Please explain the exec() function and its family

Question: What is the exec() function and its family?  Why is this function used and how does its work?
Please anyone explain these functions.
Comment: Try to read Stevens again, & clarify what it is you don't understand.
Comment: That's [Advanced Programing in the Unix Environment](https://www.amazon.com/Advanced-Programming-UNIX-Environment-3rd/dp/0321637739) by Stevens et al. (he died so a coauthor took over). This is a 1000+ page tome, so somewhat overkill for this specific question, but well worth the time and money.
Answer: Simplistically, in UNIX, you have the concept of processes and programs. A process is an environment in which a program executes.
The simple idea behind the UNIX "execution model" is that there are two operations you can do.
The first is to fork(), which creates a brand new process containing a duplicate (mostly) of the current program, including its state. There are a few differences between the two processes which allow them to figure out which is the parent and which is the child.
The second is to exec(), which replaces the program in the current process with a brand new program.
From those two simple operations, the entire UNIX execution model can be constructed.

To add some more detail to the above:
The use of fork() and exec() exemplifies the spirit of UNIX in that it provides a very simple way to start new processes.
The fork() call makes a near duplicate of the current process, identical in almost every way (not everything is copied over, for example, resource limits in some implementations, but the idea is to create as close a copy as possible). Only one process calls fork() but two processes return from that call - sounds bizarre but it's really quite elegant
The new process (called the child) gets a different process ID (PID) and has the PID of the old process (the parent) as its parent PID (PPID).
Because the two processes are now running exactly the same code, they need to be able to tell which is which - the return code of fork() provides this information - the child gets 0, the parent gets the PID of the child (if the fork() fails, no child is created and the parent gets an error code).
That way, the parent knows the PID of the child and can communicate with it, kill it, wait for it and so on (the child can always find its parent process with a call to getppid()).
The exec() call replaces the entire current contents of the process with a new program. It loads the program into the current process space and runs it from the entry point.
So, fork() and exec() are often used in sequence to get a new program running as a child of a current process. Shells typically do this whenever you try to run a program like find - the shell forks, then the child loads the find program into memory, setting up all command line arguments, standard I/O and so forth.
But they're not required to be used together. It's perfectly acceptable for a program to call fork() without a following exec() if, for example, the program contains both parent and child code (you need to be careful what you do, each implementation may have restrictions).
This was used quite a lot (and still is) for daemons which simply listen on a TCP port and fork a copy of themselves to process a specific request while the parent goes back to listening. For this situation, the program contains both the parent and the child code.
Similarly, programs that know they're finished and just want to run another program don't need to fork(), exec() and then wait()/waitpid() for the child. They can just load the child directly into their current process space with exec().
Some UNIX implementations have an optimized fork() which uses what they call copy-on-write. This is a trick to delay the copying of the process space in fork() until the program attempts to change something in that space. This is useful for those programs using only fork() and not exec() in that they don't have to copy an entire process space. Under Linux, fork() only makes a copy of the page tables and a new task structure, exec() will do the grunt work of "separating" the memory of the two processes.
If the exec is called following fork (and this is what happens mostly), that causes a write to the process space and it is then copied for the child process, before modifications are allowed.
Linux also has a vfork(), even more optimised, which shares just about everything between the two processes. Because of that, there are certain restrictions in what the child can do, and the parent halts until the child calls exec() or _exit().
The parent has to be stopped (and the child is not permitted to return from the current function) since the two processes even share the same stack. This is slightly more efficient for the classic use case of fork() followed immediately by exec().
Note that there is a whole family of exec calls (execl, execle, execve and so on) but exec in context here means any of them.
The following diagram illustrates the typical fork/exec operation where the bash shell is used to list a directory with the ls command:
+--------+
| pid=7  |
| ppid=4 |
| bash   |
+--------+
    |
    | calls fork
    V
+--------+             +--------+
| pid=7  |    forks    | pid=22 |
| ppid=4 | ----------> | ppid=7 |
| bash   |             | bash   |
+--------+             +--------+
    |                      |
    | waits for pid 22     | calls exec to run ls
    |                      V
    |                  +--------+
    |                  | pid=22 |
    |                  | ppid=7 |
    |                  | ls     |
    V                  +--------+
+--------+                 |
| pid=7  |                 | exits
| ppid=4 | <---------------+
| bash   |
+--------+
    |
    | continues
    V

Comment: Thanks for the shell reference with the find program. Exactly what I needed to understand.
Comment: Why is `exec` the utility used for redirecting the current process's IO?  How did the "null" case, running exec without an argument, get used for this convention?
Comment: @Ray, I've always thought of it as a natural extension. If you think of `exec` as the means to replace the current program (the shell) in this process with another, then _not_ specifying that other program to replace it with can simply mean you _don't_ want to replace it.
Comment: I see what you mean if by "natural extension" you mean something along the lines of "organic growth".  It seems that redirecting would have been added on to support the program replacement function, and I can see this behavior remaining in the degenerate case of `exec` being called without a program.  But it's a bit odd in this scenario since the original usefulness of redirecting for a new program -- a program which would actually get `exec`uted -- disappears and you have a useful artifact, redirecting the current program -- which is not being `exec`uted or started up in any way -- instead.
Answer: Functions in the exec() family have different behaviours:

l : arguments are passed as a list of strings to the main()
v : arguments are passed as an array of strings to the main()
p : path/s to search for the new running program
e : the environment can be specified by the caller

You can mix them, therefore you have:

int execl(const char *path, const char *arg, ...);
int execlp(const char *file, const char *arg, ...);
int execle(const char *path, const char *arg, ..., char * const envp[]);
int execv(const char *path, char *const argv[]);
int execvp(const char *file, char *const argv[]);
int execvpe(const char *file, char *const argv[], char *const envp[]);

For all of them the initial argument is the name of a file that is to be executed.
For more information read exec(3) man page:
man 3 exec  # if you are running a UNIX system

Comment: Interestingly, you missed [`execve()`](http://pubs.opengroup.org/onlinepubs/9699919799/functions/execve.html) from your list, which is defined by POSIX, and you added `execvpe()`, which is not defined by POSIX (mostly for reasons of historical precedent; it completes the set of functions).  Otherwise, a helpful explanation of the naming convention for the family — a useful adjunct to [paxdiablo](https://stackoverflow.com/users/14860/paxdiablo)'a [answer](https://stackoverflow.com/a/4205020/15168) which explains more about the workings of the functions.
Comment: And, in your defense, I see that the Linux man page for `execvpe()` (et al) does not list `execve()`; it has its own, separate man page (at least on Ubuntu 16.04 LTS) — the difference being that the other `exec()` family functions are listed in section 3 (functions) whereas `execve()` is listed in section 2 (system calls).  Basically, all the other functions in the family are implemented in terms of a call to `execve()`.
Answer: The exec family of functions make your process execute a different program, replacing the old program it was running. I.e., if you call
execl("/bin/ls", "ls", NULL);

then the ls program is executed with the process id, current working dir and user/group (access rights) of the process that called execl. Afterwards, the original program is not running anymore.
To start a new process, the fork system call is used. To execute a program without replacing the original, you need to fork, then exec.
Comment: Thank you that was really helpful. I'm currently doing a project requiring us to use exec() and your description solidified my understanding.
Answer:
what is the exec function and its family.

The exec function  family is all functions used to execute a file, such as execl, execlp, execle, execv, and execvp.They are all frontends for execve and provide different methods of calling it.

why is this function used

Exec functions are used when you want to execute (launch) a file (program).

and how does it work.

They work by overwriting the current process image with the one that you launched. They replace (by ending) the currently running process (the one that called the exec command) with the new process that has launched.
For more details: see this link.
Answer: exec is often used in conjunction with fork, which I saw that you also asked about, so I will discuss this with that in mind.
exec turns the current process into another program.  If you ever watched Doctor Who, then this is like when he regenerates -- his old body is replaced with a new body.
The way that this happens with your program and exec is that a lot of the resources that the OS kernel checks to see if the file you are passing to exec as the program argument (first argument) is executable by the current user (user id of the process making the exec call) and if so it replaces the virtual memory mapping of the current process with a virtual memory the new process and copies the argv and envp data that were passed in the exec call into an area of this new virtual memory map.  Several other things may also happen here, but the files that were open for the program that called exec will still be open for the new program and they will share the same process ID, but the program that called exec will cease (unless exec failed).
The reason that this is done this way is that by separating running a new program into two steps like this you can do some things between the two steps.  The most common thing to do is to make sure that the new program has certain files opened as certain file descriptors.  (remember here that file descriptors are not the same as FILE *, but are int values that the kernel knows about).  Doing this you can:
int X = open("./output_file.txt", O_WRONLY);

pid_t fk = fork();
if (!fk) { /* in child */
    dup2(X, 1); /* fd 1 is standard output,
                   so this makes standard out refer to the same file as X  */
    close(X);

    /* I'm using execl here rather than exec because
       it's easier to type the arguments. */
    execl("/bin/echo", "/bin/echo", "hello world");
    _exit(127); /* should not get here */
} else if (fk == -1) {
    /* An error happened and you should do something about it. */
    perror("fork"); /* print an error message */
}
close(X); /* The parent doesn't need this anymore */

This accomplishes running:
/bin/echo "hello world" > ./output_file.txt

from the command shell.
Answer: When a process uses fork(), it creates a duplicate copy of itself and this duplicates becomes the child of the process. The fork() is implemented using clone() system call in linux which returns twice from kernel.

A non-zero value(Process ID of child) is returned to the parent.
A value of zero is returned to the child.
In case the child is not created successfully due to any issues like low memory, -1 is returned to the fork().

Let's understand this with an example:
pid = fork();
// Both child and parent will now start execution from here.
if(pid < 0) {
    //child was not created successfully
    return 1;
}
else if(pid == 0) {
    // This is the child process
    // Child process code goes here
}
else {
    // Parent process code goes here
}
printf("This is code common to parent and child");

In the example, we have assumed that exec() is not used inside the child process.
But a parent and child differs in some of the PCB(process control block) attributes. These are:

PID - Both child and parent have a different Process ID.
Pending Signals - The child doesn't inherit Parent's pending signals. It will be empty for the child process when created.
Memory Locks - The child doesn't inherit its parent's memory locks. Memory locks are locks which can be used to lock a memory area and then this memory area cannot be swapped to disk.
Record Locks - The child doesn't inherit its parent's record locks. Record locks are associated with a file block or an entire file.
Process resource utilisation and CPU time consumed is set to zero for the child.
The child also doesn't inherit timers from the parent.

