LIGO Fails To Detect Gravity Waves 357
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... (Score:4, Interesting)
As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?
Re:As a matter of interest... (Score:5, Funny)
There will be less for spectators to do when gravity scores?
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Gravity waves are predicted by Einstein's theory of general relativity. Thus, if gravity waves do not exist, then general relativity is fundamentally flawed (hence the interest in testing this particular prediction of the theory).
-JS
Re:As a matter of interest... (Score:5, Funny)
Re:As a matter of interest... (Score:5, Interesting)
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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 seem
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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 e
Re:As a matter of interest... (Score:4, Informative)
A big IF (Score:3, Informative)
Unfortunately, too many physicists aren't very familiar with the theory of information. [wikipedia.org]
If one can state the one basic principle in that theory it is that to send or store information you have to spend energy, increasing the entropy in the universe. However, thermodynamics is a macroscopic phenomenon, at quantum dimensions all phenomena are reversible. In quantum dimensions one could say that time is bidirectional
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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 q
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I took my last university physics course in 1991 and my memory of the details is very, very fuzzy. But I do remember a specific conclusion that I came to based on what I was learning, and I have retained that conclusion along with a vague idea of what led me to it, which was:
Light is electromagnetic radiation. Breaking this term
Bummer (Score:5, Informative)
2) You then need to explain stuff such as Mercury's orbit precession and observed effects of double Neutron stars slowing down - the FSM stirring his planetary meatball lunch slower?
not that dramatic (Score:2)
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There are others? I've never heard of any. At least not as an overall theory. I know there are theories as to the causes of specific phenomena, but those are, to my knowledge, within the General Relativity realm.
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http://en.wikipedia.org/wiki/Alternatives_to_general_relativity [wikipedia.org]
http://en.wikipedia.org/wiki/Parameterized_post-Newtonian_formalism [wikipedia.org]
At least not as an overall theory.
I don't know what that means. How is one field theory more "overall" than another? People have attempted to apply GR to many more problems than any of the others, but that doesn't make GR any more complete than any of the others.
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Re:Bummer (Score:5, Interesting)
Personally I'd side with LIGO being wrong or not sensitive enough or something. At least until there's a bit more evidence.
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Indeed.
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Here's a graph of what LIGO has been in terms
Re:As a matter of interest... (Score:5, Informative)
What we would replace it with that could explain all of the observations that GR predicts I don't personally know, but it's a good day in physics when a theory is proved wrong because it means that we've done our job.
All that and fun too! (Score:2, Insightful)
Not only does it mean we've done our job, it's also a whole lot of fun. Suddenly there's a whole new theory (or even better, lack of one) to test. Lots of new experiments to do. More hours to spend in basement labs...
ID'ers just don't know the fun they're missing.
Re:As a matter of interest... (Score:4, Insightful)
Re:As a matter of interest... (Score:5, Informative)
We have seen binaries losing energy in a manner consisted with GW predictions, so there is a good chance the theory of GWs is correct.
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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 expe
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Does it make any testable predictions? What? No? Oh well then.
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General relativity predicts that gravity waves exist. Quantum mechanics predicts that all energetic wavelike phenomena can be thought of as made up of particles. So putting the two together suggests that gravity waves can be thought of as being made up of particles, and a good name for these particles is 'graviton'. But there are big problems with combining general relativity and quantum mechanics and there isn't a very good theory of gravitons.
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"Failure" isn't really the correct word. An experiment such as this has a certain sensitivity, meaning that they would detect gravitational waves greater than such-and-such magnitude, in a certain frequency range, etc. In scientific lingo, what you call "failure", we call a null result. Of course, it seems like I'm just renaming it with a fancier, nicer-sounding name. But really it's more than that. From a scientific standpo
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- that gravity waves don't exist (ie that GR is wrong)
- that the calculated sensitivity of LIGO is wrong by orders of magnitude
As a result, this study really doesn't tell us very much at all.
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TFA proposes a third explaination, ie: the burst was not in Andromeda, it was from a galaxy, far, far, away.
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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].
like Michaelson-Morley experiment (Score:2)
Perhaps the gravity result suggests a replacement for general relativity.
