multiple parameters are stored in the same register that have the same string representation.


How do I access a firewall log (Windows 10 w. Windows Defender)

Question: I am going through some event logs (in event viewer), and noticed I can't seem to find any firewall logs.  Windows Defender provides the firewall.  How do I get to the firewall logs that should be generated by Windows Defender, or are they not even generated?  I have searched through event viewer, the Windows Defender firewall GUI, and google searches have been unsuccessful (they generally point to older versions of Windows not using Windows Defender)!
Comment: I'm voting to close this question as off-topic because this should have been migrated to SuperUser
Answer: Applies to

Windows 10
Windows Server 2016

To configure Windows Firewall to log dropped packets or successful connections, use the Windows Firewall with Advanced Security node in the Group Policy Management MMC snap-in.
Administrative credentials
To complete these procedures, you must be a member of the Domain Administrators group, or otherwise be delegated permissions to modify the GPOs.
To configure the Windows Firewall log

Open the Group Policy Management Console to Windows Firewall with Advanced Security (found in Local Computer Policy > Computer Configuration > Windows Settings > Security Settings > Windows Firewall with Advanced Security).
In the details pane, in the Overview section, click Windows Firewall Properties.
For each network location type (Domain, Private, Public), perform the following steps:

Click the tab that corresponds to the network location type.
Under Logging, click Customize.
The default path for the log is <code>%windir%\system32\logfiles\firewall\pfirewall.log</code>. If you want to change this, clear the "Not configured" check box and type the path to the new location, or click "Browse" to select a file location.
Important: The location you specify must have permissions assigned that permit the Windows Firewall service to write to the log file.
The default maximum file size for the log is 4,096 kilobytes (KB). If you want to change this, clear the Not configured check box, and type in the new size in KB, or use the up and down arrows to select a size. The file will not grow beyond this size; when the limit is reached, old log entries are deleted to make room for the newly created ones.
No logging occurs until you set one of following two options:
To create a log entry when Windows Firewall drops an incoming network packet, change "Log dropped packets" to "Yes."
To create a log entry when Windows Firewall allows an inbound connection, change "Log successful connections" to "Yes."

Click OK twice.

Resources
If you have any more questions you can see this whole guide and read more into it here:
https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-firewall/configure-the-windows-firewall-log
Hopefully this helps you solve your problem!
Comment: Good to know that firewall logging doesn't occur unless you explicitly set it up.  I would have thought it would be set to log by default.  Would be nice if Windows and other products built more security in, like this for example.
Comment: To go into more detail on step #3, you must grant the "NT Service\MpsSvc" service write permission to the folder you choose (if the folder doesn't exist, you may have to create it and then add this permission).


Please help how to undo 301 on blog?

Question: I have a landing page on example.com and then a blog at example.com/blog.  I noticed I have very little traffic on the blog even though it has 45 posts. I went to Bing Webmaster Tools and found this 301 message "moved permanently" on H1 and title tags which I want to remove. My landing page looks perfect for title and H1 tags and the meta name on the landing page is perfect also. How can I get this on my blog too? Am I right in thinking this is why I have so little traffic?
Comment: What does 301 message "on H1 and title tags" mean? 301s are applied to entire URLs, not page elements.
Answer: As your site is at the moment, it doesn't seem like there is any problem involving a 301 status.  If a user were to enter the URL parenting4specialneeds.com/blog then with a normal browser the user will end up viewing your "About Me" page, which is located at parenting4specialneeds.com/blog/ (note the extra "/" at the end).  Internally the browser does actually receive a "301 moved permanently" response which tells it that the page should be retrieved from parenting4specialneeds.com/blog/ but this is normal, and nothing to worry about.  The browser will automatically get the right page.
If I were you, I would rethink what you want for what you are calling your "landing page", the one at parenting4specialneeds.com/.  What you have now is a rudimentary page which forces any visitor to enter an email address to go anywhere or see anything (and it's pretty "spammy" looking, besides).  You'd quickly lose a lot of visitors right there, I'd think.  The page comes after that, for those who enter an email, is even more rudimentary, and does not have any link to your blog either.  So your blog will not be getting any views if anyone were to come to your "landing page".  The general quality of your blog pages seem much better than your landing page and its successor page.  Perhaps you should use a blog page as your landing page (with a redirect!).
Comment: thanks so much for that-I will make these changes-greatly appreciate your help and your honesty!

Please help how to undo 301 on blog?


Pet person or dog person?

Question: My little sister bought a dog yesterday. But since keeping pets is not my thing (I'm too scared of them), I havent been even able to bring myself  to touch or pat it. So what should I say? That I'm not a pet person?
Comment: *I'm not a pet person*, *I'm not a cat person*, *I'm not a dog person* are all relatively "normal" utterances in casual speech, but the nature of the underlying relationship between the "attributive noun" *(pet, cat, dog)* and  the relevant "true noun" *(person)* gets increasingly uncertain as you extend the construction. If I say *"I'm not a rabbit person"*, only context would tell you whether that meant I don't want a rabbit as a pet, hate going near or thinking about rabbits, don't *eat* rabbit, or whatever.
Comment: Or if you are some strange form of human-rabbit hybrid.
Answer: If you're frightened of dogs, I would recommend you say that and not say "I'm not a dog person." Saying that you're not a dog person implies that you just dislike dogs for some reason. A dog owner might try to change your feelings by getting you to engage with their dog, because they can't imagine anyone disliking the pet that they find so appealing. If you say "I'm scared of dogs.", most dog owners will understand that is not something that you can easily change your mind about, even if their pet is a really nice dog.
Here's another example: if my friend and I are talking about food and I say "I'm not a shrimp person.", my friend might say "Oh that's just because you've never tasted MY shrimp recipe that everyone says is really good! I'll make some for you tomorrow." If I had said "I'm allergic to shrimp.", my friend might say "Oh that's too bad - I have a really great recipe for shrimp. Maybe I'll try it with chicken next time so you can taste it."
When you tell someone that you don't like something that they themselves like, it's just human nature to try to persuade you over to their point of view.
Answer: I think people would tend to say "I'm not an animal person" more than they would say "I'm not a pet person." If you are scared of pets, you are probably scared of animals in general.
In this case, I would probably say either that you're not a dog person or that you're scared of dogs. From personal experience, no matter what you say, the other person will try to convince you that their dog is different from all those other dogs out there.
Comment: I think that if you say "I'm not a dog person", you're actually inviting the other person to show you how nice dogs can be. I would recommend saying "I'm sorry, but I'm terrified of dogs." instead. I am a dog person (and a cat person), and if I know that someone is frightened by my pets, I take extra care to keep them under control just as common courtesy. I would hope that folks that keep pet tarantulas would be as considerate of me and my fear of those creepy, multi-eyed, 8-legged, venomous monsters :)

Pet person or dog person?


Relay before or after power supply

Question: My 3D printer has a 240W power supply that takes 220V AC as input and gives 12VDC/20A as output. I'd like to use a relay, controlled by a raspberry, to turn the printer ON/OFF. My question is: should I put the relay between the 220V and the power supply, or after the 12V? What are there pro and cons between the two configurations?
Comment: What's the rated output current of your supply?  Relays typically have a much lower rating for DC due to the absence of zero crossings that are present in AC.
Comment: I edited my question, please see above
Answer: Switching mains is better because:

It eliminates vampire losses by the power-supply itself which will use some power in standby. This saves a little bit on the energy bill.
It eliminates risk of fire from PSU overheating (although it may introduce it somewhere else).
It avoids heating the room.

The disadvantages include:

Thermal cycling of the PSU, PCB and components. This may lead to component or board failure.
Delay before you can print.
Comment: There's already a several-minutes delay before printing, a few seconds from the relay & PSU booting up isn't notable.  however, the cheap Chinesium power supplies that come with most printers are going to die very quickly, and replacing those will dominate the savings from vampiric losses.  Of course, they also catch fire, so the savings on buying a new house will outweigh buying new power supplies.
Comment: +1 I'd add a 240W PSU may actually not like having it's load removed and the sudden transients involved in breaking those kinds of currents may be rather detrimental to either the power supply, the load, or both. Same goes for the reconnect. Turning it off/on at the mains side should be no worse than unplugging the thing assuming the OP does not plan on doing it every minute.

Relay before or after power supply


Why do high frequency waves have high energy

Question: It is known that electromagnetic waves with a high frequency possess a greater amount of energy than waves with lower frequencies. Why is this the case? Does it have anything to do with Planck's law?
Answer:
It is known that electromagnetic waves with a high frequency possess a greater amount of energy than waves with lower frequencies.

This isn't quite true. The energy carried by an electromagnetic wave is the product of two independent factors:

the energy of each individual photon, which is given by the Planck law $$E_\mathrm{photon}=h\nu,$$ in terms of the light's frequency $\nu$ and the Planck constant $h$, and
the number $N$ of photons present in the beam.