But what about the child memory? Is a new address space created for a child?
The answers in no. After the fork(), both parent and child share the memory address space of parent. In linux, these address space are divided into multiple pages. Only when the child writes to one of the parent memory pages, a duplicate of that page is created for the child. This is also known as copy on write(Copy parent pages only when the child writes to it).
Let's understand copy on write with an example.
int x = 2;
pid = fork();
if(pid == 0) {
    x = 10;
    // child is changing the value of x or writing to a page
    // One of the parent stack page will contain this local               variable. That page will be duplicated for child and it will store the value 10 in x in duplicated page.
}
else {
    x = 4;
}

But why is copy on write necessary?
A typical process creation takes place through fork()-exec() combination. Let's first understand what exec() does.
Exec() group of functions replaces the child's address space with a new program. Once exec() is called within a child, a separate address space will be created for the child which is totally different from the parent's one.
If there was no copy on write mechanism associated with fork(), duplicate pages would have created for the child and all the data would have been copied to child's pages. Allocating new memory and copying data is a very expensive process(takes processor's time and other system resources). We also know that in most cases, the child is going to call exec() and that would replace the child's memory with a new program. So the first copy which we did would have been a waste if copy on write was not there.
pid = fork();
if(pid == 0) {
    execlp("/bin/ls","ls",NULL);
    printf("will this line be printed"); // Think about it
    // A new memory space will be created for the child and that   memory will contain the "/bin/ls" program(text section), it's stack, data section and heap section
else {
    wait(NULL);
    // parent is waiting for the child. Once child terminates, parent will get its exit status and can then continue
}
return 1; // Both child and parent will exit with status code 1.

Why does parent waits for a child process?

The parent can assign a task to it's child and wait till it completes it's task. Then it can carry some other work.
Once the child terminates, all the resources associated with child are freed except for the process control block. Now, the child is in zombie state. Using wait(), parent can inquire about the status of child and then ask the kernel to free the PCB. In case parent doesn't uses wait, the child will remain in the zombie state.

Why is exec() system call necessary?
It's not necessary to use exec() with fork(). If the code that the child will execute is within the program associated with parent, exec() is not needed.
But think of cases when the child has to run multiple programs. Let's take the example of shell program. It supports multiple commands like find, mv, cp, date etc. Will be it right to include program code associated with these commands in one program or have child load these programs into the memory when required?
It all depends on your use case. You have a web server which given an input x that returns the 2^x to the clients. For each request, the web server creates a new child and asks it to compute. Will you write a separate program to calculate this and use exec()? Or you will just write computation code inside the parent program?
Usually, a process creation involves a combination of fork(), exec(), wait() and exit() calls.
Answer: The exec(3,3p) functions replace the current process with another. That is, the current process stops, and another runs instead, taking over some of the resources the original program had.
Comment: Not quite.  It replaces the current process *image* with a new process image.  The process is the same process with the same pid, the same environment, and the same file descriptor table.  What has changed is the entire virtual memory and CPU state.
Comment: @JeremyP "The same file descriptor" was important here, it explains how redirection works in shells! I was puzzled on how redirection can work if exec overwrites everything! Thanks


Matrixing LR as MS?

Question: As I've become more comfortable with mid-side stereo recording and matrixing it as LR in post, rather than at capture time, I'm wondering about the ability in various plug-ins to encode LR into MS. Has anyone found this to have practical use?
Answer: It's quite commonly used in (music) mastering. You can obviously widen/tighten the stereo image, clean up specific "areas" of the mix, and push for more volume (like that's needed!). I've only tried it a few times for interest sake.
Just on a (mid)side note, I'm a HUGE fan of Voxengo's MSED plugin, which allows you to pass your already decoded sound inline through the plugin and work on the matrix again in post. And Michael Maroussas introduced me to the Brainworx bx_solo, which is cool cause you can solo each element of the matrix.
Comment: @andrew Spitz is tge bx_solo actually a decoder or is it just used the solo the parts of MS?
Comment: @sepulchra I think it is, not 100% sure and can't check at the moment.
Answer: I've done this before to narrow the width of a stereo recording to give it more center focus and mono punch. If you pay close attention to phase, it can be useful sometimes to EQ the M and the S of a signal differently (IE, rolling off some of the lows in the stereo field while keeping the center bass heavy). You could also use it to extract a phantom center from a stereo image.
But now I'm using various imaging tools like Waves Center, S1 Imager to accomplish this. I'm guessing that they might be encoding LR-MS-LR under the hood of these plug-ins anyway.
Comment: I wonder what would happen if you were to pull out the phantom center, then play it out of phase alongside the original LR...  Wait, is that how you pull the vocals out of a song?
Answer: I've used it on rare occasions, and I do mean RARE, to process the center part of a stereo image. The few times I've done it was when there was something in a track that obfuscates a piece's dialog in some way. If you do it carefully, you can "attenuate" a few frequencies here and there without destroying the left/right image...or replace a tiny segment with surrounding audio (kind of like clumsy manual interpolation) which can help prevent a short loopy sound, because the other "side" information gets left alone. I suppose you could edit the "side" audio instead of the "mid," but I've never done that.
Like I said, it's pretty rare that I do it. But when it works, it's a useful noise reduction trick to have in the bag.

Matrixing LR as MS?


Divisible elements in CAT(0) groups

Question: Given a group $G$ acting on a $CAT(0)$ complex $X$ by isometries can $G$ contain a divisible element, i.e. an element $g\in G$ such that $\forall n\in\mathbb N$ there is $h\in G$ such that $g=h^n$.
Comment: You need to add the condition that the action is properly discontinuous and cocompact, as required by the definition of a CAT(0) group, otherwise there are easy counter-examples.
Answer: Let $G$ be a group acting geometrically on a CAT(0) space $X$.

Show that there exists some $N \geq 0$ such that any torsion element of $G$ has order $\leq N$.
Deduce that $G$ cannot contain a divisible element of finite order.
Let $g \in G$ be an infinite order element. In particular, it must be loxodromic, ie., there exists a bi-infinite geodesic $\gamma$ on which it acts by translation. If $g=h^n$, then $h$ also acts on $\gamma$ by translation. Moreover, $\ell(g)=\ell(h)^n$, where $\ell(\cdot)$ denotes the translation length.
Show that, if $g$ is a divisible element and if we fix a point $x \in \gamma$, then there exist infinitely many elements $h \in G$ satifying $d(x,hx) \leq 1$. Hence a contradiction with the fact that $G$ acts geometrically on $X$.

In fact, the argument essentially holds in a more general setting, for instance for semihyperbolic groups.
Comment: why if $g$ has to be an infinite order element it has to be loxodromic? could it not be parabolic?
Comment: If a group acts cocompactly on a CAT(0) space, then it contains no parabolic. This statement should be available in Bridson and Haefliger's book; I should be able to find a precise reference if needed.

Divisible elements in CAT(0) groups


Does philosophy develop?

Question: Mathis Bitton in Philosophy Now currently, suggests ways in which philosophy develops against the background of historical and scientific progress. Does philosophy develop in this way?
Comment: I do not understand the wording in your first sentence. Please clarify.
Comment: Does this answer your question? [Does philosophy progress?](https://philosophy.stackexchange.com/questions/29886/does-philosophy-progress)
Comment: Picasso was once asked about the development of his art. He said, "I do not develop, **I am**." Explains a lot.
Comment: Does falsification, contradiction, and other negation count as progress? From a logical perspective, many utterances have been proven bogus, including once-accepted philosophical claims.
Comment: That might depend on whether it matters what journal you publish in. Principles of experimentation, abstract mathematics, and cosmology seem to me unquestionably exercises in philosophy even in the modern sense excluding "natural philosophy", but when those people want peer-review and an audience that will be able to interact with them, they publish in science and mathematics journals.
Comment: '(Why) is this negative outlook on the concept of philosophy misguided?' https://philosophy.stackexchange.com/questions/86865/why-is-this-negative-outlook-on-the-concept-of-philosophy-misguided/86866#86866
"It's like saying what's the point of science if we just end up with more, greater mysteries. Philosophy is a toolbox, not a destination."
Comment: To clarify my view. If modern philosophy is the leftover bit of science, then philosophy is dependent on evidence, and not speculative. Further areas of philosophy will become integrated into science as it progresses. I agree that philosophy is a toolbox, just like the scientific method. This does not mean that philosophy does not progress.
Answer: Of course philosophy develops. Otherwise philosophy would stagnate, but that's not the case. Hence I consider the deeper question to be your second question: Does philosophy make any progress?

Any answer to this question presupposes an agreement about how to
measure and how to evaluate this development. Otherwise we would
only continue the outdated discussion about "philosophia perennis".
A possible measure is to sample a class of representative problems
from philosophy and to determine the degree of their solution.
That's an approach following the paradigm of problem and solution.
Candidates for such sample of problems are – in random order - the
mind-body problem, the problem of free-will versus determinism, the
existence of universals, the ontology of the micro-world, the limits
of human knowledge, to characterize the philosophical methodology,
etc.

Later philosophers always have the advantage that they can read what
their predecessors have thought. Later generations see the
prejudices and implicit assumptions of former scholars. Hence they
have the chance to avoid the errors of the precedessors. But it is
always impossible to recognize the blind spots of oneself.
Therefore I expect the development of philosophy to go on forever.
Comment: Ok, so it is developing, but is it *progressing*? Maybe the first thing philosophers should do is decide why they are all doing this?
Comment: @ScottRowe First things first: Do you agree with the approach from no.1 of my answer and the candidates from my list?
Comment: As far as I know, the candidate problems in your list do not have consensus answers.
Comment: @ScottRowe It seems that you accept the candidates. The next step is to develop a list of key indicators how to assess a proposed solution. Any proposals?
Answer: The concept of progress (SEP) is often treated as a meta-narrative. From WP:

A metanarrative (also meta-narrative and grand narrative; French: métarécit or grand récit) is a narrative about narratives of historical meaning, experience, or knowledge, which offers a society legitimation through the anticipated completion of a (as yet unrealized) master idea.

