gravitational waves and the general theory of relativity for reptiles

merle van osdol
merle van osdol
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Topic 197897

How essential is the existence of gravitational waves to the general theory of relativity? Since Einstein refers to me as a reptile I would need the answer in layman's terms if possible at all. :-)

merle

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Bikeman (Heinz-Bernd Eggenstein)
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gravitational waves and the general theory of relativity for rep

Quote:
How essential is the existence of gravitational waves to the general theory of relativity? Since Einstein refers to me as a reptile I would need the answer in layman's terms if possible at all. :-)

The "reptile" reference may need some explanation, I guess not all here have heard about a recently published letter written by Albert Einstein to Marie Curie, at a time when Curie was going thru a very hard time as there was a public hate campaign waged against her. The letter can be found here, and the reptiles are obviously those harassing Curie. So no, Einstein would not have considered YOU a reptile ;-).

As for the question: Gravitational Waves are a necessary consequence of General Relativity. Disproving GW would disprove GR, (and would cause severe head-scratching wrt. the work that caused the Nobel prize for physics of 1993 to be awarded to R. Hulse & J. Taylor, which was an indirect confirmation of the existence of gravitational waves http://en.wikipedia.org/wiki/Tests_of_general_relativity#Strong_field_tests:_Binary_pulsars).

Cheers
HB

merle van osdol
merle van osdol
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Thanks for the reply. But I

Thanks for the reply. But I guess indirect confirmation isn't quit enough so we need to find those GW's directly. I didn't realize it would totally overturn GR.

The reptile reference I read in several places in Isaccson's biography of Einstein. I thought it was in reference to all those who couldn't understand his GR theory but I probably misread the references. I might go back and double check.

Thanks again.

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

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Mike Hewson
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RE: But I guess indirect

Quote:
But I guess indirect confirmation isn't quit enough so we need to find those GW's directly. I didn't realize it would totally overturn GR.


Shorter Answer
Yup. For example : if orbital energy is reducing in a Taylor-Hulse type inspiralling system, as observed and as predicted by GR, then where is it going ? If we want to keep conservation of energy then we need to reconcile that loss somewhere. In simple terms one can think of GR as a time delayed gravity theory. So a wiggle over there takes time to affect us here, the finite speed of light being the presumed cause of the delay ( c/w Newton which was instantaneous ).

Longer Answer + Quantum Digression
In generality the loss of energy from an emitter is due to 'radiative reaction'. Think of it like the recoil of an artillery piece when a round is fired. Thus your local TV station has to input energy to keep up the acceleration of the electrons ( ie. supply a force to yank them back and forth ) in it's antenna when they are chucking out photons. The idea is that you have to reconcile that locally, meaning : energy is conserved at the point & time of photon emission regardless of the ultimate fate of the photon after departure from the scene. Likewise energy is conserved at the receiver end of the deal with photon absorption, regardless of where the photon came from. In a sense we have the photon 'concept' in order to get around classical action-at-a-distance. It's the cheque in the mail.

Thus far we have the word 'graviton' to label the case of lumps of the gravitational field, in analogy to photons being lumps of the electromagnetic field. No one has been able to make that work on the theory side though ie. 'quantise gravity', unified field theory, theory of everything etc. Richard Feynman, one of the chaps who came up with Quantum Electrodynamics ( QED, the very successful quantum theory of electromagnetism ) was partly aiming to dispense with the field concept and just have objects traversing back and forth carrying the things you want to conserve. If you look at a Feynman diagram with all the straight & wiggly lines etc, then at the meeting points of the lines ( interaction vertices ) all relevant conserved quantities ( eg. energy, momentum, charge, various quantum counters ) must be accounted for by the properties of the objects involved at the vertex. The traditional wave aspect is preserved as due to the multiplicity of possible transmission paths and a probabilistic aspect to detections when large numbers of messengers are involved. Waves are a 'commentary' about groups of particles.

It's a very ancient duality/dichotomy the particle vs wave discussion, by no means settled. GR is a classical ( non-quantum ) theory and so we talk of propagating waves - entities varying continuously over large volumes of space.

