Has anyone ever participated in a private beta before?
2 hours later…
user434058
4:58 AM
@ACuriousMind @AaronStevens @JMac @DavidZ @peterh-ReinstateMonica Thank you for your response!
user434058
@JMac The time due for ending the suspension is the same on both the sites. And it is very unlikely that both the suspensions would have occurred at the same time. Moreover I cannot dig up any inappropriate activity by the user on Chemistry SE (which is expected, because all the mess would have been cleared). So I suppose this was a case of very closly timed suspensions on both the sites.
I was reading about about QFT from daily physics magazine I have a doubt, if the universe behaves in a way that contradicts the predictions of QFT with weak gravity (extremely unlikely), or if information is destroyed by black holes (almost inconceivably difficult to test, even with Planck energy accelerators) or by mapping the landscape, and showing it doesn't match our universe (incredibly computationally infeasible).
@YuvrajSingh... whether QFT can be applied to gravity at all is unknown.
In weak gravitational fields we can write down a quantum field that we think would work as an approximation. This is what we call an effective field theory.
The question is whether it is useful to try and describe gravity using an effective quantum field theory. And the simple answer is that no-one knows.
@YuvrajSingh... the reason we do this is that we understand QFT very well, so if we can show that gravity can be described by a QFT then that means we understand quantum gravity.
But whether this is going to work or not no-one knows at the moment.
@YuvrajSingh... the attempt to write gravity as a quantum field is called asymptotic gravity:
Asymptotic safety (sometimes also referred to as nonperturbative renormalizability) is a concept in quantum field theory which aims at finding a consistent and predictive quantum theory of the gravitational field. Its key ingredient is a nontrivial fixed point of the theory's renormalization group flow which controls the behavior of the coupling constants in the ultraviolet (UV) regime and renders physical quantities safe from divergences. Although originally proposed by Steven Weinberg to find a theory of quantum gravity, the idea of a nontrivial fixed point providing a possible UV completion...
@JohnRennie :54032223 What we solve Field Equations, what we solve exactly? :P I wrote Stress Energy Tensor, but, now I dun know what to solve $g$ or $R$
I forgot that there are two other tensors apart from that...
For example in Schrodinger's equation, we solve, $\psi$ then what we solve here?
We don't really solve the field equations in the sense that you think of solving a simple linear differential equation. Typically they are solved by guessing the general form of the metric using properties like symmetry.
Then once you have the general form you can write the Ricci tensor and scalar and write down the ten equations. Finally play with them until you manage to guess the exact form of the metric.
@AbhasKumarSinha Google for the derivation of the Schwarzschild metric as this is a good example. The spherical symmetry restricts the form the metric can take so we can guess a starting point that we can take forward.
The Schwarzschild solution describes spacetime under influence by a non-rotating massive spherically-symmetric object. Of the solutions to the Einstein field equations, it is considered by some to be one of the simplest and most useful.
== Assumptions and notation ==
Working in a coordinate chart with coordinates
(
r
,
θ
,
ϕ
,
t
)
{\displaystyle \left(r,\theta ,\phi ,t\right)}
labelled 1 to 4 respectively, we begin with...
@JohnRennie Well, more computationally inclined physicists do solve them like that! It's just not what you tend to see in a textbook or what you could do by hand. Don't fall prey to theorist myopia ;)
I was about to say I don't know of any analytic solutions that were arrived at by directly solving the ten independent equations, but solving with an ansatz is a direct solution really so I'm not sure my statement is fair.
I assume numerical solutions are done that way, but I confess I know nothing about the way numerical GR is done.
My guess is that numerical GR is mind manglingly complicated.
All numerics is pretty complicated, honestly, unless you can use one of the solvers that already exist out of the box for your particular problem
The crucial achievements there are materialized not only in papers (the algorithms used) but also in the software libraries that actually implement them
In classical mechanics we have a concept of generalized coordinates.
Say my generalized coordinates are (x,y).
My doubt is ,Is it legal to write the position vector in any vector basis say polar basis but having components which are functions of x, y and then use the Lagrangian equation?
Auch, it seems like I have misunderstood the method I am implementing for my Master's thesis, and now the programming I have done the last two months is kind of useless
lol that reminds me back in 3rd year when I tried to do exactly that, and I end up spending the next 3 hours looking up how to find the eigenvalues of a tensor and such thing does not really exists
@SirCumference That's a secret... don't tell anyone... That Astrophysics and Cosmology are same.. but it's physicist's propaganda to make them look different.
