The h Bar

obe

12:02 AM

well... sorry I was forced to say that. please forgive.

ACuriousMind

@obe You can mimic 0celo7's awkward apologies for saying silly stuff all you like, it's not the same :'(

3

obe

I know. :(

obe

12:20 AM

@ACuriousMind I don't remember him apologizing though, in fact it's difficult to imagine him saying sorry.

DanielSank

2:33 AM

What data indicates that the universe is expanding via the metric as opposed to things just moving away from each other?

Secret

3:11 AM

NbCeID 15

iopscience.iop.org/article/10.1088/0264-9381/32/17/175003/meta

Secret

NbCeID 16
http://www.nature.com/ncomms/2015/150804/ncomms8917/full/ncomms8917.html

Danu

6:46 AM

@DanielSank What do you mean?

You do understand that, if it was not metric expansion, there would be a preferred "center of the universe", right?

Think about a fixed box with balls moving around in it.

Or, maybe more generally, you wouldn't have everything moving away uniformly (homogeneous & isotropic universe).

DanielSank

7:44 AM

@Danu I never understood that. If I pop a balloon full of particles, all of the particles wind up moving away from all the others. What is the "kinematic" difference between moving particles and what astrofolks mean when they say "expansion"?

Danu

@DanielSank You cannot make everything move away from everything in the same way without expanding the volume, no.

Just think about particles in a box (no metric expansion)

Fix a single point in the box.

Now, you require that everything moves away from it---this is backed by observations of the universe.

Then pick a different point in the box. Your particles will not all be moving away from it.

Also it's not astrofolk but cosmofolk

Astronomy does not concern itself with GR (much)

(neither do astrophysicists)

DanielSank

8:37 AM

@Danu gf calls herself astrophysicist, uses GR.

Ghost

hello!

a good friend of mine is also an 'astrofolk' and uses GR as well

Danu

@DanielSank I'm just basing that on the fact that they don't teach courses on GR in most astro degrees---nothing personal :P

and of course "our own" Kyle Kanos :D

Ghost

interesting... several astro based degrees here do teach GR

hello?

lol... just saw an article about 3D-printed pancakes.... love it!

Slereah

9:01 AM

hello

Danu

@Ghost Hmm, maybe my sample size was simply too small ^^

Ghost

@Danu different places, different processes

Slereah

"Zee combined the idea of the BD theory with the Higgs-Mechanism of Symmetry Breakdown for mass generation, which led to a scalar-tensor theory with Higgs field as scalar field, in which the scalar field is massive (short-ranged). An example of this theory was proposed by H. Dehnen and H. Frommert 1991, parting from the nature of Higgs field interacting gravitational- and Yukawa (long-ranged)-like with the particles that get mass through it"

neat

Slereah

9:19 AM

"In language that rarely appeared in the unpolluted pages of Annalen der Physik, Abraham (1912c, 1056) accused Einstein’s theory of relativity of having “exerted an hypnotic influence especially on the youngest mathematical physicists which threatened to hamper the healthy development of theoretical physics.”"

My god

He denied Einstein and the Evidence!

Ahahah

This paper talks about Einstein's theory of gravitation using a variable speed of light

(this was 3 years before GR)

Good old JD, always at the forefront of physics

Physics Meta

11:00 AM

0

Q: Limitations of human learning

Are there any proposed solutions to the following problem: People can only learn a limited amount of knowledge in their lifetime. Physics and math theories are becoming more complex, and it becomes harder to understand them as time passes. On the other hand, a person's learning capacity is limit...

discussion

Slereah

He confused Physics Meta with metaphysics I think!

3

Danu

@Slereah This is pretty hilarious

yuggib

11:29 AM

@Slereah I have been always confused by the "english" usage of the word Meta. In ancient greek, is a preposition that means "after"

Danu

@yuggib You can only have a meta discussion after a discussion ;D

yuggib

and metaphysics was called like that because the books by Aristotle were anthologized after the one about physics

@Danu :-P

but the meaning it has now is of self-reference

I find it quite misleading

I knew about the greek usage first, so when I first encountered words like metamathematics

I thought it was some mystic concept beyond maths :-D

(maybe it actually is, at least in many people's opinion)

John Duffield

11:51 AM

@DanielSank : IMHO a better analogy is squeezing a stress-ball down in your fist, and then letting go. Or the raisin cake. IMHO it's important to appreciate that space isn't nothing, and that there is no evidential reason for assuming that the universe has no centre or edge.

Danu

@JohnDuffield There is very strong evidence against the universe having a center.

It's called the CMB

But its discovery came after Einstein's time, so this shouldn't come as a surprise to me.

Slereah

The Einstein universe also did not have a center

Nor does the FRWL metric

Danu

@Slereah But it's known not to be a good model

Slereah

well yes but if we must talk of EINSTEIN AND THE EVIDENCE

Danu

@Slereah The status of FLRW wasn't so obvious early on, I assume.

