Talk:Gravitational wave

Back to the basics

Any good article in a popular encyclopaedia should develop its ideas in proper sequence, beginning with the basics and progressing to ever deeper levels of complexity. What this particular article needs in its introductory level is a very simple mathematical treatment of gravitational radiation. I would supply one if I could find one. Unfortunately, all the expert sources seem pre-occupied with extreme cases, such as black holes and massive binary stars in spiral collapse, which complicates the maths. Can anyone direct me to a site that deals with gravitational radiation from small masses? Yes I know that this modest kind of radiation is too small to be measurable in scientific experiments, but likewise it's not so extreme as to be incalculable for laymen. I have searched the web in vain for hours for some such simple treatment. Lucretius 09:04, 29 October 2006 (UTC)

The mass doesn't really matter, and having black holes actually simplifies the math, if anything. I think that what you're looking for is the radiation emitted by a simple binary, in the approximation that it isn't spiralling inwards because of its loss of energy in the form of gravitational waves. This is what I put in the "Sources of Gravitational Waves" section, describing the Earth-Moon system. I don't really know much about web sources, but the best book I can recommend for this sort of thing is "Gravitation", by Misner, Thorne, and Wheeler, Chapters 35 and 36.

I agree that this article is poorly structured, and wouldn't mind seeing it put in better order. I'll be glad to help with that, if you have a good idea for an outline. As an interesting note, Steven Spielberg's next movie will involve gravitational waves as an important part of the plot. I have no idea when it's coming out, but I'll bet anything that this page gets tens of thousands of views just because of the movie, so it'd be nice to see it in good shape. --MOBle 04:48, 30 October 2006 (UTC)

Thanks for this reply. I have concerns about equations like this one, copied from the text:

${\frac {G}{c^{4}}}\,{\frac {1}{r}}\,\mu R^{2}\omega ^{2}\ll 10^{-33}\ .$

Firstly, it seems to me that this equation could be written in more 'user-friendly' terms for a laymen. When I translate the factors such as 'angular frequency' and reduced mass, and then cancel out, I get this:

${\frac {v^{2}}{c^{4}}}{\frac {GMm}{M+m}}{\frac {1}{r}}$

The result is a dimensionless number. Speaking as a layman, I would rather see a calculation of the actual energy that is radiated. The metric stuff can come later in a more advanced level of the article. I think we should remember that people who turn to this article are probably looking for a basic explanation of gravitational radiation. The only people who could really understand the article as it is now are people who have little or nothing to learn from it - they know this stuff already. Anyhow, that's how i see it. Lucretius 08:28, 30 October 2006 (UTC)

Your translation of the equation isn't quite right: $\omega R$ is only the velocity if one of the objects is stationary. The Earth is roughly 80 times more massive than the Moon, so this is a reasonable approximation, but not exact, and certainly not general. I have made a few changes, though. I think the metric perturbation is a good way of thinking of things, if we point out that this is basically the fractional change in the size of anything through which the wave passes. Still, what I wrote there may be a little too precise. We could maybe get by with more approximations.

In any event, there's a whole lot of restructuring of the article to be done, and I think it would make more sense to do that first, then work out the details. The current "Characteristics" section needs to be totally rewritten, the "Derivation" section needs to be cut down and tidied up a lot, and the rest needs reorganizing. Probably the biggest problem with this article is that most of the individual sentences are right and many are relevant, but they're all in the wrong places. As a first stab at this, here is my suggested outline, please revise as necessary:

• Leader (I think the current one is pretty good)
• Introduction -- just a rough outline of the more "user-friendly" parts of the following article
• The effect of a passing gravitational wave
  • Pictures
  • Description of the basic effect
  • Polarizations
• Sources of gravitational waves
  • Discussion of general properties of sources
  • The list of sources
  • Radiation from the Earth-Moon system
    • numbers for this system (masses, separation and orbital frequency)
    • energy loss
    • comparison of that energy loss to energy radiated by stars, or used by humans
• Gravitational wave detectors
  • The general idea
  • Laser interferometers
  • LIGO, VIRGO, GEO, TAMA, LISA
    • Einstein@Home
    • Prospects for detection
• Mathematics
  • A sentence or two about the metric
  • The metric perturbation
  • Einstein's equations
  • Linearized Einstein's equations
  • Wave solutions
  • Generation of these waves by a source
  • Simulations

