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Feb
26
answered Is the accelerated expansion of the universe consistent with conservation of energy?
Feb
25
comment Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
another relevant paper I just googled: arXiv:0804.3595
Feb
25
comment Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
@СимонТыран: ultimately, Einstein ;) I do not have any convenient links lying around that make exactly the point I was trying to make, but you could look at arXiv:0808.1081 and meditate over the beautiful graphic in this answer by Pulsar
Feb
25
comment Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
@СимонТыран: the same thing it does when you cross a black hole's event horizon
Feb
25
comment Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
@СимонТыран: time will stand still - but not at the Hubble sphere, but the cosmological event horizon
Feb
25
revised Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
added 12 characters in body
Feb
25
answered Is it valid to apply Einstein's Relativity to scenarios involving expansion of space?
Feb
25
revised Intuition as to why the orientation (of a 3D object) is not a conserved quantity?
deleted 12 characters in body
Feb
25
comment Intuition as to why the orientation (of a 3D object) is not a conserved quantity?
@DonHatch: see edit
Feb
25
revised Intuition as to why the orientation (of a 3D object) is not a conserved quantity?
describe algorithm
Feb
25
answered Intuition as to why the orientation (of a 3D object) is not a conserved quantity?
Feb
19
awarded  Taxonomist
Feb
16
comment Why does it take a projectile as long to get to its apex as it does to hit the ground?
@DavidHammen: I'm slightly ashamed to admit it, but I actually did have to draw some pictures to convince myself that this is valid reasoning ;)
Feb
15
comment Why does it take a projectile as long to get to its apex as it does to hit the ground?
note to self: verify that ascent and descent actually do not take the same amount of time once you add drag...
Feb
15
revised Why does it take a projectile as long to get to its apex as it does to hit the ground?
added 18 characters in body
Feb
15
answered Why does it take a projectile as long to get to its apex as it does to hit the ground?
Feb
15
comment Does a symmetry necessarily leave the action invariant?
if we want to apply Noether's theorem, the infinitesimal transformation needs to leave the action invariant (up to a divergence term)
Feb
11
awarded  Nice Answer
Feb
8
comment It's established that universal energy is not constant. But is the net change positive or negative?
in some sense, the total energy of a homogeneous isotropic universe is well-defined: FLRW spacetime comes with a privileged set of observers, making Pirani's expression for energy well-defined; that energy is trivially 0 in any finite volume as the density vanishes identically, yielding a vanishing total energy as limit
Feb
8
comment Why does heat added to a system at a lower temperature cause higher entropy increase?
$\frac QT = \Delta S = \Delta(k \ln\Omega) \approx k \frac 1\Omega \Delta\Omega \Rightarrow \Delta\Omega\approx \Omega \frac Q{kT}$, ie transferring heat into a system opens up a new number of microstates $\Delta\Omega$ proportional to the number of existing ones and the number of degrees of freedom we can excite given by heat $Q$ divided by average energy per degree of freedom $kT$; cf physics.stackexchange.com/questions/33372/…