Black Holes don’t eat as much as they accrete!

Posted by Ethan on May 2, 2012
(23)
More »

“This is the way I wanna die. Torn apart by angry fans who want me to play a different song.” -Regina Spektor

You’re familiar with the classic picture of a black hole: a dark, dense region at the center from which no light can escape, surrounded by an accretion disk of matter that constantly feeds it, shooting off relativistic jets in either direction.

Image credit: University of Warwick, retrieved from bordermail.com.au.

This is a pretty accurate picture of active black holes. But most black holes aren’t active, and of the ones that are, they aren’t active most of the time!

Most people think of black holes as marauders, gobbling up whatever poor stars happen to get in their way. You very likely have a picture of a black hole as though it behaves like a great cosmic vacuum cleaner, sucking up anything that dares get too close to it.

Video credit: NASA / GSFC, via YouTube.

I can’t fault you for thinking that; this is a genuine NASA video, and the picture that some very smart people have been painting for you for a long time. But that isn’t quite how the Universe works.

So, how does it work? When any object falls in close to a black hole, it experiences different forces on different parts of the object. We call these forces tidal forces, because they’re the same types of gravitational forces that cause the tides we experience here on Earth!

Image credit: Barger and Olsson.

Only, in the vicinity of a black hole, the tidal forces are much stronger than we experience on Earth. They are, in fact, much stronger than Jupiter’s innermost moon, Io, experiences, and those forces are powerful enough to constantly tear Io apart, making it the only volcanically active moon in the Solar System!

No, when you get close to a black hole, you get stretched at either end so severely, and compressed in the middle so thinly, we call the process spaghettification, one of the greatest astrophysics words ever invented!

Image credit: John Norton at Pittsburgh.

But “falling in” to a black hole, like illustrated above, practically never happens! Space is simply too big, and even for supermassive black holes — like the multi-million-solar-mass behemoth at the Milky Way’s center — the event horizon is too small. Most stars and objects that pass nearby to a black hole simply do what all other objects in the Universe do.

Gravitate! (Ha ha ha ha haaaaa!)

Animation credit: UCLA Galactic Center Group / Andrea Ghez.

Remember that space is huge, and that getting within a paltry 0.001 light years of our galaxy’s supermassive black hole won’t even disrupt the passing star, much less “vacuum it up,” as you might have thought.

“But what if the star does get close enough,” you ask, “then what happens?”

Well, they’ve actually gone and run the simulation properly, and I’ve got the results, here, to show you!

Video credit: NASA, S. Gezari (Johns Hopkins), and J. Guillochon (UCSC).

Note how, first, the star gets completely ripped apart by these intense tidal forces! But rather than acting like a vacuum cleaner and sucking it all up, most of the mass from this star doesn’t get devoured at all; quite to the contrary, most of it gets ejected back out into the space around the black hole! It’s only a small fraction of the original that gets swallowed, but that’s totally sufficient to take a quiet, supermassive black hole, and bring it back to life!

And we know this, because we just observed a super distant galaxy — more than 2 billion light years distant — just become ultra bright thanks to its supermassive black hole sneaking a bite out of an unlucky passerby! Let’s take a before-and-after look.

Image credit: NASA, S. Gezari, A. Rest, and R. Chornock, as are the next two.

The above images, from GALEX (in the Ultraviolet) and Pan-STARRS (in the visible/IR), show this distant galaxy shortly before it started snacking on its newly accreted material. The images are low-resolution because GALEX and Pan-STARRS focus on grabbing very wide fields-of-view; when you’re looking for very rare occurrences like this, you need to grab as much of the deep sky as possible!

So, that was 2009. But the next year…

The galaxy has brightened by a factor of around 350 in the Ultraviolet, and the visible/IR image has turned much bluer, an indication of the extraordinarily high energies being belched out by this suddenly noisy galaxy!

Taking a look at the before-and-after images together, you can really see the difference.

But don’t be fooled by the vacuum cleaner description; it’s not eating the entire thing that ran into it! This is, in fact, something that we may see happening for much smaller black holes that are much closer to us; the nearby galaxy Messier 83 just had a very similar outburst from a much smaller black hole!

Image credit: Optical: ESO/VLT; Close-up - X-ray: NASA/CXC/Curtin University/R.Soria et al., Optical: NASA/STScI/Middlebury College/F.Winkler et al.

Black holes aren’t giant leviathans, devouring anything that comes nearby, but nor are they dainty, steady nibblers on objects that orbit. Rather, black holes are wild, violent and inevitable, tearing anything that dares approach too closely into shreds, but coming away with a snack-sized meal whose first bite makes quite an impression!

