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Ghideon

Can gravitational waves be affected by matter?

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How are gravitational waves affected by gravitational fields and matter through which the waves pass? What interactions are predicted by mainstream theories? I’m interested in discussing such predictions even if observations or experiments are not (yet) possible.

I have some (limited) understanding of how space and matter are affected by a passing gravity wave*. But not the other way around, how matter (and gravity) can affect a gravity wave.

 

*) LIGO 

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I believe that they are affected by spacetime curvature (eg. they can be lensed) but not directly by matter - they would just pass through the Earth, for example.

There are probably more detailed answers here: https://stuver.blogspot.com/p/informational-posts.html

See the series on "The Journey of a Gravitational Wave"

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4 minutes ago, Strange said:

See the series on "The Journey of a Gravitational Wave"

Thanks! Good and quick answer! From your source: "Universe is essentially transparent to a gravitational wave."

I'll study the rest to see what "essentially transparent" means in this context.

 

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Massive explosion on earth a few months ago - strange nobody detected it - not even the super sensetive

gravity wave monitors.  Is there an explanation for that?

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14 minutes ago, fred2014 said:

Massive explosion on earth a few months ago - strange nobody detected it - not even the super sensetive

gravity wave monitors.  Is there an explanation for that?

What explosion are you referring to? (And if no one detected it, how come you know about it !?)

It may have been detected by one or more of the gravitational wave detectors but unless it looked like a black hole or neutron star merger it would have been filtered out.

But a lot of effort goes in to preventing vibration affecting the detectors so, unless the explosion was quite close, it might not have caused any detectable effects.

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Posted (edited)

From the source https://stuver.blogspot.com/p/informational-posts.html, bold by me:

Quote

As light travels through the Universe, any time that it encounters other matter, some of the light is absorbed by the matter or reflected away from its original path.  The opposite happens for a gravitational wave; it can pass through matter and come out the other side unchanged (although there are some negligible effects)!

I'm curious, what are these negligible effects?

 

 

9 hours ago, fred2014 said:

Is there an explanation for that?

Without knowing what explosions you might have in mind I'll answer regarding earth vibrations in general. As @Strange said, a lot of effort goes into making sure vibrations are not detected. LIGO uses two basic strategies to shield the detectors from Earthly vibrations, referred to as “Passive” and “Active” vibration isolation systems.

Quote

Passive Vibration Isolation

LIGO’s Passive Vibration Isolation system prevents vibrations from reaching the crucial mirror or "test mass" that reflects the laser beam that tells us whether or not a gravitational wave has passed. It does this using some fairly basic physics: principles of pendulums, and the Law of Inertia.

You can illustrate at home why a pendulum is so effective at reducing vibrations. Using the image at left as a guide, tie four heavy washers or bolts together in a line, each one separated by an equal length of string. Hold the string at the top and rapidly shake the top back and forth by a small amount (you're simulating vibrations from the environment around LIGO). You'll see that the lowest mass moves very little, if at all, compared to the top one. This is because each segment in the pendulum absorbs the vibration it 'feels' from above and prevents it from being transmitted below. In this way, this system "isolates" the bottom mass from all the "noise" you created above it.

Do-it-yourself Quad Pendulum

 

Quote

Active Vibration Isolation

As good as it is, on its own, the "Passive" system is still not good enough to enable LIGO to detect gravitational waves. To get to that level, the passive system is itself contained within an "Active" isolation system. As the name implies, active isolation is a process whereby a number of sensors in and near LIGO's interferometers detect vibrations from the environment (wind, earthquakes, traffic, etc) and send feedback signals to actuators that physically move the passive isolation system around to cancel out as many vibrations as possible before they reach the passive system. This is the same basic principle by which noise-cancelling headphones operate. This active system cannot remove all vibrations, however, and what little makes it to the quad suspension is taken care of by the passive isolation system.

Source and more info: https://www.ligo.caltech.edu/page/faq

 

Edited by Ghideon

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1 hour ago, Ghideon said:

I'm curious, what are these negligible effects?

