
LIGO gravitational wave detectors that hunt for ripples in space-time upgraded - bookofjoe
https://www.npr.org/2019/03/19/701498785/massive-u-s-machines-that-hunt-for-ripples-in-space-time-just-got-an-upgrade
======
platz
[paraphrasing]

"there's a lot of confusion about if space stretches, doesn't that also
stretch the light being mesured...

... notice that I didn't say whether space stretches or that time slows down
(in response to a passing wave) - since we don't know how exactly space and
time are mixed up, we just record that it took longer for the light to come
back, but we don't say whether it was a difference in time or a difference in
space that cause the slowdown"

The Technical Challenges of Measuring Gravitational Waves - Rana Adhikari of
LIGO
[https://www.youtube.com/watch?v=1D2j8nTjOZ4](https://www.youtube.com/watch?v=1D2j8nTjOZ4)

~~~
acqq
This article tries to explain:

[https://www.forbes.com/sites/startswithabang/2018/09/15/ask-...](https://www.forbes.com/sites/startswithabang/2018/09/15/ask-
ethan-if-light-contracts-and-expands-with-space-how-do-we-detect-
gravitational-waves/)

"Ask Ethan: If Light Contracts And Expands With Space, How Do We Detect
Gravitational Waves?"

It also links to the FAQ at LIGO:

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

which for this question ends with:

"This varying pattern of misaligned laser beams, observed and recorded over
the time it takes for the gravitational wave to pass tells us two things: 1.
How much the arms changed length during the light-beams' journeys and, 2. the
frequency at which, or how quickly the arms changed lengths (longer then
shorter then longer, etc.) in response to the gravitational wave. This
information can tell us what generated the gravitational wave in the first
place.

In this scenario, the actual wavelengths of the beams of light have no bearing
on the much more important interference pattern. The effects of the length
changes in the arms far outweigh any change in the wavelength of the laser, so
we can virtually ignore it altogether."

------
basementcat
In case anyone is interested, here is an Iron Python tutorial for finding
colliding black holes in some LIGO data.

[https://www.gw-
openscience.org/s/events/GW170104/LOSC_Event_...](https://www.gw-
openscience.org/s/events/GW170104/LOSC_Event_tutorial_GW170104.html)

------
jak92
[https://text.npr.org/s.php?sId=701498785&rid=1002](https://text.npr.org/s.php?sId=701498785&rid=1002)

------
sidcool
I attended a conference recently here in India about the Ligo India project. I
am on the team that will do the data pipelines for Ligo. The sheer engineering
feat makes my software engineering bit like a kindergartener undertaking.
Truly massive and impressive.

------
deftnerd
The detectors are a big L-shaped pipeline that lasers bounce around in.
Doesn't that just measure the X and Y axis of gravitational waves? What about
Z-axis? Shouldn't they drill down as far as they can and add another arm?

~~~
magicalhippo
I forgot to add, the reason they don't drill down is because it's hard and
expensive. The arms are 4km, and to get the required sensitivity they bounce
the light about 300 times before detecting it.

Drilling down 4km would be quite expensive, especially since you need a large
mirror assembly[1] and vibration isolation system[2] down there. The mirror
assembly and vibration isolation systems would also have to be of a different
design, further increasing cost and complexity.

[1]:
[https://twitter.com/ligo/status/601848954645401601](https://twitter.com/ligo/status/601848954645401601)

[2]: [https://www.ligo.caltech.edu/page/vibration-
isolation](https://www.ligo.caltech.edu/page/vibration-isolation)

~~~
jessriedel
It would be waaaay easier to put additional detector 90 degrees away on the
surface of the Earth than it would be to drill 4km into the ground (and
service mirrors there!).

~~~
magicalhippo
But having three arms on the surface still makes the detector blind to the
third dimension.

~~~
granos
Imagine a giant cube around the entire earth that touches in 6 places -- one
on each cube face. Pick 3 faces and place a detector where each touches the
earth -- on the surface. You can rotate the detectors around on the surface so
that you get an arm in each axis relative to the center of the earth.

