
Colliding neutron stars falsify some theories of gravity - BerislavLopac
https://arstechnica.com/science/2017/10/colliding-neutron-stars-decapitate-zombie-theory-of-gravity/
======
acqq
There were indirect proofs that the speed of gravity is close to the speed of
light since at least 1980-es, based on the 1974 discovery of a pulsar in the
binary system (1993 Nobel Prize in Physics for Hulse and Taylor) and the
measurements and analysis of that data:

[https://www.forbes.com/sites/startswithabang/2016/04/28/why-...](https://www.forbes.com/sites/startswithabang/2016/04/28/why-
does-gravity-move-at-the-speed-of-light)

"we have constrained the speed of gravity to be equal to the speed of light
with a measurement error of only 0.2%!"

But this one now is the first direct and so precise measurement that there is
no more wiggle room for those who needed it for their hypotheses.

Written in 2016 in the article I've linked:

"The best results, at the present time, tell us that the speed of gravity is
between 2.993 × 10^8 and 3.003 × 10^8 meters per second, which is an amazing
confirmation of General Relativity and a terrible difficulty for alternative
theories of gravity that don’t reduce to General Relativity!"

Now in 2017 also confirmed by the direct observation. The Einstein's General
Relativity (1915) is more than 100 years old.

Those who designed all the detectors for the gravitational waves were sure
decades ago. There were already enough consensus even then that allowed that
long work, after all, it's all based on the 1915 theory confirmed in many
different ways since. But it's so nice that now we have the real direct
measurements of the gravitational waves and the EM signals together. It's
really something comparable to the first use of the telescope 400 years ago.

~~~
nerfhammer
As as result of the recent detection gravity is now known to update at the
speed of light to within ~0.0000000000000001%, if I'm not mistaken.

------
canjobear
I found the writing in this article cutesy and incomprehensible, but the
subject matter sounds very interesting. Are there any more lucidly written
descriptions of this work? The actual paper is hard to understand.

~~~
Santosh83
Yup. [http://backreaction.blogspot.in/2017/10/new-gravitational-
wa...](http://backreaction.blogspot.in/2017/10/new-gravitational-wave-
detection-with.html)

~~~
importantbrian
This is why I love HN.

------
lstodd
There is also a recent hypothesis that doesn't require anything 'dark'

> It is shown that reduction of the gravitational mass of the system due to
> emitting gravitational waves leads to a repulsive gravitational force that
> diminishes with time but never disappears. This repulsive force may be
> related to the observed expansion of the Universe.

[https://arxiv.org/abs/1608.01541](https://arxiv.org/abs/1608.01541)

Published in MNRAS
[https://doi.org/10.1093/mnras/stw1517](https://doi.org/10.1093/mnras/stw1517)

In their other work they try to explain the EM-drive as a crude detector of
high-frequency (GHz range) gravitational wave background.

~~~
eli_gottlieb
Sounds kooky. But... the Royal Astronomical Society is publishing it?

~~~
lstodd
The math in the original article is more or less solid, no errors were found
since publication in 2016.

The EM-drive idea is an attempt at an experiment to prove/disprove that the
math has any relation to the reality.

edit: inventing undetectable dark this or dark that sounds way more kookier
than this.

~~~
acqq
From the 1608.01541 (2016) paper you linked:

"We hope that our theoretical prediction about _decreasing acceleration_ of
the Universe can be verified by observations."

But the observations actually see the opposite, the _increasing acceleration_
, verified enough that five years before that paper, the 2011 Nobel Prize in
Physics was for that discovery of the increasing acceleration, based on the
measurements of Ia supernovae:

[https://www.nobelprize.org/nobel_prizes/physics/laureates/20...](https://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html)

