
Gravitational waves from a binary black hole merger observed by LIGO and Virgo - nickcw
https://www.ligo.caltech.edu/news/ligo20170927
======
autocorr
From an observational astronomy point of view, perhaps the most interesting
thing about the report is that by including the Virgo detector with both LIGO
detectors, the uncertainty in the direction the gravitational wave event was
reduced by a factor of 10! The "error ellipse" on the sky is now ~60 square
degrees as opposed to ~600. This makes it much more feasible to do rapid
observational follow-up with other observatories at all wavelengths (radio,
optical, X-ray, etc.), to search for the counterparts.

Many observatories like the VLA, Chandra, Fermi, Gemini, have a rapid response
trigger for the detection events, where within minutes they will stop their
current operations. Observations in the EM spectrum are crucial for learning
about a huge number of questions in astrophysics, like what kinds of galaxies
do the merger events originate from, where are they located in those galaxies,
how does the luminosity decay at different wavelengths, etc. It will be
seriously exciting when the first counterpart is discovered!

~~~
c3534l
A factor of 10 would be impressive enough, but a factor of 3,628,800 is
incomprehensible.

~~~
vorotato
you could say it's astronomical

------
rcthompson
> about 3 solar masses were converted into gravitational-wave energy during
> the coalescence

This is such a staggering amount of energy to be released in such a short
time.

~~~
jobu
Didn't realize that energy could be converted into gravitational waves. It
does make sense I suppose, but I always thought gravity and gravitational
waves were just a side-effect of the way mass interacted with space-time.

~~~
WhoBeI
According to Einstein there is no gravity that pulls us to the center of
Earth. Instead mass curves spacetime in such a way to make you experience
traveling along the curvature as gravity. If the mass is rotating the
curvature will also drag on the surrounding spacetime. Imagine something like
a whirlpool and then consider what happens if two of those merge. While
merging some of their energy will cause waves in the surrounding water that
eventually fades away with range. Those waves would be the gravitational waves
- ripples in spacetime.

~~~
lomnakkus
Incidentally, I also find the "geometrical" approach a great way to look at
the whole "light vs. black holes" thing: It's not that black holes directly
"trap" light or "prevent light from escaping" per se, it's more that spacetime
gets curved so much that regardless of which direction a ray of light is
traveling it'll always find itself back inside the event horizon.

(Not sure where I first heard this explanation, but I can definitely say that
I didn't originate it!)

~~~
nerfhammer
Man, that made me realize that black holes really are literally "holes".

~~~
lomnakkus
Yup. A "hole" in space in some sense, though I must admit I haven't thought of
it such extremely concrete terms[1]. I imagine that realization went into the
naming :).

[1] I mean, we usually imagine we can retrieve things when they drop into a
hole, but that's not really a thing in this case. (Excepting Hawking radiation
and the absurd amount of computation/time you would need to reconstruct the
original state information from that. If you thought the time-of-evaporation
for a Black Hole was long, you've got another thing coming when it comes to
reconstructing the state from that evaporated radiation! Not that this has has
any practical bearing, but it's fun to think about, right?)

------
ChuckMcM
Pretty awesome, nice to see they confirmation with Virgo.

I realize that I've never really considered what it would be like to be 'near'
(say a few 10's of thousands of light years away) from an event like this.
With all that energy, at what level would it pulverize a planetary body?

~~~
sandworm101
Nothing. If it happened that close you would still feel/see nothing. These
detectors are seeing movements far smaller than the width of a proton. A
planet would have to be orbiting one of these binaries to be close enough to
potentially feel much of anything.

~~~
Florin_Andrei
It depends on the gradient.

The fundamental nature of the force that rips you apart during
spaghettification is exactly the same. And you would definitely feel that one.

It all boils down to how strong the distortion is at one extreme of your body,
versus the opposite extreme. If it's about the same, you won't feel it. If the
difference is sizable, you would feel it to some extent. If it's very large,
it's a meat grinder.

~~~
sandworm101
These two BHs just merged. So they have been spinning around each other for a
few million years already. Anything close enough for spagetification to be an
option would have been tossed away long before the final merger.

~~~
bkanber
Relativity of simultaneity. Different observers not agreeing on when things
happen is a built-in trait of the universe. In our reference frame, those
things only just got spaghettified.

------
Diederich
Can anyone expand on the meaning of this?

"we find that the data strongly favor pure tensor polarization of
gravitational waves, over pure scalar or pure vector polarizations"

..taken from the paper. Thanks!

