The h Bar

1:18 AM

I have a simple question about notation, when people say "1+1D" for instance, does this mean we're working in 1 timelike and 1 spacelike dimension? So most generally GR is done in "1+3D"?

Slereah

2:59 AM

Yes.

David Z

6:03 AM

@Charlie I've heard "3+1D" more often. I don't know how many people put the spacelike number first vs the other way around.

JingleBells

8:54 AM

Why are profits maximized when MR = MC? Doesn't that mean that the cost of producing one unit is equal to the revenue of that unit, so I'm actually at a profit of 0?

Charlie

9:06 AM

@Slereah @DavidZ I see thank you

Slereah

@DavidZ There's no standard

Usually it doesn't matter, though

Unless you're dealing with theories with more than one time dimension

Charlie

9:53 AM

do any mainstream theories do that?

Slereah

10:53 AM

It occasionally pops up in string theory

Charlie

11:24 AM

Ah ok

Jim

11:48 AM

@DavidZ Personally, I've only ever seen it as 3+1D. If anyone were to write 1+3D, that would be confused with 3 time dimensions. It would seem to me to be improper form

user434058

1:46 PM

Meta seems quiet. Seems like we aren't closing enough questions :P

Charlie

2:29 PM

It seems like the punchline at the end of every introductory book is that we were really doing differential geometry all along without knowing it

PM 2Ring

3:06 PM

Is anyone else annoyed by this recent network-wide bug:

37

Q: The list of related questions is shown empty

At the moment, any question that I check returns empty list of related questions. I see just a header called "Related" but no questions shown below that. I have checked this on multiples sites and also while not logged in. (At the moment I do not have the possibility to try it from a different co...

Or am I the only one who uses the Related questions list to fine-tune dupe target searches?

ACuriousMind

Jul 15 at 17:10, by glS

@ACuriousMind don't you find it terribly annoying? To name one thing, how do you find duplicate questions and such?

PM 2Ring

@ACuriousMind Thanks. Now that you mention it, I remember reading that in the transcripts... I sometimes forget which chatroom various conversations occurred in, especially if I wasn't a participant in the conversation.

@Charlie You might enjoy exploring Greg Egan's site.

Apr 16 '19 at 20:20, by PM 2Ring

@CaptainBohemian Have you read Greg Egan's articles on these topics? He's written novels set in ++++ and in --++ universes, and has relevant technical articles on his website, as well as material about the usual -+++ metric. He also has some articles on topics in QM.

Specifically, the Orthogonal and Dichronauts series.

Charlie

oh nice

PM 2Ring

He even wrote an app that lets you play with simple geometry in his --++ world.

Charlie

Do any/many mainstream theories use metrics where the time part and the space part don't have opposite signs?

PM 2Ring

3:22 PM

@Charlie Not that I know of. But Egan figured out a relatively simple way to make it work for his Orthogonal books.

He's a good technical writer. It's a shame that he doesn't write actual textbooks, although there are lots of technical articles on his site. But I guess writing sci-fi pays better than writing textbooks. :) Some people complain that his stories can get bogged down in too much technical detail, but most of his fans don't seem to mind.

Charlie

so he's a legit physicist?

PM 2Ring

3:38 PM

@Charlie Not exactly. He used to work as a programmer before he was able to support himself through his writing. But he does know a lot of physics & mathematics. He has a Bachelor of Science degree in Mathematics. And he's a friend of mathematical physicist John Baez

Tachyon209

3:49 PM

Hi guys. I'm an undergraduate student in 3rd year of physics. I'm looking for guidance regarding undergraduate research. Actually, I've never been involved in research ever before and every field of research being carried out by profs in my institute seem as if they need a lot of prerequisites to understand I much more prefer to read books and learn things by myself, but honestly I don't know what research is so I'm not in a position to judge.

Can someone guide me how should I find something so that I can get some experience with undergrad research?

PM 2Ring

@Charlie You can read Egan's Foundations articles on relativity & QM here: gregegan.net/FOUNDATIONS/index.html

Charlie

ah ty i'll check it out

bolbteppa

4:54 PM

It's always clear from the context, $1+1 D$ or $3+1$ or $9+1$ or $10+1$ or $25+1$ or $(D-1,1)$ or $\mathrm{SO}(p,q)$ it'll always be obvious what is meant

bolbteppa

5:07 PM

@Tachyon209 go ask them

Jim

@Tachyon209 pick a broad field of research that sounds interesting to you. Track down profs in that field. Ask to be an undergrad researcher with them. Rinse. repeat

Mad Spaces

when you drop a magnet in say bronze tube it gets slowed down and falls very slow.
this is because while falling the tube creates local Magnetfeld that is in opposite direction to the falling magnet (lenz rule)
But why does it gets slowed down? is it because of the lorenzforce? or why exactly ?

Korra

I don't want to do this but here goes

Jim

@MadSpaces I'm confused by your confusion. You correctly explain Lenz's law and then ask for an explanation of Lenz's law

Korra

0

Q: Two equations equal to zero

I just wanted to ask that under which circumstances are two equations of the form, $A\sin{x}+B\cos{x}\sin{y}+...=0$ For example, $A\sin{x}+B\cos{x}\sin{y}=0$ And $C\cos{u}+D\cos{v}\sin{z}=0$ are equal to each other?

trigonometry

Please please please take a look

Mad Spaces

5:13 PM

@Jim

I wanted to know why the resulting magnetic counter feld slows down the magnet.