There is another issue at a higher level : whereas electromagnetism involves energy ( lumps or waves ) traversing across space, gravitational disturbances are of space & time itself. A reasonable analogy is paint color varying across a canvas vs the canvas itself varying across the canvas. This comes under the rubric of 'background (in)dependence'.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

merle van osdol
merle van osdol
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RE: RE: But I guess

Quote:
Quote:
But I guess indirect confirmation isn't quit enough so we need to find those GW's directly. I didn't realize it would totally overturn GR.

Shorter Answer

Yup. For example : if orbital energy is reducing in a Taylor-Hulse type inspiralling system, as observed and as predicted by GR, then where is it going ? If we want to keep conservation of energy then we need to reconcile that loss somewhere. In simple terms one can think of GR as a time delayed gravity theory. So a wiggle over there takes time to affect us here, the finite speed of light being the presumed cause of the delay ( c/w Newton which was instantaneous ).

Longer Answer + Quantum Digression
In generality the loss of energy from an emitter is due to 'radiative reaction'. Think of it like the recoil of an artillery piece when a round is fired. Thus your local TV station has to input energy to keep up the acceleration of the electrons ( ie. supply a force to yank them back and forth ) in it's antenna when they are chucking out photons. The idea is that you have to reconcile that locally, meaning : energy is conserved at the point & time of photon emission regardless of the ultimate fate of the photon after departure from the scene. Likewise energy is conserved at the receiver end of the deal with photon absorption, regardless of where the photon came from. In a sense we have the photon 'concept' in order to get around classical action-at-a-distance. It's the cheque in the mail.

Thus far we have the word 'graviton' to label the case of lumps of the gravitational field, in analogy to photons being lumps of the electromagnetic field. No one has been able to make that work on the theory side though ie. 'quantise gravity', unified field theory, theory of everything etc. Richard Feynman, one of the chaps who came up with Quantum Electrodynamics ( QED, the very successful quantum theory of electromagnetism ) was partly aiming to dispense with the field concept and just have objects traversing back and forth carrying the things you want to conserve. If you look at a Feynman diagram with all the straight & wiggly lines etc, then at the meeting points of the lines ( interaction vertices ) all relevant conserved quantities ( eg. energy, momentum, charge, various quantum counters ) must be accounted for by the properties of the objects involved at the vertex. The traditional wave aspect is preserved as due to the multiplicity of possible transmission paths and a probabilistic aspect to detections when large numbers of messengers are involved. Waves are a 'commentary' about groups of particles.

It's a very ancient duality/dichotomy the particle vs wave discussion, by no means settled. GR is a classical ( non-quantum ) theory and so we talk of propagating waves - entities varying continuously over large volumes of space.

There is another issue at a higher level : whereas electromagnetism involves energy ( lumps or waves ) traversing across space, gravitational disturbances are of space & time itself. A reasonable analogy is paint color varying across a canvas vs the canvas itself varying across the canvas. This comes under the rubric of 'background (in)dependence'.

Cheers, Mike.

Thanks Mike,
There are parts of your answer which astounded me and opened my eyes in a new way. But there were also parts which were dark for me. I have been thinking and trying with these ideas for over 50 years. Little by little it becomes clearer. I just recently tried reading Einstein's "layman's" explanation of his theories. FaGetAbout it! :-)

PS. I think we have so much more to learn. Probably more than we can even imagine.

merle

What is freedom of expression? Without the freedom to offend, it ceases to exist.

— Salman Rushdie

edjcox
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Wonder if anyone has ever

Wonder if anyone has ever thought of this.

Hubble has imaged any number of gravitational lenses out there in the cosmos. These lens effects are essentially gravitational distortions of star light passing near massive gravitational pulls resulting in smearing of the light as it is path distorted near the field of a gravity source.

Now if gravity indeed can and does produce waves and propagates them would not these gravitational lenses be impacted by such waves and the distortions of the light be impacted to in effect cause a shimmering corresponding to the gravity wave passing?

As far as I know these images of gravitational lenses remain steady and do not exhibit any variations. Assuming we have adequate imagery of a given lens area a long term observation and time lapse recording might provide evidence of or disprove the existence of gravity waves...Observations of a number of these lens effects in various parts of the cosmos might allow one to postulate either a uniform lack of gravity waves or if images showed variations existence of them. With timing or simultaneous observations one might even be able to discern if they are uniform / simultaneous / or perhaps exhibit delays that might lead to calculations of propagation speeds. Universal assurance of a speed limit might also come out of this research..