MOBle 12:00, 5 November 2006 (UTC)

Thanks again for your reply. I think your overview looks very good. The article at present looks as if a committee worked on it and it certainly needs coherent management. You seem to know quite a bit about the topic and I for one am happy to sit in the back seat and enjoy the ride. I hope you find time to include an equation for gravitational radiation. Yes, the Einstein maths belongs at the end. Lucretius

Okay. I think I'll make this my new Wikipedia project. (Maybe simultaneously cleaning up PSR B1913+16.) I might be slow about it, though. --MOBle 00:37, 5 November 2006 (UTC)

Which Direction

In the sentence "Roughly speaking, they will oscillate in a cruciform manner, orthogonal to the direction of motion. First, east-west separated particles draw together while north-south separated particles draw apart, after which east-west separated particles draw apart while north-south separated particles draw together, and so forth." - what direction does "orthogonal" mean? The images are nice but does the radiation come from the viewpoint of the reader or from left/right/north/south? I think you better need an image in 3D. -- Nichtich 23:48, 3 November 2006 (UTC)

In the pictures, the wave is passing directly through the screen, either from behind or in front. We should try to make this clearer in the rewrite. I would welcome 3D versions of these pictures, if they didn't complicate things beyond understanding, but I'm not going to spend my time on it. --MOBle 01:08, 5 November 2006 (UTC)

massless particle waves

"Massless" must refer to infitesimal mass rather than no mass since E=M*c^2. Adaptron 11:44, 4 November 2006 (UTC)

Well, we don't really have any firm theory for the graviton, but we might by looking at linearized general relativity. If we assume that it travels at the speed of light (which we linearized gravity says it would), then the formula E=M*c^2 wouldn't apply, just as it doesn't apply to the photon. --MOBle 00:37, 5 November 2006 (UTC)

Under construction

Hi MOBle. I finally found a formula for gravitational radiation (power) and maybe you could include it in your reconstruction of the article. I took it from here [1] The formula is as follows:

${\frac {32}{5}}{\frac {G^{4}}{c^{5}R^{5}}}(Mm)^{2}(M+m)$

I don't know how this was derived but I'm hoping you'll know a derivation that is user-friendly for laymen. Is there for example a derivation based on the Lamor formula for electromagnetic radiation?:

${\frac {2}{3}}{\frac {Kq^{2}}{c^{2}}}{\frac {a^{2}}{c}}$

where K is Coulomb's constant, q is charge and a is acceleration. Using this as a model, I translate the gravitational radiation formula as follows:

${\frac {32}{5}}{\frac {GM}{R^{2}}}{\frac {GM}{cR^{2}}}[{\frac {Gm^{2}}{c^{2}}}{\frac {G(M+m)}{c^{2}R}}]$

Here m is the mass that radiates and M is the mass that imposes a speed on it. The factors are acceleration, acceleration divided by c and, in the square brackets, is the stuff left over, which appears to be the gravitational equivalent of ${\frac {Kq^{2}}{c^{2}}}$ and which features a ratio gravitational radius/R. I would be surprised if this is an acceptable derivation, but it gives you an idea of the sort of thing an amateur like me can relate to. Hopefully you can find something along these lines. Lucretius 04:36, 5 November 2006 (UTC)

I've never seen a derivation by analogy with the Larmor formula. That's nice. I would have expected a factor of two to come out of nowhere, though. I'll look into it.

I guess, if you like the energy idea, then looking at the energy given off by PSR B1913+16 would be the most interesting thing to do. Despite having a separate page for that binary, it would be good to put something here about the mathematics, for continuity. I've added a little about this to the outline above. --MOBle 00:37, 5 November 2006 (UTC)

Closer to home, if M is the Sun, m is Earth and R the mean distance between them, the radiated energy is approx 200 Watts. That's a couple of lightbulbs worth of radiation. This would be a fine example of the weakness of gravitational radiation relative to the gigantic masses involved.

