Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

LIGO and Virgo announce the detection of a black hole binary merger

ligo.caltech.edu

Post reply on HN
Print view View original

51–60 of 70 posts

Re: LIGO and Virgo announce the detection of a black hole binary merger

#51

Post by raattgift »

evanb wrote: post #32
evanb wrote: post #30

Earlier quoted context omitted.

The answer is "by the processes of general relativity". When things fall "down" their potential energy is lowered, so they get faster. When mass-energy accelerates it emits gravitational waves, in much the same way when an electron accelerates it emits electromagnetic waves. If you're satisfied that a radio works by sloshing electrons around, you should be satisfied that a black hole merger emits radiation by sloshin…

Maybe I should be more clear: at least in the generally relativistic conception of gravity, all of the mass of the black hole "comes from" the warping of spacetime already. That's why I keep dancing around the question of whether it's "really" the kinetic energy or the mass or a mix that gets converted---it's hard to distinguish and may not be meaningful to distinguish the pieces from one another, especially if I go…

Heh I should have kept reading down the thread, could have saved some typing. :-)

Re: LIGO and Virgo announce the detection of a black hole binary merger

#52

Post by raverbashing »

improbable22 wrote: post #40
raverbashing wrote: post #35

Earlier quoted context omitted.

> all of the mass of the black hole "comes from" the warping of spacetime already No, it's the other way around. The warping is due to the mass But the other answer hinted at the source: accelerating mass turns into gravitational waves as an accelerating electron turns into EM waves (remember rotation and things stopping abruptly also means acceleration) So yeah, it's the potential energy that turns into (less than N…

Certainly normal matter (like the earth) has mass and warps spacetime. But a black hole is a purely gravitational object, there need not be any normal matter involved. The mass of a black hole is something that's really only defined from a distance away. If a planet of 1 earth-mass orbits it at the same speed & circumference as we do the sun, then we say the black hole has 1 solar mass.

> But a black hole is a purely gravitational object, there need not be any normal matter involved.

There's no "purely gravitational object" it only "looks like that" from behind the event horizon

We know the escape velocity > c and that's pretty much it, for GR it's a singularity (which usually means the theory is incomplete in that circumstance) and we don't know how QM work when squeezed harder than a Neutron star

> The mass of a black hole is something that's really only defined from a distance away.

If you mean "we can sense the gravitational field of something having mass X at a distance D larger than the event horizon" I agree.

But I'd rather say "we don't know what happens there" instead of "singularity" (which is what the current theories say it's inside and from the point of view of Relativity they're not wrong)

Re: LIGO and Virgo announce the detection of a black hole binary merger

#53

Post by nerfhammer »

merraksh wrote: post #25

Six black hole merges observed in ~2 years! That's quite interesting. More observations will make for some useful statistical study. I wonder if this would give us any insights w.r.t. matter and its distribution across the Universe, and/or help us better understand/estimate dark matter/energy.

Most interesting is that these have all been medium-sized black holes which weren't known to exist previously and we don't have a theory of how they form.

Re: LIGO and Virgo announce the detection of a black hole binary merger

#54

Post by raattgift »

improbable22 wrote: post #40
raverbashing wrote: post #35

Earlier quoted context omitted.

> all of the mass of the black hole "comes from" the warping of spacetime already No, it's the other way around. The warping is due to the mass But the other answer hinted at the source: accelerating mass turns into gravitational waves as an accelerating electron turns into EM waves (remember rotation and things stopping abruptly also means acceleration) So yeah, it's the potential energy that turns into (less than N…

Certainly normal matter (like the earth) has mass and warps spacetime. But a black hole is a purely gravitational object, there need not be any normal matter involved. The mass of a black hole is something that's really only defined from a distance away. If a planet of 1 earth-mass orbits it at the same speed & circumference as we do the sun, then we say the black hole has 1 solar mass.

> But a black hole is a purely gravitational object

Are you calling compact objects from stellar collapse that have an apparent horizon something other than a "black hole"?

