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quanta
2005-Jan-24, 12:01 PM
I 'thought' I understood the basic principle used at LIGO to detect gravity waves, but perhaps not.
Assuming that the measuring 'arms' change in length as expected, shouldn't the laser used to detect the length change in the two perpendicular arms also change wavelength, therefore effectively changing the 'ruler' used to measure the length variation, rendering the exercise as futile?

Hmm, scratches head :-k

Eta C
2005-Jan-24, 02:30 PM
Well, for starters one should peruse the LIGO web site (http://www.ligo.caltech.edu/) which probably answers a lot of the questions you have.

quanta
2005-Jan-24, 09:18 PM
Well, for starters one should peruse the LIGO web site which probably answers a lot of the questions you have

One did, but thanks for the advise.
The criteria seems to be detection of path length change and the issue I addressed doesn't seem to get a mention, that I could detect.
Perhaps it's not an issue, but the question is a reasonable one.

Gsquare
2005-Jan-25, 04:13 AM
I 'thought' I understood the basic principle used at LIGO to detect gravity waves, but perhaps not.
Assuming that the measuring 'arms' change in length as expected, shouldn't the laser used to detect the length change in the two perpendicular arms also change wavelength, therefore effectively changing the 'ruler' used to measure the length variation, rendering the exercise as futile?

Hmm, scratches head :-k

You are absolutely correct, quanta. I addressed this exact issue about a year ago (on another site) and having done extensive research, I was able to convince myself laser interferometry should not be able to detect GW's.

1st, most people recognize that space "stretches" as the gravity wave passes, but assume that the light beam is independent and think, because of that, the interference pattern should change.
However, in reality, (and a fact never addressed in these LIGO sites is that) the wavelength of the laser light changes equivalently with the space change, so there should be NO interference shift.

I was able to locate only one report ( I'll see if I can find it ) by a LIGO theoretician who, while admitting the above statement IS TRUE, (light beam & space stretch equally), nevertheless, after having done a full blown GR analysis, STILL concluded an interference shift is expected. His logic (which I couldn't quite see), went something like this (paraphrasing his results):

'Yes, at the passing of a GW, space is strectched, but so is the in-flight light beam, which should result in a null interference shift. However, because of the time dilation in the frame of the wave, the shift will nevertheless be detectable'. But I couldn't see it from his calculations and when I questioned his results, I never got an answer!

If someone could show me proof of this (even without all the GR math) I'd would like to see it. :o

G^2

quanta
2005-Jan-25, 10:31 AM
Gsquare wrote:

You are absolutely correct, quanta. I addressed this exact issue about a year ago (on another site) and having done extensive research, I was able to convince myself laser interferometry should not be able to detect GW's.


Thanks Gsquare.
I think one of us should let LIGO management know their interferometers won't work.
I vote you do it (you're closest) :D


....'Yes, at the passing of a GW, space in strectched, but so is the in-flight light beam, which should result in a null interference shift. However, because of the time dilation in the frame of the wave, the shift will nevertheless be detectable'. But I couldn't see it from his calculations and when I questioned his results, I never got an answer! .....


It's a conspiracy!
Seriously though, it would be interesting to see some maths etc.

papageno
2005-Jan-25, 10:37 AM
Have a look at the AURIGA project (http://www.auriga.lnl.infn.it/).

scourge
2005-Jan-25, 11:34 AM
Thanks Gsquare.
I think one of us should let LIGO management know their interferometers won't work.
I vote you do it (you're closest) :D


Heh. Well, they've been up and running for awhile now, I'm sure it's starting to sink in, the poor devils :-({|=

Anyone have any idea what we can do with this crazy contraption once they abandon it?

Gsquare
2005-Jan-25, 06:40 PM
Gsquare wrote:

You are absolutely correct, quanta. I addressed this exact issue about a year ago (on another site) and having done extensive research, I was able to convince myself laser interferometry should not be able to detect GW's.


Thanks Gsquare.
I think one of us should let LIGO management know their interferometers won't work.
I vote you do it (you're closest) :D


Like Scourge said, .... after so long there are probably those in LIGO who are already beginning to suspect 'something is rotten in Denmark'.
Besides; I'm not going to be the one to inform a thousand engineers, physicists, technicians, and secretaries that their jobs have just been terminated [-( . Let Congress do it. :wink: He, he. :P

G^2

The more you study, the more you know;
The more you know, the more there is to forget;
The more there is to forget, the more you can forget;
The more you can forget, the more you do forget....
So why study? :P

Gsquare
2005-Jan-25, 06:48 PM
Heh. Well, they've been up and running for awhile now, I'm sure it's starting to sink in, the poor devils :-({|=

Yea, but be careful; there are plenty of 'die-hards' out there that will go down kicking, clawing and scratching that "we just haven't given it enough time"! :lol: :P



Anyone have any idea what we can do with this crazy contraption once they abandon it?

How's about putting it up for auction on E-Bay? :lol:

G^2

quanta
2005-Jan-25, 09:20 PM
papageno wrote:

Have a look at the AURIGA project.

Thanks papageno, but the operting principle is dissimilar to LIGO's interferometer, where the "ruler" is integral to the structure along its entire length. If the struture changes length in LIGO then it remains a mystery (to me) why the light used to detect changes in length should not also change. There are many publications in the referenced site, which I'm wading through.


Gsquare wrote:

.........Besides; I'm not going to be the one to inform a thousand engineers, physicists, technicians, and secretaries that their jobs have just been terminated . Let Congress do it. He, he.



It's easy. Just go up the the front desk, ask to see the person in charge of the 'long, expensive piece of redundant scrap metal thingy'.
Then just tell them you think its a piece of junk.
They will naturally be very grateful :^o

papageno
2005-Jan-26, 10:23 AM
Have a look at the AURIGA project.
Thanks papageno, but the operting principle is dissimilar to LIGO's interferometer, where the "ruler" is integral to the structure along its entire length.
That was actually my point.
I wanted to show that LIGO is not the only way.



If the struture changes length in LIGO then it remains a mystery (to me) why the light used to detect changes in length should not also change. There are many publications in the referenced site, which I'm wading through.
When I first read about LIGO, I had the same impression.
Maybe light and material structure are affected in different ways by the gravity wave (but this is hand-waving).