Gravity and space question
James
fall to earth because it is simply *following* curved space itself. But
if an object is theoretically put there, completely still, 2 metres
above the earth, why should it 'fall'? Why wouldn't it just hang there
motionless? I am not saying it might drift off into space, but why
can't it stay still in the same spot relative to earth, even though
space is curved?
The only reason i can come up with is that space itself is 'moving'
towards the earth all the time. So that the still ball isn't really
'falling' its just following Space in its movements? I hope you
understand my question and can answer.
Ian Stirling
>I understand that a moving object (like if I throw a tennis ball) would
>fall to earth because it is simply *following* curved space itself. But
>if an object is theoretically put there, completely still, 2 metres
>above the earth, why should it 'fall'? Why wouldn't it just hang there
>motionless? I am not saying it might drift off into space, but why
>can't it stay still in the same spot relative to earth, even though
>space is curved?
What if you throw a tennis ball straight up.
Consider it at the instant that it's stationary.
--
http://inquisitor.i.am/ | mailto:inqui...@i.am | Ian Stirling.
---------------------------+-------------------------+--------------------------
Things a surgeon should never say:
Better save that for the autopsy.
vanhout
away from earth is the same as the acceleration that the earth gives you
while attracting you. Because their directions are opposite, the cancel each
other out, leaving the object at its relative position to earth.
The curved space concept is good, no matter what they say about it, it's
just not necessary to use it with this problem. In the future, try to
approach every problem as simplistically as possible.
James <nos...@nospam.com> schreef in berichtnieuws
390196...@nospam.com...
> I understand that a moving object (like if I throw a tennis ball) would
> fall to earth because it is simply *following* curved space itself. But
> if an object is theoretically put there, completely still, 2 metres
> above the earth, why should it 'fall'? Why wouldn't it just hang there
> motionless? I am not saying it might drift off into space, but why
> can't it stay still in the same spot relative to earth, even though
> space is curved?
>
Joe Fischer
: I understand that a moving object (like if I throw a tennis ball) would
: if an object is theoretically put there, completely still, 2 metres
: above the earth, why should it 'fall'? Why wouldn't it just hang there
: motionless? I am not saying it might drift off into space, but why
: can't it stay still in the same spot relative to earth, even though
: space is curved?
Space is not curved, "space-time" is. Gravity
is very evident, yet the cause or mechanism is not known.
But there is definitely gravity, and what is called the
"affine connection" in General Relativity.
General Relativity uses the Principle of Equivalence
of gravity and acceleration to model local physical interactions,
and an object dropped simply continues it's prior motion without
any forces acting (it can't just be placed there "perfectly still").
: The only reason i can come up with is that space itself is 'moving'
: towards the earth all the time. So that the still ball isn't really
: 'falling' its just following Space in its movements? I hope you
: understand my question and can answer.
That is pretty much how most relativists view gravity,
as the natural motion of the inertial motion coordinates are
in freefall, i.e., they accelerate toward the center of the
Earth.
This of course is not a completely satisfactory answer,
but it is one possibility.
I consider the kinematics of the motion of free moving
objects to be near perfection in General Relativity, but the
phenomenon of "surface gravity" needs more of an explanation.
That is why I subscribe to a model I call "Divergent
Matter" where matter expands due to net internal repulsion,
which is transparent to observation, except as "surface gravity",
and the "affine connection" effects.
Divergent Matter would produce a 3-D "Principle of
Equivalence, and since things like length and diameter are
the only way we can measure distance, and distance determines
the measured rate of time flow (the interval), a four dimensional
physical model results.
All this simply means, a dropped object continues
it's prior motion, and the surface of the Earth accelerates
toward it.
But don't expect employed physicists to embrace this
concept, a least not until an experiment supports it.
Divergent Matter covers more than just gravity, it
forms a new physics and system of mechanics.
General Relativity is the portion that describes
the kinematics of objects in free motion.
