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Old 12th February 2016, 09:38 AM   #121
Pixel42
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Originally Posted by mbarrett View Post
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal. At this angle no differential length change will occur, thus cancellation, and no output signal seen. In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
Before LIGO we couldn't detect any gravitational waves, now we can. I don't think it matters if we can't detect all of them.

Welcome to the forum, btw.
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Old 12th February 2016, 09:40 AM   #122
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Originally Posted by mbarrett View Post
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal. At this angle no differential length change will occur, thus cancellation, and no output signal seen. In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
If there's only one observatory, that may be true. However, by design, LIGO is two, separated by quite a distance, and I doubt there are many directions, if any, a GW could arrive from, that would produce a null result in both. Add a third - Advanced Virgo, for example, and you'll get a signal, no matter where it comes from.

Also, I suspect that even just one observatory would be sensitive to an arrival direction that is as little as a degree from 'the blindspot' (depends on the intensity of the GW too, of course).

Finally, the 'blindspot' would be just two directions, orthogonal to the plane of the arms, right?
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Old 12th February 2016, 09:41 AM   #123
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Originally Posted by mbarrett View Post
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal. At this angle no differential length change will occur, thus cancellation, and no output signal seen. In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
Could we build this in space and somehow encompass all angles? I could see a unit built with 360 pairs of arms?
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Old 12th February 2016, 10:07 AM   #124
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Originally Posted by JeanTate View Post
A critical thinker would be critical of "bad science", but would not dump something that has been shown - repeatedly, etc - to work.


Sure. It's also possible to say: we have an extraordinarily good theory at the moment.

How do you decide?

FTFY

You don't speak for me, Maartenn100. Nor, I suspect, a great many others.

Please stop pretending that you do.


You want to 'reject' GR?!?!?!?!?

Cool!

When can I expect to read your paper on this, in Physics Reviews?
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Old 12th February 2016, 10:07 AM   #125
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Originally Posted by JeanTate View Post
If there's only one observatory, that may be true. However, by design, LIGO is two, separated by quite a distance, and I doubt there are many directions, if any, a GW could arrive from, that would produce a null result in both. Add a third - Advanced Virgo, for example, and you'll get a signal, no matter where it comes from.

Also, I suspect that even just one observatory would be sensitive to an arrival direction that is as little as a degree from 'the blindspot' (depends on the intensity of the GW too, of course).

Finally, the 'blindspot' would be just two directions, orthogonal to the plane of the arms, right?
I think it would be any direction (360 degrees) in a plane orthogonal to the arms, bisecting them. Feel free to correct me if wrong.
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Old 12th February 2016, 10:11 AM   #126
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Originally Posted by mbarrett View Post
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal. At this angle no differential length change will occur, thus cancellation, and no output signal seen. In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
(deleted)

ETA:

Nope, I'm not quite right. See my next post below...
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Old 12th February 2016, 10:12 AM   #127
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Originally Posted by surreptitious57 View Post
In science theory is the highest classification possible incorporating a body of knowledge which is as close to being
objectively true as it can be. For something to be a theory it to has be subject to the most rigorous and consistent
testing of all. Evolution is a theory. Gravity is a theory. Special Relativity is a theory. General Relativity is a theory
Electromagnetism is a theory. So does not mean just an idea or hypothesis which is what the lay definition means
Small pedantic quibble:

Evolution is observable fact -- allele frequency changes over time in many populations of organisms. Natural selection (as well as sexual selection, genetic drift, etc.) is a theory that (partially) explains this fact.

This is despite the fact that it is always (infuriatingly) called the theory of evolution. It should just be called evolution - there is nothing theoretical about it.

Part of the whole problem is that the term theory is used in different contexts for different fields. That, and laypersons (not necessarily people in this thread) confuse the terms hypothesis and theory.

Back to your regularly scheduled thread about gravity waves...
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Old 12th February 2016, 10:13 AM   #128
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Originally Posted by NoahFence View Post
Could we build this in space and somehow encompass all angles? I could see a unit built with 360 pairs of arms?
We do get exposure to many directions, as the earth rotates the detectors to different angles. But not considering that, if we're talking about the ability to have no blind spots for a single event, I expect that given the earlier note that there is another detector already at a different location and angle (and more to come), it is probably more a case of increasing the sensitivity for those rather than needing 360 pairs of arms.

