From the article What would happen if you were swallowed by a black hole? Cecil says:
That is a new one on me. How is that supposed to work? That would make the black hole a substantial part of the entire universe.
Not saying it is not so. I am no physicist. I also no gut instinct is often shown to be folly when contemplating such things. Nevertheless that just feels wrong. Generally black holes are lots smaller than the mass that formed them.
Anyone know what is up with this?
I’m not an astrophysicist (nor do I play one on TV), but I’m thinking “typo”. That should be “ten million light years across”, most likely.
Given that Cecil speculates that the entire universe resides inside a black hole, I’d say the 10 Billion is intentional. I don’t pretend to understand the argument, however.
A black hole of galactic mass would be millions of kilometers wide, not light-years. Or perhaps Cecil meant “if our entire universe”. I’m kinda interested in the latter. Just what would the interior of a universe-massed, universe-sized black hole be like? Any way to distinguish it from our actual universe? Where would the singularity be, and how long would anything have before falling into it?
Actually, the sentence was about if the galaxy collapsed into a black hole, not the Universe. If the Milky Way collapsed into a BH, according to Cecil it would be 10 b/million LY across (much smaller than current size if it’s the 10 million LY figure, much larger if it’s the 10 billion figure). That’s what I think it’s a typo if Cecil is talking about the galaxy.
The rest of the argument is “well, if a star collapsing into a BH makes a really dense object and a galaxy collapsing into a BH makes an object only as dense as gas, then having the entire Universe collapse into a BH could make the density as low as the actual density of the Universe… which implies that we’re living in a black hole.”
At least, that’s my interpretation. Other people are free to differ. 
I’m not a astro-physicist, so I can’t say about the bigger the black hole, the less dense. However, one of the ideas of a black hole is that even light cannot leave it because the black hole’s mass bends space into a closed object. Once inside a black hole, there is no exit. In this respect, our Universe is a “black hole”.
I’ll have to talk to my son and see if he can explain it to me. Most likely, my son will blather away about the very esoteric nature of the whole thing, and I’ll nod my head in feigned understanding. He’s studying to be a theoretical quantum physical chemist, and no, I have no idea what that means.
I just get the bill, and I pay the tuition.
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That was indeed an error in the original column, though I won’t speculate as to how it came about. An updated column is coming up soon.
Guess it was an error per Chronos (look forward to the updated version) but as to this our galaxy is about 100,000 light years in diameter. So, even 10 million light years (instead of billion) would still seem to be waaaay off the charts for a galactic mass black hole.
Chronos said there was an error, not that the error was simply saying billions instead of millions. Wait for the update.
I will. Ed’s worst subject is science. Always has been.
As for the density thing, it’s actually rather simple – the density of a black hole is mass per volume, same as with anything else; however, the volume used is that defined by its Schwarzschild radius, since that’s the only way we know off that volume has a meaning regarding black holes. Thus, a small (and thereby, light) black hole is an incredibly dense object – the Schwarzschild radius of the Earth is around a cm, for example. However, the Schwarzschild radius is directly proportional to the black hole’s mass, and thus, the volume it circumscribes grows with the cube of the mass; so, the volume grows much faster than the mass, and heavy black holes are much less dense than light ones.
That doesn’t answer the question, though. It could have been confusing “galaxy” with “Universe”, it could have been a misplaced decimal point, it could have been a unit conversion error, it could have been using the wrong formula, there are plenty of things it could have been. I just don’t have enough information to speculate what the cause of the error was.
As I posted about three weeks ago, if the galaxy collapsed into a black hole, it would be about 0.3 light years, or 3 x 10[sup]12[/sup] km, across.
:smack:
← walks off to Cafe Society, where his skills and knowledge are more appropriate. 
Sorry I missed that.
I saw the Black Hole article on the front page (as they rotate in and out old articles) and saw the error. I came here, scanned the one page that is displayed (only one for me anyway) and did not see your post from three weeks ago so I figured to start a thread on it.
Suppose I should have searched but the hamsters seem to hate it when I use the Board search engine so I didn’t.
Black Hole: A collection of matter whose extents are within its Schwarzschild radius.
Schwarzschild radius:
r = 2 * G * m * c^-2
r is Schwarzschild radius
G is gravitational constant = 6.67428 * 10^-11 * m^3 * kg^-1 * s^-2
m is mass
c is speed of light = 299,792,458 m * s^-1
Simplified:
2 * G * c^-2 = 2.95 km * solar_mass^-1
r = m * 2.95 km * solar_mass^-1
Mass of Milky Way galaxy:
m = 5.8 * 10^11 * solar_mass
Schwarzschild radius of Milky Way galaxy:
r = 5.8 * solar_mass * 10^11 * 2.95 * km * solar_mass^-1
r = 5.8 * 10^11 * 2.95 * km
r = 1.711 * 10^12 km = 0.123788012 * light_year
=======================================================
If the entire Milky Way were to form into a Black Hole, it’s radius would be less than or equal to 0.123788012 light years
What if the entire visible universe were to collapse into a black hole? How big would that be?
I’d ask about the entire universe, but I think we don’t know how big that is, right? (I used to think the visible universe was the same as the entire universe, but somebody here disabused me of that idea.)
Speaking of non-visible universe, here’s something to ponder:
If the universe is infinite and uniform, then no matter what the density, at some point it will fulfill the necessary density for a black hole, because a uniform universe’s mass increases as the cube of the radius, while the black hole’s radius increases directly proportionately to the mass, so eventually the radius will catch up to the mass.
Now, I’m not a physicist but I assume that given the enormous distances, something will prevent a catastrophe. Most likely dark energy since that will overcome gravity over those enormous distances.
Or, it could be we ARE in a black hole and we can’t tell that we are quickly approaching the singularity! One day we will just be toodling along and all of a
Could it have something to do with the ‘fact’ (or modeling view) that once you get down to Planck length, everything starts growing again? That is, energies and everything else start rebounding in exactly the way they would if the lengths were expanding instead of shrinking, and string theory cosmologists think a space smaller than a Planck length is indistinguishable with a space of a certain size larger than a Planck length.
I don’t think it’s what’s up, because I don’t think it would account for a less dense black hole. But I thought I’d bring it up.