Let’s see — the Tsar Bomba nuclear weapon released the equivalent of converting about 2.3 kg of matter into energy (1).
One solar mass is about 2 x 10^30 kg, so round numbers this event released the same as 10^31 Tsar Bombas, which is … a lot of energy? That number is too big to be a good intuition pump.
Let’s try again: over the course of its entire lifetime of about 10 billion years, the sun will release about 0.034% of its mass as energy (2). So one solar mass of energy is about 3000 solar-lifetime-outputs.
So this event has released about as much energy as 45,000 suns over their entire lifetime. I’m not sure how much of the energy was released in the final few seconds of merger, but probably most of it? So… that’s a lot of energy.
Yeah, it's alot alot :-). Over on Mastodon I asked Phil Plait (@badastro) if the "missing mass" in the universe might be a result of black holes converging[1]. He wrote up this event in his newsletter[2] and points out that when they merge, they emit more energy in that instant than every single start in the universe in the same instant. So kind of like an instant of double energy. Hard to fathom how much energy that is with my meager mammalian brain.
I have read somewhere that an experiencing a supernova at sun distance would be the same as holding a hydrogen bomb to your eyeball. The energy released in these events is basically unimaginable.
Assuming your 0.034% figure is correct, then one solar mass is equivalent to 2941 lifetimes of a sun's output, not 30. So 15 solar masses would be more like 44115 solar-lifetimes.
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.
I was disappointed to learn that it would require billions of solar masses of energy from a black hole merger to be able to ride the gravitational wave starting at a distance of a few Schwarzschild radii. It seems like riding a plasma jet might be better.
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.)
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.
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
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."
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.
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.
No matter how chaotic the merger looks, the event horizon must asymptotically become either spherical (Schwarzschild) or oblate (Kerr). The mass distribution inside doesn’t change this, general relativity doesn’t allow static “lumpy” horizons.
It’s wild how much happens in those milliseconds though. Numerical relativity papers like the one you shared from arxiv.org show the horizon “sloshing” before it stabilizes.
Sure, especially consider if singularities are not real. Then what's inside the event horizon is just some bunch of unknown material in some actual shape. Why wouldn't it be?
If singularities are real...same thing but more boring answer maybe? (the distribution just being: in the center).
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.
Edit: "The Kerr metric also predicts the existence of an inner and outer event horizon, with the shape of these horizons being oblate rather than perfectly spherical due to the rotation."
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?
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.
I'm not a physicist, but I took a class on special relativity in college, and I still remember some of it ... If I'm remembering it right, we still have conservation of momentum and energy in special relativity, with the caveat that these are defined differently than in classical mechanics. Specifically, E = γmc^2 and p = mvγ, where γ = 1 / sqrt(1 - v^2/c^2) and m is the invariant mass (aka the "rest mass"). [1] Note that when v=0 (so γ=1), this equation for momentum is the same as the classical p=mv, which is generally a good approximation when v << c.
So, using those relativistic definitions for energy and momentum, I think you're exactly right, at least up to the part about "since the final object is at rest". However:
- As I understand it, invariant mass, aka "rest mass" (which is equivalent to "rest energy", aka "rest mass energy"), is invariant, and it's the same before and after the collision, so the kinetic energy doesn't get "converted into rest mass energy". Rather, if the final object is at rest, then all of its kinetic energy has been radiated away; kinetic energy (E_K) is is total energy (E) minus rest energy (E_0 = mc^2, where m is invariant mass)
- I have no idea whether gravitational waves are the only way for the kinetic energy to be radiated away. I imagine other forms of energy could also be emitted.
- In order to know that the final object is at rest/has no kinetic energy (in an inertial frame), I worry that we might need to have specified more in the original question. In particular, I don't know how to handle spin. (I know that black holes have some concept of "spin", but I don't know if this is like rotational spin, or more like quantum mechanical spin, or something else, and I don't know how it figures into the black holes' total energy.) If we change the original question to say that the black holes are not spinning, then I think we can ignore this (since the collision is head-on).
