The h Bar

lucas

11:02

@JohnRennie Sorry, I don't want waste your time, but a question is bothering me for a long time. I want to ask it but I am afraid of you get uncomfortable because it is a personal stuff.

John Rennie

@lucas I'm reasonably good at answering questions about physics, but generally poor at answering questions about real life. However go ahead and ask and if I can help I will.

lucas

@JohnRennie Why do you not shave your beard?

ACuriousMind

I bet it's another adoption request ;)

Ah, damn

lucas

I like this image of you more

:-)

Secret

For a completely new subject, I tend to go cover to cover

I also did the same thing when reading journal articles, that's why it took 3x more time to do a literature search for a completely new subject

John Rennie

11:11

@lucas that picture is from over twenty years ago. I used to be in an amateur opera society and that picture is of me in costume for a production of Die lustige Witwe by Franz Lehár. I only grew the beard for the production and I shaved it off afterwards.

@lucas that isn't me. That's a different John Rennie who works for Scientific American magazine.

lucas

@JohnRennie But he looks like you too much!

John Rennie

I have blue eyes and fair hair. That picture doesn't look anything like me!

lucas

BTW, I feel nice now. Thank you very much! I don't waste your time anymore. :-)

Secret

@acuriousmind Looking at chris white's question intriguies me, can the warping of spacetime by the stress energy tensor be described as if spacetime is some kind of material with certain properties. What terms in the riemann curvature tensor prevent this "material oriented" interpretation be applied to describe the warping of spacetime?

Danu

@ACuriousMind Ew

ACuriousMind

11:20

@Secret What would be the use of that?

@Danu yep

Secret

Well, for a laymen perspective, since materials can be bent by something. Likewise spacetime can be warped. What makes spacetime cannot be considered as just another material. Or perhaps the question is, why is spacetime not a material with matter as part of its composition?

ACuriousMind

@Secret No, that's not a meaningful question. "Materials" are things that exist in space(time), i.e. they are objects with coordinates and extent. Spacetime doesn't have that. It's like trying to ask "Why is the canvas not the paint?"

Viet Tran

Does spacetime has physical properties like thermal/electrical conductivity?

John Rennie

@Secret spacetime is not a thing. It doesn't have a Young's modulus.

ACuriousMind

@VietTran Thermal, no. For electrical you might argue that the vacuum permittivity is the thing to look at

John Rennie

11:26

Spacetime is an entirely mathematical object. And the curvature isn't deformation in any everyday sense of the word.

@ACuriousMind that's a function of the QED vacuum, Shirley

Secret

hmm, I see

Viet Tran

@ACuriousMind wow haven't thought about this

ACuriousMind

@JohnRennie I said "might argue". I didn't argue ;)

John Rennie

@ACuriousMind :-)

Viet Tran

I was asked to ask my question in chat. So here I am...
For my studies I can choose a question or a problem which I can then try to solve in a project. Since I am interested in machine learning I thought that one could combine this project course with my interest. So Is there a physical problem which involves machine learning but is not too complex as filtering petabytes of data as in CERN?

John Rennie

11:29

@VietTran I'm not sure that any of our regulars work in machine learning. You might try Daniel Sank as he works for Google, though on quantum computing not machine learning.

Secret

@JohnRennie Ah that explains why slereah said spacetime is not like a fluid, and how slereah pointed out the major challenge of wormhole building is there is nothing in GR that provide a mechanism to change the spatial topology, (unlike how in kneading clay you can easily poke holes in it because the clay particles can simply be displaced away from a region to form a hole)

Secret

Argh I hate typos, it turns one ping into n pings

Emilio Pisanty

Is it just me, or is this a weird thing to put on your profile?

ᴡᴏʀᴅs

130 1 6

Not that it's wrong

But I find it weird.

0celo7

Good morning

John Rennie

@Secret See:

19

Q: Building a wormhole

We regularly get questions about wormholes on this site. See for example Negative Energy and Wormholes and How would you connect a destination to a wormhole from your starting point to travel through it?. Various wormhole solutions are known, of which my favourite is Matt Visser's wormhole becaus...

general-relativity wormholes exotic-matter

ACuriousMind

11:34

@EmilioPisanty I guess someone is letting their freak flag fly, hm?

John Rennie

The problem with building a wormhole is knowing what's going to happen at the far end.

Emilio Pisanty

@ACuriousMind Probably, yeah.

I mean useful flags are good 'n all

But wasn't it actually about the posts?

0celo7

@ACuriousMind wow

ACuriousMind

Oh, wait, that's Normal Human just with another name!

0celo7

@JohnRennie what

John Rennie

11:36

@0celo7 What?

ACuriousMind

That user was infamous on math.SE for explicitly seeking old closed posts where a single downvote could trigger auto-deletion

Apparently their math.SE account is gone?

