LIGO Fails To Detect Gravity Waves

planckscale writes "Last weekend, LIGO (the Laser Interferometer Gravitational-Wave Observatory) did not detect gravitational radiation in association with a gamma ray burst (GRB). The non-detection was actually a valuable contribution, as it helped to distinguish between competing models for what powers GRBs. The detector is due to be upgraded this year for even more accurate measurements. The interferometer is constructed in such a way that it can detect a change in the lengths of the two arms relative to each other of less than a thousandth the diameter of an atomic nucleus."

As a matter of interest...

[gnalre]

This is another failiure in the long history of trying to detect gravity waves.

As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?

Re:Of couse, they could *both* have it wrong...

[boot_img]

... but I would call this simply "bad" science - You can't use one poorly-understood phenomenon to explore another.

You are incorrect. Gravitational waves (the phenomenon) are a very clear and very well understood prediction of the theory of General Relativity. So I would say that this is as far from "bad" science as you can get. If, ultimately, gravitational waves are not detected by LIGO and its successors that would prove GR was incorrect. And that would be a huge scientific advance.

An interesting question...

[sapphire wyvern]

I looked at the Wikipedia article about LIGO and noticed this interesting question in the discussion. No one has answered it there. Apparently it's from some forum somewhere. Maybe someone here can explain the solution to this "conundrum" for me?

Just getting back to LIGO for a while (sorry if this isn't strictly on topic), I understand that two long laser beams, at 90 degrees to each other, split from one laser source originally by a semi-silvered mirror, are re-combined at a sensitive detector to see whether their wave forms are cancelling or reinforcing. A passing gravity wave will sequentially lengthen and shorten the wavelength of only one of these light beams because the space-time continuum is distorted in only the direction of travel of the gravity wave. This, it is assumed, will cause the interference of the two laser beams to vary also - causing a variation in the light level measured at the detector. I still don't see why LIGO will work because a gravity wave is indiscriminate in the way it distorts things.

Everything is embedded in our 4-space, including the laser light waves lying along the direction taken by the gravity wave. As the gravity wave compresses and then dilates space-time, the LIGO tube and the laser beam within it will compress and dilate in perfect synchrony. Even the human observers' heads will compress and dilate as the gravity wave passes! The number of light waves per unit length of the LIGO tube (the laser wavelength) will appear unchanged because the actual physical length of the tube will shorten and lengthen as the light waves do, and as the eyeballs of the experimenters do too. If the waves of the re-united beams were re-inforcing peak-to-peak before the gravity wave arrived, they will remain peak-to-peak as the gravity wave passes through also. This alteration in the length of the tube, or arm, of the LIGO experiment, together with the variation in the wavelength of the laser beam, will be completely undetectable for that reason.

It's not a case of the gravity waves being too weak to detect, their influence is universal within our frame of reference and therefore cannot be directly detected .. by definition! The above is the way I see the situation. But dozens of scientists have spent billions of dollars designing LIGO, so I have to conclude I'm completely incorrect in my reasoning. Can anyone tell me how you can measure a distortion of space-time (4-space) if you, and every tool you use to measure the distortion, including light, are part of the same space-time being distorted?

I'd be fascinated to see what's wrong with the reasoning here!

Re:Fails?

[Anonymous Coward]

Ok - I accept your analogy of the DNA sequencing to explain the usage of "indirect". What I'm not sure is whether the authors of the wikipedia article intended to use the word that way.

Let's forget wikipedia for a second. Let me ask:
1. Has a man-made grativational wave been detected by LIGO (or any other gravitational wave instrument)? If so, I'd appreciate links to authoritative sources.
2. Has a natural event which has been corroborated by other sources been detected by LIGO (or any other gravitational wave instrument)?

Both the above tests have are true for DNA sequencing - man made and natural mutations can be detected "indirectly". If neither of the two conditions hold true for LIGO, then how do we know that it even detects gravitational waves?

Ofcourse, if my question appears ignorant - please understand that I'm a general slashdot reader and not a physicist. I'd appreciate your effort in helping me understand.

Re:Bummer

[JohnFluxx]

He's talking about, for example, where we have observed two neutron stars are orbiting each other, and their orbiting speed is slowing down. If you say that it's because they are giving off 'gravity waves' and thus losing energy in that way, then the theory exactly matches the results.

Personally I'd side with LIGO being wrong or not sensitive enough or something. At least until there's a bit more evidence.

Re:Fails?

[bluFox]

From the wiki GW [wikipedia.org] and here [wikipedia.org] : The gravitational wave has been indirectly shown to exist by showing that the evolution of orbit of the binary system is in precise agreement with the loss of energy predicted by gravitational waves. Note that this was not done by LIGO. It was observed using convensional Radio telescopes. More over the LIGO [wikipedia.org] is a *direct* detector of Gravitational waves. So I would like to know if LIGO has ever detected gravitational waves. If not the claim is questionable.

