Lab Tuned to Gravity's 'Ripples'

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. "

Comments

negative outcomes?

[m874t232]

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.

Re:negative outcomes?

[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

Re:negative outcomes?

[mwvdlee]

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.

Re:negative outcomes?

[TheChrisMan]

"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.

Re:negative outcomes?

[ScentCone]

This is the alternative model. The rest of us know that such waves actually propogate via infinitely long strands of pasta.

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?

Re:negative outcomes?

[jcorno]

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.

Re:negative outcomes?

[insanarchist]

Yes, let's be absolutely sure we're correct before testing a hypothesis. After all, what are hypotheses for?

Re:negative outcomes?

[Tim C]

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?

Re:negative outcomes?

[LordVader717]

Try this. 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.

Re:negative outcomes?

[JensR]

just imagine the possibilities. Flying car anyone?
Think bigger: Anti-Gravity-Sex !

Re:negative outcomes?

[LordVader717]

These guys will sure like it.

Re:negative outcomes?

[Ana10g]

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?

Re:negative outcomes?

[sweetser]

I've got an alternative, and it does EM too, being discussed here:

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

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 a thought experiment. You have a cup of neutrinos (see, this is a thought experiment because no such cup can be manufactured). You spill the cup. The neutrinos fall, and when they reach the floor, they keep falling, through the center of the Earth, to the other side, and in about 88 minutes, back to where they started, just to repeat the cycle again. This is a SHO (simple harmonic oscillator), with a period of 88 minutes, and a wavelength of twice the diameter of the Earth. The neutrinos are acceleration in the direction of velocity which is a defining characteristic of a longitudinal wave.

doug
TheStandUpPhysicist.com

Re:negative outcomes?

[Bob3141592]

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 another?

Your post seems to imply this is an all or nothing experiment. But GR wouldn't be challenged only by a null result. If the magnitude of any detected gravity waves is significantly different from expectations, the discrepancy will have to be explained. Often, these differences are more challenging to a theory. Unexpected behavior of a newly detected but predicted phenomena is also a major challenge to existing theories. I can't wait to see what kind of information can be obtained by some analog of Zeeman splitting in gravity wave spectroscopy, if such a thing exists. That will be far more revealing than any "Yup, that's a gravity wave" result.

Re:negative outcomes?

[AvitarX]

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.

Re:negative outcomes?

[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 sensitive to static gravitational fields. Of course objects will accelerate in the direction of the gravitational field. This has nothing to do with whether the waves are parallel or perpendicular to their direction of propagation.

Note that if you did the same experiment with a test charge and a uniformly charged ball, you'd get simple harmonic motion, but electromagnetic waves are provably transverse: another flaw in your logic.

Re:negative outcomes?

[rotenberry]

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, they contain information about the beginning of the universe (long wave) or the collision of massive bodies (short wave). The advantage of a laser interferometer is that it is a wide band detector, while a Weber bar is a narrow band detector.

It was still being argued as late as the 1960's whether Einstein Theory gravatation waves could transmit energy:
http://en.wikipedia.org/wiki/Gravitational_radiati on

Re:negative outcomes?

[andylievertz]

Hypotheses only create more questions, according to Phaedrus.

Re:negative outcomes?

[Splab]

You know, that might just be it, get the porn industry involved - they would have it figured out in no time.

Re:negative outcomes?

[metamatic]

What are the alternative models if gravity waves simply don't exist?

Intelligent falling. After all, gravity is just a theory.

Re:negative outcomes?

[sconeu]

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."

Re:negative outcomes?

[Ohreally_factor]

Bah. The Hanso Foundation has already been conducted research like this for years.

Re:negative outcomes?

[carnivore302]

Nope, it is unsure Einstein even heard of the experiment at the time he wrote his famous paper.

Re:negative outcomes?

[sweetser]

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 generated by an isolated source, but both have the same cause, gravity. Nature likes to be logically consistent, so the I think the longitudinal SHO for the static gravitational field will also appear as a longitudinal gravity wave for a dynamic gravity field.

If you vibrate an electron back and forth say by heating it up in a light bulb filament, it will emit a photon perpendicular to that motion. EM is a transverse wave every way it is generated, no exceptions. To be logically consistent, I argue that gravity is a scalar or longitudinal wave, no exceptions. My proposal says EM is a transverse, spin 1 field, and gravity is longitudinal/scalar, spin 2 field, both of which travel at the speed of light.

doug

Re:negative outcomes?

[mkw87]

Think of where you posted this.....you just confused the masses.

Re:negative outcomes?

[budgenator]

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?

Re:negative outcomes?

[Camulus]

http://video.google.com/videoplay?docid=-284332758 8197143326&q=flying+car

Re:negative outcomes?

[sweetser]

Clearly /. is not the best way to communicate physics. I said very clearly, EM waves are transverse, that there is no longitudinal EM wave, yet you appear to think that is what I said.

If you read about the Gupta/Bleuler method for quantizing a 4D wave equation, they talk about 4 modes of emission: 2 transverse, one longitudinal, and one scalar. They are using the word "scalar" to describe the mode of emission, not in reference to tensors. If you are familiar with that approach to quantizing the EM field, it is the scalar mode of emission that would allow negative probabilities. Since that makes no sense, they use a "supplementary condition" to make the longitudinal and scalar modes virtual. What my proposal does is make those modes do the work of gravity, where like charges attract, due to a symmetric field strength tensor of a spin 2 field. EM is the spin 1 antisymmetric field strength tensor where like charges repel.

> SHO does not logically imply longitudinal waves.

I agree with that. Sound happens to be a longitudinal wave. The changes in air pressure occur in the direction it is moving. In my neutrino example, the neutrinos accelerate in the direction they are moving. That is why the neutrino SHO is a longitudinal SHO. This system, the neutrinos moving in a static g field, is a longitudinal wave. I think that is correct at the undergrad level.

No matter how you jiggle an electron, you will never make a longitudinal wave of light, the waves will always be transverse. What I argue is that no matter how one moves masses, it will never be a transverse wave. As Clifford Will wrote in his 100+ page review, if the data comes back to show that the gravity waves caused by the collapse of a dense source is NOT transverse, that would be a big challenge for general relativity. It would also be a piece of data in support of my rank 1 field theory. Fortunately, data will rule the day.

