Talk:Gravitational wave/Archive 3

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High Quality

This is just to say that this article really is rather well written. Accommodating, intelligently simple, enjoyable to read!

LCGT

I noticed that the LCGT page is orphaned and requests have been made to link it into appropriate pages. I believe funding has not yet been secured for this gravitational wave detector, but would it still be appropriate to add it to the list of detectors (e.g. LIGO, VIRGO, GEO600, TAMA?) It's situation is probably little different than that of the LISA project, which is mentioned. Comments? -RockyRaccoon 01:31, 2 April 2007 (UTC)

Thanks for the information. I've now added this link. Lucretius 07:23, 4 April 2007 (UTC)

Kinetic energy?

I'm interested in the following edit, which a contributor made on the grounds of being scrupulous with the facts.

'Thus the total power radiated by the Earth-Sun system in the form of gravitational waves is about 300 Watts (i.e. about five 60 Watt light bulbs). This is truly tiny compared to the total electromagnetic radiation given off by the Sun (roughly 3.86 x 10²⁶ Watts) or even when compared to the kinetic energy of the Earth's orbit (about 2.7 x 10³³ joules). It is this latter kinetic energy which supplies power for the gravitational radiation.'

The contributor says in his edit-note that gravitational energy is not derived from the Earth's kinetic energy but from the kinetic energy associated with the Earth-Sun system. Fair enough. However, I have some doubts that gravitational energy can be derived from kinetic energy at all! Kinetic energy increases as the Earth inspirals. This energy would be truly enormous by the time the Earth approaches crunch time. Yet, as I understand it, the gravitational energy typically radiated by such a system is not enough to be detected here on Earth - massive binaries are needed for appreciable radiation. I'd like to see what the math says about this. There is also this point - if energy is being lost from the system due to gravitational radiation, then that energy source is being depleted, which is certainly not the case with kinetic energy. I suspect the radiation is more correctly sourced to angular momentum, which does decrease with orbit. Also, a loss of kinetic energy would surely mean a decrease in acceleration. I don't see how the Earth's acceleration would decrease as it inspirals. But only the math can decide the argument either way. Anyone want to have a go at that? Lucretius 06:20, 28 June 2007 (UTC)

No that's no contradiction since the total energy, the sum of potential energy and kinetic energy have to be considered.

$E_{tot}=E_{kin}+E_{pot}=-E_{kin}$

The total energy is decreasing, if the kinetic energy – force and acceleration as well – increases and the orbit is getting closer to the Sun. One square meter on Earth reaches not more than 1,300 Watts of 3.86 x 10²⁶ Watts. From a distant star this fraction is of course much smaller. The gravitational radiation is again much smaller than the electromagnetic radiation. Therefore, that's next to nothing. 84.59.42.119 19:10, 28 June 2007 (UTC)

Thanks for this but I have problems with it. Firstly, I thought the total energy of an inspiralling system remains constant except for the energy lost through radiation. So there is something of a tautology in your explanation - you are basically saying "gravitational radiation is the energy taken from total energy by gravitational radiation". Secondly, the article explicitly derives gravitational radiation from kinetic energy. Thirdly, I don't understand what you mean by 'one square metre on Earth'. What kind of radiation is coming from 1 square metre? Is this thermal radiation, you mean, and what has it to do with gravity? Lucretius 23:01, 1 July 2007 (UTC)

Fat Page and archiving

The article page is too fat/wide for my computer screen. How did this happen and how can it be reversed? If anyone knows how to slim the page down, please do this.

Also this talk page is getting too long and it needs to be archived. Can anyone do this? Lucretius 22:42, 27 August 2007 (UTC)

Done and done. The wide page was caused by a huge matrix equation; which I have now split over two lines. Looks horrible; but it looked horrible to start with. :-) —Duae Quartunciae (talk · cont) 23:26, 27 August 2007 (UTC)

Thanks for this. The archiving has worked a treat but unfortunately the article page has not gone on the diet you prescribed. Lucretius 23:20, 28 August 2007 (UTC)

I have made another reduction, by removing some unnecessary explicit working. Let me know how the page width works now.