Still, it is a cool idea, and maybe could be done, maybe with lots of extra reflections going on.
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Old 12th February 2016, 10:18 AM   #129
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Originally Posted by slyjoe View Post
I think it would be any direction (360 degrees) in a plane orthogonal to the arms, bisecting them. Feel free to correct me if wrong.
You may be right, for planar waves (like light); however, GW is quadrupole, isn't it?

I vaguely remember seeing a diagram showing the estimated directional sensitivity of a LIGO detector; IIRC, it's kinda peanut-shaped, with reduced sensitivity in a plane, but not zero (due to the quad nature of GW).
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Old 12th February 2016, 10:26 AM   #130
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On that I don't know. I was thinking in planar waves.

My real point is that both arms, when deflected the same, can give you an indication there was a wave. It's just that a single (two arm) detector can't give you the direction if the direction is in that plane.

mbarrett's quote was:

Quote:
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal.
True

Quote:
At this angle no differential length change will occur, thus cancellation, and no output signal seen.
You will see an output signal - the differential length change is for determining the angle. I don't think it is for detection.

Quote:
In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
I think only the angle sensitivity is 0; the arms are still deflected.
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Old 12th February 2016, 10:29 AM   #131
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Originally Posted by JeanTate View Post
You may be right, for planar waves (like light); however, GW is quadrupole, isn't it?

I vaguely remember seeing a diagram showing the estimated directional sensitivity of a LIGO detector; IIRC, it's kinda peanut-shaped, with reduced sensitivity in a plane, but not zero (due to the quad nature of GW).
These may be the diagrams you saw:

https://books.google.co.uk/books?id=...0waves&f=false
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Old 12th February 2016, 10:33 AM   #132
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Originally Posted by Pixel42 View Post
Before LIGO we couldn't detect any gravitational waves, now we can. I don't think it matters if we can't detect all of them.

Welcome to the forum, btw.
Beyond proving gravitational waves, their chatter is equally excited at putting up more of these and using them as early warning detectors to triangulate future events so they can focus telescopes for various EM frequencies to look for crazy new stuff.
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Old 12th February 2016, 10:56 AM   #133
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So did any of the real psychics predict this awesome news?
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Old 12th February 2016, 11:17 AM   #134
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Originally Posted by Crossbow View Post
So did any of the real psychics predict this awesome news?
My fortune cookie said "Good news is on the way", so someone *must* have known when they shrink-wrapped it back in the 80's...
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Old 12th February 2016, 11:37 AM   #135
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Originally Posted by Maartenn100 View Post
GR cannot explain the observed motions of galaxies or the way the universe seems to expand. If Einstein’s model of gravity is correct, around 96 percent of the cosmos is missing. To make up the difference, cosmologists have posited two mysterious, invisible, and as yet unidentified ingredients: dark matter and dark energy. Fudgefactors in order to keep your falsified theory of gravity. A theory of gravity, falsified by the evidence. Falsified by the motion of stars and galaxies.
Similar things could be said about the neutrino until it's detection.

Dark matter behaves exactly as GR predicts.

The acceleration of the expansion of the universe is actually incorporated into GR and was done in 1917 in the form of the cosmological constant, long before the observation of the rate of expansion/acceleration of the universe.

See: https://en.wikipedia.org/wiki/Lambda-CDM_model
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Old 12th February 2016, 12:01 PM   #136
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Originally Posted by Craig B View Post
Yes, I was wondering what would have happened if we had been much closer to the event, so that let's say the arms of the LIGO momentarily changed length by a hundred metres. Would that have caused utter disruption of all material structures, including the planet, or would it just have rippled past leaving things as unaffected as is a cork floating on a pond agitated by a breeze?
I think if we were struck by a gravitational wave front that momentarily changed the distance between two objects 3000 km apart by 100m, then our proximity to the source of the gravity waves would be a far greater worry than the waves themselves.
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Old 12th February 2016, 12:50 PM   #137
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Originally Posted by Craig B View Post
Yes, I was wondering what would have happened if we had been much closer to the event, so that let's say the arms of the LIGO momentarily changed length by a hundred metres. Would that have caused utter disruption of all material structures, including the planet, or would it just have rippled past leaving things as unaffected as is a cork floating on a pond agitated by a breeze?
Great question.

