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Testing the Theory of Relativity 169

HD 456 wrote to us with an MSNBC story about the new observatories that are starting to come online for the purpose of gravity wave research. One is located outside New Orleans, the other in Washington State, the facilities will shoot lasers down 2.5 mile tunnels in an attempt to detect changes in gravity caused by black holes. Redundancy in facilities is being used to avoid having false background noise skew results. In addition to garnering more information on black holes, the scientists also hope to collect data concering the origin of matter.
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Testing the Theory of Relativity

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  • I honest to god was speaking to a friend just yesterday about this very issue. I wanted to take a laser, shoot it several miles down a tunnel, and put a large magnet to answer the question "is light matter or energy"? Damn. Guess that's one less experiment for me to worry about. Freaky. Very freaky. Deja vu freaky.

  • Hopefully that Fred Moody ( guy doesn't read the article and freak out again.

  • by Signal 11 ( 7608 ) on Thursday November 11, 1999 @11:54AM (#1541152)
    Contary to the belief of Fred Moody (as pointed out by another reader) - merely observing a black hole will not create one. Only stupid people generate black holes. Fortunately those are kept between their two ears, and they are buried with them. Relax, Fred. Just don't go near stupid people - they have a tendancy to suck.

  • How complex are these gravity waves going to be? To me, it seems that we're going to have to filter out the background noises (the redundancy seems to solve most, but not all, of that) and then try to see if the lasers were changed, and if so, by what and in what manner.

    This seems to lend itself to the kind of distributed effort seen by seti@home - which does a lot of the same things (mainly filtering out background noise & analyzing for signals). On the other hand, it might not be quite so complex. I don't know if I'm giving up on detecting extraterrestrial life from my desktop just yet, but analyzing gravity waves would be a very fun thing to have my computer doing!
  • I'm confused...
    I thought Relativity described gravity in terms of warped space, not particle/waves.
  • the facilities will shot lasers down 2.5 mile tunnels in an attempt to detect changes in gravity caused by black holes

    great... that's the kind of irresponsible science that goes on, eh? Putting black holes in 2.5 mile tunnels? Someone could get hurt!
    ------------------------------------------------ ----------
  • No, you didn't think of it first. This idea has been bandied about for rather some time. At least a decade or so.

    This isn't to say that you didn't come up with it independently. I invented it in high school, and quickly found out it was already proposed. Makes you think they already had all the good ideas eh ?

  • I wanted to take a laser, shoot it several miles down a tunnel, and put a large magnet to answer the question "is light matter or energy"?

    Never mind that these guys are trying to answer a different question, but I am intrigued: how a magnet was going to help you decide whether light is matter or energy? And what would happen if it's both?

  • by osu-neko ( 2604 ) on Thursday November 11, 1999 @11:57AM (#1541158)
    I just love how we're observing things like black holes light-years away by looking at things inside dark tunnels. Kind of reminds of observing solar neutrinoes by looking at things in a dark cave.

    Of course, since everything in the universe interacts with everything else gravitationally, I ought to be able to deduce the superstructure of the universe and the motion of anything and everything in it by observing how my tea leaves settle on the bottom of my cup. With sensitive enough instruments, of course. My grandmother, for instance... :-)


  • Must be... they haven't done it yet, and they're already using the past tense of shoot!

  • There was a tiny mistake in dialect translation during the production of this story, they aren't working on any high-level theoretical physics down in Louisiana, they are working on something much more down to earth:


    We cannot reason ourselves out of our basic irrationality. All we can do is learn the art of being irrational in a reasonable way.
  • by Thorsett ( 5255 ) on Thursday November 11, 1999 @12:11PM (#1541163) Homepage

    Despite the suggestion to the contrary in the MSNBC story, gravitational waves have already been shown to exist. Joe Taylor and Joel Weisberg and their collaborators have demonstrated, using a binary star system, that Einstein's prediction of the rate at which energy is radiated away by accelerating masses is correct to better than one half of one percent. (This was the subject of the 1993 Nobel Prize in Physics [], awarded to Taylor and Hulse.) These results also prove, for example, that gravitational waves travel at the same velocity as electromagnetic waves (ie, the speed of light, or 300,000 km/s).

    LIGO is an exciting project that may open a whole new field of gravitational wave astronomy and directly probe the properties of such exotic objects as black holes and neutron stars. But it will do it using well-established physical principles.

  • There was a subplot about this in one of the Hitchhiker's Guide books... IIRC, it was about a piece of cake which the machine analyzed and found out everything about the universe. However, anyone to whom this information was imparted would immeadiately die, because they could not deal with the fact that their entire life was but "an insignificant little dot on an insignificant little dot."

