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ZeroTheHero
ZeroTheHero
12:13 AM
@Archer Correlation is not useless: if there's no correlation between two "events" it's unlikely that one is the cause of the other. The other away around doesn't work though.
 
ZeroTheHero
ZeroTheHero
12:33 AM
Causation is actually really hard to prove. Just tell the tobacco company: not every smoker gets lung cancer, and not everyone getting lung cancer is a smoker. However by careful use of control groups one can eliminate alternative causes.
 
 
4 hours later…
Jack Rod
Jack Rod
4:17 AM
energy spectrum of universe
 
 
2 hours later…
Sir Cumference
Sir Cumference
6:36 AM
@ZeroTheHero that x axis bothers me
35000 > 45000 > 20000
 
 
3 hours later…
Slereah
Slereah
9:26 AM
The issue of proving causation is that the universe is never in the same state anyway
So how can you tell!
You don't know if the time, the color of the experimenter's shirt or whatever is happening in the nearby galaxy is influencing the experiment
 
 
1 hour later…
Jack Rod
Jack Rod
10:44 AM
@JohnRennie hi
 
John Rennie
John Rennie
@JackRod hi :-)
 
Jack Rod
Jack Rod
What is the central force in physics?
 
John Rennie
John Rennie
A central force is a force that always points towards the same point in space.
For example consider the gravitational force from a sphere. This force always points towards the centre of the sphere, so it is a central force.
 
Jack Rod
Jack Rod
OK what is the torque about this point
if I have a regular shape body
 
John Rennie
John Rennie
Do you mean something like a rectangular shaped body orbiting a point mass?
And you're asking if the central force can create a torque on the rectangular body?
 
Jack Rod
Jack Rod
10:49 AM
yes
can we expect any torque from it
 
John Rennie
John Rennie
The central force cannot exert a torque on the body i.e. the torque on the body due to the central force is zero.
The central force just produces a net force acting through the centre of mass of the orbiting body.
 
Jack Rod
Jack Rod
ok that's where I have the confusion actually our friend asked what is a central force I said a force which act along the centre of the body
 
John Rennie
John Rennie
I can see the scope for confusion, because there are two "centres" - the centre of the planet and the centre of the orbiting body.
Gravity is a bit special because mass only comes with one sign i.e. all masses are positive.
It is possible for a central field to exert a torque. For example consider the field created by a point charge. This is a central field like the field created by a point mass.
But if you put an electric dipole in the field there will be a torque on the dipole.
 
Jack Rod
Jack Rod
hmm!
 
JingleBells
JingleBells
11:05 AM
@ACuriousMind It turns out I actually absorb more and I learn better from reading. I'm just lazy most of the time so I refer to videos (which quickly give me a vague idea of the topic). But when I sit down and I really focus on a book (article, or even Wikipedia) I grasp the concepts faster and more in-depth. So I agree with you - reading is better for learning because you set your own pace to learn at and unlike with videos, you dive deep into the topic.
I watch videos when I'm lazy, tired, and need a quick intuition/introduction to a topic. But when you need a deep understanding of something, it's better to get a book, sit down and focus. After 15 minutes you get into a state where you've lost track of time and you absorb the info like a sponge.
 
JingleBells
JingleBells
11:37 AM
 
ACuriousMind
ACuriousMind
12:01 PM
how did you think a casino makes profit? :P
 
Emilio Pisanty
Emilio Pisanty
0
Q: Why do atoms oscillate at absolute zero?

The RadioactivesI have heard that average kinetic energy of atoms is directly proportional to the absolute temperature . Then why don't atoms come to a stop at absolute zero(T=0k).

atomic-physics atoms cold-atoms
presumably a duplicate?
 
ACuriousMind
ACuriousMind
 
Charlie
Charlie
12:41 PM
On a manifold there's no requirement that every element of the topology have at least one chart map right? The only requirement is that the charts provide an open cover for the manifold
This is mostly motivated by manifolds with curvature singularities, then again I'm pretty sure I've asked a related question before and the answer was more advanced than I could understand
 
ACuriousMind
ACuriousMind
@Charlie What do you mean by "every element of the topology"? Open sets? There's usually lots of open sets at aren't connected and hence don't have charts on them since they can't possibly be homeomorphic to $\mathbb{R}^n$.
 
