[ExI] Gravitational Waves Detected By LIGO!

spike spike66 at att.net
Thu Feb 11 19:29:55 UTC 2016



From: extropy-chat [mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of Giovanni Santostasi
Sent: Thursday, February 11, 2016 11:07 AM
To: ExI chat list <extropy-chat at lists.extropy.org>
Subject: Re: [ExI] Gravitational Waves Detected By LIGO!


GW interacts with matter very, very, very weakly. So unless you are really close to the event you would not be impacted. 
Few event horizons radii away from the source and the GW become ridiculously small in terms of strain. 
In the radiation zone the strain h goes down linearly with distance. I can do more precise calculations but GW are so difficult to detect exactly because they don't interact strongly with matter. 





Ja that’s the story I heard too.  My reasoning goes thus: we can estimate how much energy is released by merging black holes.  Then if we assume all that goes into the kinds of energy that interacts strongly with matter, then there would be easily detected signals everywhere, for merging black holes would be an event perhaps in the same order of magnitude likelihood as a type 1A supernova, and we see those here and there.


Ander might know the answer better than I do, but my understanding is that GWs are hard to detect, which is why we needed those big elaborate devices to find even one.


There is another kicker here: back in the early 90s, when Kip Thorne and Drever and those guys were setting up LIGO, there were good arguments at the time, some with pretty convincing-looking equations in refereed papers, that GWs would never be detected with that technology.  As I recall, the argument was that the interferometry techniques couldn’t get sufficient resolution to see GWs, which those same papers agreed are there.  They claimed LIGO was a big waste of money, that this signal couldn’t be extracted from background noise, that GWs would be difficult to detect even if it happened in the local group of galaxies, and so forth.


It has been over twenty years since Thorne’s Black Holes book came out, and I didn’t really understand it then, so I might still not be getting this.  But it feels good that those two detectors both saw the signal.  Needless to say, the headlines totally made my day.








On Thu, Feb 11, 2016 at 12:50 PM, spike <spike66 at att.net <mailto:spike66 at att.net> > wrote:

From: extropy-chat [mailto:extropy-chat-bounces at lists.extropy.org <mailto:extropy-chat-bounces at lists.extropy.org> ] On Behalf Of Anders Sandberg

>…If we assume the energy release was around 10^50 J over a second, then the power per square meter at distance d is 10^50/(4 pi r^2) Watts. So the criticial distance if the danger power is P is r=sqrt(10^50/4 pi P). If we assume a megawatt/m^2 is enough to cause biosphere damage, then the distance is 298,000 lightyears. To wipe out more advanced civilizations I would expect a much higher P; for a gigawatt the range is 9,400 lightyears - bad in the central part of a galaxy, but not even covering it…


Anders, cool, but we need to know how GW energy would interact with matter before we conclude that it would nuke biomes.  



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