Why do leptons and quarks mix?

Question: Is the fact that weak eigenstates are not mass eigenstates completely arbitrary? Or is there a deeper reason for the existence of the PMNS and CKM matrices?
Answer: Yes, there is a very good reason why leptons and quarks mix. It would be shocking if they didn't mix. Just to avoid confusions, we're not saying that leptons and quarks mix with each other: they don't because leptons are eigenstates of color or baryon charge with  different eigenvalues than quarks which are also eigenstates: this difference prevents mixing between leptons and quarks because the mass terms would violate the charge conservation (for color and/or baryon charge - the former would be an internal inconsistency; the latter would be consistent internally but it would imply that the proton would decay rapidly).
However, leptons mix with leptons and quarks mix with quarks.
The theory - whether you mean quantum field theory, string theory, or anything else that may be found in the future - produces several generations of quarks. Let's talk about upper quarks, $u_1, u_2, u_3$ - normally called $u,c,t$. For each of them, the quantum field theory produces a 4-component Dirac spinor. It is actually more correct to decompose it into 2-component spinors, but let us use the 4-component formalism here.
If those three Dirac spinors have some kinetic terms, they may always be diagonalized so that the kinetic terms have the form
i$$\sum_i \overline{u_i}\partial^\mu \gamma_\mu u_i$$
There could be some general matrix $I_{ij}$ multiplying $\overline{u_i}$ and $u_j$ but by a proper transformation of the $u$ fields, it can be set to the identity.
However, once you bring the kinetic terms into this form - by choosing the right combinations - there is not much freedom left. Generally, the underlying quantum field theory or string theory or whatever will also produce arbitrarily bilinear interactions of the form
$$-\sum_{i,j} M_{ij}\overline{u_i} u_j$$
The matrix $M_{ij}$ may be chosen Hermitean because the antihermitean part would cancel, anyway, as the product of two $u$'s is Hermitian - it gets complex-conjugated under the exchange of $i,j$. But aside from the Hermiticity, $M_{ij}$ is a completely unknown and arbitrary matrix. There is no reason for it to be diagonal; even if it is a generic Hermitian matrix with off-diagonal terms, it satisfies all symmetries and consistency conditions we normally require. In particular, it conserves the electroweak SU(2) isospin, U(1) hypercharge, SU(3) color, the angular momentum (only spin in this case), and the rest of the Lorentz symmetry (and also the baryon number). It's not surprising because from the viewpoint of the values of all these charges, the three generations of the quarks are indistinguishable, so coupling $\overline{u_1}$ with $u_2$ is as good as coupling it with $u_1$.
Now, the kinetic term is still invariant under SU(3) rotations of the three quark flavors. You may use this surviving symmetry to diagonalize the matrix $M_{ij}$. In the right basis, it is diagonal. However, you won't be able to diagonalize the lower-type quark mass matrix at the same moment.
A similar mass matrix exists for the down-type quarks. However, the Standard Model has an SU(2) electroweak isospin symmetry which is very important, at any energy scale. For a chosen triplet of fields $u_1,u_2,u_3$ of the up-type quarks for which the mass matrix was diagonal, we have their SU(2) partners - obtained by applying an SU(2) generator - $d_1,d_2,d_3$. However, the mass matrix for these three down-type quarks, which is $M^d_{ij}$, another Hermitian matrix, can be and probably will be non-diagonal once again. Right now, we don't have any symmetry left.
So it means that the mass of the three down-type quarks is expressed by a $3\times 3$ Hermitian matrix that is not necessarily diagonal, and there is no useful residual symmetry left that would allow us to diagonalize the matrix. So we must accept it as a fact: the three mass eigenstates among the down-type quarks will be different from the SU(2) partners of the three mass eigenstates among the three up-type quarks. You can't do anything about it. The CKM matrix is what relates these two bases.
The case of the leptons is analogous if one includes the masses for the neutrinos.
It would be shocking to find the matrix to be diagonal in the same basis because there is no real "need" - such as symmetry or renormalizability or anomaly cancellation - that would dictate that the off-diagonal elements of the mass matrix have to vanish. Because it doesn't have to happen, it probably won't happen - this principle is known as Gell-Mann's totalitarian (or anarchic) principle. All terms that are allowed by the symmetries and consistency will occur with nonzero coefficients - most likely with coefficients that are "natural" i.e. comparable to one (or the typical mass scale). In particular, this is also true for the off-diagonal elements of the mass matrices relating several generations of particles that are otherwise equivalent when it comes to their conserved charges. So the mixing generally occurs.
Nature seems to satisfy the anarchic principle, at least with some nonzero coefficients - not necessarily comparable to one. So it follows that it produces two pretty much random Hermitian matrices for the up-type quarks and down-type quarks, and the bases in which these two matrices can be diagonalized won't have the property that one basis may be obtained by SU(2)-rotating the other one. That's why the CKM matrix is not equal to the identity and one needs to discuss the mixing. Similarly for the PMNS matrix for the leptons. "Guessing" that the matrices should be diagonal is as unnatural as guessing that the wealth of Bill Gates is exactly 56,789,012,345 dollars. There's no reason why a "nice" guess like that should be true, so it will probably not be true. The same thing holds for the guess that the off-diagonal entries of the mass matrices vanish.
Comment: A very thorough answer, but not particularly helpful. Invoking the totalatarian principle is all well and good, but it an unsatisfactory answer to *"Why?":* it is equivalent to saying *"We don't know, that's just the way it is".* Worse, this doesn't address the question of why both strong and electro-magnetic interactions *do* respect flavor. If the explanation is *"It would be weird if they did",* then we have a weird fact on our hands...
Comment: That was a very satisfying answer. One question though: the CKM matrix *is* nearly diagonal, with the weak interaction being largest between quarks of the same generation. Is this just a happy coincidence?
Comment: @dmckee Agree. Let me also note that Strong CP Problem is an example of exactly such "weird" restriction.
Comment: @dmckee I think the reason why different flavors mix is because they have the same properties - the SU(2) isospin, U(1) hypercharge, SU(3) color and so on. Luboš Motl already noted this in his answer.
Answer: If you assume that whatever generates the mixing patterns of quarks and leptons (beyond the SM) has no underlying symmetry and that nature chose $V^{CKM}$ and $V^{PMNS}$ randomly within the set of $3\times3$ unitary matrices, then it is natural to expect mixing between families because the probability of randomly selecting $V^{CKM}=V^{PMNS}={\mathbb 1}$ is zero. From this anarchical scenario one would expect both mixing matrices to have no special pattern. This works quite well for neutrino mixing where the PMNS matrix is an archetypical $3\times3$ unitary matrix (meaning that the probability of generating within the set of unitary matrices one that has 2 large angles and one small one is quite large). The problem is that anarchy fails to explain quark mixing badly since the CKM matrix is very special, it is a small perturbation of the unit matrix.
The special mixing pattern of quarks suggest some underlying flavor symmetry (at least in the quark sector). So yes, I would believe that there is some deep reason for nature's specific choice of mixing patterns. One then has to explain why neutrinos and quarks mix so differently. There are several options one can consider: Either flavor symmetry is only present in the quark sector (this is not nice for Grand Unification) and neutrinos are anarchichal, or flavor symmetry is in both sectors but because of some mechanism it is completely scrambled for neutrinos (e.g. because of the See-saw mechanism). A third option is that each sector has its own flavor symmetry. And so on... it becomes complicated and no known model seems to stand out for the moment.
Answer: I think the short answer is that they mix, because nothing prevents them from not doing it.
The fact is that states with equal quantum numbers in general do mix. The experiment will tell you how much they mix.
If you find a very or almost zero mixing you can start thinking to add a symmetry that prevents some mixing to happen, which was the case of a lot of neutrino mass models until the mixing angle $\theta_{13}$ has been measured to be non-zero.
Answer: Short speculative answer: I think that the leptons (electron, muon, and tau) mix because they are excitations of the same state. So nature can mix them the same way She can mix different excitations of a hydrogen atom.
Comment: For a recent reference to these sorts of ideas in the literature, see Phys.Lett.B548:35-44,2002, "Search for Excited Electrons at HERA"
Authors: H1 Collaboration: C. Adloff, et al  link: http://arxiv.org/abs/hep-ex/0207038v1

Why do leptons and quarks mix?


How to make strokes disappear on an InkCanvas

Question: I am completely new to this forum, and still a beginner on WPF.
I am working on a project that requires the strokes on an inkcanvas to be animated. One of the animations required is "disappearing". I want to make the selected strokes gradually disappear with the click of a button, but appear at the end of the animation.
Since there is no opacity property for stroke, I tried using the ColorAnimation class along with storyboards. I have failed to make this code work, as I cannot target the strokes either using themselves or using their names, since they don't have any.
Right now I am thinking of implementing this system by gradually changing the color of the strokes to the color of the background, and at the end, resetting it back to its initial value. This is a costly loop, but I have no other ideas.
I would appreciate it if there are any other solutions you might share with me.
Thanks in advance.
Edit: I have not answered the comments, as I was dealing with other parts of the same project.
I have tried using the Alpha values that are stored in the DrawingAttributes, but I cannot change the value as it is not a variable. The same goes with RGB values. I have no idea on how to make the strokes disappear in a loop. I have already implemented most of the project, so I just need something to slowly make them disappear. Below you can find an example where I change the stroke itself to animate it.
 private int dropOffset = 1;
    private void DropAnimation()
    {
        m = new Matrix();
        m.Translate(0, dropOffset);

        animStrokes.Transform(m, false);
        YChange += dropOffset;
        dropOffset += 2;
    }

And in another class, I have
public void AnimateStrokes(Dispatcher canvasDispatch)
    {
        Stopwatch initial = Stopwatch.StartNew();
        while (initial.ElapsedMilliseconds < 2000)
        {
            foreach (Animation ai in AnimationList)
            {
                ai.animateSelected();
            }
            canvasDispatch.Invoke(new Action(() => { }), DispatcherPriority.Render);
            Thread.Sleep(50);
        }
        foreach (Animation a in AnimationList)
        {
            a.undoAnimation();
        }
        canvasDispatch.Invoke(new Action(() => { }), DispatcherPriority.Render);
    }

I know that it's not healthy to pass dispatcher like this, but it suffices for now.
Thanks again in advance.
Comment: You can edit the Stroke colour's alpha value, but you'll still have to loop through them. You say it's a costly loop, but have you benchmarked it or are you assuming it's too slow?
Answer: InkCanvas1.DefaultDrawingAttributes.Color = Color.FromArgb(100, 0, 255, 255);

Might be a bit late but help for others none the less! The 100 is the alpha value which basically acts like an opacity value!! Mess with that and you will be able to change how transparent your strokes are :)


Will economic discrepancy between democratic and authoritarian states decrease over time?

