Advanced LIGO has detected gravitational waves from a binaryblack hole collision/merger

In article ,
Jos Bergervoet writes:
There will be noise in the data, but why in the predictions?

Because the input parameters -- initial black hole masses and
separation and source direction, for example -- are not known
exactly. Probably some detector parameters are not exactly known
either. Also because there are two different modeling approaches to
the data. Caption text includes "Shaded areas show 90% credible
regions for two independent waveform reconstructions."

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Jos Bergervoet writes:
Possible explanations:
1) For some reason (to make the curve look more "natural"?)
someone decided to add random noise to the computed results.
And they added *different* noise for Hanford and Livingston.
To me this seems a silly eplanation.
2) The results are different polarization componentsa (after
all you only need a 45 degree tilt to see the independent
other polarization for a spin-2 field.)
3) The numerical routines generate some numerical errors
visible as small random looking "ripples" in the computed
result. This seems likely since complex curved space-time
will enforce a complicated non-uniform grid in the 4
coordinates.

4) The expected signal looks different for the two detectors,
considering their different orientation in space. This includes the
reason 2) and also things like projection effects.

[Mod. note: quoted text trimmed -- mjh]
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Space - The final frontier

Op donderdag 18 februari 2016 21:27:45 UTC+1 schreef Phillip Helbig:
In article , Nicolaas Vroom
writes:

The next thing you can do is draw a sphere with radius r0 around
Observer. If at t0 at this whole sphere there is also a BH merger than
the gravitational waves from that event will also reach the observer
at (t10,r10=0). Again a superposition and that is not what we want.
It should be mentioned that the concept of a sphere with radius r0
is an approximation.

OK. Consider that such an event lasts a few seconds or whatever. One
can get some idea about the chances of two or more overlapping (the one
at the farther distance taking place farther in the past, of course).

If you are considering one point (one direction) than in case of
overlapping both events take place at different distances and time.
However there can also be overlapping IMO when you consider two different
directions.
There can be overlapping when the distance is the same. In that case
the two events happen at the same time.
But there can also be overlapping when the distance is not the same.
In that case the two events don't happen at the same time.
It is particular this situation (assuming my understanding is correct)
that worries me.

People have done the calculations and estimates of the numbers of such
events. I don't think "confusion", as this is probably called, that is,
more than one event observable at the same time, is an issue here.

That is just my concern.
In fact all the extraterrestrial noise detected is caused by such events.

To detect a single supernova the story is different. At each instant
you can detect many supernovae simultaneous assuming their directions
come from different positions on the sphere surrounding us.

In principle the same with gravitational waves. However, since they
last much shorter than a supernova, probably only one is visible at any
given time.

IMO for supernova there is no issue of overlapping when you
consider two different directions.

Nicolaas Vroom

On 2/12/16 3:04 AM, Jonathan Thornburg [remove -animal to reply] wrote:
The event is called GW150914.

This Nature page is a good summary
http://www.nature.com/news/einstein-...t-last-1.19361
as is this LIGO page
http://www.ligo.org/science/Publicat...0914/index.php

The main discovery paper is:
http://link.aps.org/doi/10.1103/PhysRevLett.116.061102

There are also a bunch of other papers published today, all linked from
the LIGO page I gave aboe.


i have been trying to ascertain if this GW150914 event with only *two*
LIGO detectors confirms the wavespeed of gravitational waves?

it seems to me that they are *assuming* the wavespeed is the same as
light speed when they do the beam-forming math (same as what we EEs do
with an antenna array or microphone array) to determine the angle of
incidence.

that 7 ms time difference (with 3000 km distance) could be accounted for
with a different wavespeed and angle of incidence than they are telling
us. it appears that this 7 ms time difference implies a *maximum*
possible wavespeed of about 140% c.

what do you guys think? don't we need at least one more detector
operating for the next GW event to nail down that gravitation propagates
at the same speed as EM?

just wondering.

--

r b-j

"Imagination is more important than knowledge."

In article ,
robert bristow-johnson wrote:

i have been trying to ascertain if this GW150914 event with only *two*
LIGO detectors confirms the wavespeed of gravitational waves?

There hasn't been a direct measurement. The observations
imply that the speed is no more than 1.7 times the speed
of light (see http://arxiv.org/abs/1602.04188), and the
absence of gravitational Cherenkov radiation implies a
lower bound very near c, but it will take an observation
with at least three detectors to get an accurate measurement.

(On the other hand, the observations match the GR prediction
extremely well, and I expect it would take quite a bit of
work to cook up a model which made the same predictions but
had gravitational waves traveling at a speed different from c.)

Steve Carlip

robert bristow-johnson wrote:
what do you guys think? don't we need at least one more detector
operating for the next GW event to nail down that gravitation
propagates at the same speed as EM?

The energy of gravitational waves must be in quanta with an energy
of Planck's constant times their frequency, just like anything else.
(The existence of a form of radiation that wasn't quantized, or
that had a different quantization than Planck's constant times its
frequency, would lead to a paradox.)

How closely the speed of a particle approaches c depends on how many
times higher its energy is than the equivalent energy of its rest
mass. If the particle's rest mass is zero, it travels at c regardless
of its energy.

Since the received gravitational wave signal doubled in frequency in
about a tenth of a second, in agreement with black-hole theory, that
means that their speeds are the same within measurement error. And
note that the frequencies involved are very low (at least compared to
typical electromagnetic frequencies), meaning the energy of each of
the particles was extremely small. Also note that the event was about
1.3 billion light years away, so any lag was less than a tenth of a
second out of more than a billion years.

So the rest mass of the graviton must be zero or extremely close to
it. Of course this doesn't prove that it *is* zero. But then the
same is true of light itself, just as it turned out to be true of
neutrinos. It's possible that everything has rest mass and travels
at less than c. But at reasonable frequencies, electromagnetic and
gravitational waves travel so close to it that we're as yet unable
to measure any difference.

(It's also possible that one or more of electromagnetic waves,
gravitational waves, and neutrinos have an imaginary rest mass and
travel slightly faster than c. (In that case lower frequencies would
go faster.) That could lead to a causality paradox, but then so could
large parts of general relativity, so I don't think that's a good
argument against it.)
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