
Second Gravitational Wave Detected at LIGO - specialp
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.241103
======
lpage
For those curious about the future of LIGO...

At present there are two LIGO facilities - one in Hanford, Washington and
another in Livingston, Louisiana. This is necessary for both denoising (it's
unlikely that a seismic event or random perturbation would effect both
simultaneously) and triangulation via parallax.

Right now having just two facilities (that are relatively close together)
limits localization to broad regions of the sky. Additional facilities are
under way/in discussion for Europe, Japan, and India. This would significantly
improve both the sensitivity of the array and its ability to localize events
in a smaller region of the sky. Hopefully these projects get funded. LIGO
stands to resolve some of the biggest open questions we have in cosmology.

~~~
wfunction
One thing I don't understand in your post: given how ridiculously far away
these black holes are, how could you possibly even dream of triangulating them
with two facilities on Earth via parallax...?

~~~
antognini
You don't use parallax, rather you calculate the direction of the source by
using the arrival times of the signals. The nearer detector will receive the
signal slightly earlier by an amount that will typically be on the order of a
few hundredths of a second. This is essentially the same method that
geologists use to determine the epicenter of an earthquake.

~~~
Xeoncross
This seems horribly prone to manipulation by cosmic radiation, gravity, and a
million slower alterations like the rotation of the earth which when combined
produce a noticeable effect.

The assumption here is that no force has acted on the energy arriving enough
to distort two arrays on earth.

~~~
RaleyField
> seems horribly

No physicists here but you don't seem to be either. Irc from what had been
said with the first event gravitational waves pass through matter and aren't
disturbed like em waves, that's why they supposedly open new doors. Secondly,
just take it at face value, if they say so it is so, it might seem improbable
and "horribly prone to manipulation" but they probably thought of that and
have machines precise enough to compensate for whatever effects you or me
imagined e.g. "LIGO is designed to detect a change in distance between its
mirrors 1/10,000th the width of a proton"[1]. It's like your grandma, never
having touched computers, expressing opinions on the next release of some
developer tool.

[1][https://www.ligo.caltech.edu/page/facts](https://www.ligo.caltech.edu/page/facts)

------
rubidium
"“It is very significant that these black holes were much less massive than
those observed in the first detection,” Gabriela Gonzalez, LIGO's
spokesperson, said in a statement. “Because of their lighter masses compared
to the first detection, they spent more time—about one second—in the sensitive
band of the detectors. It is a promising start to mapping the populations of
black holes in our Universe.”"

from [http://arstechnica.com/science/2016/06/ligo-data-includes-
at...](http://arstechnica.com/science/2016/06/ligo-data-includes-at-least-one-
more-black-hole-merger/)

------
yread
My wife has this really stupid (or brilliant?) question - if during the
collision one solar mass turned into gravitational waves is it possible to
create mass from gravitational waves?

~~~
antognini
In principle, yes. The Einstein field equations are time symmetric, so you
could reverse the situation by pumping in gravitational waves from a great
distance in a spherically symmetric way, and have them converge in some
central region in such a way as to increase the mass of a black hole at the
center, or have it split up into two more massive black holes.

~~~
dandare
Isn't it in principle equally difficult as generating concentrial waves on the
surface of a swimming pool in such way as to eject a swimmer back on the
podium where he jumped in from?

~~~
JonnieCache
Like this? ;)

[https://youtu.be/WffR6HrEqTA?t=39](https://youtu.be/WffR6HrEqTA?t=39)

~~~
ar0b
So you could theoretically build a black whole generator by building a sphere
that could emit gravitational waves with precision?

~~~
Aelinsaar
You'd need the sphere to be more than massive enough to collapse under its own
gravity, into a black hole; such a structure wouldn't be hypothetically
possible. Building black holes with anything other than stars or giant gas
clouds (in the early universe) turns out to be hard; your black hole
generators inevitably keep collapsing into black holes or at least neutron
stars.

~~~
fizx
> You'd need the sphere to be more than massive enough to collapse under its
> own gravity, into a black hole; such a structure wouldn't be hypothetically
> possible.

