[HN Gopher] How do merging supermassive black holes pass the fin...
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How do merging supermassive black holes pass the final parsec?
Author : jnord
Score : 66 points
Date : 2024-10-24 02:12 UTC (20 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| andrewflnr wrote:
| > In some scenarios, the lightest of the three holes is ejected
|
| That's terrifying. Imagine a rogue supermassive black hole
| floating in intergalactic space.
|
| But I mostly want to know how badly self-interacting dark matter
| messes up the existing LCDM simulations that most astrophysicists
| sort of rely on?
| tivert wrote:
| > That's terrifying. Imagine a rogue supermassive black hole
| floating in intergalactic space.
|
| Why terrifying? It's literally doing nothing, far away from
| anything. Seems like the safest place for it to be.
| andrewflnr wrote:
| Not doing anything yet. If galaxies can collide, the rogue
| black hole can collide with your galaxy, and you won't get
| much warning either. (I mean, realistically you're right, in
| the same way that our galaxy colliding with Andromeda is
| scary but has negligible chance of affecting us. But,
| imagine.)
| yongjik wrote:
| Galaxies are big, including our own. Unless the rogue black
| hole is traveling near light speed, you will get at least
| tens of thousands of years of advance warning. (What you
| could do with that warning is a different matter,
| though...)
| andrewflnr wrote:
| If you were lucky enough to see the ejection happen, yes.
| But if one was already on the way?
| chowells wrote:
| You would absolutely notice the gravitational distortions
| a very long time in advance. That's a lot of mass. You
| can't miss the way it's distorting space for a very long
| way around it.
| khafra wrote:
| There are lots of easily visible stars on the other side
| of any supermassive black hole that's nearer to you than
| other galaxies are. When those stars start lensing in
| ways visible to the naked eye, you're going to know
| something weird is going on.
| The_Colonel wrote:
| > and you won't get much warning either
|
| A supermassive blackhole floating towards you would have
| very visible effects, it would be impossible to miss for
| millenias before it gets to you.
|
| It's the micro black holes which can hit you without a
| warning.
| rompledorph wrote:
| The article state that small black holes have the size of
| stars. Micro black holes should be quite big too I guess?
| The_Colonel wrote:
| That's likely a mistake in the article. "Small" black
| holes (i.e. smaller than supermassive) have star/stellar
| mass, but not size.
|
| Micro black holes are only hypothesized so far, but they
| could get very small - e.g. a black hole with Earth mass
| would have less than 1 centimeter in diameter.
|
| The size itself is not that important for spotting black
| holes, though. Even if it's as large as a star, all you
| see staring at the black hole "object" is nothing. What's
| important are the gravitational effects on the
| environment, and there the differences are stark. At a
| distance of 1000 light years, it will be difficult to
| spot a stellar-mass black hole floating through empty
| space, because its pull is strong enough only at stellar
| distances and won't produce enough disturbance in
| interstellar space for us to notice. OTOH supermassive
| blackholes will deform whole surrounding star systems
| because of its immense mass and gravitational pull. A
| micro black hole (e.g. Earth mass) passing through the
| solar systems would likely go undetected unless it
| collides with something (which is improbable). There
| could be a measurable disturbance, but it would be one-
| off and difficult to attribute to a black hole.
| ordu wrote:
| _> A micro black hole (e.g. Earth mass) passing through
| the solar systems would likely go undetected unless it
| collides with something (which is improbable)._
|
| Solar wind? Would it generate some interesting effects
| when coming too close to a black hole? All these protons
| accelerated to a near light speed, probably hitting each
| other and running away into a black hole.
| The_Colonel wrote:
| An Earth-mass black hole will have a similar
| gravitational effect on solar winds as Earth. Can we
| detect these effects on solar winds from distance? Maybe
| if we watch for them in that particular direction of
| empty space, but will we? The effects would be localized
| and short-lived at each particular place on the
| trajectory.
| usbsea wrote:
| An earth mass black hole would surely peturb various
| orbits of planets in a solar system though?
| zesterer wrote:
| There are likely several > earth-mass objects in our
| solar system that are as-yet uncatalogued, and almost
| certainly many hundreds of dwarf planet-mass objects too.
