The h Bar

2:56 AM

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Q: When are questions with multiple different possible answers good?

It is well known that we do not encourage/allow questions that solicit open-ended discussions or that can only be answered by providing personal opinions, etc. However, at least to me, it seems a bit unclear as to what is the policy around questions that are not ill-defined (in that there is a we...

John Rennie

5:03 AM

The Fermilab muon g-2 results are out today.

4

There are frustratingly few rumours about what they contain.

My guess is that if they had a 5σ signal for physics beyond the Standard Model it would have leaked, so my guess is that either there is no signal or there is just another <5σ signal.

antimony

6:14 AM

nice!

btw, i'm really trying to understand how particle velocity relates to temperature re. emission wavelength

i can totally see how an ensemble of bouncing particles eg. gas molecules, can have a distribution of kinetic energy

what i don't understand how that can act as a source for eg. IR emissions

is it because each atom/molecule is essentially a point charge?

and so those charges moving around emit an EM wave?

ie. that would be a classical view

or is it more a QM phenomena, because as the particles interact they trade off energy with eachother, which means they are jumping to higher and lower energy states

which means they will release a photon when dropping state?

John Rennie

7:47 AM

@antimony gases emit radiation because collisions excite rotational states, then those rotational states decay back to the ground state and emit a photon.

In dense systems like solids and liquids the radiation they emit is mostly black body radiation, and tis comes from thermal oscillations in the electron density in those materials.

So emission from solids is not due to changing quantum states.

However under normal circumstances gases are not dense enough to emit black body radiation, so for gases the emission does come from transitions.

At room temperature the only transitions that can be excited are rotational, so the radiation gases emit comes from rotational transitions.

antimony

8:09 AM

thank you that is a marvelous explanation :) would it therefore be fair to say that the classical model cannot explain radiation emitted from gases, but it could explain radiation emitted from solids and liquids?

@JohnRennie

hmm although i guess it could only incompletely explain emission from solids...

(if at all, since my lack of understanding on that matter still reminds ofc)

Nihar Karve

12:10 PM

the SUVAT equations finally make sense to me

Slereah

1:32 PM

Seems simple enough

What the hell is $\mathrm{String}$

ACuriousMind

@Slereah The entry in the Whitehead tower of O(n) above Spin, obviously ;)

see en.wikipedia.org/wiki/String_group

RewCie

2:51 PM

Hello Everyone

I'm in a big trouble... There's R&D teacher out here, gave me a project to do, I barely know anything about IoT

I tried to convince him, but I don't know why he wants me to do a project on IoT

Also, it requires a bit of Mechanical Engineering too

far-far away from my background

I tried telling him in both English/Hindi, I'm a C/C++/JAVA/Python guy with Machine Learning stuff.... He's such a damn big *****.

Sleb Lagnej

what if instead of doing:
var oldHeight = object.height
object.height = 12
we do
var oldHeight = object.height = 12

ACuriousMind

@SlebLagnej in most languages, the assignment chain will end up assigning 12 to oldHeight, this is not equivalent to your first version.

Sleb Lagnej

so it already exists?

(im JingleBells btw)

ACuriousMind

many languages allow you to write a = b = c, yes. But it is usually interpreted to be equivalent to a = c and b = c, while what you want is a = b, then b = c, for which there is usually no shorthand

Sleb Lagnej

wot is that profile picture

u used to be normal

ACuriousMind

3:01 PM

I regret to inform you I've never been normal

(and it's Xzar from Baldur's Gate)

Sleb Lagnej

normal is boring ;)

Shashaank

Could anybody here please help me by confirming whether $r$ is just an affine parameter for null geodesics in Schwarzschild or is it an affine parameter even for time like geodesics. I don’t think it is an affine parameter for time like geodesics and thus am confused about the argument presented in my notes that the manifold is not geodesically complete for time like geodesics cannot be extended for all affine parameter

And the problem being that r is not an affine parameter for time like geodesics thus it should not be a problem if we cannot extend time like geodesics for all values of r because my understanding is that r is not an affine parameter for time like geodesics. So what I want to confirm is that whether I am correct in saying that r is not an affine parameter of time like geodesics

@Slereah am I correct in that r is not an affine parameter for time like geodesics

John Rennie

3:34 PM

@Shashaank the coordinate $r$? If so that's a coordinate not an affine parameter.

In the geodesic equation $\tau$ is an affine parameter. Is that what you are thinking of?

Shashaank

@JohnRennie what you are saying is correct. But for radial null geodesics we can show that $r$ itself is an affine a

parameter

and the ongoing radial null geodesic is stopped at r=2GM that is it cannot be extended beyond that

Similarly the outgoing radial null geodesic begins at r= 2M ....

But if we try to see the timelike geodesics then I guess $r$ is not an affine parameter and thus it should be no problem in that we cannot extend a timelike geodesic for all values of $r$..... because $r$ is not an affine parameter for timelike geodesics in the first place.....So the argument for not being able to extend the timelike geodesic for all values of an affine parameter is not true here

@JohnRennie because $r$ is not an affine parameter for time like geodesics

So what I am asking is that like $r$ is an affine parameter for null geodesics am I correct in saying that $r$ is not an affine parameter for timelike geodesics

Slereah

4:02 PM

$r$ isn't a parameter at all, although of course, for some curves, it can be that their parameter coincides with a coordinate

whether or not it's timelike or something else will depend on how it varies along other coordinates as well

If it only varies with $r$, it will be spacelike outside the horizon, and timelike inside

Shashaank

4:14 PM

@Slereah see imperial.ac.uk/media/imperial-college/…, we can show r is an affine parameter for null geodesics. If you say no, then how can we shown that we cannot extend radial null geodesics all the way beyond the horizon.

