
LIGO Detects Gravitational Waves for Third Time - eaq
http://www.caltech.edu/news/ligo-detects-gravitational-waves-third-time-78193
======
cletus
The numbers here are just staggering:

\- Black hole merger occurred 3 billion light years away

\- _Two solar masses_ were converted to energy

\- Briefly 10^34 megatons of energy were released every second

This is hard to intuitively wrap your head around because we think of space as
constant. Something like this can distort space itself. Amazing stuff.

~~~
kstrauser
> \- Briefly 10^34 megatons of energy were released every second

That quote caught my eye too. What's the full unit on that? Is that literally
the "m" you'd plug into E=mc^2, or was there an elided "...of TNT", like we'd
use to describe nuclear weapons?

~~~
Florin_Andrei
It must be TNT equivalent. One solar mass is 1.99 × 10^30 kilograms, and we
know that 2 solar masses were converted in total, so 4 x 10^30 kilograms,
which is far less than the "megatons" mentioned, in terms of pure mass.

I wish folks would avoid mixing military units and general relativity units
like this, it's confusing.

~~~
nategri
I don't think it's that ambiguous.

"Megaton" isn't really used anywhere except for explosive yields, where it
always means TNT. As far as the 'native unit' astrophysicists will tend to use
ergs for events like supernovae.

[https://en.wikipedia.org/wiki/Erg](https://en.wikipedia.org/wiki/Erg)

~~~
kstrauser
I thought it was ambiguous in the context of an article talking about
converting mass to energy. Without running the numbers, I couldn't tell if
that was a unit of the input mass or the output energy.

------
smortaz
FYI - if you want to check out the data, the code, even an audio of the wave
checkout:

[https://notebooks.azure.com/roywilliams/libraries/LIGOOpenSc...](https://notebooks.azure.com/roywilliams/libraries/LIGOOpenScienceCenter)

It's a Jupyter notebook that anyone can clone and run.

[edit: updated link]

~~~
japhyr
It's just awesome to see these notebooks released. I'm not going to play with
them, but I love that people are able to. It makes me much more confident in
the results that are announced, and I hope this approach to doing science
becomes the norm.

~~~
jxramos
I second the wider adoption of notebooks. I'd really love to see government
budgeting offices and what not begin to make their spending and analyses
transparent through these documents. We need government on github and jupyter
notebooks :D

------
mturmon
I was at a talk by Janna Levin, astrophysicist and author of a book _Black
Hole Blues_ that describes LIGO. (E.g.,
[https://www.nytimes.com/2016/04/18/books/review-black-
hole-b...](https://www.nytimes.com/2016/04/18/books/review-black-hole-blues-
recounts-the-quest-to-find-the-cosmic-kazoo.html))

She gave a neat analogy between GWs, as sensed by LIGO, and an electric
guitar. In the sense that a distant pluck on the string is transmitted as a
wave down the string to the pickup, which senses a little wiggle in the string
and amplifies it. I thought it was a poetic analogy that gives a second
meaning to the word "instrument" in this context.

~~~
ehsankia
Except imagine you're embedded on the string itself and cannot actually sense
the string "moving through space". The way you have to measure it is by
sensing tiny changes in distance between the left and right side of the string
as it wiggles around.

The actual detail of the experiment and the precision they reach is quite
fascinating. Veratisium has a pretty good video explaining it in more laymen
terms [0]

[0]
[https://www.youtube.com/watch?v=iphcyNWFD10](https://www.youtube.com/watch?v=iphcyNWFD10)

------
eaq
The paper describing the event is available to the public at
[https://dcc.ligo.org/LIGO-P170104/public](https://dcc.ligo.org/LIGO-P170104/public)

The instrument data of this event is also available to the public at
[https://losc.ligo.org/events/GW170104/](https://losc.ligo.org/events/GW170104/)

~~~
nonbel
Thanks

>"GW170104 was first identified by inspection of low-latency triggers from
Livingston data [15–17]. _An automated notification was not generated as the
Hanford detector’s calibration state was temporarily set incorrectly in the
low-latency system_. After it was manually determined that the calibration of
both detectors was in a nominal state, an alert with an initial source
localization [18,19] was distributed to collaborating astronomers [20] for the
purpose of searching for a transient counterpart. About 30 groups of observers
covered the parts of the sky localization using ground- and space-based
instruments, spanning from γ ray to radio frequencies as well as high energy
neutrinos [21]."
[https://dcc.ligo.org/LIGO-P170104/public](https://dcc.ligo.org/LIGO-P170104/public)

