# Okay, my mind is officially blown...February 28, 2013 6:52 PM   Subscribe

'Nearby' supermassive black hole rotates at close to the speed of light For some further details than the Guardian link (though the title says it all) a NASA link.
posted by aleph (55 comments total) 29 users marked this as a favorite

It’s long been suspected that supermassive black holes in the heart of galaxies rotate faster and grow larger as they feast on gas, dust, stars and matter.

Huh. I wonder what the Associated Press thinks gas, dust, and stars are made of.
posted by Sys Rq at 7:19 PM on February 28, 2013 [12 favorites]

There's other kinds of matter.
posted by empath at 7:31 PM on February 28, 2013 [1 favorite]

Hardly a rounding error's worth.
posted by ook at 7:49 PM on February 28, 2013 [1 favorite]

Really just gas anyways. The dust is all destroyed by that point, and the stars would never make it in that close.
posted by kiltedtaco at 7:54 PM on February 28, 2013

Does anyone know what are they talking about?

Black holes have angular momentum, I think they get whatever angular momentum comes the matter they consume, so if spinning objects fall into a black hole, the black hole retains that angular momentum.

But it doesn't seem clear at all to me what they mean when they say a black hole is "spinning" at a certain "speed" expressed in miles per hour.

If you take a typical planet or star, or ball, or any other object, different parts move at different speeds. But in a black hole, everything is supposed to be collapsed down to a singularity: a single point, or a ring, depending on the angular momentum. If it's a single point then you would always have a speed of zero, regardless of the rotation rate. If it was a ring, you could calculate it, I suppose.

Now, I suppose you could do a calculation where you take the mass of a black hole, the angular momentum, and calculate the radius of the event horizon from the mass and then calculate the speed at which a point on the surface would move if it was rotating around the entire width of the event horizon at the same speed that the inner singularity was rotating.

But, and again, as far as I know there isn't actually anything stuck to the surface of a black hole.

So are they talking about how fast the singularity itself is rotating, if it's stretched out into a ring?
posted by delmoi at 7:56 PM on February 28, 2013 [4 favorites]

There's other kinds of matter.
I wonder how black holes and non-baryonic dark matter interact. Obviously black holes themselves are a type of dark matter. I would think that since dark matter interacts gravitationally with regular matter, dark matter might fall into black holes. But would that mean that black holes would then emit dark matter particles as hawking radiation?

Obviously we don't know what actually makes up (most) dark matter at this point, so we can't actually answer that.
posted by delmoi at 7:59 PM on February 28, 2013

@delmoi - been a long time since I read about black holes, but I don't see how a black hole can be exorbitantly large and also collapsed into a single point. I'm pretty sure the singularity is considered to be at the center of the black hole, not the black hole itself.

...and a quick gander at Wikipedia seems to bear this out - the singularity is at the center. Oh, and interestingly, apparently quantum theory rules out singularities, so they're still just one hypothesis.
posted by Peevish at 8:07 PM on February 28, 2013

But it doesn't seem clear at all to me what they mean when they say a black hole is "spinning" at a certain "speed" expressed in miles per hour.

They're talking about the speed of rotation at the event horizon. As far as I understand, it's actually space-time itself that's rotating.
posted by empath at 8:08 PM on February 28, 2013 [2 favorites]

Here's the math, btw.
posted by empath at 8:09 PM on February 28, 2013 [3 favorites]

Basically, if you stood 'still' at the event horizon, you'd be spinning at nearly the speed of light, because space and time itself would be whirling around you.
posted by empath at 8:10 PM on February 28, 2013 [2 favorites]

I'm pretty sure the singularity is considered to be at the center of the black hole, not the black hole itself.

No idea whether this actually contradicts your statement but the main Wikipedia black hole article claims,
It can also be shown that the singular region contains all the mass of the black hole solution.[56]

56. Carroll, Sean M. (2004). Spacetime and Geometry. Addison Wesley. ISBN 0-8053-8732-3, the lecture notes on which the book was based are available for free from Sean Carroll's website
posted by XMLicious at 8:27 PM on February 28, 2013

I'm not sure space and time would be "whirling around you" at the event horizon. Spinning Mass does a weird thing I've vaguely heard of called "frame dragging" in that space is dragged along with the spinning mass in the direction of spin. Or something like that. What (also) blows my mind from that is someone told me that it makes a trip you take around that spinning mass be a different length depending on if you traveled *with* the spin or against it. And the effect is proportional to the mass so around a black hole it would get big enough to notice in a lot of different ways.
posted by aleph at 8:29 PM on February 28, 2013 [1 favorite]

Yes but how fast does a plate of beans spin?
posted by Drumhellz at 8:46 PM on February 28, 2013 [2 favorites]

Spinning Mass does a weird thing I've vaguely heard of called "frame dragging" in that space is dragged along with the spinning mass in the direction of spin.

Right, but at the event horizon, the space on one side of you will be rotating at a much faster speed than the other side, causing you to spin. It's just a matter of your reference frame.
posted by empath at 8:55 PM on February 28, 2013 [2 favorites]

I can no longer hear the phrase "supermassive black hole" without thinking of vampire baseball.
posted by dersins at 8:56 PM on February 28, 2013 [1 favorite]

Also, an inner region is dragged more than an outer region. This produces interesting locally rotating frames. For example, imagine that an ice skater, in orbit over the equator of a black hole and rotationally at rest with respect to the stars, extends her arms. The arm extended toward the black hole will be torqued spinward. The arm extended away from the black hole will be torqued anti-spinward. She will therefore be rotationally sped up, in a counter-rotating sense to the black hole.
posted by empath at 8:57 PM on February 28, 2013

Lay impressions are worth their weight in electrons, but here goes:

I thought singularity referred to the stuff inside the event horizon, not a theoretical point at the center of the black hole. Seems like, by definition, the black hole doesn't actually have a center, just a diameter.

