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Gravitational Wave Detection Imminent?
Seumas Hyslop writes "The UK Telegraph is reporting that we may finally have equipment that are sensitive enough to measure gravitational waves, which are incredibly small and have evaded detection despite the theories that they are present as a way of explaining gravitational effects. Basically, a laser beam is split into two branches that are sent down two identical 2000 feet long tubes and back again via mirrors. Assuming the two arms remain exactly the same distance, they will cancel each other out. But the scientists think that the beams will interfere with each other owing to the effect of gravity, meaning the length of the branches is altered and a gravitational wave has been detected."
The word is imminent. immanent means something completely different.
How we know is more important than what we know.
The gravitational waves! They're all in your mind!
The opinions stated herein do not necessarily represent those of anybody at all. Deal with it.
i won't even get into it.
anyways, the purpose of the interferometer is to measure the differential gravitational strain between two remote masses. as a gravity wave passes (supposedly), two masses will be driven to oscillate in quadrature with one another. that means that relative to some fixed point, one mass will be drawn closer, and at a right angle another mass will be pushed further away. IIRC.
luckily a michelson interferometer is a great way to detect these small changes, where the remote masses are mirrors. the extremely long beam paths increase the sensitity of the device. and two remote locations are needed for local error cancellation. if you have three locations (there is a LIGO opening in louisiana soon. uh, maybe) then you can actually do gravitiational wave astronomy.
probably some LIGO person will write a better explanation, but it's late.
m
Gravitational Radiation - the cosmological reference, not the meteorology ones.
Some other gravitational wave detection projects
Some anomalies in gravity theory
and, of course, Einstein@Home
Mongrel News all the news that fits and froths
It's obvious to anyone who's read this site for a while that an admin just burned 20 or 30 of his unlimited mod points to bitch slap all of the "you spelled it wrong" posts. It wasn't regular readers with mod points.
The interferometric GW detection systems have been under development for quite a while. These include the LIGO project in the US, the GEO in the UK/Germany, and Australia and I believe Japan and Italy have their own versions. LIGO started collecting data a couple of years ago. So now the guys in the UK turned on their instrument.
So what's the big deal?.. Well, there isn't one. Today's instruments are pretty damn bad. I don't remember the numbers, but you'd have to run them for quite a few decades in a row for a good chance to observe one event (it would have to be something big falling into a black hole somewhere relatively close to us, or a major supernova, or something equally rare.) Essentially, you are trying to measure a ludicriously small displacement (10^-16 cm) of a macroscopic object.
The good thing is, technology is continuing to improve, increasing the sensitivity. Furthermore, there's hope (subject to funding) of creating a space-based version of the experiment by bouncing laser beams between three satellites millions of kilometers apart. So is the GW detection imminent?.. Considering the scale and cost of the projects, it better be, but I (being a scientist and all) prefer to steer clear of that word. So provided the funding doesn't get cut, we'll very likely detect gravitational waves in a few years. But be prepared to wait.
For more deets, check out www.ligo.caltech.edu
i will try again here. so one case of the "gravitational wave" theory is that when two black holes spiral around one another (or any two large masses), they will emit energy in the form of gravitational waves, like two boats circling in a lake. physicists would like to detect this energy.
let me digress for a second to radio. normal EM radiation is in the dipole form. which means the radiation makes charges (electrons in an antenna) oscillate up and down. gravitational waves (i think) hit us in the higher order quadrupole mode, which instead of "up and down" is more like "in and out". or taking a circle and squishing it along one axis, and then the other.
so lets say you are standing on a field. then you have two stones hanging on strings, one 100m north, and the other 100m east. when a gravitational wave passes, if you were God, you would be able to notice that the north stone was pushed closer while the east one was pushed away, then the east one was pulled toward you while the north one was pushed away.
to detect this *infinitesimally* small force, you replace the rocks with mirrors. and put the mirrors in vacuum to prevent them being jittered by air molecules and strange index of refraction effects with the air. then put the mirrors really far apart to increase the relative sensitivity to the same strain.
now take a laser beam, split it where you stand and send half the beam to each mirror. the beam then returns to you, you recombine it at the same beamsplitter, and the photons in the laser beam will interfere. whether this interference is *bright* or *dark* depends on the relative path length difference of the two arms.