| Granted, they are further out, but their influence is
| almost undetectable even over millions of years. It's
| pretty unlikely that the presence of an earth-mass black
| hole for just a few years (since it would likely be
| moving many times solar escape trajectory) would have
| much in the way of a measurable impact.
| 3eb7988a1663 wrote:
| I am reminded of a quote from Mass Effect 2. Eventually that
| black hole could hit something. Damn
| straight! I dare to assume you ignorant jackasses know that
| space is empty. Once you fire this hunk of metal, it keeps
| going till it hits something. That can be a ship, or the
| planet behind that ship. It might go off into deep space and
| hit somebody else in ten thousand years. If you pull the
| trigger on this, you are ruining someone's day, somewhere and
| sometime. That is why you check your damn targets! That is
| why you wait for the computer to give you a damn firing
| solution! That is why, Serviceman Chung, we do not "eyeball
| it!" This is a weapon of mass destruction. You are not a
| cowboy shooting from the hip!
| FredPret wrote:
| In Neal Stephenson's Seveneves (read immediately) the moon
| explodes for an unknown reason.
|
| I always imagined it as being caused by a rogue mini black hole
| zipping through.
| caf wrote:
| A rogue star having a close interaction with our solar system
| would be catastrophic enough, and we _know_ there 's lots of
| those in our galaxy.
| ardel95 wrote:
| The potential to detect Supermassive Black Hole mergers is one of
| the reasons I'm really excited about the LISA project [1], and
| hope it actually gets funded and doesn't delay too much.
|
| [1]
| https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
| libraryofbabel wrote:
| Now, maybe I'm just a jaded cynic with a stale physics masters
| degree, but isn't there something depressing about this? Like,
| faced with an interesting anomaly out there in the world, we have
| to resort to tweaking the model of dark matter that is _already_
| an invention to fit observational anomalies. We've no way to
| detect dark matter directly, and so no way to prove or disprove
| this hypothesis, so is this really progress in physics at all? I
| mean, I know, I know, modified theories of gravity have a lot of
| problems, but what are the other possibilities here? Any current
| physicists care to weigh in?
| andrewflnr wrote:
| They did mention how they're hoping to find evidence from the
| pulsar timing array. It's not exactly easy to find the
| evidence, and it won't be tomorrow, but the idea at least seems
| falsifiable.
| antognini wrote:
| I actually spent quite a while in grad school thinking about
| the last parsec problem, and although I'm not in the field
| anymore I still think about it from time to time. (My thesis
| was on gravitational dynamics.)
|
| My perception of the field (which is now about a decade out of
| date, so take it with a grain of salt), is that there is quite
| a bit of skepticism about invoking exotic physics to solve the
| last parsec problem. Galaxies are generally pretty messy
| places, and the centers of galaxies are especially messy, so
| it's hard to know if you've correctly modeled all the relevant
| physics. A lot of astronomers aren't convinced that there
| really is a last parsec problem.
|
| The main "standard" approach to solve the last parsec problem
| is from scattering stars (which the article mentions).
| Basically, every now and then stars from the galaxy wander
| close to the orbit of the black hole binary and then get
| slingshotted out of the system. This removes energy from the
| orbit, and causes the black hole binary to shrink. The problem
| with this approach if you do a naive calculation is that the
| stars have to come from a particular set of directions, called
| the "loss cone" in the jargon. And since the orbits of stars in
| galaxies are probably fairly static, once a star gets kicked
| out of the loss cone, it doesn't come back. So over time the
| loss cone empties and the black hole orbit stops shrinking. The
| question is, does the orbit shrink far enough before the loss
| cone empties, and the answer to this question has generally
| been "no."
|
| The way around this is to question how static the orbits of
| stars in galaxies really are. One of the more important papers
| on the topic found that if an elliptical galaxy is sufficiently
| triaxial (that is, sufficiently non-spherical), then
| interactions between stars in the galaxy can repopulate the
| loss cone and cause the orbit to keep shrinking. But as I
| vaguely recall, not everyone was convinced by that result.
|
| I personally have had some ideas that galactic tides might
| contribute, especially right after the merger before all the
| orbits have had time to thermally relax. But I'm not in the
| field anymore and haven't really had time to really model this
| idea and see if it would work.
