The h Bar

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6:58 AM

@Slereah thank you :)

Captain Bohemian

my computer emits so much heat flow that I feel so hot.

TaeNyFan

10:30 AM

Hi, I wanna ask how do you guys study physics textbooks? I always find that I lose interest after 2, 3 days and halt my progress. I had always wanted to finish a textbook in full but always unable to do so.

For example, how long do you guys take to finish a physics textbook? And how long do you study it everyday?

I find that I learn and absorb best from textbooks but the progress can be really slow and frustrating.

user434058

11:17 AM

@CaptainBohemian Was this an intentional joke? ;-)

Mad Spaces

1:28 PM

In a galvanic cell electrons flow from Zinc to copper obviously because Potentail
difference exists.

I have learned that putting a metal in a Electrolytliquid will cause a difference of Potential between the
metal and the liquid since there is a difference of Concentration of the metal ions along the edge layer which causes
some to go into the liquid and thus the metal will gain a small negative charge and the edge layer of the liquid surrounding the metal
will get a positive charge and one will have a potentail difference thus Voltage. however it is in equilibrium since the same amount of…

ACuriousMind

3:00 PM

@MadSpaces I'm not quite sure you and your professor are describing different things. You're describing the Galvani potential between one metal and the electrolyte, while your prof talks about the Galvani potential of two metals. In a galvanic cell, both are relevant - the potential of the cell arises from the Galvani potentials of both metals and the electrolyte.

Mad Spaces

oh. So both of them contribute, could you explain however how both of them contribute?

ACuriousMind

Well, the metals and the electrolyte are all in electronic contact with each other (through being immersed in the electrolyte), so all their galvanic potentials relative to each other are relevant

Mad Spaces

Is connecting two metals thru a cable counts as Contact voltage ? (as if we put them directly on top of eachother)

ACuriousMind

the cable is also a metal, so then you'd have the contact voltages of either metal with the metal of the cable :P

Mad Spaces

Yes i know. But i mean we usually just idealize things and consider wires to be just transport objects

Consider this Thoughtexperiment.. suppose we have no effect of Contactpotential between the metals (please dont assume here that both metals are the same or this thoughtexperiment will not work)
So we have no voltage due to contact... but we still have voltage due to the difference of voltages between each Electrode and the liquid.

Does a current flow?

Charlie

3:36 PM

Is there a special set or type or coordinate transformations that relate local inertial frames, analogous to the Lorentz group?

ACuriousMind

@MadSpaces See en.wikipedia.org/wiki/… for the relation between cell potential and the Galvani potentials

Charlie

For instance given a coordinate system that is locally inertial at $p$, is there anything special about a coordinate transformation that makes some other point $m\in\mathcal M$ locally inertial?

ACuriousMind

What do you mean by "locally inertial"? Vanishing Christoffels at $p$?

Charlie

Yes

can that phrase mean other things?

Yuvraj

Ah! How are you h Bar?

Charlie

3:39 PM

Hi @Yuvraj

Yuvraj

Hi @Charlie

ACuriousMind

@Charlie Sometimes physicists can be a little bit unclear about their phrasings, I wanted to make sure :P The mathematical name for these is normal coordinates, by the way

Yuvraj

@ACuriousMind??

Sometimes lol..

Charlie

Ahh I see, I remember hearing that phrase at some point

ACuriousMind

@Yuvraj !! (I'm not sure what your question marks are referring to :P)

Charlie

3:43 PM

visible confusion

Yuvraj

You said sometimes physicists can be little unclear about their phrasing.

Does anybody have last weekend physics stack exchange news letter link?

ACuriousMind

@Charlie Since normal coordinates are only defined in a small area around $p$, you generally can't have a transformation that turns them into normal coordinates around $q$ because the normal neighborhoods of $p$ and $q$ might not contain each other

Yuvraj

I missed the newsletter mail

Charlie

Oh

I was under the impression that normal coordinates only locally flatten the metric but that the coordinate system in general extended to the entire manifold

wait that must be the case no? I think I've misunderstood

ACuriousMind

@Charlie If the coordinate system extended to the entire manifold, then the manifold is just $\mathbb{R}^n$!

The "coordinate system" is a chart, mathematically. You usually need more than one chart.

Charlie

3:49 PM

I understand that the entire manifold doesn't look flat in normal coordinates necessarily, but I can still label any point on the manifold with normal coordinate systems?

oh

ah man I feel like I'm missing out by not knowing enough about charts

John Rennie

Shame on you Ethan Siegel. Resorting to click bait is not a good look for a respected physicist.