I don't think that's technically true. You could build a bunch of catapults on
the edge of the sphere, and when they all launch rocks at the center of the
sphere, they would eventually form a black hole. The catapults could be
arbitrarily far from each other as the radius of the sphere increases, such
that they would not really do much to each other gravitationally. You'd just
have to wait a real long time for the rocks to hit the middle.

> Building black holes with anything other than stars or giant gas clouds (in
> the early universe) turns out to be hard;

Well yes, galaxy-sized intelligently designed structures don't really happen.

~~~
Aelinsaar
I think by "Black hole generator" the other person meant a device which could
create black holes remotely through some kind of process, not just a mass that
collapses under its own gravity. In that sense, if you could find an old,
spun-down neutron star (and man wouldn't that be a fun search! Massive, but
tiny and dim...) then as you say, you could just keep adding mass until
_crush_.

~~~
fizx
Any black hole generator that doesn't itself become a black hole is
essentially throwing part of itself in the black hole (Yay conservation
laws!). You can replace the catapults with lasers or plasma guns or whatever.

~~~
Aelinsaar
Sure, but I think the original commentator was imaging a massive sphere that
could emit such powerful and precise gravitational waves, that you could
create more black holes as a result. My point was just that any mass capable
of achieving that, even hypothetically, would have long since collapses into a
black hole.

I take your point however, that you could coordinate in some way separated
masses, but at some point you'd probably run into issues with the
aforementioned galactic-scale of engineering.

------
macintux
The article linked from that page is more useful to the physics-challenged
such as myself:
[http://link.aps.org/doi/10.1103/Physics.9.68](http://link.aps.org/doi/10.1103/Physics.9.68)

------
BenoitP
How come: 14.2 + 7.5 == 21.7 != 20.8 ?

Does the gravitational wave contain 0.9 solar masses of energy?

~~~
ISL
Because the event is _extremely_ violent.

Just as a nuclear bomb converts a small amount of mass to radiant energy
(grams?), a merger can convert mass into other forms of energy, too.

Gravitational waves, of which there are probably many passing through us all
the time from many sources, carry a lot of power. The only reason we don't see
them every day is that spacetime itself is extremely stiff.

~~~
stouset
Isn't it also that power decreases over the cube of the distance to the event?
A truly gargantuan amount of power (1 solar mass equivalent) distributed
across a 3-dimensional shell dozens or hundreds of light years across should
be minuscule at any Earth-sized point along that shell.

~~~
elihu
Wouldn't it be inverse square, not inverse cubed? Or do gravity waves behave
different than other things we're used to, like light?

~~~
dsqrt
Actually the nice thing about gravitational waves is that we are sensitive to
their amplitude, which scales as 1/r, and not to their energy, which scales as
1/r^2.

------
nonbel
This signal was reported on months ago. Can anyone explain what they did to
move the GW151226 signal from 2-sigma to >5-sigma?

"The data is in fact completely open and you could analyse it yourself! In
addition to the GW150914 event there are also two others that rise somewhat
above the background ("GW151012" and "GW151226"). You can see them by eye in
the above plot. They are clearly not statistically significant enough to
announce a discovery alone, but still they are tantalising... with room for
improvement to design sensitivity (by a factor of ~2 which increases the
spatial reach by 2^3) and the construction of a third detector in India to
triangulate the signal, the future of gravitational wave astronomy is
exciting."
[http://syymmetries.blogspot.com/2016_03_01_archive.html](http://syymmetries.blogspot.com/2016_03_01_archive.html)

~~~
xxxxxxxxxxxxx
Their background analysis is data-driven; the more data they take the better
they know their background noise. They are now at the point where they can
definitely say that what looked like an unusual fluctuation above the
background noise months ago was in fact very unusual. Enough for discovery
significance.

~~~
hcs
Citation, as it says in the article:

"Two matched-filter searches used coincident observations between the two LIGO
detectors from September 12, 2015 to January 19, 2016 to estimate the
significance of GW151226. One of these searches was the off-line version of
the online search discussed previously. The off-line searches benefit from
improved calibration and refined data quality information not available to
online searches."

[http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116...](http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.241103#fulltext)

------
devy
Can someone who has expertise explain the significance of this in plain
English please?

~~~
jofer
I'm definitely not a good person to explain or comment on this, but in a
nutshell:

Black hole collisions were expected to be a fairly common occurrence, but we
had no way of knowing for certain how frequent they are when only one had been
detected. Now that another one has been detected a fairly short time
afterwards, it supports the prediction that we should observe several of them
every year.

