Microgravity parable

"Stuf4" said

I hope this analogy helps to illuminate the fundamental problem with
the widely used terminology: zero/microgravity.

- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).

Are you sure??? I may be mistaken but from the point of view of describing
forces felt by, or accelerations produced in a body accelerations ARE
equivalent to gravity.

This is highlighted by comparing Newton's law of gravity vs Newtons 2nd
law.

Newton's Law of gravity states that

Fg = -Mg * del(Mg * G / r)

Newton's Second Law states

Fi = Mi x a

where

Fg and Mg refer to gravitational force and mass
G is the universal gravitational constant
r is the radius vector
Fi and Mi refer to inertial force and mass

If you solve for a

a = -(Mg/Mi) * del(Mg * G / r)

which is testable (for Mg/Mi=1) and has been tested to in excess of one
part in 10^11 and within those limits found to be true.

This means that acceleration and gravity are essentially indistinguishable
for the body being acted upon.

Essentially

From Anthony Garcia:
"Stuf4" said

I hope this analogy helps to illuminate the fundamental problem with
the widely used terminology: zero/microgravity.

- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).

Are you sure??? I may be mistaken but from the point of view of describing
forces felt by, or accelerations produced in a body accelerations ARE
equivalent to gravity.

This is highlighted by comparing Newton's law of gravity vs Newtons 2nd
law.

Newton's Law of gravity states that

Fg = -Mg * del(Mg * G / r)

Newton's Second Law states

Fi = Mi x a

where

Fg and Mg refer to gravitational force and mass
G is the universal gravitational constant
r is the radius vector
Fi and Mi refer to inertial force and mass

If you solve for a

a = -(Mg/Mi) * del(Mg * G / r)

which is testable (for Mg/Mi=1) and has been tested to in excess of one
part in 10^11 and within those limits found to be true.

This means that acceleration and gravity are essentially indistinguishable
for the body being acted upon.

Essentially

You are only addressing a specifically defined constant acceleration.
I was speaking about acceleration in general.


As stated in a recent post, I do agree with your point about the
equivalence of gravitational mass and inertial mass. An interesting
point here is that under Newtonian physics, such equivalence can only
be attributed to some quirk of random coincidence against astronomical
odds.

A much more satisfying explanation points toward an inherent
connection between inertial mass and gravitational mass. My
expectation is that the equivalence will someday be shown to be a
necessary consequence of higher dimensional space that gets bound in
superstrings (or some such theory that supercedes superstrings).

Such a connection would be a huge stride toward understanding the true
nature of gravity/inertia.


~ CT

(Stuf4) wrote in message . com...

snip
- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).
snip

I haven't posted here for a while; I just took a look, came across
this interesting-looking thread title, and then read the above
statement. You appear to have mangled your terms somewhat:

Gravitation is certainly not the same thing as acceleration. Gravity,
on the other hand, is locally indistinguishable from acceleration. The
fact that this is so led Einstein to apply Occam's razor and postulate
that the two are in fact one and the same phenomenon, leading to
general relativity. GR has yet to be falsified. We may therefore say
that, to the best of our knowledge, gravity and acceleration are the
same thing and that NASA is correct in its use of the letter g.

Mike.

On 13 Oct 2003 01:28:51 -0700, (Mike Hanson)
wrote:

I haven't posted here for a while; I just took a look, came across
this interesting-looking thread title, and then read the above
statement.

....First mistake: you *read* something CT posted.

....Second mistake: you didn't ignore it.

....Third mistake: you didn't killfile the little trolling *******.


OM

--

"No ******* ever won a war by dying for | http://www.io.com/~o_m
his country. He won it by making the other | Sergeant-At-Arms
poor dumb ******* die for his country." | Human O-Ring Society

- General George S. Patton, Jr

(Stuf4) wrote in message . com...

snip
- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).
snip

I haven't posted here for a while. Decided to take a look, saw an
interesting-looking thread title, and came across the above statement.
You appear to have mangled your terms somewhat:

*Gravitation* is distinctly different from acceleration.

Gravity, however, is locally *indistinguishable* from acceleration.
That this is so led Einstein to apply Occam's razor and postulate that
they are one and the same phenomenon, leading to general relativity.
And since GR has yet to be falsified, one can say that, to the best of
our knowledge, gravity and acceleration are indeed the same thing (and
hence that NASA is correct in its use of the letter g).

Mike.

My apologies for the double-post - Google trouble.

Mike.

From Mike Hanson:
(Stuf4) wrote

snip
- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).
snip

I haven't posted here for a while. Decided to take a look, saw an
interesting-looking thread title, and came across the above statement.
You appear to have mangled your terms somewhat:

*Gravitation* is distinctly different from acceleration.

