# The Naked Scientists Forum

### Author Topic: How do gravitational waves go from source to detection?  (Read 2519 times)

#### lean bean

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##### How do gravitational waves go from source to detection?
« on: 16/02/2013 12:04:41 »
If gravitational waves are tidal ‘forces’ changing in position and time, then are they travelling via a pre-existing space?
In other words…
How do the gravitational waves go from source to place of detection?
If I remember rightly they are a prediction of GR and wondered how this 'trip' was accomplished in GR, so ruling out gravitons. :)
« Last Edit: 16/02/2013 12:08:36 by lean bean »

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #1 on: 16/02/2013 12:42:14 »
The 'motion' of it should be a description from ones local frame of reference, in effect where you are, measuring. From a global perspective maybe more like a jello that gets 'shaken' internally, from where distortions emanate?

I don't think it is a 'motion' really, more like some compression/elongation of the whole shebang, including the arrow as perceived from some 'locally inertial observer'.
=

Then again, define what moves :)
Motion is very confusing.
« Last Edit: 16/02/2013 12:45:52 by yor_on »

#### lean bean

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##### Re: How do gravitational waves go from source to detection?
« Reply #2 on: 16/02/2013 19:17:53 »
confusing.
Yes, they are.
Even the book 'Exploring Black Holes' gives...
From Page1 and 2 of pdf.
Quote
Gravity wave: a tidal force that propagates through spacetime.
--------
Gravity waves are essentially tidal forces that vary with time and position;
that is all they are. As a gravity wave passes over you, you are alternately
stretched and compressed in ways that depend on the particular form of the wave.
'Passes over you'. Passes...movement?
The pdf. found here...
http://www.eftaylor.com/exploringblackholes/GravWaves100707V2.pdf
in above quotes, my bold.
We are told energy is transferred by the waves, but again, what are the waves that have this property of energy?
« Last Edit: 16/02/2013 19:30:42 by lean bean »

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #3 on: 17/02/2013 00:47:51 »
I'll give you some arguments for why I find motion ill defined. First we have the concept of uniform motion, in where you are free to define whom is 'really' moving, and it being totally true as far as I can see. Then we have the concept of motion costing energy, but in a uniform motion there is no such notion :) as energy cost. Then we have a proof of motion in that we can find different speeds observing different objects astronomically which is a clear indication of the concept existing. then we have the ordinary definition of a motion in where one self is the pivot the world hinges on, and if you define yourself as being still relative some other object then you are still.

But the main point is that you can't use 'energy expended'  to define a 'motion'.
And if you can't use that?

The same thing goes for gravity, it does not expend 'energy'. there is no such thing as a 'tired gravitational potential'. In Einsteins universe, and as I read it, you have 4 dimensions that cling together, not separable. In that universe a gravitational wave seems best described as something happening inside a 'jello' consisting of those four dimensions. But the position you define to something, and the time, is only consistent with your local measurements. That we find a Lorentz transformation to allow us some theoretical description of a 'unified universe' in where everything is translatable is no surprise to me as the only thing such a description state is that there is a logic to what to we see. And without such a logic the universe would become 'magic'.

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #4 on: 17/02/2013 01:07:07 »
I sometimes wonder if it has to do with some smallest bits, relating to each other? You could imagine a universe consisting of points'. Each point finding its own definition of all other points 'time' (1)and 'position' (3), acting upon each other from their local definitions. One important point with relativity is that you need two frames for it to exist. To define something as relativistic using one frame only is not possible. That one can be used as a argument for 'reality' being using only one frame actually, ignoring the multitude created in comparisons. But if you use that you also get a concept where all frames are logically consistent with their 'own reality', and then the problem becomes to find what joins those 'frames'. Because they must have a common ground for us to be able to exist, as individuals, objects etc etc. (communicating)

And if you use 'c' you find that they have a common description of 'time', or at least the arrow. So the arrow are the same for us all. But that is also a question of scale, as you need to scale down that 'point' of observation until it presents you with a flat SpaceTime, where you no longer can discern any gravity.

#### lean bean

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##### Re: How do gravitational waves go from source to detection?
« Reply #5 on: 17/02/2013 11:08:24 »
The same thing goes for gravity, it does not expend 'energy'. there is no such thing as a 'tired gravitational potential'.

No ones saying a gravitational wave is expending energy, it's carrying energy.
So, what is meant when they say a gravitational wave can carry energy  away from an event, say binary neutron star system.What is the wave that has the property of energy?
when a gravtational wave passes through anything it leaves that thing in the same state as it was before the wave. No energy is expended by the wave...( someone correct if wrong here]

In Einsteins universe, and as I read it, you have 4 dimensions that cling together, not separable. In that universe a gravitational wave seems best described as something happening inside a 'jello' consisting of those four dimensions.
If your 'jello' consists of four dimensions, what then  is a gravitational wave in four dimensions? If a gravitational wave is said to carry energy from a system, then what is doing the carrying?
The transfer of energy/momentum in ocean waves happen via the medium of water. :)  :)
« Last Edit: 17/02/2013 11:55:07 by lean bean »

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #6 on: 18/02/2013 11:25:21 »

Yeah, it's weird, and I don't know what is correct there. You could look at as if the energy 'transfered' is a redistribution of energy, as defined relative a 'fault/gravitational wave' through some 'medium', consisting of our four dimensional reality (SpaceTime). That type of 'medium' is not a 'material' though, as it involves arrow(s), as well as space.

