
Rumours swell over new kind of gravitational-wave sighting - indescions_2017
http://www.nature.com/news/rumours-swell-over-new-kind-of-gravitational-wave-sighting-1.22482
======
saganus
This might be a naive question, but I'd like to clear it out.

Do gravitational waves travel at the speed of light?

I know the theory says nothing can travel faster than light. I also know that
photons can be seen as quanta or as waves. So my guess is that gravitational
waves travel at most, at the speed of light.

But do they? or do they travel slower? faster? Is there a doppler effect for
GWs?

I ask because I would think ripples in the space-time fabric itself might be a
bit different than light waves or other more studied phenomena.

Can anyone point me in the right direction?

~~~
bramen
The speed of light is sometimes referred to as the speed of causality, and it
seems like it's more of a fundamental speed limit on the propagation of events
or information through space.

~~~
reubenswartz
IIRC, everything moves through spacetime at c. Things with mass like people,
planets, etc, move through the time portion as well as the space portion. As
you go faster through space, you travel less through time, though at non-
relativistic speeds you don't notice (GPS satellites do have to account for
this). Electromagnetic waves have no mass, they don't travel in time, so the
entire portion of their travel takes place in space, so we say they travel at
the "speed of light."

~~~
Swizec
> Electromagnetic waves have no mass, they don't travel in time, so the entire
> portion of their travel takes place in space, so we say they travel at the
> "speed of light."

This part comfuses me. If they don't travel in time, how do they have a speed?
Light is a type of electromagnetic wave right? And it takes many years to
travel to us from a nearby star.

If we can measure or calculate the time it takes for light from some place to
reach us, does that not imply traveling through time?

~~~
erikpukinskis
Think about a wave on a lake. It may appear to be moving in time. The water
particles certainly move up and down. But if nothing is in it's "path" is the
wave really moving? It's actually just there, the wave undulates and that
creates the perception of motion, but really the thing you see moving is just
a visual effect on the surface of a field the size of the entire lake. A field
which is not moving at all.

Photons are similar. You see the peak of the wave moving around, but the wave
itself is everywhere and eternal... until other forces get involved anyway.

~~~
Koshkin
This picture is incorrect: the electromagnetic wave has a mechanical momentum
in the direction of its propagation, which means that something is moving in
that direction.

~~~
erikpukinskis
Does it have momentum before we measure it? I thought momentum was a property
of the collapse event, not a property of the wave?

~~~
Koshkin
According to the classical electrodynamics - it sure does. From the quantum
mechanical point of view, it also does - in the sense that we can always
measure it (i.e. it is an _observable_ ). The "property" in this case is not
so much a particular outcome of such measurement as much as the expectation
value; actually, I'm afraid that the use of the word "property" in this
context can only lead to confusion as it effectively conflates several
different things: the (quantum-mechanical) state, the observable, and the
particular value observed.

------
carbocation
My understanding is that black hole mergers are not expected to have any
optical-wavelength emissions, whereas neutron star mergers should have
emissions across the electromagnetic spectrum. Is that distinction part of the
excitement here?

~~~
raattgift
The matter and light radiated away from black hole (BH) mergers will all come
from the accretion discs of each BH. The accretion discs of the BH masses for
which LIGO (and Virgo) is most sensitive will generally be fairly sparse, so
the emissions will generally be fairly dim.

For a pair of similar-mass neutron stars (NS) that merge into a BH, you can
treat region of spacetime close to the merger as being filled with extremely
dense accretion discs. The density of matter leads very bright emissions, and
the available geodesics produce radiation that will escape to infinity rather
than being quickly absorbed close to the source (including within the dense
matter around the newly formed BH as it settles into an accretion disc). The
picture is slightly different where the NS masses are highly unequal.