Question: The most popular explanation for the economic discrepancy between different states is that in the authoritarian states the leaders need corrupt extractive institution to preserve their power. However, the technological progress makes it easier to retain power and fight with opposition (by using advanced tracking technics like face recognition or by implementing incentives to stay loyal such as social scores in China). Does it mean that leaders of authoritarian states will in future rely less on corrupt subordinates, and, therefore, the negative economic effects will decrease, making the discrepancy between authoritarian and democratic states narrower? If so, will the effect be large?
Comment: Sounds suspiciously like an essay question if you ask me.
Comment: @EB3112 it would be interesting to know what reason might be for asking an essay question here! It might be rational to ask a solution for a formal problem (because it is impossible to prove someone copied it and solved him/herself), but copying an essay answer from here is a direct way to get blamed for plagiarism. Regarding my motivation: as a person living in an authoritarian state, investing into emerging (=often authoritarian) countries and interested in institutional economics, I have a triple interest in the topic
Comment: We'll have to agree to disagree on the motivation and utility of collective feedback on a very specific question. Despite that, you have stated your interests are sincere. Discussion over.
Answer: As you point out, going by the typical doctrine of extractive institutions from Why Nations Fail (Acemoglu), we have this basic framework:

Disenfranchisement of private sector leads to capital flight / brain
drain
If long-lasted, this leads to slower economic growth and potentially
a failed state

However, I think we may want more granularity here. A functioning capital market acts as a way to have an efficient allocation of resources, facilitating the financing of the "best" projects, theoretically leading to better overall conditions. The similarity between China's macro picture and the developed world is more a function of its progress in capital markets rather than anything else. Extractive institutions can exist in market-oriented economies as well; government structure is exogenous in this sense.
However, what tends to happen in market economies is that blatant abuse of shareholder capital can be met with well-defined and effective laws. The execution of asset manager Lai Xiaomin earlier this year shows a unique angle into the intersection of "authoritarianism" and capital markets. Transparency and enforceability make big differences here. This was also part of the thinking that we see stated in The Principles of the Social Credit System (bear in mind IP theft and even fake food were major problems that China faced in the 90s, leading up to the draft regulations):

"Untrustworthiness is a problematic market phenomenon resulting in
both tangible fiscal losses and intangible damage to the growth of the
socialist market economy and society as a whole."

Trivium  has an excellent paper on the subject for those that want to read more.
In this way, the state has a vested interest in keeping markets reasonably functional and holding violators accountable. With somewhat rational assumptions, the state can make more money the "honest" way as fiscal revenue increases more from a thriving economy than by draconian siphoning off the private sector.
Ultimately, the state finances itself from the private sector one way or another. The question is, can it convey a reasonable pareto improvement for the public by taking in resources. If it's reasonable, the state is likely to continue, but if policy veers into the confiscatory or makes unpredictable moves it may deter growth, using the conventional political economy framework. Whether or not emerging technologies like facial recognition and credit scores will allow the perpetuation of a dictatorship is certainly possible, but its ability to narrow the gap with modern economies is questionable.
Conclusion
Again, the answer is it depends. If the states with the next-gen policy tools you envision are simply high-tech command economies, then, no -- throwing tech at it won't solve resource allocation problems. The absence of mature markets would still create big discrepancies. On the other hand, if we have authoritarian regimes with a functional capital market, then the greater visibility into the private sector could theoretically help the state optimize its rent seeking.
For further research on bureaucracies maximizing their income, see Niskanen's work.


Multi-variate normal distribution covariance matrix

Question: Let x ~ CN (0,a) and y ~ CN(0,b), with a, b positive constants. Let z = x + y. I need to find the distribution of the vector t=[x; z]. My basic understanding is that vector t is also complex Gaussian distributed and it has  mean [0;0]. I do not know how to compute the covariance matrix of t. Can anybody help me? Thank you.
Answer: Because x and y are independent random variables, Var(z) = Var(x) + Var(y) = a + b;
One way to compute the covariance Cov(x,z) = Cov(z,x) is to simply apply the definition of covariance.
Cov(x,z) = E[xz*]-E[x]E[z*], and here we have E[x]=E[z*]=0.
Cov(x,z) = E[x(x+y)* ]=E[xx*+xy*]=E[xx*]+E[xy*]. Note that E[xy*]=E[x]E[y*]=0.
Cov(x,z) = E[xx*].
Observe that Var(x)= E[xx*]-E[x]E[x*] = 1, and E[x]=E[x*]=0 in this exercise.
Finally, Cov(x,y) = a.
Having computed the variances and co variances it is then straightforward to write the correlation matrix.
Comment: OK, I see. I try to correct the answer then.
Comment: These are not the correct definitions for the Complex Normal distribution.  See https://en.wikipedia.org/wiki/Complex_normal_distribution#Mean_and_covariance.
Comment: Isn't it so that I have used the definitions for complex normal variables, while links points to definitions for complex normal vectors? If I understand correctly, x, y and z are complex normal variables, and I compute variance and covariance for normal variables (not vectors). Am I wrong? Thanks
Comment: You are wrong because you neglect to take complex conjugates.


Nethereum: How to get the list of pending transactions (haven't mined to a block yet)

Question: Currently I'm using GetBlockWithTransactionsByNumber by looping into the latest 200 blocks to get the transactions sent to my address. I can actually get my transaction through the block number but what about the pending transaction which has no block number yet (haven't mined to a block).
Answer: The txpool_content  inspection property can be queried to list the exact details of all the transactions currently pending for inclusion in the next block(s), as well as the ones that are being scheduled for future execution only.
Refernce : https://github.com/ethereum/go-ethereum/wiki/Management-APIs#txpool
This API is not directly available.So you can integrate the txpool_content API using the following code.
                  web3.currentProvider.sendAsync({
                         method: "txpool_content",
                         params: [],
                         jsonrpc: "2.0",
                         id: new Date().getTime()
                  }, function(error, result) {
                       console.log(result);
                       if (result.result.pending) {
                            if (result.result.pending[address]) {   //address -external input
                                var txnsCount =
                               Object.keys(result.result.pending[consumerAddress]).length;
                                 console.log("txnsCount: "+txnsCount);
                            }
                         }
              })

Comment: Thanks! I can now get the list of pending transactions using Nethereum's RpcRequest by passing txpool_content method.
Comment: @doohder Can you please explain/show how you were able to get list of pending transactions using Nethereum?


What is meant by a "single-letter" expression for the quantum channel capacity?

Question: The quantity $Q_1(\Phi) = max_{\rho} I_C(\rho, \Phi)$, is called one-letter capacity of channel $\Phi$. I want to know, what is meant by the term "single-letter" capacity here, often used in information theory? The term can be found for example in this article (with free access).
Comment: Hi, can you (1) edit your question to point to the abstract instead of the PDF, to help people with limited bandwidth, and (2) edit to explain how the term is used in the paper, and what you think it might mean?
Comment: There doesn't seem to be much quantum in this question given that, as recognised in the question, this is also an issue for classical channels. Second result on my google search give this answer: https://crypto.stackexchange.com/questions/71740/what-is-the-single-letter-characterization-of-the-secrecy-capacity
Answer: In a series of individual works, Lloyd, Shor, and then Devetak developed what is known as the "LSD Theorem," which gives a formula for the quantum capacity of a quantum channel. The result states that the quantum capacity $Q$ of a channel $\Phi$ is the regularization of the coherent information, written
$$Q(\Phi)=\frac{\lim_{n\rightarrow \infty}}{n}\max_\rho\{ I_C(\rho,\Phi^{\otimes n})\}=\frac{\lim_{n\rightarrow\infty}}{n}Q_n(\Phi)$$
The LSD formula is satisfying because it is analogous to Shannon's description of the classical capacity of a classical channel as the regularization of the mutual information. However, unlike in the classical case, there doesn't seem to be a way of simplifying the limit in the LSD formula, making the task of computing the capacity of a quantum channel untractable (e.g. we don't even know the quantum capacity of the qubit depolarizing channel in practical noise regimes).
Rather than altogether abandoning the study of quantum channel capacities, researchers looked for examples or classes of channels where the formula for $Q$ does simplify (for instance, this happens for the class of degradable quantum channels). Of particular note are quantum channels, which have a single-letter formula for their quantum capacity.
A "single-letter" (aka "one-shot" or "single-shot") formula is said to hold for the class of quantum channels for which to compute the full quantum capacity one only needs a single use of the channel, i.e. the channels $\Phi$ for which $Q_1(\Phi)=Q(\Phi)$, rather than needing to regularize over an unbounded number of channel uses i.e. $\Phi^{\otimes n}$ for larger and larger $n>0$.
One reason why, in general, $Q(\Phi)\neq Q_1(\Phi)$, and more broadly the issue with simplifying the regularization of the coherent information appears to stem from the fact that the quantum capacity, unlike the classical capacity, is not additive. For example, it is known that there exists a quantum channel $\Psi$ for which $$Q_2(\Psi)>0,\text{ and } Q_1(\Psi)=0,$$ demonstrating that the inequality $$Q_2(\Psi)>Q_1(\Psi)+Q_1(\Psi),$$ is strict. This surprising phenomenon, called superactivation, was discovered by Smith and Yard.
Comment: Thanks @Condo for that detailed explanation. Why is it called as "single-letter" formula?
Comment: @Micheal the term "single-letter" refers to the fact that the capacity can be calculated over a *single* use of the channel. The language comes from classical information theory, e.g. a "single-letter" expression for the mutual information is $I(X,Y)$ while a "multi-letter" expression is $I(X^n,Y^n)$ for $n>1$.
Answer: I think we have here superposition of double meaning of 'single-letter' term:

Single-letter in mathematical notation is just 'single-letter
symbol' used for mathematical quantities in equations and
expressions or as a formula name, with subscript or superscript
indices or labels, if necessary. There are already discussions
whether is a pain or gain here:
https://math.stackexchange.com/questions/24241/why-do-mathematicians-use-single-letter-variables

For this specific 'single-letter formula related to channel capacity' the source paper is Shannon's work:
C. E. Shannon, "A mathematical theory on communication," Bell System Technical
Journal, no. 27, pp. 379–423, October 1948
which is mentioned in already existing answer:
https://math.stackexchange.com/questions/164088/meaning-of-the-term-single-letter-formula
Shannon in his paper showed that the capacity of Discrete Memoryless Channels (DMCs)
$$
\{{\mathbb{A}},{\mathbb{B}},\{\textbf{P}\underset{B|A}(b|a):(a,b)\in {}{\mathbb{A}}\times {\mathbb{B}}\}\}
$$
is characterized by single letter formulae
$C=max\underset{P\underset{A}{}}{}I(A;B)$.


