Reputation
3,969
Top tag
Next privilege 4,000 Rep.
Access 'trusted user' tools
Badges
1 4 19
Newest
 Enlightened
Impact
~20k people reached

18h
reviewed Close Why did Kepler Telescope's Reaction Wheels fail?
18h
reviewed No Action Needed What is the axial tilt of a planet measured relative to?
18h
reviewed No Action Needed What is the axial tilt of a planet measured relative to?
1d
comment How is expansion different from contraction?
@punkerplunk On the first paragraph, I'm not sure what you mean that it's were 'space is contracting'. If you look at, say, the Schwarzschild-de Sitter solution (basically, simplest black hole with a repulsive cosmological constant), then space is expanding and accelerating inside the black hole similarly to how it is on the large scale: the volume of a blob of test particles initially at rest would have a positive second derivative.
1d
comment How is expansion different from contraction?
@punkerplunk If the length of the meter-stick changed between epochs while free-propagating light did not, that's observationally the same as the (red/blue)shift. If your measurement is (wavelength)/(meter-stick), it's entirely up to you to decide which 'really changed' as long as the ratio stays the same. It would stupidly over-complicate the rest of physics to say 'everything else' shrunk, so it's fairly ridiculous to do that. Anyway, if you want to talk about any details, feel free to hit me up in chat, since here it's a side-comment that got out of hand.
1d
comment How is expansion different from contraction?
@pela of course it would. My statement was trivial, too trivial to be useful: anything expanding by a finite factor is observationally equivalent to everything else contracting by the same factor, because we only measure ratios. This includes redshifts: the expansion of any wavelength can equally well be interpreted as the contraction of every meter-stick (e.g., atom) used to measure it. ... Though as I said, interpreting things this way is both very impractical and useless.
2d
reviewed No Action Needed Can we spot a gray-goo exoplanet?
2d
comment How is expansion different from contraction?
I think your question is too vague as it stands. I will say that it's consistent to interpret universal expansion as the typical large-scale intergalactic distances staying the same and everything else contracting. Highly impractical for various reasons, but consistent. ... But I don't know what phrases like 'manifestation of dimension' are supposed to mean in this context, or much of what follows.
2d
reviewed Close Why didn't we have inflation when the theory of everything and GUTS broke symmetry?
2d
reviewed Close Dull, Red, Slow Moving, Meandering Meteors?
Jul
3
comment Does the sun move
If you're wearing a shirt and you move, then of course the shirt moves. I think your confusion stems from the fact that motion doesn't make sense without (at least implicit) reference to something else to compare it to. You don't move with respect to yourself, and the shirt doesn't move with respect to you. ... You should specify something the Sun may or may not be moving with respect to (the Milky Way center, the Earth, etc.). If you don't, the question is just silly.
Jun
28
comment How does gravity interact with a photon?
If you want to talk about evidence, the most accurate and precisely tested theory of light and its interactions with matter is quantum electrodynamics, in which photons are absolutely massless. Mind, the actual experimental limits of photon mass are $m_\gamma< 10^{-18}\,\mathrm{eV}$, but it remains the case that being absolutely massless doesn't prevent interacting with our eyes, so your basic facts are incorrect, and you should probably take your own advice.
Jun
25
comment If nothing can pass the schwarzschild radius, how can black holes even start?
The short answer is that it just isn't true that nothing passes the event horizon, and this 'our point of view' is just the fact that we can use coordinates that aren't defined at the horizon. This is essentially a duplicate of Does matter accumulate just outside the event horizon of a black hole?.
Jun
25
comment Why Do Planets Revolve Faster When they Are Closer to Their Parent Star?
@QPaysTaxes no, it does not. Conservation of energy is generated by time symmetry, while conservation of angular momentum by rotational symmetry. Nothing in this answer addresses Kepler's second law at all. Those things are logically independent of one another, and indeed it is possible to imagine a conservative force without angular momentum conservation, e.g., any non-radial conservative force will do.
Jun
23
answered How does gravity interact with a photon?
Jun
5
answered Question about extreme space distortion and creation of a new dimension
Jun
5
answered Why is dark energy preferred to the cosmological constant?
May
3
comment On a log-log plot of surface gravity to planet mass, what is the meaning of the y-intercept?
That $4k_2$ should be $2k_2$ in my comment (typo).
May
3
comment On a log-log plot of surface gravity to planet mass, what is the meaning of the y-intercept?
@RobJeffries The OP is far-extrapolating gas giants, so there $\rho$ is also strongly dependent on $M$--but that shouldn't change the correspondence to mean density for any particular fixed $M$, which was actually OP's question as stated. Really, though, the OP's extrapolation is physically inappropriate, and for light exoplanets ($M\lesssim 4$ in units of $R_\oplus = M_\oplus = 1$), Seager et al. (2007) $$\renewcommand{\lg}{\log_{10}} \lg R = k_1 + \frac{1}{3}\lg{M}-k_2M^{k_3}\text{,}$$ and so $\lg g = \frac{1}{3}\lg M - 2k_1 + 4k_2M^{k_3}$.
May
3
comment On a log-log plot of surface gravity to planet mass, what is the meaning of the y-intercept?
Shouldn't that be $g = (\tfrac{2}{3}\sqrt[3]{6\pi^2})(G\sqrt[3]{\rho^2M})$? Note that the OP's question is actually mostly about $\log M\to 0$, but otherwise, I agree, the most straightforward physical thing it corresponds to is mean density, fit to the data.