Jim

because it provides a force in the opposite direction

Mad Spaces

And is this force the LORENZFORCE?

Stan Shunpike

@ACuriousMind But which are the fiction? The quantum gravity ones? :')

Jim

in as much as any force due to em fields is the lorenz force

Charlie

@Korra You shouldn't advertise your questions, and your question is confusing anyway

Mad Spaces

5:15 PM

Okay so far so good. i am however very unsure how this looks like
Because if we consider the magnet falling down thus the speed in -z direction and the resulting magnetic field is maybe +z? so how does the angle workout?

Jim

the moving charges are not moving in the direction of the magnet

Mad Spaces

Okay see this confuses me because i literally just saw a dumb video where tehy said exactly the opposite to what you said

Jim

its more complex than that simple picture. At the end of the day, the force boils down to being the lorenz force, but if you were to envision the problem on that scale, it would quickly become confusing

Mad Spaces

So its not correct to say that the charges are moving in the same direction of the moving object

well obviously i mean? they go all around the place.. but you know the object consists of charges thus there should be some truth to that senteance

Korra

5:46 PM

@Charlie can you please tell me what is confusing?

Charlie

@Korra Are you asking for specific coefficients that satisfy those equations?

Korra

@Charlie noooo

Okay, while deriving Lorentz transformations we did this

X1=0

In S1 frame

And X1-V =0 in S2 frame

Which gave us X1=λ(X1-V)

Aaagh

I meant X2-V=0

What I meant was if we have two equations, both of which equals to zero

@Charlie this is what I am asking for

Then under what conditions can we state them as equal to each other?

satan 29

6:18 PM

0

Q: Confusion regarding Young's double slit experiment in different media

Consider the Given arrangement. Point P is equidistant from S1 and S2.The glass slab in air has refractive index $\mu_{2}$, and the one in water($\mu_{1}$) has refractive index $\mu_{3}$. We need to find the relation between $\mu_{2}$ and $\mu_{3}$ such that P is the central maxima.Both glass sl...

homework-and-exercises optics waves double-slit-experiment interference

Disclaimer: I am not keen on advertising

I just want to ask what can i do to make it less likely to get closed?

@FakeMod et al

user434058

@satan29 IMO, the given answer seems wrong. Yours seems right.

David Z

7:13 PM

@satan29 A few things you could do to improve: you ask "So why exactly is this true?" but it's not totally clear what "it" refers to there. It'd be nice to clarify that. It would also be useful to present the question as showing that you've made a bit of effort to try to find out why that thing is (supposedly) true. You actually did some of that already by reverse-engineering the equation; maybe it'd help to move the part about the reverse-engineering to the end of the post.

Sir Cumference

8:44 PM

@Korra State what equal to each other?

equations cannot equal each other

Charlie

9:25 PM

This might not be a well-posed question, but when we vary a system's evolution between two points in time to minimise the action - are we varying the path through configuration space or the path through Euclidean space that it takes?

ACuriousMind

The former

Charlie

Ok I'm glad that's a much more satisfying way of thinking about it

ACuriousMind

The action lives on the spaces of generalized coordinates of Lagrangian or Hamiltonian mechanics, not on "positions"

Charlie

Ok I had to think about it for a second, but the "axes" of the configuration space include $q_1,...,q_n$ and $\dot q_i,...,\dot q_n$, right?

which would make it an $n\times n$ dimensional - I guess, manifold or whatever it turns out to be, I haven't reached that yet

I want to say it must do because the Lagrangian depends on both

ACuriousMind

"Configuration space" usually only refers to the $q^i$

Charlie

9:31 PM

oh ok

ACuriousMind

Lagrangian mechanics takes place on the tangent bundle of the configuration space, Hamiltonian mechanics on the cotangent bundle, and we call the (co)tangent coordinates $\dot{q}^i$ resp. $p_i$.

Charlie

oh that's pretty neat

ACuriousMind

But I'm sure you can find texts that use "configuration space" for one of these spaces, too

Charlie

oh right because the "vectors" in the tangent space are the set of all tangent vectors to curves

ACuriousMind

Jun 12 at 16:02, by ACuriousMind

@FakeMod Yes. I talk about the dependences between $q^i, \dot{q}^i, p_i$ at some length here

In the last paragraph of that answer I talk about how to think about the action in terms of these spaces

Charlie

9:34 PM

I have that post saved to read when I have some more of the details figure out

ah ok I will have a look

Charlie

10:02 PM

This is just a small concern, but in Goldstein's classical mechanics he introduces the variational principle in the usual way, in one dimension (with $x$ acting like a generalised time parmeter) take the actual path and modify it by some function $\eta$ with the parameter $\alpha$, $y(x,\alpha)=y(x,0)+\alpha\eta(x)$. This is fine, but then he says that the action becomes a function of alpha, $S(\alpha)$. Is this similar to saying that $y$ in $y(x(t))$ is a function of $t$ "by proxy" in a way?

Or is this slightly hand-wavy notation? What bothers me is that the action which is a functional is now a function of a scalar value

Stan Shunpike

11:38 PM

I'm having trouble finding some formulas for moment of inertia. I'm looking for the moment of inertia for the figure on the right

However, all I can find is the "Rectangular L" shape on the left

Here's an example

efunda.com/designstandards/beams/RectangularLBeam.cfm

Does anyone know what the "greater than 90" version of an L beam would be called or where I can find info on it's moment of inertia? Any help would be great.

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