Regarding a factor of 2, forgive my ignorance but I'm not sure if you mean it should or should not appear out of nowhere. There are these possibilities - if the Earth radiates 200 Watts, then the Sun might radiate another 200 Watts under the Earth's gravitational pull. Or if m=M, then 2 naturally appears. But perhaps the entire system is supposed to radiate 200 Watts, which I suppose would be radiated from the centre of mass. The location of the source of radiation is surely an important issue and maybe you could touch on that also. Or maybe it's a non-issue. I have no idea. Lucretius 02:13, 5 November 2006 (UTC)

You're right. Discussing the Earth and Moon would be more effective on this page, and I'll just put in some discussion on the PSR B1913+16 page about that system. --MOBle 12:00, 5 November 2006 (UTC)

In the section 'Effects of a passing gravitational wave', there is no mention of how quickly the pulse occurs to the ring of particles. If I understand you correctly, this pulse is 'linearly polarized' and the wave travels at the speed of light. In that case, the frequency of the pulse is determined simply by the length of the wave or the pulse, isn't it? The smaller is the length, the more rapid is the pulse - and the higher would be the energy? This 'less is more' quality is typical of an electromagnetic wave, whereas I had thought gravitational lengths were a case of 'more is more'. That's another 'layman' issue that could be cleared up. Gee, I'm glad I'm not writing this article! Lucretius 05:39, 5 November 2006 (UTC)

Also, what polarizes a gravitational wave and why is the angle of polarity different to that of an electromagnetic wave? Is there a simple explanation for this polarization? I hope so. In return for your knowledge I offer you my ignorance, which I think is fairly typical of the general reader and which the article should try to address. Asking questions is hard work, though not so hard as answering them, I guess. Feel free to ignore me. Lucretius 06:30, 5 November 2006 (UTC)

I'll try to answer these questions in the article, except for the polarization angle question. This is actually a deep consequence of the "spin" of the gravitational field. (Its spin is 2.) Now, this is an interesting and important fact, but I'm not sure how to explain this, because there are lots of issues. By spin here, I don't actually mean quantum spin. However, the quantum spin of the graviton is 2, and is a direct result of this type of spin. Anyways, it's complicated, and I don't know how to explain it in simple terms, so I'm not going to try just yet. Maybe there should be a section on quantum theory and gravitational waves.

These are good questions, and this is exactly the kind of interaction that will hopefully make this a good article. The ability to ask questions and pursue the answers is more important than the ability to answer them. Thanks, and keep 'em coming. --MOBle 12:00, 5 November 2006 (UTC)

Hey Lucretius. Now, the reason most of us go into theoretical physics is because we're no good with actual numbers. I checked to make sure that your formula for radiated power is right, and I think that 5 should be a pi. Also, I plugged in the numbers, but I get $1.162\times 10^{-5}$ Watts radiated. Could you check your math again? I'm not saying that I'm right, but I don't see my mistake.

32*G^4/c^5/pi

(M1 M2)^2(M1+M2)

1/R^5

--MOBle 23:11, 5 November 2006 (UTC)

Hi once again MOBle! You must be flat out trying to get too much done because the error in this case is yours. Firstly,the tell-tale sign of too much work is a simple reading error - you'll notice that I mentioned Earth and SUN, not moon. The numbers needed are:

R = 1.5 x 10^11 m

Sun = 2 x 10^30 kg

Earth = 6 x 10^24 kg

Punch those into the formula and the formula will jab you back with approx 200 watts.

Regarding Pi instead of 5, don't ask me. I don't know. However, the site I got it from (linked above) definitely says 5. My expectation is that 32/5 has something to do with angles, same as the 2/3 in the Larmor formula, but I could be wrong.

Regarding people getting into theoretical physics because they make simple errors in maths (sorry, math), phooey! They get into theoretical physics because they are good at maths. But maybe they get so used to complex problems that their basic maths gets a bit rusty in the process. However, in this case your error was in literacy, not maths. Sorry, my error this time - MATH! Lucretius 06:46, 6 November 2006 (UTC)

By the way, in my Larmor translation of gravitational power, these 2 things have the same units kg.m and obviously refer to each other:

${\frac {Kq^{2}}{c^{2}}}$

${\frac {Gm^{2}}{c^{2}}}$

The following bit can be understood as a ratio of half the Scwharzschilde radius to the distance R, but better still it can be understood to refer to the metric thingy:

${\frac {G(M+m)}{c^{2}R}}={\frac {v^{2}}{c^{2}}}$

where v is a speed derived from the total mass of the two bodies separated by the distance R. Apart from this metric thingy, everything in the Larmor formula has its equivalent in the gravitational formula, as far as I can tell. But the question is whether or not the system radiates this energy, or is the source either M or m separately. The exact arrangement of factors depends on what is doing the radiating. This scruple about the origin of radiated energy is relevant to the article (my way of arriving at it via Larmor is quite idiosyncratic and of course should not feature in the article, unless there is a reliable source that makes the same connection). Lucretius 07:34, 6 November 2006 (UTC)

Okay. Now I agree, but for that factor of 5/pi. Kip Thorne says it's pi, so I say we go with him.

