The h Bar

Secret

1:44 AM

@vzn I am leaning towards the skeptics side, though keep an eye for that alien possibility

I am so excited to meet aliens and try to understand the not understandable

If we have more alien probes coming, I hope we can find more insights on the quantum gravity question

Seth Taddiken

3:23 AM

Hey, I'm new here! And I have a physics question!
If we have an electron in a finite potential well, at the fundamental wavelength, is its wave-function the Gaussian-distribution function? I ask this because I recently learned that the magnitude of the wave-function is sinusoidal in an infinite potential well, but in a finite potential well the magnitude of the wave-function decays exponentially as it "tunnels" into the energy barriers that form the well... which, in my mind, seems to heavily resemble the Gaussian distribution. Thanks to anyone who read this!

More Anonymous

3:55 AM

hey all ... any thoughts on my question?

0

Q: An idea to model a one-dimensional thermometer?

Let's say I have a $1$-dimensional material with thermal expansion $\alpha$: $$ \alpha l_0 = \frac{\Delta l}{\Delta T}$$ where $l$ is the length of the system and $T$ is it's temperature. This is the same as saying: $$ \frac{l}{l_0} - 1 = \alpha T$$ One can assume this to mean if there are $...

vzn

4:23 AM

@Secret skepticism is fully warranted but hope someone reads the harvard paper & comments here before assuming its all bogus and/ or dissing/ dismissing it... maybe even someone with a phd... :)

Secret

calling that bogus is too extreme, I would not call something bogus that easily

Secret

4:42 AM

Right so I have looked at the paper, but given all the data we have, the nature of Omuamua cannot be conclusively determined. The high and narrow velocity does seemed to lean towards supporting it might be directed probe or debris that just get ejected from its home star system (referring the other paper in the link that tries to calculate the home system of Omuamua).
Either way, we need more similar things in the future to figure out

Also in regard to the general comment on bogus, I am flexible enough that even if a result is bogus, it can still be used. The only thing I cannot get any useful information from is ghosting. The paper has some detailed calculations and thus one can only be skeptical on whether it is alien probe, but one cannot call that bogus analysis unless a error is found

(Reams of rant censored and directed to the wormhole theory paper of Slereah)

It will be really cool if another Omuamua object arrives somewhere in 2020, as that year is really important for The Plan

Seth Taddiken

@Secret what do you mean by "narrow velocity"?

Secret

narrow range of velocities. It is a line near the conclusion of that paper when it discussed it as a possible solar sail fragment

Seth Taddiken

@Secret Are you talking about the CNN article? I can't find other links in here haha

Secret

yup

Seth Taddiken

@Secret I can't find the word "narrow", can you give me a keyword to use Finder on?

Secret

4:56 AM

> ‘Oumuamua’s entry velocity is found to be extremely
close to the velocity of the Local Standard of Rest, in
a kinematic region that is occupied by less than 1 to 500 stars
(Mamajek 2017).

Unparaphrased version

Seth Taddiken

@Secret Ah, thanks! So why does a velocity close tot he Local Standard of Rest (LSR) suggest that Oumuamua might be a directed probe? Wouldn't a natural object from another system be expected to have a velocity close to the LSR? Or is it so close to LSR that it seems that the object was carefully directed toward us along an efficient path?

And after a little Wikipedia read on Oumuamua, it seems like that object got really close to Earth! Is this true? Is 0.22 AU pretty close to Earth? Close enough to gather data? :O

In other words (about the first question I had), would an object that gets shot out of a system naturally be expected to be shot out so fast that its orbit is very unaligned with that expected of something conforming to LSR?

Secret

0.22 AU is closer than the orbit of mercury. As for the question about LSR, I am not an expert to answer

Seth Taddiken

5:20 AM

Hmm, do you have any idea how much information can we gather about Mercury just from instruments in orbit around Earth?

And I hadn't see that you said "or debris that just got ejected from its home star system" which made me think that you had more thoughts about the "directed probe" idea haha

Secret

Given something that is as convincing as Tabby's star is revealed to be dust and not dyson swarm, it is why I am also a bit more skeptical on Omuamua's nature

Nature have done way too many times explaining away weird things with natural reasons, so I won't just jump to conclusions so easily

Seth Taddiken

5:32 AM

@Secret I agree, but out of curiosity I wonder how well our most advanced telescopes could image Earth if they were placed 0.22 AU away from it... trying to Google it but I'm not very knowledgable about anything of relevance haha

Secret

5:44 AM

Me neither, it's definitely still much further than the moon and earth distance

Seth Taddiken

@Secret It's 85 times farther away from Earth than the moon is according to Wikipedia. We can take pretty good pictures of the Moon from Earth, but that (very very) roughly reduces in quality by a factor of 85^2 by the inverse-square law, right?