I think the former content of their profile was how far they were from reaching 20k downvotes, @EmilioPisanty - not much better, is it?

John Rennie

Right, I've been writing Intranet code for 7 hours now (since 05:00 BST) so I'm off for a well deserved hot shower and after that a laaaaaaarge lunch.

0celo7

@ACuriousMind Let $f:S^2\to M$ be smooth. If we view $S^2$ as the 1-pt compactification of $\Bbb R^2$, can we make $$\int_{\Bbb R^2}\left(\left|\frac{\partial f}{\partial x}\right|^2+\left|\frac{\partial f}{\partial y}\right|^2\right)\,\mathrm dx\mathrm dy$$ be finite/make sense

Emilio Pisanty

@ACuriousMind How do you know is the same user?

0celo7

something with stereographic projection?

John Rennie

11:39

And after that I might just have a play with my AWESOME NEW NEXUS 9 THAT I'VE JUST HAD DELIVERED :-)

ACuriousMind

I'm not sure I understand the question

0celo7

@JohnRennie I'm not sure I understand the distinction between what is "real" and what is "mathematical"

How does one tell?

ACuriousMind

If a function $f$ on the reals extends to a function on the sphere that means it's well-behaved at infinity, in particular bounded, so that integral would just be the integral of the divergence of $f$ over the sphere (removing infinity is just a point).

Emilio Pisanty

@JohnRennie you have fun with that. I'll join you when I get my awesome new HP Spectre x360 15 delivered next week.

0celo7

@ACuriousMind Bounded does not imply integrable over $\Bbb R^2$.

ACuriousMind

11:41

@0celo7 but it's integrable over $S^2$.

And the $\mathbb{R}^2$ integral would just be the integral over $S^2 - \{\mathrm{pt}\}$.

Emilio Pisanty

@ACuriousMind huh, yeah, the Wayback Machine confirms the link but the archived maths account gives 404s

John Rennie

@EmilioPisanty Wow, effectively a 15" tablet. That would be awesome for watching films :-)

ACuriousMind

You might have issues with the measure on the sphere not coinciding with the measure on the plane, but as I said, I'm not sure I understand the question, anyway

Emilio Pisanty

@JohnRennie Indeed I expect it will. A 15'' tablet with dual-core quad-thread sixth-gen i7, 512GB SSD, 16GB RAM and 4k screen.

0celo7

@ACuriousMind I don't see that

Secret

11:45

Emilio Pisanty

@ACuriousMind gone with, like, 40k rep (link). Go figure, must've had their reasons.

John Rennie

@EmilioPisanty I think I've just been top trumped. I bet you don't have a cyclotron pushbike though :-)

Secret

Wait I thought for the spacetime near an antihorizon there are no paths for light to move towards it because all the future light cones will be pointing away from the antihorizon? (thus a reversed version of what happened to light cones near a schwartzchild black hole)

thus how can we send a light pulse arbitrarily close to an anithorizon?

John Rennie

@Secret Assuming you're using regular $(t,x,y,z)$ coordinates the light cones don't rotate, they open out and asymptotically approach planes.

ACuriousMind

@EmilioPisanty I'd guess they had a different vision for math.SE than what it currently is and finally gave up in exasperation (I remember there even was a "Normal Human bot" that posted auto-comments on questions that were badly tagged, consisted only of caps or only of formulae, etc.)

John Rennie

11:47

i.e. the cone angle approaches $\pi$

Secret

ah, ok

Emilio Pisanty

@JohnRennie No, I definitely don't. Is that actually a thing? Like, an actual physical product you can own? Have you got one?

John Rennie

Oh hang on, by antihorizon I was assuming you mean a white hole like horizon. Having just read the bit of text you posted I'm no longer sure that's the case.

0celo7

If $p:S^2\to\Bbb R^2$ is stereographic projection, why is $f\circ p$ integrable

Why won't the integral be horribly divergent

John Rennie

1 hour ago, by John Rennie

https://www.kickstarter.com/projects/1989795590/the-cyclotron-bike-revolutionary‌-spokeless-smart-c

Secret

11:49

It's from the morris 1988 article from your link

John Rennie

@Secret from my link?

Secret

19

Q: Building a wormhole

We regularly get questions about wormholes on this site. See for example Negative Energy and Wormholes and How would you connect a destination to a wormhole from your starting point to travel through it?. Various wormhole solutions are known, of which my favourite is Matt Visser's wormhole becaus...

general-relativity wormholes exotic-matter

Gilmar's answer

0celo7

wait

it's $f\circ p^{-1}$

@ACuriousMind Will $f\circ p^{-1}$ even tend to $0$ at infinity?

I'm sure that's a necessary condition for the integral to converge.

John Rennie

@Secret ah, OK, I didn't read Gilmar's answer in detail because it didn't answer my question. And as a non-academic I don't have free access to the American Journal of Physics.

