Is there a black hole the size of the Solar System out there? And are we heading for it?
A black hole that size just boggles my mind, and probably a couple of other minds also.
How “much” gravity will this have? What would it be like in the center of such a monster? I know that a human bean would only last a nano-second, but…
Well, my mind is too “Boggled” to continue.
Anyone care to join me? 
I assume that by “size of the Solar System”, you mean radius, not mass. The vast majority of the mass of the solar system is in the Sun, and it’s not believed to be possible for a black hole to form with less mass than the Sun, in the current Universe (though there might be a few left over from the Big Bang).
A black hole’s radius is proportional to its mass. It’s expected that the largest black holes would have a mass of perhaps 1 billion-10 billion solar masses. The Sun, were it a black hole, would have a radius of about 3 km, so we’re looking at radii of 3 billion-30 billion km, or 20-200 AU, for the largest black holes in the Universe. Pluto’s about 40 AU away, but the Oort Cloud is perhaps 50,000 AU away, so depending on how you define “the size of the Solar System”, the largest black holes might or might not be that large.
We’re certainly not “heading for” such an object: A black hole that size would be extremely easy to detect from its interactions with other matter. Supermassive black holes live only in the cores of galaxies, and that’s mighty large even for supermassive black holes (the one in the core of our own Milky Way is 2.6 million solar masses). Such a hole would only be found in the center of one of the monster giant elliptical galaxies, and there aren’t any of those nearby (the nearest, M87, is about 55 million lightyears away, compared to 2.5 million lightyears for the Andromeda galaxy). M87’s black hole is believed to be around 6.4 billion solar masses.
The gravity of an object can be regarded as being either incredibly strong, or incredibly weak, depending on how you look at it. In particular, the tidal forces from it would be extremely weak: The tidal forces from a 10 billion solar mass black hole as you’re crossing the horizon would be about 40,000 times weaker than the tidal forces you feel from the Earth, so it would not rip you apart the way a smaller black hole would. You could, in fact, continue oblivious for hours after you crossed the horizon, without even noticing.
Thanks for the reply Chronos,
I’ve read the book “Black Holes, A Traveler’s Guide” by Clifford A. Pickover, but it didn’t include how massive (size?) a black hole can be. I might have misread your reply, so I’ll ask again: Is there a limit to how large or massive a black hole can be? Can one eventually swallow the entire Universe?
Seems that while they’re toolin’ around the Universe they could eventually swallow up each other, then vacuum up the stars and stuff like that there ?
Thanks,
Jake
Can you define what you mean by a BH’s radius? Do you mean the event horizon? Or the bound of the matter within the event horizon? I’m not sure I’m posing the question well, but my mind boggles a bit when the “size” of a BH is discussed – doesn’t the singularity not really have a size per se?
There’s no limit in principle to how big a black hole can get: If you take a big black hole, and toss some matter into it, you’ll get a bigger black hole. The expected upper bound of about ten billion solar masses is a practical limit, based on how much matter we expect to be available for any given hole to eat, not a fundamental one. And we think we’ve ruled out the possibility that the entire Universe is inside a black hole, but that was still on the table as recently as 15 years ago or so (it’s exactly equivalent to a closed universe with no cosmological constant: The Big Crunch singularity at the end is the center of the hole).
squeegee, yes, by radius I mean the event horizon. Everything inside the horizon is considered “the hole”. The actual stuff in the middle, if anything (standard black hole theory models a hole as not containing any matter whatsoever, just a topological defect in the fabric of space), falls into the category of “don’t know, don’t care”, since it can’t affect anything outside.
Thanks, Chronos, for clarifying. So when you speak of the mass of the black hole, there’s no actual matter? Wow.
Is the mass of the BH then still directly related to its start mass (from the star that collapsed into one), plus all the mass that’s dropped into it since? Setting aside accretion disks and such, if I drop a 1 solar mass thing into a 1000 solar mass BH, is it now 1001 solar masses? I think the answer is, “well, duh, of course it is”, but you know what happens when you assume.