Gravitational waves are -- just like E&M waves -- created by the entire field, rather than just the point masses -- or point charges. Even without a solid footing for deriving the equation in analogy to the Larmor formula, it might be useful to draw the analogy just for the sake of familiarity. I'll leave this up to you. I can certainly put a little about the power formula in the maths [ ;) ] section.

As for getting too much done, don't worry -- I only work on these things in spurts. (I'm actually going to LIGO for a week in a couple days, so I'm guessing I won't have any time to do Wikipedia stuff for a while.) --MOBle 08:09, 6 November 2006 (UTC)

Hey MOBle - you've made some wonderful changes, particularly to the section 'Effects of a passing gravitational wave'. The changes bring that section to life and make full use of the given graphic. Even I understand it! The Sun-Earth section is also very good - I admit to being a bit scared by trigonometry but the formula for power is intelligible even to a determined maths simpleton such as myself. Cheers Lucretius 08:39, 6 November 2006 (UTC)

Thanks. Very nice of you to say. I think this interaction is helping the article a lot.

Something else: in the section about waves from other sources it is said of two inspiralling stars that "...their orbit is about 75 times smaller than the distance between the Earth and Sun — which is actually smaller than the Sun itself". I don't understand this -does it mean that the diameter of the Sun is greater than is the distance between those other 2 stars? This would imply that the Sun's diameter is nearly 10^11 meters (ie nearly the distance between Earth and Sun). That's an amazing statistic if it's right. But I'm sure it must be wrong. As you said in the 'passing gravitational wave' section, the distance R is very large and Sun and Earth are 'very small'. But the universe is full of wonders and maybe it's right. I'm not used to scaling things visually where exponents are involved and maybe that's the problem here. Less confusing perhaps would be this: The distance between these two stars is less than the diameter of our own Sun.

Good catch. I lost two factors of two. The distance between the stars is actually three times the diameter of the Sun.

I don't want to make waves but, at the risk of being tedious, can I suggest a further change to the Sun-Earth section? Is it possible to put the power formula before the trig stuff? I say this because I think many readers would be scared off by the trig before they actually get to the bit they can understand. The simplest parts should always come first wherever possible. Trig might be simple to you but it's a foreign language to 99.9% of the human race (this could be a conservative estimate). Lucretius 10:06, 6 November 2006 (UTC)

Check. I think the changes will also help lead in to why detecting them directly is so difficult. --MOBle 10:47, 6 November 2006 (UTC)

Hi, MOBle - yes this is better. The simple formula for power will encourage the reader to keep reading and the trig is then less discouraging. Also, after the trig, comes another pleasantly simple piece of maths, the formula for amplitude, which I think rewards the average reader for persistence. However, you appear to have left G out of the formula for angular velocity and that needs fixing. I won't fix it because I don't want to start fiddling with your math(s) - fiddling with math(s) is for me like a bag of salted peanuts and it's hard to stop once I get started, which could prove disasterous for the integrity of the article.

As an aside, I'm puzzled why they call angular velocity a velocity since it appears to be the inverse of a time. That's another good reason for me not to fiddle since maybe I have misunderstood something regarding definitions. Lucretius 07:21, 7 November 2006 (UTC)

Oh. Yet another nice catch. I always think in geometrized units, so G=c=1 to me. I had to do a lot of searching to find those factors of $G^{4}/c^{5}$ and such. (I have no idea what the mass of the Sun is in kilograms, but I can always remember that it's 1.477 kilometers.) Good work.

