@Bhushan, regarding the equipment as an issue in this question, I do attach a link about!

http://www.symmetrymagazine.org/article/october-2014/to-catch-a-gravitational-wave

Can expensive science that lasts for decades sometimes become not more than a very expensive casino game?

Perhaps we still don't have that kind of equipment's to detect this waves yet.

It's amazing how many times this question has been asked, after 1993, the year Hulse and Taylor were awarded the Nobel Prize in Physics for having discovered how the period of a binary pulsar varies with time-and having explained this variation as the result of emission of gravitational waves, exactly according to the predictions of general relativity. So the question, as posed, is meaningless, since gravitational waves *have* been found. What detectors like LIGO and VIRGO and (E)LISA-and others-are after,  is to discover *other* ways of detecting gravitational waves, that will provide different insights into their properties.

I guess he meant its not directly detected Hulse and Taylor did indirect  detection.

Text from Wikipedia:

Although gravitational radiation has not been directly detected, there is indirect evidence for its existence. For example, the 1993 Nobel Prize in Physics was awarded for measurements of the Hulse–Taylor binary system that suggests gravitational waves are more than mathematical anomalies.

For technical issues, encyclopedias aren't the primary source of information (and, in particular, the term ``mathematical anomalies'' indicates that the person writing the article didn't have any idea what they were writing about, since the term doesn't mean anything: either such configurations can be proved to be solutions  of the Einstein equations-which was the case-or they can be proved not to be-which isn't the case). The distinction between ``direct'' detection and `ìndirect'' detection doesn't have any physical basis-it's just metaphysics.  The meaningful distinction is what properties of gravitational waves can be studied though one method  or another for detecting them-once one has detected them at all. For instance, the gravitational waves emitted by the binary pulsar can't be detected by Earth-bound detectors-and, probably, from satellite-based detectors, either. This places conditions on the astrophysics behind gravitational wave sources and what other signals of gravitational waves might prove interesting to detect with Earth-bound detectors.

My dear @Demetris, what do you think about this: "Scientists got it wrong on gravitational waves. So what?The team involved has been criticised for publishing results before they were peer reviewed. But this is what science is: debate, discussion, deliberation!....... But now it looks likely that the alleged evidence of both gravitational waves and ultra-fast expansion of the universe in the big bang (called inflation) has literally turned to dust." ?

http://www.theguardian.com/commentisfree/2014/sep/26/scientists-gravitational-waves-science

http://www.einstein-online.info/spotlights/gravWav

@Bhushan, regarding the equipment as an issue in this question, I do attach a link about!

http://www.symmetrymagazine.org/article/october-2014/to-catch-a-gravitational-wave

BICEP2's results don't have anything to do with the issue, since gravitational waves were, already, known to have been detected- BICEP2's results, if confirmed, would have probed other properties. In any case, it was known, already  from their preprint, that their results couldn't be taken as a signal (but a calibration of their detector, awaiting for a way of isolating the dust contribution)-and, absent a model for the electromagnetic response of interstellar dust, it's not obvious, whether it will be possible to isolate the electromagnetic signal that gravitational waves, emitted during inflation, produce, to discovery precision. A new approach will be needed, if a quantitative model for interstellar dust response can't be developed. And the fact that it's not, yet, possible to detect the gravitational waves emitted during inflation, from their electromagnetic proxies doesn't imply anything particular about inflation. Popular accounts aren't a reliable source of technical information.

I think Demetris asks a good question.  Let me summarize what I think is the current state.  All of these topics can be debated, I'm just cutting to a conservative position which I think is probably defensible:

• Decay of binary systems indicates gravitational energy is being radiated.
• Detectors that should have detected them have not.  Not talking about the early days of the detectors, but the latest and best ones.

So.... what does it not imply first:

• The lack of detection does not necessarily imply that gravitational waves do not exist, because there is some good evidence for them.
• GR is not necessarily faulty, because it gave accurate predictions of the binary decay rates (and a lot of unrelated stuff).

It could be that some of our assumptions about gravitational radiation are wrong.  To me it seems this is more likely than that there are no gravitational waves.

ALL of the detectors I have read about depend on wavelength.  If wavelengths were much longer, they wouldn't work.  As an "antenna" they are simply floating on a potential level that is indistinguishable if the wavelengths are very long.

I am not necessarily suggesting gravitational waves are lower in frequency ... but that they may have much higher propagation velocity.  Because gravity affects everything, i.e. affects the "potential" energy of everything, and because we can only detect energy differences, i.e. potential differences, then we can only detect a gravitational "wave pressure" relative to an unaffected area. 

This requires some type of signal, traveling at the speed of light, from the unaffected area.  So gravitational waves could not be used to "communicate" faster than light, because of the detection problem.

It just now occurs to me, this may constitute a sort of proof that the velocity of gravitational waves is very, very large, almost infinite, because if it is larger than c but wavelengths are short, local detection would be possible.  I can explain this in more detail if someone is interested, but otherwise won't take up the space here.

Robert: I agree with much of your answer (apart from the speculation about the propagation velocity), but there is one point that might be misleading: "Detectors that should have detected them have not." The detectors that we have had to date nowhere near had the sensitivity to detect gravitational waves from binary pulsars. They _may_ have detected black hole or neutron star mergers if such a thing happened at the right time and relatively nearby, but these things don't happen that often. They (different detectors, e.g., the use of distant spacecraft as gravitational wave detectors) _may_ have detected some strong, very long wavelength primordial gravitational waves if such waves existed, but this was highly speculative. But we have to wait, at the very least, for Advanced LIGO, to detect continuous gravitational waves from binary pulsars, or to reliably detect mergers in a much larger volume of space.

