« first day (3644 days earlier)      last day (36 days later) » 

12:05 AM
When we define an isomorphism between a vector space and its dual, for instance with the metric tensor, or (as I've just learned) with the Killing form in the case of Lie algebras, are there any further constraints placed on which element of $V\otimes V\cong V^*\otimes V^*$ we use other than it be symmetric?
In other words, does every element of $V\otimes V$ induce an isomorphism between the vector space and its dual?
 
@Charlie No. E.g. $0\otimes 0$ certainly does not induce an isomorphism.
 
hmm
I guess not :P
 
 
1 hour later…
1:17 AM
@JohnRennie Since the discussion of virtual "particles" has popped up again, do you think image "charges" could be a bit of a helpful analogy?
 
 
3 hours later…
4:16 AM
@Charlie No $o \otimes A^*$ doesn't!
 
4:41 AM
@skullpatrol Remember China was Corona free for last month? 0 cases and all? -> twitter.com/IndoPac_Info/status/1320305172321423361?s=20
 
 
1 hour later…
5:50 AM
@user76284 I can't see any way image charges would be a useful analogy for virtual particles.
 
6:07 AM
I mean not for virtual particles specifically, but for the idea of something in physics being a computational device rather than "real".
It's a very loose analogy, I know. Maybe there are other more similar examples?
 
Hi!
 
I am trying to create a copying program in C, and I want it to end after one input. My current code looks like this:
#include <stdio.h>

int main(void)
{
int c = getchar();

while(c != -1)
{
putchar(c);
c = getchar();
}

return 0;

}
In the above code, I have used -1 instead of EOF (end of file).
(Both of which yield the same results, as I have confirmed it)
Now the problem is that the above code results in a program that never terminates and keeps on copying whatever (ASCII only) input you enter. How can I tell the program to end after just the first input?
I thought that return 0; might do the work, but it doesn't. Is there any reason why?
@skullpatrol yo
 
6:23 AM
@Azmuth yup, i remember. Also their president has told the military to be prepared to go to war!
 
123
6:44 AM
Hi all.
 
hi
 
123
I don't understand that why Wikipedia inelastic collision take definition of bouncing ball. It could be wrong. Because in free fall acceleration due to gravity worked. So velocity kept changing continously
Not definition example of bouncing ball
Change in kinetic energy happening
 
@FakeMod Why aren't you using scanf()? Is there anything particularly about getchar()?
` #include <stdio.h>

int main(void)
{
int c = getchar();

while(c != '\n')
{
putchar(c);
c = getchar();
}

return 0;

}`
Now it should work
@skullpatrol I was talking about Corona thing... (Yes, that's why Mark T Esper and Pompeo are holding 2+2 talk on Indo Pacific today)
 
@Azmuth AFAIK, to use scanf, you need to specify the length of the data type of scanned input, which is char. Thus it limits the maximum length of your input. However, the more important reason for using getchar() is because I already know scanf(), but I am new to getchar().
 
yup, our nation is not only healthy, but we are getting ready to go to war
 
6:57 AM
@FakeMod Use pointers, if you are unsure about size of the data enters, it's similar variables, but just declared like this --int *var_name;
 
@Azmuth Works, thanks! But it does seem hack-ish. It would be even better if you could just make return 0 in my original code work somehow.
@Azmuth Hmmm... I haven't learnt about pointers yet. I'll give it a try.
 
1234
1234

...Program finished with exit code 0
Press ENTER to exit console.
@FakeMod the main function exits with 0 for me.
@FakeMod Can you be specific about the processor/OS/Compiler you are using? Datatypes change with change in system, so, I can guess if there is some short of problem here.
 
@Azmuth Hack-ish in the sense that it uses \n instead of EOF or -1
Hack-ish in the sense that it fails for certain inputs, like the following
my name
is fakemod

and azmuth is
helping me write my code.
 
@FakeMod EOF stands for *End of File` is used to recognize the last byte of the file in POSIX or Windows System, not end of file or enter key (notice both are different), -1 is just another parameter that, it doesn't means EOF either. If you want to emulate enter key then use \n endline character.
 
