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Gravitational waves are said to cross the Universe without being affected by anything. Has anyone got a view on whether gravitational waves (as detected by LIGO) could be affected by Black Holes? Could this provide a method for mapping the presence and frequency of intervening Black Holes within the Universe and their passage to us? 

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Are  Gravitation Waves not a  product of Black Holes.  The small ones engaging  with the large ones, or some such reaction.
My knowledge of the relationship between the two is scant indeed.

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The first detection was from coalescing black holes, yes - but how would that signal be affected by other intervening black holes, that the gravitational wave passed through or by, on it's way to our earth based detectors?

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5 hours ago, Fosforus said:

Gravitational waves are said to cross the Universe without being affected by anything.

Whoever said that was wrong. Gravitational waves interact with matter: that's how they were detected. When gravitational waves meet any matter they have an effect of stretching and squeezing it (quadrupole radiation) - this causes some heating so the waves lose some energy. The effect is tiny which is why it took about a century between predicting their existence and directly measuring them. Even then it had to be an incredibly powerful source: a pair of colliding black holes. Gravitational waves can refract, diffract etc, analogously to electromagnetic ones (with the proviso of quadrupole rather than dipole radiation, and a "charge" - mass - that can only be positive). Hence the excitement: being able to "see" gravitational waves potentially gives a whole new way of looking at the universe. The very early universe was opaque  to electromagnetic radiation: the cosmic microwave background marks the era when it became transparent, and is a limit on how far back we can look with electromagnetic radiation. The early universe was not opaque to gravitational radiation: in theory we could look right back to the Big Bang. I suppose a rough shorthand might be to say that those waves can "cross the universe without being affected by anything." Insert "much" and it becomes more accurate.

Edited by acey
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Makes sense that they will be effected by mass and a black hole is a fairly good lump of mass in whatever form. If they had no interaction with mass then we could not have detected then - LIGO moved, therefore they interacted with mass, otherwise I cannot see how they caused this movement to the mass of the earth (or LIGO) as they passed through.

It may only have been a movement of a proton width (less I think but lets keep it simple) but that is interaction.

Edited by ronin
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I think maybe the phrase "Gravitational waves are said to cross the Universe" comes from discussions about the hypothetical elementary particle the graviton which would have zero rest mass and would travel at the speed of light
  "because the gravitational force appears to have unlimited range" and obays the inverse square law like electromagnetic waves -photons-  do, which can also cross the universe ( when the universe became transparent at the time of last scattering t=300,000y )

BUT I have a problem !
All that I have read about gravitational waves and LIGO say that the waves squash and stretch space (quadrupole) and so shrink and lengthen the two (orthogonal) legs of the LIGO interferometer. Nothing about moving matter, or interacting with matter, or acting upon matter (that type of 'force' description was a Newtonian concept)
So did the gravitational wave lose energy while passing through LIGO ? I cant see where Feynman's 'sticky beads resulting in friction' analogy comes into LIGO.

Did it lose energy by moving the interferometer mirrors (40kg each ?) Would it, LIGO, have been more sensitive if the mirrors were each 80kg ?
If so does that mean that space is more difficult to distort (does it resist being bent!) in the presence of large masses ie. large gravitational fields ? Is space 'stiff' near mass, and very very stiff near black holes ??!

my head hertz !

EDIT : much head-scratching and coffee later and I think I have an analogy :-

The old rubber sheet trick : the sheet is depressed in the middle (as though there was a mass there as is usually depicted) for one leg of LIGO so it and its mirror are trying to slide down in the shrinking direction ,,, but at the same time sort of superimpsed is a second sheet that is being poked upwards by a big stick and is being seen by the other leg & mirror which are being sort of slid away from the middle  ?? does that work ?


Edited by SilverAstro
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