As for angular velocity, all you have to do is multiply by the distance from the rotation axis to get the regular velocity, $v=\omega r$. Angular velocity and angular momentum obey rules similar to the ones for regular velocity and momentum, too. There's even an angular mass -- the moment of inertia -- and the formula for rotational energy looks a lot like ${\frac {1}{2}}mv^{2}$. --MOBle 21:35, 7 November 2006 (UTC)

Thanks for this. Regarding your error, it came about because I've been asking you to put things in layman's terms, which you are not used to doing. Lucretius 07:33, 8 November 2006 (UTC)

Hi MOBle. I wonder if the section 'Sources of gravitational waves' can be better introduced. The section provides a list of objects that will or will not radiate, but the list seems a bit ad hoc because the intro involves a technical explanation (quadrupoles) that means nothing to the general reader. What is needed is an intro that briefly summarizes the criteria for radiation/non-radiation so that the general reader can evaluate each item on the list - "Yes," the reader should think about each item, "such an object fits the definition for a radiating or non-radiating object". For instance, I had thought 'acceleration' would be a key word (though it does not appear here), and I can see from the article that 'spherical' must be a key word. An intro could therefore go something like this: "Objects radiate if their motion involves acceleration, whether travelling, pulsing or spinning, provided that their motion does not describe a perfect sphere." Something like that (I'm sure you could put it better). You could then put in something about quadrupoles. There might be exceptions to the general rule defined in the intro and those could feature in the list also (eg discs). I think this would aid comprehension for the general reader. Lucretius. 58.169.160.122 09:35, 9 November 2006 (UTC)

Still under construction

Regarding my last suggestion, I've fleshed it out a bit. Here is my idea of what the section should look like (but it's just a draft and it's mostly a re-arrangement of your own words!):

In general terms, gravitational waves are radiated by objects whose motion (whether travelling, pulsing or spinning)involves acceleration, provided however that this same motion does not describe a perfect sphere or a disc. More technically and in accordance with general relativity, the quadrupole moment (or some higher multipole moment) of an isolated system must be changing in time in order for it to emit gravitational radiation.

The simplest example of a quadrupole moment changing in time is the spinning dumbbell, tumbling end-over-end (as opposed to spinning around its long axis). The heavier the mass, and the faster it's tumbling, the greater the gravitational radiation it will give off. If we imagine the two weights of the dumbbell to be massive stars like neutron stars or black holes, orbiting each other quickly, then significant amounts of gravitational radiation would be given off.

Some more detailed examples:

• Two objects orbiting each other with angular frequency $\Omega$ in a quasi-Keplerian planar orbit, have a time-varying quadrupole moment, so this system will radiate.

[Observers far from the system and in its equatorial plane will observe linearly polarized radiation (aligned with the rod) with frequency $\nu =\Omega /\pi$. Observers far from the system and lying on its axis of symmetry will observe circularly polarized radiation].

• A spinning non-axisymmetric planetoid (say with a large bump or dimple on the equator) will define a system with a time-varying quadrupole moment, so this system will radiate.

[Observers far from the system and lying in the plane of rotation will observe linearly polarized radiation. Observers far from the system and near its axis of symmetry will observe circularly polarized radiation].

• A supernova will radiate except in the unlikely event that it is perfectly symmetric.
• An isolated object in "rectilinear" motion will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.
• A spherically pulsating spherical star (non-zero monopole moment or mass, but zero quadrupole moment) will not radiate, in agreement with Birkhoff's theorem.
• A spinning disk (nonzero but stationary monopole and quadrupole moments) will not radiate. This can be regarded as a consequence of the principle of conservation of angular momentum. On the other hand, this system will show gravitomagnetic effects.

Lucretius 10:50, 9 November 2006 (UTC)

Incidentally, does a 'disc' refer to a particular type of galexy, or is it a purely imaginary object that meets the needs of a mathematical argument? Lucretius 10:57, 9 November 2006 (UTC)

Here, the disk just refers to a geometric object. A disk galaxy isn't a perfect disk, but still wouldn't really radiate much (as a galaxy) because the speeds are too low, and everything's basically Newtonian.

Hi MOBle. Yes, the latest change to 'Sources' makes good sense to me. I'm a bit sorry to see your explanation of polarization WRT angles left out and maybe this could be fitted in elsewhere. In fact, it belongs in 'Effects of a gravitational wave', perhaps as a sub-subsection titled 'Types of gravitational waves'. Another graphic would be useful in that context, showing 2 inspiralling stars and the points at which an observer would detect different types of gravitational waves.

