The h Bar

4:04 AM

@SirCumference When Rutherford moves to Cambridge, his colleagues had a problem with him because he was very loud.

After few days he talked on telephone with someone in America, during the dinner he said to professors and colleagues “today I spoke from here and everyone could here me on that whole continent” , however one of his colleagues said “I doubt you needed an apparatus for that” .

Semiclassical

5:46 AM

Please consider how badly this site failed at progressing scientific knowledge when you hear about this in the news. — the-moon-is-still-there 4 hours ago

is my favorite one

(though, in absolute fairness, I find consciousness-causes-collapse interpretations pretty distasteful, and I dislike the extent to which pop-sci treatments of QM try to pose it as such. looking at you, what-the-****-do-we-know)

9:29 AM

@JohanLiebert Are you that Kyubey? I know you’re Kyubey.

@Knight Yes I am. I told this to fakemod and Abhas but it seems you didn't noticed it!

9:49 AM

*am $\Rightarrow $ was

12:09 PM

@tpg2114 I tried what you said and got the conclusion that "No, even if we are given moment of inertia about infinitely many axes through COM then too it's not possible to give the exact distribution of mass around any given axis."

@JohanLiebert That seems consistent with what JMac's intuition said as well. I haven't tried it, so I don't know for sure -- but while it may not be possible to do it for an arbitrary shape, you might be able to do it for some specific ones (maybe exactly, or maybe it requires approximation)

For example, wheel or rotor balancing machines -- they spin up the body, measure the vibration, and then figure out where and how much mass to add or remove to get rid of the unbalance that causes vibration

That may take some approximations though, rather than exact expressions... I haven't looked into how they work

The other thing -- what if you don't use axes through the COM? If you picked different axes in other locations and measured the moment of inertia about those, could you back out the mass distribution? I suspect you might need 2 or 3 at a minimum with different orientations and origins.

@tpg2114t actually I was searching for counter examples of the statement "Given moment of inertia about infinitely many axes we can predict the exact mass distribution." And I was able to find a counter example, the case of a hollow sphere and a solid sphere (as they have the same moment of inertia).

@tpg2114 is it scientifically correct to make such conclusion from one counter example?

@JohanLiebert It depends on what your hypothesis is. If you have the hypothesis "Given the moments of inertia about the center of mass, it is possible to find unique mass distribution of any body" then yes, a single counter example disproves your hypothesis and is a valid conclusion

If you have the hypothesis "Given the moments of inertia about the center of mass, it is possible to find unique mass distribution of some bodies", then all you need to do is find 1 body where that is possible and you will have proven your hypothesis, and so that would be a valid conclusion also.

So a single counter-example will disprove the first hypothesis, while no number of counter-examples can disprove the second (unless you tested every single possible body, which can't be done since it's effectively an infinite number).

Likewise, a single example where you can calculate it will not prove the first hypothesis, while a single example where you can calculate it will prove the second

@tpg2114 I did this with axis through centre of mass because we have parallel axis theorem and perpendicular axis theorems which give moment of inertia through shifted or rotated axes.

Tyga - Ayy Macarena (Official Video)

12:24 PM

►

@JohanLiebert It's been about... 15? years since I've had to calculate a moment of inertia, so I'd have to think about the precise steps that would be needed... heh.

@tpg2114 so I guess you are about 30 years old?

I'd approach it as solving a differential equation for $dI/dr = r^2 \rho(r)$ where $\rho(r)$ is the density as a function of position. If I picked multiple non-orthogonal, non-collocated axes and tried to solve that system of equations, I wonder if I could get a unique distribution

@JohanLiebert Roughly, yeah

The differential equation approach wouldn't give an exact, closed form answer I'm guessing. Except for maybe some very specific shapes. And I'm not sure it would give a unique solution, without some assumptions added like continuous or discrete mass distributions or something

12:47 PM

Thanks for helping. I would give it a shot when I get familiar with solving the questions on moment of inertia. Thanks again.

Not sure I helped at all ;) But good on you for trying to work it out on your own -- that's the best part about learning the tools of physics! You get to try and figure out answers to your own questions

1:54 PM

@Semiclassical I actually spent several minutes replying to some now deleted conversations between me and OP. I think the underlying question could have possibly been phrased so that it was on topic. I tried to explain that the way the question was framed wasn't really appropriate for the site. It was only after they replied to that with "You're trying to kill ideas" that I decided to stop engaging, then they made the troll comment after someone else tried to help.

