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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."
Both sites are asking for public help processing the data, via a special screensaver called Einstein@Home.
--Greg
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.
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.
Now if we can only get rid of the strong local influances such as the sun and moon, then we might get some sensitivity.
The influence of these make detecting very weak waves difficult. It is like detecting the change in sea level due to a rain storm or evaporation. Local wind caused waves and tides make detecting these minute changes difficult.
The truth shall set you free!
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.
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 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.
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.