Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: rubygold on 22/04/2010 17:09:53
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considering that a gravity field in fluctuation produces a gravity wave, meaning a physical wave that travels through matter, the question is this; when a star ends and is collapsing to a neutron star, what effect will a gravity wave travelling through the in-falling mass have on events?
there is a big difference between gravitatioal-wave radiation emission and the actual gravity-wave of the " drop a stone into a pond" variety, and the reference is to the physical variety.
standard models take no notice of this, however, so perhaps the question could be; explain how a gravity wave travelling through collapsing stellar mass has no effect on the outcome?
Mod edit - I've formatted the subject as a question. Please do this to help keep the forum tidy and easy to navigate. Thanks.
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Well it's still a theory, isn't it? Even though expected to exist as it explains some binary stars interactions as seen from us. As I remember it originally comes from Einsteins equations in where he expects them to exist. Assuming that it is gravity that holds our universe together :) which might be all wrong naturally, we could talk about a '3D-field' with dips and heights as observed by us. Then that field are us, to get that wave 'moving' we need a component called the 'arrow of time' making it a 'moving picture'. Your question seems to be if not the components making what we call matter (fermions) should be interacting slightly different when f.ex 'compressed' by that wave?
Those waves should pass, and have passed, in our universe on a daily basis if our universe is infinite and gravity have no limit. But everything seems to be the same as always to us, don't it? So expecting them to do so I would presume that they do not change the interactions in matter. I'm not discussing bosons here btw.
And if that was right?
Why would it be that way?
Assuming that gravity is the background of matter, and that SpaceTimes geodesics and distances is the expression(s) we get from the same, then motion comes into the picture. Motion is relative times arrow, We use that arrow to measure it. That arrow changes with motion creating a universe of 'frames of reference'. In that universe only frames 'at rest' with each other will see the 'same sights' as I understands it, and so be able to agree on distances and time passed.
So to have it work without any interference I would expect that arrow to compensate for the gravitational wave somehow, equalizing the interference made.
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If I understand the question, Rubygold, you are wondering whether the gravity wave produced by a collapsing star (resulting from a supernova explosion) would influence the collapse itself so as to change the nature of the resulting neutron star. I would say it would probably affect the outcome very little and it is difficult to separate out the internal gravity wave from the physical source of the wave, i.e. the collapse itself. In this case the collapse is triggered as a result of the gravity overcoming the electron degeneracy. I would suppose that the collapse will originate at some point near the centre where the inward pressure is highest, although I have never looked at models of such a collapse so I am only guessing. Once the collapse is initiated I can see that there would be a cascade effect spreading out towards the surface. I would assume this would be primarily due to the nuclei moving inwards, under pressure from the mass further out, to fill the void and then subsequently collapsing because of the increasing pressure as the star's radius shrinks. However there could also be a contribution from gravitational waves moving outwards which, I would suppose, could also trigger some collapse. I can see why it may not be modelled :-)
It is hard to see how this would affect any outcome for the star but it may change the profile of a gravitation wave that is emitted as a result of the collapse. At the moment I think anyone would pleased to detect a gravitational wave at all, let alone one with a defined profile.
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how and to what extent does the gravity-wave contribute to events? it does not change the nature of it. the gravity-wave should either contribute to the formation of a neutron star or it should be shown how it does not contribute. gravity-waves are a fact of physics, notably fluid dynamics, and exist where ever and when ever a gravity field is disturbed. that other term, gravitational wave is a theory.
gravity-waves will be produced from the start of the stars' explosion and continue til the final collapse. as long as the gravity field is in fluctuation, as in a stars explosion and collapse, there will be gravity-waves produced.
current models, as complicated and detailed as they are, do not account for the effects of waves moving through the collapsing stellar mass. the effect would be of varying pressure and densities organized in shells moving inward. so. thats the picture.
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Ah, I'm sorry. I thought you did actually mean gravitational waves and you were trying to determine if these had an effect on the modelling of the collapsing star. I see you did try to explain it in your original post but I missed it completely. D'oh!