    On a somewhat related note, I asked my high school phyics teacher (possibly my best teacher throughout my 4 years) about this, and he told me that this had been thought of back when Newton came up with his original laws. Someone back then said something along the lines of "It's all a matter of decimal places." Dunno why, but there is some reason this won't work. Possibly because it yields an infinate number of simultaneous equations which would require an infinate number of infinately accurate, infinately precise data points to reconcile.
  • A few months ago, I can't remember where I read it (maybe at /.), a gravitational experiment was set up to measure the effect that an eclipse had on large pendulums like the one in the Smithsonian. Cameras were placed to detect small variations in the pendulum's drift. As I recall, the very early preliminary data suggested that pendulums within the eclipse zone had variations, whereas those outside the zone did not. Now that would be something if true ! Has anyone heard anything more about this ?
  • by Duke of URL ( 10219 ) on Thursday November 11, 1999 @12:16PM (#1541168)
    Here's a usenet relativity FAQ [] if your interested in some info.
    Here's some info on black holes [] too, while I'm at it.
  • Imagine if two probes (say Deep Space 8 and, er, 8.5) were equipped with lasers, so that they could shot at each other once they were outside major planetary influences. It's the only satisfactory solution (imho), because when you're on a planet there's no way you can say for sure that you've accounted for every microscopic movement. Granted, you've still got planet x out there somewhere, but what the heck. If the experiment was screwed up by the discovery of a new planet (or brown dwarf) I wouldn't complain too much.
  • Call me stupid, but hasn't the theory of Relativity been proven several times over? I remember at least one experiment involving atomic clocks on airplanes...
  • by cybercuzco ( 100904 ) on Thursday November 11, 1999 @12:19PM (#1541171) Homepage Journal
    This is basically a Michalson interferometer on a monster scale. The Michalson interferometer splits up a light source, in this case, a laser, into two beams, sends them down tow tunnels at right angles to each other and then recombines them and projects the result against a screen or detector. This thing is EXTREMELY sensitive to changes in the path lengths. A change in the path length of half a wavelength of the light used is enough to cause an interference pattern at the dtector. The longer the paths used, the more sensitive it is to disturbances. This thing can detect the gravitational pull of a human walking by the end of it. Needless to say they get lots of interference, like the moon, cars, trucks, squirrels etc. Which is why they need a whole bunch of them spread out. Presumably a gravity wave will hit all the detectors at the same time, since gravity moves as fast as it wants to and doesnt have to be held back by the speed of light rule. Anything that doesnt occur at both detectors at the same time is thrown out. Pretty neat really, Einstein was right about everything else, i feel confident he'll be found right once again. Damn Sylvan fissure ::pounds skull::

  • I believe you can filter this kind of noise by having multiple detectors.

    Several detectors 100s of miles apart will get the same signal from a distant black hole, but will get very different signals from local perturbations.
  • It is described in terms of warped space, but the waves this article are talking about are similar to a wave in the ocean (a transverse wave) only the ocean is space/time
  • I don't recall seeing it here, but NASA gave it a mention [] on their rather spiffing web site. If you haven't subscribed to their daily bulletins, do so now.
  • Showing that the energy flux from a particular system corresponds to the theoretical energy loss via gravitational radiation doesn't prove that the waves exist.

    It increases the likelihood that the theoretical predictions are correct, BUT, I believe to really drive the point home we need to observe localised fluctuations in the S/T continuum.

    Of course that's failed miserably so far

    Its good to get excited tho

  • by mattorb ( 109142 ) on Thursday November 11, 1999 @12:25PM (#1541179)
    I think this article is largely missing the point. GR has been amply proven, at least to the extent that LIGO isn't going to add much to the proofs -- gravity waves exist (see earlier post about a Nobel given out for this work).

    A large part of what makes LIGO interesting -- at least to those who believe it will work (see below) -- is the prospect of eventually being able to do some real astronomy with the thing; that is, the idea that particular astrophysical phenomena would send out unique and detectable gravitational wave signatures.

    Kip Thorne, one of the world's bad-asses on this subject, I think talks about LIGO in his book from a few years back, Black Holes and Time Warps. Highly recommended if you're interested in this sort of thing.

    Finally, it's worth pointing out (as the article did not) that there are real questions about the odds of getting useful data out of the thing. Admittedly, I trust Thorne's opinion on this a lot more than most, but there's definitely a pretty narrow zone where a) we'll detect lots of gravitational waves with LIGO and b) we wouldn't have detected them already. (If I'm not mistaken, there have been small-scale versions of LIGO done already.) I wish I could point you to a link on this, but I can't think of anything useful.

    Just my two cents. :-)

  • This kind of measurement can measure gravitational lensing, which shows that light has mass.

    Astronomers first attempted to measure this in an eclipse the year after Einstein published. (1928 or thereabouts). They got results, but it was later found that the measurements they made were smaller than the margin for error.

    I think that the effect is pretty much accepted now :)
  • Science is all about redundancy! That's probably for the better too. I remember reading about the atomic clocks experiment, and it did prove Einstein's theories, but this experiment (and I'm sure the many that will follow) prove Einstein's theories from different perspectives. At least that's how I look at it. :-)

    Mayhap it'll give us a more interesting glimpse of how things work (which is what it's all about, isn't it?

  • Gravitational lensing has been observed many times in astronomy - the canonical example is Einstein's Cross (a quasar split into 4 images by an intervening galaxy). However, the fact that light is bent by gravity does not imply that it has (rest) mass.

  • Sure does.

    But a little bit of theoretical work with a binary system shows that energy can be lost from the system in the form of ripples in Space/Time - our good old fashioned gravity waves.

    By analogy with electromagnetism it was then supposed that these waves take the form of discrete packets of energy - gravitons.

    So if we presume that dear old gravity and quantum mech can be married at some level, its reasonable to assume that gravitons behave both like waves and particles (I recommend books by Richard Feynman if you want to get a good feel for this - and avoid Hawking like the plague, he can't write and he gets it wrong. Its amazing what a bit of media manipulation can do).

    Richard Feynman - the greatest Physicist of the 20th Century.

  • Yep. The Total Perspective Vortex (Adams) and the Laplacian Omniscience Fallacy (Laplace). (I may not have the canonical name of the fallacy, but it's close.)

    IIRC, quantum uncertainty is the bugaboo, and not even infinitely precise measurements and calculations will save you: there's a Gedankenexperiment involving dropping an elastic, symmetrical, smooth sphere--precisely vertically--onto a similar hemisphere in a vacuum. How many times will it bounce before it goes off center?

  • Ah, but gravitational radiation was much harder than gravitational lensing to understand theoretically. Eddington, for example, believed that the waves predicted by Einstein were really just an illusory effect caused by using a non-reasonable coordinate system to analyze the problem. This was a major debate that continued for nearly half a century until Bondi settled the theoretical questions within general relativity -- and then until around 1980 when the pulsar observations provided the experimental confirmation.
  • ecials/total_eclipse/newsid_415000/41527 3.stm

    Is what you are referring to I think.

