In Cecil’s Black Hole (or ebony aperture) possibility #3, he says, and I quote, “…it is possible that the known universe is itself a black hole, with us living in it” unquote—Well, Say this were true, then, according to possibility #1 let me refresh your brains on a certain couple words…“At 100 miles away you’re heated up to 2,000,000 degrees Kelvin.” unquote— According to this, and Possibility #3, we would be nothing but cosmic dust floating around in this abyssmal universe. Or, according to possibility #2 “…spat out the other side, in a different place in space and time” unquote—If this were true, with possibility #3 still in mind, (universe we live in is itself a blackhole) then we would never get a moment’s rest, we would be getting spatted out every second to a different place and time, I am sorry to be such a nitpick, but sometimes I just let my mind wonder…
Sorry about that, the link is http://www.straightdope.com/classics/a1_200.html
Have a Coke, Hizzex; you’re overheating.
Now, your possibility #1 is true of a stellar-sized black hole. The gravity gradient decreases with the effective mass of the black hole, however. A stellar-size black hole would not merely rip you to your component atoms; it would rip the atoms to elementary particles (and possibly shred the nucleons to quarks, although QCD…but I digress). You might not even notice if you fell into a black hole of galactic mass. A universe-sized black hole…well, look up the critical value for omega (the mass density needed to close spacetime).
Possibility #2 is badly worded, to say the least. If you pass through an event horizon, you are, by some definitions, in a separate universe – that’s what the physical meaning of “event horizon” is, after all. Unless you fall through an event horizon in that universe, however – and the mathematics gets a bit flaky here, but it appears possible, at least, that you don’t have to – you won’t pass to yet another universe.
Akatsukami possibilty #1 is not refering to the tidal forces nera the event horizion of a (small) black hole caused by the steep gravity gradient but the frictional heat caused by matter being accreted by the black hole. This still holds true for larger black holes, for example a quasar is thought to be a supermassive galactic centre black hole that occasionally accretes whole stars which can be observed due to a high-energy jet caused by it’s accretion disc.
The event horizon of a black hole isn’t another universe, it still obeys the laws of physics and an observer within the event horizon can still see the outside universe even though he can’t be seen. The event horizon is merely the point of no return where the escape velocity is equal to c.
A few things about the column: an Einstein-Rosen bridge is very unstable and would collapse simply by a single photon passing through it, also the idea that the universe is a black hole seems incorrect to me as it would suggest a centre of mass that we must be moving towards (as it would presumably be a Schwarchild black hole).
If you can’t see into it, how do you know it obeys the laws of physics? 
The event horizon isn’t another universe, of course; it’s the boundary between what can be interpreted as one universe and another (depending on the exact nature of spacetime, an event horizon could be a volume, but that’s not relevant to the question). Note that, in the case of an event horizon surrounding a singularity – a black hole (again, you can mathematically derive other kinds of event horizon, but they seem to have no physical interpretation), you can’t see out of it either; all incoming photons are blue-shifted to infinity.
And would this universe be a Schwartzchild black hole? Why wouldn’t it be a Kerr-Newman black hole, where you can plot a world line that doesn’t take you through the singularity? (I can actually think of two reasons why not, but let’s see if anyone else can.)
It is incorrect to say you cannot see outside of an event horizon, yes, incoming light is highly blueshifted, but you can still observe the outside universe from within the the Schwarzschild radius.
The universe has an angular momentum of zero and a charge of zero, therefore it would have to be a Schwarzchild black hole.
The pretty loose analogy - there’s the distinction between time dependent and static solutions, for starters - that’s sometimes made between the universe and the interior of a black hole primarily applies to a closed universe. In such a case we would be moving inexoriably towards a singularity: the Big Crunch. The other version draws an analogy between the Big Bang and a time-reversed solution for gravitational collapse. In that case there’s a singularity, but in our past.
Since our universe is actually unlikely to be closed and neither is a singularity in the past much of a threat, Cecil’s concern is perhaps somewhat misplaced.
Cite?
:dubious:
rsa is correct, in that a black hole with the same mass as the Galaxy would only be about a light year across. But it’s worse than that. Whisper it quietly: [sub]Cecil - or Ed - has got mixed up between the Galaxy and the universe.[/sub]
Why? Well, the passage makes somewhat more sense if it was intended to be
This is still not perfect, but it’s getting close enough to indicate that that’s the confusion.
I don’t normally resort to equations on the board, but I’ll make a brief exception here. I’ll then try to express the point in plain English.
The back-of-the-envelope version of the argument might run as follows. A simple black hole with mass M will have an event horizon of radius
R = 2 GM/c[sup]2[/sup].
So the “density” of the hole is
3c[sup]2[/sup]/8 pi R[sup]2[/sup].
Meanwhile, the critical density of the universe is
3H[sub]o[/sub][sup]2[/sup]/8 pi.
In our universe this corresponds to a smoothed out average of about a hydrogen atom per cubic metre; which I think is the origin of Cecil’s reference to “a thin gas”. These two densities are equal when
R = c/H[sub]o[/sub].
But this is just a version of Hubble’s Law. A volume of the universe about us at critical density big enough to form a black hole will thus have galaxies at its edge receding at the speed of light. But such galaxies mark the edge of the visible universe. Conclusion: there’s just enough mass in the visible universe to form a black hole the same size as the visible universe. (Several of the assumptions here are dodgy, but the argument is only meant to be heuristic.)