you can detect changes on the order of 1/100 wavelength (actually, much less, but that's more complicated) which is about 1e-8 meters. since the interferometer is 2e3 meters long, that means you can detect a fractional change of about 1 part in 1e11. but it's actually crazy better than that due to many smart inventions the LIGO people created about locking optical cavities. you get the idea.
so then you watch your interference as a function of time, then go to your astronomy books to see what events should create gravitational waves at the frequency you have observed them.
in a nutshell.
m
ps. analogy: a radio telescope uses electronic amplifiers to measure the induced motion of electrons from EM waves : a GW telescope uses a high finesse optical cavity to measure the induced motion of masses from gravitational waves
LIGO has been operating for years. Their first "science run"---i.e., the first stretch of time during which the main purpose was not calibration, but actual data collection---happened between 2002 and 2003, if I remember correctly. For those who are interested, GEO is just one of many similar gravitational wave detectors in the world. There is TAMA in Japan, VIRGO in Italy, LIGO (two sites in the U.S., one in Hanford, WA, and the other in Livingston, LA), and there's also one other detector somewhere in Australia. LIGO is by far the biggest---each arm of the interferometer is 4 km long, compared to GEO600's 2000 feet (about 0.5 km). Plus, LIGO employs some clever little tricks such as using an optical resonating cavity, so that the light bounces back and forth several times in each arm, making the effective length more like 400 km. Pretty nifty.
Everything -- that has mass and that moves -- generates a ripple in gravity. You do it, your mom does it, too. Heck, so does any movement of Earth (e.g., techtonic plate movement, oceanic changes due to El Nino, etc).
Even though these gravitational waves generated from these local sources are weak compared to a truly remarkable astrophysical sources (e.g., mergers of blackholes), these terrestrial sources are closer; hence damned stronger compare to any expected extraterrestrial sources.
And yet, we have not detected a coherent signal of gravitational wave from local sources. This science is that hard. And that's why this is so fascinating. I think physicists have spent the last decade identifying these local sources and how the local signal would manifest itself in their experiment. I'll tell you, having seen some of the modeling, etc., detecting a gravitational wave from an orbiting pulsar is like trying to catch a person who's yelling "Yankees Rule" in the Fenway stadium via TV broadcasting. Oh that may be actually easier (since the guy would be dead on the spot by the mob of the BoSox fans).
I'm still awake and really should be sleeping, but instead I'll simplify even further, for the first year physics guy. Great description, by the way... I didn't know gravitational waves were supposed to be quadrupole.
Any accelerating charge (an electron for instance) will create an electromagnetic wave. A radio transmitter basically causes electrons in its antenna to oscillate at a particular frequency, and this produces radio waves at that frequency. Theoretically the same thing should hold for mass and gravity. If you cause a mass to accelerate (like the charge) then it should produce gravity waves (like the radio waves). Because gravity is so extraordinarily weaker than electromagnetism, the waves are correspondingly smaller, so very difficult to detect. Einstein says gravity causes space-time to curve, so passing gravity waves should stretch and squish space-time a little bit as they pass. Unfortunately you need to be able to measure distances really precisely.
An interferometer is how you do it. You send out two in phase light beams, bounce them off a mirror, then recombine them. If they travelled exactly the same distance then they should still be in phase (peaks and troughs line up) so they'll reinforce each other. If they travel slightly different distances then they won't be quite in phase anymore and the intensity of the recombined beam will be a bit less than it was originally.
So now you send the beams off at ninety degrees to each other and see if the ratio of the distances they travel changes. It will of course, due to all kinds of things, but maybe one of those things is passing gravity waves. So you have detectors on different continents and correlate their measurements. Local things (tiny earthquakes, people walking around above the detector, somebody turning on their washing machine down the street) will not be recorded by both detectors. Things like gravity waves will.
One more interesting thing you can do -- if you have more than two detectors, by watching when the waves are recorded by each detector you can measure the speed of the wave... the speed of gravity, and you can tell what direction the wave came from.
Simplified lots, and I should be sleeping, so that was probably full of errors and you should pay attention to the parent instead, but that's probably as simplified as it can get.
