[HN Gopher] LIGO detects most massive black hole merger to date
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LIGO detects most massive black hole merger to date
Author : Eduard
Score : 119 points
Date : 2025-07-14 20:06 UTC (2 hours ago)
(HTM) web link (www.caltech.edu)
(TXT) w3m dump (www.caltech.edu)
| perdomon wrote:
| What happens when black holes collide? Does one black hole
| "consume" the other? Do they become a larger black hole? Does it
| get more dense or just larger?
| chasil wrote:
| What happens inside cannot be known.
|
| As I understand it, black holes are defined by three
| quantities: mass, spin, and charge.
|
| I'm assuming that these quantities will be additive post-
| merger.
|
| Edit: "The black holes appear to be spinning very rapidly--near
| the limit allowed by Einstein's theory of general relativity."
|
| Perhaps the additive spin becomes asymptotic. Alternately, the
| gravitational waves might have departed with the energy of the
| excess spin.
| jameskilton wrote:
| My basic understanding is that they combine, basically you just
| add the masses together. That increased mass then means more
| gravity, so the event horizon is pushed further out.
| __MatrixMan__ wrote:
| They become a more massive one. The volume of a black hole
| (assuming you're measuring at the event horizon) is determined
| only by its mass, so the final density is the same as you'd get
| for any other black hole of that mass regardless of how it came
| to be.
|
| I don't know how to address the "consume" question. If you were
| pulling on a piece of fabric and two tears in it grew until
| they met each other to become one tear... would you say that
| the larger one consumed the smaller?
| a012 wrote:
| I think the GP meant "merge"
| dataflow wrote:
| > The volume of a black hole (assuming you're measuring at
| the event horizon) is determined only by its mass, so the
| final density is the same as you'd get for any other black
| hole of that mass regardless of how it came to be.
|
| Wait, really? So if you had a super massive disk that was
| just 1 electron away from having enough mass to become a
| black hole... and then an electron popped into existence due
| to quantum randomness... then it would become a sphere
| instantly? Wouldn't that violate the speed of light or
| something?
| gus_massa wrote:
| It's the https://en.wikipedia.org/wiki/No-hair_theorem ,
| but it only applies after a while, not instantly.
|
| Your disk will emit a lot of gravitational on
| electromagnetic radiation, and after a while it will be a
| nice sphere. (Unless it's rotating and it will be a nice
| somewhat-elipsoidal ball.)
|
| ---
|
| > _and then an electron popped into existence due to
| quantum randomness_
|
| I feel there is a huge can of worms of technical problems
| in this sentence that nobody know how to solve for now.
| Just in case replace the quantum randomness with a moron
| with a broken CRT used as an electron cannon.
| ars wrote:
| > and after a while it will be a nice sphere
|
| Time doesn't exist for black holes, so "after a while" is
| not something you can say about them.
| addaon wrote:
| > then it would become a sphere instantly
|
| Event horizons are non-physical. Better to think of it as
| "then a spherical event horizon would become apparent."
| When the mass within a given black-hole-shaped volume
| (spherical for non-rotating mass) is "one electron short"
| of being a black hole, then one can define a surface in the
| shape of the (future) black hole where the escape velocity
| is /just/ below the speed of light. In practice, all light
| emitted within that volume will already be captured by the
| mass, unless it's perfectly perpendicular to the (future)
| event horizon. When that extra electron is added, it
| becomes true that the escape velocity at that same surface
| is now the speed of light -- the definition of event
| horizon. But nothing needs to "form" to make this true.
| ars wrote:
| That electron would take an infinite amount of time to
| reach the edge, since time dilates to infinity with gravity
| that strong.
|
| > a sphere instantly
|
| The concept of instantly doesn't work with time dilation
| like this. What you see will be different depending on if
| you are also falling in, or if you are far away.
| gus_massa wrote:
| > _the "consume" question_
|
| My guess is that in some popular depictions black holes are
| like holes, and things fall in the holes, and even a small
| black hole can possible fall inside a bigger hole.
|
| A better image is too drops of water on a glass, add some
| black ink for bonus realism. They merge into a bigger drop.
| Except, obviously black holes are not filled with water. And
| the " _average density_ " of the new black hole is smaller
| then the " _average density_ " of both original black holes,
| unlike the density of water drops on a glass. So don't take
| this image too literaly.