| davedx wrote:
| Considering the three body problem and extrapolating I would
| expect star orbits in a galaxy to be completely chaotic on
| long time scales?
| neutronicus wrote:
| A galaxy is a pretty degenerate instance of the n-body
| problem, though.
|
| A system of one or two giant bodies orbited by a collection
| of tiny ones. Each tiny one spends most of its time in a
| very weakly-perturbed two-body problem, quasi-stably
| orbiting the giant one(s) with tiny deviations caused by
| all the other ones. So you have to do some statistics to
| see how often an assumed distribution of quasi-stable
| orbits results in the tiny bodies approaching each other
| closely enough to kick one another into meaningfully
| different orbits of the giant ones.
|
| This gives you a better idea of how long is "long" and how
| it compares to the age of the universe
| privong wrote:
| > Considering the three body problem and extrapolating I
| would expect star orbits in a galaxy to be completely
| chaotic on long time scales?
|
| That's more or less captured by the loss cone calculation.
| On a very "long time scale" the loss cone will probably be
| occasionally replenished by stars coming in from scattering
| on larger scales, but the timescale for this to become
| significant for galaxies is substantially longer than the
| age of galaxies (and also longer than the Hubble time,
| which is the ~age of the Universe). So, at least from this
| back of the envelope calculation, that doesn't solve the
| final parsec problem.
|
| See, e.g., https://www.astro.umd.edu/~richard/ASTRO620/Dyna
| mics_Lec3.pd...
| rob74 wrote:
| I don't have a physics masters degree, or anything comparable,
| but when I read "Astrophysicists have a new suggestion: Dark
| matter could sap angular momentum from the two black holes and
| nudge them closer" I also thought "Really? Is _dark matter_ now
| the stand-in explanation for anything unexplainable? "
|
| But who knows, when one day someone will come up with a better
| model that does away with the "dark matter kludge", it will
| turn out that these phenomena actually have a common root
| cause?
| red1reaper wrote:
| I mean... isn't that the point of dark matter and dark
| energy? To be a stand-in name for stuff we have to account
| for but can't detect properly? The dark here means dark as in
| "we are in the dark about that".
|
| It does not really matter if those exist or are an artifact
| of current theories, replacing relativity is not that easy
| and there are already a ton of physicists working on that
| anyway.
|
| In the meanwhile of either having instruments capable of
| detecting these or a new theory that demostrates that those
| are artifact emerge... they have to exist. That's the point
| of them.
| at_a_remove wrote:
| Dark matter does not mean "we are in the dark about this,"
| at all.
|
| It's astronomy. It means "anything that is not shining like
| a star but we can detect it via inference." _Light_ matter
| is stars. Dark matter can include MACHOS: neutron stars,
| black holes, brown dwarfs, rogue planets. No new physics
| required for MACHOs, as opposed to WIMPs.
| dguest wrote:
| More positive spin: every "kludge" you add puts more
| constraints on what dark matter could be. If we can verify
| that the angular momentum transfer under this theory is
| consistent with other dark matter models, we can build a
| better framework to test a whole suite of theories with one
| experiment.
| ykonstant wrote:
| If it exists and is ubiquitous then it makes sense to
| contribute to all sorts of disparate phenomena; are we
| surprised that "mass" pops up in all sorts of places?
|
| Besides, epistemologically it is very good to suspect and
| investigate the effects of DM on all kinds of phenomena.
| Then, if it does not exist, we will have many more points of
| reference from which to derive experimental or theoretical
| contradictions.
| exe34 wrote:
| or conversely, I wonder how many different phenomena are
| "explained" by dark matter, since it's infinitely
| parametrisable.
| elashri wrote:
| > We've no way to detect dark matter directly, and so no way to
| prove or disprove this hypothesis
|
| We can and we are already searching for dark matter directly
| [1] and indirectly [2]. The phase space is being closed every
| now and then and this is progress. This gives us information
| about where to look next.
|
| [1]
| https://en.wikipedia.org/wiki/Direct_detection_of_dark_matte...
|
| [2]
| https://en.wikipedia.org/wiki/Indirect_detection_of_dark_mat...
