PDA

View Full Version : Null result for LIGO



Argos
2009-Aug-21, 03:36 PM
In results announced today, a huge physics experiment built to detect gravitational waves has yet to find any. Rather than be disappointed by the null findings, physicists say the results were expected, and in fact help them narrow down possibilities for what the universe was like just after it was born.

Space.com (http://www.space.com/scienceastronomy/090819-gravitational-waves.html)

peteshimmon
2009-Aug-21, 06:48 PM
WHAT!

They spent millions and millions and found
nothing!

That was OK in the days of gentlemen scientists
spending their own money but is not quite the
expected thing nowadays.

:)

ravens_cry
2009-Aug-21, 07:08 PM
WHAT!

They spent millions and millions and found
nothing!

That was OK in the days of gentlemen scientists
spending their own money but is not quite the
expected thing nowadays.

:)
Let's say you been given a tub of ice cream, but you weren't told what kind it was. Someone telling you the kinds it isn't can be almost as informative as telling you what it is.

peteshimmon
2009-Aug-21, 07:20 PM
Thats the kind of wise guy that will have the
ice cream poured over him:)

That LIGO has always reminded me of something.
The Michelson Interferometer.

What was that famous for now...

Cougar
2009-Aug-25, 05:20 PM
WHAT! They spent millions and millions and found nothing!

Well, no, not really. From the article....



LIGO's null result has limited the possible strength of the [gravitational wave] background. Now that researchers know this background can't be strong enough to have been detected so far, they can put new constraints on the details of how the universe looked in its earliest moments.

"The thing that makes this exciting is that this is really the only way to probe the early universe..."

For example, the conjecture regarding cosmic strings (http://en.wikipedia.org/wiki/Cosmic_string) looks like it is headed for the dustbin of history.

flynjack1
2009-Aug-25, 07:47 PM
The article additionally states : The next phase of the project, called Advanced LIGO, will improve the experiment's sensitivity, allowing scientists to probe a volume of space about 1,000 times larger than the current project's range.

"If Advanced LIGO doesn't see gravitational waves I think people will be very surprised," Mandic told SPACE.com. "It is likely such a situation would require revision of General Relativity."

If they suspected that they would need a higher sensitivity to start with, why not build it appropriately to cover the full spectrum reasonably expected? Is this the same tool only slightly modified to give a greater sensitivity, or are they building a new testing device all together?

parejkoj
2009-Aug-25, 10:14 PM
If they suspected that they would need a higher sensitivity to start with, why not build it appropriately to cover the full spectrum reasonably expected? Is this the same tool only slightly modified to give a greater sensitivity, or are they building a new testing device all together?

LIGO took ~8 years to get to this sensitivity. It was breaking new ground in stability, cooling, laser technology, vibration damping and computing techniques to remove background noise (among other things). There were some initial doubts that they'd be able to even reach the sensitivity level they were hoping for.

LIGO was really more of a testbed. People that I talked to who worked on it around 2003/2004 said it probably wouldn't detect anything, but that it was required to verify that the techniques they were developing would work. Kind of like the Mars Pathfinder mission: it's primary goal wasn't really new science, but to show that that particular method of getting to Mars and driving around would work for future missions. The science that came out was a welcome bonus.

Advanced LIGO uses some of the same equipment, and expands on the techniques that LIGO developed. But it's more than just "slightly modified."

http://en.wikipedia.org/wiki/LIGO

KaiYeves
2009-Aug-26, 01:47 AM
Is it a bad sign that I read that as "Null result for LEGO"?

Perhaps I shouldn't play with the little coloured bricks so often...

flynjack1
2009-Aug-26, 05:17 PM
Parejkoj,
Well as is usual the science story is less than complete in explaining matters. There seems to be a problem getting science writers to write precisely but keep it interesting at the same time. Too much sensationalism too few details. Thanks for your reply.

Argos
2009-Aug-26, 05:31 PM
I can hardly wait for LISA (http://lisa.nasa.gov/), scheduled [hopefully] for 2014.

The MM experiment was mentioned somewhere above. Itīs interesting how all these projects rely on the same basic design as the MM experiment. Itīs terrifically suited for measuring flows of all kinds.

peteshimmon
2009-Aug-26, 06:16 PM
What I am curious about is that using the
Michelson-Morley set-up to look for sudden
movement of the test masses seems incongruous
if masses are moving due to space-time ripples
passing through. What does that do to the
light beams?

I am sure that millions have been spent with
due regard to these simple questions but what
is the simple answer?

Jerry
2009-Aug-27, 05:03 PM
LIGO took ~8 years to get to this sensitivity. It was breaking new ground in stability, cooling, laser technology, vibration damping and computing techniques to remove background noise (among other things). There were some initial doubts that they'd be able to even reach the sensitivity level they were hoping for.

