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Lab Tuned to Gravity's 'Ripples' 173 173

Krishna Dagli writes "One of the great scientific experiments of our age is now fully underway. Success would confirm fundamental physical theories and open a new window on the Universe, enabling scientists to probe the moment of creation itself. The experiment is trying to detect ripples created in the fabric of space-time that sweep out from merging black holes or exploding stars and detection would be a final test of Albert Einstein's General Theory of Relativity. "
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Lab Tuned to Gravity's 'Ripples'

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  • negative outcomes? (Score:3, Interesting)

    by m874t232 (973431) on Monday June 26, 2006 @08:59AM (#15604834)
    What are the alternative models if gravity waves simply don't exist?

    It's important to have alternative hypotheses, among other reasons, in order to be able to determine when you got a null result. Until the theoreticians have done their homework and provided a reasonable and plausible alternative hypothesis, perhaps we shouldn't be investing millions of dollars (euros) in these kinds of experiments.
    • by Anonymous Coward
      Of course we should be investing in this technology. Even if it does cost us millions, nay even billions. Gravity is the single most important universal effect, and the sooner we know what it is, and how to manipulate it the better. A few billion upfront for that sort of tech is peanuts.

      If the research doesn't pan out, then we will move on and create new hypothesis. You can only throw so much money at one thing at a time. If we fail, we redo.

      just imagine the possibilities. Flying car anyone?

      Baz
    • by mwvdlee (775178) on Monday June 26, 2006 @09:06AM (#15604883) Homepage
      Plausible alternative hypotheses are nice to have, but shouldn't be a requirement for the simple reason that there might not be any plausible alternatives. Or at least none concievable with current knowledge, thus further necessitating the gathering of scientific proof as it can show whether you're missing some vital piece of knowledge.
      • Your argument is predicated on the assumption that we learn something from this experiment, but I don't think we do.

        If the outcome is positive, it just confirms all existing theories (but likely won't be compelling enough to do so beyond reasonable doubt), and if the outcome is negative, we simply assume that the detection threshold wasn't good enough.

        So, I agree that confirmatory experiments are important, but this one just doesn't seem to be a good one.
        • by Open_The_Box (620252) on Monday June 26, 2006 @01:03PM (#15606579)
          OK. I wasn't going to get involved in this thread, but I really have to jump on that one.

          It's not just about confirming Einstein's theory of general relativity. Or, in fact any of the other relativistic gravitational theories - most (if not all - been a while since I checked on the basic theory and they might have come up with some new ones) of which require the existence of gravitational waves. It's not simply a case of checking that the theory is correct - there are indirect measurements which have already done this, it's about directly detecting something we're sure is there. Don't get me wrong; in part, you're correct - if the outcome is negative, then we can set an upper limit (i.e. the waves must be of lower magnitude than X at frequency Y). This in itself allows corroboration with cosmological models and provides a valuable experimental check against predictions of numerical relativity such as the strain effect on space due to the merger of black holes.

          But when a positive detection is made it will provide confirmation/empirical data on the processes involved in such violent astronomical phenomena. What are the physical processes involved in the inspiral of a binary system? Do pulsars with asymmetrical mass distribution really lose energy as gravitational waves? We know about the cosmic microwave background, what about the gravitational wave stochastic background?

          It's not just a case of "There's a peak on the trace! Well, that's our job done! Who's for tea and biscuits?" The potential gains in knowledge of astronomy, astrophysics and even particle physics are vast. Not to mention the gains in laser technologies, control systems, material science and computational analysis that such a project brings. Just by designing and building these instruments we push the boundaries of what's known. Of course there will still be tea and biscuits (well, maybe beer and doughnuts) but that's half the fun right there.

          OK. Rant over. Everyone back on your heads.
          • Not to mention the gains in laser technologies, control systems, material science and computational analysis that such a project brings.