I see some more cases of explicit working in the article, which I don't think is good in an encyclopedia. It should be enough to have the forumalae, and the values substituted, and the result obtained. Giving the working is redundant. —Duae Quartunciae (talk · cont) 23:41, 28 August 2007 (UTC)

Yes this works. I agree that there is some redundant math in the article. By all means get rid of it. I think you are referring to the wave amplitude section. Lucretius 23:46, 28 August 2007 (UTC)

Stuff to consider

My Wiki edit buttons have disappeared. I don't know what that's about but I'll motor on anyway:

I'm not really happy with the section Wave amplitudes for the Earth Sun system. Half of it belongs in the previous section about power radiated. This is the part that I would transfer to the previous section:

An alternative formulation of the quadrupole equation, which is easily related to the orbital motion of binary stars or black holes, rotating rods, laboratory HFGW generation, etc. is based upon the jerk or shake of mass (time rate of change of acceleration) and is derived by Baker

$P={\frac {G(I\omega ^{3})^{2}}{5({\frac {c}{2}})^{5}}}$

or

$P=1.76\times 10^{-52}({\frac {2r\Delta f}{\Delta t}})^{2}\ \mathrm {watts}$,

where P is the power of the GWs in watts, I is the moment of inertia of the system in kg-m², ? is the angular rate in radians per second, r is the distance between two masses in meters, ?f is a change in force in Newtons, over the time interval ?t in seconds, that is, the jerk or shake of the two masses, such as the change in centrifugal force with time as masses move around each other on a circular orbit. Please recognize, however, that ?f need NOT be a gravitational force. Electromagnetic forces are more than 10³⁵ larger than gravitational forces and should be employed in laboratory GW generation. As Weber points out: “The non-gravitational forces play a decisive role in methods for detection and generation of gravitational waves ...” The foregoing equations are also termed “quadrupole formalism” and hold in weak gravitational fields (well over 100 g’s however), for speeds of the generator “components” less than the speed of light and for r less than the GW wavelength. This last restriction may not really apply. Certainly there would be GW for r greater than the GW wavelength, but the quadrupole formalism might not apply exactly. For very small ?t the GW wavelength, ?_GW = c?t (where c ~ 3x10⁸ ms^-1, the speed of light) is very small and the GW frequency ?_GW is high. As a numerical example, we will choose r = 10 m (convenient laboratory size though usually greater than  ?_GW), ?f = 4x10⁸ N (or 400,000,000 N; for example, the force produced by a large number of piezoelectric resonators) and ?t = 2x10^-10 s (or 0.000,000,000,2 s; equivalent to about a ?_GW = 5 GHz shake frequency) so that ?_GW = 6 cm and P = 2.8x10^-13 W (0.000,000,000,000,28 watts or 0.28 picowatts). Clearly a very small HFGW power generated.

I might add that I personally would edit a lot of this out completely because much of it is concerned with excruciating technical reservations (eg beginning "This last restriction may not apply...etc). I mean who really cares?

This leaves a question about the rest of the section, which I think we should just delete because it is closely related to the stuff quoted above and it makes no sense on its own:

It is important to relate the amplitude of a GW, A, with the power, P, or more exactly with the GW flux, FGW, in Wm^-2.

$A=1.28\times 10^{-18}{\sqrt {\frac {F_{\mathrm {GW} }}{\nu _{\mathrm {GW} }}}}$

It should be recognized that A is the amplitude of the time-varying, periodic spacetime strain h. Following the proceeding numerical example we will concentrate the HFGW on a diffraction-limited area of 4x10^-3 m² or 0.004 m² for a HFGW flux of 2.8x10^-13/4x10^-3 = 7x10^-11 Wm^-2. Thus A = 2x10^-33. It is an extremely small HFGW amplitude, but possibly a detectable signal.

Really the argument needs to be a whole lot clearer than that. There is often a problem with Wiki physics articles - the writers often seem to assume that the readers know as much as they do. If I knew what the writer knew, I wouldn't bother reading his explanations. Nor would I read his stuff if his explanations are so far over my head that they require an explanation. Lucretius 02:39, 30 August 2007 (UTC)

I made a bit of a start in trying to clear it up a while ago; but bogged down pretty quickly and then just left it. The polarization formulae, for example, include a reference to a variable t, which is not defined. It is almost certainly a hang over from representations of the time varying amplitude, which is vary important for waves that are likely to be strong enough to detect, as something spirals in towards collision; but not important for the Earth Sun system. I started to write something a lot simpler for the Earth Sun system, but have put it aside for a bit. I'm not really expert in this. —Duae Quartunciae (talk · cont) 03:22, 30 August 2007 (UTC)