As for how fast it spins... I guess it depends on how quick the turtles can move. Or else it has something to do with the interface between the outside of the event horizon and whatever is on the other side of it. At one distance stuff still obeys our known version of the universe and at another distance (possibly not measurable) it slips over to another version of the universe. Time gets wierd as it hits the event horizon, then it doesn't matter any more. (...doesn't matter...see...)

Okay, I'm back with the turtles. Carry on.

Awesome post.
posted by mule98J at 9:40 PM on February 28, 2013

I thought singularity referred to the stuff inside the event horizon, not a theoretical point at the center of the black hole. Seems like, by definition, the black hole doesn't actually have a center, just a diameter.

Nope, there's definitely a point at the center. But we can't really observe that, the only observables are mass-energy, position, angular momentum, velocity and charge.
posted by empath at 9:45 PM on February 28, 2013 [1 favorite]

A rotating black hole (the Kerr solution) has a maximum possible angular velocity. (Actually a maximum angular-velocity-per-mass.) I'd guess that that's what they mean here— it's a near-extremal hole. The NASA article phrases it more clearly, if less punchily ("it is spinning almost as fast as Einstein's theory of gravity will allow").

Kind of like the speed of light, if you spun up a black hole past this limit, Nonphysical Things would happen (e.g., a singularity that's not hidden behind an event horizon), but there doesn't seem to be a way to get there from here: the closer a black hole is to the limit, the harder it is to add angular momentum.

So it wouldn't be too odd for science journalism to use speed-of-light phrasing for a near-extremal black hole.
posted by hattifattener at 9:59 PM on February 28, 2013 [3 favorites]

@delmoi - been a long time since I read about black holes, but I don't see how a black hole can be exorbitantly large and also collapsed into a single point. I'm pretty sure the singularity is considered to be at the center of the black hole, not the black hole itself.
The event horizon is just the distance from the center where light cannot escape. It's not an "actual thing". All the matter is condensed into the singularity, at least if you assume normal physics stays true inside the event horizon.
Nope, there's definitely a point at the center. But we can't really observe that, the only observables are mass-energy, position, angular momentum, velocity and charge.
A point or a ring, if it's spinning. But it seems like all real black holes should have angular momentum.
So it wouldn't be too odd for science journalism to use speed-of-light phrasing for a near-extremal black hole.
Well, it's quite confusing to read - the article guardian article makes it sound as if there is some actual "thing" physically moving at "670m mph". The NASA article just says it's spinning at close to the maximum, and actually explains how they are able to measure it (by looking at how close the accretion disk is to the black hole itself)

I guess the guardian author just decided to add some nonsense?
posted by delmoi at 11:03 PM on February 28, 2013

Or is it stationary, and the rest of the universe is spinning around it at close to the speed of light, hey?
posted by panaceanot at 11:08 PM on February 28, 2013 [1 favorite]

The paper by Risalti and collaborators, and Nature's accompanying summary for people outside astrophysics. It boggles the mind that there are a dozen news articles and press releases out and nary a one links back to the actual research, whether it's paywalled or not.

delmoi, there are several ways to answer your question, but probably you won't like any of them.

I would tell you that a black hole is an object with an event horizon --- a one-way boundary in spacetime. There's been a lot of intellectual effort expended to describe what's inside the event horizon, whether there's a pointlike singularity, or a ringlike singularity; and also whether it's possible to form a "naked" singularity without an event horizon surrounding it. The hypothesis is: there's always an event horizon. Which means that there's no way to probe the "inside" of a black hole, which means, operationally, that a black hole doesn't have an inside. A black hole has an event horizon, and usually an accretion disk. Due to time dilation, new matter falling on the event horizon takes an infinite amount of time to disappear (though it takes a finite amount of time to appear to cool off to the event horizon's temperature). What happens at this interface is certainly very complicated and the odds are good that general relativity is only approximately right. What happens "inside" the black hole is total speculation.

While the event horizon has a pretty well-defined size, I'm not sure it makes sense to talk about its rotational speed. It makes more sense to estimate the angular momentum of the black hole. And in the Nature papers, a quick skim doesn't turn up any reference to speed; that nugget seems to be present in Harvard's press release, but not NASA's.

There is a reference in the Nature summary to a theoretical maximum angular momentum for a black hole. This is one of those nuggets of knowledge that I have forgotten. I know that the speed of light is a speed limit, but not an energy limit, and my first guess was that you could put any amount of rotational energy into a black hole. But I can imagine that there's some centripetal-like effect where, if the black hole has too much spin, frame dragging in its ergosphere allows only objects which slow the spin to reach the event horizon, so that the black hole can absorb no more angular momentum. And I wouldn't be surprised if you can compute this limit using only G, M, c, and dimensional analysis, and it turns out to be the same angular momentum as a spherical shell with the mass of the black hole and the Schwartzchild radius rotating so that its equator is moving at the speed of light. General relativity is full of those sorts of coincidences. It doesn't mean that a rotating spherical shell is a good model.