Now, if you’ll excuse me, all this black hole talk has made me hungry! Where did I put the spaghetti…

(23)
More »

Comments

  1. #1 Wow
    May 3, 2012

    Ever read “Going Postal” by Terry Pratchett?

    Mr Gryle eats in the same way.

  2. #2 Peter Fred
    May 3, 2012

    Is energy conserved in a black hole? That is, does the mass-energy that is sucked in by the black hole add to the preexisting mass-energy of the black hole?

  3. #3 Wow
    May 3, 2012

    It certainly increases the strength of the gravitational field and thereby acts as if it has gained weight.

    Since we can’t get at what’s happened further inside than the event horizon, that’s the best we can do to say “yes”.

    Conservation of energy is how Hawkin figured that black holes emit radiation (in a sideways entropy-always-increases way).

    It may not be *true*, but it acts no differently from the true truth.

  4. #4 Kyle Wilson
    May 3, 2012

    One thing I have wondered for a while now is how much dark matter is being added to the galactic scale black holes. I understand that their cross section id relatively small, but given that dark matter appears to be only affected by gravitation (thus won’t be impeded by other dark matter or by normal matter as it sleets through) I’d expect a small but steady accumulation of mass from the constant flux (and probably larger than that from accretion of normal matter as the effects of heating, radiation pressure and fluid dynamics would seem likely to drive off a large fraction of the normal matter that would otherwise become part of the black hole).

  5. #5 Wow
    May 3, 2012

    An interesting thought.

    If Dark Matter had an actual theory of what it WAS, rather than just a placeholder, the answer to your question might be possible.

    It ought to give us some limit as to what that matter might *be*.

    I just wonder if those proposing DM have given it some thought or not.

  6. #6 Eric Lund
    May 3, 2012

    Kyle @5: Very likely there is a slow mass accumulation as dark matter falls in, but I don’t think it’s significantly faster, averaged over long time scales, than the ordinary matter infall. Yes, it’s true that there are no forces other than gravity to prevent dark matter from falling in. But that also means there are no forces other than gravity to cause dark matter to start falling toward the Schwarzschild radius in the first place.

    In the accretion disk surrounding an active black hole, there are fluid effects that cause some of the plasma to escape into the jets. The energy to allow this matter escape into the jets comes at the expense of other matter which falls into the black hole. So there is a significant amount of matter which is not initially on a trajectory to fall inside the Schwarzschild radius but is placed on such a trajectory. This infalling matter is orbiting in the range of 1.5 to 3 times the Schwarzschild radius, where circular orbits exist but are unstable. (No circular orbits exist inside 1.5 Schwarzschild radii because the velocity required to maintain such an orbit exceeds c.) This is unlike dark matter elements reaching this region, which will not fall further in unless perturbed by gravity. So while ordinary matter is not captured with 100% efficiency, the capture cross section is larger than for dark matter.

  7. #7 Kyle Wilson
    May 3, 2012

    In part, I’m wondering if there is some way to use this presumed increase in mass to look for a signal that might provide insight into the dark matter issue. If (for example) warm WIMPs are the real dark matter and given that the majority of the mass of the universe should be dark (by my understanding of the current data) and further given that the only phenomenon I know of that can capture such particles (given that they have to interact gravitationally in order to cluster around galaxies and provide the gravitational lensing that has been observed) would be a black hole where any particle passing within the event horizon can’t escape I’d expect that some predictions about black hole masses over time could be made that might allow some analysis to happen…I’m not a physicist (once wanted to be one, but wound up in engineering instead) so I don’t have the deep math or the contacts to dig here myself…so I thought I’d ask here and see what might be going on that I don’t know of…

  8. #8 Wow
    May 3, 2012

    There *ought* to be something possible, Kyle.

    But nobody is, as far as I can tell, trying to work out what Dark Matter is, concentrating at the moment on how much of it “must” there be.

    But your question is why scientists (and Ethan will back this idea to the hilt) do NOT sit in ivory towers and refuse to listen to “knowlessmen” (unlike most career politicians and lifelong executive types) because sometimes an idea will be raised that, if not actually new, is at least as insightful as any scientist could come up with.

    Being unable to work the answer out yourself doesn’t mean you can’t have a good question.

  9. #9 OKThen
    May 4, 2012

    Ethan, thanks, I did not know.