|
| (There are some problems to define the "density" of a black
| hole, but let's ignore all of them.)
| hnuser123456 wrote:
| They become a larger black hole, mostly conserving mass, minus
| a few percent to gravitational waves. However, their mass is
| proportional to their radius, not volume, so it gets LESS
| dense. If you laid out a bunch of black holes in a line, just
| barely not touching, and let them merge, suddenly, the whole
| sphere of space enclosing the line becomes black hole. It also
| turns out that a black hole with the mass of the universe would
| have a volume about the size of the universe.
| JumpCrisscross wrote:
| > _turns out that a black hole with the mass of the universe
| would have a volume about the size of the universe_
|
| Mass and energy.
| gjm11 wrote:
| Is that intended to be a correction? (I don't think the
| original statement needs correcting, other than by
| replacing "universe" with "observable universe" in both
| places.)
| hnuser123456 wrote:
| Up until the universe was around a few billion years old,
| its Schwarzchild radius would have been larger than even
| the co-moving (not just observable) universe's radius,
| but the initial momentum from the big bang was high
| enough to prevent collapse.
| AnimalMuppet wrote:
| That sounds suspiciously like "they were inside a region
| with enough mass to form an event horizon, but they
| escaped because they had enough momentum", which in turn
| sounds like "we can escape from inside an event horizon
| if we just move fast enough". Can you explain how that's
| _not_ what you 're saying?
| hnuser123456 wrote:
| I wish I had a straightforward answer to that. I'm sure
| the answer is some combination of cosmic inflation and
| dark energy, but by all means it appears the early
| universe either narrowly escaped, or simply is a black
| hole, that singularities are a flawed concept, that
| nothing is escaping the universe, and we are all stuck
| moving forward in time, and that the infinite future is
| the singularity.
| jMyles wrote:
| Is that true though?
|
| Can't we generalize to say that we observe that black
| holes have a similar density (which is to say, proportion
| of mass to volume) any sample of the observable universe
| sufficiently large as to be roughly uniform?
|
| In other words, doesn't this observation scale both down
| (to parts of the universe) and up (beyond the
| cosmological horizon, presuming that the rough uniformity
| in density persists), at least for any universe measured
| in euclidian terms?
|
| It's very possible that I'm wrong here, and I'd love to
| be corrected.
|
| ...I also think we have to acknowledge that "similarly"
| is doing a fair bit of work here, as we're not accounting
| for rate of expansion - is that correct?
| itronitron wrote:
| >> just barely not touching
|
| Which part of them is barely not touching?
| gjm11 wrote:
| The event horizons.
| dylan604 wrote:
| In cosmological terms, what is barely not touching? Is that
| distance measured in meters, kilometers, AUs, lightyears,
| parsecs?
| pantalaimon wrote:
| > minus a few percent to gravitational waves
|
| They actually convert up to 42% of their mass into energy,
| mostly radiation
|
| https://youtu.be/t-O-Qdh7VvQ
| foota wrote:
| I think this is over their lifetime, not when they merge?
| hnuser123456 wrote:
| For normal matter inspiraling, yes, but a black hole which
| is falling into a black hole doesn't get to glow in gamma
| rays to try to escape :) they can only lose mass/energy by
| making splashes in spacetime itself (or hawking radiation)
| ars wrote:
| My understanding is they just spiral into each other forever.
|
| From our point of view nothing can actually fall into a black
| hole, instead it time dilates into nothing. "It is true that
| objects that encounter the event horizon of a black hole would
| appear "frozen" in time"[1]
|
| So we would never actually see the black holes merge. In fact
| I'm not clear how a black hole can even form in the first
| place, since it would take an infinite amount of time to do so
| (again, from our POV).
|
| (And yes, I know that from the POV of the falling object, they
| just fall in like normal. But that doesn't help us, because
| we'll never see it.)
|
| [1] https://public.nrao.edu/ask/does-an-observer-see-objects-
| fro...
| cloudrkt wrote:
| I wonder how the singularities would merge with each other.
| Enginerrrd wrote:
| We can't REALLY answer questions about what's inside the event
| horizon, but some real work has been done on what BH mergers
| look like, though even that as I understand, is extremely
| difficult model.
|
| https://m.youtube.com/watch?v=5AkT4bPk-00
| TechDebtDevin wrote:
| What are the waves of gradient colors, gravity?