Joe Fischer
--
3
3
Paul Alan Cardinale
>
> I understand that a moving object (like if I throw a tennis ball) would
> fall to earth because it is simply *following* curved space itself. But
> if an object is theoretically put there, completely still, 2 metres
> above the earth, why should it 'fall'? Why wouldn't it just hang there
> motionless? I am not saying it might drift off into space, but why
> can't it stay still in the same spot relative to earth, even though
> space is curved?
>
> towards the earth all the time. So that the still ball isn't really
> 'falling' its just following Space in its movements? I hope you
> understand my question and can answer.
>
applying a force that deflects it from its natural path through
spacetime. When you let go of it, it is free to follow that course; and
it does.
Paul Cardinale
P.S. Ignore john reed. He doesn't understand relativity.
ande453@attglobal.net id 12jizi-0000Cu-00 for
>
> I understand that a moving object (like if I throw a tennis ball) would
> fall to earth because it is simply *following* curved space itself. But
> if an object is theoretically put there, completely still, 2 metres
> above the earth, why should it 'fall'? Why wouldn't it just hang there
> motionless? I am not saying it might drift off into space, but why
> can't it stay still in the same spot relative to earth, even though
> space is curved?
>
> The only reason i can come up with is that space itself is 'moving'
> towards the earth all the time. So that the still ball isn't really
> 'falling' its just following Space in its movements? I hope you
> understand my question and can answer.
Something that is sitting at rest above the surface of the earth
is acclerating in the local inertial frame according to General
Relativity. You need to apply a force to it to keep it there.
If there are no forces on it, it falls.
You don't need to think of "space moving", whatever that's supposed
to mean. In GR an inertial object near the surface of the earth
is falling towards the center of the earth with an acceleration
of 9.8m/s^2.
John Anderson
J. Scott Miller
>
> I understand that a moving object (like if I throw a tennis ball) would
> fall to earth because it is simply *following* curved space itself. But
> if an object is theoretically put there, completely still, 2 metres
> above the earth, why should it 'fall'? Why wouldn't it just hang there
> motionless? I am not saying it might drift off into space, but why
> can't it stay still in the same spot relative to earth, even though
> space is curved?
>
> The only reason i can come up with is that space itself is 'moving'
> towards the earth all the time. So that the still ball isn't really
> 'falling' its just following Space in its movements? I hope you
> understand my question and can answer.
Lets give this a try.
As you indicate, space is curved. It is specifically curved toward the
concentration of mass causing the curvature. In two dimensions, you might
think about it perpendicular to the surface of the extended body. So, when
you release the ball, it follows the curvature to the surface. When you throw
the ball, you are throwing along the curvature but constrained to still follow
it, so you again reach the surface.
Crude analogy and others with much more GR background may be able to improve
on it, but it gets to your question a bit.
--
J. Scott Miller, Program Coordinator Scott....@louisville.edu
Gheens Science Center and Rauch Planetarium
http://www.louisville.edu/planetarium
University of Louisville
Lord Jubjub
>I understand that a moving object (like if I throw a tennis ball) would
>fall to earth because it is simply *following* curved space itself. But
>if an object is theoretically put there, completely still, 2 metres
>above the earth, why should it 'fall'? Why wouldn't it just hang there
>motionless? I am not saying it might drift off into space, but why
>can't it stay still in the same spot relative to earth, even though
>space is curved?
>
>The only reason i can come up with is that space itself is 'moving'
>towards the earth all the time. So that the still ball isn't really
>'falling' its just following Space in its movements? I hope you
>understand my question and can answer.
>
>
If you place a ball on a frictionless slope, it will move down the slope.
--
Lord Jubjub
Ruler of the Jabberwocky, Guardian of the Wabe, Prince of the Slithy Toves
Gordon Fisher
To get the ball to "hang" there in space you have to balance two forces.
One is gravity which is pulling things towards earth and the other is is
the force you have imparted to the ball to throw it up into the air. If
you just throw it straight up, three things can happen, it will crash to
earth. It will hang in orbit or it will sail away from Earth forever. It
all depends on how hard you throw. Can you throw a ball in excess of
17,000 miles per hour?
To get the ball to orbit at 2 meters, you would need to throw it upwards
with a lot of energy to keep it from crashing to the earth because it is so
close to the earth and the pull of gravity is so great. Because of this,
at a 2 meter height, the ball would be going very fast with respect to
Earth, and would not "hang" in one spot.