What kind of database is Cassandra DB?

Question: I was taking some courses to understand better the way Cassandra works (at academy.datastax.com) and by what I gathered it came to me that it is a Key Value Database because of the way it stores data based on a hash of the key and distributes to nodes accordingly. But everywhere I look people say Cassandra is a Column-Based Database and I can't see why.
Answer: From wiki.apache.org:

Cassandra is a partitioned row store, where rows are organized into tables with a required primary key. The first component of a table's primary key is the partition key; within a partition, rows are clustered by the remaining columns of the PK. Other columns may be indexed independent of the PK. This allows pervasive renormalization to "pre-build" result sets at update time, rather than doing expensive joins across the cluster.

https://dba.stackexchange.com/users/8783

"a partitioned row store" a.k.a. a column-oriented data store. Note it is not a true "Column-store". It is column-oriented or a column-family store. The terminology of partitioned row store versus column-oriented is less important than understanding what was shared though. If you walk into Cassandra thinking it is a true column store, you'll be surprised. – https://dba.stackexchange.com/users/55511

What kind of database is Cassandra DB?


Why did my well behaved cat suddenly start biting and meowing?

Question: I'm not even sure how to begin with this. I have a very well behaved and sweet cat who does not usually show aggression. Today, he walked over to me as if he wanted to cuddle, but then proceeded to bite and meow as I have never heard before. Every time his jaw moved, he would meow as if distressed. I thought this may have been a dental problem or something, but he was at the vet just a few weeks ago and they saw no problems. I allowed him to bite without jerking back, because that usually makes him stop and become cuddly. Instead of stopping, he started to pull very hard, like a dog tugging on a bone. He kept backing up, like he was trying to lead me somewhere, but stopped and slowly walked off as soon as I began to follow him.
I have no idea what this whole episode was about and I hope I described it well enough. Does anybody have any idea what could be happening?
Comment: Have there been any other behavior changes? Is your cat eating normally? Also, check for injuries - whether he's favoring a limb, moving awkwardly, or shows distress when you touch a certain part of his body (even with short haired cats the fur can hide injuries)
Comment: I hope your cat is vaccinated for rabies and the like.  I'd really recommend a vet visit with a description of all the symptoms
Comment: Is he neutered?
Comment: How old is this cat? Is he an outgoing or indoors cat? You don't have other pets, right?
Comment: If not vaccinated against rabies, the cat, and possibly the human, could be dead by now.
Answer: Sudden unexplained  behavior change always calls for a vet visit.
Answer: Your cat might be going through cat puberty, it makes them become aggressive and start wanting to go outside more. They will also start marking their territory by peeing everywhere.
Also, if your cat is running around your house like crazy and is grooming more than usual, he might have fleas.
In conclusion, check with your vet if you're not sure!
Answer: I would agree with keshlam, watch his diet and water intake, try to determine whether he has any health issues or is suffering from pain, just in case.
But to be honest, I think maybe he was just having one of those lets be naughty days, which I find hilarious. Give it a few days and watch closely.
Please note that if it's a young male, he might be getting ready for breeding, in any case, unless you want to breed, make sure to get them neutered.
But for males, it will especially make them much more mellow and content.
Answer: Could be cat puberty, but also could be rabies. I agree with keshlam on this one, go and see a vet. If you don't have the time, try a video vet consultation with Pawsquad or vetondemand depending on where you're based.


Point of view in Eudora Welty's "A Worn Path"

Question: In "A Worn Path" by Eudora Welty, how can the story's point of view be described? How does the point of view affect what we know about the situation?
Comment: This looks like a homework question. That's not a problem *per se*, but asking us to *do your homework for you* certainly is. Please [edit] your post to focus on one key aspect (your question as it currently stands is too broad), add what ***you*** think the answer might be, and then explain what it is that you're unsure about. For further guidance, see [ask] and take our site's [Tour]. :-)
Comment: Hi and welcome to Literature Stack Exchange. People who have reviewed your question seem to think it is too broad, so I have removed the sub-question about suspense to make it a bit more focused. It would be OK to ask the question about suspense as a separate post.
Answer: The point of view in 'A Worn Path' can be described as "third-person, limited, objective". That is, it is told from the point of view of a "third person", a narrator who not themselves a character in the story; it is "limited" (as opposed to "omniscient"), describing only things seen and experienced by the protagonist; and it is "objective" (as opposed to "subjective") describing appearances and events, but not the protagonist's thoughts.
The effect of "limited" narration is to give a strong impression of what it is like to see things from the perspective of the protagonist. For example, when she comes across a scarecrow, it is described like this:

Then there was something tall, black, and skinny there, moving before her.
A first she took it for a man. It could have been a man dancing in the field. But she stood still and listened, and it did not make a sound. It was silent as a ghost.
"Ghost," she said sharply, "who be you the ghost of? For I have heard of nary death close by."

This conveys the threat to the protagonist inherent in the landscape—the figure of a man is dangerous to her—and her sense of the numinous, the possibility that she might meet the ghost of someone who recently died.
However, because the narration is objective, the reader gets no insight into what the protagonist is thinking or intending, except for what she says out loud or reveals through her actions and expressions. This allows Welty to preserve suspense as to why the protagonist is making this difficult and dangerous journey.


Re-apply for UK tier 4 student visa

Question: I applied for a UK Tier 4 General Student visa. My bank statement had my name handwritten in English and not printed, as my bank had to write it down and stamp twice, since my information on the bank system is only accepted in my country's language, which is not English. They said this is the right way to certify it since applying for a bank statement with my name printed in English was going to take time due to system routines and approvals. I applied for an English printed name version anyway, but it came out after my appointment so I had to use the bank statement with my name hand written on it.
Surprisingly, I received a refusal because my name is handwritten and not printed as the rest of the statement is.
Briefly, the Clearance Officer states:

... however, the name on the bank statement has been handwritten. This leads me to question if the bank statement is truly owned by you.
UKVI has attempted to verify your bank statement and we have been unable to verify that this account is genuine. In accordance with paragraph 39B (c) of the Immigration Rules as we have been unable to verify your offer of financial circumstances, it has been discounted for the purpose of assessing your application.
I am therefore not satisfied that you have achieved 10 points under paragraphs 10 to 14 of Appendix C and do not meet the requirements to be granted entry clearance under Tier 4 (General) Student). I therefore refuse your application under paragraph 245ZV (c) of the Immigration Rules.

I was advised to re-apply since I have fixed the problem.
Would I still be refused since my bank statement (with printed name in English) was issued before the refusal date, or there is no relation/no rule for that?
Please note that bank statement date had not passed 30 days of my new visa application and the required amount didn't drop for more than 28 days. I have no time to apply for a new bank statement and my course is starting within two weeks.
Answer: Getting an important new document that addresses the refusal reason is a good basis for making a new application.
Can't be sure you will get your visa though. Once you show a verified statement, the visa office will make a decision on all of the information available and you may be refused on some other issue.

Re-apply for UK tier 4 student visa


FTP Connection with a WAMP server

Question: I downloaded and installed the latest version of WAMP package to my PC, and what I want to do is to connect it using FTP.
How to do this?
Answer: Apache, the server that is included in WAMP, is purely a web server, not a FTP server.  You need to set up an additional FTP server in order to be able to FTP into it.  One such server is the Filezilla Server, but there are other options too.
Answer: As farfromhome stated, the WAMP server hasn't got an FTP server included. If you do want a complete package without installing a seperate FTP server such as Filezilla, I recommend removing the WAMP server and installing complete package like XAMPP. XAMPP also includes FileZilla, but will set it up for you. However do note that XAMPP isn't meant for production purposes but development purposes.

FTP Connection with a WAMP server


Why copying files in one partition is much faster than copying files to another partition?