            Those gains would be even greater if we invested directly in those areas.

            if the outcome is negative, then we can set an upper limit (i.e. the waves must be of lower magnitude than X at frequency Y)

            We have had half a dozen experiments trying to detect gravity waves, all with negative or indeterminate outcomes. And I note that neither you nor anybody else in this thread has
            • But we do invest in these areas. How do you think you get a gravitational wave detector in the first place? They don't build themselves you know. And aside from this, you need to have reasons to investigate (and therefore invest) in these technologies - this is an example of a large scale project which has the potential for practical and tangible gains in (as I posted before) laser technologies, control systems, material science and computational anaylsis. These are tangible scientific results in their own
              • And aside from this, you need to have reasons to investigate (and therefore invest) in these technologies - this is an example of a large scale project which has the potential for practical and tangible gains in (as I posted before) laser technologies, control systems, material science and computational anaylsis. These are tangible scientific results in their own right with several industrial applications and assorted spin-off tech companies.

                To the degree that the spin-off applications are valuable, the spi
                • ...have a clear, predictable, and demonstrable scientific benefit no matter what the outcome of the experiment.

                  Experimentation is as much about what we don't expect as it is about finding predicted results. The discovery of penicillin is the obvious example of this. In the long run, selecting scientific experiments for economic value over scientific value would destroy the pursuit of science. Investing only in "safe" and "predictable" outcomes means only reaping small, predictable returns.

                  Sounds like g

                • To the degree that the spin-off applications are valuable, the spin-off applications themselves will drive the development of the technologies, which can then (in a few decades) be used to conduct the physics experiments at a much lower cost. If the potential spin-offs don't justify investment in the technologies, then your justification that these are economically valuable is bogus.

                  Hmmn. I take your point. I really do. But I have to stress my earlier point - the spin-offs will never be started without

                  • I said that doing big projects is what research is all about.

                    Yes, and I think that's a very narrow view. Large scale experiments are almost the exclusive domain of a small subset of experimental physicists (and, very recently, a few biologists). Prior to the bomb, even most physics experiments were bench-top, but the cold war gave a small subset of physicists a lot of power to obtain large amounts of funding. Other disciplines really have not had the luxury of demanding to do a huge, expensive experiment
            • From the parent post:

              The potential gains in knowledge of astronomy, astrophysics and even particle physics are vast. Not to mention the gains in laser technologies, control systems, material science and computational analysis that such a project brings.

              It's important to invest in such technology for just this reason. The space shuttle, the moon landing, the Higgs-Boson experiments, etc are not just virtuosic feats or symbols of our technological growth, but they provide the means of refining the technol

      • Exactly. Before Planck, there wasn't any "plausible alternative hypothesis" to solve ultraviolet catastrophe. In that situation, what we simply say is "we dont' have a good theory to explain this... yet."
        • Bah. The Hanso Foundation [thehansofoundation.org] has already been conducted research like this for years.
        • The difference is that the photoelectric effect didn't take multi-million dollar installations to demonstrate.

          The question is and remains whether this particular experiment is a sensible use of scarce research dollars at this time, in particular since the same kind of experiment with the same promises has been carried out multiple times before.
    • by TheChrisMan (982990) <think_before_you_eat_it.hotmail@com> on Monday June 26, 2006 @09:08AM (#15604892)
      "It's important to have alternative hypotheses"

      Is it? If I remember correctly the lack of an alternate hypothesis when Michelson and Morley failed to detect the aether caused Einstein to beging pondering special relativity.
      • MM was an experiment to measure a specific quantity, and it was clear that the quantity could be measured with that apparatus. As soon as the measurement was performed, there was no issue of detection thresholds: whether the measured speed was 0 or 10^-3 m/s, either way presented a problem for the classical theories.

        Gravity wave detection is not at all analogous to that, since a negative outcome in this experiment still doesn't really tell you anything.
      • Nope, it is unsure Einstein even heard of the experiment at the time he wrote his famous paper.
      • You'll find more contemporary examples - especially those that get covered on TV and Slashdot, like the solar neutrino problem and super kameo-kande - will get through more successfully to these "omg what's a history" noobs.
    • by ScentCone (795499)
      This is the alternative model. The rest of us know that such waves actually propogate via infinitely long strands of pasta [venganza.org].

      But seriously - if things all point to a likely model, and nothing (rationally) points to an alternative, why kill yourself (and your budget) documenting hollow alternatives just so that you're sticking to academic form?
    • Why does there need to be an alternative hypothesis if there's a chance the first hypothesis is correct? It's not like this would be the first experimental confirmation of general relativity.