Question: An example of perfect diamagnet is a superconductor. This implies that when a superconductor is placed in a magnetic field of intensity B, the magnetic field $B_S$ inside the superconductor will be

Options: 1) $B_S$= -B , 2) $B_S=0$, 3) $B_S=B$, 4) $B_S \lt B$ but $B_S \neq0$

My answer is (1) , but the book is writing that inside any perfect material the magnetic field is always zero. Please explain

Meissner effect

The Meissner effect (or Meissner–Ochsenfeld effect) is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state when it is cooled below the critical temperature. The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples. The samples, in the presence of an applied magnetic field, were cooled below their superconducting transition temperature, whereupon the samples cancelled nearly all interior magnetic fields. They detected...

TBH though I know next to nothing about superconductivity, I just vaguely remembered that and googled it.

2:10 PM

why every atom has a unique spectral lines, I know spectral lines help us to identify the properties of substance , and that, s what basis Raman , but I have a question, question is every substance consist of atom, and every atom consist of electron, and electron are like fizzy cloud Tha, s what shordinger equation says, so what make the spectral lines different although every substance has same thing?

@YuvrajSingh... The energy (and other properties?) of the components and amount of those components changes with the atom/isotope though. I'm pretty sure spectral lines have something to do with discrete energy levels and the specific distribution of electrons/neutrons/protons in the atom. The fizzy clouds aren't the same shape for every atom as far as I know (but I'm remembering this from chemistry, I never really studied QM).

2:54 PM

@j Mac Thanks sir for replying let see what other say about it!

This is the way this great site has survived, I don't what to say...

... I don't really know what you're trying to say.

I'm saying that that comment is of the year 2013 and at that time people were facing problem about whether this SE is of any help or not, but due to people like @tpg2114 it has survived

3:09 PM

@Knight As far as I know that was about maintaining a certain amount of actual professional physicists on the site to maintain a high quality of questions and answers. I think it's actually quite debatable whether or not we managed to maintain a good level of professional physicists. Quite a few high status people had left around that time from what I've read.

@JMac Did they leave only because of quality was deteriorating ?

Jim

not necessarily. We move too fast. Professional physicists don't usually have the time to dedicate to this site to catch all the questions they could answer. We get a lot of graduate students, masters degrees, and retired physicists, but not professors. Some, but not a lot

@Knight It had to do with the "critical mass" problem AFAIK. There weren't enough high level physicists using the site, so high level physicists didn't find much interest in the site and would leave, making the problem even worse for the remaining high level physicists. Basically, you need enough high level questions to keep high level answers interested, and there was concern that we didn't have enough to maintain that.

But this site is manufacturing professional physicists and they will contribute

No there was a certain user. And some people were in his support and some against him( and some who didn't cared). When he was suspended due to issues related to his behavior on the site. People in his support left this site to make physics overflow. (Though it might be the partial truth as I don't know anything more than what I have read on meta.)

3:15 PM

Here comes @PM2Ring Sir

@Knight This site might be a tool to help people on their journey, but I wouldn't say it's manufacturing them at all. This site really isn't set up to teach people entire subjects or turn them into professionals. Becoming a professional generally involves being in academia first; not just looking at SE.

@JohanLiebert Who is he?

Q: Physics is rotting into a bad situation, a request for community manager intervention

-39

Physics Stack Exchange is experiencing the throes of very bad days. Lots of main and key users (compared to the total number of them) have left or are leaving. Examples of community destruction, with such users leaving, have been discussed in Physics Meta posts: Have we lost the necessary cri...

@Knight Note that the member who posted that is still an active contributor to the site.

@JMac Not merely academics can make them good physicists

@PM2Ring Yes, and he is very modest too (just like you)

3:17 PM

This was so terrible (exaggeration?) a situation that it reached the mother meta(☝).

However, we have lost a lot of experts. One member has a Nobel prize. He's still a member, but hasn't been active for several years. :(

en.m.wikipedia.org/wiki/Lubo%C5%A1_Motl

Lubos Motl too stopped posting since last year.

@PM2Ring Who's he?

@Knight I would say academics is one of the better ways to produce a good physicist, especially when compared to using a Q&A site instead. We're a good resource to help along the way, but I would not want anyone to consider us the driving factor in their education.

@Knight G. t' Hooft.

determinism quantum-mechanics string-theory quantum-interpretations black-holes aerodynamics bells-inequality

3:22 PM

Gerard 't Hooft

Gerardus (Gerard) 't Hooft (Dutch: [ˈɣeːrɑrt ət ˈɦoːft]; born July 5, 1946) is a Dutch theoretical physicist and professor at Utrecht University, the Netherlands. He shared the 1999 Nobel Prize in Physics with his thesis advisor Martinus J. G. Veltman "for elucidating the quantum structure of electroweak interactions". His work concentrates on gauge theory, black holes, quantum gravity and fundamental aspects of quantum mechanics. His contributions to physics include a proof that gauge theories are renormalizable, dimensional regularization and the holographic principle. == Personal life == He...

G. 't Hooft, Utrecht, Netherlands

4.5k 3 22 29

@PM2Ring Why did he leave?