Charlie
Charlie
Yeah I meant open sets, that makes sense too ty, I guess an example would be non-simply connected Lie groups?
Uh actually I don't see why disconnected open sets can't be homeomorphic to $\Bbb R^n$, we can define disconnected open sets in $\Bbb R^n$ which means the charts can be continuous
 
ACuriousMind
ACuriousMind
@Charlie No, an example would already be just two disjoint open balls in $\mathbb{R}^n$
 
Charlie
Charlie
What property of the homeomorphism fails on them?
 
ACuriousMind
ACuriousMind
$\mathbb{R}^n$ is a manifold, and the union of an open ball at $p_1$ and an open ball at $p_2$ is an open set that's disconnected if we choose the radius small enough
@Charlie homeomorphisms preserve connectedness properties
 
Charlie
Charlie
12:56 PM
Does it not count if the connectedness property to be preserved is non-connectedness?
 
ACuriousMind
ACuriousMind
what's your definition of a chart?
my definition says it goes between some open set on the manifold and some open ball in $\mathbb{R}^n$
 
Charlie
Charlie
Just a homeomorphism to $\Bbb R^n$
 
ACuriousMind
ACuriousMind
@Charlie right, and $\mathbb{R}^n$ is connected
 
Charlie
Charlie
Is the union of open balls in Rn not again open?
 
ACuriousMind
ACuriousMind
so you can't get a chart from some disconnected set to $\mathbb{R}^n$.
 
Charlie
Charlie
12:58 PM
oh my god the chart maps are surjective
 
ACuriousMind
ACuriousMind
@Charlie sure it is
@Charlie they're even bijective!
that's what being a homeomorphism means
 
Charlie
Charlie
that literally hadn't even occurred to me
oh man that actually changes a lot
i'm so used to this kind of image
it corrupted me
because it doesn't look surjective so it never even crossed my mind
well, ty :P
 
ACuriousMind
ACuriousMind
it's surjective onto the subsets of $\mathbb{R}^n$ :P
 
Charlie
Charlie
oh wait I wasn't going crazy
 
ACuriousMind
ACuriousMind
some people define a chart to be to some open ball in $\mathbb{R}^n$ instead of the whole (every open ball is homeomorphic to $\mathbb{R}^n$ so this doesn't really matter)
 
Charlie
Charlie
1:00 PM
ughhh
oh I actually didn't know that last fact
I assume the reasoning is then since isomorphisms are transitive if $U\subseteq \mathcal M$ is homeomorphic to some subset of $\Bbb R^n$ then it must be homemorphic to all of $\Bbb R^n$ and therefore has to be connected since $\Bbb R^n$ is
 
ACuriousMind
ACuriousMind
it's simple to see - choose polar coordinates centered at the center of the ball, the ball has some radius $R$, the map $(r,\phi,\theta)\mapsto (r/(R-r),\phi,\theta)$ is the homeomorphism
@Charlie not all subsets of $\mathbb{R}^n$ are homomorphic to $\mathbb{R}^n$, this is specific to the open ball
 
Charlie
Charlie
-oh
 
ACuriousMind
ACuriousMind
a closed ball is also a subset of $\mathbb{R}^n$ but not homeomorphic to $\mathbb{R}^n$ - the closed ball is compact, $\mathbb{R}^n$ is not
 
Charlie
Charlie
I don't suppose by some miracle the codomain of chart maps is always an open ball is it?
oh but all open subsets of $\Bbb R^n$ are homeomorphic to the open ball?
 
ACuriousMind
ACuriousMind
@Charlie Why "miracle"? You make that part of the definition
@Charlie no, I already gave the disjoint union of two open balls as an example :P
 
Charlie
Charlie
1:14 PM
Ok everything makes sense now so far, the only strange part is that disconnected open sets on the manifold can still be validly homeomorphic to disjoint open balls in $\Bbb R^n$, but I guess it's fine if we exclude them as part of the definition of the charts
 
Ratman
Ratman
I thnk you should check that also other topological properties like connectedeness or simple connectedness are shared by the two sets, if at least one is not shared there can't be a homeomorphism
as a general approach, not to this particular case
 
Charlie
Charlie
Yeah they're called topological invariants or something right?
I wasn't actually aware that connectedness and simple connectedness were distinct properties lol, it's been a while since I purposly read about topology
 