I am not a physicist, but I think I can take a crack at this. You may have stumbled across the expression “Black holes have no hair”, meaning they have only three observable properties: mass, charge and angular momentum. It is not so much that there is no actual matter as that we cannot know what is in there and for all practical purposes, it doesn’t matter. It is not known what gives rise to the property known as mass (that is, there is no theory of quantum gravity although there are some candidates) so perhaps the hole does not contain matter as we know it.
FWIW,
Rob
Isn’t a black hole just the deformation of space-time? There’s nothing there that’s like regular matter that we know and are used to, it’s just a gravitational well that ripped the fabric of space-time.
I’m pretty sure everything is speculation too. They come up with theories on how things form with the data and knowledge we have at this moment. It’s just explanations, not the actual thing. They can detect them thru its gravity and accretion disks and jets from these disks via magnetic forces a black hole creates.
There are different sizes too, Supermassive Black Holes reside in the galactic center of galaxies, where as regular black holes are from supernovae. If you can get a hold of a recent Scientific American at a local library, there was an issue within about 6months trying to explain black holes. I think it was about 4months ago, but not too sure. Check it out at your local library.
Black holes don’t actually get bigger do they? They become denser, right?
Nope. More matter entering equals a larger radius for the event horizon.
Event horizon- the area at which things start to be pulled in?
If you took a marble and put it on a sheet and measured the distance at which lighter marbles start to roll towards the original, that would be the event horizon?
But if you took a marble that was the same size as the previous one, but far heavier, wouldn’t the event horizon be larger without having actually increased the size of the marble?
I’m not necessarily arguing that this is so- just trying to understand. I know very little about astronomy. I’d always been of the understanding that black holes don’t change in size, just the amount of stuff packed in 'em. >.<
There is matter in a BH…that is how it becomes a BH. There is so much matter in a small enough space it collapses in on itself.
The “no matter” thing, I believe, is there is the singularity where all the matter is and the event horizon with nothing in between. The singularity is incredibly tiny…Planck length or smaller (so way, way, way smaller than an atom). The Event Horizon can be many miles or thousands or millions of miles from the singularity (depending on the mass present at the singularity).
I suppose one might say there is stuff in between the event horizon and singularity as stuff falls in and transits to the center but once in the event horizon there is no escaping a meeting with the singularity.
And yes, a 1000 solar mass BH that has our sun fall in to it becomes a 1001 solar mass BH (minus some stuff that may get tossed out in the process of falling in to the BH before crossing the event horizon but that is nitpicking).
Look at post #2 by Chronos.
The event horizon of a black hole is the boundary beyond which it would take more energy than going the speed of light to escape. (It would still be very difficult to escape near the event horizon, but at least it would be physically possible.)
Black holes come in all sizes. You can theoretically have a black hole of sub-atomic size. As Chronos also said, you could have a black hole equal to the size of the universe. The difference is directly proportional to the amount of matter within the event horizon.
In your example, by increasing the density you increased the mass. It’s the mass that counts, and only the mass.
Yes and no.
If (hypothetically speaking) our sun magically poofed into a black hole the earth and other planets would continue their orbits as if nothing happened. The mass is still there and the earth would orbit it just the same as if it were the sun.
Black holes are not hoovers that suck everything in. They are a mass like any other mass and orbital mechanics are the same for them as any other large mass.
That said the event horizon is the sharp line where, once crossed, the black hole singularity is always in your future. Which is a fancy way of saying you are well and truly stuck and fucked. There is no escape. Abandon all hope ye who enter there.
As Chronos noted for a truly massive black hole it is possible to cross the event horizon and not even know it. For most “smaller” black holes the tidal forces at the event horizon would spaghettifi you (gravity reduces the further you are away from the source…BH gravity can be so strong the pull on your feet versus your head [in a feet-first fall] would be so dramatically different you would be stretched out like a piece of spaghetti).
Alright, cool. Thanks for the responses.
Another question!