> The mass of a black hole is something that's really only defined from a distance away

The stress-energy in the region of spacetime where one finds a black hole totally determines the Einstein tensor in that region. Solve the Einstein Field Equations for T_munu in \Sigma \subset \M. Extract the metric tensor. Solve the geodesic equations for that region, and you have the orbit for your Earthlike planet. You can make it simpler and consider the surface stress-energy on a shell within the horizon, or even outside in the case of your orbiting Earthlike planet problem, for which we don't care about the region inside the shell.

You are skipping the first steps, treating the metric tensor as a known, or worse, something that you can extract from a single geodesic.

But let's think about that anyway. Do you really know the metric you source? Sure, your stress-energy is localized so you can deploy a large shell to partition the "inside" and "outside" stress-energy. "Outside" it's zero, and nobody will really care about your small deviations from exact spherical symmetry, so your metric is therefore Schwarzschild, right? No, we can't make that inference based on the far field outside the shell; we need an Israel junction condition. [Synge 1960, Relativity: The General Theory (Amsterdam: North-Holland), ch. IV § 6 goes into this in detail, contrasting "realist" vs "agonist" and "creator" positions; yours is the "creator" position in that you are happy with an exact solution of the EFEs, and so you might enjoy reading what Synge wrote as he walks towards the mathematics of junctions :-) .] In a model can ignore that because we get it "for free" by laying down an exact generator of the vacuum Schwarzschild solution and not worrying whether the stress-energy is physical.

Re: LIGO and Virgo announce the detection of a black hole binary merger

#55

Post by evanb »

raverbashing wrote: post #52
improbable22 wrote: post #40

Earlier quoted context omitted.

Certainly normal matter (like the earth) has mass and warps spacetime. But a black hole is a purely gravitational object, there need not be any normal matter involved. The mass of a black hole is something that's really only defined from a distance away. If a planet of 1 earth-mass orbits it at the same speed & circumference as we do the sun, then we say the black hole has 1 solar mass.

> But a black hole is a purely gravitational object, there need not be any normal matter involved. There's no "purely gravitational object" it only "looks like that" from behind the event horizon We know the escape velocity > c and that's pretty much it, for GR it's a singularity (which usually means the theory is incomplete in that circumstance) and we don't know how QM work when squeezed harder than a Neutron star…

> There's no "purely gravitational object" it only "looks like that" from behind the event horizon

It depends on what framework you're working in. If you're working in general relativity, there's a singularity and absolutely nothing else---no matter at all. If you're talking about the "real world" you can ask: Is GR reliable for these situations, especially in light of quantum-gravitational puzzles? I think you agree that we don't know the answer. But we can say what the GR answers are.

Re: LIGO and Virgo announce the detection of a black hole binary merger

#56

Post by evanb »

raattgift wrote: post #51
evanb wrote: post #32

Earlier quoted context omitted.

Maybe I should be more clear: at least in the generally relativistic conception of gravity, all of the mass of the black hole "comes from" the warping of spacetime already. That's why I keep dancing around the question of whether it's "really" the kinetic energy or the mass or a mix that gets converted---it's hard to distinguish and may not be meaningful to distinguish the pieces from one another, especially if I go…

Heh I should have kept reading down the thread, could have saved some typing. :-)

I thought your other answer was great!

Re: LIGO and Virgo announce the detection of a black hole binary merger

#57

Post by evanb »

raattgift wrote: post #50
evanb wrote: post #12

Earlier quoted context omitted.

When an apple falls to the earth, by what process is its potential energy converted to kinetic energy? Gravitational processes alone. Apple-earth collisions primarily radiate apple sauce, black hole mergers primarily radiate in gravitational energy. Minor nit: In general relativity black holes are not actually comprised of matter---they're entirely warping of spacetime. Whether that remains true in a quantum theory o…

The apple sauce line is amusing, and I'm sure you know all the following, but even in Newtonian mechanics, potential and kinetic energies are frame dependent; for example the latter is rotationally but not Galilean invariant. In modern gravitational physics you can treat components of the Einstein or stress-energy tensors as like these energies, e.g. for a family of observers, the apple-breaking kinetic energy is lik…

Yeah, I went straight to the static solutions for simplicity of discussion. From the outside, though, is it even in principle possible to tell whether a BH is a stellar-collapse Bh or if it's an eternal Schwarschild vacuum BH? Now we're outside of my realm of expertise. Do I recall properly from my GR class that once stellar collapse begins the outer shell reaches the singularity in a finite time? In that case, I think it does come down to philosophy (if you're staying within GR) or some theory that resolves the singularity to say whether the BH is "made of matter" or not.

