Why is this differentiation made, rotating and non-rotating neutron stars?  There are almost certainly no non-rotating neutron stars in the universe!

Well, you see, physicists have an awful habit of oversimplifying things.

(No, in seriousness it's a very fine question.)   The real answer is that in physics we do prefer simplifying things first.  We construct simple models first to get some insight, and then build up to a more realistic or general case and see how that modifies things.  Knowing the mass limit for simple non-spinning neutron star serves as a benchmark and from there we can look at what happens if we introduce spin or other parameters.

Likewise all black holes in nature certainly spin (most quite rapidly!), but the non-spinning Schwarzschild black hole was the first to be mathematically understood (1916, just a year after Einstein developed general relativity).  The equations for the spinning Kerr black hole were not solved until 1963.  So while the spinning black hole solution is more realistic, it is also a great deal more complicated!  

Someone doing serious modelling of astrophysical black holes will of course use the Kerr solution, but students learning general relativity will almost always be introduced to the Schwarzschild solution first, because it provides the most insight to black holes and general relativistic effects for the least amount of mathematical difficulty.  It also turns out to be an incredibly useful approximation for the gravitational field of stars and planets.  The Kerr solution can come in later, when one wants to understand how the rotation modifies the behavior.

A non-spinning black hole can be made by colliding exactly counter-rotating black holes (possible but unlikely), but another way spin can be reduced is by the Penrose Process.  In principle there is nothing that prevents an advanced civilization from harvesting enormous amounts of energy from spinning black holes, and after all that has been harvested they would be left with a "dead" non-spinning black hole.  

The other extreme for black hole spin is to have it spin as quickly as possible.  Very weird and interesting things happen in that solution, though it probably never happens.  Nature gets "almost there", with the upper limit of about 99.8% of the fastest possible rotation speed.

In principle there is nothing that prevents an advanced civilization from harvesting enormous amounts of energy from spinning black holes, and after all that has been harvested they would be left with a "dead" non-spinning black hole.

Hm. I should have thought of this method as well! As a builder of alien megastructures!
Especially as Kip Thorne describes such a method for harvesting black holes in 'Black Holes & Time Warps'. If I remember correctly.
It's been a while since I have read the book.

In principle there is nothing that prevents an advanced civilization from harvesting enormous amounts of energy from spinning black holes, and after all that has been harvested they would be left with a "dead" non-spinning black hole.

Hm. I should have thought of this method as well! As a builder of alien megastructures!
Especially as Kip Thorne describes such a method for harvesting black holes in 'Black Holes & Time Warps'. If I remember correctly.
It's been a while since I have read the book.

Some of the most amazing and physics-bending megastructures can be built around black-holes. Imagine a matrioshka brain structured around the event horizon of one! The AI(s) housed within it would view the outer universe at a warped speed of time, and might conserve energy for a more cold time in the distant future when computing can be more efficient.  Lots of interesting possibilities...

Especially as Kip Thorne describes such a method for harvesting black holes in 'Black Holes & Time Warps'.

Yes, that's an excellent book!  I love Kip Thorne's explanations and writing, and he's just all around an amazing expert on black holes (and especially gravitational waves, though that's another topic).  

Some of the most amazing and physics-bending megastructures can be built around black-holes.

I like to imagine a civilization building a set of nested spherical cities around a black hole.  Because of the curvature of space, they can fit many more spheres close around it than the faraway viewer would have guessed was possible.

I like to imagine a civilization building a set of nested spherical cities around a black hole.  Because of the curvature of space, they can fit many more spheres close around it than the faraway viewer would have guessed was possible.

Yes! Exactly! Plus you would an absolutely stupendous amount of living space and energy utility, especially if you built that sort of thing around a supermassive blackhole, which can be many times the size of the sun. That is, if you could calm the region around it down. I recall galactic supermassive blackholes are violent and greedy in nature and tend to eat stars a lot. Still, that would be a true MEGAstructure and cool as hell.

Anyway, I have some astro-news: astronomers have identified the missing baryonic matter in the universe. As some of you on this forum may know, the universe is comprised of 69% dark energy, 26% dark matter and of course 5% normal 'baryonic' matter or 'baryons'. The latter includes everything like blackholes ( ), galaxies, stars, planets and that bratty kid next door who for some reason feels the need to listen to music at 7 in the morning even though its the weekend and he has neighbors.... okay I'm getting off topic. Anyway, of that 5% of baryonic matter, all the stars and galaxies etc mentioned earlier only comprise about a third of it. The rest of the 'normal' matter was a mystery until now. Using the distant quasar 1ES 1553 9 (4 billion light-years away) as a back-light, the XMM-Newton X-Ray space telescope over a period of 18 days between 2015 and 2017 identified the signature of oxygen in the hot gas of the intergalactic medium between us and the quasar. This oxygen signature indicates that the warm-hot intergalactic matter (WHIM), which are clouds of ionized oxygen existing in super dense galaxy clusters like the Sculptor Wall, exist throughout the universe, and are as such in a sufficient quantity to account for the missing matter.  

Very cool Stellerator! Very informative post. I can understand how heavier elements can be detected in these extremely hot and diffuse clouds but how in the world did atoms created within earlier generations of stars end up in between galaxies? Please don't tell me the tag along Dark Matters is to blame. Haha.

I'm going to go listen to some Pink Floyd really loud and think about this...

I'm going to go listen to some Pink Floyd really loud and think about this...

NOOOOOOOOOOOOO- hey, actually that`s not a bad idea.

I'd say "Astronomy Domine" or "Set the controls for the heart of the Sun" *coughs*

Time, but of course, not just because Alan Pearson's antique clock chimes, like gravity waves, can be heard clear across the Galaxy, it's that lately I've quietly been hanging on... Running out of breath... waiting for something to show me the way.

I can understand how heavier elements can be detected in these extremely hot and diffuse clouds but how in the world did atoms created within earlier generations of stars end up in between galaxies?

Probably by galactic winds.    Galaxies are not entirely closed systems.  Not only is new material from the surrounding space falling into them, but AGN and supernovae can be energetic enough to blow a lot of material back out into the intergalactic medium.

My only guess was intergalactic red giants previously ejected and finally going nova with a whimper, all alone out there. Or something similar to "galactic Oort clouds".

Have any of you guys heard about the ISS springing a leak in its Soyuz MS-09 spacecraft, docked to the Rassvet module of the Russian segment? I know, I was shocked to hear about too. And even more concerned when I read about Russia claiming it was a possible inside job, on account of the hole looking suspiciously like a drill bore.



There is a lot of regurgitated and rhetorical information online and in the news about it, so it was kind of hard for me to find any reports that were not from endless link-spamming independent blogs or puddle-deep news shows, so I went to the NASA space station website and found the status reports: https://blogs.nasa.gov/spacestation/2018/08/

No mention of drill-holes or inside jobs are mentioned in the NASA article (conspiracies were not mentioned on Space.com either), but various online journalists eager to slap their label on it spread the Russians statements of 'sabotage' as far as possible.

Personally, I think the hole was caused by innocent human error - or maybe flat-earther terrorists! That last one I saw on a conspiracy website that popped up in the search engine and makes zero sense.

Ahhh... the vagaries of online research .

Scott Manley had a pretty good video blog about it, addressing and correcting a lot of the misconceptions that arose in the news reports.

[youtube]S6vqYUBoe3g[/youtube]

Scott Manley had a pretty good video blog about it, addressing and correcting a lot of the misconceptions that arose in the news reports.

This sums up my opinion of the whole matter very well.

[youtube]yaPJKNiEeiw[/youtube]