    V. interesting stuff !
  • by vlax ( 1809 ) on Thursday November 11, 1999 @12:40PM (#1541189)
    Your magnet will have no effect. Photons have no charge, and are uninfluenced by the other photons in the magnetic field. Energy does not exist apart from mass and vacuum has no mass, so it's difficult to say that photons aren't matter. Of course, it's also semantically unimportant to distinguish between matter and energy at all.
  • >Presumably a gravity wave will hit all the >detectors at the same time, since gravity moves >as fast as it wants to and doesnt have to be >held back by the speed of light rule.. NO!!! Gravity moves at the speed of light. The importance of the seperation is that local interference will presumably not be the same at both points, allowing a statistical cancellation of the error effects. (BTW, I left my account logged on in a library computer and some idiot has been doing those second-first-post messages. A -31 karma's pretty bad...maybe I should just cancel this account.)
  • This is the LISA project, which hopes to launch in 2008: see NASA's LISA homepage [].
  • analyzing gravity waves would be a very fun thing to have my computer doing!

    Where else will you find "computer, "analysing gravity waves," and "fun" in the same sentence? :P

    Not that that's a bad thing.
  • Physics isn't my phorté, but I was wondering:

    Can light have "rest" anything?

    It's October 6th. Where's W2K? Over the horizon again, eh?
  • by Hrunting ( 2191 ) on Thursday November 11, 1999 @12:50PM (#1541197) Homepage
    Other principles they will be testing:
    1. The Cat Theory
      This theory states that no matter how hard it tries, the cat will never be able to catch a moving laser.
    2. The Eyeball Theory
      This theory states that no matter how large the warning on the side of the laser, someone will inevitably see what happens when they shine it in their eye.[1]
    3. The Policeman Theory
      Shine a laser through a donut and one can theoretically throw a policeman into a brain lock as they try to defend themselves from the obvious sniper while also try to obtain the donut
    4. The Stupid People Theory
      See number 1, substituting 'stupid person' for 'cat'.
    5. The Nasal Theory
      This theory states that, whoa, dude, when you shine the laser up your nose, it glows like Rudolph, man! *puff*[2]
    6. The Austin Powers Theory
      This theory states that sharks with frickin' laser beams on their heads are more deadly than mutated sea-bass.[3]

    And yes, I want one of these in pen form.

    [1] They go blind
    [2] I in no way condone the use of illegal drugs
    [3] Being conducted in conjunction with the Darwin Society.
  • I guess I should prefix this with "I am not a physicist, but..."

    Consider this quote from the article:

    However, small earthquakes, acoustic noises and laser fluctuations can cause a disturbance at one site that could lead scientists to believe they have detected a gravity wave.

    Now recall that Hanford was one of the nation's big plutonium production sites back in the day. There's still alot of waste around the site. It is also near a river (acoustic noise).

    Certainly these factors are probably much less significant than, say, earthquake-prone California. But if I were tasked with selecting a site, I think I'd prefer somewhere flat and boring. The less disturbances the better. Like I said, though, I'm not a physicist, so perhaps they feel that they can compensate for these factors.

    Best regards,


  • I once had a degree in Physics and some knowledge of the subject. Now all I've got is the degree. Any better informed physicists want to take a shot at this?

    Einstein described gravity in terms of warped space/time, and the motion of particles through it as the shortest possible path through a curved space. His numbers worked (confirmed as early as the 1920's if I recall) so folks mostly bought it. Einstein hoped to express all forces as some form of curve in space, but that didn't work out for him - other forces are selective in their effects, while gravity happens to everybody.

    Now, these days we tend to view things in terms of particles. The reasons are less experimental than logical. We're not talking about tiny billiard balls hurling through space, quantum mechanical particles are a little too weird for that.

    Take the following well established notions:

    1- Energy does not exist apart from a mass (or alternately that mass and energy are the same thing - the two statements are pretty much the same.)

    2- Vacuum has no mass - it isn't a medium that can carry energy.

    3- Gravity, like the other three forces, transports energy.

    Given these three, we pretty much must conclude that gravity is transported by particles. It might be a particle which, like the photon, has zero rest mass, but if energy is moving through empty space, a particle, by definition, must be carrying it.

    Of course, saying so doesn't answer any questions at all. Why should the action of gravity particles distort space/time? If you have a good answer and experimental data to back it up, the Nobel committee has a sizeable cash prize waiting for you.
  • by phil reed ( 626 ) on Thursday November 11, 1999 @12:57PM (#1541201) Homepage
    What, prey tell, is the medium which allows these waves to propergate?

    The structure of space-time itself. It's not the ether, since electromagnetism propagation is different.


  • No, the gravity waves from a passing bus or 747 are actually very small, even by LIGO's standards. However, the shaking of the earth due to seismic activity or a bus (passing very close to a detector) will be detectable. Fortunately, it's easy to model a passing bus, seismic activity can be characterized, and there is a small but significant range in which gravity waves from coallscing compact objects could be detected.
  • Actually, there are "second order" effects which allow one high energy photons to interact (indirectly) with each other. But this is a very small effect.
  • by vlax ( 1809 )
    I did neglect to consider things like quantum interference, which in principle would make a difference. It would be hard to measure though unless the experiment was designed to look for it.
  • Can light have "rest" anything?
    Relativity tells us that if you're travelling at c, then time stops (for you). Therefore you're everywhere in the universe at the same time. Therefore if you go from a to be, you don't move. Therefore you're at rest.
    I rest my case.
    Anyway, when was the last time you saw a moving photon?
  • by laertes ( 4218 ) on Thursday November 11, 1999 @01:09PM (#1541208) Homepage

    As far as I recall, general relativity defines gravity as the curvature of space-time. Gravity waves are the result of changes in the curvature of space time. Gravity waves are theorised have these properties:

    Gravity waves will be accompanied by gravitons, a hypothetical particle that has zero rest mass and twice the spin of a photon.

    Gravity waves and gravitons propagate outward at the speed of light.