Question: Why is it significantly faster to copy files in one partition, than copying them into another?
Comment: Copying or moving? When moving within a partition, only the file system metadata that describes where the file is needs to be changed. Copying is often faster between separate physical hard drives.
Comment: @Bob so why cutting from a partition and pasting it to the same one is faster than pasting it to another one? so what happen in cutting process?
Comment: @AmirrezaNasiri ? "Cutting and pasting" in most file managers is an abstraction for moving. In other words, it *is* moving.
Answer: I don't think, copying on the same partition is faster. If you mean "moving", then look at the answer of Daniel Schwartz, he explains perfectly what's going on.
However if you mean a real copy-operation, I would not expect big performance differences as long as the operation takes place on the same disk. There may be some differences because of the time, the head-placement-operation will consume for harddrives (obviously not for SSDs), but I dont expect the impact of this to be very big.
Maybe there are also some differences because of different caching strategies of the OS, but also this effect, I dont expect to be very big.
What makes a great difference is copying from one to another disc. If the controler is fast, this is much faster than copying on the same disk, because either drive only has to read or to write, not to do both of the operations together (An exception of this rule may be small data, because of the hdd- and controller cache).
Please note that the OS can make links to files. In this case, the data will not be copied, but only a new reference to the data will be created (The file will also not consume double its space). This is very fast. But if you copy your files with explorer or standard shell commands, the files will be copied and not linked.
Answer: This is only an issue when copying data to and from the same hard disk drive. Data is read, cached and written back, but before it can start writing, the read/write head must be moved to empty space. Having the source and destination on seperate partions will ensure the corresponding disk sectors are physically far apart. The read/write head will constantly be moving back and forth. Often you can actually hear this.
While the head is moving, data cannot be transfered. Having source and destination on the same partition usually means less time spent moving the head around and therefore more efficient file transfers.
Comment: So why cutting from a partition and pasting it to the same one is faster than pasting it to another one? so what happen in cutting process?
Comment: @AmirrezaNasiri: If the destination is on the same partition, no files are physically moved. The file system maintains a table which is updated to reflect the fact that a certain range of bytes on the hard drive now makes up a file in a different directory. Updating this table requires very little data to be written. Moving files to a different partition involves the data itself being written to another part of the hard drive.
Answer: Let's compare to the two operations. First, copying a file from one partition to another:

The source is opened.
The destination is opened/created.
Space is allocated for the destination.
Every byte of file data is read from the source and written to the destination.
The destination file is closed.
The source file handle is set to unlink on close.
The source file handle is closed, unlinking the file.
Since that is (typically) the last reference to the source file data, the source file data has to be made free.

Now, copying a file from one folder to another on the same partition.

The source is opened.
A new hard link is created to the source file in the destination directory.
The handle to the source file is set to unlink on close.
The source handle is closed, unlinking it. The source file is still linked to the destination directory, so no free space needs to be changed.

Notice that there are fewer steps and also none of them require touching any of the file data or even the free space mapping.
Comment: @DavidSchwartz I would say the correct definition of moving is where the source file is, well, moved, "cut and pasted", to the destination. And copying would be creating a copy, leaving the source file as-is: as you describe in your first operation. I've never actually heard anyone refer to dragging and dropping as moving except when the file is actually being *moved*. So I'd say your second operation, with the hard link manipulation, is moving. Unless you're referring to copy-on-write, in which case... well, the source file would not be unlinked if that were the case.
Comment: I think what you're describing is commonly called *moving* files, not *copying*. Copying is when a _copy_ of a file is created, i.e. a second file with identical contents. After a copy operation you have 2 identical files, while after a move operation you still have 1 file, only in another location. Moving files is faster within the same partition; copying is often faster between different physical drives.
Comment: Windows users commonly use the term "move" to refer to dragging and dropping a file from one Window to another. This question was tagged *Windows*.


Updating wordpress from my Linux distro's repository

Question: I installed wordpress with apt, which ties in a lot of directories, so I suspect that using the built-in core update tool will break the installation; but wordfence constantly demands that I update using said tool -- which is probably much more up-to-date than the linux distro repository.  Do I have to wipe it out and start over in order to resolve this annoying bind?
Comment: Installing WordPress via `apt` is highly unusual, I do not recommend using a system level package manager such as `apt` to install WordPress.
Comment: Well, most Linux sysops seem to believe that installing software directly off the Internet should be highly unusual, and generally unwise.  That (and convenience) is the whole point of maintaining repositories.  Now, you may insist that "WordPress is different!" -- as do all proponents of off-repository packages -- and/or that Pop!OS & Ubuntu repositories are suspect.  While I can't confirm or deny that, in the end you have to trust someone, and I've decided to trust the people that millions of other users have chosen to trust.
Comment: In any case, I've already spent a week or two getting my wordpress site working again, so I'm not looking for advice on how to start over from scratch.
Comment: And I'm not providing advice on how to start over again, though it would take much less than a week to switch to a standard WP install, or one managed by a more appropriate package manager such as `composer`, if you do want that advice I'd suggest a new question. WP is not system level software, and isn't built with that assumption in mind, neither are WP plugins. It's not surprising that you're running into frustrations, though it is alarming that `apt` is not giving you the most recent stable version. You should report this as a major security concern to the repository maintainer.
Answer: I actually like the idea of updating wordpress via apt, as if it is done correctly it can eliminate some headache with installing and configuring the DB and web server while followinf best practice, but the question is how reliable it is to help you maintain wordpress up to date. from looking at https://packages.ubuntu.com/groovy/wordpress it seems that ubuntu LTS is stuck with wordpress 5.3.2, so a potential problem is that you will upgrade wordpress via the wordpress admin or wp-cli and one day when you will run apt update you will cause wordpress to actually be downgraded.
So if you go the apt way, you should probably commit yourself to keep using apt which might actually be OK in some specific settings.
As for wordfence.... with all due respect to them, wordpress setups can vary alot. I would use their "error" indications as a trigger to reevaluate what I am doing, but not as a bible.
Comment: Thanks, Mark.  That was my concern.  I don't really care if I have to accidentally downgrade occasionally (and temporarily), as long as it keeps working!  At the moment I'm just annoyed by the constant stream of "emergency" email messages from wordfence about the core not being up to date.  I tried to turn it off, but apparently they consider that update of primary importance.   I guess I'll just have to delete a lot of emails every day, unless you think it would be safe to use the built-in download updater.
Comment: v5.3.2 has known security vulnerabilities, upgrading to v5.3.8 would fix 16 security vulnerabilities, many of which are particularly serious, Wordfence is correct to be concerned
Comment: My Pop!OS repo gave me v5.6; wordfence wants 5.7.2 instead.  It also wanted me to add the Imagick plugin, which didn't work (many missing components it wanted me to install separately).  I'm starting to get a bit nervous about wordfence -- it acts like an exploit!
Comment: downgrading an application which stores anything in the DB is a non trivial thing with the main danger is that data structures had changed and the older version can not work with the new DB, therefor it is probably wise to avoid downgrades in all cost. As for wordfenece, it follows "crowd wisdom" regarding some security aspects like the need to run the latest and greatest as soon as it is out which might be a good idea if you are running a blog, but a stupid one if you have an "web app" based on wordpress.
Comment: If you want to have a blog and do not understand how to keep wordpress secure by yourself you should probably just follow all it advices, so yes, if apt gives you an old version you probably should not use it and stick with the wordpress upgrade tools
Comment: as for imagick, yeh you would like to have it installed as it will get you better image manipulation related feature, but having GD which is default in PHP should be enough if all you want is to have something working right away
Answer:
Do I have to wipe it out and start over in order to resolve this annoying bind?

No, since WordPress is not system level software, you can copy it, and its data elsewhere, or migrate it to a new instance installed using more standard means. You do not need to install and rebuild your site from scratch.
To get around WordFence however, you will need to update WordPress to the latest version as it wants, and do it using apt and only apt if you want to continue to use that tool. At the time of writing, that would be WordPress 5.7.2 with 5.8 due imminently. If you do this any method other than apt then the changes will be undone when apt next processes that package for an update, or a forced reinstall.
If your repository does not provide the current latest then you should contact them immediately, v5.3.2 has known security issues ( at the time of writing 5.3.8 is the newest version of the 5.3.x branch with hotfixes )


How comes the notion of an ensemble of identical states (e.g. in quantum tomography) does not contradict the no-cloning theorem?

Question: The no-cloning theorem states that it is impossible to create an independent and identical copy of an arbitrary unknown quantum state. How does that fit together with Quantum tomography, a process by which a quantum state is reconstructed using measurements on an ensemble of identical quantum states? It seems that if I want to reconstruct a quantum state, e.g. as a result of a quantum algorithm, first I have to make many copies of the (unknown) state, which I then measure. Why is there no contradiction?
Comment: Because quantum tomography does not allow you to make a perfect copy of the original state, only an approximate one.
Answer: Quantum tomography requires many copies of a quantum state, so we typically use it to describe some hardware device. For example, say you have a quantum widget where you can press a button and it will give you a pair of Bell state photons most of the time, but sometimes just gives a classical pair of photons. By pressing that button a bunch and measuring those photon pairs in a number of bases, you can statistically reconstruct density matrix corresponding to the quantum state my widget's output photon pairs.
Now the no-cloning theorem says that given a single copy of an unknown quantum state, you cannot create a perfect copy of it. Going back to the widget, if you press the button once, there's no way of deterministically determining if the pair of photons you got from that one press are in a Bell state or a classical state since you only get one measurement before you perturb your photon pair, and there's no way to copy that state to be able to make more measurements.
TLDR: Quantum tomography lets you statistically describe the outputs of a quantum source, while the no-cloning theorem limits what you learn from a single output of a quantum source.


How does Islam see those who only offer eid prayers and jumma prayers?

Question: If a person only performs eid and jumma prayers and is good with people and honest, how  does Islam see that person? Also, what could he face on the day of judgement?
Thanks in advance.
Answer: There are so many good deeds we can do. There are so many sins we might commit.
In principle each person on earth has a different combination of good deeds and sins, should we ask a question for each of these combinations?
Allah, the Judge on the Day of Judgement, has helped us by laying down principles by which he judges, and they are clear from the Quran.
In some cases we might have a good guess of someone's fate according to Quran. If he keeps up regular prayer at perfect timing, performed haj, does charity, cares for family, tries to do everything Allah advocates and tries his best to stay away from the sins, then very probably he will be successful in the Hereafter. If one denies Allah's signs, commits zina regularly, is selfish and greedy, is violent and commits regularly other sins and he never repents his sins then it is likely that he belongs to the fire.
But we are not 100% sure, because Allah is the Judge, and He is the most forgiving. See the following hadith of a prostitute who was forgiven for her (previous) sins.

