The h Bar - 2015-03-04 (page 3 of 3)

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9:08 PM

@NeuroFuzzy lol. I will never unsee Jamie

@ACuriousMind hmmmmm! I tried to read what Leandro said about Casimir effect, but exactly when I was glad that he is going to explain something, he got lost in quoting you, and all the issue went to smth. else. "but the caveats noted in the related question linked by ACuriousMind apply -- there is an interesting discussion in the comments to his answer that you should read."

@Sofia He literally just left out the technical part where you regularize the sum. There is no explanation missing.

9:20 PM

Can questions about (basic) relativity be asked here? The problem is too "complex" for a regular stack exchange question, but I would still need some help

They can be asked here but if your question could be useful to someone else in the future it should be asked as a question

I tried writing a topic about it, but it turned out like there were a lot of things that would have to be figured out "on the way", if you get what I mean

Welllll coming up with the question is half the battle!

@JamalS: Sorry, I forgot to reply to you. I've got no nice quotes coming to my mind right now, if I think of something, I'll let you know

Not a topic, a *question. Nevertheless, the problem has to do with time dilation. Judging from the twin paradox, it sounds to me (and I know this probably sounds really stupid) that none of the "times" slow down, only that each observer experience the other one's slowing down (simply because they see each other's clock moving slowly)

I'm hardly that experienced with physics (compared to people who do it professionally), which is why I'm asking this in the chat (as it seems that most questions are dedicated to more experienced users)

9:30 PM

@Max "as it seems that most questions are dedicated to more experienced users"...that's not how it looks to me.

Really? Maybe I haven't seen it all then :P

Well there are a lot of questions like that that have already been asked, too.

@ACuriousMind Up for a string theory question?

@0celo7 Sure, I can't guarantee I can answer it, though.

I looked at those, but none that I could find told me whether the formula (T' = γT) was referring to the experienced time from one observer's perspective or whether it was a relation which works both ways

experienced time being how you perceive the other person's clock

9:34 PM

Well in the twin paradox, the twin that gets into the spaceship is special

because of the head-snapping jerk when they turn around going near the speed of light in one direction to near the speed of light in the other

Yeah, causing him/her to be exposed to a large gravitational force

so when the two twins meet back in the same spot, one has aged T and one has aged T/γ. And that is NOT symmetric.

but if the two twins are passing each other, then yes, that situation IS symmetric

@ACuriousMind I'm reading 't Hooft's ST notes. He derives the string worldsheet action, which is just a 2-d QFT, right? He says to imagine an infinite string in the $z$ direction. Then we have transverse excitations in the $xy$ plane, and the excitations move at the speed of light. Then he says that because the field theory is nonlinear, if the string is strongly excited, it no longer stretches in the $z$ direction and the other tiny excitations move in the $z$ direction slower.

@ACuriousMind Why does the field theory being nonlinear prevent it from stretching due to a strong excitation?

And by symmetric, we mean that the formula is describing one's perspective, or is that incorrect?

@ACuriousMind Aleandro says "Our objective is to calculate the expectation value of the Hamiltonian in the vacuum state" and he gets indeed $\sum_k (2 \pi k)/L$, where I suppose that L is the distance between the two metallic walls. So, is an energy there. But then, he diverges: "The point of this example is to illustrate what people mean when they talk of quantum fluct. between the plates. They are indeed related to harmonic oscillator Hamilt. for each Fourier mode, but the caveats . . ."

9:36 PM

In this case, that is

1 views the 2's time to be dilated by γ, and 2 views 1's time as being dilated by γ

Also he says that strings can reorient themselves in any direction. Why? Does this have to do with nonlinearity? @ACuriousMind

"dilated by γ" meaning "each dt for the stationary person, dt/γ elapses for the moving person".

So in theory, if no turn around were to take place, they wouldn't age differently? (Thanks a lot for giving me a straight out answer to that by the way)

@0celo7 I don't know.