| MattPalmer1086 wrote:
| I wonder what would happen if one black hole shot through another
| one at high relativistic velocity, instead of spiralling towards
| one another.
| fsmv wrote:
| They would merge and produce a black hole with the sum of their
| momentums
|
| Because nothing can ever leave the event horizon black holes
| are essentially perfectly sticky.
| mkw5053 wrote:
| So, if two black holes, each with mass M, were moving at
| nearly the speed of light and collided head-on (resulting in
| a final velocity of zero), what would happen to all that
| momentum? Would the resulting black hole have a mass greater
| than 2M? If so, how and why would this occur?
| mkw5053 wrote:
| I think I'm going to answer my own question by saying both
| momentum and energy are conserved. The momentum of the
| entire system was zero before and after the collision.
| Energy must also be conserved, and since the final object
| is at rest, all the kinetic energy gets converted into rest
| mass energy, minus what is radiated away as gravitational
| waves.
| hnuser123456 wrote:
| My hunch is they would briefly pancake and much of the
| mass/energy contribution from their initial velocities
| would dissipate as incredibly high amplitude gravitational
| waves from the ring-down.
| dkural wrote:
| It would create a universe, obviously. First all the mass
| would attempt being squished at a singularity. WHILE the
| squishing continues, the first-in-line stuff would've
| already started to explode back-out inside the event
| horizon. From the inside viewpoint, this looks like the big
| bang. Once all the mass from the two black holes collide
| and loose momentum, the inside-universe no longer expands
| as fast. Things wobble a bit as all this happens, creating
| tangles and non-homogeneity. Could be caused by initial
| Planck-scale uncertainties even when having a perfect head-
| on collision.
| fooker wrote:
| > Because nothing can ever leave the event horizon black
| holes are essentially perfectly sticky.
|
| If Hawking radiation turns out to be non existent, yes.
|
| Also, we don't know if it's possible to 'crack' open a black
| hole. If anything, another black hole might be the perfect
| instrument for doing this.
| im3w1l wrote:
| When you say cracking open a black hole do you mean
| cracking the event horizon to form a naked singularity?
| fishsticks89 wrote:
| Hawking radiation occurs because black holes are sticky !!
| snowwrestler wrote:
| The escape velocity from inside the event horizon is faster
| than the speed of light, which is the highest possible speed in
| the universe.
|
| So black holes cannot approach each other faster than the speed
| of light. And if their trajectories intersect perfectly, they
| won't be able to escape each other's gravity.
|
| A black hole can't pass "through" another black hole like two
| bullets hitting each other. More like two incredibly strong
| magnets hitting each other in midair.
| ChrisArchitect wrote:
| [dupe] https://news.ycombinator.com/item?id=44555220
| mnemonk wrote:
| Man, that is some seriously interesting phenomena:
|
| "The black holes appear to be spinning very rapidly--near the
| limit allowed by Einstein's theory of general relativity,"
| explains Charlie Hoy of the University of Portsmouth and a member
| of the LVK. "That makes the signal difficult to model and
| interpret. It's an excellent case study for pushing forward the
| development of our theoretical tools."
| WrongOnInternet wrote:
| > the 225-solar-mass black hole was created by the coalescence of
| black holes each approximately 100 and 140 times the mass of the
| Sun.
|
| Does this mean that 15 solar masses were converted into energy?
| Because that's a LOT of energy.
| andrepd wrote:
| Yes! And still, gravity is so weak that that immense amount of
| energy translates to just a relative contraction of less than
| 10^-20, or about a hair's width in the distance from the Earth
| to the Moon.
| cgdl wrote:
| Do we know how far this event was from earth? Wouldn't that
| distance be the determiner of what the relative contraction
| observed on earth would be?
| sgustard wrote:
| estimated distance of 2.2 Gpc per
| https://en.wikipedia.org/wiki/GW231123
| BurningFrog wrote:
| I've always thought the event horizon for a black hole has to be
| spherical.
|
| But my physics intuition tells me that as two of them merge, the
| resulting BH should have a "peanut" shape, at least initially.
|
| And maybe it can keep having an irregular shape, depending on the
| mass distribution inside it?
