The h Bar

PM 2Ring

00:19

Oops. That should be Isle of Man. Oh well...

ZeroTheHero

02:13

Like Mann-chester?

PM 2Ring

04:16

@DavidZ physics.stackexchange.com/questions/528549/… Sorry about that. I just added the info about seeing UV as an aside, I didn't expect it to turn into a long comment thread.

And I didn't expect that question to hit the HNQ, but in retrospect I guess that was fairly likely. ;)

uhoh

06:12

1

Q: Time vs magnification in a binary lens

For a single lens: we got \begin{equation} u(t)=\sqrt{\left(\frac{t-t_0}{t_E}\right)^2 + u_0^2},\end{equation} so the magnification is \begin{equation} A = \frac{u^2+2}{u\sqrt{u^2+4}} \end{equation} Therefore the magnification for a single lens depends on three parameters: $u_0$ (minimum s...

gravitational microlensing question

user434058

07:27

Can anybody tell me why did Smoke Detector marked this as containing a potentially bad keyword. I am not able to see that "keyword". Note that the link in my message is to my post on the metasmoke website and not on PSE.

ACuriousMind

07:52

@FakeMod The reason actually tells you: the "keyword" is the href tag. For the actual reason you could ask the folks in the Charcoal HQ chat, but I'm pretty sure this is because such HTML links are a sign that the post is a template and not a post written for SE - on SE, one uses markdown for links.

Slereah

morning

ACuriousMind

mornin'

user434058

@ACuriousMind I never used that markdown.

user434058

My post was just the kind of everybody writes

ACuriousMind

Although I'm just guessing it's morning - it's been overcast and rainy here for three days straight and I'm starting to forget what morning sun looks like

user434058

07:54

Morning, however it's afternoon here...

ACuriousMind

@FakeMod Huh? By "markdown", I mean writing [text](link) for links

user434058

@ACuriousMind Forget what I said, that was a blunder...... So yeah, isn't that how we write links? I mean I did it the same way, but SmokeDetector changed my questions markdown.

ACuriousMind

Oh, right. In that case I have no idea and you should really ask the Charcoal folks if you want to know

Knight

What does this sentence means For those who might be wondering, it only take about a week for the twitching need to answer questions on the site to fade ?

David Z

08:13

@PM2Ring oh, no worries, it's not that big a deal but because of the combination of it hitting HNQ and several more comments being posted, I decided to delete them.

Tapi

@Knight not sure but I think it means that it'd take about a week for the urge to answer questions posted on this site to fade, that is basically saying that you'd be bored of answering.

user434058

09:04

@Knight After leaving, one has the desire to keep on answering questions like they used to do (inertia). But that desire will fade out after a week or so.

Knight

@Tapi @FakeMod Thank you.

user434058

You are welcome!

Knight

@DavidZ Once John Sir told me that you sleep during the night :), so seeing you in chat is like a surprise.

@user8718165 you know i know

user8718165

@Knight Hello :-) How are you?

user434058

user434058

09:12

@Knight You are welcome

user8718165

@FakeMod what if it were just a piece of paper (to disguise). LOL he'd have terribly fallen XD

user434058

@user8718165 BTW It is fake!

user8718165

@FakeMod Oh! I see.

user434058

However I am amazed by the fact that this seems so real!

user434058

Morever, you can only expect Fake stuff from FakeMod :P

user8718165

09:20

@FakeMod You're very real :P

user434058

@user8718165 You got the joke! ;)

user8718165

@FakeMod :-)

Knight

09:41

@FakeMod But I can expect something from Dhruv.

user434058

@Knight He is preparing for JEE :)

Knight

@FakeMod Okay, God knows where is he!

user434058

10:08

He is gone now! Check his account!

user434058

@Knight

user434058

Still he can chat!

user434058

Maybe a bug of the site, or a superpower given to him, who knows!!!

user434058

I am his ghost.....

user434058

!

user434058

10:10

IS ANYONE EVEN GETTING THESE MESSAGES??!?!?

user434058

This is awesome!!!! I can send messages even if my account is deleted! Wohooo.....

user434058

But alas.... All of this BS is going to trash... Waiting for ACM to send it to trash.

David Z

Only your site account was deleted, not your chat account. They're separate.

user434058

@DavidZ Wait, what?

user434058

That's why all of my older messages always get reassociated with my account! I see.

David Z

10:14

You had an account on physics.stackexchange.com, which was deleted, and you have an account on chat.stackexchange.com, which was not deleted.

user434058

Do I have to delete that separately?