The huge confirmation came with the CMB

Slereah

11:56 AM

I think pretty much all early cosmological models had no center

Because a center is a weird thing to have in GR

Danu

It goes against the spirit of GR

John Duffield

@Danu : it isn't evidence for the universe not having a centre. It's evidence for the raisin-cake expansion of space, wherein being in space can be likened to being in a fog. The expanding universe isn't like Daniel Sank's moving particles. But a raisin cake does have a centre.

Danu

@JohnDuffield That's hilarious (and wrong)---it's evidence for the homogeneity and isotropy of space---which implies that there is no center.

Slereah

the homogeneity part, specifically

yuggib

"Dios es una esfera inteligible, cuyo centro está en todas partes y su circunferencia en ninguna"

Danu

12:01 PM

right

God in the center

John Duffield

@Danu: there's no evidence that space is isotropic. This is an assumption. For all you know some guy somewhere is looking up at the clear night sky, and half of it is black.

Danu

@JohnDuffield That's, in fact, irrelevant because homogeneity already implies no center.

John Duffield

@Danu : it doesn't. Note that a gravitational field is inhomogeneous space. There is no overall gravitational field in the universe, it didn't collapse when it was dense. The fact that we're here is evidence that space is homogeneous on the large scale. But that isn't evidence that space has no centre.

Danu

@JohnDuffield It's pretty obvious that a presupposed "center" of the universe would imply that not all points are the same.

I hope you're able to understand it

John Duffield

@Danu : I understand it. It's like being in a fog. Everything around you looks the same, a white-out. But you cannot deduce from this that the fog has no centre.

Slereah

12:10 PM

ahah what

John Duffield

I would go so far as to say that the expanding universe is hard scientific evidence that space is not infinite. (How anybody can square an infinite universe with the Big Bang beats me). And if it isn't infinite and if it's flat as WMAP suggests, with no magical mysterious intrinsic curvature, that means space is somehow bounded. And that means it has a centre. The thing I don't understand is why Einstein didn't predict it.

Danu

@JohnDuffield You're falling for the beginner's fallacy of believing the Big Bang was a place, not a time.

If it's flat it would be very unnatural to assume a center (in the view of every professional physicist), regardless of what you may want to believe.

Secret

Is it possible that every points in the universe look the same, except there might be a centre (and its neighbourhood) that is not, but that region of inhomogeneity just happen to lie outside our hubble horizon?

Or is the CMB already have this information encoded in it since it corresponds to the period in the history of the universe where it just became transparent (the last scattering surface), hence any evidence of a inhomogenious pathc of spacetime must already be "projected" onto it?

and the homogenity of the CMB thus rules out the presence of a centre?

Slereah

What does a center even mean in the context of spacetime, though

Unless it has boundaries it is a rather odd idea

even with boundaries it's not that trivial to define

John Duffield

@Danu : no I'm not falling for a beginner's fallacy. And what's very unnatural, is for professional physicists to make assumptions that are unsupported by scientific evidence. Perhaps you're referring to cosmologists? Some of whom have only a sketchy understanding of general relativity?

@Secret : it doesn't rule out the presence of a centre. The CMB is akin to fog. The presence of this fog, wherein you see white everywhere you go, doesn't mean there's no centre to the fog you're in.

Danu

12:21 PM

@JohnDuffield Right... You know what's sketchy? Claiming to understand the universe without (1) any serious physics education (2) any apparent mathematical knowledge (3) any published research (4) any regards for the professionals who do fulfill all other requirements for believability (5) any willingness to learn from others.

John Duffield

@Slereah : it means what you think. See the NASA picture here. They've dropped one space dimension and they're showing the time dimension going from left to right. Each vertical grid line is a billion years. Imagine you're looking into this "stemless wine flute" from the top. Where the sides are is the edge of the universe.

Secret

Danu

@Secret Hey, look, a drawing that I do understand! Maybe John will get it too ;-)

Secret

@Slereah The left case does not work because one need to mention what all that nonspacetime "stuff" in between is

The right case will imply the presence of a small region of inhimogeneous spacetime, which we have not observed

But...

@Danu Does the CMB contains the global details of our entire universe, or just up to the hubble horizon/observable universe?

Because if that's the latter, then it is possibel that the inhomogenous region may just happen to fall outside our observable universe and thus we then cannot simply say the homogenity of the CMB rule out a centre

So does the CMB contains global details of the entire universe at the time of the last scattering event?

Danu

@JohnDuffield, I can even refute your claim in your terms. Imagine walking in a fog. If there is a center, one can simulate this by a point that generates fog and blows it outwards in all directions. If you're standing at this point, everything moves away from you. Now, move one step. If you look back at where you came from, the fog is now moving towards you. This flatly contradicts homogeneity.