METERS
try reading http://en.wikipedia.org/wiki/LIGO
maybe the european one is 2000 feet, but not the two in th US. actually, the full length of each arm is 4000 m. i've been to the facility, touched a beamtube, drove to the end. meters.
m
I heard they only wanted 10^-12 m (picometer) resolution. And they aren't planning on keeping them fixed at that relative distance; that's impossible without breaking the experiment (if you don't know why, then you don't understand the experiment). The probes are going to be moving at a speed of order meters/second relative to each other. In any case, they don't have to keep the distance fixed so long as they keep track of where the probes are wrt each other. The engineers claim that the optics is the "easy part" (a friend of mine is working on one of the "hard parts"), though it seems anything but easy to me.
...
Also, keep in mind that our wonderful politicians want to build a wind farm next to Hanford since "there's nothing there anyway"
LISA satellites need to be stable to within 1 nm per root Hz of bandwidth. (It's been a while since I worked on it, so someone else is welcome to explain what exactly this means.) Suffice it to say that this is a tractable problem, and I would argue no more difficult than the Advanced LIGO designs currently being implemented. And you get more bang for the buck in sensitivity.
Please show me a good reference for LIGO expected detection rates. This is taken from a popular book, but the numbers agree with what I remember hearing from those working on LIGO.
Supernova (within our galaxy)
1 to 3 per century
Black Hole/Black Hole Merger (300 million light-years)
1 per 1,000 years to 1 per year
Neutron Star/Neutron Star Merger (60 million light-years)
1 per 10,000 years to 10 per century
Neutron Star/Black Hole Merger (130 million light-years)
1 per 10,000 years to 10 per century
Source: Einstein's Unfinished Symphony: Listening to the Sounds of Space-Time by Marcia Bartusiak
Probably the same way they detect cosmic rays, built gigantic facilities underground and cover them in water.
How we know is more important than what we know.
Here's the deal with local sources: their masses are tiny compared to astronomical sources!
But here's a more local source that we have detected: the moon. The moon causes tidal deformations in the Earth's crust, which LIGO (disclosure: I am involved with the LIGO project) and the other large scale interferometers (GEO, VIRGO, TAMA) have to subtract out in order to see anything besides the moon.
Essentially, to make gravitational waves large, the conditions which need to be satisfied are 1) large amounts of matter 2) moving quickly. Things which satisfy this are: supernovae core collapses which are sufficiently non-axisymmetric, compact (eg. black hole or neutron star) binary system decay, and maybe some events we don't yet know of.
-Leo
The article is writing about GEO600, whose two arms are 600m which is about 2000ft.
-Leo
This is in reply to this post and a number of others on the same topic.
... so they can correlate with those sources.
:)
Major sources of noise: seismic, acoustic, photon shot noise, thermal noise.
1) Acoustic noise: the entire beam tube system is in vacuum, so the only mechanical vibrations can be coupled in through the mirror supports, which are suspended on thin wire. The pendulum created by the hanging mirror essentially creates a mechanical low-pass filter which reduces the effects of noise above about 10 Hz. The gravitational wave projects (on Earth, not talking about LISA here) are mostly interested in frequencies around a few hundred to a few thousand Hz.
2) Seismic: this can cause pretty large displacement. Each of the mirrors (on its' hanging suspension) is sitting on a system of masses and springs (three levels) which creates a third order lowpass filter which further reduces noise.
3) Photon shot noise: this rises with frequency; essentially, photons are uncorrelated random events which create a Poisson noise distribution. In a Poisson distribution, the standard deviation of count rate is equal to the square root of the count rate, so the variance is decreased by decreasing count rate at the detector. This is why the interferometric detectors operate "in null," meaning they keep the mirrors at a differential path length which is equal plus or minus integer multiples of wavelengths. This way, the output at the point where they interfere is kept dark. The idea is that it's easier to detect a difference between 0 and 1 than between 100 and 101. (There is a ton of feedback to keep the whole system in null. Read up on Pound-Drever locking to understand it.)
4) Thermal noise: the surface of the mirror is made of atoms which jiggle in random Brownian motion. This is unavoidable unless the mirror is cooled sufficiently, which is difficult to do because of how well isolate the mirrors are. However, the Brownian motion can be averaged out over a large area by making the laser's spot size large.
