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8h
comment Is the speed of a black holes gravity faster than light?
@DecompileCodex I made a typo: "aren't" was supposed to be "are". Hence the ghetto edit by deleting and re-typing the comment, since it was past the edit time limit.
9h
comment Is the speed of a black holes gravity faster than light?
The question in the title and the question in the body are two inequivalent questions that are not connected to each other at all.
1d
reviewed Approve What is faster than a supernova explosion?
1d
reviewed Approve Gravitational lensing in Newtonian physics
2d
reviewed Close Relationship between temperature and HRD
2d
reviewed No Action Needed If the Sun became a black hole, would it swallow our entire Solar system?
2d
reviewed Approve Relationship between temperature and HRD
Aug
21
comment Radial Density Profile Equation
@RobJeffries I'm not dissenting about moving to close, but I am noting that this site does need to make it clear how homework questions are to be received. I have no problem with your policy being adopted by this site, since the last time such things were considered was two years ago, and this has moved on a bit since then.
Aug
21
reviewed Approve What if our galaxy didn't have a SMBH?
Aug
21
comment Radial Density Profile Equation
@RobJeffries Hmm, I don't think this site has a coherent homework policy, but it should probably get one at some point.
Aug
21
reviewed No Action Needed What measure should I use to help optimise the design of a telescope in a cubesat?
Aug
21
answered Radial Density Profile Equation
Aug
20
reviewed No Action Needed Looking for planetary position calculation
Aug
20
reviewed No Action Needed Elemental abundances in the crust of exoplanets and accessibility to them
Aug
19
answered Gravitational lensing in Newtonian physics
Aug
18
comment An anti-black-hole-engine
This question is essentially equivalent as to whether FTL is possible. (Short answer: no. Longer answer: future not completely certain, but don't bet on it.)
Aug
17
comment Why did the big bang not just produce a big black hole?
@OlegMihailik That there is no interruption is dead obvious from the Penrose diagram. There's also no delay except in as much as the tidal forces stretch the body--but the tidal forces can be arbitrarily low if the black hole is very large compared to the body. Either the tidal forces are negligible, in which case there is no delay, or they're not, in which case there is nothing special about the horizon because a similar delay will occur at everywhere else the tidal forces are significant. ... I suggest you open a new question regarding this or related topics.
Aug
16
comment How does 'centre of mass' concept work?
@barrycarter You know, almost every time I see your name, I mentally read it as 'barycenter' and have to go back and mentally correct myself. ;)
Aug
15
comment Why did the big bang not just produce a big black hole?
@OlegMihailik It is simple, but you're just mistaken. The foot will be able to communicate with the rest of the body of a free-falling man because the rest of the body will shortly be crossing the horizon--otherwise, the man would not be free-falling. ... I suggest you look the trajectories of two infalling point-particles at a Penrose diagram of Schwarzschild spacetime; it becomes very obvious that the one that falls first can still send signals the one following it, precisely because the one following it falls in rather than staying out of the black hole.
Aug
14
comment Why did the big bang not just produce a big black hole?
@OlegMihailik Not quite that simple. Your statement would be correct if you're hovering just above the horizon (using rocket power or whatever), so that if you reach out with any part of your body, it will be cut off. But that case is where you're accelerated by the thrust keeping you hovering, so you can equivalently say it was torn away by the thrust (since the force necessary to keep stationary diverges to infinity as one gets closer). ... But if you're in a local inertial frame, i.e. freefalling and small compared to the BH, you can't detect the horizon.