
The Gravitational-Wave “Revolution” Is Underway - LinuxBender
https://www.scientificamerican.com/article/the-gravitational-wave-revolution-is-underway/
======
joeyrideout
> the distribution of the boring events is fascinating

This line needs to be made into a motivational poster, or a statistics meme.

~~~
eukaryote31
"Boeing events are fascinating, for sufficiently large values of boring"

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copperx
Airbus is stoked about that.

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wwarner
I would recommend (as I've done before) Kip Thorn's book _Black Holes and Time
Warps_
([https://www.amazon.com/dp/0393312763](https://www.amazon.com/dp/0393312763))
to learn more about LIGO.

~~~
ksangeelee
There's a lovely Sean Carrol podcast interview with Kip Thorne where he
discusses the people and the work that led to the discovery.

[https://www.preposterousuniverse.com/podcast/2018/11/26/epis...](https://www.preposterousuniverse.com/podcast/2018/11/26/episode-24-kip-
thorne-on-gravitational-waves-time-travel-and-interstellar/)

~~~
jamiek88
I can second this. The book is still worth getting too but this podcast is an
excellent discussion around the subject.

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hinkley
> Just as light can be bent and magnified when it passes through the
> gravitational fields of galaxies and other massive objects, gravitational
> waves should be warped in the same way, too.

I'm gonna need a refresher course on that.

~~~
bpchaps
Gravity propagates at the speed of light.

So, imagine you have a massive celestial body floating out in space, with a
large gravitational field. Its gravitational field is always propagating. Now,
take that celestial body, and make it completely and instantaneously
disappear. There's now a gravitational differential between the now-gone body,
and its previously propagated gravity field. You should be able to detect that
if you're close, say through tidal differences.

Very similar happens with black holes colliding, except the gravity
differential comes from the two black holes oscillating near each other, close
to the speed of light.

Edit: this obviously isn't _exactly_ how this works, since it makes a lot of
assumptions, such as the ability to instantaneously remove something. So,
don't think of this as how "things actually work", but as a model to help
build your intuition.

~~~
Filligree
Be careful with that example. You probably know this, but stars can't
disappear instantaneously, and so if you start with that assumption it's easy
to get paradoxical results from relativity.

That doesn't mean there's anything wrong with the model. It's just GIGO.

~~~
wahern
A blackhole traveling at near light speed is pretty darned close to the
analogy of a massive object instantaneously disappearing, similar to fictional
spaceships engaging their warp engines.

Of course, it's not actually disappearing, just moving, but the original point
was about detecting sharp changes in the gravitational waves. A quick Google
search tells me that gravitational wave red shifting is a thing, and I imagine
that with blackholes it's a very important phenomenon and area of study. And I
would guess that there can also be interesting second-order effects that such
a blackhole's movements have on the propagation of gravitational and
electromagnetic waves from other objects.

~~~
raattgift
> gravitational wave red shifting is a thing

Yes.

> with blackholes it's a very important phenomenon

Black holes can lense gravitational radiation emitted by background systems.

Most background systems we are likely to detect soon will involve black holes.
But these are black holes in some sort of mutual orbit, rather than black
holes simply moving across some system of celestial coordinates.

For black holes that are moving linearly at near the speed of light, the black
hole's effect on the metric elongates like a pencil, with the field weak
outside and growing strong towards the centre of the "lead" or graphite. This
is similar to Lorentz-contracting the near region around the black hole, and
one can generalize a bit and say that as the boost between an observer and any
object increases, the object thins. In the ultra-ultrarelativistic limit, the
object and all the strengthening-towards-infinity field values around it
become infinitely thin.

As one's speed relative to a black hole gets very close to c, the black hole
becomes quite easy to model as an exceptionally high-energy massless particle.

You get this effect when your small space capsule whizzes by our galaxy's
central black hole at speeds near that of light too, and your small momentary
perturbation basically affects the black hole not at all. Because Lorentz
contraction is reciprocal, whizzing a black hole -- even a large one -- at
ultrarelativistic speeds past the International Space Station is going to have
very little effect on it.

We model this with the
[https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultrab...](https://en.wikipedia.org/wiki/Aichelburg%E2%80%93Sexl_ultraboost)
metric of General Relativity and usually some gauge fixing and small
perturbations.

Tossing a large black hole past the ISS at _low_ speeds compared to light will
really mess up the neighbourhood of the solar system, but your space capsule
can pretty safely manage a slow-compared-to-light hyperbolic orbit around a
large black hole without much problem (ignoring any accretion disc and twin
"paradox" issues).

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ollybee
I recommend watching videos by Rana Adhikari if you want to know about
gravitational waves. I went down a rabbit hole about a year ago with his
video's, he is increasingly engaging. Also even in a short time his
predictions for the field are coming to pass.

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H8crilA
The fundamental thing about gravitational waves is that those are a new form
of data that the universe has always been sending towards us, and that we can
read now. It's like being able to read the radio waves coming from the
universe for the first time, not just the visible spectrum, or close to the
visible spectrum.

If future advances in this field permit we may even use those to communicate.
Who knows, perhaps someone is already talking in that language.