Charlie

Is the problem that normal coordinates are necessarily singular or undefined at certain points and so can't cover the entire manifold?

ah wait no that is wrong

John Rennie

Normal coordinates are never singular - well only at a curvature singularity where every coordinate system is singular.

ACuriousMind

@JohnRennie That entire article is just someone being smug about a pup-sci explanation of a physical theory being wrong. And the author doesn't even acknowledge that Hawking himself wrote that the analogy isn't to be taken literally in the original paper, which either shows a tremendous lack of research or is deliberate deception.

@Charlie It's just that in general they don't cover the entire manifold, like every other coordinate system

John Rennie

Yes. Siegel's popular science articles of Forbes are generally pretty good, but that one is a shocker.

ACuriousMind

3:55 PM

Remember the conversation we had about charts, and needing like 4 for the torus if you wanted to be rigorous? This is the same issue

Charlie

Ok thank you I see roughly where my thinking was going wrong, I need to cover this chart stuff in more detail once I'm done with schutz

yeah I'm getting a bit ahead of myself and misapplying things I don't really understand yet

back to the drawing board

ACuriousMind

But there's no guarantee that normal coordinates, in general, are like the torus or sphere coordinates that just have issues at single points - on an arbitrary spacetime all you can say is that they cover some part of the manifold around the point, but the part may be small or may be large

@JohnRennie Now just someone needs to go through these looking for analogies to write a "Siegel lied to us" rebuttal

John Rennie

@ACuriousMind if you read his article closely it appears his only real objection is that Hawking used the virtual particle analogy in his popular science book, A Brief History of Time, which is true. But then Hawking's target market for that book don't know that much about Bogoliubov transformations :-)

The article is still highly misleading.

ACuriousMind

Sure, I mean, regarding the facts of the analogy being untrue Siegel is correct. The problem is that he paints a pop-sci analogy as a lie (not even with the moniker lie-to-children these things sometimes get) and that he doesn't even mention that Hawking was certainly aware of the limits of the analogy

This is terrible science communication because as much as I hate pop-sci analogies sometimes personally, almost 100% of science communication would be a "lie" by this standard

John Rennie

4:10 PM

> Reporting Correction for:

Title: Yes, Stephen Hawking Lied To Us All About How Black Holes Decay
Author: Ethan Siegel
URL: https://www.forbes.com/sites/startswithabang/2020/07/09/yes-stephen-hawking-lied-to-us-all-about-how-black-holes-decay/

-

Your Name: John Rennie
Correction Request: Please remove the click bait references to Hawking lying. He did not.

In this original paper "Particle Creation by Black Holes", Commun. math. Phys. 43, 199—220 (1975), Hawking says, and I quote:

----8<----

ACuriousMind

> Forbes Staff will review your concern shortly.

I wait with baited breath

JMac

@ACuriousMind What seems so weird to me is that it's often clear that pop-sci analogies aren't meant to be true statements of reality. Like I read and enjoyed "The Elegant Universe"; but I never got the impression that I was actually learning many facts about QM/string theory. You learn some cool things to get some idea what the fields are about, but it seemed clear you weren't really learning QM or string theory.

Charlie

4:26 PM

Schutz mentions that Newtonian spacetime is Euclidean space repeated endlessly in time, written $\Bbb R^3 \times \Bbb R$. Is there a meaningful difference between this and $\Bbb R^4 = \Bbb R \times \Bbb R \times \Bbb R \times \Bbb R$?

I guess this boils down to whether the product operation is associative

bolbteppa

5:55 PM

I think the point he's trying to make is something like Euclidean space has a metric whose norm is $||(t,x,y,z)|| = \sqrt{x(t)^2 + y(t)^2 + z(t)^2}$ which tells you at each point in time you just get Euclidean space, in Minkowski space it's $\sqrt{t^2-x^2-y^2-z^2}$ so you don't get the Euclidean space norm anymore, I don't think it's about $\mathbb{R}^4 = \mathbb{R}^3 \times \mathbb{R}$ which holds in Minkowski space also

Slereah

Newtonian space has a unique projection function

ie you can define a unique time for every point

(up to translation)

Charlie

6:56 PM

Ok thank you

Sir Cumference

@Charlie I mean mathematically they're technically different, but they're probably identical with regard to representing four-positions

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