The h Bar

So, then, can we say that curvature "causes" "motion" (avoiding the whole debate about what "causes" really comes down to)?

12:32 AM

in the presence of curvature we have motion that deviates from that in SR (flat spacetime)

what "causes" it?

that's philosophy

0537

lol

@HDE226868 well...we can have motion in flat spacetime

@0celo7 Well, yes, of course.

@HDE226868 the EOM for a particle in SR is $\ddot x^\mu=0$ which can have "line" solutions

the physical effect of curvature is tidal forces

@HDE226868 do you know "why" particles in GR obey the geodesic equation?

@0celo7 Nope. It's not something we can know, is it?

It isn't physics.

12:37 AM

@HDE226868 it's just the action principle

so you have to ask why that holds :P

DanielSank

@TanMath Good idea. Then ping us here.

TanMath

@DanielSank yep...

the extrema of $\int \mathrm{d}\lambda\sqrt{\dot x^\mu\dot x^\nu g_{\mu\nu}}$ are geodesics

TanMath

@DanielSank btw we can continue with our convo if you have time to write it out...

and that's just the spacetime arc length of the path

another way to see this it to use local energy-momentum conservation on the energy-momentum tensor for dust: each dust particle moves along a geodesic

12:40 AM

Aside: My argument that curvature is responsible for motion lies in the idea that a non-zero $T_{\mu\nu}$ leads to a non-zero curvature. This, in turn, means that the metric tensor is not flat, and so the motion of particles will be influenced.

That borders on being philosophical, but so does this whole damn situation.

the old

matter tells spacetime how to curve

and curved spacetime tells matter how to move

user54412

@HDE226868 You're right, that answer is completely wrong.

@ChrisWhite I'm not even sure what the answer is trying to say

@ChrisWhite I'm concerned with my logic (bravo for understanding that whole thing, by the way).

I.e. I don't know if my reasoning is correct.

user54412

The more philosophical questions attract a lot of egregious misunderstandings, and while it is often possible to clarify everything, I find the effort too soul-draining.

12:47 AM

The age when most scientists could afford to be philosophers ended a loooong time ago.

In related news: the Subtitles for AssCreed 4 randomly capitalize words

maybe I should start Typing like this

user54412

@HDE226868 I've studied more than a little philosophy myself. Really the problem is there is as much trash popsci philosophy as there is trash popsci physics, but many people (including too many physicists) don't recognize this.

@ChrisWhite That surprises me a bit.

ah, apparently they use the old English capitalization rules

user54412

Imagine if smart people in another field kept making fun of physics for thinking there are giant rubber sheets in space that cause gravity.

12:49 AM

@ChrisWhite no, there's just a gin-clear ghostly elastic solid

I want that analogy dead and buried. Forever. I've written three separate answers on Astronomy that basically say, "This is why you misunderstand the situation."

@HDE226868 you don't like martinis I take it

@0celo7 Ask me in four years (or more).

@ChrisWhite My sympathy with philosophers just rose several magnitudes.

user54412

I can't really imagine ever finding myself in a situation that involved a martini.

12:53 AM

@HDE226868 huh?

@ChrisWhite Make one?

@0celo7 I've decided to not drink 'til 21. If possible.

I recommend 1:4 vermouth to gin.

@HDE226868 o

What if you make a trip to a country with sensible alcohol laws?

user54412

@0celo7 Saudi Arabia?

@ChrisWhite sigh

user54412

Also, the image of a disheveled, bearded grad student sipping a martini, complete with toothpick-speared olive, alone at home is... just too incongruent.

12:58 AM

You could throw a party as cover for you mixing the martini.

@ChrisWhite I'm not even a grad student and I'm not above that

Oh, also mix in a tsp of olive brine before you shake.

And a single olive is just silly. The martini is an olive delivery device.

@ChrisWhite Is that - sans martini - a self-portrait?

user54412

@HDE226868 grad student? check. disheveled? check. alone? check check check. bearded? currently, but to my discredit I've never been reported to the police for looking like a terrorist

1:23 AM

THERE ARE NO DAGGERS IN ASSCREED 4

>:(

dmckee

1:48 AM

@ChrisWhite If I try to grow a full beard, it comes in patchy around the cheekbones so I look like a madman. On the other hand I can grow a really fantastic goatee or van dyke.