For light that's far away from the quantum regime, the total quantum-mechanical energy $E=Nh\nu$ transitions over into the classical regime, where it becomes better described by the classical intensity, which is proportional to the amplitude of the electric-field oscillations in the light. In that regime, the light can carry any amount of energy you wish to put into it.
However, that property fails to be true at low energies, where the photon number $N$ is of order $1$. In this regime, quantum mechanics takes over, and the light becomes incapable of carrying less energy than the photon energy $h\nu$: it either has one photon's worth, or none at all. And, because of the Planck law, this minimal energy increases with the frequency.
The reason this is important is that if you have a biological tissue that's absorbing, say, one UV photon's worth of energy spread over a bunch of infrared photons, then the energy absorbed by each individual molecule can be quite small. However, if the light is in the UV, then it's impossible to break that energy down into smaller chunks, and it's a single molecule that needs to take the entire hit, and if the photon energy is big enough then that will take the molecule over its damage threshold.
Answer: Quantum mechanics tells you that the energy and frequency of an individual photon are related as $$E = h\nu$$ where $h$ is Planck's constant. This is part of the Quantum Hypothesis of Planck.
Answer: I don't think that an electromagnetic wave carries more energy depending of the frequency as the energy of the electromagnetic field (or better, its density) is
 $E=\frac{1}{2}\cdot \epsilon_0 \cdot {E_0}^2$
 so it depends the maximum value of the electric field $E_0$.
Differently, a single photon has the energy
 $E=h\nu $
so a single photon which frequency is higher, carries higher energy.
Comment: A wave does not "carry" photons, instead it carries energy; a wave "exchanges" energy with particles in units of photons. The two are not the same thing.
Comment: Example of a possible misunderstanding: a wave can be composed by 5 photons with high  frequency and thus energy, or by 100000 photons with low frequency and energy (each) but in total, adding the single photon ones,  the wave "has" more energy.


Balanced incomplete block design - randomizing treatment order

Question: In a balanced incomplete block (BIB) design, what is the best way to ensure each treatment appears in all positions/orders? Should I randomize treatment order within each block, so that participants within each block see the treatments in a random order?  Or, should I randomize the order of treatments in each block once (such that all participants in a block see treatments in the same order), ensuring that across all the blocks, each treatment occurs in each position?
Answer: You should randomize treatment order within each block, unless there is something special about your experiment (which you did not tell us) that dictates otherwise.
If you use your second option, results might be biased ... maybe there is some learning effect, so treatment always shown first get a disadvantage, or something else.


One word that describes a person who has an eye for detail

Question: I am looking for one word that describes a person who has an eye for detail. This also could be a person who may find it difficult to focus on the global picture as the individual's attention is focused in the details or building block, rather than on the house.
Comment: Meticulous? taking or showing extreme care about minute details; precise.
Comment: You mean: "can't see the forest for the trees"? An expression used of someone who is too involved in the details of a problem to look at the situation as a whole. Or
Comment: The expression "an eye for detail" is generally positive, while your description calls for a more pejorative phrase such as has been offered by JOSH ("can't see the forest...") and John Feltz. ("myopic"). If you truly need a phrase for "eye for detail" in its more complimentary sense, use "detail-oriented" or JOSH's "meticulous". Also, "scrupulous" or "ultraprecise" for an option that could go either way.
Comment: Not a direct duplicate of english.stackexchange.com/questions/141581/… but that question has got some negative words for people too focussed on the detail (that might apply totes second part of this question) e.g. myopic or parochial.
Answer: I think you're asking for 2 things.
If you're talking about a person who has problems seeing the big picture, because they are so focused on details, consider 'myopic' (MW, definition 2):

a lack of foresight or discernment :  a narrow view of something

A sample sentence would be

Bob is a skilled accountant, but sometimes he's so myopic that he spends all day balancing the petty-cash account to the penny, rather than finding the best way to file our taxes.
Answer: I think the word 'pedantic' should match the details of your query
Pedantic means "like a pedant," someone who's too concerned with literal accuracy or formality. It's a negative term that implies someone is showing off book learning or trivia, especially in a tiresome way.
Answer: Observant fits your description
Answer: You could use a compound adjective:
-focused, -centered, -minded, -orientated.
Meticulous and detail-oriented tend to be popular, but it all depends on the context.
Answer:
This also could be a person who may find it difficult to focus on the
global picture as the individual's attention is focused in the details
or building block, rather than on the house.

overscrupulous (adj.)

Excessively scrupulous
The overscrupulous TSA agent confiscated the monkey's two-inch-long toy sidearm —  Kyle Peterson m-w

Scrupulous means thorough, exact, and careful about details. Collins

They can be overscrupulous in moral behavior and excessively clean
and tidy. They are bound to pay too much attention to detail, to the
point that they miss the big picture (they can't see the forest for
the trees). Jason Williams; The Mental Edge in Trading (2012)

At the risk of duplicating Le Duchat's special brand of pedantry, I
will include only one passage exemplifying this overscrupulous
attention to detail. Theodore Fraser; Le Duchat, First Editor of
Rabelais (1971)

Behind those eyes, Garth knew, would bea kind of fanatic: over dogged,
overscrupulous, in the smallest detail of conduct. John Dickson Carr; The Witch of the Low Tide


Is it normal that front wheel bends when front disk brakes are applied?

Question: When I apply front disk brakes I see slight bend in the rim and wheel. Is this normal or I have a low quality parts.
Comment: That really doesn't sound right, even for low-quality parts.  It sounds more like something is wrong, like maybe the axle isn't tight enough.  Can you explain better what you mean by "bend?"  Flex to the side?  It would be best if you can include a short video, or at least a picture.  Regardless, get this maintained before you ride it again!
Comment: Welcome to the cycling corner of SE.  This doesn't sound normal, can you organise a couple of photos to show the difference?  Might need 3 or 4 hands.
Comment: Are the QR properly tightened? In the intended way?
Answer: No, not normal at all. You should have a bike shop look at it for a safety check if you are new to the sport and bike.
One thing that can cause a wheel to bend is poor spoke tension. A good bike shop should be able to fix that for you pretty quickly.
Answer: Are you talking about small bends in the rim, maybe a few mm, that are always there, but you notice them when you are braking?  In other words, when you spin the wheel, do you notice that the rim appears to wobble between the brake pads, but the wheel does not actually wobble on its axle, and the wheel does not change shape?  I'm talking about the problem where the rim is slightly bent by a few millimeters right or left of center in some places, but not up or down.
If that is the case, then you have a wheel that is not "true", which is a very common issue.  If the rim is bent far enough that braking is not as effective, then you should get the problem fixed.  It is usually possible to adjust the tension of the spokes to bring the rim closer to the ideal shape; this process is called truing the wheel.  There is lots of advice on the internet about how to true a wheel.  Here is one such video from YouTube.
Lower-quality bicycle wheels need more truing than higher-quality wheels, and may even come from the factory or bicycle shop needing to be trued, but in my experience all bicycle wheels eventually need to be trued.  Rear wheels with gears on the side have more tension in the spokes on one side than the other, which is not the case for front wheels, so rear wheels generally need truing more often than front wheels.
Comment: Additionally, disk brake wheels can be ridden when they're out of true to the point a rim brake bike would suffer from brake-rub.
Comment: Just as an FYI fixing spoke tension will also true a wheel.
Comment: With a disc brake (OP said), a bent rim won't affect braking. That's only a problem for rim brakes.
Answer: Another possibility is that the front bearings are loose and are being twisted to one side when braking.
Comment: This was it for me! Thank you :)
Answer: Of course wheels bend somewhat when braking force is applied. Otherwise they would need to be infinitely stiff, or at least so stiff it would make it stupidly heavy.
"Somewhat" means, they bend so little that you can't possibly see it, though you might be able to feel it. Trials riders certainly do, and it's one reason why they're still mostly holding out with rim brakes, unlike all other MTB disciplines.
If it does bend visibly, then probably something is very wrong indeed. Low spoke tension is the most likely reason, as the other answers have said. Properly trueing a wheel is a bit tricky, but at least you should check if any spokes are snapped yourself or at least at way too low tension (you can feel it when "plucking"/twisting a spoke; it should feel almost solid, only slightly elastic but certainly not wobbly).
Answer: It possible the wheel skewer is not tightened enough.


Eigen vector of Pauli Matrix (z-component of pauli matrix)

Question: I have had no problem while finding the eigen vectors for the x and y components of pauli matrix. However, while solving for the z- component, I got stuck. The eigen values are 1 and -1. While solving for the eigen vector corresponding to the eigen value 1 using $$(\sigma _z-\lambda I)X=0$$,
I got $$\left( \begin{matrix} 0 & 0 \\ 0 & -2 \end{matrix} \right)\left( \begin{matrix} x \\ y\end{matrix} \right)=0$$
Now, how can I find the eigen vector for eigen value 1 with this relation since it will give me only $-2y=0$
Answer: The math is telling you that $0 x = 0$, so $x$ can be any number, and $-2 y = 0$, so $y$ must be zero.  For a unit length eigenvector, we usually choose $u=\{1,0\}^T$, but $v=\{e^{i\phi},0\}^T$, where $\phi$ is a real number, is also a unit length eigenvector with eigenvalue 1.


How can I test a Singleton class with DUnit?