You are really looking at a much simpler, classical, effect. Though still complicated. I am surprised this is not modelled. And you are looking at the model from the time of the supernova explosion?? The gravity wave will manifest as a compression wave which one might assume will be spherically symmetric. I would suppose that it would certainly influence the nature of the collapse but it is hard to say whether the final outcome would differ. Perhaps it could cause a star that was close to the 5 solar masses needed to continue the collapse into a black hole to do so at a lower mass limit. This by virtue of the extra compession from the wave. Interesting point.
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thank you, i've had some difficulty expressing this idea.
as you have guessed, my original intention was aimed at blackhole formation. it was necessary to refer to the more familiar model of neutron star formation that actually has math and formulas associated with it. this part does look at the model from the time of supernova explosion, however, for simplification the final collapse to a blackhole could be considered as the essential part.
the final outcome would not differ, it would just include the wave pulsation(s).
there are consequences following from this point but the acceptance of gravity-wave compression, density distribution and propagation speed need to be envisioned before a thought experiment makes sense.
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I think it may be a fair assumption that once the neutron star had formed that it could continue to "ring" for some period of time. This may be a better starting point than to model the event from the beginning of the collapse. If the neutron star can be considered as a material with a certain density and elasticity then there is reason to suppose that wave propagation could be modelled. The ringing would continue until the energy was radiated away by either gravitational waves or as em radiation - heat from whatever damping forces are present in the neutron star. As I said before, such a pulsation could possibly cause a BH to form at a lower mass threshold than for a star that had no such pulsation. I don't know the likely time constant for the damping of such wave; is it nanoseconds, days, years?? The frequency would be high because the star would not be very large. I would guess it would damp fairly quickly but I have no evidence for such a view. If it continued for a long time maybe there could be evidence found in the radiation from pulsars because the Bremstrahlung pulse may show signs of a higher frequency modulation. I don't think I can help much here as my interest is merely an amateur one and I think you need to delve into the mathematical modelling to get anywhere.
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You know. there are several things with gravitational waves that confuse me. First of all, gravity is not energy as far as I know. If you look at it as SpaceTimes geodesics then a gravitational wave seems to me more of a distortion in the SpaceTime fabric. I've seen statements saying that gravity waves carrying with them 'energy'? If that is true then why use geodesics? Isn't those two diametrically opposed concepts?
Assuming it to 'interact' with matter becomes then a matter of interpretation. I would say that it won't 'interact' with matter more than matter is following, as well as creating, the geodesics existing. And if so, this question breaks down to if fermions coming closer to each other due to a matter wave will interact differently. Also, a matter wave doesn't 'lose energy' as I see it, it's just following the restrictions placed on it "by that the gravitational force between two objects is inversely (oppositely) proportional to the distance between the two objects squared (multiplied by itself). For example, if the distance between the two objects doubles, the force between them becomes one-fourth of its original strength." so the farther that wave 'propagate' the weaker it will be.
As for "gravity-waves are a fact of physics, notably fluid dynamics, and exist where ever and when ever a gravity field is disturbed. that other term, gravitational wave is a theory." I'm getting even more confused :) gravity waves exist mathematically, Einstein for one, even though being the one to come up with them, referred to them as 'waves traveling with the speed of thought' or was that Eddington? Never mind, neither of them expected them ever to be measurable, and so far they've been right as far as I know. LIGO might change that though? We have what we think is evidence for them in our observation of the Hulse-Taylor Pulsar where we have seen those neutron stars orbiting each other lose energy as expected through the mathematics describing matter waves, and so orbit constantly closer each other. But that they lose energy doesn't mean that a gravity wave 'wins' that energy, at least not as I see it. And if believing that it does I would be very interested in how one would define that energy. I don't know any experiments proving that concept?
So, assuming that they 'press' particles together at some point they still don't interact, there are no 'virtual photons' involved as far as I know and no exchange of 'forces'. So the only interaction possible to me would then be the particles own finding themselves now at different positions relative each other due to a different 'geodesic'. And assuming that we have them, as I did in my first post, they must be passing through us constantly, but creating no such effects, and if you look at the binary system producing them, and possibly stars close to them you should be able to see variations produced in their radiation if those waves interacted with matter in the way thought. As it seems to me that is.