Regarding the earlier detection:

>"At 11:23:20 UTC, an analyst follow-up determined which auxiliary channels
were associated with iDQ’s decision. It became clear that these were _un-
calibrated versions of h(t) which had not been flagged as “unsafe” and were
only added to the set of available low latency channels after the start of
ER8_. Based on the safety of the channels, the Data Quality Veto label was
removed within 2.5 hours and analyses proceeded after re- starting by hand."
[http://ligo.elte.hu/magazine/LIGO-magazine-
issue-8.pdf](http://ligo.elte.hu/magazine/LIGO-magazine-issue-8.pdf)

So both times humans had to take special action for the detection to "count".
I really wonder about whether the null model they are using is
appropriate/relevant here.

Also, the other thing I have been concerned about is the lack of any
corroborating evidence that these signals are truly generated by inspiraling
black holes(gamma ray bursts, etc). Apparently, in this case the above-
mentioned miscalibration has impeded that effort:

>"The event candidate was not reported by the low-latency analysis pipelines
because re-tuning the calibration of the LIGO Hanford detector is not yet
complete after the holiday shutdown. This resulted in a delay of over 4 hours
before the candidate could be fully examined. We are confident that this is a
highly significant event candidate, but the calibration issue may be affecting
the initial sky maps. We will provide an update in approximately 48 hours
which may include an improved sky map."
[https://gcn.gsfc.nasa.gov/other/G268556.gcn3](https://gcn.gsfc.nasa.gov/other/G268556.gcn3)

I can't tell from that text file whether they got corroborating evidence or
not. IANAP though.

~~~
sleavey
There have been three signals witnessed in about 12 months of observations -
of course the models need some tuning to correctly, automatically, trigger
alerts. In any case, you are referring to the _online_ triggers which look
very quickly at the data and try to guess if an apparent signal is real before
informing electromagnetic observatories to follow up. The real analysis is
conducted _offline_ in a much slower, careful way with lots of checks and
balances on the state of the instruments to rule out artificial signals.
That's one of the main reasons why it took 5 months between the first
detection and the publication of the paper announcing it.

In terms of corroborating evidence, remember that the two independent LIGO
detectors - 3000km apart - saw the event within 10ms of each other. That's
enough corroborating evidence for a lot of people. The NASA text file you link
shows no observed electromagnetic counterpart, but that's expected:
unfortunately the best models so far for black hole coalescences predict very
little or no electromagnetic emission - so although EM partners were informed,
the chances of them seeing anything were slim. Other predicted sources of
gravitational waves, like as-yet unseen binary neutron star coalescences, are
more likely to emit EM radiation and stand a chance of being witnessed by
conventional observatories as "corroboration".

~~~
nonbel
>"In terms of corroborating evidence, remember that the two independent LIGO
detectors - 3000km apart - saw the event within 10ms of each other. That's
enough corroborating evidence for a lot of people."

I don't see what the first part of the post has to do with the null
("background noise") model being inapplicable to situations where special
human intervention comes into play. Do they include any events like that in
the background timeseries or not? I am suspecting not (which renders the model
false and hence false alarm rates/sigma values meaningless), but do not know
for sure.

Second, that is just a detection. Corroboration occurs when your model
predicts multiple types of observations related to a phenomena (measured by
different types of instruments). This weakening of definitions is concerning
to me if it has infected physics. I have seen that trick used a lot by
"softer" fields such as medicine/psych (eg their definition of a replication
is just seeing "an effect" in the same direction).

------
wolfram74
Anyone familiar with this branch of astronomy want to explain why one
detection in a volume on the order of 27 billion cubic light years is
reasonable? Are they still processing data and will find more events? Is the
sensitivity highly anisotropic so the detection volume is significantly
smaller? Or are events like this just really conveniently rare that we get
about 1 every data gathering interval?

~~~
yodon
Events big enough to be detected are quite rare, but the phenomenon you are
asking about is more a financial reality than a "convenient coincidence" as
you put it. There is a pretty steep curve connecting sensitivity and cost, so
when the team that built LIGO was designing it, they used the best available
models of colliding black hole event rates to estimate the sensitivity
required to deliver a conclusive result in a reasonable amount of time.

If you're getting 10 events/second with a device like this, you probably
overpaid for sensitivity and if you're getting 1 event per century you're
probably not going to be able to maintain the operating expenses to still be
running when the detectable event occurs (and, as critically, none of the
people involved will be able to get the data they need in the time they need
it to get their PhD's, assistant profesorships, or tenured positions, so you
can't get the labor force you need for your experiment to work on it, which is
really what sets the acceptable duration of most experiments in practice).

It looks like the original estimates were pretty good, so events are coming in
at about the rate the experimenters hoped they would see them.