Question: Or it's better to use another Design Pattern?
Answer: Responded to a similar question some days ago here, mocking a Singleton. The original post is for C#.Net as regards mocking a singleton's behaviour, but should still apply.
As regards the singleton pattern, there isn't anything wrong with it per se - in many cases we want to centralize logic and data. However, there is a very big difference between a singleton and a static class. Building your singleton as a static class hard codes that implementation to every consumer in your application - which makes unit testing very difficult!
What you want to do is define an interface for your singleton, exposing the methods for your consumers to use. Your consumers in turn are passed a reference to an implementing class by whomever instantiates them [typically this is your application, or a container if you are familiar with Dependency Injection\Inversion of Control].
It's this framework, whomever is instantiating the consumers, that is responsible for ensuring one and only one instance is floating around. It's really not that great a leap from static class to interface reference [as demonstrated in link above], you just lose the convenience of a globally accessible instance - i know i know, global references are terribly seductive, but Luke turned his back to the Dark Side, so can you!
Generally speaking, best practices suggest avoiding static references, and encourages progamming against interfaces. Remember, it is still possible to apply the singleton pattern with these constraints. Follow these guidelines, and you should have no problem unit testing your work :)
Hope this helps!

singleton != public static class, rather singleton == single instance
Answer: Lack of testability is one of the major downfalls of the classic Singleton model (static class method returning an instance). As far as I'm concerned, that's justification enough to re-design any code that uses Singletons to use some other design.
If you absolutely need to have a singular instance, then Dependency Injection and writing to an interface, as suggested by johnny g, is definitely the way to go.
Answer: I'm using the following pattern when I write a static-based singletons that I can mock. The code is Java, but I think you will get an idea. The main problem with this approach is that you have to relax constructor to package-protected (which sorta defeats a true singleton).
As a side note - the code applies to ability to mock your "static" code not necessarily simply calling it
Answer: I generally only use Singletons for Flyweight objects or similar value objects.  Looking into an IoC container (as discussed above) is probably a better way to handle a shared object than a singleton.
Consider that in Smalltalk (where a lot of these patterns originated), true and false were both effectively singletons :)
Comment: ... regarding Smalltalk: yes, but true and false are _so_ testable :)
Answer: If you must use a singleton (and there are reasons to do so...but I would always try to avoid it if possible). I would recommend using a IOC container to manage it. Im not sure if there is one for Delphi or not. But in Java you could use Spring, in .NET you can use Windsor/Castle. A IOC container can hold onto the Singleton and can register different implementations for testing.
It's probably too big of a subject to get into here beyond this snippet.


In Middle Ages Europe, Physical scientists referred to the "Queen of Sciences"; what was this?

Question: We often hear scientists write of the Queen of Sciences.  To what were they referring?  And is it relevant in today's research?
Comment: Theology was "the queen of the sciences" and philosophy "her handmaiden". Later Locke declared philosophy "*the handmaiden of the sciences*" and later still Gauss proclaimed:"*Mathematics is the queen of the sciences and number theory is the queen of mathematics.*" Philosophy never got a break.
Comment: If mathematics is the queen, what discipline is the king?
Comment: it hardly matters. These are cringeworthy metaphors. You would make better use of your time actually studying the things you keep referring to as your kings and queens
Comment: @DJohnsonIt mattered to the philosophers and scientists (legitimate scholars) for centuries. Would you discount their research, credibility, contributions?  It is on their shoulders which you stand, eh?
Answer: There were at least two Queens:
In the late Middle Ages, Theology was called Queen of the sciences.  See, e.g., "How Theology Stopped Being Regina Scientiarum—and How Its Story Continues"
Subsequently in the 1700s, the title was transferred to mathematics.  E.g., Gauss who said "Mathematics is the queen of the sciences and number theory is the queen of mathematics."
(There's a lot more if you google "Queen of the Sciences"...)
Mathematics is, of course, relevant.  And in some areas -- String Theory, for example -- there seems to be no hope left but prayer...
Comment: Is there an authentic German/Latin quote of Gauss?
Comment: "Die Mathematik ist die Königin der Wissenschaften und die Zahlentheorie ist die Königin der Mathematik." -- attributed to Gauss
Answer: There is a much-quoted description of the elaborately sculpted tomb of Isaac Newton, repeated (for instance) in the April 1731 (vol 1, number 4, p.159) issue of Gentleman's Magazine, in the year Westminster Abbey says the statuary was unveiled.

... On the globe sits the figure of Astronomy weeping, with a sceptre in her hand (as Queen of the Sciences) and a Star over her head on the Pyramid. ...

Other publications with almost identical descriptions include The Political State of Great Britain, April 1731, vol. 41, p.412,413; The Present State of the Republick of Letters, April 1731, vol. 7, pp.317-320; and The Pennsylvania Gazette, 22-29 July 1731, No. 141 (p.512 in the 1968 reprint).  The Republick version has some added content, including a quotation of Pope's "All Nature and its Laws lay  in Night; God said let Newton be, and all was Light".
Astronomy is not far removed from mathematics, and so on, but Q of S can have other meanings than Theology and Mathematics.
Added 7 May 2023:  An earlier English instance is in John Gadbury's 1677 interestingly titled book The Just and Pious Scorpionist..., where the preface (flatteringly titled "To the Honest and Intelligent Reader") states

For it is the Divine Urania! and she alone ! that is the Mistris and Queen of all Natural Sciences.

Alas, it is clear that the author means Urania the muse of astrology not astronomy.


New to me Plant Questions

Question: Mornings folks!
I inherited this plant from an office that was closing down.  I'm hoping someone can help identify the plant so that I can look into proper care.
It needs to be repotted for sure as it's currently a leaning tower.
Thanks so much!
Answer: This plant looks like a Dracaena fragrans or corn plant in English. It is pretty neglected. I think the main problem with this plant is that it outgrew the container, and it needs a much bigger one with fresh potting soil. Be sure the container has good drainage, that is with holes in the bottom.
The Corn plant is further a pretty easy plant, you can forget to water it once in a while, and it can stand in a spot with indirect sunlight.
What you could do is to transplant it to a bigger container, and you could try to prune it a little (but if you don't mind the current state of the leaves, pruning is not necessary). Here some detailed care instructions found on the net.
In time this could become your favorite plant! Good luck.

New to me Plant Questions


Can i add custom field on Add account team page

Question: Is it possible to add a custom field on Account Team?
Answer: If you simply wanted to add custom fields to Account Teams, then you can go to the object settings for Account Team Member and add the custom fields.
If you wanted to display additional custom fields in the Add Team Members page, you can do so by creating a custom VF Page (with custom fields) and then override the Add Team Member(s) button to display the custom VF Page. The SF object you are want to configure is Account Team Member. From object settings page, go to Account Team Member > Buttons and Links (see screenshot below for ref).

Edit Add Team Member or Add Team Members button and override with the custom VF page (see screenshot below for ref).

In Salesforce classic, you can directly go to the Buttons & Links set-up page using the following URL (replace <My Domain> with your org domain name):
https://<My Domain>.my.salesforce.com/p/setup/link/ActionButtonLinkList?pageName=AccountTeamMember&type=AccountTeamMember&setupid=AccountTeamMemberLinks&retURL=%2Fui%2Fsetup%2FSetup%3Fsetupid%3DAccountTeamMember

Comment: Hi Arut, thanks but i found out with Winter'20 i can customize the layout too but this limited to Lightning users, but thanks for the reply.https://releasenotes.docs.salesforce.com/en-us/winter20/release-notes/rn_sales_feature_core_accounts.htm
Comment: This release notes mentions about general improvements made w.r.t to Account Teams, but doesn't mention anything about improvements made over ways to customize the related standard pages. Even with lightning experience, you can customize the default page layout but nothing out of the box for tweaking the standard page.
Comment: Do you have any idea how the same can be done in classic, cause this helped me in lightning only.
Comment: Check the updated answer and let me know if that works for you. Although, I've not tested this in SF classic, overriding the standard page in lightning should reflect in classic too.
Answer: The Customization of Add to Account Team, can be done by adding the custom field on the Account Team Member and adding the same field on multiline editor for Account team layout.
https://releasenotes.docs.salesforce.com/en-us/winter20/release-notes/rn_sales_feature_core_accounts.htm


Is it safe to write data to MAX35103 flash memory

Question: I am using MAX35103 sensor in my project and I want to store some constant data into MAX35103 flash memory using STM32. However, I've read something that it is not safe to write data to the MAX35103 flash and this will shorten the life of the chip. Isn't it really safe to do this?
Answer: It depends what do you mean by safe.
All flash memories have a specification for how many times it can be erased and written until the memory cells are worn out and can't store data any more. So they degrade a little bit each write and erase cycle.
The details of this are usually mentioned in the data sheet.
The write and erase also take time, and if for example user cuts off device power when MCU is storing new values into memory, the power cut may haopen middle of erase or write operation, so the memory contents may be left in any state and the data you were trying to store to flash is gone and it may contain no or partial data so the contents are corrupted.
So yes, writing to flash reduces the life time of the chip. Whether that is a problem, the life time depends on how often you write to flash and how many times the flash can be written.
And the system must be able to detect somehow if the flash contains valid data or has it been corrupted by power cut during a write or erase, so you know if the data is OK to use or not.
Comment: Then you look at both data sheets and compare. I have no clue what your measure of safety is.
Comment: I am storing these data into STM32 flash memory. Safe means safer than storing into STM32 microchip
Answer: FLASH memories have limited guaranteed erase cycles number.
This IC FLASH has guaranteed 20k cycles. If you erase 10 times a day it will last 5.5years. You you need to erase more frequently you need to use EEPROM instead.


error in webrat

Question:
duplicate of Error in webrat installation

hai all,
       i am getting an error when i am iinstalling webrat for my rails app.i want to use in my rails app cucumber, rspec,webrat to my app test.so please give me some solution to this error.