Re:As a matter of interest...

[Chemicalscum]

Gravitons have not been detected and there is no solid theory for them, but to be consisted with the rest of particle physics, they need to exist.

One of the results of string theory, that the proponents of string theory point out as one of its greatest successes, is the prediction of the existence of the graviton. They were not trying to derive the graviton from the theory, they found that the theory predicted an unexpected particle. When they looked at it closer they then realized that it had the expected properties of the graviton.

Re:Fails?

[Pictish Prince]

The argument is that since binary systems which include a neutron star steadily lose energy it must leave in the form of gravity waves.

Einstein predicted transverse gravitational waves. This post [google.com] questions whether LIGO is capable of detecting transverse or only longitudinal waves. The poster also points out that two waves would be generated and says that the longitudinal waves cancel at large distances.

Re:As a matter of interest...

[VernonNemitz]

Actually, ONE possible problem with the experiment has nothing to do with the sensitivity of the detector. See, there is a fundamental and unproved ASSUMPTION in Physics that gravity waves must travel at the speed of light, and therefore when a gamma-ray burst happens, we expect any gravity waves from the event to arrive at the same time as the gamma-ray photons. But if they don't have to travel at light-speed, then they can exist and be detectable, just not at the same time as the gamma rays.

Re:As a matter of interest...

[IWannaBeAnAC]

As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?

They do exist. There have been measurements done of the slowing down of a rotating binary pulsar, which is a prediction of Einstein's theory of General Relativity, where the system will emit gravitational radiation and slowly lose energy. This was the subject of the 1993 Nobel prize in Physics [nobelprize.org].

It'd be interesting..

[Anonymous Coward]

... if alternatives to GR got some more attention.

http://www.plasmacosmology.net/spec.html [plasmacosmology.net]

http://peswiki.com/index.php/PowerPedia:Tesla's_Dynamic_Theory_of_Gravity#Dynamic_theory_of_gravity [peswiki.com]

Personally I'm getting the feeling that we're getting more and more off-course with the dark energy, dark matter, 10 dimensional 'string thingy' theories.

Yes, I am saying that this is a feeling.. I'm not a(n) (astro)physicist... but somehow I have the idea that the universe just has to be more elegant than our currently collection of exotic and unwieldy theories.

All good programmers should be able to understand what I'm saying in the previous paragraph.

Re:As a matter of interest...

[Chris Burke]

See, there is a fundamental and unproved ASSUMPTION in Physics that gravity waves must travel at the speed of light

Well, if you're referring to gravity waves being limited to at most c, then that's a pretty safe assumption. It basically means you're assuming that causality exists (i.e. effects occur after causes). It would be an extremely bizarre universe if that were not true.

Though I suppose Newton considered it a pretty safe assumption that time was constant for all frames of reference. Must have seemed like a no-brainer, because in what kind of crazy universe did time pass at an objectively different 'rate' for different people?

But at least we can reason about a universe in which that isn't the case. A universe which doesn't obey causality? I'm not sure it would be possible to make sense of such a thing.

Re:As a matter of interest...

[SuiteSisterMary]

I've always been confused about why 'c' and 'causality' are considered one in the same.

Lets say you create a gravity generator. You put it 1 light year away from a gravity receiver. You also put a big honkin' flashlight on that gravity generator.

Now, lets say that gravity is instant. You turn on the gravity generator and the big honkin' flashlight. The receiver instantly notes the increase in gravity, and one year later sees the flashlight. How is causality violated? The receiver did not see the effect until the generator was activated. But it did see it faster than light.

Honest question; I'm sure there's an explanation that makes perfect sense, and I just don't know it.

Re:A big IF

[Chris Burke]

Coincidentally or not, it seems that in quantum dimensions there is no limitation in speed, information can be transmitted instantly. And, what is more, there are experimental results confirming this.

Despite experiment showing that one can have "spooky action at a distance", it is in fact impossible to transmit information this way. It turns out that any and all information was in fact transfered along with the entangled particles themselves as you separated them at sub-luminal speeds. So you can't use quantum entanglement to send information, but you can use it in a quantum-crypto system [wikipedia.org] to make sure nobody listens in on your communication.

I confess I'm not too confident on those proofs that information cannot be transfered faster than light. Until someone creates a theory that conciliates quantum mechanics with general relativity, I'm willing to believe anything. Maybe irreversible time is just an illusion created by the thermodynamic effects in our macroscopic brains...

The proofs are quite solid in showing that ftl information transfer results in a violation of causality assuming Special Relativity is true. And SR assumes causality, which is why we instead rule out ftl information travel.

So far, while clearly not compatible with SR, QM has made no moves towards trying to disprove it. And that includes the SR assumption that causality holds. While I agree that we should wait for the theory that reconciles QM and SR to arrive (especially given what new and unique views of the universe both those theories gave us), it isn't necessary for such a theory to actually overturn either one.