David Z

If you want it to be deleted, yeah, you have to do it separately. I'm not sure offhand how you would do that but it probably involves contacting SE. I could try to find more details if you want (although likely not today)

Naveen Balaji

Hey all! I have this particular question about the Einstein-Hilbert Action: When we vary the same we get two integrals-one with the variation of the metric tensor and another, the variation of the derivative of the metric tensor- now in order to kill the latter we add in the Gibbons-Hawking term. If we demand asymptotic flatness for the metric prior to the variation, would this automatically not require for the Gibbons term since the spacetime now would become Minkowski?

user434058

@DavidZ Oh thanks! But I think I won't be bothering you with such trivial things.... See you soon, till then Goodbye everyone

Slereah

@NaveenBalaji As far as I know, the main point of the Gibbons-Hawking action is because we should ideally have the action work out on any finite domain

Because, unlike some other fields, we don't have any guarantee that if we integrate over the entire spacetime (which has no boundaries), the action will be well-defined

For instance take the AdS space with topologt $\mathbb{R}^n$, its action is the integral of a constant over all space

Therefore not very well defined

I don't know if asymptotic flatness would guarantee finite action, but one thing it certainly won't cure is the presence of boundary terms on an arbitrary subset

user434058

10:24

@DavidZ Hi! But here, Shog9 says that if I delete my last account, my chat account will automatically be deleted. Doesn't seem to be working that way....

Naveen Balaji

@Slereah Hmm, my thought was that if we could demand asymptotic flatness, the definition states for the derivative in the metric tensor (wrt 'r') to die down and this would eventually kill the boundary term piece.

user434058

@DavidZ And yes, now I think I really need you to get me out of here forever... Of course, it's a trivial request, nothing serious, so take your time. Waiting for your update. Thanks :)

Slereah

@NaveenBalaji It's not impossible that it would be finite, but then you could only ignore the boundary term by taking the action over all of spacetime

ie $$S = \int_M R d\mu[g]$$

But this is true whether or not the spacetime is asymptotically flat. The only difference is whether or not that quantity is defined.

The issue is more whether $$S = \int_{U \subset M} R d\mu[g]$$ has issues with the boundary

Naveen Balaji

@Slereah Do you know any reference which talks about the same or at least discusses in detail about asymptotic flatness in relation to the 'r' coordinate? I found a chapter in Wald but that seems to focus on various other things.

Slereah

Errrr Straumann maybe?

But yes, under some conditions, you have that, for $r \ll 1$, the metric approaches Minkowski space

Something something the metric on the conformal boundary is Minkowski space

Naveen Balaji

10:34

Oh yeah, that's Penrose's definition right?

Slereah

pretty much yeah

also $r \gg 1$

not $\ll$

Novalium Company

11:01

Hello humans.

Guys, where do you live?

Slereah

11:47

Atlantis

Knight

12:25

In other people’s heart and soul.

bolbteppa

13:15

@Slereah have you worked out Kaluza Klein

Slereah

I have not

Though I have been looking into old timey unification theories

Speaking of things which are totally useless

Einstein was 100% convinced that making the metric tensor and connection generic enough would spit out all the physics

Slereah

13:34

Or at the very least gravity and EM

and to be fair, it's not a bad argument

Gravity is a symmetric tensor, EM is an antisymmetric tensor

Combine them to a general tensor and you get both!

PM 2Ring

Kaluza-Klein theory was a stepping stone towards string theory. I don't know if that's a good thing or a bad thing. ;)

Slereah

Hot take : Einstein was a huge liability when it comes to quantum gravity

As he was really against the notion that quantum mechanics was a fundamental law, he dragged a lot of other physicists away from QM when it came to work on gravity matters

bolbteppa

That Einstein stuff is very interesting

Teleparallelism

Teleparallelism (also called teleparallel gravity), was an attempt by Albert Einstein to base a unified theory of electromagnetism and gravity on the mathematical structure of distant parallelism, also referred to as absolute or teleparallelism. In this theory, a spacetime is characterized by a curvature-free linear connection in conjunction with a metric tensor field, both defined in terms of a dynamical tetrad field. == Teleparallel spacetimes == The crucial new idea, for Einstein, was the introduction of a tetrad field, i.e., a set {X1, X2, X3, X4} of four vector fields defined on all of M such...

Slereah

Teleparallelism and Kaluza-Klein are basically the only parts of it that we remember

bolbteppa

1

Q: What is basic tensor algebra in teleparallel equivalent of general relativity?

