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Darren New <dne### [at] san rrcom> wrote:
> It *is* a force, depending on your definition of "force". :) As in
> "F=ma". If you're going to argue gravity isn't a force, then magnetism
> isn't a force either, nor is pushing on something.
According to GR gravity is not a force. A force causes acceleration and
gravity doesn't.
An object in free fall does not accelerate, it's simply travelling along
its geodesic. It might *look* to us like it was accelerating, but that's
only because we are seeing a 3D slice of space-time. (It's a similar effect
that railtracks in a photograph look like they are converging even though
in reality they are parallel and never converge.)
An object "resting" on the surface of the Earth, for example, is in
constant acceleration, caused by the force exerted by the surface of
the Earth. This force makes it constantly deviate from its geodesic.
What gravity is, is a deformation of space-time. Masses deform space-time
in such way that geodesics are not linear anymore (in the cartesian sense),
and thus objects in free fall which travel forward in time will travel
along these geodesics and will look like they are accelerating in our 3D
slice view of the world.
It's not even a question of "definition" of "force". According to GR
gravity is *not* a force because it doesn't cause an acceleration, and
a free-falling object is *not* accelerating.
--
- Warp
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Warp wrote:
> It's not even a question of "definition" of "force". According to GR
> gravity is *not* a force because it doesn't cause an acceleration, and
> a free-falling object is *not* accelerating.
I agree with everything you say.
Given that, some definitions of "acceleration" include "a change in
velocity". Clearly, gravity changes an object's velocity. That's the
sort of thing I meant. If you define gravity *not* to be a "force", then
you have to define "force" differently than Newton did (obviously), at
which point I don't know what the definition would be.
Interestingly enough, I understand that if you actually do the math, the
curvature of space by mass is surprisingly (to me) tiny. For example,
the circumference of the entire sun is only about a kilometer different
than you'd expect from multiplying the radius by Pi. (I don't know how
to do that math, but I read it in a text by someone you could safely
assume did.) It seems kind of odd that such a small change in the
curvature of space would lead to such a relatively large effect.
Oh, there it is. Wow, that was a bit of searching for the right google
terms.
"""
There is a simple formula for a spherical mass M - the change in the
radius (delta r) = MG/3c^2 For the earth this turn out to be about 1.5
millimeteres
"""
Also,
"""
If you consider the universe as a whole - it has the correct mass for a
black hole having a size equal to the Hubble radius.
"""
altho I can't say I know that's true.
<http://www.physicsforums.com/archive/index.php/t-7152.html>
Cool stuff. I guess light doesn't really bend very much going past the
sun, but it still seems like the earth has a pretty large gravitational
effect for something that's less than 2mm wrong. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> Clearly, gravity changes an object's velocity.
It only looks to us like it changes its velocity, again, because we
only see a 3D slice of space-time.
This is not much different as when we see a movie of a car driving
towards the horizon: On the 2D screen it looks like its velocity is
continually decreasing, until it gets to an almost halt, even though
in reality the velocity of the car could be constant. This is because
a 2D projection of the 3D world loses information.
Due to the principle of inertia, since there are no forces acting on
a free-falling object, its velocity doesn't change.
> If you define gravity *not* to be a "force", then
> you have to define "force" differently than Newton did (obviously), at
> which point I don't know what the definition would be.
If gravity does not change the velocity of an object (because it doesn't
cause any acceleration on it) then there's no need to redefine the meaning
of "force". The only difference with Newton is that Newton assumed that
gravity is a force even though it isn't (well, not according to GR anyways,
if we can assume it's correct).
--
- Warp
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Patrick Elliott nous apporta ses lumieres en ce 2008/01/04 14:41:
> In article <477e8615$1@news.povray.org>, dne### [at] sanrrcom says...
>> Alain wrote:
>>>> That's what has me curious. How does space far from galaxies "decide"
>>>> to get larger while space between stars doesn't?
>>> It don't decide to get larger far from galaxies, galaxies prevent it to
>>> get larger.
>> Yeah. I was asking about the mechanism, tho. Is there an actual theory
>> as to *why* this happens?