If black hole collisions were very rare events, it would be harder to use them
as a natural laboratory. It's much easier to demonstrate that some small
deviation from theory is real when you have multiple observations of it.

Therefore, this second observation in a short time confirms that gravitational
waves are likely to be a useful tool to explore fundamental physical
principles.

~~~
misnome
Also, this event is much closer to what they would "Expect" e.g. a much more
common predicted size of black holes; the first event was rather on the
extreme end of things (which is why it was so much easier to see in the data).

------
kirykl
Some nice visuals to explain the concept [http://www.npr.org/sections/thetwo-
way/2016/06/15/481934630/...](http://www.npr.org/sections/thetwo-
way/2016/06/15/481934630/gravitational-waves-from-colliding-black-holes-shake-
scientists-detectors-again)

------
mudil
When gravitational waves are released from the merger of two black holes, the
combined mass of the resultant black hole is less than sum of its component
black holes, because some of the mass was released in the form gravitational
waves.

Questions.

What are the implications of this on black hole entropy and temperature? Can
black holes evaporate from gravitational radiation alone without Hawkins
radiation?

What are the implications of this on the mass of any object in the universe,
since all objects are related to each other gravitationally and the universe
is expanding? Does it mean objects are constantly loosing mass and the
universe is filled with energy from this release? Can dark energy be related
to this process? Can universe be expanding because matter is constantly lost
into the gravitational waves?

What are the implications of gravitational waves on the fabric of spacetime,
if objects are constantly leaking gravitational waves in a nonstatic universe?

~~~
akuchling
My understanding is that gravitational waves are only radiated when there's a
dipole moment: two (or more, I guess) objects in rotation around each other. A
black hole just sitting on its own doesn't radiate any gravitational waves at
all. According to
[http://www.ast.cam.ac.uk/public/ask/2519](http://www.ast.cam.ac.uk/public/ask/2519)
, the Earth and Moon should also be radiating gravitational waves (and
therefore losing a little bit of energy) but this is surely dwarfed by other
effects and not measurable.

~~~
ISL
Important pedantic point: There are no gravitational dipoles, as there is no
repulsive gravitational interaction.

A dumbbell-shaped mass is the sum of a gravitational monopole and a
gravitational quadrupole. That gravitational radiation originates only from
quadrupolar sources (which are far less-efficient radiators than an equal-
sized dipole) is one reason that gravitational waves are hard to detect.

------
eaq
Data and audio files relevant to this event are available to the public at
[https://losc.ligo.org/events/GW151226/](https://losc.ligo.org/events/GW151226/)

There are also Jupyter tutorials on processing GW signals at
[https://losc.ligo.org/tutorials/](https://losc.ligo.org/tutorials/)

------
Jerry2
How do we know that these waves they're detecting are from collisions of black
holes? How do they locate these black holes and how do they make these
conclusions that what they detect is coming from black hole Alpha & Beta
colliding?

~~~
eaq
The waveforms are predicted by our knowledge of general relativity;
specifically what the gravitational wave profile would be for two objects of
certain masses spiraling into each other.

A semi-hand-wavy way we know that the objects in this particular inspired are
black holes is that for them to be orbiting each other this fast, they must be
very close. For them to be this close, they must be very small. Together, this
implied the objects have a certain size which is smaller than their
Schwarzschild radius, which means they are probably black holes.

------
cyphar
I was lucky enough to be in an astrophysics faculty (doing a research project)
when the LIGO paper was published. Everyone was super excited and was printing
off papers and discussing the experiment, results and future of astronomy. It
was really something else to see that many clever minds so excited about a
result that many scientists involved in the field of cosmology and relativity
didn't live to see.

------
etrautmann
Interesting to note that the y-scale on figure 1 is 10^-21 (units of strain).
Measurement at that scale is absolutely insane. The power of good engineering,
incredible optics, and lots of averaging.

~~~
ISL
There's minimal averaging, as the signals are transient.