    Gravity waves compress mass in one direction perpendicular to the direction they travel, and expand it in a direction perpendicular to both the direction of compression and direction of travel.

    Gravity waves are moving ripples in space-time.

    Black holes coliding make big gravity waves.

    Gravity waves pass through matter.

    This experiment it is trying to get empirical evidence on all of the above claims. This has been a goal of some physicists since the theory was proposed in 1916. However, this goal has previously been beyond experimenters technological reach. It takes today's most sophisticated lasers and detectors to isolate a gravity wave from far away. Any local vibrations reaching either the lasers (like noise, or earthquakes) or the detectors will be easily confused with gravity waves. However, the mass of nearby objects does not interfere, just the vibrations they produce.

    As for the design of the installation: it is in the shape of an L, because (as I mentioned before) gravity waves both compress and expand matter as they pass through it. On laser moves faster, and the other slower. This is different from a Michalson inferometer, which checks if normal gravity (that is: curvature of space-time) bends light. A Michalson inferometer isn't used to determine the nature of gravity waves.

    Gravity affects all of the universe simultaneously (although it doesn't affect it much, it does affect it). Gravity waves are held back by the speed of light limit though. So, the two installations would get waves at different times, depending on the orientation of the earth to the event.

    Of course, this is all conjecture, and that's why we US taxpayers get this installation. If this had already been proven, we wouldn't need these two new observatories.

    PS: Check out the observatories homepage [] for more info!

  • A gravity wave refers to a "packet" of basically periodic warping of space. The warping of space affects the "distance" which the photons must travel before reaching the detectors. Thus, the phase of the photons will be different if there is a significant warping of space time along the path of the photons. The experiment will measure relative phase shifts of the photons to detect the gravity waves.
  • It's a nice idea but with one little flaw.

    They use laser-interferometers. To build one of those you need *two* laserbeams with exactly the same length and at exactly 90 degrees from eachother to form an L shape.
    Actually, it's one beam that's reflected down two tubes...

    What you do is that you messure the difference in lenght that occurs when a gravitywave passes.
    The wave will make the space "compress" in the direction it is travelling.

    Using three probes, one for every point in the L , it would be very hard to position them and keep them in place with enough accuracy.

    You could, of course, build two rigs in space which then could be a lot larger than the 4 kilometer ones they use for Ligo.
    (Longer = higher sensitivity)

    You would still need two to filter out satelites and instrumental errors.

  • These observations are indirect in the sense, that if you beleive GR, then the increased binding energy of the binary implies gravity waves. However, if you don't beleive GR, then the inference that there are gravity waves does not stand up. Since multiple different theories (mostly variations on GR) can accurately explain all previously observed gravitational phenomena, it makes sense to do new experiments trying to distinguish between the theories.
  • >The Eyeball Theory
    >This theory states that no matter how large the
    >warning on the side of the laser, someone will
    >inevitably see what happens when they shine it
    >in their eye.[1]

    This just reminds me of the old joke disclaimer..

    "Do not shine laser into remaining eye"


  • There are many different predictions of general relativity. That's one reason it's so nice. A relatively simples model makes lots of predictions. Many of these predictions have been verified. Others have not.

    Also, several different theories (mostly GR variations) can explain nearly all observed gravitational phenomena. Therefore doing new experiments may allow scientists to distinguish these theories.
  • Indeed. Tell your congresspeople to support LISA, a similar experiment, but to be done in space. The improved sensitivy (mainly from being off the earth) will allow many more scientific discoveries and very interesting astrophysics.
  • []

    Caltech's Press Release []

    I don't know why their writing articles on this now. LIGO has been in the works for quite a while now.

    Just to give you an idea of how sensitive these things are: my roommate spent the summer working on calculating shifts in the earth's crust caused by the sun and moon, so that the correction factors can be applied. If I remember correctly, at one point Kip Thorne (the Feynman Professor of Physics here at Caltech) was working on correcting for the change in Earth's momentum caused by raindrops hitting the surface.


  • Gravity waves move at the speed of light just like excitations of any other "massless field" (The field is the local geometry of spacetime itself). This "speed limit" is in fact necessary for the theory to be relativistic in the first place (i.e. not having a preferred frame of reference). The principle of (special) relativity o.t.o.h. is really well established, if gravity would violate it, we would most probably have seen the effects already (for example, in the original Michelson experiment which failed to find any velocity of the Earth w.r.t. the so-called "aether", which would have been a preferred frame, at any time of the year).

    Since gravity waves are a feature of any field theory of gravitation, such as GR, they had better be found or not only would GR be ruled out but also any theory of gravity based on our understanding of classical and quantum field theory. Although for LIGO to detect it, the waves will have to be pretty strong (some not-too far supernova for example might do). Next-generation experiments are expected to be able to detect waves that originate from more common sources such as close binaries (mentioned in an earlier post).

    On a more fundamental note, you never prove a scientific theory, you only (try to) disprove (falsify) it. In Einstein's words, "no amount of experimentation can ever prove me right, but a single experiment can prove me wrong."
  • Sorry, but you can't _prove_ scientific theories, only mathematical theorems.

    You can gain evidence that the theory is right and increase your confidence in it though.

    It only takes ONE REPEATABLE experiment, though, to refute a scientific theory, so that's reason enough to keep testing a theory's predictions.
  • But that only deal with Einstein's theories about time/space vs. speed.
    The prediction of gravitywaves has, as stated in an earlier post, been proven by measuring the energyloss caused by gravitywave radiation in binary star systems.
    But noone has yet detected *an actual gravitywave*!
    Only by second hand observation.
    And, of course, a new kind of instrument to look into space with is never wrong.
    Look at what the telescope, then the radio-telescope and later x-ray/gamma ray/infra-wave detectors have meant for astronomy and our understanding of the univers!
    The introduction of gravitywave observatories will most probably lead to so far unknown discoveries.

  • Well, your point of view is one reason to do the experiment. A small minority of scientists still don't beleive in gravity waves. Therefore we look and see who is right. Michelson-Morley had much lower sensitivities.