The program only returns "my name" as its output.
@Azmuth I want the program to not stop copying until the complete input is copied.
 
7:05 AM
@FakeMod \n' is end of line (different from end of file) is used to emulate enter keys only!
 
@Azmuth I am using Min-GW to compile my code on Windows.
@Azmuth Yeah, so I don't want to the copying to end when just a new line is created.
 
123
I don't understand why you guys worked on computer languages as a physicist.
 
Who told you that we are physicists?
 
@123 Higher physics requires a lot of coding and computer emulation.
 
123
:-) because this is physics stack exchange. I thought only physicist here.
 
7:10 AM
@123 nope.
 
@FakeMod You can use Ctrl+C which can be used to terminate once your job is over...
 
123
I worked on Javascript and python. But did not worked on C
 
@123 All the Physicists you see here are half coders.
 
@Azmuth True, that's how I stop it :P, but it needs to be user-friendly.
 
123
That's good news.. Excellent.
What is the benefit of C, over python and Javascript.
 
7:12 AM
@FakeMod Then what you want to do? It does copies your line, one by one...
 
123
I know is most low level language exist now. I need your opinion based on your experience.
Then I will learn C also
 
@123 C is a lower level language than Py and JS, you can alter each processor instruction, this is pretty useful when coding low level stuff like Drivers, OS and some stuff like Reverse Engineering/
 
@Azmuth I want it to stop on its own.
 
@FakeMod when you are typing stuff at the console you have to manually enter the "end of file" character by typing ctrl-Z.
 
@FakeMod Wth? Is your program AI? How it will know the user wants to stop it?
 
7:15 AM
@Azmuth I want it to stop after it copies one time. I don't want it to go on copying whatever I enter for eternity.
 
@JohnRennie Yes, in Linux it's Ctrl + D
 
123
OK. Means no limitation on C for work. I will definitely learn C also
 
@JohnRennie True, but that's not what I am trying to do, AFAIK.
 
123
Which one is better. C, C++, C sharp. What I need to learn
 
@FakeMod Okay, you want to print whatever is written in Clipboard? (copying contents from clipboard? (for windows?)
 
7:16 AM
@Azmuth No, this is just a learning project. It is not going to be used by anyone, but that does not mean that I will disregard user experience.
 
Yes, Windows, MSVC and MinGW Compilers offer that too, but you need to use pointers to copy whatever written in clipboard (of variable size) to a datatype
@FakeMod Honestly, I don't understand what are you doing? Do you have an example or some sample?
 
@Azmuth Thanks for your help, but I got to go now. I will surely follow you up.
 
@FakeMod Okay, no problem...
 
8:15 AM
@FakeMod getchar blocks until the user enters something - unless the user manually types an EOF character into the terminal, this program will indeed never terminate. Why are you using a while-loop if you just want to do this once?
 
8:34 AM
@ACuriousMind Not using while will just result in copying one character. I don't want that, I just want to copy one "input". FWIW, I did ask a question on SO.
 
Well, what is "one input"?
 
@FakeMod do you mean you want to copy one line then exit?
 
Your version considers the input to end at EOF. Azmuth already gave a version where the input ends at a newline
Apparently you want neither, so what is "one input"?
I see the commenters at SO are already telling you similar things :P
 
@JohnRennie No, I wanted something different, but now I realize that there's no concrete way to define what I mean by "one input", thus the newline solution seems the most appropriate as of now :)
 
Ok :-)
 
8:40 AM
Thank you Azmuth, ACM and John Rennie.
 
@FakeMod for future reference, we tend to take programming questions to a new room because they aren't physics. I'm always happy to answer programming questions in another room.
 
@JohnRennie Oh, sure. I suppose you are talking about the "A General Chat Room".
 
@FakeMod I do a lot of helping with programming questions, and in most cases the student creates their own room for the purpose.
The problem usually isn't of interest to anyone else, so it doesn't make sense to post it in a communal room.
 
@JohnRennie Ah, yes. So I might just invite you the next time to a personal if I have any programming questions. Thank you.
 