"What will be the final destiny of the Universe? Probably it will end in ice,
if we are to believe this year's Nobel Laureates in Physics. They have studied
several dozen exploding stars, called supernovae, and discovered that the
Universe is expanding at an ever-accelerating rate. The discovery came as a
complete surprise even to the Laureates themselves."

~~~
lstodd
I'm not a scientist. I can only post links and say "let's see, this might have
some merit".

The authors (or at least N. Gorkavyi) recently (12th Sep) announced a new
paper (literally "it's in print" whatever that means) that as far as I
understand tackles the question.

A crude translation of the announcement:

> In the second article, (in print) we show that the reverse process - growth
> of the black holes by consumption of high-frequency gravitational waves - is
> the cause of the currently observed increasing acceleration of the expansion
> of the Universe. We have even got the correct value of the cosmological
> constant, solving the problem which is about a hundred years old.

Might be the hope that you quoted was about surviving long enough to witness
the acceleration to cease increasing.

edit: spelling

~~~
acqq
Looking at who cited the paper you gave, I've found only one, and there is
actually one rebuttal of the paper that cited the former:

[https://arxiv.org/abs/1708.00799](https://arxiv.org/abs/1708.00799)

The authors of the rebuttal of that paper write:

"Since those early years, a discussion took place on the different geodesics
that can be written using local or non-local coordinates. Large accounts were
published on this debate [10, 12–14].

Two sort of claims are recurring. _Ignoring previous literature, there is a
long list of scholars who pretend having discovered repulsion._ They are
listed in [10, 12–14]. The other concerns the attribution of physical
consequences in the local environment of the falling body. For dismantling the
latter claim, we follow the analysis by Cavalleri and Spinelli [15, 16]."

"It is a coordinate effect appearing to an observer at a remote distance from
the black hole and when coordinate time is employed. Repulsion has no bearing
and relation to the local physics of the black hole, and moreover it cannot be
held responsible for accelerating outgoing particles. Thereby, the energy
boost of cosmic rays cannot be produced by repulsion."

The rebutted paper doesn't have the same topic as the one you cite, so it
doesn't have to mean too much or anything. But it hints about the way the
"repulsion" claims can be problematic:

"The main shortcoming of [17] is the assumption that repulsion, type i), is a
physical effect and not just a coordinate effect. In other words, repulsion is
only present when coordinate time is adopted and cannot explain phenomena
local to the source, and ruled by proper time."

However the lack of other responses to the paper you cite is so big at the
moment that I don't expect much of it. Is there any response to the paper you
cite, by any scientist, that you are aware of?

But I see that the guy is productive, and produced dubious papers before, so
much that he doesn't list them himself but that could be found:

[http://adsabs.harvard.edu/abs/2005AAS...206.4904G](http://adsabs.harvard.edu/abs/2005AAS...206.4904G)

The Pioneer anomaly is currently considered solved,

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

and definitely not the way he attempted to do, just like now he mentions then
"a total energy of relic high-frequency gravitational radiation in our
Universe" etc.

So I'm not holding my breath for his next paper.

Edit: thanks for the link to his Russian blog. It seems even weirder than I
could imagine, I haven't tried Google translate, but just looking, it appears
that in one post he first writes about the EM drive stuff and only then he
mentions that he apparently solved all the cosmological problems? Wow, he has
his priorities. A typical troll, the way I see him.

~~~
lstodd
NASA ADS (link is ugly and is at the arxiv page) lists three citations.

Of those, 1608.02882 (and also 1608.07136v1 which is not listed there) are a
straight attempt at rebuttal, by the same author, the other two I didn't read
yet.

As far as I understand, which is not much, that attempt was found not well
founded.

At this point I can only suggest somehow searching for discussions that
appeared back then in 2016. I don't think there was any formal (as in a
published article) rebuttal of this rebuttal, only a summary in Gorkavyi's
blog, which is in Russian and too complex for me to translate[1]

I'd wait for more papers from the original authors, in the hope that they
would generate more discussion. One has to take the academia's inertia into
account - someone who wrote a couple of dozen papers on dark stuff is
understandably reluctant to even mention and thus acknowledge a competing
hypothesis.

1\. [https://don-beaver.livejournal.com/176937.html](https://don-
beaver.livejournal.com/176937.html)

------
gregcoombe
"If the Universe were your local neighborhood, dark matter is the family that
communicates with everyone, even other family members, only by changing the
name of their WiFi network."

Ha! There are a couple of other gems in the article. Entertaining and
educational.

~~~
KGIII
That's actually pretty horrible. Gotta be honest.