Emilio Pisanty

@JohnRennie Sure, I'm sold on the prototypes existing. But can you actually buy one?

Secret

11:52

You can always sci hub it if it is necessary (Although you can choose not to because strictly speaking it is illegal)

Emilio Pisanty

If you have, then I do get top trumped.

Otherwise, I'm perfectly happy with my x360 and my Giant Escape 2.

John Rennie

I have seven of them, one for each day of the week.

(in my dreams)

ACuriousMind

@0celo7 Don't make me pull out the $L^2$-functions that don't go to 0 at infinity again

4

John Rennie

My pushbike is a ten year old clunker for which I have a totally irrational fondness.

Secret

(For me, i don't care about legal issues if I am too driven to analyse information)

0celo7

11:54

@ACuriousMind oh god

but still, why does that integral converge

Emilio Pisanty

@ACuriousMind Hmmm. You mean, like $\sum_{n=1}^\infty\chi_{[n,n+1/n^2]}$?

ACuriousMind

@EmilioPisanty Yeah, things like that. You can even get 'em smooth

Emilio Pisanty

@ACuriousMind Sure, that's pretty given

Can you make them analytic, though?

ACuriousMind

Probably not

Emilio Pisanty

@ACuriousMind Why not?

ACuriousMind

11:58

Since these functions usually are zero on an open set, and analytic functions that are zero on an open set are zero everywhere.

Emilio Pisanty

There is no analytic function in $L_2$ that doesn't go to zero at infinity?

Secret

(related non wormhole stuff) Hmm, if Morris is right about the white hole antihorizon and kerr tunnel blueshift of light arguement, then maybe Hammond's Cosmic accelerators formed by negative mass black holes are not stable at all, since right at the singularity sutff is infinitely blueshifted
http://arxiv.org/abs/1308.2683

ACuriousMind

@EmilioPisanty Well, I said "probably" because I'm not 100% sure

Emilio Pisanty

@ACuriousMind No, you definitely can. Take $\sum_{n=1}^\infty \exp\left( -n^4 (x-n)^2 \right)$.

lucas

0

A: Why can't we feel acceleration under free fall?

CASE -I:Consider the acceleration of a body with mass M when a force of 100 N is applied on a body. CASE-II : Consider the acceleration of the same body when a force of 1000 N and 900 N respectively are applied on it simultaneously in opposite directions. The acceleration in both cases will ...

0

A: Why does a free-falling body experience no force despite accelerating?

CASE -I:Consider the acceleration of a body with mass M when a force of 100 N is applied on a body. CASE-II : Consider the acceleration of the same body when a force of 1000 N and 900 N respectively are applied on it simultaneously in opposite directions. The acceleration in both cases will be...

ACuriousMind

12:01

@EmilioPisanty Ah yes, that appears to work

heather

@EmilioPisanty, just thought you'd want to know, I posted the feature request here and it was closed as a duplicate here which I placed a bounty on.

ACuriousMind

hehe

Secret

Is zero throat circumference the same as having no holes at all?

or they are still different because their genus is different?

Emilio Pisanty

@Secret zero throat circumference is what coroners usually consider definitive proof of death by strangulation.

ACuriousMind

@Secret what. What "throat"? what are you reading?

0celo7

12:04

@ACuriousMind physics porn

ACuriousMind

@EmilioPisanty Hm...I think beheading could also cause that

Secret

http://scitation.aip.org/content/aapt/journal/ajp/56/5/10.1119/1.15620
Morris and Throne's 1988 wormhole summary paper (I have my uni access, alternately I can sci hub it)

Emilio Pisanty

@heather Yeah, that's a pretty harsh response, MSE can be harsh on occasion. (prety frequent occasions, unfortunately.) You did the right thing with the bounty.

Secret

p.396 is that screenie

Emilio Pisanty

I would also post it separately under threads that solicit ideas for new privileges.

heather

12:07

@EmilioPisanty, So I should look at other feature-request links and post this link in the comments?

And perhaps on the comments for the examples I found?

Secret

Slereah said the current consensus of GR is that there is no known mechanism that allow the change of spatial topology. Therefore if the consensus holds it means for the schwartzchild wormhole the genus of the throat regardless of radius will have to be 1, otherwise topology will be changed and it will attract the question on how exactly that happened

Emilio Pisanty

@heather No. There's a couple of threads (like the one I linked to earlier) where the team asks for ideas for new privileges. Post it as an answer in there (obviously with all the relevant links).

Secret

which then brought the question on whether a zero throat radius still have genus 1

Emilio Pisanty

@heather It's also perfectly OK to add in an answer here with a proposed fine-tuning of the proposal.

heather

@EmilioPisanty, okay; looking for your first link...

