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ZeroTheHero
ZeroTheHero
12:15 AM
@Stupidquestioninc Use Gauss’ law. As long as the density depends only on $r$ AND if you are at $R>r$ then the statement holds.
 
 
4 hours later…
John Rennie
John Rennie
4:26 AM
@Stupidquestioninc see Newton's shell theorem
 
shelton Benjamin
shelton Benjamin
5:22 AM
.
 
 
3 hours later…
Stupid question inc
Stupid question inc
7:54 AM
thanks guys
 
Jack Rod
Jack Rod
8:30 AM
Can anyone suggest me a good book on Atomic and Molecular orbital physics, requirements -I mean the book should be easier to comprehend and have started from basic to a decent level
 
Azmuth
Azmuth
9:20 AM
Hey! I got my new phone Today, it's Realme C12! :-)
 
John Rennie
John Rennie
You got the text and url the wrong way round.
 
Azmuth
Azmuth
Messed up!
 
John Rennie
John Rennie
It's [link text](url)
 
Azmuth
Azmuth
ummm.... some problem
doesn't appears correct
Yay!
@JohnRennie I recieved Realme C12 today, (4G)
 
John Rennie
John Rennie
Cool :-)
 
Azmuth
Azmuth
9:23 AM
I'm still trying out to figure out how to add pattern lock...
It's very confusing, a lot of weirdo UI Animations and stuff...
 
John Rennie
John Rennie
I'm trying to convince myself I should buy a Oneplus Nord, but my Oneplus 3 continues to work just fine even though it's now over three years old.
 
Azmuth
Azmuth
6000mAh Battery, it's coolio....
 
John Rennie
John Rennie
It looks like a really nice phone.
 
Azmuth
Azmuth
@JohnRennie 3 years isn't too old, I usually prefer to stretch my phone upto 5 years
yep :) Helio G35 Processor!
 
John Rennie
John Rennie
At some point the battery will wear out, and the battery isn't user replaceable in the Oneplus 3.
 
Azmuth
Azmuth
9:25 AM
You need to go to showroom for it. Now a days, no phone has user replacable batteries
My father uses VIVO, my mother uses Mi & iPhone 11, I use C12, none are user replacable
@JohnRennie I got the phone for just 7000 INR
That's lit!
Festival offfer
It does has fingerprint sensor and all those stuff, but, I've never tried that...
I'm confused which hand's which finger to use for fingerprint lock..... :O
 
peterh - Reinstate Monica
peterh - Reinstate Monica
9:38 AM
Anybody has an idea, why my this answer got its second down?
0
A: Could an infinite number of photons fit in finite space?

peterh - Reinstate MonicaNo. Because we are in a fixed space, it gives an upper limit to the wavelength of the photons. Having an upper limit to the wavelength, we also have a lower limit to their energy, thus to their mass. Having infinite particle with a lower mass bound, would mean that we have infinite mass in a fini...

 
John Rennie
John Rennie
9:49 AM
@peterh-ReinstateMonica your answer makes the point that you couldn't build up an infinite number of photons in a box while keeping the energy finite, e.g. by combining photons hf, hf/2, hf/4, etc because at some point the photon wavelength would be too big to fit in the box.
But this seems a bit academic since the real point is that you can't have infinite energy.
I would guess the downvoters (which don't include me) thought your answer missed the point.
 
peterh - Reinstate Monica
peterh - Reinstate Monica
10:20 AM
@JohnRennie "Because we are in a fixed space, it gives an upper limit to the wavelength of the photons."
 
John Rennie
John Rennie
Yes, that's true. But it's not really the point.
 
Slereah
Slereah
11:10 AM
You could have an infinite number of photons
Simply by having photons of rapidly decreasing frequency
which is basically what happens for the infrared divergence
But since those photons aren't measurable it's not really an issue
 
peterh - Reinstate Monica
peterh - Reinstate Monica
You can not have a photon in a fixed volume where its wavelength is bigger than the average size of the volume.
 
Slereah
Slereah
true
I guess it can only be at best a half wavelength of the box
 
peterh - Reinstate Monica
peterh - Reinstate Monica
Imho this is a bikesched, my answer was correct and I refuse do delete the answer. I am considering its improvement to make it better visible, but honestly I dislike bikescheds, so I choose the leave it as it is.
 