The Max Planck Institute for Gravitational Physics at Potsdam Germany has done
many numerical simulations of NS mergers. NASA's animated one [1] where the NS
are of substantially different masses:

[https://www.youtube.com/watch?v=vw2sLcyV7Vc](https://www.youtube.com/watch?v=vw2sLcyV7Vc)

Systems of mass that are barbell-shaped (a pair of heavy masses connected via
an arbitrarily thin bar; for orbiting stars and BHs we take the limit as the
bar goes to zero volume and zero mass) will radiate gravitational waves when
they are spun about an axis on an axis perpendicular to the bar. These
spinning NSes will be radiating gravitational waves with increasing amplitude,
and these radiated GWs remove angular momentum from the rotating system,
allowing the NSes to move closer to each other. By the start of the video, the
GW radiation being emitted should eventually be detectable by instruments at
enormous distances; that radiation has allowed the two NSes to reach a
critical proximity.

At this point the smaller NS disintegrates under tidal stress and its
additional mass-energy-momentum that then begins falling onto the larger NS
causes the larger NS to collapse into a BH. A large proportion of the smaller
NS remains in the region near the new BH and is swept up into an accretion
structure alsong with s small proportion of the larger NS that did not get
trapped behind the horizon. The accretion structure is briefly very bright,
especially in gamma rays, as the matter from the smaller neutron star self-
collides until it is entrained into a dense disc. Those early gammas will be
visible at enormous distances (e.g. we can pick up extragalactic NS mergers
with sky-scanning instruments searching for gamma ray bursts [2]).

By comparison, the smaller of a pair of mass-mismatched black holes cannot
disintegrate (everything is stuck within each BH's horizon), and the larger of
the pair is likely to have a sparser accretion disc, so the amount of disc-
disc collision will be relatively low. The reshaping of the accretion material
around the merged BHs may be driven principally by the dynamical spacetime
around the BH, with only occasional collisions. While such collisions can be
arbitrarily energetic (at the point where their geodesics intersect bits of
matter may be moving ultrarelativistically with respect to each other), there
are unlikely to be enough such emissions to be reliably detectable at large
distances.

\- --

[1] [https://arxiv.org/abs/1001.3074](https://arxiv.org/abs/1001.3074)

[2] There is a timing-coincidence argument about a short gamma ray burst
detected in this way and a detection by LIGO & Virgo that is circulating
around the rumour mill. Peter Coles blogged some detail
[https://telescoper.wordpress.com/2017/08/23/ligo-leaks-
and-n...](https://telescoper.wordpress.com/2017/08/23/ligo-leaks-and-
ngc-4993/) This may be an NS-BH merger, in which the picture is again somewhat
different, and depends on the density of the BH's accretion disc both in terms
of its matter content and in terms of the momentum (e.g. if the NS and BH are
counter-rotating, collisions will be more frequent and more energetic).

~~~
mturmon
Thanks for this well informed comment. It links together some of the observing
strategies and motivations really well.

------
Koshkin
While I would have a trouble imagining a quantum of the space-time curvature
(the graviton), it is not hard to see that the changes in the curvature could
propagate in the form of waves. So, while yes, an experimental discovery of
these waves is an important event in history of science, I am left curious as
to whether it adds anything to our present understanding of Nature...

~~~
raattgift
> trouble imagining ... the graviton

Let's start with a graviton as a gauge boson, mediating the gravitational
interaction similarly to how the photon is a gauge boson mediating the
electromagnetic interaction.

First, let's start with "gauge". We can use as an analogy an air pressure
gauge, one that measures relative air pressures. Let's take it to a place with
a standard pressure (say, in conditions which are effectively STP) and tune
our pressure gauge so that it reads "0" at that air pressure. As we wander to
and fro reading our calibrated gauge, we'll see pressures that are zero,
positive or negative. If we climb a tall hill we'll see a negative reading. If
the temperature drops we'll see a positive reading.

If we contrive things so that we can take a reading of a generalized pressure
with our pressure gauge everywhere in the universe at all times, we can
construct a (classical) gauge field. In deep space, the gauge will read
strongly negative. At the bottom of the ocean, or deep in Jupiter's
atmosphere, or at the core of the sun it will read strongly positive. In the
early universe, it'll be strongly positive; in the far far distant future away
from the black hole that will dominate our patch of de Sitter vacuum, it will
read strongly negative. We'll get zero values in some places, like near the
Earth's surface through a lot of Earth's history, or in the upper reaches of
Jupiter's atmosphere through a lot of its history.

