[lyner]

Nothing is traveling in a straight line - not even light, in a Universe where there are objects with mass. Most things are "as near as dammit", of course. But not a straight straight line.

[Geezer (OP)]

Well, yes and no.

According to Hawking - A brief History of Time - quote:

“In general relativity, bodies always follow straight lines in four-dimensional space-time, but they nevertheless appear to us to move along curved paths in our three-dimensional space. The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space time, it appears to us to move along a circular orbit in three-dimensional space.”
End quote.

Photons have zero mass, (perhaps because they are only quanta of energy propagating through space-time) so they are not influenced my mass. However, objects with mass do distort space, and therefore photons are influenced, indirectly, by mass. If I understand correctly, this is why “gravitational lensing” works.

[Geezer (OP)]

i'm just standing on the sidelines, but not with scorecards. i've already said i know nothing about this topic, i was just offering a reminder that there are many researchers working on physics. it is possible that they can't see the wood for the trees, but i find this unlikely considering there are physicists all over the world in every continent (and therefore one system of thinking is not influencing them all, besides, of course, the scientific method)

Glovesforfoxes:

Consider that if a certain patent examiner in Switzerland had assumed that physicists would figure everything out, our world would be a very different place.

Now, I freely admit that I'm no Einstein, but perhaps, if I can provoke just a little lateral thinking, it might help someone figure out one of the greatest mysteries in science. I may embarrass myself in the process, but nothing ventured...

BTW, the idea that gravity is "different" in our locality has been suggested by physicists to explain the "missing matter" in the past. However, this is not generally accepted (correctly, I think) because it would require that there is something "special" about our part of the universe. I am not suggesting there is. I am questioning whether the amount of matter in a volume of space can in some way alter the gravitational effect within that volume. This would require that space has some novel properties, but as there is little agreement regarding what space actually is, and as we know matter does interact with space, why would we assume the interaction does not expose novel properties?

Perhaps there are experiments that demonstrate my question has no merit. Fine with me. I'm just hoping someone can point them out.

[Geezer (OP)]

"Its true,  on the earth 2 objects of different weight fall at the same speed in a vacuum"

No disagreement.

For clarification, here's an experiment that might help to explain my question:

On Earth, take two spheres. For convenience, they can be of equal mass. Measure the apparent attractive force between them. (Include tests to ensure they are not being attracted by electrostatic force, magnetic force, air movement etc.)

Transport the same two spheres deep into intergalactic space. Repeat the measurements.

Same results as on Earth? Case closed! Keep looking for dark matter and other explanations.

It's a simple enough experiment, just hopelessly impractical. But perhaps a more practical experiment already exists.

[Turveyd]

The spheres in a vacuum,  on earth or deep space would move the same correct but....

The Earth is a Sphere,  moving in a fixed direction,  but being pulled towards the sun at the same time,  if you stopped the earth from moving relative to the sun then it would free fall straight into the sun ( Lets not try it )

If you sped the earth up then the orbit and distance from the sun would increase and eventually we'd fly off into deep space,  then freeze.

The spheres / objects within the air given the same aerodynamic properties then the heavier object will reach a higher top speed as it's mass will counteract the resistance from the air.

Easy!!

[Geezer (OP)]

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.

[JP]

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. 

[Geezer (OP)]

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.

[JP]

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.

[Geezer (OP)]

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.

[Geezer (OP)]

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.