And if you still want to think about an object with zero spatial extent and finite angular momentum, don't forget the electron.
posted by fantabulous timewaster at 11:14 PM on February 28, 2013 [9 favorites]

I guess the guardian author just decided to add some nonsense?
More like, somewhere in the chain of press releases, a well-meaning science journalist confused a helpful analogy with a physical model. It looks like the Harvard press release uses the "speed of light" metaphor, and the Guardian "helpfully" added a number.
posted by fantabulous timewaster at 11:20 PM on February 28, 2013

There's been a lot of intellectual effort expended to describe what's inside the event horizon, whether there's a pointlike singularity, or a ringlike singularity; and also whether it's possible to form a "naked" singularity without an event horizon surrounding it.

The new black hole debate is over firewalls.
posted by empath at 11:40 PM on February 28, 2013

I think we need physicsmatt, or someone for whom astrophysics is his bread and butter, to show up and school us about these things.
posted by JHarris at 11:57 PM on February 28, 2013 [1 favorite]

Mr. Primate the Younger is giving a presentation to his class on black holes in about two hours from now...Thanks for the factoid to add that extra oomph!

I'm sure his 4th grade class full of jocks will appreciate the need for speed :/
posted by digitalprimate at 12:11 AM on March 1, 2013

So, in this case, "spin" just means spin?
posted by Obscure Reference at 5:02 AM on March 1, 2013

The new black hole debate is over firewalls.

I was all gonna chime in that dropping the packet instead of sending a TCP RST shouldn't be controversial in this day and age and then I read the article.
posted by Slap*Happy at 6:15 AM on March 1, 2013

Paging Dr. Neil deGrasse Tyson
posted by 4ster at 6:53 AM on March 1, 2013

JHarris, you are aware that physicists sleep, right?

OK, black holes, fantabulous and empath have said most of what I'm going to say, but apologies to them, I'll say it again.

Black holes are a particular solution to Einstein's equations of general relativity. In fact, a non-rotating black hole is the simplest solution other than the trivial empty space one. This solution is called the Schwarzschild metric (the metric in GR describes how distances and time differences are measured between two point. A curved metric -one that differs from point to point- is how we get gravity). All you ask here is "what does the metric look like when I drop a point mass in flat empty space?" It's a theoretical argument, but you have to start somewhere, and exact solutions to GR are hard to come by.

What Schwarzschild found is that this matter arrangement gave a completely expected result when you are far from the point mass: the gravitational field drops like 1/r^2, exactly like it does for objects like the Earth or the Sun. This is great, because we already know that standing on the surface of the Earth, we cannot tell the difference (gravity-wise) from an extended object underneath our feet versus a point object at the center with the same mass (well, in one case we'll fall as there's nothing supporting us, but hey, theory and that's not the point).

What happens when we get closer to the point mass? Well, according to Newton, gravity would get stronger and hit infinity when r (the distance) went to zero. Also according to Newton, there'd be some distance from the mass when the escape velocity (the speed I'd have to launch a rocket straight up for it to coast out to infinity) would exceed the speed of light (c). General Relativity is not like that.

In GR, once you get close enough to a point mass, eventually you will hit a point where the energy you would require to pull yourself out of the mass's gravity well is infinite. This is different than having an escape velocity > c. In the classical (Newtonian) case, if you were trapped inside the radius where the escape velocity > c, I could lower a ladder to you from the outside, and you could slowly climb your way out. The energy you'd need to expend would be finite, so it'd be hard, but you could do it. With GR, inside this specific point, the event horizon, you cannot. The isn't enough energy in the Universe to get you out. But it's important to remember that black holes are not cosmic vacuum cleaners; they don't pull with some magic force "greater than gravity." The metric of the Earth outside the surface is exactly the same as the metric of a black hole with the same mass at the center. The difference is a) we could get closer to the center of a black hole and b) once we got too close, we'd never leave.

For Schwarzschild black holes, we can even guess the radius of the event horizon. You do this by putting together the fundamental constants of nature relevant to GR in such a way that you get a distance. The strength of gravity is set in the Universe by the constant G, which has units Force*distance^2/mass^2 or velocity^2*distance/mass. The only velocity that could matter is c, the speed of light, and the only mass around is the mass of the black hole M. So, canceling units in the only way possible, the event horizon is
r = 2GM/c^2.
The two you can't get from this technique of dimensional analysis, but being a theoretical physicist is mostly about knowing where to put the factors of 2. Turns out for an Earth mass black hole, you get an event horizon at around 1 cm. (As an aside, you can do pretty well in the physics GRE if you just apply dimensional analysis a lot. A surprising amount of physics can get done with some knowledge of units and a feel for which constants are relevant to a particular problem. Doing this well is a sign you actually know what the hell is going on, and didn't just memorize a formula)

What happens if you fall inside an event horizon? You will never return. As you get closer to the event horizon, your "light cone" (the set of possible future points in spacetime you can reach while moving slower than light) starts tipping towards the center of the black hole. That is, your future starts looking like "in." Once you pass the event horizon, your light cone is oriented in such a way that what you call "the future" an outside observer would call "the center of the black hole" (well, they couldn't actually see you, but lets assume some omniscience here, this is theoretical physics and I get to set up the rules of my own games).

Does anything special happen at the event horizon? Not necessarily. For a black hole of anything other than tremendous size (probably bigger than the one in the article, which I swear I'm going to get to any month now in this comment), you'd end up with tidal forces that would pull the parts of your body closer to the black hole more strongly than the parts further away and you'd get torn apart. The technical term for this is spaghettification, and no I'm not making that up. But that's just a function of getting too close to a big compact mass; moons get "spaghettified" if they pass inside the Roche limit of a planet, for exactly the same reason.

