Slashdot Mirror
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."
In a perverse sort of way, I'm hoping that this experiment generates all the wrong data. Data that is completely the opposite of what people expect.
Think of all the fun that would be! Think of the chaos, the pontificating, the explanations, the TV specials! Think of all the dissertations that would generate! Yes sir, that would be wonderful.
If tits were wings it'd be flying around.
"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.
You can also read the story here.
The LISA experiment, which gets mentioned in passing, is really quite audacious - three spaceships orbiting the sun in a clever rotating triangle pattern, 5 million miles apart from each other, and detecting changes in distance between each other to an accuracy of 20 picometers!
In essence, it's just a really, really big version of the Michelson interferometer we all played with in 1st year physics - I remember the thrill back then of realising what tiny changes in distance you could discern with just a couple of mirrors, a lamp and something to measure the recieved intensity.
It's exciting to witness the nascence of an entirely new form of astronomy.
No, read carefully. By "change the configuration", Suen means changing the configuration of the model. So they just massage the model until it fits the observed results.
It's a classical inverse problem, he's just trying to explain it in layman's terms and making a bit of a hash of it...
Perhaps the most exciting thing about them is that we may well not know what it is we're going to observe. We think black holes, for sure. But who knows what else we might find?"
Jabba-The-Hut doing the Wild Thing.
"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."
Sounds to me like they may be changing their model to fit the data in such a way that they won't know for sure it is a match. For example, a signal roughly fits the model of a black-hole forming, but not quite. They then keep tinkering with the black-hole formation model until it matches the signal. But in reality the signal could actually be something not related to black holes. They are putting the cart before the horse it seems.
It seems they would have to match a specific electromagnetic observation(s) to the gravity wave event to verify. Otherwise it is just a guess.
I could see some justifiable confidence if the signals were complex, and were only slightly off the models. In other words, near dead-ringer matches of something that would be too much of a coincidence to be something completely different generating the same (expected) complex signal. But I doubt we are at this stage in both the models and accuracy of the signal detection.
Table-ized A.I.
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...
It is only mentioned briefly in the article, but I'll try to elaborate.
Basically gravity waves will stretch space in one direction (say x) and contract space in a perpendicular direction (y). Given this, the "easiest" way to detect gravity waves is to build a very large interferometer. LIGO is the current ongoing gravity wave interferometer, which splits one laser beam into two lasers beams, sending each perpendicularly down a vacuum "hallway" four kilometers long. At the end, the beams are reflected by mirrors. The two lasers meet again after another 4km.
The two beams are recombined afterwards. If the distances the two travel are exactly equal, then the two beams will interfere constructively. But if the lengths which the two beams are stretched/contracted by a passing gravity wave, the beams will interfere since one will be "shifted" (it had to travel a longer/shorter distance. By measuring the interference pattern between theses two beams, and hopefully physicists will be able to detect a gravity wave.
The amount that a gravity wave will shrink/extent one of the beam lines is amazingly small. Each 4km beam line will have it's distance changed by 10^-18 meters, or on the scale of attometers! Because of this, any vibration or local variation will affect the beam length. So the physics who are part of the LIGO collaboration built two such laser devices, one in Livingston, Louisiana and the other in Hanford, Washington. When a gravity wave (from outer space) travels through the earth, hopefully both sites will measure the same small variation, which will correspond to a passing gravity wave.
You can get more information about LIGO at:
LIGO's Home Page
LIGO collaboration page.
Slashdot recently had a science story about LIGO.
Would this help unify quantum gravity and GR?
No. The waves we're going to see are a prediction of the classical theory of gravitation, general relativity. This is, of course, only an approximation to some "quantum" theory, but on this level of accuracy we're going to see only classical effects.
Compare this with classical electrodynamics (which predicts electromagnetic waves, ie. light): merely detecting gravitational radiation is going to tell you just as much about quantum gravity as seeing sunlight tells you about quantum electrodynamics.
Could it give evidence to bolster string theory?
No.
The results of this experiment should be very interesting.
Yes, but not in the way you seem to be expecting.
No "new" physics is likely to come out of these experiments (at least not directly). The exciting part is, like the article says, that this is going to give us a whole new way of doing astronomy: remember that a century ago the only way to get any information from distant objects was to look at them, but there's a whole lot of objects that are sending stuff at us on wavelengths not visible to the human eye. So, the early astronomers missed many very important things of what we're now able to see.
Being able to observe the whole electromagnetic spectrum has completely revolutionized astronomy in the past 100 years. Just think of cosmic background radiation: for a long time, it was completely missed since nobody was doing astronomy with microwaves. Similarly, there are many interesting things out there that could be sending us a signal through gravitational waves (like, for example, merging black holes) - and soon we'll be able to see that signal and whatever it's telling about these events.
Of course, the resolution will really be of the sort "an event lasting t seconds was recorded...", but we can extract useful information from even this kind of observations, especially if we can combine them with others (like optical telescopes). (This way we may even indirectly discover something totally new.)
I wonder if they will be at all able to measure the speed of a graviton with this current setup. It seems as though they are having enough trouble just detecting them in the first place though. I think this is a first step towards a new branch of physics that uses gravitons in experiments. For instance, some spin-2 thermodynamics could be experimentally demonstrated if gravitrons could be isolated and easily detected. This is probably not going to happen any time soon, but LIGO is a big first step towards that goal.
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...