Narrated Abu Huraira:
  The Prophet (PBUH) said, "While a dog was going round a well and was about to die of thirst, an Israeli prostitute saw it and took off her shoe and watered it. So Allah forgave her because of that good deed."
حَدَّثَنَا سَعِيدُ بْنُ تَلِيدٍ، حَدَّثَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي جَرِيرُ بْنُ حَازِمٍ، عَنْ أَيُّوبَ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَبِي هُرَيْرَةَ ـ رضى الله عنه ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم  " بَيْنَمَا كَلْبٌ يُطِيفُ بِرَكِيَّةٍ كَادَ يَقْتُلُهُ الْعَطَشُ، إِذْ رَأَتْهُ بَغِيٌّ مِنْ بَغَايَا بَنِي إِسْرَائِيلَ، فَنَزَعَتْ مُوقَهَا فَسَقَتْهُ، فَغُفِرَ لَهَا بِهِ
  Sahih al-Bukhari 3467

So what I am trying to say is, if you want to postulate about other's judgement, don't. Because you can't know for sure, Allah is the Judge. If you are worried about your own judgement, Allah knows everything we do and everything we conceal. So do as many good deeds (such as keeping up prayers at prescribed timing) as possible and refrain from sins as much as you can, why risk it?


Is there any way that Cognito Forms can display calculations only?

Question: I created a form with Cognito Forms, taking advantage from calculation function. Is there any chance in email I can get only the fields with calculation rather than fields with data entered.
Answer: I work with support for Cognito Forms.  To include just the fields you want, you can go into the email settings for the Notification or Confirmation emails.  Click on the Message field and then look in the Toolbar for "Insert Fields".  From there, you can craft your email to have only the fields you want.
You may also want to turn off "Include Entry Details" on the email setting so that the entry fields aren't included.  Otherwise you will get the fields you entered in the Message field and then also get any other fields with data.


Password security with short passwords

Question: I know that a long password is a good password. However, some sites require short passwords e.g. exactly 8 characters long.
Under those limitations, what is a good method of creating and memorizing a (relatively) secure password?
Comment: Using special characters, lower and upper case characters. Sites that require you to have exactly 8 character password are in my opinion weak because this makes guessing a lot easier.
Comment: If it comes to bruteforcing, all 8-digit combinations can be checked in less than a minute on modern GPU. So my advise is - do not care about this password. Choose anything outside of "100 most popular passwords". Because if it comes to bruteforcing, your efforts at typing in `g%y66I;O.` just will make no difference.
Answer: With eight characters you're extremely limited.
My advice - forget the memorisation part.
Generate an eight character string using a cryptographically secure pseudo random number generator, using letters (both cases), numbers and symbols. This will give your password 52 bits of entropy, which is the maximum you can achieve using this scheme. The alternative would be to start using unicode characters if these are supported, as a much greater key-space can be achieved. However, entry of these passwords may be difficult depending on the devices used. This would enable you to achieve the 80 bits necessary for a secure password.
Use a password manager to remember the password for you - this can be a memorable password, however make sure that this has enough entropy to make it secure. This is the best way to achieve memorability and security of your passwords, because you only have to remember one password.
For example, the LastPass generator is here. However, make sure your browser supports the Web Cryptography API, otherwise unfortunately this one falls back on current time as seed and uses a standard PRNG. You should be OK with any modern, popular browser.
Answer: Honestly "password security with short passwords" is an oxymoron.
On balance, if this is a site/service that you simply must use, I would just make the password as random as you reasonably can, possibly rely on a password manager app to use a random password you can't remember easily, and most importantly make the password different from any other passwords that you use anywhere else.
As long as your sessions are protected with HTTPS, preventing a man-in-the-middle from intercepting your password when you use it, you're probably reasonably safe from a brute force attack against your password agaisnt the website over the Internet. Especially if the site detects multiple failed login attempts in a short time period and impedes (slows down) or blocks further attempts from the same IP address.
But if the credentials database at that site is ever stolen, no 8 character password will hold up for long against a brute force attack.
Answer: When a web application limits passwords to short lengths, the user has almost nothing left  in his control. The main problem of short passwords is what you already know: password guessing and password cracking.
But you never know: may be the website is hashing your password (How to securely hash passwords?), salting it (How to store salt?), using a pepper (What is the purpose of a Pepper?) or even a combination of all this (Password Hashing add salt + pepper or is salt enough?).
What is left for you as a user is:

Check if the web application is running over HTTPS
Check if the web application does not store your password in plaintext (all web applications offer the Forgotten password option: execute it and check if you receive your former password in plain text, if that is the case you can almost be sure that they store your password in plaintext, unless the reverse decryption is used, but this scenario is rather rare in web applications)
If the above mentioned two points are satisfied and you still like the website in question, and you do not publish within it private information about yourself then combine special, alphanumeric as well as lower and upper case characters to your password

Password security with short passwords


How do you enable filesystems in /proc/filesystems?

Question: When I freshly installed Oracle Linux 7 and opened /proc/filesystems, I only saw xfs. But there was still mkfs commands for ext filesystems. So I created a block device, used the mkfs command to make it ext4, and then mounted it. Mount worked, and I checked /proc/filesystems to see ext4 added. Quite bizzare to me. I don't understand why it isn't in /proc/filesystems in the first place. Is there a typical way to enable filesystems onto /proc/filesystems?
Comment: Presumably the corresponding module was not yet loaded.
Answer: Just like hardware drivers, filesystems are available as kernel modules. And just like hardware drivers, they can be automatically loaded on demand. When you attempted the mount, the module was automatically loaded so you didn't have to modprobe it manually.
If you run lsmod you'll find the ext4 driver in there. If you unmount the filesystem, you can rmmod the module, lsmod again to see it's gone, then mount and lsmod and it'll be back. And whenever that module is loaded, it will contribute to the contents of /proc/filesystems.
Comment: ok then say that I want to load a different file system, for example fuse. Then, do I do that using modprobe?
Comment: FUSE is a special case. From the kernel's point of view it's a filesystem driver like ext4 or xfs, but really it's a bridge to several different filesystems implemented in userspace, like ntfs-3g. Normally you'd just issue a `mount -t ntfs-3g` command and the necessary modules would load automatically. If you need to do it manually for some reason, `modprobe fuse` should work (assuming it's included in the Oracle Linux build - I'm not actually familiar with that distribution)


Non-inertial uniform velocity frame

Question: Suppose, an observer is on a train which has only platform to stand but no walls. The train is moving with a constant velocity with reference to railroad. While moving, the observer feels strong wind against his face (if he faces towards front). If he puts a light ball on the platform, it automatically goes backwards due to wind. So, $F=ma$ does not work for him and the frame is non inertial to him. But, the train moves in uniform velocity. How come is this possible?
Comment: According to the observer, there is a wind blowing towards them, whereas from the ground observer the train is moving into the air.
Answer: In both scenarios the wind causes air drag (force $F$) which acts on the ball. For both observers the acceleration of the ball is $$a = \frac{F}{m}$$, but the ball has different initial velocity.
For the train rider the initial velocity is zero, and for the ground observer it equals the speed of the train.
Comment: i got it.. What a silly question mine was.. thanks.
Answer: If the train is moving with a "constant velocity", how is it a "Non-Inertial" reference frame?
Also if there any body moving in a fluid, the drag force is experienced even if the object is moving with constant speed.
The drag force is typically directly proportional to the velocity of the object, so all laws hold.
You can read more about it here.

Non-inertial uniform velocity frame


How can I identify a neutral wire?

Question: I'm trying to install a WeMo light switch in the place of an existing switch. I'm unsure which wire is neutral, or if I have one at all. Below is a photo of where I'm at currently. There are two switches in this one box, one of which is a fan and the other is a light. Both have only two wires coming out of them currently. There is a cap over a bundle of wires visible in the photo, but one of each of the wires from the switches is tied into this bundle. Is this the neutral wire or should I be looking for something else?
Comment: Looks like somebody forgot to cover the box before the painter got there.  Based on the photo, it doesn't look like there's a neutral in the box. Though I could be wrong, since it's tough to see all the wires.
Answer: The tone of your post scares me a little. Meaning if you are working in this box and cannot even identify the neutral, and are asking if one of the blacks is it, you should really get some experience or read up on basic home wiring before you start pulling things apart.
Bottom line is, the bundle of spliced whites in the bak of the box is your neutral.
Comment: It's hard to gauge competence based on a brief post, but I agree it sounds like the OP doesn't understand what how a basic switch is wired, much less one that requires a neutral.
Answer: Do you have more than one switch that controls the fan?  The light?
If this is new construction, typically a red wire is used for a 3-way switch, along with the typical black, white and bare copper.

White - common
Black - "Hot"
Red - 3-way wire
Copper - Ground (Not common!)

If this was a prior re-wire, you may have a case where the person doing it did not follow code.  Also, on a single switch system, you usually connect only the black (hot) wire as the switch will either open, or close, the circuit.  The whites will usually be all nutted together in the back of the box and not connected to the switch.  You may, or may not, have a bare copper wire connected to protect you from getting shocked should something breakdown inside the switch.
Comment: "Common" is not a generic term for a neutral, or grounded, conductor. Also, a red is not nearly always associated with a 3-way switch. It is just as common to see red used as a switched wire where the black may be constant hot, or part of two switch legs to a bath or ceiling fan, or a home run feed as part of a multi-wire branch circuit.

How can I identify a neutral wire?


is it possible to program an arduino with usb/ttl adapter

Question: i want to upload sketch into arduino uno using usb/ttl adapter.i tried searching for it, but found about programming arduino using another. can anyone help me.
Comment: I'm trying to understand your question, why can't you just program it by plugging a USB lead between a PC and the board.  Are you doing something special?
Comment: This may be obvious to you, but may not be to us.  What are you connecting to each end of the USB to TTL adapter cable?
Answer: The Arduino onboard USB to serial is connected to D0 (Rx) and D1 (Tx).
And without a USB connected to the Arduino you can use a USB/TTL adapter connected to D0 and D1 like this;
<code>Arduino  - USB/TTL
Tx       - Rx
Rx       - Tx
</code>
And to get the Arduino inte "programming" mode:

Automatic way, if the USB/TTL have CTS, connect it to Arduinos reset pin.
Manual way, press the reset button when the Arduino IDE starts to upload the code.
Answer: The Uno is normally programmed by its on-board USB adapter.
The Pro Mini does not have an on-board adapter, and is normally programmed with an external USB/serial adapter. This should also be possible with Uno.