We can’t actually see black holes, because light can’t escape from them, correct? So we can only examine them by examining the gravitational affect they have on nearby celestial bodies.
If all that matters is the mass and you can increase the mass by increasing the density, what’s to say that all black holes aren’t sub-atomic in size, but that they very in densities?
Or what’s to say that the opposite isn’t true, that they really do vary in size?
I guess what I’m asking is, if all we can measure is the affect they have, how can we measure their size at all, when it’s possible for its mass to increase without actually changing how big it is?
They do change in size.
More mass = bigger.
Yes, the singularity does not change in size (depending on the theory it has no size or a Planck size but either way super tiny).
However, more mass makes the event horizon expand in size and as noted that can be anything from fractions of an inch to feet to miles to millions of miles. Any mass, in theory, can become a black hole (it is all about a certain amount of mass crunched in a tiny space…most matter is empty space…a Neutron Star which removes the space between particles is equivalent to packing Mt. Everest into a teaspoon [kidjanot] to give you an idea of the densities involved). People were worried the Superconducting Supercollider could make atomic mass black holes which would settle at the center of the earth and start feeding and growing inexorably. An atomic mass black hole is theoretically possible but the worry of one eating the earth is fantasy.
So, keep adding mass and the event horizon grows and there is no limit to the size. As long as you can keep feeding it matter it will keep growing.
The reason we say that there is no matter in a black hole is that the simplest form of the black hole equations has nothing but pure vacuum everywhere. You could say that all of the mass is in the singularity, but what makes the singularity a singularity is that it’s not an “anywhere”: It’s not that there’s something weird at that location, it’s that that location just doesn’t exist. Now, of course, it’s possible that the simplest model is incorrect. It may be that once we understand quantum gravity, we’ll realize that there isn’t a singularity after all, but that there really is some extremely exotic form of matter in a tiny lump at the center. How big that lump would be is anyone’s guess, since we can only speculate about its very existence, but the Planck length is as reasonable a guess as any.
As to the density of a black hole, assuming by that you mean the mass divided by the volume enclosed by the horizon, it actually goes down when the hole grows, not up. Again, the radius of the horizon is directly proportional to the mass, and the volume (as normal) is proportional to the radius cubed, so the density goes as the inverse square of the mass. That is to say, if you double the mass of the hole, you’ll only have a quarter of the density you had before.
If, instead, you mean the density of the lump of matter in the center, again, we don’t even know that such a lump even exists. Like I said before, don’t know, don’t care.
But I thought that simply becoming more dense, without necessarily increasing in surface area, would result in more mass.
I forgot to address this part.
A black hole, by itself, we cannot observe directly. Like the name implies there is no way to see one (I suppose maybe Hawking Radiation but if that is real it is so dim I do not think we could detect it from a stellar mass black hole).
So, we see them by their effect on other stuff. If the black hole is “feeding” (i.e. stuff is falling in to it) then the stuff falling in will heat up and should emit x-rays and the theory details what they should look like. As it happens we do see those.
Then there is the effect on orbits of nearby stuff we can see. We see something orbiting, to us, nothing it is reasonable to assume something is there we cannot see…a black hole.
I suppose gravitational lensing might be a method as well but not sure about that (watch how light bends if it transits in the vicinity of a BH).
See this is where I part ways with physicists. Or rather where they lose me.
I am nowhere near able to argue the math of it as you and some others here can but while it may make your calculations simpler it seems absurd to say the mass is not “anywhere”.
I have done these threads long enough to know things like quantum effects can have disturbingly un-intuitive results. Nevertheless the black holes are there. They have mass which we can measure. That being the case how can you say the mass is “nowhere”? Clearly it is at the center of the BH as evidenced by the orbits of things around it.
Perhaps a zero-dimensional point cannot be said to be anywhere…that is beyond me. Still, the things exist, mass is not siphoned to another universe (as far as we know) or otherwise destroyed (thought that was not possible…you can mutate matter but in the end it can all be accounted for) so the black holes are “there”. So how can a physicist keep a straight face and say the matter is nowhere?