So, what you need to do is find a place where gravity is less so the ball
can go slower with respect to earth and still orbit. The farther from
Earth you get, the slower the ball can go. At the height that the Space
Shuttle flies, the gravity is still great enough that you have to orbit
faster than the Earth. The shuttle goes around the Earth about once every
90 minutes. So, keep throwing that ball higher till you reach a point
where the gravity is reduced to the point where the ball speed and the
Earths rotation speed are equal. It's way up there.
So you can make the ball hang in the air. But the point where it can be
done is just not at 2 meters from the surface.
David Evens
One glitch: Gravity isn't actually a force in GR, and it doesn't really act
like a force even in Newtonian mechanics.
Peter Gijsemans
If the ball will hang it wont be in the air, but rather into space :-)
--
_______________________________________________________
PETER GIJSEMANS ("`-/")_.-'"``-._
Alcatel Bell Space N.V. . . `; -._ )-;-,_`)
Berkenrodelei 33 (v_,)' _ )`-.\ ``-'
B-2660 Hoboken - Belgium _.- _..-_/ / ((.'
phone : ++32(0)3.829.51.87 __((,.-'___((,/_____________
James
> When you hold an object starionary (with respect to Earth), you are
> applying a force that deflects it from its natural path through
> spacetime. When you let go of it, it is free to follow that course; and
> it does.
Thank you for all your replies I read them all and though some didn't
quite get what I meant most of you attempted to explain at least what
you knew which was appreciated a lot.
But what I was getting at is what MAKES something follow its natural
path in the first place? What MAKES it move through its natural path to
begin with?
Is it a law of physics that objects must for some reason always be
moving through space-time and if they are prevented from moving through
it, then they will restart as soon as they possibly can?
Or is it the gravity-acceleration law, and that even if something is
still, it will START accelerating (just as something moving will KEEP
accelerating)?
Thank you for any further clarification. I am nearly there don't stop
now. :)
Wojciech Rulewicz
"James" <nos...@nospam.com> wrote in message news:390B9A...@nospam.com...
| But what I was getting at is what MAKES something follow its natural
| path in the first place? What MAKES it move through its natural path to
| begin with?
down because of gravitation)
Tom Roberts
> But what I was getting at is what MAKES something follow its natural
> path in the first place? What MAKES it move through its natural path to
> begin with?
There is nothing which "makes" an object follow its natural path, that
is just the natural path which it follows.
You seem to be attempting to attempt to apply the naive notions of
causality to physics. That's invalid -- such notions of causality
were developed in our everyday lives, not in the realm of detailed
physical theories. Modern physical theories do NOT conform to such
notions of causality -- this naive causality is at best an approximation
(albeit a very good one in our everyday lives).
> Is it a law of physics that objects must for some reason always be
> moving through space-time and if they are prevented from moving through
> it, then they will restart as soon as they possibly can?
All timelike objects experience elapsed proper time (which is I suppose
what you mean by "moving through spacetime") -- there is no way to
"prevent" this, and timelike objects never "restart" because they never
"stopped" in the first place.
Tom Roberts tjro...@lucent.com
scott jones
James wrote:
>
> Thank you for all your replies I read them all and though some didn't
> quite get what I meant most of you attempted to explain at least what
> you knew which was appreciated a lot.
>
> But what I was getting at is what MAKES something follow its natural
> path in the first place? What MAKES it move through its natural path to
> begin with?
What you're asking is a very deep question: you are wanting an
'ultimate cause' to use an old-fashioned term. This is a good
description of the big aim of science and I'm sorry but we havent
finished science yet so the answer is...we dont know.
One answer is that everything pulls on everything else
(general relativity). So it is all the other matter in the universe
that decides my coffee has just fallen downwards onto my
keyboard, not up. This theory works for large masses like stars.
The other thing I can do is to point you to a lovely little book
by R.P. Feynman called
"QED: the strange theory of light and matter".
It will tell you how light moves in a straight line (usually) by 'sniffing
out' each possible step forward and taking the 'easiest' one.