As such, it depends entirely on what you mean by "progress". I have an illuminating book on the topic that explores the historical development of the idea of progress, but mine is quite old and there are many. One contemporary example seems to be The Idea of Progress (2017) (GB) by Robert Nisbet. In fact, it's a common title on Google Books. What do you mean by "progress"? From the SEP article:

Philosophical proponents of progress assert that the human condition has improved over the course of history and will continue to improve. Doctrines of progress first appeared in 18th-century Europe and epitomize the optimism of that time and place. Belief in progress flourished in the 19th century. While skeptics of progress did exist alongside its supporters from the beginning, it was not until the 20th century that theorists backed away en masse from the notion. Many 20th-century thinkers rejected the notion of progress after horrendous events such as the two World Wars, the Holocaust, and the use of nuclear weaponry... In general, writings on progress tend to bear a close relationship to the environment in which they were produced. Because of the strong connection between doctrines of progress and historical events, this article is organized by time and place.

That is to say, there is great variance in a response to the question, and some thinkers deny "progress" exists at all. Until that question is resolved, the question of how philosophy fits in the meta-narrative can't meaningfully be answered. (It also opens up the metaphilosophical box of strife requiring an adequate answer to "What is philosophy?")
Comment: Your question inherits a problem with the [impredicativity](https://en.wikipedia.org/wiki/Impredicativity) of defining progress through a definition of progress. My strategy is that Progress in the big sense simply be defined in terms of progress in the constructivist sense. If a collection of people each have knowledge, and that knowledge is built on propositions, and some of those propositions are foundational in some sense, then as each person constructs new axioms, new arguments, and new conclusions, then each has progressed in the accumulation of knowledge (or physical structures too)...
Comment: @ScottRowe And that to me is an important definition of Progress. As there are more people, more buildings, more ideas, and more history, we progress. But obviously that's only along the dimension of quantity, and there would have to be a qualitative aspect to Progress too. Are we all happier? Are we all freer? Is the world more just? Is the ecosystem healthier? Etc.
Comment: If we've gone from knowing what progress is to not knowing, is that progress? If not, maybe were going the wrong way? We should require everyone who wants to participate in Philosophy to say what their objective is, and what progress towards it would be. Otherwise, we're just debating with a bunch of agitators. Me first: my objective is to shoot down outdated and ill-considered ideas so that we can focus on things that will definitely improve people's lives, like nailing down ethics and community dynamics.
Comment: It's lovely stuff, which I heartily agree with. Like Robert Pirsig saying that the history and results of technology could "only soberly be described as upward progress." To those who disagree, I would say: define progress in a way that it can be achieved, or let's stop bringing the subject up. But then, I'm an engineer. (don't get me started on big-endian vs little-endian, a pointless idea if I ever heard one!)
Answer: Progress gets a meaning when there is a goal, a destination, otherwise it's just an abstract concept. Philosophy has, in general, lost its destination, it is applied mainly as an analytical tool to furnish our intellectual vanity.

I was asked to provide a convincing argument. I will justify the use of "analytical tool", "vanity" and I will paste a story.
analytical tool
Philosophy today: cries of alarm and prospects of progress
(very interesting)
vanity

the quality of being worthless or futile; incapable of producing any
useful result; pointless.

Most professional philosophers of today say that, with regard to the great questions, "the answer is that there is no answer."
story
(article from Philosophy Now)
Once upon a time, there was an Athenian soldier called Socrates inspired to do philosophy. Socrates taught the young aristocrat Plato. Plato taught the doctor's son Aristotle. And Aristotle taught Alexander, son of Philip, King of Macedonia. Alexander went forth and conquered most of the known, and some of the unknown, world, in the name of the Greek language, culture and philosophy. In conquering a swathe of the world, Alexander became known as "the Great". So we can see the sort of influence doing philosophy can have.
Comment: I like provocative statements if they are supported by convincing arguments - and are not just opinion-based. It would be interesting to hear some of your arguments. :-)
Comment: @Jo Wehler, I'll leave this one as Kant's : "Genius is the talent that gives the rule to art", unexplained. But, don't take it personal, one way or the other we are all into this.
Comment: No problem. - But converting your statement into a polarizing question could prompt some interesting answers. One has only to avoid that the question will be banned due to prompting "opinion-based" answers :-)
Comment: @Jo Wehler, I eventually made the try to explain myself.
Comment: +1 for the reference to Parrini
Answer: It depends entirely on how broad is your interpretation of the word progress. You can consider progress in a number of dimensions. Take, for example, accessibility. Without doubt philosophy is a vastly more accessible subject than is was fifty years ago, when, as a teenager, I had access to perhaps half a dozen out-dated books in my local library.
Even if you interpret progress narrowly as referring only to the progression of mainstream ideas, there are still different dimensions to consider. Measured purely in terms of output, there is a progressive increase in the total sum of published papers. You might say that the availability of new material is not necessarily a sign of progress if the material itself is of poor quality or doesn't lead to breakthroughs. However, there are plenty of other fields of effort in which you have to persevere before you find valuable results. Consider the tons of valueless material processed in mining for diamonds, for example. A huge effort was expended in developing string theory in physics, which has never lived up to the hopes that were had for it, but it is progress nonetheless because you have to explore new terrain in order to find your way around- think of all the time spent discounting dead-ends when looking for the source of the Nile.
Answer: No.  As a subject it will not develop.  But in the case of many people the way of thinking will change and in their case it will develop into another subject called spirituality.  Because, like other subjects philosophy is also for seeking the truth.  But "What is to be known (the Ultimate Truth) has already known" and if it develops, it will take people only to some dreamworld which is useless to laymen. And then  it can only be called change; not development.
Is Philosophy the source of all other fields of study?
If we can extend the words of Ramana Maharshi, we can conclude that the development of philosophy has already ended.  Otherwise the new subject and its benefits would not have been known.
Answer: Both the question, and the linked article misunderstand, I would argue, the nature and purpose of philosophy, as well as the extent to which scientific progress is definitional of science.
Scientific progress is a measure of the organized, systematic, rule-governed and objectively certifiable increase in the size and scope of a unified body of factual knowledge. There's no direct analog in the world of philosophy. Philosophers do create new sciences, all the time, some successful, and some less so, so there's a temptation to call that philosophical progress. But solving a philosophical problem removes it from the realm of philosophy. It doesn't change the discipline of philosophy itself.
Ultimately, the success of philosophy at any given moment in time is how well and how productively it helps us engage with the world as we experience it at that point in time. Philosophy is cyclical, because history is cyclical. If you perceive history as a linear progression, then you perceive the same in philosophy. Conversely, if you view history as a decline, you are likely to see philosophy in the same light.
Comment: It does not. It is a clarification.
Comment: History is neither. It is an active process, in which philosophy participates.  Philosophy is the foundation of knowledge, and endures. Science is a process of enquiry, not a destination.
Comment: @Meanach - it's not clear to me what in your comment disagrees with anything I said in my answer--or rather, what you perceived in my answer as incompatible with what you said in your comment.

Does philosophy develop?


What happens when a the volume of a perfect vacuum (0 psi) is increased?

Question: I'm an engineer working on a design in which a "plunger" will be pulled out of a sealed vessel, where the starting volume of the vessel is essentially zero and I need to know what will happen to the surrounding structure. If I'm asking this in the wrong place then please feel free to point me in the right direction.
Say you have a hand pump with an outlet that allows you to fully depress the plunger until the volume inside is zero. The outlet is then closed and the plunger is pulled, increasing the volume inside of the pump. At some point a "perfect" vacuum will form inside of the pump and from what I understand the pressure inside the container can never drop below the inverse of pressure outside the container (1 atm). If this is true, where does the energy that is applied to the system as you continue to pull the plunger go? If the pressure can no longer decrease inside the pump then shouldn't the plunger require zero force to pull?
Comment: (1) you won't get perfect vacuum - there are always leaks. (2) You continue to make more volume with small pressure, with you pulling from the high pressure side so, no, the force does not decrease.
Answer: First off, I'd say what you would create is a "reasonable" vacuum, not a perfect vacuum.  You're creating something on par with this box moving device.
By contrast consider a vacuum like this:

A vacuum like that is not considered a perfect vacuum, but it does pull an "ultra high vacuum," which is .00000000000098 atmospheres, and involves rather exotic techniques that are quite a lot more disciplined than a simple plunger.
Now for your purposes, you can probably just assume "zero pressure" inside your vessel, and look at what forces are applied by the 1 atmosphere of pressure pushing in on it from the outside.  Practically speaking, unless the term "turbomolecular pump" or "sorption pump" mean something to you, you're probably going to treat any atmosphere left as a negligable rounding error.
Now if you're really pushing for perfect vacuums, there's a rule in the trade: everything outgasses.  Your "perfect" vacuum will get gasious atoms inside the vessel, no matter what you do.  As you push into the ultra-high vacuum world, you get fun effects to deal with, such as hydrogen atoms diffusing through your stainless steel pipes.  You can mitigate this quite a bit, but there will always be some atmosphere.
Mind you not much atmosphere.  In the ultra-high vacuum world, molecule of gas may fly 40km back and forth within your vessel before colliding with another molecule.  But there is an atmosphere.
In your case, when pulling the plunger, remember that it's not so much that the vacuum on the inside is "pulling" your plunger in, forcing you to do work.  It's more that the atmospheric pressure on the outside is "pushing" your plunger in, and there's no pressure (very little pressure) on the inside helping keep the plunger out.  You're fighting against our atmosphere.
And in theory, if you really wanted to get down to it, you'd find that if you pulled the plunger out, creating a larger vacuum, the outer edges of our atmosphere would go up slightly, and you'd find that the work you did was effectively the work required to lift that atmosphere up.
Answer: Suppose the plunger is 1 square inch in diameter. The work you do to pull it out is 14.7 pounds (force) times the distance you pull it out. Where's the energy? It's potential energy you got by lifting a 1 square inch column of air that distance. Let go of it, and that column of air will fall back, converting potential to kinetic energy. Then when the plunger hits the end the kinetic energy will get converted to heat.
The vacuum will not be perfect, but that won't make any practical difference. If there are 3 molecules of nitrogen in there, that's practically none.
Answer: First,

from what I understand the pressure inside the container can never drop below the inverse of pressure outside the container (1 atm).