Pause and think about *regular* (non-wave-like) Earth gravity for a second. Imagine if gravity were turned off and you were floating somewhere above the Earth. Then imagine that gravity could turn on. But it turns on for a very short time (say 5 milliseconds)---you'd find yourself suddenly accelerating downwards---and after the 5ms are over it turns on in reverse and you're accelerating upwards. Imagine if the accelerating were very, very, very strong, so that whole up-and-down trip involved a 100m displacement. That's a little bit like a gravity wave (and a bit not).

If you were nowhere near anything? Not standing on the ground? No problem. You wouldn't feel anything locally, since you'd be in freefall the whole time.

If you were clinging (weightlessly) to the ground to begin with? Different question. The sudden acceleration would squeeze you violently into the ground, as if you suddenly weighed hundreds of tons.

A gravity wave is sort of like that except it's a sort of volumetric effect---it's a quadrupole distortion. The left side of your body and the right side of your body suddenly want to freefall towards one another. Your head wants to freefall away from your feet. A floating mirror 2km to your left wants to freefall towards a floating mirror 2km to your right, and if able to do so would take a 100m trip out-and-back. But---most solid objects on Earth, exposed to a gravitational force, will not deform and undeform in the direction freefall tells them to go. A great many solid objects will smash under those strains. So, yes, a very strong gravitational wave will have observable consequences---huge quadrupole-shaped tidal accelerations that can smash and break things.
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Old 12th February 2016, 03:17 PM   #138
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Originally Posted by ben m View Post
Great question.

Pause and think about *regular* (non-wave-like) Earth gravity for a second. Imagine if gravity were turned off and you were floating somewhere above the Earth. Then imagine that gravity could turn on. But it turns on for a very short time (say 5 milliseconds)---you'd find yourself suddenly accelerating downwards---and after the 5ms are over it turns on in reverse and you're accelerating upwards. Imagine if the accelerating were very, very, very strong, so that whole up-and-down trip involved a 100m displacement. That's a little bit like a gravity wave (and a bit not).

If you were nowhere near anything? Not standing on the ground? No problem. You wouldn't feel anything locally, since you'd be in freefall the whole time.

If you were clinging (weightlessly) to the ground to begin with? Different question. The sudden acceleration would squeeze you violently into the ground, as if you suddenly weighed hundreds of tons.

A gravity wave is sort of like that except it's a sort of volumetric effect---it's a quadrupole distortion. The left side of your body and the right side of your body suddenly want to freefall towards one another. Your head wants to freefall away from your feet. A floating mirror 2km to your left wants to freefall towards a floating mirror 2km to your right, and if able to do so would take a 100m trip out-and-back. But---most solid objects on Earth, exposed to a gravitational force, will not deform and undeform in the direction freefall tells them to go. A great many solid objects will smash under those strains. So, yes, a very strong gravitational wave will have observable consequences---huge quadrupole-shaped tidal accelerations that can smash and break things.
Hmm ... if you are close enough to a stellar-mass BH, the tidal force (difference in gravitational acceleration between your head and feet) will spaghettify you, even though you are in freefall.

"You wouldn't feel anything locally, since you'd be in freefall the whole time."

How, and why, does this differ from being close enough to a BH to be spaghettified?

And how would a planet fare (any difference between a gas giant and a rocky one)? A modest asteroid? A rock? A speck of dust? A molecule? An atom?? A minimal-mass white dwarf? A minimal-mass neutron star?

The part I can't get my head around is that the energy of the GWR from GW150914 was estimated as the equivalent of three sols. And its duration ~1 second (or less). For ordinary matter 'close' to the event - 100 million km, 10 billion km, even 1 light-year - GWR with humongous power would have gone through it ... what effects would it have had? Hard to imagine it'd be merely modest ...
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Old 12th February 2016, 03:44 PM   #139
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Originally Posted by jonesdave116 View Post
Ahh, damn. It's all a fake or cover up to get more funding! Knew there was something fishy.

http://www.thunderbolts.info/forum/p...hp?f=3&t=16117
Oh that is just ... astonishing!

One member there wrote "They [the LIGO team] do not seem specialists in electromagnetism and quantum physics, which is more my area." And then goes on to reference Robitaille! With apparently a straight face!!

It also seems that none of those who have posted in that thread have even read the main published paper, let alone the ~dozen which accompany it (never mind actually understanding what they read).