Teleparallel gravity represents a viable alternative to general relativity where gravitation comes from torsion rather that curvature. The theory is based on a new modified connection, and the curvature for this theory is zero and in the geodesic equations there is a term which represents a forc...

general-relativity differential-geometry tensor-calculus modified-gravity

PM 2Ring

13:54

@bolbteppa Intriguing... but to me it sounds like spooky parallel transport at a distance. ;)

Slereah

Despite the name it has nothing to do with that

it's just GR with a connection instead of a metric

bolbteppa

How do you derive the action (eq. 77) and why is it equivalent to the EH action arxiv.org/pdf/1810.12932.pdf#page=23

It says in 78 it's equal up to a divergence, but wtf

Slereah

Hecc I dunno

As far as I can tell from old unified theories, they mostly just did the action the same?

They just did whatever fancy metric and connection, and then just computed the usual EH action from this

dunno if that's the case for teleparallelism

Yuvraj Singh...

14:13

@PM2Ring hi sir!

PM 2Ring

Hi, @Yuvraj

Guru Vishnu

PM 2Ring: Hi. How are you? It's been a long time since we talked previously.

PM 2Ring

Hi. I'm tired. I should have gone to sleep a few hours ago...

Aaron Stevens

15:51

Haven't been in the chat for a while. How's it been going here?

Novalium Company

16:28

So mutual funds are essentially many people "hiring" (under a fee) a person to manage a "soup" of investments? (I'm basically hiring someone to invest with my money?)

John Rennie

@NovaliumCompany yes

The idea is that they have the time to do research that you can't do so they have a better idea of what are the best investments.

Novalium Company

yep gotchya

I'm looking over the whole internet for a simple explanation of the difference between Mutual Funds and ETFs but people like to overcomplicate things for some reason. All I understood is that ETFs are traded on the stock market (as if they are stocks).

nvm

got it

John Rennie

16:46

@NovaliumCompany I use mutual funds because I have little enthusiasm for doing all the hard work needed to figure out which stocks are best.

Novalium Company

So the core difference between mutual funds and ETFs is the way you acquire them? (ETFs are stock-like, traded on the stock market)

John Rennie

I think ETFs are a bit specialist. Ordinary Joes like me would normally buy mutual funds. I have to confess I'm not sure what the point of ETFs is.

Novalium Company

oks

user129412

17:16

What would you make of an editor writing "we will probably send your resubmission out for further review"? I got a paper back with two referee reports, one only had minor comments and strongly recommended it, the other only wrote a summary of the paper and said it was very nice, but then said it was too specialized so find a more specialized journal.

I suppose my actions are independent of what is meant by the statement, but should I interpret it as mostly positive or negative, would you say? To me it sounds a bit like, make the changes that were asked for, and if you can convince us or the referee (sounds hard) that it isn't too specialized we'll publish, else we will ask a third person

amanuel2

Anyone know what the answer is and why? I am compltely baffled on how its not y(t) = dcos(2PIft)

enumaris

17:45

weez

ACuriousMind

@Slereah Well, you can interpret our failure to have settled on a theory of quantum gravity so far in two ways - either it's out there and we've failed partly because Einstein dissuaded some smart people from looking for it, or it's not and he was right to do so :P

enumaris

oh was just gonna comment that I got to meet you in person last week :D

Slereah

@ACuriousMind It certainly wasn't where he was looking for!

Hoping really hard that pure geometry would end up giving discrete energy levels

enumaris

o/

ACuriousMind

@enumaris 'Twas nice...and I just realized I forgot to send you that follow-up mail. :D

enumaris

17:50

@ACuriousMind no worries, I sent you one, you'll see it tomorrow :P

ACuriousMind

Heh

enumaris

quantum foam was a pretty cool concept, and it's pretty "geometrical" or at least...topological...

Did anybody other than Wheeler follow up on that lol

ACuriousMind

@Slereah That just depends on your definition of "pure geometry"!

Slereah

Quantum foam is still kind of a thing?

In the general sum over topologies idea

It's not a super popular idea but it's still around

enumaris

nice

ACuriousMind

17:52

If people can call the weird stuff that goes on with schemes and stacks "geometry", we can certainly find a way to call QG geometry regardless of what it ends up being :P

enumaris

Wheeler was a pretty smart dude

Slereah

@ACuriousMind I mean sure, but he was dead set against the whole Schrodinger type QM

He didn't even like having point particles or matter fields for his unified theory

he just wanted a big manifold doing its business

ACuriousMind

...he was in the pocket of Big Manifold?

rob

@amanuel2 Half the time you talk about amplitudes you want the "distance from zero" amplitude; half the time you want the "peak-to-peak" amplitude. They're different by a factor of two.