I’m not sure. I guess it might be the small (tiny) amount of lensing ?

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As LIGO is able to detect gravitational waves, some energy has to be taken from those waves.

As electromagnetic radiation from some of the sources is comparatively extremely easy to detect, the energy absorbed by even the whole earth would be very small, with no detectable effect on the gravitational waves.

Some of the comments after https://stuver.blogspot.com/2012/07/journey-of-gravitational-wave-i-gws.html look at different ways energy could be absorbed.

 

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Ghideon,

I think your forgot the third measure, namely that there are 'two Ligo's', and a Virgo. If not all three measure the same effect at nearly the same time, it is something local. 

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3 hours ago, Carrock said:

As LIGO is able to detect gravitational waves, some energy has to be taken from those waves.

I think I agree on that, and I would like to understand more about how that works. When trying to formulate some analogy my attempts sounds too vague and opens for possibilities that the gravitational wave could be unaffected. Like: I could measure the distance of one meter without affecting "the meter", space time coordinates are unaffected by my activities*. Or: I could measure and calculate time dilation without affecting the passing of time?

Your comment is spot on regarding my question; are there (tiny) effects on gravitational waves passing trough matter, effects that are not there when the wave passes through vacuum.

 

5 hours ago, Strange said:

I’m not sure. I guess it might be the small (tiny) amount of lensing ?

Could be, I'll have to read some more before commenting.

 

2 hours ago, Eise said:

I think your forgot the third measure, namely that there are 'two Ligo's', and a Virgo. If not all three measure the same effect at nearly the same time, it is something local. 

Excellent! I was too focused on the individual setup of each site, missing that completely.

 

*) I kind of see gravitational wave as a geometrical effect. The gravitational wave does not do work, squeezing the particles. Kind of like universe expansion does not move galaxies, it is just the distance that gets bigger. I'm not convinced my view is a correct description.

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Posted (edited)
28 minutes ago, Ghideon said:

*) I kind of see gravitational wave as a geometrical effect. The gravitational wave does not do work, squeezing the particles. Kind of like universe expansion does not move galaxies, it is just the distance that gets bigger. I'm not convinced my view is a correct description.

 

Do we have equations for the energy of a gravity wave yet?  Like h.mu or something?  I assume planks constant isn't relevant here as it isn't electromagnetic and I do not know if gravitational effects are quantised or not? I wonder if there is a new constant waiting to be found that equates the energy of the gravity wave with the wavelength.

 

 

 

 

Edited by DrP
sp.

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37 minutes ago, Ghideon said:
4 hours ago, Carrock said:

As LIGO is able to detect gravitational waves, some energy has to be taken from those waves.

I think I agree on that, and I would like to understand more about how that works. When trying to formulate some analogy my attempts sounds too vague and opens for possibilities that the gravitational wave could be unaffected. Like: I could measure the distance of one meter without affecting "the meter", space time coordinates are unaffected by my activities*. Or: I could measure and calculate time dilation without affecting the passing of time?

Your comment is spot on regarding my question; are there (tiny) effects on gravitational waves passing trough matter, effects that are not there when the wave passes through vacuum.

LIGO's detection proves the GW energy is there; the GW energy would still there even without LIGO detecting it so any LIGO measurement effect is not really an issue.

The mutual coupling between gravitational waves and the earth is so weak that the energy required to stretch or compress matter is (very very very) nearly entirely lost from the gravitational waves.

I think there are no analogous effects when the wave passes through vacuum.

I've gone well beyond my minimal knowledge of gravitational waves; regard the above as plausible speculation.

 

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1 hour ago, DrP said:

Do we have equations for the energy of a gravity wave yet?

Yes. Derived from GR. See page 12 of this: http://www.tat.physik.uni-tuebingen.de/~kokkotas/Teaching/NS.BH.GW_files/GW_Physics.pdf

If I remember correctly, the gravitational waves from the first observed black hole merger radiated about 3 solar masses as energy.