| empath75 wrote:
| One way to think of non-detection is as evidence that we're
| looking in the wrong place. Another way to think of non-
| detection is as evidence that it doesn't exist.
| mr_mitm wrote:
| > One way to think of non-detection is as evidence that
| we're looking in the wrong place
|
| We aren't looking in only one place
|
| > Another way to think of non-detection is as evidence that
| it doesn't exist.
|
| Absence of evidence is not evidence of absence. Exhausting
| the parameter space until evidence is found is simply the
| normal scientific progress.
| zmgsabst wrote:
| How many places do I need to look before I start to have
| evidence my lost wallet isn't in my house?
|
| The normal spot on the shelf? ...checking my pockets?
| ...searching the bags I used to go shopping?
|
| At each step, another of my theories about where my
| wallet is gets disproven -- but none are direct evidence
| it's not in my house.
|
| We are playing the same game with dark matter: they keep
| checking spots and it keeps not being there. At what
| point does checking wrong theories start to suggest that
| the entire idea is flawed?
| mr_mitm wrote:
| When you exhausted the parameter space (assuming you can
| with our current technical abilities) or a competing idea
| with actual evidence comes along. It's not like people
| aren't trying.
| elashri wrote:
| The parameter space is usually (not only) a two
| dimensional plot of mass and the coupling force
| parameter. Those can give you a space that you would want
| to explore. There will be no one experiment that would
| give you sensitivity in all the parameter space. So you
| design experiments and collect data. If you don't find in
| the region your experiment were sensitive too (and you
| did your analysis carefully) then you establish a limit
| that says basically it cannot be in this region, look in
| other regions, then repeat.
|
| The analogy with your wallet is misleading because we
| know your habits, we don't know about nature habits.
|
| As I explained, when we exhaust the parameter space
| regions and can't find anything then this would tell us
| to give a shot to something else. Not that people are not
| doing this now already anyways.
| nkrisc wrote:
| Except you don't know which it is until you've exhausted
| the search space.
| elashri wrote:
| "Absence of evidence is not evidence of absence"
|
| William Wright [1]
|
| [1] https://quoteinvestigator.com/2019/09/17/absence/
| marcosdumay wrote:
| That's only true if you don't look.
| margalabargala wrote:
| The statements "We are attempting to directly detect dark
| matter, so far without success" and "We have no way to detect
| dark matter directly" can both be true. The first does not
| disprove the second.
| biorach wrote:
| > but isn't there something depressing about this? Like, faced
| with an interesting anomaly out there in the world, we have to
| resort to tweaking the model of dark matter that is already an
| invention to fit observational anomalies
|
| This is unnecessarily negative. Physics progresses in fits and
| starts, with plenty of blind alleys and red herrings. Maybe
| we're in a phase akin to the aftermath of the Michelson-Morley
| experiment. Or maybe it really is that complicated out there.
| pantulis wrote:
| > Maybe we're in a phase akin to the aftermath of the
| Michelson-Morley experiment.
|
| Loving this comparison, and I hope this is the case.
| horsawlarway wrote:
| I don't really find it unnecessarily negative.
|
| I find dark matter seems to fit into the same pattern as
| epicycles. We can add additional complexity to the theory to
| make our models better match observational data, and that's
| _useful_ but also strongly hints that something more basic
| about the model is fundamentally incorrect. That 's
| depressing.
| mr_mitm wrote:
| Which additional complexity has been added to dark matter?
| I'm not aware of anything. A WIMP would still explain
| everything, no? I don't see the comparison to epicycles at
| all. Then again, almost no one with a formal education in
| cosmology or partical physics at the PhD level does.
| Curious how that works.
| horsawlarway wrote:
| See - now this... this I find needlessly negative.
|
| > Then again, almost no one with a formal education in
| cosmology or partical physics at the PhD level does.
|
| This is also just wrong. I think we're seeing a number of
| established voices starting to question the underpinnings
| of dark matter as a theory.
|
| As for
|
| >Which additional complexity has been added to dark
| matter?
|
| Let's start with the fucking article we're discussing,
| where we now need to have "self-interacting" dark matter,
| as a new spin on the theory to account for the forces
| needed to explain what we're observing...