ERROR:  Error installing webrat:
 ERROR: Failed to build gem native extension.

/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/ruby.h

Gem files will remain installed in /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0 for inspection.
Results logged to /Library/Ruby/Gems/1.8/gems/nokogiri-1.4.0/ext/nokogiri/gem_make.out
Comment: Same as http://stackoverflow.com/questions/1659586/error-in-webrat-installation - duplicated question and user.
Answer: try  apt-get install libxslt-dev

error in webrat


What moisture content of wood is desirable for building an open veranda?

Question: My family is planning on building an open veranda in Europe in a climate with the average annual percentage of relative humidity of 76.0% and the standard deviation of 8.5%. The framing will be done using 6in x 6in timbers of Scots pine (typical wood there). What is recommended moisture content for materials upon delivery? The construction is supposed to begin in a few weeks when the relative humidity level is about 65%. In addition, when is the best to paint the wood: immediately after the construction or later?
Answer: Relative humidity is different than the humidity present in wood, although it does affect the humidity that is present in the wood.
Since the wood will be outside, the moisture content (MC) of the wood can be around the 15% to 20% range, although the lower the better. 20% MC is getting close to green (fresh cut lumber/timbers). If it is lower than 15%, great. I have measured the MC of wood that was under cover, exposed to the outside humidity and it measured 9% MC, this was in an old storage building with siding. Most framing lumber is considered "kiln dried" (KD) and the MC should be somewhere around 6%-10% MC. This may go even higher if the material is stored for a long time outside. Some timbers, in the US, is sold as green, or seasoned dry and will be in the higher percentages, still good to build an outside structure. If the material will be pressure treated (PT) for longevity, will be VERY wet, more than 20% unless in Europe it is KD.
If the finish product is to be painted, and it is not PT, all cuts are primed to seal against moisture intrusion. The material could be primed before it is cut and used, just prime the ends after they are cut and before it is covered by the next piece or where it joins the next piece.
If the material is PT, the wood needs to have time to loose the excess moisture and have the surface get a chance to leach out the preservative so the paint can bond to the wood, otherwise it will bubble back off. The usual time for PT to "cure" is 6 months before painting, but the ends that get covered do not need to be primed like the standard framing does or should.


Match off points into $N$ red/blue pairs with straight lines connecting pairs, so that none of lines we draw intersect

Question: Suppose we are given $2N$ points in the plane (we may assume that no $3$ are collinear). Assume that $N$ of these points are colored red, and $N$ points are colored blue. Can we match off the points into $N$ red/blue pairs with straight lines connecting these pairs, so that none of the lines we draw intersect? If this matching exists, can we find it by an algorithm?
Comment: Do the lines end at the points, or are they extended ?
Comment: If the matching exists we can definitely find it by looking at every possible matching. Obviously this will not be an efficient algorithm. Are there any other constraints on your algorithm?
Answer: Yes, such a matching always exist.
The algorithm works recursively. You either
1) find a pair of red and blue point R and B such that the number of red points and blue points on the left side of the vector RB is the same. You match RB, and then you recursively find the matchings for all the points on each side of the vector separately.
or
2) find any line that divides the plane into two non-empty planes such that the number of red and blue points on each side of the line is equal. You recursively find matchings on these points on each side of the line separately.
We need to prove that either one of these two cases must happen. Suppose case 1) does not exists. Consider any pair of node R and B. Consider the vector RB. RB divides the remaining points into those that are on the left side of the vector, and those on the right side of the vector. Given a vector RB, let RB_left be the the number of red points on the left of RB substracted by the number of blue points on the left of RB. Since case 1 does not exist, RB_left is not zero. We also have RB_left = -BR_left. Now consider rotating the vector X=RB using until it eventually becomes BR, and consider what happens to X_left. Each time the vector is collinear with a point, then X_left will increase or decrease by 1 point. Since BR_left = -RB_right, there must be some time in which X_left = 0.


partitioning a hard drive without an OS for dual boot

Question: I'm planning to buy a laptop without a preinstalled OS.I want to install a Linux distro first, then later dual boot it with linux and windows 10 once I purchase a licence. What is the best way to partition the hard drive and still be able keep a separate partition for all my media and data files without formatting it when I install windows?
Also, what are the chances of driver and uefi problems if I dual boot with ubuntu and windows 10?
Comment: Very simple actually Install Linux first with Linux create a second partition for windows and then a third for your data. I have done this before and I didn't get any problems. However I do recommend just buying a separate drive for your data. If the drive fails you will still have your data.
Comment: @DylanRz: Doesn't Windows overwrite the master boot sector/ grub? For me, I always used to install windows first and keep however much I wanted unformatted. Followed up with installing linux and merrily formatting however I wanted.
Answer: Installing Linux to then in the future install dual boot with windows 10 will give you some headaches, because Windows will create its own boot partition and will overwrite your current boot. So what you might have to do is to reinstall grub once Windows 10 is installed using your Ubuntu LiveCD.
<code>sudo grub-install --root-directory=/mnt/ /dev/sdX
</code>
The best partition?
Leave a portion of your HD without partition, this is more up to you how much space you want for Windows.
Linux: This is very personal, people might have others opinions

/boot 512Mb
/root 10Gb
/home the rest of space (keep all your media and documents here)
/swap (Is your Disk a Solid state drive? SSD? don't use swap, otherwise use double your RAM 6Gb RAM -> 12Gb swap)

it will look like this

sda XXG  0 disk
├─sda1   XXXG  0 part  (free partition for windows)
├─sda2  512M  0 part  /boot
├─sda3   10,0G  0 part  /root
└─sda4  XXXG  0 part  /home

Further recommendations:

Start using encryption. Make this an every day routine. Encrypt your /root and /home partition. Take your privacy seriously.
Linux will fulfill all your needs. No need for Windows. Install Linux as the only OS and if you happen to need windows, just go for a Virtual Machine.
Give Arch Linux a shoot. A bit complicated at the beginning but totally worth it and stable.
Use Logic volumes LMV so you can change the sizes later.
Comment: Thanks for the answer. I'm not ready to take that next big step with arch yet and I prefer lubuntu actually(the repos and LTS prevents me from trying others, lol). The laptop is an i3 and I plan to run xp on virtual box for the time being; but I'm not sure how well that will go. I don't mind formatting the linux partition when installing windows, but just to be specific, i want to create a primary partition for the OSs and an extended partition for my data. Is that the right way to go when setting up the partition tables?
Comment: Use a primary partition for the OS and extended for the rest (though it doesn't really matter now a days since Ubuntu can boot from a extended partition). Don't forget to mark this as the answer if I was helpful.
Comment: hey @fr33c0untry could you mark this as your answer? thanks.


Are the privacy features of Firefox 69.0 complementary to PrivacyBadger 2019.7.1.1?

Question: Are the privacy features (blocking of Trackers, Cookies, Cryptominers, Fingerprinters) of Firefox 69.0 complemantary to PrivacyBadger? or do they interfere with each other?
Comment: The last release of PrivacyBadger was 2 months ago (before the release of Firefox 69.0). According firefox's new ETP, with miners added, the only thing i dubt is about browser fingerprinting but if the settings are set to 'normal mode' it shouldn't be a problem since these protections do only apply for strict mode.
Answer: Firefox' new Enhanced Tracking Protection (ETP) does in fact the same what PrivacyBadger does, It blacklists Cross-Site trackers, blacklists cryptominers (although I'm not aware of one), so there is no need to use PrivacyBadger longer. Furthermore you can disable or allow third-party cookies. If you want to use PrivacyBadger anyway then I recommend Browserprivacy set to 'normal mode' otherwise there is a possibility whereby PrivacyBadger features can conflict with Firefox' new features since these protections only apply with Browserprivacy set to strict-mode. Privacy Badger does protect you against canvas-based fingerprinting, Firefox does this also and thus there is a possibility that both can conflict with each other. 1
Two security-guards that do the same is worthless and likely will override the existing security.


What is the difference between an amateur and a professional camcorder?

Question: It is known that professional photographers use SLRs & regular users use point & shoot camera. What kind of cameras do professional videographers use & whats the difference?
Comment: Based on discussion on the meta site, questions about cinematography and video equipment are off-topic.  Recommend this be closed.
Comment: This site concentrates on the photography, so there may not be enough videographers here to give an authoritative answer. One thing I can say is that it is well documented that some TV shows have been filmed using DSLRs; I'd also suggest that SLRs aren't just for professionals.
Comment: I see coward people downvoting your post because you have posted a question about video in the 'video' section. So, I will up vote you to help you take your points back.
Comment: @Altar: I think you mistake how this site works, and particularly the tagging mechanism. There is no "video section"; just a tag someone created. The existence of (or lack of) a tag has nothing to do with whether a question fits the site's defined topic. For that, see http://meta.photo.stackexchange.com/questions/28/can-i-ask-videography-related-questions
Answer: I'd regard this as off topic as it concerns video cameras not stills, but anyway a lot of professionals use the Red series of cameras:
http://www.red.com/
The most popular being the Red One:

(source: redoneutah.com)
Benefits over consumer level video cameras nicely mirrors the difference between DSLRs and Compacts, namely:

The ability to shoot video in raw, for example using the proprietary RecCode format. This makes contrast adjustments, post production and grading easier.