But my fave, from that site (no, I'm not going to give you a link) is this:
Originally Posted by Stephen Smith
It is not the intent of this paper [JT: I don't know what this refers to] to prove or disprove the existence of black holes. That has been done numerous times, especially in presentations given by Stephen Crothers. It is imperative, however, to understand what is going on in the minds of theoretical physicists. It is assumption upon assumption, building theories on the backs of other theories that can provide no experimental evidence. LIGO is not announcing some new principle of physics, it is announcing the confirmation of computer models fashioned according to presumptions. If black holes do not exist, there is no gravity wave detection.
Irony, anyone?
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Old 12th February 2016, 04:30 PM   #140
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Originally Posted by JeanTate View Post
The force is strong in young Stephen ... the force of denial ...
Stephen J Crothers was already in his mid-forties when he set out to show why the physics departments of respected research universities are often reluctant to admit PhD students who compensate for their lack of training and experience in physics and mathematics with relevant life experience in related fields such as private investigations and gardening.

The true subject of this thread is of course far more interesting. I apologize for contributing to this schadenfreudist sideshow.
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Old 12th February 2016, 05:19 PM   #141
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Originally Posted by JeanTate View Post
How, and why, does this differ from being close enough to a BH to be spaghettified?
Sorry, the bit you quoted was not about gravity waves yet, nor about black holes; I was just trying to remind the reader that that a time-varying gravitational fields can feel like either an imperceptible frame shift (if you're in freefall) or like a real force (if there's a rigid body preventing you from freefalling).

Spaghettification is about tidal strains---if you're in a place where the gravitational field accelerating your feet is very different than the field accelerating your head. (Falling into a small black hole does this readily, falling into a big black hole less so. If you fell into the Milky Way's BH you would survive the horizon crossing!) You don't need to think about strain in order to get through my what-if-Earth-gravity-turned-on-and-off example. Gravity waves are only ever stretch-and-squish strains, not directional accelerations. (That's the difference between a quadrupole and a dipole wave.)
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Old 12th February 2016, 06:49 PM   #142
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Originally Posted by Varanid View Post
Small pedantic quibble:

Evolution is observable fact -- allele frequency changes over time in many populations of organisms. Natural selection (as well as sexual selection, genetic drift, etc.) is a theory that (partially) explains this fact.

This is despite the fact that it is always (infuriatingly) called the theory of evolution. It should just be called evolution - there is nothing theoretical about it.

Part of the whole problem is that the term theory is used in different contexts for different fields. That, and laypersons (not necessarily people in this thread) confuse the terms hypothesis and theory.

Back to your regularly scheduled thread about gravity waves...
And they think a theory is a guess!!!
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Old 12th February 2016, 07:48 PM   #143
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Could you imagine being the first person, probably a low-level technician, to see the traces and put "2 & 2" together? The first human to see evidence of gravity waves!
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Old 12th February 2016, 08:01 PM   #144
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Originally Posted by 3point14 View Post
Are there any potential, practical benefits from this?
Are there any practical benefits to understanding the size and age of the universe, other than arguing with young earth creationists?

Note that your answer may say something about your state of mind.
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Old 12th February 2016, 08:04 PM   #145
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I do have a practical question which I can't answer. If there was a star/planet relatively close to this black hole merger, aside from any EM radiation issues; would there be any noticeable effects from the gravity wave?

Reading backwards; I see that I am not the first to ask this question. I am in good company.

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Old 12th February 2016, 08:12 PM   #146
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Originally Posted by Varanid View Post
This is despite the fact that it is always (infuriatingly) called the theory of evolution. It should just be called evolution - there is nothing theoretical about it.
We digress, but I wish to chastise you. This is the type of comment that creationists use ad nauseam. Please look up the scientific definition of "theory".
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Old 12th February 2016, 08:15 PM   #147
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Originally Posted by Elind View Post
I do have a practical question which I can't answer. If there was a star/planet relatively close to this black hole merger, aside from any EM radiation issues; would there be any noticeable effects from the gravity wave?

Reading backwards; I see that I am not the first to ask this question. I am in good company.

Not much, apparently:

Originally Posted by Dr. Amber Stuver
Now assume that we are 2 m (~6.5 ft) tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165 nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright). This is more than survivable.

Your Questions About Gravitational Waves, Answered
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Old 12th February 2016, 08:24 PM   #148
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Originally Posted by mbarrett View Post
From some locations in space of merging black holes, the angle of arrival of the gravitational wave at the two arms of a detector will be equal. At this angle no differential length change will occur, thus cancellation, and no output signal seen. In these directions in space (which encompass a whole plane) the observatory has zero sensitivity i.e. is blind. I see no discussion of this limiting effect.
Are you sure about that?