One reason I cut way back on posting to SPR is people decide to toss in negatives, when all I ever care about are the technical issues. I have a Lagrange density, field equations, solutions to the field equations, and experiments like the one discussed in this thread, all plugged through Mathematica. In a measurable way - there are two experiments to test the validity of my proposal, the other being light should bend 0.7 microarcseconds more around the Sun than GR predicts - the GEM theory is a better formed alternative to GR than all of string theory at this time.

Re:negative outcomes?

[Open_The_Box]

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' experienced by the beam on the way out will have changed by the time the beam returns such that the frame of reference is different for the outgoing and incoming beam in each arm. In essence, the instruments detect 'changes' in space-time but are unable to measure static states.

The schematic you mentioned from the article has additional mirrors [e + f]. 'f' is the signal recycling mirror which allows the instrument to by optically 'tuned' to the frequency of optimal detection (i.e. the frequency of your expected gravitational wave event). That's not to say that by choosing one frequency you're ignoring all the others - it just means the device is more sensitive at one chosen frequency.

Re:negative outcomes?

[m874t232]

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.

Re:negative outcomes?

[m874t232]

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 enough to finance these kinds of experiments right now, or whether we should simply wait until either theory or experimental techniques have caught up.

It's not like this would be the first experimental confirmation of general relativity.

You can't confirm hypotheses, you can simply fail to disprove them. General relativity has survived a number of experimental tests, but so have an infinite number of alternative hypotheses.

Why does there need to be an alternative hypothesis if there's a chance the first hypothesis is correct?

Because otherwise you can't interpret the results. As I was saying, there have been attempts to detect gravity waves, and they failed to measure them. So, without an alternative hypothesis, you simply don't know: is this because they don't exist? Are they too weak? Are there other possibilities?

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.

Re:negative outcomes?

[m874t232]

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 - what if the theory is correct, and there simply *isn't* any "reasonable and plausible alternative hypothesis"

I believe gravity waves exist, but people have failed to detect them in previous experiments. So, that means that either the previous experiments were insufficient to test the theory, or people don't understand the theory enough to make testable predictions. Either way, either people screwed up on the previous experiments, or they screwed up on the theory. The point is that until people have figured out which of the two is the reason previous experiments came up negative, it is difficult to justify funding expensive experiments.

Therefore my question: what are the alternative hypotheses? Another way of putting it is: how do you account for the fact that previous experiments designed to test for the existence of gravity waves didn't find them? Or, more succinctly: before we fund any more of this, we need to know how previous experimenters screwed up.

I note that nobody in this thread has given a satisfactory answer; everybody just waves their hands about how nice it would be to find them, and how it is important to identify them, both of which I agree with. But those are not sufficient reasons to conduct this experiment in light of what we already have done and what we know.

Re:negative outcomes?

[m874t232]

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.

Re:negative outcomes?

[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 case of LIGO, there was a sacrifice, however: gravity theory is somewhat less funded than it once was. Many leading gravitational physicists were consulted on this matter back when the funding for LIGO was being debated. The consensus of the community was that yes, this experiment is worth doing, even if gravity theory takes a hit

Speaking as someone who has worked in gravity theory, I think LIGO is a necessary experiment, even if it comes at the expense of some theory. There is no question of "waiting for the theory or experimental technique to catch up". Theory has gone about as far as it can without additional experimental input: there are lots of alternative gravity theories lying around, they are just currently indistinguishable from GR without better data. And the technology to build a working LIGO can't be developed from thin air: the advanced LIGO (LIGO II) experiments could not have been designed without doing LIGO I first. We have the technology now: we simply can't engineer it into a working instrument without testing it for real.

Re:negative outcomes?

[steve_bryan]

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 approximately nothing which is not a problem. But deciding you should offer sage advice is somewhere between ludicrous and nauseating.

Re:negative outcomes?

[sweetser]

I hope we agree that sound is a longitudinal wave, and the EM is a transverse wave. The changing pressure in sound is in the direction of the traveling wave. The changing E and B field for light are transverse to the direction the light wave goes.

I'm ready to concede my logic was too simple. Part of the description of the sound field or the E and B fields must include the vectors for those fields. I would have to show that all the vectors that describe gravity all are colinear for the neutrino example. I beleive that to be true, but I did not show that, so you are correct in your critique.

Here is the rank of various parts of the theory.

        Rank 0: The Lagrangian
        Rank 1: The field equations, the source
        Rank 2: The asymmetric field strength tensor.

Exactly the same thing applies for EM. The classical Lagrange density is rank 0. The Maxwell equations can be writen as a rank 1 field theory. The field strength tensor for EM is an antisymmetric rank 2 tensor, F^uv.

Re:negative outcomes?

[m874t232]

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 other areas.

Speaking as someone who has worked in gravity theory, I think LIGO is a necessary experiment, even if it comes at the expense of some theory. There is no question of "waiting for the theory or experimental technique to catch up". Theory has gone about as far as it can without additional experimental input: there are lots of alternative gravity theories lying around, they are just currently indistinguishable from GR without better data. And the technology to build a working LIGO can't be developed from thin air: the advanced LIGO (LIGO II) experiments could not have been designed without doing LIGO I first. We have the technology now: we simply can't engineer it into a working instrument without testing it for real.

OK, that's a more sensible answer than any of the other b.s. that people have posted as justification for these experiments before.

Personally, I still think it would be better to wait a couple of decades for technology to catch up with the needs of this community and to permit instruments to be built at a small fraction of the cost. Furthermore, I think theory and computational techniques also ought to catch up. I have yet to see a good, clear formulation of alternative theories to Einstein, and computational techniques seem to be weak and ad-hoc.

Re:negative outcomes?

[m874t232]

In point of fact, it is quite likely that LIGO I will fail to observe gravitational waves: its sensitivity is borderline. This was known before the project started. It was built anyway, because only in building it could LIGO II be developed, which is the real experiment. If LIGO II doesn't see them, then you can't just attribute that to failed experiment or a misestimate of what waves should be out there: it will mean something's wrong with our theories of gravity.

But the article says something different: "Researchers are extremely confident they now have the technology to detect gravitational waves." But then, "If there is a supernova in our vicinity during the next couple of months, our chances of detecting and measuring the resulting gravitational waves are good". Overall, there seems to be a disconnect between what people believe they can achieve and how they are selling these big projects to the public.

In any case, what you have said now is a reasonable position that one could use as a starting point for arguments about cost/benefit. I still think that when everything is taken into account, these kinds of projects could easily wait for another decade or two. Gravity isn't going anywhere, and I think there is a lot more theory, engineering, and computing that can be worked out and developed with modest funding before spending the enormous amounts of money of building prototypes using current (limited) technologies.