A larger sensor, the Red One has a similar sized sensor to a DSLR (same size as super 35 film) this allows better low light performance and shallower depth of field.

The ability to have interchangable lenses.

Higher resolution pictures (again to refer to the Red One it will output at 4k (4,096 x 2,304 pixels, which is at least four times full HD)).

Better build quality. All professional tools should be built to be packed up and shipped round the world and work first time on location.

There are other advantages specific to professional videography cameras, namely:

Higher framerates for [real] slow motion 24fps -> 1000fps (some consumer cameras do this, but not very well)

Wider range of inputs for mics etc.

Wider range of accessories, HD out for live monitoring etc.

Record to multiple data banks, HDDs, flash memory etc.

Another difference is that professionals will more often than not hire equipment as the cameras cost more than even the most expensive 35mm DSLRs, and they are typically only shooting for part of the year in intensive bursts. This also enables the use of specialist cameras (such as the Phantom 1000fps camera, which can only be rented, not bought).
It's worth pointing out that DSLRs which shoot video offer many of the advantages posted above, but they lack the modularity and flexibility of output that you get with a professional digital cine camera. They are also limited in terms of shooting time.
Comment: The Red One is brilliant for lots of things, but it's worth knowing that it suffers from [rolling shutter artefacts](http://en.wikipedia.org/wiki/Rolling_shutter) and that the output is quite desaturated.
Comment: It seems rolling shutter is inevitable with high resolution output, though shooting at 100fps ought to mitigate the effects to a degree. I think most people operating a Red One will be planning on grading the film anyway so desaturated output is not a problem, especially if you short in REDCODE.
Comment: This answer is old and outdated.  It could probably stand on it's own if all mention of RED One was stripped from it, but I'll give the author a chance to update it first.
Answer: One of the big differences is manual control.
Many consumer video cameras don't let you disable automatic control of simple things like focus and exposure (or more often exposure compensation).
I think that lots of cheap video hardware would be very capable of capturing clear and beautiful moving images if only one were able to control it more precisely.
As an example, consider that David Lynch's Inland Empire was shot using reasonably inexpensive but highly controlable DV cameras, and it looks great.
Comment: Indeed, manual controls are the main difference. But obviously the more money you spend the better hardware you're going to get.
Answer: I don't do much in the way of cinematography myself, although it is a field that interests me, and one I hope to break into sometime in the future. I am primarily a photographer, and my first video camera will likely be a Canon 5D Mark III (once released.) The Canon 5D Mark II was one of the first DSLR's to add video capability, and combined with the advanced lenses available to such cameras, you can achieve some of those vaunted "cinematographic" effects.
As I can't offer any specific advice (hopefully another member will be able to answer and provide more useful information than I can), I can supply some very useful links. Here are a few that I have found helpful myself over the last couple months:

Cinema5D
Cinema5D's "Canon 5D Rig Review"
VideoMaker.com's "Learn" Site

Excellent general info about cinematography

Luminous Landscapes Video DSLR vs. Camcorders

Interesting article including info on how pros are starting to gravitate towards DSLR for video (particularly the Canon 5D Mark II), and what is missing from the DSLR (combocam) setup
Answer: Worth noting that like all tools, a professional will use the right tool for the job.

If you're shooting a cinematic feature something like the Red One would be a great tool for the job. However if you're shooting news footage you'd be an idiot to take a Red One - it's designed to be used by several people at once, and is optimised for the greatest creative control of the picture by having manual controls for exposure, depth of field and focus; while an ENG (Electronic News Gathering) camera is designed for one person to put on their shoulder and get the shot in the heat of the moment, with automatic settings that make it easier to do that.
A professional cinematographer will choose the tool that best suits the job. Are you shooting as part of a crew of hundreds, or is it you and the director? Is the result going to be seen in cinemas or on YouTube? Are you after a certain look, such as shallow focus or do you need ease of use? Are you strapping it on someone's helmet when they jump out of a plane? Do you want to shoot documentary footage while keeping a low profile? Is weight a consideration, say if you're using a gimbal or drone?
Answer: Honestly in this day and age the difference in "Pro" and "Amateur" gear is whether or not you're getting paid to use it. I know some amateurs with 5D mk2s and some serious pro shooters with 60ds.
Completely agree with the comments on manual control. But even without manual control and some semblance of exposure compensation you can get amazing results. It is not about the gear, but mastering what you have in your hands to tell the story.
I just spent the last week using an audio recorder that cost less than $200 but it was so much more intuitive than the $600 one I've been using. There is no difference in the sound at all. Mentioning that as an example... realizing you were asking about cameras. And in all fairness putting Flip Video footage on a large monitor - it doesn't stand up... but don't get hung up on "Pro" and Amateur"...  get hung up on stories.


When and where did 'hench' become a verb?

Question: Inspired by this question, when did people start to use "hench" as a verb? Was there a popular fictional origin, or did this usage originally come from non-fiction discussions of villains' associates?
Comment: I'm voting to close this question because it's an English language question, not a question about SF or fantasy.
Comment: may be more appropriate for https://english.stackexchange.com/
Comment: I've seen examples as early as 1916; https://www.google.com/search?q=henchman+%22henching%22&tbm=bks&sxsrf=AB5stBjDcQqk59VzToMjid2pTBSdqZsoMg:1688405006883&source=lnt&tbs=cdr:1,cd_min:1900,cd_max:1999&sa=X&ved=2ahUKEwi1hrDlhvP_AhUSHsAKHSewBYYQpwV6BAgBEBU&biw=1396&bih=657&dpr=1.38
Answer: Valorum pointed out a case in 1916 (!) which seems to match, a translation of Plautus's Amphitryon

Mer. (aside dryly) The henchman's first try at henching pretty nearly came to grief.

Under sheer trivia, henchman is generally agreed to come from Middle English henxman, referring to one's groom, or "horse man".
Comment: Edward Woodward ordered his henchman to "go and hench something" in the dramatic reveal at the end of the 1989 movie of Agatha Christie's "The Man in the Brown Suit"
Comment: @KerrAvon2055: And Valorum added a comment indicating likely usage in the early 20th century. I'll update my answer.
Comment: Thank you for this info
Answer: It's certainly made it into Wiktionary:

(comics) To be a henchman or henchwoman, usually for a supervillain.

But it's been a verb for quite some time, in Scots.

To HENCH, v. a. To throw stones by bringing the hand along the haunch, S.

To HENCH, v. n. To halt; to limp, Gall. Roxb.-Germ. hink-en, claudicare; Teut. hinck-en, id. radically the same with Su.G. hwink-a, vacillare; Dan. hink-er, id. hinken, lameness.

To HENCH AWA', r. n. To move onward in a halting way, Fife, Roxb.

A Dictionary of the Scottish Language, John Jamieson, 1846.

In English, there seems to be confusion over the derivation of "henchman".
We have this from "The Lord Mayor's Show", London Magazine, 1844.

These Pages to the Mayor derived their name, says Blackstone, from following the haunch of their masters, and thence being called haunch-boys or hench-boys.

related to the first Scots meaning above.
Halliwell and Wright's Glossary (1844) has a full definition:

HENCHMAN. A page or attendant. Etymologists have been puzzled to find the origin of this once common word; and their attempts may be seen in Todd's Johnson. To me the simple etymology of judge Blackstone seems the most probable: haunchman, from following the haunch of his mas- ter. Bishop Percy also made the same conjecture in a note on the Northumberland Household Book. Hence it is applied to boy as well as man, hench-boy, or haunch-boy. Shakespeare speaks of "the haunch of winter," for the latter end of it. 2 Hen. IV, iv, 4. They who derive it from hengest, a horse, do not seem to have considered that it is most commonly used for a foot attendant or page. Mr. Douce, however, thinks otherwise, and he has certainly found mounted henshmen in Chaucer. See Illustrat., vol. i, 189. Still this only affects the etymology; for it seems clear that they became pages afterwards. Minshew says expressly, that "it is used for a man who goes on foot attending upon a man of honour, or great worship.

I do but beg a little changeling boy To be my henchman.

Mids. N. Dr., ii, 2

He whose phrases are as neatly decked as my lord mayor's hensmen.

Jack Drum's Entertainm., B 4.

They were excepted from the operation of the statute (4 Edw. IV, cap. 5), concerning excess of apparel:

Provided also, that henchmen, heralds, pursuivants, sword-bearers to mayors, messengers, and minstrels, nor none of them, nor players in their interludes, shall not be comprised within this statute.

Hench-boy was not uncommon:

How could they affect these filthy harbingers of hell, These proctors of Belzebub, Lucifer's hench-boys?

Muses' Looking Gl., O. Pl., ix, 187

Sir, I will match my lord-mayor's horse, make jockeys of his heach-boys, and run 'em through Cheapside.

Wits, O. Pl., viii, 420

Thus, to set the hench boys on horse- back, was to change the nature of their service. In one of Milton's MS. copies of the Ode on a Solemn Music, he had called the cherubim "Heav'n's henshmen," which, with very good taste, he afterwards expunged. See Todd's Milton, vol. vii, p. 57.
Comment: How did you post this answer while the question was closed?
Comment: @jwodder  It was in a draft,and I wasn't sure whether to post it or not. So I put my phone down for at least an hour and a half, but at some point I thought to myself "You spent all that time putting it together, why not" and hit POST.
Comment: @jwodder I had NO clue it was closed because the browser hadn't been refreshed in all that time.
Comment: @jwodder OP's meaning of "hench" only applies to superhero villains anyway, and at a stretch could be categorized under "fandom". I think ELU would have been a better site for OP to ask this in, but other such questions have been accepted because of some remote SF or Fantasy tie-in.


