The h Bar - 2022-09-29

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12:20 AM

@ACuriousMind I feel like one of the few people who thinks pigeons are nice to have around

they add a bit of nature to cities

@ACuriousMind I remember hearing a lot of city pigeons were wild ones that migrated, since buildings were similar to cliffs

4 hours later…

4:50 AM

@SirCumference I suspect you're thinking of peregrine falcons, or possibly other raptors outside the UK. They nest in cliffs and in the UK they have taken to nesting on the vertical faces of buildings. They are doing very well in fact, presumably on a steady diet of pigeon.

In the UK at least urban pigeons are descended from escaped racing pigeons. The main problem with them is the copious amounts of pigeon poop they produce. Anywhere they congregate to rest for the night is plastered with the stuff. In my city a lot of buildings have a pigeon repellent gel smeared on window ledges or anywhere else pigeons might congregate.

2 hours later…

6:34 AM

There is also the infamous "gift" from above...

7:28 AM

I think it's only happened to me once ...
That's once too often of course!

7:43 AM

It happened to a friend of mine. He was wearing a white shirt! I couldn't help but laugh at his face :P

Note: I would probably burn myself if it happened to me

8:04 AM

It's interesting how these things matter less as you get older. As a teenager I would probably also have burned the shirt (though not myself as that seems a bit extreme :-) but these days I'd take the view that the poop is sterile since it's quite acidic so I'm not that fussed.

Not that I'd go out of my way to smear myself in pigeon poop, but if it happened it would be a minor irritation rather than a catastrophe.

growing up I always heard getting hit with bird poop was supposed to be good luck

I guess it says something about how people come to terms with stuff like that

To be fair to pigeons tho, any kind of bird probably poops that much. People don't really complain about sparrows and robins in cities though

@JohnRennie I wish I could be as rational as you in such situations, my repulsion dominates and I can't think lucidly

@Feynman_00 I think it's just age (an ancient 61 years in my case) as I would have felt the same as you as a teenager.

it could be worse tbh. pigeons tend to fly off to caves when they're about to die. if they didn't we'd have to deal with dead pigeons everywhere

i don't think i've ever seen a dead pigeon tho

At least in my case everything matters less as I age. I appear to headed in a vaguely Buddhist nirvana direction. Once I get there I'm hoping it will be clear WTF quantum field theory is really all about.

8:19 AM

I'm afraid the parallels between theoretical physics and oriental wisdom have been greatly exagerated by 90's snake oil salesmen

The Tao of Physics

The Tao of Physics: An Exploration of the Parallels Between Modern Physics and Eastern Mysticism is a 1975 book by physicist Fritjof Capra. A bestseller in the United States, it has been translated into 23 languages. Capra summarized his motivation for writing the book: “Science does not need mysticism and mysticism does not need science. But man needs both.” == Origin == According to the preface of the first edition, reprinted in subsequent editions, Capra struggled to reconcile theoretical physics and Eastern mysticism and was at first "helped on my way by 'power plants'" or psychedelics, with...

70's, even!

Proof if any were needed that writing complete bollocks is not limited to those studying interpretations of quantum mechanics.

I mean that kind of nonsense is as old as physics itself

older even

One of the great mathematician of ancient greece is best remembered as a wizard

1 hour later…

9:25 AM

@Feynman_00 I once got a bucket of vomit dumped over my head (it was a freak accident) so a bit of bird poop is really not that bad

Everytime I read about supersymmetry I just imagine the words in Mario's voice

SUPER FIBER BUNDLE

1. [60 FPS] Title & Prologue - Super Mario Galaxy

The typical SUSY paper

$\mathbb{Z}_2$ graded Mario Galaxy

Is Luigi Mario's superpartner?

(that would mean he was born as Smario)

@ACuriousMind Just thinking about it makes me feel sick

9:41 AM

@ACuriousMind As you well know Mario is bigraded

user image

Ah, right, I forgot about Wario and Waluigi. Bit strange they chose to label the superpartners with W- instead of S-.

user image

Last time I checked there were no ghosts

Actually that reminds me of Polymerization YGO card

user image

Catto got polymerized

Tensor cat

9:55 AM

it's a symplectic cat

You can define a metric structure by the intersection of a conformal structure and a volume structure

I wonder if that relates to the decomposition of the Riemann tensor into Ricci & Weyl tensor

Since one is related to conformal symmetry and the other relates to changes in volume

Tho there is also the Ricci scalar involved so who's that guy

What does the Ricci scalar tracks

Also the volume

Maybe it's related to a subgroup like dilations?

