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Oct
16
comment How much would you weigh if Earth stopped orbiting the Sun?
@DavidHammen yes, I thought so to at first, that's why I answered both interpretations: freefall and held still, one after another. As to how it would be held still, that's not relevant; there are stranger unrealistic questions in introductory mechanics classes.
Oct
16
comment How much would you weigh if Earth stopped orbiting the Sun?
@DavidHammen no, what you're calculating are tidal forces. I interpreted the question as comparing the weight when the Earth is held still vs when it's orbiting, not at comparing different locations on the Earth. When it's held still, the Sun's $5.9\,\mathrm{mm}/\mathrm{s}^2$ is felt in addition to Earth's gravity (well, or subtractively, depending on location).
Oct
16
revised How much would you weigh if Earth stopped orbiting the Sun?
added 813 characters in body
Oct
16
answered How much would you weigh if Earth stopped orbiting the Sun?
Oct
16
comment What is the correct ratio of Newtonian to General Relativistic gravitational effects for Sun + single planet orbital system
David is of course correct in that this kind of comparison only makes sense for slow orbits in a weak-field approximation, though fortunately that's also the context of this question. It may be noted that for the specific case of Schwarzschild spacetime, orbits are exactly described by the effective potential; the approximation comes in when one treats the radial coordinate and proper time as if they were Newtonian, which is invalid in more general situations.
Oct
15
comment What is the correct ratio of Newtonian to General Relativistic gravitational effects for Sun + single planet orbital system
@steveOw yeah, I misread how the variables were defined too. Eh!
Oct
15
revised What is the correct ratio of Newtonian to General Relativistic gravitational effects for Sun + single planet orbital system
Rewrite; previous version based on silly misread of how variables were defined.
Oct
15
answered What is the correct ratio of Newtonian to General Relativistic gravitational effects for Sun + single planet orbital system
Oct
15
comment why do we look at dark energy as an energy?
You can apply the formula to find a mass density if you like, but the point was only that unlike the previous cases, you can't decompose dark energy into a collection of massive particles zipping about.
Oct
15
answered why do we look at dark energy as an energy?
Oct
14
comment Is it dark inside a black hole?
This is incorrect: "You wouldn't see the object in front of you since the light would not reach your eye." That would violate the equivalence principle; besides, a Penrose diagram of a black hole spacetime clearly implies that this is incorrect in without some substantial qualification.
Oct
14
answered Time period in which a planet rotates
Oct
13
comment How to leave the Universe
@PeterU the big bang happened everywhere, so the singularity is not even analogous to a single point in those cosmologies. When I said this is a perfect storm of misconceptions, I was being serious. Please stop, write out some of your assumptions, and research them (this site even had questions that addressed those very things!), because at this point we're digging a well with no end in sight.
Oct
13
comment How to leave the Universe
@PeterU: you're mistaken. In a flat or open FLRW cosmology, the universe is and has always been infinite. There is no contradiction whatsoever between that and the scale factor going to zero in the finite past (as required by Big Bang), because the scale factor is not necessarily the physical size of the universe. The essential characteristic is that the density diverges to infinity in the finite past; that says nothing about size. Sorry, but you keep on assuming one incorrect thing after another.
Oct
11
revised What is a singularity? What is at the center of a black hole? Specifically regarding space-time
Converted to MathJax + minor corrections (e.g., the Gullstrand–Painlevé chart of the Shwarzschild solution has flat space)
Oct
11
awarded  Custodian
Oct
11
reviewed No Action Needed Why objects are uniquely defined by their right ascention and declination?
Oct
11
comment How to leave the Universe
@PeterU: That implication is completely unjustified; it is quite possible for "everything that was formed from the 'Big Bang'" to be infinite. Additionally, there is no "void or Space" it is expanding into. There is absolutely nothing like that in Big Bang cosmologies at all.
Oct
11
comment Can Hubble red shift be interpreted as time dilation?
@Anixx diverging functions can have finite integrals, such as $\int_0^1 \mathrm{d}x/\sqrt{x}$. In both static coordinates and comoving coordinates, the result is finite. ... As for the rest, again, this Hubble sphere is not the entirety of the de Sitter universe. Not by a long shot. For analogous reasons as to why the external Schwarzschild metric is not the whole Schwarzschild spacetime.
Oct
11
comment Can Hubble red shift be interpreted as time dilation?
@Anixx that's a property of a particular time coordinate. Just like for a Schwarzschild black hole, things take an infinite amount of Schwarzschild time to cross, but a finite amount of proper time, or Kruskal–Szekeres time, or Gullstrand–Painlevé time, or Lemaître time, etc. The spacetime is quite well-defined beyond the horizon; it doesn't just end. ... Also, in this frame the Hubble volume is finite and static, $pi^2\alpha^3$, while in a comoving frame it's finite and expanding.