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 gravity
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Apr
20
reviewed Approve What is the volume of the universe?
Mar
30
answered gravitational time dilation multiple sources
Mar
26
revised Does matter accumulate just outside the event horizon of a black hole?
A reference on history + timeline correction and elaboration
Mar
24
comment Initially non-flat space-time makes dark matter obsolete$\dots$
What does "in a clean way" mean specifically? In the usual cosmology, the initial conditions could be chaotic and heterogeneous--but would be smoothed out due to inflation (which is necessary because large-scale homogeneity and isotropy are observed facts). And of course on the galactic scales on which dark matter is most important, the spacetime curvature is already quite heterogeneous. So I'm rather unclear about what you're trying to do...
Mar
23
comment Would time go by infinitely fast when crossing the event horizon of a black hole?
@ctrebor sorry, that's wrong--or rather, the implications you're drawing from them are wrong.
Mar
20
awarded  gravity
Mar
19
awarded  Enlightened
Mar
19
awarded  Nice Answer
Mar
16
comment Why is space and time spoken of as one thing
@JohnDuffield if all "moving through spacetime" means is that a worldline can be parametrized by time (which is what I content), then it's not only harmless in understanding relativity and completely consistent with any 'block universe' viewpoints, but also just plain trivially true.
Mar
16
comment Why is space and time spoken of as one thing
If a particle has a spatial position $\mathbf{x}(t)$, we can say that the particle moves in space, even though it also has some curve as its trajectory over its entire history (e.g., an ellipse for a Keplerian orbit). Similarly, in spacetime, we can have $x^\mu(\lambda) = (t(\lambda),x(\lambda),\ldots)$, so what's the problem in saying it moves in spacetime, even though it also has a worldline in spacetime over its entire history? (I agree with Ben Crowell regarding speed, but this more general point seems to needlessly insisting on forbidding something totally harmless and straightforward.)
Mar
15
answered Why is space and time spoken of as one thing
Mar
14
answered Does conservation of energy make black holes impossible?
Mar
14
comment What causes Earth's gravity?
"mass is caused by ... the Higgs boson" is at best a partially true oversimplification, and qualifying it enough to make it correct would make it nigh-irrelevant to gravity, because only a tiny amount of the mass of ordinary objects is due to the Higgs. It's of dubious relevance anyway, because the gravitational charge is energy rather than mass.
Mar
4
answered Distinction between metric expansion and objects just moving apart from each other?
Feb
29
revised Does the accelerating expansion of spacetime mean that the pace of time is changing?
small correction + clarification
Feb
28
comment Does the accelerating expansion of spacetime mean that the pace of time is changing?
@LocalFluff The particle horizon is the boundary of the region that the observer could receive signals from by some specific time, e.g. present. It's basically the observable universe in principle (the practically observable universe is smaller, of course). The event horizon is the boundary of the region that observer could send a causal signal to, even if one waits forever. In some sense they're the opposites of each other. Both of them are also different from the Hubble sphere, the place at which recession velocity is $c$ from the observer.
Feb
28
answered Does the accelerating expansion of spacetime mean that the pace of time is changing?
Feb
28
comment Does the accelerating expansion of spacetime mean that the pace of time is changing?
Pace of time--relative to what? You can compare two different clocks to each other (e.g., as in time dilation), but it's very unclear what comparison you're trying to make here.
Feb
27
answered Why does mass naturally move closer toward's the center of other masses?
Feb
21
reviewed Close What is the current state of gravity in the vacuum of space?