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Black Hole Merger Model

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18 years 2 months ago #26717 by albertw
Replied by albertw on topic Re: Black Hole Merger Model
Bart, I think we are in compete agreement on gravity. You are just looking more from a newtonian perspective, well with newtonian language anyway, while I'm looking more from GR. Trying to mix both leads to headaches as I've found out many times! (eg last week I convinced myself that black holes have NO gravitational effect, and I still havent sorted it out fully in my head!)

OK, I'm still confused here. Gravity IS a force. It can do work, hence it is, by definition, a force.


Not according to GR. A planet orbiting a star is taking the straight path as far as it is concerned. There is no force acting on it, its going on a straight line through warped space. If there was a force it would either act to push the planet out of orbit or pull it towards the star.

The fact that this force is a result of space-time curvature does not chance the fact that it is a force!

Oh yes it does! (cue panto!). Its not a result of space time curvature, it is space time curvature. The object always moves in a straight line unless a force acts on it. Its a subtle but important bit of Einsteins argument.

There is nothing wrong with the classical representation of gravity. Clasical physics says nothing about the cause of gravity, just about the effect of gravity. Namely that the force of gravity follows and inverse square law and that is correct.


Perfectly true.

Classical physics describes what gravity does, Einstein gave us an insight into how it does it, there is no conflict here.

GR does not fit into newtonian physics that way though. Einstein gave us GR with its spacetime ideas which have gravity built in, no force needed. Thats why he was the first to say that gravity is not a force.

As for whetehr or not gravity waves propagate, I'm not qualified to give a proffesional opinion on the matter but I have to wonder why they would not. The effects of gravity are not local, they are massively non-local working on the giga-parsec scale and above so why should the ripples be local? I may be wrong but I'm pretty sure Einstein believed in gravity waves because they do seem to be a direct prediction of his theories. Those theories have stood up well to verification so far so why assume it will be different for gravitational waves?

The gravity ripples of binary stars or black holes are indeed predicted by GR. Though detecting them will be tricky. Gravity moves at the speed of light[1] so the fluctuations will be quick. Secondly the ripple looses intensity in the order of the square of the distance so by the time it gets to us it will be very hard to detect because the effect will be small. To put some numbers on this the LIGO observatory which is designed to detect these gravity waves has two 4km tunnels which lightbeames get fired down and reflected back to an interferometer. They are expecting to detect gravity waves that distort the lengths of a path by 10^-18 m. Yep, 10 to the minus eighteen of a meter over 4km. And they are looking for big events, like the merger of black holes. IIRC they estimated a 20% chance of detecting a single event by 2010, and since they havent found anything yet want to build a more sensitive detector.

Cheers,
~Al

[1] Trust me ok! You know the rubber mat model of the solar system with the sun being a heavy ball in the middle warping the space around it? Well if the sun disppeared the rubber mat would flatten out from the center at the speed of light. In fact as the space flattened around the plaets they would go off in tangents according to newton; or in GR terms they would continue to go in a straight line as they had been doing anyway!

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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18 years 2 months ago #26718 by JohnMurphy
Replied by JohnMurphy on topic Re: Black Hole Merger Model
We're not going to agree on 'Gravity' here and we're certainly not going to solve the 'gravity problem'. Suffice to say that it still has science baffled - why should matter influence spacetime in the first place? The classical model has already been shown to be not quite the whole picture. Einsteins model will not be the whole picture either, I've read much on this subject, including Loop Quantum Gravity, all the flavours of String Theory, M Theory and Supersymmetry and none have yet been shown to be fully correct. One of the latest theories which also addresses dark matter claims that gravity leaks into our universe from other "parallel" universes, and this does away with the need for dark matter - personally I really think this one is a load of B"£$%^.

You can think of gravity as a force if that makes you feel more comfortable, to all intents and purposes it acts like a force, and we'll all still use the inverse square law because it works, but it isn't really a force so there is no point in looking for Gravitons, and gravity waves when you need to be looking for fluctuations in the curvature of spacetime. It's a subtle difference but once you get used to not thinking of gravity in the classical sense it opens up whole new ways of thinking.

When you say it does work what do you mean? You have an object moving through spacetime, it encounters a highly curved region of spacetime, it carries on moving in a straight line, however the line is now curved, so it has apparently moved in a different direction. No force has been applied.

Clear Skies,
John Murphy
Irish Astronomical Society
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18 years 2 months ago #26719 by dave_lillis
Replied by dave_lillis on topic Re: Black Hole Merger Model
You learn something new everyday, the planets are going in straight lines.
Its a perspective I hadnt thought of. :)

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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18 years 2 months ago #26720 by JohnMurphy
Replied by JohnMurphy on topic Re: Black Hole Merger Model

You learn something new everyday, the planets are going in straight lines.
Its a perspective I hadnt thought of. :)


HaHa!
Their going in straight lines through curved spacetime.

Clear Skies,
John Murphy
Irish Astronomical Society
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18 years 2 months ago #26723 by dave_lillis
Replied by dave_lillis on topic Re: Black Hole Merger Model
exactly :lol:

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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18 years 2 months ago #26724 by JohnMurphy
Replied by JohnMurphy on topic Re: Black Hole Merger Model
This has been an interesting discussion.
Albert - the force is strong in you, and yet we still have not met.
Sorry did I really just type that?
See you at the next SDAS and hopefully we can finally meet or if you're at the IAS on Monday...

Clear Skies,
John Murphy
Irish Astronomical Society
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