I don't know what you're referring to, but I don't believe that's the case. The pressure will continue to drop inside the vessel as you pull, but there's a limiting factor. No seal is perfect, and materials outgas as well, so gas will begin entering the vessel. But once you're at low pressure,

If the pressure can no longer decrease inside the pump then shouldn't the plunger require zero force to pull?

No. Atmospheric pressure is pushing against the vessel in all directions, so you have to worry about implosion. Even though the pressure might not decrease further in the vessel, you have to fight 14.7 psi of atmosphere in order to pull the plunger further. I.e. the increase in difficulty with which you have to pull may level off, but it will still be difficult.
Comment: So once that minimum pressure is reached a failure of some sort will occur, whether it be a seal breaking or outgassing of the vessel? The goal is to prevent any change in the properties of the vessel, and in this case the vessel is a very thick steel tube so I doubt implosion will be an issue.
Comment: It's not necessarily that the seal will break, it's just that air will find a way to slip through the tiniest of cracks, etc. Especially when you have moving parts, they are harder to seal up. Outgassing can be minimized by baking the vessel first, but can't be prevented. What are your requirements on "perfect" vacuum? A pressure of one millionth of atmosphere could be maintained for a long time in the research-grade equipment I use, but I couldn't say in your case.
Answer:
I need to know what will happen to the surrounding structure.

Draw a diagram of the equipment with the pressures and forces involved.  Can the structure withstand a ~1atm pressure difference across the entire face of the container?

from what I understand the pressure inside the container can never drop below the inverse of pressure outside the container

The pressure inside can never drop below 0 pressure (absolute) or below -1 atm (gauge), assuming you're operating it at 1atm.

If this is true, where does the energy that is applied to the system as you continue to pull the plunger go?

The force you apply to the plunger moves the system into a higher potential energy configuration.  It's similar to extending a spring.  If you were to release the handle, the energy would be converted back to kinetic energy as the plunger accelerates.

If the pressure can no longer decrease inside the pump then shouldn't the plunger require zero force to pull?

The interior pressure can not decrease below zero.  But the work you have to do is based on overcoming the net force on the plunger.  That depends on the pressure difference between the sides.  Unlike a spring that increases the resistance forces as it is extended, the plunger will have close to a constant force (ignoring minor leaks) as it is pulled.


Is there a word for petals falling without withering?

Question: When roses whither, the petals shrivel and dries up before falling. Is there a word like withering, but doesn't imply that the petals shrivel and dries up? Is there a word that refers to just petals falling like in the beauty in the beast?
Answer: Looking through the Glossary of botanical terms, I found the term abscission

The normal separation of flowers, fruit, and leaves from plants.

which describes the process you're referring to quite accurately.
Comment: True, but that is a scientific term - it wouldn't be used in everyday conversation. We can say that the flowers fall or that the petals fall.


AMS to Glasgow KLM

Question: My daughter and her 2 young children are arriving at AMS from Lima and then have 50 minutes to connect to Glasgow.
Is that ample time or do they have to go through security etc?  Can you anticipate delays getting to boarding?
Comment: 50 minutes are not many. You may able to get it (also because there are possibilities of flight arriving earlier). If you miss, doesn't despair. On ASM there are many connection desk to have a new flight rebooked.
Comment: Is this on a single ticket or booked separately?
Answer: This is a very tight especially with two small kids.
If this is on a single ticket, your bags will be checked through. You still may miss the connection but if you do, the airline will put you for free on the next flight to Glasgow. This may be the next day, depending on time of arrival.
If these are two different tickets, your daughter has no chance of making it. You need to rebook this.
See more details at: https://www.klm.com/travel/gb_en/prepare_for_travel/at_the_airport/connections/index.htm#p2
Comment: Glasgow is not in Schengen - they are not going through immigration control.
Comment: However passengers arriving on flights from Peru may need to wait for transfer security screening.

AMS to Glasgow KLM


How do space missions preserve their batteries for so long?

Question: If I have a set of rechargeable batteries, I count myself lucky if I can get them to last for more than 2 years. Yet there are many spacecraft out there, which use their batteries much more regularly than me, and yet they have batteries that still work after years. How are they able to maintain the lifespan of these batteries for such an extended period of time?
Comment: Do you have a specific example? Some of those "batteries" are actually super capacitors, or nuclear thermoelectric systems.
Comment: Rechargable AA batteries might not last for long, but e.g. sealed car batteries can operate for a decade or two (while being constantly recharged - much like batteries in a spacecraft would)
Comment: I'm just looking for generalities, but using super capacitors would be such an example. Most Nuclear Thermoelectric systems still use a battery to build up charge for a while (MSL/Curiosity is one such, I think most do)
Answer: This document provides a bunch of useful information on the topic. Some applicable quotes:

Space missions impose several critical performance requirements on batteries and fuel cells. Batteries required
  for space applications must be capable
  of operating in a hard vacuum and
  withstand severe launch environments
  (vibration, shock, and acceleration).
  Space applications also require batteries
  that can provide maximum electrical
  energy in minimum weight and
  volume
Long cycle life (> 30,000
  cycles) is the critical driver for orbiting
  spacecraft, and long active shelf life is
  the driver for planetary probes (> 7-10
  years). Radiation resistance and operation at temperatures as low as -80°C is
  essential for some planetary missions.
  No single battery system can meet all
  these complex requirements.
The Ag-Zn battery was the first
  choice in the early days of space missions. The Ni-Cd battery became the
  major energy storage device over the
  next 20 years because of its long cycle
  life. The Ni-H2
  battery started to play a
  role in the 80s.

Shiny table!:

There is also plenty of information about the longevity of power sources. For example, the Space Shuttle didn't use batteries:

The space shuttle
  requires a power source that can provide 6-12 kW for 2,000 hours. Batteries
  cannot satisfy this application, as the
  required battery weight would be prohibitive. The application can and has,
  however, been met with an alkaline
  fuel cell operating on hydrogen and
  oxygen stored separately in cryogenic
  tanks. International Fuel Cells is providing alkaline fuel cells for the shuttle
  orbiter applications. This system has
  been in use for the past 18 years. Plans
  are underway to replace the alkaline
  fuel cell system with an advanced
  PEM-based system. The replacement of
  the hydrazine-powered auxiliary
  power unit (APU) with an electric APU
  powered by 100-150 kWh Li-ion batteries is also underway.

I won't just copy and paste the whole thing here, but it's a good read, with lots of rich information on the topic of batteries in space.
Comment: What does "Commercial" or "1st Mission" mean in "Life in Space" column?
Answer: Generally speaking, there's a couple of things that should be done to prevent this from happening.

Order good batteries. This helps, to an extent, but isn't a sure thing.
Try to keep the thermal limits of the batteries within limits. Don't allow the batteries to get too warm, or too cool, and vigorously monitor this.
Charge the batteries fully, but don't overcharge them. This is probably one of the big reasons that consumer batteries fail. It is actually common for them to be somewhat overcharged, which is why most batteries are noticeably warmer after they are done charging. They aren't super overcharged, but this does contribute to wear and tear.
Minimize the depth of discharge. Essentially, charge your batteries when you have used less than a third of their capacity. You are much more likely to have issues if you completely discharge your batteries.

If you do all of these things, your batteries will last longer. The same goes with personal electronics, by the way. Of course, making sure your battery is charged in an optimal manner can be a very difficult and time consuming matter, which is why more systems don't do it (Every battery is a bit different).


What should I look for in lenses for long range and close up animal and natural shots?

Question: I am looking at trying to buy a long range lens for shooting animals and nature. I'm kinda new to photography and am not sure which brand or size I should be looking for. What features are important and what will I get from different price points? I also would like to get a macro lens for really close shots — is that possible with the same lens?  I have a nikon d3000.
Comment: Also see [What is a "fast lens"?](http://photo.stackexchange.com/a/13624/1943)
Comment: I've edited the question to be more general; hopefully it can serve as guidance for people with this question without getting into the murky waters of specific shopping advice.
Answer: With wildlife, 300mm is considered the minimum focal length you want (if we're really talking wildlife out in the wild, and not backyard semi-tame beasties that aren't shy of humans).  400mm is typically considered a minimum for birds.  So, it does in some respects depend on what wildlife you're most typically stalking, in terms of how large and how shy they are.  Smaller, shyer animals require longer lenses to fill a frame.  And paramount over all this talk of glass, remember that fieldcraft is probably going to be far more important. Your ability to get closer to wildlife without disturbing them is a learned skill and some folks have more talent (or drive to practice) to gain this skill than others (go google "hide" and "ghillie suit").  So again, what focal lengths you'll need rely a great deal on your fieldcraft capabilities.  Most of the stunning wildlife images you see were obtained with a lot of sweat, time, patience, and expense--don't expect that picking up a 70-300mm lens is all you need to do to get wildlife shots.  But you probably can't expect to get them without one. :)
As a Nikon D3000 shooter, your choices of lenses over 300mm will mostly boil down to either Nikon or Sigma (supposing you have a four-figure budget).  If your budget is below $1000, chances are good you can only afford a 70-300mm zoom, and all you have to worry about is whether or not you want to go Nikon or 3rd party.  Your main issue, of course, will be finding a lens with AF-S so you'll have autofocus.
Most of these lenses do not let you shoot Macro as well, despite a number of 3rd-party 70-300 lenses being labelled as "macro" lenses.  While you can get relatively close, they don't do the true 1:1 magnification of a dedicated macro lens, and if you plan on shooting very small subjects, like bugs or small flowers, chances are good you'll prefer a dedicated macro lens instead.
So, the main lens specs you'll probably be concerned with (aside from cost) are:

Focal length.  As I stated above, 300mm on a crop body is good for larger or less-shy wildlife. But if you're thinking about small birds that aren't in the backyard, then 400mm or longer is more likely to be what you want. And the cost leap will be considerable.  Say, from $600 to $1000 (third-party) or $2000 (OEM).
Image quality.  Sharpness, contrast, and distortion will probably be the main concerns. With longer lenses, you typically don't worry much about vignetting or C/A.  Zooms may be slightly softer than primes, but not always, if you can afford the four-figure price tags.
Maximum aperture.  Wildlife moves fast, and often in low light. But this is going to be the most expensive thing you can go for.  f/4 300mm lenses are astronomical. You don't want to see the prices for f/4 at 400mm. Or f/2.8. Think five figures.  You may not have a choice of going with f/5.6 or even f/6.3 with third-party lenses for this.  But because wildlife tends to be active in the golden hours, and because the longer focal lengths require faster shutter speeds to mitigate camera shake blur, a larger maximum aperture will help widen the possibilities.  The 1/focal_length rule of thumb doesn't really restrict you much at 50mm, but at 500mm, it's something completely different.
Autofocus speed.  No point in having a great, sharp supertelephoto to shoot flitting birds if it can't autofocus fast enough to grab the shot before the bird flies off.  However, given that you have a D3000--you're going to have to get an AF-S lens anyway just to have autofocus.  But pay attention to this in reviews.
Stabilization.  If you plan to handhold while you stalk critters (once you go past 400mm, chances are good you have to shoot with a monopod or tripod, simply from the weight), stabilization can help a lot with the shutter speed restrictions the longer focal length will impose upon you.  VR, for longer lenses, is worth it.
Focus lock.  Some of the longer lenses also have a "focus lock" that allows you to tell the lens to ignore a certain portion of the autofocus range in order to speed up focus locking.  This can be invaluable if you tend to shoot things that move fast and are always beyond your reach (i.e., birds in flight).
Size/Weight.  The longer or faster the lens, the bigger and heavier it gets. Some folks claim they can handhold a Sigma 150-500 OS, some folks cannot.  But handholding telephotos and supertelephotos does require a certain amount of technique and care that shorter lenses do not. There are special holds you can use (e.g., the machine gun hold, where you place your left hand on your right shoulder, and rest the lens on your left elbow), as well as a plethora of other ways (beanbags, monopods, tripods, resting on fence rails, your knee), etc. to help with stabilization.

You may also want to see the following lensrentals.com videos about supertelephoto lenses to get a sense of what the really big guns are like:

Comparing Nikon Supertelephoto Lenses
Comparing Canon Supertelephoto Lenses
Supporting Supertelephoto Lenses
Answer: The general rule of focal length for animals is that too much is never enough.  It basically comes down to how much you can afford and are willing to carry.  Many nature shots will be in remote places, so lugging the equipment there is a serious consideration.  That all said, I'd at least want to go out with a 300 mm lens (relative to a full frame sensor).  Even with that, a small bird which is quite close will still look like a small thing in the middle of a big picture.
Macro is a totally different thing from telephoto, so I wouldn't try to get one lens that does both.  Even if you can find such a thing, a lens of the size and weight required for distance animals will be unwieldy and difficult to use for macro shots.  I've gotten quite passible macro shots with a near-telephoto and extension tubes, although outright macro lenses are easier to handle in the field.
One difference between nature macro photography and other macro photography is that you generally want a little longer macro lens.  A common macro subject in nature photography is insects, which tend to buzz off if you try to get too close.  Something in the 100-135 mm range helps with that.  The downside is that it's tougher to hand-hold without apparent shaking.  For things that don't run away, like plants, something around 70 mm is easier to handle.
Comment: Ya I knew that I would have to get 2 different lenses but thank you for clarification on that....I just didn't know which 2 lenses would be the best as far as mm also which brands are better
Comment: One exception to the macro vs. telephoto lens bit is catadioptric lenses.  Some of them can focus close enough to provide near 1:1 capability, though the depth of field is razor-thin.
Comment: However, mirror lenses tend to have a smaller max. aperture (f/8), which can compromise autofocus performance so not ideal for wildlife, and the cheap-cheap ones may not be terribly sharp. But the donut bokeh is cool. :)
Answer: There are a number of guides to selecting lenses on the web -- you should read one or more to familiarize yourself with lens terminology. One you could start with is this one from gizmag.com.
Briefly, like many engineering endeavors, designing a lens (and choosing which to buy) is a matter of tradeoffs. You want to build (or buy) the best product possible, but that means weighing factors like optical quality and light gathering ability against practical matters like cost, size, and weight.
Factors you'll probably want to look at include:

sharpness: How sharp, or focussed, is the image at given focal length?
speed: What is the maximum aperture (i.e. lowest f-number)? Larger apertures allow more light through the lens, which lets you use shorter shutter speeds. That helps minimize blurring due to camera shake or subject movement.
cost: Everyone has a budget to live with.
size/weight: A large, heavy lens can be impractical on a long hike, for example.

Other factors that are also important include chromatic aberration, distortion, build quality, image stabilization, special coatings that help to keep the front and back elements clean or to reduce internal reflections, and whether the lens itself can be mounted to a tripod.
Answer: To @Caleb's answer I would add:

the accuracy and speed of the focussing mechanism.
How quiet the focussing mechanism is.

Most lenses are accurate and focus precisely, but they do it at varying speeds. For example, the Nikon 55-300mm AF-S f/4.5-f/5.6 was a nice lens for portraiture and relatively slow moving objects but not animals because: 1) the focussing mechanism was slow, and 2) at the telephoto end it would sometimes not be able to focus at all until zoomed out.
The Nikon 70-300mm is a bit more money, but is probably better suited to animal photography because the focussing mechanism is both fast and quiet.


Is capturing better than killing a monster?

Question: In Monster Hunter World is it better to capture monsters vs kill them if the quest isn't kill specific? By better I mean will it generate more rewards to capture or just different rewards?
Comment: In previous titles capturing was almost *always* the better choice for getting rare drops, but I have not played/seen enough of World to confirm if the same rule applies.
Comment: Also note that reward from killing requires carving which may be interrupted by nearby monsters especially into higher tier quests where there might be another large monster which is not related to the main quest. This potentially reduces reward for killing.
Comment: Comparing investigations of capture vs kill it seems like captures seem to give out more rewards at the end vs being able to carve out a few materials. But in terms of non specific quests/expeditions I can't tell
Answer: Capturing a monster will always give you more rewards but sometimes also different ones. Personally I'd say capturing a monster is "better" unless you're farming for a very specific item that can only be carved. You can see the rewards of monsters in your hunter's notes and if a certain Item only shows up in the "Carving" tab you won't get it from capturing a monster.
As a quick list: Here are some pros and cons for capturing a monster in MH World.
Pro

More rewards than from killing a monster
Only 20 seconds to return to camp/HQ (the hunt is faster in general since you don't have to completely deplete the monster's health)
You can usually get "breakable" items even though you didn't break the part of the monster (Tails if you're using a blunt weapon for example)
Gives you more research points than simply killing it.
Messing up a trap isn't as bad anymore since you can restock everything in your tent.

Con

Requires some setup (Traps, Tranquillizer)
Requires precision and Knowledge of the monster (When can you capture it?). When normally hunting a monster you can just smash it without much thought about when to stop.
Some Items may be carve-only (From personal experience Plates, which are very rare, have a higher droprate in carves than in quest-rewards but I don't have any numbers on that)
The monster needs to be lured towards a specific location. Traps don't work everywhere.

In general I'd say you should capture monsters if you're farming for a specific armor set and if you're still missing some specific items you can go for a normal hunt (only if the drop chance from carving is better of course).
Comment: The option that is originally abandon mission will change to exit and you can use it to skip post quest timer
Comment: Something to note, after you have captured a monster it will unlock an arena quest where you get to kill it for more resources (on top of what you got when you captured it).
Comment: In MHW there is an option to skip the post victory 20/60 sec and return to town immediately. Though in killing hunts you'd want that time to carve.
Comment: I'd also add another Pro to capturing that the hunt ends faster as you do not need to reduce the monster's HP to 0 before capturing.
Comment: @BenOng Oh I didn't know about the skipping part (Is that in the options somewhere?). I already mentiononed the quicker hunt but I'll clarify it further :)


Calls and texts in tablet with mini-USB SIM adaptor

Question: I was hoping to find out if I can use my 10.1 inch Android tablet running Android 4.1 for calls and texts using a SIM card attached through a mini-USB (or if not, how?). Surely there must be a way.
Answer: I think what you're looking for is a full-blown 3G USB dongle, not just a SIM card adapter. See, for example: Can I receive/send SMS of cell phone number with a tablet using dongle?
A SIM card adapter would simply be a device that can read the contents of the SIM card and possibly edit it. So it's pretty much like an SD card reader. To send and receive text messages and voice calls, you still need a transmitter, which is what a USB 3G dongle will provide (if your tablet doesn't have built-in 3G support). However, I'm not sure if you can use a 3G dongle to make voice calls, although SMS is pretty much a standard feature for such devices.
Answer: I have a 10.1 in tablet with a SIM card. Tablets lack the hardware to make cellular calls - Those with cellular SIM cards can only use it to access the internet.  You might use a service like Skype but I am not that familiar with those types of services.
Answer: I have a Freelander 9.3" tablet with sim slot.  I have used it to call as long as I have a bluetooth headset.  Got it from China, but the vendor doesn't carry it any more see item # CVAGW-104152.  It works great if you need to use your computer AND make a call too.  It is quite handy, but not pocket worthy...  Wished I could get a newer version for my wife.


State vector after applying CNOT

Question:
In the circuit, the CNOT gate is applied in 11 state and it should transform into 10 state. But why is the probability of getting 01 state 100 percent?
Comment: Actually, the second digit of the bat plot refers to `q0`, the result is correct, it's just a little weird.
Answer: You are right, that $CNOT |11\rangle = |10\rangle $ in the usual setting. But the reason why you see the answer as $|01\rangle$ instead is because Qiskit uses little endian convention.
See this documentation to clear thing up.