Any intelligent comments would be appreciated.

Edward J Cox CPP

Mike Hewson
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RE: Wonder if anyone has

Quote:

Wonder if anyone has ever thought of this.

Hubble has imaged any number of gravitational lenses out there in the cosmos. These lens effects are essentially gravitational distortions of star light passing near massive gravitational pulls resulting in smearing of the light as it is path distorted near the field of a gravity source.

Now if gravity indeed can and does produce waves and propagates them would not these gravitational lenses be impacted by such waves and the distortions of the light be impacted to in effect cause a shimmering corresponding to the gravity wave passing?

As far as I know these images of gravitational lenses remain steady and do not exhibit any variations. Assuming we have adequate imagery of a given lens area a long term observation and time lapse recording might provide evidence of or disprove the existence of gravity waves...Observations of a number of these lens effects in various parts of the cosmos might allow one to postulate either a uniform lack of gravity waves or if images showed variations existence of them. With timing or simultaneous observations one might even be able to discern if they are uniform / simultaneous / or perhaps exhibit delays that might lead to calculations of propagation speeds. Universal assurance of a speed limit might also come out of this research..

Any intelligent comments would be appreciated.

Edward J Cox CPP


That sounds good. I guess the issue, as usual, would be to reliably distinguish the effects of interest from all other confounding ones.

The first of those that comes to my mind is source variation. Now there are instances on record eg. the COSMOGRAIL program where some source object has fluctuated in some time pattern. The various lens elements have followed that pattern but with varying time delays and thus revealing the different path lengths. I guess one could extend that idea by sensing the variation in those path lengths over many fluctuations, and perhaps attribute that to the passage of gravitational waves. As ever the devil would be in the detail ie. is the effect large enough to disambiguate given that it is a differential or higher order* effect being sought ?

Cheers, Mike.

* A difference of differences. :-)

( edit ) NB the commentary in that article mentions inconstancy of the lens due to the passage of 'individual stars of the lensing galaxy' across the line of sight, as it were. I find it amazing that we can deduce even that ! :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

edjcox
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Thanks for reading my notes.

Thanks for reading my notes. Also I thank you for the reference. Reading the reference has led me to a possible line of exploration. As it is now the observations to date have focused on objects discernable and measurable via a given lens effect observation. Thus the article and the analysis of quasars and objects and the time differences caused by gravitational stretching and slowing. Hence the differences.

What would be intriguing would be to time plot lens observations in far apart lenses and in effect seek coincidence in these distant part of the sky observations. That would then perhaps lead to some other "x-factor" impacting the lens effect other than the already postulated smearing of a given lens. Discovery of coincidental time overlaps might lead one to have discovered a means of discerning gravitational waves...

Mike Hewson
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RE: Thanks for reading my

Quote:

Thanks for reading my notes. Also I thank you for the reference. Reading the reference has led me to a possible line of exploration. As it is now the observations to date have focused on objects discernable and measurable via a given lens effect observation. Thus the article and the analysis of quasars and objects and the time differences caused by gravitational stretching and slowing. Hence the differences.

What would be intriguing would be to time plot lens observations in far apart lenses and in effect seek coincidence in these distant part of the sky observations. That would then perhaps lead to some other "x-factor" impacting the lens effect other than the already postulated smearing of a given lens. Discovery of coincidental time overlaps might lead one to have discovered a means of discerning gravitational waves...


Absolutely. What you propose has an analogous effort at radio wavelengths, going by general moniker of 'high resolution pulsar timing arrays'. The idea here is to characterise very many pulsars in their 'usual' measurements - including their individual quirks etc - and maybe one day note that the signals from them have coherently altered across the group. This leads to the sensible conclusion that they haven't all conspired to fool us by altering their behaviour at source, but that gravitational waves have passed us by causing our local perception to change. Now the technical challenge is to be sufficiently precise with observations in order to ( eventually ) be confident about an effect.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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