This is invitation enough as far as I am concerned and I will now delete the quoted section. The parent of it might get offended but hopefully he/she will, after some friendly negotiation, agree either to euthanasia or at least to some seriously corrective surgery. The caravan moves on.Lucretius 04:43, 30 August 2007 (UTC)

Propagation

A section in the article read, "Gravitational waves have two important and unique properties. First, there is no need for any type of matter to be present in order for the waves to propagate. For example, two black holes, such as those at the centre of Galaxy 0402+379, crashing into each other, could generate powerful gravitational waves; however, this event would be completely black in electromagnetic radiation..." -This is not correct, electromagnetic waves also need no medium to propagate. This section was implying that the electromagnetic blackness of an event is due to electromagnetic waves' inability to travel through free space, while actually they travel best through empty space. I have removed this from the article.

-User: Nightvid

I think that bit was meant to describe the generation of GWs, rather than their propagation. I changed it back and fixed it up because I think that point is very important.

Also, I'm pretty sure there is really no radiation pressure -- it's not just miniscule. --JerryBomb 00:28, 27 October 2007 (UTC)

Gravitational waves have linear momentum as discussed in the article and thus they must exert a radiation pressure, for the same reason as electromagnetic radiation. ~~User: Nightvid

I agree that they have linear momentum. The question is whether or not they actually impart any of this to the matter through which they pass. I'd like to hold off on that statement until you can cite a reputable source. 131.215.123.98 03:58, 29 October 2007 (UTC)

Check Sticky bead argument. Gravitational waves do impart linear momentum to matter (as does simple gravity). However, I don't believe there is radiation pressure, because the wave is not affected by the matter which it effects. Radiation pressure in EM waves is due to the wave interaction with the matter. a photon reflecting off of an atom imparts momentum to the atom. A gravitational wave does not reflect off of matter, it goes through it, thus there is no pressure.--Astrobit (talk) 04:22, 17 April 2008 (UTC)

Explain why grav. waves can propagate through the Universe unimpeded.

What's the answer? Or is it a 50/50 call, whether they can or cannot propagate long distance? And if g.w.s don't propagate long distance, then what would be expected from our exquisitely sensitive (~10^-21)[1] interferometry observatories, surveying a large statistically adequate volume for g.w. sources?Zanardm (talk) 08:03, 20 November 2007 (UTC)

Gravitational waves propagate unimpeded because they do not get absorbed and reradiated like EM waves would. They are waves in the curvature of space time. If you just think of it in terms of gravity, the gravity of the sun doesn't get cut off behind the earth. Similiarly, a gravitational wave emitted by the sun would not get cut off by the earth or anything else.--Astrobit (talk) 03:27, 17 April 2008 (UTC)

Here's a quote I found today: "2) Because the energy released during the merger is in gravitational waves, all matter and energy is affected by it, but the waves couple weakly to the matter and energy, which means the waves don't affect it much. As the waves pass through a person or a planet, or a star, most of the energy stays in the wave. The gravitational waves must pass through a lot of matter and energy before they are attenuated (diminished) significantly" [2]--Astrobit (talk) 03:17, 20 April 2008 (UTC)

One of the clearest explanations I have heard of the reason that the path of light is changed when a ray passes close by a star (our sun being the nearest example) is that light always goes by a "straight line" in the sense that it takes the minimum path length available, but that the warping of three-dimensional space in the vicinity of a heavy mass is analogous to the warping of two-dimensional space when, e.g., a flat surface is made to bulge out or cave in. What is a straight line on a spherical map will not match a straight line made after the map has bee peeled off the globe and spread out flat, and vice-versa. That's the difference between great circle routes and routes that one might plan on the basis of a Mercator projection.

So what happens when a gravitational wave spreads out in three dimensional space would be analogous to what happens when a two dimensional surface such as a drum head is struck at one point and the impulse provided there spreads out as a wave across the drum face. A microbe on the surface of the drum might be aware only of the two dimensions, the surface of the drum. But its movement due to the drum head vibrating would be in the third dimension. Only a very small component of the basically up and down movement would contribute to horizontal movement of the microbe.