 
2 hours later…
Ankit
Ankit
12:58 PM
@JohnRennie can you help me with the comment in your this answer physics.stackexchange.com/a/260997/271783
??
 
Charlie
Charlie
1:16 PM
What is the minimum information that uniquely labels a quantum system? Is it just the Hamiltonian? It can't just be the state space since if it's infinite dimensional it's isomorphic to all others
 
ACuriousMind
ACuriousMind
@Charlie It's the algebra of observables.
 
Slereah
Slereah
@ACuriousMind You can have the same algebra for two different quantum systems
You need to also define states I think
 
ACuriousMind
ACuriousMind
it depends a bit but yes, a narrow meaning of "system" is an algebra of observables + a choice of irrep of it
 
Slereah
Slereah
Also I think it's a good idea to single out the Hamiltonian, too
Wouldn't two systems with different Hamiltonians in non-relativistic QM have the same algebra?
 
ACuriousMind
ACuriousMind
@Slereah No, since the commutators of that Hamiltonian with the other observables would be different
 
Slereah
Slereah
1:26 PM
But how would we know that it is the Hamiltonian?
For any system, both $p^2$ and $p^2 + V(x)$ are observables
 
Charlie
Charlie
There always seems to be precious little information about "algebras of observables" when I search that phrase online
 
ACuriousMind
ACuriousMind
@Charlie the technical term is more like "$C^\ast$-algebra formalism"
and yes @Slereah you're probably right that you need to mark one of the observables as the Hamiltonian
 
Charlie
Charlie
AH nice that's given something, ty
 
Slereah
Slereah
From what I can tell Reed defines a QFT as the Hilbert space, the rep of the symmetry group (so including the Hamiltonian for time), the field operator and the vacuum state
I don't know if they're all mandatory but that's how they define it at least
 
Slereah
Slereah
1:48 PM
If you want the nlab way I think it's supposed to be whatever this is :
$$\mathrm{PolyObs}(E_{\text{BV-BRST}})_{reg}[ [\hbar] ] \otimes \mathrm{PolyObs}(E_{\text{BV-BRST}})_{reg}[ [\hbar] ] \overset{\star_{\tfrac{i}{2}\Delta}}{\longrightarrow} \mathrm{PolyObs}(E_{\text{BV-BRST}})_{reg}[ [\hbar] ] $$
 
Ankit
Ankit
Does Pauli exclusion principle indicate the existence of an extremely great force ?
 
Slereah
Slereah
Do you mean the degeneracy pressure
 
Charlie
Charlie
If you mean something like a "fifth fundamental force" then no
 
Ankit
Ankit
Why ?
How do we explain Pauli's principal with any of the four fundamental forces ?
 
Charlie
Charlie
Because it doesn't manifest in the same way the other forces do
 
Ankit
Ankit
1:54 PM
Which way ?
 
Charlie
Charlie
in quantum field theory
idk if you'll get a much more satisfying answer without knowing more about that
 
ACuriousMind
ACuriousMind
Also, this xkcd is eternally relevant when talking about the fundamental "forces" :P
3
 
JMac
JMac
2:15 PM
I love the hover text on that one.
 
danielunderwood
danielunderwood
2:36 PM
That's pretty much exactly what I was told in my intro physics course
 
John Rennie
John Rennie
2:52 PM
@Ankit The adhesive force is proportional to the area of contact, and the area of contact is (approximately) proportional to the normal force.
 
Ankit
Ankit
@JohnRennie but normal force is repulsive and adhesion needs attraction .Right ?
 
John Rennie
John Rennie
@Ankit if you touch two perfectly flat clean surfaces together then they will bond to each other. The bonding energy per unit area is a constant - it's a property of the material - so the total bond energy is proportional to the area of contact. OK so far?
 
Ankit
Ankit
@JohnRennie but doesn't that bonding need an attraction ?
 
JMac
JMac
I don't know if you would call normal force "repulsive" necessarily. It's more restrictive than repulsive. It prevents solid objects from moving through one another, but it doesn't actually like repel them like a magnet or anything. The surfaces can still get close enough to form bonds and deform, just not enough to actually pass through each other.
 