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Einstein's theory of General Relativity predicts gravitational waves from certain kinds of astrophysical events. If these events take place but there are no g-waves, the theory is wrong.
diameter? (Score:3, Insightful)
Would that be a hydrogen nucleus... a uranium nucleus? Please be more specific.
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They should have just said "itty-bitty".
Re:diameter? (Score:4, Informative)
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"...of less than a thousandth the diameter of an atomic nucleus."
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In physics parlance, the "diameter of the nucleus" means ~10^-15 m, which is the diameter of a proton. Because nucleons are close-packed, nuclear diameters are less variable than atomic diameters. The cube root of 240 is only about 6, so the heaviest nuclei are less than 10 times the diameter of the lightest.
That said, this is not a blow for GR. We do know (in the perfectly ordinary sense of "know", the same way you know you had at least one great-great-grandfather) that gravitational waves exist, based o
...she could hang glide on a Dorito! (Score:5, Funny)
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Well of course they can't! (Score:4, Funny)
Oh wait, you said LIGO, nevermind.
There is no gravity (Score:4, Funny)
Of couse, they could *both* have it wrong... (Score:2)
Alternately, since no one can really say where gravity itself comes from, the concept behind LIGO could simply fail to account for how gravity really works.
Who can say that the same shortening of one side compared to the other doesn't affect the speed of light proportionately to the change in length? In that case, we could just as well have a black hole buzz our solar syste
Re:Of couse, they could *both* have it wrong... (Score:5, Interesting)
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Yeah yeah, I understand that. So why haven't we ever seen one of these flying pink unicorns?
If, ultimately, gravitational waves are not detected by LIGO and its successors that would prove GR was incorrect.
Quantum phsics already "proves" GR as wrong. We just can't articulate how.
In any case, I disagree. The wrongness of GR doesn't necessarily follow from a lack of gravi
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Yeah yeah, I understand that. So why haven't we ever seen one of these flying pink unicorns?
Because they have almost no effect on space time. The fact that we didn't see neutrinos for a long time didn't meant hey didn't exist.
Quantum phsics already "proves" GR as wrong. We just can't articulate how.
In any case, I disagree. The wrongness of GR doesn't necessarily follow from a lack of gravity waves - We simply don't
know how gravity works. It could have an underlying mechanism totally outside the scope of GR, thereby not disproving
GR but requiring a small modification to it, just as GR didn't "destroy" Newtonian physics. We still use the classic kinetics
laws juuuuust fine in day-to-day calculations. Instead, it extended the older work into a realm that Newton had virtually
no knowledge of. I don't see why the same idea can't apply here.
Quantum physics does not prove GR wrong. GR is a valid theory and will continue to be so. However, for certain circumstances (inside black holes, for example) we need both general relativity and quantum mechanics, and it is at that time that we cannot say what happens. The two theories address completely different problems and a theory like string theory is the attempt to i
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Ummm, "dark energy", anyone?
Re:Of couse, they could *both* have it wrong... (Score:5, Insightful)
Looking at not clear and poorly understood theories, there is string theory, which has changed so many times that its not even close to the original anymore. The latest on string theory is that certain parts of it mimic what we know already, but exactly how it operates no one has any idea of. Another example is quantum gravity. Again, we have a general idea, but nothing concrete. However, just because we don't know the more correct theory doesn't mean we can't use the initial theory. Newtonian mechanics did not become wrong after QM and GR. Its just not as accurate.
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Those predictions turn out to follow from almost any reasonable extension of Newtonian mechanics to relativity.
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Ok granted, I tend to take string and quantum theory with a grain of salt. Also, to a large extent, GR makes a lot of sense to me. What I suppose isn't making sense to me is the term used here. Looking over the math in the wikipedia article, it seems that we're looking for some kind of radiated energy, but nothing specifically in the equations seems to relate that what we're looking for is gravity. Or am I still missing something here?
Its not gravity. Its a wave in space time. Consider a lake. Drop a pebble into it. You'll see waves emerging from the point of contact between the water and the pebble. Spacetime is the surface of the lake and the merger is the pebble dropping into the lake. General relativity is not a theory of gravity or even forces so much as a theory of space time.