Yeah, I didn't mean to imply they should have detected binary pulsars.  So they should be continued in operation and improved.  But at some point if no waves, which is what this question asks, then I think more radical questioning of assumptions would be the first step before giving up on them.

And what if LIGO, Advanced LIGO, LISA do not detect empirical evidence for the existence of these waves? Will there be substantial scientific progress, independent from whether gravity waves will be detected or not? Some claim there has at least been/will be technical progress.... For instance, advanced techniques developed in field A may become useful in another field B.

Yeah, for years we have justified the space program that way.  But the current president didn't buy it.  :D))

Everything what exists has been found. The key question is: who found it and who will eventually be informed about it? 

Perhaps a key question is: Can the human brain and their mechanical products (e,g. scanners, machines) perceive/measure everything that exists? I would say: I don't know, because I don't know if I as a human can perceive/measure everything

There have been NO detections of Einstein's gravitational waves at all. Reports of their detection are wishful thinking. They will never be detected because they don't exist.

(1) The so-called 'linearized' Einstein field equations are meaningless, and they produce a wave equation that has no definite speed of propagation, because that speed is coordinate dependent. With a change of coordinates this speed of propagation also changes, and this can be done with "the speed of thought" (Eddington, 1923). The claim that these alleged waves travel at the speed of light is false. A set of coordinates producing the speed c is selected a priori in order to make the resultant speed c in the wave equation - a circular argument and so it is invalid.

(2) Einstein's gravitational field energy can't be localised and so his gravitational waves are a fiction. To localise his gravitational field energy Einstein introduced his so-called pseudo-tensor. First, it is NOT a tensor. But Einstein and his followers justify its use because it acts "like a tensor" strictly under linear transformations of coordinates; but that does NOT make it a tensor. Second, his pseudo-tensor is a totally meaningless concoction of mathematical symbols because by contraction it produces a first-order intrinsic differential invariant. However, first-order intrinsic differential invariants DO NOT EXIST, proven by the pure mathematicians in 1900. Thus, assuming Einstein's pseudo-tensor has mathematical validity it produces an absurdity and so it has no validity whatsoever (reductio ad absurdum). The upshot of this is that General Relativity violates the usual conservation of energy and momentum for a closed system and is therefore inconsistent with a vast array of experiments. How many experiments are necessary to invalidate a theory? One will do.

(3) The Landau-Lifshitz pseudo-tensor suffers from the very same fatal defects as Einstein's. All such 'pseudo-tensors' fail for the same reasons.

(4) The foregoing is explained in detail in the following paper:

General Relativity: In Acknowledgement Of Professor Gerardus ‘t Hooft, Nobel Laureate, http://vixra.org/pdf/1409.0072v2.pdf

The Hulse-Taylor binary pulsar system has an orbital decay which is indirect evidence of the emission of gravitational waves. I understand that no other model can describe this orbital decay as precisely and hence we have already found gravitational waves we just need to detect them directly. This indirect detection of gravitational waves led to Hulse and Taylor receiving the Nobel Prize in 1993.

http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html

e.g. http://adsabs.harvard.edu/cgi-bin/bib_query?arXiv:astro-ph/0407149

Antonia Rowlinson - I reiterate that NO Einstein gravitational waves have ever been found. One cannot use General Relativity because it violates the usual conservation of energy and momentum for a closed system and is therefore in conflict with a vast array of experiments; and so it is a false dogma. The wave equation from the linearized field equations is totally meaningless because its speed of propagation is coordinate dependent. And Einstein's pseudotensor is bunkum. So are the other 'pseudotensors'.

It's back to the drawing board for the cosmologists.

Obviously you did not study the paper I cited. Here it is again:

General Relativity: In Acknowledgement Of Professor Gerardus ‘t Hooft, Nobel Laureate, http://vixra.org/pdf/1409.0072v2.pdf

If gravitational waves are never found, it will require alternative explanation for the decay of the orbital period of pulsar. One such explanation is given in the following article.

Research Alternative explanation for orbital period decay of a pulsar

A Few Things You Need to Know to Tell if a Nobel Laureate is Talking Nonsense, 10 July 2015,

http://vixra.org/pdf/1507.0067v1.pdf

Crothers, S. J., A Nobel Laureate Talking Nonsense: Brian Schmidt, a Case Study,

16 July, 2015,

http://vixra.org/pdf/1507.0130v1.pdf

The question here is not that gravitational waves exist. Gravitational wave are a fundamental consequence of general relativity. Their existence was proved without a doubt by Hulse and Taylor.

The question is: Will an interferometer-type instrument such as LIGO, that is that is within the sensitivity to expect gravitational wave detection, actually work.

There is an experiment that could be seen is an "inverse-analogy": Michelson and Morley’s attempt to detect the effects of motion through the “ether” - which also used a interferometer-type instrument. Here it was found the expected effects due to the ether could not be found - there was no ether. A null result.

On the other hand, we know that gravitational waves exist - but what if LIGO never measures the effects due to gravitational waves? In other words, a null result.

History shows that the Michelson-Morley experiment, although null, led to a fundamental change in the way we view space and time - Einstein’s special relativity. Could perhaps a null LIGO result also lead to a fundamental change or even new physics beyond what the intentions of the LIGO experiment were built for?