Re:negative outcomes?

[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't fund the big project they say, "This field doesn't need that much money", and the scientists get squat. Typically, the only time money gets "created" for other experiments when the funders don't go with a big experiment is when the other experiments are also big ones in direct competition with it. What you claim "could" happen, generally doesn't, in the political world of science funding. That LIGO had an impact at all on the funding of the rest of the gravity community has mostly to do with (a) how unusually expensive LIGO is, and (b) how small the rest of the gravity community is. (In fact, most of the gravity community is LIGO: a major reason for funding the project was to get more money and personnel into gravity in general, even at the expense of other areas of gravity.)

Personally, I still think it would be better to wait a couple of decades for technology to catch up with the needs of this community and to permit instruments to be built at a small fraction of the cost.

This isn't Moore's Law. Much of the expense is in things like construction work, paying the staff, etc. The technology that can be improved mostly increases the cost of the project, by making more sensitive instrumentation; it's not the case that the same instruments get much cheaper. Furthermore, as I pointed out, the way that the technology improves is by actually doing it. LIGO requires all kinds of specialized tech that simply wouldn't be developed if LIGO itself weren't being built. And the tech needed for LIGO II, the experiment that counts, wouldn't be developable at all without first having done LIGO I to work the bugs out. We wouldn't be getting these amazing cost reductions in microprocessors and such if people didn't keep working on building microprocessors!

There is essentially no advantage to waiting to build LIGO. It would slow R&D, slow the pace of science, and the future price tag would not be appreciably cheaper.

Furthermore, I think theory and computational techniques also ought to catch up.

With what?

I have yet to see a good, clear formulation of alternative theories to Einstein,

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.

and computational techniques seem to be weak and ad-hoc.

Computational techniques for, say, binary inspiral are up to spec. The binary merger problem is notoriously hard. All the more reason to build LIGO, in fact: it can tell us things that about strong gravity that we can't yet predict with theory (even assuming our theory is right). That's really the point of doing experiments. And there is nothing ad hoc about either of the methods. Approximations have to be made, but that's always the case in physics. Sometimes you can control the error involved, other times you have to do the experiment and see how good they are. Both cases exist in GR.

Re:negative outcomes?

[sweetser]

> See MTW (chapter 7)

Specifically exercise 7.2. He uses an antisymmetric field strength tensor. MTW is correct, that does not work. That is not what I am trying to do, so the exercise does not apply.

Newton's theory of gravity is the simplest rank 0 field theory that can be made. We know it fails because the speed of light c is not respected. To make Newton's gravity law consistent with special relativity leads right to Einstein's field equations (I think both Feynman and Weinberg showed that, whoever, they were smart). That is the simplest rank 2 field theory to explain gravity. All efforts to quantize the theory have failed to date, and I take that as a message: it cannot be done.

I asked Clifford Will why his review paper did not include the simplest rank 1 field theory possible, which is the focus of my efforts. He repeated a line that I've seen in MTW (beginning of chapter 40 to be specific): gravity must be a metric theory.

You know why he said that? Because he's right: gravity must be a metric theory. For me, it is a question of how to implement a metric theory. Right now, the only way is to use the Riemann curvature tensor, which is composed of the difference between two derivatives of connections and the difference between two products. I try and just use a covariant derivative which is the difference between a standard derivative and a connection. A scalar potential cannot explain the bending of light around the Sun because the g00 term gets smaller than 1, while the g11 term gets larger. A 4-vector potential can do the task. Effectively, by using a 4-potential, I lift potential theory to equal footing with changes in the metric (the connection). It is the ultimate middle ground between a theory which is only about potentials (Newton's rank 0 field theory) and a theory which is only about spacetime geometry (Einstein and Hilbert's rank 2 field theory).

I am aware I should call the proposal a vector field theory. Almost as soon as I say that, the technical folks in the audience say, no cannot be, the graviton must arise from a symmetric rank 2 tensor. That's what is in the action, darn it. The field equations (rank 1) are not the field strength tensor, a reducible asymmetric tensor that splits into the spin 1, rank 2, antisymmetric tensor for EM where like charges repel, and a spin 2, rank 2 , symmetric tensor for gravity.

So I get around the problem cited in MTW by using a different tensor to do the contraction, and am consistent with the use of a symmetric rank 2 field strength tensor required for the graviton.

>I would be very skeptical of any relativistic theory with a source of gravity other than rank-2

Yup, most folks are. Why? Because every reasonable proposal to day uses that. It is also why people think a reasonable theory for gravity should be nonlinear. I had a good discussion with Clifford Will, and that the line he used - not that a linear theory was not possible, but that there has not been a linear theory that is consistent with all experiments to date (light bending around the Sun, the precession of the perihelion of Mercury, tests of the equivalence principle, the strong field tests). I have gotten the light bending right, and the precession of the perihelion was a 4 pager that turned out OK. I feel vary confident that a quadrapole is the lowest order of emission for a gravity wave for my theory, but have not done the nuts and bolts calculation myself (Thorne did it in a related paper).

Re:negative outcomes?

[BrianTung]

It's important to have alternative hypotheses...

I disagree. It's nice to have them, but it's not important. What is important is to recognize when there's a significant discrepancy between the model and observation, but you don't have to know right off the bat how to interpret that. When double refraction was first measured, it was noted that it didn't fit the model of light then widely used, but they didn't have a "backup" model to step into place. Often, it's the anomalous observation itself that suggests the model. You can't conceivably have a contingency plan for every bizarre anomaly you might encounter when testing out a theory.

Re:negative outcomes?

[Vellmont]

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 alternative hypotheses to explain its abscence. People came up with explanations after (like the aether was blocked by walls, it was dragged by the earth, etc). They were all bunk of course. But the abscence of an alternative theory doesn't mean they shouldn't have done the experiment. In their case it ultimately meant (after more and more experiments) that there was no aether, and they had to abandon the whole she-bang. It took until Einstein and special relativity to find the correct theory to explain the results of Michelson-Morley. Why does the abscence of an alternate theory mean you shouldn't do the experiment?

Re:negative outcomes?

[gpeters]

Of course, the really funny thing is, gravity wave detectors are very sensetive versions of the Michelson Morley apparatus.

Re:negative outcomes?

[m874t232]

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. Unfortunately, none of the information on LIGO that I can find does this--neither the popular news articles, nor the overview papers by project members. They all just say how nifty it would be to detect gravity waves, with no analysis of just those questions.