HN has enough people who could design a protocol and hardware to do exactly
that with wifi. HN can't adequately explain dark matter, except to say it is a
placeholder and to define how we observe the effect. Anything more than that
is conjecture and anyone who says otherwise is a dirty rotten liar.

Really.

I hate the name Dark Matter. It's a horrible name.

But, someone here understands electromagnetism well enough, pulse signaling,
electronics, binary, spectrums, radio, whatever. They could build some sort of
mesh network to do exactly that. They could probably build it with COTS and do
it for pretty cheap - and write you a damned good paper.

Don't have a damned clue what Dark Matter really is. Nope.

Edit: Hell, now that I think about it, someone here would probably set that up
on like their ham radio stuff at 600 baud just for fun and make it all operate
out of terminal.

That's a compliment, really. There'd be a Show HN and someone would tell them
it had an Emacs shortcut.

------
tynpeddler
>The light and the gravitational waves travel along the direct line of sight
to us, curling around the gravity wells of intervening galaxies along the way.
As a result, the initial burst of light and gravitational waves hid a little
gem: the time difference between the arrival of the gravitational waves and
the light. All 1.7 seconds of it.

I'm not clear on this part. Was the light slower by 1.7 seconds because it
interacted with intervening matter?

~~~
acqq
At the moment, the researches don't believe that the light was slower, but
that it took that much time for the electromagnetic "flash" to be produced,
compared to some specific point in the signal observed by the gravitational
waves detectors. The later signal is not a point-like but a long signal which
gets stronger as the stars get closer and closer to one another and then it
disappears once there are no two stars anymore.

[https://www.forbes.com/sites/startswithabang/2017/10/17/why-...](https://www.forbes.com/sites/startswithabang/2017/10/17/why-
neutron-stars-not-black-holes-show-the-future-of-gravitational-wave-
astronomy/)

"It's anticipated that these two speeds are exactly equal, and the delay of
the light signal comes from the fact that the light-producing reactions in the
neutron star take a second or two to reach the surface."

A lot about this event was modeled even before this observation, and there
will now be more attempts to make more precise models that better match what
was measured. And the best is, these models will be evaluated by the next,
planned observations.

~~~
lutorm
The same process, but in a different scenario, was observed in supernova
1987a:

"Approximately two to three hours before the visible light from SN 1987A
reached Earth, a burst of neutrinos was observed at three separate neutrino
observatories. This is likely due to neutrino emission, which occurs
simultaneously with core collapse, but preceding the emission of visible
light. Transmission of visible light is a slower process that occurs only
after the shock wave reaches the stellar surface."

This result set a pretty strong upper bound on the neutrino mass because it
must be small enough that, across the 168,000 light year distance from the LMC
to us, the neutrinos kept their lead on the photons emitted a few hours later.

A neutron star is much smaller, the process is much faster, and the event
happened much further away, so the constraints on the difference in speed
between the gravitational waves and photons should be even more tightly
constrained in this event.

------
marktangotango
Odd that the title here has been edited to _not match_ the source when usually
titles are edited to match the source. If ars uses a click bait title why
should HN fix it?

~~~
sp332
Ars Technica articles get two headlines. After A/B testing for a bit it
settles on the more popular one. It's possible that was the other title that
showed up on this article.

~~~
njarboe
For something like an article title one could even try A/B/C/D/E testing.

------
jerry40
But how do we know that light and gravitational wave were emitted exactly at
one moment? It is not obvious for me.

~~~
jerf
What is your resolution on "one moment"? At the second-or-so level, we'd think
that they _weren 't_ emitted at the same moment, as the gravity waves come
from the orbiting stars before they collide with each other, i.e., mostly
prior to the collision.

However, relative to a prediction that they should be separated by over a
year, they arrived simultaneously.

You might wonder whether perhaps we correlated the gravity waves from one
event with the light from another; this is counterindicated by the fact that
the gravity waves match theoretical predictions about colliding neutron stars,
and the resulting light data we gather matched theories about what colliding
neutron stars might look like, an event never before witnessed. The
coincidence of these two factors makes it rather likely these events are
connected, in another spectacular triumph of theory predicting events that
we've never seen before. I don't know that the popular press articles captured
or explained this aspect very well, but I think it is part of why the
astronomy community got so excited about this. Only part; there's plenty to be
excited about in this event! It may well be the most exciting single
astronomical event in my lifetime, because there were just so many bits that
came together all at once for the first time.