~~~
andrewla
I'll take a shot at this. Essentially they're saying that they've found
evidence that alternative theories about gravitation (non-Einsteinian
theories) that are strictly simpler (like classical Newtonian gravity) are not
consistent with the observations.

Here scalar means that the field that describes the phenomenon can be
described as a single scalar value at each point of space, like density, as is
the case of sound waves.

Vector is more complex; although "charge" is a scalar, electromagnetic waves
do not propagate by inducing charge in the surrounding space, but by
propagating through the free-space Maxwell equation (that is, where the charge
density is zero). So the polarization of an electromagnetic wave can be
described by a single vector at a point, and the measurement of the magnitude
of the wave will depend on how closely the measurement axis corresponds to the
polarization axis for a polarized wave. For a general wave, it can be
described as some linear combination of the orthogonal polarizations.

Although "tensor" is a general linear object of which vectors are a subset, in
this case of gravitational waves, we require a 2-tensor to describe the
polarization of the wave. Effectively that means the polarization (the
strength of the signal with respect to a given detector) can only be described
by two independent vector that describe the evolution of the wave. Since the
three detectors (Ligo Hanford, Ligo Livingston, and Virgo) are not exactly
aligned with one another, it should be possible to get good evidence that the
polarization cannot be adequately described by the superposition of two
orthogonal vectors, but instead two independent pairs of orthogonal vectors.

------
Florin_Andrei
If you turn a detector on - and, as soon as you do that, you detect some
event, chances are those events are happening, like, all the freaking time.

There's got to be some interesting implications here for cosmology.

~~~
stefco_
I'm a grad student with LIGO.

Yeah, the rate of intermediate mass black hole mergers was totally unexpected
and has caused a bunch of active discussion on formation mechanisms.

In short, intermediate mass black holes are too big to be generated from star
collapses, which means they (likely) come from smaller BH mergers. But because
these things take so long (GWs radiate energy very slowly until the bodies are
very close and fast), we didn't previously expect there to be large
populations of them. It has been surprising to see that there are enough of
them to sustain a pretty decent IMBH merger rate.

~~~
perlgeek
How likely is it that there are feeding mechanisms for black holes that make
them this large?

I guess the main problem is that you'd need friction to slow down matter
that's attracted by the black hole, so it doesn't just orbit it...

~~~
stefco_
I don't keep up with the latest material here, but there is some stuff I've
picked up.

For one thing, it's probably not ordinary matter falling into the black holes.
When ordinary matter falls into a black hole, it usually forms a bright,
glowing accretion disk. We would see that with an ordinary telescope,
especially given just how much matter would need to be accreted. (We're
talking about dozens of suns worth of mass for each of these intermediate mass
black holes!)

You can also have three-body interactions. A pair of black holes in a circular
orbit at a decent distance will merge on a timescale of billions of years. But
if a third black hole comes in and disrupts the orbit, it can knock those two
black holes closer together, at which point the GW power gets much higher and
the merge can finish on a reasonable timescale.

For these reasons, some papers I've seen suggest that there might be large
collections of black holes that are dense enough for three-body interactions
to be relatively common.

------
netcraft
I know there were a lot of people hoping this was the rumored neutron star
merger (with possibly visual observations!) but I don't think thats not still
on the table, this was just a different announcement. Still quite an
achievement and always great to get further confirmation of the observations.
Imagine what we could do with a much larger observatory.

------
gus_massa
> _Black holes produce gravitational waves but not light._

Is this correct? I thought that it would produce a lot of light, but the light
is more easily absorbed by interstellar gas and dust, so it's more difficult
to see it.

Perhaps not the black holes doesn't emit light directly, but the surrounding
gas will be extremely hot.

And when the two event horizons collide, the virtual particles should be very
confused and scape in huge numbers, in a process like the Hawking radiation,
but much bigger.????? I'm far from being an expert in this, so this is only a
conjecture and unsupported handwaving. But I would be very surprised to
confirm that the event has no emission of light.

~~~
cyphar
Black holes themselves cannot emit light (Hawking radiation is a separate
topic). Gas that is in accretion disks around a black hole can become hot and
produce light, but that process has nothing to do with the black hole itself.
By definition, light cannot escape a black hole so processes inside the black
hole do not emit light (or rather, if they do we cannot see it).