Ratman
Ratman
yes exactly
 
shelton Benjamin
shelton Benjamin
can i ask questions
 
Charlie
Charlie
1:30 PM
The rule is usually don't ask about asking, just go for it
 
shelton Benjamin
shelton Benjamin
if we give a explanation using concept of com then since the com is moving down so ball should also move down so it perform oscillatory moytion
but is there a explanation without concept of com
also why motion of com is oscillatory
@Charlie when i see such discussion i think its better to ask
 
Slereah
Slereah
2:30 PM
@Charlie It's not, but there always exists a chart of a manifold such that everything is homeomorphic to a ball
it is sometimes more convenient to use such a chart
it's easy enough to prove since every open set has open balls as a basis
 
Charlie
Charlie
I don't understand that first message, when you say "everything is homeomorphic to a ball" what does "everything" mean? All open sets on M?
What I gathered from the discussion earlier was mostly that disconnected open sets in M don't have a chart map
I assume that even if there are open sets containing disconnected components of M we can still define a chart on each disconnected component separately
 
Slereah
Slereah
I'm saying that, given a manifold $M$, there exists a collection of charts $\{ (U, \phi) \}$ such that, for every such $(U, \phi)$, $\phi(U)$ is a ball
 
Charlie
Charlie
oh
 
bolbteppa
bolbteppa
3:31 PM
"A good way to approach the subject is the way Sophus Lie did himself. A Lie group is a group with continuous (or smooth) parameters. We convert the associativity of the group law into a differential equation, and study the integrability of that differential equation."
 
Ratman
Ratman
3:52 PM
It is kind of amazing what Lie did
In calculus, we learned about the Taylor or power series. Taylor said that if we gave him
all the derivatives of a function f (x) at x = 0 (say), he could construct the function. In
contrast, Lie said that, thanks to the multiplicative group structure, he only needs the first
derivative of the group element R(θ) near the identity.
a quote from Zee i really like
 
John Duffield
John Duffield
4:43 PM
@bolbteppa : sorry, what paper?
 
bolbteppa
bolbteppa
The Born-Infeld one you linked to
 
John Duffield
John Duffield
@bolbteppa : no, but I'd say they were saying spin is a real rotation. Hans Ohanian wrote a paper about that in 1984, see staff.fnwi.uva.nl/m.renzo/materials/WhatIsSpin.pdf
 
bolbteppa
bolbteppa
It seems like they are saying a couple of things like that, it would be interesting to read someone's analysis of it
 
John Duffield
John Duffield
He referred to Frederik Belinfante's 1939 paper On the spin angular momentum of mesons.
@bolbteppa : what puzzles me is that Einstein worked with Leopold Infeld on the nature of particles, but didn't use anything from the Born-Infeld model. There was a peper they wrote together with another guy. Hang on, I'll dig it up...
The paper is The Gravitational Equations and the Problem of Motion
It's all about point-particles. I've tried to research Einstein's unification work but I haven't really gotten anywhere with it.
@AcuriousMind: what's your policy on referring to Sci-Hub?
 
bolbteppa
bolbteppa
5:02 PM
22
Q: What is the prevailing opinion in scientific community about Hans C. Ohanian's description of spin?

Charudatta ManwatkarIn the paper What is spin?, Am. J. Phys. 54 (1986) 500, by Hans C. Ohanian, spin is described as a circulating flow of energy in the wave-field of a particle. Is this the generally agreed upon explanation of intrinsic angular momentum or just a fringe theory? (A similar thread exists on Reddit,...

quantum-mechanics angular-momentum quantum-spin
 
Sir Cumference
Sir Cumference
when Euler lost his $i$ he couldn't $\mathbb{C}$ anymore
 
Azmuth
Azmuth
@Charlie lol
@SirCumference lmao!
 