Our best descriptions of how small amounts of matter move also
follow this type of theory.
The problem is the existence of two totally different descriptions.
As you can see physics is a bit messy right now.
Keep watching we'll have it figured by the weekend...
All I am doing is giving you our 'best guess yet', and of course the
next question is - well why does it work like this? To answer that
is to provide a better theory and so on and on.....
Richard Perry
> I understand that a moving object (like if I throw a tennis ball) would
> fall to earth because it is simply *following* curved space itself. But
> if an object is theoretically put there, completely still, 2 metres
> above the earth, why should it 'fall'? Why wouldn't it just hang there
> motionless? I am not saying it might drift off into space, but why
> can't it stay still in the same spot relative to earth, even though
> space is curved?
>
> The only reason i can come up with is that space itself is 'moving'
> towards the earth all the time. So that the still ball isn't really
> 'falling' its just following Space in its movements? I hope you
> understand my question and can answer.
Questions such as this remind me that there are still inquisitive and open
minds left out there.
However, the replies remind me that most of the self proclaimed physicists
in these NGs have absolutely no common sense whatsoever and should change
occupations before they hurt themselves.
The stationary tennis ball falls because, in reality, it is an aggregate of
atoms that are all in motion relative to the Earth.
I hope this clears things up for you.
Richard
J. Scott Miller
Now that is abundantly clear! Explain, if you can, how such an aggregate of
atoms falls to the Earth because of relative motion, as opposed to say,
parallel to its surface or away from it at any chosen angle.
Richard Perry
> > > I understand that a moving object (like if I throw a tennis ball) would
> > > fall to earth because it is simply *following* curved space itself. But
> > > if an object is theoretically put there, completely still, 2 metres
> > > above the earth, why should it 'fall'? Why wouldn't it just hang there
> > > motionless? I am not saying it might drift off into space, but why
> > > can't it stay still in the same spot relative to earth, even though
> > > space is curved?
> > >
> > > The only reason i can come up with is that space itself is 'moving'
> > > towards the earth all the time. So that the still ball isn't really
> > > 'falling' its just following Space in its movements? I hope you
> > > understand my question and can answer.
>
> > atoms that are all in motion relative to the Earth.
> >
> > I hope this clears things up for you.
> >
> > Richard
>
> Now that is abundantly clear! Explain, if you can, how such an aggregate of
> atoms falls to the Earth because of relative motion, as opposed to say,
> parallel to its surface or away from it at any chosen angle.
>
First reread the original question. None who replied understood it, apparently
including you.
GR says that space is curved by the presence of mass. In the case of our solar
system it is said that a planet is following what appears to be a strait line
through space. Is this correct or not?
Now in order for the trajectory of a mass to conform to GR it must first "have" a
trajectory, i.e. it cannot move in a curved path if it is not moving. This seems
clear enough to me.
This is what was being implied.
So the question arises "Why does a stationary mass fall?" or more accurately,
"How does GR account for the acceleration of a stationary mass, i.e. a mass
without a trajectory to alter?" Mathematical GR may provide this effect, but GR,
as it is argued, cannot; it is conceptually untenable.
In either case if we perceive a macroscopic object as a collection of microscopic
particles in rapid and random motion, which is true, the question evaporates.
Each particle has a trajectory to alter.
Marc Bellon
}-Richard Perry wrote:
}-> James wrote:
}-> > I understand that a moving object (like if I throw a tennis ball) would
}-> > fall to earth because it is simply *following* curved space itself. But
}-> > if an object is theoretically put there, completely still, 2 metres
}-> > above the earth, why should it 'fall'? Why wouldn't it just hang there
}-> > motionless? I am not saying it might drift off into space, but why
}-> > can't it stay still in the same spot relative to earth, even though
}-> > space is curved?
}-> > The only reason i can come up with is that space itself is 'moving'
}-> > towards the earth all the time. So that the still ball isn't really
}-> > 'falling' its just following Space in its movements? I hope you
}-> > understand my question and can answer.