Spilled vinegar on laptop, what to do?

Question: We accidentally spilt vinegar on netbook (EeePC). What should we do about it? Should we try to wash vinegar out or just let it dry?
Comment: +Nate Koppenhaver Kitchen + some bottles + 3yo daughter.
Comment: +1 how on earth did you do that? :)
Comment: Tolerably good copper etchant. Yipes!
Answer: I spilled lemonade on mine a week or 2 ago. It was a lot. My laptop immediately shut off (without being told). I immediately removed the battery and power cable (minimizing damage) and set about removing all the moisture. I have some experience taking laptops apart, but not a whole lot. I removed the keyboard and enough covers to access just about all the internal components. The keyboard was had most of the moisture and I found some drops on the pcb under the memory and various drops elsewhere on the pcb.
After soaking up as much as I could by spot blotting with a towel (don't just throw your towel on and mash away--only blot where you see drops) and letting it air dry for about a half hour, I reassembled and tried firing it up. It didn't work. It was as if the power button was inop. The power light did come on with the cable plugged in though.
The final step in solving this was spraying a lot of contact cleaner in the keyboard (had to remove the keyboard to do this) letting it soak a bit and then spraying copious ammounts of compressed air in/on it to dry it completely. After that, it's worked like a champ. If you got a lot of moisture on the pcb under the keyboard, it is probably more dangerous but still worth trying the contact cleaner & air dry method I used on the keyboard.
Here's a video to use as sort of a guide on disassembly http://www.youtube.com/watch?v=O0Jlq_mbqkI
Here's the contact cleaner I used for the keyboard: http://www.amazon.com/CRC-Electrical-Contact-Cleaner-Aerosol/dp/B003NTQCA2
Comment: Contact cleaner is definitely the way to go, it's designed to clean and protect electronic components. If you have spilled anything other than water on electronics you need to clean it, not just remove the liquid. In your case you spilled acidic vinegar, this will greatly increase the rate of corrosion/oxidisation of the electrical components, so you need to get as much off as possible.
Comment: @Hydaral, I disagree. Contact cleaner is isopropyl alcohol, it is meant for bare metal ends in electrical components to remove contaminants and ensure a better connection, it is not designed for circuit boards. Isopropyl alcohol is part water, meaning it is just as damaging to use as the vinegar itself. Penetrating fluid is the defacto method of safely lifting liquids off of circuit boards. If you do insist on using anything with Isopropyl alcohol, ensure it is 90%.
Comment: @MaQleod - I did failure analysis and repair at a contract manuf of printed circuit boards for years. Your comment is almost entirely bogus regarding alcohol. We used it all the time in the manual cleaning process. Also, our automated machines used water based solutions for cleaning at various steps in the process (see also http://www.smtnet.com/Forums/Index.cfm?CFApp=1&Message_ID=50372 for what some other folks still use). The only thing you got right was to suggest a higher concentration as this will dry faster.
Comment: @MaQleod, I have only used three different brands of contact cleaner, but all contained lubricants and protectants, and all were rapidly evaporating.
Comment: @MaQleod *> Isopropyl alcohol is part water, meaning it is just as damaging to use as the vinegar itself.* Isopropyl alcohol is isopropyl alcohol is isopropyl alcohol. it is *not* water. One is C3H8O, one is H2O. Now, isopropyl alcohol is usually sold mixed with water, and the 90%+ advice is sound - but a bit of pure water doesn't hurt, either, and is far less conductive than water with free ions (salty water, vinegar, etc.) in it. So, water itself is nowhere near as bad as vinegar either.
Answer: I think adding more liquid is not the way to go.
See here for some good tips.

Turn it off. If need be, do a hard shutdown.
Blot up excess liquid with a soft cloth. Don't use a wiping motion, that just pushes the liquid around.
Remove any cables, external drives, removable bays and external Network Cards.
Blot up the liquid that may have gotten on the removable media.

See the link for more.
Comment: Actually, adding liquid is what saved my laptop (though there is a bit more too it--see my answer)
Answer: Vinegar (acetic acid) is conductive, but has a very low conductivity rate. That said, it can still do damage as circuitry in a computer is fairly sensitive. Washing it out would be a bad idea, as the water that comes out of your tap would be far more conductive and far more damaging than the acid. The first step is always powering down and removing the battery as quickly as possible. If you flooded the device, it would be best to take as much apart as you can and remove the liquid carefully with a sterile cloth by blotting and not wiping. If there isn't much, simply blot lightly with a cloth, but don't pour it out, as it could flow over something that you would rather it didn't. You can use a light amount of penetrating fluid (such as wd-40) to get into harder to reach spots or remove moisture that the cloth is not removing. Leave the device in pieces for a while so that it dries out completely before attempting to power on again. You should also do a visual inspection to ensure that there are no blown or distended capacitors on the board.
Answer: This is just a guess, but I would:

Turn it off
Take out the battery and anything else that's removable
Let it dry, and dry anything possible with a cloth or something

Spilled vinegar on laptop, what to do?


How to stream Stored videos on red5?

Question: I'm able to LIVE Stream on red5 using Adobe Flash Media Live Encoder but I want to run stored videos as well on red5. Is it possible using FMLE or directly from the server. Kindly suggest.
Answer: You can play the stored videos as well,but they have to be in the flv format.And to convert any video to the flv format you can use FFMpeg http://ffmpeg.org/
If you need any more help,you can ask me.Cheers

How to stream Stored videos on red5?


What is meant by globally hyperbolic spacetime?

Question: Can somebody explain what is meant by globally hyperbolic spacetime?  What other kinds can there be?
Comment: Possible duplicate of [Intuitive meaning of Globally Hyperbolic](https://physics.stackexchange.com/q/229765/)
Comment: Did you google this?
Comment: @JMac I disagree that this is a duplicate because (A) this question does *not* ask for the intuitive explanation, so answers should reasonably include intuitive and formal explanations; and (B) this question additionally asks for what other types of spacetime there could be.  Just my two cents.
Answer: Wald has a nice chapter on "Causal Structure" that lays it all out very well.  He points out that there are three distinct definitions of globally hyperbolic spacetimes, but that they are precisely equivalent.  And I find Wald's to be the simplest, so that's what I'll explain here.  The basic idea is that there is a single surface that we might think of like an "inital-data slice" for which, if you know everything about all physical fields on that surface (and maybe an infinitesimal neighborhood, so that you can define derivatives), then you can (in principle) predict everything about the future of the universe and retrodict everything about its past.
Start out with the notion of causality.  Basically, a point $p$ in a spacetime is in the "causal future" of another point $q$ if $p$ can "be influenced" by a signal sent from $q$.  In GR (and mainstream physics more generally), this is basically taken to mean that $p$ can be connected to $q$ by a timelike or null curve (representing possible paths of massive or massless particles, respectively).
Next, we think of an achronal set $S$.  This is basically a slice through some part of the spacetime (we'd usually think of it as being a "spacelike" slice, though it could be null), for which no two points can be connected to each other by a timelike curve.  That is, if we have two points $p\in S$ and $q \in S$, then — at best — only a null signal from one can influence the other.  (Of course, there are usually other points that are not on $S$ that could be timelike connected to both, but such points would be in the past or future of $S$.)  The achronal set doesn't have to extend very far, but it could extend "across the entire spacetime" in some sense.
Now, we can get to the idea of a domain of dependence for an achronal set $S$.  The future domain of dependence $D^+(S)$ is the collection of points in the spacetime for which every single causal curve that goes through those points will go through $S$ (assuming the causal curves are extended far enough back in time).  Roughly speaking, this means that the only information that can affect anything in $D^+(S)$ is information about $S$ itself.  So $D^+(S)$ represents all the points that we can predict given knowledge of $S$.  Similarly, we can talk about the past domain of dependence $D^-(S)$, representing all the points that we can retrodict given knowledge of $S$.  And the full domain of dependence of $S$ is just the union of all those points:
\begin{equation}
  D(S) = D^+(S) \cup D^-(S).
\end{equation}
This is the whole set of points that can be either predicted or retrodicted from knowledge of $S$.
Finally, we can define a globally hyperbolic spacetime as a spacetime for which there exists (at least one) achronal set $\Sigma$ for which $D(\Sigma)$ is the entire spacetime.  That is, the entire past and future of the whole spacetime can be either predicted or retrodicted just from knowledge of $\Sigma$.  Such a surface $\Sigma$ is called a "Cauchy surface".

Now, the question is what good is this definition?  What other type of spacetime could there be?  Wald says

There are some good reasons for believing that all physically realistic spacetimes must be globally hyperbolic.

