Initial Tests Fail To Find Gravitational Waves

eldavojohn writes that though gravitational waves are "predicted to exist by Einstein's Theory of General Relativity, the initial tests run by the Laser Interferometer Gravitational-Wave Observatory Scientific Collaboration (LIGO) failed to find anything. It doesn't disprove their existence although it does rule out a subset of string theory. From the article, 'For example, some models predict the existence of cosmic strings, which are loops in space-time that may have formed in the early universe and gotten stretched to large scales along with the expansion of the universe. These objects are thought to produce bursts of gravitational waves as they oscillate. Since no large-amplitude gravitational waves were found, cosmic strings, if they exist at all, must be smaller than some models predict.' The scientists working in Washington and Louisiana (in tandem to rule out flukes) will now move on to Advanced LIGO which will analyze a volume of space 1,000 times larger. If they don't find any gravitational waves in that experiment, the results will be more than unsettling to many theorists."

Comments

Just because they failed to detect any

[spike1]

Doesn't mean the gravitational waves aren't there.
Maybe they've just got the detection method wrong.

Re:Linearization

[geekgirlandrea]

No, there are exact gravitational radiation solutions, and you can also predict gravitational radiation from weak-field situations where the linearized approximation is very, very accurano te (the h^2 term would be less than 10^-15 for the sun's gravitational field at Earth's orbit, for example). The decay of orbits due to gravitational radiation has been observed indirectly in PSR B1913+16, and matches the theoretical prediction. If no gravitational radiation is observed at the expected amplitudes for things like that, it will throw a lot more than just string theory into question, and would raise the obvious conservation of energy question about that pulsar.

Re:Linearization

[SleepingWaterBear]

As far as I remember from my course on general relativity, gravitational waves follow from a linearization of Einstein's field equations. Thus, if they failed to find them, it wouldn't falsify the theory as a whole but only the linear approach to the field equations.

This isn't exactly right. The equations describing gravitational waves do result from a simplifying approximation of Eintstein's equations, but it's the sort of simplifying approximation that really has to be quite accurate in many circumstances. If they don't find gravitational waves of a certain magnitude then either Einstein was wrong or, more likely, the sorts of astronomical phenomena that could create the waves don't exist.

The LISA mission

[fulldecent]

Please see the LISA mission:

http://lisa.nasa.gov/

LISA can be thought of as a giant Michelson interferometer in space. The spacecraft separation sets the range of GW frequencies LISA can observe (from 0.03 milliHertz to above 0.1 Hertz).

Maybe they can't be detected

[MrKevvy]

My own "pet theory" for this was that they would never be detected because although they do exist, they perturb the measurement device to the same degree that they do everything else, ie a gravity wave may perturb one arm of a LIGO detector, but it also correspondingly perturbs the waves of the laser beam passing through it. As a result it isn't detected.
An analogy: It would be like measuring everything in a room with a ruler, then scaling the whole room including the ruler up or down. You wouldn't see a change with the same scaled ruler; you'd have to bring one in from outside.

I bounced this idea off a few physicists (including Bruce Allen who runs the Einstein@Home project on LIGO) but they don't seem to like it. :^) Maybe it will turn out to be correct, who knows. It certainly seems to be turning out to be more difficult to detect gravity waves than was initially predicted.

Re:Linearization

[TapeCutter]

"Gravity waves should follow the same paths as light waves, and we get plenty of light waves in out gravity well."

Which always made me wonder, how do gravity waves escape a black hole?

Re:what to do, what to do

[bigmaddog]

ID it not a theory, it is a religious/political ideology being presented as a theory that aims to explain the perceived weaknesses of science in order to advance the interests of certain groups and individuals.

- This beautiful, complex interaction could not have possibly arisen spontaneously, therefore God's will.
- This makes no apparent sense/has no apparent purpose, therefore descent from God's will.
- You cannot explain something neatly, therefore God's will or the descent from it.

That's not a theory. The aim of a theory is to predict something that you can then test for. ID doesn't predict anything, there is no empirical test for God and deciding arbitrarily whether things are as God intended or not does not increase our understanding of them - it's merely a reactionary attitude advanced by old men who are afraid of change and what it means for their status.

Besides, even if you believe in God the creator, the ID advocacy of ignorance still seems bogus; God gave you all these wonderful cognitive capabilities, so why not use them to try to fully appreciate his grand work? You would be wasting God's gifts if you didn't. :p

Re:Linearization

[maxwell demon]

Well, it's easy to explain: It conveys the wrong message. While the curvature of space is indeed similar to the curvature of a rubber hose when a mass is on it, the image you get is wrong at quite a lot of counts:

• To begin with, the mass is not lying on top of the space, it's inside space.
• Next, while space is curved, that's not the main effect you see (indeed, the curvature of space near earth is so little that it's very hard to measure it). It's spacetime whose curvature is important.
• The objects are not attracted downwards something "below space", and they don't just "roll down the hill" - indeed, the space curvature picture could equally well be depicted upwards without making a difference (except that the very intuitive, but wrong notion of objects rolling "down" wouldn't work any more). Indeed, the whole point of General Relativity is that the path of an object in spacetime is straight ahead (as long as no non-gravitative force modifies it, of course). However, since the spacetime is curved, their paths don't seem straight. An image for this would be two people on Earth, starting on slightly different places on the equator, and going North. Despite the fact that their ways start out parallel, and neither makes a turn to the left or the right, they'll come closer together until they meet at the north pole, where they meet at a non-zero angle.
• Finally, the curvature is not "into another dimension"; space actually only has an inner curvature (basically, deformation along the space direction). However this is quite hard to visualize, therefore all pictures are a curved two-dimensional space embedded into three-dimensional space. However the outer curvature has no meaning at all. For example, you might depict flat space as a plain sheet of paper. However, you could depict the very same flat space also as convoluted piece of paper. It doesn't make any difference. It only makes a difference if you do something which would make the paper to get torn or to crumple (so you'd need a rubber sheet to maintain a continuous surface).

I hope those explanations did help a bit.