In fact, general relativity is a _space-time_ theory, so that you
do not have motionless objects: everything is on a space-time trajectory
with a changing time coordinate. In the Newtonian approximation, the
gravitational potential is proportional to the deviation of the
time-time component of the metric from 1.
[snip]
}-> The stationary tennis ball falls because, in reality, it is an aggregate of
}-> atoms that are all in motion relative to the Earth.
Gravity is a classical theory, which does not have to bother of the
fine structure of objects; the only thing it sees is the stress-energy
tensor, that is the density of energy and the various stresses of the object.
}-> I hope this clears things up for you.
For me, it would rather have smogged it.
}-> Richard
}-Now that is abundantly clear! Explain, if you can, how such an aggregate of
}-atoms falls to the Earth because of relative motion, as opposed to say,
}-parallel to its surface or away from it at any chosen angle.
If you know the answer, why post such a teaser ?
}---
}-J. Scott Miller, Program Coordinator Scott....@louisville.edu
}-Gheens Science Center and Rauch Planetarium
Marc Bellon.
CNRS.
J. Scott Miller
>
> >
> > > > I understand that a moving object (like if I throw a tennis ball) would
> > > >
> > > > The only reason i can come up with is that space itself is 'moving'
> >
> > >
> > >
> >
> > Now that is abundantly clear! Explain, if you can, how such an aggregate of
> >
>
> including you.
> GR says that space is curved by the presence of mass. In the case of our solar
> system it is said that a planet is following what appears to be a strait line
> through space. Is this correct or not?
> Now in order for the trajectory of a mass to conform to GR it must first "have" a
> trajectory, i.e. it cannot move in a curved path if it is not moving. This seems
> clear enough to me.
> This is what was being implied.
> So the question arises "Why does a stationary mass fall?" or more accurately,
> "How does GR account for the acceleration of a stationary mass, i.e. a mass
> without a trajectory to alter?" Mathematical GR may provide this effect, but GR,
> as it is argued, cannot; it is conceptually untenable.
> In either case if we perceive a macroscopic object as a collection of microscopic
> particles in rapid and random motion, which is true, the question evaporates.
> Each particle has a trajectory to alter.
>
>
I would suggest it is you that is not clear on GR. The following explanation
comes from the Usenet relativity FAQ. Note in particular the last paragraph.
(http://math.ucr.edu/home/baez/physics/relativity.html)
"So, what does this have to do with gravity? It is quite simple! When a mass
is present in the above space-time it distorts it so that whilst it remains
true that travelling through space causes you to travel through time,
traveling through time now causes you to move (accelerate) through space. In
other words just by existing, you are compelled to move through space - this
is gravity.
"The particular advantage of this theory of gravity (General Relativity) is
that it explains, at a stroke, all the observed properties of gravity. For
example the fact that it acts equally on all objects and substances becomes
obvious when you thing of gravity as a distortion of space-time rather than a
force.
"Imagine that you are in free space, away from any planets or stars, when
suddenly a planet is created quite close to you. You would not be aware that
anything is happening to you, you would feel no force, but you would find
that you started to accelerate towards the planet. This is just like the case
where you travel through space, you are not aware that you have also traveled
through time but people observing you are."
[end of quote]
Now, it would seem that if one could put a tennis ball in space near Earth
without any motion, that this is the same scenario of having an observer
suddenly have a planet pop up beside them. In either case, the curvature of
space-time caused by the presence of the planet results in a trajectory of the
observer or the tennis ball toward that planet.
--
J. Scott Miller, Program Coordinator Scott....@louisville.edu
Gheens Science Center and Rauch Planetarium
Jon Hurwitz
understand the same thing, and I've read the thread and found posts
irrelevant or incomprehensible. (Actually I don't even understand why a
thrown tennis ball falls to earth, so I'm in an even worse position.) I can
understand why curved spacetime can apparently accelerate an object observed
as in motion, but how in can reverse its direction completely is beyond me.
I also fail to understand why GR predicts that a ball, when released, will
drop. Does the force of gravity remains in GR by another name? I just
can't get my brain around the replacing of a force with curved spacetime.
If you now understand, can you let me know.
James <nos...@nospam.com> wrote in message news:390196...@nospam.com...