      And a null result is easy. All you need is the absence of gravity waves when you observe an event (like a collision of stars or black holes) that should produce them.
      • And a null result is easy. All you need is the absence of gravity waves when you observe an event (like a collision of stars or black holes) that should produce them.

        Unfortunately, it isn't easy. We have had multiple experiments like this, all quite expensive, and all of them failed to demonstrate gravity waves. Physicists still believe that gravity waves exist but we just need a bit more sensitivity to detect them. That's a reasonable belief, but the question is whether that kind of belief should be eno
        • by Anonymous Coward
          None of that matters if you're talking about cheap table-top experiments. But these kinds of experiments are expensive, and other science isn't getting done because these experiments are getting funding.

          This is a common fallacy. I heard it a lot back in the SSC days. When the SSC was cancelled, did all of that earmarked money go to other physics? No. In reality, much of the funding for these large physics experiments is created specifically for those experiments, and would not exist otherwise.

          In the cas
          • This is a common fallacy. I heard it a lot back in the SSC days. When the SSC was cancelled, did all of that earmarked money go to other physics? No. In reality, much of the funding for these large physics experiments is created specifically for those experiments, and would not exist otherwise.

            That's a common cop-out. In fact, there is only a limited amount of funding that can be "created"; if this funding wasn't "created" for those experiments, it could be "created" for other experiments by researchers in
            • by Anonymous Coward
              That's a common cop-out. In fact, there is only a limited amount of funding that can be "created"; if this funding wasn't "created" for those experiments, it could be "created" for other experiments by researchers in other areas.

              It's not a cop-out. Look at the history. For the very large experiments, it's almost never the case that the funding committee goes "Well, we could fund this big experiment, or we could give everyone else in the field the equivalent amount of money instead". Usually, if they don'
              • Don't be absurd. You could carpet a small moon with all the alternative theories that have been developed in the literature. See Cliff Will's book and Living Review online for a limited discussion.

                Great! So, that was what my original question was about. However, it's not sufficient just to point at "a small moon" "carpeted in theories", you also need to explain how the different theories actually differ, whether they make different predictions for this experiment, and what those predictions are. Unfortun
    • Yes, let's be absolutely sure we're correct before testing a hypothesis. After all, what are hypotheses for?
      • Hypotheses only create more questions, according to Phaedrus.
      • Yes, let's be absolutely sure we're correct before testing a hypothesis. After all, what are hypotheses for?

        What I'm saying is: when we invest a lot of money in an experiment, let's be sure we understand how the experiment and its possible outcomes relate to the hypothesis, and why similar previous experiments have failed. Just doing open-ended experiments without a prior understanding of what the possible outcomes mean is not doing science, it's voodoo or alchemy.
    • by Tim C (15259) on Monday June 26, 2006 @09:24AM (#15604984)
      It's important to have alternative hypotheses, among other reasons, in order to be able to determine when you got a null result. Until the theoreticians have done their homework and provided a reasonable and plausible alternative hypothesis, perhaps we shouldn't be investing millions of dollars (euros) in these kinds of experiments.

      That's simply not true. Right now, all our understanding of how the universe works points towards the existence of gravity waves. If we fail to detect them, then one of two things is true:

      1) The equipment was wrong
      2) The theory was wrong

      Until such time as it looks like 2) is the case, there's no basis for exploring alternative hypotheses, especially given that so far, we have no reason to doubt the current one and every reason to believe that it's either valid, or very nearly so.

      As for needing an alternative to be able to recognise a null value, that's not the case either. The current theory makes a prediction. If we don't make an observation that matches prediction within expected tolerance and we can find nothing wrong with the equipment, then the theory is most likely wrong. At that point, you can bet your life that people will be scrabbling to work out how, and what needs to be done to correct (or replace) it.

