New Gravitational Wave Discovery (Press Conference and Online Q&A Session)

Hans
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Okay, so when einstein@home

Okay, so when einstein@home will find gravity waves from a single spinning neutron star, science will have a continuous source of gravity waves to work with, instead of emissions from just a singular event like the black hole merger. Thanks for your feedback.

Betreger
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"gravity waves from a single

"gravity waves from a single spinning neutron star,"

I don't think that would work, IIRC it would lack the relativistic changes to make waves. 

AgentB
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Betreger wrote:"gravity waves

Betreger wrote:

"gravity waves from a single spinning neutron star,"

I don't think that would work, IIRC it would lack the relativistic changes to make waves. 

It depends on the distance and how wobbly a NS is, I would think.

From here First low-frequency Einstein@Home all-sky search for continuous gravitational waves in Advanced LIGO data.

We report results of a deep all-sky search for periodic gravitational waves from isolated neutron stars in data from the first Advanced LIGO observing run.

Results - The search did not reveal any continuous gravitational
wave signal in the parameter volume that was searched.

No GWs were found within the current sensitivity and data volume searched. The paper suggests the nearby NSs are all low ellipticity - aka smooth and spheroid.  This doesn't mean the GW don't exist just that they they are below a current threshold value.   

Jim1348
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AgentB wrote:No GWs were

AgentB wrote:
No GWs were found within the current sensitivity and data volume searched. The paper suggests the nearby NSs are all low ellipticity - aka smooth and spheroid.  This doesn't mean the GW don't exist just that they they are below a current threshold value.   

The search seems rather specialized to me, with the limits on the frequency range and whatever.  It would appear that they can at best detect the ellipticity of some neutron stars.  Is that such a big deal?  I suppose if one region has a lot of them and another region doesn't, that might say something about their evolution.  But I don't know that the present search can give the direction to the source or the distance, so they may not even know what neighborhood they are in.  Maybe we will learn more when they find some, but I have not seen an explanation of what else they are hoping to find out.

Mike Hewson
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The key to all GW detections

The key to all GW detections whatever their intrinsic strength is a "non-axisymmetric" process/scenario, this in turn being techno-speak for non-zero quadrupolar* radiation. It indeed could well be true that spinning neutron stars are not sufficiently radiating for us to detect at current instrument sensitivity levels. So why bother then ?

- it is still a valid scientific result to discover that ie. generate upper bounds on neutron star geometry. So no signal found ( to some threshold ) does not equal project failure. It is up to nature to behave in some way, we seek to accurately determine what that might be.

- said upper bounds feed back to the knowledge of the equation of state for whatever is posited to be the composition of such bodies. 'Neutron star' is a best guess label and is probably true for at least the outer skin. Who can say what might lie deep within? Most of what I have read does not speak confidently about the central core. It's not like we have a sample to poke and probe in the lab.

- it's not ellipticity ( oblate spheroid ) that is interesting as that is axi-symmetric. There may be mountains or bumps, the "relaxation" of which is hypothecated to cause glitches in the electromagnetic pulsar records. Star quakes if you like. It would also be epic if we could simultaneously record GW and EM changes on the one object.

- strictly speaking E@H is a component of LIGO's Continuous Wave Group. It could well be that nature has scenarios other than pulsars which may produce waves of that type. Who can say ?

- while we all like the idea of Einstein's GR being correct, as it certainly has been shown so far, maybe it isn't sometimes. How would we know if we didn't measure ?

If you like : being a new mode of observation then the approach taken is to look while making the fewest assumptions. To see what the universe will show us. We are not in the realtime/alert loops. We examine or integrate long periods of observations to tease out sufficiently periodic sources that show regularities above noise. Noise here is defined as those inputs which are not matched for a specific software signal template.

Cheers, Mike.

* Can't be monopolar as that breaches conservation of mass/energy. Can't be dipolar as that breaches conservation of momentum. It can be quadrupolar and higher orders in spherical harmonics though ....

( edit ) So no, we don't do the crash-bang stuff here at E@H. We might catch an inspiral perhaps but we are not best tuned for that ( though I'll bet someone is looking back over the records to see if there is any precursor to the particular collision under discussion here ). On the upside we are not examining the 'stochastic background' which is even weaker again, that's kind of like a low murmur of static with the whole universe as the echo chamber.

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Jim1348
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I knew someone would have a

I knew someone would have a good answer.  Thanks.

Mike Hewson
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I'd add that with the immense

I'd add that with the immense density of neutron star material the moments of inertia etc are of absolutely epic proportions. So even a 'small mountain' ~ 10cm high can allegedly give a perceptible GW signal at astronomical distances with LIGO devices.

Personally I like the degree of challenge presented for E@H to solve. It is an intrinsically hard problem where the signal is a fraction of the noise. A good analogy is perceiving the slow ebb and flow of tides at a beach, that being of slower time rate of change than the more vigorous, but shorter lived, waves. If anything I'm very relieved that gravitational waves were verified to exist at all ! :-)

I understand that some contributors may misunderstand our role, but it is quite correct to say that we are ( have been and will be ) testing General Relativity here at E@H. It is real/productive/proven science.

Cheers, Mike.

( edit ) The other record broken with the recent discovery is that it got 25% of all the world's professional astronomers out of bed focused on the one event at the same time !! :-)))

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Kavanagh
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With what units are these

With what units are these signals measured?

Richard

Mike Hewson
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Typically it is

Typically it is 'gravitational wave strain' this being a dimensionless number ie. a ratio of length over length. We deal in the order of 10-24 so that would mean approx 10-21 m over one kilometer of interferometer path. Bear in mind that when the perpendicular arms are locked or tuned as a resonant ( Fabry-Perot ) cavity, then a given photon may transit the LIGO beam tube about 200 times or 100 round trips of an arm. In effect then a four kilometer arm acts as an 800 km one. But in detail this all depends on the state of the interferometer during some period. The momentary sensitivity is subject to quite active mechanisms to keep the gadget in the sweet spot. This brings the actual ( differential arm ) distance to around 10-18 m variation with a passing wave ( most suitable geometry to source ). A proton or neutron has width of order 10-15 m !! :-))

The 'type' of length in the denominator of the ratio you could call the Euclidean or normal length in the everyday sense. The strain measures how far off Euclid/Pythagorus/Newton/etc the general relativistic correction is. 

The Gorrible Truth is that a length ratio for one observer may be perceived as a time ratio for another, or some mix of the two, depending on relative observer motions. This can always happen in The Relativities. Hence you may also hear of the term 'spacetime strain'. It all works out in the end though because what we actually measure is phase differentials ( 'fringe shifts' ) converted to varying photon counts. Hence all observers will see the same variation and thus discern, as they should, that a wave has passed. General Relativity has this important property that all observers ought agree afterwards* about what happened eg. two neutron stars had an epic collision.

Some plots give strain per square root Hertz thus having dimensions of time1/2. This is done, I presume, in analogy to other engineering discussions of wave power where one wants to normalise ( and thus compare ) total energies received per frequency channel, as accumulated in some given time period. It's a general finding over lots of physics that more frequently varying oscillations are more energetic.

Cheers, Mike.

* Not forgetting the notable corollary that in the instance of one observer falling into a black hole while another watches at a safe distance : afterwards = infinitely later ..... :-))

I have made this letter longer than usual because I lack the time to make it shorter ...

... and my other CPU is a Ryzen 5950X :-) Blaise Pascal

Kavanagh
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So, how large a signal would

So, how large a signal would a neutron star of mass m  with a mountain of 10mm height at a distance x would we receive? Sorry this isn't fair - you have a day job.

Richard

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