Problem with short circuit

Question: Comparing with short circuit how does a current flow in parallel circuits with different resistances. Shouldn't it just choose the one with lesser resistance?
Comment: If you have a water tower with 2 different diameter pipes coming out the bottom, does water only flow through the bigger pipe?
Comment: It is choosing such a route, that maximum current will flow overall. If it can flow two ways instead of one, it will.
Comment: @CHendrix While I always use the pipes analogy, it is not always obvious, *why* this analogy is good..
Comment: @EugeneSh. Water molecules = electrons.  Or would that be positrons?  The analogy breaks down when you directly compare electrons to water molecules, because electrons flow opposite of current direction.  I just do the direct comparison, but remember that electrons flow opposite of current direction.
Answer: Resistance is defined as the ratio of voltage across an object (the 'resistor') to current flowing through it. It doesn't matter if other components in a circuit are drawing more current, when a particular voltage occurs across a resistor, a certain current will flow through it - always.
So long as the voltage remains constant, adding other components in parallel makes no difference to the current an individual resistor draws. Consider the 3 circuits below:-

simulate this circuit – Schematic created using CircuitLab
In circuit A, R1 draws 5v/5Ω = 1 Amp. In circuit B, R2 draws 5V/1Ω = 5 Amps. In circuit C, V1 and V2 are replaced by a single 5V supply, and R3 and R4 are in parallel. Both resistors get 5V, so R3 draws 5V/5Ω = 1 Amp and R4 draws 5V/1Ω = 5 Amps.
In a 'short circuit' situation the total current may be limited by other components in series, or by the power supply itself. As a result the voltage drops and the lower value resistor 'steals' some of the available current from the other one. However the current will still split between them in proportion to their resistances.
Answer: Typically the resistance of conductors increase with temperature (they have a positive temperature coefficient.) What this means is that there is an "auto-balancing" effect between conductors that are connected in parallel, since as a conductor conducts more current, it begins to heat up, and become more resistive- meaning that electrons will begin to flow in the other, cooler conductor until they reach some kind of equilibrium.
Follow this link to find the proper way to calculate the total resistance of multiple conductors connected in parallel, as well as some basic information about how such circuits behave: https://www.swtc.edu/ag_power/electrical/lecture/parallel_circuits.htm

Problem with short circuit


Unicode in Content-Disposition header

Question: I am using HttpContext object implemented in HttpHandler child to download a file, when I have non-ascii characters in file name it looks weird in IE whereas it looks fine in Firefox.
below is the code:-
       context.Response.ContentType = ".cs";
context.Response.AppendHeader("Content-Length", data.Length.ToString());
context.Response.AppendHeader("Content-Disposition", String.Format("attachment; filename={0}",filename));
        context.Response.OutputStream.Write(data, 0, data.Length);

context.Response.Flush();

when I supply 'ß' 'ä' 'ö' 'ü' 'ó' 'ß' 'ä' 'ö' 'ü' 'ó' in file name field it looks different than what I have in file name it looks fine in firefox. adding EncodingType and charset has been of no use.
In ie it is 'ß''ä''ö''ü''ó''ß''ä''ö''ü'_'ó' and in firefox it is 'ß' 'ä' 'ö' 'ü' 'ó' 'ß' 'ä' 'ö' 'ü' 'ó'.
Any Idea how this can be fixed?
Comment: Is this the content of the file, or the filename itself?
Comment: @leppie, it is filename itself
Answer: I had similar problem. You have to use HttpUtility.UrlEncode or Server.UrlEncode to encode filename. Also I remember firefox didn't need it. Moreoverit ruined filename when it's url-encoded. My code:
// IE needs url encoding, FF doesn't support it, Google Chrome doesn't care
if (Request.Browser.IsBrowser ("IE"))
{
    fileName = Server.UrlEncode(fileName);
}

Response.Clear ();
Response.AddHeader ("content-disposition", String.Format ("attachment;filename=\"{0}\"", fileName));
Response.AddHeader ("Content-Length", data.Length.ToString (CultureInfo.InvariantCulture));
Response.ContentType = mimeType;
Response.BinaryWrite(data);

Edit
I have read specification more carefully. First of all RFC2183 states that:

Current [RFC 2045] grammar restricts parameter values (and hence Content-Disposition filenames) to US-ASCII.

But then I found references that [RFC 2045] is absolete and one must reference RFC 2231, which states:

Asterisks ("*") are reused to provide
the indicator that language and
character set information is present
and encoding is being used. A single
quote ("'") is used to delimit the
character set and language information
at the beginning of the parameter
value. Percent signs ("%") are used as
the encoding flag, which agrees with
RFC 2047.

Which means that you can use UrlEncode for non-ascii symbols, as long as you include the encoding as stated in the rfc.  Here is an example:
string.Format("attachment; filename=\"{0}\"; filename*=UTF-8''{0}", Server.UrlEncode(fileName, Encoding.UTF8));

Note that filename is included in addition to filename* for backwards compatibility. You can also choose another encoding and modify the parameter accordingly, but UTF-8 covers everything.
Comment: @Sergej, I tried what you said and it works fine, can you please explain why it ruined  filename, so that I have it clear in my mind before I actually implement it.
Comment: As per Ash's answer, I used this but with `UrlPathEncode`, which worked like a charm (it doesn't add the '+' signs).
Comment: Thanks for this. I am using Java and used Guava's PercentageEncoder to encode the filename:  `new PercentEscaper("_-", false).escape(filename)`
Comment: Been using Firefox 46, the only problem for me was single quotes `''`  after `filename*=`. Thanks.
Answer: HttpUtility.UrlPathEncode might be a better option. As URLEncode will replace spaces with '+' signs.
Answer: For me this solution is working on all major browsers:
Response.AppendHeader("Content-Disposition", string.Format("attachment; filename*=UTF-8''{0}", HttpUtility.UrlPathEncode(fileName).Replace(",", "%2C"));
var mime = MimeMapping.GetMimeMapping(fileName);
return File(fileName, mime);

Using ASP.NET MVC 3.
The Replace is necessary, because Chrome doesn't like Comma (,) in parameter values: http://www.gangarasa.com/lets-Do-GoodCode/tag/err_response_headers_multiple_content_disposition/
Answer: You may want to read RFC 6266 and look at the tests at http://greenbytes.de/tech/tc2231/.
Answer: For me this solved the problem:
var result = new HttpResponseMessage(HttpStatusCode.OK)
{
   Content = new ByteArrayContent(data)
};

result.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment")
{
    FileNameStar = "foo-ä-€.html"
};

When i look ad the repsonse in fiddler i can see the filename has automaticcaly been encoded using UTF-8:
Fiddler response example with encoded Content-Disposition filename using UTF-8
If we look at the value of the Content-Disposition header we can see it will be the same as @Johannes Geyer his answer. The only difference is that we didn't have to do the encoding ourselfs, the ContentDispositionHeaderValue class takes care of that.
I used the Testcases for the Content-Disposition header on: http://greenbytes.de/tech/tc2231/ as mentioned by Julian Reschke.
Information about the ContentDispositionHeaderValue class can be found on MSDN.
Comment: I encoded value for FileNameStar. It did not work. The framework already handles encoding nicely.
Answer: For Asp.Net Core (version 2 as of this post) UrlPathEncode is deprecated, here's how to achieve the desired result:
System.Net.Mime.ContentDisposition cd = new System.Net.Mime.ContentDisposition
{
   FileName = Uri.EscapeUriString(fileName),
   Inline = true  // false = prompt the user for downloading;  true = browser to try to show the file inline
};

Response.Headers.Add("Content-Disposition", cd.ToString());

Answer: I`m using Uri.EscapeUriString for converts all characters to their hexadecimal representation, and string.Normalize for Unicode normalization form C.
(tested in ASP.NET MVC5 framework 4.5)
    var contentDispositionHeader = new System.Net.Mime.ContentDisposition
    {
        Inline = false,
        FileName = Uri.EscapeUriString(Path.GetFileName(pathFile)).Normalize()
    };
    Response.Headers.Add("Content-Disposition", contentDispositionHeader.ToString());
    string mimeType = MimeMapping.GetMimeMapping(Server.MapPath(pathFile));
    return File(file, mimeType);

Unicode in Content-Disposition header


How do you read the Railo documentation?

Question: I was reading Ben Nadel's post about the isnull function and my initial thought was "wait: there's an isnull function in ColdFusion?  I didn't know that!  I wonder if there's one in Railo?" So I typed:
railo isnull
into google and this was the first link. In fact, a lot of my Railo searches have been to similar pages, so apparently this is something that I'm supposed to be able to read and understand.  Here's my question:
Q: How do you read the Railo documentation?
I'm used to seeing words written out in sentence format like Ben's post. I mean: I can see that a lot of work went into this page, so I should try to squeeze some kind of meaning out of it...  When I see for example:
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Does that mean anything to an applications programmer like myself, or is that a kind of internal documentation? I followed the link to the actual source code, but that's almost too much information.
Maybe I've identified a need.
Comment: That is not the CF documentation for the function. It is the Java API for the `IsNull` class, generated by [javadoc](http://en.wikipedia.org/wiki/Javadoc). It has a standard format to be sure, but it is written for java developers.
Answer: You read the Railo Javadoc API the same way you read any Javadoc API.

