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 10^32 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. So one solar mass of energy is about 30 solar-lifetime-outputs.
So this event has released about as much energy as 3,000 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.
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.
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.
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.
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."
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.
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.
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.
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."
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.
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)
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?
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?
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.
Does this mean that 15 solar masses were converted into energy? Because that's a LOT of energy.
One solar mass is about 2 x 10^30 kg, so round numbers this event released the same as 10^32 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. So one solar mass of energy is about 30 solar-lifetime-outputs.
So this event has released about as much energy as 3,000 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
https://www.science.org/content/article/trump-s-proposed-cut...
https://appropriations.house.gov/sites/evo-subsites/republic...
Then again, your file has less drastic reductions on nsf budget so who knows what would be the impact on ligo
Interesting that they break this news today. Props to them for playing the game.
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
"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."
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.
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.
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?
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.
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.
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...
https://youtu.be/1agm33iEAuo
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."
They actually convert up to 42% of their mass into energy, mostly radiation
https://youtu.be/t-O-Qdh7VvQ
Mass and energy.
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?
Which part of them is barely not touching?
Or in other words, black holes mergers conserve their total radius, not volume as one would get with normal matter.
https://cybersystems.dev/gtc/gtc.html
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.)
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?
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.
Time doesn't exist for black holes, so "after a while" is not something you can say about them.
> 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.
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.
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...
https://m.youtube.com/watch?v=5AkT4bPk-00