~~~
AstralStorm
Quantum entanglement is much more feasible instead.

~~~
fsh
Quantum entanglement does not allow communication. This is known as the "no-
communication theorem".

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jackcosgrove
I am a complete neophyte regarding these topics. I did notice that the
detected events (black holes merging, neutron stars merging) seem really
exotic, while I assume gravitational waves are all around us all the time. Is
our instrumentation for detecting gravitational waves simply very insensitive
compared to the instrumentation used to detect electromagnetic waves? Or are
they harder to detect for some more fundamental reason?

~~~
chousuke
As far as I understand it, the primary reason is that gravity is a _much_
weaker force than any of the others, so detecting gravitational effects is
difficult unless the scale of the event is colossal. These instruments are
extremely sensitive; the effects they detect are just that small.

~~~
User23
It may be weak, but we can still measure it quite accurately:

“...Concluding on a lighter topic, let me remind the GGP community of what I
recall as probably the most memorable moment of the first campaign. It
occurred at the GGP Workshop in Munsbach Castle, 1999, when Virtanen was
describing the effect of snow cover on the residual gravity at Metsahovi. He
showed a figure of gravity increasing by about 2 microgal over a 4-h period as
men shoveled snow from the roof of the SG station, when a member of the
audience asked why there was an interruption in the rise of gravity, Heikki
said this was a 'tea break'...” D. Crossley, in Journal of Geodynamics 38/3-4
(2004), p. 234.

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Jerry2
Prof. Sabine Hossenfelder has an interesting blog post on LIGO [1] and a
related HN thread [2].

[1] [https://backreaction.blogspot.com/2019/09/whats-up-with-
ligo...](https://backreaction.blogspot.com/2019/09/whats-up-with-ligo.html)

[2]
[https://news.ycombinator.com/item?id=20881986](https://news.ycombinator.com/item?id=20881986)

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gfodor
A revolution would imply there has been a breakthrough that would potentially
lead to new theory. However, as with most modern physics experimental
breakthroughs, it seems gravitational waves provide just further support for
existing theories (with a few minor exceptions.)

Not to say that this isn’t exciting, but it seems long overdue for us to
stumble upon something major that is unaccounted for.

~~~
ganzuul
It's telescope 2.0. Not physics 2.0.

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wwarner
Btw -- _everyone_ should see the new documentary Chasing Einstein! I just got
out of the theater and was BLOWN AWAY.