Peter Card
> GR says that space is curved by the presence of mass. In the case of our solar
> system it is said that a planet is following what appears to be a strait line
> through space. Is this correct or not?
Not exactly. The path a body follows is a geodesic - the path with the
shortest distance between two points. When talking about Relativity you
have to bear in mind we are dealing with 4 co-ordinates, the 3 space
co-ordinates plus time, so the movement of a body is represented by a
line in 4-space.
> Now in order for the trajectory of a mass to conform to GR it must first "have" a
> trajectory, i.e. it cannot move in a curved path if it is not moving. This seems
> clear enough to me.
Nope. The movement of a stationary body, in the chosen frame of
reference is described by a straight line in 4-space, parallel to the
time axis.
> This is what was being implied.
> So the question arises "Why does a stationary mass fall?" or more accurately,
> "How does GR account for the acceleration of a stationary mass, i.e. a mass
> without a trajectory to alter?" Mathematical GR may provide this effect, but GR,
> as it is argued, cannot; it is conceptually untenable.
You haven't grasped the concept. You probably need to start slowly with
some basic textbooks.
--
----------------------------------------------------------
email Peter...@jet.uk || 10001...@compuserve.com
The Experiment Formerly Known As JET - TEFKAJET
Andrew Plotkin
> Did you get an answer to this question that you understood? I'm trying to
> understand the same thing, and I've read the thread and found posts
> irrelevant or incomprehensible. (Actually I don't even understand why a
> thrown tennis ball falls to earth, so I'm in an even worse position.) I can
> understand why curved spacetime can apparently accelerate an object observed
> as in motion, but how in can reverse its direction completely is beyond me.
> I also fail to understand why GR predicts that a ball, when released, will
> drop. Does the force of gravity remains in GR by another name? I just
> can't get my brain around the replacing of a force with curved spacetime.
The rough answer is that time is a dimension in relativity. Everything is
moving through time. Fast. (It would not be totally misleading to say, "at
the speed of light".)
If you're in deep space, far from any mass, spacetime is (nearly) flat. So
your test objects all move through time parallel to each other; they
remain motionless relative to each other.
(Assuming, of course, that the objects themselves are light enough not to
bugger up the experiment! That is, light enough not to attract each other
significantly.)
If you add a heavy mass in the middle of the picture, you get curved
space. All your test objects are still zipping through time, but they now
veer towards the heavy mass. They fall.
--Z
"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the
borogoves..."
Richard Perry
> Richard Perry wrote:
>
> > GR says that space is curved by the presence of mass. In the case of our solar
> > system it is said that a planet is following what appears to be a strait line
> > through space. Is this correct or not?
>
> Not exactly. The path a body follows is a geodesic - the path with the
> shortest distance between two points. When talking about Relativity you
> have to bear in mind we are dealing with 4 co-ordinates, the 3 space
> co-ordinates plus time, so the movement of a body is represented by a
> line in 4-space.
>
> > Now in order for the trajectory of a mass to conform to GR it must first "have" a
> > trajectory, i.e. it cannot move in a curved path if it is not moving. This seems
> > clear enough to me.
>
> Nope. The movement of a stationary body, in the chosen frame of
> reference is described by a straight line in 4-space, parallel to the
> time axis.
>
> > This is what was being implied.
> > So the question arises "Why does a stationary mass fall?" or more accurately,
> > "How does GR account for the acceleration of a stationary mass, i.e. a mass
> > without a trajectory to alter?" Mathematical GR may provide this effect, but GR,
> > as it is argued, cannot; it is conceptually untenable.
>
> You haven't grasped the concept. You probably need to start slowly with
> some basic textbooks.
>
The above was not my question.
I am familiar enough with the concepts of grt, I was only relating a more coherent
version of the original question, since none who replied seemed to understand it.
Apparently my form of the question was somewhat clearer.
It reveals misconceived notions, yes, but again, not mine.
