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Does any one have any info on what causes gravity?

I know that it is caused by the curvature of spacetime around an object of mass, but why does this curvature of spacetime harness this energy and where does it come from?

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It is perhaps better to think of gravity not as "caused by" the curvature, but "is" the curvature.

Think of a flat surface, like a mattress. Think of placing a golf ball on that surface, a relatively small mass. It will curve the surface some, but not much. Now put a bowling ball on the mattress. It will cause a greater deformation of the surface (spacetime). If close enough, the bowling ball's curvature will make the golf ball roll toward it because of the curvature/ gravity.

Now some big fat butt sits down...

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FT, That does not answer my question. There is a force being applied to all objects of mass from (or is)the curvature of spacetime, this force must come from somewhere. It seems like all objects of mass are bubbles of matter in the medium of spacetime. Could this force be or be caused by dark energy?

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Wavelength - I don't think there is a need for an "external" force. Freethinker's explanation is fairly standard.

All things have a "potential" to move towards a state of rest. Because space-time is curved, objects of mass will follow the path of least resistance as long as nothing is stopping it.

In classical physics (ie, Newton) things pulled at each other. It is a good theory and it stands true for calculation in most cases. However, it does not cater for things like the speed of gravity and how gravity fields affect the passage of time. This was what Einstein solved with his relativity theory.

Here is a more detailed take on gravity:

Wikipedia: Gravity

http://en.wikipedia.org/wiki/Gravity

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Originally posted by: wavelength?

FT, That does not answer my question. There is a force being applied to all objects of mass from (or is)the curvature of spacetime, this force must come from somewhere.

This force IS gravity. Or perhaps we can say that gravity is the name we give to that force. You seem to want to assume that some OTHER force from some other place is needed.

Are you familiar with Einstein's Elevator thought experiment?

It seems like all objects of mass are bubbles of matter in the medium of spacetime. Could this force be or be caused by dark energy?

Good question. At this point it seems that "dark energy" is the name we are assigning to the interactions we are observing. Perhaps as just a way to carry on a discussion about what is being observed. As I understand it, it works more like the opposite of gravity. It is a repulsive force causing an acceleration in expansion of the universe. Perhaps the dipolar component of gravity?

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Is it equally possible that gravity is getting weaker over time? I'm talking millions or billions of years, not something recent. Does this also explain the accelerated expansion?

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Uncle

I doubt it. If gravity is now weaker then the deceleration in the past would be greater than expected, but a reduced deceleration can't explain a complete reversal to acceleration.

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If gravity is weaker now than before, we would need to find a) how much weaker and B) is it weaker all over the place. If there are local variations in the force of gravity then that could have an impact on how the universe has developed, not to mention the shape and size of things.

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Originally posted by: Uncle Martin

Is it equally possible that gravity is getting weaker over time? I'm talking millions or billions of years, not something recent. Does this also explain the accelerated expansion?

???

I thought I answered this one before? Perhaps another thread? Or another life?

As we are able to see galaxies billions of light years away, we are actually seeing how things were billions of years ago. If gravity was different at that point in time, what we observe would require different understandings of gravity's parameters in order to correlate what is going on.

But this does not appear to be the case,

Thus gravity seems to have held relatively constant.

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While changes in the strength of gravity can't explain accelerated expansion, it can't be discounted entirely. We don't have much detail for the most distant galaxies. However they DO seem to be different. It would be more surprising if they were not. If the BB theory is correct, the universe was different in a number of important ways then. Still, that makes it hard to claim with any conviction that the universe wasn't different in other ways. Models of galaxy behaviour for that time are based on the evidence, rather than convincing theory. They change with each new observation, if forces were a little different, theories of equal conviction would be forthcoming. The jury is still out. Actually, the jury will never come in. You can't disprove any effect with undefined magnitude and undefined distance in the past. Evidence can only push back the limits as to how recent, and how big.

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Blame, even though the values for cosmic constants occasionally change due to new observations, it is our accuracy or method for measurement which changes, not the actual cosmic constant.

Gravity is one cosmic constant which must be the same throughout the universe, because it defines how our universe is constructed on large scales. I have never seen any attempts to prove otherwise, and if you have any examples please post some links, it would be very interesting to read.

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Originally posted by: BlameTheEx

We don't have much detail for the most distant galaxies. However they DO seem to be different.

In what ways?

Esp regarding gravity.

If the BB theory is correct, the universe was different in a number of important ways then.

Why would that be? In what ways? What measurements support this?

Models of galaxy behaviour for that time are based on the evidence, rather than convincing theory.

Theories are based on observation/ "evidence". First something is observed, then something is proposed to explain it. This is followed by developing tests to confirm or reject the proposition. When the proposition has been shown to have enough "evidence" to support it, and it does not violate other established theories, it then becomes a THEORY.

They change with each new observation,

This is also not correct. Theories only change when "new observations" (assuming you mean those that conflict with the theory, otherwise "new observations" only add support) are confirmed (to show it was not an observational error or other anomaly) and force a change or complete rejection of the theory.

if forces were a little different, theories of equal conviction would be forthcoming.

yes the big "if".

The jury is still out. Actually, the jury will never come in. You can't disprove any effect with undefined magnitude and undefined distance in the past. Evidence can only push back the limits as to how recent, and how big.

you say "<u>undefined</u> distance in the past'. Yet the "distance in the past' IS "how recent", which you acknowledge that we DO have.

And

you say "<u>undefined</u> magnitude". Yet the "magnitude" IS "how big", which you acknowledge that we DO have.

