Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: CPT ArkAngel on 14/05/2013 00:21:35
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How to differentiate gravitational waves from tidal effects?
http://en.wikipedia.org/wiki/Gravitational_wave
http://en.wikipedia.org/wiki/Tidal_force
Its seems to be the same effect, isn't it?
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So a tide would then be the result a gravitational wave :)
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According to General Relativity, it should be. But the usual tidal force is more like a DC component of the wave. Still, the motion of the moon around the earth and the rotation of the earth produce a wave for an observer standing still on the earth.
http://relativity.livingreviews.org/open?pubNo=lrr-2009-2&page=articlesu3.html
http://math.ucr.edu/home/baez/einstein/node5.html
The problem is that it changes the density of bodies, so it is not an oscillation of space but an oscillation of matter in space.
If experiment like Ligo detects gravitational waves, it would mean either, there is no gravitational waves, or the speed of light is not constant... Ligo uses one clock and one laser... Either, it changes the density (not a spacetime wave), or it changes the speed of light (a spacetime wave). In SR, space and time are entirely reciprocal so you can't have a spacetime wave. Strictly speaking, the spacetime interpretation of GR is not compatible with SR. This is my point of view.
I think this is where Special Relativity surpasses GR.
In the case of a white dwarf and a neutron star rotating at close range, the relativity of mass in SR modifies the space-time coordinates of the center of mass acting on each body (unless each body rotation and translation are all exactly aligned, which is impossible). The center of mass coordinates forms a spiral shape that oscillates over time. This produces precession and finally degeneration of orbital motion.
The big questions:
What mediates gravity and at what speed?
My best guess is photons of one wavelength equal to the distance between elementary particles and either the speed of light or a fixed delay of Planck time (variable speed of gravity).
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Maybe, if you look at it as if gravity is a spiders net. Then you can have waves and vibrations propagating in that net. It should depend on definitions, because I think you would have to prove that there can be no such thing as as a constant 'contraction' of that 'net' as locally defined, to make it all fit a wave description.
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Choose the wrong word there, don't think of it as some Lorentz contraction for this, first of all it's a dynamic universe in 'relative motion', but you can imagine two planets/suns in a 'stable' orbit around each other in some 'flat' space otherwise. That should give you what I meant by a 'constant attraction' although somehow it came out as a 'contraction' instead :). Got a cold and headache so that might have something to do with that misprint. Although it still being a dynamic system. The best one may be to imagine some mass, 'at rest' with some other mass, to then try to define that relation from gravitational waves, if that is what you really mean?
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How would you then define changing your coordinate system from experiencing a gravity to become in a free fall, aka a geodesic?
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I had something similar, wanderings of the mind relating it to 'energy', some time ago.
See what you can make of it. http://www.thenakedscientists.com/forum/index.php?topic=37886.0
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GR is an attempt to describe gravity without knowing its sources and localizations. It is a macroscopic theory, not a complete theory. No wonder why there are singularities and even worst, rotating singularities.
Einstein started from classical physics with a world filled with the electromagnetic and gravitational fields. QM was not developed. Einstein couldn't use any theory of particles. So a point particle concept was the best approximation.
Then Einstein developed the Special Theory of Relativity, which is a theory of relative motion, including the notion of relativity of mass and energy. It comes from the observation of the constancy of the speed of light. The breakthrough is the conclusion that there is no absolute reference frame, no ether. It means that locally, space and time must be variable to allow a constant speed of light. But Einstein was not satisfied; he wanted to include the gravitational field.
Having no description of matter, Einstein’s General Relativity could only be an approximation; the cause of the gravitational effect was unavoidably unknown. But Einstein needed an explanation, he needed something like spacetime. But what is spacetime? I don’t want to believe it without understanding it.
We now know that particles are not simple point particles. Where is matter in GR? GR supposes that matter bend spacetime and spacetime bend the path of matter. Where is the proof of that? You must believe in that. There is no further explanation.
For me, the solution must be found in the relativity of elementary particles.
According to SR, if you point a beam of light toward a mirror and catch its reflection to measure its speed, you will always obtain a constant speed for light. Even if you have a wave of space-time because space and time should change so the speed of light is a constant locally; thus you should not be able to measure gravitational waves with one laser and one clock. Aaahhh!!! You might say the catch is in the inertial frame... Beyond a description of the relative motion, what is an inertial frame? How does it work? Why the constancy of the speed of light?
In my opinion, the spacetime can’t explain the effect of gravity on the density of matter without violating the second postulate of SR: the constancy of the speed of light in an inertial frame of reference. For me, no matter means no frame. Relativity is about relations of matter particles. In Einstein’s GR, matter is not the essence…
If gravity contracts space and slows down time, how it increases matter density? What is your explanation? What’s the proof of it? What if we can explain all GR phenomena without it? I didn’t find anything I can’t explain without it…
No one understands GR, even Einstein didn’t. How can you understand a world where information is limited by the speed of light? It is a big mess… What is time? Why Newton’s laws are good at the motionless limit?
In my opinion, if Einstein were here today, he would agree with me.
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Einstein knew what he did. He made a postulate about a local constant called 'c' and created SR from that. Later he found a way to relate a uniform constant acceleration to gravity and from those two comes both SR and GR. If you're asking why 'c' is then nobody has any answer to that. We have constants and they becomes limits and borders describing a universe. Science have never had a 'ultimate answer', Science is about joining what we know and can experimentally verify into a framework giving as good as explanation we possibly can, at the moment, and search for more :)
Did you look at the link?