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Your responses above helped me to formulate relevant searches. I found some interesting material and I’ll post some followups, starting with lensing.
It seems to be generally accepted that gravitational waves are lensed and the effect is not negligible. Here is a paper discussing how to use that fact to probe fundamental physics: https://arxiv.org/pdf/1612.02004.pdf

Quote

We investigate the potential of high-energy astrophysical events, from which both massless and massive signals are detected, to probe fundamental physics. In particular, we consider how strong gravitational lensing can induce time delays in multi-messenger signals from the same source. Ob- vious messenger examples are massless photons and gravitational waves, and massive neutrinos, although more exotic applications can also be imagined, such as to massive gravitons or axions. The different propagation times of the massive and massless particles can, in principle, place bounds on the total neutrino mass and probe cosmological parameters. Whilst measuring such an effect may pose a significant experimental challenge, we believe that the ‘massive time delay’ represents an unexplored fundamental physics phenomenon.

According to the paper gravitational waves are subject to gravitational lensing in almost exactly the same manner as photons.
 

Here is a speculative* paper claiming lensing have already allowed for dual observations of one event: https://arxiv.org/abs/1901.03190

Quote

We identify a binary black hole (BBH) merger that appears to be multiply lensed by an intervening galaxy. The LIGO/Virgo events GW170809 and GW170814 have indistinguishable waveforms separated by 5 days, and overlap on the sky within the 90\% credible region. Their strain amplitudes are also similar, implying a modest relative magnification ratio, as expected for a pair of lensed gravitational waves. The phase of the two events is also consistent with being the same, adding more evidence in support of both events originating from the same BBH merger.

My conclusion: gravitational lensing of gravitational waves may be a useful tool. (Even if the results in the paper above is not confirmed.)

 

*) Note that LIGO representatives does not agree: https://www.scientificamerican.com/article/has-ligo-seen-galaxy-warped-gravitational-waves/  

 

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Posted (edited)
On 5/23/2019 at 1:02 AM, Carrock said:

LIGO's detection proves the GW energy is there; the GW energy would still there even without LIGO detecting it so any LIGO measurement effect is not really an issue.

The mutual coupling between gravitational waves and the earth is so weak that the energy required to stretch or compress matter is (very very very) nearly entirely lost from the gravitational waves.

I think there are no analogous effects when the wave passes through vacuum.

I've gone well beyond my minimal knowledge of gravitational waves; regard the above as plausible speculation.

 

Gravitational waves [as I see it] are ripples in the "fabric" of the universe/space/time itself. Since mass exist in that fabric, the stretching and compressing of that mass as a gravitational wave passes through can be visualized. The vacuum you mention, is that same spacetime.

Probably the most useful sought after information or detection of gravitational waves, will be detecting gravitational waves from the BB itself. Such a discovery will not be curtailed by the 380,000 year recombination era, as it is for EMR and light and will [as I see it] go right back to the actual instant of the BB.

 

Edited by beecee

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I'm not sure LIGO extracts energy from the gravitational waves.
What it does is measure the distortion to space-time caused by the gravitational waves ( path differences ).

If you picture space-time as a three-dimensional ( I don't think you can do four ) grid or co-ordinate system; then the gravitational wave would emanate from a central point outward, alternately stretching the grid along one axis, and compressing it along another.
If there is a pre-existing 'warping' of the grid, caused by matter and its associated mass-energy, then the wave will be slightly modified in its path.
so, sure, matter affects gravitational waves.

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4 hours ago, MigL said:

so, sure, matter affects gravitational waves.

Good point. I see that my question could be better stated. I made an implicit distinction:

Affected by gravity = a gravitational wave disturbed when passing through a gravitational field in vacuum
Affected by matter = a gravitational wave disturbed when passing through matter, like straight through the earth

I see that these two of course are overlapping. 

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Continuing my post from yesterday with other effects than lensing.

On 5/22/2019 at 10:22 AM, Strange said:

I’m not sure. I guess it might be the small (tiny) amount of lensing ?