Emilio Pisanty

12:11

(for example, I think Pëkka's suggestion of non-mods having access to rude&offensive flags is definitely a non-starter, and mostly a way to make fights escalate. but too chatty? for sure, that could perfectly well be reviewable.)

Secret

Basically, if I interpret Johnrennie and Slereah's explanation about wormhole construction correctly, besides that we have no idea what's going to happen at the far end, is how to change the topology so that a region of spacetime with no hole can form a hole that is the throat of the wormhole

heather

@EmilioPisanty, I can't seem to find your link...maybe I'm looking in the wrong spot.

Secret

We knew that for spacetimes where the spatial topologies changes during their course of evolution (such as the pants spacetime) are artificially constructed examples, that is, they already have that topology change included as part of the structure of these spacetimes

But in order to construct a wormhole, we need to go from a region of spacetime with no holes to one where there is one hole, and it is not clear how this can be done

heather

@EmilioPisanty, I added the answer to the duplicate/bounty post; however, I still can't find your link. I'll keep looking...

ACuriousMind

@heather Find what link?

heather

12:21

@ACuriousMind, see Emilio Pisanty's earlier comment ("There's a couple of threads (like the one I linked to earlier) where the team asks for ideas for new privileges.")

Jim

@no_choice99 Bacon

heather

@EmilioPisanty, found the link.

Secret

@JohnRennie
The Morris and Throne 1988 paper have said nothing on how the topology of the transversible wormhole is made. They just say "here's the metric that describe this transversible wormhole, and here are the properties"

Jim

@ACuriousMind I know your motivations better than you and you still talk to me. But maybe that's because I'm an exception. And because I brainwashed you to be an evangelist for The One True Jim

ACuriousMind

::starts singing 'Ave, Jim'::

Secret

12:31

1

Q: How would you connect a destination to a wormhole from your starting point to travel through it?

Lets say it was possible to create as massive traversable wormholes as we want. Say we started from earth and wanted to cross to alpha centauri. How would I create the wormhole so that it leads directly to alpha centauri? So we start from earth and want the wormhole to start from their and e...

faster-than-light wormholes interstellar-travel

> The trouble is that GR only tells us that the local geometry will look like a wormhole. It doesn't, and can't, tell us anything about the global geometry. Referring back to the diagram above, GR tells us what the left side of the diagram looks like but can not tell us anything about the right hand side. So GR tells us that the wormhole must connect somewhere, but it doesn't tell us where. Even if we could build a Visser wormhole we couldn't predict in advance where the other end would be.

...wait a minute, spacetime is a unchanging structure. That means any change in spatial topologies has to be already there on the spacetime manifold

0celo7

@ACuriousMind Ok, what if the gradient is constant? Then you're integrating a constant over $\Bbb R^2$. The integral is defined over $S^2$ since it's compact, but what about over $\Bbb R^2$?

Jim

@Secret "unchanging spacetime" and "we can predict the future" are not the same thing

Secret

@Jim Mind elaborate?

heather

@EmilioPisanty, (I assume) you saw Jon Ericson's proposal on this page; should I post a comment on this answer as well as an answer at the end?

ACuriousMind

@0celo7 The measure on $S^2$ doesn't become the one on $\mathbb{R}^2$ under stereographic projection or something like that. What problem exactly are you trying to solve here?

Jim

12:36

the spacetime topologies may already exist in the manifold, but they are still in our future. We still can't predict where the wormhole would lead to because that spacial topology is undetermined in the present

0celo7

@ACuriousMind I'm trying to make sense of the energy functional of a map $f:S^2\to M$

This book describes it using stereographic projection

And I'm not sure why the integral is even defined

heather

@EmilioPisanty, by the way, I didn't see Pekka's suggestion...

Secret

yeah, that's the point mentioned by JohnRennie about "cannot predict what happens to the other side"

But did GR provide any mechanism on how some energy momentum at the present can lead to the evolution of a spatial hypersurface to change genus (thus forming the wormhole throat)?

I learnt from the EVEOnline discussion with Slereah and Acuriousmind yesterday (and also from Chris White's question today) that spacetime don't behave like kneading clay, thus it is unclear how one can suddenly poke holes on it

ACuriousMind

I didn't say a single thing in any "EVEOnline discussion"

Secret

sorry, then it's just me and slereah then (I often mix up many h barers because there are too many discussions)

Jim

12:43

@Secret that's fair. Wormhole production is, at best, unclear. If we knew a clear and defined way to do it, we'd be working harder to try to accomplish it

ACuriousMind

@0celo7 Uh, just form $\int \lvert\lvert \mathrm{d}f\rvert\rvert^2\mathrm{d}\mu$ for the volume measure induced by the standad Riemannian metric on $S^2$? To see how it works in the projection one would have to know what becomes of the sphere metric under the projection, which I don't remember, if indeed I ever knew it

Jim

@ACuriousMind an evil twin perhaps?