Secret

As I said, not an expert, so I am not sure

John Rennie

6:02 AM

@SethTaddiken no. The shape of the wavefunction looks superficially Gaussian but it isn't. As you say it is sinusoidal inside the well and decays like $e^{-kx}$ in the walls.

@SethTaddiken you need to know the angular resolution of the telescope to find out how much detail it can resolve. If you want to know more there's a nice description of this here.

Seth Taddiken

@JohnRennie Thanks! Also, I know that the exponential and sinusoidal parts of the wave-function line up such that they form a continuous and differentiable function, but is the wave-function piecewise nonetheless? Seems like it would be to me... since the transition over a medium doesn't "feel" very continuous.

John Rennie

@SethTaddiken well the environment is piecewise as well :-)

Seth Taddiken

@JohnRennie Ok that's what I was thinking! Thanks!

John Rennie

@SethTaddiken If you looked really closely at the walls of the well they are made up of atoms with vaguely defined edges, so the transition from the inside of the well to the wall is actually smooth not discontinuous. The wavefunction would therefore change smoothly as well.

Seth Taddiken

@JohnRennie Isn't there a fundamental length within the universe? So if you look even closer it will be piecewise? hahaha

or maybe fundamental unit of discrete energy would be better to say^

John Rennie

6:16 AM

You're probably thinking of the Planck length.

But no, the universe isn't discrete at the Planck scale.

Well, I guess I shouldn't say that since we can't do the experiment. More precisely there's no reason to suppose the universe will be discrete at the Planck length.

Seth Taddiken

Uh oh, looks like I have a pretty big misconception haha. I'm just now learning about quantum physics: the Stern-Gerlach experiment, Hidden Variable Theory (HVT), oh and the Two-Slit experiment of course... and Alain Aspect's experiments which disprove HVT -- so I don't know anything about the Planck length or any concepts beyond tunneling and entanglement.

So, what is the Planck length if not the fundamental and discrete length?

Semiclassical

note: the aspect experiments are intended to disprove local HV theories

John Rennie

@SethTaddiken to resolve smaller and smaller distances needs higher and higher energies. As you keep upping the energy there comes a point where the energy of your probe is so large that it causes a black hole to form. That happens at around the Planck length.

Since we can't see inside a black hole that means there is a fundamental limit to how small a distance we can measure. but that doesn't mean spacetime is discrete, only that our ability to resolve it is limited.

Seth Taddiken

Ah, way more advanced than I thought it to be! That's cool though... so if I had an extremely thin wire and I compressed it along its length, at the moment it began to collapse into a black hole would its length be an integer multiple of the Planck length?

My experiment is probably invalid realistically but is that the right line (no pun intended) of thought?

@JohnRennie But we do think that energy is fundamentally perfectly discrete right? (I think I'm wrong here haha but I want to be educated!)

Semiclassical

not really, no

John Rennie

6:31 AM

@SethTaddiken wrong again :-)

Semiclassical

there are systems which display energy quantization, to be sure

John Rennie

Only bound systems, e.g. your particle in a well, have discrete energy levels. Free particles have a continuous spectrum of energies.

Seth Taddiken

@JohnRennie Hmm that seems reasonable and doesn't appear to be inconsistent with anything I've learned... but I did have this idea that quantum physics suggests that the universe is fundamentally discrete... there goes my hopes of being in a simulation!

@Semiclassical Both are wrong huh?

Semiclassical

Quantization is more complicated than just "only integer multiples are allowed", basically

A lot more complicated

John Rennie

@SethTaddiken One of the problems with learning QM is that the way to best understand it requires a lots of math. Stuff you don't learn until university. You can learn it without the math, but viewed this way it looks weird.