    However, a humougous majority of scientists beleive that space time itself is being distorted in a quaesi periodic way and that this is at least theoretically observable.

  • Actually gravity isnt bounded by the speed of light because it is a phenomenon that is outside of space. Its really just the bending of spacetime itself and as such, is not limited to the speed of light. A gravity wave is a wave in spacetime where as light is a wave that travles through spacetime. A comparison would be waves in water vs waves in air. A wave in water travels much slower than a wave in air ( sound) because they are in different media, the same is true for gravity, its in a different media than light, so it obeys different speed limits

  • The whole reason why we talk about forces as being particles is that particles can move through a vacuum. Electromagnetism works across a vacuum because the force is carried by a particle: the photon. Radio waves do not need a medium to travel in because they are composed of photons.

    "Gravitons" are what gravity particles are called. In principle, it works the same as radio, except with a different particle. Of course, there are important (and unexplained) differences between radio and gravity - that's what keeps physicists employed.
  • Youre right of course, i havent been keeping up on the latest research, but on furter examination I read that indeed gravity does move at the speed of light. The implications of this are disheartening for faster than light travel, which were mostly based on using gravity to move a peice of space at huge velocities. If gravity can only move at lightspeed, it will be much harder to get around the light limit. grrr guess my warp drive will have to wait

  • Despite the suggestion to the contrary in the MSNBC story, gravitational waves have already been shown to exist.

    That's all well and good (actually, it was a very informative post), except that there was no such suggestion in the article.

    Here's what it actually said: "There is evidence that the waves exist, but technology has not been powerful enough to detect them." And that's exactly true: we have evidence that they "must" exist--because we see energy radiated away from a binary star system at the predicted rate--but we have not actually directly detected them yet, which this experiment is designed to do.

    On the other hand, I agree that the article didn't do too much to clear up the point.
  • If I recall correctly, the point of LIGO is not to prove "existance" (as pointed out, this has already been done), but rather to determine if the laser technique will work. The goal of LIGO is to create a "network" of these laser based gravity wave detectors in order to infer direction of the gravity waves... (There was a PBS show called "The Astronomers" awhile back that described this technique in pretty good detail. Heh... I remember more from a PBS show that I do from getting a BS in Astronomy. That's probably a bad thing...)
  • by kwijebo ( 38707 ) on Thursday November 11, 1999 @01:41PM (#1541227)
    You're right that LIGO is basically an enormous Michalson interferometer, but in order to increase its apparent length it measures the superposition of a laser beam that has bounced back and forth across its length many times. As you say, the longer the path used, the more sensitivity to displacements in length it has.

    But it is NOT true that LIGO can detect the gravitational pull of a human walking by the end of it. A litte background: gravitational waves are created by changes in the gravitational quadrupole moment of a system. Electromagnetic waves, in contrast, are created by changes in the electrical dipole moment of a system.

    Getting changes in the quadrupole moment of a system requires doing something funky with the angular momentum of the system. That can happen when black holes collide (they radiate a bunch of their angular momentum away), and I don't know when else it might happen. But it definitely DOESN'T happen (on any reasonable scale) in any terrestrial processes, or any that we know of in our solar system.

    It is true that a passing truck, or even a passing person, can cause mechanical vibrations that will affect LIGO's measurements. LIGO is an incredibly accurate system for measuring differences in distances between its perpendicular lengths. Gravitational waves should, according to GR, cause a change of length in one axis and not the other. Mechanical vibrations will also cause a difference in position for the mirrors, so LIGO has a VERY complicated system of mechanical dampers to minimize this effect.

    Finally, the last time I checked, most GR experts were of the opinion that gravity waves propagate at the speed of light, not at infinite speed.
  • This isn't a comment so much as a question -- I'm just curious as to what phenomena/theories you're thinking of. I couldn't think of something offhand which would a) predict all previously observed stuff and b) not imply some sort of wave propagation equivalent. Ie, it's well known (and, given the nick, you're probably aware :-)) that no scalar theory of gravity (eg, Nordstrom metric) can accurately predict gravitational deflection of light; I'm just not clear on what well-posed alternatives exist. Then again, I don't know GR that well. :-)
  • As you pointed out, quantum interference might deflect the beam. Another thing might: the energy of the magnetic field will create a gravitational field that would bend the light. Stupid question: which effect is the greater (assume a 1kW argon laser and a 10 tesla magnetic field in a volume of one cubic meter - you have four hours to give a detailed answer :-)
  • I thought the speed of sound in water was higher than in air, rather than lower. However, you and I might be talking about two different things as aquatic surface waves do travel much slower than sound in air. This makes me wander what speed air waves travel at the interface between air and vacuum (though I know there is no real interface as such, too much of a gradient)
  • Ermm. Just for future reference, you should know the difference. Pork belly = bacon. Pork Barrel = public works projects given to a congressmans district. Pork belly = delicious for all except the Jewish, the Muslim and the vegetarian. Pork Barrel = great for those who get paid for it, slightly irritating for those who don't.

    pork belly []

    pork barrel []

    we clear?

  • How long would it take for one of those mini-lasers to make someone go blind? Because I know of a lot of stupid people who have them.
  • Presumably a gravity wave will hit all the detectors at the same time, since gravity moves as fast as it wants to and doesnt have to be held back by the speed of light rule

    Actually, gravity waves do travel at the speed of light, at least according to the theory of general relativity. Of course, this is subject to experimental verification, which is one of the reasons why these gravity wave detectors are being built.