I believe I've learnt some more now. A chapter in Three Hundred Years of Gravitation* provides a catalog of wave propagation effects which I think answers my question regarding the small negligible effects. Gravitational waves can for instance be absorbed, scattered and dispersed. The effects are negligible except near the Planck era. The book contains several interesting references to detailed analysis of different kinds of matter but I’ve not yet looked at those. A page from the book:

TpD2TeeqplXSoo1hr2aKrn9a3VQR5MpbV57WQjq96o3J_l-BMgRj-uHISdka8BRABGIgJO2N0pY3d2qg3lf2bUzCaquKBiH2710qA17dg91ySJV_6p1_oeLq6wbQR5jPPG9sElaA

image.png.be1ee0beabcebeb5b1cfc98572f2e014.png

 

I think the effects mentioned above raises a new question when combined with:

23 hours ago, beecee said:

Probably the most useful sought after information or detection of gravitational waves, will be detecting gravitational waves from the BB itself. Such a discovery will not be curtailed by the 380,000 year recombination era, as it is for EMR and light and will [as I see it] go right back to the actual instant of the BB.

If the effects listed above are negligible except near the Planck era, is that a problem or a feature (or both)? Would the non negligible effects hinder scientific observations of any remaining early gravitational waves? Or would the effects be a factor that helps to expose interesting properties of an early epoch?

 

 

*) K. S. Thorne, “Gravitational radiation.” in Three Hundred Years of Gravitation, edited by S. W. Hawking and W. Israel (1987)

 

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18 minutes ago, Ghideon said:

I think the effects mentioned above raises a new question when combined with:

If the effects listed above are negligible except near the Planck era, is that a problem or a feature (or both)? Would the non negligible effects hinder scientific observations of any remaining early gravitational waves? Or would the effects be a factor that helps to expose interesting properties of an early epoch?

When I mentioned gravitational waves being detected from the BB itself, I was not that certain that it was within the capabilities of aLIGO and Virgo, but probably more probable with LISA once it is launched and set up. If such waves were to be detected, I would imagine it would be analogous to when the Higg's particle was found and referred to as the "God Particle". 

The difficulty though in detecting such waves, even for LISA would be immense. On checking for some info on the subject I found a couple of interesting articles........

https://theconversation.com/how-giant-atoms-may-help-catch-gravitational-waves-from-the-big-bang-80430

extract: "But there are limits to what LIGO can do. While gravitational waves exist with a big variety of frequencies, LIGO can only detect those within a certain range. In particular, there’s no way of measuring the type of high frequency gravitational waves that were generated in the Big Bang itself."

 https://theconversation.com/gravitational-waves-offer-glimpse-into-the-past-but-will-we-ever-catch-ripples-from-the-big-bang-54855

extract: "To make such observations will require detectors with sizes far larger than the 4km arms of LIGO. The proposed eLISA experiment will put three satellites into orbit as an equilateral triangle with sides longer than the distance from the Earth to the moon."

and the following by Lawrence Krauss: 

https://www.nbcnews.com/mach/space/these-waves-may-let-us-see-big-bang-s-earliest-n744851

Either way such detections are certainly a way off yet, if at all. But if one day instruments such as LISA do detect gravitational waves from the BB, obviously it will be a "eureka moment" and give us insight into the supposed Inflation event and the quantum foam from whence the BB speculatively evolved from.

 

 

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On 5/24/2019 at 11:22 PM, beecee said:

three satellites into orbit as an equilateral triangle with sides longer than the distance from the Earth to the moon."

Quite an engineering challenge to construct, launch and to operate it! (With zero maintenance possibilities once launched, probably better discussed in a separate thread)

 

I have a followup question regarding lensing.