ACuriousMind

@Jim Damn, I thought ADeviousMind wouldn't find me here!

0celo7

@ACuriousMind It seems straightforward?

heather

@EmilioPisanty, I posted a comment on Jon Ericson's proposal; I'm also posting an answer.

Though, there is one problem: you suggested 15k+, the page you linked to is 30k rep privileges.

Should I change my original post, or say it doesn't matter?

ACuriousMind

12:48

@0celo7 Well, that's an alright definition, but it's not the standard metric on the sphere under stereographic projection, cf this math.SE post

0celo7

@ACuriousMind Why is is alright??

ACuriousMind

@0celo7 ...because it's a definition?

0celo7

Why is that integral defined, as written there

ACuriousMind

I don't generally expect the energy functional to be finite for arbitrary functions.

Have you read on to see whether it is generally assumed the integral is finite?

0celo7

Doesn't the integral need to be finite to do calculus of variations on it?

ACuriousMind

12:50

That's never stopped any physicist :P

0celo7

I'm not a physicist

Secret

0

A: Energy Required to Rip Spacetime

Some theories do allow for "rips" in spacetime, usually some variants of quantum gravity which allow for topology change. Such a concept goes back to Penrose's quantum foam concept, who theorized that high enough curvature might allow for a change in topology at the Planck scale level, though w...

0celo7

And this is not a physics book

Secret

Ok we are back into quantum gravity territory where here be dragons
Looks like the answer requires QG once again, in that case, it will be dealt with later

John Doe

@ACuriousMind Are you familiar with this standard electrostatics question?

Jim

12:52

@0celo7 not with that attitude

0celo7

@ACuriousMind Jost generally seems to require functionals to be finite before dealing with them, it seems

ACuriousMind

@0celo7 Yeah, so what you have there is not generally finite (you gave the obvious example yourself)

What am I supposed to do about it?

0celo7

@ACuriousMind Help me

ACuriousMind

@JohnDoe No, not really

I'll take a QM question over that any day ;)

John Doe

@ACuriousMind I have none at the moment, but I'm sure lots of potential confusion in the future awaits :) I unfortunately have to learn some EM, but it's interesting as well, at times...Doing independent studying of a few things at the moment, trying to decide what to register for...

0celo7

13:00

@ACuriousMind the paper they reference doesn't seem to exist...

ACuriousMind

@0celo7 Can't do anything about that either

peterh

@Jim The only one goal of this initiative was from my side, that I tried to convince him about a possible revival of a MO-like PhysSE site, no more and absolutely no bad. Well, I've also committed mistake by formulating something badly, but that he currently ignores me, is to me obvious oversensitivity

0celo7

welp

John Duffield

@Secret: look at the stress-energy-momentum tensor. It "describes the density and flux of energy and momentum in spacetime". Look at the picture below. Note the shear stress. Then google Einstein elastic space.

peterh

@JohnRennie I've checked the night sky. It was big, and black. It didn't seem to me a purely mathematical object. :-)

Secret

13:09

@JohnDuffield The spacetime question shared by Chris and the h bar discussion that follows have revealed spacetime does not have a notion of Young's modulus, thus asking about its elasticity is meaningless

so I am guessing they name those terms shear stress is just an analogy to shear stress in materials

despite spacetime is not really a material as the discussion revealed

0celo7

@JohnDuffield what's that

Slereah

That's the diagram of the stress energy tensor on wikipedia

0celo7

never heard of that before

It's not in my books

Slereah

Basically everything he talks about is from wikipedia

Secret

@peterh It may be something, but it is definitely not a material else we woud have measured the young's modulus (and insert material physical parameters) of spacetime already

0celo7

13:15

what will happen when JD realizes the energy momentum tensor describes matter, not spacetime

Secret

On a related note, is the quantum foam some kind of stuff?

peterh

@Secret Elasticity of collisions can be measured also by the proportion of the energy converted to heat

Slereah

Everything is some kind of stuff

Some manner of thing

Some variety of whatever

0celo7

spacetime is just a rubber band thing

Secret

@peterh How would you collide two regions of spacetime given we are all bounded on it?

Slereah

13:16

But if spacetime is a rubber band

How do you explain this

Ozsváth–Schücking metric

The Ozsváth–Schücking metric, or the Ozsváth–Schücking solution, is a vacuum solution of the Einstein field equations. The metric was published by István Ozsváth and Engelbert Schücking in 1962. It is noteworthy among vacuum solutions for being the first known solution that is stationary, globally defined, and singularity-free but nevertheless not isometric to the Minkowski metric. This stands in contradiction to a claimed strong Mach principle, which would forbid a vacuum solution from being anything but Minkowski without singularities, where the singularities are to be construed as mass as in...

John Duffield

@Secret : the shear stress term is nevertheless there. Read articles such as this one from the ESA. The title is "Space-time as an elastic fabric".