~~~
antognini
Depends on what you mean by averaging I suppose, but the signal is a measure
of the average path length from several hundred reflections in the cavity.
Furthermore, the signals are generated from the average interference of many
photons. Even still, shot noise from the limited number of photons in the
cavity is a major source of uncertainty.

~~~
ISL
All true.

To give perspective on where I was coming from: from the (human) perspective
of looking at noise in 1/sqrt(Hz) units, the signals are actually less-than-
averaged if they're less than a second in duration.

It's not like a continuous-wave source where one can integrate for months and
hammer down the uncertainty to far lower strain values.

------
ars
Something I've asked before but got no answer - at these extreme masses,
velocities, and forces time dilation has got to be immense.

Yet the article makes no mention of this whatsoever.

~~~
dsqrt
Something to keep in mind is that gravitational waves are linearized solutions
to Einstein equations. That is, they can only be meaningfully defined far away
from those sources. What happens close to the black holes cannot be described
in terms of waves. You should not think of the gravitational waves as being
emitted directly by the black holes. Instead what happens is that the black
holes distort spacetime in complex non-linear ways, which turn into
gravitational waves at large distances (hundreds of Schwarzschild radii). As
such, the time dilation imprinted in the waves depends on the motion of the
"center of mass of the binary" and the cosmological redshift.

~~~
bobbles
I keep trying to picture how this would work using a 'drain in a bathtub'
image in my head where the water rushing down the plug is like a black hole
pulling things in with gravity.

Is there any way thinking like that I can get an approximate idea of what the
gravitational wave would be in that scenario?

~~~
ars
It's not like a bathtub. (And let's ignore time dilation.)

Imagine you have a large mass (sun) moving in a straight line, and an object
(sensor) 1 light minute from that mass. The gravity from the sun takes 1 light
minute to reach the sensor - that would mean that the sensor is attracted to
where the sun was 1 minute ago.

That can't work - it violates all sorts of conservation laws.

Instead what happens is that the gravitational force itself is ALSO moving in
a straight line! So when the gravity from the sun reaches the sensor it
attracts the sensor to where the sun is now because both the sun, and the
gravitational force, are moving together.

This works out very nicely.

But what happens if the sun is moving in a circle? Otherwise known as
accelerating?

The gravitational force can't know what the sun will do in the future (that it
will move).

So as the sun moves it forces the gravitational force to change - before the
force was moving in direction a, now it's moving in direction b. This change
in the gravitational force is known as a gravitational wave.

This wave, because it is accelerating, has the ability to impart change in
other objects, otherwise known as imparting energy. So gravitational waves can
carry energy! Potentially huge amounts of it.

(And since they carry energy, they themself have mass, and therefor gravity,
but these second-order effects as they are called, are too confusing, and
weak, and everyone ignores them.)

Back to the black hole - as it orbits the other black hole (as they orbit each
other), they change direction very very rapidly, causing huge gravitational
waves - the waves steal energy from the orbit, causing the two black holes to
fall into each other with smaller and smaller orbits, i.e. a spiral.

My problem is this: Near a black hole time dilation is enormous, huge gravity,
plus huge velocity. So to an outside observer the black holes appear basically
frozen and don't move. If they don't move they don't make gravitational waves,
so we should detect nothing.

I have no answer to this question.

~~~
raattgift
FWIW, for supermassive black holes, the curvature at the horizon can be
arbitrarily flat.

------
an_account_name
So, I remember hearing about LIGO when the first wave was detected, and was
excited by it - but I had never heard of it before that.

What other experiments are running that would generate similar excitement from
the science community that I probably haven't heard of yet?

------
heegemcgee
>he signal persisted in the LIGO frequency band for approximately 1 s,
increasing in frequency and amplitude over about 55 cycles from 35 to 450 Hz

Would love to hear an audio facsimile of what this might "sound" like.

~~~
eaq
For this particular event, see
[https://losc.ligo.org/events/GW151226/](https://losc.ligo.org/events/GW151226/),
near the bottom of the page

~~~
dev1n
It's truly incredible that I can sit here, plug my headphones into this little
electrical box and "hear" the gravitational sound of two black holes
colliding. If only Mr. Einstein could be here to listen.