      Think of it this way - what if the theory is correct, and there simply *isn't* any "reasonable and plausible alternative hypothesis" (perhaps because we can't think of any, perhaps because there simply aren't any). Should we *never* attempt to confirm it?
      • Think of it this way - what if the theory is correct, and there simply *isn't* any "reasonable and plausible alternative hypothesis" (perhaps because we can't think of any, perhaps because there simply aren't any). Should we *never* attempt to confirm it?

        Isn't this how religions get started?
        • But the theory is falsifiable.

          The religion is started if not new theories are developed i this one proves false. The daa gathered as it disproves gravity waves may even be key to the new theories. To come up with a new theory that matches our observation but excludes gravity waves for no good reason sounds like fringe science to me.
      • Until such time as it looks like 2) is the case, there's no basis for exploring alternative hypotheses,

        "The equipment is wrong" is an alternative hypothesis, albeit not a fully formulated one.

        What is happening right now is that, after a number of these experiments have been done in the past and failed to demonstrate the existence of gravity waves, the people involved just say "hey, it didn't work, maybe our equipment wasn't sensitive enough/faulty, so we're just going to try again".

        Think of it this way - wh
    • by LordVader717 (888547) on Monday June 26, 2006 @09:26AM (#15604999)
      Try this [wikipedia.org]. The experiment is strikingly similar to the Michelson-Morley interferometer, an experiment which also returned a null-result, trying to detect an "aether" for electrmagnetic waves.

      The problem with these kinds experiments though is that results are very easily misinterpreted, because we really have no, shall we say, "creativity" in our imagination about such fundamental physics.

      The Sagnac-interferometer (which BTW I will be building for a project) seemed to prove the presence of the aether that the Michelson-Morley experiment couldn't detect. It turned out to be a misinterpretation because they didn't quite grasp the concepts. (It turned out to be very useful anyway, as it's the basis for laser-gyroscopes)

      This makes experiments like this even more important because if you are to accept any theories as "confirmed" or develop upon them, you need to research every possible result and implication.
      • by budgenator (254554)
        I must be dense and you seem to know what your talking about; FTA I see a schematic of a device that would be very accurate in measuring minute differences in distance and time. Presumably a Gravity wave would distort time-space consistant with the lorentz transformations, which I think I understand, what I don't understand is since the time-space distortion would apply to the instruments frame of reference, wouldn't they get the same null-results that the MM experiment got?
        • Technically, you're correct. If the distortion of space-time 'compresses' one arm of the interferometer and 'extends' the other in a step (or constant offset), the amount of time taken for the light to traverse the arms and recombine at the beamsplitter wouldn't change and no phase difference would be detected between the returning beams.

          However, gravitational waves are not stepwise events and have a frequency. This means that if the beams are split and traverse the arms the 'compression/extension' experien
      • because we really have no, shall we say, "creativity" in our imagination about such fundamental physics.

        If we had no creativity then we wouldn't have this test in the first place.
    • by sweetser (148397)
      I've got an alternative, and it does EM too, being discussed here:

      http://physicsforums.com/showthread.php?t=87097 [physicsforums.com]

      The theory also predicts gravity waves, but the transverse modes of emission for a 4D wave are EM, and the longitudinale and scalar modes are the stuff of gravity. So GEM theory (gravity and EM) predicts that gravity waves will travel at the speed of light, but the polarization will not be transverse like GR predicts.

      I think gravity MUST be viewed as a longitudinal wave, not transverse. Here's
      • by Anonymous Coward
        I think gravity MUST be viewed as a longitudinal wave, not transverse. Here's a thought experiment.

        Your thought experiment proves nothing. GR predicts that a "cup of neutrinos" will oscillate back and forth; it also unambiguously predicts that gravitational waves are transverse. Therefore, neutrinos oscillating back and forth is not proof that gravitational waves must be longitudinal; a counterexample exists.

        More directly: your thought experiment has nothing to do with gravitational waves, it is only sen
        • The question for me is whether GR is logically consistent on polarization. I know that gravity waves like the ones being hunted for here arise from the "water balloon" kind of motion for an isolated source (a quadrupole moment, no dipoles allowed). And GR predicts that the wave emitted will be transverse.

          A simple harmonic oscillator is described by a few things: its period, its wavelength, and its polarization. As you correctly point out, the neutrinos are a different animal from the gravity waves genera
    • What are the alternative models if gravity waves simply don't exist?