ACuriousMind

@Slereah that's deep, man

0celo7

@Slereah WHAT

is that in Stephani?

Slereah

I... think it is?

Lemme check

peterh

@Secret You don't need to do it to calculate a thermal conversion rate.

Slereah

13:18

It is

p. 174

Secret

@Slereah How does the existence of this metric prevent spacetime from being described as something that act like a rubber band?

I see it is flat and wave like

Slereah

Ozsvath-Schucking is part of the metrics that have nop purpose outside of being counterexamples

It's Ricci flat but not flat

0celo7

You know

Slereah

And it has no waves

Since it's a static solution

0celo7

I haven't heard JD explain why Kruszal-Skeres coordinates are bad

he always says they are

or however you spell them

Slereah

13:20

He does not actually know what they are

Secret

wikipedia said it is one of the pp wave solutions, which means it can describe gravitational radiation?

> In general relativity, the pp-wave spacetimes, or pp-waves for short, are an important family of exact solutions of Einstein's field equation. These solutions model radiation moving at the speed of light.

John Duffield

@0celo7 : a photon isn't matter. It has an E=hf wave nature, and an "active gravitational mass", which means it causes gravity. In similar vein, remember what Einstein said: "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy". The stress-energy-momentum tensor describes the density and flux of energy and momentum in spacetime.

Secret

@slereah Since the Ozsváth–Schücking metric is an example of a pp wave spacetime, except being static, can any physical meaning be ascribed to it?

0celo7

Pretty sure photons don't appear in general relativity @JohnDuffield

Slereah

Well it's a Ricci flat spacetime

ACuriousMind

13:23

@Secret You need to read more closely, as always. It's about spacetimes filled with radiation, it doesn't model actual gravitational waves, just generic radiation.

Slereah

It's just empty space

Secret

ok I see

ACuriousMind

Also read the first paragraph of the pp-wave article, it explicitly says that the definition is purely mathematical and doesn't directly relate to the field equations or physical interpretations

Jim

@peterh I am impartial here. No judgments, just jokes

John Duffield

@Secret : there's an issue wherein people tend to mix up space and spacetime.

peterh

13:27

@Jim Ok-ok, no prob!

Secret

@JohnDuffield Currently that's not my concern because I am analysing the concept spacetime, not space. We will talk about this issue later (I am aware of that issue as raised by some)

0celo7

@ACuriousMind ...what

Sanya

@heather my main problem was the apparent lack of effort ;) the question being a bit weird to read comes second ...

0celo7

Which first paragraph?

ACuriousMind

@0celo7 Sigh...the first section. Titled "mathematical definition".

The second one then gives a kind of physical interpretation

John Duffield

13:28

@0celo7 : you should read stuff like this.

ACuriousMind

You know, at least in taking things very literally you are the perfect mathematician :P

0celo7

@ACuriousMind The pp spacetime @Slereah linked is a solution to the field equations.

ACuriousMind

@0celo7 yes

0celo7

@ACuriousMind That's like the 15th paragraph, dude.

ACuriousMind

I didn't say it wasn't

Secret

13:29

> This means that any pp-wave spacetime can be interpreted, in the context of general relativity, as a null dust solution

Ok that explains what slereah said

a spacetime filled with radiation

ACuriousMind

Just ignore what I said if you dislike it.

0celo7

@ACuriousMind Done.

Ahhhhh, why is the energy infinite

Maybe I have to restrict to a Sobolev space

Secret

> This stands in contradiction to a claimed strong Mach principle, which would forbid a vacuum solution from being anything but Minkowski without singularities, where the singularities are to be construed as mass as in the Schwarzschild metric

John Duffield

@Secret : strictly speaking spacetime is the mathematical manifold that models space at all times. Because it models space at all times, it is absolutely static. Nothing moves in it or through it. A worldline represents the motion of a wave through space over time, but the wave isn't moving through spacetime. It's moving through space.
This wave might be a gravitational wave. See (LIGO)[https://www.ligo.caltech.edu/page/gravitational-waves] : _"Gravitational waves are ripples in space-time (the fabled “fabric” of the Universe)..."_

0celo7

@ACuriousMind I don't understand when people say this

Secret

13:34

@Slereah
> This stands in contradiction to a claimed strong Mach principle, which would forbid a vacuum solution from being anything but Minkowski without singularities, where the singularities are to be construed as mass as in the Schwarzschild metric

What does this have to do with showing that "spacetime has some kind of elastic properties like a rubber band" is false?

0celo7

What am I supposed to do, read your mind?

ACuriousMind

@0celo7 Kind of. All communication requires that to some extent since very few sentences are actually unambiguous unless you speak Lojban.

0celo7

What is this "Lobjan"

I need to see someone speak this

I think I just found my purpose in life

ACuriousMind

@0celo7 A constructed language that is designed to make syntactic misunderstandings impossible

0celo7

@ACuriousMind ...you really think I can't google myself?