(Though as you note this is more internal documentation and most CFML developers wont need to.)

If you want Railo's CFML documentation for isNull, you visit railodocs.org/isNull instead.
I don't know why Google isn't capable of finding the correct pages. :/
For the benefit of Google I'll also throw in these other links to the isNull documentation: http://railodocs.org/function/isnull?version=current and http://railodocs.org/index.cfm/function/isnull/version/current - they all display the same content (which may be a factor in why they're not ranked properly), so if someone reminds me later I'll bug Mark about getting the URLs canonicalised.
Answer: Railo does have isNull()

It works on variables that are not strings
It works on strings

In my experience, Railo 4.2 more or less supports CFML at the same level as ColdFusion 10. It is always useful to test. I agree that as of early 2014, Railo's documentation is lacking a certain clarity.
Comment: Thanks James!  Well, I guess the saying is: find a need and fill it.  Maybe I should start posting railo examples on my personal website!


encryption algorithms

Question: I have a P2P backup network where a peer can store data and retrieve it later from another peer. What are the best security measures I can take so that data wont be read by other users which have not created it? I was going to use single key encryption and digital signatures but I am sure whether this is the correct approach.
Thanks in advance for your help
Comment: I'd strongly recommend that you read up on encryption and hashing before implementing this - there's a lot of ways you can make a crackable implementation if you don't fully understand what you're doing.
Answer: Symmetric key (single key) encryption will work. However, getting it right is a bit tricky -- be sure to learn about cipher operating modes. You should ask the user for a password, and use that to derive a master encryption key. Then you should encrypt each data block (you're splitting your data into blocks, right?) with a key based on the master key and block number.
Digital signatures won't prevent eavesdroping, but you should sign the content anyway, because you can't trust the peer to give you the correct content. You should use MACs (message authentication codes), since you will have already established symmetric key anyway, and the user is the only party that needs to verify the signature.
Answer: If, as I understood, is only the creator that retrieve the stored data, then a single key encryption mechanism should be enough because data are encrypted and decrypted only by that user, without the needing of a key exchange.
Certificates (or public/private keys)aren't necessary because no other user are involved in the encryption process.
Take in consideration also the need of use a MAC function(Message Authentication Code) to assure that the data stored wouldn't be modified.
Comment: He does however need a message validation mechanism - it just doesn't have to be a signature generated by an asymmetric encryption algorithm.
Comment: yeah that is exactly what I meant the creator is the one that should be able to decrypt the data. In such scenario, do I still need to use digital certificates or these aren't necessary? Thanks for your help
Comment: @Tzer001: yes..you don't need digital certificates
Comment: @Erik: yes you are right.. otherway message can be mistified.. thanks for the advise..i'll fix my answer.
Answer: The question is vague, but anyway here goes:
Each peer generates a unique encryption key - known only to itself. When it needs to store data, it should do e.g: Hash data with e.g. a SHA algorithm, concatenate data and hash, encrypt, store. This allows for a cheap verification of data after retrieval without having to deal with asymmetric algorithms.
To summarize: Use hashing and symmetric encryption algorithms without known weaknesses, such as SHA-2 and AES, avoid e.g. MD5 and DES. No need for asymmetric algorithms such as RSA/DSA, rather sign with an embedded hash.
Answer: The primary way of busting encryptionsm, especially in this scenario, still is brute force. It works very well and very efficient in most cases.
To crack for example the secret token used to sign some URL or the password behind an md5 hash you can just crank up a bunch of Amazon EC2 instances, just pay them for the time you need them, and distribute the brute force attack. Most algorithms work far better on GPUs rather than CPUS but its mostly more convinient because you can rent and share CPUs better.
So what does this mean? In theory you can bust into any password or key protected account in a matter of minutes if you can distribute the attack. In practice this mostly doesnt work because the authentication server limits the number of requests, therefore limiting your ability to apply brute force.
In this case it sounds like you are completly giving away the data to an potential attacker.
So if you store files on my computer and i want to crack them, I can distribute them to a huge computing cloud (that only costs me a few hundret buck for that short endevour) and bust your signature.
If this is possible, not even long digital signatuers are secure. Of course the amount of computing power needed rises exponentially with the lengh of the key but thats not really enough.
So after this introduction ... So what is the way to go?
In my opinion the best would be to generate a unique signature for each and every file and leave them at the client side. Do NOT use a shared signature.
So if i send over fileA.zip to your server I keep a fileA.zip.key on my side that enables me to decypher it. In this way you can bust one file, but my account is not comporomised.
If you do a 2048 bit key or somethig that is sotred for every file individually this should be really quite secure.
You can also think about using bcrypt where you can deliberately slow down the encryption process, to make attacks harder. but keep in mind that this also slows down the decyphering on your end. But if its backup storage or something only this would be a considerable security gain.
Comment: -1: brute force efficient? You're mixing a proven weak hashing algorithm (MD5) with perfectly good encryption algorithms such as AES. And there's still not enough computing power on this planet to effectively bruteforce a modern hashing/encryption algorithm that don't have an inherent weakness.
Comment: i agree that aes is still secure. but brute force doesnt care about the algorithm. thats why its called brute force. of course busting an md5 argument is way easier than busting an aes key because its far longer, but the principile is the same. "A device that could check a billion billion (1018) AES keys per second would in theory require about 3×1051 years to exhaust the 256-bit key space." but its very well possible that a lack of entrophy in the keyspae is found as it was the case with ibms ssl implementation and openssl. i just suggested not to use 1  key for all files for more security.

encryption algorithms


How to add holes to an extruded path object - I wish to modle a filmstrip

Question: I plan to animate a filmstrip (growing and curling around itself), using the path or bezier tool. BUT how could I get the sprocket holes as well; is there a system where you can extrude and add a regular set of boolean holes for example? Thanks in advance.
Comment: Wow! Thanks you very much for this detailed explanation.
Answer:
Start with a flat plane (film strip) and make an array of perforations (using a couple arrays) to punch holes in the plane. I have used academy 4 perf per frame here. I start with a box outside of the film plane, so that I know where it would appear o the previous film frame.

Then we array the plane, which has holes in it, and fit that to a curve. Make sure that all of the objects have their Origin Cleared at the centre (Alt-C).

BEWARE! Down't rotate the curve in the normal Z direction (up) as this will cause the film strip to pinch along the side which it would not do.

EDIT: Bonus material using a UV unwrap.
Answer: I would just use a cylinder to cut holes in a single film strip, and then use the array modifier to multiply that piece of film in a line. I don't know if this is what you're looking for, but hopefully it works.


Parse a http response from file

Question: I have a http response which I have written in a file using following calls
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, responseCallback);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, responseFile);

static size_t responseCallback(void *ptr, size_t size, size_t nmemb, void *userdata)
{
   int written = fwrite(ptr, size, nmemb, (FILE *)userdata);
   return written;
}

Now I am trying to read back that data from file using
void getAccessToken()
{
  long len;
  char *buf;
  fseek(responseFile,0,SEEK_SET); //go to beg.
  fseek(responseFile,0,SEEK_END); //go to end
  len=ftell(responseFile); //get position at end (length)
  fseek(responseFile,0,SEEK_SET); //go to beg.
  buf=new char[len];
  fread(buf,len,1,responseFile); //read into buffer

      //some operation to be performed on buf

  delete [] buf;

}

The operation that I want to perform is retrieving a particular token that I have received in the http response ( which could be done using strtok). I have been able to get the data, but at the end some junk characters are also appearing.
Also, the size of the file is 1743 bytes(the same size of data is written by curl call back function) but the buf is being filled till 3510 bytes of data. How is this possible? Any clues ?
Answer:
Is anything wrong that I am doing here ?

Yes, you're not keeping track of return values at all. I/O calls can fail, and you must check the return values before depending on the results.
Comment: Thanks unwind. I have been able to solve the basic problem of getting the junk data but the problem now that I am facing is data after performing operations is not correct

Parse a http response from file


How to remove these red lines in Illustrator?

Question:
Hi I wonder if there is a way I can get rid of these red lines in Adobe Illustrator? I can't seem to find where to disable them
Answer: You or the maker of the image has (maybe accidentally) drawn some slices. If it was not an accident you can make them invisible by selecting Hide Slices in the View menu.
In the next example I have drawn one path (=the grey rectangle) and one slice.