~~~
magicalhippo
Right, that's basically what they do now by building multiple detector
facilities. I was talking about multiple arms in a single location.

~~~
jessriedel
This isn't necessary since the wave looks the same at all locations on Earth.

~~~
magicalhippo
It can come directly from above, in which case you can't discriminate between
from above or from below as it'll hit all arms at the same time if they're at
a single location. You'd also have poor directionality.

~~~
thewakalix
Wouldn't the Spacetime Cube be just as vulnerable to the perpendicularity
problem, though? It might have more sensitivity in near-perpendicular cases,
but that's simply because there are more detectors.

~~~
magicalhippo
As mentioned, if multiple detectors are not spatially coinciding, then one can
use the time difference between detection events as well.

------
est31
I'd like to recommend this talk by Daniel Sigg about LIGO [1]. It's technical
and explains the design choices that went into building the detector. I found
it to be very interesting.

[1]: [https://www.youtube.com/watch?v=j4gE-
hSQm68](https://www.youtube.com/watch?v=j4gE-hSQm68)

------
v_lisivka
> Einstein realized that when massive objects such as black holes collide, the
> impact sends shock waves through space-time that are like the ripples in
> water created by tossing a pebble in a pond.

What? AFAIK, "like the ripples in water" describes opposite theory.

~~~
SiempreViernes
Uh, what opposite theory?

The "ripples in water" only refer to the radiation behaving like classical
waves, of which water vaves are an example.

~~~
v_lisivka
Theory of Ether, of course. Waves in Ether is like waves in water. :-/

~~~
grigjd3
The visual simile is, of course, not perfect, but the simile is not provided
for people that knowledgeable on the science in the first place. If you happen
to find a more appropriate simile that people follow, please make it known.

~~~
v_lisivka
The visual simile is Ok, but it contradicts theory of Relativity developed by
Einstein, because theory was based on _absence_ of these ripples in vacuum, as
proven by Michelson–Morley_experiment, which leaded to assumption that
c(vacuum) = const.

IMHO, author should change text to "like the ripples in fabric of pond created
by tossing a pebble in a pond", to be in line with Einstein.

~~~
AnimalMuppet
Michaelson-Morley was about a medium in which electromagnetic waves traveled
(and in fact proved that it didn't exist). We're dealing with gravitational
waves here, which travel... on... space-time itself? But it's not a medium in
the sense that the aether was supposed to be.)

At any rate, even the simile doesn't contradict either SR or GR. It's just
hard to describe what the waves are traveling on.

In particular, the problem with aether is that it was a non-relativistically-
invariant medium (that is, by moving, you could change your speed with respect
to the medium). Whatever light and gravity travel in is relativistically
invariant, but the waves-in-water analogy for how waves spread is still
perfectly appropriate.

~~~
raattgift
> We're dealing with gravitational waves here, which travel... on... space-
> time itself?

It's probably easiest to get there by considering the breaking of symmetries
of spacetime.

Let's start with the maximally symmetric spacetime: Minkowski spacetime AKA
flat spacetime. We can choose any point in the entire spacetime and measure
the same field values everywhere. Conventionally we'd normalize the
everywhere-and-everywhen-identical measurements to zero.

Let's add a spherically symmetric central mass of uniform density, giving us
the Schwarzschild spacetime. This spacetime has two new properties: firstly,
there is a location-dependence of the field-values, and secondly the spacetime
is asymptotically flat. The gravitational field values will depend on spatial
distance from the central mass, and conventionally we normalize so that they
will be larger closer to it and smaller further from it. If our central mass
emits light, we get the same result: a falling-off of the _local_ intensity of
the light with distance, and the "light field" is just the value everywhere of
the intensity of the light. At great enough distance, the measurement of
gravitational field values is practically indistinguishable from what one
would get if measuring Minkowski spacetime anywhere. Likewise, if the object
is bright, then at great enough distance we fail to gather up enough light to
see the central object: in every direction we look, we see the blackness
expected of an empty spacetime. Our field values depend on one particular
_space-like_ coordinate; we can vary the other two, or the time-like
coordinate, and the field values remain unchanged.