Seth Taddiken

6:40 AM

@Semiclassical Ok, so like @JohnRennie seemed to be saying, quantization depends on the context... and from the finite well example, it seems like energy can be more smoothly quantized and not be perfectly discrete like it would be in an infinite potential well

Semiclassical

The point is really that, in the physics context, "quantization" means "to find the quantum version of a classical system". it doesn't mean "discretization"

Seth Taddiken

@JohnRennie I'm in university, but only have Calc 1-3, Diff Eq, and Discrete Math (Discrete is surprisingly unhelpful with QM as far as I can see hahaha)

Semiclassical

(tbf, there are people who do buy into the notion of spacetime having some discrete structure. But those ideas are highly speculative and in particular do not play well with relativity)

Discrete is useful in understanding certain consequences of quantum mechanics

John Rennie

@SethTaddiken linear algebra is probably the key area you need for QM

Seth Taddiken

@Semiclassical Ok, and you seem to be saying that there is a distinction between a system being quantum vs discrete, I thought they were the same?

Semiclassical

6:43 AM

Nope

Seth Taddiken

@JohnRennie Ah, I am taking that next semester! I only know how to use Lin Alg in the context of graphical programming (easy peasy Lin Alg) and I know about Markov Chains, very cool stuff!

Semiclassical

There is some interplay between discreteness and QM, but it's a good deal more subtle than what you probably envision

and not easily understood without knowing the math

Seth Taddiken

@Semiclassical Can you hint at what the math might involve?

Semiclassical

Have you seen eigenvalue problems?

Seth Taddiken

@Semiclassical Yes! From the Markov Chains, call me Larry Page!

Semiclassical

6:47 AM

A lot of problems in QM can be understood as eigenvalue problems, though not necessarily in the sense of "$Av=\lambda v$". (in particular, you see eigenvalue problems expressing particular differential equations rather than linear systems)

Seth Taddiken

@Semiclassical In what sense? My understanding of Eigenvalue problems is that they can determine convergence of a system

Semiclassical

More generally, you'll have $Lv=\lambda v$ where $L$ is some operator on a vector space and $v$ is some element of that vector space

but that vector space could just as well be a space of functions and $L$ could be a differential operator

Seth Taddiken

@Semiclassical Ah ok, so... stability... that last part you said makes me think that the Eigenvectors are a tool used in the math but aren't so much in the direction of an intuition helper, hm?

Semiclassical

Depends on the system

But you usually do care about the eigenvectors in order to make actual computations

Seth Taddiken

@Semiclassical Is my idea that systems tend toward stability a good start... haha

Semiclassical

6:52 AM

Not really.

For one, quantum systems tend to oscillate

Seth Taddiken

@Semiclassical Aye, or instability hmmm then that's not so useful of a thought

Semiclassical

not instability---an oscillation isn't either

Seth Taddiken

So you mentioned differential equations and eigenvalues, which makes me think stability, what should I be thinking? Or is this not related to any more-intuitive ideas really?

Oh wait, eigenvalues aren't necessarily describing stability but the warping (stretching?) of a space right? Is this a better, more general idea to have? (in this context)

Avnish Kabaj

$e = mc^2$

Semiclassical

I think you're trying to map the problems onto the ones you know, and that's not a great idea

Seth Taddiken

6:56 AM

@AvnishKabaj I thought it was 2.71

Semiclassical

For one, you often deal with hermitian operators in QM

Seth Taddiken

@Semiclassical Ah I'm not there in my math knowledge

Semiclassical

which among other things means the eigenvalues are real

Avnish Kabaj

No if you differentiated the eigenvectors relative to space time you get $e = mc^2$

Semiclassical

yeah, uh, have fun with that

Avnish Kabaj

6:58 AM

A few colleagues and I had a long and draining discourse on the topic

Semiclassical

but more to the point, you're not in a situation where "going forward by one step" is equivalent to "multiplication by A"

Seth Taddiken

what situation am I in?

Semiclassical

in fact, the version you'd see in QM would be more like "multiplication by $e^{-i H \Delta t}$ where $H$ is a hermitian operator

Seth Taddiken

Ok, so the eigenvalues don't really describe stretching of spaces in that situation?

Semiclassical

nope. note that any eigenvalues of $H$ will amount to $e^{- i H\Delta t}$ having eigenvalues which are all complex numbers with unit modulus

so rotations rather than any kind of stretching

To the extent that there's "stretching" involved, it's in the sense of "the probability of observing certain outcomes may change as time goes on"

But that reflects what happens when you take superpositions of states with definite energy

Seth Taddiken

7:14 AM

Hmm... so I should not be extending my ideas of Markov Chains and space distortions and limits of systems here? At least not until I learn about hermitian operators and many other math concepts that are necessary...

Seth Taddiken

7:24 AM

@Semiclassical hey I learned about Lagrange Multipliers with "boundary value problems" in calc 3 and I know that when you have a problem where need to plug initial conditions into a Diff Eq you call it a "boundary value problem"... can the term "boundary value problems" be seen to mean the same thing in a calc 3 context as it does in a diff eq context? Or is there a greater dissonance in these cases than I am looking for...