So they've thought about it a little bit. And they are also measuring other non-detector channels like seismic activity and acoustics near the detector and wind speed and
The NSF doesn't go around giving millions to any old project
-Leo
Each of the mirrors have magnets glued to them in a quadrupole configuration (so they are not affected by stray magnetic fields) which are used to move the mirrors around. The mirrors are moved around for a few reasons:
1) The detector works in a closed loop feedback system, so they keep the mirrors they same distance (mod the wavelength) from the source (and the error signal which is used to feed back on them is the actual "gravitational wave" signal.
2) Calibration: during data collection, the mirrors are periodically driven with a known magnetic field by the same set of magnets. Because we know the mechanics of moving a mass with the force of a magnet, we know how far it went. And when we look at the error signal to keep the device in lock while it is being forced, we know the ratio between distance the mirrors travel and digital signal coming out of the detector. So after the data are collected and ready to be analyzed, a lot of people spend lots of time preparing and reducing the data, which includes coming up with a calibration from the forcing.
-Leo
Not sure about GEO600, but the LIGO interferometer uses a simple solution: build two observatories on opposite sides of the country, and if only one detects a signal, it's almost certainly spurious. I'm guessing that since TFA says GEO600 will come online at the same time, it'll just be treated as another part of the same array for those purposes.
This is sqrt(not) a sig.
Hello,
The speed of light will not change no matter what, but otherwise I agree completely with your description, we should not be able to detect a raw change in length due to gravitational waves, however we should be still able to detect a local change in gravitational field, i.e. the local metric, precisely because the change is local due to the waves.
Intuitively this might be why two interferometer arms are necessary.
Hundreds of scientists spend millions of dollars of money on an incredibly expensive
8 6.htm
method of detecting gravity waves when cheap ones somehow already exist.
Build your own gravity wave detector:
http://www.rexresearch.com/hodorhys/remag86/remag
Non sequitur: Your facts are uncoordinated.
In essence, you can think of it as doing that. Think about how you detect frequency shifts: when you tune a guitar, you listen for a frequency difference because one string is accruing phase faster than the other one. When they combine, you hear beats.
One you look at the output of a Michelson interferometer, there are differences in intensity because one arm may have accrued more phase than the other one. You can think about it in the time domain, measuring length of the arms, or in the frequency domain, because the arms (which are resonant cavities) have had their resonant frequencies shifted.
-Leo
I'm so happy for the GEO600 crew if they are in fact getting close! I've actually seen the facility - pretty amazing stuff and a very good example of how far you can push things using much brain and relatively little dollar. For example, the article didn't mention this but the arms of the interferometer do not intersect at quite 90deg due to the fact that the arms are built along the borders of farm plots.. As for filtering out noise, they filter out everything above and below particular frequencies. It's all extremely sensitive work and I'm glad somebody else is doing it!
The LSC (LIGO Scientific Collaboration) thinks a little bit different about that.
:-)
In their document "First report on the S3 analysis" (http://einstein.phys.uwm.edu/PartialS3Results) which is based on the Einstein@Home community efforts they say:
"However, the numbers of sources and their distances from us are uncertain, and in their first few years of operation it is quite possible that the LIGO and GEO instruments may not detect anything."
"So far, we have not seen any evidence for pulsar signals in the S3 data. As described earlier, this is not surprising, because LIGO is not sensitive enough to guarantee that we will see one or more pulsars."
LIGO is going to be upgraded ("Advanced LIGO"), which will improve the detection of events by the factor 100-1000.
Maybe the theory of grav-waves is even wrong, who knows...
You look like a million dollars. All green and wrinkled.
> Seismic activity?
There are huge seismic isolation system that damp any movement of the test masses, but they don't remove all the seismic noise - that is why the LIGO data is only really useful from about 40Hz upwards.
> Temperature changes?
Again the test masses are isolated and in vaccum so that temperature flucuations don't effect the sensitivty that much, however this is a problem and is a limiting factor in the overall sensitvity of the interfermeter.