~~~
privong
> If you're getting 10 events/second with a device like this, you probably
> overpaid for sensitivity

Aside from issues processing and disentangling the overlapping events in a
situation with that high of an event rate, more events would not be bad, so
I'm not sure I'd call it "overpaying". Imagine the kind of population
demographics that could be built up if we were detecting that many events.

~~~
yodon
It's not that physicists wouldn't love to capture all those events, it's that
the cost of building instruments like LIGO is nearly prohibitively high and
the cost is a strong function of the sensitivity of the instrument. If you aim
too high in your sensitivity aspirations, the cost hits a point where the
experiment simply can't be funded.

~~~
privong
> If you aim too high in your sensitivity aspirations, the cost hits a point
> where the experiment simply can't be funded.

Agreed. I misunderstood your meaning then; I'd interpreted your wording to
mean that "overpaid" was still within the bounds of reasonable expectations
for funding. "Overpaid" didn't imply "too expensive to build", to me.

------
netcraft
Great veritasium video about this latest wave:
[https://www.youtube.com/watch?v=NVKO7UCIlgs](https://www.youtube.com/watch?v=NVKO7UCIlgs)

What is involved with increasing sensitivity I wonder? Is it purely
lengthening the arms? or are there other advancements required?

Hopefully one day we can have these things in space, isolated from noise and
curvature of the earth and no need for vacuum equipment.

~~~
sleavey
There are loads of possible ways to increase sensitivity, but none of them are
easy or cheap given that the low hanging fruit was all picked off in previous
generation detectors.

Increasing arm length is the "easiest" but definitely the most expensive
option. Try finding a 40km L-shaped area that's seismically stable and free
from significant anthropogenic activity. There may only be a handful of places
in North America. However, 4km is already on the cusp of being long enough
that gravity misaligns the two mirrors are each end of each arm due to the
curvature of the Earth. Going to 40km would prompt the need for static
corrections to mirror alignment, which will increase the amount of seismic
noise that couples into the longitudinal direction in which gravitational
waves are sensed. There are other problems such as the need to either refocus
light at points along the arms (very susceptible to alignment and thermal
noise) or use much, much bigger mirrors. The Advanced LIGO mirrors are already
~40kg, ~30 x 15cm cylinders of the purest fused silica known to man circa
~2012. There is talk of increasing the mirrors to 200kg and ~50 x 25 cm, and
no facility is currently capable of producing pure enough fused silica at that
size.

An "easier" option is to increase the laser power. This gives diminishing
returns, and leads to an increase in high frequency sensitivity at the expense
of low frequency sensitivity (due to photon pressure pushing the mirrors
around noisily). However, the challenges are to make stable lasers that are
also powerful - very tricky - and to mitigate the effect that laser absorption
has on the mirrors within the interferometer - as you increase laser power,
things heat up. Hot mirrors can lens the light, misaligning it and creating
extra loss (i.e. reducing sensitivity). It's trickly to mitigate. Another
effect of higher laser power is the introduction of parametric instabilities,
where the mechanical body modes of the mirrors are amplified by the high laser
power, leading to huge spikes of noise at narrow frequencies which are
difficult to damp out.

Another is to use a different interferometer topology: instead of an L-shaped
Michelson interferometer, suggestions have been made for Sagnac
interferometers which possess an interesting property called quantum non-
demolition, which can potentially reduce the limiting noise source in Advanced
LIGO which directly increases sensitivity. Research into this is at a very
early stage and will not be seen in detector facilities for decades, if ever.

So, the short answer is: there are lots of potential methods to increase
sensitivity, but all of them are challenging and require significant R&D and
money.

------
gjem97
> These are collisions that produce more power than is radiated as light by
> all the stars and galaxies in the universe at any given time.

Astounding, especially given that these are happening at regular intervals in
our "neighborhood".