Our choice of "0" is not ideal, because "0" is only rarely the value at any
point in our gauge field that permeates all of spacetime. Instead we should
set "0" as the value in extragalactic space, because then "0"s will dominate
the field (indeed it is possible that all readings will then be non-negative).
In effect, when we set our "0" at STP we normalized the gauge field; when we
decided instead to set our "0" in extragalactic space, we renormalized it. We
could obtain an ideal renormalization if we could sample the whole of
spacetime and find the lowest reading of our pressure gauge, but we can
certainly get rid of practically all negative values by taking far fewer
samples in regions where we think the lowest readings might be.

Once we have settled on a decent normalization, we could look at the
propagation of nonzeros and study their statistics. If they follow the Bose-
Einstein statistics, we'd call them "bosons". If they follow the Fermi-Dirac
statistics, we'd call them "fermions". If they follow some other statistics,
we'd assign them yet another name. (Our choice of generalized "air pressure"
probably follows some odd statistics.)

Perturbative quantum gravity works something like this. [2]

We have a background spacetime with a metric; we have a gauge that measures
the deviation from this metric. It'll be "0" at every point where the
arrangement of stress-energy _exactly_ matches the metric, and nonzero
elsewhere. We are interested in modelling the gravitational interaction as the
arrangement of nonzero values in our field. Patterns of nonzeros around
gravitationally interacting matter themselves evolve (under a suitable
decomposition of spacetime into 3+1 space and time) and interact like
(classical) waves (made up of many molecules), and upon some study we can
determine that these waves in our gauge field form patterns that strongly
suggest they have a rotational symmetry of two, which we expect on theoretical
grounds too because the metric is a rank-2 tensor field so particles
representing the (change in the) metric field should be spin 2.

Conveniently, in a quantum gauge group theory, a particle with spin 2 is
attractive of a particle with the same charge and repulsive of a particle with
the opposite charge. (Compare with spin 1, where same-charges repel and
opposite-charges attract). [4] So we can identify the _nonzero_ numbers in our
metric gauge field with gravitons. This is amenable to study with perturbation
theory.

Unfortunately General Relativity is a non-linear theory and in our
perturbatively quantized gravity, when you have a lot of high-energy gravitons
they spawn more gravitons. We would want to apply Wilson's thinking on
renormalization and reset our gauge to "0" in a cluster of these high-energy
gravitons by finding some suitable ground value in the cluster. This is
_extremely_ successful up to a point [1], but as the energies of the gravitons
increases we have to take more measurements to find a suitable ground value,
and eventually we have to take an infinite number of measurements to find one.
This is what is meant when you read "gravity is perturbatively non-
renormalizable".

There are, as you suggest with ("... space-time curvature ..."), other values
related to the gravitational interaction that we can turn into a quantum field
[3], but most suffer a highly similar fate: in some conditions we have to do
an infinite amount of work to make our field values sensible _and_ match
observables.

Finally the gauge field that we built on the metric is fully relativistic and
generally covariant, so it works with any system of coordinates, choice of
units, slicing of spacetime into 3+1, etc. that we want, up to diffeomorphisms
(we have to remember that we chose a static background spacetime). So even
though it gives us useless readings in some regions of a spacetime containing
strong gravity, perturbative quantum gravity is a useful and standard tool.
However, it is not considered a candidate for a fundamental theory (barring
some unforessen advancement in renormalization theory) rather than an
effective theory and moreover by implication it undercuts General Relativity's
claim to be a fundamental theory too.

\- --

[1] Relativists tend to define "strong gravity" at this point, since we get
correct results from renormalization at any energy lower than it. Strong
gravity only appears very close to gravitational singularities (and in the
case of black holes, that means well inside the horizon). If we are using the
path-integral formalism then we'd find that we have "strong gravity" in this
sense in every Feynman diagram containing at least one loop of gravitons.