Computational techniques for, say, binary inspiral are up to spec.

People can't even decide whether Kopeikin's approach actually did or did not measure the speed of gravity. If computational techniques were up to spec, you'd simply do the computation and settle the question. If people can't even compute values for such a simple measurement for the standard theory, there is obviously something amiss with the state of computational gravitational physics.

Re:negative outcomes?

[stonecypher]

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.

Re:negative outcomes?

[stonecypher]

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.

Re:negative outcomes?

[stonecypher]

Nothing in academia demands that tests be postponed pending alternatives. Grandparent was just waving his hands and trying to sound smart.

Re:negative outcomes?

[stonecypher]

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.

Re:negative outcomes?

[Eldin]

If we could manipulate gravity today, we still wouldn't have flying cars. Several practical working models of flying cars already exist. (http://www.labicheaerospace.com/ and http://www.firebox.com/?dir=firebox&action=product &pid=415)But imagine the dificulty of avoiding mid-air collisions if even half the traffic on the streets in a mid-size to large city were flying around town instead. Without any existing roadways, lanes, signals, etc. We have a hard enough time avoiding accidents WITH all of those restrictions, and on the ground to boot. You'll see flying cars catch on a few years after the market gets comfortable with purely autopilot vehicles, with which the only user interaction is inputing a destination and hitting "go".

Re:negative outcomes?

[sweetser]

Neither string theory nor loop quantum gravity at this time makes a testable prediction. They are areas of study done by serious people. At this time they CANNOT fail.

In contrast, if you measure the bending of light around the Sun to second order PPN accuracy, GEM theory could be shown to be correct or a failure. If the polarization of a gravity wave is determined, GEM theory could be right or wrong. GEM theory has reached the point of being a testible hypothesis, unlike anything done in string theory or loop quantum gravity.

> If you ditch the stress-energy tensor as source, it's very difficult to convince anyone that the theory you're talking about is "gravity".

I agree, there is a VERY strong bias out there. It is a BLIND SPOT. I referenced the places in the literature you rely upon to dismiss the simplest vector theories for gravity, and they are inadequate. I don't explect you to see or admit a blind spot, because, well, it is a blind spot.

Re:negative outcomes?

[sweetser]

> It's the definition of what gravity is.

I would disagree. What data we have proves gravity is a metric theory. It cannot show what is the math behind the metric theory. Right now, the only serious player for getting at second order derivatives of a metric that transforms like a tensor is the Riemann curvature tensor. What has been overlooked is the divergence of a covariant derivative transforms like a tensor and has second order derivatives of a metric. Riemann's curvature tensor is the simplest tensor to describe curvature as a tensor. I claim there is a simpler structure that has both second order derivatives of the metric and the potential. You have the freedom to described gravity as a 4-potential effect, a metric effect, or some combination of both. With Newton, its all potential, with GR it is all metric.

I understand the GR is a self-consistent theory, the source is a rank 2 tensor, and the field equations are a rank 2 tensor. To be self-consistent, my source must be a rank 1 tensor like my field equations are a rank 1 tensor. So my work is definitely in direct conflict with GR. Too bad for me, I better keep the day job, which I will, since it is clear you are choosing to define the possibility into oblivion, instead of finding a paper anywhere in the vast literature that directly addresses the issue.

Re:negative outcomes?

[sweetser]

What is the 4-current used in EM? It is the product of electric charge density and 4-velocity. What is the 4-current used in GEM? The part for EM is exactly the same, and one subtracts from that the square root of Newton's constant G times the product of mass density and 4-velocity. G^(1/2) m has exactly the same units as electric charge. Essentially inertia decreases the effective electrical charge by an unmeasurable amount (electric charge is defined to 10 significant digits, while the mass of an electron in the same units is 16 orders of magnitude smaller).

The proposal MUST use mass if it predicts light will bend 11.5 more microarcseconds around the Sun due to second-order PPN effects than the 10.8 more microarcseconds predicted by GR. Right now we can do light bending measurements to 100 microarcseconds, so three orders of magnitude improvements are required to detect 0.7 microarcsecond difference. The fact that the Sun spins and has a quandrapole moment both make contributions on this order, so it might not be practical.

GR is a rank 2 theory. GR is our best approach to gravity at this time. GR makes predictions. GR has one weakness, quantum. I am researching a different theory. It definitely uses the masses of bodies for doing calculations. The source and the field equations are both vectors, as they must be to have a well-formed equation. The theory can be quantized, because it is in all graduate-level quantum mechanics books under the Gupta/Bleuler quantization method. All I do is alter the interpretation of two of the virtual modes, getting them to do the real work of gravity.

Mass can be part of a scalar, a vector, or a tensor, it happens all the time.

Re:negative outcomes?

[m874t232]

I have already told you that physicists DO do this, it's just not published as part of the LIGO project.

Yes, and I have already told you that it must be part of the description of such an experiment, both in the grant application and in any paper describing results; it is not sufficient for the information to exist somewhere in the literature.

That you are ignorant of this work is not a testament to the incompetence of LIGO physicists, it simply speaks to your own incompetence.

Your assumption about what I know is incorrect, but that is not relevant here. My point isn't about the possible theories that exist somewhere, my point is about the kind of information that must be supplied both as part of the preparation and as part of any presentation of a scientific experiment. You keep confirming that, not only is this information is missing in the LIGO documents, but you know so little about the scientific method that you don't even think anything is wrong.

Assuming it gets published, of course, seeing how physicists are incompetent and don't understand theories or computations; maybe the reviewers simply won't recognize your genius.

The problem with LIGO and Kopeikin isn't one of gravitational physics, the problem is one of scientific method and for procedures for using computational methods for establishing correctness of scientific theories. In that area, I am almost certainly more qualified than you, since that's my work. And these omissions are so glaring in this work that I could diagnose them even if I knew nothing about gravitational physics (I know some, but you may well know more than me).

Now your bigotry comes out. You don't have any problem with LIGO, you just have a massive chip on your shoulder against physicists.

I have a chip on my shoulder against sloppy science, in particular when it involves spending billions of dollars of tax payer money.

Look, you ass.

Insults don't make your argument any more compelling; they just reinforce the stereotype of physicists being arrogant and condescending towards other disciplines.

Re:negative outcomes?

