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Comment by thaumasiotes | Hacker News Reader

Way to headline.

The numbers in the article suggest a violation of conservation of mass:

> Today, the LIGO Collaboration announced the detection of the most colossal black hole merger known to date, the final product of which appears to be a gigantic black hole more than 225 times the mass of the Sun.

> GW231123, first observed on November 23, 2023, seems to be an unprecedented beast of a black hole merger. Two enormous black holes—137 and 103 times the mass of the Sun—managed to keep it together despite their immense combined mass

Is the explanation here "225 is a nice round number, and 240 is technically 'more than' that", or "a lot of mass evaporates into other forms of energy when black holes merge", or "during a merge, it becomes possible for matter to escape an event horizon", or what?

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the extra mass is converted into energy in the form of gravitational waves (maybe other forms too idk but this is part of it)

Entire solar masses being lost to gravitational waves, like the voltage drop across a resistor, is a humbling prospect.

I'll underscore your awe by reminding you those solar masses disappeared in only 1 tenth of a second - the length of the gravitational wave signal.

but that's the time that passed here... it sounds like a mind-warpingly different perspective might have been seen there

I think you have it backwards. From the POV of someone near the Event Horizon, other space speeds up. Galaxies begin to spin at noticeable speeds.... But the black holes would appear to be approaching each other at "normal" speeds.

Dang

I suppose nothing but gravitational waves can escape the even horizon — or, rather, gravitational waves are born near / around it, because the black holes bend the space enormously.

OTOH whatever else may be outside the black holes near the merger and count towards their mass for astronomical purposes, such as accretion discs, should be much lighter weight than what's inside the event horizon.

Gravitational waves also can not escape. Those waves carry energy, and it's actually energy that can't escape.

The waves are actually made just to the outside of the event horizon.

I always understood that the waves are "made" everywhere, but that only the waves outside the even horizon will escape.

Was my understanding wrong all along?

Sort of correct?

Time is halted inside the black hole, so the waves made inside it never show up. Static gravity does show up though, but changes do not.

> The waves are actually made just to the outside of the event horizon.

How do we feel about this vis-a-vis action-at-a-distance?

Gravity does action at a distance. That's its thing.

The reason these waves are not generated from inside the black hole is that, to us, time stops there. For example these black hole mergers aren't actually merging, they are getting closer, and then they time dilate out of existence.

> Gravity does action at a distance. That's its thing.

Why does it need to travel in waves at the speed of light? If one mass moves, a distant mass is unaffected until the information reaches it. That's the opposite of action at a distance.

Your question is confusing. Action at a distance does not imply going faster than light, it means there is some sort of field connecting the two things.

Action at a distance means there is nothing connecting the two things. That's the "distance" part of action at a distance. Modern physics rejects the concept, saying instead that forces are carried by particles from a source to a destination, and the effect of the force is the result of local [opposite of "distant"] interaction with the particles carrying the force.

Compare wikipedia:

> Under our modern understanding, the four fundamental interactions (gravity, electromagnetism, the strong interaction and the weak interaction) in all of physics are not described by action at a distance.

( https://en.wikipedia.org/wiki/Action_at_a_distance )

Or: https://en.wikipedia.org/wiki/Principle_of_locality

> This is an alternative to the concept of instantaneous, or "non-local" action at a distance.

> The idea is that for a cause at one point to have an effect at another point, something in the space between those points must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, carrying the influence.

You'll note that "action at a distance" does in fact specifically mean that information travels faster than light!

But this understanding would seem to be incompatible with the idea that the mass inside a black hole can interact gravitationally with anything outside the black hole.

I'm not seeing where any of this requires faster than light travel.

But here's something that might help: We'll use gravity for our example, and I'll be non-technical for ease of typing.

The gravitational force that leaves an object is constant and continuous, it never stops, and it never starts. It exists from before the beginning of time, and it will never stop. The only thing you can do with that force is move it. This is because it's impossible to destroy energy. If you move the mass (the energy actually if you want to be exact) then you have changed the location (but not the strength) of the gravitational force, and that CHANGE travels at the speed of light.

So the gravitational attraction of my hand has, right now, already reached the end of universe, out to infinity. When I move my hand, it sends a tiny gravitational wave that travels at the speed of light, indicating a change in where the force is.

So the gravity inside the black hole has already reached the end of the universe, when that matter starts to clump, a change in the location of the gravity is sent out saying "this gravity is now moving over here".

This is why it feels like it's moving faster than light - it's not, it's already there at the end of the universe.

This is also why the orbit of Mercury is different in relativity, the sun pulls on Mercury where it WAS in the orbit, not where it is (which would require faster than light travel). In Newtonian gravity it's instant (i.e. faster than light).

What's going on here?

What part of your comment says something about gravity that is different from what I've already said about it?

How do you look at a quote stating explicitly that action at a distance is, by definition, instantaneous, and say "I'm not seeing where any of this requires faster than light travel"?

> So the gravitational attraction of my hand has, right now, already reached the end of universe, out to infinity.

And how has the gravitational attraction of your hand gone more than 200 light years from Earth?

> How do you look at a quote stating explicitly that action at a distance is, by definition, instantaneous,

I clicked to the Wikipedia article, I did not see this quote.

> And how has the gravitational attraction of your hand gone more than 200 light years from Earth?

My hand is made of mass collected on earth. Those atoms have been rearranged into the shape of my hand, but their gravity has existed since the beginning of the universe, just in difference shapes.

> I clicked to the Wikipedia article, I did not see this quote.

No need to go to that much effort; I pulled that quote in my comment.

>> This is an alternative to the concept of instantaneous, or "non-local" action at a distance.

