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Examining Gravity Waves
Joseph "JoeDaMac" Haake writes "Sometime within the next two years, researchers will detect the first signals of gravity waves -- those weak blips from the far edges of the universe passing through our bodies every second. Predicted by Einstein's theory of general relativity, gravity waves are expected to reveal, ultimately, previously unattainable mysteries of the universe."
"Researchers WILL detect..."
On the whole, i think that's not necessarily true. There are several mathematically consistent fringe quantum-physical theorys (usually something akin to higher-order-symmetry electrodynamics) in whcih gravity waves are indistinguishable from e.m. waves, or are longitudinal-time e.m. waves.
Suen and his collaborators are using supercomputing power from the National Center for Supercomputing Applications at the University of Illinois, Urbana-Champaign, to do numerical simulations of Einstein's equations to simulate what happens when, say, a neutron star plunges into a black hole. From these simulations, they get waveform templates. The templates can be superimposed on actual gravity wave signals to see if the signal has coincidences with the waveform.
"When we get a signal, we want to know what is generating that signal," Suen explained. "To determine that, we do a numerical simulation of a system, perhaps a neutron star collapsing, in a certain configuration, get the waveform and compare it to what we observe. If it's not a match, we change the configuration a little bit, do the comparison again and repeat the process until we can identify which configuration is responsible for the signal that we observe."
They will be changing the way they observe in order to conform with what they expect to observe. Doesn't this mean that ultimately they're not really going to discover anything new? I mean, if they set up their observation so that when looking at a neutron star collapse it matches the mathematical model, what's the point? Why not just look at the mathematical model?
Karma: Chevy Kavalierma.
Well, in the mind of the commoner, Einstein is the scientist par excellence.
You can't buy t-shirts or posters with Newton. You can with Einstein. Ask people if they know a physical formula, and they'll say E=mc^2, not F=GMm/r^2 or F=ma. Say 'genius', and people think of Einstein, not Newton (although Newton probably was more ingenious than Einstein). Hell, think about it; they'd have to change the scientist icon in games like Civilization to something else...
frawaradaR anahaha islaginaR!
Scientists are extremely uptight about exact numerical analysis. We get the data and compare them to a tightly parameterized model which includes everything we know about our detector response as well as the probable sources of the events we are detecting. Good models have small numbers of parameters and many constraints compared to the richness of the data. "With enough free parameters you can fit an elephant", the saying goes, which indicates how important it is to scientists to keep the number of parameters small--no one wants to see a comment like that on a referee's report!
With regard to gravitational observatories, the data are very rich: polarization, amplitude, phase and frequency spectra will be available, possibly from several detectors with different orientations. Detector response is also extremely well understood. The theoretical physics of the sources--general relativity--is also very well understood, and models of stellar collapse, neutron star collisions, etc, contain few parameters (masses, angular momenta, impact parameter...that's about it.)
As such, we can compare model to reality and produce a statistically valid likelihood that the model is false. The Baysians in the audience will point out that relative to our prior knowledge we can also produce the probability that the model is true.
So it isn't a matter of getting something that "roughly fits"--the analysis either produces a fit within error or it does not. If it does not, we dig more deeply into the possible sources of disagreement. The data are sufficienty rich that many, many types of cross-checking and internal consistency checking will be available.
To a hardened skeptic, this of course will not do. But hardened skepticism is an anti-scientific attitude. Scientists are open-minded skeptics, who are able to keep the contingent nature of their beliefs in mind while at the same time maintaining a commitment to distinguish clearly between probable truth and probable falsehood.
--Tom
Blasphemy is a human right. Blasphemophobia kills.
You are completely right, but this can be a very dangerous thing to try to do. I work in computational fluid dynamics, and some people advocate doing this kind of CFD (tuning turbulence models to match data of very complex things usually) this leads to some bad mojo most of the time. you get codes that look good when you use them on multistage axial flow trans-sonic compressors (for example) because it was tuned to that, but it can't solve flow in an axisymetric duct! Then people think the code is great and start to trust it until it misses in a huge way on something that is a little different that what it was tuned to and everyone freaks out!
I REALLY think that the only way to do this kind of thing correctly when you don't match data, is to go back and look at the set up/first principles... Were your boundary condition assumptions fair? Did you assume anything was insignificant, was it?... that sort of thing. Tuning is something that scares the crap out of me, mostly because it sounds like a good idea to most people.
"I'll have a Guinness, no wait, make that a Coors Light" -Grad student I work with, who shall remain anonymous...
What I meant was, could this new data resolve some of the inconsistencies in physics?
Yeah, and the answer still is "not directly".