So, I don't think there's any realistic suggestion that some relevant physical model involves non-globally hyperbolic spacetime.  But its certainly possible to imagine them.  One prime source of this imagined spacetimes is where there are points that are simply "missing".  This is a weird idea that you might expect to see in Star Trek or something, and it shouldn't feel intuitive.  But just imagine that some region of spacetime ceased to exist suddenly, and then came back into existence after a moment.  When it comes back into existence, the physical fields will have some new values inside that region that may or may not have anything to do with how they were before the region blinked out of existence.  That means that anything in the future domain of dependence of this region will depend on the values of the physical fields when they came back into existence.  There's no reason to think you could have predicted those values from some much earlier achronal set, for example.  Similarly, you couldn't retrodict the values from a later achronal set.  Therefore, such a spacetime could not have a $\Sigma$ whose domain of dependence is the entire spacetime, and it's not globally hyperbolic.
Again, this isn't something that we expect to happen on physical grounds, but you can construct a mathematical model that has these features.  And if we're trying to model physics with math, then we need to specify which mathematical models we find relevant to physics.  That's all.
Comment: Nice answer mike. From your answer though, it seems like global hyperbolicity =determinism. Which kind of makes me think that in order to account for quantum indeterminism one would have to consider non globally hyperbolic spacetime's. I don't believe GR itself requires spacetime to be globally hyperbolic
Comment: @R.Rankin What you say is essentially correct.  Global hyperbolicity (GH) is basically a type of determinism.  And GR itself doesn't require GH; it's just a reasonable suggestion that you're allowed to ignore. But remember that we don't claim that GR is a perfect and complete theory.  In particular, GR itself doesn't incorporate quantum effects at all.  You have to basically take GR as is and slap quantum theory on top of it, or move on to other theories that can't really be called GR.  But to the extent that GR is a good approximation to physics, GH is a helpful guideline.
Comment: Thank you. Due to it's  nonlinearities, GR may still have some surprises for us. Using techniques of geometric and differential topology, complicated spacetimes representing particles (in the spirit of Einstein-Rosen and later wheeler) are now analyzable (see Asselmyers papers for example) then the mathematical nature of EH action takes on forms similar to wittens topological quantum field theories. Even if not right, it's a fascinating area
Comment: "*One prime source of this imagined spacetimes is where there are points that are simply "missing".......... just imagine that some region of spacetime ceased to exist suddenly, and then came back into existence after a moment.*"                                              This isn't so wierd if you consider that it describes the opening and closing of a wormhole on a manifold (a change in topology). Wheeler showed that in this situation, nothing can actually traverse the wormhole due to the expansion and consequent contraction of spacetime in the vicinity  https://www.osti.gov/biblio/4780637
Comment: Geroch "Domain of dependence" (1970) states that "A set is said to be achronal if no two of its points may be joined by a timelike curve." What about null curves? Can a null curve be considered a Cauchy surface? I am posing this question, because in A. Wall "Maximin Surfaces, and the Strong Subadditivity of the Covariant Holographic Entanglement Entropy " (2016) ,page 9 in the footnote9 it is stated that "an AdS-Cauchy surface, using the convention in which a Cauchy surface is permitted to be null".
Comment: @GeorgeFanaras You're right.  I was being too restrictive with the meaning of achronal; points in the set can be null connected.  I've edited my answer to correct that.  And in particular, it *is* possible to have a null surface that is also a Cauchy surface.  (Not every null surface will be Cauchy, but there are spacetimes in which you can find a null surface that is also Cauchy.)
Answer: Short answer:
The term globally "hyperbolic" refers to "hyperbolic differential equations". The term originates from the notion that the gravitational field equations (which are a set of "hyperbolic" differential equations) can be solved "globally" on the manifold. This means that providing initial data on some "Cauchy surface" (all of "space" at a single "time") determines a solution everywhere in the spacetime. If a Cauchy surface exists, the spacetime is globally hyperbolic. A Cauchy surface can fail to exist for various reasons, for example due to a naked singularity or a topology change. Having a Cauchy surface is equivalent to various causal structure properties, which you can read about in many places.


How old is the phrase "What's your angle?"

Question: I have a character saying this in a novel set in the 1920s. I suspect it was in use at that time, but I've been fooled before by this kind of thing. (I had to delete a sentence in which a 1920s character says, "It's out of left field" because that phrase is traceable only back to 1940 or so.)
Comment: Not many people have access to the OED. Unless you live in the UK and have a public library card, it requires a paid subscription.
Comment: silly question -- have you checked the OED for the word angle to see when it entered the language with this particular meaning/use?
Comment: Oh. I am in the US and grew up with it in the house (along with my first magnifying glass) and work in a high school -- we have a copy in the library.
Answer: Have you checked Google Ngrams?  That might help.
The Ngram indicates the phrase what's your angle first appeared in the mid-1920's, so you should be okay. However, when using this tool for such verification purposes, it's important to remember that Ngram results are based off of written documents (magazines, journals, and books); there's a chance the expression may have been used conversationally before it made its way into print. Some sayings need to reach a critical mass of usage before writers start using the phase in their work.
Comment: Ngrams are case sensitive. "What's your angle" (as opposed to "what's your angle") appears to have first been registered in the mid-1920s.
Comment: Yes, you're right about the "critical mass" concept. The "out of left field" phrase was supposedly in use in the 1930s, but the earliest written is 1949, so I decided to drop that one. I just might keep "what's your angle" in the novel, and assume it's OK.
Comment: @coleopterist: Great catch!! I've edited my answer, to include a new link to the revised Ngram search.
Answer: Old newspapers
Searching the Chronicling America newspaper archives (1836-1922) gave one result. This question was asked in a letter to the New-York Tribune of December 20, 1922:

Dear Sir: A day or so ago we had an argument as to what the five leading features for the year were in sport, whether on the part of an individual or a team. We never seemed to get anywhere with the debate. What's your angle on the top five? F.L.G.

Oxford English Dictionary
Whilst not the exact phrase, the OED's sense 9b is:

The perspective or aspect from which something is considered, regarded, or presented; a viewpoint, a standpoint. Freq. with preceding modifying word.

With pre-1930 quotations:

860   E. G. Parker Reminisc. Rufus Choate viii. 480   From whatever angle he looked at the facts, from whatever chords he struck the tones, you heard ever the same recurring strain.
1872   'G. Eliot' Middlemarch II. iii. xxiii. 13   Tacit expectations of what would be done for him by Uncle Featherstone determined the angle at which most people viewed Fred Vincy in Middlemarch.
1907   E. Wharton Let. 19 Nov. (1988) 124   As soon as I look at a subject from the novel-angle I see it in its relation to a larger whole, in all its remotest connotations.

And 9c:

colloq. (orig. U.S.). A way of approaching a task; a modus operandi, a method or scheme (sometimes with implication of dishonesty or exploitation).
1920   J. Conway in Variety 31 Dec. 8   He finally caught the proper angle... When in Rome do as the Romans do.
1921   J. Conway in Variety 18 Mar. 5,   I thought I was hep to all the angles.
Comment: I was looking through the OED and it made an interesting distinction between the use of the word [though not the phrase] from the perspective of the mathematical, and direction of approach (which could then be traced back to 1805), or the ability to catch something as in fishing. For this meaning, "to use artful means to catch" it dates to 1589 (1601 in Shakespeare).
Comment: @Dan: If I remember correctly, *angling* for fish is related to the angled shape of a hook, and the Angles, who give us the name England and English, are from a hooked peninsula in Denmark. / I also found a possible [1914](http://books.google.fi/books?id=AKDRClwkUNcC&q=%22what%27s+your+angle%22&dq=%22what%27s+your+angle%22&hl=en&sa=X&ei=tcvlUIe2BuTa4QT9joDgBg&redir_esc=y) which may be related: *"What do you hang it on — **what's your angle of approach?**" "Maybe one thing, maybe another," said Mack.*


2010 Toyota Corolla - pumping gas - the gas pump kicks off constantally

Question: In another forum someone said "Screen down the fillneck may be partialy plugged up with matter."  Is this something a neophyte can change or fix himself, or should I take it to an auto shop? How can I get to this screen to see if it is clogged or blocked?
Comment: I have a car that does this too, but only at certain stations and even then only when I put the nozzle into the filler neck as far as it could go. When I backed it out 1/2 to 1 inch it filled fine.
Answer: This is probably something you should take the car to a shop for. It sounds like a problem with the evaporative emissions system.
Your tank is a completely sealed system and when you are pumping gas it needs a way to vent the gas fumes from then tank because the gas flowing in pushes the fumes out. There are vent lines and solenoids that run to a charcoal canister which is usually in the front of the car. If there is something wrong with one of these solenoids or the charcoal canister itself it, will prevent the fumes from escaping and could cause the problem you are describing.
Answer: I finally took it in to a Toyota dealership.  They said it was the overflow control valve (OCV); it's inside the gas tank, and requires replacing the entire gas tank, about $1000.
Comment: As a side note, this sounds like something that should be covered under warranty on a 2010 vehicle.


Help! Baking soda/water stain on finished wood countertop!

Question: I soaked strawberries in a bowl with water and baking soda to clean them, ten minutes later I came back and realized the bowl was cracked and had been leaking. This stain is the result. Wooden countertops have some sort of glossy finish to them. Freaking out.  Any tips to get rid of this?
Answer: Some substances, such as those in some types of wood (and also in red cabbage), are natural pH indicators. Logwood, for example, darkens from yellowish to purple as pH changes from acid to alkaline.
It might be that the baking has darkened the wood by making it alkaline.
An easy fix for that would be to cover the whole surface with dilute vinegar (perhaps 20% vinegar, 80% water, i.e., ~1% acetic acid), which would make the pH more acidic (though it might make the whole surface a little bit lighter). To get the vinegar to stay in place, cover the counter with paper towels and spray on the vinegar. Let stand for an hour or two, but move the towels a bit to avoid missed spots.
Of course, the stain might be from strawberry juice... If vinegar fails, follow up with a similar, more dilute, chlorine bleach wash (perhaps % liquid bleach to 95% water).
Answer: To remove a baking soda water stain from a wood countertop, you can follow these steps:

Make a mild cleaner by mixing warm water and a few drops of dish soap in a bowl.
Dip a clean cloth or sponge in the soapy water and wring it out so it is damp, not wet.
Wipe the stain gently with the damp cloth or sponge. Avoid scrubbing too hard, as you might damage the wood finish.
Dry the countertop well with a clean, dry cloth.
Check for any residue left on the area. If the stain is still there, you may need to repeat the process or try other methods.
For persistent stains, you can use a vinegar solution. Mix equal parts white vinegar and water, dampen a cloth with it, and wipe the stain gently. Dry the area well afterward.
For stubborn stains, you can use a baking soda paste. Mix baking soda with water to make a paste, apply it to the stain, and let it sit for a few minutes. Then rub it gently with a soft cloth, wipe away any residue, and dry the countertop.


