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."

Comments

There's two for twice the price

[Greg Hullender]

I notice that GEO 600 actually has a US competitor called LIGO which the Telegraph article seems to have missed, but according to the New Scientist apparently they're both due to go live at the same time.

Both sites are asking for public help processing the data, via a special screensaver called Einstein@Home.

--Greg

Sure...

[Secret Rabbit]

Prof Jim Hough, a physicist at Glasgow University, has been on the gravitational wave quest for over 30 years. He said: "Given what we now know about the frequency of events that cause the emission of powerful gravitational waves and the sensitivity of the equipment we now have, I am confident that we will see things during this session."

Don't spout "I'm sure." I don't care if you're sure. Tell me when it actually happens.

I want results, not speculation. What was that about this being science again?

And how many times have we heard this before? Theory X is about to be proven. Then in the weeks, months and years to come, nothing more is heard and the media circus fades out of memory.

And even if this thing detects something, how do we know if it has actually detected what we think it has detected.

I remember hearing a story about some Experimental Physicists a while ago. They were doing some sensitive experiments and kept getting weird results; spent a lot of time trying to figure out what was wrong/checking theory/etc.

What it ended up being, is some Chemists refrigerator three floors up and at the other end of the building. The magnetics were interfering with the Physicists device.

So, who's to say that something similar might happen here. Possibly a passing train? Airplane maybe? If it even detects anything at all.

I'll wait for the real story. But, I'm certainly not going to hold my breath.

Re:hehe

[JanneM]

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.

cloud problems..

[Janek Kozicki]

I was on a lecture about gravity waves, once. This guy showed a lot of interesting stuff, like that a best gravity wave emitter is when you place four bodies each on a node of a square, and then squash/unsquash this fictional square. Then a strong wave is emitted in perpendicular direction.

also he said that some folks are trying to detect gravity waves by sending two laser beams through a very long tunnel, they bounce of mirrors and then interfere, so length of their way can be measured with high precision. Exactly like in the summary above.

And guess what? They got totally different results depending if there are clouds up in the sky or not. The beams were attracted to the clouds because of cloud's mass. Of course it means that they couldn't detect any gravity waves from far away - too strong local effects.

He also said that the only possible gravity wave detector should be placed in the space on lagrangian point.

Food For Thought

[mjinman]

Ok sounds nice and all, but heres some food for thought. In order for this to work you need both 2000ft arms to be the same EXACT length. If they are not EXACT then the whole thing wont work, but in that case how will you know? Or even better when they are making it, how do they know the gravity waves arent throwing off their measurements before its even built!

Re:Food For Thought

[ColaMan]

in order for this to work you need both 2000ft arms to be the same EXACT length.

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 :-) 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.

Re:such poor writing in the summary

[Technician]

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.

Moonbase LIGO

[Mathinker]

Wouldn't that make the Moon the perfect place to set up a detector?
Vacuum at no cost, no tectonics(?)

I'm not considering travel expenses and room and board... :-)

Forget slashdot spelling... look at the science

[tod_miller]

What if some hungry and / or amorous rabbits are enjoying the beets near the pipes?

Seismic activity?

Temperature changes?

Planes flying overhead? (sound)

I am not sure how they can remove all this tiny tremors and vibrations and details from their detection equipment. I wish they would publish a 'how it works' that deals with stuff like that. It will be on my mind all day now.

please type the word in this image: buffets
random letters - if you are visually impaired, please email us at pater@slashdot.org

Re:Forget slashdot spelling... look at the science

[stevelinton]

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.

What about other interference?

[MadCow42]

I can think of a ton of other things that would affect such a test...

What about vibrations in the ground? 4000ft of tubing and NO vibrations? Unless the tubes were also a perfect vaccuum, the resulting pressure waves in the tubes would diffract the laser beams slightly and cause variation. (believe me, I worked for a company that makes laser imaging devices).

Ok... now heat/cold? The length of the tubes, the positioning of the mirrors, deforming of the mirrors, etc. will be affected by this. Over 2000 ft, it doesn't take much of a change to have noticable influence.

I think you'd have a very sensitive instrument, that would measure 200 different factors at the same time - how do you tell whats gravity and whats not?