Let's now add a bump on the surface of the sphere, much like a large mountain
made up of the same uniformly dense material. Now our gravitational field
values will depend on where we measure relative to the bump. The field values
at a pair of adjacent points in this spacetime will differ from those we would
get in Schwarzchild spacetime. For example, if we see the bump as on the left
of the sphere-with-bump then if we measure slightly clockwise or
counterclockwise around the object we will see a change in angle reported by a
sensitive mass detector that we would not see in the case of the perfectly
smooth sphere. If our bump is much brighter than the sphere, and they are both
mutually transparent, then we can also tell that the bump is on the left, or
behind, or in front of the sphere on which it rests.

This is still an asymptotically flat spacetime: at great distances, our
measurements barely register the difference from true Minkowski spacetime. The
source of light and gravitation is pointlike at large but closer-in distances
so is asymptotically Schwarzschild. Closer in still we begin to register
observables that betray the presence of the bump. This is still a time-
independent spacetime, though. The field values (gravitational or visual) at a
given point are identical at all times; they only differ between points where
there is a difference in at least one of the _space-like_ coordinates.

Now let's add some angular momentum, and have the bump be on the equator of
the spinning sphere. Now if we choose a point in spacetime, and with a fairly
tame choice of coordinates (polar, say, or Cartesian, or their 4-d extensions
Schwarzschild coordinates or Minkowski coordinates) if we choose a point in
the equatorial plane and hold the three space-like coordinates constant then
the field value is _time-dependent_ \-- it varies with the angular momentum of
the rotating sphere-plus-equatorial-bump.

Let's make the bump bright, and the sphere it sits on dim, and both opaque.
Visually, in this time-independent spacetime, if we are in the equatorial
plane and not too far away, we will see the bump disappearing behind the bulk
of the sphere, then reappearing, brightening as it gets closer, reaching a
brightness peak when it closest to us, then dimming prior to its eclipse. If
the rotation rate is constant, then the field-values at our chosen equatorial-
plane point rise and fall according to the rotation rate. If we choose
different points anywhere in this spacetime, we can predict what the field-
values will be based on straightfoward laws.

One thing to note is that the gravitational and light measurements line up:
gravimetry and optical telescopes will agree that the "bump" is visible or
not, or is closest to us, or not.

The instant of contact during a neutron-star/black-hole (NS/BH) inspiral
corresponds to the bright bump on our dim sphere. Future to that period the
spacetime looks more and more like Schwarzschild. Past from that period, the
bump resolves into a separate roughly spherically symmetrical body (the NS) in
a mutual orbit with the BH, and further past the orbit is wider and slower. So
we have a more complicated, dynamical time-dependent spacetime; it's still
asymptotically flat (and far out but less far, it's still Schwarzschild-like):
at enormous distances one might not be able to resolve the two objects -- you
get a pretty stable point-like system with all but the most exquisitely
precise (to the point of physically implausibly precise) instruments --and at
even greater distances, this spacetime will look like empty gravity-free void.

The real spacetime around an astrophysical NS/BH merger has a lot of other
masses, and a metric expansion of space at large distances. However, we can
remove some of that "foreground" and adapt to the difference between the
expanding background and an asymptotically flat one, and reasonably predict
the field-values a given NS/BH merger would create at a point in spacetime
corresponding to a laboratory on Earth at a given time.

> waves ... which travel

The distribution of the electromagnetic (tensor) field-values generated by the
objects in the paragraphs above obey the massless wave equation. Since the
distribution of the curvature (also tensor) field-values generated by them
line up everywhere, then they too must obey the massless wave equation.