I've made some cosmetic changes to paragraph spacing throughout the article. Scientific text is less intimidating if it comes in discrete chunks (the layman's brain is quantized!). In the section titled 'Prospects' the conclusion is somewhat mystifying. What does this mean? "By directly studying the details of gravitational radiation given off by these systems, astronomers could potentially learn much which they would not be able to learn from electromagnetic radiation." Lucretius 01:21, 12 November 2006 (UTC)

Your changes look good to me. You thought the sentences about polarizations in the bullet list were good? I don't think I put those in. I just removed them because I thought they broke up the flow and maybe caused confusion, but I would have no problem with them being replaced there.

Also, I still haven't worked on anything at or below the Einstein@Home section. I think that Prospects section is yet another good, though misguided part of the old article. Looking at it now, it occurs to me that it might be a good start for a new main section on the Astrophysics we might hope to learn about with gravitational waves.

Here's an interesting scruple, though I'm not sure it is entirely relevant to the article. I've read that electric charges in a gravitational field (such as the Earth's) should radiate electromagnetic energy because they are accelerating - we should have free electrical power in huge quantities! The fact that charges do not do radiate in a gravitational field is considered a conundrum among some physicists. It occurs to me that the same conundrum applies to gravitational radiation. Imagine a gigantic junkyard in space, not accelerating either linearly or orbitally. It should not radiate gravitational energy. However, the discarded fridges, pots, pans and car tyres that make up the junkyard are all individually accelerating within the collective gravitational field, and therefore these should radiate. Therefore the junkyard should radiate. Is there a scientific explanation for this conundrum? Maybe it's not a conundrum at all - the junk isn't really moving anywhere and therefore it isn't 'really' accelerating. Maybe the same could be said of electrical charges in a gravitational field. I don't know. The conundrum is out there on the web but it could be misinformation.Lucretius 07:20, 12 November 2006 (UTC)

I like to think of electromagnetic radiation as occurring when the charge accelerates with respect to the electromagnetic field in some sense. Now, imagine you have a free charge in a gravitational field with no outside E&M field. Of course, we know that the gravitational field is just a curvature of spacetime, and the charge will just be moving along a geodesic. But then, so will the E&M field — they'll be accelerated along with each other.

An electron sitting in a trash heap, of course, isn't a free charge, but it and the surrounding E&M field have reached an equilibrium so that the trash heap and the planet it rests on don't collapse. This obviously isn't the whole story, but it's close enough, I think. The combination of classical gravity with classical electromagnetism is pretty well understood (where "classical" means: ignore quantum mechanics). --MOBle 13:59, 12 November 2006 (UTC)

Thanks for this. The stuff I read about charges radiating in a gravitational field must be misinformation. However, I'm still puzzled about the gravitational parallel - shouldn't the pots and fridges radiate in the gravitational field of the junkyard, even if they are stationary relative to each other? Therefore the junkyard should radiate. Anyway, what are the pots and fridges accelerating relative to? Is the gravitational field accelerating past them? That doesn't sound right. But they have weight and therefore they must be accelerating. All very curious.

As for polarization WRT angles, I agree that it needed to be removed from the section 'Sources' but I do think there is a place for it in 'Effects of a grav wave', preferably with a simple diagram to eliminate the need for many words. I don't have the computer nous to come up with such a diagram but maybe you do. Wave types is an aspect of the subject that is new to me and it seems quite fascinating. And clearly it is relevant since some waveforms (eg linearly polarized) would be harder to detect than others. The important thing is to explain it simply and concisely so as not to clog the flow of the article, which is now beginning to run quite smoothly.

I'm looking forward to the Astrophysic section you are contemplating. I'm also wondering what can be said about quantum gravitational radiation. There are also topics such as Hawking radiation - that's not really grav radiation but it is gravitationally induced radiation. It's up to you whether or not to pursue those lines of enquiry. You might feel there is enough to do already. Lucretius 07:27, 13 November 2006 (UTC)

Hey Lucretius. I'll think about any good way to illustrate polarizations. This topic really is very analogous to the polarization of light. Hawking radiation isn't really closely related enough to be in this article. On the other hand, it might be useful to provide a link for the main article in the leader like the link about gravity waves.