MacCow

Re:Naive question

[Anonymous Coward]

That's why there are two of them on different parts of the globe. Any gravity-wave phenomenon will affect both, so local effects can be ignored (see how I used affect and effect in the same sentence!)

Re:Naive question

[edgr]

I assume they would correlate their data with the equivalent agencies around the world. If the gravity waves came from some remote source (like a black hole), their effect could be measured anywhere on Earth. If it was a passing train, that would obviously only occur to one place at a time.

Re:Can someone please explain this (dumbed down)?

[mennucc1]

the french-italian project VIRGO has two arms , each 3000 METERS long.
BTW, whenever you here someone speaking of physics and using feets , you should doubt that s/he knows anything about the subject.

Possible problem with the whole idea?

[bloodstar]

No modding for me late at night, instead I'll ask a dumb or not so dumb question, If spacetime is being squished and expanded, would that also not locally change the speed of light, which would render detection impossible (at least with that methodology)? Which is what theory testing is about. but I'm just wondering if that's possible.

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! :)

Re:Possible problem with the whole idea?

[bloodstar]

hmmmmm, but in that case, would it be more probable to detect a red/blueshift from the local increase/decrease in the gravitational field? so, why cause an intereference pattern when you could simply detect the frequency of the beam after it has travelled the length of the array. Would the equipment be sensitive enough to detect a minute change in frequency that could be caused by that?

Re:difference

[louisfreeman]

My mistake

Let me put is this way: how do you callibrate something like this. Don't get me wrong, I'm sure they know very well what they are doing but I'm just curious.

GW doing work

[cdn-programmer]

If you can detect the passing Gravitational Wave then does this mean it has done work? If so then the waves should attenuate.

So in what form is the energy of a Gravitational Wave? With EM the energy travels in the form of a photon. Does this mean if we detect a gravitational wave that we have detected a graviton?

If so is it quantized? Also does this mean that somehow the graviton interacts with other mater? Wouldn't this unify gravitation into the EM force?

Well - I don't know enough physics to answer but I suspect that gravitational energy might actually be continuous.

However the mechanizm by which gravitational energy (which should have mass because E=MC^2 - except they are thinking "rest" mass and the rest mass may be zero) gets transfered from the gravitational wave into whatever it gets transfered into may have a consequence. If we have a pair of spinning black holes for instance then this may be a way for them to leak energy and thus they might slowly evaporate.

Re:GW doing work

[LionMan]

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.

What happened to the other experiment?

[foreverdisillusioned]

I seem to recall reading that NASA had sent up a bunch of satellites bearing very sensative equipment that were supposed to detect gravitational waves, though I don't think they were using this method of detection. Does anyone know what happened with that experiment? Do they have the results yet?

At any rate, I think I read about it on slashdot, so I suppose I could just wait a few months for a dupe.

How to discriminate signal from noise

[mu22le]

The other replies are mostly right: you use more than one detector so you can look for a signal in both of them...

then you have the fact that the signal you look for has a very well known shape (it can be calculated in our teoretical framework)

you also expect to observe it toghether with other (indipendent) signatures:
a supernova for example would be observed by all astrophisics experiments sensitive to light (common visible light telescopes, radio/gamma telescopes) and by most of the neutrino experiments around (this is no sci-fi neutrino telescopes already "saw" a supernova a few years ago)

and finally every piece interferometer apparatus is contained in a vacuum tank and suspended at the end of many pendules designed in such a way that it is as decoupled as possible from external vibrations (bunnies humping, trains, earthqakes, everything mentioned by the other /.ers)

Just a thousand billion billion billion billion..

[Ancient_Hacker]

Not new, there have been attempts to do this for at least 35 years. Detecting gravity waves is really hard. Gravity is close to 10^40 times weaker than electromagnetic attraction. So any detector you build has to be like REALLY stable against electromagnetic effects. Even then the predicted sensitivity to gravity is soooo loooow it requires you to hope for neutron starts hitting head on, or other such huge gravity wave generators. Even then, an ant walking by at 100 meters is likely to snafu the data.

My hat's off to anybody in this business, they must have a lot of time and money and patience!

And if the results are negative? What then?