SO I am evidently meant to be a mad prophet or a grand vizier.

user54412

mad prophet and grand vizier both sound like really fun occupations

I am not hunting an ocelot!

0537

2:05 AM

lol

Alfred Centauri

2:29 AM

@ACuriousMind Some (many?) 'grown ups' aren't grown up. But I am reminded of one of my favorite quotes (in two parts): "I will ask you to project the look on a child’s face when he grasps the answer to some problem he has been striving to understand. It is a radiant look of joy, of liberation, almost of triumph, which is unself-conscious, yet self-assertive, and its radiance seems to spread in two directions: outward, as an illumination of the world"

@ACuriousMind "—inward, as the first spark of what is to become the fire of an earned pride. If you have seen this look, or experienced it, you know that if there is such a concept as “sacred”—meaning: the best, the highest possible to man—this look is the sacred, the not-to-be-betrayed, the not-to-be-sacrificed for anything or anyone. "

Terry Bollinger

3:04 AM

@ChrisWhite I'd much rather make fun of physics for thinking there are tiny vibrating rubber strings that cause particles... }8^)>

3:38 AM

@TerryBollinger of course not, they're made out of carbon nanotubes

@igael perhaps you can explain the two-body problem of how space-time curvature causes two relatively stationary objects to start moving towards each other. I guarantee there's a Nobel prize in it for you. — Benito Ciaro 6 mins ago

Now I'm really confused.

People are not making sense.

@HDE226868 you're me 2 hours ago

@HDE226868 Well...I don't think I could sit down and prove that two objects attract within the GR framework.

I just did a set of simple particle-in-a-magnetic-field problems, but unit conversions have messed up my head. How dare they ask for energy in kiloelectronvolts!

If one's mass is negligible...the problem is trivial.

But if both have significant mass...that's tricky.

@HDE226868 Do you know if there's a metric for two black holes

3:56 AM

I doubt it.

Well...that's what you'd need.

@HDE226868 If one body can be treated as a test body, then set up Schwarzschild spacetime around the other one and show that radial geodesics drag the small particle inwards.

But if they have the same mass (up to a few orders of magnitude) this is no longer possible...make sense?

@HDE226868 Hmm, I should have expected something horrible.

Q: Open quantum system modelling

@0celo7 Yeah, it makes sense.

TanMath

4:53 AM

@DanielSank it is not on topic on code review

1

I have been working for a long time now on modelling an open quantum system using the Lindblad Equation. The Hamiltonian is the following: However, two other matrices are added to the Hamiltonian. One of them has all the diagonal terms equal to -33.3333i and everything else zero. Another is a ...

Here is the question..

user54412

5:13 AM

@0celo7 How else do you think they initialize merging black hole simulations?

user54412

@HDE226868 Meh. Like I said, nothing to be learned from that engagement.

John Rennie

7:09 AM

@0celo7 There is no analytic solution. The nearest is the C-metric - arxiv.org/abs/gr-qc/0609056

Stan Shunpike

Why does E&M have U(1) symmetry as opposed to some other kind? Is there a physical reason for this?

Often 87

8:35 AM

I dont get the whole concept of singularity

user116211

11:39 AM

What's the meaning of this reason: 'Hey everybody! Come look at my black holes!'???? Some user has put on a bounty on this: physics.stackexchange.com/questions/27267/… . no offence. But what the hell is this reasoning?

Q: What would the consequences of a force having a hermitian symmetry group be?

11:58 AM

@ChrisWhite I meant a closed form exact solution of the EFE for two black holes.

Of course there's stuff for simulations.

Qmechanic

12:28 PM

1

Just curious of what would happen if there was some fundamental force whose symmetry group was a group of hermitian matrices. Say the symmetry group of QCD was suddenly switched to a hermitian subgroup of SU(3). would it change anything?

particle-physics symmetry group-theory

Unclear what's been asked?

12:46 PM

@user36790 huh, that's weird. Though I suppose one could still go by the general reason - looking for an answer drawing from credible sources - but it's kind of an odd choice given that there are no (undeleted) answers on the question.

