Slashdot Mirror
Examining Gravity Waves
Joseph "JoeDaMac" Haake writes "Sometime within the next two years, researchers will detect the first signals of gravity waves -- those weak blips from the far edges of the universe passing through our bodies every second. Predicted by Einstein's theory of general relativity, gravity waves are expected to reveal, ultimately, previously unattainable mysteries of the universe."
In a perverse sort of way, I'm hoping that this experiment generates all the wrong data. Data that is completely the opposite of what people expect.
Think of all the fun that would be! Think of the chaos, the pontificating, the explanations, the TV specials! Think of all the dissertations that would generate! Yes sir, that would be wonderful.
If tits were wings it'd be flying around.
"Researchers WILL detect..."
On the whole, i think that's not necessarily true. There are several mathematically consistent fringe quantum-physical theorys (usually something akin to higher-order-symmetry electrodynamics) in whcih gravity waves are indistinguishable from e.m. waves, or are longitudinal-time e.m. waves.
You can also read the story here.
Would this help unify quantum gravity and GR? Could it give evidence to bolster string theory? The results of this experiment should be very interesting.
On the ither hand, it could be affected by the whole varying-alpha thing. If something that fundamental is wrong, I think their data will be much less useful.
The LISA experiment, which gets mentioned in passing, is really quite audacious - three spaceships orbiting the sun in a clever rotating triangle pattern, 5 million miles apart from each other, and detecting changes in distance between each other to an accuracy of 20 picometers!
In essence, it's just a really, really big version of the Michelson interferometer we all played with in 1st year physics - I remember the thrill back then of realising what tiny changes in distance you could discern with just a couple of mirrors, a lamp and something to measure the recieved intensity.
It's exciting to witness the nascence of an entirely new form of astronomy.
Suen and his collaborators are using supercomputing power from the National Center for Supercomputing Applications at the University of Illinois, Urbana-Champaign, to do numerical simulations of Einstein's equations to simulate what happens when, say, a neutron star plunges into a black hole. From these simulations, they get waveform templates. The templates can be superimposed on actual gravity wave signals to see if the signal has coincidences with the waveform.
"When we get a signal, we want to know what is generating that signal," Suen explained. "To determine that, we do a numerical simulation of a system, perhaps a neutron star collapsing, in a certain configuration, get the waveform and compare it to what we observe. If it's not a match, we change the configuration a little bit, do the comparison again and repeat the process until we can identify which configuration is responsible for the signal that we observe."
They will be changing the way they observe in order to conform with what they expect to observe. Doesn't this mean that ultimately they're not really going to discover anything new? I mean, if they set up their observation so that when looking at a neutron star collapse it matches the mathematical model, what's the point? Why not just look at the mathematical model?
Karma: Chevy Kavalierma.
Perhaps the most exciting thing about them is that we may well not know what it is we're going to observe. We think black holes, for sure. But who knows what else we might find?"
Jabba-The-Hut doing the Wild Thing.
"When we get a signal, we want to know what is generating that signal," Suen explained. "To determine that, we do a numerical simulation of a system, perhaps a neutron star collapsing, in a certain configuration, get the waveform and compare it to what we observe. If it's not a match, we change the configuration a little bit, do the comparison again and repeat the process until we can identify which configuration is responsible for the signal that we observe."
Sounds to me like they may be changing their model to fit the data in such a way that they won't know for sure it is a match. For example, a signal roughly fits the model of a black-hole forming, but not quite. They then keep tinkering with the black-hole formation model until it matches the signal. But in reality the signal could actually be something not related to black holes. They are putting the cart before the horse it seems.
It seems they would have to match a specific electromagnetic observation(s) to the gravity wave event to verify. Otherwise it is just a guess.
I could see some justifiable confidence if the signals were complex, and were only slightly off the models. In other words, near dead-ringer matches of something that would be too much of a coincidence to be something completely different generating the same (expected) complex signal. But I doubt we are at this stage in both the models and accuracy of the signal detection.
Table-ized A.I.
You are completely right, but this can be a very dangerous thing to try to do. I work in computational fluid dynamics, and some people advocate doing this kind of CFD (tuning turbulence models to match data of very complex things usually) this leads to some bad mojo most of the time. you get codes that look good when you use them on multistage axial flow trans-sonic compressors (for example) because it was tuned to that, but it can't solve flow in an axisymetric duct! Then people think the code is great and start to trust it until it misses in a huge way on something that is a little different that what it was tuned to and everyone freaks out!
I REALLY think that the only way to do this kind of thing correctly when you don't match data, is to go back and look at the set up/first principles... Were your boundary condition assumptions fair? Did you assume anything was insignificant, was it?... that sort of thing. Tuning is something that scares the crap out of me, mostly because it sounds like a good idea to most people.
"I'll have a Guinness, no wait, make that a Coors Light" -Grad student I work with, who shall remain anonymous...
It is only mentioned briefly in the article, but I'll try to elaborate.
