# Gravitational lens and gravitational waves question

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Hi everybody! I am new here and my mother tongue is not english, so please dont kick me if make spelling mistakes

Now, to the topic:

With the recent news about the finding of gravitational waves there will be plenty of discussions about it. I have a very specific question, that why I am not making it on other threads about it.

I just have a very small idea about the Theory of Relativity (the kind of things that a common person will learn if read some book about that), so my question may look like silly: didnt we already had a proof about gravitational waves with the Gravitational Lens effect? I tought that gravitational lens already was a proof about the existence of gravitational waves...because the lensing effect is created by the existence of a distortion in the fabric of space-time. Isnt it??

I am confused and probably mixing ideas and concepts that I should mix, can someone help me to understand this?

Thanks a lot

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Not bad yes we proved spacetime can be curved to produce lenses. While gravity waves involve the same principle of distortion, this is the first time we have confirmed measurement of a distortion wave.

So it's another piece of evidence, along with lenses that GR is correct

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Just to add a bit more... Gravitational lensing (and the force of gravity we feel) is caused by static curvature of space-time. Gravitational waves are a dynamic change in the curvature, spreading out like ripples from a changing source such as two black holes orbiting and merging.

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Thank you very much, all of you. Your posts were clear and answered to my question.

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• 3 weeks later...

As far as I understood, the gravitational waves detected were from 2 black holes that merged.

My question is: the waves were emitted/detectable in all directions, including straight up/down from the orbital plane?

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As far as I understood, the gravitational waves detected were from 2 black holes that merged.

My question is: the waves were emitted/detectable in all directions, including straight up/down from the orbital plane?

Yes. I think that very close to the source, the waves may not be completely isotropic but as soon as you get a reasonable distance away, the waves are the same in all directions. (note that there are components that fall off at different rates with distance)

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Does the difference in masses of dual orbiting bodies cause each to take a more eccentric path around the centre of mass than if their masses were equal? If so would the eccentricity enhance the signal for Earth detectors?

Edited by StringJunky
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Yes. I think that very close to the source, the waves may not be completely isotropic but as soon as you get a reasonable distance away, the waves are the same in all directions. (note that there are components that fall off at different rates with distance)

Are you sure that we would get the same signal from the merging BH, no matter if we are in their orbital plane or perpendicular to it? How it was in the actual/recorded case?

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Are you sure that we would get the same signal from the merging BH, no matter if we are in their orbital plane or perpendicular to it? How it was in the actual/recorded case?

Fairly sure. But I can't immediately find anything to confirm it...

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Fairly sure. But I can't immediately find anything to confirm it...

I can wait.

Anyone else?

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Interesting question. Gravity waves propogate outward in all directions.

An easy way to visualize would be a sound wave from say an explosion.

The type of wave however is a quadruple polarization transverse wave.

The spacetime densities (medium)will

however influence the rate of travel

Most animations will give you a false image of the wave travelling along a plane. (It's top tricky to model it in 3d) so images use the rubber sheet analogy

Edited by Mordred
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Interesting question. Gravity waves propogate outward in all directions.

An easy way to visualize would be a sound wave from say an explosion.

The type of wave however is a quadruple polarization transverse wave.

The spacetime densities (medium)will

however influence the rate of travel

Most animations will give you a false image of the wave travelling along a plane. (It's top tricky to model it in 3d) so images use the rubber sheet analogy

I think you missed an 'o' out, it should be quadrupole. Can you explain the two GW polarisations here. What are we looking at here?

http://webs.um.es/bussons/GW_lecture_KG.pdf

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the first image is the longitudinal or plus polarization, the second is similar to the first but rotated by 45 degrees its a cross polarization. The paper you posted has the metrics but doesnt explain that the reason the polarizations is a quadrupole is due to gravity being spin 2 staitistics.

The details on how each is determined is math intense and involves the EFE. Rather than try to post the metrics it would be better to post a decent article.

http://arxiv.org/pdf/gr-qc/0309058v2.pdf

one thing to note when the x axis + and minus contracts the y axis + and - expands by the same amount. a good animation is here.

see the gifs on the right

https://en.wikipedia.org/wiki/Gravitational_wave

Edited by Mordred
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the first image is the longitudinal or plus polarization, the second is similar to the first but rotated by 45 degrees its a cross polarization. The paper you posted has the metrics but doesnt explain that the reason the polarizations is a quadrupole is due to gravity being spin 2 staitistics.