John Rennie
John Rennie
@Ankit the two surfaces bond to each other because the atoms in each surface bond to the atoms in the other surface.
After all, the solid is only a solid because all the atoms in the solid bond together. So when you touch the two surfaces together the atoms bond just like they do in the solid.
If you're interested, this phenomenon can be used to join pieces of metal together:
Cold welding
Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion/heating at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no liquid or molten phase is present in the joint. Cold welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. Newly discovered micro- and nano-scale cold welding has already shown great potential in the latest nanofabrication processes. The reason...
 
JMac
JMac
3:02 PM
Closest example to cold welding I've really seen was gauge blocks, but even those are really cool IMO
 
John Rennie
John Rennie
@JMac Really clean glass cold welds.
 
Ankit
Ankit
@JohnRennie okay . That's clear. But my question is that doesn't the bonding need an attraction ?
 
John Rennie
John Rennie
@JMac If you draw a glass sphere over a glass surface, both have to be ruthlessly cleaned, then you get a trail of fractures where the glass has welded and been torn apart by the motion. I have seen this with my own eyes! :-)
 
JMac
JMac
@JohnRennie Is that why chemistry glassware is often a bit rough at the joints instead of like smooth?
 
Ankit
Ankit
Just like in ionic bonds there is a net attractive forces . Then what attractive forces are acting between asperities ?
 
John Rennie
John Rennie
3:04 PM
@JMac No, ordinary glassware is nowhere near clean enough to get cold welding. It's just cheaply made glassware :-)
@Ankit the same forces that make a solid solid.
 
Ankit
Ankit
@JohnRennie are you talking about ionic forces ?
 
John Rennie
John Rennie
Solids are solid because the atoms in the solids attract each other and bond together to form the solid. Without those bonds we'd have a gas not a solid.
@Ankit there are many types of bonds in solids. In metals we have metallic bonds.
 
JMac
JMac
@JohnRennie Ah fair enough. I figured there might have been some risk with chemistry stuff if it was smooth and being shoved into each other, since it would be fairly clean surfaces to prevent contamination I would think. Does the glass have to have more uniform chemical properties, or just a highly prepared surface?
 
John Rennie
John Rennie
Metallic bonding
Metallic bonding is a type of chemical bonding that rises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be described as the sharing of free electrons among a structure of positively charged ions (cations). Metallic bonding accounts for many physical properties of metals, such as strength, ductility, thermal and electrical resistivity and conductivity, opacity, and luster.Metallic bonding is not the only type of chemical bonding a metal can exhibit, even as a pure substance...
@JMac see:
134
A: Why doesn't water actually perfectly wet glass?

John RennieIn everyday life glass surfaces are always covered by a layer of, well, crud. Glass surfaces are exceedingly high energy surfaces due to the high density of polar hydroxyl groups and they attract pretty much anything. This means that outside of a colloid science laboratory you will never encounte...

 
JMac
JMac
@JohnRennie Oh cool. I think I read that answer before actually. Do you have to do it in a vacuum chamber, or just work with a limited timeframe until the surface gets contaminated again?
 
John Rennie
John Rennie
3:14 PM
In a glove box you have a few minutes before it starts getting dirty again.
 
JMac
JMac
I figured there must be a glove box involved somewhere; I just couldn't picture it any other way. (also probably important given the potential glass shards and chemicals involved)
 
 
1 hour later…
Emilio Pisanty
Emilio Pisanty
4:22 PM
1
Q: How does a black hole capture an approaching star and force it to start an elliptical orbit?

Janko BradvicaLet's assume that a standing still black hole attractor can have a star orbiting it in an orbit if the star was somehow a result of the same past proccesses as the black hole.So the star must be on a given point with right velocity and direction to be in an elliptical orbit around the BH.If the s...

gravity black-holes orbital-motion stars
c'mon, let's make this the next HNQ
it's been ages since I've made the rep cap ::grin::
 
Ratman
Ratman
4:37 PM
good evening
to everyone
 
Charlie
Charlie
evening
Is there a reasonable explanation for why the different components of the Lorentz group are homeomorphic?
 
ACuriousMind
ACuriousMind
didn't we talk about this already?
Oct 12 at 17:47, by ACuriousMind
Further, all the components of a Lie group are just "copies" of the identity component, i.e. there is some element $g_i\in G$ such that $G_i = g_i G_0$ for $G_0$ the identity component and $G_i$ some other component.
 
