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Author Topic: Dark Matter - Is gravity really constant?  (Read 9441 times)

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #25 on: 17/08/2009 20:41:10 »
I think you are saying that you believe the attractive force would be the same at both locations. That may be true, but what evidence exists to support that conclusion? If you discount the existence of dark matter, the evidence, based on observations of objects in space, says it is not true.
 

Offline JP

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Dark Matter - Is gravity really constant?
« Reply #26 on: 17/08/2009 23:41:25 »
The problem with assuming that gravity differs in different places in the universe is that, on a large scale, the universe looks the same everywhere.  If gravity differed from one point to another, you wouldn't expect that. 

You could argue that the large scale structure of gravity is the same everywhere, but it has small fluctuations that make it look like "dark matter" is present in some spots.  However, since most people would like the laws of physics to be the same everywhere in the universe, that theory isn't as nice as just assuming that gravity is the same everywhere, and that there's something we haven't managed to observe yet. 
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #27 on: 18/08/2009 00:22:46 »
The problem with assuming that gravity differs in different places in the universe is that, on a large scale, the universe looks the same everywhere.  If gravity differed from one point to another, you wouldn't expect that. 

You could argue that the large scale structure of gravity is the same everywhere, but it has small fluctuations that make it look like "dark matter" is present in some spots.  However, since most people would like the laws of physics to be the same everywhere in the universe, that theory isn't as nice as just assuming that gravity is the same everywhere, and that there's something we haven't managed to observe yet. 

I think you may have missed my point. I am not suggesting that there is anything "special" about any particular region of space. I agree, that would seem most unlikely. What I am suggesting is that the gravitational effect might vary according to the amount of matter in the "vicinity". That would not violate the concept that, on the grand scale, the universe looks the same wherever we look.
 

Offline JP

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Dark Matter - Is gravity really constant?
« Reply #28 on: 18/08/2009 03:09:10 »
Ah.  Then I guess what you're looking for is one of the theories that proposes that general relativity is incomplete.  I've heard of them briefly in passing, so Wikipedia might be a better resource:

http://en.wikipedia.org/wiki/Dark_matter#Modifications_of_gravity

It sounds like the trick is getting a new theory to agree with observed effects, such as gravitational lensing.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #29 on: 18/08/2009 04:27:21 »
Ah.  Then I guess what you're looking for is one of the theories that proposes that general relativity is incomplete.  I've heard of them briefly in passing, so Wikipedia might be a better resource:

http://en.wikipedia.org/wiki/Dark_matter#Modifications_of_gravity

It sounds like the trick is getting a new theory to agree with observed effects, such as gravitational lensing.
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Ah.  Then I guess what you're looking for is one of the theories that proposes that general relativity is incomplete.  I've heard of them briefly in passing, so Wikipedia might be a better resource:

http://en.wikipedia.org/wiki/Dark_matter#Modifications_of_gravity

It sounds like the trick is getting a new theory to agree with observed effects, such as gravitational lensing.

Correct. I hate to suggest that general relativiy might need tweaking, but I suppose that's what it would boil down to.  Thanks for the link! I'll check it out.
 

Offline Geezer

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Dark Matter - Is gravity really constant?
« Reply #30 on: 18/08/2009 05:05:33 »
Ah.  Then I guess what you're looking for is one of the theories that proposes that general relativity is incomplete.  I've heard of them briefly in passing, so Wikipedia might be a better resource:

http://en.wikipedia.org/wiki/Dark_matter#Modifications_of_gravity

It sounds like the trick is getting a new theory to agree with observed effects, such as gravitational lensing.

Prof. John Moffat has developed such a theory. Its called MOG, for modified gravity. It does not invoke the need for dark matter. According to Wikipedia (which is quite often inaccurate of course), MOG is compatible with observeable phenomena, including colliding galaxies.

Apparently Moffat and Einstein exchanged quite a few letters over the years and Moffat seems to be a well respected physicist. I'll try to read his papers, but the math will likely defeat me!

Thanks again.
 

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Dark Matter - Is gravity really constant?
« Reply #30 on: 18/08/2009 05:05:33 »

 

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