16.04 : "aptitude" at the command line

Question: After upgrading to 16.04 I tried to use "sudo aptitude update" at the command line and got the error that the command was not found.  What do I need to do to be able to use it again?
Answer: Hot tip:  You don't need 'apt-get' in 16.04, only 'apt'.   Example:
sudo apt install aptitude

Answer: You can always use
sudo apt-get update
sudo apt-get upgrade

to update your system, but if you like using aptitude
sudo apt-get install aptitude

should install aptitude.
Answer: Seems https://launchpad.net/ubuntu/xenial/+source/aptitude is available.
Just open your terminal and type sudo apt-get install aptitude
Then try again.
Thank you.

16.04 : "aptitude" at the command line


Are ocean water planets at Mars' atmospheric pressure possible?

Question: Can water planets exist whose atmospheric pressure at sea level is just above the triple point of water, say at about 690 Pa (0.1 psi)? If no, why not? Just curious. How thick or thin can atmospheres of ocean planets be?
Comment: If you look at the phase diagram of water, then just above the triple point there is just a tiny temperature window in which water is in a liquid state. So I'd say it's possible in theory, but you'd need a vary stable temperature: you could switch to either ice or vapour with a few degrees change. https://upload.wikimedia.org/wikipedia/commons/3/33/Phase_diagram_of_water_simplified.svg
Comment: @Jean-MariePrival Some of that is covered in answer(s) to [Mars' atmosphere and the triple point of water](https://space.stackexchange.com/q/17118/12102) in Space Exploration SE, perhaps you can add an answer here?
Answer: There's a couple reasons why it would be difficult, but not necessarily impossible (and I'll toss in a hypothetical at the  end).   I don't know how to answer this questions with a lot of "in theoreys" and "this is unlikelys", but I'll give it a shot.  Maybe when the James Webb Space Telescope gets up and running we'll get a better look at planet's atmospheres and maybe even identify a planet somewhat like your scenario.  For now, exoplanet atmospheres are incredibly hard to measure, so there's mostly just models and educated guesses to work with.
One reason it would be tricky is because the vapor pressure alone approaches the triple point air pressure, so this means that if the planet got too warm too often, say, mid-day noon or during summer, assuming it has seasons, then the vapor pressure, just from the ocean would probably rise above the triple point and that's before you add any additional gases.   CO2, CH4, N2, O2 if you want to get creative and get rid of a lot of the CO2 by photosynthesis and lets not forget Argon which forms from decaying potassium 40 in the planet's crust.  Earth's atmospheric Argon alone is about twice the triple point pressure.
So you'd need a planet with oceans but virtually no other gases and that might be hard to do.   Planets can lose atmosphere to space by solar wind or jeans escape, but water is a light molecule and arguably gets removed from a planet more easily than gasses like CO2, N2, O2 or Argon.  That's not to say it's impossible that a planet might be sufficiently water rich that it loses atmosphere but retains water.  I just think it would be a rare occurrence.
A second potential problem is temperature variation, which means, if the planet is too warm, too much ocean becomes atmosphere and adds too much vapor pressure and if the planet is too cold, the oceans would be largely covered by ice.   Planets are dynamic and a thin atmosphere planet should have a wide temperature range, just as Mars does.   Latitude, day/night cycle or day side/night side if tidally locked and eccentricity could all play roles in significant temperature variation, and lets not forget, the planet's internal heat and potential volcanism.   There would be a narrow range where the planet could maintain both oceans and a very thin atmosphere.
A final consideration is what you mean by triple point.   Oceans are likely briny and astronomers as a rule, are more interested in the "triple point" of briny water not pure water, though a chemist would say that triple point applies only to pure substances not salt water or possibly ammonia water.
If you consider the triple point of salt water oceans, then it goes below freezing where ice sheets become possible, and with an expected natural variation in the planet's temperature, ice sheets become very likely.  That doesn't make your scenario impossible, but I think you'd need a cold planet mostly covered by ice with narrow equatorial regions of exposed ocean - just a thought.
The triple point of briny water is discussed here - a fun read.
And a chart with some thing like the triple point of salt water in this research gate question here.
If, in theory, Mars was given oceans, by some terraforming efforts, as a result, initially the water would evaporate but over time, it's combination of water vapor and CO2 would push it's atmospheric pressure further above the triple point, and as it's south pole releases more CO2 as it presumably gets a little warmer, Mars would move away from this scenario if it had enough liquid water.  It has too much CO2, though for now, the majority of it's CO2 is frozen.
And Ammonia/Water ocean is especially interesting as the freezing point of the correct ratio can dip below the freezing point of CO2, this drops the triple point significantly, below -80 C° but it may solve the CO2 problem.  Frozen CO2 at the poles with Ammonia/Water oceans and lots of water ice.  Most of the oceans would have a permanent water-ice cover.
Mars, for example, undergoes a variation in atmospheric pressure as it goes through it's seasons by as much as 25%.  (Scroll down to the Mars section).   That's mostly eccentricity driven and happens because Mars is further from the sun during it's Southern Hemisphere winter and this allows some atmospheric CO2 to freeze, which thaws again as the planet moves back towards it's perihelion and Southern Hemisphere summer.
Certainly a planet with oceans and triple-point like conditions could be designed if you could create your own planet, but in actuality, the likely mix of compounds in the atmosphere and temperature variation and general dynamics of a planet would, I think, make such planets rare, if they exist at all.   That said, observations have shown astronomers things that they didn't expect to see many times before, so there's a chance.
Hypothetical: a Mars like planet has ice at it's poles, but warms up enough to lose it's atmosphere.  Ice is stable even in a vacuum, provided it stays below freezing temperature.
After losing it's atmosphere, the planet warms up and it develops a magnetic field.  The ice melts and now it has oceans and a very thin atmosphere.   Perhaps not impossible.  It requires a few things to go just right, so, still unlikely, I think but who knows.


Word or phrase for pile of accumulated rubble under vertical cave opening

Question: In caves whose openings are in the ceiling of the cave, there is typically a conical pile of rubble underneath the opening from where things have fallen into the hole over a long period of time. What is the name of this pile? I think it's either one or two words and may be something like 'rubble tower'.
Answer: The general term for loose rock that naturally accumulates at the bottom of a hill is scree. This isn't specific to caves, though, and doesn't suggest an accumulation directly below an opening. That said, it could be used to describe the material itself that is accumulating.


Bhagavad Gita was first recited to whom?

Question: Some people say that it was first said to Suryadev Vivasvaan and other say, it was recited to Arjuna, Sanjay and Drithrashtra? So, who is supposed to be the first to hear Bhagavad-Gita?
Comment: When Krishna says he imparted this knowledge to Vivasvaan, it could just mean the knowledge of karma yoga, not necessarily entire Gita, since Gita mentions folks by name like Bhishma, Duryodhana etc. who were there only in Dwapara Yuga ending.
Comment: but he also said that it was necessary for him to recite Gita to Vivasvaan, as he was nishpaksh (gives sunlight equally to everyone), and he was also responsible for the moral teachings of Manu
Answer: It was first recited by the Supreme Lord to Surya, best known for being the father of Mahabharata character Karna, Ramayana character Sugriva and also for giving the Akshaya patra to Yudhisthira.
From Bhagavad Gita Chapter IV:

"The Holy One said,--'This imperishable (system of) devotion I declared to Vivaswat: Vivaswat declared it to Manu; and Manu communicated it to Ikshvaku. Descending thus from generation, the Royal sages came to know it. But, O chastiser of foes, by (lapse of a) long time that devotion became lost to the world. Even the same (system of) devotion hath today been declared by me to thee, for thou art my devotee and friend, (and) this is a great mystery.'

It is clearly said that the first Bhagavad Gita was recited to Sun God. Vivaswat is Sun God.
The supreme lord recited to Arjuna too but after many ages and eons later in Dwapara Yuga.
Answer: As per my knowledge of Mahabharatha, Krishna has given the knowledge of Shrimad Bhagavad Gita to Arjuna in the middle of battlefield i.e. Kurukshethra (as written by Vyasa Bhagawan himself). Sanjaya had was provided a power to know what's happening at other places and he has listened to Gita from the court of Dhritharastra. Sanjaya also explained this to Dhritharastra.
Comment: Beginning of the war not middle of the war.
Comment: In Gita itself we find the verse which says that core part of Gita was told to Surya Dev in the very beginning..


Why are my cantilever brake pads already on rim without pulling brake lever?

Question: I am trying to put together a mtb (1998 Specialized Rockhopper) that was abandoned and given to me. Many components were missing so I'm piecing it together. I'm not very experienced with this. It did have cantilever brakes on it but no levers. I replaced all cable and housing, new gear shifter levers, and brake levers. I cannot get the brakes to work. Even with brake pads pulled out as far as possible from tire, they rub on rim without me even pulling lever. Do I perhaps have the wrong type of brake levers? How do I differentiate them? I have a drawer full of random parts. Can I tell just by looking at them which are long pull and which are short pull? Or is it my tires? Too wide? Any help or advice is appreciated
Comment: Welcome to the site! I'd like to ask for clarification if the brake cable is taut? Does this happen with the brake cable completely loosened?
Comment: Can you add some photos please?   One with the brake loose, one with the lever pressed and brake actuated.   This would be easier than asking for a description of the straddle cable's angles.
Comment: Yes in the morning I will take photos. It could be the wrong size link unit. When I have the cable loose, the brake pads don't pull to rim hard enough to make tire completely stop spinning (weak brakes). When cable is taut, the brake pads hit rim without pulling lever
Answer: Tires should not matter, as the brakes are meant to clamp the rim, not the tire. Cantilever brakes need to be adjusted in a way that the pads do not touch the rim while the lever is not pulled. If both the right and the left pad touch the rim without the cable installed, the rim is too wide, or the brake setup needs to be fundamentally different. You need to set the cable length (possibly trimming the housing) in a way that the pads to not touch the rim unless you pull the lever.
If only a single pad (left or right) touches the rim, you need to center the brake, how to do that depends on the model. A lot of brakes are only centerable if the lever mechanics is not jammed by dirt or old grease, so you might need to clean and possibly grease the lever mechanics of your brake to be able to center it.
Comment: Thank you that's good advice! I didn't think about the housing length as a possible issue. I will check that out tomorrow morning and post back results