And where did you get the differentiation, gravity waves versus gravitational waves Ruby? That was news to me :)
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yor_on, gravity waves are rather simpler than gravitational waves. The waves on the ocean are gravity waves for example. I was confused at first too. I didn't know the definition until I checked and it seems they are the waves that can form at the interface of two different media, like air and water for example. I am not sure whether these sort of waves would be so importent in a neutron star or not. I would guess compression waves in the interior would be more significant (if at all) but I would suppose both would be present for a time. I don't know but maybe a neutron star would ring like a bell with a whole set of resonances for a time.
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Thanks, that explains it.
I was stymied for a moment there, trying to figure out how gravity would differ from gravity :) As for the rest I'm still confused. It reminds me to much of the concept of potential energy. An energy existent but not seen until in an actual interaction, and in this case discussing another interaction, but now with one of the 'objects' exchanging that gravitational energy being SpaceTimes geodesics.
If gravity is an energy then you will have one force more to count on, like, the apple, Newtons head, the potential energy existing in that apple, and the sparkling energy stretching in every point between, called gravity :)
But yes, I think it should be a harmonic too, the music of the spheres :)
But I still wonder if gravity waves will interact. That a binary star system might, as described by the Hulse-Taylor Pulsar, is no mystery, that they might in their interaction create a 'propagating' distortion seems plausible to me. But I'm not sure how it would interact. It's, as far as I understand, expected to move at the speed of light right. That should mean that even if passing us by, we being real near that binary star, that wave will do so extremely quickly. So how fast can one expect particles to react to changes in their position? Instantly, nah, don't think so :) limited by 'C' then? or slower yet? After all, they are matter? So, before they can react, as a guess, the particles will be back in their 'correct position' as it seems to me? What would that do to a atom f.ex? There is naturally the question of how large that distortion would be to consider too. But if they do do :) something it should be measurable by observing objects close to the systems creating the waves? Also it seems to me as they, the distortions should change times arrow, as they as a guess wrinkle space the same way a object of invariant mass does if so, even if the geometry expressed might differ. So in that moment that gravitational wave passes you, you will not only deform, according to an observer not involved in that experience, you should also 'slow down', it all falls down to how big a 'frame of reference' can be here, doesn't it, when considering that waves 'edge' passing one through. As well if that wave really will have a width, considering its speed? A lot of imponderables to it, it seems :)
Or maybe I'm just bicycling here
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If you consider a gravitational wave to resemble a electromagnetic it will have a extension in space naturally? But then you will need those binaries to act as a prolonged transmitter, won't you? but how long does that distortion take? And is it a wave in the same manner as a electromagnetic one? Won't it be more like a 'im-pulse' as contrasted to an 'em-pulse :)
And seen my way, just a 'distortion' having nothing whatsoever to do with the wave concept? How can we know how such a distortion act? We have never measured any. I know that the normal description of it is that it is thought to move like a stream, with the 'forces' directed like ripples on it, at right angles to the direction in which they're traveling. But will it act as EM wave does with an extension in time? Why?
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If you consider it a wave acting, then it could send ripples (make the particles act as if it was doing so, I mean) that reminds me of self oscillating shock waves through matter, if having the right 'resonance', could it not? Ripping matter apart. But we haven't noticed anything like that?
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It's an interesting question rubygold. I'm assuming that the gravity wave you're thinking of is the one created by the collapse, and not an arbitrary gravity wave caused by something else passing through the system e.g. the collapse of another star.
Anyway, as the star starts to collapse, gravity waves will be produced by all parts of the volume that is collapsing, so gravity waves will be produced from the outer layers of the collapsing volume which will propagate both outwards, as we would expect, but also inwards and focussed towards the core. What is more, the gravity waves will propagate inwards at 'c' (or at least so we believe) but the collapsing outer layers apparently only reach a maximum velocity of ~0.2 'c', so it does seem as though you might get a focusing of gravity waves at the center of a collapsing star before the collapse has finished.