The details on how each is determined is math intense and involves the EFE. Rather than try to post the metrics it would be better to post a decent article.

http://arxiv.org/pdf/gr-qc/0309058v2.pdf

one thing to note when the x axis + and minus contracts the y axis + and - expands by the same amount. a good animation is here.

see the gifs on the right

https://en.wikipedia.org/wiki/Gravitational_wave

Thanks Mordred. It''s hard to visualise in 3d, polarisations of waves.

It seems you do need asymmetry for GWs, that I asked in my other question. Supernovas must also explode asymmetrically to generate GWs, it said..

Edited by StringJunky
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Thanks Mordred. It''s hard to visualise in 3d, polarisations of waves.

It seems you do need asymmetry for GWs, that I asked in my other question. Supernovas must also explode asymmetrically to generate GWs, it said..

I guess there would be a single pulse / single step along with the pulse of EMR.

If a Nova converted say 5% of mass to EMR then whilst you the observer had not noticed the Supernova because the light had not yet reached you then the mass-energy would be constant (ie all the mass energy would be in a sphere closer to the Star than you) but as the pulse of EMR passed you then a portion the mass energy would be in a shell outside your orbit around the now cooling star. According to Newton's Shell / Gauss' Law of Gravity spherically symmetrical shells outside the radius of the observer do not get felt by the interior observer. The attractive force felt by you would diminish in a step as the EMR passed - I guess; if you hadn't been burnt to a crisp and if you had time to look at your instruments and tear your eyes away from the storm-front of approaching plasma about to engulf you

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In EM, any multipole radiation is not isotropic. So this may hold for GW's, too, but I haven't been able to find anything that confirms it.

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So, the signal ("ripples") that can be detected in a place straight above/below the orbital plane of the merging black holes may be different than the signal that can be detected in a place situated at the same distance but close to the orbital plane? How different? Which one is bigger?

How was Earth situated in relation with the orbital plane of the black holes in the actual/recorded case?

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How was Earth situated in relation with the orbital plane of the black holes in the actual/recorded case?

There is no information about this (I have just looked through the paper again). The paper discuss the initial masses, final mass, final spin, constraints on the initial spin, and the distance. So I assume the effect of the orientation is not significant. Maybe it is less than the error bounds?

Edited by Strange
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That would be difficult to answer, Here perhaps this 3d image will help.

http://www.ligo.org/science/GW-Sources.php

http://www.dailymail.co.uk/sciencetech/article-3050898/When-supermassive-black-holes-COLLIDE-Best-3D-simulation-reveals-space-time-warps-cataclysmic-event.html

As you can see they can be extremely erratic

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Layman's description of a blackhole merger: "Imagine a bath emptying, with 2 soggy dumplings circling the plug hole and then forming into one as they hit the centre of the vortex"

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...

As you can see they can be extremely erratic

For some reason the distant observer position is always in (or close to) the orbital plane ...

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According to @AstroKatie (Katie Mack, an astrophysicist), the radiation is strongest perpendicular to the orbital plane

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According to @AstroKatie (Katie Mack, an astrophysicist), the radiation is strongest perpendicular to the orbital plane

Was that a joke?

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Was that a joke?

Why would it be? Is perfectly sensible.

Dr Katherine (Katie) Mack is a theoretical astrophysicist. Her work focuses on finding new ways to learn about the early universe and fundamental physics using astronomical observations, probing the building blocks of nature by examining the cosmos on the largest scales. Throughout her career as a researcher at Caltech, Princeton, Cambridge, and now Melbourne University, she has studied dark matter, black holes, cosmic strings, and the formation of the first galaxies in the Universe. Katie is also an active online science communicator and is passionate about science outreach. As a science writer, she has been published by Slate, Sky & Telescope, Time.com, the Economist tech blog "Babbage", and other popular publications. http://www.astrokatie.com/

Edited by StringJunky
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Why would it be? Is perfectly sensible.

Was the quote from her twitter feed? I would be surprised if that was right. Why do you think it was sensible? I would love to know the right answer.

GE production slows the body allowing the orbit to decay, so if it is removing force that was directed tangentially to the orbit I thought the energy given off would be an expression of this lost energy i.e at all times it is generating a wave front to the fore and these waves spread out from there.

Like a bow wave of a boat maybe.

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