>>
>>> The concensus is rather that galaxies start as spiral and degenerate
>>> into globular clusters.
>> Ah. This is something new (or more correct at least) than when I last
>> looked at it.
>>
> Actually, as I stating in the other post, its not really the consensus
> any more, at least in new studies. And imho, it also makes no sense.
> Tidal forces from a spinning black hole "could" pull stars into a
> spiral. That would make sense. But... What kind of mechanism would
> possibly make a galaxy start spinning, when it first formed *then* stop
> spinning later? Either Alain has it backwards, or if just flat out
> doesn't make any sense to me.
>
Globular galaxies DO spin, otherwise they would totaly collapse unto themself,
all the stars merging toggether. This would cause the biggest, meanest,
mega-super-novae, if colapsing slow enough, or immediately collapse into a
super-massive black hole, if the collapse get a little faster. Both will cause
the formation of a huge black hole, the first way only add some mega firework.
Dopler shift analysis of all globular galaxies have shown that they are rotating.
Blobular galaxies are a lot like the nucleus of the spiral ones. The cenario is
that after the interstellar gases are all used up to form stars and planets, the
spiraling arm slowly fade away, leaving only the nucleus visible.
--
Alain
-------------------------------------------------
Phobophobia, n., The fear of fear. The fear of those who have phobia.
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Warp wrote:
> If gravity does not change the velocity of an object (because it doesn't
> cause any acceleration on it)
Except that newton assumed that gravity caused acceleration, and GR
isn't too far different from newton at low speeds.
Are you sure this doesn't apply only to things traveling at light speed?
I suspect I just don't understand the details of GR well enough to
understand the conflicts that seem inherent in saying that gravity
doesn't cause acceleration, etc. I'll stop now.
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> Are you sure this doesn't apply only to things traveling at light speed?
Yes, I'm sure.
--
- Warp
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Warp <war### [at] tagpovrayorg> wrote:
> Darren New <dne### [at] sanrrcom> wrote:
> > Are you sure this doesn't apply only to things traveling at light speed?
> Yes, I'm sure.
Another way of thinking about it: When you drop a ball it looks to you
like it was accelerating because it's *you* who is accelerating, not the
ball. The ball is in inertial motion, you aren't.
--
- Warp
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Warp wrote:
> Another way of thinking about it: When you drop a ball it looks to you
> like it was accelerating because it's *you* who is accelerating, not the
> ball. The ball is in inertial motion, you aren't.
That's an interesting idea. Cool, thanks!
It still boggles my mind that a warp of space of < 2 mm can cause the
gravity on earth. :-)
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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Darren New <dne### [at] sanrrcom> wrote:
> It still boggles my mind that a warp of space of < 2 mm can cause the
> gravity on earth. :-)
I think there's too much experimental evidence of the GR equations being
right to not to believe it...
I suppose the geodesics "bend" quite a lot in the dimension we don't
"see" from our limited 3D perspective. Or, from another point of view,
we move in time quite "fast".
--
- Warp
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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> It still boggles my mind that a warp of space of < 2 mm can cause the
>> gravity on earth. :-)
>
> I think there's too much experimental evidence of the GR equations being
> right to not to believe it...
(holds tongue... ;-)
But seriously, yes, I believe it. It's still mind boggling. :-) I think
it's more likely there's something more complex that's well understood
by people other than me that accounts for the apparently-large results
from apparently small distortions of space.
Maybe it's just that (a) we evolved with this gravity, so of course it
seems like a lot, (b) gravity isn't really that strong, as it takes the
entire sun just to keep the earth in an annual orbit, or something like
that.
> I suppose the geodesics "bend" quite a lot in the dimension we don't
> "see" from our limited 3D perspective. Or, from another point of view,
> we move in time quite "fast".
Since GR measures time and space in the same units (modulo "c"), I would
guess it's well-understood how fast we move in time. :-)
I think I need to dig up my feynman lectures on GR for people who aren't
physicists again. (aka "Six not-so-easy pieces")
--
Darren New / San Diego, CA, USA (PST)
It's not feature creep if you put it
at the end and adjust the release date.
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