I've changed the intro a little. I think this is far more approachable than the old one. This section, however, is the one you'd be entirely suited for. Feel free to rip it apart and make any changes you want. I still haven't touched anything in or after the (new) Astrophysics section. I don't know if that will turn into anything interesting. The other thing that obviously needs to be done is a total rewrite of the Mathematics section -- though that should be straightforward -- and probably a deletion of that energy section at the end. Also, I've seen interesting treatments of the graviton in the context of gravitational waves. Maybe I could squeeze that in near the Math section. I think everything else is looking pretty good, though. --MOBle 09:10, 13 November 2006 (UTC)

Sounds

It might be useful to explain the different types (not polarizations) of sources in terms of the different "sounds" their grav. waves would make. Because grav. waves have an amplitude and frequency, just like sound waves, we can consider what they would sound like if the amplitude were high enough. (The difference in speeds doesn't really matter.) If the wave's amplitude were high enough, the output of LIGO could be plugged into a speaker and we could literally hear it. In fact, in the LIGO control room a speaker that actually is hooked up to the output (though not actually in the expectation that a signal will be heard).

For example, a spinning neutron star -- like what Einstein@Home is looking for -- would have the sound of a pure tone. It might be nice to play the tone expected from the pulsar in the Crab Nebula, for example. Merging binaries, however, would have totally different sounds, where the pitch increases to some maximum, and then there's a brief pop, and it's all over. The difference might be illuminating.

I think this might fit nicely into the article, with a little blurb about comparing grav. waves with sound waves just after the bullet points about amplitude, frequency, etc. I've put in some commented text about this. That could then tie into some follow up in the section about sources, where we could link sound files next to the relevant source.

I can make something like the sounds on this page in OGG format, if this would be helpful enough to be worth the trouble. --MOBle 09:10, 13 November 2006 (UTC)

This sounds like a great idea. Unfortunately there's something wrong with my audio and I don't know what the sounds sound like. But I'm sure they sound great. Laymen will love it. Lucretius 09:46, 13 November 2006 (UTC)

The intro is crystal clear, excites interest and needs no input from me. I found a split infinitive ('to directly measure') which I corrected by a simple expedient (I deleted 'directly'). Split infinitives are being used more frequently these days, even by quite respectable news agencies, but they are inelegant and just plain wrong. I speak as a school teacher. [:}] Lucretius 10:01, 13 November 2006 (UTC)

Perpetual motion? (Bear with me)

I recently added the following to the Perpetual motion:

• Global violation of the first law of thermodynamics through cosmic expansion and redshift: Though the laws of physics make a local violation of the first law impossible, the energy lost when a photon is redshifted due to the expansion of space does not go anywhere apparent. It may be that the reverse process is possible: create a contraction of space, possibly using gravity waves (which are, so far, only theoretical) and fill that space with photons that are then blueshifted. This may well be impossible, depending on the as-yet-undetermined nature of gravity waves. If it were possible, a machine to take advantage of it might need to be be truly enormous, using for example supernovae as the source of gravity waves. Still, unlike the previous example, the scale involved would not be greater than a star cluster and would not tend to destroy (or create) anything at the galaxy scale or beyond.

Obviously, I realize that this needs work. I know that the basic premise is solid - cosmic expansion violates the first law, destroying energy through redshift, and there is no a priori reason to be certain that the reverse could not happen. However, it may well be that the proposed mechanism (using gravitational waves) is unsound, and that some process I do not understand would mean that the apparently "created" energy would actually just be bled off from the wave itself. If this is true and well-understood, please create a section here to explain it. (Either way, the sentence above that starts "This may well be impossible" is just my way of trying to hedge on this issue while still sounding authoritative and definitely needs to be replaced by something less star-trek-y.) Any help or even response here would be appreciated. --Homunq 23:25, 14 November 2006 (UTC) ps. I also included a related entry in List of unsolved problems in physics.