~~~
lnx01
It's like the difference between an explosion of TNT and a atomic bomb, but on
a much larger scale.

Stars like our sun spend ~10billion years turning a portion of their mass into
energy. Most stars are like ours, small, dim and weak in power output. Our sun
will not go supernova and will not collapse into a black hole when it dies, it
will simply go nova and end up as a dwarf star in a nebula.

But, now imagine two black holes each a billion times as massive as the sun
turning all their mass into energy in a couple of seconds.

10billion years to convert 99% of the mass of the sun to energy versus 10
seconds to covert 2 billion times the mass of the sun to energy. Now it makes
sense that the power output is more in one second than the whole universe put
together.

Solar fusion is on the cosmic scale a very slow way to convert mass to energy.
It's so slow that we humans have been 'on the brink' of harnessing it for
power generation for decades.

Now imagine if we could build two nano-black-holes and let them collide....

~~~
onebigbug
> But, now imagine two black holes each a billion times as massive as the sun
> turning all their mass into energy in a couple of seconds.

Sure, lemme just take off my "good at socializing with apes and running for
long periods of time after antelope" hat and put on my "Cosmological scale"
hat.

Huh, I seem to have misplaced that one. And the one I'm currently wearing is
oddly well affixed.

------
shortstuffsushi
Somewhat naive questions, as I know very little about astronomy. Do black
holes "move?" How is it that they could merge if they're stationary, unless
they're pulling each other in I guess? If black holes are indeed pulling in
everything, does that mean the whole universe would eventually be one giant
black hole?

~~~
teraflop
> Do black holes "move?"

Yes, like any other massive objects, black holes can have velocity and
momentum. Two black holes, or a black hole and another object like a star, can
orbit each other in a way that _almost_ follows Newton's laws.

> How is it that they could merge if they're stationary, unless they're
> pulling each other in I guess?

This gets at what makes LIGO's findings interesting. Two black holes merge if
they fall into each other's event horizons. But Newtonian gravity predicts
that, in isolation, this would never happen; two orbiting black holes would
just maintain their elliptical orbit forever. (I'm hand-waving here, because
Newtonian gravity can't properly model black holes at all.)

The theory of general relativity predicts that the gravitational curvature of
the space around the black holes contains energy, similar to the energy in the
electric field around a charged particle. And intense changes in curvature can
create waves in the curvature of space, which carry away kinetic energy from
the black holes and cause them to spiral into each other. Under normal
conditions these waves are so unimaginably tiny that they're unmeasurable, but
during a black hole merger, they become intense enough to be (barely) detected
from billions of light-years away. This is what LIGO detected, confirming a
long-known theoretical prediction of GR.

> If black holes are indeed pulling in everything, does that mean the whole
> universe would eventually be one giant black hole?

Not necessarily. Everything in the universe attracts everything else
gravitationally, but that doesn't mean any two objects will inevitably
collide. If they have enough energy to move apart faster than their common
escape velocity, they are not gravitationally bound and will continue
separating forever.

~~~
lnx01
If we scale time to one-trillion-quadrillion years into the future, isn't is
possible that all mass in the universe eventually coalesces into a massive
universal super black hole? Or, does the expansion of the universe outstrip
that?

~~~
colonelxc
Not an expert, but the evidence is pointing towards continued expansion until
a 'big freeze'

[https://en.wikipedia.org/wiki/Future_of_an_expanding_univers...](https://en.wikipedia.org/wiki/Future_of_an_expanding_universe)

------
mudil
Here's what I don't understand.

In the first detection, they mentioned that two black holes collapsed, emitted
gravitational waves, and the resulting combined mass was less than then sum of
two previous masses because energy was spent on gravitational wave generation.
Hence it means, that due to gravitational interactions, objects leak mass.
Now, we know that every object in the universe is gravitationally related to
every other object, plus universe is expanding hence objects are constantly in
flux with each other. The question is where all the leaked mass goes? Can this
leakage account for dark matter? What about the space-time, does it function
as a storage medium for this energy that now came from the leaked mass?

Please explain...

~~~
grigjd3
It's important to note that gravitational waves themselves contain energy and
angular momentum. Relativists often use the terms mass and energy
interchangeably. When I worked in numerical relativity, we used units such
that the speed of light was 1, so E=mc^2 (really E^2=m^2c^4+p^2) simplifies to
E=m.