NormalsNotFar

8:24 AM

Hi people, when linearizing the metric around an arbitrarily curved background, $\tilde{g}_{mn}=g_{mn}+h_{mn}$, should we require the covariant derivative corrsponding to the background metric, $\nabla_m$, to be torsionless, or the one corresponding to the full metric, $\tilde{\nabla}_m$ ? or both ?

Physics Meta

9:53 AM

0

Q: I need more light on the homework policy

Please, I need to know where a problem is if one posts their homework here and it is well formatted. The actual thing is they might have tried solving before coming here in the case where they didn't have another place to go seek for some 'thinking' or solution before the real homework correction...

discussion scope questions homework-and-exercises

Slereah

11:50 AM

@NormalsNotFar Both I believe

Slereah

12:11 PM

0

Q: Geodesics of anti-de Sitter space

It is said that, given the anti-de Sitter space $\text{AdS}_2$, let's say in the static coordinates $$ds^2 = -(1 + x^2) dt^2 + \frac{1}{(1+x^2)} dx^2$$ Every timelike geodesic will cross the same point after a time interval of $\pi$. That is, if $(x_0, t_0) \in \gamma$, then $(x_0, t_0 + \pi) \...

general-relativity geodesics anti-de-sitter-spacetime

halp

Help me before I have to look into MTW

It's probably in there but I may die reading it

Slereah

12:49 PM

Hm, am I allowed to answer rudely if I answer myself

John Rennie

@Slereah no self abuse in this chat room please

Slereah

Well I meant on the site

John Rennie

Or on the site :-)

Slereah

According to WA the solution for $t(\tau)$ is

Doesn't help a lot

Sha Vuklia

Guys, question about 1b. I solved for $V/n$, and got
\[
V/n=\frac{RT/P\pm\sqrt{R^2T^2/P^2-4RTB(T)/P}}{2}.
\]
Now I have to explain why $B(T)$ would be negative at low $T$, and positive at high $T$. I’m not sure about the forces between the molecules? All I know is that a higher temperature, means higher kinetic energy, so the molecules will bump into each other faster and harder. And if we only take the ‘$+$’ solution, positive $B(T)$ would mean a lower $V/n$, so the volume gets smaller… This could be explained if there are some extra forces between the molecules due to a higher $T$, but I …

Physics Meta

1:08 PM

0

Q: What to do if my question ban is not getting removed?

I have read on the Physics S.E website that users get a chance to ask question after 6 months of question ban If you're unable to improve your existing questions, you'll get the chance to ask a new one 6 months after your last question. If that question is positively received, you may be able...

discussion questions

Physics Meta

1:24 PM

0

Q: Possibility to retract flag on comments

I have noticed that even if I mistakenly flag a comment, I cannot be able to retract the flag afterward... I think it will be a good feature to add the possibility to retract flags on comments.

discussion feature-request flagging

Sha Vuklia

1:42 PM

oh whoops, I had a minus sign wrong here. then that solves my confusion

Slereah

2:47 PM

Does anyone keep up with @0celo7

He doesn't seem to answer on skype no more

Anonymous

He changed his name to Ryan Unger on the main site now

Anonymous

Last seen Nov 9 :P

bolbteppa

3:25 PM

I think he's learned his lesson and could do with a bit of lenience

Even the justice system has a parole system, how come this system has no parole system, very authoritarian

Anonymous

@bolbteppa Well, there sort of is. Users can reply to moderators in reply to their suspension message. But it needs to be convincing.

Anonymous

I have heard of at least a handful of cases where suspension was lifted early that way.

Anonymous

However, the problem is that most of the suspensed users are either rude even in their reply message or they never write it.

bolbteppa

That's no different to someone shouting 'I'm innocent' after being 'caught' :p The real justice system goes out of it's way to set up the parole aspect in advance, and lets the defendee know in advance, what kind of a system pretending to be legitimate is more harsh than the standard Western system

I should have thought of this earlier, god, what a gigantic hypocrisy haha

As usual

Again, Ron's century ban, with no parole, I mean come on

Anonymous

In a real-world justice system everyone also has right to lawyers, etc. This isn't a real-world justice system and we shouldn't pretend as such :/

John Rennie

3:33 PM

Access to the Stack Exchange is not a right, it is a privilege. One that the Stack Exchange can withdraw as and when they choose.

bolbteppa

The parole system does not require lawyers, it's simply an in-built part of the concrete of the system

Anonymous

Anyhow, yeah, Ron's century ban struck me as really weird

Anonymous

Aug 19 at 0:17, by Emilio Pisanty

so I need to run for mod and win to scratch that curiosity itch? I'm tempted to say it's worth it.