| mr_mitm wrote:
| Which established voices do you mean? All I see is the
| usual suspects, i.e. McGaugh, Kroupa, etc.
|
| The article is also not adding complexity. We didn't get
| new evidence that required modification of DM. We got a
| phenomenon which has many potential explanations as the
| "fucking" article points out, one of which could be a
| subset of all DM theories. The final parsec problem isn't
| challenging DM at all, instead one particular flavor of
| DM could help explain the observation. If anything, it
| could constrain DM. Why wouldn't you explore this
| possibility?
| tsimionescu wrote:
| Dark matter models have different amounts and
| distributions of it for each galaxy we care to look at,
| so there is no predictive power, similar to how they had
| to add new epicycles to explain the motion of every new
| planet they observed.
| Keysh wrote:
| That's not even remotely correct.
|
| Just today I heard a talk about people looking at stellar
| motions in ultra faint dwarf galaxies, because standard
| dark-model theories predict they should have centrally
| cusped dark-matter density profiles -- and fuzzy dark-
| matter models predict different profiles, which they
| could potentially discriminate between.
| sebzim4500 wrote:
| Detecting objects by first detecting their gravitational
| influence has been successful in the past, see the
| discovery of Neptune for example.
| zmgsabst wrote:
| Michelson-Morley worked; we call it LIGO.
|
| The period after the apparent failure to the present success
| has been incredibly fruitful.
| tsimionescu wrote:
| The two are quite unrelated. Michelson-Morley was looking
| for a difference in the speed of light reaching the Earth
| from the Sun depending on the direction of motion of the
| Earh. Gravitational waves are not related to that in any
| way.
| misja111 wrote:
| The article did mention some other possibilities: stars swung
| out of the surrounding galaxy into the orbiting black holes,
| our friction due to surrounding gas disks. Both of these events
| would take out angular energy as well but it's not sure if it
| would be enough though.
| bradley13 wrote:
| I don't think you're a cynic at all. I object to the entire
| "dark matter" hypothesis, because there is zero evidence for
| it. What we have, are observations that indicate errors in our
| models. We need to look for those errors, not invent ever more
| abstruse version of "ether" to explain them away.
| marcosdumay wrote:
| When you get a more complex blank to fill, the normal thing to
| do is to improve all of your blank-filling models to adapt for
| it.
|
| There's nothing wrong with that part.
|
| I do have a problem with the single-minded insistence on
| disqualifying any exploratory study on alternatives to a model
| that gets more and more partially falsified all the time1. To
| the point that only iffy personalities that don't care for
| their careers decide to work on them. But on a situation like
| this, those other models would probably be tweaked too.
|
| 1 - I've never noticed it before, but that phrase gives me
| great "Superstring Theory" vibes.
| InDubioProRubio wrote:
| It is interesting though.. if you think about the stage of
| physics before relativity. The made up compensations
| accumulating, but these made up formulas and compensation
| strategies had a signature- a sort of kinematic model
| connection, from which one could have guessed with a
| unconventional thinker at the encompassing more correct model.
| Like a formula, whos convex hull you can trace by swinging it
| wildly.
| mpreda wrote:
| Is there an equivalent of Tidal Heating [1] taking place between
| the two black holes? It would extract kinetic energy and put it
| into.. heating the black holes.. whatever that may mean. Assuming
| there is movement and friction in the core of a black hole.
|
| [1] https://en.wikipedia.org/wiki/Tidal_heating
| isoprophlex wrote:
| A black hole only has mass, angular momentum, and charge. So
| you can only deal with those three... not deform and knead a
| physical mass of planet as in tidal heating.
| orlp wrote:
| This is often stated but it literally can't be true. It's
| true for _stationary solutions_ to the relevant equations,
| but we 're very much not living in a stationary world.
|
| Here's a simple thought experiment disproving your claim. A
| person hovers just above the origin of a supermassive black
| hole. They chuck a massively charged object into the black
| hole. If what you said is true they should observe the charge
| instantly being transported to the singularity, since a black
| hole can't have any attributes such as where charge is
| distributed within the horizon.
|
| Where it gets impossible is that someone very far away around
| the same supermassive black hole could observe a small charge
| increment. They in turn could chuck charged stuff into the
| black hole and now you've got faster than lightspeed
| communication.