[sweetser]

Sorry for my apparent lack of precision. I meant it is JUST LIKE EM. So form the 4-current JUST LIKE EM, but put in the invariant mass where you put in the invariant electric charge. These are /. miscommunication issues. I know that vectors can only be subtracted from vectors so the result transforms like a vector.

The J of EM is an electric current density. I want a J that is formed exactly like the J of EM, except where one would put an electric charge q, a Lorentz invariant, I swap in q-G^(1/2) m, also a Lorentz invariant. If both sides of the equation, the source and the fields, are densities, there is no mismatch. There is nothing complicated here, sorry.

I am certain I never discussed a mass-energy density, that is the stuff of a rank-2 tensor.

Re:ripples in fabric of space-time?

[necro81]

If this test is sucessful, what can be used with the information the scientists gain?

The development of warp drive capabilities, of course.

Moment of Creation

[Anonymous Coward]

enabling scientists to probe the moment of creation itself

In other words, scientists will get a touch of His Noodly Appendage, and a bath in His Mighty Sauce.

Re:Moment of Creation

[stonecypher]

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.

Mmmmmm.. Ripples.....

[insanarchist]

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!

Eddies in the Space/Time Continuum

[Ohreally_factor]

No, seriously, he is. Anyone have any idea on how to get him out?

Re:Eddies in the Space/Time Continuum

[TrekkieGod]

I don't care if he gets out, I'm keeping his couch.

Re:Eddies in the Space/Time Continuum

[SEWilco]

Dude, we're in it too. You go first.

Will they measure the speed of gravity as well?

[master_p]

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). 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.

Re:Will they measure the speed of gravity as well?

[Anonymous Coward]

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.

Re:Will they measure the speed of gravity as well?

[node 3]

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!"

Re:Will they measure the speed of gravity as well?

[RocketRainbow]

Yes, we can create anti-gravity stomach holder-uppers, navigate Guild ships through folds in space-time, even create invisible spaceships... but it's meaningless without a good religion and a good sniff of Melange.

Re:Will they measure the speed of gravity as well?

[r2q2]

If you read the wikipedia article it says that that variance is in line with current theoretical model's of general relativity.

Re:Will they measure the speed of gravity as well?

[anshil]

>>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 on one side until it appears on the other side.
* And if it only travels at the speed of light, since the electron with it's electric charge can never be INSIDE the barrier. Where is the charge when it does not travel instant, does this violate the symmetry of eletric charges?

Re:Will they measure the speed of gravity as well?

[steveo777]

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.

Re:Will they measure the speed of gravity as well?

[flumps]

... doesn't that rather pre-suppose two things?:

        1. The aforementioned Anti-gravity and

        2. That you will become important enough to know a Baron?

Both, I would say, are equally improbably but 2 perhaps slightly more so :P

Love the idea though rofl

Re:Will they measure the speed of gravity as well?

[internic]

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

Re:Will they measure the speed of gravity as well?

[exp(pi*sqrt(163))]

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.

Re:Will they measure the speed of gravity as well?

[infolib]

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 probability of having 0 or 2 (or 3 or 4 apples) you could try to model your uncertainty regarding the number as a Gaussian probability distribution having a very small but finite sigma. You now have 1+/- sigma apples.

I really wouldn't eat any apples arising from quantum fluctuations though. First of all you have no idea what they're made of, plenty of apple-resembling quantum fluctuations might be toxic. Secondly the apple would be the rarest and most exciting object in the observable universe, probably for our duration. Give it to the Smithsonian or something, even if the curator seems somewhat skeptical...

Re:Will they measure the speed of gravity as well?

[internic]

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 light, you will never find it on the other side, meaning it does not get there instantaneously (or faster than light).

Where a particle is between measurements is an ill-defined question in quantum mechanics (and by definition impossible to test), but I should note that when you do measure, the electron can be found inside the barrier. While the barrier in a tunneling experiment is "classically forbidden" (meaning it would take more energy than is available for the electron to enter, according to classical mechanics), in quantum mechanics there is a probability that the particle may be found inside the barrier. Depending on what the "barrier" is, it may be difficult to measure the electron there, though. In order for the electron to get to the other side of the barrier, though, it must have some probability of being found inside the barrier, because probability current in quantum mechanics follows a continuity equation.

So, the short answer to the question of whether the electron is between measurements is that it is in a superposition of many possibile position, which include positions inside the "classically forbidden" barrier.

Re:Will they measure the speed of gravity as well?

[exp(pi*sqrt(163))]

Not to nitpick or anything, but I think the number of apples might be better modeled by a Poisson distribution than a Gaussian.

Re:Will they measure the speed of gravity as well?

[stonecypher]

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 violating relativity; we have experimentally verified particle entanglement with bose-einstein condensates for up to half a second now, for example. (For a more comprehensible explanation, look up the science fiction term "ansible" - the idea is similar enough that when you go back to read about the real thing, light bulbs should start going on.)

Re:Will they measure the speed of gravity as well?

[stonecypher]

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 that this would in fact violate relativity - sure, if it's something like the Higgs Boson or the Graviton at work it would, but there are several gravitational theories which aren't obviously wrong that rely on some form of entanglement, and entangled particles can co-react at distances and speeds which relativity would otherwise prevent (that bose-einstein condensate ansible they made six years ago comes to mind.)

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.

One is reminded of the reactions received by Faraday, Dyson, Vannevar Bush and Feynman. Whereas I'm inclined to agree that we're not likely to derive any low-power neato technologies from a better understanding of gravity, I don't think it's entirely reasnable to call it far-fetched. For example, once we better understood electromagnetism, we knew how to manipulate it to cause pulses (not that it's particularly reasonable, but a carefully vibrated quantum black hole might do the same for gravity.) Once we knew how to pulse magnetism, anodizing became relatively easy. The end result? Better cookware, mag-lites and more durable car engines.

Who knows what we'll learn to do in materials science when we learn how to manipulate gravity?

Re:Will they measure the speed of gravity as well?

[master_p]

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.

Re:Will they measure the speed of gravity as well?

[master_p]

I am not so sure about that. Perhaps new ways to control currently uncontrollable forces are found...a theory that includes GR and then allows for more ways to control matter.

Re:ripples in fabric of space-time?

[RocketRainbow]

Runlevel 5 asked: "what can be used with the information the scientists gain?"

It would certainly explain the fact that there seems to be an upper limit on the rotational frequency of neutron stars (pulsars). Likewise, you can also expect to see gravity waves in the oscillation of large stellar bodies in collision, which might also give insight into gamma ray bursts.