    What's really amazing about this project is the fact that they're going to be looking for an extraordinarily weak signal in a bunch of background noise. So how do they know that what they're looking at is the real thing? Well, that's one of the main reasons for having multiple detectors. With the sites in both Washington and Louisiana, they'll be able to look at coinicdences between the signals to determine if the event was a gravity wave or simply some local fluctations due to seismic activity or fluctuations in the laser or one of the other countless things that might give rise to a false signal. I used to be skeptical that these things would be able to detect anything at all (except noise), given that the amplitude of the waves they're trying to detect is smaller than the size of an atom! But I now know (due to a class that I'm taking that's being taught by Kip Thorne) that a lot of thought has been put into all of this and I'm fairly confident that we'll see gravitational waves with LIGO. For more information on the project, check out the Caltech LIGO website [].
  • Well don't throw in the towel so quickly :-)

    Actually it might be possible to move effectively faster than light WITHIN general relativity.

    One idea is wormholes but I believe it is not clear, or rather, doubtful, that wormholes can be generated and made stable at all.

    Anyway, the speed limit is a local thing: You are not allowed to move faster than a light beam next to you, but actually if you can warp the spacetime in a clever way you could possibly travel arbitrarily fast, as seen from far away (although a light ray coming in from the behind would be even faster!)

    I remember a physicist, M. Alcubierre, describing such a "clever space-time geometry" but unfortunately I can't locate the paper on the web anymore. It's published in Class. Quant. Grav. though, in case you have access to a physics library... That paper was serious, the best thing being that the distortion needed is PURELY LOCAL, just a few meters around the "USS Enterprise". The problem was that in front of the spaceship you need a negative energy density. This is frowned upon in classical physics but possible in principle in quantum theory.

    You can also search (aka, the physics preprint archive, for the word "warp" or "warp drive"; you'll be surprised about the number of titles showing up.
  • All the way to my quantum mechanics final exam.

    I can't even remember where to begin to calculate the photon density in the field. If I did, I might be able to work that half out (although I'd probably get the wrong answer - seem to recall not doing that well on my quantum final either.)

    The interference from a field that's parallel to the beam sure as hell won't be much. Of course at the edges it won't be parallel... I think I'll need some extra paper.

    Thank God I quit physics. :^)
  • Alcubierre's homepage ( k/pub/Miguel.Alcubierre/index.html []) has a broken link to the paper itself. I can't find another copy.

    The New Scientist has an article about it here [].
  • GR is a non-linear theory. Most often the equations are linearized when discussing gravity waves. (It has been shown that gravity waves exist in the full non-linear equations, but that was a significant accomplishment.)

    Any theory which when linearized gives the same equations will "predict" gravitational waves. (I don't know if any alternatives have been shown to support gravity waves in their non-linear equations.) However, different theories would predict different this in the non-linear regime, presumablely where these gravity waves are being created. Studying the waveforms may allow you to gain some insight about the non-linearities in the equations, thus supporting one theory or another.

    This what I was refering to. I don't know of a (possiblely valid) theory that works and doesn't imply some sort of gravity waves. But I'm no expert.
  • The warp paper mentioned in my other post is at []

    Also there is a news article mentioning some of the ideas at
    [] _364000/364496.stm

  • One thing I find interesting by this fact, is that since gravitational waves travel at the speed of light, space therefore has an elasticity.

    For example, if a huge black hole were to open up where the sun is, and engulf it, the Earth wouldn't feel the effects of it for 7 minutes (I think its 7 minutes at c to the Sun?). I find that pretty cool.
  • guess you're too smart for us. what with our 'research' and all. we'll just let you take the helm now.
  • by QuantumET ( 54936 ) on Thursday November 11, 1999 @02:39PM (#1541251)
    For people interested in tests of general relativity...

    Gravity Probe B is a satellite that will be launched in a few years' time. It plans to check for one untested prediction of general relativity (the frame-dragging effect of massive spinning objects like the earth) by placing several hyperaccurate gyroscopes in orbit and measuring the change in the rotation axis of those gyroscopes from this effect.

    It's been under works for 30 years now... here's the website [] for the project.

    The whole system has to be incredibly accurate... I worked with this over the summer, and the technical details are scary (for example, the gyros used are the smoothest spheres ever made by man... if they were expanded to the size of the earth, the greatest height difference between valleys and peaks would be about 16 ft)
  • Thy're going to take an object and multiply it by the square of the speed of light to get some energy?

    Jay (=
  • Well, while you may want your congresspeople to support LISA, that doesn't mean that doing the project on Earth is a bad idea. I talked to one of the guys involved in this project a few years back and he said that while space-based projects aren't susceptible to as many noise problems, they're massively more expensive than ground-based strategies because

    1. You have to GET the thing into space, which is really frickin' expensive.
    2. You have to make sure the thing is space-proof: ie, it won't die due to the extreme cold or radiation, etc., which is also really frickin' expensive.
    3. As if that weren't enough, if the thing breaks or just doesn't work exactly as expected (these things have to be precise), you can't just go in and fix it- you have to launch a mission or build a new one. Which is, as you can guess, really really frickin' expensive.

    Comparatively, swampland is a GREAT place to put one of these things...
  • Yep, I have that book on hand. Good read. To spell out why LIGO will probably work better, it's all in the redundancy. Eventually there will be as many as 8 (?) different instruments at 4 sites (2 at each) to guarantee that it isn't background noise. Because of quantum physics and all that jazz, the wires supporting the detectors creak, even when perfectly isolated. Every part of it will actually make noise. With one detector, it's likely you'll see noise, with two, very unlikely both will creak at the exact same time. With 8, it's almost guaranteed that you'll get good data, not to mention the sites need to be spread out to triangulate. The farther they are appart, the better data you get, since gravitational waves have massive wavelengths. They also have to be perfectly synchronized to the same clock, otherwise the information is worthless. Personally, (I didn't read the article, but I understand the concept from said book) I think the data collected if any will be very valuable and greatly increase our understanding of the universe. With each step forward, from light to infrared to x-rays, we learn astounding new things that revolutionize our concept of the universe and our place in it. This should be no different.
  • I have always wondered what people who say things like this must think of scientists. Is the idea that at the research labs, PhD's sit around the bong taking hits and saying things like, "Whoa... dude... what if the universe, like, is the result of this huge explosion where there were all these particles and anti-particles and stuff? That would be awesome. Somebody start writing: I feel a paper coming on."