9:38 PM

Or maybe you can't draw a conclusion like that

@Max well... if no turn around took place no comparison would be able to take place

@0celo7 Uh, I would say that they can reorient themselves unless we find something forbidding that

@ACuriousMind Phys.SE worthy?

@0celo7 Yes, it's a good conceptual question

because they're far apart and there's no definition of "simultaneity". So you can't say who is what age at "what time" because there's no consistent notion of "what time it is".

9:39 PM

I'll just copy the relevant part from the notes into the post then.

Oh that's right! That makes a bit of sense to me actually!

@Sofia Yes. Again, I don't see a problem. He derives the expression for the Casimir energy. What do you think is missing?

@Max want to know my advice? Draw minkowski diagrams ASAP and learn why hyperbolae are sent to hyperbolae!

@0celo7 Do that! I wish more people did when asking about stuff they read.

I watched a few videos about it, but we're not quite at that level yet. But I'm guessing that it's required to develop a full understanding

We = my class

9:42 PM

@ACuriousMind 't Hooft has a curious name for the proper time: eigentime.

@Max See khanacademy.org/computer-programming/relativistic2/… and... hold on I used that to answer some question but I can't remember what the question was

@Max physics.stackexchange.com/questions/126565/…

@0celo7 Proper time is Eigenzeit in German. I believe the Dutch call it that, too, perhaps @Danu can confirm.

Bookmarking both

@ACuriousMind Better name than proper time.

@0celo7 I agree

9:43 PM

And thanks a lot :)

@ACuriousMind Title suggestion?

@Max So read the question before trying to decipher the diagram!

@Max In the diagram a trained eye can see length contraction, time dilation, and the relativity of simultaneity!

@Max because they're all consequences of special relativity.

@0celo7 The question ;) - something like "Why does the non-linearity of the string action prohibit stretching due to strong excitations?"

I think I can see time dilation by looking at the "derivative"'s roll, but none of the other

@ACuriousMind I could do that. I was hoping for something short and sweet.

9:47 PM

@0celo7 Well, I guess you could leave out the "due to strong excitations"

@Max for time dilation, I'm looking at the purple dots. If the slider is all the way to the right, the dots are collinear and you get one time. If the slider is all the way to the left, the difference in times between the dots is higher.

But informative titles trump short and sweet ones

@Max length contraction - if you look at a horizontal line through the graph, the width of a set of the collinear blue lines shrinks more the faster they're moving.

@Max simultaneity - the green dots (which emit light in in the question) emit light at the same time in one frame, but in the other frame they emit light at different times.

@Max I know it's information overload!

But it does give a great visual representation, hahaha

The problem is that I'm not that used to the time-space graph yet. In our education, we haven't even seen one (first time was today)

@ACuriousMind Done. Should I bother with anything other than the "string theory" tag?

9:51 PM

I understand it, but the fact that the blue lines "change" when speed change is still a fairly new concept to me :)

@0celo7 I'd suggest action, field-theory and non-linear-dynamics

@ACuriousMind How about QFT? How do I embed tags like that in chat?

@Max oh boy, I coded it in an annoying way so I can't really show you how it would look classically very easily...

@0celo7 Ah, okay, either field-theory or quantum-field-theory

And the code is [tag:tag-name]

Ok, how do you display code like that?

9:55 PM

Make backticks ` around it

I see.

Thanks.

"Hello benobled physicists!" LOL

@0celo7 No problem :)

@ACuriousMind "I sit here wondering about quantum mechanics from a philosophical point of view"

Don't kill him!

He knows not what he does!

Heh, he made me laugh. His "luck" that I'm out of close votes.

And nobody knows what they do :)

We all just stumble around in this weird thing called life

@ACuriousMind A comment rant would be appreciated.

Yeah, that string theory tag is weird.

0

Q: The balance of stability

I am wondering how the stability of existence works? Is there an optimal size for stability or does stability increase with the size of things? - as more and more elements are combined? I occurs to me that the social sciences seem to think that you cannot use exact mathematics on the questions o...

quantum-mechanics stability

Round 2?