More (and better) data at the turn of the century helped scientists discover the inadequacy if Newtonian mechanics,
Yes, that's the usual story. But it's not really that accurate.
the constancy of the speed of light,
Actually, this was a theoretical prediction of classical electrodynamics, not something that was first discovered by experiment. Most physicists of that era just didn't like this prediction, so they tried to interpret it through the ether idea - and then later experiments disproved this idea.
I know you've probably heard the story about how the Michelson-Morley experiment left everyone baffled until Einstein came along and explained everything by taking this observed constancy as a basic postulate of a new theory of mechanics. That's a nice story, but it's not what actually happened! There is little evidence that Einstein was even aware of the whole experiment. His first article on the special theory of relativity doesn't refer to it (some parts of it can be interpreted as evidence that Einstein was aware of the experiments, but not very convincingly).
So it's not like some "new and better data" suddenly made everyone realize there was something wrong with the current theories of physics. There were two basic theories of physics, mechanics and electrodynamics, which weren't compatible (unless you made some additional, artificial postulate, ie. ether). Einstein solved this problem by theoretical thought alone by modifying the other theory; he didn't use any experimental data (expect, of course, the data that verified classical mechanics and electrodynamics in the
first place).
So, the point of this long explanation is that scientific progress doesn't necessarily follow this simple path of "oh, here's the new data... oops, it doesn't fit our theories, we better invent new ones... oh, here's the new data..." (and, in fact, with the most fundamental theories of physics, it never does).
Right now there is a one similar, big inconsistency in modern physics: quantum mechanics and general relativity aren't "compatible". This is not completely analogous to the situation with the ether and all that: since we have succesfully made all the other classical theories (mechanics, special relativity, electrodynamics) compatible with quantum mechanics, we would expect that general relativity we could similarly quantize general relativity and get a "quantum" theory of gravity. We already know which particular feature of general relativity makes the usual quantization methods fail, so many people think this is just a question of finding the right way to do it.
(In fact, in situations in which this annoying feature of general relativity - its "nonlinearity" - isn't important, we can already make some credible calculations "combining" general relativity and quantum mechanics. The best known example is Hawking radiation.)
And, like I said in my previous post, we're not expecting this experiment to show any "quantum" effects. We have already verified general relativity on this scale (and it works - you can't see any quantum gravitational effects in the motion of planets, for example). If general relativity were to fail on this scale, we should already be able to see quantum gravitational effects in other experiments. So, the only way we could see QG in these experiments would be if GR and QM turned out to be completely wrong... and, even though you all non-physicist out there may not believe me when I say this, this is just not going to happen.
Just like I said earlier, you can safely compare this to classical electrodynamics and light: it doesn't take much experimental accuracy to verify the existence of light (ie. electromagnetic waves), but it does take a lot of work to get to the level where you get to see quantum electrodynamics in action. Similarly (this analogy is actually very close to being exact), there's a long gap between being able to merely detect gravitational waves and seeing quantum gravity in action. Even the former is very difficult to do (as should be evident), so it shouldn't be surprising that nobody expects that the latter is going to happen any time soon ("soon" quite possibly meaning many centuries or even millenia).
Like I said, there is always the possibility that we might be able to see some unexpected things through these gravitational waves, but the waves themselves will be just what classical general relativity predicts (and if they aren't, it will not mean we've hit the quantum theory of gravity; it will mean that GR is completely wrong).
And, of course, most importantly, there are a lot of interesting thing out there waiting to be discovered that just aren't the most fundamental things that exist. Not every discovery can lead to a great revolution in fundamental physics, but that doesn't make the discoveries any less exciting! The big revolutions happen so rarely that if that's all you're interested in, you're not going to get much else than disappointment from science.
(Really, a whole new kind of astronomy is being born! It's going to tell us all sorts of interesting things about the universe, even if it doesn't lead to the Theory of Everything. And that's exciting enough for me!)
wave-particle duality, and the structure of the atom.
Now this is getting closer: the Bohr model was rather directly based on experimental evidence. But the experiments were actually very misleading: they made people believe that some kind of discreteness was essential, which made them develop a theory (originally called quantum mechanics) based on some arbitrary "quantization conditions", while the real theory was actually something completely different.
Now we're stuck with the horribly misleading term "quantum mechanics" and a whole lot of people who think "discreteness" is the most essential feature of the theory. But, umm... this is getting offtopic, so I better stop right now...
The theories themselves may have had nothing to do with experimental evidence, but the verification of those theories did. Without more powerful technology, those theories would have been mere conjectures, since they don't significantly differ from classical physics in 'normal' situations. To continue the SR/GR example: I knew that Einstein developed his theory without knowing about Michelson-Morely. The important thing, as I see it, is that the Michelson-Morely experiment provided an experimental validation of one of the postulates of his theory.