Secret

13:37

@Slereah Or put the question in another way, what prevents a spacetime filled with radiation, that is static, non conformally flat from being described by any rubber band model?

ACuriousMind

@0celo7 The evidence thus far is inconclusive on that matter.

0celo7

When there's a definite word or phrase I can!

I don't know how to google math

Sanya

@ACuriousMind sounds a bit like the "metaphysic free" metaphysics of the vienna circle :o

ACuriousMind

@Sanya Let's say it attracts a...peculiar kind of people ;)

0celo7

@ACuriousMind let's learn this

Sanya

13:40

from the limited amount of knowledge I have about the field, I'd hope that no linguists get attracted ... @ACuriousMind

ACuriousMind

@0celo7 I think I'd sooner learn Klingon :P

@Sanya I don't know...it's probably interesting as a proof-of-concept language

Sanya

@ACuriousMind proof of which concept? That we can form words which will have exactly the same meaning for ALL speakers and thus have no subjectivity in conversations just because we changed the language?

ACuriousMind

@Sanya That it's possible to have a syntactically unambiguous language that's much more like a natural language than a programming language

Sanya

oh wait, I've seen that I've not read well enough :D I still remain a bit skeptic as contextuality makes life so much easier

yeah, this unambiguity ... well, I don't believe you'll get rid of that in a living language

0celo7

@ACuriousMind what is your opinion of Iggy Azalea

ACuriousMind

13:47

It doesn't get rid of misunderstandings entirely, since you can still have semantic misunderstandings

@0celo7 I don't have one

I barely know this individual exists, and that's it

Secret

0

Q: Providing an intuitive description of scalar and vector quantities in physics

Often the standard introduction to the concept of scalars and vectors in physics is something along the lines of: "A scalar is a quantity that is completely described by a single number (it has no directional dependence). A vector is a quantity that requires both a magnitude (a number) and a dire...

classical-mechanics forces vectors education scalars

0celo7

Yikes

Sanya

yeah, that's where I've gone wrong; sorry about that - still, I think the point about a practical language use depending heavily on contextuality to increase efficiency stands

Secret

Is "is this a good explanation of <insert mainstream physics concept>" an on topic qustion?
Because that's the impression the above question gave me

I felt it will likely be closed for "too opinion based"

ACuriousMind

@Sanya Well, I'm not a fan either, but I do think that linguists might care about the fact that such a language is at least possible

Sanya

13:52

got a point there @ACuriousMind

Secret

http://physics.stackexchange.com/questions/2710/what-does-foliation-mean-in-the-context-of-a-foliation-of-spacetime

It only works if said spacetime can be foilated. There are many examples of spacetimes that cannot have the time direction defined for each point and that poses a challenge to the "space at all times" explanation of spacetime, at least not in th usual intuitive sense we understood as matter in space evolving through time

so spacetime in some sense is more general than space

As for that "fabric" argument, we need to understand why a ricci flat, non conformally flat and static spacetime filled with radiation cannot be described by rubber band models (whatever they are, because I am pretty sure they might be not the rubber band concept laymans think about)

Secret

I think I felt my brain is exploding, cause in the last 3 hours I have been attempting to process so much infromatio that I have no complete understanding hence tons of half baked questions and comments

I should go back to my quantum mechanics study, at least QM is easier to wrap my head around than GR

Danu

o/

Secret

how's your day?

Danu

I heard that the exam that I thought went best resulted in the "worst" grade of this semester, which is weird.

0celo7

14:04

@Danu Any chance you've heard of this?

Danu

The grading was kinda harsh

Secret

what subject?

Danu

@0celo7 Energy functional? Sure

@Secret Mathematical gauge theory

0celo7

@Danu the one specifically in this post

Danu

After looking at what happened I still think I essentially knew everything---they just really nailed me on some details.

Secret

14:05

ok that's not suprising, it's definitely much harder than linear algebra (and in our uni inear algebra is graded extremely harshly thus most end up onyl getting passes)

Danu

Well, my other courses were harder IMO

Also this exam was easier

But yeah, they were nitpicky :P

ACuriousMind

@Danu It's weird that you haven't realized that grades are very dependent on the instructor and the grading scheme :P

Danu

I do realize that

I just really thought I did really well on this exam :P

In particular, I'm almost never wrong in knowing a lower bound for my grade---this time the result was below that

It's still no big deal (2.0) but meh

Emilio Pisanty

@heather No, I hadn't seen it - I hadn't trawled that thread for details. That does look like it's covered well enough in that answer.

@Danu what's 2.0 mean?

Danu

@EmilioPisanty Grade

0celo7

14:09

@Danu 2 on a 1-6 scale?

Danu

@0celo7 Passing grades here go like 1.0,1.3,1.7,2.0, ... , 4.0, and 5 is a fail.