Like if you had a static evenly distributed 'energy' field originally, with 'faults' created from matter, (possibly) relative motion and (definitely) accelerations. And what distributes it all is then the arrow as I see it. And it also means, if you look at it that way, that you always will need at least two frames of reference to be able to define it. So defined from only one frame it does not exist :)

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #7 on: 18/02/2013 11:41:30 »
Maybe somewhat like this, all frames of reference, (points in SpaceTime, defined in four dimensions) are locally equal. Meaning that if you superimpose two such points, they will be equal in arrow and 'distances'. What creates a dichotomy is when you compare one point to another inside SpaceTime, using your local clock and ruler. The first definition is similar to a 'static field' to me, in a very theoretical manner as there is no way to measure on its properties that I know of, more that to admit that when superimposed you must find this equality. The other is when we introduce comparisons between 'frames of reference' as defined from locality, which then becomes our dynamical SpaceTime, ever changing.

From a point of locality my frame is your frame, no matter what values you measure comparing frames of reference. But it's a sort of mindgame also, as the way we exist is just by comparing frames of reference. and not only 'me', but all constituents of whatever defined, from quarks to doves, 'compares' through defined rules, in the end coming down to 'constants' defining what 'reality' we observe.
==

What such a reasoning state though is that there should/must be some more dimension/degree of freedom from where the comparisons becomes possible, as there are differences when comparing frames of reference.

You might want to use 'c' as one definer of it, but you need something more explaining how 'c' always are 'c', locally measured, at the same time presenting us with different frames of reference when comparing. And that should be some truly wondrous 'degree of freedom' :)
« Last Edit: 18/02/2013 12:05:36 by yor_on »

#### lean bean

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##### Re: How do gravitational waves go from source to detection?
« Reply #8 on: 18/02/2013 17:37:12 »
Thanks yor_on
I give your posts some thought.
:)

#### syhprum

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##### Re: How do gravitational waves go from source to detection?
« Reply #9 on: 18/02/2013 21:30:00 »
one reads of bipolar objects as radiating energy only a few watts for objects such as the Earth/Moon system, something must become of this energy when it passes thru me causing stretching and contraction some of it must be converted to some other form presumably thermal.

#### lean bean

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##### Re: How do gravitational waves go from source to detection?
« Reply #10 on: 19/02/2013 14:08:43 »
something must become of this energy when it passes thru me causing stretching and contraction some of it must be converted to some other form presumably thermal.
Not disagreeing with you.
What made me say no energy is expended by the wave in my #5 post, was because of something I read on some other site long ago, can’t find it now,I wasn't too sure of it then and still not... thus the 'someone correct me' tag on the end.
In answer to my own question, on most sites it seems space is the ‘medium’.
I think most people have read this before , but  I wanted to see what would pop-up.

Quote
gravitational waves
Distortions of space geometry that propagate through space with the speed of light, analogous to ripples on the surface of a pond propagating as water waves.
http://www.einstein-online.info/dictionary/gravitational-waves

Quote
With space and time not as rigid background structures, but as dynamical objects (changing as the world changes in and around them), general relativity predicts fundamentally new phenomena. One of the most fascinating is the existence of gravitational waves: small distortions of space-time geometry which propagate through space as waves!
http://www.einstein-online.info/elementary/gravWav

Quote
Simple gravitational waves only squeeze and stretch space in directions perpendicular to the direction of their propagation. In the language of physics, such waves are called transversal.
http://www.einstein-online.info/spotlights/gw_waves

Quote
The effect of a propagating gravitational wave is to deform space in a quadrupolar form. The effect alternately elongates space in one direction while compressing space in an orthogonal direction and vice versa, with the frequency of the gravitational wave. A Michelson interferometer operating between freely suspended masses is ideally suited to detect these antisymmetric distortions of space induced by the gravitational waves; the strains are converted into changes in light intensity and consequently to electrical signals via photodetectors.

Ps. I’m still trying to find a site that gives information about gravitational waves losing energy and why…anyone?
« Last Edit: 19/02/2013 14:10:25 by lean bean »

#### yor_on

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##### Re: How do gravitational waves go from source to detection?
« Reply #11 on: 20/02/2013 08:43:47 »
If SpaceTime is a jello then it also must be the same to us all, or else we all see a different jello described through our local measurements. Which one is right I can't say although I prefer the later. In both your gravitational wave should be a distortion in that jello, changing the expression of energy in and around it. If I assume that this 'energy change' is related to mass, and 'propagating', then it should be a conserved property of SpaceTime, a distortion that somehow takes with it whatever amount of 'energy' that first was created in mass interacting with mass, and you better take this with a great deal of salt, as we say in Sweden. The most used description and proof being binary stars rotating around each other. Then it's not 'energy' being transported as much as it is a distortion, changing the energy values relative it as it 'propagate', possibly :)

You could also ask yourself how it 'moves'?
Does it cost energy for it?

How can it distort anything without costing energy?
« Last Edit: 20/02/2013 08:48:29 by yor_on »

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##### Re: How do gravitational waves go from source to detection?
« Reply #11 on: 20/02/2013 08:43:47 »