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jobseeker990
If gravity waves exist, does that mean we could theoretically produce a
gravity wave laser?

~~~
pdonis
Just having "waves" is not enough for a laser. You need bosons--i.e., quantum
particles with integer spin. To see the quantum aspects of gravitational waves
that would correspond to this, you would need extremely strong sources--much
stronger even than the black hole mergers that LIGO has observed. Nobody has
any expectation of observing any quantum aspect of gravitational waves in the
foreseeable future, much less being able to control such sources in order to
make a laser.

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typon
This is giving me chills after reading Dark Forest. I hope this sparks a new
era of SETI with gravitational waves as the medium. I believe a civilization
advanced enough would use gravitational waves for communication.

~~~
aqme28
What are the advantages of using gravitational waves for communication? They
seem incredibly difficult to produce or detect, and don't travel any faster in
a vacuum than electromagnetic waves.

~~~
typon
Because gravitational waves get "dimmer" inversely with distance, while
electromagnetic waves get dimmer inversely with the square of the distance.

~~~
teraflop
No, both gravitational waves and electromagnetic waves decrease in intensity
with the square of distance.

(This is true of any wave in 3D space, because the area covered by a wavefront
increases with the square of distance. The inverse square law is a consequence
of conservation of energy.)

~~~
typon
The energy of the gravitational waves will decrease with the square of the
distance, but the amplitude will not. We can detect this in LIGO. This
amplitude is also known as "strain". For more discussion check this page:
[http://spiff.rit.edu/classes/ast613/lectures/grav_i/grav_i.h...](http://spiff.rit.edu/classes/ast613/lectures/grav_i/grav_i.html)

The equation for strain is:
[http://spiff.rit.edu/classes/ast613/lectures/grav_i/eqn_stra...](http://spiff.rit.edu/classes/ast613/lectures/grav_i/eqn_strain.png)

~~~
hannasanarion
The same is true of electromagnetic waves. The amplitude of EM waves decreases
inversely with distance, and the energy decreases with the inverse square.

Your original comparison of EM waves to gravitational waves is only possible
because you looked at the power of one, and the amplitude of the other. If you
compare apples to apples, they're the same.

~~~
balfirevic
A question for typon: in light of this information, do you still believe that
advanced enough civilizations would use gravitational waves for communication?

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platz
I really enjoyed this account of the technical aspects of LIGO

The Technical Challenges of Measuring Gravitational Waves – Rana Adhikari of
LIGO

[https://blog.ycombinator.com/the-technical-challenges-of-
mea...](https://blog.ycombinator.com/the-technical-challenges-of-measuring-
gravitational-waves-rana-adhikari-of-ligo/)

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z3t4
> Within those, 20 have been black hole mergers, two have been neutron star
> mergers

And here I am on a blue planet, surfing the web.

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Khelavaster
The high-frequency gravity wave revolution has been especially interesting.
Keep an open mind.

------
narrator
Unlike Electromagnetism, gravitation is boring because it is only a positive
quantity. If you had positive and negative you could have maxwell's equations
for gravitation and gravitational circuits and conductors and stuff.

~~~
mhh__
Maxwell's equations for gravitation otherwise known as the Einstein Field
Equation(s) .

------
SiempreViernes
Summary: ”everything is going well”

~~~
izzydata
Well that's a relief.

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lolc
They're listening on a really noise channel. It seems premature to talk about
the frequency of the detected events matching some expected rate when they
keep tuning their filters to the expectations.

I recently read an article deploring the missing confirmation of the mergers
through light-based astronomy. I can't find the article now, so I'm just going
to list another one:
[https://www.newscientist.com/article/mg24032022-600-exclusiv...](https://www.newscientist.com/article/mg24032022-600-exclusive-
grave-doubts-over-ligos-discovery-of-gravitational-waves/) Has anything
changed from what is described in the article?

~~~
kryptiskt
There has been confirmation, I don't know why that article doesn't bother to
mention it.
[https://en.wikipedia.org/wiki/GW170817](https://en.wikipedia.org/wiki/GW170817)

~~~
lolc
The article does mention the 17. August 2017 event just not under the GW170817
name. From the article:

> They point out that the collaboration initially registered the event as a
> false alarm because it coincided with what’s known as a “glitch”.

