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Gravitation 'Beach'

Posted: Thu Aug 16, 2018 5:53 pm UTC
by dhartkop
Are there places in the universe where it is easier to detect or observe cosmic gravitational waves? Just as ocean waves are nearly flat as they travel over the open ocean, but bunch up and become tall before breaking near a shoreline, are there places where faint gravitational waves would become quite obvious if you could be there?

Re: Gravitation 'Beach'

Posted: Thu Aug 16, 2018 8:40 pm UTC
by SuicideJunkie
Well, you could go right to where they're being generated, but I expect it will turn out like https://what-if.xkcd.com/73/, and you'll be killed by mundane side effects of the area before you really notice the gravity waves on a macro scale.

Re: Gravitation 'Beach'

Posted: Thu Aug 16, 2018 9:33 pm UTC
by Soupspoon
That indeed might be like being sat on the flanks of La Palma to guarantee you'd notice the megatsunami its collapse would provoke[citation needed], because your boat wouldn't really notice its passage in the middle of the ocean..

The question is probably related to where would be a beach. The various kinds of waves (both lapping, ripple-kind that storm-sized are noticable and the hugely displacing 'tidal' waves, at least; and then there's actual tidal bores) make a splash at the water's edge as the medium they travel through runs out of depth.

It's getting into advanced (and theoretical) physics to imagine how the medium of gravity (Higgs field? Something else?) runs out of depth and thus concentrates and changes its oscillations and their nature into visually breaking (or overflowing) effects distinct from their open-water options.

Though that is likely carrying the 'water wave' analogy far too far. Perhaps we'd be better looking at how sound-waves might become self-reinforcing in open-air, but I think that's still on the wrong side of the analogous divide, for much the same reasons.

Re: Gravitation 'Beach'

Posted: Thu Aug 16, 2018 10:19 pm UTC
by Pfhorrest
I think the sound wave analogy does work better than the water waves because AFAIK gravitational waves are compression waves, not transverse waves. (In what dimension could they even be transverse?)

The Higgs field has nothing to do with gravity BTW. No more than any other fields, at least.

Re: Gravitation 'Beach'

Posted: Fri Aug 17, 2018 12:06 am UTC
by Soupspoon
Higgs stuff is the closest thing that comes to mind without going full on towards branes inhabiting the 11th dimension... Or whatever it is at the moment. But, yes, probably the "something else".

And EM waves are considered transverse, while gravity-waves exhibit forms of polarisation (something that a compressive wave cannot) although I'm nowhere near the confidence of understanding needed to separate the 'reality' of the waves from the good old fictional Praxis Wave.

(Obviously, the result of the rapid orbit, then collision, of two massive objects does have a plane in which to propagate itself in one manner differently to whatever happens towards the perpendiculars, but it's still a tricky and complex thing to imagine what is moving where as a result of the initial event.)

Re: Gravitation 'Beach'

Posted: Fri Aug 17, 2018 7:59 am UTC
by p1t1o
An ocean wave can be made to change speed, and when it does so, its amplitude changes. I dont think a gravity wave can travel at anything but c?

Re: Gravitation 'Beach'

Posted: Fri Aug 17, 2018 10:37 pm UTC
by Heimhenge
Yes, it's the change in speed of water waves that increases their amplitude. So gravitational waves couldn't do that. The only way I see to create an amplitude increase would be via superposition of gravitational waves from multiple sources, but that would be unlikely to occur with the correct timing.

I think getting close to the source would be the only way to "experience" gravitational waves on a macroscopic scale. Put yourself in orbit around a binary neutron star, or binary black hole, with a sufficiently tight orbit, and you might be close to the gravitation beach.

But if I understand gravitational waves correctly, the distortion might not be visually noticeable even then, since your local environment would also be distorted -- unless you were running an interferometer or atomic clock. Not sure if you would "feel" the distortions unless the waves were approaching the "spaghettification" (tidal force) levels of Δg (like when falling into a black hole). And in a free fall orbit around around one of these binary systems you'd be weightless, sure, but still feel the tidal forces. Just be sure to keep your orbit outside the event horizons. :)

Re: Gravitation 'Beach'

Posted: Fri Aug 17, 2018 10:44 pm UTC
by Pfhorrest
Could gravitational waves not be slowed by passing through masses interacting with them, much like light itself can be slowed moving through a medium?

A "gravitation beach" would then be some kind of ginormous wall of mass, somehow?

Re: Gravitation 'Beach'

Posted: Sat Aug 18, 2018 5:09 pm UTC
by Soupspoon
Heimhenge wrote:Yes, it's the change in speed of water waves that increases their amplitude. So gravitational waves couldn't do that. The only way I see to create an amplitude increase would be via superposition of gravitational waves from multiple sources, but that would be unlikely to occur with the correct timing.

Arrange a Klemperer Rosette of a sufficient number of identically-composed and carefully timed/orientated supermassive binaries, then sit at the (whole-thing) barycentre?

A bit tricky to set up, you might argue, but who said science was easy?!? ;)

Re: Gravitation 'Beach'

Posted: Sun Aug 19, 2018 5:50 am UTC
by PM 2Ring
Pfhorrest wrote:Could gravitational waves not be slowed by passing through masses interacting with them, much like light itself can be slowed moving through a medium?

A "gravitation beach" would then be some kind of ginormous wall of mass, somehow?


I don't think so, because matter is essentially transparent to gravity. However...

In Newtonian physics, the total gravitational force at a given point in a many-body system is simply the vector sum of the gravitational forces from all of the bodies. You don't reduce the strength of a vector that happens to pass through another body.

In General Relativity, the situation is similar. However, there is an important difference because gravitation in GR is the curvature of spacetime. In the low energy regime, where Newtonian physics is an excellent approximation, the curvatures due to multiple masses combine in a linear fashion, i.e., we can just add the different curvatures together.

Incidentally, the curvature mostly affects the time component, causing gravitational time dilation, the spatial components of the metric are barely affected at all unless the gravitational energy is very high. But when the gravitational energy is high, then things get tricky. We can no longer use linear approximations when the gravitational energy density of a region is high because that energy itself is a source of gravitation.

In regions of extreme gravity, we get gravitational lensing, due to the spacetime curvature being so high that its spatial effects become significant. This lensing not only affects electromagnetic radiation, it also affects any gravitational waves that happen to be passing through. So yes, we should get amplification caused by the waves getting bunched together, just like light gets concentrated.

Re: Gravitation 'Beach'

Posted: Tue Aug 28, 2018 10:20 am UTC
by speising
here's another apocalyptic scenario:
Gravitational Waves Pose a Bizarre New Threat That Could Destroy Earth

To be clear, we can file this into the 'very maybe probably not' category of potential apocalypse scenarios.

Re: Gravitation 'Beach'

Posted: Tue Aug 28, 2018 4:05 pm UTC
by SuicideJunkie
We can certainly say it isn't new.
And there are a lot of bigger targets that would have been noticeably hit if it was not-unheard-of.

Might make for a mad scientist movie plot tho.