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5:29 AM
@Hotlab when we are using natural units we are in effect writing the values of physical quantities as multiplies of $G$ (and $c$ and $h$ and $e$ etc). But if we write some quantity as a multiple of $G$ then the error in our quantity can't be less than the error in $G$. Emilio is pointing out that the error in $G$ is large by the standards of metrology.
 
 
2 hours later…
8:10 AM
Are coordinates the dual of ordinates
 
 
2 hours later…
10:20 AM
contraordinates
 
10:49 AM
Will virtual particles be detected in the next LHC run?
 
@Avantgarde virtual particles can't be detected because they don't exist
 
@JohnRennie It was a joke!
 
Ah, OK, clearly my irony detector needs recalibrating :-)
 
Evidently
You need some lunch
 
11:25 AM
Even worse, I'm not even sure on-shell particles exist
Feynman had that whole argument that since to detect a particle you required some interaction vertex, and no particle in such a vertex is exactly on shell, none of the particles we actually witness are on shell
 
12:05 PM
@Slereah yes
re: abscissa and ordinate
 
Hm
Thinking about that GR thing
$\gamma(i), i \in \mathbb{N}$ is an infinite sequence, but then again so is $\gamma(i), i > k$
Hence for any neighbourhood $U$ of the limit point which contains the point $p_k$, there is a subset $U' \subset U$ which contains either $p_k$ or $p_{k'}$ such that $p_k \ll p_{k'}$
Hopefully this shall be useful
Though that doesn't mean that there's a timelike curve in $U'$ that joins the two, tho
bloody hell
 
12:42 PM
arxiv.org/pdf/gr-qc/9406053.pdf seems to have a more detailed proof
 
@Slereah I'd like to see two ordinates duelling with swords. Or banjos.
 
12:58 PM
@Slereah what now
 
Still thinking how to produce a liquid spacetime
Liquid spacetime:
 
@RyanUnger not much
Just thinking aloud
 
Define a metric such that it approximate a fluid density. It should be expected that like a river flowing downstream, the metric at each point in the spacetime manifold should structured in such a way such that there are vector fields carrying geodesics towards areas where the stress energy tensor is largest
actually... that does not sound very different from how ordinary spacetime behave at all, hmm...
Surely there should be a way to define something that is more generalised than frame dragging such that any matter placed in those region of spacetime will be carried elsewhere even when at rest, as if it is on a stream
and since the EFE are 4th order PDEs, there is more than enough complexity to produce eddies and vortices in spacetime without needing e.g. a black hole at the centre of those eddies
73
Q: Turbulent spacetime from Einstein equation?

ChamIt is well known that the fluid equations (Euler equation, Navier-Stokes, ...), being non-linear, may have highly turbulent solutions. Of course, these solutions are non-analytical. The laminar flow solutions (Couette flow for example) may be unstable to perturbations, depending on viscosity. ...

2
 
1:21 PM
@JohnRennie In physics.stackexchange.com/questions/488007/… The OP had already seen your dupe target, but it doesn't really explain "But what actually causes the reaction to go in the first place? It's like saying this ball will go down the hill because it will lose energy - what gives it the nudge required?"
In the comments, Cosmas mentioned tunneling, which sounds reasonable, and it certainly applies to alpha decay, but then:
Beta decay doesn't have anything to do with tunneling. — Ben Crowell 1 hour ago
I expect a good answer would at least mention W bosons, and why weak interactions are relatively rare. I would've written an answer myself, mostly because the answers on the new question are pretty bad, but I don't think I have the expertise to do it justice.
 
1:49 PM
@PM2Ring yes, I agree. I've voted to reopen though it will need four other reopen votes.
 
2:34 PM
@JohnRennie Thanks. I've also voted to reopen.
 
"Almost every nation on Earth has fallen under the yoke of tyranny -- the metric system. From Beijing to Buenos Aires, from Lusaka to London, the people of the world have been forced to measure their environment in millimeters and kilograms. The United States is the only major country that has resisted, but we have no reason to be ashamed for using feet and pounds."
"Esperanto died, but the metric system continues, this weird, utopian, inelegant creepy system that we alone have resisted."
 
