Slashdot Mirror
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."
Bring out your gravity surfboard and roll on!
bash$
Maybe the gravitational waves changed the i into an a.
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
No the headline was completely correct. The scientists are thinking that current technology can actually read gravitational waves and we're actually detecting it all the time in our minds! So not only is it immanent but it's immanently immanent!
Both sites are asking for public help processing the data, via a special screensaver called Einstein@Home.
--Greg
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
Punative down-moderations like this are done by the admins, not regular readers. Regular moderators get five points at a time, not the twenty or so that would have been required to reduce all the spelling flames so far so quickly.
I usually mod down language gripes (and dupe complaints) whenever I can, and I'm sure many other moderators do too. Yes, we know there was a misspelt word there, and yes, we know there was a similar post a while ago. So what? No need to point it out. Over. And. Over. Again.
Trust the Computer. The Computer is your friend.
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).
Follow the guidelines for moderation. Moderate UP not DOWN. And browse at -1 Newest Posts First, No Threading. I metamoderate three times daily and anything that is modded offtopic or flamebait or troll that isn't obviously so will earn you black mark that will reduce your chances of being selected as a moderator again.
How we know is more important than what we know.
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
Do you seriously think they might have forgotten about callibration? Do you think whoever is in charge of this thing is that dumb? By all means, if you do, pick up a telephone, call them and shout "Remember to do some form of callibration!!!". Be sure to be very emphatic. Science will thank you.
Thnaks to all teh braev suols who wer willign to bern kamra to piont out teh diferense btween "immanent" and "imminent". Othrewise we all wuold haev to RFTA and haev a maeningful dicsussion. Tihs is Slasdhot, and we ca'nt haev taht heer! (Stewpid atricles!)
It's not an error, don't you know that in US english on the Internet any vowel can replace any other ?
The few readers who will actually know what "immanent" means will also know that it was actually supposed to be "imminent", so no harm done. The rest will just see it spelled as usual.
Live with the times !
May contain traces of nut.
Made from the freshest electrons.
in order for this to work you need both 2000ft arms to be the same EXACT length.
:-) and you'd take a long term average of the results to find a distance that has the highest peaked output and call that the centre baseline.
I presume that they have some way of adusting one path - you simply adjust it to peak brightness / least inteference. Then when something happens, it'll be a different distance either way and you'll see a null, or at least a drop.
If you can't get a peak because the damn thing is jiggling all over the place, then it's working
You are in a twisty maze of processor lines, all alike.
There is a lot of hype here.
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
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
What I'm thinking is the following, We all know the speed of light is constant for a material (or vacuum). From our frame of reference we will not notice the distortion in spacetime. Our yardstick will shorten and lengthen with the compression and expansion of the waves. which would make it impossible to detect the changes. Of course, I'm probably just not knowledgeable enough to know what's going on here, but then again. I'm curious to see if this idea has been addressed.
If no one has thought of this idea yet, I just did and I claim it! :)
"The bass, the rock, the mic, the treble. I like my coffee black, just like my metal" - Mindless Self Indulgence
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
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.
This is a reply to this post and some of its' ancestors.
Gravitational waves are predicted to weakly interact with everything which is matter-energy. For that matter, gravity interacts with itself (which is why GR predicts black holes and other such singularities). However, in the weak-field regime (that is, space-time is flat except for a deviation which is orders of magnitude less, meaning we can take the leading term in the expansion, so the theory is linear), gravitational waves just pass through everything. Since they pass through things, their energy falls off like the square of distance from the source. In the strong-field interacting picture, they certainly should exhibit non-linear exotic behaviour, but those are precisely the parts of GR we are trying to probe with LIGO.
The exchange between matter-energy and curvature (gravitational waves) that you are thinking about is from the latter to the former, but just think about the former being turned into the latter - that is the prediction of the source of gravitational waves. However, it works both ways.
On the levels at which LIGO hopes to detect gravitational waves, we will see about 10^53 gravitons. I am quoting this figure without understanding where it comes from, since we certainly don't have a quantum theory of gravity. But gravity is predicted to be quantum in nature as well, but we won't see the quantization from where we stand.
One of the ancestors addressed the issue of measuring while your meter stick is being squashed and expanded, and another about the local speed of light. These issues are related. One of the postulates (argue argue whatever) of GR is that the speed of light is constant in every frame, and it has the same constant value compared between all frames. Light is the perfect meter stick (or clock) for making measurements with.
I had the same thought about measuring the arm lengths as you did for a while until I started taking GR. Here's how the thought goes: "If space is being stretched, and a meter stick is sitting in front of my face, I will always see the meter stick as being one meter long." Here's what GR predicts: the proper distance between free test masses sitting in space as a gravitational wave passes by will exhibit the increase in the X, decrease in the Y and vice versa oscillation pattern. To measure this, you need to use something free, like the mirrors at the ends of the beam tubes (they are really only free in one dimension). To measure distance, you can't use a meter stick, because it is not an ideal measuring device which you need to measure space with in GR. The ideal device is light. To think about it without resorting to a meter stick increasing and decreasing, think about the light travel time. Since light has a constant speed in all frames, if the proper distance is what is really increasing (disregard what happens to the meter stick, since it is made up of fallible matter and might stretch along with space, but light won't), then it will take longer to go down one arm and shorter down the other. Therefore one arm will add phase relative to the other, which will no longer perfectly interfere at the end.
-Leo
There are multiple layers of noise removal, of course. One trick is that the mirrors are suspended in such a way that
any disturbance will likly make them vibrate at a very specific fequency. Signals at that frequency are ignored.
That's the most rediculous thing I've ever heard!
Bogtha Bogtha Bogtha
> 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.
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 see a few posts saying, "Well, we haven't seen gravitational waves yet, so maybe they don't exist." To that, I have several responses:
1. There's no reason why we should have seen them yet; they're so weak that even LIGO I probably won't see them. (LIGO II probably will, if the equipment works as designed.)
2. Gravitational waves have already been detected indirectly: the 1993 Nobel Prize was awarded to Taylor and Hulse for this discovery. They observed a binary star system whose orbits were inspiralling at exactly the rate that general relativity predicts for a binary system that is losing energy via gravitational waves. That rate also gives the rate at which energy is leaving the system, and allowed them to infer the speed of gravitational waves: the speed of light, to within a few percent --- also as predicted by general relativity.
3. Even if general relativity in particular is wrong, pretty much any field theory compatible with special relativity contains wave solutions propagating at the speed of light, for demonstrable reasons of logical consistency. This holds for both classical and quantum theories (e.g. Maxwell's equations, general relativity, the Standard Model of particle physics, etc.), theories of quantum gravity like string theory, and so on. You basically have to throw out all of relativity and go back to Newtonian physics to get field theories without wave solutions.
actually, local sources of GWs are highly unlikely to produce signals which are detectable. the bigger problem is local sources of vibration: trucks driving on the road, heating and air-conditioning fans, planes flying overhead, etc etc.
saulson's book has an example calculation of what would be needed to generate detectable GWs in the lab. take two steel balls, mass 1000 kg each, 1 meter apart. rotate them around their common center of mass at a frequency 96 Hz (about 600 rad/s). the strain that generates is about (1/r)*1e(-35) where "r" is the distance from the generator to the detector. in comparison, a typical pair of neutron stars, 1.4 solar masses each, 20 km apart, and rotating at about 400 Hz. if the pair is in the Virgo cluster, about 15 megaparsecs away, the strain at the earth would be about 1e(-21). this sort of order of magnitude stuff can't just be handwaved without a few approximate equations.
i did my phd research with ligo, so i have somewhat of an insider's view.
This is a tautology.