Milky Way "starquake"
19 Feb 2005 21:26:22 UTC
Topic 187732
(moderation:
So....a magnetar shifted a bit, unleashing huge quantities of energy
http://news.bbc.co.uk/1/hi/sci/tech/4278005.stm
Presumably such an event would also generate gravity waves. Would the current detectors be able to detect them if the event were in the right part of the sky, or does nobody know?
Milky Way "starquake"
)
Yes, such an event generates gravitational waves. I don't think that particular event, energetic though it was, would be detectable by LIGO. (And LIGO wasn't running in science mode on December 27 when it went off.) But I'm at home and don't have my references handy to check. Ian Jones might know, since one of the co-authors is at his university and he wrote a paper on similar events.
In any case, that's not the sort of event that Einstein@Home is searching for. Magnetars are indeed spinning neutron stars, but the key is the length of the signal. Short signals (called bursts), such as from a big quake, are computationally cheap to search for and are done separately. The really big job is looking for a signal that is essentially always turned on, such as from a mountain on a spinning neutron star, and isn't associated with anything that ordinary telescopes have observed previously. That's what Einstein@Home does, and why we in the LIGO Science Collaboration need and very much appreciate your spare CPU cycles.
This is a good point to introduce myself. I'm a member of the Collaboration and do some theory work as well as data analysis. So is Ian Jones, who posted some links to web sites a while back. I'm thinking of setting up a page tailor-made for Einstein@Home, but that's going to take a while since I'm awfully busy during the semester. In the meantime, I'll drop in now and then and be glad to answer some specific questions.
Oh, and here is a handy feature of the board if you don't already know: At the bottom of each post are buttons to give a + or - to its rating. You can only do it once. If enough people + a post, like Ian's post about science web sites, you can set your preferences to pick it out of the many threads. We'll certainly need that feature until the Science Board gets split off.
And since I'm talking web sites, let me plug the Center for Gravitational Wave Physics, where I work and Ian used to. You might find the Outreach link of most interest. The Research link also has some digests of forefront technical papers under Highlights.
Enjoy-
Ben
> Ben, thank you so much for
)
> Ben, thank you so much for your informative reply. I appreciate you taking the time. I am really optimistic that LIGO will detect gravity waves, and sooner rather than later. I read recently that nobody really knows how strong, in terms of wavelength and frequency, the waves will be - is this true, and if so are there theoretical lower and upper limits?
Also, in terms of interferometry, is the baseline between LIGO and the German detector long enough to be able to pinpoint the source of the waves? The Einstein@home screensaver displays the location on the sky of where it is searching, so how is this directionality achieved? This seems to imply that it is an active system rather than just a passive one of detecting waves and then trying to pinpoint them. Or have I misunderstood?
You are certainly welcome to my spare CPU cycles and I am proud to be a member of Einstein@home. And SETI@home for that matter, although in nearly six years of belonging to the latter I have yet to detect a candidate signal!! Never mind, it's the searching that's important.
I'd like to wish all those working on the project the very best of luck. We do live in truly exciting times.
Best wishes
Andrew
I would like to draw
)
I would like to draw attention to the following article from the Southhampton Group website:
http://www.cc.rochester.edu/college/rtc/Borge/overview.html
One would not expect gravitational waves travelling over vast distances to have a Faraday effect and rotate as polarized electromagnetic waves do because gravitational waves presumably are not charged. However this article suggests that there may be an anistropic orientation to the fabric of spacetime. Can gravitational waves be polarized? Could it be that polarized gravitational waves will show rotation along the anistropic axial cones that Borge's describes? Would our observations be able to show any diferences between GWs arriving to earth from directions parallel and perpendicular to the anistropic axis?
I have a feeling that no one
)
I have a feeling that no one really knows offhand.
> Yes, such an event
)
> Yes, such an event generates gravitational waves. I don't think that
> particular event, energetic though it was, would be detectable by LIGO. (And
> LIGO wasn't running in science mode on December 27 when it went off.) But I'm
> at home and don't have my references handy to check. Ian Jones might know,
> since one of the co-authors is at his university and he wrote a paper on
> similar events.
>
> In any case, that's not the sort of event that Einstein@Home is searching for.
> Magnetars are indeed spinning neutron stars, but the key is the length of the
> signal. Short signals (called bursts), such as from a big quake, are
> computationally cheap to search for and are done separately. The really big
> job is looking for a signal that is essentially always turned on, such as from
> a mountain on a spinning neutron star, and isn't associated with anything that
> ordinary telescopes have observed previously. That's what Einstein@Home does,
> and why we in the LIGO Science Collaboration need and very much appreciate
> your spare CPU cycles.
>
> This is a good point to introduce myself. I'm a member of the Collaboration
> and do some theory work as well as data analysis. So is Ian Jones, who posted
> some links to web sites a while back. I'm thinking of setting up a page
> tailor-made for Einstein@Home, but that's going to take a while since I'm
> awfully busy during the semester. In the meantime, I'll drop in now and then
> and be glad to answer some specific questions.