[2] [https://arxiv.org/abs/gr-qc/0206071](https://arxiv.org/abs/gr-qc/0206071)

[3]
[https://www.wikiwand.com/en/Canonical_quantum_gravity](https://www.wikiwand.com/en/Canonical_quantum_gravity)

[4] One might ask, "is there oppositely-gravitationally-charged matter
anywhere"? It's an OK question, and people have discussed it seriously. Sabine
Hossenfelder has touched on this a few times on her blog, including
[http://backreaction.blogspot.com/2017/04/why-doesnt-anti-
mat...](http://backreaction.blogspot.com/2017/04/why-doesnt-anti-matter-anti-
gravitate.html) although while the photon has no electromagnetic charge, the
graviton itself (in perturbative quantum gravity) has gravitational charge
(this reflects the non-linearity of General Relativity).

------
legohead
The idea of two neutron stars colliding gives me shivers. To see something
like that would be mind boggling..

------
MilnerRoute
"We are working hard to assure that the candidates are valid gravitational-
wave events, and it will require time to establish the level of confidence
needed to bring any results to the scientific community and the greater
public. We will let you know as soon we have information ready to share."

\-- Statement from Ligo
[http://www.ligo.org/news/index.php#O2end](http://www.ligo.org/news/index.php#O2end)

"This did not, in fact, blow my sox off."

\-- Astronomer Peter Yoachim
[https://twitter.com/PeterYoachim/status/901175225819176961](https://twitter.com/PeterYoachim/status/901175225819176961)

------
colordrops
I wonder if the gravitational wave would be physically noticeable if one were
near the event.

------
whoopdedo
> swell ... wave

Someone had fun writing that headline.

~~~
ChuckMcM
No doubt they will "crest" just before the announcement :-). And opinions will
"undulate" over whether or not they are valid results.

Perhaps we just need more drama in science classes to be more inclusive, "Will
this acid change the Ph level of the solution? Or will its buffering protect
it? Find out after the break ..."

~~~
qubex
You will never believe what happens to this falling feather in a vacuum!

------
smhost
As a non-science person who occasionally watches PBS documentaries, could this
have any implications for Hawking radiation?

~~~
snissn
Potentially this neutron - neutron star merger or ones that are detected in
the future will result in the formation of a black hole and in that event we
may get more data to help refine models of black holes and those refined
models may give us insight into the mechanics of black hole radiation - but it
would be a bit indirect and we'd have to get lucky. Also Hawking radiation is
"very slow" and may never be measured directly in interstellar black holes

------
andrewflnr
Normally, the scientific community is pretty careful about not revealing
results before they're fully baked (press notwithstanding). Seeing how that
control has broken down for this incident, is it correct to infer that
astronomers have pretty much lost their minds over the possibility of
capturing a neutron star merger?

Edit: calm down, people, I'm excited too.

~~~
evanb
LIGO didn't announce because they want to be sure. But one of the most
valuable aspects of LIGO is as a trigger for optical astronomy of transient,
time-sensitive events. So, when they see something, they have to tell a wider
astronomical community---the other observers who look in non-gravitational
channels---though they might not be ready to make the information public. Each
observatory comprises hundreds of people. Suddenly your message isn't
contained to just the hundreds of people loyal to LIGO, but to thousands of
people. Moreover, these observatories tend to be publicly owned and funded,
and transparency and modern open-science practices often mean live updating of
the status of these observatories. Finally, there are the scientists who have
nothing to do with this physics, but fought hard for some observation time and
had their scheduled observations interrupted for something high-priority.
These people can infer (or are often told directly) why their allocation was
preempted.

------
perseusprime11
Someday could we ride these waves to Interstellar travel?

~~~
Panoramix
No. The effect of the waves is so small that it required a gargantuan project
to detect the strongest events. The LIGO detector is possibly the most
sensitive instrument of any kind humans have made.