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It is difficult to discuss this subject usefully without making use of the specific details of the phenomena. One such specific detail is the specific nature of a gravitational wave, as I understand it, specifically as to its polarization characteristics. I understand that according to Einstein's calculations, a gravitational wave should vibrate at right angles to the direction of propagation, but not in the manner of an electromagnetic wave. Instead, the displacement is quadripolar: the "force", or whatever, oscillates in a plane at right angles to the direction of propagation, between a state in which the phenomenon thrusts outward in 2 opposite directions while simultaneously thrusting inward in the two directions which are at right angles to the former, and then reversed for the second half of the cycle. If this is so, then the simplest vibrating mass structure which is capable of emitting gravitational waves is the quadrupole. Therefore, if a spherically symmetric object undergoes a collapse, it cannot radiate a quadrupolar phenomenon. If, however, the object that collapses is irregular in shape, then there is the possibility of emitting such waves. This could happen, for example, if the object were spinning rapidly, and thus were more massive along the equatorial plane than along the polar axis. Its quadrupole moment in that situation is different from zero, and in a collapse there might be a change in the quadrupole moment. In such a situation, the strongest emission would be away from the equator, with zero emission along the polar axis.
This does not rule out some kind of gravitational effect inside the object, if for no other reason than the fact that gravitation is tied up with the metric of the space, which in turn is tied up with the effects of movements, so that the non-flatness and inhomogeneity of space within the object could well affect the details of its collapse as the matter moves through nonuniform and even changing space.
The energy in gravitational waves (or lack thereof): It is an interesting question where the energy that is expended when lifting an object against gravity, actually goes. In electromagnetism, when two charged objects of the same sign are brought into proximity, work must be done. The energy expended can be found within the altered field, by comparing the integral over all space of its square before, and after, the motion. Energy in the field per unit volume is proportional to the square of the intensity. With gravity we might expect a similar phenomenon to exist: bringing two massive bodies together results in a more intense local field (using the Newtonian conception). If the square of the intensity is integrated before and after, a change will be noted. Unfortunately, this change is of the wrong sign, because bringing the two bodies together did not require work, it yielded work. A possible answer to this puzzlement may be found in taking an Einsteinian view, which says that time runs slower at positions of lower gravitational elevation. That implies that quantum mechanical objects, which vibrate according to E = hν, also vibrate slower. That implies that their total energy is less, and therefore their mass is less. Thus, lifting an object to a higher elevation would increase its mass (as seen from the original position), so that the energy in question would reside within the increased mass of the object rather than in the field.
Applying this concept to gravitational waves suggests that when a gravitational wave is emitted by the sinusoidal oscillation of an object's quadrupole moment, work is done against the emitting object's own mass, which varies with time; but that the work so done is not conserved, and that although work was expended, the object ends up having its original mass configuration, thereby violating, as far as can be told from within that reference frame, the conservation of energy; however, when the wave encounters other objects, those objects' masses undergo sinusoidal variation, and if those objects are free to move, work can be extracted from them. So of course the question is, did or did not energy actually enter the field (in view of the previous analysis of the square of field strength, which suggests that it doesn't)? Here, we need some serious mathematics, and the answer may depend upon just how we define things and what sort of reference frames we choose to measure them in.
Another point: It is a general rule that wave action is most effectively emitted when the emitting object measures about 1/4 to 1/2 of the wavelength. When there are orbiting binary stars, the period of the wave equals one half the period of the orbit (quadrupolar action). If the wave moves at the speed of light and the period of the orbit is a matter of say, months (in many cases it would be much longer), the wavelength is very, very, long compared to the distance between the stars, so that the resulting wave would be expected to be extremely weak. For significant emission, the stars must orbit within a time that is a significant fraction of the time required for light to travel from one to the other. The stars thus would have to be moving at a significant fraction of the speed of light. That condition will be found only under the circumstance where there is very intense gravity capable of holding such a system together, as, for example, if both stars were dense neutron stars and they were in fairly close proximity.
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Nearly everyone is making the same mistake I did when first replying to rubygold. It is "gravity waves" that the question was about and NOT "gravitational waves". Check out the definition!!
In any case, rubygold, are you sure you mean gravity waves (surface waves) or isn't it more significant to consider compression waves in the interior (really sound waves)?
