LIGO, High Gear Science Run Options

[The Messenger]

http://www.spaceref.com/news/viewpr.html?pid=19142

ST. LOUIS, Mo. -- The quest to detect and study gravitational waves with the NSF-funded Laser Interferometer Gravitational-Wave Observatory (LIGO) is now in the fourth month of its first sustained science run since achieving its promised design sensitivity, project personnel announced at the annual meeting of the American Association for the Advancement of Science (AAAS). ...

Now that the LIGO is sensitive enough to detect changes in distance a mere thousandth the diameter of a proton, Marx adds, the science return should be even greater. Recent results from the Swift satellite pinpointing the location of short gamma-ray bursts (GRBs) have also heightened astronomers' interest in the results from LIGO's current observational run.

That level of sensitivity is, in my opinion, the most incredible technical achievement since the VLA.

The very long gamma ray associated with supernova/hypernova 1996aj should also be of great interest.

[edstrick]

LIGO can potentially detect 1) predicted types of sources and 2) unpredicted types of sources. It cannot detect some types of sources that are predicted to have most of their energy at wavelengths much longer, frequencies much lower, than it's high frequency sensativity range.

The proposed LISA space-interferometer gravity wave mission would search for much lower frequency sources and is predicted to observe many to the point of having considerable confusion sorting out things like white-dwarf binaries and cosmic backgrounds.

The problem with LIGO, Mark 1, is it's sensativity is so low that the PREDICTED frequency of detectable events of PREDICTED type, the classic being binary-neutron star "in-spirals", is considerably lower than 1 per year. So they're searching down in the noise level for barely detectible, if at all, events.

We COULD get locky... and have a predicted type of source do it's thing so close there's an obvious signature well above the noise...but don't count on it.
We COULD get luckier, and have predicted type of events happen much more often or have unpredicted types of events happen often enough and close enough they also stick out above the nose. Again... don't count on it.

LIGO, Mark 2... The second generation detector system, is intended to have high enough sensativity so that there are multiple events per year that are clearly detected.

[Guest_Richard Trigaux_*]

We COULD get locky... and have a predicted type of source do it's thing so close there's an obvious signature well above the noise...but don't count on it.
We COULD get luckier, and have predicted type of events happen much more often or have unpredicted types of events happen often enough and close enough they also stick out above the nose. Again... don't count on it.

Translation: LIGO may detect nothing in all the run. But if so, it would not be an evidence that gravity waves don't exist.


In SETI (I alway compare to SETI, as basically it is the same problem: sort out signals from noise) they also try to find recognizable patterns, such as pulses, multiple pulses, or single frequencies shifting (with the motion of the emitter). The problem they have to face is that there is so much records that, statistically, any pattern you search for is likely to appear from mere random noise. So you have to consider the overall probability of a pattern match in all the data, and it can be much higher than expected. Despites this they do have some alarms which occurence at random are very low (one billionth or less). If they don't claim such alarms as positive detection, it is only because they have further tests to accept these alarms, for instance that they happen several times on the same place.

With LIGO there is only one set of data (there is no direction) but still a long time and many frequencies. So if we look in depth into the data, what is likely to happen is something for instance like a whistle of shifting frequencies which probability to appear from random noise is equal or higher to the probability of coming from a (far) black hole spiraling. Such a result would "encourage to build a more sensitive experiment", but I would prefer a clear result, a strong enough event which probability to appear from noise is very low. But we still need to be lucky...


Of course if a clear signal is detected, this will much more encourage to build more sensitive detectors, as gravitationnal waves would be a new and completelly different window on the universe, where we directly see the movements of large/dense bodies.

[The Messenger]

The data being reduced on Einstein at Home computers (right now) is from science run 4 (S4). It may be necessary to process all of the data before a determination can be made as to whether an event has been witnessed. I know, for example, that about a third of the way through run three, they determined new baseline criteria (based upon the analsysis of noise levels), and restarted crunching the numbers.

Richard's analysis is consistent with what I have read elsewhere - the odds witnessing a gravity wave in this data are small.

Their is much more optimism about the current run (S5), if the current level of sensitive can be maintained. In fact a number of scientist near the project have stated that a null result through ~2010 should be considered a 'successful' non-detection, and GR theory would have to be slated for revision.