Energy and momentum are always conserved in EVERY physical process. We can distinguish three types of collisions: “sticky” ones, in which the kinetic energy decreases (typically, it is converted into heat); “explosive” ones,
where the kinetic energy increases; and elastic ones, in which the kinetic energy is conserved. Since the total energy (rest plus kinetic) is always conserved, it follows that rest energy (and hence also mass) increases in a sticky collision, decreases in an explosive collision, and is unchanged in an elastic collision. The resulting black hole in other words would have way more of a mass than 2M since you're talking about a 'sticky' collision in the above instance. You can see an example of why this is in Griffiths' text (Introduction to Elementary Particles (which I highly recommend)) -- page 101 contains a great example of what happens to the mass of particles in 'sticky' collisions: https://www.hlevkin.com/hlevkin/90MathPhysBioBooks/Physics/Q...
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.
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.
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.
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?
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.
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.)
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.
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?
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.
I don't have an answer either. But in my amateur opinion, of the available options, I lean toward "is a black hole". If all the mass we can see adds up to a black hole the size of what we can seen, then if you add all the stuff outside the light cone, it should add up to enough mass to make a black hole radius that includes the distance out to there.
But that leaves us with black holes forming inside a black hole, which I have absolutely no idea what to do with.
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?
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)
Thanks, I was kinda curious why it wasn't full of spam. it's running on a super neglected rpi, really need to wipe it and spend a month refreshing myself on basic webhosting stuff
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?
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.)
> 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?
The analogy I like goes something like this. Imagine you're paddling in a canoe on the river. You approach a waterfall. If you do nothing you'll get consumed by the waterfall. So you try to paddle away from the waterfall, but as you get closer to the edge of the waterfall, the current gets stronger.
The event horizon is the imaginary line across the river which once passed, even if you paddle as quickly as you can, you won't be able to get away from the waterfall. Once you pass that line, you're bound to reach the waterfall eventually.
Now, thanks to Maxwell and Einstein, we know there's a maximum speed that anyone can paddle, the speed of light, and so we define the event horizon to be relative to this speed.
You can calculate the event horizon for just about anything. The main difference between a black hole and everything else, is that for a black hole the event horizon is larger than the object itself.
For example, the event horizon of a neutron star with a mass of 1.4 solar masses and a radius of 10km is about 4.1 km, well inside the neutron star. Thus you don't get the "black hole effect", since once you pass the surface of the neutron star the matter above you pulls you away from the center.
The river analogy is actually not far off what they try to use as an analog for testing black hole predictions, effectively a large water tank with a drain hole. Sixty Symbols did a video on this way back[1], and this thesis[2] goes into the details. Some are going beyond water using liquid helium to simulate quantum black holes this way[3].
Long before your disc neared the mass where it would form an event horizon, the matter it's made of would collapse into neutron star material, which would form a sphere.
Perhaps if it were exceptionally wide the whole disc wouldn't collapse. Maybe only the parts near it's center. In that case you'd end up with a large ring around a neutron star. Add a bit more mass and maybe it's now a ring around a black hole. The gravity of the ring might distort the event horizon in some way, I'm not sure quite how, but probably its possible to get a non-spherical hole in situations where the objects distorting the shape are still in the universe.
But as for the matter lost into the hole, it's gone. If the hole were to retain some shape based on what's "inside" of it, that would be the kind of information leak that the laws of physics do not permit.
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.
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.
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.
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.)
This is true, but it's not an observable distinction. It's true that in some sense those two black holes "haven't yet collided", but at this point they're well past the point of last observability and have now red shifted and time dilated into the invisible background. All the interesting stuff happens before that.
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.
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.
> 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.
Let’s see — the Tsar Bomba nuclear weapon released the equivalent of converting about 2.3 kg of matter into energy (1).