> My hand is made of mass collected on earth. Those atoms have been rearranged into the shape of my hand, but their gravity has existed since the beginning of the universe, just in difference shapes.

So, as you acknowledge, zero information about your hand has gone much distance from the earth. Someone 1000 light years away who could resolve gravitational information into an image with perfect detail wouldn't be able to perceive your hand, you, or anything related.

Because gravity isn't a non-local force.

>> This is an alternative to the concept of instantaneous, or "non-local" action at a distance.

Notice the word "alternative", instantaneous action at a distance is a concept that was suggested at one point in the development of physics and discarded once relativity was figured out. Action at a distance these days is always at the speed of light.

> So, as you acknowledge, zero information about your hand has gone much distance from the earth.

No, not zero. All the gravity from the atoms in my hand is already out there.

> Someone 1000 light years away who could resolve gravitational information into an image with perfect detail wouldn't be able to perceive your hand, you, or anything related.

Correct, they would see the location of the atoms that make up my hand today, as those atoms looked 1000 years ago. (Probably in plants, and water.)

But remember: They do see the gravity from those atoms!!! Meaning the gravitational force my hand exerts, already exists 1000 years away, just in a different shape.

> Because gravity isn't a non-local force.

Are you saying gravity is a local force? Because that's not true.

Rather confused. 225 solar masses isn’t gigantic by any means

it's above what's considered possible to create by usual star collapse means

https://en.wikipedia.org/wiki/List_of_most_massive_stars lists only 2 stars more massive than that

So if you have two black holes within each other's event horizons, but they're too big to collide, what's supposed to happen instead?

The situation you describe is impossible. "If you have a very large positive number that is less than zero, what happens?"

What's the contradiction in the black hole setup?

I'm not even sure what it would mean for two black holes to be too big to collide, or where that became some kind of constraint.

I thought it was just thought that it would take too long for them to spiral into each other for it to have happened enough times in our universe

You're imagining some moment where two points merge.

These aren't points; they are (literally) opaque volumes of space, and once their outer limits broach, they have collided.

By analogy, two warships can collide, even though their centers of mass don't.

It would be difficult for two warships to collide without some of the material in one warship touching some of the material in the other one.

But there is no matter at an event horizon. That's just an imaginary line in space. It's opaque, but not solid.

If the black holes were moving fast enough, it should be possible for their event horizons to cross and then uncross, although that would immediately raise the question of what would happen to matter in the zone of overlap. Perhaps "fast enough" would exceed the speed of light?

> If the black holes were moving fast enough, it should be possible for their event horizons to cross and then uncross,

No, that is not possible.

...because?

Time dilation is infinite at the even horizon, so anything that touches it, freezes for good.

Infinities are typically indications of where a theory breaks down, not of something real.

Infinity there isn't abrupt, but steadily grows. Anything that approaches just does it slower and slower, and remains always finite, only exponential.

The event horizon is not an object that can touch something, so why is that an objection to the idea of two event horizons crossing and uncrossing?

It's time itself that stops there. No matter what you want to move, you will have a problem moving it. It might be easier to understand that Democritean idea of space as literal nothingness is a bit old, in modern science space is field, i.e. solid matter (universe), particles move in it like sound for which steel is the most transparent medium, and nothingness is the most impenetrable, because there is no foothold there, we think it's empty because we can move freely in it.

Also most black holes have matter on the event horizon, because something fell on them. Maybe it can even touch, because this matter is frozen slightly above even horizon.

Time has also stopped for photons, which does nothing to stop them from moving from place to place.

I don't see a way to read your comment that allows for the possibility that a black hole might move, which is something they do.

I'm riiiiiiight on the edge of concluding that you are a poet and the only thing you know about the words is the way they sound. Is there more to it than that?

Technically time is exponentially dilated, this introduces large relativity of synchronism: the same thing takes different time in different reference frames. You aren't completely wrong to say black holes will fly through each other, but this doesn't take the same time in all reference frames. For a distant observer it takes infinite time after their event horizons touch, and Hawking radiation evaporates everything before that.

How black holes can move is an interesting question, but as you can see, in reality there are no infinities, since stuff slows down before that. Maybe infinities could exist for an observer inside black hole, but an observer outside of black hole sees only large slow down.

I think you're playing fast-and-loose with the notion of "colliding".

First define what you mean by "colliding".

By analogy, I'm defining it as the Event Horizons intersecting, at which point nothing in side "either" black hole has any physical meaning to us ever again. For all we know, the insides could be chocolate ice cream on one half of the now-double-sized Event Horizon, and pure neutronium on the other. It's meaningless to even pretend we know anything about that volume.

Collision occurs when there is some sort of interaction between massy particles that try and fail to occupy the same space. Whatever stops you from passing through the floor counts as "collision".

An event horizon has no mass or other existence and cannot collide with anything. Within the black hole, there is mass somewhere, but generally not at the event horizon. If you're not comfortable assuming that, we can make it a definition - take this to be an example where none of the internal mass lies within the region of overlap.

What is the obstacle to the black holes separating again?

If one is within the other's event horizon, they have already collided (and are now surrounded by a common event horizon).

I have read elsewhere that all black holes are imploding, and there will be a "bounce" followed by matter emerging from the event horizons.

GR doesn't say any such thing, and anything beyond GR is speculation.

Nothing is too big to collide, the issue here are the initial masses which are bigger than expected from core-collapse stars

I don't know if there's ever been a more perfect setup for a your mother joke, but sometimes art is the brush strokes you don't make.

I'll counter Debussy ("the space between the notes ..." and all that :)

... and give it a go: "Yo mama is so big she can't even collide with a black hole" (or something ...)

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