@Secret Of course we can, by definition, not say anything about things outside the observable universe from an experimental perspective.

You can assume all you like about what's outside of what we can observe, but that'd be the biggest cop-out possible.

Secret

12:31 PM

@Danu So the CMB only maps the observable universe at the time of the last scattering event, and thus the homogeneity of the CMB only told us that "within the observable universe", there is no centre?

and the observable universe is the thing that is experimentally relevant to us, while anything outside is pretty much unreachable and not measurable?

John Duffield

@Danu : I give you a reference to Einstein describing a gravitational field as inhomogeneous space. I point out that WMAP indicates that space is flat. And that the FLRW metric assumes the homogeneity and isotropy of space. If all you can offer in return is featherspitting hubris because you have no adequate response, well, it doesn't look too prefessional, now does it?

Danu

@Secret Of course; one can philosophically even take the stance that things outside the observable universe don't really exist, although this is not a very popular point of view among physicists.

Secret

Hmm, ok

John Duffield

@Danu : this "refutation" is nothing of the sort. I'm not saying the fog is blowing outwards. Particles are not blasting out from the centre through space. Space itself is expanding. It's like this gin-clear ghostly elastic solid. With an innate pressure. It expands.

Danu

@JohnDuffield Obviously FLRW assumes homogeneity and isotropy. The point is that we measured things in the Universe and found that it is, indeed, well-described by FLRW. It's not that hard: This is how all of science works, in case you hadn't ever noticed.

I don't think references to Einstein are relevant because Einstein did not understand how to properly describe our universe by means of GR.

John Duffield

12:36 PM

@Danu : and suppose I snapped my magic fingers, such that you were no longer in Denmark, but instead were standing on a planet 46 billion years away. Looking up at the clear night sky. Wondering why half the sky is black?

Danu

@JohnDuffield I've put enough effort into talking to you about this; I'll stop responding.

You're just demonstrating once more that you're unwilling to listen to anything else than your preconceived ideas.

@JohnDuffield I'm not in Denmark. No snapping needed.

Also, years are not a measure of distance.

Bye for now.

John Duffield

@Danu : you're the one who is unwilling to listen, you're the one running away from the references that you can't counter. And by the way, a light year is a measure of distance.

If anybody is interested, see this answer which relates to the above.

Danu

@JohnDuffield light year, yes.

Secret

@Danu @Slereah
That CMB question I asked above shows how my mind is more of a mathematician than a physicist

When I chat with my physics professors and peers, often they have to state explicitly that "blahblahblah is experimentally relevant" in order for me to be able to understand why the very general mathematical case is ruled out

That is, at least for the past few years until the present, my way of learning physics is like this:

1. Observed a phenomenon, obtain some data
2. Try to find some patterns in the data

Secret

One example of the above is back in my 2nd year, I do the particle in a box problem as if it is a ODE problem for 3 times, and that rule out the trivial cases and other physically irerelevant cases by substituting the assumptions or constraints one by one into my general maths equation

It is only after this I can do this problem like a physicists did, by skipping over the physically irrelevant cases and still be confident that I am not cheating or overlooking or missing out something

So basically, as of now, to me physics is like maths but with a special type of constraints that are based on observations and cannot be derived from axioms, which is needed to make the equations work in the physical context

correction: "back at physical intuition" should be "bad at physical intuition"

Secret

12:55 PM

In short, my mathematical oriented mindset means physics to me is a sub(multi)set of maths that there exists a type of constraints that are derived from an object called "experiments" and this is needed to ensure the solution the problem remains closed in physics under the usual operations

and this is why I generally struggle in physics, because I am bad at figuring out those experimentally derived constraints

thus end up writing pages and pages of algebra when doing problems

ACuriousMind

Um, @Secret, this isn't your diary, it's a chat. We talk to each other here, and posting giant walls of texts isn't really conducive to that.

Secret

I am just trying to tell you guys about my learning methods, which might help on refining the answer when I ask questions later in physics in the chat

ACuriousMind

@JamalS, haven't seen you here in a while, how have you been?

Secret

I am not sure how to shorten them more though to get the point across. For one thing I am not a native speaker thus my English is not perfect. For another, the timing out of the messages means I usually cannot fix my previous message and end up need to clarify it on th next block of message

ACuriousMind

@Secret That's supposing the people you ask these questions read and memorize what you write there even if they may not even have been here when you posted it.

Slereah

1:15 PM

It's a gh-gh-ghost D:

Ghost

boo!

ACuriousMind

It's not even a ghost-of-ghosts, not that scary

Ghost

not scary at all

ACuriousMind

Ah, a little bit scary since even normal ghosts mean quantization will be a bit tricky

Secret