10:02 PM

@0celo7 That's strangly reminiscent of the idea that we might be sitting in a "false vacuum" with some expectation values, and that existence as we know it could collapse if someone manages to locally lift one of the VEV fields out of its local minimum, so that it settles in another one.

@ACuriousMind @ACuriousMind he says "The point of this example is to illustrate what people mean when they talk of quantum fluctuations between the plates. They are indeed related to harmonic oscillator Hamiltonian. for each Fourier mode, . . ." I expected him to say, "here! we have energy in the vacuum". Or, alternatively to say, "we got smth. but we can't say if there is or isn't energy in vacuum". (Though the fact is that the plates attract/repel one another.) But he got lost in a caveat.

@ACuriousMind Interesting.

@0celo7 Also, tunneling means that existence could collapse without us doing something, anyway

I suspected a crackpot when I first heard this, but I believe Hawking said something like that about the Higgs, and I cannot find something wrong with it from a QFT perspective.

@ACuriousMind Doomsday weapon: lift Higgs field out of false ground state.

And I'm not decided whether I'm intrigued, terrified or both by that idea

10:05 PM

@ACuriousMind The first one 404'd.

You must have hurt his feelings.

@Sofia Leandro seems to me to be a careful theoretician. Saying "There is energy in the vacuum" would add nothing of rigorous meaning to what he already said, but increase the possibility of people getting weird ideas about the vacuum and energy, so he doesn't say that.

0

Q: How can I find a numerical solution for Einstein's field equation?

If I am given accurate measurements for the mass of an object how would I go about finding numerical solutions for Einstein's field equation? Would I start by solving Newton's equation for a large radius and deriving Christoffel symbols by using the gravitational acceleration according to Newton...

general-relativity gravity field-theory

@0celo7 No, he replied with a smiley in his reply to my comment. He seems aware that his understanding of physics is lacking, and is just floating "test ballons" to see if his ideas make any sense.

Do people even try?

@0celo7 Many, it appears, don't.

10:12 PM

@ACuriousMind Does the second paragraph make sense to you: physics.stackexchange.com/questions/7781/… ?

@0celo7 You mean the third, right? The second is pretty good.

@ACuriousMind I didn't count the first sentence as a paragraph.

The third block of text.

I realized that directly after typing :D

Yeah, it's non-sensical to me, too

@ACuriousMind @ACuriousMind but what you say, vis-à-vis the fact that the plates attract/repel one another? It's a fact. Well, if I seem to press you, I don't have such an intention. As I said, it's for a long time that I suspect the vacuum has a hand in entanglements, as I suspect that it has a hand in making the field felt between the capacitor plates. But, of course, I may be wrong.

@ACuriousMind I suspect one of two things: 1) It's wrong. 2) He made some crazy observation that is technically true but no one knows because it is totally irrelevant and does not contribute to an understanding of the subject. (And he's bad at explaining.)

10:19 PM

@Sofia I don't know what you want me to say. I'm fully satisfied by the description of the Casimir effect provided by Leandro (except that, of course, the derivation of the force is missing, but that'd just be differentiating the energy with resepct to the distance between the two plates)

@0celo7 I don't understand most these "Stuff moves through space-time at the speed of light"-arguments, and I suspect they are mostly nonsense, but I am not sure

@ACuriousMind I'm confused by the comments here: physics.stackexchange.com/questions/168296/…, starting at "Okay, first" by Chris Drost.

Actual distance seems like it should be a scalar quantity, but $ct-x$ isn't a scalar, I don't think.

Is he missing a Lorentz factor or something?

@0celo7 "Actual distance" = "Spatial distance you see". That's not an invariant.

People in that comment thread are basically just confused about what they mean with "actual" and "distance"

(The question kinda provokes that confusion, though)

@ACuriousMind I'm not sure what he's talking about. I'll just delete my comments because I probably misunderstood him.

0

Q: Does the speed of light vary depending on what it travels through?