If you have a enormous black hole, then it was generally thought that you'd see nothing interesting at the event horizon. The actual metric curvature there could be quite low, and you'd fall into your doom without even knowing it. Hell, we could be inside a Universe-sized black hole right now, falling to our future at the center, and have no idea. However, as empath brought up with firewalls, new work from the intersection of gravity and quantum field theory (which often use black holes as thought experiments) has very recently brought this into question. If I haven't died of old age or had to do real work by the end of this post, I'll maybe talk about that.

So that's a simple black hole: mass at the center. Now, I can spin that point particle. You might say "what does it mean to spin a point?" Well, spinning in GR can be defined relative to the metric at infinity (you sometimes hear this poetically referred to as moving relative to the "fixed stars"). Now any mass that spins in GR drags the metric around with it. Earth does it, as was measured by Gravity Probe B after a many years of experimental effort. Earth, not being particularly heavy or spinning exceptionally fast, doesn't drag spacetime around with it that much.

What does frame dragging mean? Well, just as with the tilting light-cone, being in a rotating metric means that "the future" starts looking an awful lot like "rotating in one particular direction." You could expend energy to avoid rotating, but it would be noticeable. Normally, if you are at rest with respect to the fixed stars, your inner ear will tell you you are at rest and you don't feel dizzy. This is because there are no forces at play on you to keep you bound on an arcing trajectory. So, if it's at night and your looking at the stars (the fixed stars) and you see them stationary, then you are also not spinning (ignore the rotation of the Earth for a moment, as I said, it's actually not that large as these things go). If you were in a region of spacetime being dragged by a rotating black hole, seeing the fixed stars stationary in your field of view would mean you would feel like you are spinning. If you move with the rotating frame, you'd start seeing the stars pinwheel about you, but your inner ear would say "nope, stationary." I expect this is when the vomiting would start.

So that effect happens with any rotating mass. However, a pointlike rotating mass allows you to get close enough to start seeing weird shit, just as a pointlike stationary mass lets you get close enough to die horribly inside the event horizon. A rotating black hole, described by a Kerr metric instead of a Schwarzschild, still has an event horizon, but it also has an outer boundary called an ergosphere. Inside the ergosphere (which looks like a slightly flattened ball oriented along the axis of rotation, as opposed to the perfectly spherical event horizon), the frame dragging is so great that "the future" is "rotating." It takes infinite amount of energy to not rotate with the black hole here. Unlike the event horizon, you can leave the ergosphere using finite energy, and there are theoretical ways of "mining" a Kerr black hole for energy by throwing things through the ergosphere and collecting them at the other end (they'll be moving faster if you do it right). This slows down the rotation of the black hole, because there is no such thing as a free lunch.

You can also have charged black holes, by throwing in electrically charged objects, but these are even more complicated, and we have enough to deal with now. For completeness a charged non-rotating black hole is the Reissner-Nordstrom metric, and the charged rotating black hole is Kerr-Newman. No-hair theorems say that from outside the event horizon you can only have 3 pieces of information about a black hole: mass, charge, and rotation.

OK, so a black hole is a particular theoretical solution to the field equations of GR. Do they exist?

Well, it's possible that they wouldn't. After all, these solutions are theoretical exercises that were done because solving GR for realistic mass configurations is really hard, especially without computers. And, as I've said, as long as you stay away from the event horizon, the solutions we found are really useful. The Kerr metric describes the Earth's metric really well; you just throw out the solution inside the Earth's radius, because clearly that's not physical.

Going towards the singularity is dangerous for the theory too, so you might say "this is a sick theory here, ignore the answer." It has infinite mass density, and that looks a little suspicious. Theoretical physics makes a lot of assumptions often in order to get mathematically tractable solutions, and we joke about "spherical cows" as a result a lot. But we are not idiots, and we know that often we have to toss out part of our solution because it's just not physically relevant. Maybe the event horizon and the singularity at the center is part of that. For a while after the discovery of these solutions, that was a reasonable assumption: some physical process would prevent the formation of black holes, so don't worry about it.

Turns out this isn't true. We found objects that clearly have a mass compacted into a small enough region that they cannot not have an event horizon. This can happen in one of two ways. Black holes of stellar mass can be formed when a particularly massive star runs out of fuel to support nuclear fusion. Without the heat source at the center, there's no radiation pressure to push out against gravitational collapse. The infalling layers of star-stuff results in a supernova, a massive explosion leaving a core remnant behind. Smaller stars get compacted into neutron stars, where the protons and electrons at the center of the ex-star get pushed together into neutrons through weak nuclear forces and the incredibly high gravitational forces are balanced by Pauli exclusion forces (neutrons, being fermionic particles, hate having to share quantum states, so they resist being piled on top of each other).

However, for stellar masses with cores above 3-4 solar masses, even this is not enough, and the core collapses in on itself and becomes a black hole. Cygnus X-1 is an x-ray source that is pretty much confirmed to be a black hole. The x-rays comes from matter infalling towards the event horizon. The infalling matter gets heated by collisions and starts emitting x-ray light (I think. We need an astrophysicist here for this).

Another way of forming black holes would be very small quantum fluctuations in the early Universe. If you just pile enough matter in a small region through random chance, it could form a black hole, and start eating up everything around it. These are "primordial black holes" and I may come back to these.