~~~
pavel_lishin
I love that the data-as-sound is so cute:
[https://losc.ligo.org/s/events/GW151226/GW151226_template_sh...](https://losc.ligo.org/s/events/GW151226/GW151226_template_shifted.wav)

~~~
jerf
Truly an unbelievably jaunty little sound considering that it represents the
_entire mass of our solar system being converted into energy_. Yow.

~~~
lisper
Indeed. And to put that into perspective:

"The amount of matter converted to energy in the atomic bomb dropped on
Hiroshima was about 700 milligrams, less than one-third the mass of a U.S.
dime."

(From [http://discovermagazine.com/2010/jul-aug/24-numbers-
nuclear-...](http://discovermagazine.com/2010/jul-aug/24-numbers-nuclear-
weapons-bomb-stockpile-peace))

------
JumpCrisscross
One of my favourite things to do, when I'm back in California, is attend SLAC
Public Lectures [1]. (There is a disturbing paucity of scientific cultural
institutions in New York.) The most recent one was by Dr. Brian Lantz about
LIGO [2].

[1] [https://www6.slac.stanford.edu/community/public-
lectures.asp...](https://www6.slac.stanford.edu/community/public-
lectures.aspx)

[2]
[https://www.youtube.com/watch?v=EMzoQAmK8Dc](https://www.youtube.com/watch?v=EMzoQAmK8Dc)

------
eggy
Is there any correlation between the fundamental vibrating of atoms in a
vacuum at absolute zero, which I think is known as the zero-energy state, and
gravitational waves? Meaning, I know quantum mechanics describes why they
vibrate, but do gravity waves have any play in this, or effect? So exciting to
have lived long enough to see some more theory become real with measurements!

------
noselfrighteous
I have a side question. If gravity (i.e. warping of space time) propagates at
the speed of light. Then does that mean that the Alcubierre warp drive is
fundamentally incapable of supra-light speeds?

------
scrumper
What does the last line of the abstract refer to, about "deviations from
general relativity"? Is it simply a statement that this (and other)
observations of gravity waves are another tool for verifying GR's predictions?

~~~
ISL
Yes. These gravitational wave detections are the only direct measurements
anyone's ever made in regions of such strong gravity. If strong-field gravity
betrays any failing of the theory of GR, we'd expect to find it in
gravitational-wave waveforms.

That these waveforms (the first one, in particular) appear to follow the basic
GR plan at the time of the merger is actually kind of a bummer. As a physicist
who makes precision tests of gravity for a living, I'd really hoped that we'd
see something unexpected. Instead, the new thing that we learned is that GR
continues to be an accurate model, something that was perhaps unexpected on
its own.

~~~
eaq
I'm not so surprised that there are no GR deviations in these early days of
detections that are just becoming significant over the instrument noise.

GW physicists are hard at work on the future generations of GW detectors... In
50 years, a signal like GW150914 might have an SNR of >1000, so we'll see all
kinds of detail that may hold more information than what we've seen so far.

~~~
ISL
People have been testing GR really hard for a century, especially in the last
fifty years. Nobody's convincingly found a single chink in the armor.

Clifford Will's "The Confrontation between General Relativity and Experiment"
is an excellent introduction to the state of the art (this is the stock
reference for everyone in the field).

[http://relativity.livingreviews.org/Articles/lrr-2014-4/](http://relativity.livingreviews.org/Articles/lrr-2014-4/)

------
jakeogh
Gem YT chan:
[https://www.youtube.com/watch?v=MOVArup3jfg](https://www.youtube.com/watch?v=MOVArup3jfg)

------
ksec
How far are we from creating Gravity Shockwave Generating Division Tool? A.k.a
Goldion Crusher.

------
jayess
Could black holes lose their mass through the propagation (generation?) of
gravitational waves?

~~~
roywiggins
It seems that it happens when they merge, since:

> The cataclysmic event saw the black holes, one eight times more massive than
> the sun, the other 14 times more massive, merge into one about 21 times
> heavier than the sun. In the process, energy equivalent to the mass of the
> sun radiated into space as gravitational waves.

So the final black hole had rather less mass than the sum of the original two.

[https://www.theguardian.com/science/2016/jun/15/gravitationa...](https://www.theguardian.com/science/2016/jun/15/gravitational-
waves-detected-from-collision-of-second-set-of-black-holes-ligo)

------
peter303
I thought the shaking was Santa on the roof!

