21,662 reputation
13471
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location Austin, TX
age 35
visits member for 4 years, 8 months
seen 8 hours ago

I am a Ph.D. general relativist working as a software engineer. I like to still go and do physics as a hobby, and to keep up my skill and knowledge.


14h
comment Is the Universe rotating?
@ACuriousMind: if the universe were asymptotically flat, we could say "the universe rotates if the universe has a net ADM angular momentum"
1d
awarded  Nice Answer
Jul
3
awarded  Curious
Jul
2
comment Is Einstein still alive? If so how can I talk to him?
physics.stackexchange.com/search?q=causality
Jul
2
comment Calculating euler number of disk
$t^{a}\nabla_{a}t^{b}$ is a vector. you could write this as $\left({\vec v}\cdot {\vec \nabla}\right){\vec v}$. How on Earth are you working through Polchinski if this is over your head?
Jul
2
comment Calculating euler number of disk
@user238194: for your first question, I'd project your first expression into tangent and normal components. For your second question, what is the boundary of a hemisphere? What do you know about geodesics on spheres?
Jul
2
comment Calculating euler number of disk
Is your manifold a disk in 2-dimensional flat space? For a d dimensional submanifold of a D dimensional space, you generically have $D-d$ normal vectors and $d$ tangent vectors.
Jul
2
comment Calculating euler number of disk
@user238194, not if $n_{a}$ is the unit outward normal. In 2d, this is $n_{a} = (x/r)dx + (y/r)dx$, which does not have zero derivative.
Jul
2
answered Calculating euler number of disk
Jul
2
comment Do metric theories with torsion contradict solar system observations?
@CuriousOne: that's actually kind of my point. Measurements like gravity probe B and just ordinary solar system constraints should put constraints on the size of the torsion tensor, which explicitly causes non-geodesic motion for spinning particles.
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
@CuriousOne: they're GR because the dimension of the manifold is an input into the theory.
Jul
2
awarded  Tag Editor
Jul
2
wiki created modified-gravity description
Jul
2
wiki created modified-gravity excerpt
Jul
2
asked Do metric theories with torsion contradict solar system observations?
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
@CuriousOne: physical initial conditions don't produce results that violate causality. They just don't. Compact hidden dimensions ARE GR. Is Einstein-Cartan consistent with known solar-system tests, given the spin-orbit coupling that Torsional theories have?
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
@CuriousOne: show me a contradiction with GR, or show me an alternate theory that is capable of replicating all of the high precision tests of GR. People have worked on alternatives, they all jsut don't work. And people know that the theory will have to break down eventually, how and where is the problem. You're arguing about a strongman.
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
@CuriousOne: sounds like a great reason to understand it, considering that it's dramatically better than anything else we have. You're seriously arguing against a strawman here. Everyone understands that you have a big, fundamental problem with the quantum gravity thing and with singularities. No one has a solution to that problem, though. And there are lots of beautiful hints embedded in what we already know about the theory, much less what we might dig out.
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
And Slerah, I think I get what you're saying now, and my answer is that your underlying Lagrangian doesn't have a global Lorentz symmetry, so, why should you expect your theory to have one? My guess is that if you try to perturbatively do a time evolution on a system like this, you're going to eventually just get a caustic that causes the evolution to stop.
Jul
2
comment Closed timelike curves in the spin-2 gravity formalism
@CuriousOne: You can get nonsensical solutions with crappy initial conditions in Newtonian mechanics, too. Unphysical toy models can be great tools for understanding physical models, at the end of the day, anyway.