Wait no that don't work

10:34 AM

The split is usually done as a product $$g = \det{g} \frac{g}{\det{g}}$$

Maybe I should look at how it works out if I compute the Riemann tensor of this

4 hours later…

2:08 PM

Riemann tensor is for the deviation of a vector in some direction while the Ricci tensor is the deviation of a volume

Are there intermediary ones in between

The deformation of the volume of a $p$-brane or something

isn't that just the induced Ricci tensor on the brane as a submanifold?

no need for some new fancy object

Trying to figure out ways to do measurements of specific tensors

There are probably papers on the topic somewhere but for some reason people mostly seem to write about the Riemann tensor

I guess measuring the deformation of a volume is more challenging than a length

at least in GR

Can't just dunk it in a vat of water

2:25 PM

What if you have

An EM cavity

Maybe the generated waves will depend on the Ricci tensor

Though I guess it doesn't depend on the volume as much as the characteristic length in that direction

Thermodynamics process may depend on volume variations though

what kind of energy does a volume at a given temperature radiate if the metric changes

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It certainly depends on it, somewhat

2 hours later…

4:39 PM

Where are you getting these equations from?

@Slereah which paper?

I just googled relativistic version of Stefan-Boltzmann

Ah I see... I thought you didn't like thermo in GR?

Also is there an agreed upon covariant generalization of Maxwell Boltzmann Distribution

?

Hell if I know

Just need some process of GR that involves only volume

4:58 PM

Temperature is bad enough just in SR. physics.stackexchange.com/q/729183/123208 Mind you, if a cup of coffee hit you at 0.9c, it would feel very warm. :)

@PM2Ring I constructed a relativistic version of this arguement

and while I can't solve the equations I am convinced is a need of 4 temperature

like a vector

1

Q: The equivalence of the Stosszahlansatz and the usual Boltzmann entropy arguments?

Question Below I show one can use the Boltzmann Stosszahlansatz to independently arrive at the Maxwell Boltzmann distribution without using the usual phase space arguments (Assuming $*$ equation has a unique solution). This makes me that suspect there is a deeper relation between the two? Can one...

thermodynamics statistical-mechanics phase-space kinetic-theory

5:16 PM

I think a 4-temperature has problems because you can't make it Lorentz invariant. So why even bother? ;) Just use the rest frame temperature. If you see a blackbody with a non-isotropic spectrum, you need to redshift or blueshift it to "decode" its restframe temperature.

You can't exactly stick a thermometer into a relativistic coffee cup. Your thermometer will explode.

6:06 PM

@Sleerah Following our last discussion, I just realized that if I have lightlike vectors u and v then $u+v$ and $u-v$ are automatically orthogonal, no need to Gram-Schmidt orthogonalize...no?

why would they be orthogonal

$(u+v)\cdot (u-v)=u^2-v^2=0-0=0$

true enough

But I fear there is something related to GS orthogonalization about this because our lecturer also said something like this...Is there a different problem where you actually need to do this?

Relativisticcucumber

does anybody know if there is a solution set to david tong's qft problem sets floating around anywhere?

6:18 PM

@Slereah Also is there a way to prove the uniqueness of the construction you mentioned if it is at all unique(modulo some multiplicative constants or other trivialities)?

Gram Schmidt?

It is not unique, no

It depends on the order you perform the construction

6:36 PM

No not the GS part...that u+v and u-v are a pair of timelike and spacelike vectors constructed from u and v two linearly independent lightlike vectors

6:54 PM

I mean an addition of two vectors is unique

so yes it is unique

7:21 PM

I have this exercise where I need to find how a gradient $\nabla f$ transforms under a $\pi$ rotation and according to the book my result $\style{display:inline-block; transform:rotate(180deg)}{\nabla f}$ is wrong

2

lol

1 hour later…

8:53 PM

0

Q: How to look up "all things astrophysical" about A=9 (or other light isobars)?

All the light isobars (group with same atomic number) are interesting and quirky, but let's use A=9 as an example. 9C and 9Li beta decay to particle unbound states in 9B and 9Be (except to 9Be's ground state), and those unbound states decay through both p/n + 8Be and 4He + 5Li/5He (which both the...

astrophysics stellar-astrophysics nucleosynthesis

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