@Slereah who wrote that
Th metric system is awful
 
Maybe we should write a GR book in Imperial Units
 
2:50 PM
A scheduled chat meeting is about to start
 
@AvnishKabaj it starts in an hour at 16:00 UTC. But the chat session has fallen into disuse anyway.
 
True true
 
$$G = 1.07 \times 10^{-9} \text{ft}^3 \cdot \text{lb}^{-1} \cdot s^{-2}$$
 
We can also say
It's always a chat meeting
 
Too bad the imperial system doesn't have any stupider unit for time
 
2:53 PM
Did you beat his ass???
 
Well he came back
So no need
How many barleycorn per second does light travel
 
I would say 3
 
that's a pretty big barleycorn
 
Nobody got the Bolzano-Weierstrass editor joke
Don't make real analysis jokes in a physics room is the lesson
 
Ravel's famous Bolzano
 
3:10 PM
You threatened me?
How dare you
 
A duel it is
 
3:28 PM
@Slereah Galois?
 
Yes
 
vzn
@Secret nice find! more fluid paradigm™! elements of this press release 2015 cited reminds me of a conversation in here with bolbteppa perimeterinstitute.ca/news/turbulent-black-holes
> Perimeter Faculty member Luis Lehner explains why it might make sense to treat gravity as a fluid. “There’s a conjecture in physics – the holographic conjecture – which says gravity can be described as a field theory,” he says. “And we also know that at high energies, field theories can be described with the mathematical tools we use to describe fluids.
> So it’s a two-step dance: gravity equals field theory, and field theory equals fluids, so gravity equals fields equals fluids. That’s called the gravity/fluids duality.”
 
3:48 PM
@RyanUnger I was in high school when Australia went metric. It was a bit annoying having to learn both systems, and doing numerous conversion exercises in junior maths classes, but it wasn't too bad. I tend to think in metric for most things, although I still think of people's heights in feet & inches.
 
The metric system is just another example of the European superiority complex
Feet and inches got us to the moon
 
The UK went metric when I was at school.
 
@JohnRennie is it true what they say about the Cambridge lawns
 
I have to confess I was not sorry to see the back of those how many ounces in x pounds questions.
@RyanUnger what do they say?
 
@JohnRennie I google that every time
@JohnRennie pristine and you’re not allowed to walk on them
 
3:51 PM
@JohnRennie can you describe me quantization of compact sympletic manifold
 
Lol
 
@RyanUnger And crashed the Mars Climate Orbiter ;)
 
@RyanUnger The Cambridge colleges have lots of money and the upkeep is generally excellent. Likewise Oxford of course.
 
@JohnRennie please, I couldn't get what quantization means.
 
My college, Peterhouse, didn't ban you from walking on the lawns. I don't recall other colleges having bans though I couldn't swear they didn't.
 
3:53 PM
@JohnRennie Princeton has a lot more money but the grass is shameful!
 
@akhilkrishnan no I can't. Next question?
 
The math department has a giant mud pit right out front
 
@JohnRennie Not to mention mixed base multiplication & division problems, especially using pre-decimal currency. Even though Oz went decimal in 1966, we still had some of those in our primary school text books, and our teacher thought it would be good mental exercise for us to know how to do such exercises.
 
@ john rennie how to avail scholarship to get in to german university. Does publishing a research article do the job?
 
@PM2Ring I guess it made sense if you were going to have to deal with those units all your life. Luckily I didn't :-)
@RyanUnger at least you have 100 cents to the dollar from the outset.
We had 12 pence to the shilling and 20 shillings to the pound sterling.
 
3:57 PM
Nice
 
I suppose it must have made sense to someone at some point.
 
I have found some positives about living here
$1.50 oysters at happy hour at a French place
And 10pm happy hour at a decent bar near campus
 
I've never been partial to oysters ...
 
I can eat dozens
 
One of my grandfathers was a carpenter by trade. He went to school in the country, in a school with one classroom. He had to leave school at the end of 6th grade to work on the farm. But he was good at mental arithmetic, and visually judging sizes. He could look at a rectangular board, estimate it's length & width, accurate to an eighth of an inch, and tell you its area in square feet & inches, down to the 1/64 of a square inch.
 