LIGO was really more of a testbed. People that I talked to who worked on it around 2003/2004 said it probably wouldn't detect anything, but that it was required to verify that the techniques they were developing would work. Kind of like the Mars Pathfinder mission: it's primary goal wasn't really new science, but to show that that particular method of getting to Mars and driving around would work for future missions. The science that came out was a welcome bonus.

By 2004, LIGO and other detectors had been up and running, so some of the earlier optimism had crested. I've extracted an interesting slice:

http://chaos.swarthmore.edu/courses/Phys130_2004/Papers/q91b07.pdf

B F Schutz
Max Planck Institute for Gravitational Physics, The Albert Einstein Institute, Potsdam, Germany
and
Department of Physics and Astronomy, University of Wales College of Cardiff, Cardiff, UK
5 November 1999

The first decade of the new millennium should see the first direct detections of
gravitational waves. This will be a milestone or fundamental physics and it will open the new observational science of gravitational wave astronomy.

Designs are already in hand at the existing detectors for their first upgrades. LIGO expects
to move to LIGO II, a collaboration with the GEO groups, by 2006. VIRGO should also be
upgraded by then, again using some GEO technology. We will see below that, while the first
Gravitational wave detectors could detect gravitational waves, the second generation will have a much greater assurance of success.
Note that the current generation of detectors have achieved or surpassed the levels of sensitivity expected by Schutz.

http://www.iop.org/EJ/abstract/0264-9381/19/7/001/
Editorial
2002 Class. Quantum Grav. 19 doi: 10.1088/0264-9381/19/7/001

The early days of gravitational wave detection were characterized by hope: the hope that nature would surprise us with unexpected strong sources of gravitational waves. In fact nature did have a surprise for us, but it was not the enormous population of colliding black holes for which we had hoped. Instead it was the Hulse-Taylor binary pulsar. This proved the existence of gravitational waves and defined a class of source which has become a major target for detection. More than this, it gave us the knowledge that a broad spectrum of gravitational waves definitely exists. Today we know that this spectrum awaits us like an unexplored continent, like Australia before European settlement. We know that the Great South Land exists, but its rivers and mountains, deserts and plains wait to be explored. We create and perfect our detectors with the knowledge and anticipation that we are privileged to be the first human beings to explore this unknown spectrum.

Likewise:

http://www.science.psu.edu/journal/Fall2002/Finn-FA02.htm


We are now reaching a truly exciting era when these waves will be directly detected on Earth, opening a brand new window to the universe."
"We all expect that the detection of gravitational waves is not far off," says Barry Barish, the Linde Professor of Physics at the California Institute of Technology and director of the Laser Interferometer Gravitational Wave Observatory (LIGO). "Sensitively searching for these waves and exploiting their science will require an intimate partnership between experiment and theory. The exciting new Penn State Center promises to provide just that missing link and we look forward to its crucial contributions to our field."

http://www.phy.anl.gov/events/Colloquium/classified/abstracts-2001/jun07.html


Gravitational wave detectors are rapidly reaching a sensitivity that will allow observation to confront theory. As that goal is reached, a community of gravitational wave phenomenologists is emerging, eager to tackle the interpretive challenges that these new observations will pose.

And in 2006, with a new generation of more sensitive detectors hitting the street, the optomism remains:

http://www.brightsurf.com/news/headlines/25124/GEO600_starts_continuous_search_for_Gravitational_ Waves.html

GEO600 starts continuous search for Gravitational Waves
June 27, 2006

Sensitivity vastly improved


The joint German-British Gravitational Wave Detector GEO600 has now entered an 18-month run of continuous measurement. Researchers are optimistic that they will be able to observe a never before seen phenomena the Gravitational Wave which is one of the great untested predictions of Einstein's General Theory of Relativity. Gravitational Waves can be used to do "ark astronomy", studying those aspects of the Universe for which ordinary astronomy using light (and the rest of the electromagnetic spectrum) can provide limited information.

The sensitivity of the GEO600 detector has been continuously improved since the start of test runs in 2002. "We could only reach out towards a small fraction of our own galaxy, the Milky Way, in those days. Today our sensitivity has increased by a factor of 3000 and we can detect events in distances many times greater than the distance between us and our galactic neighbour, the Andromeda Galaxy" Karsten Danzmann explains.

Professor Jim Hough of the Institute for Gravitational Research at the University of Glasgow adds "I am optimistic about the chances of a detection over the next eighteen months." When Ladbrokes offered odds of 500-1 against the detection of gravitational waves by 2010, Professor Hough was one of many who were quick to place their bet and the odds fell to 2-1 in days, before the book was closed. The bookmakers could well find themselves paying up in the next 18 months.