The title is pretty much self explanatory. Does the speed of light vary depending on what it is traveling through? The speed of light through vacuum is 299792458 meters per second but does it change if it would say travel through air or water? If so why and if not why?

speed-of-light physical-constants

I don't...*::sigh::*

@0celo7 He's not as clear as could be wished, but I think he's just saying that the proper distance, which is invariant, is not the distance you see.

Do they mean the effective speed or literally the speed of photons?

Comment time!

10:30 PM

My comment still stands:

8 hours ago, by ACuriousMind

Let me say again that I wish more people would downvote these questions

@ACuriousMind I downvote regularly.

@0celo7 Very good. ;) I wasn't implying anything about you specficially, though.

You're nicer than I am with the votes. But that's not a bad thing, in itself.

@ACuriousMind Is it correct to say that charge is unbounded because there is nothing preventing us from having more charge?

You can see my votes?

@0celo7 The vote totals are at the right bottom of every user page. No one can see individual votes

@0celo7 Yes, the space of most theories is a Fock space with states of arbitrary particle numbers in it, after all.

1,500 downvotes

I need to get on it!

0

Q: What is the nearest neighbor distance in Titanium, Scandium Aluminum?

What is the nearest neighbor distance in Titanium ? Is there database ?

crystals x-ray-crystallography

haha what

10:37 PM

@0celo7 I said it's not a bad thing to be nicer than me!

@ACuriousMind You vote more than me both ways.

@ACuriousMind OH, does he mean what is the atomic distance in those metals?

That's true. There isn't enough voting both ways around here, I think.

@0celo7 Yes, I believe so

At what rep can you see VTCs on other people questions? Is it 3k because that's when you can VTC too?

@0celo7 did you vote for close this question? I voted for close, and saw that other people did it too, but under the question I saw only a (1) near the word close.

@Sofia Uh, I flagged.

I don't think I can VTC, can I?

10:50 PM

@0celo7 nope, 3k needed

@Sean yep, 3k needed

I did a mad rush to get 2k so I could edit...

I hate spending a lot of time on questions and only getting one or two votes.

@ACuriousMind but why don't I see near the word close the actual # of voters for close?

My string question got 7 views in one hour.

@Sofia You do. 0celo7's flag is not a close vote.

@ACuriousMind , you mean that I did. Yes, I did.

10:52 PM

@0celo7 That is the fate of many a technical question

@ACuriousMind It's not even that technical.

@Sofia Uh...no, the present tense is right here. You asked me "Why don't I see..." and I answered "You do".

@0celo7 It is. You'd have gotten more views with a less informative title, but none from anyone who'd be of any help to you.

I've gotten 110 total views on a repeatedly edited 10 days old question, so I know how you feel ;)

@ACuriousMind ah! Pedant Prussian education! many "a" technical question. Why not the simple "many technical questions"?

@ACuriousMind strictly by the canonical rules! Oh, pedantry! :-D ! (I don't criticize)

@Sofia Hehe. Yes, I'm a pedant (and mostly proud of it ;) ). The "many a thing" instead of "many things" is an archaic construction, I think, and it adds a bit of pathos to a statement

@ACuriousMind you are lovely! And you are an enthusiast. It's very nice.

10:59 PM

Why, thank you!

good question :-)

Holy crap this guy from my calc class is on the news. He got arrested for ISIL recruiting.

My compactification question only has like 30 views, so I feel your pain

@Sean Ali Moh's answer is correct, as far as I can tell, though. Symmetry broken by a ground state is not uncommon.

@0celo7 That's scary.

@ACuriousMind Well isn't that the whole idea of symmetry breaking?

11:12 PM

@0celo7 Exactly

11:31 PM

Heh, mathematicians know how to play fun games.

some say math is just a game

the part of the game applicable to reality is called physics :D

@ACuriousMind "Why", malicious you are! But, tell me, what was the question of Ali Moh with symmetry breaking? I remember that I had to say smth. about it.

@Sofia It is this one by Sean

11:59 PM

@HDE226868: Did you know there is another HDE here?

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