~~~
nthcolumn
I have no idea what these people are talking about but I always thought as a
small child that c should = 1. That square really bugged me - why squared, why
not cubed or halved or more realistically some bally awkward number... Sounds
like fun this numerical relativity!

~~~
Chronos
It's squared because energy is work, work is force times displacement
(distance), force is mass times acceleration, acceleration is velocity per
time, and velocity is vector distance per time. So you get:

mass × distance × ((distance ÷ time) ÷ time)

mass × distance^2 ÷ time^2

mass × (distance ÷ time)^2

------
castis
When a gravitational wave hits the earth, does the planet oscillate in place
for the duration, or is our position in the cosmos displaced, or something
else altogether?

~~~
dwaltrip
The actual space in which the planet resides stretches and shrinks as the
gravitational wave passes through it. The fabric of space itself is the medium
that the wave travels through.

However, the affect is incredibly tiny, even though it was generated by two
black holes colliding. The size of the distortion experienced here on Earth is
1000x smaller than the width of a proton! It's mind boggling.

I think I remember hearing that there is immense distortion in the area
immediately around the collision, but I'm not certain.

~~~
asmithmd1
The resolution of the detector is 1/1000th the width of a proton. The signal
they see is much bigger: the length of 4km long arms stretch and then shrink
about 1/10 the width of a proton. How do they know it is really space/time
changing and not an earthquake? They have two detectors over 1000 miles apart
from each other:

[https://en.wikipedia.org/wiki/LIGO#/media/File:Simplified_di...](https://en.wikipedia.org/wiki/LIGO#/media/File:Simplified_diagram_of_an_Advanced_LIGO_detector.png)

~~~
dwaltrip
Ah that's right, I was going off of memory. Thanks!

------
nsxwolf
Does the calculation of how long ago this event occurred account for the speed
at which the universe is expanding? Does it need to?

~~~
ahnitz
We do account for the expansion of the universe in fact. We estimated that
this source was at about z ~ 0.2 (see
[https://en.wikipedia.org/wiki/Redshift](https://en.wikipedia.org/wiki/Redshift)).
Roughly speaking this means there'll be only ~20% effect as the scale of the
universe (a) has increased by (1+z) over the time it has traveled.

------
nurettin
How does LIGO separate vibrations caused by a nearby truck from whatever
reading that is required for gravitational waves?

~~~
CodeCube
This is a complete guess, as I don't have any firsthand knowledge ... but I
can only imagine that they have a whole array of seismographs on the premises,
which they can use to clear noise from the main LIGO readings they are
interested in.

Curious to know if this is the case from anyone who happens to know one way or
the other :)

 _edit: the more I think about it, the more I think that random vibrations
from passing trucks would be irrelevant ... it doesn 't detect vibrations,
it's measuring the speed of light between two points_

~~~
lnx01
Actually, I think it's measuring the intensity of light between two points. It
has two perpendicular lasers of some wavelength which then interfere with one
another. If they interfere perfectly you measure a zero, if the interference
is off by some amount you measure a deviation in intensity from the i^ or the
j^ direction.

You can eliminate passing trucks, earth tremors, mining, asteroid impacts etc
simply by applying a band-pass filter that excludes measurement frequencies
outside of the range predicted by the equations.

No expert though, just guessing.

~~~
sleavey
No filtering is applied to data containing such anthropogenic/geological noise
after it is measured to remove such effects. In fact, such data is usually
junked and not used for analysis. The sites have thousands of witness channels
which listen for things like trucks, ground motion, magnetic storms, etc. that
could possibly influence the mirrors in the way a gravitational wave would. If
the same signal appears in both the gravitational wave channel and some
auxiliary sensor, it's thrown away.

Instead, the mirrors are highly isolated from the ground (suspended from
pendulums, motion damped by actuators, that sort of thing) so that such
effects do not have significant impact on the motion of the mirrors.

In any case, seismic noise can't be fully isolated and creates a sensitivity
wall below around 10Hz. To get sensitivity much below 10Hz, you have to go to
space (look up LISA).

------
shawkinaw
This is awesome. I spent a summer in high school at LIGO Hanford (Washington),
it's so cool to see positive results starting to come out of it.