Bass

same user, same thing here: physics.stackexchange.com/questions/10932 '3 up votes......'

1:02 PM

@Bass thanks for pointing it out.

1:21 PM

@DavidZ have you seen this one?

6

vs

1

Caching?

yeah, that what HDE reckons, it sounds plausible

Have they got a deleted, upvoted question that you can see?

Yeah

Yeah, then that should be it then.

though it was deleted a long time ago

1:23 PM

huh

~10/20/30/40 days?

Yeah, a month ago

That sounds too long for caching, right?

Yeah

huh

It should be ~1 hour for meta, I think. Or 24 hours at most

I'll make some inquiries

Q: List of communities with base css updates completed

1:25 PM

Excellent, I knew I'd get the curiosity bug in you

Just noticed this, too

67

The design team has been rolling out design updates to sites as part of network-wide update to a new base css framework. The updates allow us to: Use .svg sprites for retina displays Fix layout bugs globally More easily add new features to all of our sites in the future Synthesize healthy, n...

discussion design announcements

We're still "on deck" along with... zero other sites.

Yeah, I haven't yet gotten a straight answer from the team about what's going on with that. I'll see if I can get a status update at least.

@Qmechanic Yes.

Q: Is there a known timeline for the deployment of the new CSS?

1:49 PM

@DavidZ In case you need to show them that there's some pressure from the community

0

Some six to eight weeks ago, Stack Exchange announced a programme to re-design the CSS and tweak the site styles throughout the network, including the completely redesigned profile pages. Since then, we've made some big requests to come with the re-design, so we've remained "on deck" for some tim...

support design

Q: Is there a known timeline for the deployment of the new CSS?

Cool, couldn't hurt.

If I hear anything, I'll post it as an answer there, or ask the team to do so.

Physics Meta

2

Some six to eight weeks ago, Stack Exchange announced a programme to re-design the CSS and tweak the site styles throughout the network, including the completely redesigned profile pages. Since then, we've made some big requests to come with the re-design, so we've remained "on deck" for some tim...

support design

0

Q: Model, Theory or Law?

I think there is a lot of issues caused by mislabeling a hypothesis. I wonder if the moderators could in some way denote the level of certainty in the post.

feature-request

Qmechanic

2:13 PM

1

Q: Speed and multiple images

Why an object (or something else) create multiple images of itself when it is moving fast? An earphone spinning in circle for example.

optics speed vision eye perception

Unclear what's been asked? (Note that the tags are mine and may not properly reflect OP's question.)

@Qmechanic No, I don't think so. OP is asking why we tend to see "afterimages"/multiple images of something moving fast. If any, it is off-topic for being more about human perception than physics, but since one sees the same effect with camera recordings - motion blur - it's resonable close to being a physical phenomenon I'd let it slide.

On one hand, we should watch out for optical effects that happen to occur in both eyes and cameras for different reasons, but on the other hand, there is some amount of physics involved in how an eye works.

I think that question, as written, is a little off topic, but it could easily be edited to make it clearly on topic. IMO.

Slereah

4:20 PM

One week 'til I am back to work

Woe is me

Robots again?

Slereah

Nope

Web stuff

@ACuriousMind Stupid question: why are the intervals with rational endpoint a a basis for R? How would I, for instance, get the open set (0,π)?

Something something limit of converging open sets with rational endpoints?

@0celo7 Yes. Take a sequence of rationals $x_n$ with $x_n\to\pi$ such that $x_n < \pi\forall n$, then take the union $\bigcup_{n\in\mathbb{N}} (0,x_n)$. That's $(0,\pi)$.

Mark Mitchison

5:45 PM

@TanMath Looks like your Lindblad equation is not trace-preserving, correct? Because you have imaginary (i.e. anti-Hermitian) terms in the Hamiltonian. You might want to check whether qutip normalises the state automatically. If so you will get the wrong result.

Jacobadtr

Hello,

I have a question regarding projectile motion; When you consider air resistance, would you still expect a symmetrical trajectory?

I haven't studied this, but have been asked to solve some DE's that describe projectile motion and the asymmetry is quite pronounced

Nevermind, "no" seems to be the answer.

6:03 PM

In the Senate in jeans and a sweater -- embarrassingly underdressed.