    Kyle
    arXiv.org has some interesting papers on this topic

    Dark matter and dark energy accretion onto intermediate-mass black holes, Mar 8 2012 says,
    “if cold dark matter has a nonvanishing pressure, the accretion of dark matter is large enough to increase the black hole mass well beyond the present observed upper limits. We conclude that either intermediate-mass black holes do not exist, or dark matter does not exist, or it is not strictly collisionless.” http://arxiv.org/pdf/1111.5605v2.pdf

    Dark Matter Accretion into Supermassive Black Holes, Feb 14, 2008 says,
    “The accretion of dark matter into black holes located in the center of galaxies and their halos was studied… . We found that dark matter contributes to no more than 10% of the total mass accreted by black hole seeds… comparable with other independent estimates, indicating that most of the accreted mass by seeds is baryonic in origin.” http://arxiv.org/pdf/0802.2041v1.pdf

    But I defer to any experts to put such research in perspective.

  10. #10 Trebor
    May 4, 2012

    @OKThen
    Caution should be used when mining arXiv.org for ‘papers’.
    As anyone can submit anything there, so unless you already know the topic well it can be hard to tell the junk from the interesting.
    And there certainly is a heck of a lot of junk there.

  11. #11 OKThen
    May 4, 2012

    Trebor
    Yes and..

    As Igor and Grihka Bogdanov published five related papers simulatneously in five diferent peer reviewed journals in 1996 (including the Annals of Physics).

    “The significance of the Bogdanov affair was hotly debated among physicists for the next few months, with most superstring theorists taking the position that this was just a case of a referees being lazy… Leaving aside the issue of whether the Bogdanovs are hoaxers or really believe in their own work. This episode definitely showed that in the field of quantum gravity one can easily publish complete gibberish in many journals, some of them rather prominent… The breakdown in refereeing is thus a serious threat to the whole academic research enterprise.” pg 213-210, Not even Wrong by Peter Woit 2006

    So yes we must be careful in deciding the junk (interesting or not) from the high value (interesting, insightful and clear headed) research. Having said that..

  12. #12 OKThen
    May 4, 2012

    “One unusual thing about the Bogdanov papers.. they were never submitted to the online preprint database.. Fewer and fewer physicists ever look at the print journals these days.”

    Even as an amateur, to continually learn, I must read the questionable even in credible sources. But it is always important that I accept something because I understand it and it makes sense to me (not because of some authority). As well, I am always ready to be proved wrong (“thank you for destroying one of my favorite misunderstandings”); because that means I understand something a little better.

    All understanding and knowledge is tentative.

  13. #13 Denier
    May 5, 2012

    If a 100 pound weight fell into a black hole, would the black hole get 100 pounds heavier or 1 pound heavier?

    Before you rush to the 100 pound heavier answer siting conservation of this or that, let me explain why this is confusing to me.

    A proton is made up of 2 up quarks and 1 down quark. If you total the rest mass of those 3 quarks, it makes up only ~1% of the total mass of the proton. The other ~99% of the proton’s invariant mass is made up of other things including virtual particles. In quantum chromodynamics(QCD) the term for the 1% is ‘current quark’ mass, while the term for the total with all of the virtual particles is the ‘constituent quark’ mass.

    Virtual particles rely on vacuum. There is no vacuum in an infinitely dense black hole. So if a weight fell in, does the black hole get the current quark mass, or the constituent quark mass? And why?

  14. #14 OKThen
    May 5, 2012

    “If a 100 pound weight fell into a black hole”

    The simplicity of your assumption implies a simple answer. Since 100 lb weight (i.e. a 45.3 kg mass) fell in; then it fell in.

    Your after the fact concern of how much of the mass is rest mass of elementary particles versus energy of various types (e.g. including relativistic mass as the object crosses the event horizon of the black hole) is irrelevant; because it is after the “fact” of your assumption.

    But the more serious question is: Is the “fact” of your assumption of a 45.3 kg object (or even a star) being completely swallowed by a black hole a physically reasonable assumption?

    And I think Ethan’s lesson in this post is “But that isn’t quite how the Universe works.” i.e. black holes don’t swallow stars or even 45.3 kg objects in that simple manner; because that simple manner is physically not possible.

    So Ethan or someone else correct me if I misunderstand.

    As well Ethan, it seems to me there is a follow-on lesson or two for you to give us in another post, in which you explain the various theoretically understood ways that a black hole can correctly (i.e. physically)increase in mass. I assume that some assumptions create unphysical infinities in the mass of black holes and that some kind of quantum assumptions are needed to prevent general and special relativity from resulting in infinite mass increases of black holes.

    Yes, I defer to all experts; please educate me.

  15. #15 Denier
    May 6, 2012

    After taking a night to think about it, the answer I’ve come up with is that there is no infinity dense part of a black hole. It is a slight of hand trick played by the universe.

