# [ExI] Gravitational Waves Detected By LIGO!

Giovanni Santostasi gsantostasi at gmail.com
Fri Feb 12 14:40:24 UTC 2016

```The reason they are called waves is because the calculation of the bending
of space time is done under the assumption the detection is performed far
away from the source. It is a linearization process that simplifies the
very complicated Einstein equation (that by the way we don't know how to
solve fully even computationally).

As with EM radiation if you are too close to the source you get all kind
of non linear effects (even more so with gravity) and you don't get the
nice linear waves (2 waves with amplitude A sum up to 1 wave with amplitude
2 A) you get when you do the calculation in the radiation field.
When you solve the Einstein equations at a large distance from a source
like two mutually orbiting masses then you get a solution that looks like a
wave with a certain frequency and frequency derivative (and you can deduce
the speed of this wave from the constants involved). The strain produced by
the waves decays with 1/r.

Tidal forces are proportional to 1/r^3 so they decay very fast as you move
away from the source.
So tidal forces are part of the same phenomenon of space-time warping but
not gravitational waves per se.

On Fri, Feb 12, 2016 at 2:40 AM, Tomaz Kristan <protokol2020 at gmail.com>
wrote:

> Spike ... and everybody else.
>
> I have the following problem. Merging of two distant black holes bends
> those mirrors. More or less like the Moon or a plane flying above, also
> bends those V or L shape structure. We call it - the tidal force as a
> function of time. And this is routinely dismissed as not a gravity wave,
>
> Now .. in a Newtonian world, a merging of two distant black holes would
> still be detectable as a tidal force function developing in time.
>
> And a big enough tidal force oscillation can kill you as well.
>
> My wrong prognosis was only, that they will not announce this detection
> yet. But they did.
>
>
>
> On Fri, Feb 12, 2016 at 7:30 AM, Giulio Prisco <giulio at gmail.com> wrote:
>
>> On Thu, Feb 11, 2016 at 9:35 PM, John Clark <johnkclark at gmail.com> wrote:
>>
>> > And the critics were correct, the old
>> > LIGO wasn't sensitive enough to detect gravitational waves unless you
>> were
>> > unrealistically lucky and 2 black holes happened to merge very near to
>> Earth
>>
>> I wouldn't call that "lucky." The astronomers (and the rest of
>> humanity) would have been killed immediately by a close black hole
>> fusion event.
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>
>
>
> --
> https://protokol2020.wordpress.com/
>
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