> Planes flying overhead? (sound)
This has been a problem in the past! A plane was approach to Pasco Airport not to far from LIGO Hanford and was recorded by one of the environmental monitors which cause a slight disturbance in the laser readout. This on its own wouldn't have been a problem, but this disturbance at Hanford occured at exactly the same time (taking into account the travel time) as a random noise spike at the Livingston site. On initial inspection this looked like it could be a detection as there was a coincident signal observed at both sites. However is was soon discovered, by observing the environmental channels, that the "signal" at Hanford was due the plane and the "signal" at Livingston was just random noise. Because of this the "signals" from planes are monitored and vetoed out from the data prior to analysis.
Indeed, correct. Just to put a tangible front on that very valid theoretical answer, light, RF, etc. (the propagation of electromagnetic radiation) does not need a medium to wiggle around in. This is why it can traverse space so efficiently. Instead of a medium, the electic and magnetic fields wiggle each other.
Furthermore, EM emission of "stuff" is a bit weird. I assume you speak of a "photon" as stuff, however a photon can be simply thought of as a packet of energy. But a photon is simply a wave and vice versa. This is certainly an easily confused subject in which this link provides a bit of insight
And yet, we have not detected a coherent signal of gravitational wave from local sources. This science is that hard. And that's why this is so fascinating.
That is the really weird part. The people at fourmilab have a video of a basement torsion bar experiment that demonstrates that objects create their own space-time curvature.
But there's no way of demonstrating that such curvature will ripple across space-time.
Vintage computer adverts: http://www.vintageadbrowser.com/computers-and-software-ads
... a gravitational wave generator patented by the NSA. I guess reverse engineering all those UFOs paid off.
[ObDisclaimerForTheClueless: No, I don't really believe they reverse engineered UFOs. The patent's real though. Who knows, it might even work.]
Human/Ranger/Zangband
I'm not sure where you get the idea that other gravitational wave detectors have "failed". I suppose you could regard an inability to build a sufficiently sensitive detector as a "failure" of sorts, but it's not the sort of failure that would cause you to draw any conclusions about the underlying theory because nobody really expected those detectors to find anything. Those experiments have been more about techniques in constructing large interferometers than they have been about astronomy. Moreover, gravitational radiation has been observed indirectly through its effects on binary pulsars. The loss of angular momentum in a binary pulsar system has been observed to be consistent with the predictions of General Relativity. It is possible, of course, that GR is wrong, and there is some other effect that is producing the angular momentum loss, but it would be a rather surprising coincidence if the rate of loss from this hitherto unsuspected mechanism were precisely what we would have expected from gravitational radiation.
As for the disagreement between GR and quantum mechanics, this is a long-standing theoretical issue. That is, you don't need observations to tell you that GR and QM don't work together; they are incompatible even at a theoretical level. Observations of gravitational waves, or lack thereof, are pretty much irrelevant to that debate. What this means in practical terms is that like classical mechanics GR is a useful approximation in the non-quantum regime, but it breaks down when you get into regimes where quantum effects are important. Physicists have been trying to figure out a new, quantum mechanics-compatible theory of gravity for decades, so it's not like they're waiting for the results of this experiment to come in before they get started on it. However, it turns out that coming up with a good, testable quantum theory of gravity is rather harder than it looks.
The real significance of these gravitational wave observatories is not in theoretical development of quantum gravity, but rather in their role as a new window on astrophysical phenomena. When astronomers began to observe in the radio we learned a lot about known astrophysical pheonomena, and we discovered entirely new phenomena that we never suspected. It seems reasonable to expect similar discoveries from gravitational wave astronomy when it becomes a reality. For example, once ground based detectors develop sufficient sensitivity, some astronomers hope to learn a lot about the makeup of neutron stars from the gravitational wave signature of binary neutron star coalescence. And the LISA experiment, if it ever flies, will have sensitivity in the frequencies that contain the relict gravitational radiation from the Big Bang. Just as the Cosmic Microwave Background has taught us a lot about the early universe, we hope to make similar discoveries from the Cosmic Gravitational Wave Background.
-rpl
Just as a point of information: mathematicians have defined "immanent" to be a certain generalization of the determinant (of a matrix).
http://planetmath.org/encyclopedia/Immanent.html
You're thinking of Gravity Probe B http://einstein.stanford.edu/, another tour de force of ultra-precise instrumentation. That mission has now finished and they're analysing the data.