A better explanation of Cecil’s point might be to say that the heavier a black hole is, the bigger it is. And it turns out that a black hole containing all the mass in the visible universe would be about 10 billion light years across. Which is just about the size of the visible universe. Furthermore, if there were more than enough mass out there to form a black hole of that size, there’d also be enough mass about that gravity would eventually stop the universe expanding and the whole lot will start collapsing back in. The eventual Big Crunch at the end of time would be very similar to falling into the singularity at the centre of a black hole. In a sense, the universe would be so heavy, it’d entirely implode into a black hole.
Except that the critical density isn’t really well-described as a “thin gas”, it’s more like a thin vacuum. I think that Ed conflated two things here: A cosmic-mass black hole would have a radius of tens of billions of lightyears, and a galaxy-mass black hole would have the density of a thin gas. But, of course, galaxy != Universe.
I certainly remember reading in an old cosmology textbook that a galactic mass (though doing a few back of the envelope equations it would have to beseveral times heavier than the Milky Way, which is not unreasable) would have the density (inside it’s event horizon) of a trillionth of that of water.
I think we have all forgotten about “sapghetti man”.
Who’s he you say? He is our hero when he gets close to the “coal-coloured opening”. Gravity is so high that at certain point the gravity on your head is so much higher than that of your feet that you’re pulled apart, thinning out like spaghetti. THAT’S what would happen to our guy (aside from the heat, time stopping and other minutiae)
I am working on a public (humorous) lecture about black holes, and did some math to calculate the tidal force (in Earth gravities) that a person my height (roughly 180 cm) would experience at various distances from a 3 solar mass black hole (a typical stellar mass black hole).
At 6400 km (the radius of the Earth) the differential force (the force that feels as if it is trying to separate your feet from your head) is about 0.5 g. For comparison, the tidal force you feel from the Earth is about one-millionth of a g at that distance. That’s roughly the force a drop of water exerts resting on your skin.
At 2000 kilometers from the BH, the force is 18 g. That’s enough to cause you to black out.
At 1000 km, it’s 144 g. Ouch.
At 100 km, the tidal force is 150,000 g. That is enough to snap your femur longitudinally, so at this point things go downhill fast, har har.
At 10 km, the force is 150,000,000 g.
At the event horizon, 3 km from the center, the tidal force is 5 billion g. That is pretty big (gulp!).
Interestingly, I don’t think it’s enough to rip an electron from a proton, though. I calculated the tidal force across an atom, and I got a number much smaller than the force holding the electron on. I was quite surprised by this! If someone wants to check my math, I’d appreciate it.
Anyway, the time it takes to freefall from 2000 km to the event horizon (assuming you started from very far out) is less than 0.1 seconds, so you probably won’t have time to pass out. You may now commence having nightmares.
Not if the black hole was big enough-
I believe you could saunter up to a GM BH and cross the event horizon almost without noticing-
the tidal stress is strong enough to tear stars apart, but probably not people or spaceships.
SF worldbuilding at
http://www.orionsarm.com/main.html
Well, I did say it was 3 solar mass black hole. That is part of the calculation; you have to specify the mass, the size of the object (a longer object will feel bigger tides than a smaller one), and your distance from the “center” of the black hole. For a million solar mass BH, the radius is a million km, so you can’t get 3 km away.
And I think I found an oops. The “radius” of a black hole with 3 solar masses would be 9 km, not 3 (3 is for a mass of the Sun). Nuts, I’ll have to fix that. :smack:
It’s not the tidal forces that do that, it’s the heat. An isolated hydrogen atom falling into a (stellar mass) black hole wouldn’t be pulled apart… But it probably wouldn’t have been isolated in the first place. If there’s a bunch of stuff falling in (as is the case in every black hole known), then it all gets pretty heated up in the process, and I’m pretty sure it gets hot enough to ionize hydrogen.
To be fair, there’s a pretty big selection effect here: Non-accreting black holes are almost impossible to detect, so we have only a very shaky notion of how many of them there are. There may be quite a few black holes where a hydrogen atom could fall in in peaceful isolation.
Right. I guess I can put this in context: I go through several ways a black hole can kill you. The first is simply falling in. The second is spaghettification. I then go through the math (well, not really, I just list the forces as I did above) to show that tides get to be something of a problem. Later, I mention getting near the accretion disk.
So I can be more specific: a single hydrogen atom falling into a black hole should make it to the event horizon intact, unless I have made an error in my math. I think that’s pretty cool.
From Cecil’s article:
“If our entire galaxy collapsed into an ebony aperture (I am getting tired of typing black hole), said BH would be about ten billion light years across, with the average density of a thin gas.”
How is it possible for a galaxy roughly 90,000 light years across to collapse into a black hole 10 billion light years across? Shouldn’t the resulting black hole be MUCH smaller than the galaxy is when spread out?
We’ve already covered that, Whack-a-Mole. There was an editorial goof. Cecil clearly was discussing two different cases, namely, a black hole with the mass of the Galaxy, and one with the mass of the Universe. Ed got confused, though, and conflated the two cases. A Universe-mass black hole would be (of order) 10 billion lightyears across; a Galaxy-mass hole would be about a tenth of a lightyear across.
Sorry…I missed that somehow. Why doesn’t Cecil go back and correct such an obvious error? If not edit the original article than add an addendum correcting the mistake.