In practice, we divide up spacetime into time-and-space, and the tensors into
time-varying vectors and scalars. We choose an idealized background --
Minkowski or Schwarzschild -- and then "correct" it to match what our less-
and-less symmetrical systems generate. At reasonable distances, we can ignore
most post-Newtonian corrections, and do perturbation theory on linearized
gravity. In that, we take a view of a detector at a point (here on Earth, say,
sufficiently far away) gathering up a field value that is the result of some
past configuration of the inspiralling-binary source. The detector-result is
essentially a deviation from the value one would expect from the background
that would be sourced by the inspiralling binary in a much more symmetrical
(i.e. Schwarzschild or even Minkowski) configuration. The signal is,
importantly, periodic.

The theoretical object which encodes gravitational waves is the _difference_
between an idealized time-independent maximally-symmetrical metric and the
dynamical time-dependent metric as inferred from actual measurements, minus
all contributions to the real metric other than that of the target binary.
That is, a perturbation field. One would write this as g_{\mu\nu} =
\eta_{\mu\nu} + h_{\mu\nu} + O(\\{h_{\mu\nu}}^2) + ... where g is the real,
measurable metric, \eta is the Minkowski metric (although one could use a less
symmetrical background if one wished), h is the perturbation field, h^2, h^3,
... are higher-order perturbations caused by general-relativistic effects. If
those higher-order effects are small because the binaries are moving slow
compared to light and aren't too far away in the universe, then we can ignore
them, and \eta combines with h linearly.

Electromagnetism is a linear theory in this limit too, handily, and there are
analogies that can be drawn in this limit. One of those is that gravitational
radiation propagates in a wave-like way very similarly to electromagnetic
radiation, and further that we can analogize between the electric and magnetic
parts of an EM wave and the squash and strain parts or + and x parts of a
gravitational wave.

PS: I didn't have time to proof-read this before submitting, and hope I won't
have to come back in some hours and apologize for howling errors or crazily-
hard-to-follow leaps. :-)

~~~
v_lisivka
It's very good post, except for the fact that you missed the point of
discussion completely.

Minkowski space-time is math. Time is used as coordinate. In reality, we
cannot walk 10 seconds to the left.

Math is important (major) instrument for physicist, because it's allows us to
make predictions for things, which we cannot see with our senses, but here we
are talking about nature of the vacuum.

In short, about 100 years ago, the dominant theory was Theory of Ether. Ether
was imagined like completely transparent gas or fluid, which transfers light
and EM waves like water transfers sound waves.

This theory was replaced by Theory of General Relativity, because ToGR makes
much more precious predictions than ToE. The one of the key experiments was
Michelson–Morley experiment, which tried to answer is Ether is moves with
Earth, so we are moving with our Ether, or is it static, so we are moving
trough it at high speed (because of rotation around Sun and around Milki Way
center). However, experiment found no disturbances at all, which leads to
assumption that there is no medium for EM waves at all.

With time, we have more and more evidence that vacuum is not empty space. (I
will use word vacuum instead of Ether, because, like with atom or +/-, it is
well-known therm).

1) QM in general. Especially Heisenberg principle of uncertainty. Small
particles displays something like Brownian motion: their position is uncertain
when they at rest, except that we cannot see that with eyes because, photons
are too massive.

2) _Linear_ Sagnac interferometer demonstrates that light is captured with
medium, which leads to assumption that physical vacuum is attached to objects
like atmosphere is attached to planet. If so, then it's impossible to find
distributions of physical vacuum in well isolated room, because vacuum will be
still, like air in cabin of airplane. Only major events, like massive
explosion, can shake air in the isolated cabin while plan travels through air.

3) Hubble constant is not a constant. It's measured with high precision, but
it has different values for different frequencies. If these values are
plotted, then they for exponential curve. It's means that light is just ages
with distance traveled, i.e. it loses some energy to the medium.

4) LIGO found gravitational waves at 1E-18 precision, which means that
Michelson-Morley experiment had much lower accuracy than necessary.

And so on.