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This is not significant (Score:5, Informative)
This article basically says that because LIGO is known to not be sensitive enough to measure past a certain distance from Earth (which encompasses the Andromeda galaxy, in whose direction this burst occurred) and because no detection was seen, the burst was not caused from a source in the Andromeda Galaxy.
I suppose that after spending all this money its not a bad thing that LIGO can actually produce some useful results (though I doubt they were amazingly useful). Advanced LIGO should be able to do the job - but not for another 5-6 years. At that point, the minimum event rate is supposed to be around 1/year and we should finally get some sort of positive detection.
Personally I'm hoping Advanced LIGO does work, because otherwise all this money will have gone to waste and the field of gravitational wave astronomy will be even more damaged than it already is. The thing is, many people in astronomy who are not affiliated with LIGO are not excited by it. Maybe that interest will be rekindled when Adv LIGO actually works, since right now its more of an engineering problem than an astronomy or physics problem. More people are interested in LISA which (if it ever launches) should have many more interesting sources. Its amusing seeing LIGO people try to point out the flaws of LISA while trying to explain why LIGO doesn't work, but then maybe I'm biased since I am working on LISA (though I have worked on LIGO in the past).
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Go LISA (Score:3, Informative)
The result is ambiguous... (Score:2)
LIGOs? (Score:2, Funny)
would they be called LIGOs?
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Re: S/N (Score:2)
An interesting question... (Score:4, Interesting)
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?
I'd be fascinated to see what's wrong with the reasoning here!
Re:An interesting question... (Score:5, Informative)
This part isn't correct. The laser beam will be redshifted and change its wavelength, however it will still travel at the speed of light, c. Since the distance between the two ends is less, it will travel that distance in a shorter time.
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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.
Basically, as a gravitational wave passes through a section of space time, that section will dilate and contract. To the light between the mirrors (which is not affected because light always travels at the same speed in a vacuum) there is now an additional distance to go. It may make more sense to imagine the light as a stream of photons that have zero volume and thus can't be stretched. When it hits the mirror, it will now be out of phase with the Fabry-Perot cavity and will exit, thus generating a signal
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If it travels 300000000 m/s in a vacuum, it would suddenly be travelling at 299999999.999 dilated-m/s.
So, to the observer it would appear that light goes slower?
Would the converse apply, that if space-time is compressed, light would appear to go faster than the speed of light?
So, if it were possible to have a permanently "compressed" area of space-time, light would travel faster than the speed of light in a permane
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The observer is dilating/compressing as well, so the photons will stay the same distance apart (if what you are saying is the case), so the observer will notice no change (as long as the observer is affected by the gravity wave in the same way as the test equipment - which would happen in this case).
What dlevitan was saying was the opposite. The observer/test equipment will dilate/compress, but the photons are unaffected by the gravity wave, so the light goes at the same speed (relative to an unknown met
Re:An interesting question... (Score:5, Informative)
So, by the same definition, a piece of space is lengthened or shortened _iff_ light spends a longer or shorter time traveling it. The speed of light never changes, but due to conservation laws its _frequency_ changes.
Very approximately, imagine the pulse of light starting at the far mirror. The EM wave makes (say) 100 oscillations in 100 seconds (totally out of scale with the real experiment, but that's not important). If the length between the mirrors is constant, the 100 wave peaks will hit the close mirror in 100 seconds. But if the distance between mirrors changes (eg, due to a gravity wave compressing space) _during_ the 100-pulse emission, the last peaks will have less space to travel than the first peaks. This means that the close mirror will be hit by 100 peaks in, say, 90 seconds. So the frequency of the wave went from 1 Hz to 10/9=1.1Hz. The waveform was deformed (compressed), but its speed was constant. (Note that the effect happens _only_ if the space changes shape _during_ the pulse. If it changes, say, between two 100-oscillation pulses spaced apart, you'll still get the travel time difference, but not the frequency shift. LIGO uses continuous lasers, though.)
The LIGO can't actually measure the change because it's much smaller than in this example. So it sends the lasers in perpendicular directions, and reflects them back. Because gravity waves stretch space differently in each direction (except if their direction happens to exactly bisect the angle between the arms), a passing gravity wave will force the two beams to go slightly out of phase. The difference between the two beams is (barely) detectable for big waves.