The main effect that could be noticed in the kind of physics experiment hoped to detect gravitational waves would stretch the apparatus in three dimension, just as a colored dot on the surface of a vibrating drum head would be alternately stretched and restored. So the object "stretched" in three dimensions would give back that energy as the gravitational wave moved beyond it, just as a bobber floating on a pond acquires potential energy as a ripple lifts it momentarily, but then loses that potential energy when the ripple passes beyond it.

O RLY?

A spinning disk 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.

Wouldn't it depend on the axis of spin? Obviously it wouldn't radiate if it were spinning about the normal axis, but it should radiate spinning on any other axis. --70.131.60.191 (talk) 07:47, 30 December 2007 (UTC)

I think the meaning is clear enough and I personally don't see any reason to add more words to it. The section includes the example of a spinning dumbell and that makes the general significance of the axis quite clear for other cases as well. This is my last entry for 2007. Happy New Year to all! Lucretius (talk) 08:53, 31 December 2007 (UTC)

I think it's clear, too. Clearly wrong. A spinning disk will, in fact, radiate, depending on the axis of rotation. So why don't you pick up your fat Starbucks-holding hands and "add more words" so that it's not wrong, Che. Because I'm not impressed by your faux-professional opinion about leaving wrong information in the article. Stop reading the Communist Manifesto for a second and take a fucking science class. --75.63.48.18 (talk) 09:58, 3 February 2008 (UTC)

I agree with Lucretius. If you think of a spinning disc you typically think of it spinning about it's normal axis. Usng a circular disc as an example is pretty common in physics, usually because the equations are relatively easy to solve. If you wanted more clarity, you could talk about a spinning top instead, same basic principle. However, whatever the original source is for this (*hint*) probably used a spinning disc for the example.--Astrobit (talk) 04:11, 17 April 2008 (UTC)

Gravitational wave detector

Can somebody comment on Reginald T. Cahill's October 2007 article in Progress in Physics 4, 63-68 (arXiv:0707.7772)? He claims to have developed an effective optical-fiber gravitational wave detector. The journal's website states it is a peer reviewed publication. Tcisco (talk) —Preceding comment was added at 14:47, 4 January 2008 (UTC)

Hello... I apologize for being unfamiliar with Wiki editing but I thought I would post this report, from February '08, of the stregnth and detectability of gravity waves from different sources through different, and new, kinds of G.W. detectors. http://arxiv.org/abs/gr-qc/0204090v1 —Preceding unsigned comment added by 128.228.93.202 (talk) 13:55, 15 May 2008 (UTC)

If you are familiar with this stuff, why not get an account and add a section?

http://www.ptep-online.com/index_files/2008/PP-13-12.PDF is really interesting!P0M (talk) 17:33, 15 May 2008 (UTC)

It's better not to ask the reader to guess

Also, the arms will be at 60 degree angles to each other

There has to be a reason why it is sixty degrees. Need the average well-informed reader be made to guess?P0M (talk) 05:57, 29 March 2008 (UTC)

Looks like it's simply a cost thing. Read the article on LISA. I debated about removing this note from the GW article, since it does seem a bit superfluous, but left it in for now. --Astrobit (talk) 04:05, 17 April 2008 (UTC)

Gravitational waves and black holes

It was theorized that gravitational waves could cause photons to escape from the event horizon of black holes. Since, in black holes, the event :horizon is the point beyond which no photons can escape, the entrapped photons 'orbit' the singulatity just inside the event horizon. If the :gravitational wave is at its peak (opposite of trough) when it reaches the event horizon, it would give the space-time at the event horizon a more :positive curvature. This could cause some of the photons at the event horizon to escape from the black hole. This would not occur often enough to :cause substantial differences in the black holes mass, but it is still a way that energy can escape from the event horizon.

I removed the above section again. Wikipedia is not the place to communicate your musings. If it cannot be backed up with proper references, it should not be part of the article.

SwordSmurf (talk) 16:01, 8 June 2008 (UTC)

mountain on neutron star?