| elashri wrote:
| > above the origin of a supermassive black hole
|
| I spent couple of minutes trying to understand your thought
| experiment and was puzzled why can't I understand it. It
| seems that it is probably because I don't understand what
| you mean by "just above the origin of supermassive black
| hole"?
|
| I feel that you have something interesting to say but not
| clear.
| orlp wrote:
| Mental typo, I meant horizon, not origin.
| elashri wrote:
| The no-hair theorem (scientific term for what you are
| descriping) essentially states that black holes can be
| completely described by only three externally observable
| classical properties: mass, charge, and angular momentum.
| The internal distribution of matter or charge within the
| black hole is irrelevant from the outside once it crosses
| the event horizon.
|
| The charge of an object thrown into a black hole doesn't
| need to "instantaneously" reach the singularity. In fact,
| it doesn't even "travel" in the sense that the observer
| would see a time delay based on where the charge is
| inside the horizon. The EM field generated by this
| charged object can still be observed outside the horizon
| before the object crosses the horizon. In classical GR,
| the exterior of a charged black hole is governed by the
| Reissner-Nordstrom metric [1], and it already includes
| the influence of charge. Once the object enters the
| horizon, the outside observer will perceive the black
| hole's external field to have changed. The field
| adjustment is instant for the outside observer because,
| from their point of view, the object never actually
| crosses the event horizon (due to infinite time dilation
| at the horizon from their perspective). The charge
| appears to have been "absorbed" by the black hole when
| the object is still outside the horizon.
|
| So no-hair theorem doesn't imply that properties such as
| charge or angular momentum must be "smeared"
| instantaneously to the singularity inside the black hole.
| The theorem only describes how these properties manifest
| externally, not their behavior inside the horizon. Once a
| charge object passes the event horizon, the information
| about its charge is not causally connected to any
| observer inside the black hole (except in QM contexts,
| where the information paradox becomes relevant). However,
| the charges affects the external metric of the black hole
| immediately and completely as seen by external observers.
| Also it doesn't say that the charge or mass needs to be
| uniformly distributed or behave in any specific way
| inside the event horizon. It only states that from the
| outside, black holes look like point-like objects
| characterized by mass, charge, and angular momentum. The
| specifics of how charge is distributed inside the black
| hole's event horizon aren't visible to outside observers
| and therefore don't affect the validity of the theorem.
|
| [1] https://en.wikipedia.org/wiki/Reissner%E2%80%93Nordst
| r%C3%B6...
| orlp wrote:
| Again, the no-hair theorem only talks about stationary
| solutions:
|
| > The no-hair theorem (which is a hypothesis) states that
| all *stationary* black hole solutions of...
|
| https://en.m.wikipedia.org/wiki/No-hair_theorem
|
| > The field adjustment is instant for the outside
| observer
|
| And as my above thought experiment shows, any instant
| changes like such could be used for FTL communication.
| elashri wrote:
| What is unique about your thoughts experiment that makes
| it applies only to dynamical phases?
|
| I am really not getting what you are trying to say?
| orlp wrote:
| The point I'm trying to make is that everyone always
| cites the no hair theorem but completely leaves out the
| "stationary solution" part, leading to misconceptions. My
| thought experiment highlights that one _can 't_ treat a
| black hole as a particle that only has mass, charge and
| angular momentum in a dynamic world, because such a
| treatment must inherently lead to instantaneous updates
| to large regions of spacetime in response to 'stimuli'
| (in the form of mass, charge or angular momentum) for
| super massive black holes, which is impossible.
| elashri wrote:
| I understand that this is valid for stationary solution
| which is what an astronomical black hole would be most of
| the time. What I don't get is why do you assume that this
| would violate causality. In GR the idea that changes must
| "instantaneously update" large regions of spacetime due
| to supermassive black holes ignores relativistic effects
| like time dilation. From the perspective of an external
| observer, as an object falls into a black hole, time
| appears to slow down near the event horizon. The
| infalling object's influence on the black hole's external
| field (e.g., its charge or mass) is perceived gradually
| by distant observers.
