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?

[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?

[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?

[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?

[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?

[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?

[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?

[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?

[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: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.

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?

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?

[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?

[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: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.

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

[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

[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).