It is related to what you asked about, how to differ between gravitational waves and normal gravity creating tides.
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What I mean is that it is not only the distance but space itself that shrinks, including the particles, so it should be impossible to measure true gravitational waves (because time slows down to keep a constant speed of light). If they measure gravitational waves, it will be shrinkage due to gravitational force or its equivalent in GR. Fortunately, GR predicts a higher level of detection that if it is due to the classical gravity, for an observer on earth. So they should be able to differenciate them...
Einstein once said that he no longer understands his own theory since mathematicians has invaded it.
I'm quite sure you have already read it somewhere, Yor_on... :o)...
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Nope, that was new one to me, but I'm sure he might have said it. Some of the results possible from manipulating the mathematics are quite 'out of here' :) Then again, we all have our own ways of reading it. For myself I'm keeping the arrow as a local constant, which makes slowing 'time' down locally a no no, seen through my eyes. But that is as it is with relativity, it's as much philosophy as mathematics and experiments :)
What I meant though is that you can differ between a gravitational wave and 'ordinary' gravity in relativity, although it takes some time to assimilate the ideas around it, and I think I can promise you will forget them after a while too :) I know I do at times. I'm still not happy with the defining of a gravitational wave as having a 'energy' for example, but that may depend on what the idea of a 'energy' mean in this context. Einstein himself changed his mind, two times I think, on the question of gravitational waves existence, in the end agreeing to them being possible.
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As in "no-yes then No - Yes. :) It has to be one of the trickiest subjects there are as some also define 'space' to be able to do ftl (black hole), at the same time as a gravitational wave is supposed to propagate at 'c', as far as I know. That one goes back to how to define a 'space'. Einstein defined gravity as the metric, me, I want it to be 'space' :)
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Yes, time does not slow down local observer measuring his own time. Time slows down relatively. An observer on top of a mountain and sending a beam of light to a reflector at the bottom of a valley has no way to measure the time dilation with one clock. At the reflector location, time is dilated by the same amount as length is contracted, for the observer. (before, i used 'locally' meaning time dilation has a reciprocal length contraction locally, so the speed of light is constant, locally... sorry for the confusion)
I think the concept of force may not be an eternal one, but I think it is not at this stage that it should disappear, if ever it has to disappear...
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Hmm, that one was slightly tricky to me?
Imagine yourself setting up a atomic clock, locally 'at rest' with you. You then measure a distance by a laser, its light reflected back to your local detector. Splitting the distance relative the amount of time measured by your clock, locally ticking, you should get 'c'. No matter where you point that laser, at a black holes event horizon or at the moon. The clock won't care for where you point the laser. You only need one clock and that distance, something reflecting the light back to you. The clock won't change its oscillations depending on where you point the laser?
If you by it mean using two 'light clocks', one where the mirror is and one local at rest with you, you can assume that the clock near a event horizon to 'tick' a lot slower than your own clock. If you solely use the remote clock to define a light speed, instead of using your local clock, sending a (two way) laser beam, you now would define light to exceed 'c'. Furthermore you should find that a day, defined from that clock, should cover a lot of days and nights locally. So you can't use that clock, because doing so you no longer define the arrow from your frame of reference.
Maybe you mean it to be as the light clocks example, and thinking of it as time at that remote location really tick slower though? If you use NIST you can find time dilations on Earth, at centimeters. But your 'proper time' won't speed up because you stand up. According to all experiments you can do it never change its pace for you. And you joining a event horizon will find the light clock perfectly synchronized with your wristwatch.
Using light clocks as our example, ticking at 'c', you then have to assume 'c' to be a variable as it ticked slower at that remote location, relative your local clock. And now we're only using two light clocks for this, no two way measurement with a laser. But joining that clock at the event horizon, as in joining a same 'frame of reference', that exact same local clock that gave you the time dilation suddenly gives you the exact same 'chunks of time' (oscillations) as the 'remote light clock' does. So how do you prove that proposal?
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What I'm aiming at is that the way we do all direct experiments is locally, defining a time relative that clock being 'at rest' with us. It's all about local definitions. And that is also the way we find a constant light speed in a vacuum.
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How to differentiate gravitational waves from tidal effects?
http://en.wikipedia.org/wiki/Gravitational_wave
http://en.wikipedia.org/wiki/Tidal_force
Its seems to be the same effect, isn't it?
Not exactly. A gravitational wave is the propagation of tidal effects. You can't have gravitational waves without tidal forces but you can have tidal forces without gravitational waves.
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There is a added difficulty, if wanting to define what I would call 'time pockets', and that's relative motion and accelerations. Frames of reference is just positions,(local time and then x,y,z) and one can imagine it as laying a grid over some locally defined space to define points.
But introduce different relative motions, mass, and accelerations and your grid won't fit mine. That we can transform one grid to another is the same sort of logic making it possible to define a time symmetry to me. It's about the logic giving you a arrow, past present and future, Without some principles and logic making a arrow definable, also making a ball bounce able to be played back and forward, unable to define which is which ideally, we lose the logic. When we do so we introduce randomness macroscopically, with or without probability. With probabilities able to define from statistics you will have a new 'logic' universe, although not ours. Without probabilities, us finding it unable to make statistics showing us patterns, you have magic, but wild magic. A sorcerer in such a universe would have nothing to support his magic on, every incantation giving him a new effect. And yes, I do like fantasy :)
But we have a logic, to QM and to relativity, as statistics and Lorentz transformations. So it will be very hard to define it as time pockets, as they will be observer dependent. Not that I know if that was your intention CPT, it's just me covering as many bases I can remember here.