I do not think that the two arms are for detecting direction. They are far too small for that, which is why there is another detector many miles away which did detect the signal some 7 milliseconds after the first.

The two arms are to eliminate interference, I think.

However two detectors cannot give direction in a three dimensional universe so we still do not know where the event happened.

I read that now there are plans for others in India, maybe China and elsewhere so in the future we should be able to direct telescopes to where something happened.
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Old 12th February 2016, 08:33 PM   #149
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Originally Posted by Cl1mh4224rd View Post
Not much, apparently:
I'm talking of asking intelligent questions. Do you include yourself in the not much apparently?

Quote:

I suggest you read that again, aside from answering some stupid questions by a reporter I don't see the answer to my question.
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Old 12th February 2016, 09:42 PM   #150
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Quote:
Could you imagine being the first person, probably a low-level technician, to see the traces and put "2 & 2" together? The first human to see evidence of gravity waves!
no need to imagine....read his story.


Quote:
Marco Drago saw the first gravitational wave on 14 September 2015.
http://www.sciencemag.org/news/2016/...tational-waves

I'd hardly call him a low level technician tho. He certainly knew enough to go about it in the correct manner.
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Old 12th February 2016, 09:57 PM   #151
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Originally Posted by Elind View Post
I do not think that the two arms are for detecting direction. They are far too small for that, which is why there is another detector many miles away which did detect the signal some 7 milliseconds after the first.

The two arms are to eliminate interference, I think.
Yes

Originally Posted by Elind View Post
However two detectors cannot give direction in a three dimensional universe so we still do not know where the event happened.
I agree. I think four would be the minimum for 100% certainty.
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Old 12th February 2016, 10:50 PM   #152
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Originally Posted by macdoc View Post
no need to imagine....read his story.

http://www.sciencemag.org/sites/defa...?itok=2yhMNadZ


http://www.sciencemag.org/news/2016/...tational-waves

I'd hardly call him a low level technician tho. He certainly knew enough to go about it in the correct manner.
Anyone here NOT running seti@home since they had a PC?
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Old 12th February 2016, 11:25 PM   #153
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Originally Posted by Maartenn100 View Post
That's all? Just that? That's enough 'signal' to interprete this as 'huge evidence' and 'a ripple in spacetime'?
Of course.

You predicted that these mirrors would move only in a very specific circumstance. You arranged the mirrors very carefully to rule out movement from any other circumstance. You observed that the mirrors moved in exactly the circumstance that you predicted.

But somehow you have no idea why the mirrors moved.
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Old 13th February 2016, 01:39 AM   #154
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Originally Posted by Elind View Post
Anyone here NOT running seti@home since they had a PC?
Well, I run einstein@home on my PC. You can probably imagine, they are all over this!
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Old 13th February 2016, 02:49 AM   #155
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Originally Posted by Elind View Post
I suggest you read that again, aside from answering some stupid questions by a reporter I don't see the answer to my question.

The answer I quoted in my previous post was in response to the following question:

Quote:
How far away do you have to be from this kind of black hole merger to live to tell the tale?

While not identical to...

Originally Posted by Elind View Post
If there was a star/planet relatively close to this black hole merger, aside from any EM radiation issues; would there be any noticeable effects from the gravity wave?

...the answer I quoted does appear to address your question. Here it is again:

Originally Posted by Dr. Amber Stuver
Now assume that we are 2 m (~6.5 ft) tall and floating outside the black holes at a distance equal to the Earth’s distance to the Sun. I estimate that you would feel alternately squished and stretched by about 165 nm (your height changes by more than this through the course of the day due to your vertebrae compressing while you are upright).

Did you have something else in mind when you said "relatively close"? Did you have something else in mind when you said "noticeable effects"? Or did you actually mean gravity waves rather than gravitational waves? If so, that would make your question off-topic (not to mention nonsensical).

If you were looking for a simple yes/no answer, that would obviously depend on the sensitivity of the detector. Is 165 nm something a person would notice? No. Other, more sensitive detectors? Quite possibly.

Please clarify.

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Old 13th February 2016, 03:03 AM   #156
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Originally Posted by smartcooky View Post
I agree. I think four would be the minimum for 100% certainty.