One of the most interesting things we can do with gravity waves is look back beyond the cosmic microwave background and watch the early gravitational shape of the universe, perhaps detect a sort of cosmic gravity wave background. It's something we've never done before, so it's a sort of "let's see what we find when we turn this thing on" experiment - we could find all sorts of things about the shape and evolution of the universe which might in turn make a tremendous difference to the way we interpret earth-bound physics.

There is no danger from gravity waves and no apparent engineering purpose (not even warp drive) because they are astonishingly small - even a 4m long laser can't detect them (yet! - some technological improvements are on the way). This is because gravity is such a weak force that the only detectable gravity waves are caused by extremely massive bodies moving at extremely high speeds; even then, the strongest waves are easily able to dissipate to "nothing" before we would ever notice them. (In numbers, the best gravity wave LIGO could ever expect to see would cause the scientist's beautiful assistant to have her dimensions perpendicular to the wave oscillate at an amplitude of 10^-21m.) So it's not just a matter of understanding and engineering gravity waves, rather of using them to confirm or falsify key elements of our physical and cosmological theories.

Of course, theoretical physics has some interesting and wholly unexpected practical outcomes... Your computer uses quantum mechanical transistors - your webcam uses a quantum mechanical CCD (photoelectric effect) and medical tomography, using astronomical algorithms, continues to save lives.

Not to mention

[maillemaker]

How cool it would be to fly like superman. :)

Steve

Wow, I need to get laid

[Raleel]

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

[mike449]

I am surprised nobody mentioned Einstein@home - http://einstein.phys.uwm.edu/.
This experiment uses distributed computing to process their results,
and you can participate.

wrong

[m874t232]

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.

let me be more precise

[m874t232]

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.

Re:let me be more precise

[Open_The_Box]

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.

Re:let me be more precise

[m874t232]

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 provided a compelling argument why this experiment should be any different. How many billions of dollars are to be sunk into these kinds of fishing expeditions?

The potential gains in knowledge of astronomy, astrophysics and even particle physics are vast.

Until physicists get their house in order, I think large scale experimental physics like this (gravity wave detectors, particle physics, fusion) should get defunded. There are enough areas, both within physics and outside physics, that have yielded far more tangible scientific (not to mention, practical) results and that would use funding far more effectively.

Re:let me be more precise

[Open_The_Box]

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 right with several industrial applications and assorted spin-off tech companies.

As to how many billions of dollars it takes - quite a lot. But the practical outcomes I've listed are what you get. Along with international co-operation - many countries working together for a common goal.

And another thing. It's gravitational wave detection. Not gravity wave detection, which is something completely different.

Large scale experiments are what research is all about. There comes a point in research where a table-top experiment just won't do.

Re:let me be more precise

[jkauzlar]

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 technology involved and solving smaller problems along the way. I can't think of any specific examples except for Tang, but I imagine the space shuttles have contributed far more to the average everyday experience of human life than just photographs of the earth from space. Sophisticated national defense is an enormous product of such otherwise useless experiments. Computers themselves were once mind-bogglingly expensive laboratory experiments with few practical applications, as were most of the high technology we see around us.

The early experiments in quantum mechanics eventually became applicable to very small-scale microprocessor technology. Einstein's theories-- well, umm, you know.. But you see my point-- the fact that light bends in space leading to the technology which clinched us the second world war? Pretty remarkable.

Re:let me be more precise

[m874t232]

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 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.

As to how many billions of dollars it takes - quite a lot. But the practical outcomes I've listed are what you get. Along with international co-operation - many countries working together for a common goal.

Strange as that may be for you to believe, but people don't just collaborate internationally on big physics projects, and useful spin-off technology doesn't just come from big physics projects (and I suspect that dollar-for-dollar, large scale physics projects are one of the least productive projects when it comes to valuable spin-offs).

So, compared to this experiment, many other projects that could be funded with this money not only yield all the practical benefits you list, but in addition have a clear, predictable, and demonstrable scientific benefit no matter what the outcome of the experiment.

Large scale experiments are what research is all about.

I guess according to you, the the millions of researchers in the world that make do with small budgets just aren't doing real research; it's only when you have figured out how to milk the taxpayers out of a few billion dollars for a single experiment that you graduate to real research, right?

There comes a point in research where a table-top experiment just won't do.

We have funded these kinds of experiments for decades, all with negative outcomes. So, there also comes a point at which investing ever more in the same kind of large-scale experiment that yield no results won't do anymore. It seems to me that we have reached this point when it comes to direct detection of gravity waves.

Therefore, again, my question: what's the justification for doing this particular experiment, where previous experiments have failed? Simply saying "it has more sensitivity" isn't good enough--you need to explain why this level of sensitivity should be good enough when the same kind of experimentalists previously argued that the previous level of sensitivity ought to have been good enough then.

And another thing. It's gravitational wave detection. Not gravity wave detection, which is something completely different.

I think you're smart enough to figure out which of the two (valid) senses of "gravity wave" I'm using (and if you have ever bothered to read the original papers, you'll understand why this ambiguity is unlikely to go away even in English).

Re:let me be more precise

[spyinnzus]

There are many more benefits of the large scale experiments that engineers can't see. Who does the experiments? College Professors, for the most part, or people aiming to be college professors In my experience getting a BS in Physics, the good teachers were the professors who were teaching one or two classes a quarter and working on this type of research. The lecturers with masters in the field and never did big research projects have always been horrible teachers. So universities and governments need to fund these professors to do what they want to do, whether prove GR works or Standard Model works, and in return they not only give us the results of these studies, but they teach the next generation of scientists, engineers, and even liberal art majors. It's why developed countries have great universities, because what good professor wants to teach in a university that wants him to teach four classes and in a country that won't fund his project? There are very few non-academics that work on a research project, and if they are they are usually the technicians or engineers who keep the experiment running by building/refining gadgets.

Re:let me be more precise

[SlowMovingTarget]

...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 gambling, doesn't it? The payouts to society increase exponentially with the outlay. But, I forget; economics isn't about culture or society.

If you must take the short-sighted view: big projects mean lots of jobs.