      There are already alternative theories, such as bosons named gravitons. That might just be a variation in interpretation of wave-particle duality, but since quantum gravity isn't the same thing as general relativity it passes muster. There are other gravitational alternatives proposed, usually flawed and/or not well accepted by the scientific community. And what impact this experiment wil have on the Higgs particle question, one way or anot
    • by rotenberry (3487)
      Gravitational waves will exist in any theory of gravitation that requires the effect of gravity to propagate at a finite speed. Newton's theory of gravity assumes that the effect propagates at an infinite speed, so this theory does not predict gravitational waves.

      It should be noted that the primary purpose of the detection of gravitational wave since at least the 1970's has been the both the detection and interpretation of the information contained in these waves. Depending on the frequency of the waves, th
    • What are the alternative models if gravity waves simply don't exist?

      Intelligent falling [theonion.com]. After all, gravity is just a theory.

    • Who allowed you to operate a keyboard before you shook the cobwebs out of your head? Do you really thing Kip Thorne reads slashdot with his Wheaties in the morning to glean your wisdom from it? One of the dangers of articles that popularize science for the multitudes who can't be bothered to study enough to understand even approximately what is involved, is that some readers get the illusion that they actually know something. Just a hint, from what you've written it seems safe to infer you know approximatel

    • It's important to have alternative hypotheses...Until the theoreticians have done their homework and provided a reasonable and plausible alternative hypothesis, perhaps we shouldn't be investing millions of dollars (euros) in these kinds of experiments.

      Huh? I simply don't understand this idea at all. Historically science hasn't come up with alternate explanations before they do an experiment with an expected result. Michelson-Morley fully expected to confirm the existence of the aether wind, and had no a
    • Wait, let me get this straight. Did you really just say that if we don't have an alternative theory, we shouldn't test the one we have?

      You should read up on the history of physics. Pay particular attention to the phlogiston, Antoine Lavoisier and the solar neutrino problem; all three are cases where it wasn't until we had disproving experimental data that we had enough information to even begin to formulate alternate theories.
  • by Anonymous Coward on Monday June 26, 2006 @09:07AM (#15604889)
    enabling scientists to probe the moment of creation itself
    In other words, scientists will get a touch of His Noodly Appendage [flyingspag...onster.org], and a bath in His Mighty Sauce.
    • Given that the flying spaghetti monster is the creator, I wish he'd create a new joke. This one's as old as time immemorial. You might as well dust off the hot grits jokes.
  • by insanarchist (921436) on Monday June 26, 2006 @09:12AM (#15604912)
    In later tests, the scientists plan to add sour cream and cheddar to the ripples in an effort to test gravity's potential for inter-galactic tastiness!
  • by Ohreally_factor (593551) on Monday June 26, 2006 @09:19AM (#15604946) Journal
    No, seriously, he is. Anyone have any idea on how to get him out?
  • by master_p (608214) on Monday June 26, 2006 @09:23AM (#15604978)
    Right now we are uncertain of the exact speed of gravity. Some measurements resulted in speed between 0.8 and 1.2 times the speed of light (according to this [wikipedia.org]). If the speed of gravity is greater than the speed of light, does that violate the general relativity? There are many consequences.

    It is important that we find what gravity is, because if it is a wave of particles, then maybe there is a possibility to find a way to shield gravity away. Shielding gravity would be a major step towards space exploration.
     
    • How cool it would be to fly like superman. :)

      Steve
    • by Anonymous Coward on Monday June 26, 2006 @09:47AM (#15605132)
      Right now we are uncertain of the exact speed of gravity.

      We are always "uncertain" about the exact value of any physical quantity, because no quantity can be measured with infinite precision.

      There is very little doubt that the speed of gravity is equal to the speed of light.

      Some measurements resulted in speed between 0.8 and 1.2 times the speed of light

      The Taylor-Hulse pulsar measurements have measured the accuracy of that speed to within a few percent, much better than the 20% figure you cite. Furthermore, most of the gravitational physics community is convinced that the experiment mentioned did not measure the speed of gravity (as the Wikipedia article alludes to).