How does a black hole affect gravity waves?
Initially I thought the waves would pass through* since gravity cannot be shielded. Even close to a black hole an observer must be affected by changes in gravitation due to some gravitational event behind the black hole from the observers point of view. But since the waves follow the curvature inside the black hole as light would do, and nothing entering a black hole may get out again, there should be no gravity waves passing through a black hole. So how are the changes in gravity "communicated" to the other side of a black hole? I believe the solution is the lensing effect. If gravity waves bends as photons then there would be some parts of the waves that follows a path from behind a black hole to an observer on the front side. Can someone confirm or explain?

My reasoning is that gravitational waves behaves similar to:

Quote

There are infinitely many images of each star, corresponding to light rays that circle the black hole several times before coming toward you.

http://hubblesite.org/explore_astronomy/black_holes/encyc_mod3_q11.html

Probably GR would predict what happens but that math is far beyond my level, at least now.

 

Here is a thought experiment I tried**, it also shows gravitational waves should not pass through a black hole:

If gravitational waves could pass through a black hole and since there is a small but non-zero absorption by matter then it could in principle be possible to get information about matter distribution in a black hole. For instance it could be possible to tell if matter is more evenly distributed, or contained in a small dense region in the centre.
But as far as I know it should not be possible, not even in principle, to get that information, or any other information***, from the inside of a black hole. It does not matter whether it is possible to engineer such a measurement or of we believe matter is in the centre or not. What matters is that any experiment that would reveal properties of what's inside the event horizon is wrong according to mainstream science. 
Conclusion: The thought experiment tells me that no, gravitational waves are not allowed to enter, and then exit, a black hole. 

 

*) A part of a wave crosses the event horizon and later emerges again at some other point.

**) I haven’t yet found this one online so I'm not sure it is correctly performed. Corrections & improvements are welcome!

***) except for properties mass, angular momentum and electrical charge, accessible from outside the event horizon.

 

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I would imagine gravitational waves would affect a BH as they do matter, and that the EH would stretch and elongate as similar to when BH's collide.

I'm sure I have seen illustrative diagrams on that but as yet have not found them. Note: BH's and gravitational waves are simply geometry of spacetime.

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Gravity is the geometry of space-time.
Gravitational waves are the 'ringing' or 'ripples' emanating from a sudden change to this geometry.
These changes, like all other information are constrained to travel at a specific speed, c , otherwise causality is violated.

As such, they would behave like EM waves ( as per your link Ghideon ). The BH would act as a 'sink', absorbing all intercepted waves, and modifying the path of approaching ones.

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On 5/21/2019 at 10:37 PM, Strange said:

What explosion are you referring to? (And if no one detected it, how come you know about it !?)

It may have been detected by one or more of the gravitational wave detectors but unless it looked like a black hole or neutron star merger it would have been filtered out.

But a lot of effort goes in to preventing vibration affecting the detectors so, unless the explosion was quite close, it might not have caused any detectable effects.

Sorry I lost this thread  I don't visit here too often - I don't recall the details - I was responding to a TV news broadcast not long before - if I remember rightly now

it occurred somewhere in the northern hemisphere - possibly arctic/russia - (I don't recall sorry)

If I remember correctly it was only recognised long after the fact and was quite a substantial blast hence my curiosity.

(It may have been volcanologists or geologists that spotted it in their records)

 

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35 minutes ago, fred2014 said:

Sorry I lost this thread  I don't visit here too often - I don't recall the details - I was responding to a TV news broadcast not long before - if I remember rightly now

it occurred somewhere in the northern hemisphere - possibly arctic/russia - (I don't recall sorry)

If I remember correctly it was only recognised long after the fact and was quite a substantial blast hence my curiosity.

(It may have been volcanologists or geologists that spotted it in their records)

 

A conventional explosion or seismic event would not have triggered the gravitational-wave detectors since it would be outside the coincidence window — gravitational waves propagate at c, so there is (at most) a very short delay (~10 ms or less) between signal arrival of the Hanford and Livingston LIGO stations. (The time lag and order of arrival is how they can assess the approximate direction the signal came from)

Conventional explosion would propagate at the speed of sound, and be ignored, since the signals would be too far apart in time.

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