John Rennie

3:50 PM

@Blue I believe a suspended account can't be deleted, so my guess is that they don't want his account deleted. Maybe there are some legal implications if the account is deleted.

Anonymous

@JohnRennie Sure, I understand that. But that's not the point. Initially Ron was suspended till 2016 for over an year. Then suddenly in 2018 they decided to extend it to 2092. What caused that extension? That's the curiousity itch we're talking about :P

Anonymous

*2026

John Rennie

@Blue I suspect the intention is that the account is never going to be unsuspended. 2026 is only eight years away, so they extended the suspension long enough to make it functionally indistinguishable from never.

Anonymous

Yeah, but it would be really unprofessional to change suspension length on a whim, and that too after over a year after the initial suspension. It's not like he had any site activity during that period. The only legit reason I can think of is that he had mailed something inappropriate regarding his suspension to the SE team (in 2018). But then we should not speculate. :)

John Rennie

The thing about words like unprofessional is that they are largely meaningless. People use them to mean something I disapprove of.

Anonymous

4:05 PM

I can agree with that :)

John Rennie

Calling something unprofessional is just a lazy way of criticising it when you can't be bothered to properly analyse the situation.

Anonymous

Well, then every usage of the word unprofessional could be attributed to laziness. :P And I wouldn't agree with that. It's more like "something I wouldn't expect of a professional company like Stack Exchange".

Anonymous

"and something I wouldn't expect others to expect of SE as well"

John Rennie

@Blue in many jobs there are clear professional guidelines: lawyers, doctors, etc, etc and in those cases unprofessional has a clear meaning. But I don't think it has any meaning when applied to the SE.

I guess the question is whether there's a reason for extending the suspension or whether they're just being spiteful. And no-one but the SE can answer that for certain.

Anonymous

@JohnRennie Gotta get hired by SE! :D

John Rennie

4:10 PM

@Blue I suspect that working for the SE would be really good fun.

Anonymous

@enumaris Please apply to SE on our behalf and let us know wtf went on with Ron's account ;)

Anonymous

Forget OpenAI

Anonymous

Or we could collect $50 million.... :D

Sir Cumference

0celos only got 3 or so months to go

Anonymous

stackoverflow.com/company/work-here

Anonymous

4:15 PM

Phew, most of the jobs seem to be in the Sales and Marketing department

Anonymous

Only 3 engineering jobs

Anonymous

That's....bad

Anonymous

Looks like they have a not-so-big team

danielunderwood

Looks like those engineering jobs aren't really software either

2 are ops and the last looks like management

Anonymous

Yeah, meh. Let's just buy SE and fill it with AI stuff

Anonymous

4:18 PM

AI mods and all

Anonymous

I thought they'd at least have some Data Science jobs

vzn

0celo7, what a memorable character, recently cited him in my blog. his cohort in crime BaSe disappeared also. RIP! hbar? aptly or inaptly named? maybe just too many strictly enforced rules to be compared to a bar ... oh and googling bars, look at this! o_O forbes.com/sites/neilhowe/2016/03/16/…

@Blue hmm yeah wonder if anyone is doing datascience at stackexchange, seems really ripe for it. seemed to happen in past but maybe not active. the company seems to be scaling back in some ways, heard about layoffs around this year. found this with google. he maybe worked in 2013 and wrote it up in 2016. One year as a Data Scientist at Stack Overflow/ Robinson varianceexplained.org/r/year_data_scientist

danielunderwood

4:40 PM

SE may be pretty heavy on recruiting on their own as well. Most companies want people familiar with their product and SO gives them just the data to pick out who they want

And the fact that people have jobs profiles on SO

vzn

4:51 PM

Would flooding the deserts help stop global warming? / nbc news

enumaris

Apply to SE...

where's this comming from lol

Semiclassical

"“The concern I have is that Silicon Valley seems very excited about these moonshots, when maybe what we need is a bunch of Boeing 737s,” said Armond Cohen, executive director of the Clean Air Task Force, a clean energy think tank and lobbying organization. “And to do it on a 10- to 15-year development cycle, not one that takes 30 years.”

that's the kind of thinking I prefer tbh

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