Basically gravity waves will stretch space in one direction (say x) and contract space in a perpendicular direction (y). Given this, the "easiest" way to detect gravity waves is to build a very large interferometer. LIGO is the current ongoing gravity wave interferometer, which splits one laser beam into two lasers beams, sending each perpendicularly down a vacuum "hallway" four kilometers long. At the end, the beams are reflected by mirrors. The two lasers meet again after another 4km.
The two beams are recombined afterwards. If the distances the two travel are exactly equal, then the two beams will interfere constructively. But if the lengths which the two beams are stretched/contracted by a passing gravity wave, the beams will interfere since one will be "shifted" (it had to travel a longer/shorter distance. By measuring the interference pattern between theses two beams, and hopefully physicists will be able to detect a gravity wave.
The amount that a gravity wave will shrink/extent one of the beam lines is amazingly small. Each 4km beam line will have it's distance changed by 10^-18 meters, or on the scale of attometers! Because of this, any vibration or local variation will affect the beam length. So the physics who are part of the LIGO collaboration built two such laser devices, one in Livingston, Louisiana and the other in Hanford, Washington. When a gravity wave (from outer space) travels through the earth, hopefully both sites will measure the same small variation, which will correspond to a passing gravity wave.
You can get more information about LIGO at:
LIGO's Home Page
LIGO collaboration page.
Slashdot recently had a science story about LIGO.
I wonder if they will be at all able to measure the speed of a graviton with this current setup. It seems as though they are having enough trouble just detecting them in the first place though. I think this is a first step towards a new branch of physics that uses gravitons in experiments. For instance, some spin-2 thermodynamics could be experimentally demonstrated if gravitrons could be isolated and easily detected. This is probably not going to happen any time soon, but LIGO is a big first step towards that goal.
I wonder if they will be at all able to measure the speed of a graviton with this current setup.
No.
For instance, some spin-2 thermodynamics could be experimentally demonstrated if gravitrons could be isolated and easily detected.
No (see my earlier post on quantum gravity and gravitational waves).
According to current theory, there is absolutely no way we could even begin to dream about detecting individual gravitons, much less confine them. These experiments aren't a step towards this "goal" any more than any other experiment out there: according to current theories of physics (yes, the most fundamental ones), confinement of gravitons is an absolutely unimaginable task for all foreseeable technology.
Compare this with neutrinos. They only interact weakly, through the "weak force" (and, of course, they also interact gravitationally), as opposed to, say, protons and electrons, which interact through the strong force and the electromagnetic force. The important difference is that the weak force is, well, like the name says, weak: the probability of any interaction of neutrinos (with any known form of matter) is much, much lower than the probability of protons or electrons interacting with each other.
Remember that quantum mechanics only predicts probabilities. Imagine a neutrino traveling towards the Earth: it has a very low probability to "collide" with any particles of the Earth, so most likely it will just shoot right through the Earth like it's just empty space. A proton or an electron wouldn't do that (or, at least, this would be very, very improbable): they can also interact through the two stronger forces, which means that they would have a much higher probability to collide with some particle of the Earth. For this reason it takes some incredibly complicated arrangements to detect neutrinos (you need a huge detector to get even a single neutrino collision per day).
Now, while neutrinos can interact through the weak force (and gravitationally), the problem with gravitons is that they can only interact through gravitation. And gravitation is much, much weaker than even the weak force! The difference is actually many, many orders of magnitude larger than the difference between the electromagnetic force and the weak force. So we're not goint to see any gravitons for a very, very long time!
Maybe some time in the future we'll be able to build some galaxy-sized detector in intergalactic space and finally see some gravitons... but, unless the "coming" theory of gravity predicts some totally new effects (and it might), it's really that far off.
If you didn't see it, crosscheck this article with the experiment to measure the speed of gravity that took place last month. There they observed the effect of Jupiter's gravitational field on incoming radio waves.
Does anyone know if results have been published yet?
... Two large gins, two pints of cider. Ice in the cider.
In case anyone is interested, "gravity waves" also refer to the buoyantly driven waves in the atmosphere and ocean.
If gravity moved at the speed of light, then the Earth, in it's orbit, would "see" the Sun where it was about 8 minutes ago. Over geologic time, this changes the orbit. Over the life of the solar system, Earth's orbit isn't stable. So, gravity much act faster. One estimate of the speed of gravity is that it must be at least 10^15 times faster than light. That means that the wavelengths may be very short. So, using light interferometry to detect them may be futile. There may still be things to learn from the experiment, however, even if it isn't about gravity.
-- Stephen.
dude--joke. reference to bloom county? no? didn't think so. probably never read it...
Liberty uber alles.
Its actually very insightfull :)
If Google really cared they would fix Android Chrome to reflow text, instead of discriminating
If some big masses wiggle at some distance and you don't believe in infinite propagation speeds, you expect to have something that looks like gravity waves. The question is whether they will be those predicted by Einstein or whether they will behave differently.