Gravity, however, is locally *indistinguishable* from acceleration.
That this is so led Einstein to apply Occam's razor and postulate that
they are one and the same phenomenon, leading to general relativity.
And since GR has yet to be falsified, one can say that, to the best of
our knowledge, gravity and acceleration are indeed the same thing (and
hence that NASA is correct in its use of the letter g).

This point regarding the equivalence theory has been addressed more
than once on this thread...

One easy way to determine whether you are accelerating due to gravity
or not is to look out the window of your spacecraft to see if there
are any stars or planets nearby.

(I've suggested elsewhere that the root of this confusion in
terminology is a misunderstanding of the equivalence principle.)


~ CT

(Stuf4) wrote in message om...
From Mike Hanson:
(Stuf4) wrote

snip
- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration.
snip

*Gravitation* is distinctly different from acceleration.

Gravity, however, is locally *indistinguishable* from acceleration.
snip

This point regarding the equivalence theory has been addressed more
than once on this thread...

One easy way to determine whether you are accelerating due to gravity
or not is to look out the window of your spacecraft to see if there
are any stars or planets nearby.

You misunderstand. First of all, one does not 'accelerate due to
gravity': objects in freefall do not accelerate by definition, since
they are inertial (no forces act upon them). Instead, objects in
freefall travel in what is commonly described as a 'straight line in
curved spacetime'. This is hard for many people to accept because
Newtonian physics - which is very intuitive and taught to most people
at school - describes gravity as a force. There is no such voodoo-like
force-at-a-distance in general relativity.

Instead, the forces and accelerations being referred to are scrictly
local ones. Standing on the ground, a man has weight: he feels a force
pushing up on the soles of his feet. Out in space and accelerating at
9.81 m/s^2, the man also has weight: he feels exactly the same force
pushing up on the soles of his feet. These two 'forms' of weight are
qualitatively identical, and this is where you have gone wrong:
looking out of the window doesn't count. The key word is 'locally',
and the question is: can you distinguish between the first and second
cases *if you don't know where you are*? And the answer is: no.

(I've suggested elsewhere that the root of this confusion in
terminology is a misunderstanding of the equivalence principle.)

You are right. This may help (and look out for the word 'local'):

http://www.npl.washington.edu/eotwash/equiv.html

Mike.

Mike Hanson wrote...
Out in space and accelerating at
9.81 m/s^2, the man also has weight: he feels exactly the
same force pushing up on the soles of his feet. These two
'forms' of weight are qualitatively identical, and this is
where you have gone wrong: looking out of the window
doesn't count. The key word is 'locally', and the question
is: can you distinguish between the first and second
cases *if you don't know where you are*? And the answer is: no.

You ask whether one can distinguish between gravity and acceleration. But
the question of whether one can distinguish between 'orbital microgravity'
and a 'microgravity field' is a different question.

It's interesting that one can make this distinction by observing a
'floating' particle which is tapped very lightly. In orbit, the particle
will oscilate in space when tapped (when the movement is viewed over an
orbital period), in a freefall or microgravity field the particle will
continue in a straight line. (Using local frames of reference of course.)

Those with a better understanding of orbital mechanics will no doubt tidy
up my post.

- Peter

Stuf4 wrote:
From Mike Hanson:

(Stuf4) wrote


snip

- Gravity is *distinctly different* from acceleration.

While gravity has a property of acceleration, it is *not*
acceleration. A 'g' is a unit of acceleration standardized upon a
particular case of acceleration due to gravity (the gravitational
acceleration at the surface of the Earth).

snip

I haven't posted here for a while. Decided to take a look, saw an
interesting-looking thread title, and came across the above statement.
You appear to have mangled your terms somewhat:

*Gravitation* is distinctly different from acceleration.

Gravity, however, is locally *indistinguishable* from acceleration.
That this is so led Einstein to apply Occam's razor and postulate that
they are one and the same phenomenon, leading to general relativity.
And since GR has yet to be falsified, one can say that, to the best of
our knowledge, gravity and acceleration are indeed the same thing (and
hence that NASA is correct in its use of the letter g).


This point regarding the equivalence theory has been addressed more
than once on this thread...

One easy way to determine whether you are accelerating due to gravity
or not is to look out the window of your spacecraft to see if there
are any stars or planets nearby.

(I've suggested elsewhere that the root of this confusion in
terminology is a misunderstanding of the equivalence principle.)


~ CT

Note...I recently posted a reply pointing out that the Principle of
Equivalence is *not* rooted in an equivalence between inertial and
gravitational mass. Rather, it comes from the equivalence of inertial
reference frames...those that are accelerating or equivalent to those in
a gravitational field. Also, the nature of inertial mass is not fully
understood and it is only postulated that inertial and gravitational
mass are the same. Experiments carried out show this to be so to within
great precision, but not infinite precision.