One solar mass is about 2 x 10^30 kg, so round numbers this event released the same as 10^31 Tsar Bombas, which is … a lot of energy? That number is too big to be a good intuition pump.
Let’s try again: over the course of its entire lifetime of about 10 billion years, the sun will release about 0.034% of its mass as energy (2). So one solar mass of energy is about 3000 solar-lifetime-outputs.
So this event has released about as much energy as 45,000 suns over their entire lifetime. I’m not sure how much of the energy was released in the final few seconds of merger, but probably most of it? So… that’s a lot of energy.
(1) https://faculty.etsu.edu/gardnerr/einstein/e_mc2.htm
(2) https://solar-center.stanford.edu/FAQ/Qshrink.html
Yeah, it's alot alot :-). Over on Mastodon I asked Phil Plait (@badastro) if the "missing mass" in the universe might be a result of black holes converging[1]. He wrote up this event in his newsletter[2] and points out that when they merge, they emit more energy in that instant than every single start in the universe in the same instant. So kind of like an instant of double energy. Hard to fathom how much energy that is with my meager mammalian brain.
[1] https://mastodon.social/@badastro/114852139083587160
[2] https://badastronomy.beehiiv.com/p/the-biggest-black-hole-me...
I have read somewhere that an experiencing a supernova at sun distance would be the same as holding a hydrogen bomb to your eyeball. The energy released in these events is basically unimaginable.
Probably here:
https://what-if.xkcd.com/73/
And it’s even more astonishing — the supernova at 1 AU would be the same as a billion hydrogen bombs at your eyeball.
Assuming your 0.034% figure is correct, then one solar mass is equivalent to 2941 lifetimes of a sun's output, not 30. So 15 solar masses would be more like 44115 solar-lifetimes.
Derp yes, pesky off-by-100 errors :) Fixed, thanks.
Is it physically limiting for a theoretical civilization to harness and use such energy?
Into what form of energy is that mass converted?
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.
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?
estimated distance of 2.2 Gpc per https://en.wikipedia.org/wiki/GW231123
That's how fast the millennium falcon goes
At 10 times the Schwarzschild radius Space literally stretches and contracts by 10-100%
I was disappointed to learn that it would require billions of solar masses of energy from a black hole merger to be able to ride the gravitational wave starting at a distance of a few Schwarzschild radii. It seems like riding a plasma jet might be better.
(Just planning my next trip.)
Much better off just chucking 90% of your mass into the blackhole to get a hella kick.
Yes. Black hole mergers are the highest energy events in the universe in terms of watts.
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...
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...
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
I wonder why Bezos doesn’t just pick up the tab, he likes space, right?
> I believe the proposed budget is being marked up tomorrow (July 15th, 12:00)
Interesting that they break this news today. Props to them for playing the game.
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
Keep an eye on whether the final FY 2026 appropriations bill keeps LIGO at two sites. Until then, it’s a real risk, but salvageable.
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."
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?
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.
Simulation: https://www.youtube.com/watch?v=Tr1zDVbSjTM
Everything is a fluid in the end and at scale right??
No matter how chaotic the merger looks, the event horizon must asymptotically become either spherical (Schwarzschild) or oblate (Kerr). The mass distribution inside doesn’t change this, general relativity doesn’t allow static “lumpy” horizons.
It’s wild how much happens in those milliseconds though. Numerical relativity papers like the one you shared from arxiv.org show the horizon “sloshing” before it stabilizes.
Is it even sensible to talk about a "mass distribution" inside of an event horizon?
Sure, especially consider if singularities are not real. Then what's inside the event horizon is just some bunch of unknown material in some actual shape. Why wouldn't it be?
If singularities are real...same thing but more boring answer maybe? (the distribution just being: in the center).
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.
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...
Here is an animation from MIT/CalTech of what a merger looks like:
https://youtu.be/1agm33iEAuo
Does the black hole's spin deform the event horizon?