@FakeMod assuming I'm around I'll be happy to help.
6
 
 
6 hours later…
2:53 PM
Here's a cool German word: Fisimatenten
I'm not sure how common of a word it is in Germany, though.
Listed as "colloquial"
 
it is
 
it is colloquially common?
You're making a big fisimatenten about this.
 
3:13 PM
Happy dushera to everyone
4
 
3:27 PM
@PrateekMourya Happy Dushera! :-)
@FadedGiant heyo!
Do you know Particle Physics too?
Do you know any proof/argument about existence of Strangelets?
 
@Azmuth where is loong?
 
123
4:04 PM
Hi guys. Yo
Is there ny mood to discuss physics.
My bad to become a physics and math teacher.
 
and what would you like to discuss about that?
 
@ACuriousMind you have not reacted to my yesterday comment on gravitons!
 
yesterday, by ACuriousMind
I don't know what it means for a particle to be "absent", it's not a pupil that can be absent from class or anything :P
I don't know what other reaction you expect from me since you did not clarify anything after that.
 
4:32 PM
@JackRod what did you ask about gravitons?
This thread?
yesterday, by Jack Rod
why QM in curved space doesn't work?
 
@JohnRennie yes!
@ACuriousMind I did if you can notice carefully!
@JohnRennie just below it there is one more question which I was discussing with acm
 
Quantum field theory is based around defining a vacuum state for a quantum field, then particles are created by adding energy to that vacuum state.
But the problem with curved spacetime is that the vacuum state ceases to be well defined. Different observers in a curved spacetime define the vacuum state differently.
 
can gravitation field interact with matter in the absence of gravitons?if yes then does energy conservation hold?
This one was second
 
The graviton is the quantum of spacetime curvature. You can't have gravity without gravitons.
 
@JohnRennie
 
4:39 PM
So it doesn't make sense to ask about gravitational fields without gravitons.
 
Ok how can I detect graviton?
Using qm
@JohnRennie
 
Gravitons interact so weakly that it is impossible to detect them in a collider like the LHC.
In principle gravitons could be produced in collisions and carry away energy, and that could be detected. In fact this would be easily detected since they are the only spin 2 particle.
But in practice you could run the LHC for thousands of years without producing any gravitons.
 
Hmm! I some time feel that I am mixing gr with qm like I am talking about the deflection of single graviton even in future if we have theory which unify them what will be the most important thing it should have
@JohnRennie
 
The main problem at the moment is that the quantum theory of gravitons cannot be renormalised so it's impossible to use it for doing calculations.
No-one knows how to get around this. String theory was supposed to solve it, but that's looks less likely now.
 
Do you think virtual particles are more towards string theory? Which has no sense
 
4:54 PM
This might be of interest to anyone that works with magnetic moments or hyperfine structure:
5
Q: Rotational magnetic moment and ab-initio calculations

2ndQuantizedRotating molecules have an associated magnetic moment that can couple with both nuclear and electronic spin magnetic moments, thereby impacting the hyperfine energies of rotating molecules. Can one compute such rotational magnetic moments from ab initio calculations?

 
123
Hi all
 
@JackRod I don't think virtual particles exist in string theory.
I don't know enough about string theory to be sure.
 
123
How much i need to study to come to the understanding level of string theory
 
@123 start by learning quantum mechanics, then learn quantum field theory, then you you're just about ready to start learning string theory.
 
123
Hi @JohnRennie , I learn't QM not QFT in terms of math, But ideas known to me.
@JohnRennie Sir, why i always think about basic physics. Which still have too many unanswered questions?
 
4:59 PM
Are there many unanswered questions in basic physics? I would have thought not.
 
123
@JohnRennie Yes The questions, which i always discussed in this forum.
Like i asked in the morning. Example of inelastic collision taken by wikipedia is bouncing ball. But bouncing is under gravity have changing K.E continuously. Is should be constant.
 
5:33 PM
@123 The inelastic collision is at the moment when the ball is touching the ground where it has no potential energy. It doesn't matter for the collision what happens when the ball is up in the air.
 
@123 KE shouldn't be constant when something is going up or down and no energy is being added or taken away. The higher it is the less KE it has because it gains gravitational potential energy. The energy is only lost on the bounce (ignoring air resistance).
 