The supermassive black holes at the center of galaxies like ours (and like the one in the article this post is about - see, I finally made it there!) are a bit mysterious. There are issues with forming them either through early core collapse or as a primordial black hole, but regardless, they clearly formed early on in the Universe's history, and somehow are related to the seeding of the formation of large galaxies. It is an area of active research though, and not mine, so I don't want to say too much more about it. But that makes the observations of this very large and very fast rotating black hole interesting: we don't know as much as we'd like about how they form, how they grow, and finding unique examples helps test various theories.

Black holes will eat dark matter just like anything else. We have no idea what dark matter is, but we do know it interacts gravitationally. Various quantum field theory theorems tell us that the gravity we experience is the unique gravity-force that can exist (there can only be 1 spin-2 field in the Universe, apparently). So dark matter, whatever it is, experiences gravity and so cannot return from inside an event horizon.

Black holes could BE dark matter. Maybe. It's unlikely. Black holes have the properties we need for dark matter, if enough primordial black holes were formed in the early Universe. However, black holes also radiate through Hawking radiation (another thing I should come back to... sigh), and so eventually evaporate. The temperature of the Hawking radiation is inversely proportional to the mass, so big black holes radiate less and live longer. For primordial black holes below about an asteroid's mass, they would have evaporated by now, and we would see the flash of gamma rays they'd produce in the sky. For black holes between 10^-8 and 100 solar masses, our searches for MAssive Compact Halo Object (MACHO) dark matter would have found them via gravitational lensing. There's a narrow window still allowed, but it's generally viewed as a disfavored model.

Supermassive black holes are impressively large (millions of solar masses), but not that important for the dynamics of the galaxy right now. They are not the major component of matter even in the galactic bulge. There are hundreds of billions of stars in the Milky Way and similar galaxies, and more than an equivalent amount of dark matter. Unless you get very close to the central black hole, your motion is governed more by the rest of the stars around you. Once you get close though, things get fun. We can actually see stars rotating around our own black hole, with galactic rotation periods of a few months (we take ~200 million years to go around the Milky Way). Also, clearly, there is some relation between formation of big galaxies and the black hole at the center, so it's not like they don't matter at all. But the scales can be confusing, and it's important to remember that black holes are huge compared to us, but galaxies are huge compared to black holes.

There are also accretion disks of matter infalling into these supermassive black holes, just as with Cygnus X-1. Through an interaction of magnetic fields and this rotation (which is different from the intrinsic rotation of the black hole), you tend to get jets of material ejected along the axis of rotation (this material gets thrown out before it hits the event horizon). Sometimes, if a particularly juicy tidbit of matter falls in (some collection of stars maybe), the jets can get very active, and if we happen here on Earth to be sitting along the axis of rotation of a far away galaxy, we can see that as an "active galactic nucleus." Particularly old and powerful AGNs are quasars, some of the most energetic events seen in the Universe, that occurred at very light redshift (a very long time ago), when presumably the center of the forming galaxies were experiencing more collisions and thus more jet activity. Again, not my research, so I shouldn't go too far off the beaten path here, but very interesting.

Our own Milky Way has two structures to the Galactic North and South of the center, extending some 8 kiloparsecs up and down (8 kpc is about how far we are away from the center, but in the plane of the Galaxy, not away from the center). These are called the Fermi Bubbles (after the Fermi telescope which collected the gamma ray spectrum where Doug Finkbeiner at Harvard found the bubbles). These are likely to be related to some energetic event in the last few million years of our Milky Way (so relatively recently as these things go), that caused jets of material to be ejected from the accretion disk.

The black hole that is spinning so very fast is unusual in its rotation. This makes it likely that it got that rotation during a collision with another black hole that had large relative angular momentum. It's actually hard to get things with large angular momentum to get eaten by a black hole, sort of by definition things with lots of orbital angular momentum tend to avoid the center, so you need something with lots of internal angular momentum (like a spinning planet, star, or black hole). Collisions of black holes would be spectacular events.

When two objects collide, they send out waves in the gravitational metric. As these ripples pass by, they actually cause distances to be measured differently (though only slightly). I always picture it as a real-life version of the "matrix-bending" Neo shows off at the end of the The Matrix, but that's just me. We can detect gravitational waves with experiments like LIGO, which built two very accurate laser-based distance measurements at right angles to each other. If a gravity wave passes by, the distances down the two kilometer long pipes would seem to change relative to each other. There were plans for a space based version that would be more accurate, called LISA, but in the current funding environment, that's unlikely. Regardless, LIGO should be getting to the accuracy to see the waves from colliding black holes in the Universe soon now.

I haven't even really touched on a lot of the things I find most interesting about black holes: the source of Hawking radiation, entropy, the holographic principle, and the recent stuff about the firewall. However, ostensibly this is a discussion of a supermassive galactic black hole, so I think this is enough to get started.

So... what questions in the thread did I miss?
posted by physicsmatt at 7:43 AM on March 1, 2013 [41 favorites]

delmoi: Obviously black holes themselves are a type of dark matter.
???

"Dark matter" isn't called merely that because it doesn't radiate light, but because it doesn't seem to interact with "normal" matter in any observable way except gravitationally.

And black holes emit light: Hawking Radiation.
posted by IAmBroom at 7:58 AM on March 1, 2013

The new black hole debate is over firewalls.
Hmmm, that's fascinating.