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uhh, this seems appropriate (Score:2)
"Hooray, I'm useful. I'm having a wonderful time." --Dr. Zoidberg
not surprised (Score:2)
No one has detected gravitational waves... Yet (Score:5, Insightful)
Graviatational radiation (Score:3, Insightful)
1. Forces due to massive bodies (gravity) to propagate at the speed of light, and
2. Energy to be conserved
must also have gravitational radiation. Information propagates at infinite speed in Newton's theory of gravity, so there is no gravitational radiation.
Re:Fails? (Score:5, Informative)
I'll spell it out for you. This is not a failure of gravitational wave detection technology.
What you apparently do not understand is that this device can detect gravitational waves. However, it did not detect gravitational waves that correlated with a gamma wave burst originating in Andromeda. Normally such bursts arise from well known phenomena, such as a collision of black holes. But in this case, the collision could not have been from one of these well known phenomena.
What the article suffers from is bad writing. It should have been put in the positive--something like "the gamma-ray burst originated from a novel mechanism". Now, because astrophysicists can not account for the burst, they must go back and (1) study other similar phenomena and/or (2) revise astrophysical theory to explain the heretofore inexplicable gamma ray burst. Why is this burst inexplicable at this point? Because they did not detect gravitational waves that correlated with the burst.
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Re:Fails? (Score:5, Informative)
Re:Fails? (Score:4, Insightful)
can it? (Score:3, Insightful)
That has never been demonstrated. For all we know, gravitational waves may simply not exist.
Re:Fails? (Score:5, Informative)
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Layne
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If gravity is confined to the speed of light, the Sun should have lost its planets long ago. For example, simple Newtonian math tells us that the Sun and Jupiter "KNOW" about each other right NOW or in a very short amount amount of time, not 43 minutes later. the Earth and the Sun "feel" each other's gravity instantaneously, not with an eight minute delay. The sun and the center of our galaxy communicate by gravity without
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For the electromagnetic interaction, relativity has been experimentally shown, but gravity is still pretty much a mystery. We know that matter somehow gives rise to an acceleration we call gravity. There is no way to tell the difference between this acceleration due to gravity and the acceleration of matter by some means. There is no experiment you could do inside, if you were sealed into a closed rocket, to tell whether your cabin was being accelerated through space at
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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.
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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 wav
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Yeah, imagine somebody trying to use this sort of argument to defend a theory with political ramifications...
"... fails to detect Global Warming, but that's a valuable contribution anyway, as it helped to distinguish between competing models for what causes GW"
The head would be still alive flying in the air looking at the rest of the body being ripped into smaller and smaller pieces...
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As predicted, the bites are already being ripped out of my flesh...
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"The absence of an observation" is not logically equivalent to "The observation of an absence"
I believe the point isn't that they failed to make an observation (Absence of an observation) but rather that they made the observation, and didn't detect anything.
This [theoretically] would prove that given current hypotheses about gravitational waves, are either a) not accurate or b) not measurable with current equipment
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In this case, lack of data is a great conclusion to the experiment.
Normally, it does detect gravity waves, but it was already known that this particular GRB was special. There are competing theories for the cause of these particular bursts, and in some of them you would expect gravity waves to be detected, and in some of them you wouldn't. So this measurement is very helpful.
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Or maybe it would not have: nobody has ever detected gravity waves, and they may simply not exist. That's why LIGO was built in the first place.
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Actually, it's the opposite. Prior to GR, Newton's theory of gravitation predicted that gravitational effects travel instantaneously. After Einstein developed the theory of Special Relativity which, among other things, forbids energy/information from traveling faster than the speed of light, he spent the next ~10 years developing a theory of gravity which was consistent with this (in physics-speak, we say that
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So, apart from the fact that all this is total conjecture, and even scientists can't decide how fast gravity waves travel - let's say that gravity waves travel at C + 5%. Now, if the gamma waves DID originate at Andromeda M31, that's 2.2 million light years away, so that means that we're about 110,000 years too late to catch the gravity wave passing.
What a pointless experiment.
Even if the speed of gravity waves is 0.001% away from C, that's 22 years between the gamma waves passing and the gravity