ACuriousMind

German university grades are from 4 (lowest pass) to 1 (best pass) in 0.33 steps (rounded). A 5 is a fail.

Danu

Lol

Secret

Ours is simpler: HD DN CR PS F

0celo7

How is that simpler

Danu

14:11

So on a 1-10 type of scale 2 should be like... 7.5?

Maybe 8

0celo7

Oh wow a 75%

In America we go by percents

So much easier

Danu

Okay

ACuriousMind

@Danu Eh, that highly depends on what the 10 on the other scale is

Secret

@0celo7
high distinction, distinction, credit, pass, fail
Each represented by a percentage range e.g. HD is 85-100%

Danu

@ACuriousMind Grade conversion is always a mess

I mean, I know got about 80% of the points for this test :P

So I just based it on that---I think there was not a significant manipulation involved in the grading of this exam

ACuriousMind

14:12

@Danu I've had exams where I got a 1.0 with 70% and ones where I got a 2.3 with 85%.

Grades just make no sense :P

Danu

@ACuriousMind Right...

Sanya

1.0 with 18/46 points @Danu @ACuriousMind :D

ACuriousMind

@Sanya lololol

Danu

@Sanya That's gotta be a terrible exam

ACuriousMind

Some people like to pose impossible exams for some reason

Danu

14:14

I think the worst I had was like... 1.3 with 56 out of 120

Sanya

introduction lecture for QM :D

well, gotta go^^

0celo7

14:33

I once got a 6 on an essay in German school

or maybe it was a 5

It was bad

We had to write a detective story and I wrote about a GTA-style shootout

Secret

Sometimes when I ask or answer a question, it is incomplete in my view. This is because I don't fully understood some terminology being used in some of the questions and answers

However, some of the questions and answers I gave are structured in a way so that when the reader reads them, they become complete. This is because the reader knew about the meaning and subtlety of those terminologies, thus when they read the answers or questions, their knowledge base will automatically fill in the blanks

A general format on these types of questions is for example:

"Why is <insert concept I don't understand> result in <a result that I understand>?"
The answer will then give an answer, which in general can be said as

"because <explanation that contains some terms I don't understand>"

I can then google the stuff I don't understand in the answer, and then use them to work out what I am asking

that is, I often rely on the knowledge base of the answerer to complete the question for me, and then they will often give me an answer which usually contains concepts that is easier to be understood to me

The end result is that I can often be seen asking a question which is sensible to the answerer, but I actually don't understand the question, except the answer and why it answers the question

Put it in another way, me and the answerer provide both the question and the answer

So in some cases in my real life, I can often get answers I want from some question from a field I don't fully understood because the terminology will explain itself in the perspective of the answerer who is within the field

and sometimes, people will misunderstood me as someone from the field

despite I knew almost nothing from that new field

(attempt to make a quanutm joke) So in a sense, most of the time when I ask questions, I don't really know what happens in part of the question, but I knew that combining me and the answerer's knowledge base, the answer will be one that address the question and I can then reverse engineer the answer to understand it

That is I only know what happens in the whole system of question-answers, but I know almost nothing about the details of the question

John Rennie

14:54

@peterh I didn't say the night sky was a mathematical object, I said spacetime is a mathematical object, where spacetime is the object whose geometry we calculate by solving the Einstein equations. Spacetime is a manifold equipped with a metric - pause here for 0celo7 to complain about the vagueness of this statement.

Whether the night sky is a manifold equipped with a metric I don't know, and neither does anyone else.

peterh

@JohnRennie With my deepest respect, I agree your argument, but the difference seems to me more phylosophic as physical.

John Rennie

@Secret: some of the entries in the stress-energy tensor can be interpreted as a shear stress, but (a) this has nothing to do with whether spacetime has a shear modulus and (b) it isn't really a shear stress anyway.

Secret

The above wall of test thus, might be one of the many reason that explains acuriousmind's observation that I don't understand what I am asking

Because most of the time, I don't understand what I am asking, but I understand what I am trying to ask, and combining with the answer, will often be something that I understand

@JohnRennie Ok

John Rennie

To get an idea of where the odder entries in the stress-energy tensor come from have a look at:

7

A: Intuitive understanding of the elements in the stress-energy tensor

For any matter/energy distribution we can in principle assemble it from point particles. So the stress-energy tensor of the whole system can be expressed as a sum of the stress-energy tensors of the point particles. The reason this helps is that the stress-energy of a point particle is very simpl...

Slereah

that's not a point particle, that's a perfect fluid!

John Rennie

14:59

@peterh I utterly and profoundly disagree. Physics is the business of making approximate mathematical models to predict the results of experiments, and actually we're pretty frakking good at it! But believing your model is what's really there is a mistake that has been made over and over since the first human burned someone for heresy.

Transcript for