However, massive bodies experiencing rapid acceleration emit gravitational
waves. This is something that happens due to a black hole's mass and not it's
event horizon properties (in principle you can also detect neutron star
interactions). But this is emitted due to the motion of the objects
themselves, not from accretion disks (which may not be present for some black
holes). So the event itself does not produce light.

~~~
gus_massa
If you have two charged black holes, orbiting closer until they collide, then
they must emit electromagnetic radiation. I'm not sure how high is the
frequency of the orbits just before the collision and how much energy they can
dissipate in this way, but it's definitively not 0.

Also, he Hawking radiation is an intrinsic property of the black holes, you
can't disconnect it. I doubt that anyone has calculated if the Hawking
radiation increase or decrease during a black hole - black hole collision. My
_guess_ is that it increase and if you are nearby you would see a flash of
light. The technical detail that the light doesn't come from inside the black
holes is not meaningful. The whole black hole - black hole collision also
includes all the Hawking-like radiation emitted.

------
c-smile
Why black hole merges happen so frequently? We just started registering them
and got three events already...

Mass of Universe is estimated to be of 6e51kg. Mass of Sun is 1e25kg. This
event is about 50 Sun masses. So that one event is about morphing 1e-26 part
of Universe. I suspect that matter collapsing into black holes happens
significantly more frequently than that. Do we have enough non-black-hole
matter available for our (humanity) life time?

Yet worth to mention that black holes "return" mass by not only
Hawking–Zel'dovich radiation but also by such events - this particular merge
emitted 3 solar masses as energy.

------
ssijak
How do they know exact sizes of the 2 black holes, and the remaining size
also? Seems like they say it as 100% truth, which maybe it is, but my
uneducated mind cannot think how they could calculate that. Only those sizes
fit in the mathematical model of the gravitational waves compared to the
observations?

~~~
MAGZine
It is trivial to obtain the size of objects, so long as you have some
information about a satellite orbiting it.

It's how we get the mass of many interstellar objects, and it's how we know
that dark matter exists.

[http://astro.physics.uiowa.edu/ITU/glossary/keplers-third-
la...](http://astro.physics.uiowa.edu/ITU/glossary/keplers-third-law/)

------
MikeGale
Don't you look forward to the time when these events need to named like this
GW1712231509236732?

------
dpcx
A coworker earlier mentioned a paper questioning the validity of the findings
around gravitational waves. Can anyone point me to them and (as a non-
scientist) explain why this paper would think those findings are inaccurate?

~~~
batmansmk
Can you coworker provide some info instead?

~~~
dpcx
He could provide information about the paper, sure. The explanation, I'm
fairly certain, he could not.

------
komali2
I'm googling but it's all above my head - what's a gravitational wave, and how
do they detect them, if anybody is willing to take a bit of time to explain to
a turbo-layman? :D

~~~
aeleos
There are many good videos on youtube that do a good job explaining what
gravitational waves are and how we detect them. Many of these were made in
response to the first detection, and they do a pretty good job explaining it
in laymans terms. I personally really like veritasiums video here.
[https://www.youtube.com/watch?v=iphcyNWFD10](https://www.youtube.com/watch?v=iphcyNWFD10)

------
ricardobeat
What are the practical applications of technology that could be derived from
these discoveries? Any chance of us harvesting this energy?

~~~
stefco_
Hi, I'm a grad student working on LIGO.

There is no practical way to harvest this energy. Gravitational waves interact
very weakly with matter. This is part of why they are useful for observations:
GWs can travel from the core of violent events unmodified, unlike light, which
gets blocked by surrounding matter. You can't see the center of a core
collapse supernova with light, but you can see it with GWs and neutrinos. They
also won't get bent by EM or gravitational fields, meaning that the direction
they come from in the sky points straight back to the source. And unlike very
high energy gamma rays, they don't pair produce into electrons.

This all makes them great for astronomy and totally useless for extracting
energy.

~~~
chebucto
What happens to that energy, though? Energy is conserved, even with
gravitational waves, correct?

So, what happens to the energy - does some of it get converted into 'normal'
potential energy by changing the (amount of gravity? gravitational fields?) of
the stuff that it passes through - the weak interaction that you refer to?

And, if it interacts very weakly, does all of the energy eventually get used
up this way - do GWs get 'used up' before they travel across the universe?

(Probably a meaningless question): can GWs hit the edge of the universe, and
if so, what happens then?

~~~
stefco_
> What happens to that energy, though? Energy is conserved, even with
> gravitational waves, correct?

Yup. It's like ocean waves carrying energy. The difference is that it's hard
to interact with GWs, so instead of "riding" them and capturing their energy,
you find them mostly passing through you. They have tons of energy, but very
little of it is dissipated back into matter.

------
ivoras
So, what are the implication on General relativity if it's shown that gravity
changes move in waves, with a finite speed?

~~~
vorotato
The speed of gravity is the speed of light, because it's the speed of
causality. That's a pretty well understood thing, and isn't really something
being explored by these.