      If the speed of gravity is greater than the speed of light, does that violate the general relativity?

      Yes. It also violates special relativity and the laws of cause and effect.

      It is important that we find what gravity is, because if it is a wave of particles, then maybe there is a possibility to find a way to shield gravity away.

      Gravity being "a wave of particles" does not imply that it can be shielded, and gravitational wave detectors are unlikely to tell us anything about that issue.

      Even if it were possible to "shield gravity" (very unlikely), it is almost certainly impossible to do it with any realistic technology, because we already have a thorough understanding of gravity on the scales that our technology can reach in the forseeable future.

      A little realism: LIGO and its kin may teach us something new about gravity near neutron stars and black holes, but the most likely outcome is that it will simply serve as a telescope to probe astrophysical phenomena not detectable in visible light. It is very farfetched to think that it will lead to antigravity or any Star Trek type applications.

      • >>If the speed of gravity is greater than the speed of light, does that violate the general relativity?
        >Yes. It also violates special relativity and the laws of cause and effect.

        What about the tunneleffect, does it violate the laws of cause and effect?
        If the electron tunnels through a barrier, at what speed does it tunnel?
        There are mainly 2 possibilities:
        * If it is instant it's greater than the speed of light.
        * If it is not instant (for example speed of light) where the hell is it, when it vanished
        • As has already been pointed out, quantum mechanics does not allow particles (or information) to travel faster than light. Essentially, if a particle is localized on one side of a barrier, it takes time for it to tunnel through to the other side (this is the time for the wavefunction to spread in non-relativistic QM, or the time for disturbances in the quantum field to propagate in quantum field theory) so that if you measure again before the particle could have gotten through the barrier at the speed of l

      • Excellent post. I only wish you'd made it while logged in so that I could look for your posts in the future.

      • no quantity can be measured with infinite precision.
        There is one apple on my desk. Not 0.99. Not 1.00002. Exactly one. I measured.
        • There is one apple on my desk. Not 0.99. Not 1.00002. Exactly one. I measured.

          Actually, due to quantum fluctuations there's all kinds of particles and their composites being created and annihilated at your desk at any moment. There is an extremely small but finite probability that an apple has come into existence since your measurement. (Or that there was actually two apples, but vacuum fluctuations interfered destructively with the photons signalling the existence of one of them). Given this small prob

      • If the speed of gravity is greater than the speed of light, does that violate the general relativity?

        Yes. It also violates special relativity and the laws of cause and effect.


        Whereas I'm certainly not defending the grandparent post, as the poster was a huge douchebag, I should point out that in fact there are several things whose effects can cross a distance in a timeframe shorter than that which light would also take. Given that we're not entirely sure of the nature of gravity, we don't actually know tha
      • because we already have a thorough understanding of gravity on the scales that our technology can reach in the forseeable future
        Thanks for the reply, but I have to disagree on this one thing: I do not think we know enough about why gravity exists. We only know what is the effect of gravity, but we have no idea why mass bends spacetime. Maybe once we know that, Star Trek like devices will be a reality.
    • Shielding gravity would be a major step towards space exploration.
      Not to mention a potential solution for our current obesity epidemic.

      Yes, it's a dream I have. The dream to one day find myself in a situation where I can use the phrase, "bring in that floating fat man, the Baron!"
    • If you read the wikipedia article it says that that variance is in line with current theoretical model's of general relativity.
    • It would be a major step in kung fu movies. No more wires, and you could finally pull off all those sweet moves at home.
    • Sigh. This experiment has nothing to do with the speed of gravity, which nobody believes is anything other than the exact speed of light. What you're quoting was someone saying "wow, we've proven that our believed speed is correct within 20%, and the frame of proof is perfectly centered."

      If the speed of gravity is greater than the speed of light, does that violate the general relativity?

      No. But still, it isn't faster than light. Also, there are several things which are faster than light without violatin
  • I really read that as gravity nipples. No, I don't know what a gravity nipple is.. maybe an inverse black hole or something. But by God, my lab would be tuned to them, that's for sure!
  • You can participate (Score:5, Informative)

    by mike449 (238450) on Monday June 26, 2006 @10:22AM (#15605370)
    I am surprised nobody mentioned Einstein@home - http://einstein.phys.uwm.edu/ [uwm.edu].
    This experiment uses distributed computing to process their results,
    and you can participate.
  • "enabling scientists to probe the moment of creation itself."