>
> Oh, and here is a handy feature of the board if you don't already know: At the
> bottom of each post are buttons to give a + or - to its rating. You can only
> do it once. If enough people + a post, like Ian's post about science web
> sites, you can set your preferences to pick it out of the many threads. We'll
> certainly need that feature until the Science Board gets split off.
>
> And since I'm talking web sites, let me plug the href="https://einsteinathome.org/%3Ca%20href%3D"http://cgwp.gravity.psu.edu/">http://cgwp.gravity.psu.edu/">Center for Gravitational Wave Physics[/url],
> where I work and Ian used to. You might find the Outreach link of most
> interest. The Research link also has some digests of forefront technical
> papers under Highlights.
>
> Enjoy-
> Ben
>
> Ooops...I hit the wrong
)
> Ooops...I hit the wrong button. I just joined yesterday, so I'm not clear on all the procedures. Anyway, what I wanted to know is what is the nature of the data that our computers are crunching. I know that it is "about" gravity waves and relies on data from pulsars. But what specifrically are our computers "computing"?
>
Andy, that's a lot of
)
Andy, that's a lot of questions. I'll make a start on answering them, but with time constraints I may have to hold myself to just one topic a day.
Let's start with the directionality, since that's what everyone sees as soon as the screensaver comes up. That will partly answer the other things you were asking, too.
Andy wrote:
> Also, in terms of interferometry, is the baseline between LIGO and the German
> detector long enough to be able to pinpoint the source of the waves? The
> Einstein@home screensaver displays the location on the sky of where it is
> searching, so how is this directionality achieved? This seems to imply that it
> is an active system rather than just a passive one of detecting waves and then
> trying to pinpoint them. Or have I misunderstood?
In general, the directionality of a gravitational wave interferometer is not that great. They're really ears, not eyes like normal telescopes. The analogy even goes to frequency - we're searching for things at a few hundred cycles per second rather than hundreds of trillions like visible light, so you can actually make sound files of the data. The corresponding wavelengths are 1000 kilometers or so, and since directionality normally means having a detector (or spacing between multiple detectors) many wavelengths long, it's fairly bad.
But long-lived periodic signals - the ones Einstein@Home is searching for - are another matter. Even if you start of with something (bump on a rotating neutron star, whatever) that's a perfect sine wave, it won't stay that way by the time it gets into the data stream. The detectors are stuck to the Earth, which spins in little circles every day and big circles every year. That motion changes (Doppler shifts) the frequency of the signal in a complicated way that is a function of time and also of position on the sky. For example, a source located over the North Pole will not be Doppler shifted by the daily rotation but will be affected by the Earth's orbital motion. And this doesn't depend on having multiple detectors with a long baseline between them, although that's good for other things; it just depends on the Earth moving.
Those complicated Doppler shifts are where the angular resolution comes from. The data analysis has to compensate for the Doppler shift to make any signal as sinusoidal as possible, which helps pull it out of the noise (with a Fourier transform). It has to do one Doppler shift for one sky location, then Fourier transform to see if there's something there; another Doppler shift for another sky location, then Fourier transform; and so on. For an in-depth search, even a small change in sky location makes the Doppler shift different enough to completely wash out any signal if done wrong. The net result is a lot of sky positions to search.
There's the rub: A more sensitive search needs more sky positions, which means more CPU cycles. Thus Einstein@Home.
Hope this helps,
Ben
By the way, I realized I left
)
By the way, I realized I left out something important from my earlier post about the starquake mentioned in the title of this thread.
I wrote:
> In any case, that's not the sort of event that Einstein@Home is searching for.
> Magnetars are indeed spinning neutron stars, but the key is the length of the
> signal. Short signals (called bursts), such as from a big quake, are
> computationally cheap to search for and are done separately. The really big
> job is looking for a signal that is essentially always turned on, such as from
> a mountain on a spinning neutron star, and isn't associated with anything that
> ordinary telescopes have observed previously. That's what Einstein@Home does,
> and why we in the LIGO Science Collaboration need and very much appreciate
> your spare CPU cycles.
That's true as far as it goes, but....
In order to have a quake like that, the magnetar had to have some sort of deformation beforehand. For example, a big chunk of the crust may have been held up by the intense magnetic field, and when it slipped back into place it made the quake and gamma rays. That deformation, spinning around day after day and year after year, is the sort of thing that Einstein@Home is searching for. But Einstein@Home is searching the whole sky for that sort of thing. There are probably a lot of neutron stars out there that telescopes (gamma-ray, radio, optical, etc) don't see for various reasons, such as the field being too weak or not pointing towards us so the light doesn't get beamed in our direction. But gravitational waves get emitted in all directions, so LIGO and company stand a chance of seeing things the ordinary telescopes can't at all.
Hope this helps,
Ben
Thanks for your time Ben -
)
Thanks for your time Ben - that's clarified things a lot.
>
maybe we should have a new
)
maybe we should have a new board called "the professors' corner".