    Differing scientific opinion, based on facts, is certainly incredibly important to scientific reasoning. Differing scientific opinion in the absence of facts is just annoying.
  • There have been plans to do this for a while, with a group of satelites in orbit around the sun. The acronym for it is LISA. The big advantage is that the arms of the interferometer can be a lot longer, which is the simplest way to make it more sensitive. Here [] is the first page google threw back at me.

    And you thought $3.5e8 was a lot of money...

    The big risk with building gravity wave detectors is if they don't detect anything. This would be a really surprising result if it were true, so no one would believe it unless more experiments with bigger instruments were done to confirm it. But getting money to repeat an experiment which didn't work the first time would be next to impossible, so everyone could be left in a frustrating situation.

  • In general using space probes would be exceedingly difficult as you don't have nearly the control that you do when you're in an earth-based observatory. The way LIGO works is by detecting very small vibrations in very delicate suspended masses. Laser interferometry is the only way to get measurements on small enough distances to detect gravitational waves. We're not actually trying to measure the deflection of the beams, but rather the motion of these masses. There's an absurd amount of calibration and fine-tuning going on that's difficult to replicate in a space probe that's going to be launched off.

    In fact, though, there are plans to set up a series of probes in space as part of LIGO to check for lower-frequency waves than the ground-based observatories will detect. I believe the first of the probes is supposed to be launched within a decade or so, but I can't recall all the details.
  • You slam "these scientists" for thinking "too small," yet here you are making a world of half-witted judgements. But the irony is likely lost on you.

    "START" reads "the first observable event";
    "END" reads "the last observable event."

    There are no assumptions about what comes "before" and "after," or what this even means in a human sense. It is undefined because we cannot (yet) know. This is very different from START being the absolute beginning of all things.

    Physicists are (surprisingly?) flexible about the nature of the universe..
  • Well, do not take me as an antipopular-opinion-everything. My opinion is entirely just that -- opinion. I have no facts about what I say, but feeling I do.

    And I do believe that the Big Band theory is correct; it is simply not whole. The process earth inside of the shell has defined the shell as being the universe -- completely without understanding and beyond the fact that the platform exists, sitting on someone's desk, inside of a corporate network on some strange world.

    Gosh, I hope we are on something like LINUX or Open Source. I would be really embarrassed if we were being run on a Windows system. I am getting nervous about a universal abend here.

    Damn it Jesse, stop thinking!

  • Ummm... in short, no.

    If you want to believe special relativity, EVERYTHING moves at most at the speed of light. Everything. Period. Gravity can indeed be characterized as bending or warping spacetime, but that doesn't exempt it at all. Information, in the form of gravity waves or whatever, cannot travel faster than the speed of light; if you allow any sort of transmission of information or energy (yes,gravity waves carry energy) faster than the speed of light you get yourself into really nasty paradoxes.

    The idea is that gravity waves coming from far away will look like the same signal everywhere just delayed a bit, whereas interference from close by will only appear on a single detector or can else be triangulated to a nearby location. Hence we can filter out unwanted signals.
  • The universe does this for us... We can observe the bending of light around super-heavy objects (black holes) already... no need for magnets and lasers...

    as far as I know at least
  • Dunno why, but there is some reason this won't work.

    Three things:

    One, lack of information. This includes both lack of knowledge about the laws of physics, and the inability to gather the necessary physical data, both because there is so much of it and because it would require observation at the sub-atomic level.

    Two, as others have pointed out, you run into problems with the fact that the process of measurement affects the measurement. This encompasses both Heisenburg uncertainty, and it also includes the fact that the computer that holds this information would have to be included as part of the universe. Since it is not possible to make a computer that is "outside" the universe, you cannot model the whole universe without encountering a problem of infinite recursion, sort of like telling a computer to back up the contents of an entire hard drive onto that hard drive. Once it gets to the backup file, it will go into an infinite loop.

    Third, you run into problems of chaos theory and spontaneous order. The problem is that a small error in measurement can lead to an enourmous change in the outcome. This is one reason why there is a limit to the accuracy of weather forecasting, for example. No matter how accurate the measurements we make, there will still be a small error, and that error will become larger as you try to predict farther out into the future. So if you make a small error in one measurement, that small error can cause all of your predictions to be off.

  • by Wah ( 30840 )
    it'll give us a chance to escape...
  • There is a proposal to do just what you describe. It's called LISA, short for Laser Space Interferometer Antenna. ESA has a LISA web page [] that gives all the gory details. Note that the big win from going into space is not getting away from the planet's gravity, as you might guess at first, but getting away from mechanical vibrations induced by being in contact with the ground. Gravitational wave disturbances produced by movements within the earth are negligible, even to a device with the LIGO's extreme sensitivity.

    As you might imagine, the major obstacle to LISA is the expense of the thing. I suspect that the success (or lack thereof) of LIGO will have a big impact on whether LISA ever gets funded. However, all the signals that we know are out there (coalescing neutron stars and the like) are likely to be too weak to detect with LIGO I (only the most optimistic estimates give an appreciable event rate with LIGO I). If LIGO II gets funded, then it will almost certainly see some events, and that could renew interest in LISA. LIGO II isn't scheduled to begin installation until 2004 (assuming it gets funded at all), so I expect we won't see a space-based gravitational wave observatory until sometime after then.


  • Seriously, though....

    If they were to find that there were some frame-dragging effects, wouldn't this throw a humongous monkey wrench into the whole 'there is no aether' thing?

    The way I was taught, *There was no "aether"*

    To me, any indication of reference frame dragging would indicate otherwise. Maybe not *exactly* the same thing that the MM experiement "disproves", but still.....

  • by wass ( 72082 )
    Hahaha, thanks for the link, I got a real kick out of reading that article.