> I'll assume you favour keeping unitarity in any solution to the AMPS firewalls problem, and would happily ditch the apparent validity of the EFT outside the horizon.

You're damn right. Unitarity, unitarity, unitarity:

https://arxiv.org/abs/1602.01473 https://arxiv.org/abs/1603.03055 https://arxiv.org/abs/1606.04948 https://arxiv.org/abs/1606.04951 https://arxiv.org/abs/1709.01932

Re: LIGO and Virgo announce the detection of a black hole binary merger

#58

Post by raattgift »

dwaltrip wrote: post #47
russdill wrote: post #21

Earlier quoted context omitted.

That's assuming they are "naked" and not surrounded by rotating disks of gas and dust.

AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward (and emitting significant amounts of light in the process). Even then, they only actively feed in that way for a short period of time -- I think something like 10k years. All of the LIGO observations have been of more basic stellar mass black holes merging together.

> AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward

Stellar binaries are extremely common, and there is a reasonably large supply of binarys where one star has become a compact object. Their companion stars often drop lots of matter onto them, resulting in a reasonable supply of black holes. Diskoseismologists and others working on Swift have catalogued hundreds of stellar mass black hole accretion disks.

Examples from Swift:

http://adsabs.harvard.edu/abs/2013ApJ...769...16R https://arxiv.org/abs/1112.2249 (preprint version)

Swift also spotted ASASSN-14li which was a star being shredded by an SMBH and forming an early accretion structure. The event has been followed up by other observatories (notably Chandra and the European very long baseline interferometry network). ASASSN-14li is an easy google search term (the trick is knowing the term in the first place :-) ), hopefully you will enjoy some of the hits. :-)

Re: LIGO and Virgo announce the detection of a black hole binary merger

#59

Post by dwaltrip »

raattgift wrote: post #58
dwaltrip wrote: post #47

Earlier quoted context omitted.

AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward (and emitting significant amounts of light in the process). Even then, they only actively feed in that way for a short period of time -- I think something like 10k years. All of the LIGO observations have been of more basic stellar mass black holes merging together.

> AFAIK, only supermassive black holes at the center of galaxies have disks of material that is falling inward Stellar binaries are extremely common, and there is a reasonably large supply of binarys where one star has become a compact object. Their companion stars often drop lots of matter onto them, resulting in a reasonable supply of black holes. Diskoseismologists and others working on Swift have catalogued hundr…

Ah, interesting. That makes a lot of sense. Would it be correct to say that if both objects in a binary pair are SMBHs, they would very likely not have an accretion disk, as the companion would be unable to send over any material?

Re: LIGO and Virgo announce the detection of a black hole binary merger

#60

Post by improbable22 »

raattgift wrote: post #54
improbable22 wrote: post #40

Earlier quoted context omitted.

Certainly normal matter (like the earth) has mass and warps spacetime. But a black hole is a purely gravitational object, there need not be any normal matter involved. The mass of a black hole is something that's really only defined from a distance away. If a planet of 1 earth-mass orbits it at the same speed & circumference as we do the sun, then we say the black hole has 1 solar mass.

> But a black hole is a purely gravitational object Are you calling compact objects from stellar collapse that have an apparent horizon something other than a "black hole"? > The mass of a black hole is something that's really only defined from a distance away The stress-energy in the region of spacetime where one finds a black hole totally determines the Einstein tensor in that region. Solve the Einstein Field Equat…

My first point about mass was that some are tempted to imagine the mass of the BH as residing in the matter of the neutron star or whatever that formed it. This is misleading, it is better to think of that matter as no longer existing, and just deal with the fact that pure Schwarzschild looks identical to a lump of matter, from a distance.

How you measure the mass, well my orbit example is admittedly crude, ADM mass is I think the right asymptotic concept.

By purely gravitational object I mean this: everything we're discussing here about merging and waves and accretion disks all concerns only the exterior. This is all vacuum Einstein, pure gravity. (Whether we can say anything sensible about the interior is another whole different rabbit-hole.)

Post reply on HN