@Knight I don't know. I have some guesses, but it was before I became active on the Physics stack.

@PM2Ring Your guesses must be very close to reality, can you please tell those

@Knight Maybe he got sick of all the low-quality homework questions... Actually, I think he got sick of a couple of trolls that have since been banned. ACM or John Rennie probably know more details. However, it's against the policy of this room to gossip about suspended people, since they aren't here to defend themselves.

I think he got sick of a couple of trolls that have since banned. How can someone troll a man like him? I'm not asking about trollers (if that's the right term) but why Nobel Laureate Mr. Hooft got trolled?

3:34 PM

Let's simulate a country and use AI to tell us the best way to run it? Like, AI creates the perfect government system.

ainism

in Tavern on the Meta on Meta Stack Exchange Chat, 7 mins ago, by Machavity

If you ever think SE is a mean place, just remember Aviation.SE was nice to this guy

no communism, no capitalism, no socialism... let's just let AI tell us the best way to do stuff so to maximize living standards, minimize suffering and increase productivity.

@Knight See HOW to BECOME a BAD THEORETICAL PHYSICIST by Gerard 't Hooft.

@NovaliumCompany Sure. Some of us have been advocating that for decades. There are a couple of problems, though. 1. Developing a sufficiently powerful AI. 2. Determining that you can trust the AI.

@PM2Ring hi sir! How are you?

Hi, @Yuvraj.

3:42 PM

@PM2Ring Thank you so much for giving me that link.

@PM2Ring sir have you seen my question? I posted in the room.

@PM2Ring But why such a good man got trolled?

@Knight It's great, isn't it.

@Knight Because trolls troll people.

I don't think there is any rationale behind it, other than being a troll.

@PM2Ring Yes it was great.

3:43 PM

in Tavern on the Meta on Meta Stack Exchange Chat, 2 hours ago, by PM 2Ring

BTW, I'm melting here. We finally had a few days of solid rain here in Sydney, which has cleared up the smoke haze. But today the temperature got up into the high 30s. It's now after midnight, but it's still around 28°C here, and rather humid.

@tpg2114 I really liked that comment of yours. I have uploaded a pic of it.

@PM2Ring then what should we consider flat-earthers then? (😂)

@PM2Ring Well, someone should work on those problems right? Trusting the AI is easy because you don't employ it to rule. You simulate a country on a powerful computer, let the AI do stuff and see the results. Then it's your choice whether to employ that technique on the government.

@PM2Ring Where can I read more about the topic?

@Knight That was a really long time ago... Who knows what else is buried away on Meta. Fortunately, the site today doesn't have nearly as many challenges as it did back then.

@PM2Ring this one sir!

why every atom has a unique spectral lines, I know spectral lines help us to identify the properties of substance , and that, s what basis Raman , but I have a question, question is every substance consist of atom, and every atom consist of electron, and electron are like fizzy cloud Tha, s what shordinger equation says, so what make the spectral lines different although every substance has same thing?

3:45 PM

Or at least, we have different ones now than we did then

@NovaliumCompany Your idea has merit, but a sufficiently sneaky AI can still screw you if it wants to.

@tpg2114 People like you kept it alive by staying yourselves hungry but giving your bread to the needy ones.

@PM2Ring Wow, the first time I give a reasonable idea, yay.

@NovaliumCompany See recent reports about AI giving minorities more jail time than non-minorities, and AI giving minorities worse health care than non-minorities for a good starter...

@NovaliumCompany Today we got all good people here.

3:47 PM

I've been reading this: amazon.com/Where-Good-Ideas-Come-Innovation/dp/1594485380 I guess it helps.

Unfortunately, AI needs data to be trained on and data comes from the programmers. So AI ends up with all of the implicit and explicit biases of the people who built it baked in

@NovaliumCompany Two of them are missing, John Rennie and ACM

@NovaliumCompany Apart from Yudkowsky? Lot's of sci-fi writers have explored the concept over the years. Eg, John Varley's Steel Beach

Also see reports about facial recognition failing to recognize minorities because it was trained on mostly non-minorities...

Yes it became very controversial

3:48 PM

Well, I'll think about the idea. It's an interesting topic.

@PM2Ring Btw, he never replied to my email. Guess I really was a bit rude :P

Just remember -- theory is nice and all, but implementation is reality. Most governments humankind has come up with are nice and functional, in theory, but fall apart in the implementation.

@tpg2114 Yeah, I'm not saying it's perfect. It's just something that popped up in my mind and I decided to share.

@NovaliumCompany You might enjoy this comic: smbc-comics.com/index.php?db=comics&id=2569

@PM2Ring xD good one

I don't know how the AI measures happiness but sure xD if it measures dopamine in the brain, then give me some drugs.