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Apr
16
comment Our universe the surface of a 4-dimensional sphere?
(v1) Could you clarify: how would your question not be answered by "there is a speed limit in the universe, $c$, so any signal you get from anything far away has been emitted in the past"? This is true regardless of the shape of the universe, so I'm confused as to why you bring it up.
Apr
16
comment Our universe the surface of a 4-dimensional sphere?
The microwave background is uniform and there is no center in any of the four spatial geometries of the Friedmann-Robertson-Walker family of solutions. The sphere is just one of those. The universe is very close to flat as far as we can see, so there's no strong reason to believe it's a sphere (and it might now be an FRW on the very large scale).
Apr
11
comment total amount of energy in the universe
possible duplicate of Total energy of the Universe
Apr
9
comment Why “Dark Energy” is called energy instead of force?
@MHK: Thinking a bit naively, if the box increases its volume by $\Delta V$ and it still has the same energy density $\rho$, then energy content increased by $\rho\Delta V$. But the pressure remained constant, so the work done is $p\Delta V$, where $p$ is the pressure. If $\rho = -p$, as it is for a cosmological constant, energy increase is balanced by negative work. I say "naively" partly because "the total amount of energy in the Universe" is (usually) not a well-defined quantity in GTR in the first place.
Apr
9
answered Why “Dark Energy” is called energy instead of force?
Mar
27
reviewed Approve suggested edit on Does leaning (banking) help cause turning on a bicycle?
Mar
27
reviewed Approve suggested edit on PSF Measurements In Fluorescence Imaging
Mar
27
reviewed Approve suggested edit on If you put a latex balloon in a vacuum, how much would it expand?
Mar
27
reviewed Approve suggested edit on how does a helicopter get forward thrust?
Mar
27
reviewed Approve suggested edit on Experimentally Verifying a Clock's Accuracy
Mar
27
reviewed No Action Needed S. Weinberg, “The Quantum theory of fields: Foundations” (1995), Eq. 9.2.15
Mar
25
reviewed Approve suggested edit on If I bury a cylinder (top end sealed) in sand, how much force do I need to pull it out?
Mar
24
reviewed Reopen Doesn't the Schwarzschild metric combined with Hawking radiation imply that nothing ever gets past the event horizon of a black hole?
Mar
24
reviewed Leave Open Aerodynamics and thermodynamic
Mar
23
comment Why is the Hodge dual so essential?
@ZachMcDargh: Yes and no. It's not needed for Maxwell's equations, but the info it incorporates is necessary for EM to be fully predictive--$H\propto\star F$ of standard EM relates $(\mathbf{E},\mathbf{B})$ with $(\mathbf{D},\mathbf{H})$, which is necessary, and as an important side-effect, connects the light cone with the causal structure of spacetime. In H&O's book, they investigate a general $H_{ab}\propto\kappa_{ab}{}^{cd}F_{cd}$ and some of the assumptions on $\kappa$ for which the light cone even exists, and for which which metric could be derived from it, up to some conformal factor.
Mar
22
answered Why is the Hodge dual so essential?
Mar
19
reviewed Approve suggested edit on Current Carrying Coil - Torque
Mar
19
reviewed Approve suggested edit on Can the two electromagnetic field tensors be combined into a more general tensor?
Mar
15
answered Is there a fourth component to the electric field and magnetic field?
Mar
15
comment Why does the speed of light $c$ have the value it does?
This is mistaken; $c$ does not determine electric permittivity and magnetic permeability. Electromagnetism is described by two constants; if you don't wish to either of $\epsilon_0,\mu_0$ and you take $c = 1/\sqrt{\epsilon_0\mu_0}$ to be one of them, then a natural choice for the other would be the impedance of free space $Z_0 = \sqrt{\mu_0/\epsilon_0}$. So 19th century physics moved from three parameters to two, not three to one.