Hi Homunq. You are putting forward personal speculation and it doesn't belong here. Your basic premise is wrong - energy isn't destroyed by redshift: it's simply spread out more thinly. You could argue that it is converted into work. But it isn't destroyed. Lucretius 04:11, 15 November 2006 (UTC)

Just a few comments, Homunq:

1. You might want to look at Peacock's "Cosmological Physics". Basically, you need to consider the total energy of a system -- both the photon (stress-energy tensor) and the gravitational field (which you neglect, though I don't honestly know of any good treatment of this for an expanding universe). More basically, Lucretius is right.
2. If you devised a way to take energy from a grav. wave and put it in a photon, you would still have the problem of creating the gravitational waves. This would require energy, so you wouldn't have a violation of the first law.
3. You need to have reliable sources for anything you put on Wikipedia. It's good of you to add things you find interesting, but you need to be able to point to an article published in a peer-reviewed journal, or a book from a credible publisher. Unless you have those for these remarks (which I doubt), you should probably revert your edits. (Of course, if you do have them, you should have cited them in the articles anyway.)

--MOBle 11:23, 15 November 2006 (UTC)

Consider this fact also, Homunq: if redshift breaks the first law then so would a police siren. Do you seriously think the police would break the law? Also I don't understand the distinction you make between 'local' and 'global'. Light comes from definite locations. It does not come from the entire universe. I have heard that entropy does not apply to the universe as a whole. Lucretius 12:50, 15 November 2006 (UTC)

The following discussion is an archived debate of the proposal. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the debate was PAGE MOVED per discussion below. There was some old page history located at the target, which is now merged in. -GTBacchus^(talk) 07:23, 25 November 2006 (UTC)

Requested move

Gravitational radiation → Gravitational wave — The article is primarily about the waves, rather than the radiation they carry with them. The title should really deal with the main content of the article. Currently, most pages link to "gravitational wave" or "gravitational waves", which just redirects to "gravitational radiation". MOBle 19:11, 18 November 2006 (UTC)

Survey

Add  * '''Support'''  or  * '''Oppose'''  on a new line followed by a brief explanation, then sign your opinion using ~~~~.

Discussion

Add any additional comments:

Hi Moble. Do you mean you intend moving sections of the article that deal with waves to a new article 'Gravitational Waves'? If so, yes this would make sense to me. Lucretius 08:04, 20 November 2006 (UTC)

BTW, what would you then do with the section 'Wave amplitude from the Sun-Earth system'? Lucretius 08:12, 20 November 2006 (UTC)

Hey Lucretius. No, I've requested that an admin move the whole thing to "gravitational wave", just so the title is really the main content of the article, and there are fewer redirects. The "radiation" name refers to energy carried by the waves (in some sources, also to the momentum and angular momentum), whereas the article is clearly mostly about the waves themselves. This is just some cleanup. The request is found on this page. --MOBle 16:04, 20 November 2006 (UTC)

Yes I see no problems at all with that move. Don't know if anyone else is following the great work you are doing here and I hope that doesn't discourage you. Anyway, you have my vote for this change. Lucretius 08:23, 21 November 2006 (UTC)

The above discussion is preserved as an archive of the debate. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

My opinion

A gravitational wave detector (such as the Weber bar, Laser Interferometry) in free fall cannot detect gravitational waves, just as it cannot detect its own motion in the plane of one of its laser arms. This has already been proven in the Michaelson-Morely and Kennedy-Thorndike experiments. Research such is this is like tossing federal or other grant money down a rathole.

The contraction these detectors purport to measure is Lorentz contraction, which could only be measured by a remote detector if instruments could be referenced to an inertial reference frame that is close to (or accelerating with) the source of the gravitational waves. In most cases, and for most purposes, this is impractical. To measure anything significant in terms of gravitational waves at all, you would need instruments that can measure distances between planetary size objects, such as the Earth and its Moon. A sophisticated gravity wave detector based on ranging to massive bodies, parked at one of the Earth-moon LaGrange points might be able to accomplish this, but no other research I have seen done in this field is worth even so much as a wooden nickel in terms of funding. — Preceding unsigned comment added by 70.106.60.44 (talk • contribs)

This is interesting but I assume either you are wrong or the issue is complex enough to be argued for and against (why would intelligent people throw money down an obvious rathole?).

You say that the detectors are in freefall. By this I understand you to mean that the detectors are accelerating in the gravitational field of the wave source and therefore they and their surrounds are all subjected to the same relativistic effects and these effects cannot therefore be measured locally. But the gravitational pull on the detectors has nothing to do with the radiation that is being measured (distance from the source excepted). Perhaps I have misunderstood your argument. Lucretius 03:01, 25 November 2006 (UTC)