[Ralph Spoilsport]

IANAP, but I like to read books on science, esp. physics, astronomy, cosmology, evolutionary biology, etc. Some people read cheapie novels, I'll read the latest thing from Kaku, Green, Dawkins, Darling, etc. Not that they are necessarily the best books on any given subject at any given time, but for the most part they are fairly accurate. It seems that in string theory gravity is solved in higher dimensions, but the instruments to test that are some time off from development, and so, in terms of testability, we're stuck where we've always been - somewhere between Einstein (relativity) and Bohr (quantum theory). And while everything in terms of matter seems to favour Quantum theory, Relativity is still on top of gravity, as we have yet to find a gravity wave or even a graviton. Therefore, IMHO, we have to come to ask an interesting question: What If Quantum Theory Simply Doesn't Work With Gravity? String Theory might have an explanation, but we're a long way off from being able to test String Theory's ideas about gravity, and (most importantly) a failure of Quantum Physics on Gravity is not necessarily an indication of String Theory's notions. So, if it this test fails (like all the other Gravity Wave Detectors has) when will scientists give up and figure out a new understanding of gravity? This test seems like a good one, so what will happen if it fails? And furthermore, given its expense, how can it be repeatable outside of its own instrumentation? I'm not being a troll - just asking honest questions and trying to get a better conversation in this article beyond a bunch of juvenile carping about spelling errors. RS

Re:IANAP, but...

[EricsProjects]

Time wouldn't have to change. If space stretched, the wavelength would correspondingly change.

Imagine a wave traveling across a lake. What if you were able to stretch the lake? The wave length would be longer.

Now, what if there were two waves traveling across the lake. You wanted to observe the interference pattern of the waves, but since they are BOTH traveling across the same lake, they are both affected the same.

My point being that you can't measure changes in space if the ruler you are using to measure changes also.

To the gravitational wave doubters:

[Anonymous Coward]

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.

Re:Can someone please explain this (dumbed down)?

[drauh]

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.

Re:Can someone please explain this (dumbed down)?

[bloosqr]

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.

I have a question for you, the obvious conclusion from what you say is that it seems to me you lose stable orbits now. That is all gravitational orbits should suffer from the same problem as classical electron-nucleus orbit decay. That is, since any 2 body orbiting system is accelerating its total angular momentum should decay, so I looked this up and in fact not only is this true, that is apparantly exactly what the 1993 nobel prize was given out for. Now I have two questions, if this is true, there should be "derivable" an equivalent of maxwell's equations for gravitational waves using the exact same argument that was used in electrodynamics (i.e. reference frame tricks). Also using the orbital decay analogy, w/ point masses shouldn't there also be a "quantization" due to the gravitational potential ala quantum mechanics. i.e. two hypothetical electroneutral point masses (say neutrons) gravitationally orbiting around each other have a "bohr" radius of ?? at the lowest allowed eigenstate? I'm curious what that number is. Is this below the strong/weak nuclear force length scale?

If they get a result....

[DynaSoar]

...then it behooves them to offer alternative explanations for their results. "In experimental philosophy we are to look upon propositions iffered by general induction from the phenomena as accurately as or very nearly true, notwithstanding any contrary hypotheses that may be imagined, till such time as other phenomena occur, by which they may either be made more accurate or liable to exception." -- Isaac Newton, Rule IV of "Rules of Hypothesizing", in Mathematical Principles of Natural Philosophy (1687).

The experimental design is that of the Michealson-Morley experiments. That hypothesis still stands (ie. they failed to reject the null hypothesis, a very different thing than supporting the alternative hypothesis, and the beast of proving the null hypothesis is imaginary). If they get results, it'll be on them to show the effect is due to something other than that which has so far been unable to be detected but previously theorized and hypothesized as causing the same effect they expect to find.

Still awaiting the technology capable of testing it is the hypothesis that ether flows along the lines of a gravitational field, and so must be tested simultaneously parallel and perpendicular to gravity. Getting a vertical structure big enough but stable enough to do this is far harder than getting two perpendiculars.

Keep in mind that in science "out of favor" and "disproven" are not the same, but in peoples minds they are taken as such. Read "The Golem" by Collins & Pinch for many entertaining examples, including the M/M experiments.