And finally:

http://www.ligo.caltech.edu/docs/P/P080011-A.pdf

Progress towards Gravitational Wave Astronomy

Maria Alessandra Papa

circa 2008


Various types of gravitational wave signals could be detected now that would not
challenge the basic understanding that we have of astronomy, astrophysics or cosmology.
In two years the enhanced detectors will have increased the volume of Universe that we can see by a factor of ∼ 8. In six years this volume will have increased by a factor of 1000. At that point cherished beliefs will have to be questioned if the data do not reveal any gravitational wave signal.

So the 'continent waiting to be explored' may be Atlantis after all.

There are more than 650 signatories on the lastest LIGO collaboration paper, and that does not include thousands of man hours and trillions of megaflops of CPU time donated through Einstein-at-home analysing the data. Gravitational waves are the most expensive no-shows in history, (and that includes M.Jackson's cancelled last tour.)

Assuming another six years of null results, an interesting question emerges: If non-detection down to the floor level does not disprove the theory, what result could? GR predicts the Earth's orbit is very slowly decaying, our best, most sensitive instruments indicate ithe orbit is expanding. How odd is that? Six years from now, will many still claim "GR has survived every test"? Is there an energy budget, gamma rays or something emitted from the Hulse-Taylor pulsar that we somehow missed?

Kwalish Kid
2009-Aug-27, 05:49 PM
What is your point? LIGO already has "a sensitivity that will allow observation to confront theory." It's just not the sensitivity that is expected to give results on GR.

mike alexander
2009-Aug-27, 09:36 PM
What could be more fun than finding out that something assumed to be there is really a pooka? The nonexistence of gravitational waves would be the neatest thing to hit scientific inquiry since Harry Hess and plate tectonics.

And remember, the last null experiment in physics that kicked off lots of fun was the ether drift experiments, again done with an interferometer.

Jens
2009-Aug-28, 02:05 AM
"If Advanced LIGO doesn't see gravitational waves I think people will be very surprised," Mandic told SPACE.com. "It is likely such a situation would require revision of General Relativity."


I'm sort of wondering, is this really a representation of the consensus? If for some reason Advanced LIGO doesn't find anything, will people really think it's necessary to revise GR, or will they just say that they need more sensitive detectors? How seriously can we take this statement?

Jens
2009-Aug-28, 02:07 AM
What is your point? LIGO already has "a sensitivity that will allow observation to confront theory." It's just not the sensitivity that is expected to give results on GR.

I think the point he's trying to make is that although people are now saying that LIGO wasn't even intended to detect gravity waves, that looking back, it appears that people were expecting it to do so. Granted, they are not necessarily the same people.

parejkoj
2009-Aug-28, 03:24 AM
If Advanced LIGO doesn't detect anything, it will definitely be a big deal.

LIGO, at it's peak sensitivity, which was only reached a couple years ago, could only detect significant events (like two black holes merging) out to a thousand light years or so. There are very few known candidates within that distance that could produce a measurable signal.

Advanced LIGO should be able to detect much fainter signals; the quoted numbers above are >10 times the sensitivity, which means >10 times the distance (gravitational waves fall off as 1/r, not 1/r2 like light does), which means >1000 times the volume. If Advanced LIGO doesn't detect anything in a few years of operation, it will certainly be a cause for alarm.

All the plots I'd seen from presentations on LIGO suggested that the first detections should be expected from Advanced LIGO, but probably not from the earlier runs. Jerry likes to suggest that *everyone* thought LIGO would detect something, but he has his on pet gravity theory to push.

Oh, and for those interested in some actual LIGO science, here's an article from 2007 about how the LIGO team looked for a signal from one possible source (http://arxiv.org/abs/astro-ph/0703419).

cjameshuff
2009-Aug-28, 06:01 AM
LIGO, at it's peak sensitivity, which was only reached a couple years ago, could only detect significant events (like two black holes merging) out to a thousand light years or so. There are very few known candidates within that distance that could produce a measurable signal.

Right. There was a chance of something big enough happening near enough for it to detect, but it wasn't expected. It would have been great if it did, but hope for something doesn't imply it's expected...and media looking for a story or people with axes to grind won't necessarily make the distinction.



Advanced LIGO should be able to detect much fainter signals; the quoted numbers above are >10 times the sensitivity, which means >10 times the distance (gravitational waves fall off as 1/r, not 1/r2 like light does), which means >1000 times the volume. If Advanced LIGO doesn't detect anything in a few years of operation, it will certainly be a cause for alarm.