    It's the moment before that I want to know about... Oh, wait...

  • If gravity waves cause spacetime to flex in a, er, wavelike fashion, then what if the wavelength of the light passing over those waves is also flexed? thus, as we and all our physical measuring equipment are also flexing then when a difference occurs the light, equipment and the field it's situated in flex too. So there will be nothing to measure. As in if you were trying to measure the expansion or contraction of a piece of metal due to a temperature change using a ruler constructed of the same metal as th
  • After reading TFA, I have a question. Are there any physics buffs out there who can answer this? The article explains that the GEO 600 works by splitting a LASER beam with a semi-transparent mirror bla bla bla....

    My question is this. What would happen if you shot photons at a semi-transparent mirror just one at a time. Can the exact number of photons that takes one route over the other be predicted? I assume that it should be 50/50 if the mirror is 50% transparent, but the likelihood of any given pho
    • by Anonymous Coward
      What would happen if you shot photons at a semi-transparent mirror just one at a time. Can the exact number of photons that takes one route over the other be predicted?

      It can be predicted only statistically, in the sense that you can't predict the "exact" number of heads that you will get if you flip a coin N times.

      I assume that it should be 50/50 if the mirror is 50% transparent, but the likelihood of any given photon taking one route over the other should be random unless maybe if the mirror is polarized?
  • True story: I was out at Caltech one summer, and was passing by the astronomy faculty lounge looking for a drink... I walked in and I noticed a curious contraption - it was rectangular, the size of a lunchbox (looked very much like a car battery charger, or power supply), but on the top it had a motor-driven bar with a metal sphere on each end. It was labeled "gravitational wave generator."

    To this day, I'm not sure if it was a joke or a real device used for tuning LIGO... still a funny thing to find layi
    • I'm sure that any gravity waves it produce would be overwhelmed by the mechanical vibrations, intermitant air currents, and EMF noise it would make, short answer it must have been a joke.
      • I'm sure that any gravity waves it produce would be overwhelmed by the mechanical vibrations, intermitant air currents, and EMF noise it would make, short answer it must have been a joke.

        I'm not so sure... all of that stuff dies out with at least inverse-distance-squared (faster for higher order moments) on its own, however EMF and mechanical vibrations are dampened considerably by ordinary matter, whereas gravity waves are affected much less... I would imagine that a device like this would be used se
        • There are some geologists that should be very interested in methods like Dr. Gold's. One problem they run into is the frequencies that are gereated are just to long to see some structures well and high frequencies just don't carry well enogh to pass through a structure and still get to their instruments. I'm not a geologist, but intend to build a seismometer some day, toying with ideas about soundcards and linux to record the data, some day.
    • It was a joke. All physical objects are such generators.
      • All physical objects are also "sound" generators, in the appropriate environment, but that does not mean that they are specifically designed to generate certain frequencies of sound, as real-life sound generators are... This device had a frequency control on it, etc.
        • It's a joke for people who have a basic understanding of physics. Also, no, objects are not all sound generators; a still object isn't generating sound (Air currents whacking off of it? It's the air that's the generator.)

          The reason it's funny is that anyone who has sense knows that that kind of thing is way, way outside of our technology, and that it's only such an object on a technicality. To use your own example, if someone put a sign on a brick that said "sound generator," that would be essentially tr
  • Hello,

    When asked if gravity waves [wikipedia.org] could transmit energy, RPF in 1957 had the following argument :

    Feynman's argument

    Later in the Chapel Hill conference, Feynman -- who had insisted on registering under a pseudonym to express his disdain for the contemporary state of gravitation physics -- used Pirani's description to point out that a passing gravitational wave should in principle cause a bead on a stick (not oriented parallel to the direction of propagation of the wave) to slide back and forth, thus heating
  • I worked on this in the 90's. I'm glad to see that its finally up and running...

A fail-safe circuit will destroy others. -- Klipstein

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