Slereah

@ACuriousMind I think he might have been still heady from his success with GR!

"I got famous making physics from geometry, perhaps it can work again!"

ACuriousMind

17:56

Ah, a case of hammer-nail syndrome

Slereah

it's too bad, really

He spent like 30 years working on stuff nobody remembers

Could have joined the quantum revolution instead!

But I guess you never know what horse will win the race

Aaron Stevens

This is not a physics question and better be answered by a biologist. Reactions happening in living cells with the help of many molecules. — verdelite 22 mins ago

Biophysics isn't real physics. Everyone knows that

ACuriousMind

@AaronStevens Relevant SMBC

(click the red button)

amanuel2

@rob Huh? Care to explain further?

Slereah

If you're a cosmologist, do you study $10^{80}$ atoms?

enumaris

18:02

you study manifolds

Slereah

I'm afraid my love of manifolds doesn't really align with mainstream cosmology

enumaris

oh man, I got up at 4 today cus of jet-lag...now I'm feelin a bit sleepy already and it's not even afternoon yet lol

Slereah

Cosmology is a lot of studying matter

and even observations

enumaris

which is all manifolds

we are all manifolds on this glorious day

ACuriousMind

@Slereah Just brand yourself as a Rebel Cosmologist, then

Slereah

18:04

Imagine being a real cosmologist

and just looking at stars!

enumaris

srsly

so did Betelgeuse explode yet

I hope I didn't miss the memo

Slereah

Or doing experimental GR and just doing a bunch of interferometry

Well, those light circles look like the correct light circles

Relativity confirmed

enumaris

Interference is tight

rob

@amanuel2 My two minute version of that comment involves making a diagram, which is tough in chat.

JMac

@AaronStevens What's really funny is that it seems like you could make just as strong of a case that they are really asking a chemistry question; not a biology one; if you felt it was worth arguing which field such questions "belong" in.

Slereah

18:06

I say it belongs on the cooking Stack Exchange

JMac

@Slereah True. ATP is really just food going through your body, so I can't see why not.

Aaron Stevens

@JMac Right. Biophysics is a tricky thing to define sometimes, as it exists at the interface of traditional scientific classifications. As a first order approximation, I see energy (physics) and ATP, molecular motors, etc (biology), so I think "biophysics".

JMac

@AaronStevens It's one of those things where you could make a case for a bunch of different fields (spoiler alert: science disciplines are actually interconnected). Unless there's no physics in the question, what matters is where the OP chose to ask it. Seems fair to assume they asked it on Physics for a reason

Aaron Stevens

@JMac Right. I feel like the fact that I felt confident enough to even write up a somewhat intelligible answer means that it belongs here. I still don't know anything about biology, tbh :P

amanuel2

@rob you want to create a seperate room?

JMac

18:35

@AaronStevens On that answer, it is basically saying the binding point on the molecular motor doubles as the ATPase?

Aaron Stevens

18:59

@JMac I believe so... Although I might be wrong.

The binding of ATP to the motor does create a sight change in the structure of the motor, I assume it is this conformational change that lowers the activation energy

JMac

@AaronStevens It really makes a lot of sense from what I understand. You wouldn't want the energy to release on it's own; but you would want a consistent way to release it when powering the motors. Having the release on the motors themselves totally seems reasonable; but at the same time biology doesn't care about the best solutions, so it's just interesting that it seems to work out so nice.

Scientist Smith YT

19:37

I guess Chinese inventors are close to inventing a perpetual mobile or a perpetual motion machine. And that they promise to soon test there EM-Drive in space. So a quantum engine that doesnt need an external energy source. Where they take advantage of the principals of photosynthesis and quantum states. And I do have to be honest. This is all both theoretically and physically possible with the advancements in technology and the rate at which we are accelerating.

But that's my scientific opinion. I'd like to hear other peoples opinions though

This machine consists of a magnetron generating microwaves. And a resonator which accumulates the energy of their vibrations. This design allows them to convert radiation into traction.

This is going to be on a spaceshuttle

JMac

20:54

@ScientistSmithYT I'm not holding my breath for either of those. The science behind EM-drives is murky at best; and perpetual motion has it no better. I wouldn't call either of them "physically possible" given that if they do exist, they might fundamentally change assumptions about physics, and there's no clear evidence yet that they do exist.

enumaris

21:12

perpetual motion...ppl still on that?

ACuriousMind

sure, perpetually :P

JMac

@enumaris As long as people have magnets, basements, and overconfidence in their understanding, I think we'll always see people trying to make perpetual motion machines.

enumaris

sounds legit

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