State vector after applying CNOT


Are french fries considered shalem?

Question: For the purposes of priority in making a blessing, is a french fri considered a shalem food or is it considered a broken piece of potato?
Comment: @DoubleAA I don't know if that is a direct analogy, unless you are making a shehakol on your french fries.
Comment: Since it has a name, i would think it's shalem.
Comment: That sounds like an answer.
Comment: Is bread considered Shalem or a broken up piece of wheat?
Comment: What difference would it make?
Answer: The reason "Daver Shalem" gets priority can be for one of two reasons
1) The food item was created that way ("ברייה") an example could be a apple or even a grape.
2) That by taking the "Daver Shalem" before the other item you are beautifying the mitzvah of making a bracha (Hidur Mitzvah) Thus if you had a large slice of bread and a small roll you would make the bracha on the roll since that has an advantage of being "shalem" (Think of  a formal meal like a wedding. Normally a roll is put in front of each person not several slices of bread since the roll is more esteemed ("mechubidik"*??) )
Since a french fry on its own while technically "shalem" was a) not created that way b) a single french fry is not considered more esteemed to serve over a piece of potato I would say that it does not take any preference over the broken potato based on shalem**.
* mechubidik - apologies for the "yeshivish" but a good translation escapes me at the moment. MY editors please suggest an appropriate translation.
** there may be a case to say that in this case you would go for the bigger of the 2 items but I haven't seen that specifically brought down.
Sources:
Yerushalmi Perek 6 Halacha Aleph
SA 168:1 + Mishna Berurah
Laws of Brachos Rabbi Forst
Comment: mechubidik = esteemed
Answer: There is a trick that you can do on Yom Tov whereby you can take a broken piece of matzah, and burn the broken edge, rendering the piece whole for the purpose of lechem mishna.
Since the fry is surrounded on all sides by crust, I would say that it would be whole just like the formerly broken matzah.
Comment: Can you bring a source for this ? I have heard of it but in recent years I have heard from dayanim that it in fact does not help.
Comment: It is mentioned here but I don't know the primary source.
http://www.chabad.org/library/article_cdo/aid/1108296/jewish/Lechem-Mishneh-The-Two-Shabbat-Loaves.htm
Comment: Thanks but even there the source, Avraham Hirch, is one person and not any source in the poskim.

Are french fries considered shalem?


ssh Tunnel (-w) "Tunnel device open failed." on mac os

Question: if i do
macos - ssh server1 -A
server1 - sudo -HE ssh root@server2 -w 0:0

all work fine
tun device up success
ifconfig -a

#contain

tun0: flags=4240<POINTOPOINT,NOARP,MULTICAST>  mtu 1500
        unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 500  (UNSPEC)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

but if i try to do it from macos (10.15.7 (19H15))
sudo ssh root@server2 -w 0:0
Tunnel device open failed.
Could not request tunnel forwarding.
Welcome to Ubuntu 20.04 LTS (GNU/Linux 5.4.0-42-generic x86_64)

with -vvv
debug1: sys_tun_open: /dev/tun0 open failed: No such file or directory
Tunnel device open failed.
Could not request tunnel forwarding.

it's fail(
please tell me what's wrong?
------ update -------
Now everything is sad, mac os does not contain Tun Tap software, found the tuntaposx project but it does not work, Apple "remakes" KPIs (kernel programming interfaces) https://developer.apple.com/support/kernel-extensions/
I have not found alternatives yet.
And it is not yet clear how realistic this is - if ssh relies on files, objects of tun / tap adapters
Answer: Enable tunneling on server2 by editing the config file /etc/ssh/sshd_config
Set PermitTunnel yes
Then restart SSH daemon

ssh Tunnel (-w) “Tunnel device open failed.” on mac os


magento payment/order flow

Question: I'm implementing magento plugin and I need place order programatically.
After payment is authorize (i use sales_order_save_commit_after event) i create invoice and shipping(programatically). After that order is set to processing.
Now I want to confirm order and request confirmation to payment gateway.
I assume that magento should do this automatically. But I dont know when.
When (and where) confirmation to payment gateway is triggered? Where can I find detailed information about this process?
Answer: I think you should use sales_order_invoice_pay because all payment methods use $order->pay(),,in this event you can check total paid amount and all other things.
Also, it is very difficult to explain the payment flow in one answer, but I would suggest just going through the below links, may be you will get some better understanding.
Link 1
Link 2
Link 3
Comment: Sorry Keyur, answers was very helpful but this is not direct answer for main question. When and  where confirmation request is triggered? I made few things like create shipment, create invoice and one of these things trigger confirmation request to payment gateway and change overall status of order to completed

magento payment/order flow


SQL Server kicking off multiple SSIS processes for same job

Question: I've seen this bug before and I don't know what made it go away in the past.  I have a test server that has three jobs that each kick off their own SSIS package.  In production this works fine and there are only ever three ISServerExec processes in Task Manager, but on this one test server the number just goes up and up until it runs out of RAM.  When I look at the Integration Services Reports it shows that each job is getting kicked off again before it finishes running.  I'm on SQL Server 12.0.5000.0.
Comment: Have you considered upgrading to the latest cumulative update? You may find that this issue was addressed by one of the updates. Worth looking into, at any rate.
Answer: I found the fix but I can't explain what changed to make the fix necessary.  I added the "SYNCHRONIZED" parameter to the package parameters of the Agent job step and then the job acted normally, waiting for the package to finish and marking the job as successful only when it actually was.


Changing the Excel 2013 Sheet Selector to a Vertical Orientation

Question: Is it possible, and if so, what is the best way to change the orientation of the sheet selector bar at the bottom of the window in Excel 2013 to be vertical rather than horizontal? I have a large number of sheets, and a different orientation would make selecting them easier.
Answer: I haven't heard of any feature like this and can't see it anywhere in the settings - I doubt this is currently possible in Excel 2013. I'd suggest either reducing the length of your sheet names, or else look at combining tabs to improve efficiency.
Think of it like a Database - unless sheets are all displaying distinctly different types of data, it's good to hold all their data on one sheet. If you have twelve different sheets showing twelve months of data, I'd consider making one annual sheet and a dynamic monthly sheet that can retrieve a month's data at a time as you need it to.


Analytical solution for the diffusion equation

Question: Is there a way to compute the solution $u: \mathbb{R}^n \times [0,\infty) \rightarrow \mathbb{R}$  of the heat/diffusion equation
$ u_t - \Delta u = 0, u(x,0) = \exp(h \cdot x) $ with $h \in \mathbb{R}^n$?
I don't want to use the fundamental solution or advanced methods here, if there is another simple possibility.
Integrating this with respect to the variable $t$ leads to an integral over $\Delta u$ and there i'm stuck with my ideas.
I know the method of Fourier-series, fundamental solutions and Separation of Variables. But there has to be an easy way to get the solution. Does someone know such a simple method?
Comment: Please, consider edit your question, in order to add additional information, containing your thoughts in the problem, for example, you can add what attempts you have tried to solve the problem, what is your background in this area, etc...
Comment: Symmetry considerations should convince you that it stays a function of $h \cdot x$. This reduces the problem to the heat evolution of $e^{x}$ on $\mathbb R$, which has an easily found solution.
Comment: I forgot to explain that i'm searching for a simple way, i.e. without fundamental solutions. There is no other information given for this problem. I have no idea how to start. Maybe integrating?
Comment: I just need an idea, not a full computation.
Comment: Thank you. You mean symmetry of the initial values?
Answer: Let $\hat h=h/|h|$ be the normalized $h$. Every point $x\in \mathbb R^n$ can be written as $x= r \hat h+ y$ where $x\cdot y=0$. Since $u(x,0)=u(x+y,0)$ for any such $y$, the uniqueness of solution implies that $u(x,t)=u(x+y,t)$ holds for all $t>0$. Therefore, $u(x,t)=u(r\hat h,t)$. This is what Anthony Carapetis wrote in a comment already.
Now that $u$ depends only on one space coordinate (projection onto the direction of $h$), the solution is found from the one-dimensional problem $U_t=U_{rr}$, $U(x,0)=e^{|h|r}$. The convolution with Gaussian kernel (fundamental solution) can be evaluated explicitly by completing the square in the exponent.
Answer: $\newcommand{\+}{^{\dagger}}%
 \newcommand{\angles}[1]{\left\langle #1 \right\rangle}%
 \newcommand{\braces}[1]{\left\lbrace #1 \right\rbrace}%
 \newcommand{\bracks}[1]{\left\lbrack #1 \right\rbrack}%
 \newcommand{\dd}{{\rm d}}%
 \newcommand{\isdiv}{\,\left.\right\vert\,}%
 \newcommand{\ds}[1]{\displaystyle{#1}}%
 \newcommand{\equalby}[1]{{#1 \atop {= \atop \vphantom{\huge A}}}}%
 \newcommand{\expo}[1]{\,{\rm e}^{#1}\,}%
 \newcommand{\floor}[1]{\,\left\lfloor #1 \right\rfloor\,}%
 \newcommand{\ic}{{\rm i}}%
 \newcommand{\imp}{\Longrightarrow}%
 \newcommand{\ket}[1]{\left\vert #1\right\rangle}%
 \newcommand{\pars}[1]{\left( #1 \right)}%
 \newcommand{\partiald}[3][]{\frac{\partial^{#1} #2}{\partial #3^{#1}}}
 \newcommand{\pp}{{\cal P}}%
 \newcommand{\root}[2][]{\,\sqrt[#1]{\,#2\,}\,}%
 \newcommand{\sech}{\,{\rm sech}}%
 \newcommand{\sgn}{\,{\rm sgn}}%
 \newcommand{\totald}[3][]{\frac{{\rm d}^{#1} #2}{{\rm d} #3^{#1}}}
 \newcommand{\ul}[1]{\underline{#1}}%
 \newcommand{\verts}[1]{\left\vert #1 \right\vert}%
 \newcommand{\yy}{\Longleftrightarrow}$
Use Laplace Transform $\pars{~\mbox{this is a}\ {\large\tt\mbox{hint}}\ \mbox{for}\ n = 1\ \mbox{but it can
be straightforward generalized}~}$:
$$
0
=
\int_{0}^{\infty}
\bracks{{\rm u}_{t}\pars{x,t} - {\rm u}_{xx}\pars{x,t}}\expo{-st}\,\dd t
=
-{\rm u}\pars{x,0} + s\,\tilde{\rm u}\pars{x,s} - \tilde{\rm u}_{xx}\pars{x,s}
$$
where $\tilde{\rm u}\pars{x,s}$ is the Laplace transform of ${\rm u}\pars{x,t}$. Now, you have a 'Helmholtz like' equation with a source:
$$
\tilde{\rm u}_{xx}\pars{x,s} - s\,\tilde{\rm u}\pars{x,s} = -{\rm u}\pars{x,0}
$$
which is easier to solve than the original one. We hope you can take fom here.


