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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."
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
Both sites are asking for public help processing the data, via a special screensaver called Einstein@Home.
--Greg
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
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.
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.
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.
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
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.
That's the most rediculous thing I've ever heard!
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