$\text{wut}_{\alpha\beta\delta}^{\sigma\phi}T^{\beta_{\sigma}}_{\delta} = F^{\varnothing^{\varnothing}}_{\varnothing^{\alpha}}$ is my response to the above comment
@PM2Ring I think once upon a time moderators were able to merge accounts on their own, but they removed that feature after one merge too many between accounts that didn't actually belong to the same person and thereby exposed PII of one user to another
@AbhasKumarSinha I'm saying it is analogous - just like EM waves propagating in vacuum without charges are allowed solutions of Maxwell's equations, gravitational waves propagating in vacuum without mass is also an allowed solution of Einstein's equations. You need charge/mass to generate the waves, but not for them to propagate.
It is possible to associate a mass with the curved spacetime. This called the ADM mass. But pinpointing exactly where this mass is located is impossible.
(note that a common variant of Schwarzschild is that the solution is valid up a certain radius and then you have a massive body sitting there instead of going all the way to the event horizon)
That is, the actual solution for a massive body that's not a black hole is Schwarzschild in the exterior of the massive body, and something else for its interior.
@AbhasKumarSinha the common way out if this is to say that the mass is located at the singularity, though note that while this makes intuitive sense it is mathematically meaningless.
In general relativity, a vacuum solution is a Lorentzian manifold whose Einstein tensor vanishes identically. According to the Einstein field equation, this means that the stress–energy tensor also vanishes identically, so that no matter or non-gravitational fields are present. These are distinct from the electrovacuum solutions, which take into account the electromagnetic field in addition to the gravitational field. Vacuum solutions are also distinct from the lambdavacuum solutions, where the only term in the stress–energy tensor is the cosmological constant term (and thus, the lambdavacuums...
@JohnRennie You will agree that $R_{\mu \nu} - \frac12 R g_{\mu \nu}$ on RHS is purely a mathematical thing... nothing to do with reality... it's abstract... But, where does Gravity entered here?
atleast it's derived without Gravity into consideration...
They say that gravity is technically not a real force and that it's caused by objects traveling a straight path through curved space, and that space becomes curved by mass, giving the illusion of a force of gravity.
That makes perfect sense for planetary orbits, but a lot less sense for the expr...
The Schwarzschild geometry is an idealised black hole. It wouldn't perfectly describe a real black hole but it would be an excellent approximation to a real black hole.
An electron would have a Schwarzschild radius smaller than the planck length, so it's unclear whether GR could describe the geometry near such a small object.
Basically it shrinks to a point, but the problem is that the geometry becomes undefined for a point mass, so GR can describe the evolution as long as the radius is greater than zero by even the tiniest amount, but it cannot descrbe what happens when the ball radius becomes zero.
The Schwarzschild geometry is the $r=0$ limit of the OS geometry, but with the point $r=0$ removed. i.e. it covers the whole spacetime except for the point $r=0$.
In the OS geometry the mass is obviously collapsing towards $r=0$, so in the limit it makes sense to say all the mass is in a point mass with infinite density at $r=0$.
So by analogy we'd say a Schwarzschild black hole with a mass M has that mass as a point mass at $r=0$. But the point $r=0$ is excluded from the Schwarzschild metric, so where then is the mass?
@user1271772 Area51 is currently basically dead and it's highly unlikely that SE will launch any new sites, cf. meta.stackexchange.com/a/344244/263383. If you want to start to follow proposals there and expect anything to come of it you're probably several years too late.
@user1271772 Recently not. A51 was always disliked at the company. They did not update it since a decade. Meanwhile, they've continuously said, that they can't update it, because it is so old. ;-) In theory, the A51 works, but only very rarely are new sites created ( stackexchange.com/sites#newest )
@user1271772 Btw, it is a like a beta, the differences: 1. rep levels are much lower than even on the betas 2. the site is invisible on the stackexchange.com
@user1271772 On the Engineering SE, I created the first question. :-) I could see a completely empty SE site! For some seconds only. It was clear that I must create a question on the spot, so I could not create a screenshot.
@user1271772 But the private beta phase lasts only for some days, and then we lose the wonderful privileges that we got.
@user1271772 Btw, the also the Physics SE was created on the A51 by Tobias Kinzler. He is a physicist from Switzerland.
Now the SE has no idea, what could they do to this whole network. My impression is, that if they could change the past, they should have not ever created the SE network et al. Or they had created only IT-related SE sites.
Maybe superuser, serverfault would exist, but no more. At the time, I've read from some SE bosses a very strong urge that "we don't divide the SO", i.e. they view the site network as the SO, with some extras.
@FakeMod It's actually the subject of the 11th highest voted meta thread on physics meta. physics.meta.stackexchange.com/questions/10581/… Featuring an answer by the author of the books himself.