John Duffield
John Duffield
@bolbteppa : I'd say Anna's answer gives the prevailing opinion. The electron is treated as a point-particle, even though the evidence of the Davisson-Germer experiment and the diffraction experiments by George Paget Thomson and Andrew Reid proved the wave nature of matter, which is why de Broglie got his Nobel prize.
I think it's worth reading about Larmor precession: en.wikipedia.org/wiki/Larmor_precession
 
bolbteppa
bolbteppa
Well I still don't really know what's going on with this really, it certainly looks like a contradiction for every QM book to say spin has no classical analogue then to see classical EM having something they call spin
 
John Duffield
John Duffield
As far as I can tell Schrodinger plus others like de Broglie and Charles Galton Darwin proposed a realistic electron in the 1920s, but the Copenhagen guys who were their rivals promoted Yakov Frenkel’s point-particle electron. The Copenhagen guys "won".
 
bolbteppa
bolbteppa
5:11 PM
If you don't assume they are point particles, nothing makes sense, it breaks special relativity to try to model them as rigid bodies because a 'push' on one part takes time to propagate (because of relativity, it's instant to Newton) to the other parts so it simply can't be a rigid body, if they are not rigid bodies they are made of 'stuff' and there's no reason why you can't zoom in to find out what the stuff is. The only way to bypass this is string theory
 
John Duffield
John Duffield
I think it only makes sense if you don't model them as point particles. Read that Wikipedia article on Larmor precession. If you know about the gyroscopic precession of a boomerang, you can liken that to the circular motion of an electron in a uniform magnetic field.
 
bolbteppa
bolbteppa
Belinfante–Rosenfeld stress–energy tensor
In mathematical physics, the Belinfante–Rosenfeld tensor is a modification of the energy–momentum tensor that is constructed from the canonical energy–momentum tensor and the spin current so as to be symmetric yet still conserved. In a classical or quantum local field theory, the generator of Lorentz transformations can be written as an integral M μ ν = ∫ d 3 x...
 
Azmuth
Azmuth
@bolbteppa Oh yeah. I totally forgot this one, thanks for reminding me :)
 
bolbteppa
bolbteppa
So the question is, if the Belinfante tensor always has a 'spin' term, even for classical fields, why doesn't this immediately mean the whole concept of spin arising as a quantum concept in the representation theory is wrong
 
John Duffield
John Duffield
@bolbteppa : I learn something new every day. Thanks.
I don't know the answer to qyour question I'm afraid. But I do like this from Feynman: “Suppose we take the example of a point charge sitting near the center of a bar magnet, as shown in Fig. 27–6. Everything is at rest, so the energy is not changing with time. Also, E and B are quite static. But the Poynting vector says that there is a flow of energy, because there is an E × B that is not zero. If you look at the energy flow, you find that it just circulates around and around..."
I have to go I'm afraid. Time for a glass of vino with the Mrs.
 
JingleBells
JingleBells
6:08 PM
You know how we boil water to turn it into steam so it rotates a turbine that produces electricity? Why don't we go to Mount Everest and boil the water there for free? :D
 
Faded Giant
Faded Giant
🙄
 
JingleBells
JingleBells
😎
 
skillpatrol
skillpatrol
🤔
 
JingleBells
JingleBells
6:27 PM
Hmm, instead of covering a 100x100 meter area with solar panels, why don't have one really strong and heat resistive solar panel in the middle and one big lens on top of it?
 
skillpatrol
skillpatrol
🙄
 
JingleBells
JingleBells
😎
 
 
2 hours later…
ACuriousMind
ACuriousMind
8:35 PM
@JohnDuffield We prefer no direct links to copyright-violating material
 
Charlie
Charlie
9:09 PM
I can't imagine the answer being no :P
 
skillpatrol
skillpatrol
thanks for your imaginative comment :P
but I don't think now is the time
2 hours later...
user image
4
:-)
 
Physics Meta
Physics Meta
9:59 PM
0
Q: How can I get the order of question in a search to be by date?

BuzzI frequently so a search with the following: [cosmology] answers:0 . I would like the questions ordered by the date they were asked with the most recent first. How can I make this happen?

feature-request
 
 
1 hour later…
peterh - Reinstate Monica
peterh - Reinstate Monica
11:00 PM
@skillpatrol Right, but in programming, you can fix the code before release. There is no way to fix a book if it is already printed.
 
peterh - Reinstate Monica
peterh - Reinstate Monica
11:29 PM
@ACuriousMind Ok, calculating absolute energy density is problematic in QFT. But, actually not this is what I want to calculate. I want to calculate the difference between the energy density of my pet higgs field and the vacuum state. Does it not make things easier?
 
Stupid question inc
Stupid question inc
How was this explaination derived in first paragraph
why F is that way?
I know it's from the limit but doesn't seem obvious to me
I mean as the limit of distance goes to infinity the force becomes approximately same as the total mass of that bounded mass system
but how you derive this?
sorry I am in hurry cya
 

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