Introduction says "The simplest example of a strong source of gravitational waves is a spinning neutron star with a small mountain on its surface" Is a Neutron star known to have mountain? Better example will be "Binary neutron stars". Am I right? - manya (talk) 11:52, 27 June 2008 (UTC)

I think you are surely right. Why not change the article?P0M (talk) 16:39, 27 June 2008 (UTC)

OK. Done. - manya (talk) 04:54, 30 June 2008 (UTC)

dogmatic deletion reverted

Another editor just deleted an entire paragraph on the basis that s/he "believed" that the opposing evaluations reported were irrelevant. Rather than deleting things on the basis of personal opinion, the best course is always to discuss the matter in the "talk" section first. P0M (talk) 16:55, 2 February 2009 (UTC)

Users Privalov and Eldereft seem to reject my passage regarding the existence of gravitational waves. The main problem seemed to be that I described Prof. Peebles's authoritative assertion as "dogmatic." Semantically, the word "dogmatic" must carry negative connotations, such as arrogance or ignorant opinionativeness. Eldereft wanted to know who claimed that Professor Peebles was being dogmatic. Privalov had even been brought to dismiss the thoughts of the scientists Peebles and Shapiro as being "irrelevant" according to his belief. Therefore, I have removed the pejorative word "dogmatic" and replaced it with the semantically less inflammatory word "disagree." It would seem that the thoughts of the eminent scientific professors Peebles and Shapiro should have some weight in the article, especially as they regard the actual existence of the gravitational waves which are the topic of the article.Lestrade (talk) 14:54, 4 February 2009 (UTC)Lestrade

Just to be clear, I am assuming the paragraph you are debating is:

Although most scientists would agree with Professor P. J. E. Peebles, who declared "Gravitational waves exist…," some scientists disagree. Professor Irwin I. Shapiro more cautiously wrote: "Should we now conclude that the existence of gravitational radiation has been established? Probably not."

This is the quote that Eldereft brought up on Wikipedia talk:WikiProject Physics. This paragraph troubles me for two reasons. First, its location does not truly fit. Skepticism about the existence of gravitational waves is appropriate in the detector section if it relates to the detectors or detection. A statement such as 'theorists predict that the largest likely gravitational waves expected has a magnitude of A, while the best current detectors can measure down to B' is very appropriate. And if B is low enough compared to A then a parenthetical statement such as the paragraph may be appropriate to add color. The location of the paragraph, though, interupts the flow and development of this section in my opinion. Second and more important it has the appearance of arguing by authority. Statements such as 'Scientists A says that ...' has no relevance to science other than as possibly a footnote or in a history section.

A very quick perusal of the article suggests to me that there is much room for addition of scientific skepticism. It would be nice, for instance to have a cleared deliniation of what is known by evidence and what is theorized (based on a solid foundation of GR, though it is) but not proven. I think the information is there it just needs to be reorganized and renamed. As an example 'Radiation from other sources' contains mostly a description of the experimental evidence for gravity waves using Huse-Taylor binary. In my opinion as an 'experimentalist' such information is very important and should be placed somewhere prominent with a descriptive title.

In any case, I am glad that Eldereft led me to this article, since I enjoyed it and have even learned a little. Keep up the good work and focus the energy you have and I think it will be even better. TStein (talk) 16:57, 4 February 2009 (UTC)

TStein's Room for addition of scientific scepticism

Is it permissible to add a section on scientific scepticism, or is there no room? Is it possible to include CalTech's Dr. Jesse L. Greenstein's words, as follows?

The detection of gravitational waves bears directly on the question of whether there is any such thing as a "gravitational field," which can act as an independent entity. … this fundamental field hypothesis has been generally accepted without observational support. Such credulity among scientists occurs only in relation to the deepest and most fundamental hypotheses for which they lack the facility to think differently in a comparably detailed and consistent way. In the nineteenth century a similar attitude led to a general acceptance of the ether …

This was published in Astronomy and Astrophysics for the 1970s, Report of the United States National Academy of Sciences, Washington, DC, 1972.Lestrade (talk) 00:30, 15 February 2009 (UTC)Lestrade

I would appreciate it if you didn't plaster my name in the front of these titles ;). I don't own skepticism nor am I an authority at all on this article. (My knowledge of gravitational waves is very poor.)

I applaud skepticism and wish that the editor's of this article had more time to include scientific skepticism. (By which I mean a better discussion of what is known (and unknown) experimentally and to what degree of certainty based on data.) Your proposed addition does not fit that criteria, though. It is an argument by authority not by data.

As a counter example for something being accepted without direct observational support consider that it was well accepted that Earth revolved around the Sun for centuries before that movement was directly measured using parallax. TStein (talk) 05:40, 16 February 2009 (UTC)