I know that an event horizon has a temperature (bigger == colder; I dimly remember computing that a solar-mass black hole would have a temperature of fifty or a hundred nanokelvin). I know that an outside observer sees stuff falling onto an event horizon go through an infinite redshift, so that it appears to take an infinite amount of time to reach the horizon. It's an interesting question to think about what this means: if you looked at a black hole up close, without being confounded by its accretion disk, would you see an event horizon? or would you see a spherical shell of very redshifted stuff just outside of the computed location of the event horizon? or could you tell the difference? Well, it takes a finite amount of time for the infalling redshifted room-temperature stuff to appear to have cooled off to the finite temperature of the event horizon. Presumably after that the infalling stuff appears to have come into thermal equilibrium with the horizon --- it can't appear to get colder than the horizon, because that's not how thermodynamics works --- and an outside observer can't distinguish them any more.

But I've never considered what this finite time-to-fall would look like from the perspective of the infalling observer. The conventional statement is that, for the infalling observer, the event horizon is not a special place. But I've just argued that the outside observer will see the infalling observer come into thermal equilibrium with a very high-entropy, finite-temperature something before crossing the event horizon. If one observer sees an event occur outside the horizon, then all observers must agree that the event occurred outside the horizon: the infalling observer, tootling in her rest frame towards the horizon at room temperature, must find herself in equilibrium with some high-entropy something whose temperature is higher than hers. Sure sounds like a wall of fire to me.

On preview: holy crap, physicsmatt. I was going to apologize for writing a longish comment that strayed a little from the subject of the link, but I guess that bridge has been crossed.
posted by fantabulous timewaster at 8:05 AM on March 1, 2013 [2 favorites]

FT, it's how I roll.

Also, I was working really hard not to run off to the quantum issues in black holes and stick to the astrophysics at hand. As usual, I was only marginally successful.
posted by physicsmatt at 8:13 AM on March 1, 2013

The black hole that is spinning so very fast is unusual in its rotation.
Why do you think this? What fraction of the maximum angular momentum would you expect more often?

I would think that an accretion disk could carry much more angular momentum than the maximum allowed for the black hole, and as it falls in, the black hole would spin up as much as it was allowed. Do you have a different model in mind?
posted by fantabulous timewaster at 8:16 AM on March 1, 2013

"Dark matter" isn't called merely that because it doesn't radiate light, but because it doesn't seem to interact with "normal" matter in any observable way except gravitationally.
As physicsmatt points out, you would class them as MACHOs, which would if distributed right do the job of dark matter. If you've got small enough black holes in a spread out enough distribution you might be able to get them to behave that way - their cross-section could be so low that they would look collisionless I'd expect (much as stars in a galaxy are collisionless in that there's very little chance even in a galactic merger that two would hit each other - the probabilities are that they'll just go right past each other).
Anyway, as physicsmatt also pointed out though, this idea is strongly disfavoured now (and probably for a couple more reasons than he said).
So I might call a primordial black hole dark matter, but I don't think anyone would class a supermassive one as such, especially when it seems fairly clear they've eaten up a lot of baryonic (that is, normal, atom-y stuff) matter which we still want to account for as baryonic, even if it's long gone. Stellar mass black holes for the same reason would probably be treated as baryonic in budgeting for cosmic mass, since they started off as baryonic matter (as stars). We have constraints from nucleosynthesis at the big bang which limit how much baryonic matter we can have, and it's too low to account for dark matter, so considering origins like that and keeping track of the baryonic budget are very important.
posted by edd at 8:32 AM on March 1, 2013 [1 favorite]

IAmBroom, as I said, black holes are certainly a dark matter candidate. Though we say that dark matter "doesn't interact except gravitationally" that's an experimental statement. It should be "we know that dark matter interacts gravitationally, we know it does not have un-surpressed strong, weak, or electromagnetic interactions, due to direct attempts to measure them. However, we don't know that it's interactions are zero." Measuring something to be zero is very hard, usually, you can just put an upper bound which is very small. Most particle models of dark matter, (WIMPS and axions for example), have non-zero interactions with matter, but they are suppressed compared even to the weak force. Also, our limits on dark matter interactions assume it is somewhat evenly spread across the galaxy. If it was clumped up in a MACHO (or even in a snowball-sized mass), the interactions could be larger without violating direct bounds. Black holes would certainly qualify, were it not for the gravitational lensing searches. Also, large black holes, as Fantabulous said, are extremely cold, so we would be hard pressed to find such dark matter.

Fantabulous, thanks for asking. I ended up looking a few things up, and learned a bit, which is always good. First, the public link to the arXiv version of the Nature article is here. I followed one of the cites and found this, which makes me think that this is not necessarily as unusual of a spin on a black hole as I'd thought (see what happens when you assume). However, the authors acknowledge that it could be that the observations are biased to high spin black holes for some reason, so who knows. Also, again, this is an area of active research that isn't mine, so I'm definitely not up on the literature.

Basically, I thought that, in order for the accretion disk to really spin the black holes up, they'd have to be ordered: all the material coming in with the same spin. Now, the disk of a galaxy indicates that this could happen, but I didn't think it would be sufficient. If all galaxies have fast-spinning black holes, then they must have been fed (as the 2nd paper says) during a period of "prolonged, ordered accretion." Which apparently has implications for the formation of galaxies. Interesting.

edd is here, and generally he knows more about this sort of thing than me, so I'll wait to see if he has something to add.
posted by physicsmatt at 8:38 AM on March 1, 2013 [1 favorite]

They're extremely dense and possess such a powerful gravitational tug that not even light can escape.

