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location Austin, TX
age 35
visits member for 4 years, 7 months
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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.


2d
comment Is Einstein still alive? If so how can I talk to him?
physics.stackexchange.com/search?q=causality
2d
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?
2d
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?
2d
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.
2d
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.
2d
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
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.
Jul
1
comment Closed timelike curves in the spin-2 gravity formalism
The causality of what? I don't see what you're doing. If you're considering the geodesics of the entire metric, then you have literally changed nothing. If you're trying to consider the dynamics of something other than the geodesics of test particles, I'm also a bit at a loss.
Jul
1
comment Closed timelike curves in the spin-2 gravity formalism
The CTC's show up in the geodesic equations of particles. The gravitational field itself does not evolve non-causally (or at all) in this metric.
Jul
1
comment Gravitational time dilatation calculation
@RandyWelt, the schwarzschild radius for a Kerr black hole is not $2M$, it is the largest root of $r^{2} + a^{2} - 2Mr =0$.
Jun
30
comment What is the velocity of Sun due to Earth alone?
The average speed of the earth is easy to figure out from knowing what the length of a year is. If you're using realistic numbers and units, the motion of the sun will be very slow, and possibly chasing numerical error depending on how careful you're being with your doubles and floats.
Jun
29
comment Why is gravity so hard to unify with the other 3 fundamental forces?
@Kitchi: there IS something that works out for electromagnetism, but you quickly run into a fundamental problem. General relativity works geometrically because gravity obeys the equivalence principle -- the old classic observation that a bowling ball falls at the same rate as a ball bearing. the other forces all have different charges for different objects. If they are "geometric forces", why is this?
Jun
26
comment What is the meaning of this definition of potential energy?
@Caneholder123: aaaaaaah, I think I understand what you're misunderstanding. $\mu$ is a placeholder for $x$, $y$, or $z$ in that second equation. It is not a particle index. $F_{\mu} = - \nabla_{\mu}U$ is shorthand for $ F_{x} = \frac{\partial U}{\partial x}$, etc.
Jun
26
comment What is the meaning of this definition of potential energy?
@Caneholder123: yes. You're taking the derivative with respect to one configuration variable,w tih the other five held constant.
Jun
25
comment Classical trajectories that are not a minimum of the action
and I'll admit that this is a snarky response that technically answers the OP's question, but is kind of unsatisfying.