My answer, which I should have posted directly was: No two masses are ever at rest
relative to one another. This would not have been a fair answer without providing
justification, and would have had no meaning without such. It would have been
equivalent to saying "because that's the way it is". I chose a non relativistic
validation of the proposition instead, designed mostly to render the question moot, but
partly as a statement I think, that there are always alternative approaches to any
problem. Grt is a masterpiece of the intellect, true, but not necessarily the only
approach, and I think if one is given the option to convey a lesson, then this one is
much more valuable than the one sought, unless of course one has entered into training
for the purpose of becoming a conformist.
Note* My earlier faux pas, admittedly hasty, was not directed against those who replied
per se, but against their inability to grasp the question that was posted. My first on
the NGs, I believe, and hopefully my last.
Regards
Richard
Jon Hurwitz
Andrew Plotkin <erky...@eblong.com> wrote in message
news:8fhjct$k5v$2...@slb7.atl.mindspring.net...
Thanks for trying to explain. That's an interesting slant (no pun
intended) and I'll think I'll need to think about it. A lot!
Jonathan
Joe Fischer
: Did you get an answer to this question that you understood? I'm trying to
: understand the same thing, and I've read the thread and found posts
: irrelevant or incomprehensible. (Actually I don't even understand why a
: thrown tennis ball falls to earth, so I'm in an even worse position.) I can
: understand why curved spacetime can apparently accelerate an object observed
: as in motion, but how in can reverse its direction completely is beyond me.
: I also fail to understand why GR predicts that a ball, when released, will
: drop. Does the force of gravity remains in GR by another name?
Yes, the inertia (inertial mass), (inertial resistance
to acceleration), of the tennis ball.
: I just can't get my brain around the replacing of a force with curved
: spacetime.
And you won't as long as you remain convinced that
Euclidean space is a cubic reference frame that causes all
objects to follow straight lines mapped on that Euclidean
space.
General Relativity goes half way, saying that the
motion due to gravity is relative to the position and other
physical attributes of the interacting and affected objects,
but it does not provide a "cause" like Newtonian gravitation
uses "attraction".
Once Euclidean space, with an interacting attribute
that causes objects to move in straight lines is presumed,
then an "attractive FORCE" is needed to explain why objects
fall or follow curved paths.
Without Euclidean space, objects move, and the size
of the objects, and the distance of separation relative to
the rate of flow of time (the time interval relation to the
distance or length interval), determine the kinematics and
the quantity of mass determines the dynamics.
To understand General Relativity, it is necessary
to understand the Principle of Equivalence of gravitation
and inertia, the establishment of coordinates based on
the position of objects in inertial motion, space-time
geometry, parallel transport, Schild's ladder math, the
concept of affine connections, and all the math involved.
To have a sample of this, without the math, and
possibly with another wrong impression;
Kenneth Edmund Fischer -
Divergent Matter GUT of Gravitation
http://www.iglou.com/members/kefischr
Joe Fischer
Andrew Plotkin
> In article <8fhjct$k5v$2...@slb7.atl.mindspring.net>,
> Andrew Plotkin <erky...@eblong.com> wrote:
>>The rough answer is that time is a dimension in relativity. Everything is
>>moving through time. Fast. (It would not be totally misleading to say, "at
>>the speed of light".)
>
I said it was rough. :-)
> When
> I try to think out what I mean by 'move', it's something like "be at one
> place at one time and be at another place at another time". Under this
> definition, the concept of moving through time seems not to make any sense.
You're right. A more accurate visualization is a static four-dimensional
diagram, where each object is a long "world-line" that exists, er, all at
once.
World lines like to follow geodesics. ("Follow" itself implies
motion... maybe I should say "World lines like to *lie along*
geodesics".) And as you know, Bob, a geodesic is the curve which is "as
straight as possible" through curved space.
To visualize an object "moving along" its worldline is an intuitive
hook. We're familiar with the idea of a ball rolling over a curved
surface, and veering left or right because of that. "Geodesic" is a less
familiar term.
Brendan Lawlor
This is because gravity pulls on everthing, and does so because
gravity is any particle's food-finder cannabilising exterior
energies,including radiation as well, so as to increase its own
internal energy and thereby evolve into heavier and more elaborate atomic
particles (via boosting the order of its innermost core).