Lab Tuned to Gravity's 'Ripples' 173
Krishna Dagli writes "One of the great scientific experiments of our age is now fully underway. Success would confirm fundamental physical theories and open a new window on the Universe, enabling scientists to probe the moment of creation itself. The experiment is trying to detect ripples created in the fabric of space-time that sweep out from merging black holes or exploding stars and detection would be a final test of Albert Einstein's General Theory of Relativity. "
negative outcomes? (Score:3, Interesting)
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? (Score:5, Interesting)
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? (Score:5, Interesting)
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:ripples in fabric of space-time? (Score:5, Interesting)
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.
Re:negative outcomes? (Score:2, Interesting)
http://physicsforums.com/showthread.php?t=87097 [physicsforums.com]
The theory also predicts gravity waves, but the transverse modes of emission for a 4D wave are EM, and the longitudinale and scalar modes are the stuff of gravity. So GEM theory (gravity and EM) predicts that gravity waves will travel at the speed of light, but the polarization will not be transverse like GR predicts.
I think gravity MUST be viewed as a longitudinal wave, not transverse. Here's 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? (Score:3, Interesting)
Re:negative outcomes? (Score:2, Interesting)
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:let me be more precise (Score:3, Interesting)
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:negative outcomes? (Score:2, Interesting)
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.