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Mar
24
awarded  Fanatic
Mar
16
comment In theory, is there anywhere in the universe where velocity=0?
To be fair to the pre-Einstein folk, the principle of relativity goes back to Galileo, and plenty of people didn't agree with Newton that space was absolute. (And for Newton himself, Galilean invariance of physics is actually a theorem in Principia, but he thought that space must be absolutely anyway for conceptual reasons.) Note that having space relative is enough for the answer to this question to be morally the same, even if time was thought to be absolute.
Mar
15
comment Is there a connection between black holes and dark matter/energy?
I suppose technically some MACHOs could be black holes, but certainly not supermassive ones, and MACHOs need to be lighter than astrophysically reasonable black holes in order to explain an appreciable fraction of dark matter anyway.
Mar
13
comment visualisation of the universe's expansion
+1, but the loaf analogy is no less accurate in general; it's just different. For the case of an infinite flat FRW universe, an infinite loaf is better than the balloon. However, the balloon analogy is more accurate finite positive-curvature FRW universe. ... So it depends on what cosmological model you're trying to describe.
Mar
11
revised Looking for help in understanding how black holes can move
clarification and response to comment
Mar
11
comment Why is there so much methane in space?
I'm unclear of what specifically you're asking, because methane is one of the simplest molecules around. Since carbon is one of the most common elements in the universe (either by mass or by number), while hydrogen is number one, it's not too surprising that a lot of $\mathrm{CH}_4$ is around. Just like there's a lot of oxygen around, so $\mathrm{H}_2\mathrm{O}$ is common, too...
Mar
10
answered Does rotation affect gravitational lines of force
Mar
10
answered Looking for help in understanding how black holes can move
Feb
28
reviewed No Action Needed Exoplanet Temperature Calculations
Feb
27
awarded  Custodian
Feb
27
reviewed Leave Closed Why would a star’s position in the sky change relative to another star right next to it?
Feb
27
reviewed No Action Needed When will a day on Earth & Mars be the same length?
Feb
26
comment Temperature of the Sun's rays
For a stark limiting case, see photon gas. That's not the situation here, but it illustrates quite plainly the danger of absolute matter/light distinctions on this issue.
Feb
24
answered Temperature of the Sun's rays
Feb
24
reviewed Approve What's the relationship between Moon phase and the Milky Way visibility?
Feb
21
reviewed No Action Needed Can Magnetars destroy planets?
Feb
21
comment Do planets repel?
@HDE226868 If you conceptually split mass into three categories: inertial, passive gravitational (how it falls in grav field), and active gravitational (what grav field it produces), then for antimatter we can only test the equivalence of the first two (and there were some antihydrogen tests for it), but we can't make enough antimatter for tests involving the third. ... However, having it be different is certainly very bizarre and would kill any notion that gravity is mediated by a long-range spin-2 field (of which GTR is a particular case).
Feb
21
comment Do planets repel?
In GTR, gravitational repulsion is equivalent to violation of the strong energy condition, which does actually happen on cosmological scales, but is irrelevant in the solar system. In regards to antimatter, Rob Jeffries is of course completely correct theoretically (if antimatter repels normal matter, pretty much everything in fundamental physics is wrong), though of course we haven't actually had enough antimatter to check.
Feb
19
reviewed No Action Needed Why did the big bang not just produce a big black hole?
Feb
19
comment What is our universe expanding into?
The Big Rip is entirely hypothetical and inconsistent with the standard ΛCDM cosmological model. It's theoretically possible, but I severely doubt that "most" of the astrophysics community will agree that it's remotely likely.