moving large amount of MYSQL data to another host

Question: I have about a large MYSQL DB(3 TB) that I need to move to another host, which is not yet up.  what's the best approach?
I plan to either zip the mysqldump or the binary, what is the choice of the zip software?  I want to be able to create many minizips so that I can incrementally download them to my client box. current host os is redhat.
how should I download the files?  is ftps built in from ssh server the best way?
lastly, most importantly, the host itself already runs out of diskspace, has a couple hundred GB to buffer.  how do I go about zipping up the mysqldumps?
Answer: You can pipe the mysqldump across ssh (enabling compression might make it faster depending on the data and server capabilities) and save it to your local machine (or directly to the destination server). You can also pipe the mysqldump command through gzip/bzip2/xz so the file it produces on the local end of the pipe is considerably smaller.
Using the binary database is not the recommended way to backup/restore (from what I know).
Comment: the database is huge, how do I handle connection issues in the process of piping mysqldump and be able to resume the process?
Comment: @user, you can dump one table or database at a time. Does your network have problems that prevent you from keeping a connection open indefinitely?
Comment: yes I was thinking about downloading to my local pc where connection is not stable. the whole db has only a couple of tables.  these are large tables.   what compression should I use if most of the data is english text?
Comment: If you've got a relatively fast connection and/or slow server, use gzip. If you've got a slow connection/fast server, xz (or bzip2 if you don't have xz).
Answer: the best solution for you to copy your data to another Database hosted on another server dump your data as insert table by table
Answer: If your local filesystem only has a few hundred GB available, and your DB is 3TB... I don't see how you can do a dump.
You can try doing it over ssh to another host as Chris S suggested.
You can also mount the disk of a remote machine via NFS or Samba and dump directly to there.
If you want to split the dump file into multiple pieces, you can use the "split" command (read the man page for options... default option is to split in pretty small pieces, and you could end up with several thousand small chunks...).
There isn't really a "best" way to do the transfer, but if it's over a local network, pretty much anything should be fine. If you're doing it over a long distance (I mean like many thousand kilometers) or a high latency link, I don't recommend generic ssh for large file transfers.
Comment: should I use tar or gz prior to split?
Comment: Well, tar will let you bundle a group of files together into a single "tar file", so it's kind of the opposite of "split". Not sure why you would want that after you have already split it up. Just split it into the sizes you want (of course, this is after you've done the dump, so you still have your original problem), and then gzip those with max compression (-9 option).
Comment: so I split the dump .sql file, and then gzip them, how do I join them together later?
Comment: You can simply "cat" the files into one big file again after you gunzip them ("cat xaa xab > newfile" for example, will join those two files into one). Note that this is all a lot of extra work and hassle. It certainly would be more logical to find a way to dump your mysqldump directly to a remote filesystem, if it's at all possible.
Comment: dump mysqldump to a remote filesystem is possible, however, I also want a local(by local I mean home pc local) copy, downloading a large file over several days can be risky given the stability of my internet connection and pc.


Why do people usually say credit card instead of saying credit or debit card?

Question: For example, when I place an order on the phone or on a web site, they always say 'What is your credit card number?' It's like they assume I only have a credit card when in reality I have a debit card.
I know that debit cards can be charged as credits, and the merchant has to manually collect the funds and pay a fee. The money still ends up being taken from your account, but it will take up to a day or two.
Comment: Brevity, probably.
Comment: The vendor is probably running your card as a credit card, regardless of what you may call it, and doesn't care about the distinction you are making. If it can be processed via the VISA network, it's a credit card as far as they are concerned.
Comment: Credit cards are much older than debit cards, especially debit cards that can be used outside ATMs.
Comment: Why do people usually say fewer words?
Comment: I don't get why this question got several downvotes.
Answer: Because many debit cards swing both ways
Behind the curtain, on the merchant side, there are two fundamentally different ways to run a charge.

As a credit card transaction: with a signature (in the US), cashback not allowed, about 3% merchant costs +-, somewhat better consumer protection, and transaction fees never on you.
As an ATM/debit card transaction: with a PIN, cashback permitted, about 1% merchant costs +-, worse consumer protection, and transaction fees sometimes on you.

ATM cards capable of doing both have a major credit card logo on them, such as Visa or Mastercard.  If you beg and wheedle your bank, you can get them to send you a plain ATM card without a Visa/MC logo, that is not able to run as credit.
It is very different for the merchant, and lots and lots of merchants are not set up for ATM cards.  That's why the "credit card mode" on debit cards is such a big deal, it works at car rentals, pizza places, the Renaissance Faire... Of course all these subtle distinctions are invisible to you, which means the bank product managers all did their jobs well.
Heck, if you can fog a mirror and have a Social Security number that isn't blacklisted, you can be a credit card merchant, from outfits like Square.  You sure can't say that about an ATM account.
I did mention consumer protection.
ATM-card mode is basically designed to be like writing a check, which has virtually no consumer protections at all if you get ripped off.
Credit card mode on an ATM card has very limited consumer protection, because VISA required this to use the network.  In this case, they must accept credit card like disputes, but they keep your money until it's resolved, and it's not an auto-win.
Real credit cards have much better consumer protection.  In that case you do not need to pay the charge until your dispute fails, and the threshold is much more in your favor.  Visa and Mastercard get a lot of market pressure from American Express, whose liberal dispute policy all but guarantees your dispute will succeed.
If you want to reduce potential ripoffs, and especially if you are living hand to mouth emptying your bank account most months (don't do that), then do not use debit at all.  Use real credit cards and pay 'em off every month.  With debit cards "credit mode" is too vulnerable: a waitress steals your card numbers, merchant double charges you, any other kind of fraud, PIFF a bunch of money disappears from your bank account and now you're up the creek.
Comment: For some transaction the vendor won't take a debit card in either mode. You run into this when renting a car, or paying for a hotel room.
Comment: You don't even need to be able to fog a mirror to get credit card offers.  For about a decade, I got credit card offers for the husband of the previous owner of my house - and he'd died years before I bought the house.
Answer: That depends on the country. In France, people typically say "blue card" or "banking card", which makes no assumption on whether the card is credit or debit. E.g.:

("carte bancaire" = "banking card")


Where Are Units and Values Specified in Questions and Answers

Question: When describing the unit (e.g. GB) of a value (e.g. 32) in questions or answers where should the unit be placed?
Answer: I normally write units without a space, but looking at your two answers, I think I prefer the look with the space. It might be different in context though — omitting the space helps keep the association which doesn't matter unless there is surrounding text:

There is a lot of debate over whether binary prefixes will ever catch on. 1GiB ≈ 1.074GB, except when 1GB really means 1GiB eg when used for RAM. 1GB + 1GB = 2GB.
There is a lot of debate over whether binary prefixes will ever catch on. 1 GiB ≈ 1.074 GB, except when 1 GB really means 1 GiB eg when used for RAM. 1 GB + 1 GB = 2 GB.

Ultimately I don't think this is anything more than a style issue, and we shouldn't try and impose a specific style on the site. If you are editing a post anyway, and it has a mixture of styles, by all means make it consistent one way or the other of course.
Answer: With a Space After The Value
Examples:

32 GB
2.8 GHz
256 kbps

...according to SI Brochure: The International System of Units (SI) [8th edition, 2006; updated in 2014] which reads as:

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule are for the unit symbols for degree, minute, and second for plane angle, °, ', and ", respectively, for which no space is left between the numerical value and the unit symbol.
This rule means that the symbol °C for the degree Celsius is preceded by a space when one expresses values of Celsius temperature t.
Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. Only when the name of the unit is spelled out would the ordinary rules of grammar apply, so that in English a hyphen would be used to separate the number from the unit.
In any one expression, only one unit is used. An exception to this rule is in expressing the values of time and of plane angles using non-SI units. However, for plane angles it is generally preferable to divide the degree decimally. Thus one would write 22.20° rather than 22° 12', except in fields such as navigation, cartography, astronomy, and in the measurement of very small angles.
Answer: Without a Space After The Value
Examples:

32GB
2.8GHz
256kbps


Herbs that Would Relax the Mind but Not Decrease Blood Pressure?

Question: Recently, I have been wondering whether there are herbal remedies that would relax the mind from situations related to stress but NOT decrease blood pressure. So far, it has been easy for me to find lists of herbs that have relaxing effects such as from this website, http://www.kitchentablemedicine.com/herbalsedative/, but when I have investigated their effects further I have found that all of them contribute some degree to lessening blood pressure. The reason I am asking this is because a member of my family has naturally low blood pressure and by taking relaxants they have found that it causes them headaches when their blood pressure falls from an already low level. I would be much obliged if someone could clarify whether such remedies are possible or impossible to obtain.
Answer: We cannot recommend medical treatments on this site.  It's dangerous, especially in a situation where we're talking about symptomatic hypotension.  I can offer you suggestions on where to go for answers:
First, I recommend a relationship with a primary care physician.  A very low baseline BP should be assessed, and precautions discussed with the patient.
Second, I would recommend your relative bring up this concern about relaxation herbal therapies to the PCP.  Most M.D. and D.O. practitioners are not well versed in alternative medicines (even those for which there is mounting evidence of efficacy) but some are.  If they are not, I might recommend seeing an N.D. or other practitioner with thorough knowledge of herbal treatments of mental health.
Third, regardless of whom else he/she sees, psychology/counseling/therapy is an extremely beneficial treatment for stress/anxiety that doesn't involve taking any substances, herbal or otherwise.  It may be able to help get to the root of the problem, rather than just treating symptoms.
Comment: -1: Questions where part of the answer is *see a doctor* are off-topic on Health. [Please do not answer questions which are off-topic.](https://health.meta.stackexchange.com/q/753/8212)


Can I use a steam engine to extract items from chests or automatic crafting tables?

