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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."
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.
Just callin' it like I see it.
As a matter of interest what would be the consequences to modern physics if Gravity waves do not exist?
There will be less for spectators to do when gravity scores?
Of course it doesn't exists, there is only Intelligent Falling.
1) General Relativity as formulated by Einstein (and a lot of other similar derivates - are there many?) would be in serious doubt. An exam question I had was take GR and show gravity waves exist - you basically show how the wave equation falls out of the formulas and these things carry momentum out of a system.
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?
The Singularity is closer than you think
Quant
It would be a serious blow to the picture in General Relativity of gravity warping space-time itself if we go for too long without detecting gravitational waves using length measurements as an interferometer does. This is especially true if we ever improve the accuracy of our measurements to the point where we can predict that we should observe gravitational waves but don't.
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.
licet differant, aequabitur
For all the people arguing over whether or not this is a failure of LIGO or not...it doesn't really say much at all. Initial LIGO (which is currently running) is more of a proof of concept sold as a viable project. But if you look at the expected rates of detection, the absolute high end for all binary sources is less than one event/year. The low end is between 4 events every 10000 years and 4 events every 100 years. The other source types are not any better.
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).
... 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.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.
> 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.
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.
Yes. This is no failure in the dectection technology. People at LIGO have estimated what they can detect and what they cannot. This puts an upper bound in the energy of the gravitational waves that were emitted by the GRB source. If it emitted more they would have detected them. This shows GRBs theories have a long way to go. We dont even know the total intrinsic amount of energy of a GRB source. If the source radiates in a polar pattern (like a lighthouse) we only see a small fraction of the GRB sources that exists (when the beam is directed toward the earth), in this case the intrinsic amount of energy is much smaller. If the GRB radiates like a star in all directions the intrinsic amount of energy is MUCH bigger. We can estimate the maximum size of the source bases in the timing of the event (if it has very fast variations it must be smaller because the information to coordinate this variation cannot propagate faster than light). But we dont know much more. This "failed" experiment is as important fot GRBs theory as the "failed" experiment in detecting the aether wind by Michelson and Morley was for the birth of Relativity. It shows we must review our theories. Airton da Fonseca Granero
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.
There have been no direct detections of gravitational waves so far. There have been indirect detections (most robustly with the various binary millisecond pulsars, whose orbits slowly decay due to their radiating energy away in gravitational waves), but no direct detections. However, this was not really seen as an issue, as gravitational wave searches before LIGO suffered from the problem that there were no known sources strong enough for them to detect with good probability. You have to start somewhere, and there is always the chance of either good luck, say a close supernova, or some unknown source that is stronger than expected, but I believe that this is the first actual event whose gravitational waves, by a reasonable model, had a chance of being detected with existing equipment. One such non-detection means nothing - maybe the Gamma Ray Burst occurred way behind the Andromeda Galaxy, for example. If this is consistently repeated, we will eventually conclude that there is something wrong with our physics or our astrophysics, but it is much too soon for that.
In fact the length of the space between the mirrors (and any length whatsoever) is _defined_ as the time light spends traveling between the two. This is the definition of distance in GR. It works because the speed of light is constant for everyone everywhere (in GR); the same thing causes all the other funny effects of relativity, for instance the same object having different lengths for different observers.
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.
"I think I am a fallen star. I should wish on myself."