Christ, what an asshole.
posted by Smedleyman at 8:38 AM on March 1, 2013

Presumably after that the infalling stuff appears to have come into thermal equilibrium with the horizon --- it can't appear to get colder than the horizon, because that's not how thermodynamics works --- and an outside observer can't distinguish them any more.
I don't think there's a problem with that. The infalling object has some apparent temperature, and the horizon is a different object with a different apparent temperature.
The infaller doesn't have to see the Hawking radiation in the same way either I think - there's probably some direct link to the Unruh effect that physicsmatt can explain?
posted by edd at 8:42 AM on March 1, 2013

yeah, part of the screwy things with Hawking radiation is that the infalling observer doesn't see it (if I remember right). As with the radiation from a boundary seen by an accelerating observer (Unruh radiation), it's only the person standing at "infinity" away from the black hole who sees the radiation, someone falling in on a geodesic doesn't. The reason that this radiation occurs at all is that the quantum states that an observer near the black hole would call "the vacuum" are not "the vacuum" at infinity; they're the quantum states of a thermal bath. So if you're falling freely into a black hole, you see a vacuum near you, it's just that far away it looks like radiation. Nutty, I know.

I think the usual story is that the falling observer and the external observer's account of what happens only start radically diverging once the event horizon is crossed. At which point it doesn't matter if one person thinks the other is annihilated in a flash of radiation, since they're never getting back together to compare notes anyway. The firewall issue is a more subtle effect, and I'll have to do a bit of homework to figure out how to best explain it here, so I won't now. I had actually thought a few months ago about posting on the black hole firewall, but got stymied by trying to explain it intelligibly (also because all the understandable articles for laypeople on it were written by people I know in real life, so I can't make a FPP using them).
posted by physicsmatt at 8:51 AM on March 1, 2013

Not sure I do have much to add physicsmatt - this might fall in a gap between your knowledge and mine. I was certainly aware there were the sorts of strong hints you referred to for fast rotating holes, but I wasn't aware what implications it would have for galaxy formation.
posted by edd at 8:52 AM on March 1, 2013

I fell within the event horizon of physicsmatt's super-dense comment and my brain suffered spaghettification.
posted by yoink at 9:26 AM on March 1, 2013 [3 favorites]

Yay...Physicsmatt is awake!
IIRC...the xrays come from matter being sheared apart at the event horizon...or it makes gamma rays which heat up the rest of the accretion disk and that makes xrays...SciAm article maybe?
The question I think everyone wants an answer to is exactly what part of the black hole is 'rotating at the speed of light' ...probably this 'ergosphere' (a new one for me) right? Also...since it seems like the (inner juicy) center of the black hole is whipping it up to this maximum velocity or at least maintaining it, would it be fair to imagine that this velocity is being achieved at all latitudes? I.e. non-differential rotation (as opposed to the differential rotation of things like the sun, or Jupiter) in which case, jesus god, wow.
Black holes are weird.
Also...I'm pretty sure you can post stuff by people you know as long as you didn't work on it yourself (pretty much how interesting stuff from projects gets on the blue)...modz?
posted by sexyrobot at 9:26 AM on March 1, 2013

The infalling object has some apparent temperature, and the horizon is a different object with a different apparent temperature.
Well, that's okay if you have only one infalling object. But an outside observer could drop stuff onto the black hole from all directions, wait for it to redshift below the horizon temperature, and then not be able to see the low-temperature horizon for all of the lower-temperature stuff that hasn't fallen in yet. In the limit that you watch forever, you expect a new, featureless event horizon at the slightly lower Hawking temperature of your slightly more massive black hole, not an event horizon with some cooler blotches on it.

I would totally love to move this derail to a seperate post on the wall of flame.

sexyrobot, did you see my first comment? I think there's not a rigid thing moving near the speed of light.
posted by fantabulous timewaster at 9:52 AM on March 1, 2013

sexyrobot, actually, the x-rays are emitted in the accretion disk, which extends much further out than the event horizon. The way to think about it is that as gas fall in, it gets more dense and thus hotter, and eventually starts emitting higher and higher energy photons, getting up to the x-ray spectrum eventually. This has nothing in particular to do with a black hole, any accretion disk will do something similar. The difference is that a black hole is so massive that the accretion disk can be very large and fast moving AND the black hole is so small (compared to its mass) that the gas can get closer to the center (and thus move faster and get hotter) before it hits "the surface" (the event horizon in this case).

The ergosphere is actually non-spherical for precisely the latitude-dependent issue you were asking about. On the equator, the ergosphere is outside the event horizon, and as you approach the poles, it comes closer and closer, touching the event horizon at the poles themselves.

The thing that is moving in the article appears to be the gas in the accretion disk itself, as they measured doppler shifts of emitted light. At the singularity, there'd be "infinite" rotational speed, but in the same way there's infinite density. That's singularities for you, and the fact that they are all protected from view by the event horizon makes asking sensible questions about them tricky. You can ask "what does that infinity mean?" or "what would you see when you look at a singularity?" but right now we don't have good answers. It's likely that quantum gravity comes in to play as you really near the singularity inside the event horizon, but we don't have a theory for that, and we can do no observation experiment on a black hole (if we had one available) that reveals the details we want to know about the singularity.

As for the posting stuff, the problem is that I'm friends with a bunch of these people, so it's more than "this is a guy I see at conferences, let me post from his website." I had asked the mods way back when, and they were generally against me making posts of this, which I totally understand. I figured one of the metafilter science squad would get around to it eventually, and hey, they did. Sorta. If you wanted to make a separate post on the firewall, a good place for a popular science explanation would be here.