Charlie
Charlie
ah yes
ty :P
 
Prateek Mourya
Prateek Mourya
5:36 PM
Can we apply binomial expansion on infinitesimal s
 
Ankit
Ankit
5:57 PM
@EmilioPisanty what does that mean ?
Can anyone present here explain me Pisanty's statements ?
 
JMac
JMac
Translation: Lets make this a hot network question (i.e. goes on the main stack exchange site getting more attention). I haven't gotten the reputation cap (+200 in a day) in a long time.
 
Ankit
Ankit
@JMac how does a question become HNQ ?
 
JMac
JMac
An algorithm that determines "hotness" based on a bunch of factors like number of upvotes, number of answers, upvotes on answer, etc
 
Ankit
Ankit
@Jmac so it looks like politics !!!!
So is it all upon you guys whether you want to make a question HNQ ?
 
ACuriousMind
ACuriousMind
for details on the algorithm, see meta.stackexchange.com/q/60756/263383
 
JMac
JMac
6:08 PM
@Ankit No not at all, I think Emilio was excited about a question they thought was good and figured giving it a little help would be nice. It kinda makes sense since HNQ questions can be a pretty poor representation of the site when it does it's own thing.
 
Ankit
Ankit
I think he was being too greedy for reputations. He knew that he has answered correctly and thus tries to make it HNQ for getting more upvotes ?
Had he not answered the question or the question was a little bit difficult or say out of his understanding then would have he done the same thing ?
 
ACuriousMind
ACuriousMind
@Ankit It was a joke - no one here can just "make" that question HNQ
 
Ankit
Ankit
@ACuriousMind it doesn't seem so !!!!
 
JMac
JMac
Emilio really just wants rep to max out a bunch of times (I don't think he cares about the actual rep number). It was just a playful comment about that. I don't think it's worth scrutinizing him.
 
Ankit
Ankit
I thought this site is a proper site for knowledge sharing but now I think there is politics here too 🤔😡
 
ACuriousMind
ACuriousMind
6:14 PM
@Ankit Though Emilio probably really wanted to say that would be nice question to go HNQ, he mostly certainly isn't motivated by "greed", and that's a rather uncharitable thing to say. Look at his amount of answers and the amount of bounties he's offered, do you really think he needs to rely on a few random upvotes from chat to bolster his reputation?
 
JMac
JMac
Yup. I'm a Canadian but for some reason the deep state hired me to spread my political malice on this site.
 
ACuriousMind
ACuriousMind
(and, but that's beside the point, of course there are politics here - we just call it Physics Meta :P)
 
JMac
JMac
The best kind of politics. Fielding random complaints and having arguments over often trivial aspects of the site.
 
ACuriousMind
ACuriousMind
6:36 PM
@JMac These joints are actually called a ground glass joint and I think it's just to increase friction so that the joints have a tighter grip
 
Jake Rose
Jake Rose
Is there a generalisation of the kronecker delta like $\delta^\sigma_\rho \delta^\mu_\nu = \delta^{\sigma \mu}_{\rho\nu}$?
 
ACuriousMind
ACuriousMind
I'll bet you find some people writing it like that but I think most people just write your l.h.s.
 
Jake Rose
Jake Rose
Would mathematicians be angry if I did the rho?
rhs?*
it feels super obvious to write it like that
 
Sir Cumference
Sir Cumference
7:06 PM
being a physicist means ignoring the angry mathematicians :P
8
 
bolbteppa
bolbteppa
7:34 PM
Kronecker delta
In mathematics, the Kronecker delta (named after Leopold Kronecker) is a function of two variables, usually just non-negative integers. The function is 1 if the variables are equal, and 0 otherwise: δ i j = { 0 if i ≠ j ,...
Generalization is discussed there
 
 
1 hour later…
JMac
JMac
8:57 PM
Anyone here ever play the puzzle video game Antichamber?
 
ACuriousMind
ACuriousMind
9:28 PM
I seem to remember watching a making-of of it but I didn't actually play it :P
 
JMac
JMac
It's pretty cool. It had a nice use of weird spatial mechanics, like an infinite staircase that went away if you turned around. I was reminded of it because I just started playing Manifold Garden, and it takes that element to the next level with really cool infinite repeating areas. I'm really liking it so far
 

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