4:25 PM
I didn't have the heart to tell him that the error on (a+d)(b+d) is (a+b)d
 
4:53 PM
0
Q: How to add corrected URL when I can't comment?

u2berggeistLooking at this question: Change of system of coordinates for the stress matrix The comment by Samuel Weir (first one) links to a url that is is not displaying the equations (at least on all of my browsers). I found a link to the same basic page (written by the same author) that is displaying co...

 
5:33 PM
-1
Q: There should be an aerogel tag

user79161I asked a question about pertaining to Aerogel, and could find the tag for aerogel. Aerogel is an innovative technology, I mean before Kistler made it, who would have imagined that jellies were more than just a delicacy. Aerogel makes it mark in active research in the recent years. As Physics.S...

 
@PM2Ring So I finished Mort today. The resolution to the climax practically killed me. I remembered thinking early in the book 'what happens if you turn an hourglass over?'; but when death basically deus ex machina's the plot with it, it was hilarious
 
5:49 PM
Ugh
 
Is metric the one where $\hbar = c = 1$
 
6:27 PM
Nope that’s imperial
 
I use natural units in math
$\pi = e = i= 1$
 
@JMac It's a bit messy, but it works. ;) Wait 'til you read Reaper Man...
A few weeks ago on xkcd I said that in Fahrenheit and Rankine, between the freezing and boiling points of water there are 180° or π radians.
@SirCumference i++ doesn't really make sense in Python, because Python variables are just nametags, they aren't containers that hold values of a specific type. If you haven't read it yet, you should take a look at nedbatchelder.com/text/names.html
Also, Guido originally did not want assignment in Python to be an expression, because in C that can quickly lead to convoluted unreadable messes. OTOH, with Python 3.8 we get assignment expressions, which has been a rather controversial addition to the language.
@JohnRennie That beta+ decay question is now open, and I just posted an answer. Any feedback & constructive criticism will be appreciated. ;)
 
7:16 PM
Oh great. I just got a critical comment from JD. I guess I can consider it as an endorsement. ;)
This doesn't help, because it introduces a W boson. The W boson is said to have a mass of 80.379 GeV/c². Saying it's only a virtual particle that's "off shell" doesn't get us anywhere. — John Duffield 5 mins ago
 
@PM2Ring Both you and rob seem to have written correct answers that don't actually answer the question. The OP already accepts that the resulting nucleus has lower energy than the nucleus we start from (even if they sort-of seem as if they don't in the 2nd paragraph).
Instead, they seem to be imagining that there is an intermediate state with higher energy in between the nucleus needs to tunnel through (look at their picture!)
To answer the actual question one needs to a) explain whether or not there actually is a potential barrier between the two nucleus states (there probably is) b) explain that quantum processes are not stopped but just dampened by such barriers, i.e that they can tunnel through them.
 
7:35 PM
@ACuriousMind Thanks for the feedback. The core of my answer is:
> However, if the energy conditions of the nucleus are suitable, there's a small probability that the W boson isn't re-absorbed but instead decays.

Whether the energy conditions are suitable depends on the configuration of the nucleus.
Frankly, I was hoping someone else would write an answer. I only wrote mine because I'm unhappy with the answers by Michael W & Arpad S
 
@PM2Ring Apart from commiting the cardinal sin of speaking as if Feynman diagrams depicted actual processes of emission and absorption, I think OP is mainly confused that a "suitable" energy condition is one like in the picture, where the final state of the nucleus has less energy than the initial state but there's no "downhill" way in between.
 
I didn't mention tunneling because Ben C said it wasn't involved in beta decay, but he's declined to write an answer himself.
 
That would be what I said in a) - if there's no barrier, then that's also an answer. But I think neither showing there's no barrier nor talking about tunneling doesn't actually answer the question as posed
 
I know that weak processes have a low probability, and that's modeled by the high masses of the weak bosons. But I'd love to read an answer by someone who understands this stuff better than I do. :)
 
8:15 PM
just a small thing to ask. when should one consider deleting a post?
 
@user79161 Assuming you mean the aerogel post physics.meta.stackexchange.com/questions/11353/… I wouldn't worry about it. "Bad" meta questions can still be a reference for someone; and I don't see much benefit in removing it.
 
yes, it is that one. thanks:)
 
 
2 hours later…
10:43 PM
Hilarious quotation from this question:
> First of all, I'm a mathematician so please try to use coordinate-free notations.
3
 

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