## Gravity Question

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### Gravity Question

Came up with this on my own, forgive me if this is horrendously wrong.

Is gravity a byproduct of matter's resistance to the flow of space time, and is that why gravity can slow the passage of time? Does matter create a 'depression' in space/time?

obsessive_writer

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im not sure i got the first bit about resistance but the second bit sounds pretty much the GR definition of gravity i.e. gravity is the curvature of space-time and matter creates this curvature.
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evilbeanfiend

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I don't think it's so much of a resistance, though that is an interesting way to look at it, as it is a displacement. Like placing an ice cube in water, you see the water warp around the ice. But yea, like Evilbeanfiend said, thats pretty close to general relativity's explanation. I guess check wikipedia for a bit more indepth description.

masterroller720

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Well, inertia is certainly a resistance to accelerations. And somehow your amount of inertia also causes bends in spacetime. So there should be some connection, though I'm not aware what the link between the two is.
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ArmonSore

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ArmonSore wrote:Well, inertia is certainly a resistance to accelerations. And somehow your amount of inertia also causes bends in spacetime. So there should be some connection, though I'm not aware what the link between the two is.

Not quite: gravitational mass (the quantity proportional to how much you "bend" space-time) and inertial mass (the quantity proportional to your resistance to acceleration) are distinct concepts. It just so happens that they appear to have the same value for all kinds of matter we know about. Why this is the case is not fully understood, although there are various attempts at explaining it (including some fairly interesting, if left-field ideas about inertia being due to, essentially, ultra-high-frequency electromagnetic fields created by the oscillation of matter due to it's gravitational mass). Indeed, we're not actually totally sure why matter has mass at all - the Higgs mechanism is likely to be the answer, but we don't actually have any evidence for it yet (maybe by 2009, we will).
aoanla

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Not quite: gravitational mass (the quantity proportional to how much you "bend" space-time) and inertial mass (the quantity proportional to your resistance to acceleration) are distinct concepts. It just so happens that they appear to have the same value for all kinds of matter we know about.
Yeah, yeah. We've heard it all before. This is more than a coincidence.
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Vaniver

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I thought the point of GR was that it's no longer a coincidence that gravitational mass is equal to inertial mass because gravity isn't a force anymore?

In Newtonian you have a=F/m=GMm/r^2/m=GM/r^2 which is indpt of m, and seems awfully convenient, but in GR it's just a natural consequence of the metric.

SpitValve
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I agree with Vaniver, it's too nice to be by chance.

And I'd like to know more about what SpitValve is talking about.
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ArmonSore

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SpitValve wrote:I thought the point of GR was that it's no longer a coincidence that gravitational mass is equal to inertial mass because gravity isn't a force anymore?

In Newtonian you have a=F/m=GMm/r^2/m=GM/r^2 which is indpt of m, and seems awfully convenient, but in GR it's just a natural consequence of the metric.

Of course. General relativity was built from the equivalence principle, i.e. free-falling observers are in inertial motion.
shill

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ArmonSore wrote:I agree with Vaniver, it's too nice to be by chance.

And I'd like to know more about what SpitValve is talking about.

Well, basically the way GR works is that gravity isn't a force, it's something else. To fully understand it involves playing with 4x4x4x4 tensors, which is just not fun, and it's my excuse why after two papers on it I still have to talk around the issue a little bit, and might confuse myself while talking.

But _basically_ you have a thing called a metric which defines geodesics, which are the "straight lines" that a particle will follow. The metric is affected by mass around it, causing these geodesics to bend. Your geodesic right now is pointing straight down - that's the path you would automatically float if the ground wasn't in the way.

Because the metric depends on the mass around you and not the mass of the moving object, you automatically have intertial mass and gravitational mass being equilavent - it's no longer a separate axiom, it's part of the essential nature of the theory.

SpitValve
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Interesting. I can see why an object's mass causes gravity, without any care at all for the mass of the object affected by that gravity.

But I'm still not understanding why your mass causes a change in spacetime, while also defining how you'd react to regular old forces. Why does this single quantity we call inertia do both things?
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ArmonSore

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ArmonSore wrote:But I'm still not understanding why your mass causes a change in spacetime, while also defining how you'd react to regular old forces. Why does this single quantity we call inertia do both things?

That's the bit you have to assume from experiment...

SpitValve
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Is the mass that causes gravity in GR the rest mass, or the apparent mass?
I was useful Yesterday.
-Paul McCartney.

ArmonSore

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The latter seems like it would cause far less problems, and doesn't require an inertial frame of reference, so I'm guessing the second.
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Vaniver

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ArmonSore wrote:Is the mass that causes gravity in GR the rest mass, or the apparent mass?

Neither. It's the stress-energy tensor, which is frame-independent. So part of it is the rest mass, but the other part is the non-vanishing kinetic energy. Its definitely not the apparent mass, because then you could do all sorts of silly things like go fast enough to turn into a black hole.
GMontag

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