A minimum of three is what I've been reading.

http://math.ucr.edu/home/baez/physic...radiation.html - "In order to determine the source of the signal, a third detector, far from either of the first two, would be necessary. Timing differences in the arrival of the signal to the three detectors would allow triangulation of the angular position in the sky of the signal."

http://arxiv.org/pdf/1202.4031.pdf - "Although a reasonable detection of a signal can be made with two detectors alone, useful triangulation can only be done with three or more detector sites in the network."

http://www.icra.it/MG/mg12/talks/gw4_zanolin.pdf - "GW detectors are capable to find source location with a few degrees resolution
• at least three detectors are required"

Last edited by Cl1mh4224rd; 13th February 2016 at 03:06 AM.
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Old 13th February 2016, 03:14 AM   #157
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Taken from Paul LaViolette's site , I refer you to a part of his initial response . There are many who consider that this is the guy who has taken Relativity into Sub Quantum Kinetics and overcome the several weaknesses of Einstein's relativity theories
QUOTE
This gravity wave was seen on September 14, 2015 in LIGO’s raw signal data as a 35 Hertz oscillation which sped up to 250 Hertz. The scientific community in their highly elated state leapt onto the bandwagon to proclaim that they had just observed the fusion of two black holes, having masses of 36 and 29 solar masses respectively.

But how much of what they say is really true? As Max Wallis has pointed out to the physicsworld.com editor:

“Interpretation of G-wave signatures is not unique. The merging Black Holes idea is problematic, if only because point BHs don’t get ‘close’ spatially. The models assume the merging objects are of the size of the Schwartzschild radius. The same type of signals would come from merging two megamasses. The two cannot be neutron stars, assert the modellists, but they retain the outdated claim that neutron stars cannot exceed twice the sun’s mass. Einstein himself did not believe in Black Holes; physicists should at least accord him respect in allowing that this discovery of gravitational waves implies asymmetric rebalancing of large condensed masses – his quadrapole formula – and opens up a way to investigate their structure, instead of the unphysical assumption of structureless point singularities.”
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Old 13th February 2016, 04:03 AM   #158
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Malbec, it is difficult to see which words in your post are quotes, and which originate with you. Could you edit, please, or post again making this more clear. if there are quotes in there, could you link to the original, please.
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Old 13th February 2016, 04:23 AM   #159
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Originally Posted by Cl1mh4224rd View Post
A minimum of three is what I've been reading.

http://math.ucr.edu/home/baez/physic...radiation.html - "In order to determine the source of the signal, a third detector, far from either of the first two, would be necessary. Timing differences in the arrival of the signal to the three detectors would allow triangulation of the angular position in the sky of the signal."

http://arxiv.org/pdf/1202.4031.pdf - "Although a reasonable detection of a signal can be made with two detectors alone, useful triangulation can only be done with three or more detector sites in the network."

http://www.icra.it/MG/mg12/talks/gw4_zanolin.pdf - "GW detectors are capable to find source location with a few degrees resolution
• at least three detectors are required"

I agree that is what they say, but I can't see how... and this is my point



Imagine 1, 2 and 3 are three GW detectors on the surface of the earth and "S" is the source of a gravitational wave

#1 detects it
#2 detect is 2ms later
#3 detects it 5ms later

This is a straight-forward problem in 3D trig. If I can define a point in 3D space that is X, Y and Z distances from the three corners of a triangle, there is always going to be another point in 3D space, on the other side (through the plane) of the triangle that can be at those same three distances... the source of the GW could be at an alternate location "A".

How can they determine which it is without a 4th detector?

If they think they can, what am I missing?
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Old 13th February 2016, 05:07 AM   #160
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Originally Posted by Elind View Post
Are you sure about that?

I do not think that the two arms are for detecting direction. They are far too small for that, which is why there is another detector many miles away which did detect the signal some 7 milliseconds after the first.

The two arms are to eliminate interference, I think.

However two detectors cannot give direction in a three dimensional universe so we still do not know where the event happened.

I read that now there are plans for others in India, maybe China and elsewhere so in the future we should be able to direct telescopes to where something happened.
The two arms, offset by 90 degrees, are to get the differential signal - the gravitational wave will move one mirror in a different direction than the other, causing the light pattern to interfere and generating a signal. I suspect that they can estimate some kind of entry cone or plane based on the specific way the mirrors moved, and by combining these estimates across three observatories, they can pinpoint a location in space.
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