    If a 100 pound weight were thrown at a black hole and crossed through the event horizon, the common center of mass of the 100 pound weight and all mass that had ever crossed the event horizon would exist in the center where the singularity is supposed to be. I say “supposed to be” because there is no singularity. The mass will be really dense, but there is no point in a black hole that is infinitely dense or where time-space is infinitely curved.

    The trick is played via gravitational time dilation. If you were outside a black hole with two synchronized clocks and tossed one of them towards the black hole, the clock headed towards the black hole would appear to slow down. The closer to the black hole it got, the slower it would appear from your perspective to run. Because the time-space of a singularity is infinity curved it would take the clock an infinite amount of time to get to the singularity.

    It is not just because the clock has to travel a distance in space. Even the very center of the mass that collapsed to form the black hole in the first place never gets to the singularity. It too is subject to gravitational time dilation and from the perspective of the outside universe, the collapse to singularity takes an infinite amount of time to happen.

    No black hole in our universe can have a singularity or will ever have a singularity. Black holes will either evaporate or the universe will end before a singularity can form.

  16. #16 drongs_real
    May 7, 2012

    All of these people explaining to each other, what a black hole does.
    You people mock science with this.

    If you can not see one, yet believe in one, you are a joke. There is no factual proof of a black hole in existence. Just as you have no clue what the atmosphere of any planet contains, other than earth’s. You can only assume.

    Black hole people are idiots.

  17. #17 Hate_Retards
    May 7, 2012

    drongs_real, there is no “end” to the universe

  18. #18 CB
    May 7, 2012

    @ drongs_real

    Proof is for math. In science we make assumptions, hopefully reasonably correct ones, and from there try to work out the implications and see if they meet up with empirical reality (assuming such a thing exists), and if so provisionally use that as a basis for understanding the world — this is what you might call “belief” but it is completely different than blind faith. By making many such predictions and tests we can be relatively sure that we are on the right track.

    And this is how we actually do have quite a few clues about the atmospheres of other planets. And how we’re able to have this conversation. Because it works. Assuming of course that reality exists and you aren’t just a figment of my imagination. Assuming I exist.

  19. #19 kenny
    May 7, 2012

    I loved this post!
    What a great little blog – just chanced upon it from reddit.
    Are you really an astrophysist?
    you write well!

    and great pic of you by the way.

    I learned so much, onward!

  20. #20 Michael Jones
    May 8, 2012

    I was wondering whether it would be plausible that a black hole is not a hole at all and more like an extremely small “dead star” meaning that it is just a inconceivably small object with the mass of the star and its remaining energy compacted into a not infinite small space (otherwise how would one black hole be larger than another) but a small space. Its gravity would be immense in that point in space and everything you explained would make more sense. Some black holes(dead stars) are more or less flatter than others because of the speed of the star in which it was traveling. The “dead star” keeps its revolutions per second but since the star has condensed into such a small object it is going at such a higher speed which centrifugal force would affect it. It would also play into my other theory that this universe is in a revolving cycle in which black holes or “dead stars” all eventually combine to form such a small point in space that it explodes causing the big bang. Please elaborate on anything that is wrong so I can learn. Thank You for your time.

  21. #21 cb
    May 8, 2012

    I was wondering whether it would be plausible that a black hole is not a hole at all and more like an extremely small “dead star” meaning that it is just a inconceivably small object with the mass of the star and its remaining energy compacted into a not infinite small space (otherwise how would one black hole be larger than another) but a small space.

    Ooh, I know this one! Many black holes are in fact the cores of dead stars that collapsed on themselves in supernova. The “size” of a black hole is not the actual size of the singularity but the Schwarzschild Radius — that distance at which gravity is so strong that the escape velocity is the speed of light — so once you go inside that there is no escape even for light. This radius depends on the mass, and that’s how black holes can have different “size”.

  22. #22 Wow
    May 9, 2012

    “If a 100 pound weight fell into a black hole, would the black hole get 100 pounds heavier or 1 pound heavier?”

    100 pounds.

    What do I win?

    “I say “supposed to be” because there is no singularity.”

    So do physicists. The laws of physics (that say there WILL be a singularity) break down well before then, therefore there’s no need for a singularity to exist.

  23. #23 Stephen
    May 10, 2012

    Black hole graphics are getting better and better.

    Another whole side of black hole physics is the reasoning that leads to the holographic principal, eg, the book The Black Hole Wars. This is so weird, i’m forced to conclude that i live in an alien Universe. I’m the normal one. My guts are inside, not encoded on my skin.