Slices are drawn by using the Slice tool. Slices occur in the layers panel as objects. It's here the object named Slice. It's selected and the selection occurs as a colored square in the layers panel.
Select your unwanted slices in the layers panel (click the circle, see how the square appears) or by clicking with the slice selection tool (it's hidden below the Slice tool) and press DEL.
To know more of slicing see the documentation. Start for ex. here https://helpx.adobe.com/ca/illustrator/using/slicing-cutting-tool-gallery.html


From relativistic equation to find Dirac matrices

Question: Is this possible and then how?
$$((\gamma \otimes \mathbf\sigma)\bullet\mathbf p)(\gamma^\prime\otimes\mathbf 1_2) = \gamma\gamma^\prime\otimes\sigma \bullet \mathbf p $$
where $\gamma$ and $\gamma^\prime$ are using to factorize later we see $\gamma^0=\gamma^\prime\otimes1_2$,$\gamma^1=\gamma\otimes\sigma_1$,$\gamma^2=\gamma\otimes\sigma_2$ $\gamma^3=\gamma\otimes\sigma_3$  where $\gamma^1,\gamma^2,\gamma3$ are Dirac Matrices, $ \mathbf\sigma $ is Pauli Spin Matrix, p is momentum four vector $p=(E/c,-p_1,-p_2,-p_3)$ and $1_2$ is 2x2 unit matrix i.e$$1_2 = \left(\begin{matrix} 1 & 0  \\ 0 & 1\end{matrix}\right)$$ $$\sigma_1 = \left(\begin{matrix} 0 & 1  \\ 1 & 0\end{matrix}\right)$$ $$\sigma_2 = \left(\begin{matrix} 0 & -i  \\ i & 0\end{matrix}\right)$$$$\sigma_3 = \left(\begin{matrix} 1 & 0  \\ 0 & -1\end{matrix}\right)$$
I know about matrix mixed product( ($A\otimes B)(C \otimes D)=AC\otimes BD$ ) but I can't understand how this possible I shall be happy to get any answer.
Here I put this Image from Where I have got this query.
Thanks.
Answer: I think this may be an answer.I am not quite sure about that.
$$(\gamma^\prime\otimes 1_2)\{(\gamma\otimes\pmb\sigma)\}\bullet \pmb p\} =\{(\gamma^\prime\otimes 1_2)(\gamma\otimes\pmb\sigma)\}\bullet \pmb p$$
[Here I assume $(\gamma^\prime\otimes 1_2)$ just like scaler]
$$=\{\gamma^\prime\gamma\otimes1_2\pmb\sigma\}\bullet \pmb p$$
[By using mixed product theorem $(A\otimes B).(C\otimes D)=AC\otimes BD$]$$=(\gamma^\prime\gamma\otimes\pmb \sigma)\bullet p$$
Similarly
$$\{(\gamma\otimes\pmb \sigma)\bullet p\}.\{\gamma^\prime\otimes 1_2\}=\{(\gamma\otimes\pmb \sigma).(\gamma^\prime\otimes 1_2)\}\bullet \pmb p=(\gamma\gamma^\prime\otimes\pmb\sigma 1_2)\bullet p= (\gamma\gamma^\prime\otimes\pmb\sigma)\bullet p$$
Note:I am not sure that there exist any such type  rule to exchange inner product with direct product.
Comment: Well I'm glad you found what you were looking. You might want to see if you can start with Klein-Gordon relativistic wave equation and generate the Dirac equation - which is how Dirac found the $\gamma$ matrices - or the Dirac Clifford algebra
Comment: Hi baponkar, please don't paste screenshots of math. We have MathJax built into the site for this reason (and you even used it in your question!).
Comment: @CinaedSimson I'm currently trying to reconstruct his original reasoning, but It's still unclear how he deduced the chiral representation of the $\gamma$s (or any other equivalent representation, for that matter). All I have obtained so far - starting from "factoring" the Klein-Gordon equation - is the anti-commutation relation $\{\gamma^\mu, \gamma^\nu\}=2g^{\mu\nu}\mathbb{I}_4$. It's hard to believe that he just guessed correctly, by "extending"
the Pauli matrices. There are quite a few ways of doing that


Difference between key encipherment and data encipherment?

Question: In the context of SSL/TLS certificates, what is the difference between key encipherment and data encipherment? What are some examples that highlights the difference?
Comment: This does not provide an answer to the question. Once you have sufficient [reputation](https://security.stackexchange.com/help/whats-reputation) you will be able to [comment on any post](https://security.stackexchange.com/help/privileges/comment); instead, [provide answers that don't require clarification from the asker](https://meta.stackexchange.com/questions/214173/why-do-i-need-50-reputation-to-comment-what-can-i-do-instead). - [From Review](/review/low-quality-posts/118983)
Comment: I think for the confusion to end,you need adapt transparency way of distributing private keys to your customers than using PDF which is not accurately performing well with your clients.
Answer: Key encipherment means that the key in the certificate is used to encrypt another cryptographic key (which is not part of the application data). This is used within TLS in the RSA key exchange, where the pre-master secret (from which the symmetric encryption key is derived) is generated by the client, then encrypted with the servers public key and send to the server and decrypted there with the servers private key.
Data encipherment means that the key in the certificate is used to encrypt application data. This is not used in TLS. But certificates are not only used for TLS (for example also in S/MIME, VPN, signing of documents ...) so there might be use cases where this is needed.
Comment: 1) for HTTPS and any other SSL/TLS, up to 1.2 it is best, and for 1.3 it is required, to use 'forward secret' key-exchanges (DHE=Diffie-Hellman or ECDHE=Elliptic Curve DH _ephemeral_) which use the certificated key for signing and not either kind of encryption and thus requires digSign in KU. (And as Steffen says, you don't normally choose KU -- or EKU; the CA does based on its 'product line'.)
Comment: Based on what you said. are my statements below true. 1) If i want to host a website with https on a server, buy a cert with Key Encipherment. 2) if i want to do message signing (i.e. payload signing such that it is not tampered with - but the payload itself is visible as clear text), buy a cert with Data Encipherment 3) If i want to use the same cert to do both 1) and 2), buy a cert with both Key Encipherment and Data Encipherment. 4) If i omit both usages when purchasing the cert, is it still usable to do 1) and 2) above ?
Comment: @joedotnot: 1) yes 2) signing is not encryption, i.e. you need key usage DigitalSignature. Apart from that you usually not buy a certificate with a specific key usage but a certificate for a specific purpose and the key usage is set by the issuer accordingly.
Comment: Reference https://ldapwiki.com/wiki/KeyEncipherment
Answer: Key encipherment is Use when a certificate will be used with a protocol that encrypts keys. An example is S/MIME enveloping, where a fast (symmetric) key is encrypted with the public key from the certificate. SSL protocol also performs key encipherment.
Data Encipherment is used when when the public key is used for encrypting user data, other than cryptographic keys.


How can a Photon have a "frequency"?

Question: I picture light ray as a composition of photons with an energy equal to the frequency of the light ray according to $E=hf$. Is this the good way to picture this? Although I can solve elementary problems with the formulas, I've never really been comfortable with the idea of an object having or being related to a "frequency". Do I need to learn quantum field theory to really understand this?
Comment: Possibly related: http://physics.stackexchange.com/q/18563/2451 and http://physics.stackexchange.com/q/18062/2451
Answer: All you need is quantum mechanics, i.e. that nature in the microcosm is dual,sometimes it can manifest wave properties and sometimes particle properties.
It depends on the measurement/experiment if the wave or the particle nature will manifest itself. Electrons manifest this duality: in the two slit experiment their wave nature appears governed by the de Broglie wavelength. Photons do the same too, displaying the wavelength/frequency associated with the collective classical electromagnetic wave.
The classical electromagnetic wave is built out of photons in a consistent way, and you could study this link if you are interested in this more complicated problem.
Answer: It is the quantum wave field $\psi$ (probability amplitude) for a particle which may have oscillations. So for a wave pulse advancing smoothly in some direction the oscillations inside the pulse correspond to the photon frequency and wavelength meanwhile the general movement of the pulse would be associated with the smooth movement of the particle. quite different. So the photon itself doesn't wiggle with the photon frequency, just the underlying abstract quantum field used to describe where we can find it.
Feynman said wave particle duality and quantum mechanics are very difficult to fully comprehend but we have to somehow get to grips with the dilemmas as the quantum mechanics results are outstandingly correct and accurate.
Answer: No, sometimes photons exhibit properties of a particle, and other times it exhibits properties of a wave, therefore having a "frequency" and at the same time being a particle.
Answer: I like what Juliana Mortenson discovered. She proposed that Planck originally derived the energy equation as $E = h \cdot$  measurement interval $\cdot f$. In later papers, Planck "assumed" the measurement interval to be 1 second to correspond to the dimensions of frequency and then attached it to $h$ so that units of $h$ became joules-sec. Originally though, $h$ should have had units joules/osc with measurement interval in sec and frequency in osc/sec. If you use this logic, every single wave of light has energy h in joules. Over 1 second, you would get $E=hf$, as expected.
Note that in modern physics, the units of f were changed from cycles/sec to 1/sec to satisfy $E=hf$. Makes no sense.
With $E=hft$, it is easy to see why ultraviolet light with its higher frequency has more energy in one second than infrared with its relatively lower frequency. It is also easy to determine the mass and momentum of a single wave of light.
I studied physics and electrical engineering years ago (1970s). This correction to Planck's constant makes sense to me. However, someone with current credentials and access to Planck's early papers needs to check this out. I believe this is very important and should be either exposed as a hoax or trumpeted as a breakthrough that possibly addresses Einstein concerns about hidden variables.
Comment: This sounds highly suspect. I took a look at the paper you mention and I cannot find any support for its conclusions.
Comment: I looked for confirmation as well, without success. Mortenson may have found a paper by Planck earlier than 1900, before he dropped the time variable. In any case, I feel that her contention should be explored.  So I raised a separate question:  http://physics.stackexchange.com/questions/305200/e-hf-vs-e-hft-does-plancks-constant-have-a-hidden-variable

How can a Photon have a "frequency"?