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Atomic-s Beautifully written. Brings me closer to understanding how it's thought to 'work'. And thanks Graham, I see what Ruby was talking about now, treating a star as a fluid, or as a density with, what I would call 'shock waves' propagating, but what apparently is called 'gravity waves' in fluid dynamics. and that was confusing :) even though logical.
So why not, it's all densities, isn't it? But one still needs to formulate the description for how those gravity waves Ruby was thinking of would come to be, and there I think we did a good job ::)) Well, at least you :)
In this case we seem to need a quadrupole moment, and an 'uneven' sphere if I got Atomic-s correctly, for those 'gravity waves' to be given the chance to propagate? And there should be a very good chance of that, shouldn't it?
And that question of energy and time Atomic-s :)
So sweet.
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And 'work' too, all of of them questions that really puzzles me.
"That implies that quantum mechanical objects, which vibrate according to E = hν, also vibrate slower. That implies that their total energy is less, and therefore their mass is less. Thus, lifting an object to a higher elevation would increase its mass (as seen from the original position), so that the energy in question would reside within the increased mass of the object rather than in the field."
That was an ingenious solution Atomic-s :)
You turned it upside down for me here.
I will need to think about that one..
Maybe we should have a thread on 'what are gravitational waves in Einsteins equations'?
So to not to branch out to much from Ruby's question and reflections?
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thanks to all and graham.d for doing so much working this out.
if taken from when the stellar mass begins its final collapse (to either a neutron star or a blackhole), it can be considered as acting initially from there by gravity. it is a part of a chaotic system. and the idea of sound waves contributing to wave patternization (?) is a welcome addition! waves causing a ringing in the core of neutron star would imply a considerable amount of pressure from these waves.
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a lot of material is covered in these posts, well explained too. i intend to go over them and answer what i can.
the sound waves will modulate the mass waves. they travel at different speeds and sometimes in different directions. looks like a fun time on the keyboard :)
looking forward to reassesing einsteins equations, too.
the surface gravity wave is the correct interpretation, but remember that an ocean wave has depth and width (or length). how it falls back will depend on the conditions of how it was thrown off.
if these waves do ring a neutrons' core, do you think they could also be expected to be strong enough to provide the push (needed?) to cause a spatial contraction event?
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I think that if the mass is near critical for collapse I think it likely that it could be pushed over the edge by a local compression, probably near the centre of the star. I also think it likely that compression waves would be spherically symmetric or at least circularly symmetric if rotating. Whether such an effect is insignificantly small or large, I don't know.
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okay, good :)
now, if we're done on compression waves contributing their share, and i wanted to take this further in the direction of blackhole collapse, should another thread be started or could we go into it here? maybe another thread ?
actually, the previous part about mass and gravity fields being in different places made me think of this: after the first explosion of a star it pushes a substantial layer out (it will return) and retains the core.
where is the field in this scenario?
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I don't think too much of a supernova explosion does return to the remnant star. It is the stuff of the universe of which planets and, ultimately, life is born. A lot of the heavier elements etc. The bang is quite big :-)
From far enough out the graitational field will be of the remnant star plus the ejected material so apart from some disturbance to the field the net graviational pull will not change much. Closer in the ejected material will now have gone out further so the net field will be mainly from the remnant star itself and anything orbiting close in or returning (most will have achieved escape velocity I think). Is this what you meant?
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yes, it does. i was daydreaming along the lines of planetary nebula. sorry, i changed topic without signaling, doh :)
graham.d, your previous questions are correct. it is the core collapse of a supernova where the gravity and sound waves should be calculated. my apologies to the others also.
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as i was surfing the net today, there was an article about a double-peaked resonance detected from neutron stars and black holes. couldn't find it again when i went back but it was in a physical review journal, article from british columbia.
could this be sound and gravity waves travelling through these objects?
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This is beyond where I can make a useful contribution I'm afraid. It is worth having a look at the abstracts of papers given here as some may be of interest:
http://www.mssl.ucl.ac.uk/~sz/Conference_files/partec.html
There was a converence at Whistler in BC which also spawned a lot of papers, which may be where you picked up your reference. The double peaked resonance you refer to may be speaking of magnetic resonances and not be likely to be caused by mechanical means.