|
| For supermassive black holes, this process might seem
| slow due to the massive scale of spacetime curvature near
| the event horizon, but there is no requirement for
| instantaneous changes. In fact, relativistic causality
| ensures that no information can propagate faster than
| light, so updates to the black hole's charge, mass, or
| angular momentum are constrained by the speed at which
| signals (gravitational or electromagnetic) can travel.
| WJW wrote:
| I think time would pass incredibly slowly for an observer
| that close to the black hole? Since the event horizon
| provides a singularity where time slows to zero, it makes
| sense that any non-zero speed along it would go to
| infinity for an outside observer.
|
| I doubt you could usefully exploit this behavior, because
| any charges would still need to travel to and away from
| the black hole at no more than light speed, and because
| time slows down the further you get to the event horizon
| the shortest path "around" a black hole would probably
| not go through it.
| saagarjha wrote:
| You can't actually observe objects going into a black hole.
| They just become redshifted and fall slower and slower from
| your perspective towards the event horizon.
| isoprophlex wrote:
| The singularity is not the same thing as its event horizon.
|
| That said, it's the no hair _theorem_. It could of course
| still be wrong...
| ben_w wrote:
| The object will be observed asymptotically _decelerating_
| as it approaches the event horizon, never gets observed
| crossing by a distant observer.
|
| IIRC the event horizon expands to encompass the object just
| before it gets to the original horizon, due to the
| Schwarzschild formula.
|
| Also IIRC, the electric field is blurred out by the
| geodesics by this point, as if it were from the interior.
| But that's based on what I've heard, I have yet to derive
| useful results from the Einstein field equations, even
| though I think I should give it a go and I can follow them
| well enough to code a simple simulation...
| gizmo686 wrote:
| Gravitational waves have a similar effect of sapping the
| kinetic energy of orbiting black holes. Instead of heating the
| black holes, the energy is emitted as gravitational radiation.
| Having said that, now that we have actual observations of the
| gravitational radiation emmited from merging black holes, we
| can be pretty confident in our understanding of the magnitude
| of this energy loss. And that understanding is that it does not
| become significant until well into the last parsec.
| vlovich123 wrote:
| But however insignificant it is, wouldn't it always be there
| sapping angular momentum & over a long period of time drag
| any two objects closer and closer together? What would be the
| repulsive force counteracting this?
| Salgat wrote:
| So the question isn't how they merge, but how they do it so
| quickly.
| antognini wrote:
| Yes, dynamical friction can "quickly" (on the order of
| 100 Myr) bring the two black holes to a distance of ~1 pc
| to each other. Gravitational wave radiation can "quickly"
| cause a merger if they are within a bit less than ~0.1 pc
| of each other. But how they go from 1 to 0.1 pc on a
| timescale of ~100 Myr is unknown.
| ardel95 wrote:
| It would be. Which is why any pair of orbiting bodies will
| eventually collide.
|
| It's just that for black holes this effect is insignificant
| (a merger would take much longer than the age of the
| Universe) until they get close to each other, much closer
| than 1 parsec.
| euroderf wrote:
| OT but, is there any good reason why "parsecs" are used as
| distance units rather than "light-[time units]" ? The latter
| enjoys broad familiarity in the general community.
| lou1306 wrote:
| Not a physicist but it seems to be part of astrophysics
| culture, and it's a good "base measure" for interstellar space
| (i.e., if something is < 1pc away it's likely within your same
| star system). A parsec is ~3.26 lys, for context.
| hnuser123456 wrote:
| At the stellar density in our region of the galaxy, sure. The
| density is hundreds of times higher closer to the center.
| vikingerik wrote:
| It's customary in astronomy because it correlates more directly
| with observation. An object 10 parsecs away exhibits a parallax
| shift of 1/10 arcsecond.
| sulandor wrote:
| supposedly not besides being a good shibboleth
| dguest wrote:
| link to papers:
|
| - https://arxiv.org/abs/2401.14450
|
| - https://arxiv.org/abs/2311.03412
|
| - https://arxiv.org/abs/2311.18228v1
| Jean-Papoulos wrote:
| "heh it's probably dark matter"
|
| Saved you a click.
| foobarkey wrote:
| Every time the math doesn't check out throw in some dark matter
| :)
| mxmilkiib wrote:
| Diaspora, by Greg Egan
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