I think so?
https://archive.ph/VrzwW
Edit: "The Kerr metric also predicts the existence of an inner and outer event horizon, with the shape of these horizons being oblate rather than perfectly spherical due to the rotation."
I wonder what would happen if one black hole shot through another one at high relativistic velocity, instead of spiralling towards one another.
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.
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?
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.
Correct. If you're curious about the 'essence' of what black holes are I actually just did a write-up on them which you can find here: https://photonlines.substack.com/p/an-intuitive-guide-to-bla...
I'm not a physicist, but I took a class on special relativity in college, and I still remember some of it ... If I'm remembering it right, we still have conservation of momentum and energy in special relativity, with the caveat that these are defined differently than in classical mechanics. Specifically, E = γmc^2 and p = mvγ, where γ = 1 / sqrt(1 - v^2/c^2) and m is the invariant mass (aka the "rest mass"). [1] Note that when v=0 (so γ=1), this equation for momentum is the same as the classical p=mv, which is generally a good approximation when v << c.
So, using those relativistic definitions for energy and momentum, I think you're exactly right, at least up to the part about "since the final object is at rest". However:
- As I understand it, invariant mass, aka "rest mass" (which is equivalent to "rest energy", aka "rest mass energy"), is invariant, and it's the same before and after the collision, so the kinetic energy doesn't get "converted into rest mass energy". Rather, if the final object is at rest, then all of its kinetic energy has been radiated away; kinetic energy (E_K) is is total energy (E) minus rest energy (E_0 = mc^2, where m is invariant mass)
- I have no idea whether gravitational waves are the only way for the kinetic energy to be radiated away. I imagine other forms of energy could also be emitted.
- In order to know that the final object is at rest/has no kinetic energy (in an inertial frame), I worry that we might need to have specified more in the original question. In particular, I don't know how to handle spin. (I know that black holes have some concept of "spin", but I don't know if this is like rotational spin, or more like quantum mechanical spin, or something else, and I don't know how it figures into the black holes' total energy.) If we change the original question to say that the black holes are not spinning, then I think we can ignore this (since the collision is head-on).
[1]: https://en.wikipedia.org/wiki/Mass_in_special_relativity#Rel...
To reiterate, I'm not a physicist. I may be off base here, but that's my understanding.
Energy and momentum are always conserved in EVERY physical process. We can distinguish three types of collisions: “sticky” ones, in which the kinetic energy decreases (typically, it is converted into heat); “explosive” ones, where the kinetic energy increases; and elastic ones, in which the kinetic energy is conserved. Since the total energy (rest plus kinetic) is always conserved, it follows that rest energy (and hence also mass) increases in a sticky collision, decreases in an explosive collision, and is unchanged in an elastic collision. The resulting black hole in other words would have way more of a mass than 2M since you're talking about a 'sticky' collision in the above instance. You can see an example of why this is in Griffiths' text (Introduction to Elementary Particles (which I highly recommend)) -- page 101 contains a great example of what happens to the mass of particles in 'sticky' collisions: https://www.hlevkin.com/hlevkin/90MathPhysBioBooks/Physics/Q...
> Energy and momentum are always conserved in EVERY physical process.
Veritasium recently claimed otherwise https://www.youtube.com/watch?v=lcjdwSY2AzM
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.
They would cancel each other out and disappear, like a snake eating its own tail.
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.
> 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.
Hawking radiation occurs because black holes are sticky ‼
Huh nice analogy
When you say cracking open a black hole do you mean cracking the event horizon to form a naked singularity?
The answer would likely be worth a Nobel prize or two.
> black holes are essentially perfectly sticky
Black Hole brand adhesive: when you absolutely, positively need something stuck down for eternity.
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.
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?
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.
> 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.
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.)
Mass alone doesn’t do it. You need energy, namely the CMB, to push the observable universe close to its Schwarzschild limits.
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.
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?
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.
I don't have an answer either. But in my amateur opinion, of the available options, I lean toward "is a black hole". If all the mass we can see adds up to a black hole the size of what we can seen, then if you add all the stuff outside the light cone, it should add up to enough mass to make a black hole radius that includes the distance out to there.