    If he's so worried about us pesky humans playing God, why is he publishing on an online medium? I mean, weren't we playing god 100 years ago when Faraday and Maxwell were working out the basics of E&M, and Shockley/Bardeen/others again with solid state research to bring us the transistor, which directly leads to the computer on his desk.

    In fact, metallargusts of antiquity were playing God as well, so we shouldn't use metals, lest we upset the delicate balance of the universe. Wait, he does denounce the combustion engine. I guess this implies he walks/bikes/etc wherever he goes, so as not to upset the ecosystem.

    He seems to be the only clear-thinker among us, if it weren't for him I'm sure we'd all be up a paddle.

  • by wass ( 72082 )
    we'd all be up a paddle

    Haha, i shouldn't be so hasty to submit my words. Of course, I meant to say, "We'd be all up a certain river without a paddle"

  • ...the technical details are scary...

    Indeed. Gravitational effects tend to be miniscule, so any noise in the system tends to kill you. Only by filtering out virtually everything can you hope to see the signal from gravitational physics. To give an example from LIGO, the displacements of the test masses in LIGO are roughly 10^-16 cm, or about 10^-8 of the diameter of a hydrogen atom. An interesting bit of trivia Kip Thorne mentioned in one talk he gave was that the test masses behave classically in LIGO I, but by the time LIGO II comes along quantum mechanical effects will start to become measurable. That means that if LIGO II comes to pass, it will be the first time that the quantum mechanical behavior of a macroscopic object has been observed directly. Amazing stuff indeed.


  • The Austin Powers Theory
    This theory states that sharks with frickin' laser beams on their heads are more
    deadly than mutated sea-bass.

    I assume you meant to say "laser" beams, right?

  • lack of information. This includes both lack of knowledge about the laws of physics, and the inability to gather the necessary physical data

    This experiment is there to increase our knowledge of the laws of physics. With your reasoning, all experiments would be pointless. Either we already know what the experiment could possible tell us, or we don't, in which case we can't do the experiment.

    you run into problems with the fact that the process of measurement affects the measurement

    Yes, and if I stand on a scale, that will influence my mass as well. That doesn't mean that the measurement is pointless. The conclusion of the Heisenburg principle isn't "all measurements are pointless". The conclusion is "there is a limit on what you can measure simultaneously".

    you run into problems of chaos theory and spontaneous order. The problem is that a small error in measurement can lead to an enourmous change in the outcome.

    Yes, and that's true for a lot of problems. But you can also calculate how much errors in your measurement influence the final result. Weather forecasting is a perfect example. There's a limit, but todays limit is further than yesterdays, due to the ability to use more data, and better precision. Nowadays, you see 5 day forecasts. I certainly remember the times you got only 3 days, and the forecast for the third day was much less detailed than you get nowadays for 5 days.

    Limits are nothing more than limits. As long as you know what your limits are, you can still do a lot. Limits don't mean "all measurements are pointless". Limits also chance over time.

    -- Abigail

  • vacuum has no mass - it isn't a medium that can carry energy.

    Ummm, well how about the Voyager spacecraft travelling through space? They have mass, E=mc^2, so they thus have energy, and they surely appear to be 'carried' through this vacuum medium. Of course, one can claim that space isn't a pure vacuum but has a few hydrogen particles per cubic meter, but then again that is what light (and gravity) is travelling through from the sun anyway.

    I think you may have meant to accord with the notion that space (vacuum) has no ether, thus the logical way of explaining energy being transferred through it is as a particle, because what can the E&M or gravity waves oscillate to carry them forward?

    Sorry if I'm misinterpreting your point. It just strikes me as a not-well-understood notion (from my point of view, that is). I cannot wait to get to grad school and do E&M again (as well as GR and QM)...

  • I'm kind of surprised that they left out the whole reason for producing LIGO. Its primary purpose is not to prove that gravitational waves exist. We're pretty darn sure of that already. The stated goal of LIGO is to devise a way to make a gravitational wave "telescope". The idea is that you can cross index electromagnetic phemonema with gravitational waves and build up a database of types of celestial events, eventually to the point where you can tell what was happening on the other side of the galaxy without searching for electromagnetic signs. Because gravitational waves can pass through objects relatively uneffected, they have the potential to make observations of phenomena that are farther than our telescopes can reach or that are interfered with by other events. Check out to get it straight from the horse's mouth.
  • I didn't mean to imply LIGO was a bad idea. I'm glad LIGO is being funded, and hope LISA will be funded similarly. I'm not involved with either, but I've heard people who are say the pricetag on LISA might actually be less than for LIGO. I suspect that is due in large part to the fact that LIGO has already developed lots of the technology they would need for LISA. Personally, I'm skepitcal that LISA would be as cheap as advertised, but the scientific possibilities more than make up for that.
  • by shogun ( 657 )
    Well not much really, since you also would not notice that anything has happened for those 7 or 8 minutes...
  • I saw a talk, given by a prominent astrophysicist at Harvard, at university last year concerning the event rates for the current and forthcoming generations of LIGO. Apparently, for this incarnation of LIGO, with the sensitivity of the electronics available, there will be approximately 2 events recorded over the life of the detector. Yes, two events, ever. This takes into account the probability that a sufficiently massive gravitational interaction (two black holes ramming into each other, or the inspiral of two neutron stars) occurs sufficiently close to the Earth that we can detect it.

    Y'all should know, though, that the next generations of LIGO are already "in the slot", and they promise to increase the event rates quite a bit. The whole trick is to isolate the mirrors as much as possible from disturbances that aren't gravity waves. Fortunately, there are very clever people working on the problem as we speak.

    BTW...perhaps the best isolation would be to shoot the whole Michelson interferometer mess up into orbit. I'm pretty sure that some guys at NASA are working on this too. It's worthwhile to check out the LIGO page. Set

The only function of economic forecasting is to make astrology look respectable. -- John Kenneth Galbraith