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12553
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location Austin, TX
age 34
visits member for 3 years, 9 months
seen 13 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.


15h
comment Which of these two textbook equations of geodesic deviation is correct?
@Peter4075: that's the best thing to do when something isn't clear. Do it the long way, and then make sense of the short way argument about it.
15h
comment Axioms behind entropy!
@CuriousOne: yes, yes. Obviously, that's true. But you need both. That's all anyone is saying, rather than it all just being experiment. Blind experiment is pointless stampkeeping, blind theory is delusional ignorance.
19h
comment Who foots the (magnetic) energy bill?
@AlanSE: but, of course, you know that's not what's going on -- you move some charges, and then exert a force on those moving charges.
19h
comment Who foots the (magnetic) energy bill?
Also, energy and magnetism is generally a subtle thing, and you seem to be confusing force and work in your analysis. I think this question is too broad and changeable in interpretation for you to get an answer that you will be satisfied with.
20h
comment Who foots the (magnetic) energy bill?
@bobie: because this is chasing a moving target.
20h
comment Who foots the (magnetic) energy bill?
Magnetic forces cannot make stationary objects move. the force is proportional to the velocity of the object.
20h
comment Which of these two textbook equations of geodesic deviation is correct?
@Peter4075: they're two copies of the SAME set of four functions.
1d
answered Work done by gravity on a ball & the ball on earth
1d
comment Axioms behind entropy!
@CuriousOne: clearly both are required. And if something is predicted before it's observed, then blind experimentation wouldn't have done much. And there have been many things that theorists came up with that experimentalists wouldn't have thought of on their own (and yes, many more things that experimentalists found that theorists were not expecting)
1d
comment Axioms behind entropy!
@CuriousOne: you're being facetious. First, maybe something else would have been there. Second, theory can't predict the mass of the higgs, even if it was a near certainty that the Higgs was going to be there. Third, science certainly has a place for inductive and deductive reasoning. That's the whole process. YOu use induction to build a model, then you try to find a way to derive that model from first principles. Then, you make new, as to yet untested predictions from that theory, and see if you can break it. When you break it, you inductively build a model, ad infinitum.
1d
comment How far has a black hole to be in order for its tidal forces to disintegrate earth?
In fact, a ten solar mass neutron star would be a lot more destructive, because then, you'd have that NS supernova immanently happening. :)
1d
revised Which of these two textbook equations of geodesic deviation is correct?
deleted 4 characters in body
1d
comment Which of these two textbook equations of geodesic deviation is correct?
@Peter4075: all that's going on with that is the fact that the Riemann tensor is defined by $\nabla_{a}\nabla_{b}\omega_{c} - \nabla_{b}\nabla_{a}\omega_{c}$. It's an arbitrary choice whether you put the fourth index that you contract on $\omega$ first or last, so this could be either equal to $R_{abc}{}^{d}\omega_{d}$ or $R^{d}{}_{abc}\omega_{d}$. Using the properties of the Riemann Tensor, you can juggle indices around, and show that these two definitions are equal to each other, except that they differ by a minus sign.
1d
comment Which of these two textbook equations of geodesic deviation is correct?
@JohnRennie: yes, but there is an additional convention on the Riemann tensor, depending on whether $\nabla_{[a}\nabla_{b]}\omega_{c} = R_{abc}{}^{d}\omega_{d}$ or whether $\nabla_{[a}\nabla_{b]}\omega_{c} = R^{d}{}_{abc}{}\omega_{d}$. But looking at these two expressions again, the second one is wrong, as it is always identically zero.
1d
answered Which of these two textbook equations of geodesic deviation is correct?
1d
comment Which of these two textbook equations of geodesic deviation is correct?
There are two different conventions for the Riemann tensor, and the tensors can differ by a minus sign between the two conventions. Without telling us which version the two books are using, this question, as written, is unanswerable, since the two expressions you give also differ by a minus sign.
Aug
26
comment Can a human size object move so fast that it ceases to be observable?
@AdamDavis: well, you could also make an argument that there is a finite recession speed where all incident visible light will be reflected back to a viewer at a redshifted frequency too low to be visible anymore.
Aug
26
revised A curious case of Relativistic Velocity Addition
added 2 characters in body
Aug
26
comment A curious case of Relativistic Velocity Addition
@lurscher: my first draft had broken latex, I fixed it, and then you edited. We must have been concurrently editing it.
Aug
26
comment A curious case of Relativistic Velocity Addition
@lurscher, you broke my latex