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Tormod

I can only agree that gravity defines how our universe is constructed. My point though, is that we don't know how the universe was constructed. We have some informed guesses based on the evidence, but if new evidence comes in those will be happily modified. God knows, we have had to do as much lots of times in the past. It would be colossal arrogance to believe that this time we have finally got it totally right, and no more revisions will be necessary.

I can't see why gravity has to be a constant. It could just as easily follow some sort of a curve. Just as long as the variation is of a nature that can't be detected as yet. Have you any evidence that a reasonable model of the universe could not be created with variable gravity?

Freethinker

Evidence for early galaxies being different:

http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/07/text/

You should have already read that article. It was the basis for my thread "Distant Galaxies. How far, how big?".

As to:

"If the BB theory is correct, the universe was different in a number of important ways then."

Do you really have to ask???

How about smaller, younger, higher density of matter, lots more gas available for star formation, lower ratio of heavy elements? Now please don't tell me any of that is controversial enough to need references!

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Originally posted by: BlameTheEx

I can only agree that gravity defines how our universe is constructed. ... It would be colossal arrogance to believe that this time we have finally got it totally right, and no more revisions will be necessary.

Absolutely. We agree on that. I would never argue such a thing - I just pointed out that I am not aware of any research in the area. If anything, it is a display of my own lack of insight.

I can't see why gravity has to be a constant. It could just as easily follow some sort of a curve. Just as long as the variation is of a nature that can't be detected as yet. Have you any evidence that a reasonable model of the universe could not be created with variable gravity?

No, I don't, but I don't think it is possible that it happened. Gravity may well have changed over the years, but for the change to be curve-like it would need to have changed in the same way all over the universe, simultaneously - like the observed change in the alpha constant and the alleged change of the speed of light.

Since we haven't got any insight at all into what happened before the BB (or even what exactly the BB was) I am not going to argue that I know where gravity comes from. Yet some physicists speculate that gravity comes from gravitons, some suggest the Higgs particle is what it takes - if this is true, then most likely gravity will have the same properties all over the universe, because even minute changes in the way particles work and interact can turn them into something else completely.

If gravity is changed *by* the expansion of the universe, then the force of gravity would be a result of the shape of the universe, not a cause for it. But then again, it's like pulling a rubber band - is the measured tension at any point a result of me pulling it or of the inherent properties of the way the rubber band stretches when pulled? Both, most likely. I have no good explanation for this apparent paradox, if it is one.

If I remember correctly, Hawkings writes about gravity in his Universe in a Nutshell, where he argues that gravity could be a 5th dimension force which permeates the other four dimensions (ie, our space-time). Gravity would then reside on a brane-world above our own, yet be separate from it. In this case it could be argued that if gravity has an effect on our universe but is not a part of it, then any change in the force of gravity would have an impact on our universe. I have no idea how brane world physics works, though, so this probably has no impact on our discussion.

I still have problems with local differences in gravity, though. If gravity was different in distant parts of the universe, then we could see things like different expansion rates there - local bubbles of expansion which differs from the one we would be in. Yet we do not observe that, AFAIK. It would be a very interesting find, though.

As for the article you refer to, I posted the same story back in June (although written by another source):

http://www.hypography.com/article.cfm?id=34251

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Tormod.

Are you concerned with a particular location, or a particular distance? If it's location then i would say that much variation is implausible. The universe is much the same in any direction. Variation over distance is another matter. The further off you see an object, the further back in time.

Gravity could be non-linear in strength. That is double the mass might not result in exactly double the acceleration. Clearly it must be very close indeed at the strengths we can test, but what about relativistic effects? There are different possibilities when the escape velocity approaches C. Most, if not all are dependent on which point of view is valid. Different observes will see different things due to time dilation, but which one, if any, will see the results we can extrapolate from our measurements?

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Blame, I don't know of any research which suggests that gravity changes over time or at relativistic speeds. Gravity is not affected by acceleration, AFAIK, it is what causes it - for example, if you drop an object at any point within the gravitational well of the Earth, it will fall towards earth at 9.8m/s^2. The mass of the object is not relevant, only the proximity to a massive object. The relative masses of the objects (say between two galaxy clusters or between a star and a black hole) will have an impact on the motion, but the gravitational force remains constant.

Since the gravitational constant is considered a fundamental property of our universe I don't think even small variations are possible.

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Originally posted by: BlameTheEx

Freethinker

Evidence for early galaxies being different:

You throw this out as if it is proof. Yet you fail to show in any way that it is.

Just posting a link is of no value.

Show us ANYWHERE in that article that it says that early galaxies had different laws from galaxies as we know them today. ANYTHING ANYWHERE.

Yes the site mentions "a zoo of oddball galaxies". But it gives specific reasons for the SHAPE variations "These oddball galaxies chronicle a period when the universe was more chaotic. Order and structure were just beginning to emerge.". This in no way shows that different laws and metrics existed back then as you have suggested. It only shows that the earliest galaxy formations followed the same "Laws" we do today, but in slightly different starting conditions. There is nothing to suggest any difference in gravity, time dilation,. ...

"If the BB theory is correct, the universe was different in a number of important ways then."

Do you really have to ask???

How about smaller, younger, higher density of matter, lots more gas available for star formation, lower ratio of heavy elements? Now please don't tell me any of that is controversial enough to need references!

If all you are asserting is that the laws were the same but matter had to go thru the various stages (primary galaxies/ stars forming, exploding, causing heavier elements and 2nd generation stars...), then you are correct, nothing more to ask/ prove. And I have misunderstood your position.

But you seem to be asserting that the basic laws and metrics were very different. THAT still needs proof/ references. And the site you list does not support it.

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