| itishappy wrote:
| It's only spherical in a Schwarzschild (non-rotating) black
| hole. A rotating black hole is called a Kerr black hole, and
| stuff gets weird, such as there being an oblate event horizon,
| a weird outer horizon called an ergosphere where spacetime gets
| dragged along such that it's impossible to stand still and you
| can accelerate objects using the black hole, a weirder inner
| horizon called the Cauchy horizon where time travel is
| possible, and a singularity in the shape of a ring. Your
| intuition is correct that during a merger it would be weirder
| still.
|
| https://en.wikipedia.org/wiki/Kerr_metric
|
| https://arxiv.org/pdf/0706.0622
|
| https://en.wikipedia.org/wiki/Ergosphere
|
| https://en.wikipedia.org/wiki/Cauchy_horizon
|
| Edit: Updated the bit about about horizons as I research a bit
| more. It's complicated, and I'm still not positive I have it
| exactly right, but I think it's now as good as I can get it.
| AnimalMuppet wrote:
| Could you (or anyone) tell what the radius of the ring
| singularity is, in terms of mass and angular momentum? I
| haven't been able to find that.
| itishappy wrote:
| The math seems to suggest R=a, or simply the spin in terms
| of length. It's certainly an oversimplification, as the
| answer will depend on the choice of metric.
|
| Here's the best resources I've been able to find on the
| question. Roy Kerr himself responded to the Quora question:
|
| > There is no Newtonian singularity at the Center of the
| earth and there is no singularity inside a rotating black
| hole. The ring singularity is imaginary. It only exists in
| my solution because it contains no actual matter. When a
| star collapses into a black hole it keeps shrinking until
| the centrifugal force stabilizes it. The event shell forms
| between the star and the outside. In 57 years no one has
| actually proved that a singularity forms inside, and that
| includes Penrose. instead, he proved that there is a light
| ray of finite affine length. This follows from the "hairy
| ball theorem".
|
| The stack overflow answer seems to describe the problem in
| terms I can better understand:
|
| > It seems unlikely to me that you're going to be able to
| formulate a notion of diameter that makes sense here.
| Putting aside all questions of the metric's misbehavior at
| the ring singularity, there is the question of what
| spacelike path you want to integrate along. For the notion
| of a diameter to make sense, there would have to be some
| preferred path. Outside the horizon of a Schwarzschild
| black hole, we have a preferred stationary observer at any
| given point, and therefore there is a preferred radial
| direction that is orthogonal to that observer's world-line.
| But this doesn't work here.
|
| https://physics.stackexchange.com/questions/471419/metric-
| di...
|
| https://www.quora.com/What-is-the-typical-diameter-
| roughly-o...
| CGMthrowaway wrote:
| Simulation: https://www.youtube.com/watch?v=Tr1zDVbSjTM
| captainkrtek wrote:
| Here is an animation from MIT/CalTech of what a merger looks
| like:
|
| https://youtu.be/1agm33iEAuo
| chasil wrote:
| Does the black hole's spin deform the event horizon?
|
| I think so?
|
| https://archive.ph/VrzwW
| kens wrote:
| A month ago, the proposed NSF budget would shut down one of the
| two LIGO observatories in the US, wrecking its ability to
| triangulate the location of events such as this black hole
| merger. A shutdown would also severely damage the noise margins
| and detection rate. Does anyone know if the shutdown is still
| planned? (I couldn't find any recent info.)
|
| https://www.science.org/content/article/trump-s-proposed-cut...
| ac794 wrote:
| I believe the proposed budget is being marked up tomorrow (July
| 15th, 12:00). Currently the NSF budget is set to be ~$7
| billion, a 23% cut compared to FY2025. I'm not sure how this
| affects LIGO exactly.
|
| https://appropriations.house.gov/sites/evo-subsites/republic...
| amarcheschi wrote:
| I had read something less recent than what you posted, but in
| that is said about 40% of ligo funding would be cut
| https://www.science.org/content/article/trump-s-proposed-
| cut...
|
| Then again, your file has less drastic reductions on nsf
| budget so who knows what would be the impact on ligo
| amarcheschi wrote:
| I was last week at an event in Pisa at virgo ego (basically
| ligo's cousin). It was to celebrate the 10th anniversary of
| finding gravitational waves iirc. There were an actress reading
| from the book the director of the Italian program wrote
| accompanied by the sound of waves made with sax. I can't
| describe it with words but it was truly moving.
|
| There were also moments dedicated to interviewing a science
| communicator and the director of the virgo center, and he was,
| let's say, quite angry at the thought of ligo losing funding.
| Rightfully so
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