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General chat for Physics SE (physics.stackexchange.com). For M...
yst 15:20
Okay thanks, I think I have to think about this some more
yst 15:18
The geodecics seem to exist when I don't have a metric
yst 15:18
@ACuriousMind But doesn't a general affine connection give rise to a geodesic? What exactly are these geodesics?
yst 15:15
Why would anyone consider these when they don't have the interpretation of the shortest line between two points
yst 15:15
If I were to take an affine connection say $\Gamma_A$ and another affine connection $\Gamma_B$, and I were to plot on the surface of the sphere two geodesics that arise from these two connections, they would in general be different.
yst 15:11
@ACuriousMind Let's stick to the sphere example. If I have different affine connections, will I get different geodesics?
yst 15:07
But how does one physically understand what these geodecics are unless the connection is the Levi-civita one?
yst 15:04
I then have a notion of geodesics
yst 15:02
So then say, if I have an affine connection
yst 15:02
This is before connections or metrics come along
yst 15:01
Say if I had a surface of a sphere. It is defined by an equation $x^2+y^2+z^2=R^2$.
yst 15:00
I think I see my issue
yst 14:59
By shape of a surface I mean say the surface of a hill
yst 14:58
The shape a surface takes is determined by the metric
yst 14:56
But in GR, I don't know what manifold I have. One uses the metric to determine the shape of the manifold don't they?
yst 14:54
As in say if I was starting from scratch and I only had access to what affine connection I was using. Could I determine the geometrical shape by embedding it in an $\mathbb{R}^n$ just by knowing the affine connection?
yst 14:52
What if I don't know what the shape is and I just have an affine connection?
yst 14:50
So if a gave you a general affine connection, one could in principle plot the manifold?
yst 14:47
@Slereah I'm just a bit confused about how the geometry of a surface in question isn't entirely determined by the affine connection chosen. It seems weird that one has geodesics arising from an affine connection, but one doesn't even know what shape they are talking about
yst 13:18
As in, does a generic affine connection give rise to some sort of geometry that I can in principle plot in Mathematica or something
yst 13:11
How does one visualise a manifold that just has an affine connection and not a metric?
Fri 18:17
I'm a big fan of performing personal 2 in to 3 out scattering experiments
Thu 20:50
Some people would call it the most normal
Thu 20:50
What is weird about hydrogen lol
Thu 18:15
It's difficult to define because I assume many people think they understand it when they just have a superficial understanding of tensor index gymnastics
Thu 18:08
@Interstellar It's difficult to define because there are different levels of understanding. But say enough to give a short course of introductory lectures confidently up to deriving the EFE.
Wed 19:29
@Slereah Just out of curiosity, how long did it take you to learn GR?
Jul 20 21:13
@RyderRude I’m currently reading it again. I forgot how many details it contained
Jul 20 16:29
@JohnRennie Have you read Dirac's biography titled The Strangest Man?
Jul 19 23:51
Well that's how he got sent two years forward
Jul 19 23:48
And it's more complicated because of WWI
Jul 19 23:48
I mean, I'm not going to copy and paste the whole biography...
Jul 19 23:40
@ACuriousMind Well in this case, it is useful
Jul 19 23:37
above
Jul 19 23:37
That was meant to be part of the text I wrote
Jul 19 23:35
This is all from Farmelo's biography
Jul 19 23:29
and he started his degree 2 years before normal age IIRC
Jul 19 23:29
He learned GR when he was an engineering undergraduate I think
Jul 19 23:29
That was when Dirac was just starting his PhD
Jul 19 23:28
" At that time, Cunningham and Eddington were streets ahead of the majority of their Cambridge colleagues, who dismissed Einstein’s work, ignored
it or denied its significance."
Jul 19 23:28
Well GR is substantially more difficult than E&M, and that would have been even more true back then
Jul 19 22:29
According to this biography, Dirac learned GR before E&M lol
Jul 19 21:59
I don't see how one cannot
Jul 19 21:54
@ACuriousMind What is your favourite period of physics?
Jul 19 21:51
@RyderRude Have you read Paul Dirac's biography by Graham Farmelo?
Jul 19 18:35
@ACuriousMind That representation for $SO(3)$ that Dirac came up with is incredible (I've only studied the $SO(3)$ part of that paper)
Jul 19 18:26
@RyderRude This (en.wikipedia.org/wiki/Infeld%E2%80%93Van_der_Waerden_symbols) was from 1933 so I imagine if was understood by then
Jul 19 00:36
Since IIRC, the relationship between $SL(2,C)$ and the Lorentz group was known then
Jul 19 00:35
Actually, I imagine things were understood fully by the 30s
Jul 19 00:34
Or maybe it was I'm not sure