FT, I'll have to think a bit more carefully about your thought experiment, but one thing is that the firewall people are discussing in immediately inside the event horizon; the outside observer still doesn't see it, so it's different from the example you have in mind. I think you are actually thinking of a separate thought experiment people have brought up about a black hole, or at least a closely related version. The common one is: "if the infalling observer doesn't see Hawking radiation, but the stationary observer at infinity does, then for a very small hot black hole, the outside observer should see the infalling observer catch on fire before getting close to the event horizon while the infalling person themselves think that everything is a-ok." I don't recall the resolution at the moment (or even if there is one). Black holes are weird.
posted by physicsmatt at 10:08 AM on March 1, 2013

Physicsmatt, that was a bravura performance!

the Fermi Bubbles (after the Fermi telescope which collected the gamma ray spectrum where Doug Finkbeiner at Harvard found the bubbles) [...] are likely to be related to some energetic event in the last few million years of our Milky Way (so relatively recently as these things go), that caused jets of material to be ejected from the accretion disk.

Further derail - there's been some back and forth about the possibility that these bubbles may be signatures of dark matter annihilation (most recently). Not sure how serious they are ...
posted by RedOrGreen at 10:17 AM on March 1, 2013

OK, something I can speak with authority about:

The bubbles themselves as large-scale structures are almost certainly not due to dark matter annihilation, but likely due to activity in the core of our galaxy involving the black hole. That would eject charged particles to the Galactic North and South, and interactions of those particles (electrons and positrons it now seems) with the Galactic medium would cause the gamma rays we see from the bubbles.

However, Dan Hooper (one of the paper's authors) has argued that there is evidence for dark matter annihilation in the very center of the Galaxy, closer in than the 8 kpc-long bubbles, and spherically symmetric (which is what you expect from dark matter annihilation, as we have reason to believe the dark matter halo is relatively spherically symmetric, unlike our disk-y visible Galaxy). Normally, you'd have trouble looking for dark matter annihilations inside the bubble regions themselves, because, you know, there's a bubble there. So even though Dan thinks there'd be evidence of dark matter annihilation at low latitudes near the core, the bubble background frustrated his search for them.

Tracy Slatyer though, is, along with Doug Finkbeiner, the person you go to if you want to figure out the Galactic center in gamma rays. She and Dan used templates derived from their understanding of the bubble shape to remove the backgrounds and find out if there is an additional contribution in this region of the Galaxy that is consistent with Dan's claim of annihilation closer in to the core. They claim there is, which is important because there are few stars and sources of astrophysical uncertainties out where they looked, which removes some of the counterarguments for the source of the original gamma ray signal Dan found. Of course, there could be new sources of uncertainty, for example, the bubbles themselves. So the story is far from over, and we'll see what the reaction of the community is. I'm just really enjoying that you found this paper, since I've been hearing about it for a few months from Dan.
posted by physicsmatt at 10:34 AM on March 1, 2013

Thanks to all who are adding such great stuff here! And correcting my misunderstandings.
posted by IAmBroom at 11:39 AM on March 1, 2013

Thank you so much physicsmatt! Extremely interesting!
posted by JHarris at 11:54 AM on March 1, 2013

Christ, what an asshole garbage disposal.
posted by homunculus at 9:04 PM on March 1, 2013

Physicsmatt, that was a bravura performance!
2nded.

Christ, what an asshole garbage disposal.

Yeah, I immediately rethought that. It'd be looking at it from the other end, wouldn't it?
So, Christ, what a colon. Or Christ, what a rectum. (Rectum? I accelerated 'em to 80% of the speed of light!)
posted by Smedleyman at 10:20 PM on March 1, 2013 [1 favorite]

So I read that SciAm article about the "firewall" and I can't get the idea out of my head that the singularity moving to meet the event horizon reminds me a lot of the inflation of our own universe. And like physicsmatt said, our own universe could be inside a universe sized black hole. I wish I had the math...
posted by runcibleshaw at 12:45 AM on March 2, 2013

So anything that that crosses the event horizon of a black hole takes an infinite amount of time (or the age of the universe anyway) to cross it, right? But, if some black holes can evaporate before the end of the universe, then what happens to the things that were crossing the event horizon?
posted by runcibleshaw at 2:26 AM on March 2, 2013

runcibleshaw: no - you fall in in a finite time. It's only an outside observer that sees you slow since the outbound light rays have more and more trouble climbing out. Your image to the outside slows but you don't.
posted by edd at 2:57 AM on March 2, 2013

"Dark matter" isn't called merely that because it doesn't radiate light, but because it doesn't seem to interact with "normal" matter in any observable way except gravitationally.
Actually, that's exactly why it's called that Where do you think the name comes from?
posted by delmoi at 4:01 AM on March 3, 2013

But, if some black holes can evaporate before the end of the universe, then what happens to the things that were crossing the event horizon?

That's actually a really profound question, and Leonard Susskind wrote a book about it called Black Hole War. The question is whether information is lost when things fall into a black hole, or whether the Hawking radiation somehow contains the information that fell into the event horizon.

The answer is kind of important, because if either there is no Hawking radiation, or it doesn't contain the information that went into the black hole, causality is broken. You have effects without causes, or causes without effects. You can't reconstruct the past or project the future, even in theory, given perfect knowledge. The answer is really complex, and has kind of fascinating repercussions on the nature of reality.
posted by empath at 6:00 AM on March 3, 2013 [1 favorite]

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