Energy content of EM radiation drops off as 1/r^2, and energy content of gravitational radiation also drops off as 1/r^2. Energy content is proportional to the square of field strength, and field strength, electromagnetic or gravitational, drops off as 1/r. The falloff of the radiation is the same, the difference is in the detectors: our gravitational wave detectors detect the field strength, not the energy content, while light sensors usually detect light by using its energy to move electrons over a potential barrier. An antenna, on the other hand, produces a voltage proportional to the EM field strength.

parejkoj
2009-Aug-28, 02:14 PM
Ahh. That was the difference -- I'd forgotten the details.

Don Alexander
2009-Aug-29, 02:33 AM
May I also point to another nice result (http://arxiv.org/abs/0711.1163).

From this, it seems the sensitivity is better than parejkoj stated, they could have detected a neutron star merger out to about 3.5 Mpc, which is a lot more than 1000 light years.

Since they did not detect anything, they rule out a NS merger for the short GRB 070201, further strengthening the case for it being a magnetar superflare in Andromeda.

Jerry
2009-Sep-20, 05:34 PM
All the plots I'd seen from presentations on LIGO suggested that the first detections should be expected from Advanced LIGO, but probably not from the earlier runs. Jerry likes to suggest that *everyone* thought LIGO would detect something, but he has his on pet gravity theory to push.

Oh, and for those interested in some actual LIGO science, here's an article from 2007 about how the LIGO team looked for a signal from one possible source (http://arxiv.org/abs/astro-ph/0703419).


Gravitational waves are one of the most robust predictions of Einstein’s general theory of relativity, but have only been observed indirectly, as the dominant energy-loss mechanism in the binary pulsar PSR1913+16. A new generation of laser-interferometric gravitational wave detectors is currently under construction, which should permitdirect observations of these waves.

http://lanl.arxiv.org/abs/gr-qc/9605027v1


The era of gravitational wave astronomy is hopefully not too much in the far future and there is an international collaboration between experts from various different backgrounds to solve the relevant problems and march to this new epoch.


http://lanl.arxiv.org/abs/gr-qc/9506086v1


Both LIGO and VIRGO are scheduled for completion in the late 1990s, and their first gravitational-wave searches are likely to be performed in 2000 or 2001. LIGO alone, with its two sites which have parallel arms, will be able to detect an incoming gravitational wave, measure one of its two waveforms, and (from the time delay between the two sites) locate its source to within a ∼ 1o wide annulus on the sky. LIGO and VIRGO together, operating as a coordinated international network, will be able to locate the source (via time delays plus the interferometers’ beam patterns) to within a 2-dimensional error box with size between several tens of arcminutes and several degrees, depending on the source direction and on the amount of high-frequency structure in the waveforms...

It is now 35 years since JosephWeber initiated his pioneering development of gravitational-wave detectors [23] and 25 years since Forward [108] and Weiss [16] initiated work on interferometric detectors. Since then, hundreds of talented experimental physicists have struggled to improve the sensitivities of these instruments. At last, success is in sight. If the source estimates described in this review article are approximately correct, then the planned interferometers should detect the first waves in 2001 or several years thereafter, thereby opening up this rich new window onto the Universe.

I am looking for, but have not found anyone in the field writing anything pessimistic about the current generation of GW detectors prior to 1996. Not that there were not skeptics; the prior generation of experimenters were equally optomistic, because, as we all know, GR has never failed.

Jerry
2009-Sep-22, 04:51 AM
Searches for gravitational waves from known pulsars with S5 LIGO data


http://lanl.arxiv.org/abs/0909.3583v1


We present a search for gravitational waves from 116 known millisecond and young pulsars using data from the fifth science run of the LIGO detectors. For this search ephemerides overlapping the run period were obtained for all pulsars using radio and X-ray observations. We demonstrate an updated search method that allows for small uncertainties in the pulsar phase parameters to be included in the search. We report no signal detection from any of the targets and therefore interpret our results as upper limits on the gravitational wave signal strength.

These telescopes aren't working.

antoniseb
2009-Sep-23, 02:42 PM
... These telescopes aren't working.
Do you mean they are broken and giving erroneous results? or do you mean that the physics of gravity waves interacting with matter (or their very existence) is misunderstood by the experimenters and that the telescopes are working very nicely?

Jerry
2009-Sep-25, 03:17 AM
Do you mean they are broken and giving erroneous results? or do you mean that the physics of gravity waves interacting with matter (or their very existence) is misunderstood by the experimenters and that the telescopes are working very nicely?
Yes. Gravity waves may exist, but are dissipated in unanticipated ways - such as flipping neutrinos. Gravitational waves may not exist, the telescopes may not be sensitive enough, the frequency ranges might be wrong. It is interesting that gravitational lensing is a common phenomenon, but some of the other widely anticipated effects of general relativity are no shows.

I still think that the gravitational field of Mercury is providing important clues to this puzzle. The Messenger fly-by is just four days away - don't miss this one!