Question: The BuildCraft wiki is rather light on information, but I need to know: is it possible to use a steam engine to extract items from chests or automatic crafting tables without the engine blowing up? If so, how?
Comment: If you mean running an item from a cobblestone generator, for example, and running it through a chest, and then through an automatic crafting table, yes it's possible. There are a few guides on youtube if I remember correctly, although I've personally never tried
Answer: Wooden pipes extract items from chests (and perhaps automatic crafting tables; I haven't tried). Engines can then be attached to the wooden pipe.
I recommend watching the videos on the official YouTube channel to gain a basic, working knowledge of the mod.
Comment: Yes, they will work with auto-crafting tables also, but using this setup, they will explode after a while.
Comment: @John — I've never had that problem. Are you connecting more than one engine to the pipe?
Comment: No, I'm not.  Which type of engine do you use?  I use redstone engines.
Comment: If you have not done so already, I would recommend updating to 2.2 and upgrading your engines. Redstone engines are the weakest, and overheat easily. A combustion engine combined with a cooling source will solve the exploding problem. A steam engine should work as well, as long as you use more than one.
Comment: @Domocus: redstone engines work the best when they are overheated, they won't explode
Answer: A wooden pipe powered by a redstone engine is all you really need to extract items from chests or auto crafting tables.  Using a combustion or steam engine on a wooden pipe is kind of overkill, and I don't think it makes the pipe extract items any faster.  And unlike the other engines, redstone engines will never explode so they don't need cooling.


If I transfer songs to a zip drive and reload, can I get them out of "Purchased"?

Question: I no longer have access to the email address associated with my Apple ID, so I'll need to create a new one pretty soon. All possible ways to recover my current Apple ID have been exhausted, so that isn't the issue. I'm wondering if I transfer purchased songs to a zip drive or burn them onto a CD, will I then be able to transfer them back to iTunes but not as purchased content and therefore be able to play them with my new ID? I'm not very technology savvy, so I hope this isn't a dumb question.
Answer: This is a bit of a grey area around fair use, but there is software out there that will allow you to remove the drm from iTunes songs. Given that the songs they currently sell do not have any drm, I think it's not a particularly immoral option.
If you want to stay on the clearly bright side of the law, burning the songs to a cd and then re-ripping them will remove the drm and is supported through iTunes. You will lose some quality though.


Hamiltonian constraint in spherical Friedmann cosmology

Question: I'm taking a GR course, in which the instructor discussed the 'Hamiltonian constraint' of spherical Friedmann cosmology action. I'm not quite clear about the definition of 'Hamiltonian constraint' here. I searched internet but cannot find a satisfying answer. Could anybody help me a bit?
Answer: When you decompose Einstein's equation into a Hamiltonian formalism using the ADM approach, you get four equations to solve:  Two equations of motion in the three-metric of your initial spacelike section, and its extrinsic curvature in the enveloping four-dimensional spacetime.
Additionally, you get two constraint equations, the Hamiltonian constraint:
$16\pi\rho = {}^{3}R - K^{ij}K_{ij} + K^{2}$,
and the momentum constraint:
$8\pi j^{i} = \bar \nabla_{j}\left(K^{ij} - {}^{3}g^{ij}K\right)$
Any more detail than this takes considerable effort.  It is a chapter in Wald, and it took up a whole chapter in my dissertation, which I can point you to on the arxiv if that's what you feel you need.
Comment: Thanks for your helpful answer! but I think what Muphrid offered is more like what I want to know.
Comment: A nice answer, but I think the question is more about the physical meaning of the Hamiltonian constraint than its mathematical definition. One perhaps could add: the name 'Hamiltonian constraint' comes about from the fact that one can apply the Hamiltonian equations on this constraint (function,) and derive the equation of motion of its dynamic variables. I hope you agree.
Comment: @John: or alternately, that the Hamiltonian constraint is literally that the Hamiltonian is zero.
Answer: The Hamiltonian constraint comes from but one component of Einstein's equations.
$$\underline G(a) = 8 \pi \underline T(a)$$
The Einstein tensor $\underline G(a)$ deals with the metric and its derivatives--terms involving curvature.  The stress-energy tensor $\underline T(a)$ describes the matter and energy in a point of space.  This equation relating the two contains the great physical content of Einstein's theory--it is the relationship between matter/energy and spacetime curvature.
The Hamiltonian constraint comes from looking at the time-time component of this equation.  That is
$$\text{Hamiltonian constraint: } \underline G(e_t) \cdot e^t = 8 \pi \underline T(e_t) \cdot e^t$$
This is referred to as a constraint because it involves no time derivatives in the equation--only spatial derivatives are buried or hidden within $\underline G(e_t)$.  It is called Hamiltonian because it is closely related to energy and the overall Hamiltonian formulation of GR.
The Hamiltonian constraint is important because any description of a system at some initial time must obey the constraint to be physically possible, and it should do so at all times.  Hence, ensuring that some initial data obeys the constraint is a key consideration when solving Einstein's equations using, for example, numerical simulations.


Are stative verbs always inchoative when used with an imperative?

Question: Wikipedia says that stative verbs are always inchoative when used as imperatives. However, negative imperatives are used to exhibit prohibition in "including the giving of prohibition," and saying "Believing that is prohibited" does not seem to have the inchoative aspect. So in a sentence like "don't think badly of this" does it have the inchoative aspect?
Comment: In other words, does _Don't think badly of this_ mean "Don't start to think badly of this"?
Comment: Once you shift from 'have an opinion' to 'change your opinion', the notion of stativeness (of say 'think' = 'hold an opinion') becomes blurred.
Comment: Wikipedia says that it is Dowty who says that stative verbs are always inchoative when used as imperatives.
Comment: If you are attending to my wounds as I'm lying on the ground and I squirm as you touch a tender spot, you might command "Be still." Which I'd interpret not as, "start being still" but rather, "Remain [or continue] being still."
Answer: To comply with the use of a stative verb one must either start, stop or continue (not in the sense of resume) one's current state. As inchoative refers only to a start, I consider Wikipedia to be inaccurate in this respect.
An example:
See the screen! One way you can be complying is if you've continued the state (seeing the screen) that you were in when you read that (presuming that someone didn't read it to you). So a stative verb is clearly not always inchoative when used as an imperative.
"Don't think badly of this!" would only imply a start (thinking well or having no opinion either way) if the listener was already thinking badly of it, so the answer to your question is "No."
Comment: 'Remain standing!' is obviously a non-inchoative imperative.
Answer: The use of the term 'inchoative aspect' in the post deserves special admiration. Before we move on to the larger issue, let us examine the terms, ASPECT, STATIVE as apposed to DYNAMIC, IMPERATIVE and INCHOATIVE.
ASPECT is difficult to define. Roughly, on a superfluous level, it expresses how an action, event or state denoted by a verb relates to the flow of time. English tense system is made of the stuff of TIME (present/past/future) and ASPECT(simple/continuous/perfect). Time refers to 'when' and ASPECT to 'how'.
Stative verbs  describe a state of being/perceiving/mind/relationship. Stative verbs like 'know','believe' or 'love' are static or unchanging through out their entire duration and do not require imput or energy like DYNAMIC (action) VERBS.
Strictly speaking, inchoative verbs are literally those set of verbs that express a state of change, require no causing agent and seem to suggest that something is occurring spontaneously of its own accord. Cf.* The leaves turned brown. However, they have nothing to do with inchoative aspect of our discussion.
Imperative is a sentence type standardly used for the communication of a demand.  The speaker desires ACTION expressed in the sentence to take place. When in negative the speaker intends that something shouldn't happen or is not desired.
* Run.
* Do not play in the Sun.
IMPERATIVE VERBS ARE TENSE LESS. Imperative are usually odd with statives. It is believed that  they are restricted to dynamic verbs. When stative verbs are used in imperatives the interpretation is ,nevertheless, dynamic.
Interestingly, in English a verb that expresses a state can also express the entrance into a state. This ENTRANCE INTO A STATE /ITS PROBABILITY/CONDITIOAL DESIRABILITY is meant by INCHOATIVE ASPECT. Inchoative is an aspect that expresses the beginning of an event or state of a verb. English does not have a separate form to express the inchoative. But the expression - be about to- is a marker of inchoative meaning.
Dowty gives some test to decide if a verb is stative. One of them says that statives don't occur as imperative unless used in an inchoative manner as we have explained already. Some examples.
* Know Greek by Friday. * Remain seated.   * Know that we went yesterday. * Look elated.* Please remember where the money was hidden.
In the above examples, statives are coerced by the imperative mood  into dynamic interpretation, or in other words, into inchoative aspect.
It is true that there are forty or so stative verbs and not all of them have an inchoative aspect.
But once it is admitted that stative verbs have inchoative presence in imperatives, it matters little whether the sentence is positive or negative. The verb is inchoative in both:* Love me / Don't remain seated long.
John Lawler nicely explained in his comment what,"Dont think badly of this" is suggestive of. Being negative, it is inchoative as well. It is an invitation to start anew.
However, day by day the borderline between stative and dynamic is getting blurred. McDonald tweaked standard usage of stative verbs to call attention to itself -"I'm lovin' it." , and  the shops post signs saying, "Enjoy your beverages outside." However, Dowty is very much right though.
https://www.usingenglish.com/forum/threads/80691-stative-verbs-vs-dynamic-verbs?s=7d998d4ad770a2e4e5d144b9366ec831
Reference may be drawn to the uses by J K Rowling as shown in the link . So it is proof enough that statives are gradually getting free of inchoative stigma, and even embracing continuous.
Answer: Inchoative when used as main verbs.
When used otherwise they permit other readings.