Slereah

I mean sure, you can parametrize a null geodesic with $r$

Or with a parameter that is equal to $r$ at $\gamma(\tau)$, anyway

But that's just a coordinate, and that coordinate stops at the horizon

There's no reason to think that you can continue that beyond the horizon

You'd have to show it by patching them together

bolbteppa

> "The Muon g-2 experiment at Fermilab sees fundamental particles called muons behaving in a way not predicted by the Standard Model of particle physics. These results confirm an earlier experiment performed at @BrookhavenLab"

ACuriousMind

4.2 sigma, we're getting closer

Shashaank

As far as I remember there was an answer by @JohnRennie which stated that Schwarzschild coordinates are good beyond the horizon as well and it’s not correct to state that the Schwarzschild coordinates are good only for r>2GM like some books do. They are good for r<2GM, only their interpretation changes.

John Rennie

@ACuriousMind Frustratingly close :)

Shashaank

4:26 PM

@Slereah so already have that the Schwarzschild coordinates exist for r<2GM, isn’t it

John Rennie

@Shashaank coordinates are just labels we attach to points in spacetime. As long as they aren't singular they are good everywhere. Their physical significance may vary but who cares? At the end of the day any coordinate system is just a set of labels we use for computing the things we can actually measure.

bolbteppa

pbs.twimg.com/media/EyYhczDWEAMc6qZ?format=jpg

physics.aps.org/articles/v14/47

I'd post the big comic here but the other one didn't even work so...

Shashaank

@JohnRennie so if we do already have Schwarzschild coordinates parameterizing point inside the horizon as well, why do we bother about Kruskal extension. Why do the books state that we need to extend the manifold to r<2GM and only then a Kruskal extension describes the physics inside r<2GM.....when like you said Schwarzschild coordinates already described the physics inside r<2GM

Slereah

you can have Schwarzschild outside the horizon, you can have Schwarzschild inside the horizon

(Gee why wouldn't he be called Bob)

But not at the horizon itself

You can see why by looking at the determinant of the metric

If you want you can do an intermediate patch in between the two, which would give you some coordinates valid on the horizon

Dvij D.C.

4:50 PM

Where is the insane influx of all the g-2 questions?!

bolbteppa

Muon g-2 experiment finds strong evidence for new physics

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John Rennie

4.2σ ... so close ...

bolbteppa

That is a short summary they put out, there's an hour long scientific conference also uploaded

Dvij D.C.

Yep! With the LHCb R(K) result and this Fermilab result, it really looks like muons are having some secret affairs with some very interesting people.

John Rennie

It's looking good for BSM physics. Unless there's some systematic error that has evaded attention.

bolbteppa

4:53 PM

It's hard not to think of all those papers 'explaining' the faster than light signals a few years ago

Dvij D.C.

Since it is by two completely different collaboration, I am not sure there is a common systemic error. But both depend on the theoretical hadronic calculations from SM which might be a common source of systemic error. Don't know enough.

bolbteppa

(It worked!) according to the comic it's not really two completely different collaborations right, it's some of the same equipment at least

Dvij D.C.

@bolbteppa I meant LHCb with their R(K) result and Fermilab with their g-2 result.

Both hint at unknown interactions of muons.

Shashaank

@Slereah @JohnRennie so do we do the Kruskal extension to just have good coordinates on the horizon as well ( where we previously didn’t have any good coordinates) or is there any other reason as well

bolbteppa

That video cites this ~ 200 page paper calculating the muon value...

Slereah

5:39 PM

different coordinates are good for different purposes!

Having coordinates that span the entire spacetime can be practical for some calculations

bolbteppa

Scientific Seminar: First results from the Muon g-2 experiment at Fermilab

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So basically I don't think you're going to find this result derived in Peskin and Schroeder

Muon g-2

Muon g−2 (pronounced "gee minus two") is a particle physics experiment at Fermilab to measure the anomalous magnetic dipole moment of a muon to a precision of 0.14 ppm, which will be a sensitive test of the Standard Model. It might also provide evidence of the existence of entirely new particles. On 7 April 2021, physicists reported that results from recent studies involving the particle challenged the Standard Model and, accordingly, may require an updating of currently understood physics.The muon, like its lighter sibling the electron, acts like a spinning magnet. The parameter known as the...

My question is, what does 'geometric unity' say about this :p

https://www.math.columbia.edu/~woit/wordpress/?p=12292
> To find out more, I’d suggest watching the first talk from the FNAL seminar today, by Aida El-Khadra, who lays out the justification for the muon g-2 Theory Initiative number, but then looking at a new paper out today in Nature from the BMW collaboration. They have a competing calculation, which gives a number quite consistent with the experimental result... So, the situation today is that unfortunately we still don’t have a completely clear conflict between the SM and experiment. In future years the experimental result will get bette…

Faded Giant

7:58 PM

Hm, our gamma spectrometry thinks that it has found Eu-155 in concrete samples. Which doesn't make sense; the values are more than 100 times higher than expected. So I guess it's a false positive caused by spectral interference. But I don't see the origin for that.

Faded Giant

8:10 PM

Probably X-rays. That's why I cannot find anything useful in the gamma catalogue.

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