But that leaves us with black holes forming inside a black hole, which I have absolutely no idea what to do with.
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?
> 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
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)
I think this is over their lifetime, not when they merge?
>> just barely not touching
Which part of them is barely not touching?
The events horizon.
Or in other words, black holes mergers conserve their total radius, not volume as one would get with normal matter.
Maybe I should've linked my toy simulator in my initial comment.
https://cybersystems.dev/gtc/gtc.html
BTW, the php in /chat2 seems to be broken, if you didn't know already. Great simulation, too.
Thanks, I was kinda curious why it wasn't full of spam. it's running on a super neglected rpi, really need to wipe it and spend a month refreshing myself on basic webhosting stuff
In cosmological terms, what is barely not touching? Is that distance measured in meters, kilometers, AUs, lightyears, parsecs?
In terms of creating a row of black holes where the space between each black hole is small relative to the size of the event horizon of each.
The event horizons.
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?
> 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.)
> 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?
The analogy I like goes something like this. Imagine you're paddling in a canoe on the river. You approach a waterfall. If you do nothing you'll get consumed by the waterfall. So you try to paddle away from the waterfall, but as you get closer to the edge of the waterfall, the current gets stronger.
The event horizon is the imaginary line across the river which once passed, even if you paddle as quickly as you can, you won't be able to get away from the waterfall. Once you pass that line, you're bound to reach the waterfall eventually.
Now, thanks to Maxwell and Einstein, we know there's a maximum speed that anyone can paddle, the speed of light, and so we define the event horizon to be relative to this speed.
You can calculate the event horizon for just about anything. The main difference between a black hole and everything else, is that for a black hole the event horizon is larger than the object itself.
For example, the event horizon of a neutron star with a mass of 1.4 solar masses and a radius of 10km is about 4.1 km, well inside the neutron star. Thus you don't get the "black hole effect", since once you pass the surface of the neutron star the matter above you pulls you away from the center.
The river analogy is actually not far off what they try to use as an analog for testing black hole predictions, effectively a large water tank with a drain hole. Sixty Symbols did a video on this way back[1], and this thesis[2] goes into the details. Some are going beyond water using liquid helium to simulate quantum black holes this way[3].
[1]: https://www.youtube.com/watch?v=kOnoYQchHFw
[2]: https://arxiv.org/abs/2009.02133
[3]: https://pirsa.org/25010083
Long before your disc neared the mass where it would form an event horizon, the matter it's made of would collapse into neutron star material, which would form a sphere.
Perhaps if it were exceptionally wide the whole disc wouldn't collapse. Maybe only the parts near it's center. In that case you'd end up with a large ring around a neutron star. Add a bit more mass and maybe it's now a ring around a black hole. The gravity of the ring might distort the event horizon in some way, I'm not sure quite how, but probably its possible to get a non-spherical hole in situations where the objects distorting the shape are still in the universe.
But as for the matter lost into the hole, it's gone. If the hole were to retain some shape based on what's "inside" of it, that would be the kind of information leak that the laws of physics do not permit.
> 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.
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.
> 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.
Simulations linked in others comment:
https://www.youtube.com/watch?v=Tr1zDVbSjTM
https://www.youtube.com/watch?v=1agm33iEAuo
The second one is much more correct, notice how it freezes before they actually merge because they time dilate out to infinity.
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.
I think the GP meant “merge”
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.
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...
This is true, but it's not an observable distinction. It's true that in some sense those two black holes "haven't yet collided", but at this point they're well past the point of last observability and have now red shifted and time dilated into the invisible background. All the interesting stuff happens before that.
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.
Chirps or it didn't happen
Somebody please call the DOJ and their dogs.
[dupe] https://news.ycombinator.com/item?id=44555220
I wonder how the singularities would merge with each other.
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
What are the waves of gradient colors, gravity?
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