Re:let me be more precise

[Open_The_Box]

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 the initial driving project. Sure, once the spin-offs have been spun off they'll take on their own economic/technological momentum, but you need the initial boost. It's also a big 'if' at the end there. Does the sum of the outcomes/advantages of such a project justify investment? Let me just state up front that, my feeling is that it does. It's not just whether you get back the capital investment. You've got material gains in the form of industry and spin-offs. You've got the (much underestimated) continuation of scientific endeavour in the form of scientists who work on projects they find stimulating - who teach other younger scientists, who teach other etc... and so on, continuing the tradition of training people to think in a scientific method. You've got the gains in knowledge already mentioned. And so much more - not all of it judgeable in terms of economic costing. You put money in, you get research and overflow into the economy and prosperity in the local area and international recognition in a field of research and knowledge, out.

Strange as that may be for you to believe, but people don't just collaborate internationally on big physics projects, and useful spin-off technology doesn't just come from big physics projects (and I suspect that dollar-for-dollar, large scale physics projects are one of the least productive projects when it comes to valuable spin-offs).

Oh, I do believe that people collaborate internationally on other projects of varying types. And believe me, I'm well aware that large physics projects do not spin out as many companies/technologies as they could. But how many small research projects have to be funded so that you get one decent spin-off? A hundred? A thousand? There's also the fact that large scale projects such as this one are essentially sub-divided into smaller research blocks, each of which concentrates on a separate (potentially spin-off-able) area with the added bonus that, at the end, you get a gravitational wave detector. I understand that there are other ways of funding research, and other things to be researched, and I understand that smaller research groups need funding too - indeed, it would be a callamity of massive proportions if all funding went on large projects, but that doesn't mean we shouldn't fund the large ones.

I guess according to you, the the millions of researchers in the world that make do with small budgets just aren't doing real research; it's only when you have figured out how to milk the taxpayers out of a few billion dollars for a single experiment that you graduate to real research, right?

Pardon? At which point did I say anything that even remotely sounds like that? I said that doing big projects is what research is all about. Research should be big! Even the smaller projects shold be funded properly. It should be grand and it should be well funded. All of it. And just because a project is small doesn't mean it will never become big. It's where the GW detectors started out - in a small lab. I don't know any researcher who, given the chance, wouldn't like to take their project to its ultimate conclusion. In the case of gravitational wave detection (which is essentially astronomy, or at the very least experimental 'telescope' design/research) this requires a large footprint installation and ultrasensitive equipment run by experts. Any idea how much a telescope costs? Quite a lot I believe. And I know many peopl

Re:let me be more precise

[m874t232]

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 "right now, just because we want to see what happens"--that attitude is almost unique to experimental physics.

The cold war is over, other disciplines have grown up, and other disciplines don't have an inferiority complex anymore. In the future, projects like LIGO II will have to be justified relative to other possible big funding efforts in computer science, biology, ecology, etc.; both politicians and other scientists will demand it, and, mark my words, funding for projects like LIGO II will get increasingly difficult.

But how many small research projects have to be funded so that you get one decent spin-off? A hundred? A thousand?

I think you live a rather protected life. In most disciplines other than experimental physics, you must justify your research in terms of societal and practical relevance, and they often include requirements for commercialization. That's not because those other sciences don't have fundamental questions to ask--they just don't get the funding to do it. So, in short, the answer is, that many funding agencies aim for one spin-off for every single small research grant.

And the only reason I picked up on it is that the distinction is a pet hate of mine. [...] But just to clarify for anyone else, gravity waves are fluctuations in gravity itself. Gravitational waves are a quadrupole phenomena which result from localised changes in gravity from a suitably massive source generating ripples in the background curvature of space-time (where space-time is curved due to the presence of mass).

Actually, the ambiguity that usually upsets people is that "gravity wave" in English also refers to a phenomenon in fluid dynamics. I don't think the original German papers even make the distinction that you are making.

Re:let me be more precise

[m874t232]

Apparently, they did not teach you critical reading and thinking as part of your BS: I didn't argue against research funding, I questioned whether "large scale" research funding (i.e., funding for megaprojects--someone else chose the terminology for this discussion) is the right kind of funding, instead of equal amounts of total funding divided up among lots of small projects. And if physics can't deliver good scientific results with many small projects, then that's a problem with physics as a discipline; there are many other scientific displines that not only deliver excellent fundamental scientific results, but also demonstrably better spin-offs.

We have already established that, unlike other disciplines, physicists are unwilling to community the reasons or justifications for experiments like LIGO clearly and accurately to scientists of other disciplines or the general public, instead relying on statements amounting to "trust us, we know why it's important" and "it's too compliated, you wouldn't understand", and the always popular but completely erroneous "this will confirm Einstein's theories". That is perhaps another reason these people shouldn't get funding until they change their tune.

Re:ripples in fabric of space-time?

[metamatic]

There is no danger from gravity waves and no apparent engineering purpose (not even warp drive) because they are astonishingly small [...]

You could say the same about atoms, but I think the people of Hiroshima and Nagasaki would beg to differ.

uh.. about creation...

[NewToNix]

"enabling scientists to probe the moment of creation itself."

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

what if it cancels itself out?

[cyclomedia]

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 the one you are measuring. Geddit?

Re:what if it cancels itself out?

[Roskolnikov]

First, I don't even pretend to understand physics, but have had a few bad experiences with gravity (3 story fall, fun, landing, bad).

which leads me to ask;

if your measuring from point A (emitter) to point B (receptor) and time/space is curved during the test by what we will presume to be
a gravity wave.

The straight (hah) line distance between the points might become shorter but the path followed by the laser should curve.

Timing this should result in the same time regardless of curvature much like light travelling in fiber optics?

Are they looking for shift in the wavelength of the laser ?

Re:what if it cancels itself out?

[Roskolnikov]

thanks, I get it, silly me I was thinking one beam.

Question for a Physics Buff

[uniqueUser]

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 photon taking one route over the other should be random unless maybe if the mirror is polarized? If it can not be predicted, is this the limitation of accuracy for the GEO 600?

Re:Question for a Physics Buff

[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?

It's random, and is 50/50 for a 50% transparent mirror.

If it can not be predicted, is this the limitation of accuracy for the GEO 600?

Uncertainty in the number of photons arriving at the detector is a limitation of the experiment's accuracy: it's called "quantum shot noise". However, that uncertainty would be present even without a mirror to split them. Perhaps the mirror exacerbates the effect.

Re:Question for a Physics Buff

[dltaylor]

Waves, not particles, grasshooper. Interference (hence "interferometer") is a wave effect. This is an experiment where you do not try to see which way the photon went (which would collapse the wave), so, in very simplistic terms, it went both ways. The question is whether "both ways" took the same amount of time. If not, something differentally shortened, or lengthened, (depending on your point-of-view) the arms.