~~~
jerry40
Yes, I know that coincidence of two such events is highly unlikely, but to
"compare" paths of gravitons and photons we need to imagine how much time
divides 2 emission events. Seconds? Minutes? Hours? Perhaps it isn't very
important but it's still interesting.

------
chmike
Good. Space Time Quantification (STQ) theory predicts and justify the
existence of dark matter particles. It can also predict and justify how all
elementary particles react with each other. It provides the equivalent of a
mendlejev table for elementary particles. These are conclusion based on the
single premice that space measurement is quantified by a unit a. See
[http://www.scirp.org/journal/Articles.aspx?searchCode=August...](http://www.scirp.org/journal/Articles.aspx?searchCode=Auguste++Meessen&searchField=authors_complete&page=1&SKID=0)

------
ianai
It sounds more like a confirmation of GR. Mass-energy tells spacetime how to
curve through gravity, and spacetime tells light where to travel.

~~~
ffn
Yes, the article even has a section where they straight up say "Einstein was
right".

The title of the article "Colliding neutron stars falsify some theories of
gravity" is accurate, but the theories of gravity that it falsifies isn't the
popular mainstream one (Einstein's) that, let's face it, we've all been
secretly hoping to take down. Instead, it falsifies some of the new challenger
(MOND) theories.

I, for one, welcome our continuing Einsteinian Relativistic overlord theories
of gravity (at least when it comes to dark matter)

~~~
yourapostasy
> ...the popular mainstream one (Einstein's) that, let's face it, we've all
> been secretly hoping to take down.

Please ELI5 what benefits follow from taking down the GR-based gravity theory?

~~~
ianai
Well I for one hope there’s a loop hole for FTL. Absolutely all evidence
currently points to c as a hard limit.

~~~
snarfy
Not a loop hole, but possibly a worm hole.

------
jackconnor
Dark matter disproven, wow this is amazing. This is literally one of the most
important shake ups in modern astrophysics.

~~~
Sniffnoy
You seem to have read the article backwards. It's saying that it falsifies
certain modified gravity theories (including TeVeS and STVG), not that it
falsifies dark matter.

------
smitherfield
Clickbait title, but interesting article.

I Am Not a Physicist, so I need a real one to explain why I'm wrong, but I've
thought for a while the cleanest explanation for dark matter is

1\. The universe is inside a black hole.

2\. Dark matter is matter which is outside the black hole, which explains why
it has mass but no other observable properties.

~~~
kobeya
> The universe is inside a black hole.

This is trivially true, btw. The Schwarzschild radius of the observable
universe is less than the presumed plausible size of the universe as a whole.
Or alternatively, a "black hole" is a region of space that nothing can escape,
not even light, and ongoing and accelerating spatial inflation causes that to
be true on an even smaller scale -- most of the universe is unreachable by
light emitted today.

> Dark matter is matter which is outside the black hole, which explains why it
> has mass but no other observable properties

Except that it interacts as if it was superimposed over objects in this
universe, not from outside of it.

~~~
brador
If the universe is inside a black hole what's outside the black hole?

~~~
losteric
Our part of the universe*... The presumption is that neutron stars are the
results of gravity overcoming nuclear forces, and black holes are the results
of some other force being overcome, and therefore our visible universe is the
inside of a greater collapse... Or the explosive eventual death of a black
holes...

It's a novel idea, but it's just turtles all the way down

~~~
kobeya
No, that's not really what I'm saying. A black hole is a just a region that
light can't escape from. It is entirely possible for the inside of the black
hole to be composed of normal, everyday matter, non-singularity matter. The
Schwartzchild radius scales linearly with mass, an unusual relationship that
means that for any density the O(r^3) volume will eventually be overtaken by
the O(r) Schwartzchild radius.

~~~
cygaril
I think you mean the O(r^3) Schwartzchild radius will eventually overtake the
radius r?

~~~
kobeya
Yes, thank you.