123
what about relation KE (Before Collision) = KE (After Collision)
We did not use Change in KE.
 
what change in KE?
both "before collision" and "after collision" are states at the same height above the ground, namely 0
one is the instant where the ball is moving down hitting the ground, the other is the instant where the ball is moving up after hitting it (but has not yet moved)
 
@123 KE before collision =/= KE after collision if the collision in inelastic. Some of the energy goes to other places, like heat
 
123
I am asking about phenomenon fall under gravity is kept changing its KE. Work-done is in place. Means Work-Energy Theorem. KE means we don't allow speed change.
 
5:49 PM
It's very unclear what you are trying to say.
 
123
6:24 PM
Sorry for the late reply. I mean, If something is falling under gravity means velocity is not kept constant it is changing continuously which is not allowed in inelastic collision, if velocity is changing $W = \frac{1}{2}m\vec{v_f}^2 - \frac{1}{2}m\vec{v_i}^2$
How it is possible take this situation for inelastic collision. If velocity changing how can possibly momentum conserved.
 
@JackRod here @FadedGiant
 
123
Hi @Azmuth
 
@123 It's still not clear what you're trying to say. What does the "falling under gravity" have to do with the collision itself?
The idea is not that the entire motion of the ball is an example of inelastic collision, it's that each of its contacts with the ground is.
 
123
@ACuriousMind OoKay, It means you agree example is not compatible with the inelastic collision.
 
No, it means that I think you have misunderstood what the example is supposed to be :P
 
123
6:30 PM
It is using just to give an idea what happened in elastic and inelastic collision
I am thinking about inelastic collision, Momentum (Before Collision) = Momentum (After Collision) and KE (Before Collision) = KE (After Collision) + Heat
For this we need to take an example when there is no acceleration.
 
@123 The situation leading up to the collision isn't the collision. Elastic and inelastic only describes what happens as an immediate result of the impact. The fact that the ball changes kinetic energy to potential and vice versa before and after the collision doesn't change the nature of the collision.
 
123
@JMac Yes But we don't take PE in the system of inelastic collision.
 
@123 Why would you, PE doesn't change during the collision, which happens at the instant where it bounces.
 
123
If it is an immediate result of the impact then why we care the comes to the same height or not
Sorry guys i asked questions here. Because no forum or book talked about this.
 
The change in height shows that kinetic energy is not conserved in the bounce, since the max height is where there is no KE and only PE, it gives a visual indicator of the kinetic energy after each bounce in height instead of velocity right after the bounce.
 
123
6:40 PM
But, always thank you all for the support and sharing your experience.
@JMac So the take away what i come up. We only need to take immediate velocity just before and after collision for both elastic and inelastic not the velocity at which the ball released. And see the effect in terms height of the ball. Is that true?
 
vzn
@JackRod String Theory: Virtual Particles dummies.com/education/science/physics/…
there is also this which doesnt seem to have much insight
4
Q: How are virtual particles explained from the perspective of string theory?

mcchucklezzI somewhat can wrap my head around virtual particles in terms of quantum field theory, (as much as possible), and I'm just curious how string theorists would describe them.

 
@123 Yeah, the height of the ball is a good indicator of the energy of the ball has immediately before and after each bounce, because neglecting air drag, the height varies based on the kinetic energy the ball had; since it converts all the kinetic energy to potential at the top of the arc. Collisions really only have to account for velocity right before and right after; though frictionless masses on a horizontal surface is usually an easier example than one that accelerates due to gravity
 
123
@JMac Ooooooooooo... I see
Why this information not written in books?
That's why i am not happy with the book. It always remain unanswered questions for each topic.
Also wikipedia have no words on this.
It means just before collision the at max velocity what is possible if just after collision velocity same it will be elastic collision and if it losses some of energy the velocity decreased (inelastic collision).
Does momentum conserved in this situation. also what about perfectly inelastic collision.
 
 
2 hours later…
9:10 PM
Hi I am back after 2 years
4
Aaaaaaand
dead
 
welcome back
 

« first day (3644 days earlier)      last day (36 days later) »