I do wonder if a "ripple" could resemble an acoustic compression wave and interact with the apparatus such that the "high-density" and "low-density" phases of the ripple lengthened then shortened the arms during the photon's flight time to provide a net "no-effect". Would depend on the wavelength of the ripple, I suspect.

Gravity Wave Generator - At Caltech

[QuantumFTL]

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 laying around :)

Re:Gravity Wave Generator - At Caltech

[budgenator]

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.

Re:Gravity Wave Generator - At Caltech

[stonecypher]

It was a joke. All physical objects are such generators.

Re:Gravity Wave Generator - At Caltech

[QuantumFTL]

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.

Re:Gravity Wave Generator - At Caltech

[QuantumFTL]

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 several miles away from the array, at least, at which point nothing but the gravity waves would survive.

Also, note that there are methods that can pull out a constant signal at very, very low signal to noise ratios, given enough integration time. An astrophysicist I worked with at Cornell, the late Dr. Thomas Gold, once devised a method whereby he calculated that a pile driver, being operated constantly for about a week could be detected at the opposite side of the globe.

It's still possible it's a joke, of course, but it's not a slam-dunk by any means.

Re:Gravity Wave Generator - At Caltech

[stonecypher]

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 true too, but obviously a joke, because a brick isn't a real sound generator.

Please don't make the argument for ignorane. It's ugly. I promise I won't wound you by answering your questions in the future. Douche. I can't imagine why you'd ask for clarification if you're just going to talk back to the person who explained it to you, but I won't bother you again.

By the way, that frequency control? It was just a disconnected knob (something you should recognize as a like mind.) The Daleks had knobs which didn't actually do anything too. So did the set of the Enterprise. In the attempt to make a believable object, sometimes we give it pseudo-convincing controls. It wouldn't be a very funny gravity generator if it didn't even *try* to look like a scientific instrument, now, would it?

For someone with a physics related nick, you sure don't seem to know much about physics.

Re:Gravity Wave Generator - At Caltech

[budgenator]

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.

Detector location

[SEWilco]

The detector is in Germany, visible on Google Maps photos. The beam arms are WNW of the arrow, along a trapezoid-shaped field with a power line tower (look for the shadow) in it. The beam arms form a 93 degree angle and resemble a road at the default zoom. The GEO 600 visitors page has a different photo.

Re:Detector location

[SEWilco]

Here is a corrected Google Maps link which includes the lost "Germany" specifier. Click on "Satellite" for the photographic view.

Can someone explain Feynman's thought experiment ?

[HuguesT]

Hello,

When asked if gravity waves 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 the bead and the stick by friction. This heating, said Feynman, showed that the wave did indeed impart energy to the bead and stick system, so it must indeed transport energy, contrary to the view expressed in 1955 by Rosen.

As I understand it, the movement of the bead is caused by the gravity wave causing local changes in spacetime metric, i.e. locally contracting and expanding. Assuming this, why aren't the stick *and* the bead both moving ? If the stick is also locally contracting/expanding, then it may remain locally motionless with respect with the bead, and hence no friction, and therefore no heat and no energy.

Is it the case that the gravity wave is generating local acceleration and not the kind of contraction/expansion one sees in both special and general relativity ? if so, why is the LIGO experiment supposed to work ? LIGO relies on contractions and expansions (at 90 degree angle ), not local changes in acceleration.

Sorry if this is not clear.

Re:Can someone explain Feynman's thought experimen

[Anonymous Coward]

If the stick is also locally contracting/expanding, then it may remain locally motionless with respect with the bead, and hence no friction, and therefore no heat and no energy.

The stick does not contract and expand, for the same reason that if you push/pull on its ends a little bit, it doesn't contract/expand. Or rather, it resists the compression/expansion, because the molecules try to be at a certain distance from each other: they have a fixed equilibrium distance that they try to maintain. (If you push/pull too hard, of course the rod will deform.) The beads, on the other hand, can slide "freely", or almost freely, if there's only a little static friction.

This is related to the reason why atoms don't expand when the universe expands.

Re:ripples in fabric of space-time?

[stonecypher]

If this test is sucessful, what can be used with the information the scientists gain? It may become possible to predict future ripples, but the nature of such phenomena would suggest that they can't be avoided or blocked.

The issue is one of context. We have a whole bunch of data that we're not entirely sure how to interpret, and although we have a lot of very convincing extrapolations, we really can't work with them until we know some of the foundational material is correct. There are dozens of examples of where scientists have built large cloud castles on assumptions, only to have them ripped out from underneath; my personal favorite is the theory of phlogiston, which is really quite an elegant theory, up until you discover that oxidation is the adding of an element rather than the extraction of an element. A whole bunch of impressive work was lost because it was invalidated when that theory went away.

By comparison, what we're doing here is essentially akin to trying to find out what burning really does. We're attempting to confirm frame relativity. Once we have said confirmation, then we can say "okay, that means that this interpretation of this data is correct, and that interpretation of that data is correct, and that means some other thing." We are providing ourselves an attempt at a proof of context by which to confirm other beliefs and extrapolations.

What direct use will this have? None. But it's the foundation of a lot of different direct stuff. Recent examples of such a thing include the exploratory work done into phase duality and uncertainty locii, which eventually led to our understanding the charge field effect, and thusly to our being able to manufacture LEDs (and a whole bunch of other crap.)

As an aside, whoever marked this question off-topic is an asshole. It's not only on-topic, but a damned smart question for someone who isn't an expert in the field.

At last :-)

[Goth Biker Babe]

I worked on this in the 90's. I'm glad to see that its finally up and running...

Re:Can someone explain Feynman's thought experimen

[HuguesT]

OK, then how are the LIGO arms suppose to extend ? It seems to me (naively) that either distances expand or contract and then there is no relative motion, or there is relative motion and rigid bodies dont contract/expand. Unless we are talking about a second order effect.

Re:Can someone explain Feynman's thought experimen

[NexusJedi]

Perhaps if the distortion of space-time caused the distance between the stick and the earth to change unevenly along the length of the stick, it would cause the stick to appear to tilt from the beads perspective. Thus, what would actually cause the bead to move is the earth's gravitational field. Would that still count as transmitting energy? I don't really understand this stuff, so it probably wouldn't even work that way, anyway.

Re:Can someone explain Feynman's thought experimen

[HuguesT]

Ok, thanks, it makes much more sense now.